Preparation strategies of the positively charged nanofiltration membrane: A comprehensive review

Nanofiltration (NF) membrane with the positively charged property has received more attention because of its extensive applications in water softening, heavy metal removal, mineral extraction, dye recovery, etc. As a result, the number of publications on positively charged NF membranes has been rapidly growing in recent years. Despite great progress, a comprehensive review is not yet available. Therefore, this study critically reviewed the positively charged NF membrane in terms of fundamental concepts, preparation strategies, applications, challenges of the positively charged NF membrane. The merit and demerit of the preparation strategies for positively charged NF membrane were particularly discussed. It is pointed out that the phase inversion method using electropositive polymers as membrane matrix is suitable for the large-scale fabrication of positively charged hollow fiber NF membrane with high packing density. In addition, NF membrane with improved charge density can be obtained through secondary interfacial polymerization using positively charged electrolytes and alcohol as post-grafting monomers and solvents, respectively. Furthermore, the typical applications and challenges of the positively charged NF membrane were summarized and discussed. In summary, this comprehensive review provides a solid basis to direct the optimization and future development of the positively charged NF membrane.