IRON ADDITION DECREASES NET MERCURY METHYLATION IN LABORATORY WETLAND MICROCOSMS

Wetland restoration and construction have received considerable attention in the past few decades. Benefits include habitat for threatened and endangered species, improved water quality, flood mitigation, aquifer recharge, and aesthetic and heritage values. One potential drawback is the formation of monomethylmercury (MMHg), which occurs via a process mediated by sulfate-reducing bacteria. Of all the forms of mercury, MMHg poses the greatest toxicity concern to humans and wildlife. To realize the benefits of wetland restoration and construction, it is necessary to identify a means for minimizing MMHg formation in wetland sediments. For this study, laboratory flow-through micocosms were used to test the efficacy of adding iron to wetland sediments as a means for decreasing the production of MMHg. One set of microcosms was designed to simulate an engineered treatment wetland such as those built adjacent to wastewater treatment plants. The second set of microcosms was designed to simulate a tidal salt marsh. The results of the study suggest that iron decreases net mercury methylation in wetland sediments open to the atmosphere and, therefore, are consistent with results from previous studies with closed microcosms. The results further support the theory that the mechanism by which iron decreases the formation of methyl mercury is not through a decrease in sulfate-reduction rates, but rather via a decrease in the activity of free S(-II). This decrease in free S(-II) likely corresponds to a decrease in the availability of Hg(II) to sulfate-reducing bacteria. Until now, no engineering approach has been available for minimizing the formation of MMHg in wetlands. Iron amendments show promise as a technique for reducing net mercury methylation in wetland sediments.