Effects of natural particles on photo-reduction of divalent mercury in everglades waters

Abstract

Photo-reduction of divalent mercury (Hg(II)) in waters plays an important role in the air-water exchange of Hg and biogeochemical cycle of Hg in general. As previous studies on photo-reduction of Hg(II) have mainly focused on dissolved Hg species, the effects of natural particles on photo-reduction of Hg(II) remain largely unknown, except the presumed light attenuating effect through light absorption and scattering. Considering the prevalence of particulate Hg due to adsorption of divalent and elemental Hg species on aquatic particles that are often photochemically active, natural particles may play a more direct role in Hg photo-reduction. By using incubation experiments with Everglades waters and additions of isotopically labelled Hg(II), we studied the effects of particles on photo-reduction of Hg(II) in natural waters. The effect of natural particles on Hg(II) photo-reduction was not observed between filtered or unfiltered Everglades waters, probably because of the low particle concentrations (<3 mg/L). When suspended particles isolated from original water was used to amend its concentration to 6.9 times the ambient Everglades waters, photo-reduction of Hg(II) was significantly enhanced. Given that the particles in Everglades waters are often semiconducting in nature, particulate Hg(II) may undergo heterogenous photo-reduction and lead to higher Hg(II) photo-reduction. However, in Everglades waters with both suspended and settling particles, high concentrations (∼100 mg/L) of particles did not result in enhanced Hg(II) photo-reduction. In this case, the enhancing effects of particles on Hg(II) photo-reduction were likely offset by inhibiting effects due to the higher irradiation attenuation and lower Hg(II) partition coefficients of the settling particles with larger sizes. This study highlights the direct involvements of particles in photoreaction of Hg species in natural waters and calls for more mechanistic research on heterogenous photo-reduction of Hg species on particles’ surfaces.