Adsorption Of Cu²⁺ By Graphene Oxide With Different Degrees Of Oxidation And Defects

Although Cu2+ is considered as an essential micronutrient for life, it can be toxic beyond permissible limits. There are many techniques to removal Cu2+. Generally, adsorption has been and still is the most often used and studied method because it is cheap, effective and easy-adaption. A lot of materials are well-known for efficient removal of Cu2+ from aqueous solution. Among them, graphene oxide(GO) is a promising adsorbent owing to its high surface area(theoretical value of 2620 m2·g-1). GO could be efficient to remove various types of metal ions from water through electrostatic interaction because of its abundant number of oxygen-containing functional groups, such as epoxy, hydroxyl, and carboxyl, which are negatively charged.To explore the relationship between the structure of GO and the adsorption characteristics of Cu2+ onto GO, a series of graphene oxides(GO) with different degrees of oxidation(GO1, GO5 and GO6) and defects(GO1–GO4) were prepared via improved Hummers methods and characterized by Fourier transform infrared spectroscopy(FTIR), Raman spectroscopy, X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), elemental analysis(EA) and atomic force microscopic(AFM). The effects of p H, initial concentration of Cu2+, contact time and ionic strength on the adsorption of Cu2+ onto GO were investigated by batch adsorption experiments.The results showed that oxygen-containing groups(i.e., epoxy, hydroxyl, and carboxyl) were generated in GO1-GO6 following oxidation and the C/O atomic ratios of GO1-GO6 were 2.00, 1.96, 2.00, 1.97, 2.59 and 4.64, respectively. The interlayer spacing of GO1-GO6 were 0.97, 0.92, 0.91, 0.89, 0.86 and 0.75 nm, respectively. The thicknesses of GO1-GO6 were 1.1, 1.2, 1.3, 1.0, 1.2 and 1.2nm, respectively. The order of oxidation degree of GO is GO1≈GO2≈GO3≈GO4>GO5>GO6 and the order of the extent of defect within the GO materials is GO6<GO5<GO1<GO2<GO3<GO4.The adsorption equilibrium of Cu2+ onto GO1-GO6 can be reached within 120 minutes and the adsorption capacities of Cu2+ on GO1-GO6 were greatly enhanced with the increase of the p H, temperature and initial concentration of Cu2+. The adsorption capacities of Cu2+ on GO1–GO6 were 91.6,90.3,92.3,90.4,78.7 and 48.8 mg/g, respectively, at a temperature of 298 K, a p H of 5.5 and with an initial Cu2+ concentration of 50 mg/L. The results indicate that the available adsorption sites of GO were independent of the extent of the defects but increased as the degree of oxidation increase. Therefore, the adsorption capacities of Cu2+ onto GO were enhanced with an increase in oxidation but were independent of defect within GO. The adsorption isotherms of Cu2+ onto GO with different degrees of oxidation and defects were well-described by the Langmuir adsorption model, and the sorption processes fit well to a pseudo-second-order kinetic model. It is suggested that Cu2+ sorption onto GO results from homogeneous monolayer chemisorption, which is independent of both the degrees of oxidation and defect within the GO materials. The thermodynamic parameters calculated from temperature-dependent sorption isotherms indicated that Cu2+ sorption on GO was endothermic and spontaneous processes.The results of adsorption–desorption experiments showed that the desorption percentages of Cu2+ eluted from GO, which was enhanced by concentration of HCl, was 97.6% with using 1mol/L HCl. Furthermore, the adsorption efficiency of GO for Cu2+ decreased about 11% in five consecutive adsorption–desorption cycles, meaning that GO is qualified for practical application due to its good reusability.