Core-shell structured cobalt–nickel bimetallic sulfide with dual redox cycles to activate peroxymonosulfate for glyphosate removal

Glyphosate is a widely used herbicide in agriculture. Its residues pose a great threat to ecological balance and human health. The potential impact of glyphosate and its by-products on the soil and water environment is of great concern. Here, a core–shell structured cobalt–nickel bimetallic sulfide NiCo2S4/Co9S8/NiS (NCS) is synthesized by a simple two-step hydrothermal method, and used as a novel peroxymonosulfate (PMS) activator for degradation of glyphosate. The effects of various reaction conditions on the degradation of glyphosate in the NCS-PMS system are investigated. The NCS is used to activate PMS by the dual redox cycle of Co2+/Co3+ and Ni2+/Ni3+ under its core–shell structure to efficiently degrade glyphosate. The optimized NCS achieved 82.2 % removal of 10-4 mol⋅L-1 glyphosate in 35 min with an initial pH = 3.9. The removal performance is 25.7 and 1.5 times higher than that of monometallic sulfides Ni3S2/NiS and Co3S4 prepared by the same process of NCS, respectively. The radical quenching experiments reveal that NCS interacted with PMS to generate SO4•− and •OH radicals, O2•− as well as 1O2 together to achieve efficient degradation of glyphosate. The main intermediates of glyphosate degradation by NCS-activated PMS are identified by liquid chromatography-quadrupole time-of-flight mass spectrometry, and possible degradation pathways of glyphosate are proposed and the relative toxicity of degraded products is evaluated. This work provides a novel insight to produce high-performance multiphase catalysts for glyphosate degradation as well as practical applications.