Vendula Smolikova PhD student vendula.smolikova@vub.be Acknowledgements: This work was supported by the project PROFISH CZ.02.1.01/0.0/0.0/16_019/0000869. The DGT passive samplers can not only overlap the lack of knowledge in the biogeochemical cycling of trace elements and their bioavailability for living organisms but they also represent a useful tool for long-term monitoring of trace metal concentrations in-situ. The field application of new DGT designs utilizing different sorbents is a crucial part of the technique validation. The salinity gradient found in estuaries represents an ideal location for evaluation of the DGT technique performance which may be hampered by the complex composition of seawater in contrary to freshwater. The potential of Diffusive Gradients in Thin films (DGT) technique as a monitoring tool for uranium in the aquatic environment V. Smolikova1,2, P. Pelcova2, A. Ridoskova2, M. Leermakers1 1Department of Analytical, Environmental and Geochemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium 2Department of Chemistry and Biochemistry, Mendel University in Brno, Zemedelska 1, 613 00, Brno, Czech Republic INTRODUCTION • 2 campaigns in 2019 and 2020, • 6 replicates of DGTs with each sorbent per station, • S01–HEM deploymen for 24 h, • HZ1 deployment for 28 days, • water samples collected at each station for comparison of the dissolved uranium concentration with the DGT-determined U concentration. MATERIALS & METHODS An improper selection of the binding phase can lead to providing skewed results that do not correspond to reality. The results of this study demonstrate that thorough testing of the DGT technique in a natural environment is a crucial prerequisite for obtaining reliable data and application of the DGT as a speciation tool. Using selective sorbents (such as Diphonix) may provide a DGT design that has the potential not only to quantify the labile fraction of uranium that may pose a risk to biota but may be used as a long-term monitoring passive sampler in seawater. CONCLUSIONS RESULTS OBJECTIVES How can be the performance of various DGT designs influenced by utilizing sorbents with different functional groups? Is any of the DGT design able to provide time- weighted average concentrations of U over the long deployment time in seawater? • Four tested sorbents: • Chelex -100, • Diphonix, • PIWBA, • Lewatit FO 36.UO22+OH-UO22+CO32-UO22+PO43-UO22+ Cl- UO2 2+ Downstream Upstream • Uranium shows typical conservative behaviour with increasing salinity along the Scheldt estuary. • Chelex-100 and Lewatit FO 36 are not suitable for application in seawater (salinity > 5 and 18, respectively). Both sorbents are not selective for U and their binding sites are therefore saturated by other competing ions occurring in the complex matrix of seawater. • PIWBA and Diphonix sorbents gels provide overall comparable results with very good agreement between the DGT-determined U concentration and dissolved U concentration at all stations. • Only DGTs utilizing Diphonix sorbent are able to provide reliable and accurate results for U determination even even after 28 days of deployment in seawater.station HZ128 PSU Salinity U (µg L−1) Station 0.0 1.0 2.0 3.0 4.0 5.0 0 10 20 30 2020 0.0 1.0 2.0 3.0 4.0 0 10 20 30 2019 HEM S04 S07 S01 S09 S12 S15 S22 U (µg L−1) Salinity HEM S04 S07 S01 S09 S12 S15 S22 Station Diffusion of labile U species from the solution 0 2 4 6 8 10 12 14 16 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 S22 S15 S12 S09 S07 2020 0 5 10 15 20 25 30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 HEM S22 S15 S12 S09 S07 S04 S01 2019 0.0 1.0 2.0 3.0 2 28 U (µg L−1) Deployment time (days) Diphonix Chelex-100 Dow-PIWBA Lewatit FO 36 Dissolved Chelex-100 Chelex filter Diphonix PIWBA Lewatit FO 36 Dissolved Salinity