Joshua C. Portner George R. Aiken Amber J. Hiranaka Michelle T. Dvorak Khuyen T. Tran Douglas E. Latch Jeffrey D. Jeremiason 2015 <p><span>This study examined the kinetics of photoreduction of Hg(</span><small>II</small><span>) and photodemethylation of methylmercury (MeHg</span><small>⁺</small><span>) attached to, or in the presence of, dissolved organic matter (DOM). Both Hg(</span><small>II</small><span>) and MeHg</span><small>⁺</small><span><span>&nbsp;</span>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(</span><small>II</small><span>) bound to reduced sulfur groups on DOM while an indirect mechanism is supported for photodemethylation of MeHg</span><small>⁺</small><span><span>&nbsp;</span>bound to DOM. UV spectra of Hg(</span><small>II</small><span>) and MeHg</span><small>⁺</small><span><span>&nbsp;</span>bound to thiol containing molecules demonstrate that the Hg(</span><small>II</small><span>)–S bond is capable of absorbing UV-light in the solar spectrum to a much greater extent than MeHg</span><small>⁺</small><span>–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</span><small>⁺</small><span><span>&nbsp;</span>and Hg(</span><small>II</small><span>); 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</span><small>⁺</small><span><span>&nbsp;</span>than in experiments with DOM and thiols; rates in the presence of DOM alone were the fastest supporting an intra-DOM mechanism. Hg(</span><small>II</small><span>) 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</span><small>⁺</small><span>. Utilizing the difference in photodemethylation rates measured for MeHg</span><small>⁺</small><span><span>&nbsp;</span>attached to DOM or thiol ligands, the binding constant for MeHg</span><small>⁺</small><span><span>&nbsp;</span>attached to thiol groups on DOM was estimated to be 10</span><small>^16.7</small><span>.</span></p> application/pdf 10.1039/C5EM00305A en Royal Society of Chemistry Photoreduction of Hg(II) and photodemethylation of methylmercury: the key role of thiol sites on dissolved organic matter article