Simultaneous photocatalytic Cr(VI) reduction and ciprofloxacin oxidation over TiO2/Fe0 composite under aerobic conditions: Performance, durability, pathway and mechanism

Abstract

Coexistence of organic compounds and heavy metals in aquaculture waters is a widespread problem. In this study, a rapid reduction of Cr(VI) and oxidation of ciprofloxacin (CIP) in a simultaneous manner was firstly reported through the photocatalysis of TiO2/Fe0 composite under aerobic conditions. The significantly enhanced removals of Cr(VI) and CIP by TiO2/Fe0 composite due to a synergetic effect between oxidation and reduction processes. Within 60min UV light irradiation, the simultaneous removals of Cr(VI) and CIP by TiO2/Fe0 composite were 97.7 and 94.6%, respectively, whereas the corresponding values by TiO2 were 57.9 and 68.7%. CIP oxidation process could be significantly accelerated under acidic and aerobic conditions. Intriguingly, a low Pb(II) concentration could induce an enhancement of CIP oxidation, whereas a negative effect on both Cr(VI) reduction and CIP oxidation was observed at a relatively high concentration. A low concentration of ethylene diamine tetra-acetic acid (EDTA) did positively affect both Cr(VI) reduction and CIP oxidation processes, whereas a negative effect on CIP oxidation was found at a relatively high concentration. A possible reaction pathway and mechanism between CIP oxidation and Cr(VI) reduction was proposed. CIP was attacked by HO produced from the decomposition of water and photo-excited holes through substitution and hydroxylation processes, leading to the production of a series of intermediates, whereas the Cr(VI) was converted into Cr(III) through the capture of photo-excited electrons and Fe0 reduction.