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

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>i.e.</i>, pH, initial metal ion concentrations, and adsorbent dosage). Furthermore, analysis of variance revealed that quadratic equations well predicted experimental data with an <i>R</i> ² value of >0.99 and a <i>p</i>-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.