Reduced Graphene Oxide-Hybridized Polymeric High-Internal Phase

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Reduced graphene oxide hybridized polymeric high internal phase emulsions for highly efficient removal of polycyclic aromatic hydrocarbons from water matrix Yipeng Huang, Wenjuan Zhang, Guihua Ruan, Xianxian Li, Yongzheng Cong, and Fuyou Du Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00005 • Publication Date (Web): 04 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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Reduced graphene oxide hybridized polymeric high internal phase emulsions for highly efficient removal of polycyclic aromatic hydrocarbons from water matrix Yipeng Huanga,1, Wenjuan Zhanga,1, Guihua Ruana,b,*, Xianxian Lia, Yongzheng Cong a,b

, Fuyou Dua,b, Jianping Lia,b

a

College of Chemistry and Bioengineering, Guilin University of Technology,

Guangxi 541004, China b

Guangxi Colleges and Universities Key Laboratory of Food Safety and Detection,

Guangxi 541004, China 1

These authors contributed equally.

1

ACS Paragon Plus Environment

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ABSTRACT: Reduced graphene oxide (RGO) hybridized polymeric high internal phase emulsions (RGO/polyHIPEs) with open-cell structure and hydrophobicity have been successfully prepared using 2-ethylhexyl acrylate and ethylene glycol dimethacrylate as monomer and crosslinker. The adsorption mechanism and performance of this RGO/polyHIPEs to polycyclic aromatic hydrocarbons (PAHs) were investigated. Adsorption isotherms of PAHs on RGO/polyHIPEs show that the saturated adsorption capacity is 47.5 mg/g and the equilibrium time is 8 h. Cycling tests show that adsorption capacity of RGO/polyHIPEs remains stable in 10 adsorption-desorption cycles without observable structure change in RGO/polyHIPEs. Moreover, the PAH residues in water samples after purified by RGO/polyHIPEs are lower than the limit values in drinking water set by European Food Safety Authority. These results demonstrate that the RGO/polyHIPEs have great potentiality in PAH removal and water purification.

Keywords: reduced graphene oxide, high internal phase emulsion, porous polymer, polycyclic aromatic hydrocarbons, adsorption

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INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are a large class of ubiquitous and toxic environmental and food processing contaminants as a result of incomplete combustion or pyrolysis of organic matter and geological processes.1 Due to their toxic, mutagenic and carcinogenic properties, PAHs have been listed as one of the most critical pollutants by European Food Safety Authority (SFSA) and the United States Environmental Protection Agency (US-EPA).2 To protect human and other animals from the poison of PAHs as much as possible, developing highly efficient methods to eliminate or monitor the PAHs in environmental systems and food sources is urgently needed. At present, nanoparticles,3 mesoporous organosilica,4 organo-bentonite,5 expanded clay aggregate,6 metal-organic frameworks,7 and various carbon materials (including porous carbon, actived carbon, and graphene/graphene oxide)8-10 have been used as adsorbents to remove PAHs. Compared with other materials, graphene-based adsorbents attracted considerable attention in PAH removal due to their impressive adsorption capacity deriving from the high specific surface area and large π-electron system in basal plane. However, the two-dimensional (2D) planar graphene and graphene oxide (GO) sheets are incompetent to practical applications since they are inconvenient to recover from liquid systems. To obtain materials with better performance on sorption capacity, 2D GO should be assembled into three-dimensional (3D) porous structure.11-15 Currently, 3D 3


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graphene

monoliths

mainly

includes

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hydrogels,14,16

aerogels,17,18

and

graphene-polymer composites.12,15,19 Though these materials showed very high adsorption capacity to many contaminants, the high-cost of these sorbents (use of a mass of GO or graphene) and easy collapse of 3D structure have tremendously hindered their large-scale production. It is well known that GO is amphiphilic and can be used as a Pickering stabilizer.20 To fabricate 3D porous structures and reduce the use of GO, the assembly of GO with organic monomer and crosslinkers via high internal phase emulsions (HIPEs, where the volume fraction of the internal phase is higher than 74%) to synthesize highly porous monoliths with well-defined structures is a good choice.21 The first GO hybridized polymeric HIPEs (GO/polyHIPEs) was reported by Wang and co-workers in 2013.21 Since then, several groups also reported similar

GO/polyHIPEs

or

RGO/polyHIPEs.

