Inhibition of precipitation and aggregation of metacinnabar (mercuric sulfide) by dissolved organic matter isolated from the Florida Everglades

Precipitation and aggregation of metacinnabar (black HgS) was inhibited in the presence of low concentrations (greater than or equal to 3 mg C/L) of h umic fractions of dissolved organic matter (DOM) isolated from the Florida Everglades. At low Hg concentrations (less than or equal to 5 x 10(-8) M), DOM prevented the precipitation of metacinnabar. At moderate Hg concentrati ons (5 x 10(-5) M), DOM inhibited the aggregation of colloidal metacinnabar (Hg passed through a 0.1 mu m filter but was removed by centrifugation). A t Hg concentrations greater than 5 x 10(-4) M, mercury formed solid metacin nabar particles that were removed from solution by a 0.1 mu m filter. Organ ic matter rich in aromatic moieties was preferentially removed with the sol id. Hydrophobic organic acids (humic and fulvic acids) inhibited aggregatio n better than hydrophilic organic acids. The presence of chloride, acetate, salicylate, EDTA, and cysteine did not inhibit the precipitation or aggreg ation of metacinnabar. Calcium enhanced metacinnabar aggregation even in th e presence of DOM, but the magnitude of the effect was dependent on the con centrations of DOM, Hg, and Ca. Inhibition of metacinnabar precipitation ap pears to be a result of strong DOM-Hg binding. Prevention of aggregation of colloidal particles appears to be caused by adsorption of DOM and electros tatic repulsion.