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An assessment of thermodynamic reaction constants for simulating aqueous environmental monomethylmercury speciation
Author: Loux, Nicholas T.
Source: Chemical Speciation and Bioavailability, Volume 19, Number 4, November 2007 , pp. 193-206(14)
Publisher: Science Reviews 2000 Ltd
Abstract:
Monomethylmercury (CH3Hg+) is both the most ecologically significant and the least well characterized species of mercury in environmental settings. Our understanding of the environmental speciation behavior of this compound is limited both as the result of lesser available laboratory data (when compared to inorganic mercury) as well as the uncertainties associated with our understanding of the properties of environmental ligands. A careful examination and synthesis of data reported in the technical literature led to the following findings: (1) a 25°C, zero ionic strength bicarbonate ion complexation constant estimate is remarkably close to an earlier reported value at 0.4 M: CH3Hg++HCO3−⇔CH3HgHCO3, log10K=2.6 (±0.22, 1 SD), (2) three 25°C zero ionic strength reaction constants reported by DeRobertis et al. (1998) were confirmed to within ∼±0.1 log10K units: CH3Hg++OH−⇔CH3HgOH, log10K=9.47; 2CH3Hg++H2O⇔(CH3Hg)2OH++H+, log10K=−2.15; CH3Hg++Cl−⇔CH3HgCl, log10K=5.45, (3) ``best estimate'' literature complexation constants corrected to zero ionic strength include: CH3Hg++F−⇔CH3HgF, log10K=1.75 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg++Br−⇔CH3HgBr, log10K=6.87 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg++I−⇔CH3HgI, log10K=8.85 (20°C corr. Schwartzenbach and Schellenberg, 1965); and CH3Hg++SO42−⇔CH3HgSO4−, log10K=2.64 (25°C, DeRobertis et al., 1998), (4) literature reported values for simulating monomethylmercury complexation with the carbonate ion may be too low: CH3Hg++CO32−⇔CH3HgCO3−, log10K=6.1 (Rabenstein et al., 1976; Erni, 1981), and (5) ``best estimate'' constants for simulating methyl mercury complexation with reduced environmental sulfur species include: CH3Hg++S2−⇔CH3HgS−, log10K=21.1; CH3Hg++SH−⇔CH3HgSH, log10K=14.5 (H++SH−⇔H2S, log10K=6.88; Dyrssen and Wedborg, 1991); CH3Hg++RS−⇔CH3HgSR, log10K=16.5 (H++RS−⇔RSH, log10K=9.96; Qian et al., 2002); and CH3Hg++CH3HgS1−⇔(CH3Hg)2S, log10K=16.32 (Schwartzenbach and Schellenberg, 1965; Rabenstein et al., 1978; and Erni, 1981).Keywords: Monomethylmercury; speciation; complexation; sulfhydryl; hydroxyl
Document Type: Research article
DOI: http://dx.doi.org/10.3184/095422907X255947
Affiliations: 1: U.S. EPA/ORD/NERL/ERD, 960 College Station Road, Athens, GA 30605-2700, USA
Publication date: 2007-11-24
- Chemical Speciation & Bioavailability covers a rapidly expanding area in environmental science.
Research on the interactions between the chemical forms and behaviour of toxic compounds and their subsequent biological uptake, metabolism and ecological fate involves many scientific fields. These studies are often published in discipline-specific journals, leading to inadequate review and information scatter. This situation hinders both the development of an international community of experienced colleagues and the open flow of information and discussion. Additionally, the importance of speciation and bioavailability research to the development of pollution law and control technologies is being increasingly appreciated by environmental regulatory agencies throughout the world.
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