Heavy metal adsorption by graphene oxide modified with 5-amino-3(2-thienyl)pyrazole using central composite design/response surface methodology (CCD/RSM)

Abstract

In this study, graphene oxide (GO) was chemically functionalized with 5-amino-3(2-thienyl)pyrazole (5-ATP), introducing oxygen-, nitrogen-, and sulfur-containing groups, to enhance adsorption performance and develop a multifunctional adsorbent (5-ATP-GO) for efficient removal of Cd(II), Hg(II), and As(III) from aqueous solutions. The GO and 5-ATP-GO composites were characterized by Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller analysis, Raman spectroscopy, and zeta potential analysis. The performance of 5-ATP-GO for heavy metal removal was evaluated by design of experiment to optimize operational parameters and assess adsorption capacity. Central composite design/response surface methodology analysis was applied for the optimization of adsorbing conditions, (i.e., pH, initial metal ion concentrations, and adsorbent dosage). Furthermore, analysis of variance revealed that quadratic equations well predicted experimental data with an R² value of >0.99 and a p-value of <0.05. Experimental optimization variables were a pH of 7.25–8.55, an initial metal ion concentration of 43.45–49.66 mg L⁻¹, and an adsorbent dose of 10–10.50 g L⁻¹. Results showed that most of the adsorption occurred within the first 30 minutes, during which only 0.2 g L⁻¹ of 5-ATP-GO successfully removed 79.8% of Hg(II), 86.5% of Cd(II), and 75.1% of As(III) ions from the solution. These rapid kinetics were further supported by high adsorption capacities of 213.5 mg g⁻¹ for Hg(II), 280.1 mg g⁻¹ for Cd(II), and 450.95 mg g⁻¹ for As(III), underscoring the superior uptake potential of 5-ATP-GO toward toxic metal ions. Furthermore, adsorption kinetics and isotherm studies revealed that the data were well fitted to the pseudo-second-order kinetic model and Freundlich isotherm, indicating that the adsorption process follows a heterogeneous chemisorption mechanism. Finally, comparative experiments with pristine GO and other conventional adsorbents confirmed the superior removal efficiency and enhanced performance of the 5-ATP-GO composite.