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Arai, Y. , E. J. Elzinga, and D. L. Sparks. 2001. X-ray absorption spectroscopic investigation of arsenite and arsenate adsorption at the aluminum oxide-water interface. J. Colloid Interf. Sci. 235:80-88

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X-ray Absorption Spectroscopic Investigation of Arsenite and Arsenate Adsorption at the Aluminum Oxide– Water Interface

Yuji Arai*, Evert J. Elzinga, and Donald L. Sparks

Abstract

We investigated the As(III) and As(V) adsorption complexes forming at the γ-Al₂O₃ / water interface as a function of pH and ionic strength (I), using a combination of adsorption envelopes, electrophoretic mobility(EM) measurements and X-ray absorption spectroscopy (XAS). The As adsorption envelopes show that 1) As(III) adsorption increases with increasing pH and is insensitive to I changes (0.01 M and 0.8 M NaNO₃) at pH=3-4.5, while adsorption decreases with increasing I between pH = 4.5 and 9.0, and 2) As(V) adsorption decreases with increasing pH and is insensitive to I changes at pH=3.5-10. The EM measurements show that As(III) adsorption does not significantly change the EM values of γ-Al₂O₃ suspension in 0.1M NaNO₃ at pH = 4-8, whereas As(V) adsorption lowered the EM values at pH = 4-10. The EXAFS data indicate that both As(III) and As(V) form inner-sphere complexes with a bidentate binuclear configuration, as evidenced by a As(III)-Al bond distance of ≈ 3.22 Å and a As(V)-Al bond distance of ≈ 3.11 Å. The As(III) XANES spectra, however, show that outer-sphere complexes are formed in addition to inner-sphere complexes, and that the importance of outer-sphere As(III) complexes increases with increasing pH (5.5 to 8) and with decreasing I. In short, the data indicate for As(III) that inner- and outer sphere adsorption co-exist whereas for As(V) inner-sphere complexes are predominant under our experimental conditions.

Key Words: Arsenic; γ-Al₂O₃; adsorption mechanisms; XAS; electrophoretic mobility; oxide-water interface.

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