Enhanced sorption of trivalent antimony by chitosan-loaded biochar in aqueous solutions: Characterization, performance and mechanisms

Contamination of aquatic systems by antimony (Sb) is a worldwide issue due to its risks to eco-environment and human health. Batch sorption experiments were conducted to assess the equilibrium, kinetics and thermodynamics of antimonite [Sb(III)] sorption by pristine biochar (BC) and chitosan-loaded biochar (CHBC) derived from branches of Ficus microcarpa. Results showed the successful loading of chitosan onto biochar surface, exhibiting more functional groups (e.g., CO, -NH2, and -OH). Langmuir model well described the Sb(III) sorption isotherm experimental data, and the maximum sorption capacity of Sb(III) by CH1BC (biochar loaded with chitosan at a ratio of 1:1) was 168 mg g-1, whereas for the BC it was only 10 mg g-1. X-ray photoelectron spectroscopy demonstrated that CH1BC oxidized 86% of Sb(III) to Sb(V), while BC oxidized 71% of Sb(III). Density functional theory calculations suggested that the synergistic effect of exogenous hydroxyl and inherent carbonyl contributed to the enhanced removal efficiency of Sb(III) by CHBC. Key mechanisms for Sb(III) sorption onto CHBCs included electrostatic interaction, chelation, surface complexation, π-π interaction, and hydrogen bonding. Overall, this study implies that CHBC can be a new, viable sorbent for the removal of Sb(III) from aquatic systems aiding their safe and sustainable management.