Wenjun Gui, Yun Shi, Yanjun Cui
The extensive discharge of synthetic dyes from textile and pharmaceutical industries has created an urgent demand for eco-friendly and cost-effective purification technologies. In this study, a magnetic bio-based hybrid microgel adsorbent (SAFBCA) was fabricated by incorporating Fe3O4/biochar (FBC) and Aspergillus niger (A. niger) into a sodium alginate (SA) hydrogel matrix. Physicochemical characterization confirmed the formation of porous microspheres with a moderate specific surface area (8.22 m2 g⁻1) and sufficient magnetic responsiveness (9.31 emu g⁻1) for facile recovery. The adsorbent achieved a Langmuir maximum adsorption capacity of 120.22 mg g⁻1 for methylene blue (MB). Adsorption kinetics were best described by the pseudo-second-order model (R2 = 0.997), and the equilibrium data followed the Langmuir isotherm (R2 = 0.991). Thermodynamic analysis (ΔH° = -27.42 kJ mol⁻1; ΔG° < 0 at 298-318 K) confirmed a spontaneous, exothermic adsorption process. Comparative studies revealed removal efficiencies of 65% (SA), 70% (SA/A. niger), 78% (SA/FBC), and 83% (SAFBCA), indicating supplementary contributions of both FBC and A. niger encapsulation to the adsorption process. The composite exhibited strong affinity for cationic pollutants, achieving removal efficiencies of 83% for MB and 90.3% for Cu2+, while showing lower removal for anionic Cr2O72- (56.6%) and Congo Red (21.5%). Mechanistic analyses revealed that MB uptake proceeds via electrostatic attraction, hydrogen bonding, and π-π interactions. This study presents a magnetically recoverable composite integrating renewable polysaccharides, waste-derived biochar, and fungal biomass, which retains ∼76% regeneration efficiency after five cycles and offers a sustainable platform for cationic dye remediation.