Robust and lightweight biofoam based on cellulose nanofibrils for high-efficient methylene blue adsorption

Robust and ultralight biofoams had been successfully prepared using readily available and biocompatible cellulose nanofibrils (CNFs) as the matrix. The γ-glycidoxypropyltrimethoxysilane (GPTMS) was first added into the CNFs suspension to act as a crosslinker to form covalent linkages between cellulose chains. Then the gelatin was incorporated into the networks via reacting with the epoxy groups on GPTMS and forming hydrogen bonding with CNFs. The content of gelatin had a significant influence on the properties of the obtained foams. With the introduction of a small amount of gelatin, the foams exhibited significantly enhanced mechanical properties and stability in water in comparison with the CNF foams without GPTMS and/or gelatin. The foams containing the appropriate ratio of gelatin to CNFs possessed the high porosity (99.16%), ultralow density (0.0077 g/cm3), good mechanical properties, and abundant functional groups (hydroxyl and carboxylate groups). These features made it an ideal adsorbent toward methylene blue (MB) and the maximum adsorption capacity could reach up to 430.33 mg/g. Additionally, the cationic MB could be removed from the mixed cationic/anionic dye solutions with high selectivity, possibly due to the strong electrostatic interactions between MB and the adsorbent. Furthermore, the recycling test demonstrated the good reusability of this biofoam. Therefore, this work provides an environmentally benign method to fabricate robust CNF-based foams, which can be considered as green adsorbents for the treatment of water pollutants.