Adsorption of glycine at the anatase TiO2/water interface: Effects of Ca2+ ions

Adsorption reactions of amino acids (AAs) on TiO₂ nanoparticles (NPs) play an important role in the available nutrients in soils and sediments. The pH effects on glycine adsorption have been studied, but little is known about its coadsorption with Ca²⁺ at the molecular level. Combined attenuated total reflectance Fourier transform infrared (ATR-FTIR) flow-cell measurements and density functional theory (DFT) calculations were used to determine the surface complex and corresponding dynamic adsorption/desorption processes. The structures of glycine adsorbed onto TiO₂ were closely associated with its dissolved species in the solution phase. The presence of Ca²⁺ exerted different influences on glycine adsorption within pH 4−11, thus affecting its migration rate in soils and sediments. The mononuclear bidentate complex at pH 4−7, involving the COO⁻ moiety of zwitterionic glycine, remained unchanged in the absence and presence of Ca²⁺. At pH 11, the mononuclear bidentate complex with deprotonated NH₂ can be removed from the TiO₂ surface upon coadsorption with Ca²⁺. The bonding strength of glycine on TiO₂ was much weaker than that of the Ca-bridged ternary surface complexation. Glycine adsorption was inhibited at pH 4 but was enhanced at pH 7 and 11.