Institute of Biogeochemistry and Pollutant Dynamics, Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland.
Environ Sci Technol. 2012 Jun 19;46(12):6654-62. doi: 10.1021/es3008112. Epub 2012 Jun 8.
The application of Hg isotope signatures as tracers for environmental Hg cycling requires the determination of isotope fractionation factors and mechanisms for individual processes. Here, we investigated Hg isotope fractionation of Hg(II) sorption to goethite in batch systems under different experimental conditions. We observed a mass-dependent enrichment of light Hg isotopes on the goethite surface relative to dissolved Hg (ε(202)Hg of -0.30‰ to -0.44‰) which was independent of the pH, chloride and sulfate concentration, type of surface complex, and equilibration time. Based on previous theoretical equilibrium fractionation factors, we propose that Hg isotope fractionation of Hg(II) sorption to goethite is controlled by an equilibrium isotope effect between Hg(II) solution species, expressed on the mineral surface by the adsorption of the cationic solution species. In contrast, the formation of outer-sphere complexes and subsequent conformation changes to different inner-sphere complexes appeared to have insignificant effects on the observed isotope fractionation. Our findings emphasize the importance of solution speciation in metal isotope sorption studies and suggest that the dissolved Hg(II) pool in soils and sediments, which is the most mobile and bioavailable, should be isotopically heavy, as light Hg isotopes are preferentially sequestered during binding to both mineral phases and natural organic matter.
Hg 同位素特征在环境 Hg 循环示踪中的应用需要确定单个过程的同位素分馏因子和机制。在这里，我们研究了 Hg(II)在不同实验条件下的批处理系统中被针铁矿吸附的 Hg 同位素分馏。我们观察到，相对于溶解的 Hg，针铁矿表面上的轻 Hg 同位素发生了质量依赖性的富集（ε(202)Hg 的值为-0.30‰至-0.44‰），这与 pH 值、氯离子和硫酸根浓度、表面络合物的类型和平衡时间无关。基于先前的理论平衡分馏因子，我们提出 Hg(II)被针铁矿吸附的 Hg 同位素分馏受 Hg(II)溶液物种之间的平衡同位素效应控制，这种平衡同位素效应通过阳离子溶液物种在矿物表面的吸附来表达。相比之下，形成外配位络合物和随后向不同的内配位络合物的构象变化似乎对观察到的同位素分馏没有显著影响。我们的研究结果强调了溶液物种在金属同位素吸附研究中的重要性，并表明土壤和沉积物中最具流动性和生物可利用性的溶解 Hg(II)库应该具有较重的同位素组成，因为在与矿物相和天然有机质结合的过程中，轻 Hg 同位素优先被隔离。
