Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. II Modelling the extent and rate of reaction

A model of the reaction of metal ions with a variable charge surface was modified and applied to goethite that had been formed in the presence of silicate. The data included the effects of initial concentrations of Ni, Zn and Cd ranging from 1 to 100 μm, of periods from 2 h to 42 d, of temperatures from 5 to 35°C, of pH from about 4 to about 8, and of background electrolyte from 0.01 to 1.0 M. The observed effects of concentration and pH were explained by assuming that the reacting surface sites were heterogeneous and that they reacted with the MOH⁺ ions in solution. The heterogeneity was described by assuming that the affinity of the sites for MOH⁺ ions decreased logarithmically as the log of the amount adsorbed increased. This gave a Freundlich relation between sorption and surface activity. The effects of increasing the concentration of the background electrolyte were explained as being caused by decreased activity of MOH⁺ ions in solution counteracted by decreases in the electric potential of the surface. To explain the effects of time and temperature, it was assumed that an initial rapid adsorption reaction was followed by slow diffusion of metal ions into the goethite. Using these assumptions, a comprehensive description of the data was obtained.