Thiol-functionalized magnetite/graphene oxide hybrid as a reusable adsorbent for Hg2+ removal
© Bao et al.; licensee Springer. 2013
Received: 25 October 2013
Accepted: 12 November 2013
Published: 19 November 2013
A thiol-functionalized magnetite/graphene oxide (MGO) hybrid as an adsorbent of Hg2+ was successfully synthesized by a two-step reaction. It exhibited a higher adsorption capacity compared to the bare graphene oxide and MGO due to the combined adsorption of thiol groups and magnetite nanocrystals. Its capacity reached 289.9 mg g-1 in a solution with an initial Hg2+ concentration of 100 mg l-1. After being exchanged with H+, the adsorbent could be reused. The adsorption of Hg2+ by the thiol-functionalized MGO fits well with the Freundlich isotherm model and followed pseudo-second-order kinetics.
KeywordsMercury ion Magnetite Adsorption capacity Graphene oxide Hybrid
Due to the development and expansion of industry, pollution of heavy metals in water supplies increases in the recent years. The pollution is seriously threatening the ecological systems as well as human health. Among them, mercury is one of the most hazardous elements due to its toxicological and biogeochemical behavior[1, 2]. A lot of adsorbents have been employed to extract Hg2+ from the industrial wastewaters. For example, thiol-functionalized adsorbents exhibited a specific binding capability toward highly toxic heavy metal ions including Hg2+ due to the existence of the thiol groups[3–6]. While for iron oxides, their adsorption mechanism was attributed to the complexation of Hg2+ and surface hydroxyl group at the iron oxide/water interface[7–9]. Iron oxide nanocrystals can further enhance the adsorption capacities because of their high specific surface area[6, 10]. Another advantage of using iron oxide-based adsorbents is that they can be easily extracted from wastewater by applying an external magnetic force. However, few research works have reported on adsorbents with both adsorption effects. The emergence of graphene oxide makes such combination possible due to its abundant functional moieties (hydroxyl and carboxyl groups)[11, 12], which enable possible metal oxide deposition and functional organic group grafting on its surface[13–15]. In this work, we deposited Fe3O4 nanoparticles on graphene oxide and then grafted thiol groups on the Fe3O4/graphene oxide (MGO). The thiol-functionalized MGO exhibited relatively high Hg2+ adsorption capacity. The adsorbent could be separated from the water solutions easily and reused after it was exchanged with H+.
Chemicals and materials
Natural graphite (500 mesh), 98 wt.% H2SO4, 5 wt.% HCl aqueous solution, 30 wt.% H2O2 aqueous solution, acetone, and Na2CO3 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 1-Methyl-2-pyrrolidone (NMP), ferric acetylacetonate (Fe(acac)3), potassium permanganate (KMnO4), NaHCO3, 1-ethy-3-(3-dimethyllaminopropyl) carvodiimide hydrochloride (EDC), and 2-mercaptoethylamine (MEA) were purchased from Aladdin Reagent Company (Shanghai, China). Other reagents used were of analytical grades without further purification. Deionized water was used in all the processes of aqueous solution preparations.
Preparation of MGO
Graphene oxide (GO, 100 mg) was dispersed in 30 ml of NMP by ultrasonication at room temperature, and the mixture was heated to 190°C under an argon atmosphere. Fe(acac)3 (1.413 g, 4 mmol) was dissolved in 20 ml of NMP and added dropwise in about 1 h to the GO/NMP solution under vigorous stirring. The stirring was continued for another 4 h after the dropping was finished. After being cooled to room temperature, the mixture was washed three times using acetone and water alternatively. The precipitate was collected by magnetic separation and was then dispersed in water by ultrasonication. The resulting black powder was collected by freeze-drying.
Synthesis of thiol-functionalized MGO
MGO (10 mg) was dispersed in 10 ml of deionized water by ultrasonication in an ice bath. EDC of 50 ml and a Na2CO3-NaHCO3 (1:9) buffer solution were added to adjust the pH of the system to approximately 9. After carboxyl groups on MGO were activated in 1 h, a solution containing 100 mg of MEA was added dropwise to the system. With the protection of argon, the reaction lasted for 24 h. The precipitate was collected by magnetic separation and was then dispersed in water by ultrasonication. The resulting black powder was collected by freeze-drying.
The effects of the initial concentration of Hg2+ and adsorption time on the final adsorption capacity were tested to obtain the saturated adsorption capacity and dynamic adsorption curve. Thiol-functionalized MGO powder was added to 25 ml of water solution with different concentrations of Hg2+. NaOH was used to adjust the pH of the solution. While the temperature was kept stable by using a water bath, the samples were placed on a standard rocker and oscillated for given hours. The supernate was collected by magnetic separation for reproducibility test. After washing with diluted HCl (0.25 N), the thiol-functionalized MGO was re-immersed in the solution with an initial Hg2+ concentration of 100 mg l-1 and oscillated for 48 h.
The X-ray diffraction (XRD) pattern was taken on a D/MAX-RB diffractometer using Cu Kα radiation. Investigation of the microstructure was performed by transmission electron microscopy (TEM, JEOL JEM-2010 F, JEOL Ltd., Akishima, Tokyo, Japan). Water bath sonication was performed with a JYD 1800 L sonicator (100 to 2,000 W, ZhiXin Instrument Co., Ltd, Shanghai, China). Hg2+ concentration was determined by using a DMA-80 direct mercury analyzer (Milestone S.r.l., Sorisole, Italy).
Results and discussion
Thiol-functionalized MGO with magnetite nanoparticles was successfully synthesized using a two-step reaction. Thiol-functionalized MGO exhibited higher adsorption capacity compared to the bare graphene oxide and MGO. Its capacity reached 289.9 mg g-1 in the solution with an initial Hg2+ concentration of 100 mg l-1. The improved adsorption capacity could be attributed to the combined affinity of Hg2+ by magnetite nanocrystals and thiol groups. After being exchanged with H+, the adsorbent could be recycled. The adsorption of Hg2+ by thiol-functionalized MGO fits well with the Freundlich isotherm model and followed pseudo-second-order kinetics. The scheme reported here enables rational design of the surface properties of graphene oxide and can be used to synthesize other functionalized composites for environmental applications.
Superconducting quantum interference device
This work was supported by the Jiangsu Environmental Protection Project (no. 2012005).
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