Selective Adsorption of Naphthalene in Aqueous Solution on

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Selective Adsorption of Naphthalene in Aqueous Solution on Mesoporous Carbon Functionalized by Task-specific Ionic Liquid Jie Zhou, Bin Yang, Zhongjian Li, Lecheng Lei, and Xingwang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5040144 • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Industrial & Engineering Chemistry Research

Selective Adsorption of Naphthalene in Aqueous Solution on Mesoporous Carbon Functionalized by Task-specific Ionic Liquid Jie Zhoua, b, Bin Yanga, Zhongjian Lia, Lecheng Leia, Xingwang Zhanga* a Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China b Department of Environmental Science and Engineering, Hangzhou Dianzi University, Hangzhou, 310018, China

ABSTRACT: As a large group of persistent organic pollutants, polycyclic aromatic hydrocarbons (PAHs) commonly coexist with chlorinated organic compounds in industrial wastewater. Preferential adsorption of PAHs from wastewater and final effluent is essential in pretreatment processes. In this study, a novel nitrogen doped mesoporous carbon (NMC) with high surface area (2130 m2/g) and bimodal pore size distribution (~2nm and ~9.4nm) was synthesized via solvent evaporation induced self-assembly process using task-specific ionic liquid (TSIL) as nitrogen source. The selective adsorption of naphthalene (NAP) with pentachlorophenol (PCP) on NMC in aqueous solution was investigated compared with conventional ordered mesoporous carbon (OMC). Kinetic studies revealed that the NAP adsorption process achieved equilibrium within 5 min and followed a pseudo-second-order rate equation. The adsorption capacity of NMC for NAP was 3.2mmol/g, which was about 13 times that for PCP at pH 10 in a bi-solute system. The equilibrium adsorption of NAP onto

ACS Paragon Plus Environment


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NMC in single- and bi-solute system could be well described by Freundlich isotherm model. The higher adsorption capacity and selectivity of NMC for NAP compared with OMC was ascribed to its unique bimodal pore system and π–π dispersive interactions. Our research also provided a new methodology for the synthesis of N-doped mesoporous carbon materials based on TSILs.

KEYWORDS: N-doped mesoporous carbon; Task-specific ionic liquid; Selective adsorption; Naphthalene; Pentachlorophenol

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are a large group of persistent organic pollutants, and listed as US-EPA and EU priority pollutants.1,

2

Natural sources

originate in terrestrial coal deposits, volcanic eruptions and forest fires. Anthropogenic sources of PAHs include combustion of fuel, exhaust gas from vehicles, and especially industrial wastewater is significant compared to natural sources in the environment.3,

4

PAHs are usual resistant to chemical/biological

degradation, persistent in environmental mobility, and have a tendency for bioaccumulation in human and animal tissue.5 Therefore, PAHs pose significant impacts on human health and the environment due to their carcinogenicity, mutagenicity, and toxicity, even at extremely low concentrations.6 In general, PAHs cannot be efficiently removed by traditional physicochemical processes such as flocculation, sedimentation, filtration or ozonation.7 Up to now, many new technologies have been developed for removing PAHs from wastewater,


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such as adsorption,8, 9 chemical oxidation10 and photooxidations.11 Adsorption has been considered as one of the most competitive technologies for PAHs removal from effluents because of its high efficiency and simplicity. PAHs commonly coexists with other chlorinated phenols in industrial wastewater, so preferential adsorption of PAHs from industrial wastewater is essential in pretreatment processes in order to improve the biodegradability of subsequent effluents.12 Moreover, it is obviously important to remove PAHs from final effluent preventing them from being discharged into the water body. As a consequence, highly selective adsorbent materials are gaining much attention in recent years. Activated carbon has been widely used for PAHs adsorption,13 but its irregular pore size distribution is not favorable for selective adsorption of PAHs in aqueous solution. Nowadays, molecularly imprinted polymers are notable attributed to their high affinity towards a given target molecules,14, 15 however, the synthesis strategy of molecularly imprinted polymers is complex because it includes copolymerizing functional monomers, cross-linking agent, polymerization initiator, and template or print molecule. . Surfactant impregnation has also been adopted for modification of different adsorbents to improve their adsorption selectivity and performance such as activated carbon,16 chitosan hydrogel beads17 and bentonite.18 However the impregnated surfactant is apt to fall off from the adsorbents during the process. Ordered mesoporous carbons (OMCs) are interesting alternatives for removing organic pollutants from effluents due to their outstanding properties such as high specific surface area, tunable pore structure and high thermal and mechanical



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stability.19 It has been reported that OMCs provide high adsorption capacity towards many organic and inorganic compounds.20,

21

The adsorption performance and

selectivity could be improved by adjusting the functional groups on the surface of OMCs.22, 23 Incorporation of heteroatoms (e.g., N, B, and S) onto the carbon can modify

its

surface

and

physicochemical

properties.24-26

Among

them,

Nitrogen-containing carbons as a kind of fascinating materials have attracted worldwide attention recently.25,

