Partitioning of humic acids between aqueous solution and hydrogel. 3. Microelectrodic dynamic speciation analysis of free and bound humic metal complexes in the gel phase

The hydrogel/water partitioning of the various species in the cadmium(II)/soil humic acid (HA) system is studied for two types of gel, using in situ microelectrodic voltammetry. Under the conditions of this work, with HA particles of ca. 25 and 125 nm radius, the CdHA complex is shown to be close to nonlabile toward a 12.5 µm radius microelectrode. This implies that its kinetic contribution to Cd2+ reduction at the medium/microelectrode interface is practically negligible. The polyacrylamide (PAAm) gels equilibrate with the aqueous medium under significant sorption of HA at the gel backbone/gel medium interface, which in turn leads to induced sorption of Cd(II) in the form of immobilized gel-bound CdHA. The rather high total Cd content of the PAAm gel suggests that the binding of Cd2+ by the hydrophobically gel-bound HA is stronger than that for dispersed HA particles. Still, the intraparticulate speciation of Cd(II) over Cd2+ and CdHA corresponds to an intrinsic stability constant similar to that for simple monocarboxylate ligands such as acetate. Alginate gels are negatively charged, and their free [Cdaq2+] is higher than that in the medium by the corresponding Donnan coefficient. On top of that, Cd2+ is specifically sorbed by the gel backbone/gel medium interface to reach accumulation factors as high as a few tens. HA and CdHA accumulate in the outer 20 µm film of gel at the gel/water interface of both gels, but they do not penetrate into the bulk of the alginate gel. Overall, the gel/water interface dictates drastic changes in the speciation of Cd/HA as compared to the aqueous medium, with distinct features for each individual type of gel. The results have broad significance, for example, for predictions of reactivity and bioavailability of metal species which inherently involve partitioning and diffusion into diverse gel layers such as biointerfacial cell walls, biofilm matrices, and mucous membranes