Why Phytate Persists: Mechanisms of Exceptional Binding and Stability on Soil Mineral Surfaces

Phytate, a six-phosphate compound that stores plant phosphorus, is a stable component of soil phosphorus, yet mechanisms underlying its persistence remain unclear. Here, we combined adsorption experiments, spectroscopy, X-ray scattering, and molecular modeling to investigate phytate interactions with five soil minerals: goethite, hematite, quartz, montmorillonite, and kaolinite. Iron oxides exhibited up to 5-fold greater adsorption capacities than quartz and clays. Natural organic matter caused negligible desorption of mineral-bound phytate, whereas phytate displaced adsorbed natural organic matter, demonstrating competitive and stable phytate-mineral associations. Adsorption capacities normalized to orthophosphate binding-site densities implied multiphosphate coordination for phytate on iron oxides, but binding beyond nominal orthophosphate sites on montmorillonite. Molecular dynamics simulations captured one to three phosphate groups in phytate associations with iron oxides; metal cation screening of electrostatic repulsion facilitated binding on negatively charged quartz and montmorillonite. Infrared spectroscopy revealed coupled vibrational responses of phosphate and inositol ring moieties on goethite, but primarily phosphate responses on montmorillonite. X-ray total scattering determined a feature at 2.7 Å, supporting inner-sphere coordination on iron oxides and consistent with quantum mechanics-based energy calculations, whereas quartz and clay features indicated interfacial reorganization. These findings establish mineral-specific mechanisms of phytate stabilization, with implications for persistence and bioavailability in soils.