27

They can enhance chemical, electrical, and

functional properties in a wide range of applications because of the inclusion of nitrogen atoms in the carbon architecture. Homogeneous incorporation of nitrogen into the carbon motif and surface can be realized by in situ doping of carbons using nitrogen containing precursors.28 Some nitrogen-doped mesoporous carbons have been prepared using ethylenediamine,29 aniline,30 polyacrylonitrile31 and aromatic polyamide32 as nitrogen source and mainly periodic mesoporous silicas (SBA-15 or MCM-48) as hard templates. But such nanocasting technology is industrially unfeasible because it is low-yield, complicated, and high-cost.33 Therefore, the soft template approach34 toward nitrogen-doped ordered mesoporous carbons with open frameworks is desirable. Ionic liquids (ILs) are a class of materials that are liquid at room temperature with high synthetic flexibility.35 Significant attention has been paid to the application of ionic liquids as neoteric media for material synthesis.36 The possible combinations of cations and anions are enormous, which would offer quite a lot of opportunities for tailor-made mesoporous carbon materials from ILs. Many ILs are composed of an organic, nitrogen-containing cation and a bulky inorganic anion.


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ILs will be a potential functional precursor when dicyanamide is used as the anion, not only because the cross-linkable dicyanamide anion might lead to C−N condensation, but also because ILs featuring dicyanamide anions consist entirely of C, N, and H atoms and possess high nitrogen content.37 ILs with dicyanamide anions have been presented to be versatile precursors for synthesis of nitrogen doped carbons through nanocasting approaches.38 Therefore, TSILs are expected to be excellent precursors to synthesize functional N-doped mesoporous carbons with controlled pore architectures. To date, knowledge about the selective adsorption of PAHs from wastewater containing PAHs and ionizable chlorinated organic pollutants is still scare. Naphthalene is a typical polycyclic aromatic hydrocarbon and is easily found in the industrial wastewater; therefore naphthalene could be a representative of PAHs pollutants. Pentachlorophenol is a typical ionizable chlorinated organic chemical. Its pKa value is close to 7.0 (Table 1), and thus it can exist either as neutral or dissociated species in wastewater. In this study, a novel N-doped mesoporous carbon (NMC) was synthesized via solvent

evaporation

induced

self-assembly

process

using

TSIL

(1-butyl-4-dimethylaminopyridinium dicyanamide) with cross-linkable anion as nitrogen source. And selective adsorption of NAP with PCP on NMC from an aqueous solution was investigated. The adsorption kinetics and equilibrium of NAP on synthesized mesoporous carbons from aqueous solution have been carried out by



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fitting the adsorption data to different kinetics and isotherm models respectively.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals including acetone, NaN(CN)2, AgNO3, CaCl2, NaN3, phenol, formaldehyde, HCl, NaOH, tetraethyl orthosilicate (TEOS), methanol and ethanol

were

purchased

from

China

Sinopharm

Reagent

Co.

Ltd..

4-Dimethylaminopyridine and 1-iodobutane used in the synthesis of ionic liquid was purchased from TCI.

Pluronic F127, as soft template, PCP and NAP (99.9% purity)

were supplied from Sigma. Chemical properties of NAP and PCP are listed in Table 1. Millipore water was used in all experiments. Table 1.Chemical Properties of Naphthalene and Pentachlorophenol Adsorbate

MW (g/mol)

Cs(mg/L)

Log Kow*

pKa

Naphthalene

128.2

31.7

3.36

/

Pentachlorophenol

266.4

14

5.25

4.74

MW: molecular weight; Cs: water solubility; Kow: octanol-water partition coefficient.

2.2. Preparation of N-doped ordered mesoporous carbon.

2.2.1.

Synthesis

([BDMAPy][N(CN)2],

of

1-butyl-4-dimethylaminopyridinium

dicyanamide

BDMAPy-dca). Structure of BDMAPy-dca is shown in

Chart S1 in the Supporting Information. Classical alkylation and ion exchange reaction were used to synthesis the functional ionic liquid. Synthesis route of BDMAPy-dca is presented in Figure S1 (Supporting Information). Detailed synthetic procedures are provided in experimental section of the Supporting Information. 1H NMR spectra of BDMAPy-dca is also available in Figure S2.


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2.2.2. Synthesis of functional resol precursor. A base-catalyzed solution polymerization process39 was modified to synthesize the soluble functional resol precursor using phenol, BDMAPy-dca and formaldehyde. In a typical synthesis, 20.0g of phenol was melted at 42 °C to liquid, and then 4.26g of 20 wt% NaOH solutions was dripped into the round-bottom flask under vigorous stirring. After homogeneous mixing, 35.4g of 37 wt% formalin was put into the solution dropwise, followed by adding 1.5g BDMAPy-dca at once. Keep the reaction at 70 °C for 1h with stirring. The pH value of the mixture was adjusted to around 7 using 1.0M HCl solution after the mixture was cooled down to ambient temperature. Water in the mixture was removed by a rotary evaporator under vacuum. 20 wt% ethanol solution of functional resol was obtained by dissolving the yellowish transparent resol in absolute ethanol. Finally the sodium chloride precipitation was separated by centrifugation.

