Surface charge and permeable gel descriptions of the ionic strength influence on proton binding to seaweed biomass

Abstract

The development of physicochemical models for the description of cation binding to algal biomass is an essential issue for the practical application of seaweeds as biosorbents in waste water treatment. These models should be able to account for the polyelectrolytic properties of these materials, and to reasonably describe and predict the ionic strength effect. Data regarding the proton binding equilibria of Sargassum muticum, Cystoseira baccata and Saccorhiza polyschides in NaCl and KNO₃ were reported on a preceding paper. These data are re-analysed in this work, in order to compare two different electrostatic models (surface charge and Donnan models), representing two opposite idealised interface structures (planar surface and permeable gel). Simple analytical equations relating the apparent pK with both the dissociation degree and the ionic strength are derived for each case. The experimental data are reproduced with similar accuracy by the surface charge and the simplest Donnan model (rigid volume), although the latter seems to yield slightly better results. The comparison of the geometric parameters involved in each model (Donnan volume and specific area) with independent experimental measurements is not straightforward. Therefore, both models seem to be almost equivalent, within their respective assumptions. Nevertheless, Donnan has the advantage that provides a simple way to account for activity coefficient effects and non-specific binding.