Structure-property relationships of functionalized cellulose nanofibers and their composites for heavy metal ions and dye removal: A review

Water pollution by heavy metal ions and synthetic dyes is a critical challenge for ecosystems due to their toxic and difficult-to-decompose nature. Cellulose nanofibers (CNFs) have emerged as a sustainable material for wastewater purification due to their abundant availability, high surface area, and abundance of carboxylic (specifically TEMPO-oxidized) and hydroxyl groups, which enable targeted chemical modification. This article reviews progress in surface modification strategies for CNFs and their mechanisms for improving selectivity and adsorption capacity for the removal of charged pollutants. In heavy metal ion removal, their effectiveness and selectivity are enhanced through organic modification (grafting of electron-rich ligands such as L-methionine, EDTA, and tannic acid), inorganic modification (incorporation of magnetite or graphene oxide), and structural modification into high-porosity 3D aerogels. Heavy metal ion removal is driven by chemical and physical mechanisms, including chelation, ion exchange, electrostatic interaction, chemical precipitation, and adsorption-reduction. Meanwhile, grafting specific functional groups, silanization to impart hydrophilic/hydrophobic properties, and the synthesis of nanocomposites with nanoparticles (ZnO, TiO₂, Fe₃O₄) or metal-organic frameworks have been implemented to remove cationic and anionic dyes. The main mechanisms involve electrostatic interactions, hydrogen bonding, coordination, and enzymatic/photocatalytic degradation, which are influenced by pH, adsorbent dosage, porosity, and the initial concentration of charged-pollutants. Furthermore, 3D printing technology has emerged as a fast, accurate, and customizable solution for membrane fabrication. Optimization of porosity, formulation, and functional group selection is key to designing high-performance CNF-based membranes. Challenges and future research directions for sustainable air purification solutions are also discussed.