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

•Ultrasound-assisted citric acid extraction of heavy metal from sewage sludge is evaluated.•Ultrasound treatment can enhance heavy metal extraction rate, but not extraction efficiency.•The presence of Fe3+, Al3+ and Ca2+ can improve the extraction efficiency of Cu by citric acid.•After citric acid extraction, the sludge has a good manure value and can be used for soil amendment.

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.