FeS redox power motor for PDS continuous generation of active radicals on efficient degradation and removal of diclofenac: Role of ultrasonic

Diclofenac (DCF), as a typical representative of PPCPs, has potential ecotoxicity to the water environment. In this study, ultrasound (US) enhanced ferrous sulfide (FeS)-activated persulfate (PDS) technology (US/FeS/PDS) was used to degrade DCF. By comparing the degradation effects of US, US/PDS, FeS/PDS and US/FeS/PDS systems on DCF, this study confirmed the synergy and strengthening effects of US. The influences of single-factor experimental conditions on the US/FeS/PDS system were investigated and optimized. The FeS catalysts before and after the reaction were characterized and analyzed by X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS). The heterogeneous reaction proceeded on the surface of FeS, and a small part of FeS2 was formed on FeS surface. During the reaction, the proportion of S2− on the catalyst surface decreased from 51% to 44%. Correspondingly, the proportion of Sx2− increased from 21% to 26%. It indicated that S2− was oxidized into Sx2− in the reaction, and the loss electrons of S2− caused the reduction of Fe3+ to Fe2+on the FeS surface, which promoted the cycle between Fe2+ and Fe3+ in turn. Furthermore, SO4− and ‧OH were the main active free radicals, of which the contribution rate of ‧OH was about 34.4%, while that of SO4− was approximately 52.2%. In US/FeS/PDS, the introduction of US could promote the dissolution of iron on the FeS surface. US contributed to the formation of a redox power motor between S2—Sx2- and Fe2+-Fe3+, which continuously decomposed PDS to generate sufficient active SO4− and ‧OH radicals, thereby efficiently and continuously degrading DCF. Finally, the related mechanism of DCF degradation by US/FeS/PDS was summarized. Overall, US/FeS/PDS can not only efficiently degrade and remove DCF, but also has potential application value in organic pollution removal and wastewater purification. [Display omitted] •US/FeS/PDS can efficiently degrade and remove DCF.•US and FeS/PDS showed a good synergistic effect (S = 3.56).•US could accelerate the dissolution of FeS to continuously to release Fe2+.•US helped to form a redox power motor between S2–Sn2- and Fe2+-Fe3+.•The ·OH (34.4%) and SO4− (52.2%) make contribution to DCF removal.