An environmentally sustainable development of ultrasound-assisted chemical process: the use of Fenton's reagent and ultrasonic transducers to remove methyl tertiary butyl ether from drinking wate...

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AN ENVIRONMENTALLY SUSTAINABLE DEVELOPMENT OF
ULTRASOUND-ASSISTED CHEMICAL PROCESS: THE USE OF FENTON’S
REAGENT AND ULTRASONIC TRANSDUCERS TO REMOVE METHYL
TERTIARY BUTYL ETHER FROM DRINKING WATER SUPPLIES
by

Hung-Li Chang

A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(ENVIRONMENTAL ENGINEERING)

December 2006

Copyright 2006 Hung-Li Chang

ii
Acknowledgements
I am deeply grateful to my advisor, Professor Teh Fu Yen, for his consistent
support and encouragement during my unexpected long period of study at the
University of Southern California. I would like to thank Professors Massoud M.
Pirbazari, Katherine S. Shing, Joseph S. Devinny, and Jiin-Jen Lee for being my
committee members, and for their valuable comments.
I wish to express thanks to all of my colleagues at the California Air
Resources Board and the Department of Health Services, drinking water program, for
their friendly assistance. I am indebted to Dr. Jessie Tu, Shu-Fang Orr and Virginia
Huang. They provided needed technical help on MTBE analyses.
Lastly, I sincerely express my deepest gratitude to my wife, May, for her
patience, understanding, and assistance, and to my parents, Yu-Kuang and Yu-Chen,
for their support and encouragement.

iii
Table of Contents

Acknowledgements ......................................................................................................ii
List of Tables................................................................................................................v
List of Figures .............................................................................................................vi
Abbreviations ..............................................................................................................ix
Abstract ......................................................................................................................xii
Chapter 1: Introduction ................................................................................................1
1.1 MTBE...............................................................................................................1
1.1.1 Properties.............................................................................................................................4
1.1.2 Health Risks.......................................................................................................................14
1.1.3 Treatment Technologies ....................................................................................................15
1.2 Environmental Regulations............................................................................22
1.2.1 Clean Air Act.....................................................................................................................26
1.2.2 Safe Drinking Water Act ...................................................................................................28
1.2.3 California Drinking Water Standards ................................................................................29
1.2.4 Other States and Countries ................................................................................................30
1.3 Ultrasound Assisted Oxidation Process .........................................................32
1.3.1 Directional Drilling............................................................................................................32
1.3.2 Static Inline Mixer .............................................................................................................36

Chapter 2: Theoretical Background ...........................................................................38
2.1 Ultrasound......................................................................................................38
2.1.1 Factors Affecting cavitation...............................................................................................48
2.1.2 Cavitation Mechanism .......................................................................................................54
2.1.3 Ultrasonic Transducer........................................................................................................59
2.1.4 Ultrasonic System Types ...................................................................................................70
2.2 Fenton’s Reagent............................................................................................76

Chapter 3: Investigation of Hydroxyl Radicals Generation .......................................81
3.1 Introduction ....................................................................................................81
3.2 Materials and Experiment Methods ...............................................................83
3.3 Results and Discussion...................................................................................85

iv
Chapter 4: Degradation of MTBE By Ultrasound-Assisted Chemical Process .........86
4.1 Introduction ....................................................................................................86
4.2 Materials and Experiment Methods ...............................................................91
4.2.1 Materials............................................................................................................................91
4.2.2 Experimental Method ........................................................................................................92
4.3 Results and Discussion...................................................................................94
4.3.1 Degradation of MTBE.......................................................................................................94
4.3.2 Possible Decomposition Mechanism .................................................................................96
4.3.3 Optimization of Experimental Conditions .........................................................................98
4.3.4 Reactions of Carbonate and Bicarbonate.........................................................................104
4.3.5 Field Applications............................................................................................................106

Chapter 5: Application of Hydrodynamic Cavitation Reactor in Sustainable Chemical
Process Treating MTBE in Groundwater.................................................................117
5.1 Introduction ..................................................................................................117
5.1.1 MTBE in Groundwater....................................................................................................117
5.1.2 Ultrasound-Assisted Chemical Process............................................................................121
5.1.3 Ultrasound .......................................................................................................................122
5.1.4 Hydrodynamic Cavitation Reactor...................................................................................123
5.2 Materials and Experiment Methods .............................................................133
5.2.1 Materials..........................................................................................................................133
5.2.2 Pentagonal Ultrasonic Liquid Processor..........................................................................133
5.2.3 Static In-Line Mixer ........................................................................................................135
5.2.4 Experimental Method ......................................................................................................135
5.3 Result and Discussion ..................................................................................136
5.3.1 Pentagonal Ultrasonic Liquid Processor..........................................................................136
5.3.2 In-line Mixer....................................................................................................................138

Chapter 6: Conclusion..............................................................................................142
6.1 The Use of UACP in the Treatment of MTBE.............................................142
6.2 Application of Acoustic Cavitation in Sustainable Chemical Process.........145
6.3 Recommended Future Work ........................................................................148

Bibliography.............................................................................................................149

v
List of Tables
Table 1.1: Octane Numbers, Blending Vapor Pressures, and Water
Solubility of the Most Important Oxygenates 2
Table 1.2: Possible Sources of MTBE in Groundwater 4
Table 1.3: Estimated Solubility of MTBE in Water at 55 Degrees Fahrenheit 6
Table 1.4: Overview of State MTBE Bans 25
Table 2.1: Comparative Oxidation Potentials 78
Table 4.1: Chemical and physical properties of MTBE, BTEX, and TBA. 88
Table 4.2: Maximum Use Levels for NSF Certified Drinking Water
Treatment Chemicals 100
Table 5.1: Saturated Vapor Pressure, Density for Water 125

vi
List of Figures
Figure 1.1: Chemical structure and property of MTBE. 5
Figure 1.2: Example of partitioning of MTBE and 2 gasoline hydrocarbons
between the aquifer material and water. (Source: U.S. Geological
Survey) 10
Figure 1.3: Example of difference in migration of water, MTBE, and BTEX.
(Source: National Water Quality Assessment Program, U.S.
Geological Survey) 11
Figure 1.4: MTBE’s high solubility and relative recalcitrance to
biodegradation causes rapid migration of the compound in
groundwater. (Source: U.S. EPA, 1997) 12
Figure 1.5: A MTBE-content gasoline spill could lead to MTBE seep in
groundwater. (porosity = 0.25) 13
Figure 1.6: Advanced oxidation process. 19
Figure 1.7: Field treatment of groundwater by UACP and horizontal
directional drilling. 35
Figure 1.8: Static in-line mixers. 37
Figure 2.1: Frequency ranges of sound. 39
Figure 2.2: Ultrasound examination during pregnancy. (High-frequency: 1-5
megahertz) 40
Figure 2.3: Ultrasound frequency vs. relative strength. (Source:
www.Bransoncleaning.com) 41
Figure 2.4: Cavitation erosion on a propeller. 43
Figure 2.5: Generation of an acoustic bubble. 46
Figure 2.6: The relationship among ultrasound frequency, energy per
cavitation, and number of cavitations. 52

vii
Figure 2.7: Interiors of collapsing bubbles. 56
Figure 2.8: Interfacial regions between the cavitation bubbles and bulk
solution. 57
Figure 2.9: Bulk solution where the radicals, being produced in the interior
of bubble and in the interfacial region. 58
Figure 2.10: Piezoelectric transducers in commercial application. 62
Figure 2.11: Basic equivalent circuit for a magnetostrictive transducers. 64
Figure 2.12: Cross-section through Galton’s whistle (dog whistle). 66
Figure 2.13: Cross-section through Hartmann’s whistle. (A- Cylinder, B-
Resonant cavity, Source: Gooberman, 1969) 67
Figure 2.14: Cross-section through a jet-edge system (liquid whistle). 69
Figure 2.15: Ultrasonic cleaning tank. 71
Figure 2.16: High intensity ultrasonic processor. 74
Figure 2.17: Typical laboratory settings for carrying out sonochemistry. 75
Figure 2.18: The stoichiometry of Fenton’s reagent and ultrasound. 77
Figure 3.1: ESR spectrum of DPPH. 82
Figure 3.2: UV-VIS spectra of radical scavenger (DPPH) under the
following conditions: 84
Figure 4.1: Experimental setting of the UACP. 93
Figure 4.2: MTBE degradation and its by-products. 95
Figure 4.3: Summary of the degradation pathway of MTBE (Church et al.,
1997). 97
Figure 4.4: MTBE removal rate vs. molar ratio of hydrogen peroxide to
MTBE. 102
Figure 4.5: Breakpoint chlorination curve. 107

viii
Figure 4.6: The most popular groundwater well-head treatment – only
disinfection then directly send to distribution system. 112
Figure 4.7: UACP can be easily add onto current setting as part of well-head
treatment. 113
Figure 4.8: UACP can be used to treat raw water at the start of a treatment
chain. 114
Figure 4.9: UACP can be used as a polishing step at the end of treatment
chain. 115
Figure 4.10: UACP can be added to the distribution systems or storage
reservoirs to solve DBPs problem. 116
Figure 5.1: Percentage of groundwater systems and surface water systems.
(Source: EPA Drinking water and groundwater statistics for
2004) 119
Figure 5.2: Percentage of population served by groundwater systems and
surface water systems. (Source: EPA Drinking water and
groundwater statistics for 2004) 120
Figure 5.3: Saturated vapor pressure for water. 126
Figure 5.4: A static in-line mixer. 128
Figure 5.5: Design parameters for static in-line mixers. 129
Figure 5.6: A typical relation between the cavity length and the cavitation
number. 131
Figure 5.7: The pentagonal ultrasonic liquid processor. 134
Figure5.8: MTBE degradation by UACP. 137

ix
Abbreviations
AOP : Advanced Oxidation Process
AQMD : South Coast Air Quality Management District
ARB : California Air Resources Board
AWWA : American Water Work Association
BrO
3
–
: Bromate
BTEX : Benzene, Toluene, Ethyl benzene, and Xylene
CERCLA : Comprehensive Environmental Response,
Compensation and Liability Act
CO : Carbon monoxide
CO
2
: Carbon dioxide
CO
3
2−
: Carbonate ion
Cu
+2
: Copper ion
DBP : Disinfection By-Product
DHS : California Department of Health Services
DIPE : Diisopropyl ether
DLR : Detection Limit for Purposes of Reporting
DMPO : 5,5-dimethyl-1-pyrroline N-oxide
DPPH : 2,2-diphenyl-1-picrylhydrazyl
DPPH
2
: 1,1-diphenyl-2-picrylhydrazine
e
–

(aq)
: Solvated Electron
EPA : Environmental Protection Agency
ESR : Electron Spin Resonance
ETBE : Ethyl tertiary-butyl ether
Fe or Fe(0) : Iron
Fe
+2
or Fe(II) : Ferrous iron

x
Fe
+3
or Fe(III) : Ferric iron
GAC : Granular Activated Carbon
GC/MS : Gas Chromatograph / Mass Spectrometer
H : Atomic Hydrogen
H
2
O
2
: Hydrogen peroxide
HAA5 : Haloacetic. Acids
HCO
3
−
: Bicarbonate ion
HDD : Horizontal Directional Drilling
HO
2
· : Hydroperoxyl Radical
IARC : International Agency for Research on Cancer
IDSE : Initial Distribution System Evaluation
IR : Infrared Spectroscopy
kHz : Kilohertz
KMnO
4
: Potassium permanganate
LRAA : Locational Running Annual Average
LUST : Leaking Underground Storage Tank
MCL : Maximum Contaminant Level
MDL : Method Detection Limit
mHz : Megahertz
MON : Motor Octane Number
MTBE : Methyl tertiary butyl ether
NAAQS : National Ambient Air Quality Standard
NaOCl : Sodium hypochlorite
NFPA
: National Fire Protection Association (Fire
Diamond)
NOM : Natural Organic Matter
NSF : National Sanitation Foundation

xi
O
3
: Ozone
OH⋅ : Hydroxyl Radical
OxyFuel : Oxygenated Fuel
PCE : Tetrachloroethylene
PES : Photoelectron Spectroscopy
PRB : Permeable Reactive Barrier
psig : Pounds per square inch gage pressure
RFG : Reformulated Gasoline
RON : Research Octane Number
SDWA : Safe Drinking Water Act
SMILES : Simplified Molecular Input Line Entry System
SPE : Solid Phase Extraction
SPME : Solid Phase Microextraction
TAME : Tertiary-amyl methyl ether
TBA : Tertiary butyl alcohol
TCE : Trichloroethylene
TiO
2
: Titanium dioxide
TSCA : Toxic Substances Control Act
TTHM : Total Trihalomethane
UACP : Ultrasound-Assisted Chemical Process
UST : Underground Storage Tank
UV : Ultraviolet
UV-VIS : Ultraviolet-Visible Spectroscopy
VOC : Volatile Organic Chemical

xii
Abstract

Methyl tertiary butyl ether (MTBE) is used as a fuel additive to reduce air
pollution. However, the threat to public health from MTBE contaminated
groundwater prompted the legislature to ban MTBE. Through the years, numerous
methods have been developed to treat MTBE, and many of these advanced treatment
processes are complicated and costly. The present research investigates the
development of a simple and cost-effective process for the degradation of MTBE.
Chain reactions involving ultrasound and Fenton’s reagent are the epitome of
this system. Fenton’s reagent can be continuously supplied by sonolysis of water and
naturally occurring metal salts in groundwater. The laboratory test results have
shown complete MTBE degradation under the ultrasound-assisted chemical process
(UACP) within one hour. For subsurface operations in the field, ultrasonic
transducers attached on the double-wall pipe and horizontal directional drilling
technology can greatly increase the effective area of in-situ remediation.
A whistle type in-line mixer installed in a re-circulating flow loop without
external electric power was used to create hydrodynamic cavitation and induce
oxidation in UACP. Experimental data have shown 13 percent addition to 75 percent
degradation of MTBE by Fenton’s reagent in five minutes.
The groundwater systems usually only have a water pump and a disinfection
process. Once there is a MTBE plume approaching, the only choice for them is to

xiii
shut down the production wells unless they have enough funding. Equipment wise,
UACP only needs chemical pump and ultrasonic transducer. This allows for low
cost, easy installation, convenient maintenance, and easy decommissioning.
Therefore, UACP with in-line mixer can be used as a drinking water package plant to
benefit small water systems in rural communities.
Furthermore, UACP can be placed at the start of a treatment chain to avoid
contamination breakthrough, placed at the end of a treatment chain as a polishing
step, or added to the distribution systems to solve disinfection by-product problem.

1
Chapter 1: Introduction

1.1 MTBE
Methyl tertiary-butyl ether (MTBE) is a gasoline oxygenate. While ethanol
use dominates the fuel market in certain areas, and other oxygenates (see Table 1.1)
such as ethyl tertiary-butyl ether (ETBE), tertiary-amyl methyl ether (TAME),
diisopropyl ether (DIPE), tertiary-butyl alcohol (TBA), and methanol are used,
MTBE is by far the most common and the cheapest. Use of MTBE as a gasoline
additive began in the United States since 1979 when it was introduced as an anti-
knocking agent to maintain adequate octane ratings during the phase out of alkyl lead
additives.
Since the primary producer of MTBE has successfully convinced Congress
that Reformulated Gasoline (RFG) with oxygenates can reduce air pollution to meet
Clean Air Act goals, MTBE use expanded dramatically in the mid-1990s. MTBE
was selected by some gasoline producers as a means of producing “oxygenated fuel”
(oxyfuel) that allowed the more complete combustion of gasoline hydrocarbons.
While solving the smog problem, MTBE has quietly entered into the groundwater.
Many research results (Zogorski et al., 1999) have shown that MTBE is an
environmental recalcitrant compound due to its high solubility in water.

2
Table 1.1: Octane Numbers, Blending Vapor Pressures, and Water Solubility of
the Most Important Oxygenates

Research
octane number
(RON)
Motor octane
number
(MON)
Blending
vapor pressure,
bar
Water solubility,
wt-%
methanol
133 99 5.2 miscible
ethanol
130 96 1.5 miscible
TBA
105 95 1.0 miscible
MTBE
117 101 0.55 4.3
ETBE
118 101 0.28 1.2
TAME
112 98 0.14 2.0

Source: U.S. EPA, 1997

3
MTBE is considered a human carcinogen defined by the U.S. EPA. The
major health concern beside its chronicle effect, MTBE has a distinctive taste and
odor like ether that can be smelled at relatively low concentration. In general, MTBE
may be detected in odor form at levels as low as 15 ppb, but the actual range of
detection may vary widely depending on an individual’s sense of smell and on the
purity and temperature of the water. (U.S. EPA, 1997)
MTBE can get into groundwater from point source such as leaking
underground storage tank (LUST) or non-point sources such as vehicles, leaking
underground fuel storage tank and leaking petroleum pipeline, as shown in Table 1.2.
MTBE has also been found to get into surface water like lakes and reservoirs from
the use of motorized watercraft with two-stroke engine. It may also travel into these
waters in polluted runoff, or it may be deposited from the atmosphere by rainwater or
other pathways (Richard, 2000). The case of using MTBE is a bad example for
sustainable development that environmental experts always emphasized.

4
Table 1.2: Possible Sources of MTBE in Groundwater
Point Sources Non-point Sources

Refineries
Pipelines
Storage Tanks
Accidental Spillage
Homeowner Disposal
Emissions during fueling

Vehicle emissions
Vehicle evaporative
losses
Atmospheric deposition
Urban runoff
Recreation watercraft

1.1.1 Properties
The physical and chemical properties of MTBE govern its fate and transport
in the subsurface environment. MTBE is an aliphatic ether with the structural
formula CH
3
OC(CH
3
)
3
, as shown in Figure 1.1. It has a molecular weight of 88.15
g/mole. The Henry’s law constant from MTBE at 25°C is 0.0197 (mole fraction
vapor phase)/(mole fraction liquid phase). MTBE is miscible in gasoline, alcohol,
and other ethers. It has a specific gravity of 0.74 (20°C) and as a result, will float on
water. Its pure phase aqueous solubility ranges from 42,000 - 54,000 mg/L.