2.2.3. Preparation of N-doped mesoporous carbon. N-doped mesoporous carbon was prepared by the solvent evaporation induced co-assembly of functional resol, F127 and TEOS, followed by thermal polymerization, carbonization and desilication.39 In a typical preparation, 9.6g of F127, 60g of ethanol and 6.0g of 0.2M HCl solution were mixed with stirring. After F127 dissolved, 12.48g of TEOS and 30.0g of 20 wt% ethanol solution of functional resol were added into the solution successively. The transparent mixture was poured into Petri dishes after the solution was stirred for 2h at 40 °C. The dishes were open for about 5h at ambient temperature in order to evaporate ethanol in the solution. Thermal polymerization of the EISA product was carried out in an oven at 100 °C for 24h. The amber condensation



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polymer was scraped off from Petri dishes, and carbonized at 600 °C for 6h with a heating rate of 1 °C /min under nitrogen atmosphere. The char was grinded till it could pass through 200 mesh sieves. The silica in char was removed by 20 wt% HF solution for 24h with stirring. The final materials were denoted as NMC. The schematic illustration of synthesis procedure for NMC is shown in the Figure 1.

Figure 1. Schematic illustration of synthesis procedure for nitrogen-doped mesoporous carbon (NMC) by task-specific ionic liquid

For comparison, the pure ordered mesoporous carbon (OMC) was also synthesized. Its synthesis procedure was exactly the same as that of the NMC, except that the precursor was 20 wt% resol and without ionic liquid.40

2.3. Characterization. The small angle X-ray diffraction patterns of the mesoporous samples was recorded with an ARLSCINTAGX′TRA diffractometer using Ni filtered


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Cu-Kα radiation (λ=1.5406Å) over a 2θ angle range of 0.5~6° with a 0.04° step size and 2s step time. The morphologies of all the synthesized samples were observed by a transmission electron microscope (TEM) instrument (FEI Tecnai G2 Sphera) operating at 200 kV. Each sample was dispersed in ethanol with ultrasonic bath for about 10 min and a drop was deposited on a Cu grid. Nitrogen adsorption-desorption isotherms were performed at 77 K with an ASAP 2020 analyzer. Before adsorption measurement, the samples were degassed for at least 6 h at 250°C under vacuum. The specific surface areas (SBET) were determined by the Brunauer-Emmett-Teller (BET) method using adsorption data in a relative pressure range from 0.02 to 0.3. The pore size distributions (PSD) were obtained from the adsorption branches of the isotherms by means of Barrett-Joyner-Halenda (BJH) model. The total pore volumes (Vt) were calculated from the amount of nitrogen uptake at a relative pressure P/P0 of 0.99. The micropore volume was estimated by the t-plot method. Elemental analysis (EA) of elements C, N, and H was performed with a Flash EA 1112 Elemental Analyzer. X-ray photoelectron spectroscopy (XPS) measurement was accomplished in a Kratos Amicus XPS instrument with a monochromatic Al-Kα X-ray source. The sample was evacuated at high vacuum before analysis and then put into the chamber. The high resolution O1s and N1s spectra of the synthesized samples were recorded.



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A batch equilibration technique referring to method previously reported41 employed to determine the point of zero charge (PZC) of the mesoporous carbons. Firstly, 10mL of 0.01 M NaCl electrolyte solution was added into vials. The initial pH values (pHinitial) of the NaCl solution in vials were adjusted in the pH range from 2 to 12 by 0.1 M solution of HCl or NaOH. Then 5 mg of mesoporous carbon was mixed with the NaCl solution. The vials were sealed and agitated for 24 h. The final pH values (pHfinal) of the solutions in vials were determined after the mixture was filtered. The PZC was presented from a plot of pHfinal vs. pHinitial. The PZC is the point that pHinitial is exactly equal to pHfinal. 2.4. Adsorption Experiments. NAP and PCP were dissolved in methanol to prepare stock solutions. NAP and PCP solutions were prepared by adding a certain amount of stock solution in methanol into background solution containing 0.01mol/L CaCl2 and 200mg/L NaN3 in distilled water. The effect of solution pH on the NAP and PCP removal was investigated in the initial pH range of the solutions from pH 3 to 11(Figure S3). The pH was adjusted using 0.1M solution of HCl or NaOH, and measured by a pH meter. The NAP and PCP percent removals were calculated as follows:

Removal (%) 

C0  Ce  100 (1) C0

where C0 and Ce (mg/L) are the concentrations of NAP and PCP in filtrate at initial and at equilibrium, respectively.


