Persulfate activated by non-thermal plasma for organic pollutants degradation: A review

Organic pollutants in water bring serious harm to human health and ecological environment. As one of the advanced oxidation technologies, non-thermal plasma technology has attracted extensive research and attention because it can quickly and efficiently treat refractory organic pollutants in the water environment. However, low energy utilization has been the bottleneck of the development of this technology. Plasma coupling catalysis has become the main way to solve this problem. Traditional plasma-catalysis technology was used to produce more OH, but it has an extremely short half-time (2 × 10−8 s) and unable to completely oxidize organic pollutants. Compared to OH, SO4− has a longer half-time (3–4 × 10−5 s), which has sufficient reaction time with pollutants. The physical and chemical properties such as ultraviolet light, electrons and free radicals in plasma can be used to activate persulfate for producing double oxidized free radicals (SO4− and OH), thus improving the energy utilization efficiency of plasma and removal efficiency of organic pollutants. Hence, this review aims to summarize the development of non-thermal plasma for persulfate activation regards to organic pollutants degradation. Factors influencing pollutants degradation by plasma/persulfate were elaborated in detail. Effect of additives and microbubbles on activation of PS by plasma was summarized. The catalytic effect of persulfate and other additives in plasma system was compared. The mechanism, pathway and toxicity variation of organic compounds degradation by plasma/persulfate were cleared. The in-depth evaluation index includes energy efficiency, TOC, COD and persulfate utilization were recounted. Finally, the conclusion and future prospects were put forward. Although the study of plasma activated persulfate has been very extensive, its mechanism has not been thoroughly studied. For example, the specific activation of persulfate by plasma has not been reported, and the specific dominant pathway of the generated free radicals on pollutants is still unclear. In addition, the impact of sulfate ions generated by activating persulfate on subsequent tail water has always been a problem that needs to be addressed.