Influence of particle size on phosphate incorporation and release mechanisms in MgAl Layered Double Hydroxides

Abstract

Phosphate-intercalated MgAl-LDH phases were successfully synthesized using three distinct synthetic approaches, and their phosphate release properties were thoroughly investigated. By tuning the synthesis parameters, the particle size was modulated from 2.0 ± 0.3 µm (urea method) to 75 ± 10 nm (flash co-precipitation). Following anion exchange, each phosphate-loaded phase exhibited a phosphorus content consistent with the stoichiometry of a pure HPO₄²⁻-containing LDH, while preserving both particle size and morphology. Structural characterizations revealed that the intercalation modes and basal stacking configurations varied with particle size. Simulations of 31P MAS NMR spectra identified up to five distinct phosphorus environments, indicating a complex structural heterogeneity. The mechanism of phosphate incorporation was found to be strongly influenced by the size of the LDH precursor particles, whereas the total phosphate content correlated primarily with the M(II)/M(III) molar ratio. Our results demonstrate that smaller particles exhibit enhanced phosphate release, with release kinetics governed by a combination of mechanisms including partial matrix dissolution, anion exchange with carbonate, and phase-specific dissolution. These findings provide new insights into the design of LDH-based materials for controlled phosphate delivery applications.