Organic matter isolated from the Florida Everglades caused a dramatic increase in Hg release (up to 0.35 mM Hgdiss) from cinnabar (HgS), an insoluble solid (log Ksp of -52.4) under most environmental conditions. Hydrophobic (humic and fulvic) acids dissolved more Hg than hydrophilic acids and other non-acid fractions of dissolved organic matter (DOM). Cinnabar dissolution was inhibited by divalent cations such as Ca+2, but was independent of oxygen content in experimental solutions. Dissolution experiments conducted in DI water (pH = 6.0, I = 0.01 M) had no detectable (>2.5 nM) dissolved Hg in solution. The presence of various inorganic (chloride, sulfate, or sulfide) and organic ligands (salicylic acid, acetic acid, EDTA, or cysteine) did not enhance the dissolution of Hg from the mineral.
Aromatic carbon content in DOM (determined by 13C-NMR) correlated positively with enhanced cinnabar dissolution. Zeta potential measurements indicated significant sorption of negatively charged organic matter to the negatively charged cinnabar (pHpzc = 4.0) at pH 6.0. Based on amounts of mercury, sulfide, and sulfate in solution, and the interaction of humic substances with sulfide in solution, the mechanism of cinnabar dissolution seems to be oxidation of sulfide by DOM and subsequent Hg complexation by DOM. Enhanced solubilization of cinnabar by DOM could be an important mechanism for mobilizing Hg in aquatic environments.
The above abstract was submitted to AGU for the 1998 spring meeting, in Boston, MA, May 26-29, 1998.
This paper was publised in the Environmental Science & Technology, 1998, Volume 32, No. 21, pp. 3305-3311. This paper was also cited imder RESEARCH WATCH page 510A, "Dissolution of cinnabar by DOM - Dissolved organic matter (DOM) interacts with cations in natural waters and soils and affects solubility, mobility, and toxicity of many trace metals. M. Ravichandran and coworkers found that organic matter isolated from the Florida Everglades caused a dramatic increase in mercury release from cinnabar (HgS). Hydrophobic acids dissolved more mercury than hydrophilic acids and other DOM nonacid fractions. Possible mechanisms of dissolutions include surface complexation of mercury and oxidation of surface sulfur species by the organic matter. DOM enhances mercury release from cinnabar under oxic and anoxic conditions. Enhanced cinnabar solubility has geochemical significance: In mercury-contaminated areas and where conditions favor HgS formation, humic substances could enhance mercury mobilization and affect bioavailability."