Heat-activated peroxydisulfate and peroxymonosulfate-mediated degradation of benzotriazole: Effects of chloride on kinetics, pathways and transformation product toxicity

Abstract

The impact of chloride (Cl⁻) in heat-activated peroxydisulfate (PDS) and peroxymonosulfate (PMS) processes for benzotriazole (BTA) degradation was investigated. Results showed that 0.42 mM BTA could be degraded by PDS and PMS under 70 °C in presence and absence of Cl⁻. The PMS-mediated BTA degradation rate increased with increasing Cl⁻ concentration up to 1000 mg/L, while a further increase of Cl⁻ concentration decreased the BTA degradation rate. In contrast, Cl⁻ inhibited PDS-mediated BTA degradation at concentrations tested between 100 and 10,000 mg/L. Radical scavenging experiments indicated that BTA degradation was mainly driven by hydroxyl and sulfate radicals in PDS and PMS systems without Cl⁻. However, reactive chlorine species (RCS) significantly boosted the PMS system for BTA degradation in presence of Cl⁻. Variation in pH substantially influenced the PMS system, but not the PDS system, whether in presence and absence of Cl⁻. By LC-MS/MS analysis, forty-two transformation products (TPs) were identified resulting from BTA degradation. Based on the TPs, polymerization, hydroxylation, benzene ring-opening, and carboxylic acid formation were hypothesized to be the main degradation mechanisms in absence of Cl⁻, whereas chlorination, triazole ring-opening, and nitration were the additional degradation steps in presence of Cl⁻. These findings help understand the influence of Cl⁻ on BTA removal rate and degradation pathway in saline wastewater. Moreover, more chlorinated TPs were found in PMS/Cl⁻ system than in PDS/Cl⁻ system, which was also reflected in absorbable organic halides (AOX) and end-product toxicity analyses. The PMS/Cl⁻ process also produced other undesirable by-products, such as chlorates which were not detected in the PDS/Cl⁻ process. This shows that PDS and PMS-based advanced oxidation processes can notably differ in terms of toxic by-product formation. Thus, they need to be critically evaluated before applying for organic pollutant degradation under saline conditions.