Water Desalination through a Zeolitic Imidazolate Framework Membrane by Electro- and Thermo-Osmosis: Which Could Be More Efficient?

Pressure-driven reverse osmosis (RO) is commonly used for water desalination. Due to high pressure gradient (Delta P) involved in RO, however, there is continuous quest for other alternative techniques. In this study, electro- and thermo-osmosis are examined by atomistic simulations for water desalination through a zeolitic imidazolate framework (ZIF) membrane. Under an electric field of 1.5 V/nm, water flux is predicted to be 2.8 x 10(4) kg/(m(2).h), corresponding to a flux at Delta P = 40 bar in RO; but salt rejection is rather poor (50 %). With a temperature difference of 20 K between pure water and seawater, water flux of 5.7 x 104 kg/(m(2).h) is achieved in thermo-osmosis, which is equivalent to a flux at Delta P = 80 bar in RO; and salt rejection is up to 98 %. From the analysis of potential of mean force, it is revealed that the free energy barrier for water flow in thermoosmosis is low (2.8 kcal/mol), thus leading to a high water flux. The number of hydrogen bonds for water in the membrane is estimated to be 2.5 and the lifetime is about 0.9 ps. This study suggests that thermo-osmosis might be superior to electroosmosis, and it could be a promising alternative technique to RO for water desalination.