As (III) immobilization on gibbsite: Investigation of the complexation mechanism by combining EXAFS analyses and DFT calculations

Abstract

The complexation of aqueous As(III) species on gibbsite was investigated as a function of pH. Theoretical calculations and X-ray absorption fine structure spectroscopy (XAFS) were combined to elucidate the structure of arsenite surface complexes on synthetic gibbsite. Several adsorption sites were evaluated using the self-consistent charge corrected density-functional based tight-binding (SCC-DFTB) method. The formation of bidentate–binuclear, bidentate–mononuclear, monodentate–mononuclear, and monodentate–binuclear complexes by means of both acid–base and non-dissociative mechanisms was studied in detail. The SCC-DFTB calculations showed the bidentate–binuclear/acid–base complex as the most thermodynamically stable geometry for As(III) bonding to gibbsite surface, estimating As–O and As–Al distances of 1.75 and 3.24 A˚ , respectively. EXAFS results also demonstrated As(III) complexation to three oxygen atoms in the first shell, at a distance of 1.77 A˚ , and to aluminum in the second shell at a distance of 3.21 A˚ , characteristic of bidentate–binuclear configuration, at pH 5.0, 7.0 and 9.0. Another As–Al interaction, attributed to the monodentate–binuclear complex due to its distance of 3.49 A˚ , was shown from EXAFS results to provide a minor contribution to As(III) sorption on gibbsite. Therefore, results from theoretical calculations and xperimental measurements confirmed the occurrence of inner-sphere complexation during the As(III) adsorption on gibbsite, in a pH range of 5–9. Hence, the higher As(III) mobility in the environment, when compared to As(V), was suggested to be related to the protonation of the As(III) adsorbed complexes. This protonation would restore the neutral H3AsO3 molecule, which could be then released from the mineral surface. These results might be useful to predict and control arsenic mobility in aqueous environments, particularly where Al oxy-hydroxides are often found.