Arsenic Speciation in Sulfidic Waters: Reconciling Contradictory Spectroscopic and Chromatographic Evidence
Arsenic Speciation in Sulfidic Waters: Reconciling Contradictory Spectroscopic and Chromatographic Evidence
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DOI：
10.1021/ac1024717
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发表时间：
2010-12-15
影响因子：
7.4
通讯作者：
Wallschlaeger, Dirk
中科院分区：
文献类型：
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作者：
Planer-Friedrich, Britta;Suess, Elke;Wallschlaeger, Dirk
近年来，分析方法的发展表明，可溶性砷硫物质构成了硫化水中溶解砷的主要部分。然而，关于这些化合物的确切化学性质正在进行激烈的争论，因为在较高（mmol/L）浓度下产生的x射线吸收光谱（XAS）数据表明在这些水中存在（氧）硫砷酸盐，而在较低（μ mol/L至nmol/L）浓度下的离子色谱（IC）和质谱数据表明存在（氧）硫砷酸盐。在本文中，我们将连接并解释这两种明显不同类型的结果。地球化学模型实验表明，在完全缺氧条件下，硫砷酸盐是亚砷酸盐与硫化物的主要反应产物。然而，硫砷酸盐对氧化极不稳定，当暴露于大气氧中时，例如在等待色谱自进样器分析时，会迅速转化为硫砷酸盐。只有当整个色谱过程在手套箱内进行时，才能消除这个问题。我们还表明，硫砷酸盐对样品稀释是不稳定的，当使用ICP-MS等超灵敏检测器时，通常在色谱分析之前使用样品稀释。这种不稳定性有两个主要原因：如果在稀释过程中pH值发生变化，那么由于在pH范围内不同的稳定区域，单个砷硫物种之间的平衡会迅速重新排列；如果在稀释过程中pH值保持不变，那么溶液中OH-和SH-的比例就会改变，从而改变潜在的物种形成平衡。通过分析未稀释的样品可以避免这个问题。我们的研究表明，如果不完全排除氧气，硫砷酸盐在IC分析中表现为硫砷酸盐，如果样品在碱性缺氧介质中稀释，则表现为亚砷酸盐。这也指出硫砷酸盐是硫砷酸盐形成的必要中间体。
In recent years, analytical methods have been developed that have demonstrated that soluble arsenic-sulfur species constitute a major fraction of dissolved arsenic in sulfidic waters. However, an intense debate is going on about the exact chemical nature of these compounds, since X-ray absorption spectroscopy (XAS) data generated at higher (mmol/L) concentrations suggest the presence of (oxy)thioarsenites in such waters, while ion chromatographic (IC) and mass spectroscopic data at lower (mu mol/L to nmol/L) concentrations indicate the presence of (oxy)thioarsenates. In this contribution, we connect and explain these two apparently different types of results. We show by XAS that thioarsenites are the primary reaction products of arsenite and sulfide in geochemical model experiments in the complete absence of oxygen. However, thioarsenites are extremely unstable toward oxidation, and convert rapidly into thioarsenates when exposed to atmospheric oxygen, e.g., while waiting for analysis on the chromatographic autosampler. This problem can only be eliminated when the entire chromatographic process is conducted inside a glovebox. We also show that thioarsenites are unstable toward sample dilution, which is commonly employed prior to chromatographic analysis when ultrasensitive detectors like ICP-MS are used. This instability has two main reasons: if pH changes during dilution, then equilibria between individual arsenic-sulfur species rearrange rapidly due to their different stability regions within the pH range, and if pH is kept constant during dilution, then this changes the ratio between OH- and SH- in solution, which in turn shifts the underlying speciation equilibria. This problem is avoided by analyzing samples undiluted. Our studies show that thioarsenites appear as thioarsenates in IC analyses if oxygen is not excluded completely, and as arsenite if samples are diluted in alkaline anoxic medium. This also points out that thioarsenites are necessary intermediates in the formation of thioarsenates.