Synergistic effect of novel pyrite/N-doped reduced graphene oxide composite with heterojunction structure for enhanced photo-assisted reduction of Cr(VI) in oxic water: Specific role of molecular oxygen.

To avoid severe aggregation and synergistically utilize the intrinsic and photocatalytic reducibility, pyrite (FeS) was loaded onto N-doped reduced graphene oxides (N-rGO) to fabricate a novel FeS/N-rGO heterojunction catalyst for enhanced chromium (Cr(VI)) reduction in oxic condition to simultaneously investigate the specific effect and role of dissolved oxygen (DO). Characterization results showed that strong interaction and combination of FeS and N-rGO not only achieved the uniform distribution of FeS, but also increased the defects, and exposed more functional groups. Meanwhile, the Type II heterojunction was formed in FeS/N-rGO, which facilitated the separation efficiency of photo-generated carriers and electrons, endowing FeS/N-rGO a superior photocatalytic activity. Cr(VI) was almost completely reduced via FeS/N-rGO within 60 min under irradiation (Cr(VI) = 10 mg/L, dosage = 0.2 g/L), 3 times that of pristine FeS (18.7 %). Trapping and Electron Spin Resonance (ESR) experiments indicated that photo-generated e and derived O species from photoactivation of dioxygen (DO) were the key reactive species for the enhancement of photo-assisted Cr(VI) reduction, rather than reductive Fe and S species. Although the photocatalysis of FeS/N-rGO cannot directly generate hydroxyl radicals (OH), the oxidative OH ascribed to superoxide radicals (O), photo-induced holes and free DO preferentially consumed by Fe and S with stronger reducibility. Hence, as compared to the anoxic condition, the reduction rate of Cr(VI) was slightly decreased, but still could be totally removed within 60 min in the oxic conditions. Due to the excessive amount of FeS/N-rGO, Cr(III) after reduction would not be influenced by oxidative species and maintain stability under oxic condition. This study provided a facile modification strategy for FeS based composites and uncovered its working mechanism for Cr(VI) decontamination.