Refining Thermodynamic Constants for Mercury(II)-Sulfides in Equilibrium with Metacinnabar at Sub-Micromolar Aqueous Sulfide Concentrations
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- 作者：A. Drott ; E. Bj枚rn ; S. Bouchet ; U. Skyllberg
- 刊名：Environmental Science & Technology (ES&T)
- 出版年：2013
- 出版时间：May 7, 2013
- 年：2013
- 卷：47
- 期：9
- 页码：4197-4203
- 全文大小：264K
- 年卷期：v.47,no.9(May 7, 2013)
- ISSN：1520-5851
文摘
An important issue in mercury (Hg) biogeochemistry is to explore the influence of aqueous Hg(II) forms on bacterial uptake, and subsequent methyl mercury formation, under iron(III) and sulfate reducing conditions. The success of this is dependent on relevant information on the thermodynamic stability of Hg鈥搒ulfides. In the present study, we determined the solubility of a commercially available HgS(s) phase, which was shown by X-ray diffraction to be a mixture of 83% metacinnabar and 17% cinnabar. At aqueous sulfide concentrations between 0.060 and 84 渭M, well below levels in previous studies, we report a solubility product (log Ksp 卤 SE) of 鈭?6.8 卤 0.1 (HgS(s) + H+ = Hg2+ + HS鈥?/sup>, I = 0, T = 25 掳C, pH 6鈥?0, n = 20) for metacinnabar. This value is 0.7 log units higher than previous estimates. Complementing our data with data from Paquette and Helz (1997), we took advantage of a large data set (n = 65) covering a wide range of aqueous sulfide (0.06 渭M鈥?40 mM) and pH (1鈥?1). On the basis of this, we report refined formation constants (卤SE) for the three aqueous Hg(II)鈥搒ulfide species proposed by Schwarzenbach and Widmer (1963): Hg2+ + 2HS鈥?/sup> = Hg(SH)20; log K = 39.1 卤 0.1, Hg2+ + 2HS鈥?/sup> = HgS2H鈥?/sup> + H+; log K = 32.5 卤 0.1, Hg2+ + 2HS鈥?/sup> = HgS22鈥?/sup> + 2H+; log K = 23.2 卤 0.1. Our refined log K values differ from previous estimates by 0.2鈥?.6 log units. Furthermore, at the low sulfide concentrations in our study we could rule out the value of 鈭?0.0 for the reaction HgS(s) + H2O = HgOHSH(aq) as reported by Dyrss茅n and Wedborg (1991). By establishing a solubility product for the most environmentally relevant HgS(s) phase, metacinnabar, and extending the range of aqueous sulfide concentrations to sub-micromolar levels, relevant for soils, sediments, and waters, this study decreases the uncertainty in stability constants for Hg鈥搒ulfides, thereby improving the basis for understanding the bioavailability and mobility of Hg(II) in the environment.