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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.
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