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

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• 作者：Graziele Duarte^a ; ^g ; Virginia S.T. Ciminelli^a ; ^g ; ^{ciminelli@demet.ufmg.br} ; Maria S.S. Dantas^a ; ^g ; Helio A. Duarte^b ; ^g ; Igor F. Vasconcelos^c ; ^g ; Augusto F. Oliveira^d ; ^g ; Kwadwo Osseo-Asare^e ; ^f ; ^g
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 期刊代码：27_00167037
• 类别：ear
• 出版时间：15 April, 2012
• 卷：83
• 期：Complete
• 页码：205-216
• 文件大小：921 K

摘要

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 脜, respectively. EXAFS results also demonstrated As(III) complexation to three oxygen atoms in the first shell, at a distance of 1.77 脜, and to aluminum in the second shell at a distance of 3.21 脜, 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 脜, was shown from EXAFS results to provide a minor contribution to As(III) sorption on gibbsite. Therefore, results from theoretical calculations and experimental 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.