5

Figure 1.1: Chemical structure and property of MTBE.

6
Table 1.3: Estimated Solubility of MTBE in Water at 55 Degrees Fahrenheit
MTBE Solubility (mg/L)
Pure MTBE 51,200
15% v/v MTBE in gasoline (oxygenated gasoline) 7,700
10% v/v MTBE in gasoline (reformulated gasoline) 5,100
1% v/v MTBE in gasoline (octane enhanced gasoline) 510
Source: U.S. EPA, 1997

MTBE has lower solubility in water when other petroleum hydrocarbons are
present, as shown in Table 1.3. Therefore, dissolved MTBE does not partition
greatly to air from water. MTBE has lower K
ow
than most chlorinated compounds,
such as TCE, PCE, and PCB. It means that MTBE is not likely to accumulate to a
significant degree in human or animal fat tissue. (U.S. EPA, 1998)
MTBE is made by combining methanol and isobutylene. Isobutylene is a
petroleum refining by-product, making it an inexpensive and easy to obtain
component. Methanol is a derivative of natural gas. As an octane enhancer, MTBE
is used at concentrations up to 8% by volume. When used at higher levels (11% to
15% by volume) it is considered an oxygenate and meets the requirements of the
Oxyfuel and RFG Programs.
MTBE has a boiling point of 55 - 56 °C and forms azeotropes with water
(52.6 °C) and methanol (51.3 °C). An azeotrope is a mixture of two or more

7
compounds. The ratio of the compounds in an azeotrope is exactly the same in both
the vapor form of the mixture and liquid phase. Due to the uniformity of liquid and
vapor, chemical composition of the azeotrope cannot be changed by simple boiling
or distillation. In order to break the azeotrope, the following techniques may be
used:
1. Use azeotropic distillation, by adding in a third component which will
generate a lower boiling heterogenous azeotrope that can be broken by
phase separating the two immiscible liquids and decanting.
2. Use extractive distillation where a third compound is added near the top of
a distillation column that reduces the volatility of one compound over
another. For example, if you wanted to separate water from a type of
alcohol by boiling, but they were at their azeotropic mixture, if you added
salt to the mixture that would raise the boiling point of the water so
alcohol could be boiled out of it.
3. Use liquid-liquid extraction to separate the compounds by segregating
them into two different liquid phases.
4. Use a membrane to separate them.
5. operate two distillation columns at different pressures, and recycle material
between these two distillation columns. One column will remove a pure
component in the bottom of one distillation column, the other will remove
a pure component in the bottom of a second distillation column.

8
Being an ether, MTBE is a Lewis base. The tert-butyl group is easily cleaved
off under strongly acidic conditions and high temperature.
MTBE can get into groundwater through leaking underground and above
ground fuel storage tanks, pipelines, refueling spills, automobile accidents damaging
the fuel tank, consumer disposal of gasoline, emissions from two-stroke marine
engines, storm water runoff, and even precipitation mixed with MTBE in the air.
MTBE and other gasoline components such as benzene, toluene, ethyl
benzene and xylenes (BTEX) have the potential to threaten local public water supply
wells when they leak from the underground storage tanks (UST). MTBE does not
significantly partition into soil matrixes compared to the other petroleum
hydrocarbons, as shown in Figure 1.2. Once it leaks into soil, then groundwater,
MTBE moves much faster than other petroleum hydrocarbons, as shown in Figure
1.3. This problem is becoming more series. Many water agencies are experiencing
drastic decreases in the water supply they can deliver to their customers because they
are forced to shut down wells due to MTBE contamination.
MTBE contains a tertiary carbon bond which microbes can not break it.
Thus, like the chlorinated solvent compounds trichloroethylene (TCE) and
tetrachloroethylene (PCE), MTBE has very slow biodegradation rate and it has been
found to persist in groundwater. Therefore, cases of MTBE plumes extending long
distances in the groundwater basin have been documented nationwide. Some
monitoring wells have shown little overall reduction in MTBE concentration over

9
several years which also suggests that MTBE is relatively persistent in ground water.
In contrast, studies of surface water (lakes and reservoirs) have shown that MTBE
volatilizes relatively quickly.
In California, MTBE comprises 11% of Reformulated Gasoline, and has been
observed in groundwater at Port Hueneme (See Figure 1.4), Lake Tahoe, and Santa
Monica areas. In 1996, two of drinking water production wells in the city of Santa
Monica were contaminated with MTBE at levels as high as 610 ppb. These two
wells, representing 50 percent of the city’s drinking water supply, were shut down
and the city began purchasing replacement water. This incident was the first major
water contamination which brought public attention to MTBE.
Some MTBE plumes have originated from very small spills, such as gasoline
spill during a car accident. Ten gallons of gasoline may contain up to 3 kg of MTBE.
If such an amount were to reach the groundwater table, it could lead to the
contamination of groundwater up to 50 μg/L level, as shown in Figure 1.5. A real
case in Maine, the gasoline spill of a single turned-over automobile led to MTBE
transport through groundwater and to the contamination of more than 20 domestic
wells.

10

MTBE Benzene Ethylbenzene
MTBE Benzene Ethylbenzene
solid phase
dissolved phase
1 liter of aquifer = 2.0 kg sand + 0.25 kg water

Figure 1.2: Example of partitioning of MTBE and 2 gasoline hydrocarbons between
the aquifer material and water. (Source: U.S. Geological Survey)

11

2.0
Elapsed
time
since
release
Travel distance (in miles)
0
3.3 years
Water
MTBE
Benzene
Ethylbenzene
10 years
30 years
1.0 1.5 0.5 1.25 0.25 0.75 1.75
continuous point source

Figure 1.3: Example of difference in migration of water, MTBE, and BTEX.
(Source: National Water Quality Assessment Program, U.S. Geological
Survey)

12

Figure 1.4: MTBE’s high solubility and relative recalcitrance to biodegradation
causes rapid migration of the compound in groundwater. (Source: U.S.
EPA, 1997)

13

Figure 1.5: A MTBE-content gasoline spill could lead to MTBE seep in
groundwater. (porosity = 0.25)
GROUNDWATER
AQUIFER
1/10 MILE
1/10 MILE
30 FEET

14
1.1.2 Health Risks
MTBE is not classified as a human carcinogen in low exposure levels by the
International Agency for Research on Cancer (IARC). However, it has been shown
to cause kidney lesions in animals. As an ether, MTBE acts as an emulsifier,
increasing the solubility of other harmful components of gasoline, such as the known
carcinogen benzene. Therefore, it may increase the risk of contamination by other
compounds.
The majority of the human health-related research conducted to date on
MTBE has focused on effects associated with the inhalation of the chemical. When
research animals inhaled high concentrations of MTBE, some developed cancers or
experienced other non-cancerous health effects. To date, independent expert review
groups who have assessed MTBE inhalation health risks have not concluded that the
use of MTBE-oxygenated gasoline poses an imminent threat to public health.
However, researchers have limited data about what the health effects may be if a
person swallows or ingests MTBE.
EPA has concluded that available data are not adequate to estimate potential
health risks of MTBE at low exposure levels in drinking water but that the data
support the conclusion that MTBE is a potential human carcinogen at high doses.
MTBE often ends up in drinking water when fuel storage tanks leak near populated
areas. Aside from health risks, MTBE negatively affects the taste and odor of
drinking water, even at very low concentrations.

15
Under the federal Superfund law - Comprehensive Environmental Response,
Compensation and Liability Act (CERCLA), MTBE has been listed as a hazardous
substance. It is also considered a potential human carcinogen by the U.S. EPA. As
with any chemical, MTBE has exposure risks. After the introduction of gasoline
containing MTBE for oxygenation, consumers of MTBE in water reported
experiencing adverse health symptoms, such as headaches, nausea, dizziness and
breathing difficulties.
Although MTBE is not an immediate health threat, it is too noxious to drink.
MTBE tastes and smells like paint thinner. In addition, this pollutant worries water
agencies because it is scattered throughout the environment. There are many
lawsuits against petroleum companies that manufacture and distribute MTBE.

1.1.3 Treatment Technologies
In the past few years, many environmental remediation techniques have been
develop to remove MTBE from our groundwater. Unlike many other petroleum
hydrocarbons, MTBE is difficult to remove from water. There are generally three
available technologies that have been shown to be effective in removing MTBE from
drinking water. They are aeration, adsorption using activated carbon, and oxidation.
If the concentration of the contaminants is high, a treatment chain is often
installed. The first device is used to remove the heavy contaminant load while the
second provides a polishing step to assure full removal of the contaminants and to

16
prevent breakthrough problem. Aeration is often the first method used while
activated carbon is often used as the polishing step. If contaminants are present in a
pure product state in the well, a recovery method is also necessary. This will reduce
the size of the water treatment equipment needed.
Activated carbon has enormous surface area within each granule. One pound
of activated carbon has a surface area greater than the size of a football field.
Activated carbon is a material that attracts many types of organic contaminants to its
surface. Once the removal capacity of the carbon is used up, then it may be returned
to the manufacturer for regeneration or can be disposed. Activated carbon can also
foster the growth of bacteria by concentrating other organics on its surface. A final
concern with activated carbon is the possible release of contaminants after they have
been initially adsorbed. This action is known as desorption or dumping. This could
occur to MTBE, because MTBE has high solubility in water and lower affinity to
carbon compared to other organic contaminants.
To address breakthrough and desorption, the overall amount of activated
carbon could be divided into two treatment tanks and the two devices installed in
series. In such an arrangement any breakthrough from the first unit can be adsorbed
by the newer carbon in the second unit. The advantage of activated carbon treatment
compared to other methods is that the water is less likely to become contaminated by
dust and other airborne contaminants. The disadvantage is that carbon attracts
organic matter from the water and thus typically supports an elevated level of

17
bacteria on its surface. In addition, periodic laboratory testing should be done of
both the raw and finished water to determine treatment effectiveness. The frequency
of this monitoring would be determined based on variability and duration of the past
sampling record and other site specific conditions. Where activated carbon is used,
the carbon will lose its removal capacity and will need to be replaced in time. A
monitoring program will be needed to predict the expected longevity of each new
carbon recharge.
Aeration treatment consists of passing large amounts of air through the
contaminated water. The efficiency of the device is improved by breaking up the
bulk of the water into many small droplets. The goal is to allow the contaminants to
volatilize into the air followed by off-gas treatment. When aeration is used, two
operational problems are possible:
1. If there are elevated levels of iron or manganese in the water, rusty precipitate
staining of fixtures and clothing is likely. Iron/manganese pretreatment may
be necessary.
2. Bacterial slime may grow in aerators requiring continuous or periodic
chlorination. The advantage of aeration is that there is no disposal or
regeneration of the treatment media necessary.
Advanced oxidation process (AOP) use appropriate combinations of
catalysts, ultraviolet light, ozone, hydrogen peroxide, and other chemical oxidants to
transform contaminants, as shown in Figure 1.6. Oxidizing chemicals could include

18
potassium permanganate, (KMnO
4
), hydrogen peroxide (H
2
O
2
), ozone (O
3
) and
sodium hypochlorite (NaOCl). AOPs can have multiple uses in water treatment.
Examples include oxidation of synthetic organic chemicals, color, taste-and-odor-
causing compounds, sulfide, iron, and manganese and destruction of DBP precursors
prior to the addition of chlorine.
Certain organic contaminants will chemically react with oxygen and oxygen-
like compounds. After the oxidation treatment, the treatment by-products may have
a lower level of hazard and further treatment may still be necessary. For example,
the use of ultraviolet radiation to break down MTBE could then be followed by either
hydrogen peroxide or ozone to oxidize the by-products of the UV breakdown.
In AOPs, hydroxyl radicals can be produced by the following reactions:
• Ionizing radiation on water
• Hydrogen peroxide with ozone (H
2
O
2
/O
3
)
• Fenton reagent (H
2
O
2
and Fe
+2
)
• Fenton-like reagent (H
2
O
2
and Fe(0) or Fe
+3
)
• Direct photolysis of H
2
O
2
by UV light (H
2
O
2
/UV)
• Titanium dioxide (TiO
2
) and ozone
• Ozone with ultraviolet radiation (UV/O
3
)
• UV/H
2
O
2
/O
3

19

OH·
Photocatalytic
Redox Processes
H
2
O
2
/O
3
/UV
H
2
O
2
/UV
Chemical Oxidation
O
3
/UV
Non-thermal
Plasmas
Electron Beams
γ - rays
Supercritical
Water Oxidation
Electrohydraulic
Cavitation & Sonolysis

Figure 1.6: Advanced oxidation process.

20
When hydrogen peroxide (H
2
O
2
) is added to ozonated water, it reacts with the
molecular ozone, which accelerates the formation of hydroxyl radicals. Therefore, in
an ozone-H
2
O
2
process the goal is to increase the concentration of hydroxyl radicals,
which is a stronger oxidizer than molecular ozone, and consequently rapidly reduce
the concentration of molecular ozone. Therefore, hydrogen peroxide is added to an
ozone process if it is used as an oxidation process but not as a disinfection process,
which relies on the prevalence of a high concentration of molecular ozone.
The ozone-H
2
O
2
process is used for the destruction of taste-and-odor causing
compounds, color removal, and destruction of micropollutants, such as volatile
organic compounds, pesticides, and herbicides. Stoichiometric analysis suggests that
the optimum H
2
O
2
-to-ozone ratio is approximately 0.3:1 (mg/mg). However, pilot-
and full-scale studies have shown that the optimum ratio is more on the order of
0.5:1 to 0.6:1 mg/mg (Liang, 1999).
Currently, the conventional design of an ozone-H
2
O
2
treatment process is one
in which hydrogen peroxide is fed as a liquid to the influent water and an ozone-rich
gas is fed through fine-bubble diffusers at the bottom of a contactor.
Considering the complexity of the reaction chemistry between ozone,
hydrogen peroxide, natural organic matter, and other water constituents, it is not clear
whether such a conventional design is the optimum design for an ozone-H
2
O
2

treatment system. Innovations in engineering design may be able to improve the
efficiency of the process at lower ozone and hydrogen peroxide doses.

21
In the presence of UV light, hydrogen peroxide decomposes to form hydroxyl
radicals. Addition of hydrogen peroxide to the influent of a UV irradiation process is
currently being used for the destruction of micropollutants from groundwater, but it
can also be used for the same purposes as other AOPs, which include the destruction
of taste-and-odor-causing compounds and the removal of color. The reaction
between UV and hydrogen peroxide to form hydroxyl radicals is substantially slower
than that between ozone and hydrogen peroxide. However, in many groundwater
remediation efforts, the simplicity of a UV irradiation system has been favored over
the complexity of an ozone generation and feed system.
Due to the slow hydroxyl-radical formation reaction in UV-H
2
O
2
systems, the
process must be operated with an excess of high concentration of hydrogen peroxide
(5 to 20 mg/L hydrogen peroxide residual). Therefore, for this process to be used in
drinking water treatment, either the process should be modified to utilize less
hydrogen peroxide or a treatment process should be installed downstream to quench
the hydrogen peroxide residual to acceptable levels (<0.5 mg/L) before the water is
put into the distribution system. The various options available for quenching the
hydrogen peroxide residual include chlorine, thiosulfate, sulfite, or granular-activated
carbon.

22
1.2 Environmental Regulations
Exposure to MTBE from contaminated drinking water has been the focus of
most of the regulatory activity to date. MTBE is a chemical compound that is
manufactured by the chemical reaction of methanol and isobutylene. In mid-90’s,
MTBE is produced in very large quantities and is almost exclusively used as a fuel
component in gasoline. It is one of a group of chemicals commonly known as
oxygenates because they raise the oxygen content of gasoline. MTBE is a volatile,
flammable and colorless liquid that is relatively soluble in water. When in
groundwater MTBE moves more quickly than other fuel components. MTBE has a
typical odor reminiscent of diethyl ether, leading to unpleasant taste and odor in
water. MTBE is used in organic chemistry as a cheap solvent with properties
comparable to diethyl ether but with a higher boiling point and lower solubility in
water. It is also used medically to dissolve gallstones.
MTBE has been used in U.S. gasoline at low levels since 1979 to replace
tetra-ethyl lead to increase its octane rating and help prevent engine knocking. Since
1992, MTBE has been used at higher concentrations in some gasoline to fulfill the
oxygenate requirements set by Congress in Clean Air Act amendments; however,
since 1999, in California and other locations MTBE has begun to be phased out
because of groundwater contamination, citing unproven health effects. The Energy
Policy Act of 2005 drops the federal requirement for oxygen content in reformulated
gasoline. (California Air Resources Board, 2004)

23
The clean-up of all MTBE in the U.S. in the form of groundwater and soil
contamination is estimated to cost as much as $140 billion, including removing the
compound from aquifers and municipal water supplies and replacing leaky
underground oil tanks. Recent state laws have been passed to ban MTBE in certain
areas, as shown in Table 1.4.
California and New York, which together accounted for 40% of U.S. MTBE
consumption, banned the chemical starting January 1, 2004, and as of September,
2005, twenty five states had signed legislation banning MTBE.
In 1995 high levels of MTBE were unexpectedly discovered in the water
wells of Santa Monica, California, and the U.S. Geological Survey reported
detections Subsequent U.S. findings indicate tens of thousands of contaminated sites
in water wells distributed across the country. The California law actually defines a
“non-leaky” tank as one that leaks less than five gallons a day, in contrast to
European regulations. As per toxicity alone, MTBE is not classified as an hazard for
the environment.
EPA had a blue-ribbon panel of leading environmental experts to study the
public health issues raised by the discovery of MTBE in groundwater and drinking
water supplies. The Agency has concluded that most of this contamination is from
leaking gasoline storage tanks. In March 1998, EPA placed MTBE on its
contaminant candidate list, which is used to set Agency priorities for possible
regulatory action. So far, MTBE has not been identified as regulated constituent

24
under EPA’s underground storage tank program, but such a listing remains a real
possibility.
Although the extent of MTBE as a health risk is debatable, a transparent
disadvantage is that MTBE ruins the taste of water even at low concentrations. The
leakage problem is partially attributed to the lack of effective regulations for
underground storage tanks, but spillage from overfilling remain important upset
scenario. The EU does not predict the same leakage problem in Europe. The MTBE
concentrations used in Europe are lower (usually 1.0–1.6%).
Advocates of both sides of the debate in the United States sometimes claim
that gasoline manufacturers have been forced to add MTBE to gasoline by law.
However, this is incorrect, since any oxygenate would fulfill the law. In 2003,
California was the first state to start replacing the MTBE with ethanol. Several other
states started switching soon afterward.
Higher quality gasoline is also an alternative. Iso-octane itself is used.
MTBE plants can be retrofitted to produce iso-octane from isobutylene, which is a
lighter-than gasoline hydrocarbon and thus more difficult to sell. Iso-octane is the
ideal gasoline, being the standard reference for the octane rating.
In the long run, diesel fuel is also an alternative, although it requires a major
switchover to diesel-run cars. There are several varieties of biodiesel; both oxygen-
containing methyl ethers and no-oxygen alkyl biodiesels are available.

