Catalytic activation of persulfate by mechanically-treated natural pyrite: Revisit on the dominant role of sulfur vacancies and size effect

• Effect of milling speed on the surface characteristics of FeS 2 was evaluated for PDS activation. • The effect of ball milling on the structure-activity relationship of pyrite was investigated. • Ball milling caused appearance of surface sulfur vacancies (SVs) in enhancing activation of PDS. • Size effect was crucial for the degradation of SMX, while SVs would act as an auxiliary. • The system efficiently treated a molasses-processed wastewater with COD removal of 770 mg/L. Mechanical treatment has been widely used to modify the surface properties of nature pyrite minerals and improve their catalytic performances; however, the mechanistic relationship still keeps unclear. In this study, we have systematically investigated the effect of milling speed treatment on the surface characteristics of commercial natural pyrite and evaluated the activation ability of persulfate (PDS) for the degradation of sulfamethoxazole (SMX). The results demonstrated that increase in the milling speed of 200-600 rpm could dramatically enhance the degradation and mineralization of SMX, while lower speed (≤ 200 rpm) exhibited marginal effect. The high milling speeds could reduce the content of sulfate and S(-II) in the pyrite and lead to the occurrence of sulfur vacancies (SVs) for promotional activation of PDS. Nevertheless, the SVs content was not in accordance with increased speed and size effect would mainly contribute to the efficient degradation of SMX. The milling pyrite/PDS system could efficiently treat a molasses-processed wastewater with COD removal of 770 mg/L, suggesting slow-release function of pretreated pyrite and it broaden application potential. In addition, based on the novel conclusion of the mechanism, this study provided a green, efficient and economical pyrite mechanical activation method and its application.