Magnetic chitosan-modified apple branch biochar for cadmium removal from acidic solution

A magnetized functional biochar (MCBC) with excellent adsorption properties for Cd²⁺ removal from aqueous solutions was prepared using apple tree branches as feedstock and chitosan as the surface modifier. Batch adsorption experiment was carried out to examine adsorption performance of biochar before and after modification. Characterization techniques were employed to analyze physicochemical differences of biochar and to investigate Cd²⁺ adsorption mechanisms. The results showed that after chitosan modification and iron oxide loading, the surface of MCBC possessed abundant oxygen-containing groups and amino groups, which served as available active sites for Cd²⁺ removal. At a solution pH of 5.0, Cd²⁺ concentrations ranging from 5 to 300 mg/L, and a temperature of 298 K, the saturation adsorption capacity of MCBC (166.42 mg/g) was significantly higher than that of chitosan-modified biochar (132.28 mg/g) and original biochar (50.58 mg/g). The removal processes were effectively characterized by Pseudo-second-order model and Langmuir model. The primary mechanisms of Cd²⁺ adsorption included coordination, precipitation, and cation-π interactions. The removal processes were spontaneous, endothermic, and primarily driven by chemical interactions. After five regenerations, the removal amounts of MCBC for Cd²⁺ remained above 90%. The study suggested that magnetic chitosan-modified biochar was effectively applied for remediating cadmium-contaminated water.