The kinetic spectrum method applied to the ion exchange of Cd(II) at Mg(II)-montmorillonite

Abstract

In this work the ion exchange kinetics of Cd²⁺ at Mg²⁺-montmorillonite have been investigated. These kinetics are fast, i.e., the exchange in diluted suspension is complete after 30 s, therefore, the stopped flow method is applied. The data were analyzed by the kinetic spectrum method, because the reaction cannot be described by classical kinetic models. Two processes were observed: i) A fast one with a mean pseudo-first-order rate constant of 20 s⁻¹. This process is assigned to the exchange at easily accessible sites at the outer surface. ii) A group of slow processes with a distribution of pseudo first order rate constants between 0.1 s⁻¹ and 3 s⁻¹. These processes are assigned to the exchange in the interlayer space and have a mean energy and entropy of activation of 20 kJ mol⁻¹ and −180 J mol⁻¹ K⁻¹, respectively. The rate controlling step is the hindered diffusion at the internal surfaces of the montmorillonite.