Fenton-like activity of iron sulfides for the generation of reactive oxygen species and degradation of various organic pollutants has been extensively investigated due to its abundance in the natural environment. However, their Fenton-like activity is usually unsatisfactory due to the limited exposure of surface ferrous reactive sites. In this work, a new strategy to enhance the Fenton-like activity of iron sulfides, using pyrite (FeS2) as a model, was developed based on the heat treatment of FeS2 by water steam. It was found that the FeS2 heat-treated by water steam (Heat-FeS2) exhibited much higher heterogeneous Fenton activity in the degradation of alachlor (ACL) than its parent FeS2 prepared from hydrothermal reaction (Fresh-FeS2). At an initial pH of 6.3, the rate of degradation of ACL by Heat-FeS2 Fenton system was 0.48 min?1, which is ~23 times higher than that of Fresh-FeS2 Fenton system. Electron spin resonance analysis and benzoic acid probe experiments confirmed the production of more hydroxyl (?OH) and superoxide radicals (?O2?) in Heat-FeS2 Fenton system than Fresh-FeS2 Fenton system. The increased Fenton-like activity of Heat-FeS2 can be attributed to the increased content of highly reactive surface bonded Fe2+/Fe3+ species, higher amount of leached Fe2+, and optimal reaction pH due to stronger acidification of Heat-FeS2. Characterization studies by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy showed that heat treatment remarkably promoted the transformation of lattice Fe2+ to surface reactive Fe2+, allowing the exposure of more surface reactive Fe2+ and leaching of Fe2+; simultaneously, heat treatment enhanced the generation of surface SO42?, creating a highly acidic surface. The surface Fe2+ percentage in the surface total iron was raised from 13% in Fresh-FeS2 to 29% in Heat-FeS2. Fe2+ leaching from Heat-FeS2 was 0.23 mmol·L?1, much higher than that (< 0.02 mmol·L?1) for Fresh-FeS2. The change in the surface Fe and S species in the Heat-FeS2 system during the Fenton-like reaction was monitored by XPS to elucidate the enhanced Fenton oxidation mechanism. The characterization results showed that after Fenton reaction with H2O2, the surface contents of Fe2+ and Fe3+ species on Fresh-FeS2 and Heat-FeS2 were remarkably raised, while the surface content of S22? species was reduced, confirming the crucial role of S22? in the reductive cycle of Fe3+ to Fe2+. These findings increase understanding of the oxidative transformation and corrosion of iron sulfides and its relevant transformation and degradation of toxic organics in natural environments. The results of this work also provide an efficient Fenton-like oxidation method based on iron sulfides for highly efficient degradation of organic pollutants (e.g. ACL) in aqueous solution.
