Retention behavior of dissolved uranium during ultrafiltration: Implications for colloidal U in surface waters

While uranium (U) in natural waters is very soluble and highly mobile as U(VI), a colloidal, ultrafilterable, form of U was, at times, reported to be important. Laboratory experiments were thus carried out to examine the behavior of dissolved U during ultrafiltration (1 kDa, Amicon S10Y1) of both natural (containing colloids) and spiked artificial river waters (containing no colloids). In addition, the distribution of dissolved and colloidal U was determined for samples collected across a salinity gradient in Galveston Bay (Texas, USA) using ICP-MS. Results of laboratory experiments showed that the constant permeation model can be used to predict the ultrafiltration behavior of U in both natural and synthetic river waters. Most importantly, we found that low molecular weight (< 1 kDa) U can indeed be retained by a 1 kDa membrane by as much as 30–60%. This behavior is similar to that previously reported for SO4, a major anion in sea water, and suggests an artifactual retention of dissolved U through preferential rejection by negatively charged membranes. Concentrations of total dissolved U increased from 2.9 ± 0.9 nmol kg− 1 in the Trinity River freshwater endmember to 8–9 nmol kg− 1 in higher salinity estuarine waters of Galveston Bay. The annual export flux of dissolved U from the Trinity River was estimated to be 6.3 × 104 moles, corresponding to a weathering rate of 0.75 moles-U/km2/yr in the Trinity River basin. Colloidal U, derived using the ultrafiltration permeation model, accounted for ∼ 15% of the total dissolved U in river waters but was negligible in the higher salinity coastal waters. Therefore, apparent colloidal U concentrations calculated from the concentration difference between initial and permeate solutions, or measured directly from the retentate solution under low concentration factors (ratio of initial volume to final volume of retentate), can be significantly overestimated compared with truly colloidal U concentrations derived from the ultrafiltration permeation model. The association of dissolved U with nanoparticles and macromolecular organic matter in higher salinity seawater seems minimal and most dissolved U in seawater should be in the form of anionic U with a molecular weight < 1 kDa.