New surface cross-linking method to fabricate positively charged nanofiltration membranes for dye removal

BACKGROUND: At present, most commercial nanofiltration (NF) membranes are negatively charged. An effective way to prepare a positively charged NF membrane is to cross-link polyimide (PI) substrate with polyethyleneimine (PEI). However, the lack of an efficient process for selecting proper cross-linking conditions is an important issue. Therefore, the major objective of this work is to develop an efficient method to cross-link only the top surface of the membranes. RESULTS: The optimum cross-linking condition was found to be at a cross-linking temperature of 70 °C, cross-linking time of 80 minutes and stirring speed of 500 rpm. Comparing the membrane before and after cross-linking, the molecular weight cut-off changed from 17520 Da to 458 Da and the membrane surface changed from positive from negative. The cross-linked membrane showed high rejection (>95%) for basic dyes with different molecular weights and relatively high antifouling properties to these dyes (flux recovery ratio > 92%, total flux decline ratio < 26.7%, irreversible flux decline ratio < 7.3%). CONCLUSIONS: Stirring speed was found to greatly influence the membrane performance. It was found that there exists an optimum stirring speed at which the cross-linker distributes evenly on the membrane surface for the most effective cross-linking to take place. The newly developed surface cross-linking method also may have the potential to modify other types of membrane surfaces.