Enhancement of ball-milling on pyrite/zero-valent iron for persulfate activation on imidacloprid removal in aqueous solution: A mechanistic study

Imidacloprid (IMI) removal by persulfate (PS) oxidation activated by ball-milled FeS2/Fe0 composite was investigated in terms of reaction conditions and mechanisms. The ball-milled FeS2/Fe0 with a mass ratio of 1:9 exhibited the highest activating efficiency for PS among the investigated mass ratios of 9:1, 4:1, 1:1, 1:4 and 1:9. Increasing dosages of ball-milled FeS2/Fe0 and PS promoted IMI removal. At initial pH of 4.6 and initial IMI concentration of 0.15 mol/L, the optimal dosage of ball-milled FeS2/Fe0 and PS were 0.1 g/L and 2.5 mmol/L, respectively, removing 99.8% of IMI after 60-min reaction. The ball-milled FeS2/Fe0 could also activate PS effectively in a wide pH range from 3 to 9. Characterizations including Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) revealed that the high activation efficiency was related to FeS produced on Fe0 surface. Activation efficiency of PS was enhanced by accelerated electron transfer mediated by increased ferrous species, ultimately, the removal efficiency of IMI was improved. The released Fe2+ activated PS to generate SO4-·and·OH. SO4-· was detected to be predominant reactive oxygen species in IMI degradation and three different pathways for IMI degradation were proposed. This study developed a efficient method to activate PS by FeS2 modify Fe0 prepared by ball milling a mixture of Fe0 and FeS2, which can provide some new insights into the degradation of organic pollutants.