Kinetics and thermodynamics studies of copper exchange on Na-montmorillonite clay mineral

The kinetics of Cu ion exchange on Na-montmorillonite clay has been investigated at 20, 30, and 40 degreesC in water, methanol, and ethanol. The reaction is endothermic in nature. Solvent effects on the reaction rate have been discussed. The thermodynamic activation parameters were calculated and discussed in terms of solvation effects. A multiple reaction rate order equation was used to describe the adsorption process. Lower rates and higher activation energies (E-a) were observed in aqueous solution than in either of the alcohols. The Ea values ranged from 20.88 kJ mol(-1) in water to 9.20 kJ mol(-1) in ethanol, while at 20 degreesC the rate constant (k) varied from 0.111 ppm(-1) s(-1) in water to 0.205 ppm(-1) s(-1) in ethanol. The main factor influencing the rate of the adsorption process is the mobility of the adsorbed Cu cations, which is apparently larger in alcohols than in water, due to the difference in the molar activation energy of the solvent. The determined isokinetic temperature indicates that the reaction is enthalpy-controlled, where the interaction between solvent and clay surface plays an important role. A reaction mechanism that describes the solvent effect on the rate of Cu ion exchange is proposed. (C) 2003 Elsevier Science (USA). All rights reserved.