25
Table 1.4: Overview of State MTBE Bans
State MTBE Ban Schedule
California MTBE ban starting January 1, 2004
Colorado MTBE ban started April 30, 2002
Connecticut MTBE ban starting October 1, 2003
Illinois MTBE prohibited by July 2004
Indiana MTBE limited to 0.5% by volume, starting July 23, 2004
Iowa 0.5% MTBE by volume cap, already in effect
Kansas MTBE limited to 0.5% by volume, starting July 1, 2004
Kentucky MTBE ban starting January 1, 2006; beginning in January 1, 2004,
ethanol encouraged to be used in place of MTBE
Maine The goal is to phase out gasoline or fuel products treated with
MTBE by January 1, 2003
Michigan MTBE prohibited by June 1, 2003
Minnesota All ethers (MTBE, ETBE, TAME) limited to 1/3 of 1.0% by
weight after July 1, 2000; after July 1, 2005, total ether ban
Missouri MTBE limited to 0.5% by volume, starting July 1, 2005
Nebraska MTBE limited to 1.0% by volume, starting July 13, 2000
New York MTBE ban starting January 1, 2004
Ohio MTBE ban starting July 1, 2005
South Dakota 0.5% MTBE by volume cap, already in effect
Washington MTBE ban starting December 31, 2003

26
1.2.1 Clean Air Act
The Clean Air Act Amendments passed by Congress in 1990 mandated the
use of oxygenated gasoline in certain geographical areas not currently meeting the
National Ambient Air Quality Standard (NAAQS) for carbon monoxide. The main
source of carbon monoxide emissions is the combustion engine. While many types
of these engines are used in off-road equipment such as lawnmowers, chainsaws and
other gasoline powered equipment, the primary source of ambient carbon monoxide
in most areas is motor vehicles.
Designed to increase combustion efficiency, oxygenated gasoline was viewed
by the government as a practical way to help reduce carbon monoxide emissions.
Two programs have been implemented to effectuate the goals under the Clean Air
Act Amendments. The Oxygenated Fuel Program (Oxyfuel Program) is in effect
during fall and winter months in certain urban areas. The Reformulated Gasoline
Program (RFG Program) requires the use of oxygenated fuel on a year-round basis in
the most polluted urban areas. The Oxyfuel Program requires a minimum oxygen
content of 2.7% and the RFG Program requires 2% oxygen content in non-attainment
areas. The two most common ways of boosting oxygen levels to the required
Oxyfuel levels are to add either 15% MTBE or 7.5% ethanol to gasoline. Currently,
the South Coast Air Quality Management District (AQMD) is one of the few places
in California with violations of the federal ambient air quality standard for carbon
monoxide (CO). When a district is federal non-attainment for CO, the federal Clean

27
Air Act requires the gasoline to be oxygenated for the time of the year when
violations are expected. Currently, this time period is October through February.
There is debate as to whether and what extent the use of oxygenated fuels has
reduced carbon monoxide levels. Although there has been a decline in urban
concentrations of carbon monoxide over the past 20 years, credit for this reduction is
generally attributed to stricter vehicle emissions standards and improved emission
control technology. Cars are built to run cleaner than in the past. Nonetheless,
studies reported by the National Science and Technology Council’s Committee on
Environment and Natural Resources indicate that there is some level of carbon
monoxide reduction associated with the oxygenated gasoline program. EPA
literature further supports the claims of carbon monoxide reduction from the use of
oxygenated gasoline. EPA notes that a 10% reduction was found in several cities
implementing the winter oxygenated gasoline program. Studies show that use of
oxygenates has also reduced total hydrocarbon exhaust emissions. Moreover, the
oxygenated fuel program showed more significant emissions reductions on older,
less technologically advanced vehicles. Despite this research, there are still questions
regarding the net environmental benefit of using oxygenates, and in particular, of
using MTBE.
In 1979 ARCO began to use MTBE instead of alcohol to blend its gasoline.
After EPA approvals in 1981 and 1988, MTBE could be used in higher quantities, up
to 15% by volume. At this level MTBE is considered a blending component rather

28
than an additive. In addition to enhancing the octane content, MTBE serves to
increase oxygen content, which has lead to its use to comply with emission
requirements under the Clean Air Act.
The Energy Policy Act of 2005, passed in the House on April 21, 2005, did
not include a provision for shielding MTBE manufacturers from water contamination
lawsuits. This bill also includes a provision that gives MTBE makers, including
some major oil companies, $2 billion in transition assistance as MTBE is phased out
over the next nine years. Due to opposition in the Senate, the conference report
dropped all MTBE provisions. The final bill was passed by both houses and signed
into law by President Bush. The lack of MTBE liability protection is resulting in a
switchover to the use of ethanol as a gasoline additive, which is in limited supply in
April 2006. Some traders and consumer advocates are blaming this for an increase in
gasoline prices.

1.2.2 Safe Drinking Water Act
The U.S. Environmental Protection Agency (EPA) has not set a formal health-based
drinking water standard for MTBE in the federal Safe Drinking Water Act (SDWA).
EPA reviewed available health effects information on MTBE in its 1997 Drinking
Water Advisory guidance and decided that there was insufficient information
available to allow EPA to establish quantitative estimates for health effects and as
such would not set health advisory limits. The EPA’s public water supply program

29
has made a nonhealth-based recommendation to limit MTBE in drinking water to
20-40 μg/L. This recommendation is based on preventing taste and odor complaints.
MTBE has a very low odor threshold at 20 μg/L, while the threshold for taste is
40 μg/L.
EPA is continuing to study both the potential health effects and the
occurrence of MTBE, and it is on the Contaminant Candidate List. As a means of
gathering occurrence information, beginning in 2001, EPA requires all large drinking
water systems and a representative sample of small systems to monitor and report the
presence of MTBE.
On March 20, 2000, EPA announced the beginning of regulatory action under
the Toxic Substances Control Act (TSCA) to significantly reduce or eliminate use of
MTBE in gasoline while preserving clean air benefits.

1.2.3 California Drinking Water Standards
California has the most stringent environment regulations among the other
states. The U.S. EPA has issued a drinking water advisory of 20 to 40 parts per
billion MTBE that protects the consumer from unpleasant taste and odor and from
potential health effects. The California Department of Health Services (DHS) has
established a primary maximum contaminant level (MCL) of 13 ppb for MTBE. The
MCL addresses concerns about the potential for cancer based on the carcinogenic
effects of MTBE observed in laboratory animals. In the development of a

30
primary MCL for a contaminant, DHS has considered the public health risks,
technical feasibility and costs associated with compliance. California’s secondary
MCL for MTBE is 5 ppb. The purpose of the secondary MCL is to protect the public
from exposure to MTBE in drinking water at levels that can be smelled or tasted.
On March 25, 1999, California Governor Gray Davis has declared that MTBE
presents a threat to public health and ordered the California Environmental
Protection Agency and Air Resources Board to devise and carry out a plan to begin
immediate phase out of MTBE from California gasoline, with 100% removal
achieved no later than December 31, 2003.
After MTBE was phased out in California, other alternative oxygenates will
be used in blending. Actually, Oxygenate is not really necessary for blending in
gasoline to meet air quality requirement. For example, the Chevron refinery has
successfully made some clean-burning gasoline without adding any oxygenate. At
the same time, there are many alternative candidates on market which are Ethyl tert-
Butyl Ether (ETBE), Tert-Amyl Methyl Ether (TAME), Diisopropyl Ether (DIPE),
Tert-Butyl Alcohol (TBA), Methanol, and Ethanol. (U.S. EPA, 1998)

1.2.4 Other States and Countries
In Europe, other compounds are also available as oxygenate additives for
gasoline, such as ethanol and TAME. Reasons for using MTBE include economical
considerations, as some of the production is obtained by adding methanol to

31
isobutylene produced as a by-product of other processes. Ethanol has been
advertised as a safe alternative by the agricultural interest groups in the USA and
Europe. Its lack of toxicity is not different from MTBE, but as a polar solvent, it
drives off nonpolar hydrocarbons from the gasoline, a problem that MTBE does not
cause. Volatile hydrocarbons from gasoline are known carcinogens and the main
contributor to photochemical smog. EU’s agricultural subsidies have produced an
oversupply of wine, and the excess low-quality wine is being refined to ethanol fuel
in Europe. This gives rise to political motives for supporting ethanol over MTBE.
However, the political stability of the supply is a major advantage for ethanol and
other biofuels.
Currently, there is no plan exist to eliminate the use of MTBE in Europe,
Asia, and many Third World countries. Instead, Europe had reduced aromatics in
gasoline from 45 to 35% by volume in 2005, which could even increase MTBE
consumption. The groundwater contamination situation is better in Europe because
double-walled storage tanks are common.

32
1.3 Ultrasound Assisted Oxidation Process
Ultrasound Assisted Chemical Process (UACP) uses ultrasonic irradiation
initiating free radical reactions at the cavitation centers to get synergistic effect.
Fenton’s reagent (H
2
O
2
+ Fe
2+
) also produces hydroxyl free radicals that non-
selectively destroy most organic matters. Laboratory scale experiments were
conducted to determine the effectiveness of UACP degradation of MTBE.
Beginning with an initial concentration of 100 mg/L of MTBE, the degradation
efficiency was 78% over the first one minute or even sooner when Fenton’s reagent
was added, and close complete MTBE removal was achieved in one hour of
sonication. The experimental results also showed that the degradation by-products
including tertiary butyl alcohol (TBA), acetone, and acetic acid could slow down the
reaction. However, once copper ion is introduced into the system, these radical
competitors will form organocopper intermediate and precipitate out.

1.3.1 Directional Drilling
Horizontal Directional Drilling (HDD) is conventionally defined as a
procedure for drilling a nonvertical hole through the earth. It was originally
developed for sidetracking a fish, drilling kill wells, correcting crooked-hole
problems, and later preventing the well from crossing lease lines, but now the HDD
has many more applications including reduced well spacing, in situ oil shale
retorting, coal gasification, in situ leaching in the mineral industry, heating tar sands,

33
enhanced oil recovery (EOR) and microbial enhanced oil recovery (MEOR). (Short,
1993)
Recently, HDD has transitioned from a recovery technology in oil industry to
a primary tool for the cleanup of contaminated sites. It has been successfully applied
on soil vapor extraction, groundwater extraction, soil venting, biosparging, air
sparging, ultrasound cleanup technologies, and site stabilization projects.
Technological advantages can lead to faster, more efficient cleanup.
Logistical advantages relate to the ability to reach previously inaccessible areas, or
being able to conduct cleanup activities without disrupting the site. Operating
advantages mostly derive from the efficiency of horizontal wells in delivering
treatment technologies directly to their intended targets. Unlike vertical wells, which
have a limited radius of influence, horizontal wells can intersect an entire
contaminant plume at right angles, creating a total treatment barrier against
migration. In fact, most contamination is relatively shallow and spreads horizontally
through heterogeneous soil pathways that are usually horizontal. Vertical well
technology is optimized to go deep, and in doing so minimizes contact with the
contaminated zone. Horizontal wells can be designed to maximize contact, with up
to hundreds of linear feet of treatment available from a single riser pipe. (Joshi,
1991)
A single horizontal extraction well can eliminate up to a dozen pumps from a
treatment system, compared to a vertical well network. HDD techniques appear

34
similar to those used in conventional vertical drilling. Both processes use boring
tools and drilling mud to install casing and well screen. (Joshi, 1991)
Most subsurface in situ remediation technologies may be installed using HDD
(Kobus, 1996). In-situ ultrasound treatment technique has been used for the
enhancement of oil recovery. For subsurface operations in the field, a robotic self-
powered mining head containing in excess of 200 ultrasonic transducers can be
attached on the inner wall of the double-wall pipe. The drilling bit combines
mechanical, hydraulic, and pneumatic systems to move through the contaminated
zone, as shown in Figure 1.7. In addition, this new technology of HDD can greatly
increase the effective area of the chemical-assisted ultrasound treatment. Many soil
microorganisms possess enzymes that catalyze the destruction of hydrogen peroxide
by converting it to oxygen. Therefore, UACP can also enhance in-situ
bioremediation. (Watts, 1993)

35
H
2
O
2
Ultrasound Transducers
Horizontal directional drilling
)))
)))
)))
)))
)))
)))
)))
)))
)))
)))

Figure 1.7: Field treatment of groundwater by UACP and horizontal directional
drilling.

36
1.3.2 Static Inline Mixer
Static inline mixer can be used as a mechanical ultrasonic transducer, because
it can convert the kinetic energy of a high-velocity liquid stream into intense mixing.
This conversion is created by a physical phenomenon known as “jet-edge tone.” It is
essentially a passive device, in which a stream of process fluid is forced through an
elliptical orifice. From there it passes over a blade-like obstacle as shown in Figure
1.8. Between the orifice and blade, the jet of liquid sheds vortices perpendicular to
the original flow vector. The shedding pattern creates a steady oscillation in the
sonic range within the liquid. The stress from these oscillations forces the liquid to
cavitate in the ultrasonic frequency range. The high level of cavitation, shear, and
turbulence within the mixing chamber shatters product particles and emulsifies
liquids. A liquid transducer can use either a simple “edge tone” principle or a more
complicated valve oscillator to cavitate liquids. (Suslick, 1988)
The use of inline mixer as part of UACP is to benefit small water systems
where electricity is not needed to run the ultrasonic transducer. Diesel engine or
natural gas engine can be used to pump the water until the flowrate reaches a certain
level, cavitation bubles will be formed inside the pipe. This particular pipe is made
out of stainless and anti-corrosion material. The inner design is similar to static
mixer and whistle. Liquid transducers can be installed in the inner wall of the
double-wall pipe. This add-on type equipment will benefit small water treatment
plant in remote area, because it is less expensive and easy to operate.

37

Figure 1.8: Static in-line mixers.

38
Chapter 2: Theoretical Background

2.1 Ultrasound
Ultrasound is sound with a frequency beyond human hearing that is above 18
kilohertz (kHz), as shown in Figure 2.1. Some animals, such as dogs, dolphins, bats,
and mice have an upper frequency limit that is greater than that of the human ear. As
a result, the dog whistle is used to call to a dog, and ultrasound can be used to
frighten away rodents and insects. Some children can hear high-pitched sounds that
older adults cannot hear, as in humans the upper limit pitch of hearing gets lower
with age. Therefore, there is a high-pitch cell phone ring tone available for teenagers
to download which parents and teachers cannot hear it. This ultrasonic hearing effect
was first discovered by divers exposed to a high-frequency sonar signal.
Ultrasound has widely applications in medical imaging. It can be used to
visualize muscles, tendons, internal organs, and a fetus during pregnancy at a high
frequency of 1 to 5 megahertz, as shown in Figure 2.2. Low-intensity, high
frequency ultrasound does not alter the state of the medium through which it travels
and is commonly used for nondestructive evaluation and medical diagnosis, as shown
in Figure 2.3. However, a study on rodent fetus brains that are exposed to ultrasound
showed signs of damage.

39

Figure 2.1: Frequency ranges of sound.

40

Figure 2.2: Ultrasound examination during pregnancy. (High-frequency: 1-5
megahertz)

41

Figure 2.3: Ultrasound frequency vs. relative strength. (Source:
www.Bransoncleaning.com)

42
As for other medical uses, more powerful ultrasound sources may be used to
clean teeth in dental hygiene tooth, breakup kidney stones, and generate local heating
for occupational therapy, physical therapy and cancer treatment.
Ultrasonic cleaners are used at frequencies from 20-40 kHz for jewelry,
lenses, watches, dental instruments, surgical instruments and industrial parts. In
industry, ultrasound is important for emulsifying cosmetics and foods, welding
plastics, and large-scale cleaning.
In late 1800s, researchers discovered that the surface erosion from the ship’s
propeller was due to the enormous turbulence, heat, and pressure produced when the
cavitation bubbles imploded on the propeller surface, as shown in Figure 2.4.
Cavitation not only occurs during turbulent flow, but also under ultrasonic irradiation
(Suslick, 1988). High power ultrasound, both above and below the cavitation
threshold, can be utilized to enhance chemical reactions in order to promote a faster
reaction time or to alter the molecular structure of a product. Ultrasound transducers
operating below 100 kHz are usually employed for this purpose. In addition,
ultrasound can produces both oxidizing (OH⋅) and reducing radicals (H⋅) under
certain conditions.

