The pH-sensitive sorption governed reduction of Cr(VI) by sludge derived biochar and the accelerating effect of organic acids

Abstract Reduction coupling immobilization is one of the most commonly adopted strategies for the remediation of Cr(VI) contamination. Biochar is a carbon-rich material with abundant active functional groups for sorption and reduction reactions. In previous reports, phytomass derived biochars and organic functional groups have been emphasized, while the performance of sludge derived biochar (SBC) has often been understated. In the present study, a 30 d kinetic study proved that the removal route involved the sorption of Cr(VI), reduction to Cr(III) and immobilization of Cr(III), and that the sorption process was the primary and rate determining step. As a result of the SBC alkalinity, the solution pH increased, and sorption was largely inhibited, which then governed the overall removal ratio. The FTIR spectra suggested the involvement of hydroxyls in these processes. Low molecular weight organic acids accelerated the removal process in the early phase and improved the reduction process.