Mercury Sulfide Dissolution in Environmental Conditions: Thermodynamic and Kinetic Approaches

Abstract

Considering the lack of feasible techniques to differentiate dissolution and readsorption 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 vii 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.viii water, soil, and sediment and can be methylated to the toxic MeHg through microbemediated (e.g., sulfate reducing bacteria, SRB or iron reducing bacteria, IRB) and abiotic processes (7, 9, 10). Because of its lipophilic and protein-binding properties, MeHg is then easily accumulated by aquatic biota through the food web (7).All Hg forms, in particular the organomercury species, are highly toxic substances (11).Acute Hg exposure can produce permanent damage to the nervous and other systems to cause a range of symptoms such as paresthesia, ataxia, sensory disturbances, tremors, renal toxicity, myocardial infarction, and even death (12). Chronic Hg exposure was considered to mainly occur from the consumption of contaminated fish and other aquatic organisms (13,14). Rice is another important pathway for human exposure to MeHg in recent years. 2This was first discovered in Guizhou province, China, and it potentially exists elsewhere (15-17). The toxicity of Hg depends on the chemical form and the sources of exposure (11).The most dangerous mercury species is MeHg, which is obtained mainly from the diet and can be almost completely absorbed into blood and then be distributed to other organs in the human body, such as brain, kidney, liver, hair, and other tissues within a f...