The role of superoxide anion to Cr(VI) reduction by pine biochar

It has been reported that Cr(VI) can be reduced by biochar because of its redox activity. Considering the anionic form of Cr(VI), we hypothesize that the reduction in aqueous phase is significant. However, the contribution of different reactive oxygen species in the biochar-Cr(VI) reaction system has not been distinguished. Herein, we quantitatively identified Cr(VI) adsorption and reduction in biochar systems. The reduction content of Cr(VI) was 1.5 times higher in untreated conditions than in anaerobic conditions. The disappearance of ·O2- under anaerobic conditions illustrated that ·O2- may be involved in the reduction of Cr(VI). Quenching of ·O2- resulted in a decrease of Cr(VI) reduction by 34%, while 1O2 was negligible, probably due to the stronger electron-donating capacity of ·O2-. The degradation of nitrotetrazolium blue chloride (quenching agent of ·O2-) confirmed that the reduction process of ·O2- mainly occurred in the liquid-phase. Boehm titration and quantification of ·O2- further elucidated the significant correlation (P < 0.05) between phenolic groups and the formation of ·O2-, which implied that phenolic groups acted as the primary electron donors in generating ·O2-. This study highlights the importance of the liquid-phase reduction process in removing Cr(VI), which provides theoretical support for biochar conversion of Cr(VI).