Study On Preparation, Modification Of Graphene Oxide Nanosheets And Their Adsorption Performance On Cadmium Removal From Aqueous Solution

Graphene is a novel two-dimensional sheet of carbon nanomaterial which is constructed from a monolayer of carbon atoms in six-member ring. Because of the specific dimensional structure graphene shows exceptional mechanical, electrical, optical and chemical properties. After the recognition of multifunctional properties, it has attracted extensive attention of the scientific community. As a chemically exfoliated graphene derivative, graphene oxide nanosheets inherit some excellent properties of graphene, such as a large surface area, high electron mobility and strong chemical stability. In contrary from graphene, graphene oxide nanosheets contain abundant oxygen-containing functional groups on their surfaces and edges. Consequently, graphene oxide nanosheets possess a strong adsorption capacity for heavy metals, organics and other pollutants. In addition, graphene oxide nanosheets are easy to be modified and can be functionalized.In this paper, graphene oxide nanosheets were prepared according to a modified Hummer’s method and purposively employed to adsorb Cd(Ⅱ) in aqueous solutions. The effects of some important factors on the adsorption reaction were studied, such as temperature, initial pH, dosage of adsorbent and so on. The adsorption characteristics and reaction mechanisms of graphene oxide nanosheets for the removal of Cd(Ⅱ) were also explored. Unfortunately, graphene oxide nanosheets were difficult to be separated and recycled from aqueous solutions. So a modification method was put forward in order to solve this problem. Thioglycollic acid was chose to modify graphene oxide nanosheets, and thus thiol modified graphene oxide nanosheets material was prepared. Characterization techniques including AFM, TEM, XRD, FITR, Zeta potential, XPS and Raman were used to characterize graphene oxide nanosheets and thiol modified graphene oxide nanosheets.The thiol modified graphene oxide nanosheets and graphene oxide nanosheets were laminated structure. Crumpled silk wave-like structures were obviously seen on the surface of graphene oxide nanosheets and thiol modified graphene oxide. Besides, the number of folds on the surface of graphene oxide nanosheets was more than thiol modified graphene oxide nanosheets. A multilayer structure can be clearly seen at the edge of the materials. There were a large number of oxygen-containing functional groups on the surface of graphene oxide, such as hydroxyl, carboxyl, carbonyl and so on. But in thiol modified graphene oxide nanosheets, the carboxyl of thioglycolic acid and the hydroxy graphene oxide nanosheets could lead to esterification. And then the thiol was connected on to the surface of graphene oxide.The effects of temperature on Cd(Ⅱ) removal by graphene oxide and thiol modified graphene oxide were found negligible. When the temperatures were 293.15 K,300.15 K and 313.15 K, the Cd(Ⅱ) removal efficiencies by graphene oxide were 90.94%,91.64% and 92.67%, respectively. But the Cd(Ⅱ) removal rate by thiol modified graphene oxide were remained 62.47%,63.80% and 64.30%, respectively.The Cd(Ⅱ) removal efficiency was found to follow in three stages of adsorption with the order of increase of adsorbent dosage as 0.02,0.05,0.10,0.20,0.30,0.40,0.50,1.00, 2.00,5.00 g/L. The first stage is a sharp increasing stage followed by a slow increasing stage. And the last is a stable stage. The adsorption data of Cd(Ⅱ) on graphene oxide and thiol modified graphene oxide were fitted well in Langmuir adsorption isotherm model.The initial solution pH was one of the most important parameters of Cd(Ⅱ) adsorption on graphene oxide and thiol modified graphene oxide. It was observed that the removal efficiency increased sharply when the pH value increased from 2.00 to 5.00. However, the removal efficiency of Cd(Ⅱ) remained nearly constant when the was pH> 5.00. Graphene oxide nanosheets presented excellent adaptability for Cd(Ⅱ) removal in different pH values. The removal efficiency of Cd(Ⅱ) could reach up to 90.05%, when the solution pH was 4.00. In contrast, the removal efficiency of Cd(Ⅱ) on thiol modified graphene oxide was increased from 7.32% to 73.97% when the pH increased from 2.00 to 6.00. But, the removal rate decreased to 63.63% when the pH value continued to increase to 7.00. Then, it began to further increase again when the pH was8.00 or higher. Finally, the removal efficiency of Cd(Ⅱ) reached up to 96.86%, when the pH value was 10.00.