Biochar conductivity and electron donating capability control Cr(VI) bioreduction.

Biochar can facilitate Cr(VI) bioreduction, but it is still undetermined which biochar property control this process. We observed that the apparent Cr(VI) bioreduction by Shewanella oneidensis MR-1 could be identified as a fast and a relatively slow processes. The fast-bioreduction rates (r) were 2-15 times higher than the slow-bioreduction rates (r). In this study, we investigated the kinetics and efficiency of biochar in promoting Cr(VI) reduction by S. oneidensis MR-1 in the neutral solution using a "dual-process model" (fast and slow processes), and analyzed the mechanisms of biochar concentration, conductivity, particle size and other properties on these two processes. The correlation analysis of these rate constants and biochar properties was carried out. The fast-bioreduction rates were associated with higher conductivity and smaller particle sizes of biochar, which facilitated the direct electron transfer from Shewanella oneidensis MR-1 to Cr(VI). The Cr(VI) slow-bioreduction rates (r) were mainly determined by the electron donating capability of biochar and independent of the cell concentration. Our results suggested that Cr(VI) bioreduction was mediated by both electron conductivity and redox potential of biochar. This result is instructive for biochar production. Manipulating biochar properties to control fast and slow Cr(VI) reduction may be helpful to effectively remove or detoxify Cr(VI) in the environment.