Doctoral Dissertations

Date of Award

12-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kula C. Misra, Michael E. Essington

Committee Members

Otto Kopp, Ralphy Turner, Chris Cox

Abstract

Adsorption of mercury on variable charge surfaces of silica, gibbsite, and kaolinite was investigated as a function of solution composition. A popular surface complexation model, the triple layer model (TLM) was used to describe, interpret, and predict mercury adsorption on these surfaces. Kaolinite was modeled as a composite of Al and Si oxide layers represented by gibbsite and silica, respectively, in this study. Mercury adsorption on all three surfaces was a function of pH; adsorption increased with increasing pH, reached a maximum, and then decreased. Evaluation of experimental adsorption data, aqueous speciation modeling of mercury in a 0.1 M nitrate background, and surface complexation modeling revealed that both hydroxy species of mercury, Hg(OH)2° and HgOH + are responsible for the overall retention of mercury by the variable charge surfaces. Below a pH of 5, both Hg(OH)2° and HgOH + undergo complexation on the inner-sphere plane; above a pH of 5, HgOH + does not form in solution, and the retention of mercury is primarily due to the ligand exchange of Hg(OH)2° for the hydroxyl ions on the surface. The kaolinite edge was appropriately modeled as a composite of SiOH and AlOH sites utilizing the surface complexation constants derived from individual sorbent experiments. Ionic strength did not appear to have any effect on mercury adsorption on any of the surfaces. In all the cases, the non-specifically sorbing sodium and nitrate ions were assumed to form ion pairs on the outer-sphere plane, thereby having no effect on the inner-sphere adsorption of mercury. However, other metals, such as calcium, nickel, and lead were determined to have undergone inner-sphere complexation, and hence influenced mercury adsorption on the surfaces of silica, gibbsite, and kaolinite. The competition invoked by calcium was much less compared to nickel and lead, which typically undergo strong specific retention. The competitive effects were much more pronounced on silica compared to gibbsite or kaolinite, which reflected a lack of preference of SiOH sites for mercury over other metals. No single set of adsorption reactions could predict competitive sorption data equally well for all three substrates; this pointed toward a limitation associated with using surface complexation modeling as a predictive tool, particularly in complex systems under high competition. In the presence of chloride, the adsorption edge of mercury shifted to a much higher pH. A comparison of the experimental sorption data with the aqueous speciation and surface complexation model curves revealed that retention of mercury in the presence of chloride is primarily controlled by adsorption of Hg(OH)2°. Inner-sphere surface complexation of a mercury-chloride species, HgClOH°, was assumed to control the adsorption of mercury below pH 7. Other mercury halide complexes were not adsorbed. In the presence of sulfate, mercury adsorption on silica and gibbsite decreased slightly because of the competition invoked by inner-sphere complexation of specifically adsorbing sulfate. In the presence of phosphate, mercury adsorption on kaolinite and silica was reduced to a greater extent. However, for gibbsite, mercury adsorption increased with increasing pH, and then decreased above a pH of 5.5. This increase in adsorption reflected an electrostatic attraction of the gibbsite surface (negatively charged due to specific adsorption of phosphate) for HgOH +. This study showed that both solution chemistry and surface properties govern adsorption of mercury onto variable charge surfaces.

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