Stabilizing Chromium from Leather Waste in Biochar

Disposal of chrome-tanned leather waste provides an environmental challenge, with land-based methods risking leaching of chromium into the environment. We investigate the production of biochar from leather as an alternative means to dispose of leather waste. Chrome-tanned leather is heated at 500-1000 degrees C in an environment excluding oxygen to form biochar. The char is leached in 1 M HCl for 15 h, and the leachate is analyzed for Cr to confirm that Cr does not leach from char formed at or above 600 degrees C. The char is analyzed by X-ray absorption spectroscopy (XAS) for chemical state and structure. X-ray absorption near edge structure (XANES) analysis shows that the leather and biochar contain Cr as a mixture of Cr sulfate and Cr carbide, with the proportion of Cr as carbide increasing from 0% for untreated leather to 88% for char formed at 1000 degrees C. Modeling of the extended X-ray absorption fine structure (EXAFS) spectra shows that the atomic near-range structure is consistent with that of chromium carbide for the high-temperature samples. Biochar produced from chrome-tanned leather waste contains highly dispersed chromium present as a stable, carbide-like structure (provided sufficiently high temperatures are used). This material, rather than being an environmental problem, may be used for soil remediation and carbon sequestration.