Nitrogen-Doped Reduced Graphene Oxide as a Bifunctional Material

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Nitrogen-doped reduced graphene oxide as a bifunctional material for removing bisphenols: synergistic effect between adsorption and catalysis Xiaobo Wang, Yanlei Qin, Lihua Zhu, and Heqing Tang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01059 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 8, 2015

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Environmental Science & Technology

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Nitrogen-doped reduced graphene oxide as a bifunctional material for

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removing bisphenols: synergistic effect between adsorption and catalysis

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Xiaobo Wanga,b, Yanlei Qin a, Lihua Zhub, Heqing Tang*,a

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a

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission

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and Ministry of Education, College of Resources and Environmental Science, South-Central

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University for Nationalities, Wuhan 430074, P. R. China

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b

College of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China

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Nitrogen modified reduced graphene oxide (N-RGO) was prepared by a hydrothermal method.

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The nitrogen modification enhanced its adsorption and catalysis ability. For an initial

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bisphenol concentration of 0.385 mmol L-1, the adsorption capacity of N-RGO was evaluated

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as 1.56 and 1.43 mmol g-1 for bisphenol A (BPA) and 1.43 mmol g-1 for bisphenol F (BPF),

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respectively, both of which were about 1.75 times that (0.90 and 0.84 mmol g-1) on N-free

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RGO. N-RGO could activate persulfate, producing strong oxidizing sulfate radicals. The

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apparent degradation rate constant of BPA on N-RGO was 0.71 min-1, being about 700 times

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that (0.001 min-1) on N-free RGO. In mixtures of various phenols, the degradation rate

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constant of each phenol was linearly increased with its adsorption capacity. A simultaneous

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use of N-RGO and persulfate yielded fast and efficient removal of bisphenols. The use of

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N-RGO (120 mg L−1) and persulfate (0.6 mmol L−1) almost completely removed the added

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bisphenols (0.385 mmol L−1) at pH 6.6 within 17 min. A mechanism study indicated that the

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adsorption enriched the pollutant, and the catalytically generated sulfate radicals rapidly

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degrade the adsorbed pollutant, accelerating in turn the adsorption of residual pollutant. 1

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KEYWORDS: Nitrogen-doped; Reduced graphene oxide; Adsorption; Catalytic oxidation;

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Persulfate.

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INTRODUCTION

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Endocrine disrupting compounds (EDCs) are crucial emerging contaminants.1,

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Their

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widespread occurrence in water sources give rise to concerns over adverse effects to wildlife

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and humans even at low exposure levels.3 EDCs are a large group of chemicals, including

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pesticides, natural hormones, and industrial chemicals.4 Bisphenol A (BPA) and bisphenol F

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(BPF) as representative EDCs were widely used in the production of various plastic

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products,5-8 and detected in food, water, paper and plastic products.9, 10 They have attracted

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particular attention because they possess high and moderate estrogenicity, respectively.11 The

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pollution resulted from BPA and its structurally similar analogues can last for several decades

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once they are released into water or soil.10 Therefore, to develop simple efficient methods for

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removing the bisphenols is a tremendous challenge.

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Various methods have been reported to remove BPA and its analogues from water, such as

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adsorption,12,

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microbial degradation,14,

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ultrasonic degradation,7,

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degradation3, 17 and chemical remediation.18, 19 Among these methods, the adsorption method

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is the easiest one to operate, but it does not decompose the pollutants and requires an

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appropriate after-treatment. Advanced oxidation processes (AOPs) are the most efficient

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method in terms of full decomposition of the target organic pollutants. Similar to Fenton and

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Fenton-like processes,18-24 the method based on persulfate (PS) activation is a recently

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emerged AOP, because the PS activation results in generation of strongly oxidizing sulfate

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radicals (SO4•−, E0= 2.6 V).25, 26 PS activation is able to be achieved by heating,27 UV-light 2


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photocatalytic

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irradiation,28 transition metal ions and metallic oxide,26, 29, 30 and even activated carbon.31

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Hanci et al. investigated a thermally activated PS oxidation process for BPA removal, and

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found that BPA (88 µmol L-1) was completely degraded in 120, 30 and 20 min when the

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degradation was carried out at 50 °C, 60 °C and 70 °C, respectively, whereas only 26% of

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BPA was removed after 120 min at 40 °C in the presence of PS (10 mmol L-1).32 Sharma et al.

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reported that an UV/S2O82- process resulted in a BPA removal of ~95% BPA after 240 min of

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irradiation with initial additions of 0.22 mmol L-1 BPA and 1.26 mmol L-1 PS.33 Huang and

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Huang made an identification of produced powerful radicals involved in the mineralization of

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BPA by a UV-PS/H2O2-Fe(II,III) two-stage oxidation process, and found that the BPA

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degradation rate constant in the UV-PS process was 0.038 min-1 at BPA and PS concentrations

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of 0.05 mmol L-1.34 It is certain that the use of AOPs is not economic for removing EDCs at

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low levels. In contrast, the adsorption method should be the most economic one for removing

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EDCs at low levels. However, the adsorption capacities of adsorbents are also limited by the

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ultra-low concentration of the target EDCs due to the adsorption-desorption equilibrium. If

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the adsorbent itself is also a catalyst for the activation of oxidizing agents, its catalysis will

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make the adsorbed EDCs be decomposed, lead to further increase the adsorption of the

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residual EDCs. To develop such a system with both strong adsorption ability and strong

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catalytic ability is a new strategy for removing ultra-low level EDCs from water.

