The utility of L-cysteine-functionalized graphene oxide for the adsorptive removal of 4-nitrophenol from environmental water: An experimental and theoretical study

Abstract

Industrial wastewater contains numerous hazardous chemicals, such as phenolic compounds, which pose significant threats to the aquatic life and the environment. In this work, L-cysteine-functionalized graphene oxide (GO@L-Cys) was prepared and employed for the adsorption of 4-nitrophenol (4-NP) in a batch-mode system. The adsorption process was optimized by varying key parameters, including contact time, adsorbent dose, salt effects, temperature, initial pH, and adsorbate concentration. The as-prepared material was characterized using SEM, FTIR, XRD, UV–Visible, and nitrogen adsorption–desorption analysis. Kinetic models [pseudo-first order (PFO), pseudo-second order (PSO), Elovich, and intraparticle-diffusion] and isotherms models (Langmuir, Freundlich, and Sips) were applied, revealing that the PFO model and Sips isotherm best described the adsorption behavior. The experimental maximum adsorption capacity at 323.15 K was determined to be 718.67 mg g⁻¹, with the adsorption occurring endothermically and spontaneously, as confirmed by thermodynamic analysis. The adsorption mechanism was further explored through both experimental analyses—including FTIR spectroscopy before and after adsorption, the effect of initial pH,—and theoretical approaches, including density functional theory (DFT) and molecular dynamics (MD) simulations. Additional efforts were made to describe the molecular interactions, bonding characteristics, and electron density distributions using Visual Molecular Dynamics (VMD), Multiwave function, and reduced density gradient (RDG) analysis. Theoretical results indicated that the high adsorption capacity of GO@L-Cys for 4-NP is mainly attributed to hydrogen bonding, van der Waals forces, and π–π interactions. Overall, GO@L-Cys is demonstrated to be an efficient material for the removal of phenol residues from environmental water.