Adsorption of heavy metals using cysteine-modified pillared montmorillonite: Mechanisms and characteristics

Montmorillonite enhanced its adsorption performance through the modification with organic and inorganic materials. Among various modifiers, cysteine was known for its significant ability to bind heavy metals due to the presence of thiol and amino functional groups. This study used X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and a series of adsorption experiments to examine the interaction between FeCl ₃-AlCl ₃ pillared montmorillonite (P Mt) and cysteine (Cys), and to investigate the adsorption characteristics and mechanisms of the composite of pillared montmorillonite and cysteine (P-Mt-Cys) toward heavy metals. The XRD and FTIR results indicated that cysteine intercalated into the layers of the pillared montmorillonite and, through its functional groups, dominated the surface of the P-Mt-Cys composite. The pillaring material slowed down the rate at which cysteine entered the montmorillonite and established pH 5.0 as the optimal electrochemical condition for cysteine intercalation. The adsorption of cysteine onto P Mt followed a pseudo-second-order kinetic model, and the Dubinin-Radushkevich (D-R) isothermal adsorption model provided a good fit for the data. In the pH range of 5.0–5.5, P-Mt-Cys exhibited superior adsorption of Cu(II) compared to unmodified Mt. and P Mt. The adsorption of Cu(II) on P-Mt-Cys reached equilibrium within 120 min. The Freundlich isothermal adsorption model best described the multilayer adsorption of Cu(II), with a maximum adsorption capacity of 27.02 mg g ⁻¹ at an equilibrium concentration of 81.91 mg L ⁻¹ . The adsorption process was exothermic and spontaneous, and the adsorption amount decreased as the temperature increased. Additionally, both the enthalpy change and entropy change were negative. These findings established the foundation for understanding the adsorption characteristics of heavy metals and offered a reference for further studies on the Mt-Cys-heavy metal system.