Molecular-Scale\nStudy of Aspartate Adsorption on Goethite\nand Competition with Phosphate

Abstract

Knowledge of the interfacial interactions\nbetween aspartate and\nminerals, especially its competition with phosphate, is critical to\nunderstanding the fate and transport of amino acids in the environment.\nAdsorption reactions play important roles in the mobility, bioavailability,\nand degradation of aspartate and phosphate. Attenuated total reflectance\nFourier-transform infrared (ATR-FTIR) measurements and density functional\ntheory (DFT) calculations were used to investigate the interfacial\nstructures and their relative contributions in single-adsorbate and\ncompetition systems. Our results suggest three dominant mechanisms\nfor aspartate: bidentate inner-sphere coordination involving both\nα- and γ-COO^–, outer-sphere complexation\nvia electrostatic attraction and H-bonding between aspartate NH₂ and goethite surface hydroxyls. The interfacial aspartate\nis mainly governed by pH and is less sensitive to changes of ionic\nstrength and aspartate concentration. The phosphate competition significantly\nreduces the adsorption capacity of aspartate on goethite. Whereas\nphosphate adsorption is less affected by the presence of aspartate,\nincluding the relative contributions of diprotonated monodentate,\nmonoprotonated bidentate, and nonprotonated bidentate structures.\nThe adsorption process facilitates the removal of bioavailable aspartate\nand phosphate from the soil solution as well as from the sediment\npore water and the overlying water.