Effect of Fe³⁺ as an electron-transfer mediator on WO₃-induced activation of peroxymonosulfate under visible light

The effect of ferric ions (Fe³⁺) on the degradation of organic pollutants through the photocatalytic activation of peroxymonosulfate (PMS) was investigated. In the presence of Fe³⁺, the degradation of 4-chlorophenol (4-CP) was significantly enhanced in the PMS/tungsten oxide (WO₃) system under visible light irradiation. The enhanced degradation efficiency by Fe³⁺ is primarily ascribed to the role of Fe³⁺ as an electron-transfer mediator, as it induces a cascadal electron transfer from WO₃ to Fe³⁺ to PMS. The production of the sulfate radical (SO₄[rad]⁻) and its significant contribution to the degradation of 4-CP in the Fe³⁺/PMS/WO₃ system were verified via electron paramagnetic resonance (EPR) spectroscopy and degradation experiments using radical scavengers (methanol and tert-butyl alcohol), respectively. The addition of Fe³⁺ to the PMS/WO₃ system also significantly increased the degradation rate of other organic pollutants, such as phenol, 2,4-dimethylphenol, bisphenol A, sulfamethoxazole, sulfanilamide, and propranolol. The degradation efficiency of the Fe³⁺/PMS/WO₃ system was largely maintained in multiple degradation cycles by the regular addition of PMS only. In addition, the Fe³⁺/PMS/WO₃ system exhibited a higher degradation efficiency than the conventional cobaltous ion (Co²⁺)/PMS system at equal transition metal ion concentrations. The addition of Fe³⁺ for the purpose of enhancing the degradation efficiency is not restricted to the PMS activation system. This method can be applied to the activation of other oxyanions, such as peroxydisulfate (S₂O₈²⁻) and bromate (BrO₃⁻), for wastewater treatment.