X-ray absorption fine-structure spectroscopy study of photocatalyzed, heterogeneous As(III) oxidation on kaolin and anatase

Abstract

We have used X-ray absorption fine-structure spectroscopy (XAFS) to investigate As(III) adsorption on bulk Georgia kaolin (KGa-1b), Wyoming smectite (SWy-2), anatase (TiO2), gibbsite [Al(OH)3], and amorphous silica (am-SiO2) with the goal of identifying the oxidants responsible for As(III) oxidation in KGa-1b. After 30 h of reaction, ≤30% of the added As(III) was oxidized to As(V) in KGa-1b and anatase experiments, whereas only ≃10% of the added As(III) was oxidized in experiments using bulk SWy-2 and gibbsite. As(III) oxidation was negligible in experiments using am-SiO2. The proportion of As(III) oxidized by KGa-1b increased with increasing equilibration time, decreased significantly with increasing pH (up to pH 8), and decreased slightly with increasing ionic strength. Although Mn has been proposed as the potential oxidant of As(III) in KGa-1b, it can account for only ≃2% of the As(V) produced under the conditions of our experiments because of its low concentration in this material (3 ppm). Proof that a Ti- containing phase rather than a Mn-containing phase is the primary oxidant of As(III) in KGa-1b is provided by experimental results showing that As(III) oxidation in slurries of KGa-1b and anatase is strongly dependent on both light and the concentration of oxygen, whereas the extent of As(III) oxidation by Mn(IV) is independent of oxygen concentration. The reaction of As(III) with natural or synthetic Ti-containing phases could provide new remediation strategies for arsenic. The high Ti content of the standard reference kaolins KGa-1 and KGa-1b is a result of the unique genesis of this clay deposit and may not be representative of soil kaolins in general.