Dye mineralization under UV/HO promoted by chloride ion at high concentration and the generation of chlorinated byproducts.

Chloride ion (Cl) may promote or inhibit the oxidation of specific organic compounds treated by hydroxyl radical based advanced oxidation processes (HR-AOPs) depending on the reactivity of chlorine radicals towards the organics. However, the effects of high contents of Cl on the removal of total organic compounds (TOC) in high salinity organic wastewater treated by HR-AOPs were unclear. The removal and mineralization of azo dye Orange II (OrgII) by UV/HO process with Cl at high contents under various pH conditions were investigated. As the pH conditions increased higher than pH 5, TOC removal rates increased slightly possibly related to the increase of O production and the reduce of futile decomposition of HO into O. Cl at relative high concentration (1000 and 2000 mM) significantly promoted the mineralization of dyes with TOC removal increasing by 10 %-40 % under both acid and alkaline conditions. The proposed mechanism is that the reaction of Cl with OH would decline the decomposition of HO into O by inhibiting the reaction between OH and HO, and the generated chlorine species (Cl and Cl) could further promote the oxidation of dye molecules into intermediates and be helpful for the subsequent mineralization process. In addition, HO and Cl can slowly react to give HClO and ClO, which may partly contribute to the decolorization and mineralization of OrgII. Meanwhile, an appropriate relative proportion between Cl and OH depending on Cl contents and pH conditions is important to enhance the TOC removal. However, the formation of various chlorinated byproducts especially under alkaline condition may increase the risk of environmental pollution accidents. The results demonstrate the promotion of TOC removal by UV/HO under certain high contents of Cl and provide new insight into the application of HR-AOPs to the pretreatment of high salinity organic wastewater.