Heavy metal chemical extraction from industrial and municipal mixed sludge by ultrasound-assisted citric acid

Citric acid chemical extraction technology in combination with ultrasonication was used to remove heavy metals from industrial and municipal mixed sludge. The removal efficiencies of Zn, Ni, Cr and Cu reached 53.5%, 40.2%, 35.4% and 13.1%, respectively, at citric acid concentration of 0.2 M assisted by ultrasound for 20 min. The extracted Zn, Ni and Cr mainly came from acid-soluble, reducible and oxidizable fractions, and Cu mainly came from oxidizable and reducible fractions. The presence of Fe3+, Al3+ and Ca2+ improved the extraction efficiency of Cu from the sludge significantly. The extracted sludge could be used for soil amendment. 2015 (C) The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.