Redox stability of As(III) on schwertmannite surfaces

Highlights: • Schwertmannite is a better scavenger than goethite in identical sorbent:solute ratio. • Partial As(III) oxidation to As(V) by schwertmannite and goethite in anoxic media. • Arsenic-rich surface precipitates form on schwertmannite with decayed morphology. • Bidentate binuclear binding of both As(V) and mixed As(III)–As(V) on schwertmannite. • Dissolved ferrous iron has negligible control on As(III) uptake and redox stability. -- Abstract: As(III)-enriched mine discharge often drains through Fe(III)-mineral abundant land covers which makes the understanding of its fate and redox behaviour extremely important. We therefore conducted batch kinetic and equilibrium studies at pH 3.0 ± 0.05 in anoxic media coupled with spectroscopic and microscopic examinations at variable conditions to understand possible As(III) binding mechanisms and the redox stability of As(III) on schwertmannite, a prominent ferric mineral in acid mine drainage environments. Schwertmannite acted as an efficient scavenger for As(III) compared to goethite at identical sorbent:solute ratios. As K-edge X-ray absorption near-edge structure (XANES) demonstrated partial oxidation of sorbed As(III) to As(V) on both the minerals depending on the Fe(III)/As(III) ratios (goethite acted as a better oxidant than schwertmannite). Sorbed As(III) and As(V) coordinated in a bidentate binuclear binding mechanism with As(III)/As(V)–O and As(III)/As(V)–Fe interatomic distances as 1.78/1.69 and 3.37/3.31 Å, respectively. Scanning (SEM-EDX) and transmission (TEM) electron microscopic, and IR spectroscopic measurements revealed the formation of As-containing surface coatings by sorbed As on schwertmannite