A novel montmorillonite-based soil amendment for Cd/REEs immobilization and nutrients sustained release

The development of clay mineral-based soil amendments has drawn significant concerns in modern agriculture. However, little attention was paid to multifunctional amendments enabling both soil remediation and nutrients sustained release. In this work, a novel montmorillonite (Mt)-based amendment (denoted as KMt-P) was developed by modifying Mt with alkali followed by decoration with phosphate, and was further applied to remediate Cd/REEs contaminated soil. The XRD pattern of KMt-P demonstrated the presence of various silicophosphate and aluminophosphate phases. ²⁷Al and ²⁹ Si MAS NMR results indicated the destruction of crystal structure of Mt. FTIR and XPS analyses confirmed the formation of Si-O-P, P-O-P, and Al-O-P bonds in KMt-P, which, as we envisioned, should be conductive to the immobilization of Cd and sustained release of nutrients. The addition of KMt-P to soil (1 wt%) could effectively reduce the bioavailable Cd in soil by 91.3% within 30 days. Sustained release of multiple nutrients was also achieved: the concentration of soluble K, Si, and P increases by 15, 3, and 135 times, respectively, in comparison with the control treatment. The increased K and P contents arose from the added modifiers, and that of Si could be ascribed to the destruction of Mt crystal structure under alkali attack. Application of only 1% KMt-P could reduce the concentrations of 17 REEs in the soil by 92.01%–98.89%. In general, KMt-P has proven a promising multifunctional amendment enabling both Cd/REEs immobilization and sustained release of nutrients.