Factors influencing the oxidation, reduction, methylation and demethylation of mercury species in coastal waters

Abstract

The objective of this study was to examine the redox reactions and other transformations of mercury (Hg) species in surface waters, and the factors determining the rates of these reactions. For the redox studies completed at the Chesapeake Biological Laboratory (CBL), two isotopes ( 199Hg II and 202Hg 0) were added into different types of filtered water (fresh to seawater) to examine the oxidation and reduction reactions. Further studies of both the redox reactions and methylation/demethylation reactions of Hg were conducted with unfiltered water on board research vessels during cruises in May and July 2005 on the Chesapeake Bay and shelf. While CH 3 199Hg II was added to allow the examination of demethylation, 201Hg II was used to examine both reduction and methylation, and 202Hg 0 was used to examine oxidation. Overall, the results showed that both Hg oxidation and reduction were simultaneously occurring and were photochemically mediated in the waters investigated. In contrast to the previously assumed “unreactive” nature of Hg 0, the studies found that the magnitude of the rate constant for Hg 0 oxidation was greater than that for reduction, indicating its importance in estuarine and coastal waters. In addition, both experiments at CBL and on board ship showed that Hg II reduction was similar in magnitude, suggesting that biotic processes were relatively unimportant. While no measurable methylation occurred during the incubation period during the on board studies, concentration of CH 3 199Hg II decreased over the time during the experiments. It appeared that the demethylation processes were not dominantly photochemically driven, but could be microbially mediated. Further studies are needed in order to help better understand Hg redox and transformations in natural water systems.