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As can be seen from Figure S3, NMC showed independent of pH effect for NAP removal and the PCP removal decreased significantly with increasing initial pH of the solution from pH 3 to 11. It was found that the highest PCP removal (98.52%) was attained at PCP initial concentration of 8mg/L and pH 3. The pH of PCP solution was adjusted above 7 in the following adsorption experiments because of the high adsorption potential and low solubility of PCP. Consequently, the initial concentration span of PCP can be expanded in two orders of magnitude (from 0.4 to 40 mg/L). In order to investigate the adsorption kinetics of NAP and PCP onto the mesoporous adsorbents, 16 mg of carbon was added into 200 mL NAP solution of initial concentrations 25 mg/L in a 250 mL flask and stirred continuously at 25±1°C. And 25 mg of carbon was mixed with 200 mL PCP solution of initial concentrations 20 mg/L in a 250 mL flask and stirred continuously at 25±1°C and pH 7. Samplings were performed by fast filtration using a 0.45 µm PTFE syringe filter at different time intervals until the equilibrium reached. The NAP and PCP uptake at any time, qt (mmol/g), was calculated as follows: qt 

(C0  Ct )V WM

(2)

Where C0 and Ct is the concentrations of NAP and PCP in filtrate (mg/L) at initial and at certain time, t (min), respectively. V is the volume of the solution (L) and W is the mass of adsorbent (g). M is the molar mass of NAP and PCP (g/mol). All adsorption isotherms in single- and bi-solute systems were conducted at 25±1°C. PCP solutions with concentration range from 0.4 to 40 mg/L were prepared



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by adding a certain amount of PCP stock solutions in methanol into background solution. Single-solute adsorption isotherms were performed by mixing 60 ml of PCP solutions of different concentrations (0.4~40 mg/L) and NAP solutions (0.3~30 mg/L) with 3 mg of the mesoporous carbons in Erlenmeyer flasks at pH 7. Bi-solute adsorption isotherms were carried out by mixing 60 ml of PCP and NAP solutions with different equivalent molar concentration with 3 mg of the mesoporous carbons at pH 7 and 10. The flasks were parafilmed and agitated at 200 rpm for 24h to reach equilibrium. Samplings were done by fast filtration after 24h. Analyses of NAP and PCP concentrations were carried out by a reverse-phase HPLC (Agilent 1260, XDB-C18, 150 mm ×4.6) with UV detector at a wavelength of 220 nm. Mobile phase was a mixture of water and ethanol (90/10, v/v) with a flowrate of 1 mL/min. The retention time for NAP and PCP is about 2.7 and 4.0 min, respectively. The experiments were performed with quadruplicates. NAP and PCP concentrations were quantified with an external standard method. Experimental uncertainties were assessed in flasks without the mesoporous carbons, which indicated that total uncertainty was less than 5% of the initial concentrations. The NAP and PCP uptake at equilibrium, qe (mmol/g), was calculated as follows: qe 

(C0  Ce )V WM

(3)

Where Ce is the concentrations of NAP and PCP in filtrate at equilibrium (mg/L).

3. RESULTS AND DISCUSSION 3.1. Structural Properties of OMC and NMC. It has been proposed that structural


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properties and surface chemistry are the two important factors affecting the adsorption behavior

of

mesoporous

carbons.42

The

pore

size

distributions

and

N2

adsorption-desorption isotherms and of OMC and NMC are presented in Figure 2. The structural parameters of the mesoporous carbons are given in Table 2. Figure 2 demonstrated that OMC has mesoporous structures because it showed a type IV isotherm and was composed completely of mesopores (Table 2). The OMC also has uniform bimodal mesopores, which centered at about 2.4 nm and 6.5 nm (Figure 2). In addition, a pronounced broad H1 hysteresis loop at high relative pressure is shown in the adsorption-desorption isotherm of OMC. Well-defined steps appear at P/P0 = 0.4~0.8, which is related to nitrogen filling of the mesopores on account of the capillary condensation.

1600

5

1400

Volume STP (cc/g)

1200

4

3

dV/dlog(D) (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

1000 800

600 400

200 0.0

2

0.2

0.4

0.6

0.8

1.0

P/P0

NMC OMC

1

0 0

10

20

30

40

50

Pore size (nm)

Figure 2. Pore size distributions of OMC and NMC. Inset: N2 adsorption-desorption isotherms of the samples.



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Table 2. Structural Properties of OMC and NMC sample

SBET (m2/g)

Vtot (cm3/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

DT (nm)

OMC

1421

1.48

0

1.48

4.16

NMC

2130

2.19

0.25

1.94

4.22

SBET, BET surface area; Vtot, total pore volume at P/P0 = 0.99; DT, the average pore size calculated from 4Vtot/SBET.