43

Figure 2.4: Cavitation erosion on a propeller.

44
Ultrasonic irradiation differs from traditional energy sources, such as heat,
light, or ionizing radiation, in duration, pressure, and energy per molecule. Because
of the immense temperatures and pressures and the extraordinary heating and cooling
rates generated by cavitation bubble collapse, ultrasound provides an unusual
mechanism for generating high-energy chemistry. As in photochemistry, very large
amounts of energy are introduced in a short period of time, but it is thermal rather
than electronic excitation. The chemical effects of ultrasound are diverse and include
substantial improvements in both stoichiometric and catalytic chemical reactions. In
some cases, ultrasonic irradiation can increase reactivities by nearly a millionfold
(Ley, 1989).
The dynamics of cavity growth and collapse are strikingly dependent on the
local environment. As ultrasound passes through a liquid, the expansion cycles exert
negative pressure on the liquid, pulling the molecules away from one another. If the
ultrasound is sufficiently intense, the expansion cycle can create cavities in the
liquid. This will occur when the negative pressure exceeds the local tensile strength
of the liquid, which varies according to the type and purity of liquid. Normally,
cavitation is a nucleated process; that is, it occurs at pre-existing weak points in the
liquid, such as gas-filled crevices in suspended particulate matter or transient
microbubbles from prior cavitation events. Most liquids are sufficiently
contaminated by small particles that cavitation can be readily initiated at moderate
negative pressures (Chen, 1998).

45
During such oscillations the amount of gas or vapor or even volatile organic
chemicals (VOCs) that diffuses in or out of the cavity depends on the surface area,
which is slightly larger during expansion than during compression. As shown in
Figure 2.5, cavity growth during each expansion is, therefore, slightly larger than
shrinkage during the compression. Thus, over many acoustic cycles, the cavity will
grow. The growing cavity can eventually reach a critical size where it can efficiently
absorb energy from the ultrasonic irradiation. This critical size depends on the liquid
and the frequency of sound. (Suslick, 1988)
The dynamics of the motion of the bubble is characterized to a first
approximation by the Rayleigh-Plesset equation. In the absence of mass transport
across boundary, a spherical bubble of radius, R, in an infinite domain of liquid
which has temperature, t, and pressure p
∞
(t) far from the bubble. It also assumed that
the contents of the bubble are homogeneous and the pressure, p
B
(t) within the bubble
are always uniform.
( ) ( )
R
S
dt
dR
R dt
dR
dt
R d
R
t p t p
L
L
L
B
ρ
ν
ρ
2 4
2
3
2
2
2
+ +






+ =
- ∞

or
( ) ( )
dt
dR
R R
S t p t p
dt
dR
dt
R d
R
L
L L
B
ν
ρ ρ
4 2
2
3
2
2
2
- - - =






+
∞

where S = surface tension of the liquid
ν
L
= kinenatic viscosity of the liquid
ρ
L
= saturated liquid density

46

Figure 2.5: Generation of an acoustic bubble.

47
The Rayleigh-Plesset equation was first derived and used by Rayleigh in
1917, and later in 1949, Plesset applied the equation to the problem of traveling
cavitation bubbles (Brennen, 1995).
Once the cavity has overgrown, either at high or low sonic intensities, it can
no longer absorb energy as efficiently. Without the energy input the cavity can no
longer sustain itself. The surrounding liquid rushes in, and the cavity implodes. It is
the implosion of the cavity that creates an unusual environment for chemical
reactions. The chemical effect of ultrasound on aqueous solutions have been studied
for many years. The primary products are molecular hydrogen (H
2
) and hydrogen
peroxide (H
2
O
2
). Other high-energy intermediates may include HO
2
(hydroperoxyl
radical), H (atomic hydrogen), OH· (hydroxyl radical), and e
–

(aq)
(solvated
electrons). The sonolysis of water, which produces both strong reductants and
oxidants, is capable of causing secondary oxidation and reduction reactions
(Motherwell, 1992). The principal products from the ultrasonic irradiation of water
are H
2
O
2
and H
2
(Suslick, 1988):
The sonolysis of simple hydrocarbons creates the same kinds of products
associated with very high temperature pyrolysis. Most of these products derive from
a radical chain mechanism. Several mechanisms have been proposed for this
sonochemical cleavage, which is usually described as a mechanical breakage of the
chains induced by shock waves or solvent flow created by cavitation during the
ultrasonic irradiation of liquids.

48
2.1.1 Factors Affecting cavitation
Cavitation is a three-step process consisting of nucleation, growth, and
collapse of a gas- or vapor- filled bubble in a body of liquid. During bubble
implosion, intense heating of the bubble occurs. The localized hot spots have a
temperature of roughly 5000°C, pressures of about 500 atmospheres, and lifetimes of
a few microseconds. It is this high temperature and pressure that affect chemical
reactions. Effect of various factors on the intensity of cavitation are listed as below:
• Temperature
• Static pressure in the liquid
• Bubble size, number of nuclei, and frequency of the applied sound field
• Intensity of the sound field
• Attenuation characteristics of bubbles
• Surface tension of the liquid
• Vapor pressure
• Concentration and diffusion rates of dissolved gases
• Viscosity, cohesive strength, and density of the liquid
• Heat transfer rate in the fluid
• Local distribution of sound intensity and pressure in the vicinity of a
collapsing bubble

49
2.1.1.1 Temperature
The cavitation process is temperature-dependent because several of the
important characteristics of a liquid that affect cavitation intensity are themselves
temperature-dependent. The principal ones are vapor pressure, surface tension, the
diffusion rates of gases that are dissolved in the liquid, and the solubility of air and
other gases in the liquid (Thompson and Doraiswamy, 1999)
·

It is well known that the kinetics of most chemical reaction increases with
increasing temperature, however, an increase in the ambient reaction temperature
results in an overall decrease in the sonochemical effect. As the reaction temperature
is raised, the equilibrium vapor pressure of the system is also increased, which
decreases the cavitation threshold and leads to easier bubble formation. However, the
cavitation bubbles are formed contain more vapor. The collapse of vapor-filled
bubbles cushions the implosion and reduces the ultrasonic energy. In general, the
largest sonochemical effects are observed at lower temperature when a majority of
the bubble contents is gas. Therefore, to determine the extent to which changes in
temperature are aspirate to affect the cavitation intensity, it is necessary to consider
its relative effect on reaction kinetcs, vapor pressure, surface tension, and other
characteristics such as ultrasonic intensity (Thompson and Doraiswamy, 1999).

50
2.1.1.2 Static Pressure in the Liquid
Increasing the ambient reaction pressure generally results in an overall
increase in the sonochemical effect because of the decrease in the vapor pressure of
the mixture. Decreasing the vapor pressure increases the intensity of the implosion,
thus increasing the ultrasonic energy produced for cavitation (Thompson and
Doraiswamy, 1999). However, ultrasonically induced cavitation can be suppressed
completely by the application of sufficient external pressure. It appeared that
operating at pressures of 200 psig and above increased the cavitation threshold in the
system to a level where the cavitation bubbles could not be produced or were
produced in such small quantities. Moreover, a reduction in the static pressure in a
liquid results in surface tension forces becoming predominate in determining the
critical bubble size for cavitation.

2.1.1.3 Bubble size, Number of nuclei, and Frequency of the applied sound field
The most important effect of frequency on the cavitation process is to
determine the maximum size of a bubble nucleus. Those will grow to a size which
results in a catastrophic collapse. Bubbles larger than this size will not grow. At very
high frequencies, the cavitational effect is reduced because either (1) the rarefaction
cycle of the sound wave produces a negative pressure which is insufficient in its
duration and/or intensity to initiate cavitation or (2) the compression cycle occurs
faster than the time required for the micro-bubble to collapse (Thompson and

51
Doraiswamy, 1999). Therefore, an increase in the frequency of the applied sound
wave requires an increase in its intensity in order to remain above the threshold value
needed for cavitation. Most sonochemical reactions are carried out in the low
frequency range between 20 kHz to 50 kHz.
Frequency impacts the diameter of the cavitation event, as shown in
Figure 2.6. Low frequencies result in large diameter cavitations and higher
frequencies result in small diameter cavitations. The energy per cavitation follows
the same trend. However, the number of cavitations per unit volume is high with
high frequency systems and low with low frequency systems. The combination of
energy per cavitation and number of cavitations is total energy and this is equal for
both frequencies.
Because of this relationship, low frequency systems (20-40 kHz) have done
well in treating environmental contaminants which need more energy.

52

Figure 2.6: The relationship among ultrasound frequency, energy per cavitation,
and number of cavitations.

53
2.1.1.4 Intensity of the Sound Field
Generally, the sonochemical effects will increase with increasing ultrasonic
intensity. However, there is a threshold level of intensity for the sound field. The
intensity of ultrasound should be larger than this threshold to occur the cavitation.
Moreover, high intensities of impressed sound are accompanied by all alteration of
the wave shape. Due to high vibration amplitude of sound wave, decoupling occurs
when the source of ultrasound does not contact with the medium, particularly if large
numbers of cavition bubbles build up at or near the emitting source of the transducer.
It results in a heavy loss of power transferring from the source to the medium
(Mason, 1999). Therefore, it is difficult to obtain significantly more vigorous
cavitation by an increase in the level of the applied sound field. There usually exists
an optimum ultrasonic intensity in most sonochemical systems.

2.1.1.5 Choice of Solvent
The properties of solvent include vapor pressure, surface tension, and
viscosity, which can significantly affect the occurrence and intensity of cavitation.
Cavities are more readily formed when a solvent with a high vapor pressure, low
viscosity, and low surface tension is used. However, the intensity of cavitation is
benefited by using solvents with opposing characteristics which is low vapor
pressure, high viscosity, and high surface tension (Thompson and Doraiswamy,
1999). Moreover, when using the extremely volatile solvent, diethyl ether, which has

54
a vapor pressure of 0.73 atm at 25°C, the cavitation was inhibited (Luche, 1998).
Therefore, to achieve higher sonochemical effects as a result of more intensive
collapsed of cavitation bubbles, selecting of solvent is frequently dictated by the type
of chemistry involved.

2.1.2 Cavitation Mechanism
In ultrasonic degradation of MTBE, chemical reactions occur at three
different regions as shown in Figures 2.7, 2.8, and 2.9 (Chen, 1998). Yen and his
research group have done many researches on ultrasound applications. The
principles of ultrasonic upgrading heavy oil and degrading pollutants are all similar
(Chen, 1998).
Liquids irradiated with ultrasound can produce bubbles. These bubbles
oscillate, growing a little more during the expansion phase of the sound wave than
they shrink during the compression phase. Under the proper conditions these
bubbles can undergo a violent collapse, which generates very high pressures and
temperatures in microscopic scale. This process is called cavitation. Because
compression of a gas generates heat, little heat can escape from the cavitation
bubbles during collapse. The localized hot spot has a temperature of roughly
5000° C (9,000° F), a pressure of about 500 atmospheres, a lifetime considerably less
than a microsecond, and heating/cooling rates above 10 billion °C per second. For a
rough comparison, the temperature of the surface of the sun, the pressure at the

55
bottom of the ocean, the lifetime of a lightning strike, and a million times faster
cooling that a red hot iron rod plunged into water (Suslick, 1988).
In aqueous sonochemistry, chemical reactions occur at three different regions:
(1) Interiors of collapsing bubbles where extreme conditions of
temperature and pressure exist, which induces chemical reactions
yielding products that are typical of pyrolysis or combustion
reactions in the gas phase. In aqueous solutions, thermal
decomposition of water vapor inside the bubbles produces
hydroxyl radicals and hydrogen atoms.
(2) Interfacial regions between cavitation bubbles and bulk solution.
The semi-volatile and nonvolatile solute accumulated in this
region can undergo thermal decomposition and then induce radical
reactions. Locally condensed hydroxyl radicals in this region have
been reported (Suslick, 1988).
(3) Bulk solution where the radicals, being produced in the interior of
bubble and in the interfacial region, the survive migration from
the interface can undergo radical reactions with solute present in
the bulk solution.

56

Gas phase
OH·
By-products
MTBE
MTBE
By-products

Figure 2.7: Interiors of collapsing bubbles.

57
Gas phase
OH·
By-products
MTBE
MTBE
By-products

Figure 2.8: Interfacial regions between the cavitation bubbles and bulk solution.

58

Gas phase
OH·
By-products
MTBE
MTBE
By-products

Figure 2.9: Bulk solution where the radicals, being produced in the interior of
bubble and in the interfacial region.

59
Ultrasound has potentially important applications in both homogeneous and
heterogeneous catalytic systems. In addition, ultrasound is well suited to industrial
applications. Because the reaction liquid itself carries the sound, there is no barrier
to its use with large volumes. In fact, ultrasound is already heavily used industrially
for the physical processing of liquids, such as emulsification, solvent degassing, and
solid dispersion. The extraordinary temperatures and pressures reached during
cavitational collapse, combined with the exceptionally high rates of cooling, may be
used to degrade many environmental contaminants.

2.1.3 Ultrasonic Transducer
An ultrasonic transducer is a device that converts energy into ultrasound, or
sound waves above the normal range of hearing for humans. Technically a dog
whistle is also an ultrasonic transducer that converts mechanical energy in the form
of air pressure into ultrasonic sound waves.
Transducer is a device that converts variations in one energy form into
corresponding variations in another, usually electrical form (Payne, 2002). One
widely used class of transducers consists of devices that produce an electric output
signal, such as microphones, phonograph cartridges, and photoelectric cells. Other
widely used transducers accept an electric input, such as loudspeakers, light bulbs,
and solenoids. Moreover, transducer is sometimes applied to devices producing an
output in the same form as their input, such as transformers and filters.

60
Theoretically, ultrasonic transducers convert energy in one form to another into
acoustic energy and can be roughly divided into mechanical transducers and
electromechanic transducers.

2.1.3.1 Electromechanic Transducer
Electromechanical transducers convert electrical into acoustic energy. There
are at least five types of such transducers in current use: moving coil, electrostatic,
piezoelectric, ferroelectric and magnetostrictive types (Gooberman, 1968).
Piezoelectric and magnetostrictive types are the most common design in commercial
application.

Piezoelectric Transducer
Certain polar molecules form crystals which lack centers of symmetry. If a
plate cut from such a crystal is mechanically deformed a voltage develops between
two faces of the plate and this phenomenon is known as the direct piezoelectric
effect. Conversely, if a voltage is applied between the two faces the plate will
deform because of the inverse piezoelectric effect.
The conversion of electrical pulses to mechanical vibrations and the
conversion of returned mechanical vibrations back into electrical energy are the basis
for ultrasonic testing. The active element is the heart of the transducer as it converts
the electrical energy to acoustic energy, and vice versa (Gooberman, 1968).

61
Theoretically, there are two general classes of piezoelectric materials: those that are
naturally piezoelectric and those that are piezoelectric only after they have received
special treatment. Examples of the first class are naturally occurring crystals of
quartz and tourmaline, artificially grown crystals of ammonium and hydrogen
phosphate, Rochelle salt, lithium sulfate and lead niobate. The other class of
materials is those that possess an enhanced electostrictive effect and can be made to
show piezoelectric properties after being subjected to a special treatment which
causes them to become polarized. Barium titanate and lead zirconate-titanate are
examples of electrostrictive materials.
Piezoelectric transducers are not frequency limited. The active element of
most acoustic transducers used for commercial applications is a piezoelectric
ceramics, as shown in Figure 2.10. Piezoelectric crystals made from quartz crystals
and magnetostrictive materials were primarily used in early 1950 (Gooberman,
1969). When piezoelectric ceramics were introduced they soon became the dominant
material for transducers, because they have good piezoelectric properties and easy to
manufacture into a variety of shapes and sizes. They also operate at low voltage and
are usable up to about 300
o
C.
Today’s piezoelectric ultrasonic transducers are reliable, efficient devices
which can be used with confidence in all power ultrasonic applications. The fact is
that piezoelectric transducers are capable of providing a wider range of frequency
and waveform characteristics and higher electrical conversion efficiency.

62

Figure 2.10: Piezoelectric transducers in commercial application.

63
Magnetostrictive Transducers
The name magnetostriction or piezomagnetism is given to the phenomenon
that when some materials are magnetized a change in dimensions occurs. This
change can be either positive or negative in a direction parallel to the magnetic field
and is independent of the direction of the field (Gooberman, 1969).
Two materials most commonly used for constructing magnetostrictive
transducers are pure nickel and iron-cobalt alloys (49 % iron, 49 % cobalt, and 2 %
vanadium). Figures 2.11 shows the design of magnetostrictive transducers which
consist of a large number of nickel plates or laminations arranged in parallel with one
edge of each laminate attached to the bottom of a process tank or other surface to be
vibrated. A coil of wire is placed around the magnetostrictive material. When a flow
of electrical current is supplied through the coil of wire, a magnetic field is created.
This magnetic field causes the magnetostrictive material to contract or elongate and
thereby obtain a usable amount of acoustic power output. However, due to physical
size limitations of the magnetostrictive transducer, it is inherently limited to operate
at frequencies below approximately 30 kHz. Because frequency is dependent on the
length of the transducer, higher frequency requires a shorter and shorter length.

64

Figure 2.11: Basic equivalent circuit for a magnetostrictive transducers.

