Adsorption of amino acids by montmorillonite and gibbsite: Adsorption isotherms and spectroscopic analysis.

The adsorption of glycine, cysteine and glutamic acid by gibbsite and montmorillonite was studied as a function of amino acid concentration and solution pH. Zeta potential measurements, X-ray diffraction (XRD), and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopic analysis were performed to deduce the mechanism(s) of adsorption and to observe changes in the basic structure of the minerals after interaction with amino acids. The ATR-FTIR analysis revealed that both NH 3 + and COO− groups of amino acids were very vital for interactions with the surface of gibbsite and montmorillonite. XRD analysis revealed that the formation of mineral-amino acid composites decreased the d 001 value of montmorillonite from 17.95 to 14.2 (glycine), 15.49 (cysteine), and 15.22 Å (glutamic acid). This observation suggests that amino acid adsorption induced structural stress on montmorillonite and expanded its interlayer space. Also, the adsorption of glycine and cysteine to gibbsite and montmorillonite increased their zeta potentials at pH 5.0 while glutamic acid decreased the zeta potentials. Thus, amino acid-induced increase/decrease in the surface positive/negative charge of the minerals indicates that chemical bonds were formed between glycine, cysteine, glutamic acid and the minerals. The findings of this study can serve as a reference for studies related to understanding the bonding interactions and distribution of different amino acids in soils. • More amino acids were adsorbed by gibbsite than by montmorillonite. • Amino acids adsorbed mainly through their amino and carboxylate groups. • Adsorption of amino acids to montmorillonite induced expansive phase formation. [ABSTRACT FROM AUTHOR]