Nitrogen-doped carbon nanosheets from polyurethane foams...

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Nitrogen-doped carbon nanosheets from polyurethane foams and removal of Cr(VI)Jiaqi Duan1,2, Baohua Zhang1, Huailin Fan2,3, Wenzhong Shen2 and Shijie Qu2,♠

1Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China2State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China3University of Chinese Academy of Sciences, Beijing 100049, China

Received 14 December 2016Accepted 10 February 2017

♠Corresponding AuthorE-mail: [email protected]: +86-351-4053091

Open Access

pISSN: 1976-4251 eISSN: 2233-4998

Carbon Letters Vol. 22, 60-69 (2017)Original Articles

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Copyright © Korean Carbon Society

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AbstractNitrogen-doped carbon nanosheets with a developed porous structure were prepared from polyurethane foams by hydrothermal carbonization following ZnCl2 chemical activation. Scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared spec-troscopy, solid state 13C nuclear magnetic resonance (NMR) spectra and X-ray photoelectron spectroscopy were used to characterize the nitrogen-doped carbon nanosheet structure and composition. The removal of Cr(VI) by the N-doped carbon nanosheets was investigated. The results showed that the maximum removal capacity for chromium of 188 mg/g was found at pH=2.0 with PHC-Z-3. pH had an important effect on Cr(VI) removal and the optimal pH was 2.0. Moreover, amino groups and carboxyl groups in the nitrogen-doped carbon nanosheet played important roles in Cr(VI) removal, and promoted the reduction of Cr(VI) to Cr(III).

Key words: polyurethane forms, nitrogen-doped carbon nanosheet, Cr(VI) removal

1. Introduction

Today, increasing amounts of dyes and heavy metal ions from industry waste water are being discharged into water bodies, adversely affecting the health of humans, animals and plants [1]. To protect the natural environment and non-renewable natural resources, it is essential to control the concentrations of heavy metal ions and dyes before discharge. The removal of pollutants in water is an important issue for human health and social develop-ment. One of these pollutants, chromium, is widely used in chromate manufacturing, leather tanning, wood preservation, and electroplating industries, and generally exists as hexavalent Cr(VI) and trivalent Cr(III) [2]. Cr(III) is harmless at trace levels, whereas Cr(VI) is con-sidered to be a highly hazardous substance because it is toxic after both acute and chronic exposures [3,4]. Due to its mutagenicity, carcinogenicity, genotoxicity and bioaccumulation through the food chain, Cr(VI) can cause severe health and environmental problems. The World Health Organization (WHO) has recommended that the maximum allowable concen-tration of Cr(VI) in drinking water should be less than 0.05 mg/L. The permissible limit of Cr(VI) for industrial waste water to be discharged to surface water bodies is 0.1 mg/L [5,6]. Therefore, for both human health and environmental protection, it is imperative to keep Cr(VI) concentrations in potable water to an acceptable level and to remove it from indus-trial waste water before discharging [6].

To control its concentration, Cr(VI) in solution is usually removed by one or more methods, including adsorption [7], ion exchange [8,9], electrochemical treatment [10], reduction-precipitation [11], membrane separation [12] and solvent extraction [13,14], etc. Among these methods, adsorption has the advantage of low cost, easy operation and

DOI: http://dx.doi.org/DOI:10.5714/CL.2017.22.060

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Three-dimensional porous graphene materials for environmental applicationsMuruganantham Rethinasabapathy, Sung-Min Kang, Sung-Chan Jang and Yun Suk Huh

KCS Korean Carbon Society

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pISSN: 1976-4251 eISSN: 2233-4998

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VOL. 22 April 30 2017

N-doped carbon nanosheets from PUFs and removal of Cr(VI)

61 http://carbonlett.org

ization and chemical activation with ZnCl2. The functionalized nanosheet materials demonstrated a high Cr(VI) removal rate, due to their high nitrogen content and well developed porous structure.

2. Experimental

2.1. Materials

The PUFs were collected from dumped sofas. Glutaric dialdehyde, hydrochloric acid, zinc chloride and potassium bichromate were purchased from Sinopharm Chemical Reagent Co. Ltd., China.