65
2.1.3.2 Mechanicl Transducer
Mechanical transducers convert the kinetic energy of stream of fluid into
acoustic energy. For example, a dog whistle may generate sound by passing a gas or
liquid through an orifice or over an edge, which can be known as whistle and
satisfactory for generation of energy in suitable materials. The passage of the gas
generates vortices, spaced periodically, which propagate the sound wave. While a
simple circular hole applied for this purpose, more stable results may be obtained in
various ways. Those devices may be used in either gas or liquids, where their low
impedance allows better coupling (Carlin, 1960).
Galton whistle is basically consists of a jet which sends out a stream of gas
against a small cavity. The principle of Galton’s whistle is shown diagrammatically
in Figure 2.12. A stream of compressed air is pumped through a passage formed by
the gap betwen two concentric cylinders. After it impinges on to the wedge-sharped
edge, resonate cavity is formed by a cylinder and plunger. The air stream sets the
cylinder end into oscillation. This small ultrasonic whistle produces high intensity
sounds at frequencies that are essentially inaudible to humans.
Hartmann whistle is similar to Galton whistle but works on a different
principle. Figure 2.13 shows a schematic cross-section of the whistle. A jet of air
flows through the cylinder A, which consists of a nozzle (cylinder A) where a jet of
compressed air emerges. At the exit, the air velocity increases and exceeds that of
sound giving rise to a shock wave a short distance out from the end of the cylinder.

66

Figure 2.12: Cross-section through Galton’s whistle (dog whistle).

67

Figure 2.13: Cross-section through Hartmann’s whistle. (A- Cylinder, B-Resonant
cavity, Source: Gooberman, 1969)

68
At this point, the shock wave is unstable and causes the air within cavity to
oscillate. This device is capable of producing high frequency, high amplitude
pressure and velocity perturbations. Therefore, the Hartmann generator has been
suggested as a precipitator of dust or fog, and also been used for signaling and active
flow control applications.
The principle of Jet-edge device is shown diagrammatically in Figure 2.14. A
stream of fluid is pumped through a circular hole in a plate of thickness. Because of
viscous forces at the fluid and solid boundary, vortices are set up which travel with a
velocity until they reach the far side of the plate where they radiate a pressure pulse
in all directions. The pressure pulse radiated by one such vortex will travel back to
the other side of the plate where it will provoke another vortex to form. Because the
jet is convective unstable, the interaction between jet and wedge produces a dipole
pressure field on the nozzle. Therefore, the oscillating jet produced by the orifice
causes sound produced by the blade. Furthermore, the stability of this system can be
improved by placing a suitably resonant wedge-ended plate near the hole.
Jet-edge generators are suitable for both gases and liquids and provide a
stable and simple means for generating fairly high powers suitable for bulk
processing of fluids by ultrasonic energy such as manufacturing emulsions and
dispersions (Gooberman, 1968).

69

Figure 2.14: Cross-section through a jet-edge system (liquid whistle).

70
2.1.4 Ultrasonic System Types
2.1.4.1 Bath Type
Ultrasonic baths were originally manufactured for cleaning purposes. Typical
ultrasonic cleaning tank has the transducers attached to the bottom, as shown in
Figure 2.15. The reaction vessel is typically immersed in the fluid contained in the
bath. However, the tank itself can be used as the reaction vessel but would require
additional mechanical agitation. Moreover, the tank walls would be exposed to the
reactants, making them susceptible to surface corrosion (Thompson et al., 1999).
When indirect sonication is used, the ultrasonic power which reaches the
reaction vessel is relatively low as compared to other ultrasonic systems, such as a
probe. In addition, obtaining reproducible results may be difficult because the
amount of power reaching the reaction mixture is highly dependent on the placement
of the sample in the bath. The results can also vary with time as the bath warms
during operation (Lickiss and McGrath, 1996). Another disadvantage using a bath
system is that the coupling fluid surrounding the reaction vessel will eventually
increase in temperature, making the maintenance of isothermal conditions difficult.
Cooling coils can be placed within the bath, but they will have an effect on the sound
field and may reduce the amount of power reaching the vessel (Thompson et al.,
1999). Moreover, it is essential to determine the optimum conditions for each bath
and to replace the reaction vessel in the same place, due to the different
characteristics of every bath.

71

Figure 2.15: Ultrasonic cleaning tank.

72
2.1.4.2 Probe Type
Probe systems, also called horn systems, are being more frequently used for
sonochemical research in the laboratory, because they are possible to achieve much
greater vibrational amplitudes. As shown in Figure 2.16, a probe type high-intensity
ultrasonic processor can deliver large amounts of power directly to the reaction
mixture, and it can be regulated by varying the amplitude to the transducer.
Transducers used in modern power ultrasonic systems are almost without
exception based upon the pre-stressed piezoelectric design. In this construction, a
number of piezoelectric elements are bolted between a pair of metal end masses. The
piezo elements would be a pre-polarized lead titanate zirconate composition, which
exhibit high activity coupled with both low loss and ageing characteristics. They are
ideally suited to form the basis of an efficient and rugged transducer.
Several experiments have been carried out to determine the sound field
characteristics in a probe system. The localized area of ultrasonic intensity in a fluid
is highly dependent on the power delivered to the transducer. Contamine et al.
(1994) observed that when the power delivered to the system is low, the distribution
of ultrasonic intensity is characteristic of a standing wave in the axial direction.
However, as the power delivered to the system increases, the wave pattern dissipates
and the intensity becomes higher near the probe tip and decreases axially. In the
radial direction, it indicated that at low powers the intensity is slightly higher at the
center of the reactor but is comparable over the cross section of the reactor.

73
However, as the power delivery increases, the ultrasonic intensity increases at the
center of the reactor and dissipates in the radial direction. At an input power of 200
W, the active region in the radial direction is equal to that of the horn (the remaining
radial direction had negligible activity). A minimum liquid height of 1 cm must be
maintained in the reaction vessel, below which the transducer does not function
properly, as shown in Figure 2.17 (Ratoarinoro et al., 1995).
Using probe systems have following disadvantages: (a) Erosion and pitting of
the probe tip may contaminate the reaction solution. (b) Because of geometric losses
and acoustic decoupling problems, it is impossible to transmit an intensity cavitation
filed more then 2-5 cm beyond the end of probe. (c) The probe system can not
transmit to large process volume. (d) Larger transducer has to displace. (e) The
higher intensity increases the stress on material, more likely to fall. Therefore, the
probe systems are not suitable for lager commercial scale.

74

Figure 2.16: High intensity ultrasonic processor.

75

Figure 2.17: Typical laboratory settings for carrying out sonochemistry.

76
2.2 Fenton’s Reagent
The metal-ion-catalyzed decompostion of hydrogen peroxide was discovered
by H. J. H. Fenton in the 1890s. Fenton’s chemistry in a laboratory setting is well
understood and has been studied for more than 100 years. Today, Fenton’s Reagent
is used to treat a variety of industrial wastes containing a range of toxic organic
compounds, such as phenols, formaldehyde, BTEX, and complex wastes derived
from pesticides, wood preservatives, plastics additives, and rubber chemicals. Many
metals have special oxygen transfer properties which improve the utility of hydrogen
peroxide. By far, the most common of these is iron which, when used in the
prescribed manner, results in the generation of highly reactive hydroxyl radicals
(OH⋅). Hydroxyl radicals play the most important role in this process. The chain
reaction can be described in simple aqueous solution as shown in Figure 2.18. Fe
3+

is the major species in the solution when hydroxyl radicals are generated. In the
presence of organic substances, the hydroxyl radicals produce organic free radicals
(R⋅) through hydrogen abstraction, which can undergo oxidation by Fe
3+
(R
i
⋅),
dimerisation (R
j
⋅), or reduction by Fe
2+
(R
k
⋅). Fe
2+
is regenerated as a result of this
process (Walling, 1975). In addition, many reports suggested that iron level and pH
were found to be far more significant than H
2
O
2
dose, and when the iron levels were
low, there was virtually no effect attributable to H
2
O
2
. Table 2.1 shows the oxidation
potential values. Hydroxyl radical is a very strong oxidant and it can be created by
ultraviolet light, ozonation, and ultrasound.

77

Ultrasound Fenton’s reagents
↓ H
2
O → OH·+ ·H
→ H
2
O
2
+ H
2
↓ MTBE + OH·
→ TBA
→ acetone
→ acetic acid
→ carbon dioxide
↓ H
2
O
2
+ Fe
2+
→ OH·+OH
–
+Fe
3+
↓ OH·+ Fe
2+
→ Fe
3+
+ OH
–
↓ OH·+ R
i
H → H
2
O + R
i
·
↓ OH·+ R
j
H → H
2
O + R
j
·
↓ OH·+ R
k
H → H
2
O + R
k
·
↓ R
i
·+ Fe
3+
→ Fe
2+
+ product
↓ 2 R
j
· → product (dimer)
↓ R
k
·+ Fe
2+
→ Fe
3+
+ R
k
–
→R
k
H
↓ R·+ Cu
2+
→ product + Cu
+
↓ Cu
+
+ Fe
3+
→ Cu
2+
+ Fe
2+

Figure 2.18: The stoichiometry of Fenton’s reagent and ultrasound.

78
Table 2.1: Comparative Oxidation Potentials
Species Volts
Fluorine 3.0
Hydroxyl Radical 2.8
Ozone 2.1
Hydrogen peroxide 1.8
Potassium Permanganate 1.7
Hypochlorous Acid 1.5
Chlorine Dioxide 1.5
Chlorine 1.4
Oxygen 1.2

79
Chemical oxidation using Fenton’s Reagent is most effective under very
acidic pH (e.g., pH 2 to 4) and becomes less effective under moderate to strongly
alkaline conditions and when free radical scavengers are present. The reaction is
strongly exothermic and can evolve substantial gas and heat. The oxidative reactions
are extremely rapid and follow first-order kinetics. The complexities of reactions
that occur when hydrogen peroxide is added to groundwater environment make it
difficult to describe the reaction kinetics explicitly.
In the field, application of Fenton’s Reagent is performed in sequential steps.
Step one usually comprises a pH adjustment. An acid is injected to temporarily
reduce the pH of the groundwater to below pH 4. This step may also serve to bring
naturally occurring ferrous iron into solution. Additionally, Fenton’s reagent is very
corrosive and should be performed in stainless or lined reactors.
Hydrogen peroxide is a chemical agent widely used for the synthesis of
organic and inorganic peroxides, peracids and lactones. It is also used as a purifying
and bleaching agent, as well as for the epoxidation, hydroxidization and oxidation of
organic sulfurous and organic nitrogenous compounds. For in situ bioremediation,
hydrogen peroxide can be used as a major source of active oxygen to treat certain
cases of serious pollution. It is also used for detoxifying, deodorizing and bleaching
wastewater. As a powerful oxidizing agent, it destroys the microorganisms
responsible for the formation of foul-smelling, toxic or corrosive products, and
decomposes the toxic compounds present in some industrial effluents. In addition, as

80
a source of free radicals, hydrogen peroxide acts as an initiator in the ultrasound
assisted chemical processes.
By itself, H
2
O
2
is a rather poor microbiocide compared to chlorine, bromine,
ozone, and other commonly used disinfectants. Consequently, it is not approved by
regulatory agencies as a stand-alone disinfection treatment. However, there are a
number of technologies which use H
2
O
2
as part of the treatment program. These
include: UV/H
2
O
2
; ozone/H
2
O
2
; and quaternary ammonium salts/H
2
O
2
.

81
Chapter 3: Investigation of Hydroxyl Radicals Generation

3.1 Introduction
There are many literatures studying the source of the hydroxyl radicals
formed from the Fenton’s reagents. Spectroscopic methods that have been applied to
measure hydroxyl radicals including electron spin resonance (ESR), photoelectron
spectroscopy (PES), infrared spectroscopy (IR), and ultraviolet-visible spectroscopy
(UV-VIS). ESR is a reliable tool in measuring free radical concentration. The free
radical spin densities were determined by double integration of the first-derivative
curve, and comparing them with a spin trapping agent such as 5,5-dimethyl-1-
pyrroline N-oxide (DMPO) or 2,2-diphenyl-1-picrylhydrazyl (DPPH), as shown in
Figure 3.1. Scientists have investigated the origin of hydroxyl radicals in the Fenton
reaction by using DMPO as a spin trap. The hydroxyl radical was generated by
addition of Fe
2+
ion to H
2
O
2
labeled with oxygen isotope
17
O. ESR spectra show that
the percentage of
17
O-labeled hydroxyl radical trapped by the DMPO was the same as
in the original hydrogen peroxide. This experimental result indicates that the trapped
hydroxyl radical was derived exclusively from hydrogen peroxide. Likewise, the
complementary reaction with ordinary H
2
O
2
and
17
O-labeled water also showed that
none of the hydroxyl radical was derived from water (Lloyd et al. 1997).

82

Figure 3.1: ESR spectrum of DPPH.

83
DPPH was used as reactive traps for short-lived hydroxyl radicals. When
DPPH reacts with free radicals, 1,1-diphenyl-2-picrylhydrazine (DPPH
2
) will be
generated with a color change from violet to light yellow in the solution. Both DPPH
and DPPH
2
have absorption maximum at 520 nm. Under room temperature, the
following five conditions: hydrogen peroxide only, ferrous ion only, hydrogen
peroxide with 60-minute sonication, Fenton’s reagent, and Fenton’s reagent with 60-
minute sonication, were studied.

3.2 Materials and Experiment Methods
A Branson’s ultrasonic cleaning tank with 20 kHz frequency was used. The
reaction temperature was kept at room temperature. The reaction solution was
prepared by mixing a fixed volume of saturated solution of DPPH in methanol with
500 ml methanol water (60:40 in volume) binary solvent mixture to obtain an initial
DPPH concentration close to 2 × 10
-5
M.
Ferrous ion stock solutions containing 56 mg/L were prepared by dissolving
FeSO
4
⋅7H
2
O
(s)
(Aldrich Chemical Company) in deionized water. The hydrogen
peroxide and ferrous ion concentrations tested were 3.4 mg/L and 5.6 mg/L, because
the optimum molar ratio of H
2
O
2
: Fe
+2
for Fenton’s reagent is 1:1 (Ray, 2000).
Ferrous ion was determined by the phenanthroline method. Ferrous ion test solution
was prepared by mixing HCl, phenanthroline solution, ammonium acetate buffer, and
distilled water with a ratio of 1:40:20:39.

84

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
450 500 550 600
Wavelength (nm)
Extinction

Figure 3.2: UV-VIS spectra of radical scavenger (DPPH) under the following
conditions:
Red line: DPPH only
Blue line: DPPH + H
2
O
2

Orange line: DPPH + Fe
+2

Green line: DPPH + H
2
O
2
+ sonication (60 minutes)
Purple line: DPPH + Fenton’s reagent
Brown line: DPPH + Fenton’s reagent + sonication (60 minutes)

85
3.3 Results and Discussion
The conversion of DPPH to DPPH
2
occurs in all five conditions. The
experiments conducted with DPPH gave proof of free radical formation in the
solution. As shown in Figure 3.2, the amount of radicals generation from more to
less is in the following order: Fenton’s reagent with 60-minute sonication > Fenton’s
reagent > hydrogen peroxide with 60-minute sonication >> ferrous ion only >
hydrogen peroxide only.
The UV-VIS spectra show that the addition of small amount of hydrogen
peroxide had a very limited effect on the production of free radicals. However, the
amount of free radicals increased significantly when ferrous ion was added. In
addition, ferrous ion itself can convert DPPH to DPPH
2
by the simple redox reaction
as the following equation.
DPPH + Fe
+2
+ H
+
→ DPPH
2
+ Fe
+3

The optimum molar ratio of hydrogen peroxide to ferrous ion appears to be
1:1. At this ratio, more than 90 percent of DPPH was converted to DPPH
2
after 60
minutes of sonication.

86
Chapter 4: Degradation of MTBE By Ultrasound-Assisted Chemical Process

4.1 Introduction
Methyl tertiary butyl either (MTBE) is the most common oxygenated fuel
additive used in reformulated gasoline because its low cost, ease of production, and
favorable transfer and blending characteristics. MTBE was used to reduce engine
knocking and help gasoline burn more cleanly since 1979. MTBE use expanded
dramatically in the mid-1990s with the implementation of the Clean Air Act
Amendments of 1990, which mandated efforts to reduce carbon monoxide emissions,
as well as ozone levels in areas with severe air pollution. MTBE has caused public
concern because it contaminates groundwater and surface water. MTBE moves
faster in water than other gasoline components and makes water undrinkable.
Groundwater contamination is most commonly associated with releases from leaking
underground storage tanks. Surface water contamination has been attributed to
evaporative emissions from poor fuel-handling practices and exhaust emissions from
gasoline boats and personal watercraft. Removing MTBE from a drinking water
supply presents a challenge, because MTBE has a low Henry’s constant and high
solubility in water.

87
MTBE is formed in a reaction between isobutene and methanol. As shown in
Table 4.1, the partial pressure of MTBE is higher than other fuel components and it
will therefore volatilize more readily. Volatilization and condensation are
complementary processes that describe the exchange of a volatile compound between
water and air. Volatilization refers to the movement of compound from water into
air whereas condensation refers to the movement from air into water. Equilibrium
can be established between concentration of a compound in air and water as
described by the Henry’s Law constant which is temperature dependent. A
compound with a value of 0.05 or larger is considered to volatilize easily from water.
In comparison with BTEX components, MTBE tends to partition strongly from the
gas phase into the water phase. Consequently, dissolved MTBE tend to stay in the
water phase and not adsorb to subsurface solids.
Conventional pump-and-treat system, which often associated with treatment
technologies such as air stripping and liquid-phase granular activated carbon (GAC),
may not be cost-effective for MTBE removal. Advanced oxidation processes (AOPs)
which utilize the principle that environment pollutants can be chemically or
physically oxidized to CO
2
and H
2
O have shown promise to treat MTBE-
contaminated groundwater (Liang, 1999). AOPs include the application of O
3
, H
2
O
2
,
UV, non-thermal plasmas, electron beams, γ rays, supercritical water oxidation,
electrohydraulic cavitation and sonolysis, and they may be used either individually or
in combination to achieve the desired degradation efficiency.

