The adsorption of copper(II) onto goethite is qualitatively difiting adsorption edge is reached. If the metal:substrate ratio ferent from that of other metal-mineral systems. At pH 5.0 the decreases below this limit there is no further shift in the adsorption isotherms are sigmoid rather than hyperbolic, and the adsorption edge (1-5). corresponding graphs of C/N versus C (where C is concentration By contrast, Benjamin and Leckie (1) noted that the fracand N adsorption density) have distinct minima. At pH 5.5 the tional adsorption of Cu(II) on silica increased with increasisotherms have the more usual shape. The experimental data can ing Cu(II) concentration. They suggested that this unusual be fitted by a model in which monomeric CuOH / and dimeric result was caused by the onset of precipitation, as the solu-Cu 2 (OH) 2/ 2 compete for surface sites, the dimer adsorbing more tions were close to saturation. McLelland (6) found a similar strongly to the surface. As the concentration increases the proportion of the dimeric species increases, which leads to enhanced effect for the adsorption of Cu(II) on goethite, but in this adsorption. At pH 5.5 adsorption of the monomer is sufficiently case the Cu(II) concentrations were below 10 05 M, and strong that normal behavior is observed at low concentrations, the pH values at least two units below those needed for before there is sufficient dimer to compete effectively for sites. precipitation. Evidence of anomalous behavior can also be Data are presented for adsorption of Cu(II) to goethite at pH 5.0 seen in the study by Balistrieri and Murray (7) of adsorption and pH 5.5, at temperatures between 25 and 70ΠC. Adsorption is of Cu(II) on goethite in 0.1 M NaNO 3 , where above pH endothermic for both monomer and dimer. Enthalpies of adsorp-5.0 a greater fraction of Cu(II) was adsorbed from a 1.8 1 tion range from 50 to 91 kJ mol 01 , and entropies from 240 to 470
10 06 M solution than from a 3.2 1 10 07 M solution.