tween the metal ions and the surface and in the treatment The influence of temperature and adsorbate concentration on of the interaction between the adsorbing species and the the sorption of Zn(II) and Pb(II) by goethite was studied at fixed charged surface ( 6 -13 ) .
pH. Proton stoichiometry, measured by direct titration, was found
In earlier work (5) we showed that the James-Healy to depend on the identity of the metal ion, the pH, and the tempermodel for adsorption of hydrolyzed species (7) correctly ature. For Zn(II) the proton stoichiometry ranged from 1.55 at predicted both the direction and magnitude of the shift in the pH 6.5 and 10ΠC to 1.95 at pH 7.5 and 70ΠC, while for Pb(II) at adsorption edge with temperature for adsorption of Cu(II), pH 5.50 the value varied from 1.05 at 10ΠC to 1.4 at 70ΠC. Three Pb(II), and Zn(II) onto goethite. However, it failed to acadsorption models-the Langmuir two-site model, the surface count adequately for the dependence on cation concentration. precipitation model of Farley, Dzombak, and Morel (FDM), and the BET model-were applied to the data, and the fitted parame-
This study investigated the sorption of Pb(II) and Zn(II) ters were used to determine the enthalpy and entropy of adsorponto goethite at constant pH and at several temperatures tion. Although the models assume rather different sorption reacbetween 10 and 70ΠC, over a range of concentrations. Proton tions, and therefore generate correspondingly different equilibrium stoichiometry was measured directly in the same expericonstants, the estimated enthalpies of adsorption were all positive, ments. We have analyzed the results using three simple modbetween 4 and 30 kJ/mol. There was also fair agreement between els, the Langmuir two-site model (which has been used to the various estimates of the adsorption capacities. However, the evaluate adsorption of both anions and cations to soils (14models yield very different estimates of entropies of adsorption, 16)), a surface precipitation model ( 6), and a BET model. these being positive (and generally in the range 100 to 160 J K 01 The last two models seek to allow for the formation of mol 01 ) for the Langmuir and BET models but negative (in the multilayers on the surface, which are likely to occur at high range 080 to 0160 J K 01 mol 01 ) for the FDM model. α§ 1996 adsorbate:adsorbent ratios. The results are discussed with Academic Press, Inc.