Ferrous sulfate (FeSO₄) and calcium polysulfide (CaS₅) stabilization are practical approaches to stabilizing hexavalent chromium (Cr(VI))-contaminated soil. The leachability and stability of Cr(VI) and Cr are important factors affecting the effectiveness of stabilized Cr(VI)-contaminated soil. This study compared the leachability and stability of Cr(VI) and Cr in Cr(VI)-contaminated soil stabilized by using FeSO₄ and CaS₅. The contaminated soil was characterized before and after stabilization, and the effectiveness of FeSO₄ and CaS₅ stabilization was assessed using leaching, bioaccessibility, alkaline digestion, sequential extraction, and X-ray diffraction tests. Results showed that FeSO₄ and CaS₅ significantly reduced the leachability and Cr(VI) content in the contaminated soil. The acid-buffering capacity and stability (leachability, bioaccessibility, speciation distribution, and mineral composition) of the Cr(VI)/Cr and Cr(VI) content of CaS₅ were better than those of FeSO₄. This study demonstrated that CaS₅ had a better effect than FeSO₄ on the stabilization of Cr(VI) in Cr(VI)-contaminated soil. The CaS₅ significantly enhanced the stabilization and immobilization of Cr(VI) and reduced its leachability and toxicity.

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.