Modeling the Effect of Temperature on Adsorption of Lead(II) and Zinc(II) onto Goethite at Constant pH

The influence of temperature and adsorbate concentration on the sorption of Zn(II) and Pb(II) by goethite was studied at fixed pH. Proton stoichiometry, measured by direct titration, was found to depend on the identity of the metal ion, the pH, and the temperature. For Zn(II) the proton stoichiometry ranged from 1.55 at pH 6.5 and 10°C to 1.95 at pH 7.5 and 70°C, while for Pb(II) at pH 5.50 the value varied from 1.05 at 10°C to 1.4 at 70°C. Three adsorption models-the Langmuir two-site model, the surface precipitation model of Farley, Dzombak, and Morel (FDM), and the BET model-were applied to the data, and the fitted parameters were used to determine the enthalpy and entropy of adsorption. Although the models assume rather different sorption reactions, and therefore generate correspondingly different equilibrium constants, the estimated enthalpies of adsorption were all positive, between 4 and 30 kJ/mol. There was also fair agreement between the various estimates of the adsorption capacities. However, the models yield very different estimates of entropies of adsorption, these being positive (and generally in the range 100 to 160 J K-1 mol-1) for the Langmuir and BET models but negative (in the range -80 to -160 J K-1 mol-1) for the FDM model.