Dissolution of malachite in aqueous ethylenediaminetetraacetate solution

The kinetics of malachite dissolution in aqueous ethylenediaminetetraacetate (EDTA) solution has been investigated in the temperature range of 298 to 318 K. The dissolution rate of malachite determined under the present set of experimental conditions was found to be independent of agitation speed. The dissolution rate increased with increasing EDTA concentration, but leveled off at higher concentrations. At constant EDTA concentration, an increase in dissolution rate was detected at higher temperatures. A dissolution mechanism involving Langmuir-type EDTA adsorption was proposed, in which the dissolution rate of malachite is controlled by the removal of Cu(II)-EDTA complex from the malachite lattice. The proposed mechanism can explain the dependency of the dissolution rate on EDTA concentration. The activation energies determined at pHs 5, 7.5, and 10 were found to be 51.4, 50.2, and 57.5 kJ mol⁻¹, respectively. The calculated enthalpy changes of EDTA adsorption equilibrium were −43.2, −35.2, and −45.0 kJ mol⁻¹ for pHs 5, 7.5, and 10, respectively. These values are in agreement with the proposed mechanism. Formerly Graduate Student, Department of Metallurgy, Kyoto University, is Formerly Professor, is Professor Emeritus, Department of Metallurgy, Kyoto University.