Superoxide-mediated Hg(II) reduction by dissolved organic matter in surface waters

Hg(II) reduction is a key process in regulating air-water exchange of Hg and its methylation. Dissolved organic matter (DOM) is an important factor in the photoreduction of Hg(II), yet the underlying mechanisms, particularly the involvement of superoxide, remain poorly understood. Herein, the contributions of dark and photochemical processes to reduction of Hg(II) were distinguished in surface waters (seawater and freshwater) based on measurements of purgeable Hg(0) using a purging-gold trap-cold vapor atomic fluorescence spectrometer, highlighting the importance of DOM in Hg(II) reduction. In surface waters, superoxide contributions to photoreduction were quantified, accounted for 38-55 % of DOM-mediated Hg(II) reduction for newly introduced Hg(II) and 28-33 % for pre-equilibrated Hg(II). Using Suwannee River Fulvic Acid Standard as a DOM model, we further confirmed that Hg(II) reduction was suppressed by the superoxide scavenger, with dissolved O₂ significantly enhancing Hg(II) reduction. For newly introduced Hg(II), superoxide accounted for 29 % and 58 % of the total reduction under dark and light conditions, respectively, whereas its contribution decreased to 22 % and 37 % for pre-equilibrated Hg(II). Superoxide generation by seven DOM samples was quantified using 3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide as a probe. Phenolic OH-blocked DOM significantly inhibited superoxide generation by 76 %, which consequently resulted in approximately 80 % inhibition of Hg(II) reduction. These findings underscore the importance of phenolic-OH and superoxide in DOM-mediated dark/light reduction of Hg(II) and provide a deeper insight into Hg(II) reduction in diverse aerobic environments.