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

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 (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.

Deciphering the Distribution and Crystal-Chemical Environment of Arsenic, Lead, Silica, Phosphorus, Tin, and Zinc in a Porous Ferrihydrite Grain Using Transmission Electron Microscopy and Atom Probe Tomography

“…However, Qui and Pichler (2014) showed that in the presence of As 3+ and As 5+ species, species of both ions adsorbed almost equally up to a pH of 5 and that adsorption of As 3+ species was favored over those of As 5+ species at higher pH values, in accordance with other studies . Adsorption of As 5+ species on ferrihydrite occurs predominantly as inner sphere bidentate binuclear (“bridging”) complexes with two adjacent edge-sharing Fe 3+ -octahedra. , Adsorption complexes involving As 5+ species can convert to amorphous ferric arsenate surface precipitates with increasing amount of As on the surface …”

Removal of As3+, As5+, Sb3+, and Hg2+ ions from aqueous solutions by pure and co-precipitated akaganeite nanoparticles: adsorption kinetics studies

“…On the other hand, in the case of acid mine drainage, whose pH can be 2, these pollutants are present, and in nature, mineraloids are the most efficient materials in their decontamination. 22 The amount of metal ion adsorbed on the pure and coprecipitated akaganeites at time t, was given by the following equation:…”

Multi-element effects on arsenate accumulation in a geochemical matrix determined using µ-XRF, µ-XANES and spatial statistics

“…However, directly measuring reaction mechanisms that are important for preventing adverse environmental impacts of trace elements is confounded by the association of minerals, organic matter, non-crystalline inorganic solids and biota into complex soil assemblages. Consequently, studies aimed at ISSN 1600-5775 # 2019 International Union of Crystallography determining mechanisms of trace-element binding in soils and other geochemical systems have often used model analogs of isolated soil components, including synthesized minerals or extracted humic substances (Adra et al, 2013;Brown & Sturchio, 2002;Dixit & Hering, 2003;Ehlert et al, 2018;Fritzsche et al, 2011;Grafe et al, 2001;Masue et al, 2007;Mikutta & Kretzschmar, 2011;Otero-Fariñ a et al, 2017;Raven et al, 1998;Silva et al, 2010;Violante & Pigna, 2002;Zeng et al, 2008). The multicomponent complexity of soil makes it difficult to discriminate bonding mechanisms at the level of specificity that can be determined from model systems (Kizewski et al, 2011).…”