It was indicated that the mesoporosity of NMC was still preserved after BDMAPy-dca incorporation because NMC presented a similar type IV isotherm, which was further proven by the bimodal mesopores centered at around 1.9 nm and 9.4 nm. Although a small quantity of micropores presented in NMC, the mesopores still were prominent. The larger mesopores of NMC around 10nm might be resulted from removal of the larger micelles generated between the functional resol and F127 surfactant during the EISA and thermal treatment. The smaller pores were possibly formed within the carbon walls because the mesoporous skeleton was partially distorted after N incorporation. The increase in the BET specific surface area and total pore volume of NMC might be ascribed to decomposition of nitrogen and oxygen functional groups of TSIL on the surface. Small-angle XRD patterns of OMC and NMC are shown in Figure 3. The OMC showed intense (1 0 0) diffraction peaks, which demonstrated that OMC had well-ordered hexagonal mesoscopic structure. In the pattern of NMC, the distinct intensity decrease of (1 0 0) reflection could be observed, which might be caused by the incorporation of TSIL. It was suggested that NMC still possessed ordered mesostructure, however, TSIL decreased the structural ordering of NMC. The phenomenon was very consistent with the following TEM morphologies (Figure 4).


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100 OMC

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 NMC

0

1

2

3

4

2θ (degrees)

Figure 3. Small-angle XRD patterns of the OMC and NMC

Furthermore, the structures of synthesized mesoporous carbons were verified by TEM characterizations. The representative TEM images of OMC and NMC are shown in Figure 4. As shown in Figure 4a, parallel channels were clearly observed on OMC, confirming the existence of well-ordered mesoporous structure. For NMC sample, ordered strip-like mesochannels could also be found (Figure 4b), which indicated that NMC maintained the ordered mesostructure as pure OMC. But NMC also had some less ordered arrays. The surface of NMC was slightly graphitized and some of the channel was collapsed. The disorder of the graphite sheets was possibly caused by the presence of nitrogen heteroatoms in the skeleton of NMC. The TEM morphologies were in agreement with the results of pore size distribution (Figure 2) and XRD (Figure 3).



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Figu ure 4. TEM im mages of (a) OMC and (b) NMC.

3.2.. Surface Chemistry of Carboon Adsorbents. The surface chhemistry off the messoporous caarbons was characterizzed by EA, XPS and PZC. P The buulk and surrface elem mental conttents from EA E and XP PS are listed d in Table 3. The dataa indicated that therre was no nitrogen n preesent in OM MC while th here was a small amoount of nitro ogen pressent in NM MC (EA:0.667%, XPS:00.99%), dem monstratingg that a few w of

nitro ogen

conttaining funnctional grooups formeed in the carbon maatrix and ssurface by the BDM MAPy-dca incorporation . n and oxyggen on the surface off the The possibble functionnal groups of nitrogen syntthesized meesoporous carbons c weere also stud died by XP PS spectrosccopy. The high resoolution O1ss and N1s spectrum s off NMC are presented in Figure 55. Two sam mples exhibited high oxygen conntent (OMC C: 6.76%, NMC: N 5.82% %). It might derive from m the preccursors withh oxygen coontaining fuunctional gro oups. The deconvolute d d O1s specttrum reveealed the exxistence of four types of oxygen functionalities at bindding energiees of ~5331 eV (O1, quinine), ~533 ~ eV (O2, carbonyll), ~535 eV V (O3, C-O--C), and ~537.5


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eV (O4, chemisorbed oxygen of carboxylic or water).43 And it was reported that the XPS N1s spectrum can be fitted by three components at binding energies of about 397.7, 399.9, and 401.3 eV.44-46 The three peaks are relevant with three various nitrogen configurations: pyridinic-N (N1), pyrrolic-N or pyridonic-N (N2), and graphitic N species (N3). The detailed XPS analysis of synthesized mesoporous carbons are listed in Table 4, including calculated N-doped and oxygen types (atom %). As shown in Table 4, carbonyl is the predominant oxygen containing functional group on OMC and NMC, and graphitic-N and pyridinic-N are two main nitrogen containing functional groups on NMC. The oxygen and nitrogen containing functional groups (i.e., C-O, C=O, COO-, C=N and C-N) might rim on the edge of connected mesoporous network of NMC. The large specific surface area, bimodal mesopore, oxygen containing groups and especially different C–N bonding configurations might make NMC a potential adsorbent for organic compound.

Table 3. Elemental Analysis of Adsorbents Total elemental analysis (atom %)

Elemental analysis by XPS (atom %)

sample C

H

O

N

C

N

O

OMC

88.61

1.37

10.02

⁄

93.24

⁄

6.76

NMC

88.71

1.31

9.31

0.67

93.19

0.99

5.82



O1s

Intensity (a.u.)

