Phosphorus-modified Metakaolin As a Soil Amendment for Immobilizing REEs and Reducing Their Bioaccumulation

Rare earth elements (REEs) as emerging soil contaminants have attracted increasing concerns. Efficient, economical, and eco-friendly soil amendments for REEs-contaminated farmland in-situ remediation are exigently needed. Herein, phosphorus-modified metakaolin (MK-P) was synthesized as a soil amendment to immobilize REEs in soil and reduce their bioaccumulation in water spinach (Ipomoea aquatica Forssk.). MK-P was developed by the combination of mechanochemical and thermal treatments using metakaolin and KH2PO4 as raw materials. Our results indicated that the aluminum potassium phosphate and active Al-O-P sites were generated within MK-P, facilitating the immobilization of REEs and the controlled release of phosphorus. The application of 3% MK-P significantly reduced the content of available REEs in soil by 76.7%-99.2% and reduced the REE contents in the shoots (edible parts) of water spinach by 43.0%-57.3%. Correlation analysis suggested that the enhancement for REE immobilization using MK-P could be ascribed to the increase of soil pH and available P. Furthermore, the application of MK-P led to the amelioration of soil fertility, which subsequently increased plant yield by 14.8%-20.1% under MK-P treatment. These findings suggest that MK-P can be used as a promising soil amendment to minimize REE bioavailability in REEs-contaminated soil and improve soil quality.