Nanocellulose-based adsorbents for water remediation: role of surface functionalisation and isocyanate chemistry.

Water contamination by synthetic dyes, heavy metals, and emerging micropollutants has intensified the search for sustainable, high-performance adsorbents. Nanocellulose (NC) - available as cellulose nanocrystals, nanofibrils, and bacterial nanocellulose - has become a leading bio-based platform for water remediation owing to its high specific surface area, abundant surface hydroxyl groups, and amenability to chemical modification. The performance of NC sorbents is governed largely by surface functionalisation, yet existing reviews treat the available modification chemistries unevenly. In particular, isocyanate (carbamation) chemistry is almost invariably reduced to a single entry in a list of modification methods, despite being one of the few strategies able to combine, within a single platform, covalent crosslinking, tunable surface hydrophobicity, and the introduction of orthogonal reactive handles for ligand conjugation. This review addresses that gap. After surveying NC types, preparation routes, target pollutants, and adsorbent architectures (hydrogels, membranes, and aerogels), it provides a critical, water-remediation-focused analysis of NC isocyanate chemistry: the carbamation mechanism and competing side reactions, the reactivity of aromatic versus aliphatic isocyanates, site-selectivity in TDI and IPDI, structure-property relationships, documented applications in oil-water separation, dye removal, and heavy-metal sorption, and the role of isocyanates as molecular bridges in multifunctional composites. The toxicological profiles of diisocyanates and emerging non-isocyanate polyurethane (NIPU) alternatives are critically assessed, and the principal research gaps and priorities are identified.