Effects of exchange cations and layer-charge location on cysteine retention by smectites

This study investigates the complexes formed between amino acids, which are the natural degradation products of organic matter, and smectites. Thus, t he adsorption and desorption behavior of cysteine and Na-, Ca-. Cu-homoioni c smectites with different layer-charge location, a montmorillonite. and a beidellite, were studied. The clay samples were treated with Na, Ca, and Cu 1 N solutions and then with a 0.2 M cysteine solution. To test smectite-cy steine stability at acidic pH, the solids obtained were repeatedly treated with distilled water acidified to pH = 5. All treated samples were characte rized by thermal, X-ray diffraction, chemical, and infrared analyses. The r esults showed that: I) Na- and Ca-rich smectites adsorbed and retained smal l amounts of cysteine, and did not show interlayer cation-cysteine complexe s, whereas the amino acid was strongly retained in the interlayer by Cu-ric h smectites; 2) d(001)-values for Na- and Ca-rich smectites showed little o r no expansion, whereas for the Cu-rich smectites the intercalation of the organic molecule produced a swelling of the structure: 3) the interaction m echanism of homoionic smectites with cysteine in an aqueous medium occurs b y weak interactions, (e.g., van der Waals interactions, hydrogen bonding, d ipole-dipole interactions, and other electrostatic forces such as entropy-d riven hydrophobic bonding). and/or by complexes involving interlayer cation s and organic ligands. The formation of a stable chelate complex with the s aturating ion permits cysteine to be adsorbed by Cu(II)-rich smectites and to be resistant to migration in soils and groundwaters.