Efficient Removal of Cadmium (Cd²⁺) from Aqueous Solutions by Chitosan@<i>fig</i> Branch Biochar: Adsorption Performance and Enhanced Complexation

In this study, <i>fig</i> branch biochar (FBB) was modified by chitosan to improve the Cd²⁺ adsorption performance from an aqueous solution. The surface area, pores, and functional groups of chitosan-modified <i>fig</i> branch biochar (CMFBB) were characterized by DTG-TGA, BET, SEM-EDS, FTIR, and XRD. In addition, the impact of various conditions on the Cd adsorption performance of the biochar was analyzed, including dosage, initial pH, concentration, temperature, reaction time, and coexisting cations. The adsorption kinetics, isotherms, and practical applications were also investigated. Under the optimized conditions including pH 6, adsorbent dose 2 g L^-1, reaction time 90 min, Cd²⁺ concentration 100 mg L^-1, and 25 °C, the Cd²⁺ adsorption capacity was 62.25 mg g^-1, representing a 78.26% increase compared to FBB. The adsorption data for Cd²⁺ by CMFBB were found to be well-described by the pseudo-second-order kinetic and Langmuir isothermal models, indicating that a monolayer chemical adsorption process occurred. The Cd²⁺ adsorption was a spontaneous endothermic process, and the coexisting cations exerted negligible influence on the adsorption process. After five adsorption-desorption cycles, CMFBB maintained an adsorption efficiency of 92.3%, demonstrating excellent regeneration capability. The quantification of adsorption mechanisms suggested that physical adsorption, cation exchange, precipitation, complexation, and π-π interactions accounted for 3.70, 2.50, 33.04, 58.76, and 2.0% of the total adsorbed Cd²⁺ in CMFBB, respectively. Compared with FBB, the level of CMFBB increased 26.52% in complexation. This work implies that CMFBB has great potential as an effective absorbent for treating Cd²⁺ polluted water.