Enhanced peroxymonosulfate activation by MoS2/NiCo2S4 composite catalyst for efficient elimination of tetracycline hydrochloride

Compared to metal oxide, owing to the sulfur's lower electronegativity, metal sulfide is a promising catalyst. Molybdenum disulfide (MoS2) has been used as a catalyst or co-catalyst in the technology of peroxymonosulfate (PMS) activation due to its conductivity and rich active sites. In this work, MoS2/NiCo2S4 composite was synthesized successfully as an effective catalyst to activate PMS for removal of tetracycline hydrochloride (TCH). Catalyst screening indicated the composite with a Mo:Co molar ratio of 0.5:1 (0.5-MoS2/NiCo2S4) exhibited the best PMS activation performance. The results showed that 98.3% of TCH (15 mg/L) was removed, and the removal efficiency of total organic carbon (TOC) was 54.5% within 90 min under conditions of an Oxone dosage of 400 mg/L and a catalyst dosage of 300 mg/L. The main active substances were found to be sulfate radicals (SO4•−), hydroxyl radicals (•OH), superoxide radicals (O2•−) and singlet oxygen (1O2) in 0.5-MoS2/NiCo2S4/PMS system based on the active species quenching tests and electron paramagnetic resonance (EPR) analyses. A potential activation mechanism for the 0.5-MoS2/NiCo2S4/PMS system was proposed in the following two aspects: one is that molybdenum and sulfur species enhanced the cyclic transformation of Co3+/Co2+ and Ni3+/Ni2+ in the reaction system, and the other is the synergy of Co3+/Co2+, Ni3+/Ni2+ and Mo6+/Mo4+ cycles in the PMS activation. In summary, owing to its high activity and reusability, the 0.5-MoS2/NiCo2S4 composite is a promising catalyst for wastewater treatment.