Ultra/nano filtration (UF/NF) is widely explored for ion separation in aqueous solution, which can be effective, by the electric and steric effect. The uranium extraction from unconventional resources such as seawaters constitutes an important secondary source of uranium for nuclear fuel production. Commercial inorganic UF/NF membranes of different molecular weight cutoff (MWCO) have been investigated for separation of uranium from other salts in natural or synthetic seawaters and freshwaters. Various physical-chemical parameters, such as pH value, temperature, pressure, salt concentration, solution composition, are studied for their influence on different metal (U, Ca, Mg, Na, etc.) rejections. Speciation diagrams implemented with PHREEQC software have helped to understand the rejection mechanism. The pH value and salt concentration are two key parameters determining rejection since the high salinity shields the electric rejection. Experimental conditions for an efficient separation of U to Na was determined to concentrate the uranium. Filtration experiments with natural or synthetic seawaters and freshwaters show that the presence of CaCl2 or MgCl2 hinders the reject of every specie including U (VI). Three hypotheses based on the filtration experiments and theoretical simulations have been proposed to explain this phenomenon. This study examines the membrane electric effect to reject solution species and demonstrate the potential for uranium concentration by UF/NF.