Isotopic Fractionation and Overall Permeation of Lithium by a Thin-Film Composite Polyamide Reverse Osmosis Membrane

A small, but significant fractionation of lithium isotopes was observed during reverse osmosis experiments performed with a thin-film composite polyamide membrane (FilmTec FT-30 membrane). Relative to the feed solution, the heavier lithium isotope 7Li was depleted in the permeate from 4.3 to 10.6 per mil (±3.3 per mil) during six experimental runs. The observed isotopic fractionation might be the result of the slightly different permeabilities of 6Li and 7Li across the FT-30 membrane. The heavier of two solute isotopes has a slightly lower mobility due to its slightly greater mass. Therefore, the heavy isotope permeates slightly more slowly inside the membrane than its isotopically lighter counterpart. As a result, more of the heavier isotope is retained on the high-pressure side of the membrane during each run. The magnitude of isotopic fractionation generally increased with increasing water permeation rates. Additionally, experimental evidence shows that the FT-30 membrane exhibits essentially identical solute rejection with NaCl and LiCl under the same operating conditions