The immobilization of zinc (Zn), cadmium (Cd) and nickel (Ni) using nanoparticles (NPs) was investigated. Two non-calcareous and calcareous contaminated soils were incubated with Al2O3, SiO2 and TiO2 at 1 and 3% wt for 30 days at 25 °C in field capacity moisture and then were fractionated by the sequential extraction procedure. After application of NPs, a significant increase in metals was observed in residual (RES) fraction. The maximum reduction in exchangeable (EXC) Cd fraction was measured in soil treated with 1% Al2O3 (38.3%) and 3% SiO2 (56.1%) for non-calcareous and calcareous soils, respectively. The highest decrease in EXC Zn fraction in non-calcareous and calcareous soils was 28.8% for Al2O3 (3%) and 57.1% TiO2 (3%) treatments, respectively. Interestingly, non-calcareous soil showed a higher capacity to reduce Ni in available fractions and it decreased on average by 14.0% and 11.0% for non-calcareous and calcareous soils, respectively. In general, SiO2 NPs were an effective sorbent for immobilizing three metals in calcareous soils, while in non-calcareous soils the maximum reduction in mobile fraction of Cd and Zn occurred in the presence of Al2O3 NPs, and the shift from mobile to stable fractions of Ni was higher in soils containing SiO2 NPs. Similar to experimental data, the model predicted that NPs could reduce metal in EXC, carbonate (CAR), oxide (OX) and organically bound (OR) fractions. Results suggested that NPs can be effectively used for metal immobilization in multi-metal-contaminated soils and that surface complexation modeling (PHREEQC) could describe different fractions of metal in soils.
