AsIII-enhanced oxidation by coexisting MnIII-phenolic complexes during arsenic contaminated groundwater treatment by MnO2

Abstract

Manganese oxides (MnO2) are widely utilized in arsenic(AsIII)-contaminated groundwater remediation; however, As(III) oxidation is greatly affected by co-occurring substances. This study discusses the oxidation pathways of As(III) oxidation-adsorption in presence of co-solutes (e.g., Mn(II), Fe(II), and 2,4-dichlorophenol (DCP). Mn(II) species competed with As(III) for adsorption and oxidation sites at MnO2 surface, resulting in a decrease in As(III) removal. As(III) oxidation in Mn(II) and Mn(II)/MnO2 systems was primarily occurred via electron-transfer with Mn(III) intermediate, however, Mn(III) species were rapidly oxidized into the less reactive oxidant Mn(IV). In the presence of DCP, As(III) oxidation was enhanced due to stabilization of Mn(III) species by complexation with DCP. Fe(II) enhanced the adsorption of Arsenic and DCP by MnO2, as Fe-(hydr)oxides formed, creating extra adsorption sites on MnO2. Three DCP byproducts were also identified by means of LC/MS analysis in the Mn(II)/MnO2-DCP system, with reaction As(III) oxidation pathways are proposed. The enhanced As(III) oxidation by Mn(II)/MnO2 system, in presence of DCP resulted due to a series of chemical reactions with hydroxyl and phenolic radicals generated in system, in addition to electron transfer with DCP-Mn(III) complexes. These results provide new insights into the environmental behavior of As(III) within mixed water systems containing metals (Mn/Fe) and phenolics.