Temporospatial evolution and removal mechanisms of As(V) and Se(VI) in ZVI column with H2O2 as corrosion accelerator

Abstract

Enhanced removal of As(V) and Se(VI) by zero valent iron (ZVI) has been recently revealed by using H2O2 as the corrosion accelerator, however, the detailed performance of such enhanced removal in ZVI column as well as the underlying mechanism is still unclear. In this study, the temporospatial evolution of As(V) and Se(VI) along a self-designed ZVI/H2O2 column in down-flow mode was systematically investigated. The variations of concerned aqueous parameters (pH, ORP, H2O2, Fe2+, As, and Se) were monitored at different positions along the column throughout the experiments. Results showed the corrosion degree of ZVI decreased with the depth of the column, as confirmed by SEM and XRD analyses of the solid samples from different layers. The retention of As and Se also decreased along the column, suggesting the uptake of As(V) and Se(VI) was highly dependent upon the ZVI corrosion evolution. In the initial stage, the influent H2O2 was mostly consumed by ZVI in the top layer. With the continuous corrosion of ZVI, the breakthrough of H2O2 would activate the ZVI at lower positions, resulting in the reactive zone continuously shifting downward along the column. The reduction of As(V) and Se(VI) to aqueous As(III) and Se(IV) was significantly inhibited at the positions in the presence of H2O2, whereas favorably enhanced in the presence of abundant Fe2+. The retention of As(III) in the lower part of the column was observed while that of Se(IV) was negligible, as related to the different effects of pH on the adsorption of As(III) and Se(IV). In addition, the evolution of different oxidation states of As and Se retained in the column were identified by XPS, further demonstrating the comprehensive mechanisms of As(V)/Se(VI) removal involving reduction and adsorption in the ZVI/H2O2 column.