Interaction with low molecular weight organic acids affects the electron shuttling of biochar for Cr(VI) reduction

Biochar can act as "electron shuttle" in soil redox reactions. It is possible that biochar accepts the electrons from low molecular weight organic acids (LMWOAs) in soil and then transfer them to the acceptors, e.g., Cr(VI). This study evaluated the interaction between seven soil LMWOAs and peanut shell biochar (BC) as well as its effect on the electron shuttling of biochar for Cr(VI) reduction. Both redox reactions and sorption process occurred during the interaction of biochar and LMWOAs, which altered the contents of Cr(VI) reduction-relevant groups (i.e., CO and CO) on the surface of biochar. The redox reactions were more important to the electron transfer between biochar produced at 400℃ (BC400) and LMWOAs due to the repeated cycle of reduction-oxidation of surface functional groups. The reduction rate of Cr(VI) by LMWOAs mediated by BC400 was 1.10-7.09 × 10^-3 h^-1, among which tartaric acid had the best reduction efficiency due to its highest reducing capability. For biochar produced at 700℃ (BC700), the sorption process of LMWOAs was the key factor to the direct electron shuttling process through the conjugated structure of biochar. The reduction rate of Cr(VI) by LMWOAs mediated by BC700 was significantly higher and ranged 7.40-864 × 10^-3 h^-1, with the oxalic acid having the best reduction efficiency due to its highest sorption capacity by BC700. The results obtained from this study can help to establish the linkage between biochar and LMWOAs in soil electron network, which better explains the multifunctional roles of biochar during the redox processes such as Cr(VI) reduction in soil.