Chemistry, generation and regulation of reactive species in persulfate-based advanced oxidation processes

Over the past two decades, persulfate-based advanced oxidation processes (PS-based AOPs) have been extensively investigated in water purification and remediation of contaminated soil and groundwater. Considerable progress has been made in the development of PS activation methods, the identification of reactive species (RS) and the understanding of reaction mechanisms. The primary RS includes free radicals (e.g., sulfate radical, hydroxyl radical, and superoxide radical) and nonradicals (e.g., singlet oxygen, high-valent metal-oxo, mediated electron transfer, and surface-bound radical), which have own merits, limitations and application scenarios. However, insufficient mechanistic understanding of RS transformation pathways and their regulatory strategies has seriously hindered the implementation of PS-based AOPs in practical applications. Herein, this review focuses on the recent advances in strategies and mechanisms for the regulation of radical/nonradical RS in PS-based AOPs. Specific attention has been paid to catalyst modification and process control to achieve the controlled reaction pathways. Regulatory directions are classified into three categories based on RS features: (1) between radical pathways, (2) between radical and nonradical pathways, and (3) between nonradical pathways. Moreover, chemical properties, generation pathways, merits and limitations of each RS are introduced in-detail. Finally, future challenges and prospects in this field are proposed. This critical review provides an insightful understanding of the chemistry, controlled generation and regulation of the RS, and provides scientific guidance for the on-demand allocation of the RS in specific treatment scenarios.