Natural organic matter-enhanced atmospheric Hg(0) uptake involves thiol-induced unusual electron transfer from Hg to carboxyl moiety.

Natural organic matter (NOM)-mediated non-photochemical oxidation of elemental mercury (Hg(0)), as an important process controlling Hg cycle, is still not well understood, particularly with respect to its electron transfer and environmental significance relevant to Hg(0) uptake by seawater. Here, we studied the uptake of atmospheric Hg(0) and its subsequent aqueous non-photochemical oxidation in seawater in the presence of NOM, by using extracellular polymeric substances and thiol compounds as NOM models. We observed NOM enhanced Hg(0) partition into seawater and its aqueous oxidation. Our results demonstrated that NOM mediated Hg(0) oxidation through a non-radical but thiol complexation-induced pathway, where electrons were transferred from Hg(0) to carboxyl moiety, resulting in the reduction of carboxyl accompanying Hg(0) oxidation. Thiol complexation with Hg(II) decreases the reduction potential of Hg(0) oxidation (Hg(0)→Hg(II)+2e) and therefore carboxyl group can act as an electron acceptor to be reduced to aldehyde/alcohol, as revealed by thermodynamic calculation and mass spectrometry analysis based on thiol models. This unusual Hg(0) oxidation pathway accompanied by the reduction of carboxyl to alcohol was also validated for NOM using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analysis. This NOM-mediated Hg(0) oxidation provides important environmental implications on the atmospheric Hg(0) deposition over environmental surfaces.