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

Field application of a novel active-passive sampling technique for the simultaneous measurement of a wide range of contaminants in water

A first test of the field capabilities of a novel in situ sampling technique combining active and passive sampling (APS) was conducted in the sea. The proof-of-concept device uses a pump to draw water into a diffusion cell where dissolved target substances are accumulated onto sorbents which are selective for different classes of contaminants (i.e., metal cations, polar and non-polar organic compounds), simultaneously. A controlled laminar flow established in the diffusion cell enables measurements of contaminant concentrations that are fully independent from the hydrodynamic conditions in the bulk solution. APS measurements were consistent with those obtained using conventional passive sampling techniques such as organic diffusive gradients in thin films (o-DGT) and silicone rubber (SR) samplers (generally < 40% difference), taking into account the prevailing hydrodynamic conditions. The use of performance reference compounds (PRC) for hydrophobic contaminants provided additional information. Field measurements of metal ions in seawater showed large variability due to issues related to the device configuration. An improved field set-up deployed in supplementary freshwater mesocosm experiments provided metal speciation data that was consistent with passive sampling measurements (DGT), taking into account the hydrodynamic conditions. Overall, the results indicate that the APS technique provides a promising approach for the determination of a wide range of contaminants simultaneously, and independently from the hydrodynamic conditions in the bulk solution.

Assessment of the DGT technique in digestate to fraction twelve trace elements

This study proposes an evaluation of the diffusive gradients in thin films technique (DGT) for studying trace elements in digested sewage sludge samples. Twelve elements were monitored by Chelex (Al, Cd, Co, Cr (III), Cu, Fe, Mn, Ni, Pb) and zirconia-DGT (As, Mo, Se) samplers exposed from 4 h to 9 days. Twenty-four hours' deployment time was suitable for most of the studied elements. However, short deployment led to insufficient element accumulation or non-establishment of steady state while long deployment (from 18 to 144 h depending on the element) led to saturation of the binding gels and/or competing effects with other major elements. In addition, this study showed that the matrix of the digested sewage sludge lowers the accumulation of some trace elements in the DGT samplers, leading to labile concentrations underestimation of roughly 10-30% (depending on the element). Moreover, compared to the conventional total dissolved elements measurement, DGT technique allowed to quantify 7 out of 12 labile elements whereas only 3 out of 12 dissolved elements were quantified. These results highlight the potential of DGT technique to assess labile trace elements in digestate samples, provided a careful adaptation of the deployment time as well as an evaluation of the matrix effect is performed.

Influence of the settling of the resin beads on diffusion gradients in thin films measurements

Binding resin beads used in DGT (diffusion gradients in thin films) tend to settle to one side of the resin during casting. This phenomenon might be relevant for metal accumulation when partially labile complexes dominate the metal speciation, especially after recognizing the important role played by complex dissociation in the resin domain. The influence of the inhomogeneity of the binding agent distribution on metal accumulation is here assessed by numerical simulation of DGT devices with binding beads in only one half of the resin disc, as a reasonable model of the standard resin discs. Results indicate that a decrease in mass accumulation of less than 13% can arise in these inhomogeneous devices (as compared with an ideal disc with homogeneous dispersion of the resin beads) when complexes with stability constant K<10(2)m(3)mol(-1) (K<10(5)Lmol(-1)) dominate the metal speciation. The loss increases as K increases, but the percentage of mass loss always remains lower than the volume fraction of resin disc without beads. For very labile or inert complexes, the impact of the inhomogeneous distribution of binding resin beads is negligible. As kinetic dissociation constants of complexes can be estimated from the distribution of the metal accumulation in a DGT device with a stack of two resin discs, the influence of the inhomogeneity on the recovered kinetic constant is also assessed. For the cases studied, the recovered kinetic dissociation constant, kd,recovered, retains the correct order of magnitude, being related to the true kd by kd≈f(-1)kd,recovered, quite independently of K and kd values, being f the fraction of volume of the resin disc where resin beads are dispersed.

