The adsorption of Cr(VI) from water samples using graphene oxide-magnetic (GO-Fe3O4) synthesized from natural cellulose-based graphite (kusambi wood or Schleichera oleosa): Study of kinetics, isotherms and thermodynamics

Graphene oxide material has been studied widely as a novel adsorbent due to its unique surface structures, excellent physicochemical properties, strong affinity and high efficiency for the metal ions adsorption [Pang H, Wu Y, Wang X, Hu B, Wang X. Recent advances in composites of graphene and layered double hydroxides for water remediation: a review. Chem - Asian J 2019;14:2542–52; Liu X, Ma R, Wang X, Ma Y, Yang Y, Zhuang L, et al. Graphene oxide-based materials for efficient removal of heavy metal ions from aqueous solution: a review. Environ Pollut 2019;252:62–73]. The present report involves Cr(VI) adsorption studies using graphene oxide-magnetic (GO-Fe3O4) based on natural graphite made from kusambi wood from Timor Island, Indonesia. GO with large specific surface area and abundant functional groups was synthesized using Hummers modification method, while GO-Fe3O4 was synthesized using in situ co-precipitation method. Characterization of GO and GO-Fe3O4 material was carried out using FTIR, XRD and SEM-EDX. The maximum adsorption capacity is optimum at the use of 0.1 g adsorbent GO-Fe3O4, under conditions: pH 2 with contact time for 80 min and temperature of 298 K. Nine kinetic models, five ishotherm models and two adsorption thermodynamics were used to study the mechanism of Cr(VI) adsorption on the surface of GO-Fe3O4. The results of kinetics and isotherms modeling show that the adsorption mechanism of Cr(VI) on GO-Fe3O4 follows pseudo-second (PSO) order kinetics with a high regression coefficient (>0.99) and was well described by Langmuir isotherm with adsorption capacity of 3.197 mg/g. The thermodynamics data revealed the exothermic nature, a decrease in the randomness or irregularity on the surface of adsorbent during adsorption process and takes place through physisorption. Studies on isosteric enthalpy values show that the adsorption of Cr(VI) on the surface of GO-Fe3O4 is strongly influenced by the amount of adsorbate which tends to decrease in adsorption capacity as the amount of Cr(VI) ions increases.