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Carbon-based materials as adsorbents have attracted much attention, such as graphene,

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activated carbons, carbon nanotubes and porous carbon.35-38 Especially, graphene, a

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two-dimensional flat structure with sp2-hybridized carbon configurations, takes a promising

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advantage of strong adsorption ability to chemicals with benzene rings through strong π−π 3



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interactions. Xu et al. reported that graphene had high adsorption capacity of 0.798 mmol g-1

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toward BPA due to both π−π interactions and hydrogen bonding.12 Pei et al. clarified that 1, 2,

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4-trichlorobenzene and 2, 4, 6-trichlorophenol were adsorbed on graphene and graphene oxide

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(GO) mainly via π−π interaction.39 Recently, we investigated the adsorption of phenolic

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compounds on reduced graphene oxide (RGO) and found that their adsorption on RGO was

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dependent on the π−π interaction between the aromatic molecules and RGO, which was

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influenced by both the reduction degree of RGO and the chemical structure of the phenolics.40

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Because of this correlation, RGO showed strong adsorption ability toward chemicals bearing

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with one or more benzene rings. It should be noted that the chemical structure bearing with

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one or more benzene rings is the most important characteristic of most EDCs. We expected

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that RGO would be function as an excellent adsorbent for removing EDCs from water.

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Graphene can be used as a catalyst carrier to improve the catalytic activity of other

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materials. Kavitha et al. found that zinc oxide nanoparticles decorated graphene nanosheets

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exhibited good photocatalytic activity for the degradation of methylene blue in ethanol under

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UV irradiation.41 An et al. decorated nano-BiFeO3 on graphene for efficient photocatalytic

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degradation of tetrabromobisphenol A.22 We anticipated that graphene could catalytically

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activate PS. However, we found that graphene itself was very poor in its catalytic

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performance for the activation of PS in preliminary experiments. Therefore, it is necessary to

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improve its catalytic ability. Nitrogen doping is a way for this purpose. It has been confirmed

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that doping the carbon network with N heteroatom can introduce electrocatalytic active sites

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and enhance the electrical conductivity of graphene. Qu et al. demonstrated that N-graphene

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had better electrocatalytic activity than platinum for oxygen reduction.42 Lin et al. also 4


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reported that nitrogen-doped graphene showed high electrocatalytic activity toward oxygen

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reduction in alkaline solutions.43 Therefore, we believed that the N modification could also

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enhance the catalytic ability of graphene for the PS activation. We anticipated that N-RGO not

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only strongly adsorbed, but also degraded the adsorbed bisphenols in the presence of PS.

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To efficiently remove low-level EDCs from water, in the present work, N-RGO was

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prepared by hydrothermal treatment of GO with ammonia at 180 °C. The enhanced adsorption

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and degradation of typical bisphenols BPA and BPF were achieved by simultaneous use of

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N-RGO and PS. It was found that N-RGO could strongly adsorb and in situ rapidly degrade

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BPA and BPF in the presence of PS, and the degradation of the adsorbed bisphenol molecules

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released more sites (on N-RGO) for re-adsorbing residual bisphenols in solution. Such a

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cycling of adsorption-degradation-adsorption provides a new strategy for removing EDCs at

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low levels.

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MATERIALS AND METHODS

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Chemical reagents. Graphite powder (SP1 graphite) was purchased from American Bay

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Carbon (Bay City, MI). BPF was provided by Aladdin Chemistry Co., Ltd. BPA, phenol (PE),

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4-methylphenol (4-MP), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), potassium

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persulfate (PS) and other chemicals were bought from Sinopharm Chemical Reagent Co., Ltd.

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(China). All the chemicals were of chemical grade or higher and used as received without

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further purification. Double-distilled water was used in the present study.

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Preparation of GO and N-RGO. GO was prepared according to a modified Hummers

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method (see SI for more details).44 N-RGO was prepared by using GO as a precursor.

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Typically, 22.5 mL of above GO suspension (0.1 g GO) was dispersed into 45.5 mL of water, 5



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Then 2 mL of 30% ammonia was added with magnetically stirring. The solution was then

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transferred into a Teflon-lined autoclave and heated at 180 °C for 24 h. The solids were

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isolated by filtering with 0.22 µm filter and washed with distilled water several times to

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remove the excess ammonia. At the end, the product was dispersed in water as N-RGO

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dispersions. For a comparison, N-free RGO was prepared similarly but without adding

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ammonia or with addition of NaOH instead of ammonia. The resultant RGO samples were

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referred to as H-RGO and S-RGO, respectively.

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Characterization. Surface morphology was observed on a Hitachi S-4800 scanning

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electron microscope (SEM). X-ray photoelectron spectroscopic (XPS) analysis was conducted

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on a VG Multilab 2000 spectrometer (Thermo Electron Corporation) with Al Kα radiation as

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the exciting source (300 W). The binding energies of the recorded XPS spectra were corrected

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according to C 1s line at 284.6 eV. After subtracting the Shirley-type background, the

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core-level spectra were deconvoluted into their components with mixed Gaussian-Lorentzian

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(20:80) shape lines using the CasaXPS software. X-ray diffraction (XRD) pattern was

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obtained on a diffractometer with Cu Kα radiation (PANalytical B.V. X’Pert PRO), operated at

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40 mA and 40 kV. Fourier transform infrared (FT-IR) spectra were measured on a Bruker

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VERTEX 70 IR spectrometer. Raman spectra were recorded on a Thermo Fisher DXR Raman

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spectrometer employing a 532 nm laser. Brunauer-Emmett-Teller (BET) surface areas were

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determined on a MICROMETERS ASAP 2020 apparatus with nitrogen adsorption/desorption

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at 77 K. The average particle size of the samples was measured with a Malvern Zetasizer

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Nano-ZS90 analyzer.

