Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Mineralogical Controls on the Bioaccessibility of Arsenic in Fe(III)− As(V) Coprecipitates Katrin Ehlert, Christian Mikutta,* Yuan Jin, and Ruben Kretzschmar Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, CHN, ETH Zurich, 8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: 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 (BAAs) was evaluated using the gastric-phase Solubility/Bioavailability Research Consortium in vitro assay (SBRC-G). The coprecipitates contained 1.01−4.51 mol kg−1 As (molar Fe/Assolid: 1.00−8.29) and comprised varying proportions of X-ray amorphous hydrous ferric arsenates (HFAam) and As(V)-adsorbed ferrihydrite. HFAam 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 BAAs of coprecipitates formed at pH ≤ 4 was highest at formation pH 3 and 4 and controlled by the intrinsically high solubility of the HFAam component, possibly enhanced by sorbed sulfate. In contrast, the BAAs 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 Ascontaminated soil and mine-waste materials.
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(“HFO”).1,2,14,15 The latter group may contain materials with molar Fe/Assolid ratios up to 4, which was proposed as the theoretical As(V) adsorption capacity of ferrihydrite16 (Fe5HO8·4H2O). The structure of these materials, frequently referred to as “AFA”, “As-rich HFO” or “arsenical ferrihydrite” in the literature, is not fully understood and may vary substantially depending on environmental conditions. In order to separate Fe(III)−As(V) mineraloids with molar Fe/ Assolid ratios
