Thermal activation of persulfates for organic wastewater purification: Heating modes, mechanism and influencing factors

Thermal activation of persulfates (TAP) is a facile and easy-to-operate advanced oxidation technology for on-site decontamination of organic pollutants. Sulfate (SO4•−) and hydroxyl radicals (•OH) are the typical reactive oxygen species (ROS) in the processes of TAP. To date, numerous TAP systems have been developed to degrade recalcitrant contaminants in water. However, a comprehensive analysis of different thermal activation modes, particularly mechanisms and controlled generation of ROS, has not yet been reported. In this work, we present an overall review on different TAP systems. Specifically, diverse heating approaches such as thermal heating, microwave-induced heating and photothermal heating have different features in TAP and generate varying performances. Microwave heating for persulfate activation usually performs better than thermal heating at the same temperature, due to the intensified generation of radicals and boosted oxidation kinetics. Photothermal heating is cost-effective and eco-friendly via using sustainable solar energy. Moreover, the most appealing aspect of TAP is the promise of leveraging industrial waste heat to establish upscale integrated systems. Solution pH and background factors (e.g., anions and dissolved organic matters) have complicated impacts because of radical scavenging and the production of less reactive or reductive species. In addition, the presence of homogeneous and heterogeneous catalysts can enhance the activity of TAP systems at low temperature, while the synergistic contribution of catalytic oxidation will be less significant at high reaction temperature. Finally, conclusions and challenges of TAP systems in actual water remediation are presented.