Unveiling the neglected impacts of halide ions on phenol removal in UV/H2O2 system: Light shielding and oxidant utilization efficiency

Abstract

Halide ions may diminish organics' removal performance of high-salinity organic wastewater pretreated by advanced oxidation processes (AOPs). However, their impact on oxidant utilization efficiency and the specific mechanisms remain unclear. In this study, the degradation performance of the UV/H₂O₂ process under high concentrations of Cl⁻ and Br⁻ was comprehensively evaluated, focusing on phenol mineralization, H₂O₂ utilization efficiency, and halogenated byproduct formation. Cl⁻ and Br⁻ significantly inhibited phenol mineralization, with Br⁻ exhibiting more pronounced inhibition. The decreased formation rate of hydroxyl radicals (•OH) in the presence of halide ions resulted from the accumulation of intermediates and halogenated byproducts shielding UV light, as indicated by a 1–3 times increase in absorbance at 254 nm. The mathematical model proved that a higher proportion of reactive radicals (•OH and halogen radicals) reacting with H₂O₂ would result in a greater amount of H₂O₂ invalidly decomposing into oxygen. H₂O₂ utilization efficiency increased by >17 % in the presence of Cl⁻ at pH 8, but significantly decreased in the presence of Br⁻. The distinction was attributed to over 95 % of Br• reacting with H₂O₂, which is much higher than the fraction of Cl•. Furthermore, elevating the pH from 3 to 8 mitigated the inhibitory effects and alleviated the acute toxicity of halogenated byproducts. These novel findings provide critical insights for the efficient treatment of high-salinity organic wastewater with UV-activated and H₂O₂-based AOPs.