Iron-Mediated Photochemical Decomposition of Methylmercury in an Arctic Alaskan Lake

Sunlight-induced decomposition is the principal sink for methylmercury (CH₃Hg⁺) in arctic Alaskan lakes and reduces its availability for accumulation in aquatic food webs. However, the mechanistic chemistry of this process in natural waters is unknown. We examined experimentally the mechanism of photochemical CH₃Hg⁺ decomposition in filter-sterilized epilimnetic waters of Toolik Lake in arctic Alaska (68° 38′N, 149° 36′W), a region illuminated by sunlight almost continuously during the summer. Results from in situ incubation tests indicate that CH₃Hg⁺ is not decomposed principally by either direct photolysis (i.e., no degradation in reagent-grade water) or primary photochemical reactions with dissolved organic material. The preeminent role of labile Fe and associated photochemically produced reactive oxygen species is implicated by tests that show 1) additions of Fe(III) to reagent-grade water enhance CH₃Hg⁺ photodecomposition, 2) strong complexation of ambient Fe(III) with desferrioxamine B inhibits the reaction in lake water, and 3) experimental additions of organic molecules that scavenge hydroxyl radicals specifically among reactive oxygen species (dimethylsulfoxide and formic acid) inhibit CH₃Hg⁺ degradation. Lake-water dilution and Fe(III) addition experiments indicate that Fe is not the limiting reactant for CH₃Hg⁺ photodecomposition in Toolik Lake, which is consistent with prior results indicating that photon flux is a major control. These results demonstrate that CH₃Hg⁺ is decomposed in natural surface water by oxidants, apparently hydroxyl radical, generated from the photo-Fenton reaction.