Precipitation and aggregation of metacinnabar (black HgS) was inhibited in the presence of low concentrations (≥3 mg C/L) of humic fractions of dissolved organic matter (DOM) isolated from the Florida Everglades. At low Hg concentrations (≤5 × 10-8 M), DOM prevented the precipitation of metacinnabar. At moderate Hg concentrations (5 × 10-5 M), DOM inhibited the aggregation of colloidal metacinnabar (Hg passed through a 0.1 μm filter but was removed by centrifugation). At Hg concentrations greater than 5 × 10-4 M, mercury formed solid metacinnabar particles that were removed from solution by a 0.1 μm filter. Organic matter rich in aromatic moieties was preferentially removed with the solid. Hydrophobic organic acids (humic and fulvic acids) inhibited aggregation better than hydrophilic organic acids. The presence of chloride, acetate, salicylate, EDTA, and cysteine did not inhibit the precipitation or aggregation of metacinnabar. Calcium enhanced metacinnabar aggregation even in the presence of DOM, but the magnitude of the effect was dependent on the concentrations of DOM, Hg, and Ca. Inhibition of metacinnabar precipitation appears to be a result of strong DOM-Hg binding. Prevention of aggregation of colloidal particles appears to be caused by adsorption of DOM and electrostatic repulsion.
