A study on dissolution properties of the sludges from Cr(VI) reduction-precipitation processes

In this article, dissolution characteristics of Cr(VI) reduction-precipitation sludges in the solutions containing mineral acids, organic complexing materials or both are examined. For this purpose, the effects of pH, concentration of complex forming agent and contact time on dissolution of metals from two different sludges obtained by using sodium sulphite and ferrous sulphate as reducing agents are studied. In addition, pollution potentials of the sludges are determined by applying the toxicity tests of TCLP, SPLP, USEPA-1979, and USEPA-19X0. It was found that pH, contact time, concentration and type of complex forming substances are effective parameters on dissolution of metals from the sludges. In mineral acid solutions at pH 3, especially in the presence of organic complexing substances, chromium leached out from hydroxide sludges exceeds the concentration limits of USEPA for hazardous wastes. The amounts of chromium dissolved in the citric and ascorbic acid solutions and the amount of iron dissolved in the EDTA solution were found to be higher than in the other test solutions. Also, TCLP test shows that chromium hydroxide sludge obtained from sulphite reduction precipitation process of Cr(VI) is a potential hazardous solid. According to the results of this study, considering the probable presence of complexing agents in the disposal sites, toxicity test methods should be modified.