Synergetic effect of heteroatoms doping and functional groups of graphene-chitosan magnetic nanocomposite on enhancement of heavy metal sorption

Abstract

The efficient removal of heavy metal ions from aqueous solutions remains a significant environmental challenge. In this study, a nitrogen (N) and sulfur (S) co-doped graphene (G) wrapped chitosan(CS) magnetic nanocomposite (NS-MGC) was synthesized via a scalable hydrothermal method. Thiourea acted as both a reducing and doping agent, while chitosan, a biodegradable and cost-effective biopolymer, provided additional functional groups for metal binding. The incorporation of Fe₃O₄@SiO₂ nanoparticles enabled rapid magnetic separation, enhancing recyclability.

The NS-MGC nanocomposite demonstrated exceptional adsorption capacities of 421.78 mg·g⁻¹ for Zn²⁺, 318.44 mg·g⁻¹ for Cd²⁺, and 169.58 mg·g⁻¹ for Pb²⁺. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption, while equilibrium data were best fitted to the Freundlich isotherm, confirming multilayer adsorption on heterogeneous surfaces. The nanocomposite demonstrated excellent stability, retaining high efficiency over multiple reuse cycles. Mechanistic investigations revealed that electrostatic interactions, ion exchange, and surface complexation predominantly governed the adsorption process. The adsorption efficiency was largely influenced by the basicity of N-donor sites and the acidity and ion-exchange ability of –COOH⁺ groups, ensuring high adsorption even at elevated metal ion concentrations. These findings establish NS-MGC as a highly efficient, scalable, and reusable adsorbent for heavy metal removal in aqueous environments.