Molecular structures of dissolved and colloidal AsV-FeIII complexes and their roles in the mobilization of AsV under strongly acidic conditions.

The effect of high concentration of iron (FeIII) on the speciation and mobility of arsenic (As) under strongly acidic conditions remains unclear. This work studied the redistribution and speciation of AsV and FeIII 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 FeIII 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 FeIII concentration. The FTIR results suggested that AsV 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 FeIII 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.