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Adsorption. Typical adsorption experiments were conducted by dispersing N-RGO (120 6


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mg L-1) in solutions of BPA and BPF (0.175-0.385 mmol L-1, pH 6.6) at 298 K. At regular

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time intervals, a small volume of the solution was sampled and immediately centrifuged at a

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speed of 16000 rpm to remove the solid particles by using an EBA-21 centrifugal (Hettich,

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Germany), and the pollutant concentrations in the supernatant were determined by high

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performance liquid chromatography (HPLC). All measurements were repeated three times

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and the results were reproducible with relative errors less than ±5%.

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Catalytic degradation. The degradation experiments were carried out in a 100 mL beaker

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at 298 K. N-RGO (120 mg L-1) was dispersed in 50 mL solution of 0.385 mmol L-1 BPA and

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BPF, followed by adjusting pH to 6.6. The suspension was magnetically stirred in dark for 10

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min to achieve the adsorption/desorption equilibrium between the solution and N-RGO. Then,

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a specified amount of PS was added into the solution to initiate the degradation. During the

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degradation, solution pH was not controlled. After the degradation, the solution pH was

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decreased to about pH 3.0. After the reaction was started, solution samples (1.0 mL) were

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taken at given time intervals, quenched immediately by adding Na2S2O3 solution, and

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centrifuged at 16,000 rpm for 3 min on an EBA-21 centrifugal (Hettich, Germany) to remove

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N-RGO. The BPA and BPF concentration in the supernatant was determined.

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Two sets of quenching experiments were performed to determine the radical species

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formed in the N-RGO/PS/BPA system by using methanol and KI as quenching agents of SO4•-.

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Prior to the PS addition, methanol and KI was added into the reaction solution and the final

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concentration was 300 and 40 mmol L-1, respectively.

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Analytic methods. The concentrations of BPA and BPF were analyzed with HPLC on a

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HPLC system (Agilent 1200 infinity series) with a G1315D 12600 DAD detector with a 7



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detection wavelength of 230 nm. An amethyst C18-P column (5µm, 4.6 × 150 mm) was used

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as separation column. The mobile phase was a mixture of methanol and water (60:40, v/v).

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The flow rate was set at 1.0 mL min-1 and the injection volume was 20 µL. Aromatic

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degradation intermediates were identified by gaseous chromatography-mass spectroscopy

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(GC-MS) on a Thermo scientific TRACE 1300 GC Ultra sys-tem with the electron ionization

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mode (see SI for more details). Total organic carbon (TOC) was assayed on a TOC analyzer

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(Elementar, Germany).

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RESULTS AND DISCUSSION

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Characterization of N-RGO. The SEM observation (Fig. S1) showed that GO, H-RGO

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and N-RGO looked like crumpled wave-like sheets, demonstrating typical graphene

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structures.40, 45-47 It also indicated that nitrogen modification did not destruct the intrinsic

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structure of graphene. The crumpled sheets of GO seemed to be unfolded because of the

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existence of plentiful oxygen-containing groups on the surface of GO. The folding of the

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crumpled sheets of H-RGO and N-RGO was increased. Under lower magnification times, the

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SEM observation demonstrated that the aggregation degree of N-RGO was less than H-RGO.

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The average particle size (d) of N-RGO and H-RGO in their water dispersions was measured

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as 579 and 764 nm, respectively, which also evidenced that the aggregation degree of N-RGO

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was less than H-RGO. The nitrogen sorption isotherms of H-RGO, N-RGO and activated

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carbon (AC) at 77 K were compared (Fig. S2). The BET analysis revealed that the specific

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surface area of H-RGO and N-RGO was 265 and 317 m2 g-1. The increased BET area of

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N-RGO was possible due to the introduction of nitrogen and the doping-induced unfolding of

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graphene structure. It should be noted that although the BET specific surface area of N-RGO 8


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was lower than that of AC (624 m2 g-1), the adsorption of the concerned bisphenols was

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stronger on N-RGO than on AC (see the discussion below for more details). (a)

(b)

(c) 1 pyridinic N 2 pyrrolic N 3 quaternary N

1

CPS

CPS

3

1

CPS

2

1 C-C,C=C 2 C-OH, C=N 3 C-O-C 4 C=O, C-N 5 O-C=O

3 2

5

3

4

2

1 600

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400

B.E. / eV

200

0

406

404

402

400

398

396

394

292

B.E. / eV

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288

286

284

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B.E. / eV

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Fig. 1. (a) Wide survey XPS spectra of (1) GO, (2) H-RGO, (3) N-RGO. (b) XPS N1s

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envelop of fresh N-RGO. (c) XPS C1s envelop of fresh N-RGO.

