Preparation of chitosan-EDTA bifunctionally modified magnetic walnut shell biochar and study on its copper ion (Cu(ii)) adsorption performance.

Abstract

This study aims to address the limitations of existing adsorbents in treating copper(ii)-containing wastewater by developing a high-performance and highly practical adsorbent. A chitosan-EDTA bifunctional modified magnetic walnut shell biochar (E-CMBC) was synthesized <i>via</i> a three-step method: magnetic modification of walnut shell biochar with Fe₃O₄ nanoparticles, surface coating with chitosan, and grafting of EDTA <i>via</i> amide bond coupling. The physicochemical properties of E-CMBC were characterized using field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, Thermogravimetric (TGA) analysis and zeta potential measurement, and its adsorption performance for copper(ii) was systematically evaluated through batch experiments. Key results demonstrate that the maximum adsorption capacity of E-CMBC for copper(ii) reaches 130.8 mg g^-1, with 52.4% of the equilibrium capacity achieved within 15 min; the adsorption process conforms to the pseudo-second-order kinetic model and the Langmuir isotherm model, confirming that chemical chelation is the dominant mechanism, and thermodynamic analysis indicates that the adsorption process is spontaneous and endothermic, with entropy increase serving as the primary driving force. Notably, E-CMBC maintains high adsorption efficiency within the pH range of 3-5, exhibits strong selectivity against coexisting ions (Na⁺, K⁺, Ca²⁺, Mg²⁺), enables rapid magnetic separation within seconds, and retains 91.13% of its initial adsorption capacity after five adsorption-desorption cycles. By integrating the combined advantages of EDTA (high selectivity), chitosan (abundant complexation sites), and magnetic biochar (easy separation), this study fills the gaps in existing literature and provides an efficient, sustainable, and engineering-applicable solution for heavy metal pollution control and high-value utilization of agricultural waste.