2.2. Preparation of polyurethane-derived po-rous carbon

The preparation steps for the polyurethane-derived porous carbon are shown in Fig. 1. The detailed synthesis procedure is as follows. First, 5 g of PUFs, 1.5 mL glutaric dialdehyde (50%) and 50 mL de-ionized water were added into a 100 mL Teflon autoclave. Then, the mixture was subjected to hydrothermal carbonization at 200°C for 10 h. The products were recovered as a solid residue by vacuum filtration and dried in the oven at 120°C, and designated PHC. Finally, the solid samples were heated under a N2 flow of 30 mL/min at a heating rate of 2°C/min and kept at either 400, 600, or 800°C for 1 h. The resultant products were designated PHC-x, where x is the carbonization temperature.

At same time, the hydrothermal product was chemically activated using ZnCl2. The hydrothermal product was thoroughly mixed with ZnCl2 (ZnCl2/PHC at a weight ratio of 1-4) by grinding in an agate mortar. The mixture was heated to 650°C under N2 flow with a heat rate of 2°C /min and kept at 650°C for 1 h. The obtained material was leached with excess amounts of 1 M HCl solution to remove the residue of zinc compounds, then washed with distilled water until it reached neutral pH, then was dried at 120°C. The resultant samples were denoted as PHC-Z-y, where Z represents the use of ZnCl2, and y represents the weight ratio of ZnCl2/PHC.

high efficiency [15]. Various types of adsorbents are used to remove Cr(VI), such as activated carbon [16-18], magnetic materials [19-22], composites [23,24], and so on. Among these materials, porous carbons have notable performance in removing Cr(VI) because they have well developed porous structures and large internal surface area. Various carbon precursors also provide convenient and diverse methods for preparing the carbon materials.

It has been reported that functional groups on the acti-vated carbon surface, especially nitrogen-doped groups, have a great influence on Cr(VI) removal because of electrostatic interaction between the functional groups and chromium ions [25-30]. Functionalized graphene oxide has exhibited high efficiency for Cr(VI) removal [31]. Used as sorbents, reusable polypyrrole hollow nanofibers with a capsular wall have shown a significantly high adsorption capacity of 839.3 mg/g for Cr(VI) at pH=2 (from an initial concentration of 200 mg/L) [6]. However, these materials have relatively complex synthetic routes, involving templates, grafting, and so on. It would be advantageous to develop a simple method of syn-thesizing N-doped carbon materials for the removal of heavy metal ions.

Polyurethane foams (PUFs) are extensively utilized in the modern polymer industry due to their high strength, satisfac-tory abrasion resistance, and inertness against oxygen and ozone [32-34]. With their increasing production, more and more PUFs waste is being generated at the same time. Using PUFs as a carbon precursor is a promising way to both recycle PUFs and prepare carbon materials [32,34]. In our previous work [35], ni-trogen-doped carbon materials were prepared successfully from PUFs through the hydrothermal carbonization method. The original morphology and nitrogen groups of the polyurethane were preserved when introduced as crosslinking agents. And the new carbon material successfully adsorbed CO2 from air.

Generally, the structure and surface chemical composition of carbon materials determine their application properties. Re-cently, carbon materials have received considerable attention because of their novel morphologies, which endow them with distinct physical and chemical properties.

In this study, a porous carbon nanosheet was synthesized using PUFs as the carbon precursor, by hydrothermal carbon-

Fig. 1. The synthetic procedure for forming the polyurethane-derived porous carbon.

Carbon Letters Vol. 22, 60-69 (2017)

DOI: http://dx.doi.org/10.5714/CL.2017.22.060 62

was provided by a monochromatic Al Kα (1486.6 eV) source. Total chromium concentration was analyzed by Thermo

iCAP6300. The Cr(VI) concentration was analysed by Lambda 25. Survey scans were received using a 100 eV pass energy, while high resolution scans of specific elements were obtained using a 20 eV pass energy. Solid state 13C nuclear magnetic resonance (13C NMR) spectra were acquired on a broad-bore Bruker Advance III 600 MHz NMR spectrometer (14.09 T) operating at a frequency of 151.00 MHz. The experiments were conducted on a Bruker 4-mm double-resonance MAS probe at room temperature, and samples were filled in 4 mm cylindrical ZrO2 rotors with a spinning frequency of 9 kHz. CP/MAS procedure (cross-polarization/magic angle spinning) was employed for GlcNH2 and GlcNAc measurement, while the HPDEC procedure (high-power decoupling) was used for the residues collected at 750°C. The 13C NMR spectra were acquired with a single pulse length of 2.4 μs, a recycled delay between two consecutive scans of 2 s, acquisition time of 0.0225 s and a contact time of 2.0 ms. Chemical shifts in the 13C NMR spectra were referred to the carbonyl line of glycine (with a signal at 176.0 ppm from tetramethylsilane). The collected 13C NMR data were processed using the Topspin 3.1 software (Bruker, USA).

