Use of column chromatography for the study of metal ion binding to oxygen bleached softwood pulp

The control of metal ions has become of increased importance when aiming at more closed and environmentally friendly pulping processes. In this work an ion exchange model was applied to study binding of different metal ions to oxygen bleached softwood pulp. By using a column chromatographic technique in the experiments even quite small differences in metal ion affinities can be detected. In the column experiments a method of competition was used with an excess of respective metal ions in comparison to the total binding capacity of the pulp studied. A reliable measure of how strongly different metal ions are bound to pulp is obtained from the number of microequivalents of the elements bound in a sorption experiment. By combination of several sorption experiments the following order of affinity was obtained for oxygen bleached softwood pulp: Pb2+>> Cu2+>> Cd2+> Zn2+, Ni2+> Ba2+> Ca2+, Mg2+, Sr2+>> K+, Na+ A's can be expected from coordination chemistry point of view, the alkali metal ions are most weakly bound to the pulp studied, followed by the alkaline earth metal ions. Of divalent metal ions lead and copper are clearly most strongly bound. In the ion exchange reactions one divalent metal ion replaces two monovalent ions from the functional groups in pulp. The operating capacities (loads) were also determined in this work and were clearly lower than the total binding capacity of the pulp reported from potentiometric pH titrations. Thus all of the functional groups are not coordinated by metal ions at the conditions used in the sorption experiments.