Heterogeneous Catalytic Oxidation of As(Ⅲ) on Nonferrous Metal Oxides in the Presence of H_2O_2

摘要

The oxidation of As(Ⅲ) (arsenite) to As(Ⅴ) (arsenate), a critical pretreatment process for total arsenic removal, is easily achieved using chemical oxidation methods. Hydrogen peroxide (H_2O_2) is widely used as an environmentally benign oxidant but its practical use for the arsenite oxidation is limited by the strong pH dependence and slow oxidation kinetics. This study demonstrated that H_2O_2-induced oxidation of As(Ⅲ) can be markedly enhanced in the presence of nonferrous metal oxides (e.g., WO_3, TiO_2, ZrO_2) as a heterogeneous catalyst working over a wide pH range in ambient reaction conditions. In particular, TiO_2 is an ideal catalyst because it is not only active and stable but also easily available and inexpensive. Although the photocatalytic oxidation of As (Ⅲ) on TiO_2 was intensively studied, the thermal catalytic activities of TiO_2 and other nonferrous metal oxides for the arsenic oxidation have been little investigated. The heterogeneous oxidation rate increased with increasing the TiO_2 surface area and [H_2O_2] and weakly depended on pH whereas the homogeneous oxidation by H_2O_2 alone was favored only at alkaline condition. The oxidation rate in the TiO_2/H_2O_2 system was not reduced at all in the absence of dioxygen. It was not retarded at all by OH radical scavengers but markedly inhibited by hydroperoxyl radical scavengers. It is proposed that the surface complexation of H_2O_2 on TiO_2 induces the generation of the surface hydroperoxyl radical through an inner-sphere electron transfer, which subsequently reacts with As(Ⅲ). The catalytic activity of TiO_2 was maintained without showing any sign of deactivation. The heterogeneous catalytic oxidation is proposed as a viable method for the preoxidation treatment of As(Ⅲ)-contaminated water under ambient conditions.