A new and innovative binder KMP is developed for solidification/stabilization (S/S) of soils contaminated with heavy metals such as lead (Pb), zinc (Zn), and cadmium (Cd). The KMP consists of acid-activated phosphate rock, monopotassium phosphate, and reactive magnesia. However, effectiveness and immobilization mechanisms of KMP in actual field contaminated soils have not been investigated. This study presents a systematic investigation of strength, leachability, and microstructural properties of KMP stabilized field clayey soil that has been contaminated with Pb, Zn, and Cd at a smelter site. Several series of laboratory tests are conducted that include unconfined compression tests (UCT), toxicity characteristics leaching procedure (TCLP), modified European Communities Bureau of Reference (BCR) sequential extraction procedure, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS) studies. The TCLP and UCT results show that the leached Pb, Zn, and Cd concentrations decrease while unconfined compressive strength and dry density of the stabilized soil increase with increasing binder content and curing time. The sequential extraction results indicate that large percentages of Pb, Zn, and Cd are transferred from the exchangeable fraction to residual fraction after the KMP stabilization. The pore-size distribution reveals that the mean diameters and volumes of interaggregate and intra-aggregate pores decrease with an increase in the KMP content. The XRD and SEM/EDS analyses demonstrate the formation of magnesium phosphate-based products and heavy metal phosphate-based products in the stabilized soil, and they are the primary mechanisms for strength increase and heavy metal immobilization, respectively.

PH values significantly affect leachability of stabilized/solidified heavy metals contaminated soils. The diffusion coefficient (D_e) and the retardation factor (R_d) are essential for quantitative assessment of the impact of stabilized/solidified contaminated soils on surrounding environment. Diffusion of Cd leached from a new binder stabilized/solidified Cd-contaminated soil are investigated in this research. A series of tests including desorption test and semi-dynamic leaching test conduct to determine the obvious diffusion coefficient (D^obs) and the R_d, respectively. Desorption test results show that leached concentrations of Cd increase with the increase of its initial concentrations and a desorption curve fit well in the Freundlich desorption/adsorption model. The semi-dynamic leaching test results show that the accumulated heavy metal mass has a linear-relationship with time on a logarithmic scale. The results demonstrate that the D^obs of Cd calculated at 2.11×10^-12, the retardant factor of Cd determined as 109.7 and the D_e is therefore determined as 2.32×10^-10 m²/s.