The sorption of Na+, Ca++, Mg++ and Mn++ on cellulose fibres: Prediction and analysis of sorption equilibrium with electrostatic models

The aim of this work is to formulate a reasonably complete model able to predict the effect of ionic strength, pH and ionic concentrations on the binding of metal ions to cellulose fibres. Potentiometric titration was used for characterization of the properties of acidic groups in cellulose fibres. By potentiometric titration it is possible to determine the charge and surface potential as well as the pK values of dissociating species of cellulose fibres. Sorption of several ions on fibres was determined by AAS. By combining potentiometric titration and metal ion isotherms it has been possible to formulate an equilibrium model based on Donnan equilibrium distribution of ions and unspecific electrostatic double layer interactions. In this study sorption isotherms of Na+, Ca++, Mg++ and Mn++ on unbleached softwood kraft pulp were determined. Distribution diagrams of ions as a function of pH were also determined. Titration and sorption data were analyzed with FITEQL and MINEQL computer programs in order to evaluate the electrostatic properties of fibres. The results show that a combination of specific adsorption of hydrogen ions with unspecific Donnan distribution of all other cations is able to predict sorption of metal ions on cellulosic fibres.