3. Results and Discussion

3.1. Characterization of the samples

3.1.1. SEM analysisSEM images of the samples are shown in Fig. 2. It can be ob-

served that the PHC produced a ripple structure, compared with the image of the PUF. As the carbonization temperature increased, the coarse and aggregated surface appeared. It is worth noting that relatively hierarchical nanosheets appeared on the surfaces of the PHC-Z series. The nanosheet structure became more clear with the increasing ratio of ZnCl2/PHC. PHC-Z-3, which has a thickness of 50 nm, was the clearest in the PHC-Z series. The nanosheet struc-ture was broken when the ratio of ZnCl2/PHC reached 4. Compared with the aggregated structure of the PHC series, this suggests that ZnCl2 acted as both an activating agent and template.

After carbonization, the ZnCl2 was leached by HCl, and the nanosheet structure was retained.

A high ratio of ZnCl2 to PHC caused excess activation and the nanosheet structure was destroyed.

2.3. Cr(VI) adsorption

A series of different concentrations of Cr(VI) solution were prepared by dissolving K2Cr2O7 in distilled water. The concen-tration of Cr(VI) was measured by ultraviolet–visible spectros-copy at λ=540 nm via photometric diphenylcarbohydrazide method. The color indicator was prepared by dissolving 200 mg of 1,5-diphenylcarbohydrazide (DPC) into 50 mL acetone, which was then diluted into 100 mL solution with distilled water.

The adsorption experiment was conducted in aqueous solu-tion at 25°C in a 50 mL beaker with magnetic stirring. During the test, a 100 mg sample was put into a 25 mL solution with different Cr(VI) concentrations.

The adsorption capacity qe (mg/g) was calculated according to the following equation.

(1)

where C0 and Ce (mg/mL) represent the concentration of equi-librium chromium ions before and after adsorption, respective-ly; V (mL) is the solution volume; W (g) represents the mass of the adsorbent.

2.4. Characterization

Nitrogen adsorption-desorption isotherms of the carbon ma-terials were measured at –196°C by a Micromeritics ASAP 2020 adsorption apparatus (USA). The adsorption branch isotherms were adopted to calculate the surface areas and pore size distri-butions of samples using the Brunauer-Emmett-Teller method (P/P0 of 0.01 to 0.2) and non-local density functional theory method (slit pore model). The total pore volumes (Vtotal) were counted based on the adsorbed amount at a P/P0 of 0.99. Micro-pore volumes (Vmicro) were calculated using the t-plot method. The scanning electron microscopy (SEM) images were obtained with an S-4800 field emission-scanning electron microscope (Hitachi, Japan) manipulated at 1 kV. Fourier transform infrared (FT-IR) spectra of the samples were obtained on a Nicolet FT-IR 380 spectrometer by the conventional KBr pellet technique. The thermogravimetric behavior of the sample was carried out under Ar flow from 30 to 800°C with a 10°C/min heating rate using a thermogravimetric analyzer (TG-DTA 8120; Rigaku, Japan). The X-ray photoelectron spectra (XPS) were recorded with an ESCALAB 250 (Thermo Electron, USA); the X-ray excitation

Fig. 2. Scanning electron microscopy images of (a) polyurethane foams, (b) PHC, (c) PHC-400, (d) PHC-600, (e) PHC-800, (f ) PHC-Z-1, (g) PHC-Z-2, (h) PHC-Z-3, and (i) PHC-Z-4.



carbonization temperature, the yields of PHC-400, PHC-600, and PHC-800 based on PHC were 82.8%, 56.16%, and 39.01%, respectively.

The calculated special surface areas and pore volumes of the PHC series were less than 40 m2/g and 0.06 cm3/g, respectively, which were much lower than that of the PHC-Z series. PHC-Z-3 had the maximum specific surface area and pore volume, which were 950 m2/g and 0.91 cm3/g, respectively. When the ratio of ZnCl2/PHC was increased to 4, the specific surface area and pore volume dropped. This may have been due to over-activation by excessive ZnCl2 so that the pore structure was collapsed during the carbonization step.

