Multilayer graphene oxide and its thio-functionalized derivative as adsorbents of furfural in a comparative approach

Multilayer graphene oxide (mGO) was synthesized from the oxidation of graphite using a modified Hummers' method. Subsequently, mGO was functionalized with dimethyl sulfoxide (DMSO) and potassium thiocyanate to obtain the new thiol-functionalized multilayer graphene oxide (mGO-SH). Characterization analyses using Infrared Spectroscopy (FTIR), Raman Spectroscopy, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Dynamic Light Scattering (DLS), and Thermogravimetric Analysis (TGA) were performed to unravell the properties of mGO and mGO-SH. Both materials were then evaluated as adsorbents for the organic contaminant furfural through a comparative approach. Batch studies on kinetics, equilibrium, thermodynamics, and adsorbent regeneration were conducted to investigate the affinity of mGO and mGO-SH for furfural. Initial adsorption assessment showed that graphene oxide was unable to adsorb furfural. After functionalizing mGO, it was found that the contaminant was adsorbed, and kinetic studies indicated that the system reached equilibrium after 5 minutes. The equilibrium adsorption capacity (q_e) was 636.93 mg.g⁻¹, and the kinetic constant k₂ was 0.01575 g.mg⁻¹.min⁻¹ for furfural. Moreover, the pseudo-second-order model best fit the experimental data. Equilibrium studies showed a maximum monolayer adsorption capacity of 722.12 mg.g⁻¹ for furfural, and the Langmuir–Freundlich isotherm provided the best fit for the adsorption in the evaluated systems. Thermodynamic experiments revealed that all systems were spontaneous [ΔGº < 0] and evidenced the physical nature of furfural adsorption by mGO-SH [ΔGº = -11.74 kJ.mol⁻¹]. Adsorbent recycling experiments showed that thiol-functionalized graphene oxide had a removal rate of over 40% after five cycles.