Removal of heavy metals from industrial sludge with new plant–based washing agents

Washing is one of the techniques for permanent removal of heavy metals from industrial sludge, for which washing agents are a key influence factor. However, high–efficiency, eco–friendly, and inexpensive agents are still lacking. In this study, the solutions derived from the three plant materials including Fatsia japonica, Hovenia acerba and Pterocarya stenoptera were employed to remove Cd, Cu, Pb, and Ni from industrial sludge. The effects of washing solution concentration, pH, washing time and temperature on metal removal were investigated. The metal removal efficiencies were found to increase with increasing solution concentrations or washing temperatures, decline with increasing pH, and presented various trends with increasing washing time. Among the three agents that derived from H. acerba showed relatively high removal for Cu (75.81%), Pb (63.42%), Ni (27.52%), and Cd (56.99%). After washing, environmental risks of residual metals were markedly diminished in sludge, attributable to decrease in their exchangeable forms. Furthermore, the applications of the plant washing agents increased sludge organic carbon, alkali–hydrolysable nitrogen, available phosphorus, and available potassium. Fourier transform infrared spectroscopy analysis suggested that the hydroxyl, carboxyl, ether, and amide may be the main functional groups in the three plant materials binding the heavy metals. Overall, the agent derived from H. acerba appears to be a feasible washing material for heavy metals removal from sludge. •Heavy-metal removal from sludge using plant washing agents was evaluated.•The agent from Hovenia acerba effectively removed heavy metals from sludge.•The washing agents tended to moderate changes in the chemical properties of sludge.•The sludge is suitable as manure and can be used for soil amendment after washing.