According

to

these

reports,

GO/polyHIPEs and RGO/polyHIPEs may possess great potential in adsorption,22 separation,23 catalysis,24 and energy storage.25 In these studies, styrene (St) and divinylbenzene (DVB), the easiest system to form stable HIPEs, were frequently used.21,23,25,26 Attempts have been made to fabricate GO/polyHIPEs utilizing other monomers and crosslinkers. Wang et al.22 reported a GO/polyHIPEs using acrylic acid and N,N′-methylene bis(acrylamide) as monomer and crosslinker. The resulting GO/polyHIPEs exists in hydrogel form, which is quite different from other reported GO/polyHIPEs. Hu et al.24 synthesized metallic oxide nanoparticle-decorated GO/polyHIPEs using melamine formaldehyde as the polymer precursor, and the resultant GO/polyHIPEs generates close-cell structure. These studies indicate that the 4


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properties of resultant GO/polyHIPEs are rather different when different monomers and crosslinkers are used. Extending the synthesis of GO/polyHIPEs or RGO/polyHIPEs using other monomers and crosslinkers is also significate to meet different application requirements. The aforementioned researches also reflect the fact that quite a number of reported GO/polyHIPEs or RGO/polyHIPEs are close-cell structure with poor permeability.21,24,26 In many applications, open-cell structure GO/polyHIPEs or RGO/polyHIPEs with good permeability are more welcomed. To obtain open-cell structure, pyrolysis has been used to remove the organic matrixes.21 But in this way, the functions of organic matrixes were disappeared. White et al.23 prepared open-cell structural GO/polyHIPEs via the addition of surfactant in the emulsions. However, this work still cannot get rid of the use of St and DVB as monomer and crosslinker, and the extremely low GO quantity (~0.02 wt%) leads to the unsatisfied application performance. Hence, using other monomers and crosslinkers for the preparation of GO/polyHIPEs with open-cell structure and appropriate high GO quantity is still needed. Currently, most of studies merely focus on physical and mechanical properties of the resultant polymers. The potential applications are mainly predicted from the characterization results. Probing the physical and mechanical properties are certainly important but clarifying the chemical properties and carrying out the potential application of such materials are also essential to develop such material. Herein, RGO/polyHIPEs with open-cell structure were successfully fabricated using 2-ethylhexyl acrylate and ethylene glycol dimethacrylate as monomer and 5


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crosslinker, and the resultant RGO/polyHIPEs were used as sorbent for PAH removal. The chemical and physical properties of the monolithic RGO/polyHIPEs were systematically investigated. The resulting RGO/polyHIPEs not only exhibit enhanced PAH removal efficiency than many conventional PAH sorbents, but also address the recycle issues raised by powder-like and 2D GO based sorbents. More importantly, the 3D RGO/polyHIPES are flexible and robust. Our work offers a new strategy to prepare monolithic sorbents with good permeability for highly efficient removal of PAHs from water matrix.

EXPERIMENTAL SECTION

Reagent and Solutions. PAH standards, including naphthalene (Nap), fluorene (Flu), phenanthrene

(Phe),

benz[a]anthracene

anthracene

(BaA),

(Ant),

chrysene

fluoranthene (Chr),

(Fla),

pyrene

benzo[b]fluoranthene

(Pyr), (BbF),

benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), dibenz[a,h]anthracene (DahA) and benzo[g,h,i]perylene (BghiP), were purchased from Sigma-Aldrich (Shanghai, China). Graphite powders (99.95%), ethylene glycol dimethacrylate (EGDMA, 98%), 2-ethylhexyl

acrylate

polyvinylpyrrolidone

(EHA, (PVP,

99%), K-30),

sorbitan

monooleate

azo-bisisobutyronitrile

(Span

80),

(AIBN),

and

ethanediamine (EDA, 99%) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). Sulfuric acid (98%), sodium nitrate (99%), potassium permanganate (99%), hydrogen peroxide (30%), hydrochloric acid (37%), dichloromethane (CH2Cl2, 99.5%) and absolute ethanol (99.7%) were supplied by 6


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Chengdu XiYa Chemical Technology Co., Ltd. (Sichuan, China). Acetonitrile (ACN) and methanol of high performance liquid chromatography (HPLC) grade were obtained from OmniGene LLC. (USA). AIBN was recrystallized prior to use. Ultrapure water (UP water) was used throughout the experiments. PAHs mixed stock solutions were prepared by dissolving each PAH standard in ACN with the concentration of 20 µg/mL for Nap, 4 µg/mL for Flu, Fla, BbF, DahA and BghiP, and 2 µg/mL for the other PAHs. The stock solutions were stored in a refrigerator at 4 °C for further use.

