(376au) Hydrophilic ZSM-5 Zeolite Membrane for Forward Osmosis

Forward osmosis (FO) is a promising membrane separation process in water treatment, osmotic membrane bioreactor, food processing and power generation. In these applications, FO has advantages with low energy consumption and less membrane fouling compared with reverse osmosis, nanofiltration and ultrafiltration. Although various types of polymeric membranes and mixed matrix membranes has been studied for FO operations, pure inorganic membranes for FO were rarely reported.

ZSM-5 is one of the most popular synthetic zeolite which is attracted attention as inorganic membrane material. ZSM-5, aluminum-containing MFI-type zeolite have been studied as membrane materials to separate water from alcohols and organic acids.　This type of zeolite possesses hydrophilicity and molecular sieving property. In this study, we demonstrated the potential of a hydrophilic ZSM-5 membrane for FO operations. This is the first report about ZSM-5 membrane for FO operations in our best knowledge.

ZSM-5 membrane was prepared on the outer surface of tubular a-alumina support (outer diameter, 10 mm; inner diameter, 7 mm; length, 30 mm; average pore size, 700 nm) by a secondary growth method. The support was seeded by using dip-coating method. The seeded support was set in a PTFE-lined autoclave with synthesis solution having the molar composition of Al₂O₃:240SiO₂:53.3Na₂O:8000H₂O, and then hydrothermal treatment was carried out at 453 K for 12 h under stationary condition. After the crystallization, obtained membrane was washed with boiling water for several hours and dried at 383 K overnight.

FO performance of the ZEM-5 membrane was investigated as follows. NaCl aqueous solution and distilled water were used as the draw solution (DS) and the feed solution (FS), respectively. The DS and FS were fed to the outside and inside of the tubular ZSM-5 membrane. Each flow rate was adjusted to be 20 mL min^-1. The fluxes of water and salt were calculated based on the changes of the volume of DS and of Na concentration in FS.

Dependencies of the of water and salt fluxes on the NaCl concentration in DS were investigated. The water flux increased from 0.96 to 2.2 L m^-2 h^-1 with increasing NaCl concentration from 2.0 to 9.0 wt% at 313 K. In contrast, the salt flux was almost constant ca. 0.6 g m^-2 h^-1 despite increasing DS concentration. The salt flux was very low even in the case of using 9.0 wt% DS. In addition, the water flux increased with rising membrane temperature from 298 to 333 K, and the salt flux was almost constant despite changing temperature.

The FO performance under acidic condition was also investigated. pH value of DS was adjusted to be 4.0 by adding hydrochloric acid. As a result, the water and salt fluxes hardly changed under acidic condition compared with those obtained under neutral conditions, suggesting that the ZSM-5 membrane kept its performance under such acidic condition. Consequently, we found the possibility for FO operation with zeolite membrane for the first time.