Extraction of heavy metal from sewage sludge using ultrasound-assisted nitric acid

The effects of nitric acid concentration and sonication time on the removal efficiencies of heavy metals from sludge were investigated. Nitric acid concentrations were varied from 0 to 0.65 M and sonication time from 0 to 20 min. The extracted metals Cu, Zn, and Pb were determined. The results indicated that the removal efficiencies of Cu, Zn, and Pb increased with increases in nitric acid concentration and sonication time. The optimal nitric acid concentration for heavy metals extraction was 0.325 M, while a maximal sonication time of 20 min resulted in maximal heavy metals removal efficiencies. The removal efficiencies of Cu, Zn and Pb reached 9.5%, 82.2%, and 87.3%, respectively, at the optimal concentration of nitric acid assisted by ultrasound for 20 min. Additionally, the residual contents of Zn and Pb in sludge met Chinese legal standards, while Cu was not significantly removed regardless of the nitric acid concentrations and sonication time. The order of heavy metal removal efficiencies was Pb > Zn > Cu. During heavy metal extraction, contribution proportions of ultrasonication and nitric acid to the extraction were 18–22% and 78–82%, respectively. Ultrasound alone was not effective enough to remove heavy metals from sludge and the role played by nitric acid predominated over that of ultrasound. However, ultrasound was synergistic with nitric acid when these were used together to extract heavy metals from sludge. Possible mechanisms of heavy metal extraction are also discussed.