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

Abstract

Abstract 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. © 2018 Society of Chemical Industry