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Synthesis of a multifunctional graphene oxide-based magnetic nanocomposite for efficient removal of Cr(VI) Dongfang Wang, Guilong Zhang, Linglin Zhou, Min Wang, Dongqing Cai, and Zhengyan Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01293 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017
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Synthesis of a multifunctional graphene oxide-based magnetic nanocomposite for efficient removal of Cr(VI) Dongfang Wang,†,§ Guilong Zhang,†,‡ Linglin Zhou,†,§ Min Wang,†,§ Dongqing Cai,*,†,‡ Zhengyan Wu*,†,‡ †
Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science,
Chinese Academy of Sciences, 350 Shushanhu Road，Hefei，Anhui 230031, People’s Republic of China §
University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui
230026, People’s Republic of China ‡
Key Laboratory of Environmental Toxicology and Pollution Control Technology of
Anhui Province, Hefei Institutes of Physical Science, Chinese Academy of Sciences, 350 Shushanhu Road, Hefei, Anhui 230031, People’s Republic of China
1
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ABSTRACT: A novel magnetic nanocomposite was synthesized using graphene oxide (GO), polyethylenimine (PEI) and Fe3O4 to removal hexavalent chromium (Cr(VI)) from water and soil. Therein, GO was functionalized with plenty of –NH2 by the modification of PEI through amidation reaction, and the resulting GO/PEI reacted with FeSO4·7H2O and NaBH4 to obtain RGO/PEI/Fe3O4 (the optimal one is designated as ORPF) through oxidation-reduction reaction. ORPF could effectively adsorb Cr(VI) through electrostatic attraction, and the adsorbed Cr(VI) ions were partly reduced to trivalent chromium (Cr(III)) with low toxicity by RGO (π electron). Afterward, the resulting ORPF-Cr could be removed from water conveniently by a magnet, achieving the maximum Cr(VI) removal capacity of 266.6 mg/g. Importantly, ORPF, once carried by sponge particles, could efficiently remove Cr(VI) from soil and the resulting mixture could be facilely collected by a magnet on a filter net. Besides, leaching experiment suggested that, when supported by a filter paper, ORPF was able to decrease the leaching amount of Cr(VI) ions and meanwhile reduce them to
Cr(III).
This
work
provides
a
promising
approach
to
remediate
Cr(VI)-contaminated water and soil using a nanocomposite, which has a huge application prospect.
2
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INTRODUCTION Hexavalent chromium (Cr(VI)), a stable state of chromium, is commonly presented in wastewaters from factories of electroplating, metal finishing, pigment, tannery, chromium mining, and so on.1,2 Cr(VI) is rather toxic and can cause liver damage, pulmonary congestion, severe diarrhea, and skin irritation.3-6 Trivalent chromium (Cr(III)), another stable state of chromium, displays obviously lower toxicity, carcinogenicity, solubility, and mobility compared with Cr(VI).7 Therefore, it is crucial to remove Cr(VI) from wastewater or reduce Cr(VI) to Cr(III) prior to discharge into the environment.2,7 Until now, several works for the removal of Cr(VI) have been reported mainly through physical (electrical enrichment and washing), chemical (reduction and adsorption), and biological (plant enrichment) methods.8-11 Therein, the physical method has disadvantages of complex-procedure and high-cost, and the biological method needed rather a long time, which greatly hindered the application of them in Cr(VI) treatment. In contrary, owing to the simple procedure and high efficiency, chemical methods have great potential for the removal of Cr(VI) and thus are attracting more and more attention.12 Recently, among the various chemical methods, fabrication of nanomaterials with functions of both adsorption and reduction to remove Cr(VI) becomes a research hotspot.13-18 However, these nanomaterials were commonly used to remove Cr(VI) from aqueous solution rather than soil, because the lack of ideal carriers made them difficult to be separated from soil.19 Furthermore, these nanomaterials were rarely used to control the leaching of Cr(VI) in soil and at the same time reduce Cr(VI) to Cr(III).20 Therefore, it is important to fabricate a nanomaterial-carrier system with performances of adsorption, reduction, and collectability simultaneously, and explore the corresponding application in soil. 3
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Graphene oxide (GO), a two-dimensional carbon nanomaterial, possesses an ultralarge specific surface area and abundant oxygen-containing groups (e.g., -OH and -COOH), and has been used as a highly efficient adsorbent to remove heavy metal ions.21-23 Polyethylenimine (PEI) could introduce a large number of -NH2 to GO through amidation reaction, so that the removal ability of GO for Cr(VI) could be effectively enhanced.4 Although GO/PEI has high adsorption and reduction abilities for Cr(VI), the resulting mixture could not be easily collected from water and soil after treatment towards Cr(VI). Herein, Fe3O4 was incorporated with GO/PEI to obtain RGO/PEI/Fe3O4 (the optimal one is designated as ORPF) which displayed a high removal ability on Cr(VI) through adsorption and reduction, and an excellent magnetic collectability from aqueous solution. Noteworthily, when ORPF was loaded in porous sponge, the resulting mixture could be conveniently collected from soil after treatment towards Cr(VI). Additionally, ORPF could be also loaded in filter paper to control the leaching of Cr(VI) and meanwhile reduce Cr(VI) to Cr(III). The removal mechanism and the optimal condition were investigated. This work provides a facile, efficient,
and
environmentally-friendly
approach
for
the
remediation
of
Cr(VI)-contaminated water and soil.
