Enhanced Dissolution of Cinnabar (Mercuric Sulfide) by Dissolved Organic Matter Isolated from the Florida Everglades

Abstract

Organic matter isolated from the Florida Everglades caused a dramatic increase in mercury release (up to 35 μM total dissolved mercury) from cinnabar (HgS), a solid with limited solubility. Hydrophobic (a mixture of both humic and fulvic) acids dissolved more mercury than hydrophilic acids and other nonacid fractions of dissolved organic matter (DOM). Cinnabar dissolution by isolated organic matter and natural water samples was inhibited by cations such as Ca2+. Dissolution was independent of oxygen content in experimental solutions. Dissolution experiments conducted in DI water (pH = 6.0) had no detectable (<2.5 nM) dissolved mercury. 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 mercury from the mineral. Aromatic carbon content in the isolates (determined by 13C NMR) correlated positively with enhanced cinnabar dissolution. ζ-potential measurements indicated sorption of negatively charged organic matter to the negatively charged cinnabar (pHpzc = 4.0) at pH 6.0. Possible mechanisms of dissolution include surface complexation of mercury and oxidation of surface sulfur species by the organic matter.

Integrated investigations on the adsorption mechanisms of fulvic and humic acids on three clay minerals

“…UV-vis spectroscopy analysis of all samples and controls without clays in batch adsorption experiments was conducted with a UV759S spectrometer (Shanghai Precision & Scientific Instrument, China) equipped with a quartz cell of 1.0 cm path length. Specific UV absorbance (SUVA) at 254 nm (SUVA 254 nm = UV 254 nm /TOC) was used as an index of relatively aromaticity of dissolved organic carbon (DOC) [21,22].…”

Evaluating the role of re-adsorption of dissolved Hg2+ during cinnabar dissolution using isotope tracer technique

“…However, previous studies showed that dissolution of mercury sulfide (e.g., cinnabar) can also serve as a continuous source of inorganic Hg in natural environment due to the fact that a variety of environmental factors can facilitate its dissolution. These factors include the presence of iron(III) in acidic water [4], sulfide in water [5], and dissolved organic matter (DOM) [6][7][8]. The enhanced dissolution of cinnabar could be an important process controlling Hg cycling in aquatic environments as this process would make the originally stable Hg more reactive and bioavailable, increasing the possibility of Hg transport, methylation and bioaccumulation, posing a great risk to humans and wildlife.…”

“…A variety of environmental factors are expected to enhance or inhibit cinnabar dissolution via affecting the fate of cinnabar dissolution products. These factors include pH, redox potential (Eh), and Hg binding ligands [4,7,12,13]. Sulfide (S 2− ), one of the cinnabar dissolution products, could be eliminated from the system via oxidation to SO 4 2− at pH 5-8 under aerobic condition [4,10,12,14,15] or conversion to HS − and H 2 S under anaerobic condition [2].…”

“…The former pathway may play a more important role as quicker dissolution of cinnabar was observed in the presence of O 2 [4,10,13]. Hg 2+ , the other product, could bind with organic ligands, in particular thiol-containing moieties in dissolved organic matter (DOM) [7,11,16]. The binding process is expected to reduce the amount of free Hg 2+ in aqueous phase and thus enhance cinnabar dissolution.…”

The Basis for Ecotoxicological Concern in Aquatic Ecosystems Contaminated by Historical Mercury Mining

“…In anoxic, sulfidic sediments, cinnabar can dissolve and become available for methylation (Benoit et al 2001). Organic acids from vegetation can enhance the dissolution of cinnabar (Ravichandran et al 1998) and increase the transport of colloidal Hg from former mining sites (Slowey et al 2005).…”