Modulating the >Co(II)/Co(III) redox cycling via confinement of cobalt with WS₂ for the ultrafast sulfite activation

Sulfite autoxidation in combination with the cobalt-based heterogeneous activators, has recently emerged as the efficient sulfate radical (SO₄^•-) generation process for organic micropollutant abatement in the water and wastewater treatment, yet the sluggish >Co(II)/Co(III) redox cycling currently compromises the efficacy of radical generation and the potential applications. Herein, regarding that the reductive W(IV) species in WS₂ can modulate the >Co(II)/Co(III) redox cycling in the advanced oxidation processes, confinement of cobalt with WS₂ (Co-WS₂) is designed and characterized. The Co-WS₂/sulfite process achieves an ultrafast tetracycline (TC) abatement (~100 % abatement of TC within 1 min) under circumneutral conditions with lower dosage of sulfite and activator, outperforming the current cobalt-based heterogeneous counterparts. The dominant reactive radicals are identified as SO₄^•- and hydroxyl radical (HO^•), which are quantified to be 9.7 μM and 4.5 μM, respectively. The superior radical generation efficiency and the concomitant TC abatement rely on the excellent redox properties and electron transfer capability of Co-WS₂. The inter-transformation of >Co(II)/>Co(III) can be accelerated via the involvement of the reductive W(IV) species with the redox-reversibility of the W(IV)/W(VI) couple in the presence of sulfite. The TC degradation intermediates and the corresponding pathways are also proposed according to the ultra-performance liquid chromatography and quadrupole-time of flight mass spectrometry (UPLC-QTOF-MS) analysis. In addition, the influences of the reactant dosage, coexisting anions (HCO₃^-, HPO₄^2-, Cl^- and NO₃^-), humic acid and the various real water matrices on TC abatement are thoroughly explored. Especially, the Co-WS₂/sulfite process is advantageous owing to the negligible effect of the coexisting anions on the TC abatement. This study provides a novel heterogeneous activator for significantly improving sulfite activation efficacy to achieve the efficient organic micropollutant abatement.