N1s Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

Graphitic-N Pyridinic-N

Carbonyl

Ether

Pyrrolic-N 392

394

396

398

400

402

404

406

408

410

412

526

528

Binding energy (eV)

530

532

534

536

538

540

Binding energy (eV)

Figure 5. Representative XPS N1s and O1s spectra of NMC Table 4. Detailed XPS Analysis of Carbons: Calculated N-doped and Oxygen functionalities content (atom %) Element ID, functionality

OMC

NMC

N1, pyridinic (~397.8eV)

̶

0.21

N2, pyrrolic/pyridonic (~399.5eV)

̶

0.17

N3, graphitic (~401.3 eV)

̶

0.61

O1, quinone (~531.0 eV)

0.28

0.00

O2, carbonyl (~532.8 eV)

6.06

5.46

O3, ether (~535.0 eV)

0.42

0.36

O4, chemisorbed oxygen (~537.3 eV)

0.00

0.00

The PZC measurement of the mesoporous carbons in the pH range from 2 to 12 is presented in Figure 6. OMC had the lower pHPZC value of about 2.5, and NMC exhibited a higher pHPZC value of around 4.0. The pHPZC value of the NMC was higher than that of OMC after nitrogen containing functional groups originated from BDMAPy-dca were formed on the mesoporous carbon surface. It might make the NMC surface less negative charged at pH above 7, which would result in weaker electrostatic repulsion between PCP- and NMC than OMC in neutral or alkaline solution.


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12

10

OMC NMC

8

pHfinal

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6

4

2 2

4

6

8

10

12

pHinitial

Figure 6. Determination of the PZC of OMC and NMC.

3.3. Adsorption Kinetics Studies. To predict adsorbate uptake rate and determine the equilibrium time, the pseudo-first-order (Eqs 4)47 and pseudo-second-order (Eqs 5)48 kinetic models were applied to describe the kinetics of the adsorption process. log(qe  qt )  log qe 

t / qt 

k1  t 2.303

(4)

1  t / qe  1 / v0  t / qe k 2  qe 2

(5)

Where qe and qt are the amounts of organic pollutant adsorbed (mmol/g) at equilibrium and at time t (min), respectively, k1 is rate constant of first-order adsorption (min-1), and k2 is the rate constant for pseudo-second-order model (g/(mmol·min)). The corresponding parameters and correlation coefficients for the kinetic models are given in Table S1. The pseudo-second-order model fitted the adsorption kinetic data of NAP and PCP on mesoporous carbons well (Figure 7) and the regression



coefficients are close to unity (R2>0.995), suggesting that the adsorption process on the mesoporous carbons can be described by the pseudo-second-order model. OMC and NMC both displayed much higher initial sorption rate and adsorption capacity of NAP than PCP. The initial sorption rate v0 of NAP on OMC was 6.04 mmol/(g·min), which was almost 32 times that of PCP. And it took less than 5min to reach the adsorption equilibrium. The equilibrium adsorption capacity of NMC for NAP and PCP were 2.95 mmol/g and 0.65 mmol/g, respectively, which were higher than that of OMC. It was inferred that the incorporated nitrogen functional groups on NMC derived from BDMAPy-dca could improve the adsorption of NAP and PCP significantly. The enhancement might attribute to the intensification of π-π dispersion interaction which is discussed in detail in section 3.5.

3.5

0.8

NAP 3.0

PCP

2

NMC: t/qt=1/7.09+t/2.9484 R =0.99511

0.7

2

NMC: t/qt=1/0.475+t/0.64928 R =0.99819

0.6

2.5

2

2

OMC: t/qt=1/0.185+t/0.51406 R =0.99869

OMC: t/qt=1/6.04+t/2.63906 R =0.99791

qt(umol/g)

qt (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

2.0

OMC NMC

1.5

0.5 0.4 0.3

OMC NMC

1.0 0.2

0.5

0.1

0.0

0.0

0

50

100

150

200

0

50

Time (min)

100

150

Time (min)

200

Figure 7. Pseudo-second-order kinetics for adsorption of NAP and PCP on carbons.

3.4. Adsorption Isotherms in Single-Solute Systems. In order to have a comprehensive understanding of the adsorption of NAP and PCP on the mesoporous carbons, in this study, adsorption isotherms of NAP and PCP were investigated in


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single-solute systems. Langmuir and Freundlich models were employed to fit the adsorption isotherm data. Langmuir isotherm model assumes monolayer adsorption onto a surface including a finite number of adsorption sites.49 The Langmuir isotherm model can be expressed as follows:

qe 

K L  qm  C e 1  K L  Ce

(6)

Where Ce is the solute concentration at equilibrium (mg/L), qe is the amount of absorbate adsorbed at equilibrium (mmol/g), qm is the monolayer capacity of the adsorbent (mmol/g), KL is the Langmuir adsorption constant related to the energy of adsorption (L/mg), respectively. Freundlich model is an empirical equation and suitable for describing the adsorption on heterogeneous surfaces. It is supposed that the active sites with strong binding force are occupied by adsorbate primarily and then the adhesion decreased with the increase of site occupation.50 The Freundlich isotherm is given by the following equation:

qe  K f  Ce1/n

(7)

Where Kf (mmol∙(L/mg)1/n/g) and n are Freundlich constants, which are relevant to adsorption capacity and energy distribution of the adsorption sites. They can indicate how favorable the adsorption process.



