Revealing the double-edged roles of chloride ions in Fered-Fenton treatment of industrial wastewater

Abstract

The speciation of reactive species and the degradation of pollutants during Fered-Fenton (EF-Fere) treatment of industrial wastewater are strongly affected by chloride ions (Cl−). The generation of active chlorine at the anode has been considered as the main positive effect of Cl– since it can contribute to pollutants degradation, however, the detrimental effects of Cl− were largely neglected. In this study, for the first time, the double-edged roles of Cl– in Fered-Fenton treatment of high salinity wastewater were systematically investigated. First, HClO can act as the scavenger of H2O2, thereby inhibiting the production of •OH from Fenton’s reaction. The steady-state concentration of •OH was only 1.0 × 10-13 M in the presence of 142.5 mM Cl–, being much lower than that of the conventional EF-Fere (8.3 × 10-13 M). The formation of singlet oxygen (1O2) via this scavenging reaction was confirmed by the electron paramagnetic resonance (EPR) test, but it can hardly contribute to furfuryl alcohol (FFA) decay due to its fast deactivation in water. The dominant roles of •OH and HClO in pollutants degradation during Cl–-mediated EF-Fere process (EF-Fere-HClO) were disclosed, the performance highly depends on the structure of target pollutants. The selective oxidation by HClO can efficiently destroy some electron-rich compounds, whereas the degradation of many electron-deficient contaminants still relies on the •OH-mediated pathway. Besides, the generation of toxic chlorinated intermediates from HClO-mediated oxidation can cause new environmental concerns. Finally, the treatment of industrial wastewater was performed to guide the practical application of the EF-Fere-HClO process.