ABSTRACT

Precipitation and aggregation of metacinnabar (black HgS) was inhibited in the presence of low concentrations (less than 3 mg C/L) of humic fractions of dissolved organic matter (DOM) isolated from the Florida Everglades. At low Hg concentrations (less than 5x10^-8 M). DOM prevented the precipitation of metacinnabar. At moderate Hg concentrations (5x10^-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 5x10^-4 M, mercury formed solid metacinnabar particles that were removed from solution by a 0.1 mm 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

INTRODUCTION

Metacinnabar (black HgS) is highly insoluble in water:

Due to its low solubility, HgS is considered to be an important sink for mercury in sulfidic environments. Also, it is generally assumed that dissolved organic matter (DOM), which complexes many trace metals strongly, has no effect on the formation of HgS and other Hg-sulfide complexes. In this lab study, we reacted organic matter isolated from the Florida Everglades with supersaturated solutions of Hg and sulfide. We observed that metacinnabar precipitation was inhibited at low Hg concentrations and aggregation of colloidal HgS was inhibited at high Hg concentrations. The results presented may have important implications for the geochemical modeling and bioavailability predictions of mercury in aquatic environments.

METHODS

DOM from two locations (F1, 2BS) in northern Everglades, FL, and Suwannee River, GA, were isolated into hydrophobic acids (HPoA), hydrophilic acids (HPiA), hydrophobic neutrals (HPoN), and hydrophilic neutrals (HPiN) and into humic (HA) and fulvic (FA) acids by XAD-8/4 resin method. The isolates were characterized in terms of their elemental compositions, reduced sulfur content by X-ray absorption near-edge structure spectroscopy (XANES), specific UV absorbance (SUVA, the ratio between UVA at 254 nm and DOC conc.), carboxyl group content (by titration), and functional group content (by ¹³C NMR).

Stock solutions of organic matter isolates and model compounds, prepared in distilled water, were deoxygenated with nitrogen in a glove box. Various concentrations of mercury and sulfide were added to these solutions and adjusted for pH (7.0 0.1) and ionic strength (0.01 M NaNO₃). Reaction flasks were shaken in an orbital shaker at 200 rpm for 24 h. Samples were then filtered (0.1 µm) and analyzed for total dissolved Hg concentrations on a cold vapor atomic absorption spectrophotometer (detection limit = 2.5x10^-9 M).

Precipitation reactions were also conducted with filtered water samples collected from the F1 (DOC = 38.9 mg/L) and 2BS (DOC = 17.6 mg/L) sites. To check the effect of polyvalent cations (e.g., Ca⁺²) on dissolution, these samples were passed through Na-saturated cation exchange resins and then reacted with Hg and sulfide. In the case of isolated organic matter (F1-HPoA), the effect of calcium was studied by adding various concentrations of CaCl₂ in the presence of Hg and sulfide.

In one experiment conducted with 10 mg C/L F1-hydrophobic acid, 5x10^-5 M Hg, and 10^-3 M sulfide, the sample was centrifuged at 18 000 rpm (RCF = 38 274). After 8 h, mercury concentration was measured in the supernatant solution (operationally defined as “dissolved” Hg). The precipitate that settled in the tube, operationally defined as “colloidal” Hg, was analyzed by X-ray powder diffraction. Electrophoretic mobility measurements were made on metacinnabar that precipitated in the absence of organic matter and the colloidal metacinnabar that settled in the centrifuge tube.

This poster was presented at the American Geophysical Union (AGU) meeting, March 21-25, 1999, Anaheim, CA.