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/Fe-0 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/Fe-0 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/Fe-0 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/Fe-0 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 Fe-0 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 SO(4)(-center dot)and center dot OH. SO4-center dot 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 Fe-0 prepared by ball milling a mixture of Fe-0 and FeS2, which can provide some new insights into the degradation of organic pollutants.