| Summary: | Laboratory simulation cells were employed to evaluate the effectiveness of a sand cap for containing highly mercury-contaminated lake sediments and the resultant impacts on the mercury fate beneath the cap. Sand cap can delay and reduce the migration of mercury into the overlying water; however, due to the small partition coefficient of mercury between sand and water, a capping material with higher partition coefficient is better for the long-term containment of mercury. In the following study, efforts were made to identify and investigate an active capping material which works effectively at containing and inhibiting the methylation of mercury. Based on information from literature and our primary experimental results, iron sulfide (FeS) was selected for the following study.
By amendment of laboratory synthesized iron sulfide (Syn-FeS) into sediment slurries spiked with Hg(II) under anoxic conditions, the inhibition effects of FeS on the methylation of mercury were investigated. A commercial iron sulfide (CIS), which was a mixture of several iron-sulfide species, was also investigated in the experiment. Experimental results showed that both Syn-FeS and CIS were good inhibitors of Hg(II) methylation. It was found that MeHg production was not correlated to total dissolved mercury in pore water.
Via batch sorption experiment, the interaction between aqueous Hg(II) and Syn-FeS was studied under anaerobic conditions. The study included effects of the pH of both initial Hg(II) solution and equilibrium suspension on sorption, mechanism of interactions between Hg(II) and FeS, and the stability of immobilized mercury regarding oxidation. Experimental results showed that FeS works effectively at immobilizing aqueous Hg(II) via mostly precipitation reaction and also some adsorption on the solid surface.
Finally, in the simulation cells, Syn-FeS and CIS were amended into uncapped sediment or sand cap to investigate their effectiveness at containing mercury and inhibiting the transformation of mercury to MeHg. Results show that, with the addition of iron sulfides, the release of mercury into overlying water was reduced and the methylation of mercury was inhibited.
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