Effect of copper(II) species on the photooxidation of arsenite under UV-C irradiation

Abstract

The effect of Cu(II) on the photooxidation of arsenite (As(III)) to arsenate (As(V)) under UV-C (254 nm) irradiation and the role of Cu(II) in the As(III) oxidation processes were mechanistically investigated. The presence of Cu(II) enhanced the oxidation of As(III), and the positive effect of Cu(II) increased with increasing Cu(II) concentration and pH. However, other metal ions, such as Al(III), Cr(III), Ni(II), and Mn(II), had little effect on the photooxidation kinetics of As(III). The UV absorption spectra of the As(III) and Cu(II) solutions indicated that not only As(III), but also Cu(II), can absorb 254 nm light. The absorption of 254 nm light by Cu(II) can induce electron transfer from Cu(II) to oxygen, which accelerates the production of hydroxyl radicals (radical dotOH) as a primary oxidant for As(III) oxidation and generates Cu(III) as another primary oxidant. The photooxidation of As(III) in the absence of Cu(II) was limited under anoxic conditions, but was observed in the presence of Cu(II). This result suggests another role of Cu(II) as an alternative electron acceptor, which enhances the production of radical dotOH and Cu(III) by mediating electron transfer from As(III) (or Cu(II)) to oxygen. Therefore, both radical dotOH and Cu(III) are involved in the oxidation of As(III) to As(V) as primary oxidants in the presence of Cu(II). The photooxidation of As(III) with Cu(II) was not completely inhibited in the presence of excess tert-butyl alcohol (TBA, an radical dotOH scavenger), in contrast with the case without Cu(II). This result confirms the generation of Cu(III) and its involvement in the As(III) oxidation processes in the presence of Cu(II). The present results imply that Cu(II) can act as both a UV-C photosensitizer and an electron acceptor in photochemical water treatment processes under UV-C irradiation and significantly affect the kinetics of pollutant oxidation.