3.1.4. Fourier transform infrared The FT-IR spectra of samples are shown in Fig. 4. The broad

band appearing at 3419 cm–1 was attributed to the –OH groups or the –NH stretching vibration. The bands appearing at 2966 cm–1, 2925 cm–1, and 2852 cm–1 were attributed to C–H stretch-ing vibrations in aliphatic –CH, –CH2, and –CH3, respectively. The weak bands at around 1197 cm–1 and 1100 cm–1 might be ascribed to –C–O stretching. There were two adsorbed bands at 1514 cm–1 and 1446 cm–1 in the PHC series, which could be as-cribed to the secondary amine group and aromatic C=C stretch-ing. The PHC-Z series had relative strong peaks at 1625 cm–1 and 1387 cm–1, which could be ascribed to C=N stretching and N-C=O– skeletal vibration.

3.1.5. 13C nuclear magnetic resonanceTo further analyze the functional groups of the materials, 13C

NMR was performed, and the spectra of PHC, PHC-400, PHC-600 are shown in Fig. 5. The chemical shifts at 17, 75, and 142 ppm could be attributed to C–N, C–O, and C=N, respectively. This was consistent with the results based on the FT-IR. There was only one strong peak at 128.19 ppm in PHC-600, which could be attributed to C=N or aromatic C=C. This indicates that a higher carbonization temperature can cause further aromatiza-tion and condensation of carbon, and as a result, the 13C NMR signal of PHC-600 was weak. It could not be detected in PHC-800.

3.1.2. ThermogravimetricThe thermal loss curves of the PUFs and PHC are shown in

Fig. 3. A great weight loss appeared between 245 and 400°C for PUF, and appeared from 330 to 400°C for PHC. The resi-dues of the PUF and PHC were 50.11% and 58.83% at 400°C. The weight loss curves of the PUF and PHC were relatively gentle above 400°C and their yields were 43.73% and 42.16% at 800°C This suggests that the thermostability of PHC was higher than that of PUF, because polycondensation may have occurred between a hydrolysis intermediate of the PUF and glutaric dialdehyde under the hydrothermal process, which may cause aromatization, so that the thermostability of the PHC was improved.

3.1.3. Primary parametersThe elemental compositions and pore structure parameters

of PHC, PHC-400, PHC-600, PHC-800, PHC-Z-1, PHC-Z-2, PHC-Z-3, and PHC-Z-4 are listed in Table 1. The nitrogen content was decreased by carbonization temperature, or by the ratio of ZnCl2 to PHC, and PHC-Z-3 had the highest ni-trogen content. While the yield was markedly decreased with

Fig. 3. Thermogravimetric analysis curves of hydrothermal products. PUF, polyurethane foams.

Table 1. Primary parameters of samples

SampleStructure parameter Elemental composition (%)

SBET (m2/g) Vtotal (cm3/g) C H O N

PHC 2.65 0.01 38.77 5.35 54.02 1.85

PHC-400 6.84 0.02 71.56 7.03 18.12 3.29

PHC-600 5.09 0.01 64.7 3.82 26.17 5.31

PHC-800 32.40 0.05 82.7 4.55 6.66 6.09

PHC-Z-1 473.24 0.29 71.3 4.81 17.28 6.61

PHC-Z-2 471.97 0.33 69.08 4.59 20.56 5.76

PHC-Z-3 950.54 0.91 54.13 6.10 32.54 7.21

PHC-Z-4 278.21 0.11 60.02 2.28 30.95 6.57

BET, Brunauer-Emmett-Teller.



series were similar at a pH of 1–3, but it was greatly decreased at a pH of 4–6 in the K2Cr2O7 aqueous solution with a concentra-tion of 400 mg/L. The optimum pH for the maximum removal of chromium is 2.0, which resulted in the removal of 99.595 mg/g Cr(VI) by PHC-Z-3.

It was observed that a green color, indicating the presence of Cr(III), existed in the solution at pH=2 after 12 h of adsorption. To account for this, the following reduction reactions of Cr(VI) by electrons might occur [37]:

Cr2O72–+14H++6e–↔2Cr3++7H2O

HCrO4–+7H++3e–↔Cr3++4H2O

To investigate the chromium ion states in solution, induc-tively coupled plasma (ICP) analysis was used to determine the Cr(III) and Cr(VI) contents in solution after treatment with PHC-Z-3 at different pHs and initial concentrations. The results are shown in Fig. 7.

