Isotherms of adsorption of Ni(II) on silica from 0.01 M NaClO 4 volving different numbers of surface sites, heavy metal ions, are almost linear with a log-log slope of 0.95, and the percentage anions of the supporting electrolyte, and protons have been of uptake at a given pH is insensitive to the initial Ni concentraoften invoked (1-5). The formation constant of each surface tion. At higher ionic strengths ( Β§0.1 mol dm 03 ) two kinds of species is an adjustable parameter in the model. In spite of behavior are observed. For low Ni concentrations the adsorption the progress in analytical techniques, for most systems of is approximately equal to that observed at lower ionic strengths.
interest the speciation is obtained as a result of curve-fitting For higher Ni concentrations (but still much lower than the conroutines rather than from direct measurements. Without a centration of surface sites) the Ni adsorption from 0.1 and 0.3 mol direct evidence of existence of particular surface species, dm 03 NaClO 4 is lower than the adsorption from 0.01 M NaClO 4 by a factor of 2 (pH 9) to 3 (pH 8). When water is replaced by models with possibly few species and thus few adjustable a mixed solvent (e.g., 2% aqueous DMSO, THF, methanol, glycparameters are more plausible. For certain systems (6) a erol) the Ni adsorption isotherms obtained in the presence of 0.1 model with single surface reaction can be successful in mol dm 03 NaClO 4 are linear with a log-log slope of 0.95. These matching experimental data covering a broad range of ionic adsorption isotherms are rather insensitive to the nature and constrength, but sometimes as many as five different surface centration of the organic cosolvent. It is impossible to explain reactions were employed (2).
the Ni adsorption curves over the entire studied range of initial On the other hand, in data sets covering many orders of concentrations and ionic strengths in terms of surface complexmagnitude of initial concentrations of heavy metal ions, the ation model, unless the Boltzmann factor is properly corrected. uptake curves often shift toward higher pH values when the