Biochar as both electron donor and electron shuttle for the reduction transformation of Cr(VI) during its sorption

Biochar has been demonstrated to be a promising sorbent and its redox activity can be involved in environmentally relevant redox reactions. In this study, the electron transfer for the reduction transformation of Cr(VI) during its sorption by biochar were evaluated. Biochar derived from peanut shell at 350 °C could effectively remove Cr(VI) from solutions, accompanied by the reduction of Cr(VI) to Cr(III) which was more obvious at the strong acidity (pH = 2), compared to that at the weak acidity (pH = 4). The O-containing functional groups, e.g. CO and CO, were the electron donor moieties of biochar for the Cr(VI) reduction. Biochar could also act as electron shuttle, enhancing Cr(VI) reduction by lactate, especially at the weak acidity (pH = 4) where the reduction rates of Cr(VI) by lactate together with biochar were about 2 and 9 times higher than those by either biochar or lactate alone, respectively. The mediated electrochemical oxidation analysis showed that biochar could increase the electron donating capacity of lactate by up to 23 times at pH = 4. The O-centered radicals, e.g. semiquinone-type radicals, were related with biochar as electron shuttle, which was confirmed by the electron paramagnetic resonance analysis. As a result of electron acceptance from lactate and/or biochar, Cr(VI) was reduced into CrOOH evidenced by X-ray diffraction analysis. Our results indicated that biochar could act as both electron donor and electron shuttle for the reduction of Cr(VI) during the sorption process, making it an alternative for removal of toxic Cr(VI) from wastewaters.