Study of metal ion binding to unbleached softwood kraft pulp by column chromatography

The binding of metal ions to unbleached softwood pulp from a Finnish pulp mill was studied using a column chromatographic method. The ion exchange mainly takes place by complexation, where divalent metal ions are coordinated to functional groups in the pulp phase. The following affinity order was obtained by combining the results from different sorption experiments: Pb(2+) >> Cu(2+) >> Cd(2+) > Zn(2+) > Ni(2+) > Ba(2+) > sr(2+) approximate to mn(2+) approximate to ca(2+) > mg(2+) >> Rb(+) > K(+) > Na(+) > Li(+) Rather large differences were observed in the binding strength (to pulp) for the divalent metal ions: lead, copper, cadmium, zinc and nickel. The differences in affinities within the alkali and alkaline earth metal groups were much smaller. The operating capacities in the sorption experiments were determined by acid-base titrations of the released hydrogen ions in the metal ion loading step and by measuring the concentration of metal ions sorbed to the pulp using an inductively coupled plasma optical emission spectrometry (ICP-OES) technique. The operating capacity results (21-58 mu eq/g o.d.) were clearly lower than the total binding capacity of the pulp determined by a potentiometric titration of acidified pulp suspension. In the ion exchange reaction one divalent metal ion was found to be replaced by two hydrogen ions from the functional groups in the pulp.