Effects of Sulfide Concentration and Dissolved Organic Matter Characteristics on the Structure of Nanocolloidal Metacinnabar

Understanding the speciation of divalent mercury (Hg(II)) in aquatic systems containing dissolved organic matter (DOM) and sulfide is necessary to predict the conversion of Hg(II) to bioavailable methylmercury. We used X-ray absorption spectroscopy to characterize the structural order of mercury in Hg(II) DOM sulfide systems for a range of sulfide concentration (1-100 mu M), DOM aromaticity (specific ultraviolet absorbance (SUVA(254))), and Hg(II)- DOM and Hg(II) DOM sulfide equilibration times (4-142 h). In all systems, Hg(II) was present as structurally disordered nanocolloidal metacinnabar (beta-HgS). beta-HgS nanocolloids were significantly smaller or less ordered at lower sulfide concentration, as indicated by under-coordination of Hg(II) in beta-HgS. The size or structural order of beta-HgS nanocolloids increased with increasing sulfide abundance and decreased with increasing SUVA254 of the DOM. The Hg(II) DOM or Hg(II) DOM sulfide equilibration times did not significantly influence the extent of structural order in nanocolloidal beta-HgS. Geochemical factors that control the structural order of nanocolloidal beta-HgS, which are expected to influence nanocolloid surface reactivity and solubility, should be considered in the context of mercury bioavailability.