It can be seen that Cr(III) existed in all of the solutions, which confirms that a reduction reaction occurred between the Cr(VI) and Cr(III) in the acid solution. The maximum Cr(III) content was found at about pH=2. This means that the transformation between Cr(VI) and Cr(III) occurred in acid solution, and that excess acid might restrain this conversion, especially in the 400 mg/L initial concentration.

3.2. Cr(VI) removal from solution by nitrogen-doped carbon nanosheet

Based on the differences in the special surface areas and pore volumes of the PHC series and PHC-Z series, it can be inferred that the PHC-Z series would have the better Cr(VI) removal ability, and this can be confirmed in the discussion about the effect of the initial concentration on Cr(VI) removal ability. The main discussion about the influence of various factors on the sorbents will center on the PHC-Z series.

3.2.1. Effect of initial pH on Cr(VI) removal In general, the pH level of the solution has a great influence

on the adsorption of heavy metal ions by sorbents in aqueous solutions. Depending on pH, Cr(VI) normally exists in the an-ionic form as Cr2O7

2– , HCrO4–, or CrO4

2–. When the pH value of the solution is below 1, the primary specie is H2CrO4. When the pH value of the solution is between 2 and 6, Cr2O7

2– and HCrO4

– ions coexist in equilibrium. In alkaline media (pH>8), the primary state of Cr(VI) is as chromate anions [36].

The effect of pH value (1.0–6.0) on the Cr(VI) removal be-havior of the PHC-Z series was investigated, and the results are shown in Fig. 6. The Cr(VI) removal properties of the PHC-Z

Fig. 4. Fourier transform infrared spectra of (a) PHC series and (b) PHC-Z series.

Fig. 5. 13C nuclear magnetic resonance structure of samples.

Fig. 6. The removal efficiency depends on the pH of the initial solution.



PHC-Z-3-Cr.The spectra for O 1s, N 1s, and Cr 2p are shown in Fig. 8.

The Cr 2p spectrum displays three bands at 577, 578.56, and 586.8 eV, which are attributed to Cr(III) 2p2/3, Cr(VI) 2p2/3 and Cr(III) 2p1/2, respectively. This indicates that the Cr(VI) was partially reduced to Cr(III), and both of them were adsorbed by the PHC-Z-3.

As shown in Fig. 8c, the O 1s spectrum can be separated into three peaks at binding energies of 532.94, 532.11, and 531.38 eV, which can be attributed to C–O–C, –OH and C=O, respectively. It can be observed that the peaks of the three kinds of oxygen in PHC-Z-3-Cr remained, whereas the rela-tive intensity of the peaks changed. The relative intensity of both the peaks of C–O–C and –OH decreased, while the C=O peak greatly increased.

The N 1s spectrum in the PHC-Z-3 could be divided into two peaks at binding energies of 398.15 and 400.50 eV, cor-responding to the nitrogen of –N= and –NH+, respectively. Compared with PHC-Z-3, the relative intensity of both amino groups –N= and –NH+ decreased in the PHC-Z-3-Cr. And, a new peak at 399.19 eV, which corresponds to –NH– also appeared.

Based on the above analysis, the mechanism of Cr(VI) re-moval is presented in Fig. 9. Because of the abundant H+ sur-rounding the surfaces of the samples, the amine groups are easily protonated, to give a positive electron at low pH level. This is beneficial for adsorbing HCrO4

– and HCr2O72– through

At the same time, chelation occurred between the chromium ions and the –NH2 and –OH groups on the PHC-Z series, which enabled more of the ions to be adsorbed on the sorbents [31]. The negative hydroxyl ions in solution increased with increasing pH, which made it hard for the functional groups to interact with the chromium ions, so that the reduction reaction was restrained [38].

It is also worth noting that chromium concentration also played a role in the adsorption at low initial concentration, such as 100 and 200 mg/L, so that the change in pH had little influ-ence on the adsorption capacity and just promoted conversion between Cr(VI) and Cr(III).

On the other hand, the change in pH had an obvious influence on the adsorption ability of the PHC-Z-3 at high initial concen-trations, such as 400 and 800 mg/L. The appropriate acid content promoted the reduction reaction and adsorption for Cr(VI) and Cr(III). PHC-Z-3 had the main role in the catalytic conversion at low pH.

