COMPARISON OF ION EXCHANGE AND DONNAN EQUILIBRIUM MODELS FOR THE PH-DEPENDENT ADSORPTION OF SODIUM AND CALCIUM IONS ONTO KRAFT WOOD PULP FIBERS

Abstract

ABSTRACT Management of nonprocess element (NPE) accumulation in pulp washing operations requires equilibrium models that predict the distribution of metals between the wash liquor and the pulp fibers. The overall goal of this study was to assess models for predicting the multi-component adsorption of hydrogen ions (H+), sodium ions (Na+), and calcium ions (Ca+2) onto bleached and unbleached kraft pulp fibers over a pH range of 2.7–11. As part of this study, binary equilibrium constants for hydrogen and metal ion exchange on carboxylate sites in bleached pulp (0.041 meq/g dry pulp) were measured at 25°C, with log K Na/Ca = −1.604 ± 0.119, log K H/Ca = 0.633 ± 0.087, and intrinsic dissociation constant pK io of 3.64 ± 0.46. Ion exchange and Donnan equilibrium models adequately predicted the multi-component equilibrium data for competitive adsorption of H+, Na+, and Ca+2 onto bleached kraft wood pulp fibers. The ion exchange model was fully predictive, whereas the Donnan model required that the solution pH be known. At pH 2.7–6, the Donnan model predicted the adsorption of Na+ and Ca+2 onto both bleached and unbleached wood pulp fibers better than the ion exchange model. The ion exchange model assumed that residual carboxylate in the pulp served as the only site for the competitive binding of hydrogen and metal ions. In contrast, the Donnan model assumed a non site-specific distribution of metal ions between charged fiber and external solution phases and a carboxylate site specific adsorption of hydrogen ions. Above pH 6, both models failed to predict that the calcium adsorption on unbleached brownstock pulp increased beyond the carboxylate site capacity, suggesting that other functional groups within the brownstock pulp with intrinsic dissociation constant values higher than carboxylate were providing additional binding sites for calcium.