Liberation and recovery of Cr from real tannery sludge by ultrasound-assisted supercritical water oxidation treatment

Efficient and sustainable treatment of real chromium-containing tannery sludge is attracting an increasing concern due to its potentially severe impacts on eco-system and human-beings. In this study, a greener alternative was studied for the removal and recovery of chromium from tannery sludge by ultrasound-assisted supercritical water oxidation approach. Supercritical water oxidation or ultrasonic treatment was respectively conducted to explore the detailed experiment parameters on organic destruction and chromium liberation. Ultrasound-assisted supercritical water oxidation treatment was then investigated to further explore the mechanism for Cr liberation. Experimental results indicate that supercritical water oxidation or ultrasonic treatment cannot effectively remove Cr from tannery sludge, while almost all Cr (99.6%) will be liberated to supernatant at optimized experimental conditions by ultrasound-assisted supercritical water oxidation treatment, with TOC removal efficiency over 99.9%. This indicates that toxic substances in sludge will be completely removed after ultrasound-assisted supercritical water oxidation treatment, and liberated Cr in the supernatant can be re-utilized after purification. This process is an efficient alternative for toxicity removal and chromium recovery from real tannery sludge contaminated by high concentration of Cr and refractory organics.