The future of microporous membranes modified by functionalized carbon nanotubes (CNTs) for water and wastewater treatment: A review

The membrane-based process is a promising technology for augmenting freshwater supply and addressing global water scarcity. Its continuous advancement depends on developing novel nanomaterial-incorporated membranes, particularly those containing carbon nanotubes (CNTs). CNTs, widely used as fillers in polymeric membranes, enhance permeability, selectivity, and antifouling properties. Their unique fullerene structure contributes to increased surface porosity, improving filtration performance. However, their inherent hydrophobicity leads to poor dispersion in aqueous systems and agglomeration within membrane structures. The formation of percolation clusters in CNTs-based membranes significantly affects transport properties by introducing additional resistance pathways and altering selective layer dynamics. Previous studies indicate that CNTs concentration beyond a critical percolation threshold can lead to reduced permeability due to increased tortuosity [71]. The interplay between CNTs aspect ratio, dispersion state, and polymer compatibility determines the extent of these effects. To optimize CNT loading and avoid detrimental agglomeration, researchers have developed an array of novel strategies for surface functionalization and explored different types of controlled dispersion techniques [106]. To mitigate this, researchers have functionalized CNTs with polar surface groups before incorporating them into polymeric matrices. While numerous studies have examined CNTs functionalization for wastewater treatment, a comprehensive review focusing on their integration into microporous polymeric membranes is lacking. This review consolidates findings on various CNTs types as nanofillers, analyzing their impact on filtration efficiency (e.g., flux enhancement of 30-70 % and pollutant rejection rates exceeding 99 % for specific contaminants) and membrane durability (e.g., improved mechanical strength and up to 50 % longer operational lifespan). Additionally, we evaluate the trade-offs associated with emerging CNTs-based membrane fabrication techniques, particularly in scalability and cost-effectiveness. Finally, we explore the future potential of CNTsbased hybrid membranes tailored for water and wastewater treatment applications.