Adsorption of chitosan combined with nicotinamide-modified eupatorium adenophorum biochar to Sb³⁺: Application of DFT calculation

The pollution and harm of Sb³⁺ to aquatic systems is a global problem, so Sb³⁺ removal from the water environment to make sure environment safety and human beings wellbeing is of urgency. This study explored the effect of chitosan combined with nicotinamide-modified eupatorium adenophorum biochar (CEBC) on adsorbing Sb³⁺ through batch adsorption experiments. The experiments indicated CEBC's maximum adsorption capacity to Sb³⁺ is 170.15 mg·g^-1. Meanwhile, the capacity of the original biochar (EBC) is only 9.97 mg·g^-1. Compared with EBC, CEBC contains more functional groups, such as CO, -OH and -NH₂. In addition, the pseudo-second-order kinetic model and the Langmuir model are fit to describe the kinetics and isotherms of adsorption of CEBC to Sb³⁺, which suggests that the adsorption of CEBC to Sb³⁺ is dominated by monolayer chemisorption. Density functional theory (DFT) calculations confirmed that the chelation between -NH₂ and Sb³⁺ is of significance in the adsorption process of CEBC. DFT calculations also found that the newly added -OH and CO in EBC have a synergistic enhancement effect on the absorption of Sb³⁺. The mechanism of CEBC absorbing Sb³⁺ includes electrostatic interactions, pore filling, Л-Л interactions, hydrogen bonding, functional group complexation, chelation, and oxidation. CEBC has an excellent anti-interference ability for inorganic anions (NO₃^-, SO₄^2- and Cl^-) and can also use the coexisting HA to improve its adsorption performance. In addition, CEBC has better mitigation of Sb³⁺ on the performance of Sb³⁺ about its secondary release and good reproducibility, which indicates that CEBC is a viable Sb³⁺ adsorbent.