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 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.

Partitioning of humic acids between aqueous solution and hydrogel. 2. Impact of physicochemical conditions

The effects of the physicochemical features of aqueous medium on the mode of partitioning of humic acids (HAs) into a model biomimetic gel (alginate) and a synthetic polyacrylamide gel (PAAm) were explored. Experiments were performed under conditions of different pH and ionic strength as well as in the presence or absence of complexing divalent metal ions. The amount of HA penetrating the gel phase was determined by measuring its natural fluorescence by confocal laser scanning microscopy. In both gel types, the accumulation of HA was spatially heterogeneous, with a much higher concentration located within a thin film at the gel surface. The thickness of the surface film (ca. 15 μm) was similar for both types of gel and practically independent of pH, ionic strength, and the presence of complexing divalent metal ions. The extent of HA accumulation was found to be dependent on the composition of the medium and on the type of gel. Significantly more HA was accumulated in PAAm gel as compared to that in alginate gel. In general, more HA was accumulated at lower background salt concentration levels. The distribution of different types of HA species in the gel body was linked to their behavior in the medium and the differences in physicochemical conditions inside the two phases.

Partitioning of humic acids between aqueous solution and hydrogel: concentration profiling of humic acids in hydrogel phases

The partitioning of the natural polyelectrolyte humic acid (HA) from an aqueous dispersion into a model biomimetic gel (alginate) and a synthetic polyacrylamide gel (PAAm) is explored. In both gels, the spatial distribution of HA in the gel body, as measured by confocal laser scanning microscopy, is markedly nonhomogeneous. A striking feature is the enhanced accumulation of HA in a thin film of thickness ca. 15 μm at the surface of the gel body, resulting in average local concentrations that are, for PAAm and alginate respectively, a factor of 10 and 4 greater than that in the bulk solution. The time dependence of accumulation in the surface film is predominantly controlled by the diffusive supply of HA from the aqueous medium, with a time constant on the order of 10(3) s for both gels. The concentration of HA within the bulk gel body differs significantly from that in the bulk aqueous medium: substantially higher for PAAm but much lower for alginate. The results are significant for understanding the nature and rate of sink/source functioning at permeable phases in contact with aqueous media, e.g., biofilms and gel-like layers at biological interfaces or employed in chemical speciation sensors.

Investigation of trace metal binding properties of lignin by diffusive gradients in thin films

The binding behavior of lignin for Pb, Cu, Co, Mn, Cd and Ni was studied using the diffusive gradients in thin films technique (DGT). Samplers with different structures of diffusive gel were used in the well-stirred systems containing known concentrations of metals along with (a) 10, 20 and 40 μM lignin and; (b) 0.64 and 6.47 μM Suwannee river fulvic acid+40 μM lignin at an ionic strength of 0.01 M (NaNO(3)) and pH=7. Diffusion coefficients of lignin complexes in acrylamide gels were estimated and found to be less than 5% of the equivalent coefficients for the uncomplexed metal ions. These values were used to calculate concentrations of labile metals from DGT measurements in solutions, where lignin could discriminate metals in the order of Pb(+2)>Cu(+2)>Cd(+2)>Ni(+2)>Co(+2)>Mn(+2). Stability constants (LogK) were calculated using Visual MINTEQ II and WHAM V software. The K values were compared with the stability constants from titration of Pb and Cd with 10 μM lignin aqueous samples and with those of humic substances in natural waters. The constants obtained from measurement of complexing capacities might bias the real corresponding values unless two line regression analyses on titration data are considered. The DGT study of fractionation of metal species at varying ratios indicated that the proportion of organic complexes decreased with increasing ratios and gradually more metals were exchanged with inorganic phases. Speciation of Pb and Cd is affected by the concentrations of FA, Cd is dominantly bound with FA while Pb is evenly partitioned between the ligands. The comprehensive knowledge of metal-lignin complexes sheds some light on in situ operational speciation information that can be achieved by DGT.