Influence of speciation during membrane treatment of uranium contaminated water

Membrane treatment can be used to selectively remove chemical species, such as uranium complexes from mine effluent. However, uranium speciation depends on different chemical factors such as inorganic and organic reaction partners, temperature, and pH, complicating a deeper understanding of the underlying mechanisms. Currently, membrane separation is not employed in the treatment of uranium-bearing effluent from the former uranium mines in Germany. In this study the potential of membrane separation for selective uranium removal was assessed for two real water samples. Speciation for such complex chemical conditions was determined with two independent methods in parallel: predictive modeling using the speciation codes “PhreeqC” and “EQ3/6”, and direct measurement using cryo-TRLFS (time-resolved laser-induced fluorescence spectroscopy). Different nanofiltration membranes and reverse osmosis membranes were characterized in a first step, for their potential rejection, and pure water flux. The best performing membrane was then employed in cross-flow experiments and reached retentions over 99% and U/Na-selectivities of 200. Uranium retentions showed a low dependency on feed uranium speciation. Continuing research is necessary for an exact determination of separation mechanisms for each membrane.