[Preparation of Chitosan-modified Biochar and Its Adsorption Mechanism for Cd²⁺ in Aqueous Solution]

Chitosan-modified biochar （CBC） was prepared as a low-cost and highly efficient adsorbent for Cd²⁺ in aqueous solutions. Batch adsorption experiments were conducted to evaluate the adsorption performance. Characterization experiments with SEM-EDS， FTIR， and XPS were used to analyze the surface microstructure and chemical composition of the adsorbent. The results showed that the adsorption performance of CBC was remarkably improved by the introduction of surface functional groups （-OH， -C=O， and -NH₂）. The pseudo-second-order kinetic model and Langmuir model were better for describing the kinetics and isotherms for Cd²⁺ adsorption onto CBC， indicating that the adsorption rate was determined by the active sites and controlled by monolayer chemisorption. The adsorption process was endothermic spontaneous， and the key mechanisms involved complexation， precipitation， cation exchange， and cation-π bonds. After five instances of adsorption-desorption cycles， the adsorption capacity of CBC for Cd²⁺ still remained above 80% of the initial adsorption capacity， indicating that CBC had a favorable recyclability. The current work embodies the concept of green chemistry， and the prepared chitosan-modified biochar was a promising adsorbent for the removal of Cd²⁺ in wastewater and soil.