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 (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 ( 0.1 mu m) 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.