Unveiling the versatile performance of transition metal sulfides in peroxymonosulfate activation

Advanced oxidation processes (AOPs) are promising treatment options for environmental remediation to reduce chemical oxygen demand (COD) and emerging contaminants. Transition metal sulfides (TMSs) are a class of common catalysts in AOPs, but actually, they also widely exist in the environment in the form of sulfide minerals and seriously contaminated water bodies as black-odorous substances. Therefore, it is crucial to find out their roles during water decontamination using AOPs. This study investigated different mechanisms and performance of four common TMSs (FeS, CuS, CoS and MnS) in peroxymonosulfate (PMS)-based AOPs for the removal of a representative micropollutant tetracycline (TC). The results showed that CoS/PMS was the most efficient with 100 % TC removal within 1 min, followed by FeS/PMS (100 %, 10 min) with the apparent rate constant (kobs) of 0.45 min−1. TC removal by CuS (kobs = 0.10 min−1) and MnS (kobs = 0.02 min−1) took 30 min with 96 % and 82 % removal, respectively. Interestingly, the dominant reactive species were different in these systems, i.e., high-valent iron (Fe(IV)) in FeS/PMS, mainly radicals (•OH, SO4•−, O2•−) and singlet oxygen (1O2) in CuS/PMS, and Co(IV) and 1O2 in CoS/PMS. In comparison, TC was removed in MnS/PMS mainly through cooperative adsorption, reduction and oxidation by MnS. Low valent sulfur species, such as S2− and S22−, may participate in the valence cycles of metals or TC reduction. The above four systems had a wide pH (3.6–11.1) adaptability and a strong anti-interference ability to co-existing Cl−, SO42− and NO3−, but could be hindered by CO32−, PO43− and humic acid. This study provides novel insights into the TMS-based catalysts in AOPs, which will pave the way for advancing AOP technology.