Mineralogical Controls on the Bioaccessibility of Arsenic in Fe(III)–As(V) Coprecipitates

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...