Development of inorganic silica reverse osmosis membranes by using a counter-diffusion chemical vapor deposition method

Silica hybrid membranes have been developed for use as reverse osmosis (RO) membranes by using a counter-diffusion chemical vapor deposition (CVD) method. A silica source (phenyltrimethoxysilane; PhTMOS) and O₃ were provided at opposite sides of a porous alumina substrate at 300°C for 90 min. The RO permeation test was conducted for 100 mg L^-1 NaCl at 3.0 MPa. The highest NaCl rejection was 94.2% for a total flux of 1.7 kg m^-2 h^-1. The module length is an important factor in obtaining highly selective RO membranes. The short module (6 cm) was better because of the higher O₃ concentration in the module. The decomposition conditions of phenyl groups on the silica surface are discussed for the hydrolysis powder of PhTMOS. According to FT-IR measurements, phenyl groups remained on the silica surface after O₃ treatment for 90 min at 300°C, whereas the number of silanol groups decreased by approximately 30% upon O₃ treatment at 300°C. The high Na⁺ rejection can be explained by the reduction of the silanol groups in the membrane.