Cross-Linking in Keratin-Based Films Enhances Cationic and Anionic Dye Adsorption

Environmental concerns have increased with industrialization, and the growing demand for dyes has seen an exponential rise across industries like textile, leather, printing, etc. Most of the dyes are extremely toxic to the environment and thus affect both marine and terrestrial life. Sustainable dye removal technologies are, therefore, an urgent need. Most of the adsorbents selectively adsorb either cationic or anionic dyes, limiting their applicability. A simple yet effective approach is being reported to develop chemically cross-linked films from waste protein (human hair keratin) for both cationic and anionic dye adsorption. Keratin-based bioplastic films have been fabricated using waste human hair, and a comparative study with pristine keratin films was conducted. The cross-linked films exhibit improved surface morphology and mechanical properties compared to the pristine films by reducing the microcracks within the films. Secondary structural information using FT-IR and XRD suggests conformational changes in the protein from a predominant α-helix to β-sheet structure during film formation. The cross-linked films maintain thermal stability up to 200 °C and are biodegradable in the presence of soil bacteria. Batch adsorption studies of the films were performed to study the dye adsorption with three model dyes, i.e., methylene blue (MB), rhodamine 6G (R6G), and bromophenol blue (BB). Cross-linked films are able to adsorb both cationic and anionic dyes with higher efficiency and capacities compared with pristine films. The adsorption capacities increased upon cross-linking by ∼10% for methylene blue, ∼50% for rhodamine 6G, and ∼70% for bromophenol blue. Kinetic modeling and the modified Weber-Morris diffusion model demonstrate that intraparticle diffusion is the rate-limiting step of the dye adsorption process. While a full life cycle assessment would be required to precisely estimate the environmental impact, keratin-based films could be a promising pathway toward sustainability. This study demonstrates a strategy for developing an efficient and versatile dye-adsorbing material from waste-derived protein.