Mechanistic Insights into Mercury Photoreduction: Effects of Dissolved Organic Matter and Inorganic Carbon in Seawater

Atmospheric mercury (Hg) is primarily derived from marine evasion of Hg(0) produced through the photoreduction of Hg(II) in seawater. Although the mechanisms of Hg(II) photoreduction in seawater have been extensively investigated, the coupled effects of dissolved inorganic carbon and specific dissolved organic matter (DOM) structures remain unresolved. Here, we report the pseudo-first-order rate constants (<i>k</i>_r) for the gross photoreduction of Hg(II) in the presence of thiols in deionized water (DIW), phosphate buffer solution (PBS), and artificial seawater (ASW) under UV-A irradiation. For aliphatic thiols, <i>k</i>_r was below the detection limit in DIW and PBS but increased to 0.081-0.50 h^-1 in ASW, exhibiting a concentration-dependent trend. In contrast, aromatic thiols yielded <i>k</i>_r up to 1.5 h^-1 in DIW, PBS, and ASW, with no evident concentration dependence. In the absence of thiols, aromatic DOM decreased <i>k</i>_r possibly by reoxidizing Hg(I) through the production of intermediate oxidants. Notably, <i>k</i>_r values for Hg-glutathione in PBS containing NaHCO₃ were comparable to those in natural seawater, whereas other sea salts produced <i>k</i>_r below detection limits, likely because HCO₃^- and CO₃^2- scavenge Cl^• and Cl₂^•-, thereby enhancing the Hg(II) reduction rate. Overall, a two-step reversible Hg(II) reduction model involving the Hg(I) intermediate is proposed to account for the dynamic redox equilibrium of Hg, underscoring the critical roles of aliphatic thiolic DOM, nonthiolic aromatic DOM, chloride, and bicarbonate in modulating <i>k</i>_r variability in seawater.