From seaweed biorefinery residues to functional films: how H2O2/UV-assisted pigment removal reshapes polymer networks and material performance

Seaweed biorefinery residues are often regarded as cellulose-rich side streams; however, their conversion into functional films depends on how extraction and post-treatment steps preserve the residual polymer matrix. This study evaluated H 2 O 2 /UV-assisted pigment removal as both an optical-upgrading strategy and a matrix-modifying treatment for residue-based films. Solid residues remaining after furcellaran extraction from Furcellaria lumbricalis , alginate extraction from Saccharina latissima , and protein extraction from Ulva lactuca were processed by extrusion and hot pressing before and after pigment removal. The materials were characterized through compositional, spectroscopic, morphological, scattering, thermal, optical, mechanical, water-affinity, and barrier analyses. All untreated residues formed self-supporting films, but performance was strongly residue-specific. Furcellaria lumbricalis residues yielded the most cohesive films, with the highest tensile strength (8.0 ± 0.5 MPa), lowest water vapor permeability (2.4 ± 0.1 × 10 −13 kg m·Pa −1 ·s −1 ·m −2 ), low water uptake (12.1 ± 1.2%), and effective light shielding. Saccharina latissima residues produced more extensible but weaker films, whereas Ulva lactuca films showed higher water affinity and permeability. Pigment removal improved whiteness, but its functional impact depended on matrix architecture. Treated Saccharina latissima failed to form a continuous film, while treated Ulva lactuca showed the highest permeability and water uptake. These findings indicate that H 2 O 2 /UV-assisted pigment removal should not be regarded only as an optical treatment, since its impact depends strongly on the organization and composition of the residual polymer matrix: when cohesive protein-polysaccharide domains were preserved, film integrity and water resistance were maintained; when these domains were depleted, processability and barrier performance deteriorated. The results provide a structure-function basis for selecting seaweed biorefinery residues as sustainable film-forming matrices with packaging-relevant functionality.