Thermodynamic Understanding of the Competitive Adsorption of Arsenate and Phosphate on Clay-Fe Oxyhydroxide Complexes

Minerals such as clay and Fe oxyhydroxides are key in controlling phosphorus (P)-induced nonpoint-source pollution and arsenic (As)-related heavy metalloid contamination, yet understanding their cooperative role in concurrently immobilizing both As and P remains challenging. This study comprehensively investigates the interfacial behavior and thermodynamic mechanisms of competitive As/P adsorption on clay-Fe oxyhydroxide complexes. Batch experiments and characterization revealed that complex formation favors immobilization of both As and P, with a preferential affinity for As over P, highlighting a priority for heavy metal immobilization while supporting nutrient conservation. Single-molecule analysis identified Fe active sites as reaction hotspots for As/P immobilization. Thermodynamic quantification showed that the interfacial binding energy for As (-31.0 kJ/mol) was substantially higher than for P (-23.3 kJ/mol), indicating stronger immobilization capabilities. In soil incubation experiments, the immobilization efficiency of As and P exceeded 80%, with Fe oxyhydroxides-driven adsorption being the primary retention mechanism. Pearson correlation analysis confirmed that Fe oxyhydroxides are key regulators of contamination, with clay content enhancing their effectiveness. This research refines our understanding of competitive retention processes of As and P, from molecular to soil-scale interactions, providing insights for sustainable nutrient management and pollution control in soils and water bodies.