Impact of ionic strength on Cd(II) partitioning between alginate gel and aqueous media

Alginate gel is representative of polysaccharide-based components of cell walls which contain a large number of negatively charged functional groups. The structural charge gives rise to a Donnan potential in the gel, which impacts significantly on the partitioning of ions between the aqueous medium and the gel. We measured the Donnan potential and partitioning of Cd2+ in alginate gel as a function of ionic strength in the range 1-100 mM. The Cd2+ partition coefficient between gel and medium, as measured by in situ microelectrode voltammetry, reaches values between 10 and 100 in the 0.1-1 mM ionic strength range, and agrees well with Donnan partition calculations based on the charge density of the gels. The total Cd(II) concentration in the gel correlates approximately linearly with the free [Cd2+]gel. The results imply that metal ion activities in the biopolymer gel phase may generally differ drastically from those in the bulk medium to an extent that strongly depends on ionic strength. This feature must be taken into account in estimations of exposure conditions for predictions of bioavailability.