Development of nanosorbents derived from functionalized n-layered graphene nanosheets for dye removal

Abstract

In this work, n-Layer Graphene Oxide (nGO) was synthetized from graphite oxidation via modified Hummers method. Afterwards, the nGO was functionalized with diethylenetriamine (DETA) and FeCl3 at 180 oC to obtain the novel n-layer amino-iron oxide-functionalized graphene oxide (nGO-NH2-Fe3O4). Infrared (FTIR) and Raman spectroscopies, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Thermogravimetric Analysis (TGA) were performed to characterize both nGO and nGO-NH2- Fe3O4. These materials were then evaluated as adsorbents of the cationic dye Methylene Blue (MB) and the anionic Remazol Black B (RB) and Drimaren Red (DR) through a comparative approach. Adsorption experiments were performed in batch, and studies of kinetics, equilibrium, thermodynamic and sorbent regeneration were carried out to investigate the affinity of nGO, nGO-NH2-Fe3O4 and the amino-functionalized nGO supported in wood waste charcoal (nGO(NH)R/C) towards the MB, RB and DR. Analysis of the point of zero net charge (pHPZC) and pH effect showed that the nGO-NH2-Fe3O4 pHPZC was 8.2, hence the MB adsorption was higher at pH 12.0; however, the adsorption of RB and DR was not influenced by pH changes. Kinetics studies indicated that the system reached the equilibrium state after 5 min, 90 min and 120 min for MB, RB and DR, respectively. Adsorption capacity at equilibrium (qe) were 971.43±3.13 and 177.86±10.32 mg/g; and kinetic constant (kS) of 0.053±0.025 and 0.0024±0.0007 g/mg/min, for MB and RB, respectively. The pseudo-second-order model was better fitted to the experimental data. Equilibrium studies showed maximum monolayer adsorption capacity (qm) of 2319.20±269.37, 264.17±15.04 and 219.75±78.55 mg/g for MB, RB and DR, respectively; furthermore, Langmuir-Freundlich isotherm best fitted the adsorption for almost all system evaluated. Thermodynamic experiments revealed that all systems were spontaneous [Delta(Go) < 0] and evidenced the physical nature of the adsorption of MB [Delta(Ho) –150 kJ/mol] by nGO-NH2-Fe3O4, whilst the removal of RB and DR was characterized as chemisorption, with Delta(Ho) 92.83 kJ/mol and 66.43 kJ/mol respectively. Recycling experiments showed that the nGO-NH2-Fe3O4 maintained the MB removal rate above 95% after ten cycles. Amino-nGO was then anchored on vegetal charcoal with the objective to ease sorbent separation from aqueous medium. Despite the relatively lower adsorptive capacity towards MB dye (qm 52.98 mg/g) in comparison to nGO-NH2-Fe3O4, nGO(NH)R/C showed a quick decantation in comparison to nGO. Phytotoxicology studies on lettuce seeds evidenced an elevated germination and elongation when exposed to nGO-NH2-Fe3O4 suspension differently from pristine material, nGO. All the results showed sorbent high adsorption capacity and outstanding regeneration capability and evidenced the employment of novel nGO-NH2-Fe3O4 as a highly efficient adsorbent of textile dyes. Aiming to ease the separation of the adsorbent from the effluent after the adsorption, nGO(NH)R was supported in wood waste charcoal. Sedimentation tests showed that nGO(NH)R/C fully decanted within one day, differently from nGO, that decanted after four days. Furthermore, adsorption experiments with MB were carried out for nGO(NH)R/C and showed an equilibrium time of 20 min, maximum adsorption capacity in the monolayer of 54 mg/g and a spontaneous and exothermic behavior.