THE EFFECT OF THE IONIC-STRENGTH ON THE ADSORPTION-ISOTHERMS OF NICKEL ON SILICA

Isotherms of adsorption of Ni(II) on silica from 0.01 M NaClO4 are alm ost linear with a log-log slope of 0.95, and the percentage of uptake at a given pH is insensitive to the initial Ni concentration. At highe r ionic strengths (greater than or equal to 0.1 mol dm(-3)) two kinds of behavior are observed. For low Ni concentrations the adsorption is approximately equal to that observed at lower ionic strengths. For hig her Ni concentrations (but still much lower than the concentration of surface sites) the Ni adsorption from 0.1 and 0.3 mol dm(-3) NaClO4 is lower than the adsorption from 0.01 M NaClO4 by a factor of 2 (pH 9) to 3 (pH 8). When water is replaced by a mixed solvent (e.g., 2% aqueo us DMSO, THF, methanol, glycerol) the Ni adsorption isotherms obtained in the presence of 0.1 mol dm(-3) NaClO4 are linear with a log-log sl ope of 0.95. These adsorption isotherms are rather insensitive to the nature and concentration of the organic cosolvent. It is impossible to explain the Ni adsorption curves over the entire studied range of ini tial concentrations and ionic strengths in terms of surface complexati on model, unless the Boltzmann factor is properly corrected. (C) 1997 Academic Press.