Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, ETH Zurich , CHN, CH-8092 Zurich, Switzerland.
Environ Sci Technol. 2014 Apr 1;48(7):3822-31. doi: 10.1021/es405221z. Epub 2014 Mar 14.
Binding of arsenite (As(III)) to sulfhydryl groups (Sorg(-II)) plays a key role in As detoxification mechanisms of plants and microorganisms, As remediation techniques, and reduced environmental systems rich in natural organic matter. Here, we studied the formation of Sorg(-II)-As(III) complexes on a sulfhydryl model adsorbent (Ambersep GT74 resin) in the absence and presence of ferrihydrite as a competing mineral adsorbent under reducing conditions and tested their stability against oxidation in air. Adsorption of As(III) onto the resin was studied in the pH range 4.0-9.0. On the basis of As X-ray absorption spectroscopy (XAS) results, a surface complexation model describing the pH dependence of As(III) binding to the organic adsorbent was developed. Stability constants (log K) determined for dithio ((AmbS)2AsO(-)) and trithio ((AmbS)3As) surface complexes were 8.4 and 7.3, respectively. The ability of sulfhydryl ligands to compete with ferrihydrite for As(III) was tested in various anoxic mixtures of both adsorbents at pH 7.0. At a 1:1 ratio of their reactive binding sites, R-SH and ≡FeOH, both adsorbents possessed nearly identical affinities for As(III). The oxidation of Sorg(-II)-As(III) complexes in water vapor saturated air over 80 days, monitored by As and S XAS, revealed that the complexed As(III) is stabilized against oxidation (t1/2 = 318 days). Our results thus document that sulfhydryl ligands are highly competitive As(III) complexing agents that can stabilize As in its reduced oxidation state even under prolonged oxidizing conditions. These findings are particularly relevant for organic S-rich semiterrestrial environments subject to periodic redox potential changes such as peatlands, marshes, and estuaries.
亚砷酸盐（As(III)）与巯基基团（Sorg(-II)）的结合在植物和微生物的砷解毒机制、砷修复技术以及富含天然有机质的还原环境系统中起着关键作用。在这里，我们研究了在还原条件下，亚铁氢氧化物作为竞争矿物吸附剂存在和不存在时，巯基模型吸附剂（Ambersep GT74 树脂）上 Sorg(-II)-As(III) 配合物的形成，并测试了它们在空气中氧化的稳定性。在 pH 值 4.0-9.0 范围内研究了 As(III)在树脂上的吸附。基于砷 X 射线吸收光谱（XAS）结果，开发了一个描述有机吸附剂上 As(III)结合的 pH 依赖性的表面络合模型。确定的二硫（(AmbS)2AsO(-)）和三硫（(AmbS)3As）表面络合物的稳定常数（log K）分别为 8.4 和 7.3。在各种无氧混合物中测试了巯基配体与亚铁氢氧化物竞争 As(III)的能力，混合两种吸附剂的反应性结合位点的比例为 1:1，在 pH 值为 7.0 时，两者对 As(III)的亲和力几乎相同。通过 As 和 S XAS 监测，在 80 天内水蒸汽饱和空气中 Sorg(-II)-As(III) 配合物的氧化表明，配合的 As(III)被稳定，不易氧化（t1/2 = 318 天）。因此，我们的研究结果表明，巯基配体是高度竞争的 As(III)络合剂，即使在长时间的氧化条件下，也可以稳定其还原氧化态的砷。这些发现对于有机 S 丰富的半陆生环境特别重要，这些环境会经历周期性的氧化还原电位变化，如泥炭地、沼泽和河口。
