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.

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.

Donnan effects in metal speciation analysis by DET/DGT.

Cross-linked polyacrylamide gels have found wide application in analytical techniques, such as diffusional equilibration in thin film (DET) and diffusive gradient in thin film (DGT). In these applications, the assumption is made that the gel matrix is effectively uncharged and chemically inert with respect to the species of interest. Recent data has shown significant nonideal behavior at low ionic strengths, which strongly suggests a finite structural charge within the gel matrix. The present investigation explores the possible ramifications of fixed charged sites within the gel on metal speciation analysis by DET and DGT. The results indicate that structural charge within the gel layer will give rise to different DET equilibrium concentrations than in the sample because of Donnan partitioning. The steady-state diffusion of ions through a diffusive gel (DGT) is also influenced by the gel charge, but the net effect will depend on the details of the speciation. The results indicate that for submillimolar ionic strength solutions the quantitative interpretation of DGT data benefits from combination with DET.