Whitened Lignin-rich Cellulose Nanofibrils: Breaking the Color Constraint while Retaining High Lignin Content and Functionality

Lignin-containing cellulose nanofibrils (LCNFs) offer intrinsic UV shielding, improved hydrophobicity, and thermal stability, yet their dark appearance limits applications requiring high whiteness and faithful color rendering. Here, we report a strategy that breaks the long-standing linkage between lignin richness and dark coloration by combining lignin pre-enrichment with chromophore-selective oxidative whitening. Specifically, dilute H2SO4 pretreatment enriches lignin by removing acid-labile carbohydrates, and subsequent alkaline H2O2 bleaching eliminates chromophores with controlled lignin loss. Mechanical fibrillation then yields whitened LCNFs (wLCNFs) with high whiteness of 70-82 while retaining 20.20-25.20 wt % lignin; notably, the highest lignin wLCNF contains more lignin than the native feedstock (25.20 vs 23.40%). Within a moderate processing window, wLCNF films show ∼120 MPa tensile strength, reduced water vapor transmission, and strong UV blocking. As a proof-of-concept, adding 5 wt % wLCNF to a commercial sunscreen raises SPF from 15.8 to 37.6 without visible tinting.