Binding of transition metals by soluble and silica-bound branched poly(ethyleneimine): Part I: Competitive binding equilibria

Competitive binding of sulfuric acid and copper, nickel and zinc sulfates by soluble and silica-bound branched poly(ethyleneimine) has been studied at 25 °C. The binding equilibria were measured by batch experiments using potentiometric and spectroscopic methods. The results were correlated with a chelation model based on a one-step proton displacement and complexation reaction. The calculated results have been compared with those obtained by means of the NICA adsorption model. ing to proton titration results, the silica-supported BPEI is less basic than the soluble polymer. Anchoring on the silica matrix also decreases the metal binding strength of BPEI, but the affinity relative to acid binding remains practically unchanged. The chelation model was shown to explain satisfactorily the proton displacement data and the dependence of metal binding on concentration and pH. Moreover, the estimated parameter values characterizing the average number of protons displaced by the metal cation are in reasonable agreement with the values determined experimentally. The H/Me ratio was higher for nickel than for copper, and the difference can be explained by the complex structures inferred from the absorption spectra of the soluble and supported BPEI. At the pH range of practical interest, a comparable correlation was obtained with the NICA model. A satisfactory agreement with the experimental data was also found, when the models were used to predict the simultaneous binding of two metals. The results of the supported BPEI showed that copper can be separated at pH 4.5 with good capacity and selectivity in the presence of 100-fold concentration of nickel or zinc. In the presence of high zinc concentration, a marked competition effect wais observed in nickel binding at pH 5.5 and only a moderate selectivity could be obtained.