Concentration dependent regulation of amorphous calcium phosphate precursor transformation by citrate adsorption

Abstract

Phosphorus bioavailability in soils is largely governed by the nucleation and transformation of calcium phosphate (Ca-P) minerals, processes influenced by organic acids like citrate exuded by plant roots. Despite extensive research, the concentration-dependent effects of citrate on Ca-P crystallization remain unclear. This study explores how varying citrate concentrations regulate the nucleation, morphology, and phase transitions of Ca-P under weakly alkaline conditions. Real-time pH monitoring indicated that low citrate concentrations (≤ 1 μM) accelerate nucleation by promoting the transformation from amorphous calcium phosphate (ACP) to crystalline hydroxyapatite (HAP), whereas higher citrate levels (≥ 2 μM) inhibit nucleation, extending induction times markedly. Morphological analyses revealed distinct crystal shapes: flake-like structures at low citrate concentrations and dense spherical aggregates at higher levels. Raman spectroscopy and XPS characterization confirmed that citrate adsorbs onto Ca-P surfaces and modulates the electronic states of calcium and phosphorus. At low concentrations, citrate likely donates electron density to surface ions, facilitating nucleation, while at higher concentrations, electron-withdrawing carboxyl groups dominate, stabilizing intermediate amorphous phases and hindering crystallization. These findings elucidate the molecular mechanism behind citrate’s dual regulatory role in ACP transformation, providing insights into phosphorus cycling in the rhizosphere and strategies for improving phosphate fertilizer efficiency in alkaline soils.