Reaction mechanism of hexavalent chromium with cellulose

It has been suggested that lignocellulosic materials are cost-effective materials for remediation of toxic Cr(VI) pollution; however, inconsistent interpretations of the reaction mechanism are frequently found in the literature. To clarify the reaction mechanism, the reaction of Cr(VI) with individual components of lignocellulosic materials must be better understood. Thus, the purpose of this work was to investigate the reaction mechanism of Cr(VI) with cellulose, which is one of the main components of lignocellulosic materials. The Cr(VI) reaction was evaluated at pH 2 and the results showed that Cr(VI) could be reduced to Cr(III) by cellulose. As indicated by the FT-IR spectra, the hydroxyl groups in cellulose were the reactive sites for Cr(VI) reduction, giving rise to the formation of carboxyl groups. The resultant Cr(III) was either bound to cellulose or released into solution. As revealed by the Cr K-edge EXAFS of cellulose after its reaction with Cr(VI), Cr(III) was bound to cellulose by forming bidentate–mononuclear complexes with carboxyl groups, which resulted from the oxidation of hydroxyl groups. The first-order rate constant of Cr(VI) reaction with cellulose was determined to be 3.84 × 10−7 s−1 at pH 2. The slow reduction of Cr(VI) by cellulose was attributed to the existence of H-bonding networks in cellulose, which restrict the accessibility of Cr(VI) to the hydroxyl groups in cellulose.

► Cellulose can reduce toxic Cr(VI) to less toxic Cr(III).
► The hydroxyl groups in cellulose are responsible for Cr(VI) reduction.
► The resultant Cr(III) ions form bidentate inner-sphere complexes with carboxyl groups in cellulose after Cr(VI) reaction.