Natural zeolite clinoptilolite-phosphate composite Membranes for water desalination by pervaporation

Natural zeolite membranes, directly sectioned from mineral deposits, have been previously reported to show molecular sieve characteristics in selective separation of water and hydrated cations. Although a high removal of cations was observed, the low water flux was a limitation for any potential industrial applications. In this study novel clinoptilolite-based phosphate composite membranes were prepared by dry pressing of mixed powder materials followed by high temperature autoclave steaming and characterized by XRD, SEM-EDX. Their performance for pervaporative water desalination was examined using different levels of salinity in a temperature range of 25-95 °C and feed side pressure of 1 atm. At 1400 ppm Na feed salinity and 95 °C, a water flux of 15 kg/m2 h and over 95% removal of Na+ were obtained. The water flux achieved was about 10 times higher than the flux obtained using natural zeolite rock membranes. Introducing phosphate into the composite membranes appears to provide not only mechanical strength, through the possible chemical bonding between zeolite particles and the in-situ phosphate cement, but also to create a chemically favorable interface between zeolite crystals. Such interface might facilitate the desalination process bypassing the technical difficulties of ion leakage/diffusion through the inter-crystal spaces commonly associated with synthetic zeolite membranes.