Immobilization in Clay Minerals and Related Materials

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

The entrapment of various enzymes and proteins by clay minerals proceeds by weak interactions including electrostatic interactions, hydrogen and van der Waals bonding. Additivity of these various attractive forces renders the adsorption irreversible in some cases, but usually a leaching of enzyme is observed under working conditions. In order to fix the enzyme irreversibly at the surface of the clay layers different processes have been tried. In order to fix invertase on bentonite, Monsan and Durand [90] previously treated the clay mineral with a coupling agent, 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) phosphate (K= 48.6) oxalate (K — 35.6) acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

Jackbean urease was immobilized on kaolinite and montmorillonite [98]. The amounts of urease required for maximum immobilization were 70 and 90 mg g 1 of kaolinite and montmorillonite, respectively. The Km values of immobilized urease (25.1-60.8 mM) were of the same order of magnitude as that of free urease (29.4 mM) but one order of magnitude higher than those of soil urease (1.77-2.90 mM). Immobilization of urease on clay surfaces leads to increases in the kinetic constants. 

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