Development and Performance of the Diffusive Gradients in Thin-Films Technique for the Measurement of Technetium-99 in Seawater

Diffusive Gradients in Thin-films technique for uranium monitoring along a salinity gradient: A comparative study on the performance of Chelex-100, Dow-PIWBA, Diphonix, and Lewatit FO 36 resin gels in the Scheldt estuary

Monitoring of uranium in the environment using the Diffusive Gradients in Thin-films (DGT) technique gains in importance as it can provide unique information about the bioavailability of the element and allows its long-term in-situ measurement. Hence, in this study, four DGT binding phases (Chelex-100, Dow-PIWBA, Diphonix, and Lewatit FO 36 resins) were evaluated for uranium monitoring to assess the robustness of their performance in estuarine and marine environments. These DGTs were deployed along the Scheldt estuary (Belgium and the Netherlands) over four campaigns between 2014 and 2021. The DGT performance (ratio of the DGT-determined vs. dissolved U concentration in grab water sample) varied with the water salinity. The Chelex-100 DGTs generally provided good performance in freshwater (median ratios close to 1.0), but an inverse correlation with the increasing salinity was observed (median ratios 0.7 at the stations with salinity >5). The Lewatit FO 36 DGTs provided good performance in the salinity range 0-18 (median ratios 1.0). However, a strong negative influence was observed at stations with high salinity levels (>18, ratio 0.6) and during the long-term deployment in seawater (ratios <0.5 over deployment periods ≥2 days). The Dow-PIWBA and Diphonix DGTs provided overall similar results with excellent performances along the whole salinity gradient (median ratios 1.1 and 1.0, respectively). Nevertheless, the long-term deployment trial in seawater (salinity ∼27) revealed the robustness of Diphonix DGTs that provided outstanding results even after 28 days of deployment (ratio 1.0). The differences in the performance of tested DGT resins were mostly given by the changes of U speciation along the salinity gradient. The speciation modelling of U showed that calcium uranyl carbonate complexes dominate along the Scheldt estuary (from 97 to 86% seawards) with increasing fraction of UO₂(CO₃)₃^4- (from 2 to 14%) towards the mouth.

New holder configurations for use in the diffusive gradients in thin films (DGT) technique

This study reports two new holder configurations that can be used in the diffusive gradients in thin films (DGT) technique, including a dual-mode holder and a new flat-type holder.

Application of DGT to high pH environments: uptake efficiency of radionuclides of different oxidation states onto Chelex binding gel

The DGT Chelex binding phase has not been tested for binding efficiency over the extreme high pH range (i.e., 10 to 13). Here, we examined the uptake efficiency of the gel-encapsulated Chelex cation exchange resin binding phase when in direct contact with solutions of radionuclides of different oxidation states over the circumneutral to high pH range (∼7 to 13). Results show that the Chelex binding gel is suitable for Eu(3+) for circumneutral pH, for UO2(2+) up to at least pH 10.7 and for NpO2(+) up to at least pH 11.7. Application may be appropriate at higher pH values but testing of complete solution deployment units will be required. This work provides the framework to use DGT as a tool for the study of high pH radionuclide systems.