Efficient degradation of tetracycline in FeS-based SR-AOPs process at basic pHs: The overlooked role of metal complexation and redox reaction in persulfate activation

Herein, mackinawite (FeS) was employed for activation of persulfate (PS) to remove tetracycline (TTC). The effect of the coexistence of Cu(II) on TTC degradation was explored at different initial pHs. Nearly 100% removal in 5 min at an initial pH of 3 was achieved with/without Cu(II). No less than 90% of TTC removal was achieved in a 60 min reaction at initial pH of 5, 7 and 9 where the Fe(II) leaching was small. More surprisingly, compared to FeS/PS/TTC system, up to a three-fold increase in degradation rates of TTC was observed in the FeS/PS/TTC-Cu system at initial pH of 9. Cu(I) produced by TTC-Cu associated with PS activation at initial pH of 5, 7 and 9 was thought to result in the apparent increase in TTC degradation efficiency. By further comparing the TTC degradation processes involving oxidizing complexed Cu(II), oxidizing non-complexed Cr(VI), non-oxidizing complexed Zn(II) and Pb(II), respectively, we determined the contribution of the complexation and redox processes of metal ions to TTC degradation. In addition, free radicals were detected in the FeS/PS/TTC-Cu system, in which SO4•− played the dominant role in the TTC degradation. Meanwhile, the structural Fe(II) was presumed as the primary form of Fe(II) improving to the activation of PS under basic conditions based on the theoretical calculations. These findings emphasize the importance of the complexation and redox reaction of metals at basic pH in the FeS/PS system, which may broaden the application of FeS/PS system.