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

✍ Scribed by Yuji Arai; Evert J. Elzinga; Donald L. Sparks

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 122 KB
• Volume: 235
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.2000.7249

✦ Synopsis

We investigated the As(III) and As(V) adsorption complexes forming at the γ -Al 2 O 3 /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 and 0.8 M NaNO 3 ) 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 -Al 2 O 3 suspension in 0.1 M NaNO 3 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 outersphere 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 coexist whereas for As(V) inner-sphere complexes are predominant under our experimental conditions.