1.4

4.0 3.5

PCP

NAP

1.2

3.0

1.0 2.5 2.0 1.5

OMC NMC Langmuir ------- Freundlich

1.0 0.5

qe(mmol/g)

qe(mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

0.6

OMC NMC Langmuir ------- Freundlich

0.4

0.2

0.0

0.0 0

5

10

15

0

5

10

Ce(mg/L)

15

20

25

30

Ce(mg/L)

Figure 8. Langmuir and Freundlich isotherms for single-solute adsorption of NAP and PCP on carbons.

Single-solute isotherms for NAP and PCP by NMC and OMC are presented in Figure 8. Isotherm data were fitted by both Langmuir and Freundlich models (Figure 8). All the correlation coefficient, R2 values and the constants obtained from the two isotherm models used for adsorption of NAP and PCP on the carbons are given in Table S2. The Freundlich model presented the higher R2 values which were greater than 0.96, representing that the adsorption of NAP and PCP on the mesoporous carbons could be well described by Freundlich model rather than Langmuir model. All the 1/n values calculated from the Freundlich model were below one, indicating that adsorption of NAP and PCP on the carbons was favorable. The results agreed with the previous works which reported that the Freundlich model gave a better fit than the Langmuir model on the adsorption of NAP and PCP using various adsorbents, such as activated carbon,51 and exchanged Al-MCM-41 materials52. This deduced that the heterogeneity on the surface of the mesoporous carbons played an important role in NAP and PCP adsorption. The adsorption capacity of NMC for PCP (1.3mmol/g) was higher than that of OMC (0.9mmol/g). This improvement was mainly attributed to the synergistic effect of weaker electrostatic repulsion and stronger π-π dispersion


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interaction between PCP- and NMC than OMC at pH 7. And the adsorption capacity of NMC for NAP (3.5 mmol/g) was also much higher than that of OMC (2.5 mmol/g).

3.5. Selective adsorption of naphthalene. Two different solution pH values were employed in this study to investigate the effect of nitrogen-doping on selective adsorption of NAP in bi-solute solution. Bi-solute isotherms for NAP and PCP on NMC and OMC at pH 7 and 10 are presented in Figure 9. Isotherm data were fitted by Freundlich model (Figure 9). Respective fitting parameters were given in Table S3. The higher R2 values obtained by the Freundlich isotherm were close to unity, representing that the Freundlich model also had good fits for NAP and PCP on the carbons in bi-solute system at pH 7 and 10. All the 1/n values fitted from the Freundlich model were below one, showing that adsorption of NAP and PCP on the carbons in bi-solute system was favorable as well. It was exciting that significant selective adsorption was observed between NAP and PCP in bi-solute system when the solution pH value varied from 7 to 10. The adsorption capacity of NAP on OMC and NMC was 1.6 mmol/g and 2.1 mmol/g, which were about 2.2 and 1.5 times of PCP on OMC and NMC at pH 7, respectively. Interestingly, the adsorption capacity of NAP was more than 13 times of PCP on NMC and 10 times of PCP on OMC at pH 10. It indicated that NMC had a great potential to adsorb NAP preferentially from industrial wastewater.



2.5

2.5

NMC pH 7

OMC pH 7 NAP PCP

NAP PCP

2.0

qe(mmol/g)

qe(mmol/g)

2.0

1.5

1.0

1.5

1.0

0.5

0.5

0.0

0.0 0

50

100

0

150

50

100

150

Ce(umol/L)

Ce(umol/L)

4.0

4.0

OMC pH 10

3.5

NMC pH 10

3.5

NAP PCP

3.0

3.0

2.5

qe(mmol/g)

qe(mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

2.0 1.5

2.0 1.5

1.0

1.0

0.5

0.5

0.0

NAP PCP

2.5

0.0

0

50

100

150

0

50

100

150

Ce(umol/L)

Ce(umol/L)

Figure 9. Competitive adsorption isotherms of NAP and PCP on carbons at pH 7 and 10. (Solid lines show the calculated curves, based on the Freundlich adsorption isotherms.)

3.6. Adsorption Mechanism. It was reported that the electrostatic and π-π dispersion interaction are the main interactions between carbonaceous adsorbent and adsorbate.53,

54

Therefore, the

adsorption behavior of the synthesized mesoprous carbons for NAP and PCP are affected by the solution pH and surface chemistry to a great extent.22 In this study, when the solution pH is higher than pKa value of PCP, PCP is dissociated to PCP-, electrostatic interaction between PCP- and carbon surface occurs and would be significant.55,56 Moreover, NAP is a typical non-ionized POPs, and can exist in the molecular state in aqueous solution with a wide pH range. Under the circumstance, π-π dispersion interactions between the NAP aromatic rings and the mesoporous carbons are primary.


