Comparative Analysis of Rhodamine B Adsorption Kinetics and Mechanism on Graphene Oxide and Reduced Graphene Oxide

This study investigates the adsorption behaviour Rhodamine B (RhB), a cationic dye, from aqueous solution onto synthesized graphene oxide (GO) and reduced graphene oxide (rGO). Systematic examinations of various parameters, including pH, contact time, catalyst dosage, and dye concentration, were performed. Graphene oxide was synthesized utilizing the well-established Hummer’s method, while rGO was produced by reducing GO with hydrazine hydrate under ultrasonication. A combination of characterization techniques, including FTIR, XRD, Raman spectroscopy, FE-SEM, and Zeta-potential measurements, was employed to analyze the structural and surface properties of both materials. The findings from the adsorption studies indicated that equilibrium was reached within 6 minutes for both GO and rGO. The optimum dye removal efficiency for GO was observed at pH 4, achieving a remarkable removal percentage of 98.12%, whereas rGO exhibited a 92.86% removal rate at pH 8. Kinetic modelling indicated that the adsorption process followed a pseudo-second-order model, suggesting that chemisorption predominates the mechanism. Isotherm analysis indicated a superior fit with the Langmuir model for both adsorbents, implying monolayer adsorption. GO demonstrated a higher adsorption capacity (19.92 mg/g) compared to rGO (5.186 mg/g), which is attributed to the more significant density of oxygen-containing functional groups present in GO. Reusability studies showed that both adsorbents maintained significant activity over six cycles, with rGO exhibiting enhanced stability. Density functional theory (DFT) calculation further supported the experimental results by confirming the favorable adsorption energy and electronic interactions between RhB molecule and GO/rGO surfaces. Additionally, a phytotoxicity evaluation through seed germination experiments indicated that treated water had minimal harmful effects, thus highlighting its environmental safety for potential repurposing.