Fabrication of ultra-light graphene-based gels and their adsorption of methylene blue

• Porous 3D network, ultra-light graphene oxide (GO) gels were fabricated.
• GO–SA gel was reduced to RGO–SA gel by glucose.
• MB adsorption was researched including kinetic, isotherm and thermodynamic study.

Porous 3D network, monolithic graphene oxide (GO) gels were fabricated using a simple process of aqueous gel precursor freezing, solvent exchange, and ethanol drying. The randomly oriented GO sheets were consolidated by cross-linked sodium alginate (SA). The obtained GO–SA gel was reduced to RGO–SA gel by glucose. GO–SA and RGO–SA gels had low densities of 16.79 and 12.93 mg/cm3, respectively. The effects of contact time and temperature on the adsorption of methylene blue (MB) onto the gels were investigated. The kinetic adsorption fit the pseudo second-order and intra-particle diffusion models. The isotherm data followed the Langmuir and Freundlich models. The maximum adsorption capacity from the Langmuir isotherm equation reached 833.3 mg/g for GO–SA indicating that it is a good adsorbent for MB. The MB absorption mechanisms can be different for these two gels. Electrostatic interaction is the main interaction between the negative surface oxygen containing groups of GO and cationic MB, but π–π stacking is the primary interaction between the aromatic rings of RGO–SA and MB molecules.