Magnetically Recoverable and Adsorptive Alginate Hydrogel Composite Beads Reinforced with Cellulose Nanocrystals

The World Health Organization estimates that approximately 1.9 million deaths occur annually due to the consumption of contaminated drinking water, underscoring the urgent need for sustainable, biobased materials for water purification. This study reports the synthesis and characterization of magnetically responsive, alginate-based hydrogels designed for pollutant adsorption and magnetic recovery. Magnetic cellulose nanocrystals (mCNC) were produced by coprecipitating Fe3O4 nanoparticles onto cellulose nanocrystals (CNCs) and subsequently embedded into an alginate hydrogel matrix at two loading concentrations (0.1 and 1% w/v). The resulting mCNC–alginate hydrogels, along with CNC–alginate and pure alginate hydrogels as controls, were fabricated in both thin film and bead forms. A comprehensive characterization was performed to evaluate the structural, magnetic, and mechanical properties. Scanning electron microscopy (SEM) revealed changes in surface morphology and pore structure, while Fourier transform infrared spectroscopy (FT-IR) confirmed chemical compatibility. Vibrating sample magnetometry (VSM) demonstrated superparamagnetic behavior of mCNC-loaded hydrogels, with a saturation magnetization of 9.85 emu/g for the 1% mCNC sample. Rheological characterization showed that CNC and mCNC incorporation significantly enhanced hydrogel rigidity and storage modulus, although no significant magnetorheological response was observed. Adsorption performance was evaluated using methylene blue (MB) as a model pollutant, given its relevance to the removal of persistent contaminants, such as per- and polyfluoroalkyl substances (PFAS). While pure alginate beads achieved the highest maximum adsorption capacity (1357 mg/g), CNC- and mCNC-loaded hydrogels demonstrated improved percent removal at intermediate concentrations. Kinetic modeling indicated that adsorption followed both the pseudo-first-order (PFO) and fractal-like pseudo-first-order (FL-PFO) models, consistent with a physisorption process influenced by diffusion limitations. These results highlight the potential of mCNC–alginate hydrogels as scalable, magnetically retrievable, and environmentally benign materials for advanced water purification. Their enhanced mechanical stability, tunable magnetic properties, and biobased composition make them promising candidates for sustainable water treatment technologies.