Recent advances in persulfate-assisted TiO2-based photocatalysis for wastewater treatment: Performances, mechanism and perspectives

Heterogeneous photocatalysis using TiO2 is a prospective approach to effectively degrade persistent organic contaminants in water. However, the fast recombination of photo-induced electron/hole pairs hinders the practical application of TiO2. The introduction of persulfate to TiO2 photocatalytic system can boost the separation of electron/hole pairs and simultaneously generate sulfate radicals (SO4•−). This paper first reviews the application of persulfate-assisted TiO2-based photocatalysis for wastewater treatment and systematically summarizes the corresponding photocatalytic mechanisms. TiO2 modification with carbon materials, transition metal-based catalysts, noble metals, and organic semiconductors expands the TiO2 optical absorption spectrum into the visible light region and improves the charge separation. As a result, the persulfate activation efficiency is enhanced, and the degradation rate of organic pollutants is increased. Magnetic transition metal-based catalyst is an attractive candidate for the construction of magnetically separable TiO2-based composite photocatalysts because M(n−1)+/Mn+ (M refers to transition metal) redox cycles can promote persulfate activation to generate SO4•−. Moreover, the reusability of TiO2-based catalysts and the effect of key operational parameters (catalysts dosage, persulfate dose, pollutant concentration, solution pH, and coexisting matters) on process efficiency were summarized and analyzed. The prospective applications of persulfate-assisted TiO2 photocatalysis (utilization of discontinuous solar energy, assessment of intermediate toxicity, and prevention of secondary pollution) in water treatment were also proposed.