Factors affecting the flux of macromolecular, labile, metal complexes at consuming interfaces, in water and inside agarose gel: SSCP study and environmental implications

DGT/DET gel partition features of humic acid/metal species

Gel layer based sensors are increasingly employed for dynamic trace metal speciation analysis in aquatic and soil media, in which humic and fulvic acid species are generally known to be relevant. In DGT (diffusive gradient in thin film), polyacrylamide hydrogels are commonly used for the diffusive gel layer. Various effects of the presence of humic species on the amount of metal detected by DGT have been observed, but the role of the different metal/humic species is still unknown. Recently it was shown that in the absence of metal, humic acid accumulates significantly in the polyacrylamide hydrogel. Here we analyze the extent of this accumulation in the presence of cadmium under various conditions of ionic strength and total humic and fulvic acid concentrations. At millimolar ionic strength level, DET data show significant accumulation of cadmium(II) in the gel phase, on top of some Donnan enrichment. The results are essential for the interpretation of DGT/DET data on metals in the presence of humics, especially in freshwaters.

Accumulation of humic acid in DET/DGT gels

Gel layer based sensors are increasingly employed for dynamic trace metal speciation analysis in aquatic and soil media. In DGT (Diffusive Gradient in Thin film), polyacrylamide hydrogels are commonly used for the diffusive gel layer. While some effects of humic and fulvic acids on the DGT detection of metal species have been observed, the gel permeation properties of the actual humic and fulvic acid compounds themselves have not been analyzed thus far. Here we show with DET (Diffusive Equilibrium in Thin film) that these natural complexing agents do enter the gel layer, and that humic acids even appear to accumulate in the gel, with enrichment factors typically on the order of 10. The results have consequences for the interpretation of DGT-data on metal fluxes from aquatic media containing humics and fulvics.

Trace metal speciation and bioavailability in surface waters of the Black Sea coastal area evaluated by HF-PLM and DGT

Trace metal speciation in seawater from the Bulgarian Black Sea coast was studied in situ by hollow fiber permeation liquid membrane (HF-PLM) and by diffusion gradients in thin-film gels (DGT). The concentrations of Cd, Cu, Ni, and Pb determined by HF-PLM were lower than those measured by DGT, in agreement with their analytical windows, e.g., free metal ions provided by the HF-PLM and dynamic (mobile and labile) species by the DGT. The obtained suite of data was further used to evaluate the bioavailability of these metals to the microorganisms, which was then compared with experimental results of metal uptake to green microalga Chlorella salina. Uptake fluxes of the Cd, Cu, Ni, and Pb to C. salina, were predicted from the measured HF-PLM concentrations and laboratory experimentation in artificial seawater, in agreement with theoretical considerations. The HF-PLM and DGT appear to be promising analytical techniques for speciation and bioavailability studies in complex environmental media and allow improved understanding of the role of different chemical species in metal bioavailability (and impact) in seawaters.

Metal speciation by DGT/DET in colloidal complex systems

Gel-layer-based sensors are increasingly employed for dynamic trace metal speciation analysis in aquatic media. In DGT (diffusive gradients in a thin film), two different types of polyacrylamide hydrogels, designated as "open pore" and "restricted", are commonly used for the diffusive gel layer. While both gels are known to be fully permeable to metal ions and small complexes, colloidal particles with radii from tensto hundreds of nanometers have generally been assumed to be excluded from the gel. Here we show, however, that for dispersions with Pb(II) as the probe metal and monodisperse latex particles as metal-binding agents, relatively large particles do enter the gel to a significant extent. By complementing DGT flux analysis with diffusive equilibration in a thin film accumulation data for the colloidal complex systems, it is demonstrated that with radii up to 130 nm permeation for particles is substantial. The consequences for interpretation of dynamic speciation data furnished by gel-based sensors are discussed.