Kinetics and equilibrium of cadmium complexation with heterogeneous complexants: A stopped-flow study at Na-illite

Rapid kinetics and equilibrium of the ion exchange of Na+ by Cd2+ at illite are studied by the stopped-flow technique and DPASV, respectively, as an example for reactions at heterogeneous surfaces. For the first time both kinetics and equilibrium are analyzed for the same system by kinetic- and affinity-spectra, taking into consideration the heterogenity of binding sites. It is possible to calculate model independent kinetic spectra by means of the CONTIN program; the isotherm is evaluated by Sips' distribution function. Monomodal distribution functions are found, characterized by mean values and half widths at half height in the range between 0.2 and 0.3 log(k/s(-1)) for the kinetics and 0.16 log(K/L mol(-1)) for the affinity. From both analyses the mean ion exchange coefficient is determined as 2.2 +/- 0.4 mol liter(-1) and 1.5 +/- 0.4 mol liter(-1), respectively. The kinetics are rapid; only one process is observed that is assigned to the binding of Cd2+ at the outer surface. The high activation-enthalpy of 35 kJ mol(-1) proves that the exchange reaction of 2 Na+ by Cd2+ at the binding site is the rate-determining step and not the diffusion to the surface. The large negative activation-entropy of -105 J mol(-1) K-1 indicates that this rate-determining step is of associative nature. (C) 1996 Academic Press, Inc.