Determination of equilibrium constants for sorption of metal ions to pulp by a batch method

Optimal management of metal ions in closed cycle pulp and paper mills requires equilibrium models that predict the distribution of metal ions between the pulp phase and the aqueous phase. The goal of this study was to determine the equilibrium constants for sorption of metal ions to pulps.Metal ions are bound to acid groups like carboxylic and phenolic hydroxyl groups in the solid pulp phase mainly by ion exchange reactions. In this work the ion exchange equilibrium constants (selectivity coefficients) were determined by a batch sorption method for pairs of metal ions to unbleached thermomechanical pulp, unbleached and oxygen-delignified softwood kraft pulps over a rather broad pH range. The study involved the following metal ions: Pb, Cu, Cd, Zn, Ni, Mn, Ba, Sr, Ca, Mg, Rb, K, and Na. The concentrations of these metal ions were determined by the inductively coupled plasma optical emission spectrometry (ICP-OES). The stoichiometric ratio of the ion exchange reaction between two divalent metal ions was found to be close to 1:1. Of the metal ions studied lead and copper were most strongly bound and the alkaline metal ions most weakly bound to the pulps. The pulp samples showed a clearly better selectivity than a synthetic strongly acidic cation exchanger to heavy metal ions.