MgO-Loaded Magnetic Crab Shell-Derived Biochar for Efficient Synergistic Adsorption of Heavy Metals and Dye: Characterization, Adsorption Performance and Mechanistic Study

The preparation of highly efficient adsorbents capable of simultaneously removing dyes and heavy metals is of great importance. Crab shell-derived biochar (BC) was successfully modified with magnesium and iron oxides (magnetic MgO@BC) via a simple impregnation–carbonization method. A series of characterizations revealed that magnetic MgO@BC possessed hierarchical porous structure with abundant oxygenated functional groups and good magnetic separability. The results of batch adsorption experiments showed that the actual maximum adsorption capacities of magnetic MgO@BC were 301.06, 1344.11 and 3232.10 mg/g for Cd2+, Pb2+ and CR, respectively. In addition, the adsorption of Cd2+, Pb2+, and CR exhibited minimal influence from pH and coexisting ions, except for Cd2+ adsorption, which was significantly affected by divalent cations. For Cd2+ and Pb2+ adsorption, the Langmuir model provided good fits for the adsorption isotherms, whereas CR adsorption was more suitable for the Freundlich model. The adsorption kinetic fitting results indicate that Cd2+ adsorption aligned well with the pseudo-first-order model, while Pb2+ and CR fitted better with the pseudo-second-order model. Regeneration tests revealed that after four cycles, Cd2+, Pb2+ and CR still maintained 85.87%, 52.43%, and 96.09% removal efficiencies, respectively. SEM, FTIR, XRD, and XPS results demonstrated that the mechanism for CR adsorption involved π-π interactions, electrostatic attraction, and hydrogen bonding. The adsorption mechanism of heavy metals was primarily governed by ion exchange, cation-π interactions, surface coordination, and coprecipitation mechanisms, where Pb2+ exhibited stronger and more preferential adsorption behavior. Binary adsorption experiments confirmed competitive and synergistic effects depending on pollutant pairs. This study offers a novel perspective on the preparation and mechanism of biochar materials for the efficient and synergistic removal of dyes and heavy metals.