Role of halide ions on organic pollutants degradation by peroxygens-based advanced oxidation processes: A critical review

Halide ions are common in wastewater but their roles in advanced oxidation processes (AOPs) and the degradation of pollutants are reportedly highly variable. This review seeks to reconcile conflicting data on the degradation of pollutants in the presence of halides by peroxygens-based AOPs reported in the literatures: peroxymonosulfate (PMS)-, peroxydisulfate (PDS)-, and hydrogen peroxide (H2O2)-based AOPs, and electrochemical advanced oxidation processes (EAOPs). Seven common substituent groups on the aromatic ring (i.e., hydroxy groups (-OH), amino groups (-NH2), alkyl groups (-R), olefinic compounds, carboxyl groups(-COOH), nitro groups (-NO2), halogens (-X)) are considered, and halide ions were found to affect contaminants' degradation in all cases, either negatively or positively. The key influencing factors such as substrate properties and operating parameters (e.g., halides dosage, ionic strength, pH, and activation method) are examined. Each individual variable may affect degradation rates, but many of these effects are a combination of these variables in experimental or natural systems. Thus, the research work on the effect of halides on the simultaneous removal of more organics is needed. This present study details the unresolved challenges to provide a path for engineering challenges on AOPs. According to the factors affecting the performance of halides during decontamination processes, better AOP technologies should be selected to diminish adverse effects and take advantage of positive effects to deal with target pollutants in the saline environment, so as to achieve the purpose of reducing the operating costs and emission reduction.