Production of ultra pure water by desalination of seawater using a hydroxy sodalite membrane

A hydroxy sodalite membrane has been synthesized and employed in pervaporative desalination of seawater at 2.2 MPa feed pressure over a temperature range of 303–473 K. The membrane's performance was also tested with aqueous solutions of NaCl and NaNO3 at 473 K and 2.2 MPa cross membrane pressure, evaluating the effect of salt concentration on permeation flux and solute rejection. In total the membrane was used for over 100 h. In all cases the membrane showed salt rejection better than 99.99%; the collected permeate was ultra pure water with a resistance of 18.2 MΩ (0.1 μS/cm) at 25 °C. Flux through the membrane in seawater desalination showed a steep increase with feed temperature, exceeding the flux values obtained for pure water. Increasing the salt concentration in NaCl and NaNO3 aqueous solutions gave opposite trends in flux through the membrane. In case of NaCl, the flux increased with increasing salt concentration in the feed, whereas the opposite was found for NaNO3 solutions. The latter is attributed to a decreased water activity, i.e. driving force for water permeation, and a blocking effect. It is speculated that in case of NaCl ion exchange between the zeolite and solution brings about compositional changes in the sodalite cage modifying the interaction with water, increasing the water mobility.