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Functional groups and delocalized electrons determine the surface chemistry characteristics on the carbon adsorbents. It was deduced that the presence of the carboxyl groups hindered the adsorption of phenolic compounds.57 The π electrons on the surface of carbon might be localized by oxygen containing functional groups, and thus, lower the dispersive forces with NAP and PCP π-electron. The surface chemical property of nitrogen-doped material is also associated with the presence of the lone-pair electrons.42 Graphitic N generated from substituting C atoms in graphene with electron-rich N atoms would offer two electrons to the π electron system, while carbon atom only contributes one π electron. Pyridinic N could also raise π electron density near the Fermi level.58 So nitrogen functional groups on the surface of NMC might contribute extra electrons to the π system to intensify the π-π dispersion interaction. Therefore, the selective adsorption of NAP and PCP on OMC and NMC from aqueous solution involves a complex interaction between π-π dispersion and electrostatic interactions, especially in a bi-solute system. Electrostatic repulsion between PCP- and the negatively charged surface of OMC and NMC resulted in smaller adsorption capacity of PCP than that of NAP in single-solute and bi-solute system. Moreover electrostatic selective adsorption between OH- and PCP- decreased the adsorption capacity for PCP greatly when solution pH increased from 7 to 10. In this case, the π-π dispersion forces became more significant because the accumulation of negative charge on the surface of the mesoporous carbons resulted in electrostatic repulsions rather than attractions. The highly selective adsorption of NAP in bi-solute system on NMC was mainly attributed to intensification of π-π dispersion interaction not only because NAP is rich in π electron, but also because NMC might contribute



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extra electrons to the π system than OMC.

4. CONCLUSIONS A novel nitrogen-doped mesoporous carbon (NMC) with high surface area (2130 m2/g) and bimodal pore size distribution (~2nm and ~9.4nm) has been prepared for the first time by soft template method using task-specific ionic liquid (1-butyl-4-dimethylaminopyridinium dicyanamide) as nitrogen source. The selective adsorption of NAP as a model of PAHs with PCP on NMC was investigated in comparison with OMC in aqueous solution. The adsorption capacity of NMC for NAP was about 13 times that for PCP at pH 10 in bi-solute system. The higher adsorption capacity of naphthalene on NMC compared to OMC is ascribed to its unique bimodal pore system and π–π dispersive interactions. Kinetic studies revealed that the sorption process achieved equilibrium within 5 min and followed a pseudo-second-order rate equation. Freundlich isotherm model well represented the equilibrium adsorption of NAP onto NMC. ■ASSOCIATED CONTENT

Supporting Information Available: Chemical structure, detailed synthetic procedure and 1H NMR spectra of task-specific ionic liquid as nitrogen source for NMC, effect of solution pH on NAP and PCP removal by NMC, parameters and correlation coefficients for kinetic models, single-solute isotherm models and bi-solute isotherm model. This information is available free of charge via the Internet at http://pubs.acs.org/.


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■AUTHOR INFORMATION

Corresponding Author *X. Zhang. E-mail:[email protected]

■ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China (Project No. U1162128, 21276231 and 21406044), the Natural Science Foundation of Zhejiang Province (LY12E06001) and the Planned Science & Technology Project of Zhejiang Province (2013C31016). ■REFERENCES (1) Manoli, E.; Samara, C. Polycyclic aromatic hydrocarbons in natural waters: sources, occurrence and analysis. TrAC, Trends Anal. Chem. 1999, 18, 417-428. (2) Liu, X.; Zhang, G.; Jones, K. C.; Li, X.; Peng, X.; Qi, S. Compositional fractionation of polycyclic aromatic hydrocarbons (PAHs) in mosses (Hypnum plumaeformae WILS.) from the northern slope of Nanling Mountains, South China. Atmos. Environ. 2005, 39, 5490-5499. (3) Mastral, A. M.; García, T.; Murillo, R.; Callén, M. S.; López, J. M.; Navarro, M. V. Measurements of Polycyclic Aromatic Hydrocarbon Adsorption on Activated Carbons at Very Low Concentrations. Ind. Eng. Chem. Res. 2002, 42, 155-161. (4) Anbia, M.; Moradi, S. E. Removal of naphthalene from petrochemical wastewater streams using carbon nanoporous adsorbent. Appl. Surf. Sci. 2009, 255, 5041-5047. (5) Morales, L.; Martrat, M. G.; Olmos, J.; Parera, J.; Vicente, J.; Bertolero, A.; Ábalos, M.; Lacorte, S.; Santos, F. J.; Abad, E. Persistent Organic Pollutants in gull eggs of two species (Larus michahellis and Larus audouinii) from the Ebro delta Natural Park. Chemosphere 2012, 88, 1306-1316. (6) Sun, Y.-X.; Hao, Q.; Xu, X.-R.; Luo, X.-J.; Wang, S.-L.; Zhang, Z.-W.; Mai, B.-X. Persistent organic pollutants in marine fish from Yongxing Island, South China Sea: Levels, composition profiles



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Selective Adsorption of Naphthalene in Aqueous Solution on

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