88
Table 4.1: Chemical and physical properties of MTBE, BTEX, and TBA.
Property MTBE Benzene Toluene
Ethyl-
benzene
Xylene TBA
Molecular
Weight, g/mol
88 76 92 106 318.5 74
Specific Gravity 0.74 0.88 0.87 0.87 0.86 0.78
Vapor Pressure,
mm Hg at 25°C
251 95 28 9 8 40
Henry’s Law
Constant at 25°C
0.02 0.22 0.24 0.35 0.31 0.00025
Log K
oc

(Adsorption)
1.05 1.9 2.0 2.5 2.6 1.57
Water Solubility,
mg/L at 25°C
43,000 1,780 535 161 146 Miscible
Odor Threshold,
ppm
0.053 0.5 0.16 6 20 21
Taste Threshold,
μg/L
20-40 500 NA NA NA NA

89
Ozonation system has problem with bromate occurrence. The use of ozone is
known to convert bromide ion into bromate ion (BrO
3
–
) in the coastal area, and other
by-products such as alcohols, ketones, and carboxylic acids are also known to occur
from reactions with organic contaminants (Kang et al., 1998). BrO
3
–
formation is
one of the key disadvantages of using ozone as the source of hydroxyl radicals.
Ozone is also an important alternative disinfectant to chlorine. During ozonation,
both bromate and disinfection by-products (DBPs) are formed. In order to minimize
the formation of bromate, many operational parameters and hydroxyl radical
enhancers/scavengers have being considered. As for the UV system, high turbidity
and dark color in the groundwater would significantly reduce the efficiency of UV
radiation.
An ultrasound-assisted chemical process (UACP) involving Fenton’s reagent
and ultrasound were developed to treat MTBE in groundwater. Hydroxyl radicals
generated from Fenton’s reagent can be continuously supplied by sonolysis of water
and naturally occurring metal salts in groundwater. In addition, UACP has no
by-products as seen in other systems. Chen et al. (1990) successfully used a
combination of ultrasound and hydrogen peroxide to completely destroy TCE and
PCE in groundwater.
The only concern will be copper ion in the system. Copper is utilized in
water treatment as coagulant and used to inhibit algae growth in reservoirs. The
average concentration of copper in natural water is about 10 μg/L. Based on

90
Walling’s studies (1975), such level of copper is sufficient to create Fenton’s reagent
when hydrogen peroxide exist. In these systems radical oxidation by Fe
3+
is an
electron-transfer process, but with copper ion (Cu
2+
), it involves either ligand transfer
or an organocopper intermediate.
Ultrasonic cavitation has been used to degrade a variety of contaminants,
such as chlorinated hydrocarbons, pesticides, phenols, and explosives. In general,
these pollutants were transformed into short-chain organic acids, carbon dioxide, and
inorganic ions over durations ranging from minutes to hours. Because MTBE is a
relatively non-polar compound, application of ultrasound with Fenton’s reagent may
be feasible for treating MTBE in groundwater. Competition of free radical reactions
among MTBE, intermediate products, and other scavengers presumably slowed the
reaction. Experimental results have shown that those organic oxidation by-products
can be efficiently removed by adding copper ion to the system.

91
4.2 Materials and Experiment Methods
4.2.1 Materials
Ferrous and copper ion stock solutions containing 560 and 130 mg/L,
respectively, were prepared by dissolving CuSO
4
⋅5H
2
O
(s)
and FeSO
4
⋅7H
2
O
(s)

(Aldrich Chemical Co., Milwaukee, Wis.) in deionized water. The ferrous and
copper ion concentrations tested were 5.6 and 1.3 mg/L, respectively, because these
ion concentrations have been shown to be effective in treating MTBE (Ray, 2000).
The concentrations of copper in groundwater were measured by atomic absorption
spectrophotometry. Ferrous ion was determined by the phenanthroline method.
Ferrous ion test solution was prepared by mixing HCl, phenanthroline solution,
ammonium acetate buffer, and distilled water with a ratio of 1:40:20:39. The ferrous
ion concentration was determined photometrically at 510 nm by an UV-VIS
spectrophotometer (Model: Hewlett Packard 8452A).
The hydrogen peroxide and ferrous ion concentrations tested were 3.4 mg/L
and 5.6 mg/L, because the minmum molar ratio of H
2
O
2
: Fe
+2
is 1:1 (Ray, 2000).
The impact of bicarbonate ions on the efficacy of ionization was tested at three
concentrations: 50, 100, 200, and 2000 mg/L as NaHCO
3
(40, 80, 160, and 1600
mg/L as CaCO
3
). A stock solution of 2,000 mg of NaHCO
3
per liter was used to
prepare the test solutions.

92
4.2.2 Experimental Method
In laboratory scale, probe type ultrasonicators (10 to 500 watts per square
centimeter) are the most reliable and effective tools. The probe generating the high
frequency vibrations is usually in direct contact with the solution being treated. For
field application, bath type reactors or continuous flow reactors using ultrasonic
transducers that create cavitation are commercially available in modular units with
20 kHz frequency. On the other hand, ultrasonic cleaning baths are easily accessible,
comparatively inexpensive, and usable on moderately large scales.
The initial concentration of MTBE studied was 2 mM (approximately 176
mg/L). Initially, the dose of Fe
+2
was kept constant at excess amount and the dose of
H
2
O
2
was varied to achieve different H
2
O
2
/MTBE molar ratios. Figure 4.1 shows the
experimental setting.
MTBE and its degradation by-products were analyzed by EPA standard
method 524.2 that is measurement of purgeable organic compounds in water by
capillary column gas chromatography/mass spectrometry (GC/MS). The method
detection limit (MDL) is about 1 ppb with other organic substrate present. In order
to analyze for disinfection by-products (DBPs) in treated water, Solid phase
extraction (SPE) or solid phase microextraction (SPME) can also be used to extract
the water samples.

93
Argon
Ice bath
Stirring bar
Ultrasonic processor

Gas phase
OH·
By-products
MTBE
MTBE
By-products
Cavitation bubbles
Localized high temperature
and high pressure

Figure 4.1: Experimental setting of the UACP.

94
4.3 Results and Discussion
4.3.1 Degradation of MTBE
Fenton’s reagent reacts with MTBE very rapidly, usually in less than a
minute, 75% MTBE already been converted to TBA and acetone, as shown in Figure
4.2. The experimental results have also shown almost complete MTBE degradation
through 60 minutes sonication. This reaction results in a ratio of moles of ferrous
iron to hydrogen peroxide reacted which may be much smaller or much larger than
the stoichiometric ratio dependent on whether radical scavengers, such as organic
substrates, carbonate, and bicarbonate, are present in the system. Hydrogen peroxide
itself also can auto-decompose in aqueous solutions with accelerated rates upon
contact with mineral surfaces as well as carbonate and bicarbonate. In addition,
excessive dosages of hydrogen peroxide may also act as a scavenger.
MTBE degradation in UACP was due to its oxidation by hydroxyl radicals.
Hydroxyl radical production is initiated by the reaction of ferrous iron with hydrogen
peroxide. Ferrous iron can be introduced into the aqueous solution directly or they
can be provided from the dissolution of naturally occurring iron by H
2
O
2
.

95

Figure 4.2: MTBE degradation and its by-products.

96
4.3.2 Possible Decomposition Mechanism
Based on the GC/MS results, TBA and acetone are principal degradation
intermediates. Church et al. have proposed MTBE decomposition mechanism, as
shown in Figure 4.3. The degradation of MTBE has two stages, a fast one which
involves the reaction of ferrous iron with hydrogen peroxide (Fe
+2
/H
2
O
2
stage) and
another, relatively, slower stage which involves ultrasound and the reaction of copper
iron with hydrogen peroxide (Cu
2+
/H
2
O
2
stage). As for the byproducts of MTBE
degradation such as TBA and acetone, they are also consumed by the hydroxyl
radicals.
Degradation of MTBE with hydroxyl radical was inhibited by dissolved
natural organic matter (NOM) and degradation by-products, such as TBA and
acetone. To address the role of TBA and acetone in free radical scavenging capacity,
experimental determinations and theoretical calculations have been carried. There
are many complex reactions involving inhibition and competition phenomena.
Walling, et al. concluded that TBA has 40 times higher scavenging capacity
compared to acetone. Therefore, adding copper ion to react with TBA and forming
precipitation is a critical step to enhance MTBE degradation rate.

97

MTBE
acetone
carbon dioxide
tert-butyl formate
microbial
conversion
tert-butyl alcohol
acetone
carbon dioxide
tert-butyl alcohol
chemical
conversion

Figure 4.3: Summary of the degradation pathway of MTBE (Church et al., 1997).

98
MTBE oxidative degradation by hydroxyl radicals follows two major
pathways. Both pathways are initiated with hydrogen atom abstraction at either the
methoxy group or any of the three equivalent methyl groups in the MTBE molecule
leading to carbon centered radicals (Chang, 2000). These reactions are summarized
in reactions 4.1 and 4.2.
O H COCH ) (CH OH COCH ) (CH
2 2 3 3
%) 71 (
3 3 3
+ ⋅ ─ ─ ─ ─ ─ → ─ ⋅ +
- n abstractio α
(4.1)
O H COCH ) (CH CH OH COCH ) (CH
2 3 2 3 2
%) 29 (
3 3 3
+ ⋅ ─ ─ ─ ─ ─ → ─ ⋅ +
- n abstractio β
(4.2)
The branching ratio of reactions 4.1 and 4.2 was found to be 71:29 (Cooper,
2002). The higher ratio for reaction 4.1 due to that attack at the formyl group is more
likely because of the charge distribution and electrophilic character of hydroxyl
radicals. The rate constant for the reaction of hydroxyl radical with MTBE has been
reported in many literatures at relatively close values. Hardison (2000) reported a
value of 2 × 10
9
M
-1
s
-1
. Another value of 1.6 × 10
9
M
-1
s
-1
was given by Buxton
(1988) in his review.

4.3.3 Optimization of Experimental Conditions
The main variable for operating UACP is contact time. The chemical
dosages can be determined from a pilot study. In other AOPs systems, such as
O
3
/H
2
O
2
or UV/H
2
O
2
, using H
2
O
2
for destruction of organics, the ratio of H
2
O
2

dosage to target compound is usually between 50 to 500 to achieve optimum results.
Therefore, AOPs usually have higher capital and operating costs.

99
Hydrogen peroxide is certified for use in drinking water by the National
Sanitation Foundation (NSF) under ANSI/NSF Standard 60 Drinking Water
Additives-Health Effects. As shown in Table 4.2, food grade hydrogen peroxide
solution is commercially available as a 35% by mass solution in water, and different
purity of hydrogen peroxide may be used for MTBE removal at different maximum
use levels when followed by chlorination of the treated water.
The experimental results revealed that a H
2
O
2
: MTBE = 0.05:1 can reach
75% removal rate, and higher degradation efficiency was achieved when the molar
ratio of H
2
O
2
/MTBE equal to 50-500, as shown in Figure 4.4 and Table 4.3.
It appears that an excess amount of Hydrogen peroxide in the solution would
slightly retard the decomposition of MTBE while a decreased amount of H
2
O
2
down
to less than 3 mg/L would be a limiting factor for the Fenton’s Reagent. The same
finding has been reported in many other hydroxyl radical related research reports.
The reason for this effect may be due to the greater chance of self-decomposition of
hydrogen peroxide at higher concentrations according to the following reaction.
H
2
O
2
+ OH⋅ → H
2
O + HO
2
⋅
The resultant hydroperoxyl radicals, HO
2
⋅, are much less reactive than the
original hydroxyl radicals. Additionally, the direct reaction of hydroxyl radical with
hydroperoxyl radical will reduce the concentration of both radicals in solution.

100
Table 4.2: Maximum Use Levels for NSF Certified Drinking Water Treatment
Chemicals
Chemical Function Max. Use
Copper Sulfate
*
Algicide 16 mg/L
Ferrous Sulfate Coagulation/Flocculation 400 mg/L
Hydrogen Peroxide 18% Dechlorination/Disinfection
Oxidation
**

5.8 mg/L
166 mg/L
Hydrogen Peroxide 31% Dechlorination/Disinfection
Oxidation
**

3.4 mg/L
97 mg/L
Hydrogen Peroxide 35% Dechlorination/Disinfection
Oxidation
**

3 mg/L
85 mg/L
Hydrogen Peroxide 40% Dechlorination/Disinfection
Oxidation
**

2.6 mg/L
75 mg/L
Hydrogen Peroxide 50% Dechlorination/Disinfection
Oxidation
**

2.1 mg/L
60 mg/L
Hydrogen Peroxide 60% Dechlorination/Disinfection
Oxidation
**

1.75 mg/L
50 mg/L
Hydrogen Peroxide 70% Dechlorination/Disinfection
Oxidation
**

1.5 mg/L
43 mg/L
*
The finished drinking water shall be monitored to ensure that levels of copper
do not exceed 1.3 mg/L.
**
Hydrogen peroxide may be used for the treatment of organic contaminants,
with or without ozone, ultra-violet (UV) light or other advanced oxidation
processes, at a higher concentration level when followed by chlorination of the
treated water. Chlorine residuals shall not exceed 4 mg/L, the EPA proposed
maximum residual level.
Source: NSF International (www.nsf.org)

101
HO
2
⋅ + OH⋅ → H
2
O + O
2

UACP is considered as an environmentally sustainable process which means
it would treat target contaminants without sacrificing natural resources or creating
secondary pollutants. Therefore, UACP using 1000 folds lower dosage of hydrogen
peroxide to reach 75% removal goal, then the synergistic effect involving ultrasound
would take care of the rest 25%.

102

50
500
0.05
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100 1000
Hydrogen peroxide / MTBE
MTBE removal, %

Figure 4.4: MTBE removal rate vs. molar ratio of hydrogen peroxide to MTBE.

103
Table 4.3: Summary of Hydrogen Peroxide Chemistry in UACP.
Pollutants pH Optimal Mole Ratio
(H
2
O
2
: Pollutant)
Reaction
Time
Catalysts
MTBE Neutral 0.05:1 (w/60 minute
sonication)
50-500:1 (w/o
sonication)
Minutes to
hours
Fe
+2
and
Cu
+2

TBA Neutral 0.05:1 (w/60 minute
sonication)
50:1 (w/o sonication)
Minutes to
hours
Cu
+2

TCE, PCE Acid 0.05:1 (w/60 minute
sonication)
50:1 (w/o sonication)
Minutes to
hours
Fe
+2

TOC, BTEX Acid 0.05:1 (w/60 minute
sonication)
50:1 (w/o sonication)
Minutes Fe
+2

Iron,
Manganese

Neutral 1:1 Minutes to
hours
None
Sulfides

Neutral 1:1 Minutes Fe
+2

104
4.3.4 Reactions of Carbonate and Bicarbonate
Reactions of hydroxyl radicals with bicarbonate (HCO
3
−
) and carbonate
(CO
3
2−
) produce carbonate radicals as given by the following reactions (Chen, 1998).
OH⋅ + HCO
3
− → H
2
O + CO
3
−
⋅ k = 8.5 ×10
6
M
-1
s
-1

OH⋅ + CO
3
2−
→ OH
−
+ CO
3
−
⋅ k = 3.9 × 10
8
M
-1
s
-1

Carbonate radicals was found to react with hydrogen peroxide and its
conjugate base to produce hydroperoxyl conjugate base based on the following
reactions:
CO
3
−
⋅ + H
2
O
2
→ HCO
3
−
+ O
2
−
⋅+ H
+
k = 8.0 × 10
5
M
-1
s
-1

CO
3
−
⋅ +HO
2
−
→ HCO
3
−
+ O
2
−
⋅ k = 3 × 10
7
M
-1
s
-1

Also carbonate radical was found to react with hydroxyl radical and go
through self combination reaction. According to American Water Work Association
(AWWA), carbonate radicals were found to react with natural organic matter (NOM)
according to the following reaction:
CO
3
−
⋅ + NOM → products k = 50 (mg of C/L)
-1
s
-1
(4.2×10
3
M
-1
s
-1
)
The various components of carbonate system are interrelated by the following
equilibrium in aqueous systems (Stumm & Morgan, 1996)
CO
2(aq)
+ H
2
O ↔ HCO
3
−
+ H
+
K
6
= 10
-6.352
(k
f1
/k
b1
) (4.46×10
5
/1×10
12
)
pKa = 6.352
HCO
3
−
↔ CO
3
2−
+ H
+
K
7
=10
-10.33
(k
f2
/k
b2
) (4.67×10
1
/1×10
12
)
pKa = 10.33
105
At high pH values the concentration of aqueous carbon dioxide (CO
2(aq)
) will
be very small compared to the carbonate (CO
3
-2
) and bicarbonate (HCO
3
-
) (Stumm
and Morgan, 1996). Therefore, reactions of CO
2(aq)
with other species, mainly its
reaction with hydroxyl radical, will be neglected.
If the concentration of either carbonate or bicarbonate is known initially, then
the initial concentration of the other forms of carbonate system can be calculated
based on the following relations for a closed system (Stumm and Morgan, 1996):

[ ]
[ ]
[ ]








+ +
=
+
+
H
K
K
H
1
C
HCO
7
6
T -
3

[ ]
[ ] [ ]








+ +
=
+ +
6 7
2
7
T 2 -
3
K K
H
K
H
1
C
CO

[ ]
[ ]
[ ]








+ +
=
+
+ 2
7 6 6
T
2(aq)
H
K K
H
K
1
C
CO

where C
T
= [HCO
3
-
] [CO
3
-2
] [CO
2(aq)
]

106
The scavenging of hydroxyl radical by carbonate and bicarbonate ions is well
documented (Chen, 1998). The concentration of bicarbonate ion used ranged from 0
to 2000 mg/L. There is no significant inhibition when low concentration of
bicarbonate ranged from 0 to 200 mg/L presented in solution. Bicarbonate
concentration can affect the degradation rate only when bicarbonate level reaches
2000 mg/L, while typical bicarbonate levels are 100-500 mg/L in groundwater well.
This result also shows that hydroxyl radicals react with MTBE in the interiors of
collapsing bubbles where extreme conditions of temperature and pressure exist, but
not in the bulk solution (Kang, 1998).

