Evaluation of a titanium dioxide-based DGT technique for measuring inorganic uranium species in fresh and marine waters

A new diffusive gradients in a thin film (DGT) technique for measuring dissolved uranium (U) in freshwater is reported. The new method utilises a previously described binding phase, Metsorb (a titanium dioxide based adsorbent). This binding phase was evaluated and compared to the well-established Chelex-DGT method. Batch experiments showed quantitative uptake (100±3%) of dissolved U by Metsorb and an elution efficiency of 95% was obtained using a mixed eluent of 1 mol L−1 NaOH/1 mol L−1 H2O2. The mass of U accumulated by Metsorb was linear (R2≥0.98) with time across the pH range 3.0-8.1, validating the DGT measurement. The measured effective diffusion coefficients were highly dependent on pH, ranging from 2.74-4.81×10−6 cm2 s−1, which were in reasonable agreement with values from the literature. Ionic strength showed no effect on the uptake of U, and thereby on diffusion coefficients, at NaNO3 concentrations ≤0.01 mol L−1, but caused the U concentration to be underestimated by 18% and 24% at 0.1 mol L−1 NaNO3 and 0.7 mol L−1 NaNO3, respectively. Deployment of Metsorb-DGT in synthetic freshwater resulted in reliable measurement of the dissolved U concentration (CDGT/CSol=1.05), whereas Chelex-DGT significantly underestimated the dissolved U concentration (CDGT/CSol=0.76). Metsorb-DGT was found to give reliable results after 8 h deployments in synthetic seawater but experienced competition effects with longer deployments. The Chelex-DGT was unable to measure U at all in synthetic seawater. A field deployment in a freshwater stream (Coomera River) confirmed the utility of the Metsorb-DGT method for measuring U in natural freshwaters, but performance of field deployments may require further evaluation due to the possibility of major changes in uranium speciation with pH and water composition. We recommend a filtered sample, of any water in which DGT measurements are to be made, be used to determine the appropriate diffusion coefficient under controlled laboratory conditions.