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The wide survey XPS spectra of GO, H-RGO and N-RGO (Fig. 1a) gave the C1s peak at

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284.5 eV and O1s peak at 532.5 eV, and N-RGO also produced a N1s peak around 400 eV,

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indicating the doping of GO by N. The XPS analysis gave the surface atomic percentages of C,

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O and N elements in the samples (Table S1). It was found that the O/C atomic ratio was 0.48,

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0.24, and 0.16 for GO, H-RGO and N-RGO, respectively, suggesting the part reduction of GO

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induced by the hydrothermal treatment. Fig. 1b illustrated the XPS N1s envelop of N-RGO,

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which could be deconvoluted into three components: pyridinic N (N in 6-member ring) at

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398.6 eV, pyrrolic N (N in 5-member ring) at 399.8 eV, and graphitic N (N in graphene basal

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plane) at 401.5 eV.48, 49 The atomic percentages of pyridinic N, pyrrolic N and graphitic N

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were evaluated to be 53.2%, 37.7% and 9.1%, respectively. Fig. 1c showed the XPS C1s

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envelop of N-RGO, which was centered at 284.5 eV with a tail at higher binding energies,

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indicating the existence of carbon atoms connected to N and O heteroatoms. The C1s XPS

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envelop of N-RGO was able to be fitted to five components at 284.4, 285.6, 286.5, 287.8, and 9



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289.1 eV, corresponding to C=C/C-C in aromatic rings (68.3%), C-OH and C=N (14.4%),

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epoxy C-O-C (8.1%), C=O and C-N (6.1%), and carboxyl COOH groups (3.1%),

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respectively.45, 50, 51

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Raman spectra of carbonaceous materials generally exhibit D and G bands. The D band is

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a signature for disorder-induced vibrational mode, being attributed to structural defects on the

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graphitic planes. The G band is commonly observed for all graphitic structures and assigned

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to the E2g vibrational mode being present in the sp2 bonded graphitic carbons.50 By

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comparing the Raman spectra of GO, H-RGO and N-RGO (Fig. S3a), it was found that the G

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band was red shifted from 1606 cm-1 for GO to 1592 cm-1 for H-RGO and 1587 cm-1 for

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N-RGO, indicating a restoration of the conjugation structure of GO after the hydrothermal

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treatment. The greater downshift of the G band in N-RGO might be related to the

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electron-donating capability of N heteroatoms, signifying the successful N modification.43, 45

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In addition, the intensity ratio of the D and G bands (ID/IG) was increased from 0.93 for GO to

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1.04 for H-RGO and then to 1.08 for N-RGO. The increased ID/IG ratio was attributed to the

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decreased average size of the sp2 domains, and the incorporation of N heteroatoms.45, 50-52

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FTIR was employed to analyze the existence of functional groups in the samples. In the

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spectrum of GO (Fig. S3b), the peaks at 1726, 1400, 1221 and 1054 cm-1 corresponded to

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C=O (COOH) stretching vibration, O-H deformation vibration, C-O (epoxy) and C-O (alkoxy)

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stretching vibration, respectively. The broad band at 3412 cm-1 was considered as the

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vibration of O-H and/or water molecules. After the hydrothermal treatment, the peaks for all

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of the oxygen-containing groups were decreased significantly in intensity for H-RGO and

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N-RGO compared with GO, indicating exhaustive elimination of the oxygen-containing 10


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groups.47, 50, 51, 53 We also noted that the broad band at 3412 cm-1 became much weaker. It

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hinted that the hydrophobicity of H-RGO and N-RGO might be increased because of part

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reduction of GO.

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XRD patterns of GO, H-RGO and N-RGO were recorded (Fig. S4). GO represented a

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main reflection at 2θ=10.9°, being attributed to the (002) crystalline plane of GO and

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corresponding to a c-axis interlayer spacing of 0.813 nm. This indicated that GO was

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successfully fabricated without a graphite peak. For H-RGO, this peak completely

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disappeared and a broad peak of the graphite (002) plane at 24.3° was observed with an

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interlayer space of 0.367 nm. These results demonstrated the graphitic crystal structure was

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recovered and the framework of the reduced sample was composed of few-layer stacked

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graphene nanosheets. In comparison with H-RGO, the XRD pattern of N-RGO showed that

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the (002) peak was shifted to 25.3° with an interlayer space of ∼0.355 nm. This was attributed

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to the reduction effect and nitrogen modification.45, 46 The examination of the peak at 2θ = 43°,

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which could be indexed to the (100) reflection, provided some hints about the evolution of the

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structure in the graphene sheets. In GO, the sp3 bonding between carbon and oxygen resulted

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in the vertical displacement of carbon atoms from the planar arrangement of sp2 bonded

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graphene sheets, weakening the intensity of the in-plane 100 reflection. Compared to GO, the

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stronger intensities of the (100) reflection in H-RGO and N-RGO suggested a restoration of

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the in-plane ordering of carbon atoms and their associated sp2 bonding.54

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Adsorption kinetics and isotherms of bisphenols on N-RGO. The adsorption of the

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bisphenols on N-RGO and H-RGO was increased rapidly in the first 1 min, and the

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adsorption/desorption equilibrium was reached within several minutes (Fig. S5). A maximum 11



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adsorption capacity qm was obtained. The fast adsorption was attributed to the single

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nanosheet structure of RGO with sp2 hybrid orbital, which made bisphenols contact

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immediately the surfaces of RGO through strong π-π interaction.40 The qm values on N-RGO

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were estimated as 1.56 and 1.43 mmol g-1 for BPA and BPF, respectively, both of which were

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about 1.75 times higher than that (0.90 and 0.84 mmol g-1) on H-RGO. Therefore, the

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adsorption ability of RGO was much enhanced by nitrogen modification.

