Conductometric study of cellulose phosphate screening with divalent Ca2+ ions

Despite the relevance and abundance of studies on ion exchange processes, the current understanding of actual ionic interactions with charged moieties on the stationary phase is wanting in specificity. The Gibbs–Donnan and Donnan–Manning theories accurately describe membrane separation processes and ion selectivities but are incapable of describing the distinction between chemical ion association and condensation and physical electrostatic ion capture and retention. This is particularly relevant for pH-dependent processes, including membrane regeneration, when the chemical association of protons significantly alters the surface potential. In this study, we demonstrate how conductometry can provide both qualitative and quantitative information on the ion association and protonation behavior of phosphorylated cellulose pulp. Combined with an expanded model capable of distinguishing between associated and retained ions, we can simulate the conductometric data with reasonable agreement and, on that basis, predict the ion association behavior of the phosphorylated cellulose fibers in a ternary system of protons, Na+, and Ca2+ ions.