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 °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 °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 °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.