Humic substances from sewage sludge compost as washing agent effectively remove Cu and Cd from soil

• Humic substances (HS) from compost showed good surfactant properties.
• HS effectively removed Cu and Cd from contaminated soil.
• Cu and Cd removal efficiency was higher for sandy clay loam than for clay.
• Soils washing with HS enable decrease of Cu and Cd ecological risk (on Er index).

Although commercially available biosurfactants are environmentally friendly and effectively remove heavy metals from soil, they are costly. Therefore, this study investigated whether inexpensive humic substances (HS) from sewage sludge compost could effectively remove copper (Cu) and cadmium (Cd) from highly contaminated sandy clay loam (S1) and clay (S2). The optimum HS concentration and pH were determined, as well process kinetics. Under optimum conditions, a single washing removed 80.7% of Cu and 69.1% of Cd from S1, and 53.2% and 36.5%, respectively, from S2. Triple washing increased removal from S1 to almost 100% for both metals, and to 83.2% of Cu and 88.9% of Cd from S2. Triple washing lowered the potential ecological risk (Eri) of the soils, especially the risk from Cd. HS substances show potential for treating soils highly contaminated with heavy metals, and HS from other sources should be tested with these and other contaminants.