Date of Award

August 2013

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Shangping Xu

Committee Members

Timothy Grundl, Weon Shik Han

Keywords

Arsenate, Chromate, Zeolite

Abstract

Sorption represents an important strategy in the remediation of groundwater contamination. As a naturally-occurring mineral with large cation exchange capacity, zeolite is negatively charged and has been widely used as an inexpensive and effective sorbent for the removal of positively charged contaminants. The negative charges of zeolite, however, make it generally ineffective in the sorption of anionic contaminants such as chromate and arsenate. In order to improve the capacity for sorption of anionic species, the surface charge of the zeolite must be modified. Cationic surfactants can be used to alter the surface charge of the minerals so that the negatively charged heavy metals can be removed.

The adsorption equilibrium and kinetics data for adsorption of As(V) and Cr(VI) from an aqueous solution onto a green solvent modified zeolite (GSMZ) were determined through batch experiments. A natural zeolite from St. Cloud New Mexico was modified by the surfactant HDmim, from the imidazolium group of chemicals, which are considered as "green solvents". The effects of ionic strength and solution pH on the sorption capacity of As(V) and Cr(VI) on GSMZ were evaluated. Our results indicate that pH has little effect on the removal of both As(V) and Cr(VI) on GSMZ. Zeta potential tests show that for the pH range tested (4-9) the surface charge of the modified zeolite is consistently positive. The removal of arsenate and chromate by GSMZ does not appear to be dependent on speciation at different pH. Meanwhile, competition by chloride ions at different ionic strength was found to have an impact on sorption capacity. The Langmuir competition model was applied to experimental adsorption data to determine the extent of competition between the heavy metal ions and chloride anions. Compared to previously modified zeolites, GSMZ performed well, with a sorption maximum for chromate of about 26 mM/kg and a sorption maximum for arsenate of about 12 mM/kg.

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