Advanced Oxidation Processes for Wastewater Treatment: Formation of Hydroxyl Radical and Application

Advanced oxidation processes (AOPs), defined as those technologies that utilize the hydroxyl radical (·OH) for oxidation, have received increasing attention in the research and development of wastewater treatment technologies in the last decades. These processes have been applied successfully for the removal or degradation of toxic pollutants or used as pretreatment to convert recalcitrant pollutants into biodegradable compounds that can then be treated by conventional biological methods. The efficacy of AOPs depends on the generation of reactive free radicals, the most important of which is the hydroxyl radical (·OH). The authors summarize the formation reactions of ·OH and the mechanisms of pollutants degradation. They cover six types of advanced oxidation processes, including radiation, photolysis and photocatalysis, sonolysis, electrochemical oxidation technologies, Fenton-based reactions, and ozone-based processes. Controversial issues in pollutants degradation mechanism were discussed. They review the application of these processes for removal of different kinds of toxic pollutants from wastewater, including aromatic compounds, dyes, pharmaceutical compounds, and pesticides, with emphasis on the parameters assessed, removal effectiveness, and the degradation mechanisms of pollutants. The authors discuss issues associated with practical wastewater treatment and offer suggestions for the direction for future researches.