Competitive Adsorption of Phosphate and Organic Acids on Fe and Al Oxy(Hydroxides): XAS Simulation Insights

Understanding how organic acids alter phosphate (P) adsorption on Fe- and Al-(oxyhydr)oxides is crucial for predicting P retention in soils and enhancing phosphorus use efficiency. This study integrated batch experiments, micro−XRF mapping, XANES spectroscopy, and ab initio multiple scattering calculations to explore the competitive adsorption of P with organic acids (acetic, oxalic, citric, and humic) on hematite, goethite, boehmite, and kaolinite. Adsorption isotherms indicated that P affinity followed the order hematite >> kaolinite > boehmite > goethite, while carboxylic acids competitiveness ranked humic >> citric > oxalic > acetic. XANES supported strong inner-sphere bidentate binuclear Fe−O−P and Al−O−P complexes on the minerals, characterised by strong hybridization features. However, acetate pre-adsorption displaced surface Fe and Al neighbours by approximately 0.5 Å, weakening hybridization and lengthening P−O bonds. These findings offer mechanistic insights into how organic acids control P retention and mineral surface reactivity in soil.