Adsorption of glycine on montmorillonite in aqueous solutions

Glycine was adsorbed on the surface of montmorillonite from aqueous solutions of variable glycine concentrations (0.001–0.3 mol L− 1) and pHs (2–12) at room temperature. The reaction products were characterized using X-ray diffraction and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy. The results indicate that adsorption is highly pH dependent, increasing at acidic pHs. Glycine adsorption is dominated by complexation of the carboxylate group of zwitterionic glycine onto the edge surface at low concentrations of glycine. When the edge surface is saturated, the adsorption occurs by cation exchange in the interlayer space. ATR-FTIR results point to an adsorption by hydrogen bonding between the NH3+ group and the basal oxygen of the interlayer surface. The dependence of glycine adsorption with its aqueous concentration fits for a Freundlich equation. The measure of the desorbed interlayer cations shows that the 68% of the K+ was exchanged with glycine. The K+–glycine exchange reaction produces a decrease of the smectite interlayer space from 14.5 to 12 Å and an arrangement of the smectite layers. Intercalated glycine is present as both glycinium and zwitterionic forms. Since only glycinium contribute to structural charge balance, total adsorbed glycine can exceed K+ released. These findings are of relevance for the evaluation of distribution and reactions of free amino acids in natural environments.