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Fig. 2 gave the adsorption isotherms of BPA and BPF on N-RGO and H-RGO, which

246

were well fitted with the Langmuir adsorption model. From the obtained Langmuir

247

parameters (Table S2), it was found that for either BPA or BPF, the adsorption capacity of

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N-RGO was about 1.5 times higher than that of H-RGO. It indicated that the adsorption

249

capacity of RGO was much increased by N doping. This is attributed to the possible

250

contributions from at least three aspects: (1) ammonia gas facilitates to exfoliation of

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graphene by introducing ammonia in hydrothermal reaction, and the existence of N functional

252

groups reduces the aggregation of the product;46, 55 (2) the N functional groups are Lewis-base

253

sites that are active for binding acidic bisphenol molecules;56 (3) the reduction effect of

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ammonia in the doping process increases the reduction degree of RGO as confirmed by the

255

XPS analysis (the atomic ratio of C/O was 6.15 for N-RGO and 4.21 for H-RGO), and the

256

increased reduction degree of GO will increase the π-π interaction between aromatics and

257

RGO as reported previously.40 Moreover, the effective site of the sp2 carbon of graphene has

258

abundant free-flowing π electrons, which can be activated by conjugating with the long-pair

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electrons from the doped N atoms.52 This also increases the π-π interaction. Therefore, the

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adsorption capacity of N-RGO toward bisphenols was increased with increasing the π-π 12


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interaction. 1.6

1.6

a

b

2

qe / mmol g-1

1.4

1.2

qe / mmol g-1

1.2

1.0

1.0

0.8

0.8

1

1 0.6

0.6 0.05

262

2

1.4

0.10

0.15 0.20 ce / mmol L-1

0.25

0.05

0.30

0.10

0.15 0.20 ce / mmol L-1

0.25

0.30

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Fig. 2. Adsorption isotherms of (a) BPA and (b) BPF on (1) H-RGO and (2) N-RGO at 298 K.

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PS-activating ability of N-RGO. The PS-activating ability of N-RGO was investigated

265

by degrading bisphenols in the presence of both N-RGO and PS. In the degradation process,

266

both the adsorption of N-RGO and the oxidation of PS activated by N-RGO contributed to

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BPA removal in the N-RGO/PS system.

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Fig. 3 presented the removal of BPA in different systems. In this figure (and other similar

269

figures below), the real degradation of the pollutant started at time of zero, at which PS was

270

rapidly introduced into the reaction solution. Before the time of zero (with negative values of

271

time), the removal of the pollutant was achieved by a pre-adsorption with a time period of 10

272

min. Therefore, the total removal of BPA consisted of the contributions from both the

273

adsorption and the oxidative degradation. When only PS was added, little BPA removal was

274

observed (curve 1). When different modified RGOs were used, the adsorption removal of BPA

275

was 28.4%, 35.3% and 50.0% for H-RGO, S-RGO and N-RGO, respectively (curves 2-4).

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After PS was further introduced into the system, only a very slight increase in the BPA

277

removal was observed in the cases of H-RGO and S-RGO (curves 2 and 3). It indicated that 13



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neither H-RGO nor S-RGO could efficiently activate PS. However, the pre-adsorbed BPA and

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the residual BPA in solution was almost fully removed by the oxidative degradation within 7

280

min when N-RGO was introduced (curve 5 and Fig. S6). The data fitting in the oxidation

281

stage gave an apparent rate constant (k) of 0.71 min-1 for the degradation of BPA in the

282

N-RGO/PS system. We also investigated the removal of BPA in the AC/PS and Fe3O4/PS

283

systems (Fig. S7), and found that the BPA removal was very slight in these systems. Therefore,

284

the N modification significantly improved the adsorption ability and PS-activating ability of

285

RGO, and N-RGO was most efficient among the concerned catalysts. 1

1.0

0.8 2 3

c / c0

0.6 4

0.4

Adsorption

Degradation

0.2 5

0.0

286

-8

-4

0

4

8

Time / min

287

Fig. 3. Kinetic data of the adsorption and degradation of BPA (0.385 mmol L-1) in the

288

presence of (1) PS, (2) H-RGO + PS, (3) S-RGO + PS, (4) N-RGO, and (5) N-RGO + PS.

289

Reaction conditions: initial PS concentration 0.6 mmol L-1, catalyst load 120 mg L-1, pH 6.6,

290

and temperature 25 °C. PS was added into the reaction solution at time of 0 min.

291

Effect of several parameters on the catalytic activity of N-RGO. Fig. 4a showed the

292

effect of ammonia concentrations in the preparation of N-RGO on the adsorption and catalytic

293

activity of N-RGO (120 mg L-1) in the presence of 0.6 mmol L-1 PS. The adsorption ability of

294

N-RGO was increased with increasing the ammonia concentration. For example, the value of 14


Page 15 of 33


295

qm was increased from 0.9 mmol g-1 for the zero addition of ammonia to 1.27, 1.49, and 1.56

296

mmol g-1 for the addition of ammonia at concentrations of 10, 40 and 400 mmol L-1,

297

respectively (Table S3). After PS was rapidly introduced, the degradation of BPA was

298

accelerated by increasing the ammonia concentration. For example, the k value was increased

299

from 0.001 min-1 for the zero addition of ammonia to 0.07, 0.46, and 0.71 min-1 for the

300

addition of ammonia at concentrations of 10, 40 and 400 mmol L-1, respectively (Table S3).

0.8

80

c / c0

0.6

0.4

Adsorption

Degradation

0.2

0.0

301

-8

-4

0

4

8

(b)

6 60 4 40 2 20

0

8

Time / min

0

100

200

300

400

0

Nitrogen content of N-RGO (%)

100

ammonia concentration (mmol L-1) 0 100 2 1000 10 400 40

(a)

BPA removal (%)

1.0

ammonia concentration / mmol L-1

302

Fig. 4. (a) Kinetics of the adsorption and degradation of BPA in the system of PS and N-RGO

303

prepared by adding ammonia at various concentrations. (b) Effect of ammonia concentration

304

on the BPA removal (the adsorption removal and the degradation removal) and nitrogen

305

content of N-RGO samples. Reaction conditions: initial BPA concentration 0.385 mmol L-1,

306

initial PS concentration 0.6 mmol L-1, N-RGO load 120 mg L-1, pH 6.6, and temperature

307

25 °C. PS was added into the reaction solution at time of 0 min.

