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

Abstract

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 Cd(2+) 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 Cd(2+) by the hydrophobically gel-bound HA is stronger than that for dispersed HA particles. Still, the intraparticulate speciation of Cd(II) over Cd(2+) 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 [Cd(aq)(2+)] is higher than that in the medium by the corresponding Donnan coefficient. On top of that, Cd(2+) 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.

The Effect of Supramolecular Humic Acids on the Diffusivity of Metal Ions in Agarose Hydrogel

“…In contrast, the values of diffusion coefficients were lower in magnitude (e.g., 4.75 × 10 −11 m 2 ·s −1 [ 51 ] and 2.12–3.12 × 10 −11 m 2 ·s −1 [ 52 ]). Our value of D g for pure agarose hydrogel is lower in comparison with the diffusion coefficients published in [ 47 , 48 , 49 , 50 ]. It is necessary to account for different types of hydrogels and the different methods of the determination of the diffusion coefficient.…”

“…Their values for Cu(II) ions ranged between 6.05 and 6.51 × 10 −10 m 2 ·s −1 and decreased strongly if metal ions were complexed with humic and fulvic acids. A similar decrease in diffusion rate caused by complexation of metal ions with humic substances and other ligands was observed in [ 48 ]. Wang et al [ 49 ] studied diffusion characteristics of agarose hydrogel used in DGT.…”

“…It is necessary to account for different types of hydrogels and the different methods of the determination of the diffusion coefficient. Results published in [ 47 , 48 , 49 , 50 ] were determined using the DGT technique, which is characterized by the diffusion in thin film. The method used in this work belongs to the diffusion in semi-infinite medium, where the diffused particles penetrate into hydrogel and their concentration on the end of hydrogel sample is unchanged during the experiment.…”

Quantitative Observation of Electric Potential Distribution of Brittle Polyelectrolyte Hydrogels Using Microelectrode Technique

“…With microelectrode, the electrodes have also been adopted to study the electric signals in and out of neuron and muscle cells. [18][19][20][21][22][23] Naturally, Some studies have been performed to use the microelectrode technique (MET) to measure the 3D electric potential of polyelectrolyte hydrogels 5,[24][25][26][27][28][29][30] .…”

“…Electrode probes have been widely used to study the physical chemistry properties of polymer solution, such as pH, impedance, potentiometric titration, etc. With microelectrode, the electrodes have also been adopted to study the electric signals in and out of neuron and muscle cells. − Naturally, some studies have been performed to use the microelectrode technique (MET) to measure the electric potential of polyelectrolyte hydrogels. ,− …”