Bulk cross-linked hydroxyethyl cellulose-silica composite membrane for acid-stable nanofiltration

This work focused on the construction of an acid-resistant composite nanofiltration membrane with the selective layer of bulk cross-linked hydroxyethyl cellulose (HEC)-silica and a support of porous polypropylene. Membranes were fabricated through in-situ sol-gel reaction and slot-die coating method employing tetraethyl orthosilicate (TEOS) as the silica precursor and cross-linking agent of HEC. Membrane structure and performance were tuned by controlling the hydrolysis time of TEOS and differentiating TEOS and HEC contents, and systematically characterized in terms of physico-chemical property, permeation attribution and acid stability. It was found that membrane performance was strongly correlated with the relative contents of C-O-Si and Si-O-Si bonds of the formed HEC-silica selective layer. High content of C-O-Si bond formed via condensation reaction of HEC and TEOS endowed the selective layer with denser network structure and good rejection ability. Both insufficient and excessive hydrolysis led to a poor rejection ability of formed membrane. The optimized HEC-silica membrane possessed an average pore size of about 1.43 nm within the scope of nanofiltration membrane, a high pure water permeability coefficient of around 10.5 LMH/bar, and a medium rejection of 85.2% to 500 mg/l Na2SO4 aqueous solution at 5.0 bar. No changes in both chemical structure and separation performance occurred with the HEC-silica composite membrane after being soaked in aqueous H2SO4 solution of 4.9 wt% for 7 days, while evident structure destruction and serious performance degradation occurred with the compared polyamide-based commercial membrane NF270. Long-term soaking test using 15.0 wt% H2SO4 aqueous solution for 30 days also demonstrated that the acid resistance HEC-silica membrane was comparable with those reported acid stable nanofiltration membranes. Furthermore, the HEC-silica membrane also maintained its water permeation and solute rejection abilities during a 30-day continuous filtration of aqueous solution of 4.9 wt% H2SO4 and 50 mg/l polyethylene glycol fraction (PEG2000). The good acid resistance of the HEC-silica composite membrane was attributed to its bulk cross-linked network structure composed of C-O-Si, Si-O-Si and C-C bonds.

Automated summary

This work focused on constructing an acid-resistant composite nanofiltration membrane with a dense network structure and good acid stability, which is suitable for the field of nanofiltration.