Cr(VI) Reaction Mechanism of Black Carbons Derived From Cellulose and Lignin.

Black carbon (BC) is one of the main soil components responsible for immobilizing toxic Cr(VI) in soil. To understand the reaction mechanism between Cr(VI) and BC, BCs were prepared by heating cellulose and lignin at 400, 500 and 600 °C and the reactions of BCs and their precursors with Cr(VI) were studied. Cellulose exhibited an insignificant Cr(VI) reduction capacity while that of lignin was the highest among all the materials. As the heating temperature was increased, the Cr(VI) reduction of cellulose-derived BC increased due to increasing surface area and aromatic carbon content. Comparatively, the Cr(VI) reduction of lignin-derived BC decreased with increasing heating temperature, because of the loss of surface functionality. The results of Cr K-edge XANES suggested that Cr sorbed on lignin and BCs were predominately in the trivalent form. The phenolic groups of the materials were the dominant drivers for Cr(VI) reduction and the resultant carbonyl/carboxyl groups provided binding sites for Cr(III) resulting from Cr(VI) reduction. Cr(III) is bound to the surfaces through surface complexation and precipitation as revealed by the corresponding Cr K-edge EXAFS.