Reduction of Cr(VI) by crop-residue-derived black carbon.

Burning crop residues is a common postharvest practice on farmland, leading to the accumulation of black carbon (BC) in the soil. To understand the potential role of BC in immobilizing toxic Cr(VI) in soil, this study evaluated the Cr(VI) sorption kinetics at pH levels ranging from 3 to 7 and examined the reaction mechanism of Cr(VI) with BC derived from burning rice straw. The BC samples, after reacting with Cr(VI), were analyzed using Cr K-edge X-ray absorption spectroscopy. The results showed that Cr(VI) was sorbed and subsequently reduced to Cr(III), which was bound to the BC surface through surface complexation and precipitation. As indicated by the diffuse reflection infrared Fourier transform spectra, the phenolic groups of BC are the dominant drivers of Cr(VI) reduction, giving rise to carbonyl/carboxyl groups on the BC surface. The reaction rate of Cr(VI) with BC increased from 10(-3.62) to 10(-1.65) h(-1) as pH was decreased from 7 to 3 because Cr(VI) sorption and reduction both occur faster at low pH. These results suggest that BC derived from burning crop residue is an effective reductant for Cr(VI) and may play an important role in determining the fate of Cr(VI) in BC-rich farmland soils.