Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. I. Adsorption and diffusion of metals

SUMMARY The reactions of Ni, Zn and Cd with goethite were studied over a range of initial metal concentrations (10−6 to 10−4M), pH values (4 to 8), reaction times (2h to 42d) and temperatures (5 to 35°C). The adsorption of metals increased with pH, reaction time and temperature. Adsorption of Ni increased relative to Zn and Cd with increasing time and temperature. The initially rapid adsorption of metals within a few hours was followed by a much slower reaction linearly related to time1/2, interpreted as diffusion–controlled penetration of goethite. The pH–dependent relative diffusion rates (Ni > Zn > Cd) were influenced by both affinity for goethite surfaces and by ionic radius. Diffusion coefficients of the three metals ranged from about 10−19to 10−20cm2s−1. The corresponding activation energies of diffusion were also calculated (Ni 35, Cd 55, Zn 90 kJ mol−1). Our view about the reactions of heavy metals with goethite involves (i) adsorption of metals on external surfaces, (ii) solid–state diffusion of metals from external to internal binding sites, and (iii) metal binding and fixation at positions inside the goethite particles. The general parameters of these processes are related to the hydrolytic properties (pK values) and the ionic radii of the metals. The results show that goethite may be an efficient sink for trace metals.