308

The significant dependence of the adsorption and catalytic properties of N-RGO was also

309

clearly seen from Fig. 4b, which compared the effects of the ammonia concentration on the N

310

content (obtained by XPS analysis, Table S4), the adsorption removal and the degradation

311

removal of BPA. The good similarity of the three curves in shape suggested that the N 15



312

functional groups on the surface of N-RGO were attributed to the greatly improved activity

313

and worked as active sites for both the adsorption and the catalysis. N-RGO may act as an

314

electron-transfer mediator, inducing the catalytic decomposition of PS and then the catalytic

315

degradation of BPA. The N functional surface species, such as C=N, are rich in electrons and

316

thus have a great potential to activate PS. Moreover, N functional groups conjugating with the

317

long-pair electrons can activate the effective site of the sp2 carbon of graphene with abundant

318

free-flowing π-electrons.52

319

The influence of hydrothermal temperature for preparing N-RGO was studied in the range

320

of 100-220 °C (Fig. S8). The change of the hydrothermal temperature had little effect on the

321

adsorption ability of N-RGO, but remarkably influenced its ability of activating PS. The

322

apparent degradation rate constant k was varied from 0.31 min-1 for the hydrothermal

323

temperatures of 100 °C to 0.66, 0.71, and 0.41 min-1 for the hydrothermal temperatures of 140,

324

180 and 220 °C, respectively. The hydrothermal temperature was optimized at 180 ºC. The

325

wide survey XPS spectra of N-RGOs prepared by different hydrothermal temperatures were

326

recorded (Fig. S9) and the surface atomic percentages of C, O and N elements in the samples

327

were obtained (Table S5). It was noted that more N could be doped into the graphene

328

networks by increasing hydrothermal temperature from 100 °C to 180 °C. When the treatment

329

temperature was up to 220 °C, the N content was decreased somewhat. Meanwhile, the

330

oxygen level was decreased gradually with the increase of reaction temperature. Therefore,

331

the dependence of the catalytic properties of N-RGO on hydrothermal temperature may be

332

related to the enhanced N doping when the hydrothermal temperature was not higher than

333

180 °C. 16


Page 16 of 33

Page 17 of 33


334

The degradation rate constant of BPA was increased with increasing the initial

335

concentration of PS (Fig. S10). The k value was increased from 0.004 min-1 for 0.1 mmol L-1

336

PS to 0.007, 0.21, and 0.71 min-1 for 0.2, 0.4 and 0.6 mmol L-1 PS, respectively (Fig. S6).

337

S2O82- is the origin of driving force for the degradation of BPA. It is reasonable that higher

338

S2O82- concentrations lead to increased degradation of BPA. However, the k value was

339

decreased slightly to 0.53 min-1 for 0.8 mmol L-1 PS, which was possibly because that the

340

generated SO4•− might react with superfluous PS to form SO42−.25, 57

341

The effect of initial solution pH was investigated on the BPA adsorption and degradation

342

in the system of N-RGO-PS (Fig. S11). BPA has a pKa of 9.5. When pH > pKa, the adsorption

343

was increased with decreasing pH value; when pH < pKa, the adsorption was almost

344

indifference to solution pH. The phenomenon could be explained by the influence of pH on

345

the interaction types between the (dissociated) phenolics and the surface of N-RGO.40 As for

346

the oxidative degradation itself (after being excluded the possible effect of the adsorption), the

347

catalytic ability of N-RGO for activating PS toward the degradation of BPA was little

348

influenced by the solution pH. Therefore, at the concerned pH values (pH 3-7), both the

349

adsorption ability and catalytic ability of N-RGO was little influenced by solution pH.

350

TOC removal. During the adsorption and degradation of BPA in the presence of PS and

351

N-RGO, it was found that not only the strong absorption peak of BPA at 276 nm in the UV-vis

352

absorption spectra (Fig. S12a) was decreased intensity, but also the absorption at 207 nm

353

being attributed to benzene ring was decreased intensity, suggesting that BPA was mineralized

354

to an extent. Therefore, the TOC analysis was carried out to evaluate the mineralization of

355

BPA (Fig. S12b). It was found that the pre-adsorption caused a TOC removal of 50%, and the 17



Page 18 of 33

356

combination of the pre-adsorption (10 min) and the degradation (30 min) achieved a TOC

357

removal of more than 90%.

358

Radical formation. It was reported that SO4•− was the dominant free radical generated

359

from the decomposition of PS at pH values from 3.0 to 7.0,26 but hydroxyl radicals (•OH)

360

might be also produced.25,

361

coumarin to fluorescent 7-hydroxycoumarin with a maximum emission at 456 nm ,58 we used

362

coumarin as a probe to examine whether •OH was generated or not. However, no formation of

363

7-hydroxycoumarin was observed, indicating that •OH, if any, was not the dominant free

364

radical generated in the N-RGO-PS system.

365

26

Because •OH can quantitatively convert non-fluorescent

The SO4•− formation was further investigated by using other scavengers. It has been

366

reported that methanol can react with SO4•− rapidly (kM = (1.6-7.7) × 107 M−1 s−1).25,

59

367

However, when methanol was added into the N-RGO-PS system, the BPA degradation was

368

decreased slightly (Fig. S13). It was possible that the hydrophilic methanol was difficult to be

369

extensively adsorbed on the N-RGO surface, but the generated SO4•− radicals were bounded

370

on the N-RGO surface. Therefore, KI was used instead of methanol, because KI can react

371

with surface-bound free radicals.60, 61 Indeed, the addition of KI almost completely depressed

372

the oxidation of BPA. Therefore, the surface-bound SO4•− (SO4•−ads) played a dominant role

373

and only a minimal amount of free SO4•− was generated in the catalytic oxidation process.

