Freeze-induced chemical crosslinking of quaternized polysaccharides enables robust cryogels for continuous-flow removal of Congo red.

Continuous-flow adsorption utilizing macroporous cryogels is pivotal for the industrial-scale remediation of dye-containing wastewater. However, fabricating robust polysaccharide-based cryogels remains challenging due to the kinetic conflict between ice crystal growth and polymer crosslinking. Herein, we report a freeze-induced chemical crosslinking strategy for constructing robust quaternized polysaccharide-based cryogels. Using bis(vinylsulphonyl)methane (BVSM) as a crosslinker and microfibrillated cellulose (MFC) as a reinforcer, we successfully engineered a robust, macroporous quaternized agarose (QA) cryogel. Crosslinking experiments combined with density functional theory (DFT) calculations suggest that the β-hydroxyl group adjacent to the quaternary ammonium moiety provides a unique reactive site that lowers the activation barrier for Michael addition to BVSM, thereby enabling efficient covalent crosslinking under cryo-concentrated conditions. The resulting QA/MFC cryogel exhibits a highly interconnected macroporous structure with an average pore diameter of 30 ± 9 μm, a porosity of 88%, superhydrophilicity, and exceptional elasticity. Owing to the dominant contributions of electrostatic attraction and hydrogen bonding, the cryogel presents a high Congo red (CR) adsorption capacity of 1475 mg/g. Fixed-bed experiments further demonstrate efficient continuous-flow removal of CR and favorable fitting by the Thomas and Yoon-Nelson models. This work establishes a versatile strategy for preparing polysaccharide-based cryogels and provides mechanistic insight into freeze-induced chemical crosslinking of quaternized polysaccharides for water purification.