Investigation of diffusion and binding properties of uranium in the diffusive gradients in thin-films technique

Environmental context Monitoring uranium concentrations and speciation in aquatic systems is important for pollution control and for environmental studies. Although an in situ speciation technique based on diffusion and uptake of uranium has been developed, known as DGT, there were uncertainties over some parameters affecting the accuracy of the measurements. This study resolved those uncertainties by investigating diffusion and binding properties of uranium in DGT and provided confidence in monitoring uranium in the environment. Rationale Diffusive gradients in thin-films (DGT) ha been used for uranium speciation measurements since 2006, but interpretational difficulties have arisen due to uncertainties in the pH dependence of U diffusion coefficients (D) within the diffusive gel. This work tested the hypothesis that differences in measured D values between laboratories are due to methodological artifacts. Methodology The properties of uranium binding and diffusion of U within DGT were systematically investigated at different conditions between pH 3–8 and ionic strength from 1 to 500 mmol L−1. Previous uncertainties were attributed to loss of U by adsorption within the diffusion cell, identified by mass balance calculations, and a binding efficiency dependent on the type of binding gel. Results The diffusion coefficient in an agarose cross-linked polyacrylamide gel measured using Metsorb DGT of 5.26 ± 0.17 × 10−6 cm2 s−1 agreed well with both values obtained using a pre-conditioned diffusion cell (< 7% difference) and by modelling. The binding efficiencies for Chelex DGT (BChelex = 0.86–0.89) and Fe-oxide DGT (BFe-oxide = 0.64 were smaller than for Metsorb DGT (BMetsorb = 1). No significant differences were found for BChelex at different pH. Discussion This work demonstrated that the diffusion coefficient of uranium is independent of pH and the binding gel used, provided the binding efficiency (B) is taken into account. These findings indicate that DGT equipped with either Metsorb or Chelex binding gels should be capable of measuring U for a wide range of environmental conditions provided the appropriate diffusion coefficients and binding efficiencies are used.