MATERIALS AND METHODS Materials: 1-Ethyl-3-(3-dimethylaminoprophy) carbondiimide hydrochloride (EDC·HCl), N-hydroxyl succinimide (NHS), and polyethylenimine (PEI, M.W. 1800) of analytical grade were obtained from Aladdin (Shanghai, China). Raw graphite of chemical grade and other chemicals of analytical grade were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). Deionized water was used throughout this work. Soil and sand were taken from Dongpu Island (Hefei, China). 4
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Preparation of GO, PEI, and Fe3O4 composites: Fabrication of GO: GO was synthesized using natural flake graphite by modified Hummers method.24 In brief, 1.0 g of graphite powder and 2.0 g of NaNO3 were mixed with 46 mL of concentrated H2SO4 in an ice-bath. Under stirring, 6.0 g of KMnO4 was added slowly to the mixture with the temperature lower than 20oC, and the resulting mixture continued to be stirred (300 rpm) for 1 h. After that, the suspension was transferred into an oil-bath (35±5oC) and stirred (300 rpm) for 40 minutes and then diluted with 96 mL of deionized water. Subsequently, the suspension was kept in an oil-bath at 95±5oC for 4h, and 200 mL of deionized water was added. Then, 6 mL of H2O2 (wt. 30%) was slowly added to the resulting suspension to reduce the residual KMnO4. Such mixture was filtered and the remained stuff (actually GO sample) was washed with 250 mL of HCl (v/v,10%) aqueous solution and 500 mL of deionized water. After that, the as-obtained GO was re-dispersed in deionized water and ultrasonically treated for 5 h and then centrifuged (4500 rpm) for 10 min. For further purification, the supernatant was dialyzed for one week to remove the residual salts and acids and the homogeneous GO solution was obtained. Preparation of GO/PEI composite: GO (51 mg), NHS (0.5 mmol), and EDC·HCl (0.25 mmol) were added to 40 mL of carbonate buffer (pH 9.5). After stirring for 10 h at room temperature, different amounts of PEI were added to the system respectively. After reaction for 12 h at room temperature, the resulting system was centrifuged and the residue was washed with deionized water for several times. The optimal GO/PEI composite (WGO:WPEI=51 mg:0.7 g) was obtained through Cr(VI) removal experiment and designated as OGP.25,26 Preparation of ORPF composite: OGP (34 mg), FeSO4·7H2O and NaBH4 with the molar ratio of FeSO4·7H2O to NaBH4 of 1:3 were added to 50 mL of deionized 5
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water.27,28 After stirring for 30 min, the resulting system was centrifuged and the residue was washed with deionized water for several times to obtain RGO/PEI/Fe3O4 composite. The optimal RGO/PEI/Fe3O4 composite (WOGP:WFe=1:0.8) was obtained through Cr(VI) removal experiment and designated as ORPF. Removal of Cr(VI) from aqueous solutions: Different samples (4.2 mg) were added to 30 mL of Cr(VI) aqueous solution respectively. After being shaken for 24 h, the solid and liquid were magnetically separated, and the residual concentration of Cr(VI) was measured by DPC spectrophotometric method.29 The influence of initial Cr(VI) concentration, pH, contact time, and temperature on the removal efficiency of Cr(VI) were investigated. After that, the removal capacity (qe, mg/g) of Cr(VI) was calculated according to equation (1): qe=(C0–Ce)V/m
(1)
where V (L) is the volume of the solution, m is the weight of sample (g), C0 and Ce are the initial and equilibrium concentrations of Cr(VI). Removal of Cr(VI) from soil suspension: 5 g of dry soil was added to 30 mL of Cr(VI) solution (80 mg/L, pH 4) in a centrifuge tube (50 mL). A given amount of ORPF was loaded in a number of sponge particles (total sponge weight of 4 mg, particle diameter of 2 mm) and the resulting ORPF/sponge particles were put into the soil-Cr(VI) suspension. The resulting system was shaken for 24 h and then the ORPF/sponge/Cr particles were collected by a magnet on a plastic filter, the rest was centrifuged (4500 rpm) for 5 min and the concentration of Cr(VI) in the supernatant was measured. All experiments were performed in quintuplicate. The removal efficiency (REsoil) of Cr(VI) was calculated according to equation (2): REsoil (%)=(C0-Ct)/C0×100%
(2)
Where the C0 and Ct are the initial and remaining Cr(VI) concentrations (mg/L) 6
