189 - Photodemethylation of Dimethylmercury in Natural Waters

Photodemethylation of dimethylmercury (DMHg) could act as an important source of monomethylmercury (MMHg), especially in oceanic waters where DMHg concentrations commonly are in the same range as the concentrations of MMHg. Yet, only two studies have previously investigated DMHg photodemethylation in water, with conflicting conclusions on whether this process occurs or not. To address this, we have studied photochemical demethylation of an isotopically labeled DMHg tracer (together with an isotopically labeled MMHg tracer) in different natural waters in the presence of artificial UV radiation as well as natural sunlight. We observed photodemethylation of the DMHg tracer in all water types tested (water from the Baltic Sea, the Arctic Ocean, a forest stream, artificial seawater, and purified water) using a UV lamp, and identified MMHg as the primary degradation product. Complementary experiments where selected water types were exposed to natural sunlight confirmed that DMHg is subjected to photodemethylation. To learn more about the underlying mechanisms of DMHg photodemethylation, we also manipulated the salinity, DOC, and O2 in samples. As these manipulations affected DMHg and MMHg photodemethylation rates differently, this indicates differences in the underlying reaction mechanisms. The general comparability between DMHg and MMHg photodecomposition rates in our study does, however, suggest that MMHg formation from DMHg photodemethylation could constitute a large source of MMHg in marine surface waters, available for biotic uptake.

Authors: Johannes West, Sonja Gindorf, Andrew Graham, Sofi Jonsson