Competitive sorption of Na+ and Ca2+ ions on unbleached kraft fibres—A kinetics and equilibrium study

In a binary system the sorption of Na+ and Ca2+ by kraft pulps follows a second order kinetics, and the isotherms follow the Langmuir model. The sorption capacity is dependent on pH and temperature. The thermodynamic parameters indicate that the sorption is exothermic and spontaneous. In a single-component system, the sorption capacity of the kraft pulp for Na+ and Ca2+ increases with the degree of dissociation of ionisable cellulosic components of the kraft pulp fibres. In a binary system the exchange of Na+ and Ca2+ is highly dependent on pH. SEM and X-ray analysis indicated that at pH above 11.0 calcium forms a precipitate bound to the surface of the fibres.

Graphical abstractCa2+ has a much higher sorption affinity than Na+ in the binary systems. It is evident that Ca2+ forms precipitate and possible ionic pairs of monovalence at high pH.Figure optionsDownload full-size imageDownload as PowerPoint slide