Design and fabrication of silica-based nanofiltration membranes for water desalination and detoxification

Abstract

Inorganic membranes typically have higher mechanical, thermal and chemical stability than their polymeric counterparts. Therefore, in this work a recent transport model was used to investigate the potential of porous inorganic membranes in water desalination. Salt rejection was predicted using the Donnan-steric pore model, in which the extended Nernst-Planck equation was applied to predict ion transport through the pores. The solvent flux was modeled using the Hagen–Poiseuille equation by considering the electroviscous effect. This model showed that inorganic NF membranes cannot achieve the selectivity of the traditional reverse osmosis (RO) membranes, and can only approach the perm-selectivity of the less robust but thinner and more flexible polymeric NF membranes. Nevertheless, inorganic NF membranes with pore size of about 1.5 nm and ζ-potential between 5 and 20 mV allow for a good compromise between water flux and salt rejection. Therefore, silica-based membranes with such properties were fabricated by sol-gel deposition. Since we have recently reported that the chemical and hydrothermal stability of unsupported microporous silica membranes can be highly enhanced by TiO2-doping, two sols were used for membrane deposition: a 5%TiO2-doped silica polymeric sol and a pure silica reference sol. The 5%TiO2-doped silica membrane showed water permeability (2.3 liters per square meter per hour per bar (LMH bar−1)) more than 30 times higher and higher salt selectivity than the pure silica membrane. As predicted by our model, at the 5%TiO2-doped silica membrane had approached without reaching salt rejection of commercial polymeric membranes with similar water flux, but it showed good performances when compared with the already reported inorganic NF membranes. Moreover, the new membrane presented nearly complete retention towards two model micropollutants, namely caffeine and ibuprofen. By investigating limits and potential of microporous inorganic membranes water desalination and detoxification, this work provides new knowledge for their rational design.