Thiol-functionalized magnetite/graphene oxide hybrid as a reusable adsorbent for Hg2+ removal

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.


Background
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 Hg 2+ from the industrial wastewaters. For example, thiol-functionalized adsorbents exhibited a specific binding capability toward highly toxic heavy metal ions including Hg 2+ due to the existence of the thiol groups [3][4][5][6]. While for iron oxides, their adsorption mechanism was attributed to the complexation of Hg 2+ and surface hydroxyl group at the iron oxide/water interface [7][8][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][14][15]. In this work, we deposited Fe 3 O 4 nanoparticles on graphene oxide and then grafted thiol groups on the Fe 3 O 4 /graphene oxide (MGO). The thiol-functionalized MGO exhibited relatively high Hg 2+ adsorption capacity. The adsorbent could be separated from the water solutions easily and reused after it was exchanged with H + .

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 Na 2 CO 3 -NaHCO 3 (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.

Adsorption experiment
The effects of the initial concentration of Hg 2+ 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 Hg 2+ . 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 Hg 2+ concentration of 100 mg l −1 and oscillated for 48 h.

Results and discussion
GO was prepared from natural graphite using modified Hummer's method [16,17]. Fe 3 O 4 nanoparticles were deposited on graphene oxide by decomposition of Fe(acac) 3 in NMP solution ( Figure 1, step A) at 190°C [18].  Figure 2b). The TEM image (Figure 2b) of the product showed that GO was decorated with magnetite aggregates with a size of several tens of nanometers. In the synthesis process, carbon monoxide was generated at a relatively high temperature and partially reduced Fe 3+ to Fe 2+ . Then, the magnetite nanocrystals nucleated and grew at the oxygencontaining defects sites such as carboxyl, hydroxyl, and epoxy groups [14]. Finally, MGO was obtained. Thiol functional groups were grafted on the MGO by the reaction between MEA and carboxyl groups on GO activated by EDC (Figure 1, step B). Energy-dispersive X-ray spectroscopy (EDAX) analysis (Figure 2c) indicated the appearance of the sulfur element, indicating that the thiol groups were successfully grafted on MGO. Thus, the thiol-functionalized MGO was obtained after the reaction. The magnetic properties of the thiol-functionalized MGO were investigated using a superconducting quantum interference device (SQUID) magnetometer. Figure 3 shows the hysteresis loop of the thiol-functionalized MGO hybrids at room temperature (300 K). The saturation magnetization was 22.0 emu g −1 , which was much smaller than 92.0 emu g −1 , the saturation magnetization of bulk Fe 3 O 4 [19]. The reduction in the value of saturation magnetization could be attributed to the rather small size of magnetite and GO in the hybrids [20,21]. The remnant magnetization and coercivity for thiol-functionalized MGO were 0.74 emu g −1 and 11.89 Oe, respectively, which were ascribed to the superparamagnetic state of magnetite nanocrystals due to the size effect. Such superparamagnetic state of the adsorbent with small remnant magnetization and coercivity at room temperature could enable the adsorbent to be readily attracted and separated by even a small external magnetic field [22]. In fact, the thiol-functionalized MGO dispersed in water solution was easily extracted from water with a magnet (Figure 3b).
The adsorption kinetics of Hg 2+ by the thiol-functionalized MGO is shown Figure 4a. The initial Hg 2+ concentration  was 10 mg l −1 . The adsorbed capacity (Q) of Hg 2+ per unit mass was calculated using the following equation: is the amount of Hg 2+ adsorbed per unit of adsorbent (mg g −1 ); C 0 (mg l −1 ) and C t (mg l −1 ) refer to the initial concentration of Hg 2+ and the concentration of Hg 2+ after the adsorption, respectively; W (g) is the weight of thiol-functionalized MGO; V (ml) is the volume of the whole solution system. After a 48-h adsorption, the solution reached a state of equilibrium. Even GO alone had a certain adsorption capacity of Hg 2+ , which was due to the formation of exchanged metal carboxylates on the surface of GO [23], while the adsorption capacity of thiol-functionalized MGO was higher than those of GO and MGO. The improved adsorption capacity of thiol-functionalized MGO could be attributed to the combined affinity of Hg 2+ by magnetite nanocrystals and thiol groups. To determine the mechanism of Hg 2+ adsorption from an aqueous solution by thiol-functionalized MGO, the pseudo-firstorder and pseudo-second-order kinetic models were applied to interpret the adsorption data. The pseudosecond-order kinetics was presented as [24] where K 2 is the pseudo-second-order rate constant (g mg −1 ) and Q t is the amount of Hg 2+ adsorbed per unit of adsorbent (mg g −1 ) at time t. The t/Q t versus t plot shown in Figure 4b indicated that the adsorption of Hg 2+ by thiol-functionalized MGO followed the pseudo-second-order kinetic model, but not the pseudofirst-order kinetic model (Additional file 1: Figure S1a). K 2 and Q e were calculated to be 6.49E − 4 g mg −1 min −1 and 30.94 mg g −1 , respectively. To understand how Hg 2+ interacted with thiol-functionalized MGO, different adsorption isotherm models were used to fit the adsorption data. The data of Hg 2+ adsorption were fit with the Freundlich isotherm model, which can be expressed as [25] logQ e ¼ logK þ 1 n logC e where K and n are the Freundlich adsorption isotherm constants, which are related to the relative adsorption  capacity of the adsorbent and the degree of nonlinearity between solution concentration and adsorption, respectively. K and 1/n values can be calculated from the intercept and slope of the linear plot between logC e and logQ e . Based on the plot shown in Figure 5a, n and K were calculated to be 1.02 and 10.54, respectively. However, the data did not fit the Langmuir isotherm model very well (Additional file 1: Figure S1b), indicating that the adsorption of Hg 2+ by the adsorbent was not restricted to monolayer formations [26]. To test the reproducibility of the adsorbents, they were immersed in an aqueous solution with an initial Hg 2+ concentration of 100 mg l −1 for 48 h with oscillation. The adsorption capacity for the first-time immersion was calculated to be 289.9 mg g −1 . After being washed with diluted HCl, thiol-functionalized MGO was applied to repeat the exact same adsorption test. The obtained adsorption capacities were 282.4, 276.8, and 258.1 mg g −1 for the second-, third-, and fourth-time immersion, respectively, which were corresponding to 97.4%, 95.5%, and 89.0% of initial adsorption capacity. It indicated that the adsorbents could be reused.

Conclusion
Thiol-functionalized MGO with magnetite nanoparticles was successfully synthesized using a two-step reaction. Thiolfunctionalized 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 Hg 2+ concentration of 100 mg l −1 . The improved adsorption capacity could be attributed to the combined affinity of Hg 2+ by magnetite nanocrystals and thiol groups. After being exchanged with H + , the adsorbent could be recycled. The adsorption of Hg 2+ by thiol-functionalized MGO fits well with the Freundlich isotherm model and followed pseudosecond-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.

Additional file
Additional file 1: Figure S1. (a) Adsorption kinetics fits with the pseudo-first-order model (red line) and (b) adsorption isotherm fits with the Langmuir isotherm model (red line).