Reusable Green Aerogels from Cross-Linked Hairy Nanocrystalline Cellulose and Modified Chitosan for Dye Removal

A novel biopolymer-based aerogel was developed by freeze-drying a hydrogel, synthesized from cross-linking bifunctional hairy nanocrystalline cellulose and carboxymethylated chitosan through a Schiff base reaction. The nanocelluloses, bearing aldehyde and carboxylic acid groups, facilitated the cross-linking with chitosan through imine bond formation while providing negatively charged functional groups, and chitosan was modified to accommodate carboxylic acid. The potential of this bioaerogel in environmental remediation was examined in a model system comprising methylene blue, a cationic dye. Electrostatic complexation between acidic groups on the anionic aerogel with the dye resulted in time-dependent dye adsorption, with long-time equilibrium dye concentration fitting well to the Langmuir isotherm, yielding a maximum adsorption capacity of ∼785 mg g^-1 and equilibrium constant K ∼ 0.0089 at room temperature. Dynamics of adsorption was modeled by numerically solving the unsteady-state diffusion-adsorption mass balance in a 1D spherical coordinate, which attested to a diffusion-controlled process with a Langmuir adsorption time constant τ_ads ∼ 7.6 s. To the best of our knowledge, this bioaerogel exhibits the highest removal capacity as yet for any reusable adsorbents prepared from biopolymers. Successful adsorption-regeneration cycles proved an excellent reusability, and the adsorption capacity remained constant over a wide pH range (e.g., pH > 7). This work may pave the way toward ultralight green functional materials.