Investigating the electron-scale adsorption mechanisms using DFT calculations and experimental studies in self-assembly magnetic biochar gel incorporated with graphene nanosheets for enhanced Sb(III) removal

Environmental contamination posed by trivalent antimony [Sb(III)] in water has been globally recognized as a complex challenge, garnering considerable public concern. To enhance the adsorption efficiency of pristine biochar (BC) for Sb(III), a novel Fe/graphene-loaded biochar (FeGB) gel was synthesized through a facile in-situ self-assembly method. This study aimed to investigate the adsorption performance and elucidate the electron-scale adsorption mechanism for Sb(III) by the FeGB-gel. The Sb(III) adsorption isotherm data fitted well with the Langmuir model, and the maximum Sb(III) adsorption capacity of FeGB-gel (113.1 mg g⁻¹) was significantly higher compared to that of BC (28.6 mg g⁻¹). Spectroscopic investigations revealed that surface complexation and π–π stacking were the key mechanisms for Sb(III) adsorption. Electrochemical analyses confirmed an enhanced electron-accepting capacity (0.815 mmol e^- g⁻¹) of FeGB-gel, linked to the formation of Fe-related functional groups (Fe–O and Fe–O–OH), which contributed to a stronger Sb(III) oxidation capacity than BC (78.5% v.s. 49.3%). Density functional theory calculations highlighted that the presence of defects on graphene nanosheets enhanced the anchoring of FeO_x on biochar, thereby elevating the Sb(III) adsorption energy of FeGB-gel to −1.96 eV. Additionally, the projected density of states profile suggested that the enhanced adsorption of FeGB-gel could be attributable to the orbital hybridization of Sb-p, O-p, and Fe-p/d orbitals (i.e., Fe–O–Sb bonding), which strengthened the electron transfer and chemical interaction during the Sb(III) removal process. The functionalization of biochar surface characteristics with Fe/graphene offers possibilities for a diverse range of biochar-based adsorbents and their application in addressing numerous environmental concerns.