Phosphate-intercalated CaFe-layered double hydroxides: Crystal structure, bonding character, and release kinetics of phosphate

The nitrate-form of CaFe-layered double hydroxide (CaFe-LDH) was synthesized via co-precipitation method, and its phosphate-intercalates were prepared by ion-exchange reaction. According to X-ray diffraction analysis, the CaFe-LDHNO₃^- compound and its H₂PO ₄^--intercalate showed hexagonal layered structures, whereas the ion-exchange reaction with HPO₄^2- caused a frustration of the layer ordering of LDH. Fe K-edge X-ray absorption spectroscopy clearly demonstrated that the CaFe-LDH lattice with trivalent iron ions was well-maintained after the ion-exchange with HPO₄ ^2- and H₂PO₄^-. Under acidic conditions, phosphate ions were slowly released from the CaFe-LDH lattice and the simultaneous release of hydroxide caused the neutralization of acidic media. Fitting analysis based on kinetic models indicated a heterogeneous diffusion process of phosphates and a distinct dependence of release rate on the charge of phosphates. This study strongly suggested that CaFe-LDH is applicable as bifunctional vector for slow release of phosphate fertilizer and for the neutralization of acid soil.