Interaction between divalent metal ions and oxygen-delignified kraft pulps

The interaction of the metal ions Ca2+ and Mn2+ with oxygen-delignified hardwood and softwood pulp has been investigated at 25 degreesC in a 0.02 mol/L (Na)Cl-medium in the pH range from 3 to 6. The measurements were made by high precision potentiometric titrations, in which pH was measured, and by batch experiments where the metal ion content in the pulp and surrounding solution were determined. The data were evaluated by applying either a "classical" complex formation model or a distribution model based on Donnan theory. The acid-base properties of the pulps were ascribed to the fibre-bound carboxylic acid groups (=COOH) and were described by an intrinsic equilibrium constant and a capacitance value for the electrical double layer. Formation of the complex Me(=COO)(+) gave a good description of the divalent metal ion content in pulp, manganese ions showing a slightly higher affinity for the pulp than calcium ions. The two independent experimental methods gave the same results within the margin of statistical error. The uncertainty was somewhat larger for the results from potentiometric titrations than those from the batch experiments. There was a systematic deviation between the amount of divalent metal ions found in the pulp and the amount calculated from a model based on the Donnan theory. The divalent metal ion content in pulp predicted by the model, was 10-45% lower than the experimentally determined amount.