Preparation of PVP Modified GO (PVP-GO). Graphite oxide were prepared by modified Hummers’ method27 as described in supporting information. Aqueous GO suspension (3.0 mg/mL) was obtained by ultrasonicating the graphite oxide dispersion for 1 h. PVP-GO was prepared according to a previous report with some modification.28 Briefly, a certain amount of PVP (PVP/GO = 200/30, w/w) was added to GO suspension, followed by magnetic stirring for 12 h, then centrifuged at 15000 rpm/min for 20 min to discard the unbound PVP. The collected slurry was dispersed in UP water to form 5.0 mg/mL PVP-GO aqueous suspension.

Preparation of RGO/polyHIPEs. An oil phase consisting of 470 µL of EHA, 430 µL of EGDMA, 100 µL of Span 80, and 1.5 wt% (with respect to the total mass of EHA and EGDMA) of initiator AIBN was added to a 3.0 mL of aqueous phase containing 9.0 mg PVP-GO in a 10 mL polypropylene centrifuge tube. The mixtures were emulsified with an IKA MS-3B homogenizer (IKA, Germany) at 3000 rpm for 5 7


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min to form HIPEs (75 vol% internal phase). HIPEs with different GO quantity and EHA/EGDMA ratio were prepared as described in supporting information. The HIPEs were polymerized at 60 °C for 18 h to yield yellow-brown GO/polyHIPEs. After cooled to room temperature, the unreacted components were washed away with ethanol and UP water several times, respectively. GO was reduced by EDA (GO/EDA = 1/5, w/w) at 85 °C for 12 h to form RGO/polyHIPEs.29 Finally, RGO/polyHIPEs were freeze dried at -50 °C and < 10 Pa for 24 h using a freeze dryer (Beijing Boyikang Laboratory Instruments Co., China). RGO/polyHIPEs contained 0.35%, 0.70%, 1.05% and 1.35% of RGO are denoted as RGO0.35/polyHIPEs, RGO0.70/polyHIPEs, RGO1.05/polyHIPEs and RGO1.35/polyHIPEs, respectively.

Material Characterization. Atomic force microscopic (AFM) images of PVP-GO nanosheets were obtained using a SNL-10 tapping mode (Bruker icon AFM, Germany). The sample for AFM imaging was prepared by spin coating the PVP-GO dispersion onto a highly oriented pyrolytic graphite substrate. Fourier-transform infrared (FT-IR) spectra of the GO, PVP-GO, polyHIPEs, and RGO1.05/polyHIPEs were collected in KBr pellets using an IS10 FT-IR spectrometer (Thermo Fisher Scientific Co., USA). X-ray photoelectron spectroscopy (XPS) spectra were obtained with an Axis Ultra DLD (Kratos Ltd, U.K.) paired with a monochromatic Al Kα X-ray source (1486.6 eV). The sample for XPS collection was prepared by firstly smashing the monolithic polymer into powder, then the powder was compressed onto an Al foil. Surface morphologies of polyHIPEs and RGO1.05/polyHIPEs were observed by a SU5000 field emission scanning electron microscope (SEM, Hitachi Ltd, Japan). All 8


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samples were placed onto carbon-coated lacy substrates and were sprayed with gold before observation.

