Molecular structures of dissolved and colloidal As^V–Fe^III complexes and their roles in the mobilization of As^V under strongly acidic conditions

The effect of high concentration of iron (Fe^III) on the speciation and mobility of arsenic (As) under strongly acidic conditions remains unclear. This work studied the redistribution and speciation of As^V and Fe^III at Fe/As molar ratio of 1–14 and pH 1.5–2.0 in the dissolved, colloidal, and solid phases. Results showed that the elevated Fe^III induced the decomposition of the precipitated poorly crystalline ferric arsenate by forming dissolved (< 3 kDa) and colloidal (3 kDa–0.1 µm) As–Fe complexes. The fraction of particulate As (> 0.1 µm) decreased from 70–90% to less than 20% when the Fe/As molar ratio increased from 1 to 14. The particle size of the bulk samples decreased significantly with the increase of Fe^III concentration. The FTIR results suggested that As^V in dissolved/colloidal As–Fe complexes dominantly occurred as HAsO₄²⁻ species. The EXAFS results indicated that each HAsO₄²⁻ coordinated with approximately two Fe atoms in dissolved/colloidal As–Fe complexes at Fe/As ≥ 2. The findings suggest that high aqueous Fe^III concentration can promote the mobility of As by forming dissolved/colloidal Fe–As complexes in acidic waters, potentially accelerating As transport from source to downstream in acid mine drainage systems.