It was reported that Hg(II) reduction to elemental mercury (Hg(0)) is involved in the electron transfer from reduced humic substance (HS) to Hg(II) in anoxic environments; however, it is largely unknown which functional moieties in HS are responsible for Hg(II) reduction. In the current study, we conducted Hg(II) reduction experiments with four different reduced HSs. The order of Hg(II) reduction efficiency was determined to be: 9,10-anthrahydroquinone-2,6-disulfonate (AH2QDS) > soil humic acid > Suwannee River humic acid > Suwannee River fulvic acid, when Hg(II) reacted with an equal amount of reduced HS (1 mg L-1). This order is consistent with the order of organic radical contents in reduced HS, with a strong positive linear correlation between %Hg(0)/THg and organic radical contents (R2=0.91). It suggests that organic radicals are critical moieties to reduce Hg(II). Additionally, we carried out Hg(II) reduction experiments under different HS concentrations. Hg(0) production decreased when HS concentration was higher than 1 mg L-1 for 10 nM Hg, or organic carbon to Hg ratio was larger than 5000. This supports recent literature results that suggest potential Hg(0) oxidation by thiol groups in reduced HS. Overall, the production of reduced HS through the bacterial respiratory reaction in anoxic water and sediment may exert significant effects on the biogeochemical cycle of Hg.

Mercury reduction by reduced humic substances under dark and anoxic conditions: Importance of organic radicals

246th National Meeting (2013)

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