Mechanochemically synthesized S-ZVIbm composites for the activation of persulfate in the pH-independent degradation of atrazine: Effects of sulfur dose and ball-milling conditions

Surface passivation and an increase in pH may restrain the efficiency and capacity of the zero-valent iron (ZVI) activated persulfate (PS) process. Mechanochemically sulfidated ZVI (S-ZVI(bm)) shows an extraordinary increase in the activation of PS. Ball-milling avoids the complex liquid reactions of traditional methods of activation and is easier to upscale. However, the effects of the sulfur dose and milling conditions have not been sufficiently investigated in the application of S-ZVI(bm) to the PS activation process. S-ZVI(bm) was pH-independent compared with similar catalysts. The best performance was obtained for S-ZVIbm with 25% sulfur synthesized at 300 rev min(-1) for 5 h. Almost complete degradation of 50 mu M ATZ was obtained after 40 min reaction with 6 mM PS and 0.3 g L-1 S-ZVI(bm). Structural and species characterizations revealed the effects of the milling conditions and showed that sulfide (S2-) might be the predominant sulfur species. The primary facilitation mechanisms of SZVI(bm) were the mitigation of passivation, an increase in hydrophobicity, an increase in electron transfer and the accelerated regeneration of Fe2+. Fe-57 Mossbauer spectrum confirmed that the Fe-0 left in S-ZVI(bm) was about two times higher than that in reacted ZVI(bm). Fe-0 acted as the predominant PS activator, but FeS2, FeS and SO32- also contributed to PS activation. The degradation pathway of atrazine was clarified based on the detection of intermediates and density functional theory calculations. S-ZVI(bm) is a promising catalyst for the activation of PS used to remediate refractory contaminants and is amenable to upscaling from the laboratory scale.