Procedure for PAHs Adsorption. To better mimic the multifarious PAHs in environmental samples, the PAH mixed standards were used in the adsorption experiments. Typically, 0.2 g of monolithic RGO/polyHIPEs was added to 10 mL of PAH solution (ACN and UP water consist the solvent), followed by gently shaking in a shaking incubator at constant temperature. Percentages of ACN in the solvent were varied from 1% to 40% for the investigation of the effect of ACN on adsorption. PAH solutions with concentrations of 1 to 8 (1 is defined as the PAH mixed solution containing 50 ng/mL of Nap, 10 ng/mL of Flu, Fla, BbF, DahA and BghiP, and 5 ng/mL of Phe, Ant, Pyr, BaA, Chr, BkF and BaP) were used in the adsorption to investigate the effect of initial concentration. At predetermined time intervals, 250 µL of the solution was taken out for HPLC analysis of the remaining PAHs. The adsorption quantity at time t, qt (mg/g), and the removal efficiency, R, were calculated according to the equation (1) and (2), respectively.

qt = (C0 ─ Ct) V / m

(1)

R% = Ct / C0 100

(2)

where C0 and Ct (mg/L) are the concentration of PAHs initially and at time t, respectively; V is the volume of the solution (L); and m is the mass of RGO/polyHIPEs (g).

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Procedure for PAH Desorption. The PAH-containing monoliths were immersed into 2.0 mL of ACN-CH2Cl2 solution (1/1, v/v) for 1.0 h with gently shaking, then the elution solution was collected in another container. This process was repeated twice, and the eluate was mixed together for the HPLC analysis of the eluted PAHs. The desorption percentage, D, was calculated as follows:

D% = C1V1 / (qm) 100

(3)

where C1 (mg/L) is the concentration of PAHs in the eluate; V1 (L) is the volume of the eluate; q (mg/g) is the PAH quantity absorbed in the adsorbent; and m (g) is the mass of the sorbent.

PAHs Analysis by HPLC. The quantitative analysis of PAHs was performed on a LC-20A liquid chromatography (Shimadzu, Japan) equipped with a RF-10AXL fluorescence detector (FLD). Chromatography separation was achieved on a ZORBAX Eclipse Plus C18 column (4.6 × 250 mm, 5 µm, Agilent) at 25 °C with a flow rate of 1.5 mL/min. The mobile phase was composed of UP water (A) and ACN (B). The elution gradient was conducted as our previous method30 with some improvement: 0-5 min, 50% B; 5-35 min, 50-90% B; 35-40 min, 90-50% B; 40-45 min, 50% B. All samples were filtered by 0.22 µm PTFE syringe filters prior to HPLC-FLD analysis, and the injection volume was 10 µL. For analyses of PAHs with trace level, the samples were firstly dried under nitrogen gas at ambient temperature then dissolved in ACN to achieve an enrichment factor of 10. The detecting wavelengths, calibration curves and detection limits of the PAHs were showed in 10


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Table S1.

RESULTS AND DISCUSSION

HIPE Stability. It is well known that the GO nanosheets are amphiphilic, which provides GO nanosheets with the capability of acting like surfactants at interfaces of water and oil. Previous reports have demonstrated that GO tends to stable oil-in-water (O/W) emulsions.21 To form water-in-oil (W/O) emulsions, inversely, a proper modification of GO is indispensable. Herein, we use PVP to modify GO sheets because PVP chains can not only slightly enhance the hydrophobicity of GO but maintain good dispersibility of GO with monolayer in aqueous phase (Figure. S1). As shown in Figure 1A, both GO and PVP-GO can emulsify the oil phase consisting of EHA and EGDMA; however, use of only GO or PVP-GO as emulsifier is unfeasible to form stable HIPEs, which leads to phase separation after standing several hours. To obtain stable HIPEs as well as forming open-cell structure after polymerization, we try different surfactants including series of OP, Tween and Span. Among these surfactants, only Span type can stabilize HIPEs. Thus, Span 80 is used in further preparation. Stable HIPEs with 75% internal phase is obtained and successfully polymerized into monoliths at 60 °C when PVP-GO and 10% of Span 80 is used. Interestingly, HIPEs stabilized by unmodified GO and Span 80 fail to polymerize into monolith (Figure 1B), demonstrating the necessity of PVP modification. Further study shows that HIPEs only stabilized by Span 80 demands a higher fraction (30%) of surfactant to form stable HIPEs and yield a monolithic polyHIPEs. This phenomenon 11


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reveals the fact that most HIPEs stabilized only by surfactant consume >30% of surfactant,31 and proves that the solid PVP-GO can greatly reduce the use of surfactant. The ratio of monomers (EHA) to crosslinkers (EGDMA) is another factor affecting the HIPE stability. As shown in Figure 1C, the viscosity of the Pickering HIPE is relatively low when the molar ratio of EHA to EGDMA (REHA/EGDMA) equals to 1/3, and the emulsion can flow down when the centrifuge tube is inverted. Paste-like emulsions with higher viscosity are acquired by increasing REHA/EGDMA. Although HIPEs with REHA/EGDMA = 1/2 remain stable at room temperature, the aqueous phase and organic phase is ready to separate in the thermal polymerization process. Homogeneous GO/polyHIPEs are obtained only when REHA/EGDMA ≥ 1/1. In this status, we control REHA/EGDMA = 1/1 in later studies (Figure 1D).

