Release behavior of chromium from tannery sludge

The release behavior of chromium from tannery sludge was assessed in this study. Fundamental properties of sludge were characterized. Total Cr content was 17,200 mg/kg. Trivalent chromium, Cr(III), was the predominant species in the sludge. The total Cr and Cr(VI) concentrations in the leachate of TCLP were 5.02 and 0.25 mg/dm3, respectively. Solubility of Cr from sludge was the lowest in the neutral range. Both pH and redox potential significantly affected release of Cr from the sludge. When sludge suspension was continuously aerated with oxygen for 600 hours, pH increased from 7.1 to 7.3, and redox potential increased from −300 to 295 mV, respectively. Dissolved Cr concentrations were lower than 7 mg/kg during the aeration period. The equilibrium solubility experiments were first conducted when the pH value was maintained at 4.2, and varied redox potential (0, 150, 250, 320, 470 mV), dissolved Cr concentrations decreased with increased redox potential. When the pH value was maintained at 8.0, and varied redox potential (−220, −100, 0, 150, 250 mV), dissolved Cr concentrations were relatively low. A linear correlation was observed between dissolved concentrations of Cr and Fe. Combining with results from sludge fractionation experiment, we can conclude that precipitation-dissolution reactions controlled the release behavior of Cr.