374

Possible degradation pathway of BPA. By using GC-MS, four main intermediates were

375

identified as phenol, 4-hydroxyacetophenone, 4-isopropenylphenol and BPA-o-catechol

376

(Table S6), being similar to the observation of Zhang et al. for the BPA degradation in a

377

heterogeneous Fenton-like system (CuFeO2/H2O2).18 Based on the identified intermediates 18


Page 19 of 33


378

and the previously reported results,18, 62 a degradation pathway of BPA was proposed (Scheme

379

S1).

380

Reusability of N-RGO. The reuse experiment was carried out to detect the recovery

381

performance of N-RGO (Fig. S14). After the adsorption and degradation of BPA was finished,

382

the used N-RGO was collected by vacuum filtration, washed with methanol and water to

383

neutral pH, and re-dispersed in the fresh BPA solution. Then, 0.6 mmol L-1 PS was added, and

384

the second cycle of degradation was conducted. These steps were repeated several times.

385

After the fifth run, the BPA removal was still about 90%, indicating the excellent reusability

386

of N-RGO.

387

Effect of adsorption on BPA and BPF degradation. As discussed above, the bisphenols

388

were firstly adsorbed to the surface of the catalyst and then degraded in situ by the

389

surface-bound radicals generated from the activation of PS. In the whole oxidation process of

390

BPA, the adsorption and oxidation facilitated one another, greatly promoting the total

391

degradation. Because that the adsorption and the oxidation influence each other, it is difficult

392

to clearly investigate the effects of the adsorption of N-RGO on the oxidation process.

393

Here we designed a set of experiments to differentiate them, in which a series of phenols

394

were used as probes. It was observed that the adsorption capacity of phenolics on N-RGO was

395

deceased in the order of BPA > BPF > 2, 4-DCP > 4-CP > 4-MP > PE (Fig. S15), and the

396

adsorption capacities of BPA and BPF were much higher than other phenolics. For example,

397

the adsorption capacities were 1.56 and 1.43 mmol g-1 for BPA and BPF, being about four

398

times that of PE (0.39 mmol g-1). By using PE as a co-existing phenol, the adsorption and

399

degradation kinetics of BPA and PE in their individual solutions and mixture solution were 19



400

studied as shown in Fig. 5. In the presence of PS, the k values of BPA and PE in their

401

individual solutions (the single pollutant system) were 0.71 and 0.09 min-1, respectively. In

402

the mixture of BPA and PE, the k value of BPA was changed little, but that of PE was

403

decreased to 0.011 min-1. By using BPF instead of BPA, similar observations were obtained

404

(Fig. S16). 1.0

4 0.8

3

c / c0

0.6

0.4

Adsorption

Degradation

0.2

0.0

405

2 1 -8

-4

0

Time / min

4

8

406

Fig. 5. Kinetics of the adsorption and degradation of PE (0.385 mmol L-1) and BPA (0.385

407

mmol L-1) in single- and bi-pollutant systems in the presence of N-RGO (120 mg L-1) + PS

408

(0.6 mmol L-1) at pH 6.6 and 25 °C. Curves: (1) BPA in the system of BPA, (2) BPA in the

409

system of BPA + PE, (3) PE in the system of PE, (4) PE in the system of BPA + PE. PS was

410

added into the reaction solution at time of 0 min.

411

The above experimental results indicated that a high adsorption capacity was favorable to

412

the oxidative degradation of phenols. BPA and BPF could be enriched quickly onto N-RGO,

413

which accelerated their oxidative decomposition by sulfate radicals. Because the adsorption is

414

so important to the oxidative decomposition of phenols on the surface of N-RGO, the

415

degradation of the phenols in their mixtures was primarily determined by their individual

416

adsorption behaviors. By using PE, 4-MP, 4-CP and 2, 4-DCP as co-existing phenols, the 20


Page 20 of 33

Page 21 of 33


417

adsorption and degradation kinetics of the phenolics in their individual single-pollutant

418

solutions and mixture solution were studied (Figs. S17 and S18). After PS was rapidly

419

introduced into the reaction systems, the k values of BPA, BPF, 2, 4-DCP, 4-MP, 4-CP and PE

420

in the single-pollutant system were 0.71, 0.537, 0.517, 0.215, 0.155 and 0.09 min-1,

421

respectively. In general, the k values of phenolics tended to be increased with the adsorption

422

capacities increased. By plotting k of the phenolics against their adsorption capacities (qe) on

423

N-RGO (Fig. 6a), a fairly good linear correlation was found between these two parameters for

424

the adsorption and degradation of the phenolics in their single-pollutant solutions. In their

425

mixture solution (BPA + 2,4-DCP + 4-MP + 4-CP + PE), the degradation rate constant ( km,

426

here the subscript m represents the mixture solution) values of BPA, 2, 4-DCP, 4-MP, 4-CP

427

and PE were 0.365, 0.065, 0.034, 0.02 and 0.011 min-1. (In the BPF-containing mixture

428

solution (BPF + 2,4-DCP + 4-MP + 4-CP + PE),

429

4-CP and PE were obtained as 0.225, 0.058, 0.038, 0.018 and 0.012 min-1, respectively.) By

430

plotting km against qe (qe was obtained in the single-pollutant system), a fairly good linear

