Anisotropic Cotton-Stalk-Derived Hydrothermally Treated Cellulose–Chitosan Aerogels Toward Anionic Dye Adsorption and Water-in-Oil Emulsion Separation

Transforming agricultural residues into functional porous materials provides a sustainable strategy for wastewater remediation. Herein, cellulose was separated from cotton stalks via formic acid-assisted hemicellulose extraction and sodium chlorite delignification, and then sulfuric acid hydrolysis. Chitosan-assisted hydrothermally treated cellulose (CC) was prepared via hydrothermal treatment in the presence of chitosan. Anisotropic CC/chitosan composite aerogels were prepared via glutaraldehyde crosslinking and unidirectional freeze-drying. The hydrophilic CC/CS aerogel exhibited an oriented porous structure, a low density of 0.03 g cm−3, and a porosity of 85.33%. For Congo red (CR) adsorption, the equilibrium data were described well by the pseudo-second-order kinetic and Langmuir isotherm models, with a calculated maximum adsorption capacity of 483.09 mg g−1. Electrostatic attraction, hydrogen bonding, and pore-mediated retention jointly contributed to CR uptake. To realize oil–water separation, methyltrimethoxysilane (MTMS) vapor modification was applied to prepare hydrophobic aerogel (M-CC/CS). M-CC/CS presented an initial water contact angle (WCA) of around 134°, and the WCA remained above 115° after 600 s of water droplet exposure. The aerogel showed absorption capacities of 16.22–40.13 g g−1 toward various oils and organic solvents. Under gravity, M-CC/CS separated immiscible oil/water mixtures at a flux of 565.47 L m−2 h−1 and several water-in-oil (W/O) emulsions with efficiencies above 99.9% while maintaining high separation efficiency over 10 cycles. This work demonstrates a cotton-stalk-derived aerogel platform whose hydrophilic and hydrophobically modified forms can be used for dye adsorption and oily water treatment, respectively.