4.3.5 Field Applications
UACP can generate hydroxyl radicals to destroy many organic substrates
including MTBE, BETX, TCE, PCE, and DBPs. Although Hydroxyl free radical can
kill germs, its half-life time is too short to form disinfectant residual in treated water.
Therefore, UACP can still earn some disinfection credits, because UACP can react
with NOMs which would reduce chlorine usage and bring the breakpoint of
chlorination to a much lower dosage, as shown in Figure 4.5. In the mean time,
UACP can therefore dramatically decrease the formation of DBPs.
107

Figure 4.5: Breakpoint chlorination curve.
108
UACP can also be used for removal of iron and hydrogen sulfide, enhancing
the taste/odor removal efficiency of AOPs, and dechlorination when using reclaimed
water for groundwater recharge. The followings are UACP’s potential applications:
• Minimization of DBPs formation;
• Wellhead disinfection and oxidation
• Pretreatment for VOC removal;
• Oxidation of iron and manganese;
• Prevention of regrowth in the distribution;
• Removal of taste and odors through chemical oxidation;
• Improvement of coagulation and filtration efficiency;
• Prevention of algal growth in sedimentation basins and filters;
• Removal of color;
• Oxidation of desulfurization for gasoline and diesel;
• Regeneration of active carbon;
• Resource recovery: cracking and upgrading of fossil fuels; and
• Chemical modification of asphaltene and kerogen.
In-situ ultrasound treatment has been used for the separation of oil from
petroleum-contaminated soil. For subsurface operations in the field, a robotic self-
powered mining head containing in excess of 200 ultrasonic transdusers can be
attached on the inner wall of the double walls pipe. The drilling bit combines
mechanical, hydraulic, and pneumatic systems to move through the contaminated
109
zone. In addition, this new technology of HDD can greatly increase the effective
areas of the UACP.
UACP can be applied to in-situ permeable reactive barriers (PRB) by
installing the wall made of iron and copper with ultrasound transducers attached.
(Chang, 2000). In addition, it can also enhance bioremediation, because many soil
microorganisms can possess enzymes that detoxify hydrogen peroxide by converting
it to oxygen. Some aquifers have low metal salts content or high alkalinity, it will
cause less free radical produced from Fenton’s reagent. In order to better the
performance of the UACP, it might be necessary to inject some ferrous ion into
aquifers. Furthermore, some market available oxidative enzymes like liginin
peroxidase from white rot fungi can be used as replacement for hydrogen peroxide in
UACP.
According to the EPA’s 2004 inventory data, there are approximately
170,000 public water systems in the United States, out of which 85% are small water
systems. EPA classifies small water system as a community water system serving
less than 3,300 people. The small water system only has a water pump and well-head
treatment which is disinfection process only, as shown in Figure 4.6. It often has
financial, managerial, and technical problems. Once there are contamination plumes
moving toward the small water system, the only choice is to shut down the
groundwater well. The other option would be to install a treatment process which
will take care of the problem but it requires more funding and space. As shown in
Figure 4.7, UACP can be easily add onto current setting as part of well-head
110
treatment. By installing UACP, not only does it protect small water systems but it
also prevents groundwater resources from being wasted.
For some extremely impaired water sources which have either high
concentration of contaminants or multiple contaminants, UACP can collaborate with
other treatment technologies such as GAC, air stripping, and other AOPs. An add-on
or portable/removable device would increase operation flexibility of a drinking water
treatment chain. For example, UACP can be used to treat groundwater at the start of
a treatment chain or as a polishing step at the end as shown in Figures 4.8 and 4.9.
UACP creates hydroxyl radicals which can also decompose disinfection by-
products (DBPs) or used as an alternative disinfectant. The Stage 2 DBPR was
proposed in August 2003 and promulgated on December 15, 2005. This rule builds
on the other DBP rules to further reduce exposure to DBPs and decrease potential
cancer, reproductive, and developmental risks.
Under the Stage 2 DBP rule, drinking water suppliers are required to conduct
an evaluation of their distribution systems, known as an Initial Distribution System
Evaluation (IDSE), to identify the locations with high disinfection byproduct
concentrations. These locations will then be used by the systems as the sampling
sites for Stage 2 DBP rule compliance monitoring. Compliance with the maximum
contaminant levels for two groups of disinfection byproducts (TTHM and HAA5)
will be calculated for each monitoring location in the distribution system. This
approach, referred to as the locational running annual average (LRAA), which
determine compliance by calculating the running annual average of samples from all
111
monitoring locations across the system. Therefore, UACP can be added to pumping
stations or storage reservoirs in the distribution systems where have shown high
DBPs concentration to meet the Stage 2 DBP rule, as shown in Figure 4.10.
In addition, EPA proposed maximum chlorine residual level is 4 mg/L.
However, EPA now requires dechlorination of reclaimed or recycled water when
they are used for groundwater recharge. UACP offers many benefits as a
dechlorinating process. Unlike dechlorination methods based on sulfur dioxide or its
salts, UACP leaves behind no additional salts to further contaminate the effluent.

Figure 4.6: The most popular groundwater well-head treatment – only disinfection then directly send to distribution system.
Chlorine
Solution Tank
Production Well
Injection valve
Pump
Distribution System
DISINFECTION
112

Figure 4.7: UACP can be easily add onto current setting as part of well-head treatment.
Fenton’s reagent
Production Well
Injection valve
Pump
Distribution
System
DISINFECTION
113

Figure 4.8: UACP can be used to treat raw water at the start of a treatment chain.

Production Wells
Distribution
System
DISINFECTION UACP
Treatment Chain:
Combination of the following
treatment processes:
Air Stripping Towers
GAC filters
AOPs (UV, Ozone, etc.)
114

Figure 4.9: UACP can be used as a polishing step at the end of treatment chain.
Production Wells
Distribution
System
DISINFECTION UACP
Treatment Chain:
Combination of the following
treatment processes:
Air Stripping Towers
GAC filters
AOPs (UV, Ozone, etc.)
115

Figure 4.10: UACP can be added to the distribution systems or storage reservoirs to solve DBPs problem.

Production Wells
Distribution System
DISINFECTION UACP
Treatment Chain:
Combination of the following
treatment processes:
Air Stripping Towers
GAC filters
AOPs (UV, Ozone, etc.)
UACP
116
117
Chapter 5: Application of Hydrodynamic Cavitation Reactor in Sustainable Chemical
Process Treating MTBE in Groundwater

5.1 Introduction
5.1.1 MTBE in Groundwater
Methyl tertiary butyl ether (MTBE) has been used initially as an octane
enhancer to reduce carbon monoxide emissions. However, this environmental
recalcitrant compound has been detected from many groundwater wells because it is
highly soluble in the water and it migrates quickly when it leaks. Beside chronicle
health effect, most people can smell MTBE in water at relatively low concentrations.
Therefore, California’s secondary maximum contaminant level (MCL) is 5 ppb
which is also the odor threshold. The water quality report for MTBE is as high as
210 ppm at certain wells. At this stage, scientists are still assessing the health risks
of MTBE; all groundwater needs to be treated to ppb level, which is a challenge for
current available technologies.
Many researches had shown that MTBE can be degraded rapidly in
atmosphere, but really slow in biodegradation, even with many different
enhancements (EPA, 1998). Due to the fact that MTBE has high solubility and low
Henry’s constant, conventional air stripping towers and GAC filters can only have
low efficiency and require extra off-gas treatment. As for advanced oxidation
118
processes, ozonation system could lead to excessive bromate conversion in the
groundwater.
Groundwater is a very important drinking water source. One of the giant
aquifers in the United States is called Ogallalo aquifer which underlies parts of eight
states. The main inputs of substances into the groundwater occur from the land
surface through the soil zone by infiltration. A contaminant plume either from point
source or non-point sources can spread with time with the groundwater flow and lead
to a long-term and large scale effects on drinking water supply.
There are approximately 170,000 public water systems in the United
States. EPA classifies these water systems according to the number of people they
serve and the source of their water. According to EPA’s 2004 inventory data, 78 %
of community water systems are using groundwater as their main water source, but
these water systems only serve 33% population and they are usually very small water
companies, as shown in Figures 5.1 and 5.2. These small water systems usually only
have a water pump and a disinfection process which is not better than the treatment
facilities in the third world countries. Once there is a MTBE plume approaching
these small water systems, the only choice for them is shut down the groundwater
wells if they have no enough funding or space to install an advanced treatment
process.

119

Groundwater
systems
78%
Surface
water
systems
22%

Figure 5.1: Percentage of groundwater systems and surface water systems. (Source:
EPA Drinking water and groundwater statistics for 2004)
120

Population
served by
surface
water
systems
67%
Population
served by
groundwater
systems
33%

Figure 5.2: Percentage of population served by groundwater systems and surface
water systems. (Source: EPA Drinking water and groundwater statistics
for 2004)
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5.1.2 Ultrasound-Assisted Chemical Process
The ultrasound-assisted chemical process (UACP) involves Fenton’s reagent
and ultrasonic cavitation. Fenton’s reagent is a combination of hydrogen peroxide
and ferrous salt, and it is an effective oxidant. It was developed in the 1890s by
H.J.H. Fenton. Ferrous Iron (Fe
+2
) is oxidized to ferric iron (Fe
+3
) by hydrogen
peroxide to a hydroxyl radical and a hydroxyl anion. Fe
+3
is then reduced back to
Fe
+2
by the same hydrogen peroxide (disproportionation) to a peroxide radical and a
proton.
Fe
+2
+ H
2
O
2
→ Fe
+3
+ OH· + OH
−

Fe
+3
+ H
2
O
2
→ Fe
+2
+ HO
2
· + H
+

In the net reaction the presence of iron is truly catalytic and two molecules of
hydrogen peroxide are converted into two hydroxyl radicals and water. The
generated radicals then engage in MTBE degradation reactions.
Ultrasonic irradiation initiated a free radical reaction at the cavitation centers,
which eventually can degrade MTBE. The addition of small amount of hydrogen
peroxide and metal salts in this process is considered as a free radical initiator.
Fenton’s reagent can be continuously supplied from sonication of water and naturally
occurring metal salts in the groundwater. Chain reactions involving ultrasound and
Fenton’s reagent are the epitome of this system. Hydroxyl radicals play the most
important role in attacking groundwater contaminants. In these systems radical
oxidation by Fe
3+
is an electron-transfer process, but if Cu
2+
is involved, then the
mechanism is either ligand transfer or an organocopper intermediate.
122
5.1.3 Ultrasound
Ultrasound is defined as high-frequency sound waves, which are above the
range of human perception. Usually, they start at 20 kilohertz (kHz) and go up into
the megahertz range. Ultrasonic technology, which is one of the most
environmentally friendly processing techniques, is now being increasingly used for a
wide range of different applications. The principles of ultrasonic degrading
environment pollutants which involve the generation of hydroxyl radicals in the
solution are similar to other AOPs (Chen 1998). Degradation of MTBE by using
Fenton’s reagent with ultrasound enhancement in aqueous solution has been
performed in both batch and continuous flow modes (Chang et al. 2000). The
degradation of MTBE was characterized to two stages, a fast one which involved the
reaction of ferrous iron with hydrogen peroxide (Fenton reagent) and another,
relatively, slower stage which involved the additional hydrogen peroxide generated
by sonolysis of water.
Liquids irradiated with ultrasound can produce cavitation bubbles. These
bubbles oscillate, growing a little more during the expansion phase of the sound
wave than they shrink during the compression phase. Under the proper conditions
these bubbles can undergo a violent collapse, which generates very high pressures
and temperatures in microscopic scale. It has been experimentally shown that the
localized high temperatures of 2000–5000 K and pressures up to 1000 atm has been
created inside and on the surface of the collapsing cavitation bubbles (Suslick 1988).
123
Ultrasonic transducers may be classified as either gas, liquid, or electro-
mechanical devices, among which, electro-mechanical transducer is the most popular
used in the medical and chemical industry. Electro-mechanical transducer, or so
called piezoelectric transducer, is usually to be referred to that convert electrical
energy into sound. Piezoelectric crystals can change size when an electric voltage is
applied, and produce high frequency sound waves. However, piezoelectric
transducers are expensive, consuming electricity and require considerable
maintenance.

5.1.4 Hydrodynamic Cavitation Reactor
In devices such as propellers and pumps, cavitation is an undesirable
occurrence. It causes a great deal of noise, damage to components, vibrations, and a
loss of efficiency. Cavitation erosion occurs when bubbles or cavities collapse on the
eroded surface. Cavitation is the formation and collapse, within a liquid, of cavities
or bubbles that contain vapor or gas.
Normally, cavitation originates from changes in pressure in the liquid brought
about by turbulent flow or by vibration, but can also occur from changes in
temperature, such as boiling. The boiling point is defined as the temperature at
which the saturated vapor pressure of a liquid is equal to the surrounding
atmospheric pressure. For water, the vapor pressure reaches the standard sea level
atmospheric pressure of 760 mmHg (1 atm) at 100°C. Since the vapor pressure
increases with temperature, it follows that for pressure less than 760 mmHg, the
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boiling point will be lower than 100°C, as shown in Table 5.1 and Figure 5.3.
Cavitation occurs whenever the local pressure in a fluid drops below its saturated
vapor pressure.
Although cavitation is undesirable in many circumstances, the shock wave
induced by cavitation can be used for many different applications. For example,
ultrasonic cleaning devices use sound waves and use the collapse of the cavitation
bubbles to clean surfaces. In industry, cavitation is often used to homogenize, or mix
and break down suspended particles in a colloidal liquid compound, such as paint
mixtures, or milk. Many industrial mixing machines are based upon this design
principle. It is usually achieved through impeller design, or by forcing the mixture
through an annular opening that has a narrow entrance orifice with a much larger exit
orifice. This method can be controlled with hydraulic devices that control the size of
the inlet orifice. The outer surface of this type of mixing valve, upon which the
cavitation bubbles are driven against to cause their implosion, undergoes tremendous
stress, and is often constructed of super-hard or tough materials such as stainless
steel. Cavitating water purification devices have also been designed, in which the
extreme conditions of cavitation can break down pollutants and organic molecules.

125

Table 5.1: Saturated Vapor Pressure, Density for Water
Temperature
(°C)
Temperature
(°F)
Saturated
Vapor Pressure
(atm)
Saturated
Vapor Pressure
(mmHg)
Saturated
Vapor Density
(g/m
3
)
0 32 0.00603 4.58 4.85
20 68 0.02304 17.54 17.30
40 104 0.07274 55.30 51.10
60 140 0.1966 149.40 130.50
80 176 0.4672 355.10 293.80
100 212 1.000 760.00 598.00

126

760 mmHg(1 atm) at 100°C 760
0
100
200
300
400
500
600
700
800
0 20 40 60 80 100
Temperature, °C
Vapor Pressure, mmHg

Figure 5.3: Saturated vapor pressure for water.

127
Our mother nature also knows how to use the power of cavitation. Both
snapping shrimp and mantis shrimp use a pair of large raptorial appendages to strike
hard objects with such high speeds that cavitation bubbles form between the
appendage and breaking shells. Based on high-speed imaging, force measurements
and acoustic analyses, it is evident that a short, intense flash of light is emitted as the
bubble collapses, indicating that extreme pressures and temperatures of at least
5,000 K must exist inside the bubble at the point of collapse (Patek et al. 2005). This
phenomenon has been defined as shrimpoluminescence which is similar to
sonoluminescence.
A static in-line mixer usually applied in water and wastewater treatment
plants for blending purpose, as shown in Figures 5.4 and 5.5. It converts the kinetic
energy of a high-velocity liquid stream into intense mixing. This conversion is
created by a physical phenomenon known as jet-edge tone. It is essentially a passive
device, in which a stream of process fluid is forced through an elliptical orifice.
From there it passes over a blade-like obstacle. Between the orifice and blade, the jet
of liquid sheds vortices perpendicular to the original flow vector. The shedding
pattern creates a steady oscillation in the sonic range within the liquid. The stress
from these oscillations forces the liquid to cavitate in the ultrasonic frequency range.
Therefore, a static in-line mixer can be used as a hydrodynamic cavitation reactor or
mechanical ultrasound transducer to create cavitation bubbles.

128

Figure 5.4: A static in-line mixer.

129

Figure 5.5: Design parameters for static in-line mixers.
130
The cavitation number expresses the relationship between a flow’s pressure
and kinetic energy, and is used to characterize the potential of the flow to cavitate.
The cavitation number (σ) can be expressed as the following equations.
2
2
1
V
p p
v
ρ
σ
- =
where
ρ = density of the fluid (kg/m
3
)
p = local pressure (Pa)
p
v
= vapor pressure of the fluid (Pa)
V = velocity of the fluid (m/s)
The cavitation number, or so called the Euler number, is a dimensionless
value common used for analyzing fluid flow dynamics problems where the pressure
difference between two point is an important variable. It can be interpreted as a
measure of the ratio of pressure forces to inertial forces. For an in-line mixer, the
resistance of a turbulent flow to cavitation is given by its cavitation number (σ).
u
d
d u
v d
p
p
p p
p p
≈
- - = σ

The parameters p
d
, p
u
, and p
v
are the downstream, upstream, and vapor
pressures, respectively, and the approximation holds when p
u
> p
d
>> p
v
, as they do
under the experimental conditions. An increase in upstream pressure should
decrease cavitation number σ and increase cavity length, as shown in Figure 5.6.
131

Cavity Length
Cavitation Number

Figure 5.6: A typical relation between the cavity length and the cavitation number.

