Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, CHN, ETH Zurich , 8092 Zurich, Switzerland.
Environ Sci Technol. 2018 Jan 16;52(2):616-627. doi: 10.1021/acs.est.7b05176. Epub 2018 Jan 4.
X-ray amorphous Fe(III)-As(V) coprecipitates are common initial products of oxidative As- and Fe-bearing sulfide weathering, and often control As solubility in mine wastes or mining-impacted soils. The formation conditions of these solids may exert a major control on their mineralogical composition and, hence, As release in the gastric tract of humans after incidental ingestion of As-contaminated soil. Here, we synthesized a set of 35 Fe(III)-As(V) coprecipitates as a function of pH (1.5-8) and initial molar Fe/As ratio (0.8-8.0). The solids were characterized by synchrotron X-ray diffraction, FT-IR spectroscopy, and electrophoretic mobility measurements, and their As bioaccessibility (BA) was evaluated using the gastric-phase Solubility/Bioavailability Research Consortium in vitro assay (SBRC-G). The coprecipitates contained 1.01-4.51 mol kg As (molar Fe/As: 1.00-8.29) and comprised varying proportions of X-ray amorphous hydrous ferric arsenates (HFA) and As(V)-adsorbed ferrihydrite. HFA was detected up to pH 6 and its fraction decreased with increasing pH and molar Fe/As ratio. Bioaccessible As ranged from 2.9 to 7.3% of total As (x̅ = 4.8%). The BA of coprecipitates formed at pH ≤ 4 was highest at formation pH 3 and 4 and controlled by the intrinsically high solubility of the HFA component, possibly enhanced by sorbed sulfate. In contrast, the BA of coprecipitates dominated by As(V)-adsorbed ferrihydrite was much lower and controlled by As readsorption and/or surface precipitation in the gastric fluid. Bioaccessible As increased up to 95% with increasing liquid-to-solid ratio, indicating an enhanced solubility of these solids due to interactions between Fe and the glycine buffer. We conclude (i) that natural Fe(III)-As(V) coprecipitates exhibit a particularly high solubility in the human gastric tract when formed at pH ∼ 3-4 in the presence of sulfate, and (ii) that the in vitro bioaccessibility of As in Fe(III)-As(V) coprecipitates as assessed by tbe SBRC-G assay depends critically on their solid-phase concentration in As-contaminated soil and mine-waste materials.
X 射线无定形的 Fe(III)-As(V)共沉淀是氧化含 As 和 Fe 的硫化物风化的常见初始产物，通常控制着矿山废物或受采矿影响的土壤中 As 的溶解度。这些固体的形成条件可能对其矿物成分产生主要控制作用，因此在人类偶然摄入受 As 污染的土壤后，这些固体在胃中的 As 释放情况也受到影响。在这里，我们通过同步辐射 X 射线衍射、傅里叶变换红外光谱和电泳迁移率测量，合成了一系列 pH 值（1.5-8）和初始摩尔比 Fe/As（0.8-8.0）的 35 个 Fe(III)-As(V)共沉淀。这些共沉淀物的特征是 X 射线无定形的水合铁砷酸盐（HFA）和 As(V)吸附的水铁矿的比例不同。使用胃相可溶性/生物利用度研究协会体外测定法（SBRC-G）评估了共沉淀物的 As 生物可利用性（BA）。共沉淀物中含有 1.01-4.51 mol kg As（摩尔比 Fe/As：1.00-8.29），并包含不同比例的 X 射线无定形水合铁砷酸盐（HFA）和 As(V)吸附的水铁矿。在 pH 为 6 时检测到 HFA，随着 pH 和摩尔比 Fe/As 的增加，其分数减少。总 As 中可生物利用的 As 含量为 2.9%至 7.3%（平均值为 4.8%）。在 pH≤4 时形成的共沉淀物的 BA 在形成 pH 为 3 和 4 时最高，由 HFA 成分的固有高溶解度控制，可能因吸附的硫酸盐而增强。相比之下，主要由 As(V)吸附的水铁矿组成的共沉淀物的 BA 要低得多，受胃液中 As 的再吸附和/或表面沉淀控制。随着液固比的增加，可生物利用的 As 增加到 95%，这表明由于 Fe 和甘氨酸缓冲液之间的相互作用，这些固体的溶解度增加。我们得出结论：(i)在存在硫酸盐的情况下，当在 pH 值约为 3-4 时形成时，天然 Fe(III)-As(V)共沉淀物在人类胃中表现出特别高的溶解度；(ii)通过 SBRC-G 测定评估的 Fe(III)-As(V)共沉淀物中 As 的体外生物可利用性取决于受污染土壤和矿山废物中 As 共沉淀物的固相浓度。
