Extension of Donnan theory to predict calcium ion exchange on phenolic hydroxyl sites of unbleached kraft fibers

Sorption of Na+, Ca2+ ions from aqueous solution onto unbleached kraft fibers—kinetics and equilibrium studies

“…This pH effect was due to the fact that the fiber charge increased with pH [7], resulting in more sites for ion exchange and sorption.…”

“…Unbleached kraft fibers carry a charge when suspended in water due to the presence of ionizable acidic groups carried over with the hemicellulose lignin and cellulose in the fiber wall [7]. The main ionizable groups in unbleached kraft fiber are carboxylic acids, which are of great importance for sorption of metal ions onto fibers.…”

“…Carboxylic acids are weakly dissociated and have pK a ∼ 4-5. The phenolic groups of lignin are also ionizable but at a higher pH (pK a ∼ 7.3-10.5) [7][8][9][10]. The presence of the ionizable groups of fiber is highly related to the cation sorption capacity of the fibers.…”

“…Previous studies indicate that Donnan theory, which is based on the balance of the system free energy, can be used to determine the distribution coefficient of ions in fiber slurries at low concentrations [7]. The pronounced influence of pH and ionic strength of solution on the distribution of metal ions have been reported [7,[11][12][13][14][15][16][17][18].…”

“…The pronounced influence of pH and ionic strength of solution on the distribution of metal ions have been reported [7,[11][12][13][14][15][16][17][18]. The nature of the sorption depends also on the structure of kraft fibers, which is highly complex consisting of layers of crystalline microfibril bundles of cellulose.…”