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

ABSTRACT

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.