Aspartic acid interaction with Ca-montmorillonite: adsorption, desorption and thermal stability

Abstract

Aspartic acid, one of the most abundant amino acids in nature, was adsorbed on Ca-montmorillonite at pH 7.0. The adsorption was a fast reaction process; 78% of the aspartic acid was retained by the clay surfaces at the end of a 15 min reaction period. The maximum amount adsorbed reached 84% (56.2 μmol/g clay) after 2 h. X-ray diffraction analysis showed that aspartic acid was intercalated in Ca-montmorillonite. The d 001 spacing of the aspartate—Ca-montmorillonite complex remained constant (18.1 Å) with increasing temperature from 25 to 150°C. However, the d 001 spacing of the aspartate—Ca-montmorillonite complex decreased linearly from 18.1 Å to 10.3 Å with increasing temperature from 150 to 600°C. The purification treatments of the montmorillonite did not affect its ability to adsorb aspartic acid and the nature of the aspartate—Ca-montmorillonite complex formed. Adsorbed aspartate was extractable by 1.0, 0.5 or 0.05M KCl solutions and completely desorbed after two washings with water. At pH 7.0, aspartic acid appeared weakly bound on the clay surfaces. Hence, the data indicate that aspartic acid was intercalated into the montmorillonite by the formation of an outer-sphere complex with the exchangeable Ca through a “water bridge”, and an H-bonding of the protonated amino group to the structural oxygen of the siloxane surfaces.