Adsorption and kinetics study on Rhodamine B removal over graphene oxide and reduced graphene oxide

This study investigates the adsorption behavior of 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, adsorbent dosage, and dye concentration, were performed. Graphene oxide was synthesized utilizing the well-established Hummers' method, while rGO was produced by reducing GO with hydrazine hydrate under ultrasonication. A combination of characterization techniques, including FTIR, XRD, Raman spectroscopy, FESEM, 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 at 10 min for GO and 15 min for rGO. The optimum dye removal efficiency for GO was observed at pH 4, achieving a removal percentage of 99.94%, whereas rGO exhibited a 99.83% removal at pH 8. Kinetic modelling indicated that the adsorption process followed a pseudo-second-order model, suggesting that chemisorption predominates the mechanism. The adsorption process was best described by the Redlich-Peterson (R-P) isotherm model, with β values of 0.811 and 0.880 for GO and rGO, respectively, indicating that the adsorption behavior is governed by heterogeneous surface interactions. The maximum experimental adsorption capacities were 9.99 mg g⁻¹ for GO and 4.80 mg g⁻¹ for rGO. Reusability studies showed that both adsorbents maintained significant activity over four 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.