Kinetic and equilibrium analysis of metal ion adsorption onto bleached and unbleached kraft pulps

Graduation date: 2001Most metal ions have negative impacts on pulp mill operations. The\ud concentrations of metal ions on pulp fibers and in washwaters rise significantly with\ud increased wastewater recycling. The development of technology to remove these metal\ud ions requires an understanding of how metal ions are bound to pulp components. It is\ud also desirable to predict distribution of metal ions between the pulp fibers and the\ud washwaters.\ud The adsorption isotherms for eight metal ions (Ca, Ba, Mn, Zn, Pb, Cd, Ni, Na)\ud were measured on bleached and unbleached (brownstock) kraft pulps at neutral pH and\ud temperatures ranging from 25 to 75°C. On bleached pulps, the metal ion adsorption\ud increased rapidly with increasing metal ion concentration in solution and then leveled off.\ud At neutral pH, the adsorption on bleached pulp was stoichiometric to the carboxylate\ud sites, whereas the adsorption on unbleached pulp was not, especially at high metal ion\ud concentration in solution and low temperature.\ud The pH isotherms specify the adsorption isotherms of sodium and calcium on\ud wood pulps as pH ranging from 2.5 to 11.0. The pH isotherms on bleached pulp with\ud only COOH functional groups (pK[subscript a] of 3.77) were saturated at pH 4 and above, whereas\ud those on brownstock pulp with both COOH and PhOH (pK[subscript a] of 10) functional groups\ud increased in two steps, at pH 4 and 8.\ud The brownstock pulp is heterogeneous material. Therefore, only the empirical\ud Freundlich model was applied to the data. To predict the metal ion adsorption on\ud bleached pulps, two fundamental equilibrium models were developed: the multi-component\ud ion exchange and the Donnan equilibrium models. The ion-exchange model\ud better predicts the metal adsorption at neutral pH, whereas the Donnan equilibrium model\ud more accurately predicts the pH isotherms.\ud The adsorption kinetics of Ba⁺² and Ni⁺² were measured on wood pulps as a\ud function of mixing speed, initial metal ion concentration, and temperature. The adsorption\ud of metal ions reached equilibrium rapidly. The intraparticle diffusion model, based on first\ud principle with a linear relationship assumption between adsorbed and free metal ion\ud concentration, satisfactorily predicted the adsorption kinetics at low metal ion\ud concentration in solution