Green synthesis of reduced graphene oxide and their use on column adsorption of phenol from olive mill wastewater

Abstract

A novel reduced graphene oxide (RGO) synthesis using Verbena officinalis as a green, reducing agent was out-reaching. RGO was encapsulated in sodium alginate (SA) by cross-linking (SA-RGO beads). SA-RGO beads have been used to treat olive mill wastewater (OMWW). This effluent is rich in phenolic compounds that inhibit biological degradation and are toxic at high concentrations. The newly prepared SA-RGO beads were characterized by scanning electron microscopy (SEM),energy-dispersive X-ray spectroscopy (EDS), X-ray diffractometer (XRD), BET surface area analyzer, and Fourier transforms infrared spectroscopy (FTIR). In addition, the organic compounds of Verbena officinalis were determined by HPLC to reveal the reduction mechanism of graphene oxide (GO). Finally, batch and fixed-bed column adsorption tests were performed to assess the performance of SA-RGO beads. The kinetics, isotherm models and adsorption thermodynamics show that the pseudo-second-order and Freundlich best present phenol adsorption and the endothermic reaction. Thomas and Yoon-Nelson's models satisfactorily described fixed-bed column adsorption. The optimized adsorption parameters were 3.68 g L−1 of adsorbent dosage, pH of 4.0, adsorption time of 135 min, and temperature of 25 °C. The reusability of SA-RGO beads and regeneration experiments with HCl (0.5 M) were performed in fixed-bed reactors. The results showed a phenol adsorption capacity of 994 mg g−1 for an initial concentration of 4000 mg L−1.