3.2.2. XPS characterization of Cr(VI) removalIn order to clarify the mechanism of the adsorption of Cr(VI)

on PHC-Z-3, XPS analysis of the PHC-Z-3 was conducted be-fore and after chromium removal (denoted as PHC-Z-3-Cr) at a solution pH=2. Fig. 8a shows the XPS survey spectra of PHC-Z-3 and PHC-Z-3-Cr. The typical binding energy patterns of C 1s, O 1s and N 1s appeared in the XPS spectra for PHC-Z-3, and the typical binding energy patterns of Cr 2p appeared for

Fig. 7. ICP analysis of chromium concentration in solution with the sorbent PHC-Z-3 at different pH and with initial concentrations of (a) 100 mg/L, (b) 200 mg/L, (c) 400 mg/L, and (d) 800 mg/L.



3.2.3. Effect of initial concentration of on Cr(VI) removalThe effects of initial Cr(VI) concentration on Cr(VI) re-

moval by PHC-Z-1, PHC-Z-2, PHC-Z-3 and PHC-Z-4 were investigated at an initial pH=2 with an adsorbent dose of 4 g/L. The results are shown in Fig. 10a. When the initial Cr(VI) concentration was increased from 200 to 1200 mg/L, the amount of Cr(VI) removal by the PHC-Z series increased from 45 to 188 mg/g, while the removal efficiency decreased from 98% to 54%. The Cr(VI) removal capacity of PHC-Z-1, PHC-Z-2, and PHC-Z-4, which was 136, 162, and 127 mg/L, respectively, almost reached maximum at 600 mg/L. The Cr(VI) removal capacity of PHC-Z-3 approached the maxi-mum of 188 mg/L at 800 mg/L initial concentration, which was the best Cr(VI) removal amount for the entire PHC-Z series. This may be attributable to PHC-Z-3 having the high-est surface area and high N content.

electrostatic attraction. When Cr(VI) was adsorbed on these protonated amine

groups, the redox reaction between Cr(VI) and Cr(III) was pro-moted by the existence of these protonated amine groups. Some of the adsorbed Cr(VI) ions were reduced to Cr(III) with the oxidation of hydroxyl groups, which caused the formation of additional carboxylic groups [31]. Because of these reductions and chelation, the most common form of chromium was Cr(III) on the surface of the sorbents. When the acidity of the solution was decreased, the surface amine groups had difficulty being charged. In this case, the electrostatic attraction between the ad-sorbent and Cr(VI) ions decreased, and the removal efficiency dropped. This could also explain why PHC-Z-3 had the best Cr(VI) removal ability, because it contained the highest amount of nitrogen groups.

Fig. 8. (a) X-ray photoelectron spectra survey, (b) Cr 2p of PHZ-3-Cr, (c) O 1s, and (d) N 1s of PHC-Z-3 and PHC-Z-3-Cr.

Fig. 9. The mechanism for removal of Cr(VI).



4. Conclusions

In summary, Nitrogen-doped porous carbon nanosheets based on PUFs were prepared by hydrothermal carbonization and ZnCl2 activation for chromium removal in aqueous solution. The nanosheet structure contributed to a high specific surface area, and resulted in more nitrogen groups being in contact with ions in the aqueous solution. Meanwhile, the content of the ni-trogen groups was an important factor in the Cr(VI) removal. The maximum removal capacity for chromium of 188 mg/g was found at pH=2.0 with PHC-Z-3. Compared to other excellent sorbents [9,38], the removal ability of PHC-Z-3 was higher, and the procedure was more simple.

Conflict of Interest

No potential conflict of interest relevant to this article was reported.

Acknowledgements

The authors gratefully acknowledge the financial support of this work by the National Basic Research Program of China (No. 2012CB626806), International Science & Technology Coopera-tion Program of China (No. 2011DFA51980), National Science Foundation of China (No. 21276266 and U1510122), Shanxi Coal Based Key Scientific and Technological Project (MD2014-09) and Zhengzhou Tobacco Research Institute of CNTC project (GY-KF-[2014]-001).

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Fig. 10. Effect of initial concentration on the removal of Cr(VI) by samples (a, b).

Fig. 11. Effect of contact time on the removal of Cr(VI) by samples.



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