Supercritical water oxidation of tannery sludge: Stabilization of chromium and destruction of organics

• We report simultaneous destruction of organics and recovery of chromium in tannery sludge by SCWO.
• Destruction of COD increases with higher temperature and oxidant, with maximum value 96.5%.
• Recovery efficiency of Cr from sludge to ash reaches more than 98% under all conditions.
• Recovered chromium in solid ash is in amorphous structure under temperature 500 °C.
• Leaching toxicity of heavy metals like Cr is greatly reduced through stabilization.

The supercritical water oxidation (SCWO) of industrial tannery sludge was investigated to understand the simultaneous destruction of organic pollutants and recovery of high content chromium. Experiments were performed in a batch reactor at temperatures of 350–500 °C, reaction time of 150–300 s and different oxygen ratios, to exhibit the effect of operation conditions. Results showed that removal efficiency of chemical oxygen demand (COD) increased with higher temperature, larger oxidant amount and reaction time; a maximum value of 96% was obtained. Meanwhile, destruction yield was much higher under supercritical conditions than that in subcritical water. In addition, removal efficiency of Cr from sludge reached more than 98% under all conditions; higher temperature played a positive role. Further, leaching toxicity tests of heavy metals in solid products were conducted based on toxicity characteristic leaching procedure. All heavy metals except nickel showed a greatly reduced leaching toxicity through their stabilization. The chromium oxide recovered in ash was amorphous below 550 °C, so that the structure of Cr could not be identified by X-ray diffraction pattern. Special attention should be paid on nickel as its leaching toxicity increased due to the corrosion of reactor surface under severe reaction conditions.