Adsorption Isotherms for Divalent Metal Ions on Unbleached Wood Pulp, 1997 Minimum Effluent Mills Symposium Proceedings

Closure of water consumption in total-chlorine free (TCF) bleach plants will require recycle of wash waters, leading to the accumulation of metals in both the wash waters and the pulp. The binding of divalent metal ions onto unbleached wood pulp (LP-Samoa #2 brownstock, KAPPA number 18.5) was studied. Specifically, adsorption isotherms, which quantify the adsorption capacity of a particular metal ion on the pulp as a function of the metal ion concentration in the solution contacting the pulp at equilibrium, were measured for a series of five divalent metal ions, including the alkaline earth metals barium (Ba +2 ) and calcium (Ca +2 ), and the transition metals cadmium (Cd +2 ), manganese (Mn +2 ), and zinc (Zn +2 ) at near neutral pH (5 to 7) and temperatures of 25 to 75 °C. Saturation adsorption capacities for the five metals ranged from 2 to 10 mg metal per gram of dry pulp (0.04 to 0.09 mmol per gram dry pulp) at metal ion concentrations above 1000 mg/L. Adsorption isotherm data was empirically fit to the Freundlich model. The adsorption capacity exceeded the carboxylic acid group (COOH) concentration in the pulp (0.054 mmol -COOH per gram of dry pulp), implying other residual chemical constituents in the pulp were also active sites for metal ion exchange. The adsorption capacity decreased by 50% as the temperature increased from 25 to 75 °C, and the heat of adsorption was negative and ranged from -4 to -13 kJ/mol. The kinetics of metal ion uptake onto the pulp were also measured in batch experiments, and equilibration was achieved within 3 hr.