Efficient Photo-Assisted Reduction of Cr(Vi) Via Activated Carbon Mediated Z-Scheme Fes2/Α-Feooh/C Heterojunctions: Specific Contribution of Reductive Species and the Fate of Oxygen

Abstract

Herein, pyrite (FeS2), goethite (α-FeOOH) and activated carbon (C) were facilely combined to construct a C mediated Z-scheme heterojunction in novel FeS2/α-FeOOH/C composites for enhanced visible light assisted reduction of Cr(VI) in oxic wastewater. Characterization tests indicated the strong interaction and combination among FeS2, α-FeOOH and C with the generation of Fe-O-C and C-S-C coordination groups, ensuring the enhanced electron transport capacity during photocatalysis. The increased amount of surface oxygenic functional groups like -C-OH, -COOH and Fe-OH, and the decreased agglomeration degree of FeS2 and α-FeOOH after combination strengthened the absorption and conversion of light and Cr(VI) adsorption, resulting in synergistic effect for enhanced Cr(VI) reduction. Additionally, C mediated Z-scheme heterojunctions with lower resistance was successfully constructed to facilitate the transfer and separation efficiency of photo-generated carriers, further improving Cr(VI) reduction capacity of FeS2/α-FeOOH/C. Consequently, almost 100% of Cr(VI) was reduced via FeS2/α-FeOOH/C in 60 min with initial Cr(VI) of 10 mg/L under visible light, 4 times that of pristine FeS2. Quenching, EPR and control experiments confirmed photocatalysis made more contribution than single reductive reagents (FeS2) for Cr(VI) reduction and the activation of dissolved oxygen (DO). The specific contribution rate of reductive species like e-, Fe2+, S22- and •O2− for Cr(VI) reduction was detailly demonstrated and •O2− was proved to make a predominant role under aerobic conditions. Due to the excessive amount of FeS2/α-FeOOH/C, oxidative species like H2O2, 1O2 and •OH ascribed to activated DO and photo-generated h+ preferred to reacted with Fe2+ and S22- with higher reducibility and-only inhibited the reduction Cr(VI) rate, the total Cr(VI) removal efficiency and reduced Cr(III) kept stable under oxic conditions.

Keywords: pyrite/goethite/activated carbon, Z-scheme heterojunction, photo-assisted reduction, hexavalent chromium, reactive oxygen species transfer pathways