Removal of Pb, Zn, Ni and Cr from industrial sludge by biodegradable washing agents: Caboxyethylthiosuccinic acid and itaconic-acrylic acid

Chemical washing is considered an effective technique for removing heavy metals (HMs) from industrial sludge. However, traditional washing methods have the potential to cause secondary environmental pollution. In this study, a new chemical washing strategy that circumvents the risk of secondary contamination by using two organic acid polymers-Caboxyethylthiosuccinic acid (CETSA) and itaconic-acrylic acid copolymer (IA-AA) were introduced to remove Pb, Zn, Ni and Cr from industrial sludge. The quantity of HMs removed from the sludge increases as a function of increasing agent concentration, Solid-to-liquid ratio, and washing time, while various trends were observed in response to increasing pH. In addition, response surface analysis showed that CETSA concentration significantly affected HM removal; and IA-AA concentration, pH, and washing time showed evident impacts on washing efficiency. Optimal washing specifications using CETSA, with respect to agent concentration, pH, and washing time, were 66.76 g/L, 2.76, and 248 min, respectively; while those of IA-AA were 100 g/L, 2, and 360 min, respectively. CETSA and IA-AA removed 69.04% and 98.31% of total HMs, respectively, and decreased the labile fractions of Zn and Ni. Fourier transform infrared spectroscopy analysis suggested that the hydroxyl, carboxyl, carbonyl might contribute to the heavy metal extraction. Overall, CETSA and IA-AA should be seriously considered as reliable washing agents for use in HM removal from sludge.