Temperature and Pressure Effects of Desalination Using a MFI-Type Zeolite Membrane

Zeolites are potentially a robust desalination alternative, as they are chemically stable and possess the essential properties needed to reject ions. Zeolite membranes could desalinate "challenging" waters, such as saline secondary effluent, without any substantial pre-treatment, due to the robust mechanical properties of ceramic membranes. A novel MFI-type zeolite membrane was developed on a tubular α-Al₂O₃ substrate by a combined rubbing and secondary hydrothermal growth method. The prepared membrane was characterised by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and single gas (He or N₂)permeation and underwent desalination tests with NaCl solutions under different pressures (0.7 MPa and 7 MPa). The results showed that higher pressure resulted in higher Na⁺ rejection and permeate flux. The zeolite membrane achieved a good rejection of Na⁺ (~82%) for a NaCl feed solution with a TDS (total dissolved solids) of 3000 mg·L^-1 at an applied pressure of 7 MPa and 21 °C. To explore the opportunity for high salinity and high temperature desalination, this membrane was also tested with high concentration NaCl solutions (up to TDS 90,000 mg·L^-1) and at 90 °C. This is the first known work at such high salinities of NaCl. It was found that increasing the salinity of the feed solution decreased both Na⁺ rejection and flux. An increase in testing temperature resulted in an increase in permeate flux, but a decrease in ion rejection.