A COMPARATIVE STUDY OF ARSENATE AND PHOSPHATE ADSORPTION ON NANO-BALL ALLOPHANE

抄録

Adsorption behavior of arsenate on hydroxides of Fe and Al with Fe-OH and Al-OH groups has been widely studied and also compared with that of phosphate. In this study, we for the first time compared the adsorption of arsenate and phosphate on natural allophane, until initial adsorbates concentration up to 70 μM with allophane/solution ratio of 50 mg/105 mL. For both arsenate and phosphate, the isotherms fitted well to Langmuir equation, and calculated maximum adsorption, Xm, decreased with increasing pH. A parameter related to adsorption strength, K, was greater for arsenate than for phosphate at pH from 5 to 8. Molecular orbital calculations with a model cluster of allophane at B3LYP/6-31+G(d,p) level showed that, for both monodentate and bidentate adsorptions, Al-OAs(P) bond length was shorter for arsenate than for phosphate, and ΔE value of the adsorption reaction was lower for arsenate than for phosphate. These supported the observed greater K value for arsenate than for phosphate. Because previous studies on gibbsite showed stronger adsorption for phosphate than for arsenate, the inverse results on allophane was attributed to the effect of Si tetrahedra attached to gibbsite sheet which may altered the nature of the Al-OH group of allophane. The strong adsorption and high removal ratio of arsenate shown in this study, especially at lower concentrations, suggested the applicability of aluminum silicate materials such as allophane as an excellent adsorbate for arsenate in water environments.