Formation of Nanocolloidal Metacinnabar in Mercury-DOM-Sulfide Systems

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• 作者：Chase A. Gerbig ; Christopher S. Kim ; John P. Stegemeier ; Joseph N. Ryan ; George R. Aiken
• 刊名：Environmental Science & Technology
• 出版年：2011
• 出版时间：November 1, 2011
• 年：2011
• 卷：45
• 期：21
• 页码：9180-9187
• 全文大小：881K
• 年卷期：v.45,no.21(November 1, 2011)
• ISSN：1520-5851

文摘

Direct determination of mercury (Hg) speciation in sulfide-containing environments is confounded by low mercury concentrations and poor analytical sensitivity. Here we report the results of experiments designed to assess mercury speciation at environmentally relevant ratios of mercury to dissolved organic matter (DOM) (i.e., <4 nmol Hg (mg DOM)p>鈭?p>) by combining solid phase extraction using C₁₈ resin with extended X-ray absorption fine structure (EXAFS) spectroscopy. Aqueous Hg(II) and a DOM isolate were equilibrated in the presence and absence of 100 渭M total sulfide. In the absence of sulfide, mercury adsorption to the resin increased as the Hg:DOM ratio decreased and as the strength of Hg-DOM binding increased. EXAFS analysis indicated that in the absence of sulfide, mercury bonds with an average of 2.4 卤 0.2 sulfur atoms with a bond length typical of mercury-organic thiol ligands (2.35 脜). In the presence of sulfide, mercury showed greater affinity for the C₁₈ resin, and its chromatographic behavior was independent of Hg:DOM ratio. EXAFS analysis showed mercury鈥搒ulfur bonds with a longer interatomic distance (2.51鈥?.53 脜) similar to the mercury鈥搒ulfur bond distance in metacinnabar (2.53 脜) regardless of the Hg:DOM ratio. For all samples containing sulfide, the sulfur coordination number was below the ideal four-coordinate structure of metacinnabar. At a low Hg:DOM ratio where strong binding DOM sites may control mercury speciation (1.9 nmol mgp>鈥?p>) mercury was coordinated by 2.3 卤 0.2 sulfur atoms, and the coordination number rose with increasing Hg:DOM ratio. The less-than-ideal coordination numbers indicate metacinnabar-like species on the nanometer scale, and the positive correlation between Hg:DOM ratio and sulfur coordination number suggests progressively increasing particle size or crystalline order with increasing abundance of mercury with respect to DOM. In DOM-containing sulfidic systems nanocolloidal metacinnabar-like species may form, and these species need to be considered when addressing mercury biogeochemistry.