Influence of sediment redox conditions on uranium mobilisation during saline intrusion

- Seawater intrusion promotes U mobilisation from oxic and reduced sediments.
- Initially reducing conditions result in stronger binding of U to the sediment.
- Mobilisation was pH dependent and kinetically controlled.
- Fe(III) phases formed during Fe(II) oxidation retard U release temporarily.

In the UK, several coastal nuclear sites have been identified as vulnerable to future sea level rise. Legacy contamination at these sites has accumulated in sub-surface sediments at risk of future seawater inundation and intrusion. Porewater salinization, changes in pH and the influx of oxygen into sediments may impact the stability of sediment associated uranium (U). In this study, saturated column experiments were performed to compare the mobilisation of U from oxic and reduced sediments into seawater under environmentally relevant flow conditions. Uranium release profiles were independent of the initial geochemistry of the sediments. Uranium release from the sediments was kinetically controlled, showing relatively slow desorption kinetics, with release initially limited by the impact of the sediments on the pH of the seawater. Significant U release only occurred when the pH was sufficiently high for the formation of U-carbonate complexes (pHoxic 6.3; pHreduced 7.5). Uranium was more strongly bound to the reduced sediments and after 400 pore volumes of seawater flow, release was more extensive from the initially oxic (46%) compared with initially nitrate reducing (27%) and iron reducing (18%) sediments. The products of iron cycling appeared to act as a buffer limiting U mobilisation, but the on-going dissolution of the Fe-phases suggests that they did not form a permanent protective layer.