Recent developments in functionalized polymer NF membranes for biofouling control

Nanofiltration (NF) membranes have recently received growing interest in many sectors as an eco-friendly and effective separation technology. Nevertheless, biofouling resulting from the accretion and growth of bacteria over the NF membrane surface constitutes the major drawback affecting the NF membrane separation systems and prohibiting their use, especially in long term. In fact, under well-moderated growth conditions, many types of microorganisms occurring in aqueous environments (e.g., bacteria, fungi, algae, and protozoa) are prone to adhere onto the surface of the nanofiltration membrane, resulting in the formation of a biofilm, which in turn reduces the membrane's performance. Functionalization of NF membranes with specific materials/nanomaterials is one promising strategy to surmount this inherent problem. The objective of this review is to provide first a thorough study of the biofouling phenomenon of nanofiltration membranes by establishing the main mechanisms causing membrane biofouling and the proposed strategies for its control and monitoring. Next, recent developments in functionalizing polymer NF membranes with nanomaterials including carbonaceous materials, metal and metal oxides, proteins, enzymes, etc. as a powerful strategy for biofouling control are elucidated and discussed in detail. Further, physical and chemical fabrication methods for nanofiltration composite membranes and their practical applications are addressed as well as current issues and futures perspectives are also highlighted.