Photoreduction of Hg(ii) and photodemethylation of methylmercury: the key role of thiol sites on dissolved organic matter.

This study examined the kinetics of photoreduction of Hg(ii) and photodemethylation of methylmercury (MeHg(+)) attached to, or in the presence of, dissolved organic matter (DOM). Both Hg(ii) and MeHg(+) are principally bound to reduced sulfur groups associated with DOM in many freshwater systems. We propose that a direct photolysis mechanism is plausible for reduction of Hg(ii) bound to reduced sulfur groups on DOM while an indirect mechanism is supported for photodemethylation of MeHg(+) bound to DOM. UV spectra of Hg(ii) and MeHg(+) bound to thiol containing molecules demonstrate that the Hg(ii)-S bond is capable of absorbing UV-light in the solar spectrum to a much greater extent than MeHg(+)-S bonds. Experiments with chemically distinct DOM isolates suggest that concentration of DOM matters little in the photochemistry if there are enough reduced S sites present to strongly bind MeHg(+) and Hg(ii); DOM concentration does not play a prominent role in photodemethylation other than to screen light, which was demonstrated in a field experiment in the highly colored St. Louis River where photodemethylation was not observed at depths ≥ 10 cm. Experiments with thiol ligands yielded slower photodegradation rates for MeHg(+) than in experiments with DOM and thiols; rates in the presence of DOM alone were the fastest supporting an intra-DOM mechanism. Hg(ii) photoreduction rates, however, were similar in experiments with only DOM, thiols plus DOM, or only thiols suggesting a direct photolysis mechanism. Quenching experiments also support the existence of an intra-DOM photodemethylation mechanism for MeHg(+). Utilizing the difference in photodemethylation rates measured for MeHg(+) attached to DOM or thiol ligands, the binding constant for MeHg(+) attached to thiol groups on DOM was estimated to be 10(16.7).