Synthesis of graphene oxide/citric acid/sodium alginate composite aerogel beads for high adsorption of methylene blue

Abstract

In this study, porous three-dimensional graphene oxide/citric acid/sodium alginate (GO/CA/SA) aerogel microspheres were prepared via a cross-linking and freeze-drying method using graphene oxide (GO), citric acid (CA), and sodium alginate (SA) as raw materials, and were applied for the adsorption of methylene blue (MB). The GO/CA/SA aerogel beads were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TG), scanning electron microscopy (SEM), zeta potential, energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Adsorption experiments revealed that the adsorption capacity of GO/CA/SA for MB reached a very high value of 994 mg g⁻¹. The adsorption behavior conformed to the Langmuir isotherm model. For a low MB concentration of 100 mg L⁻¹, adsorption onto GO/CA/SA essentially reached equilibrium within 120 min, achieving an adsorption capacity of 175 mg g⁻¹ and a removal efficiency of 87.5%, following a pseudo-second-order kinetic model. Combined with thermodynamic parameters, the adsorption process was identified as a spontaneous exothermic reaction dominated by chemisorption. Further analysis by XPS, FT-IR, and zeta potential confirms that the adsorption mechanisms of MB onto GO/CA/SA included hydrogen bonding, π–π interactions, and electrostatic forces. Moreover, in the presence of the anionic dye methyl orange (MO), the neutral dye rhodamine B (RhB), and humic acid (HA), GO/CA/SA retained good adsorption performance and selectivity toward MB. Using ethanol for desorption, GO/CA/SA were reused for five adsorption–desorption cycles with a MB removal >90% and an adsorption capacity >180 mg g⁻¹. GO/CA/SA aerogel beads demonstrate great potential as efficient adsorbents for the removal of MB from wastewater.