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ADSORPTION OF TRACE LEVELS OF ARSENIC AND SELENIUM
FROM AQUEOUS SOLUTIONS BY CONDITIONED
LAYERED DOUBLE HYDROXIDES
by
Megha Dadwhal
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMICAL ENGINEERING)
May 2010
Copyright 2010 Megha Dadwhal
Object Description
| Title | Adsorption of trace levels of arsenic and selenium from aqueous solutions by conditioned layered double hydroxides |
| Author | Dadwhal, Megha |
| Author email | dadwhal@usc.edu; meghadadwhal@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemical Engineering |
| School | Viterbi School of Engineering |
| Date defended/completed | 2009-10-29 |
| Date submitted | 2010 |
| Restricted until | Unrestricted |
| Date published | 2010-01-15 |
| Advisor (committee chair) |
Tsotsis, Theodore T. Sahimi, Muhammad |
| Advisor (committee member) | Pirbazari, Massoud M. |
| Abstract | Arsenic (As) and Selenium (Se) are found in water in the form of oxyanions. Relatively high concentrations of As and Se have been reported both in power plant discharges, as well as, in fresh water supplies. The International Agency for Research on Cancer currently classifies As as a group 1 chemical, that is considered to be carcinogenic to humans.; The focus of the present work is a systematic study of the adsorption of As and Se by conditioned layered double hydroxide (LDH) adsorbents. Conditioning the adsorbent significantly reduced the Mg, Al dissolution observed with uncalcined and calcined LDH. The adsorption rates and isotherms have been investigated in batch experiments using particles of four different particle size ranges. As(V) adsorption is shown to follow a Sips-type adsorption isotherm. The As(V) adsorption rate on conditioned LDH increases with decreasing adsorbent particle size; the adsorption capacity, on the other hand, is independent of the particle size. A homogeneous surface diffusion model (HSDM) and a bi-disperse pore model (BPM) - - the latter viewing the LDH particles as assemblages of microparticles and taking into account bulk diffusion in the intraparticle pore space, and surface diffusion within the microparticles themselves - - were used to fit the experimental kinetic data. The HSDM estimated diffusivity values dependent on the particle size, whereas the BPM predicted an intracrystalline diffusivity, which is fairly invariant with particle size.; The removal of As(V) on conditioned LDH adsorbents was also investigated in flow columns, where the impact of important solution and operational parameters such as influent As concentration, pH, sorbent particle size and flow rate were studied. An early breakthrough and saturation was observed at higher flow rates and at higher influent concentrations, whereas a decrease in the sorbent particle size and a decrease in influent pH resulted in an increase in the bed volumes treated at breakthrough. Both the HSDM and BPM were shown capable of predicting the column behavior.; Analysis of power plant effluents from various power plants in the greater Los Angeles area shows that, in addition to As and Se, these streams also consist of ions like fluorides, nitrates, chlorides, carbonates, sulphates and phosphates. In this study we have also tried to understand the effect of each of these individual ions on the As and Se removal capacity of LDH. For that, we carried out binary isotherm experiments with varying concentrations of As and of the individual competing ions in order to better understand the competition for the available adsorption sites. We fitted the data using the extended Sips isotherm equation. It was observed that the As adsorption capacity decreases in the presence of various individual competing ions in the order of PO42- > SO42- > CO32- > Cl- > NO3- > F-. For the Se removal from aqueous solutions in the presence of individual competing ions, calcined LDH proved more effective than conditioned LDH.; We also studied the regeneration of LDH and it was shown that even after regenerating the LDH 15 times, the adsorption capacity only decreased by 10%, making it a very economical adsorbent. The TCLP test carried out on As- and Se-saturated LDH, showed that it can be categorized as a non hazardous waste. |
| Keyword | arsenic; selenium; layered double hydroxides; homogeneous surface diffusion mode; bidisperse pore model |
| Geographic subject (county) | Los Angeles |
| Geographic subject (state) | California |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m2799 |
| Contributing entity | University of Southern California |
| Rights | Dadwhal, Megha |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@dots.usc.edu |
| Filename | etd-Dadwhal-3453 |
| Archival file | uscthesesreloadpub_Volume44/etd-Dadwhal-3453.pdf |
Description
| Title | Page 1 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@dots.usc.edu |
| Full text | ADSORPTION OF TRACE LEVELS OF ARSENIC AND SELENIUM FROM AQUEOUS SOLUTIONS BY CONDITIONED LAYERED DOUBLE HYDROXIDES by Megha Dadwhal A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMICAL ENGINEERING) May 2010 Copyright 2010 Megha Dadwhal |