Figure 1 (A) Emulsions stabilized by GO, PVP-GO, GO and Span 80, and PVP-GO and Span 80 (from left to right); (B) HIPEs stabilized by GO and Span 80 (left), and PVP-GO and Span 80 (right) after incubating at 60 ⁰C for 18 h; (C) HIPEs stabilized by PVP-GO and Span 80 with different REHA/EGDMA (3/1, 2/1, 1/1, 1/2 and 1/3 from left to right); (D) the resultant GO/polyHIPEs with different REHA/EGDMA (1/2, 1/1, 2/1 and 3/1 from left to right) at 60 ⁰C for 18 h.

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Sample Characterization. Figure 2 shows FT-IR spectra of GO, PVP-GO, polyHIPEs, GO1.05/polyHIPEs and RGO1.05/polyHIPEs. The absorption peaks at 3440 (νO−H), 1722 (νC=O), 1400 (νC−OH), and 1047 (νC−O−C) cm-1 in Figure 2A confirm that the carboxyl, hydroxyl, and epoxy groups have been successfully introduced to the GO sheets.32 New bands at 2969-2889 (νCH3, CH2), and 1662, 1290 (νN−C=O) cm-1 in Figure 2B prove that PVP chains have been modified onto the GO sheets.33,34 The FT-IR spectrum of GO1.05/polyHIPEs appears the absorption peeks at 3304 cm-1 (νO−H, N−H),

1736 cm-1 (νOC=O) and 1662 cm-1 (νNC=O), revealing the hybridization of

PVP-GO, EHA and EGDMA. There are no apparent differences between FT-IR spectra of GO1.05/polyHIPEs and RGO1.05/polyHIPEs due to the strong absorption peaks deriving from the organic matrixes greatly interfere the direct analysis of the reduction of GO. Hence, high resolution C1s XPS for GO1.05/polyHIPEs and RGO1.05/polyHIPEs are further collected (Figure 3). Upon reduction, C1s peak associating with carbons singly bonded to oxygen (C─OH and C─O─C, 286.5 eV)35,36 decrease obviously in RGO1.05/polyHIPEs, which demonstrates the reduction of hydroxyl and epoxy groups on GO. The remaining intensity ascribed to C─O and O─C=O in the C1s XPS of RGO1.05/polyHIPEs can be attributed to the oxygen-containing groups in EHA and EGDMA that cannot be reduced by EDA.

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Figure 2 The FT-IR spectra of GO (A), PVP-GO (B), polyHIPE(C), GO1.05/polyHIPEs (D) and RGO1.05/polyHIPEs (E).

Figure 3 High resolution C1s XPS of GO1.05/polyHIPEs (A) and RGO1.05/polyHIPEs (B). 284.6 eV: carbons in unoxidized, aromatic sp2 structures (C─C, C=C); 286.5 eV: carbons singly bonded to oxygen (C─O,); 287.9 eV: carbons in amide groups (N─C=O); 288.9 eV: carbons in carboxyl, ester groups (O─C=O).35, 36

Surface morphologies of the polymers were observed by SEM. As shown in Figure 4, both polyHIPEs and RGO1.05/polyHIPEs exhibit open-cell structure because the presence of a small fraction of surfactant.31 Adjacent voids are interconnected by pore throats. The open-cell structure is conducive to good permeability and fast mass transfer. Compared with the smooth pore surface in polyHIPEs, a rather rough surface of micro-nanoscale papillae is observed in RGO1.05/polyHIPEs.

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Figure 4 The SEM images of polyHIPEs (A, B) and RGO1.05/polyHIPEs (C, D). Scale bars are 200 µm.

According to the N2 adsorption-desorption isotherms (Figure S2), the BET specific

surface

area

(P/P0

Reduced Graphene Oxide-Hybridized Polymeric High-Internal Phase

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