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respectively. Removal of Cr(VI) from sandy soil: 350 g of sandy soil was mixed evenly with 30 mL of Cr(VI) solution (100 mg/L, pH 4) to obtain a soil/Cr(VI) mixture. A given amount of ORPF was loaded in a number of sponge particles (total sponge weight of 20 mg, particle diameter of 2 mm) and the resulting ORPF/sponge particles were put into the soil/Cr(VI) mixture. Then the system was stirred (200 rpm) for 24 h at room temperature and dried at 50oC overnight, and the resulting ORPF/sponge/Cr particles were collected by a magnet. Subsequently, the remaining soil/Cr(VI) mixture was placed in 1 L of deionized water and the resulting system was shaken for 24 h to make Cr(VI) dissolved completely. Afterward, the resulting system was centrifuged (4500 rpm) for 5 min and the concentration of Cr(VI) in the supernatant was measured. All experiments were performed in quintuplicate. The removal efficiency (REsandy soil) of Cr(VI) were calculated according to equation (3): REsandy soil (%)=(C0V0-CtVt)/(C0V0)×100%
(3)
Where the C0 and Ct are the initial and remaining concentrations (mg/L) respectively, V0 (30 mL) is the initial volume of Cr(VI) solution, and Vt (1 L) is the volume of added deionized water. Effect of ORPF on the leaching behavior of Cr(VI): Dry sand (50-100 mesh) was mixed with dry soil (50-100 mesh) at a certain weight ratio (Wsand/Wsoil=7:3), and the resulting sand-soil mixture (20 g) was put into a centrifuge tube (60 mL) with a hole (diameter of 2 mm) at the bottom. ORPF with a given amount was loaded in a filter paper and the resulting system was placed on the top of the sand-soil mixture, then 10 g of the sand-soil mixture was placed on the top of the filter paper. The leachate was collected after being sprayed with 30 mL of Cr(VI) solution (100 mg/L, pH=4.6) on the top of the system, and then centrifuged for 5 min (4500 rpm). Finally, 7
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the concentration of Cr(VI) in the leachate was measured. All experiments were performed in quintuplicate. The removal efficiency (REleaching) of Cr(VI) was calculated according to equation (4): REleaching (%)=(C0-CL)/C0×100%
(4)
Where the C0 and CL are the initial Cr(VI) concentration and the Cr(VI) concentration (mg/L) in the leachate respectively. Characterizations: The morphology was observed on a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (Sirion 200, FEI Co., USA), an H-800 transmission electron microscope (TEM) (Hitachi Co., Japan) and an atomic force microscopy (AFM) (MultiMode V, Veeco Co., USA). Magnetic behavior was measured by a superconducting quantum interference device (SQUID) magnetometer (Bruker Biospoin GmbH, Germany). The structure and composition analyses were conducted using a TTR-III X-ray diffractometer (XRD) (Rigaku Co., Japan), a Fourier transform infrared (FTIR) spectrometer (iS10, Nicolet Co., USA), and an X-ray photoelectron spectroscope (XPS) (ESCALAB 250, Thermo-VG Scientific Co., USA). The thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed by a thermogravimetric analyzer (Q5000IR, TA Co., USA). The concentration of Cr(VI) was measured using a UV-vis spectrophotometer (UV 2550, Shimadzu Co., Japan) at a wavelength of 540 nm.
RESULTS AND DISCUSSION Removal of Cr(VI) from aqueous solution: Owing to the nanosheet structure and surface groups such as -OH and -COOH, GO can adsorb a certain amount of Cr(VI) ions to the surface of GO nanosheets and thus remove them from aqueous solution. Additionally, GO could reduce Cr(VI) to Cr(III) because of the π electrons, which also facilitates the removal of Cr(VI). As shown in Figure 1A, PEI 8
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modification could effectively improve the removal performance of GO for Cr(VI), wherein the qe of GO/PEI increased significantly with the increase of PEI amount, reached the maximum value at WGO:WPEI of 51 mg:0.7 g and decreased afterwards. This was probably because PEI modification could introduce plenty of -NH2 to the surface of GO, and the -NH2 tended to be protonated to -NH3+ under acidic condition, which was favorable for the adsorption for negatively charged Cr(VI) (e.g., Cr2O72-, HCr2O7-, CrO42-, and HCrO4-) through electrostatic attraction.30,31 The -NH2 amount increased greatly with the increasing PEI amount initially (