431

correlation was also found between these two parameters in the mixture system. These solidly

432

confirmed that the pre-adsorption of the pollutants was a critical step (as the rate-determining

433

step) in the process of their degradation, which was easily explained by considering the

434

attacking from the surface-bound SO4•− radicals in the N-RGO-PS system.

the km values of BPF, 2, 4-DCP, 4-MP,

435

It was noted that the km values of each pollutant in the mixture solution were decreased in

436

comparison with that in the single-pollutant solutions. This is attributed to that the co-existing

437

organic pollutant will compete with it in the consumption of the generated SO4•− radicals. It

438

was evident that the km value of BPA (or BPF) was much greater than those of the other tested 21



Page 22 of 33

439

organic pollutants in the mixture. That is, in comparison with the single-pollutant system, the

440

degradation rate of BPA (or BPF) was only mildly depressed but that of the other co-existing

441

phenolics were depressed greatly in the mixture solutions. Figure 6b compared such relative

442

depression in the apparent degradation rate constant (km/k) in the BPA- and BPF-containing

443

mixtures. Interestingly, the relative depression degree (1 – (km/k)) was only 0.48 and 0.58 for

444

BPA and BPF, being much less than that for other phenolics (0.88, 0.84, 0.87 and 0.88 for 2,

445

4-DCP, 4-MP, 4-CP and PE in BPA-containing mixture and 0.89, 0.82, 0.88, and 0.87 in

446

BPF-containing mixture). This difference may reflect the synergistic effect between

447

adsorption and catalytic degradation: the easier degradation permits more adsorption of the

448

pollutant in the solution, which in turn maintains the easier (faster) degradation. 0.6

0.8

(b)

(a)

BPA

0.6

BPF

BPF

0.4

km / k

k / min-1

2, 4-DCP

BPA

0.4

0.2 0.2

PE

4-MP

4-MP

2, 4-DCP

4-CP

4-CP PE

449

0.0 0.0

0.5

1.0

q / mmol g-1

1.5

2.0

0.0 0.0

0.5

1.0

q / mmol g-1

1.5

2.0

450

Fig. 6. (a) A correlation between the apparent degradation rate constant (k) and the adsorption

451

capacity (qe on N-RGO) for various phenolics in their individual single-pollutant solution.

452

The line is only for eye guide. (b) A correlation between the relative depression of the

453

apparent degradation rate constant (km/k) and the adsorption capacity (qe on N-RGO) for

454

various phenolics in their mixture solution of (solid circles) BPA-containing mixture (BPA + 2,

455

4-DCP + 4-MP + 4-CP + PE) and (open circles) BPF-containing mixture (BPF + 2,4-DCP +

456

4-MP + 4-CP + PE). Here km represents the apparent degradation rate constant of a 22


Page 23 of 33

457


corresponding pollutant in the mixture solution.

458 459

Mechanistic insights. A mechanism was for the adsorption-decomposition of bisphenols

460

on N-RGO in the presence of PS was proposed as follows. Firstly, bisphenols in solution was

461

adsorbed on the surface of N-RGO due to the strong π-π interaction. Secondly, the oxidizing

462

agent was catalytically activated by N-RGO, producing surface-bound SO4•− radicals. Thirdly,

463

the adsorbed bisphenols were in situ degraded by the adjacent SO4•− radicals, releasing the

464

pre-occupied sites as free sites. Fourthly, the released free sites on N-RGO allowed a new

465

adsorption of bisphenols in solution, and then an adsorption-degradation cycle was started

466

again. The above-mentioned cycles were repeated continuously until all the added bisphenols

467

were degraded. The N-RGO surface provided good adsorption sites for bisphenols and good

468

catalytically active sites for the activation of PS. The synergistic effect between the adsorption

469

and the catalysis caused an efficient and rapid degradation and mineralization of bisphenols.

470

In conclusion, N-RGO was prepared by a hydrothermal treatment of RGO. The

471

heterogeneous system of N-RGO and PS could efficiently adsorb and oxidize bisphenols.

472

More than 97% of the added bisphenols was completed removed within 17 min. The

473

promoted adsorption and catalytic performances of N-RGO were attributed to the doping of N,

474

which functioned as active sites for both the adsorption of bisphenols and the activation of PS,

475

producing surface-bound SO4•− radicals. The mechanism study clarified the synergistic effect

476

between adsorption and catalysis on N-RGO. The adsorption-degradation-adsorption recycle

477

system containing N-RGO and PS provides a new strategy for removing EDCs at low levels.

478

ASSOCIATED CONTENT 23



479

Supporting Information

480

Preparation of graphene oxide, identification of aromatic degradation intermediates, and

481

adsorption models. Supporting figures (Figs. S1-S18), Scheme S1 and tables (Tables S1-S6).

482

This material is available free of charge via the Internet at http://pubs.acs.org.

483

AUTHOR INFORMATION

484

Corresponding Authors

485

Tel/fax:

486

[email protected] (H. Tang).

487

ACKNOWLEDGMENTS

488

The authors acknowledge the funding support from the National Natural Science Foundation

489

(Grants Nos. 21377169 and 21177044).

490

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491

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32


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666 667

TOC

668 669

Adsorption Bisphenols

CO 2 + H 2O

Catalysis 1.0

N -RGO

PS RGO+PS

c / c0

0.8 0.6 0.4

N-RGO

Adsorption

Degradation

0.2 0.0

N-RGO+PS -8

-4

0

Time / min

670

33


4

8

Nitrogen-Doped Reduced Graphene Oxide as a Bifunctional Material

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