Carboxyl-Modified Nanocellulose Cryogels with a Wide Range and Fast pH-Responsive Switchable Wettability for Oil/Water Separation

In recent years, wettability materials with pH-responsive have attracted increasing attention in oil/water separation applications. However, these materials were limited by the pH range and infiltration time. Herein, a simple operational procedure is proposed to prepare banana nanocellulose cryogels with pH-responsive switchable wettability to realize these outstanding performances. Alkyl-modified cryogels (BCNC-MS) are obtained by adding methyltrimethoxysilane (MTMS) to the banana nanocellulose (BCNF) suspension. BCNC-MS are soaked in the carboxyl-modified solution to produce pH-responsive cryogels (BCNC-MS-SA). The carboxyl-modified solution is made from succinic anhydride (SA), (3-aminopropyl)triethoxysilane (KH550), and N,N-dimethylformamide (DMF) in a molar mass ratio of 1:1:18. The key to achieving the pH-response is the protonation and deprotonation of the carboxyl groups. SEM demonstrates that the modification keeps the three-dimensional porous structure of the cryogel, and the results of EDS, FTIR, and XPS show the success of alkyl and carboxyl modifications. BCNC-MS-SA can realize hydrophilic/underwater oleophobic (θ_water = 0°) and hydrophobic/underwater oleophilic (maximal θ_water = 135°) wettability transitions after treatment with different pH solutions. Compared with other pH-responsive oil/water separation materials, BCNC-MS-SA performs well in pH = 1 and pH = 13 environments, and the shortest infiltration time is only 3 s. With a porosity of 93.80%, BCNC-MS-SA possesses excellent adsorption capacity (10–40 g/g), oil/water separation efficiency (> 92%), and adsorption cycle performance (15 cycles) even for viscous oils. Moreover, BCNC-MS-SA has satisfactory stability. Cryogels are made of banana nanocellulose, and they are inexpensive and can be easily degraded. BCNC-MS-SA has great potential in practical applications such as oil removal and purification of oily wastewater.