Application of ultra/nano filtration membrane on mining uranium from seawaters

Uranium exists in seawater at a concentration of 3.3 μg/L, forming highly stable Ca-UO₂-CO₃ and Mg-UO₂-CO₃ complex. Though dilute, this amounts to an estimated 4.5 billion tons of uranium. The objective of this work is to test the possibility to separate Uranium from other salts in natural and recomposed water systems by ultra/nano filtration process realized with inorganic membranes, and to understand the rejection mechanism, the interaction of membrane with solution species.Various physical-chemical parameters, such as MWCO, pH value of solutions, temperature, applied pressure, ionic strength/salinity, solution composition, were evaluated for their influence on metal rejections. Electric repulsion and steric effect are two mechanisms deducing the interaction of solution species and membrane surface thus the rejections. Speciation distribution of each solution was studied with phreeqC software. Concentrations of retentate and permeate were measured by ICPOES and ICPMS for determining the rejections. It was concluded that electric repulsion and attraction is the main mechanism of interaction between the membranes and solutions species. pH is the principal factor which influences the rejection of each species. There is a selectively big reject of U(VI) to Na at pH 3 and 8.25 in solutions of U and NaHCO₃. MWCO does not influence the rejections of U or Ca except that the IEP of different membranes can be slightly different. The salinity influences a lot the rejection of U in solutions presenting NaHCO₃ and NaCl (1-35 g/L). U rejection decreases with the increase of salinity, but is still considerably rejected until salinity 10 g/L. With the CaCl₂ solutions of increasing concentration at pH 3 and the same for MgCl₂ at pH 3, the salt concentration influences a lot the rejection of both Ca and Mg. Both these experiments prove that high salinity shields the electric repulsion, by consequent decreases the rejection. On the other hand, U can be rejected from sodium in solutions contained only U and Na from NaHCO₃, NaCl or Na₂SO₄ at pH 8. U can be separated from sodium salts and be concentrated in a concentration experiment, so as Ca and Mg in the solutions respectively of CaCl₂ at pH 3 and of MgCl₂ at pH 3. Finally, filtration experiments with complicate natural and recomposed solutions including seawaters, Rhone River and theses solutions doped with U were conducted. In all the natural or recomposed seawater solutions, none of the species (U, Na, Mg, Ca, K) was rejected. However, it was proved that with the inorganic membranes, the presence of CaCl₂ or MgCl₂ hinders the rejection of any species in solutions including U.