Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor's Name

Yong Cai

First Advisor's Committee Title

Committee chair

Second Advisor's Name

Rudolf Jaffé

Second Advisor's Committee Title

Committee member

Third Advisor's Name

Bruce McCord

Third Advisor's Committee Title

Committee member

Fourth Advisor's Name

Martin Quirke

Fourth Advisor's Committee Title

Committee member

Fifth Advisor's Name

Georgio Tachiev

Fifth Advisor's Committee Title

Committee member

Keywords

Mercury sulfide, Dissolution, Re-adsorption, Isotope tracer technique, Dissolved organic matter, Everglades, Hg speciation, PHREEQC, Hg methylation

Date of Defense

11-8-2016

Abstract

Mercury (Hg) is a global contaminant of ecosystems and human health risk, with complicated biogeochemical processes. Mercury sulfide (HgS) dissolution has been suggested as a key process in Hg cycling, as it could potentially increase the pool of inorganic Hg (iHg) for the production of methylmercury (MeHg). Despite previous sporadic observations of enhanced HgS dissolution under certain conditions, much remains unclear on mechanisms of HgS dissolution. The objective of my research was to advance the mechanistic understanding of HgS dissolution, concerning re-adsorption of released Hg, effects of thiol-ligands, and Hg speciation.

Considering the lack of feasible techniques to differentiate dissolution and re-adsorption processes, I first developed an efficient method using isotope tracer and isotope dilution techniques to investigate the re-adsorption of released Hg during HgS dissolution. The HgS dissolution rate with consideration of re-adsorption was two times the rate calculated from detecting Hg alone in the presence of O2, indicating the importance of Hg re-adsorption during HgS dissolution. I further examined the role of Hg-ligand complexation in HgS dissolution and Hg(II) re-adsorption using a thermodynamic adsorption method, selecting L-cysteine (Cys) as a model compound for low molecular weight ligands and Waskish fulvic acid (FA) for natural dissolved organic matter (DOM). My results suggest that the presence of Cys enhanced HgS dissolution through the decreased re-adsorption of Hg-Cys complex, whereas Waskish FA inhibited HgS dissolution, possibly because of the adsorption of FA on HgS surface that covered dissolution sites.

I further employed a geochemical modeling method to study Hg speciation and the relation of iHg speciation to MeHg, aiming to provide a methodological example for potentially evaluating the implications of Hg species distribution during HgS dissolution on MeHg production. I applied geochemical model PHREEQC to the Florida Everglades, a well-studied wetland with model input parameters available, to determine the distribution of iHg in surface water at different sites. The modeling results suggest that sulfide and DOM govern iHg speciation, and the Hg-sulfide and Hg-DOM species are related to MeHg in environmental media but not fish, suggesting the importance of iHg speciation in MeHg production and the complexity of Hg bioaccumulation.

Identifier

FIDC001206

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