132
The general concept of sustainable development is to develop sustainable
treatment processes that meet the needs of the present without compromising the
ability of future generations to meet their own needs. Environmentally sustainable
design is becoming part of mainstream. To develop processes that are
environmentally sustainable as well as economically sound, a mechanical ultrasonic
transducer, such as an in-line mixer, was incorporated into UACP which requires less
chemicals and land use.
The goal of this study was to determine the effectiveness of oxidizing MTBE
with Fenton’s reagent and enhanced by ultrasonic liquid transducer which creates a
lot of collapsing cavitation bubbles in the system. The mechanism producing this
behavior is the inducement of the growth and collapse of cavitation bubbles driven
by the high frequency acoustic pressure fluctuations. Cavitation bubble collapse
produces extremely high local pressures and temperatures. Such conditions are
believed to produce hydroxyl radicals which are strong oxidizing agents. We have
applied hydrodynamic cavitation to contaminated water by the use of static in-line
mixer to trigger widespread cavitation and induce oxidation in the bulk solution.
Experiments were conducted in recirculating flow loops. Temperature, pH,
pressures, and flow rates were controlled and systematically varied. Samples of the
liquid were taken and the concentration of MTBE measured with a GC/MS.
Experiments were conducted in parallel with a pentagonal ultrasonic liquid processor
for comparison.
133
5.2 Materials and Experiment Methods
5.2.1 Materials
All chemicals used were at least reagent grade. Hydrogen peroxide (30%
solution) was certified ACS grade and used as received from Fisher Scientific
(Pittsburgh, PA). All solutions were prepared in deionized water. Test solutions of
MTBE at 100 mg/L were prepared in deionized water and stored in aluminum foil
wrapped volumetric flasks at 4°C. Calibration standards were prepared by serial
dilution of the stock solutions. Test samples for MTBE degradation studies were
prepared from appropriate stock solution immediately before use.
Hydrogen peroxide solution was prepared by diluting 30% H
2
O
2
by deionized
water. Ferrous Sulfate solution for Fenton’s reagent was prepared by dissolving
FeSO
4
⋅7H
2
O solids in deionized water.

5.2.2 Pentagonal Ultrasonic Liquid Processor
The outside of the tube is pentagonal, whereas the inside is cylindrical.
Transducers were placed on three opposing sides, of the outer surface, as shown in
Figure 5.7. This design can increase the intensity and focus of the ultrasonic
irradiation. Once the Fenton’s reagent enters the tube it will flow downward, using
the force of gravity to move the chemical. As it flows it will be continually mixed by
the transducers.
134

Figure 5.7: The pentagonal ultrasonic liquid processor.
Overall diameter: 12 inches
Process chamber diameter: 6 inches
Overall length: 30 inches
Chamber capacity: 4 gallons
Construction: 316 Stainless
Operating frequency: 40 kHz
Shipping weight: 60 lbs.
135
The tube will go through a series of bottlenecks to concentrate the intensity of
mixing. The final portion of the tube will be used for cooling. A cooling coil will be
wrapped around the outside. The treated water can then be re-circulated by an
external pump.

5.2.3 Static In-Line Mixer
A whistle type in-line mixer installed in a recirculating flow loop without external
electric power was used to create hydrodynamic cavitation and induce oxidation in
UACP.

5.2.4 Experimental Method
The total volume of the test solution was 15 liter, and the solution passing
through the ultrasonic transducer is circulated by a pump. A booster pump provided
a minimum pressure of 10 pounds per square inch gage pressure (psig) at all times.
MTBE samples were analyzed by the U.S. EPA Standard Method 524.2, a
purge-and-trap technique combined with gas chromatography/mass spectrometry
(GC/MS). Curves of MTBE concentration versus time were then created and
compared with control and initial values. MTBE is a regulated drinking water
contaminant in California. MTBE has a primary MCL of 13 micrograms per liter
(μg/L), established in 2000, that addresses health concerns, and a secondary MCL of
5 μg/L, established in 1999. The secondary MCL addresses taste and odor concerns.
A detection limit for purposes of reporting (DLR) is 3 μg/L.
136
5.3 Result and Discussion
5.3.1 Pentagonal Ultrasonic Liquid Processor
The Pentagonal Ultrasonic Liquid Processor is a continuous flow reactor. It
equipped with 30 highly efficient piezoelectric transducers bonded to each of the five
sides of the process tube to assure maximum energy in the process. This design
assures uniform ultrasonic activity through the volume of the processor. The tube is
constructed of type 316 stainless steel for compatibility with acidic Fenton’s reagent.
In addition, multiple processors can be used in series to increase ultrasonic exposure.
Fenton’s reagent reacts with MTBE very rapidly under high intensity
ultrasonic field. The decomposition efficiency reached 74% in less than a minute.
The experimental results have also shown 90% MTBE degradation through five
minutes sonication, as shown in Figure 5.8.
137

Fenton’s Reagents
Ultrasound
0
10
20
30
40
50
60
70
80
90
100
-1 0 1 2 3 4 5
Sonication Time, minutes
30 transducers (4 gal Tank)
in-line mixer (4 gal tank)
Concentration, ppm

Figure5.8: MTBE degradation by UACP.

138
5.3.2 In-line Mixer
A static in-line mixer converts the kinetic energy of a high-velocity liquid
stream into intense mixing. This conversion is created by a physical phenomenon
known as “jet-edge tone.” It is essentially a passive device, in which a stream of
process fluid is forced through an elliptical orifice. From there it passes over a blade-
like obstacle. Between the orifice and blade, the jet of liquid sheds vortices
perpendicular to the original flow vector. The shedding pattern creates a steady
oscillation in the sonic range within the liquid. The stress from these oscillations
forces the liquid to cavitate in the ultrasonic frequency range. The high level of
cavitation, shear, and turbulence within the mixing chamber shatters product
particles and emulsifies liquids. Even this technology does not appear to be
appropriate.
The following are advantages over mechanical in-line mixers:
• Lower equipment cost.
• Lower installation cost.
• Lower energy requirement
• Zero maintenance.
• Longer life.

In a typical water treatment plant, the purpose of the static in-line mixer is to
provide a thorough and complete mixing of the raw water and coagulant chemicals.
As the MTBE-contaminated water enters the in-line mixer, its velocity is
139
dramatically increased. There is also a corresponding decrease in vapor pressure,
which causes vapor bubbles to form in the mixer. As the water flows through the
mixer, its velocity is again decreased and the vapor pressure is again increased,
resulting in an implosion of the bubbles. Bubble formation and implosion causes the
cavitation effects. The intense energy release and turbulence associated with
cavitation causes the degradation of MTBE. Experimental data have shown 13
percent addition to 70 percent degradation of MTBE by Fenton’s reagent in five
minutes.
Although UACP is a less powerful oxidation process than other AOPs, it
requires less equipment to feed and maintain a water production system. In most
cases, a chemical pump and static in-line mixer are all that is required. UACP can
easily be incorporated into the existing drinking water treatment system which might
include air stripping tower or granular activated carbon filter. For example, a
drinking water treatment plant was designed for treating PCE, and TCE. When the
fast-moving MTBE towards to the groundwater wells, the water supplier can easily
add UACP to the existing treatment processes. Once MTBE has been completely
gone and PCE and TCE contamination problem still remain, UACP can be either
decommissioned or used as a polish unit.
Although conventional well-head treatment can also be easily modified by
increasing air flow rate for air stripping tower or using different grade of activated
carbon for the filters to treat MTBE, the capital cost and operation cost still much
higher than UACP.
140
UACP using in-line mixer as its ultrasound source could benefit small water
systems where electricity is not needed to run the ultrasonic transducer. Diesel
engine or natural gas engine can be used to pump the water until the flow rate
reaches certain levels, cavitation bubles will be formed inside the pipe. This
particular pipe is made out of stainless and anti-corrosion material. The inner design
is similar to static in-line mixer and liquid whistle. Liquid transducers can be
installed in the inner wall of the double-wall pipe.
Cavitation could result in accelerated corrosion in piping systems. When
cavitation bubbles hit the metal surface, they rapidly collapse or implode, producing
a shock wave sufficiently strong to erode the pipe. The inside of a pipe can be
affected by chemical corrosion and erosion. Turbulent flow in the pipe can cause
cavitation, particularly along elbows and T-joints.
To avoid cavitation from turbulent flow, the piping system design would
usually incorporate a minimum number of bends and elbows and the velocity of the
fluid flow in the pipe should not be too high. Some prevention methods for reducing
cavitation in piping systems include improving the flow lines within the pipe,
allowing bends to have larger angles, and changing pipe diameters gradually rather
than abrupt changes. Other methods include slowing the flow rate to reduce
turbulence, reducing the amount of dissolved oxygen, changing the pH, and changing
the pipe material to a different metal or alloy.
A pressure relief valve might need to be installed at the down stream of the
UACP to prevent water hammer in piping systems. Water hammer is a pressure
141
surge or wave caused by the kinetic energy of a fluid in motion when it is forced to
stop or change direction suddenly. It depends on the fluid compressibility where
there are sudden changes in pressure. Moving water in a pipe has kinetic energy
proportional to the mass of the water in a given volume times the square of the
velocity of the water. For this reason, most pipe-sizing charts recommend keeping
the flow velocity at or below 5 ft/s (1.5 m/s).
142
Chapter 6: Conclusion

6.1 The Use of UACP in the Treatment of MTBE
MTBE is the abbreviation for the man-made compound methyl tertiary butyl
ether. In the mid-1990s, MTBE was used as oxygenate in the reformulated gasoline
(RFG) to reduce carbon monoxide emission. However, while MTBE is not really
solving the smog problem, it has contaminated hundreds of groundwater wells across
the country. Consequently, many drinking water suppliers have had to shut down a
number of their water wells or monitor for MTBE as required by regulatory agencies.
Many water quality data have shown that MTBE usually comes fast and leaves fast
pending on the groundwater flow and well hydraulics.
MTBE contamination can be cleaned up in groundwater using existing
technologies such as air stripping, granular activated carbon (GAC), and advanced
oxidation process (AOP). However, these methods are often costly and time
consuming. Air stripping usually needs a higher air to water ratio to remove MTBE
from groundwater, because MTBE has high solubility in water and low Henry’s
constant. In addition, MTBE does not stick well to carbon; therefore, high volumes
of the contaminated water must repeatedly pass through a GAC system before MTBE
is effectively removed.
UACP involves ultrasonic cavitation and a modified Fenton’s reagent which
is a solution of hydrogen peroxide, copper ion, and ferrous ion without adjusting pH.
143
UACP can effectively oxidizes a wide range of compounds including MTBE, BTEX,
TBA, TCE, PCE, DBPs as well as natural organic matter. A synergistic effect has
been observed in the chain reactions involving formation of hydroxyl radicals.
UACP needs only very low dosage of hydrogen peroxide and metal ions to
initiate the reaction, because they both can be continuously supplied by sonolysis of
water and naturally occurring metal salts in groundwater. In UACP, radical
oxidation by Fe
3+
is an electron-transfer process, but if Cu
2+
is added, then the
mechanism is either ligand transfer or an organocopper intermediate. Under bench-
scale conditions, the MTBE degradation efficiency was 78 percent over the first one
minute or even sooner when Fenton’s reagent was added, and complete removal was
achieved in one hour of sonication.
The strategy of using UACP in the treatment of MTBE is to form a
multi-barrier treatment with other EPA approved treatment technologies. A multi-
barrier treatment, or so called treatment chain, is a set of independent treatment
processes placed in series, and each barrier should effectively reduce the contaminant
by a significant fraction of the total required reduction. UACP can be placed at the
start of a treatment chain to protect the whole treatment chain and prevent
breakthrough problem with GAC filters, when the raw water contains high or
fluctuated concentration of MTBE. UACP can also be placed at the end of a
treatment chain as a polishing step to treat the extremely impaired source which
contains multiple contaminants and unknown contaminants prior to direct usage in a
144
domestic water distribution system. In addition, UACP can be added to the pump
stations or storage reservoirs to control DBPs hotspots in the distribution system.
UACP is considered as an environmentally sustainable process which means
it would treat target contaminants without sacrificing natural resources or creating
secondary pollutants. The chemicals used in UACP are environmental benign.
UACP using 1000 folds lower dosage of hydrogen peroxide compared to other AOPs
to reach 75% removal goal, then the synergistic effect involving ultrasound would
take care of the rest 25%. In the mean time, ferrous iron can be continually provided
from the dissolution of naturally occurring iron (Fe
0
) by H
2
O
2
in the groundwater.
A whistle type in-line mixer installed in a recirculating flow loop without
external electric power was used to create hydrodynamic cavitation and induce
oxidation in UACP. Experimental data have shown 13 percent addition to 75 percent
degradation of MTBE by Fenton’s reagent in five minutes.
145
6.2 Application of Acoustic Cavitation in Sustainable Chemical Process
Designing an environmentally sustainable process to remove MTBE from
groundwater without compromising natural resources is our ultimate goal. UACP
combines the benefits of a conventional well-head treatment with low consumption
of man-made chemicals to maintain a physical, chemical, and biological environment
that contributes positively to health, prevents illness, and assures protection of the
public. In terms of required equipment, UACP only needs chemical pumps and
ultrasonic transducers. This allows for low cost, easy installation, convenient
maintenance, and easy decommissioning.
Environmental pollution and natural resources shortages are both global
issues today. Although MTBE has been phased out in many states of the United
States, it still is the most popular fuel additive in many European and Asian countries
due to the low production cost and fuel price. There are approximately 170,000
public water systems in the United States. EPA classifies these water systems
according to the number of people they serve and the source of their water.
According to EPA’s 2004 inventory data, 78 percent of community water systems are
using groundwater as their main water source, but these water systems only serve
33 percent population and they are usually small or very small water systems (served
less than 3,300 people). A small water system only has a water pump and a
disinfection process which is no better than the current treatment facilities in the
developing countries. Once there is a contamination plume approaching these small
water systems, the only choice for them is to shut down the groundwater wells unless
146
they have enough funding or space to install an advanced treatment process. By
installing UACP, it can protect many small water systems and prevent wasting
groundwater resources.
Usually, cavitation is undesirable because it produces extensive erosion of the
rotating blades. The cavities form when the pressure of the liquid has been reduced
to its vapor pressure; they expand as the pressure is further reduced along with the
flow, and they collapse violently when they reach regions of higher pressure.
However, UACP using hydrodynamic cavitation reactor as its ultrasound
source could benefit small water systems where electricity is not needed to run the
ultrasonic transducer. An in-line mixer can create cavitation in UACP and it
functions like a mechanical ultrasound transducer. Diesel engine or propane gas
engine can be used to pump the water until the flow rate reaches certain level, and
cavitation bubbles will be formed by passing through the static in-line mixer. This
particular mixer and piping systems are made out of stainless and anti-corrosion
material. The inner design is similar to liquid transducer which can be installed in
the inner wall of the double-wall pipe.
Furthermore, UACP has shown that it can be used for other applications such
as color removal, iron and manganese removal, taste and odor control (oxidation of
sulfides). It also supplies supplemental dissolved oxygen for bioremediation. For
some extremely impaired water sources which have either high concentration of
contaminants or multiple contaminants, UACP can collaborate with other treatment
technologies such as GAC, air stripping, and other AOPs.
147
For subsurface operations in the field, multiple ultrasonic transducers
attached on the inner wall of the double-wall pipe and horizontal directional drilling
(HDD) technology can greatly increase the effective area and prevent further
migration.
Therefore, UACP is a global solution for groundwater systems. UACP also
provides a theoretical background for using hydraulic in-line mixer to treat
groundwater contamination.
148
6.3 Recommended Future Work
The intensity and frequency of ultrasound affect the degree of chemical
reaction. An empowered whistle-type mechanical ultrasound transducer need to be
further studied. They may enhance the efficiency of the UACP. The effect of
cavitation number on the efficiency of degradation also needs to be determined.
Precipitation occurred in the copper ion catalyzed Fenton’s reagent, especially
when copper reacts with TBA. The precipitation study which includes element
analysis and toxicity test will provide knowledge for the modification on the UACP.
UACP with an automatic feedback system controlling ultrasound intensity
and chemical dosage would make it easier to operate and monitor the treatment
chain. A modified UACP using on-site generation of hydrogen peroxide or solid
hydrogen peroxide would hopefully reduce cost as well as storage and transportation
problems.
Further studies of the potential environmental impact for the use of other
oxygenates like ethanol, should be conducted. Many states have replaced MTBE
with ethanol. The compatibility of ethanol with the various compounds used for
connections, seals, joints, pipes, and coatings in hoses, engines, and underground fuel
tanks should also be investigated.
149
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Abstract (if available)

Abstract Methyl tertiary butyl ether (MTBE) is used as a fuel additive to reduce air pollution. However, the threat to public health from MTBE contaminated groundwater prompted the legislature to ban MTBE. Through the years, numerous methods have been developed to treat MTBE, and many of these advanced treatment processes are complicated and costly. The present research investigates the development of a simple and cost-effective process for the degradation of MTBE.

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The feasibility studies on sonochemical processes for treating used oil: toxin reduction for eliminating recycle interference

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Creator Chang, Hung-Li (author)

Core Title An environmentally sustainable development of ultrasound-assisted chemical process: the use of Fenton's reagent and ultrasonic transducers to remove methyl tertiary butyl ether from drinking wate...

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Environmental Engineering

Publication Date 10/04/2006

Defense Date 09/07/2006

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Fenton's reagent,hydrogen peroxide,in-line Mixer,MTBE,OAI-PMH Harvest,sustanable development,ultrasound

Language English

Advisor Yen, Teh Fu (committee chair), Pirbazari, Massoud M. (committee member), Shing, Katherine S. (committee member)

Creator Email hunglich@yahoo.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m82

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Document Type Dissertation

Rights Chang, Hung-Li

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

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