Pyrolysis-temperature depended electron donating and mediating mechanisms of biochar for Cr(VI) reduction

Biochar could be involved in environmentally relevant redox reactions, and their redox-active moieties may change with pyrolysis temperature. In this study, pyrolysis-temperature depended electron donating and mediating ability of biochar for Cr(VI) reduction were evaluated. All biochar derived from peanut shell at 400-800 °C effectively reduced Cr(VI) into Cr(III), and the reduction capability decreased as the pyrolysis temperature increased (400-600 °C), and then increased (600-800 °C). The electron donating moieties transformed from the -OH functional groups at lower pyrolysis temperature (<600 °C) to the functional groups associated with conjugated structure at higher temperature (>600 °C). Biochar could mediate the reduction of Cr(VI) by lactate, with the reduction rates of Cr(VI) increased up to 16.3 and 345 times that by either biochar or lactate alone, respectively. The redox ability of biochar was the premise, but its conductivity was the dominant factor, for the mediating reduction of Cr(VI). The mediation capability of biochar increased with elevated pyrolysis-temperature due to its increased conductivity related to the growth of conjugated clusters and carbon defects. Our results indicated that pyrolysis-temperature affects the formation of functional groups and conjugated carbon structure of biochars which have a distinguishable influence on the electron donating and mediating ability for Cr(VI) reduction.