Copolymer-Templated Synthesis of Nitrogen-Doped Mesoporous

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Copolymer-Templated Synthesis of Nitrogen-Doped Mesoporous Carbons for Enhanced Adsorption of Hexavalent Chromium and Uranium Yang Song, Guoyu Wei, Maciej Kopec, Linfeng Rao, Zhicheng Zhang, Eric Gottlieb, Zongyu Wang, Rui Yuan, Gang Ye, Jianchen Wang, Tomasz Kowalewski, and Krzysztof Matyjaszewski ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00103 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Nano Materials

Copolymer-Templated Synthesis of Nitrogen-Doped Mesoporous Carbons for Enhanced Adsorption of Hexavalent Chromium and Uranium Yang Song,†,‡,# Guoyu Wei,†,# Maciej Kopeć,‡ Linfeng Rao,§ Zhicheng Zhang,§ Eric Gottlieb,‡ Zongyu Wang,‡ Rui Yuan,‡ Gang Ye,*,† Jianchen Wang,† Tomasz Kowalewski,‡ Krzysztof Matyjaszewski*,‡ †

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear

and New Energy Technology, Tsinghua University, Beijing 100084, China ‡

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh,

Pennsylvania 15213, United States §

Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road,

Berkeley, California 94720, United States

Abstract: Polyacrylonitrile (PAN) homopolymer and polyacrylonitrile-block-poly(n-butyl acrylate) (PAN-b-PBA) block copolymer were synthesized via supplemental activator reducing agent (SARA) atom transfer radical polymerization (ATRP) and used as precursors to nitrogendoped nanocarbons. Carbonization was performed at two different temperatures (500 and 800 ºC) to fabricate nanocarbons with different structural properties and nitrogen contents.

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Copolymer-templated nitrogen-doped carbons (CTNCs) had larger surface area and higher mesoporosity than PAN homopolymer-templated nanocarbons (PANCs), due to the presence of PBA block acting as a sacrificial porogen. Adsorption performances of PANCs and CTNCs for Cr(VI) and U(VI) species were systematically studied. Due to the well-defined structure and larger surface area, CTNCs showed stronger adsorption ability than PANCs. The nitrogen atoms incorporated into the carbon framework led to higher electrostatic attraction for Cr(VI) anions at low pH and complexation with U(VI) cations at high pH. Theoretical maximum adsorption capacities of CTNC-500 on Cr(VI) and U(VI) were 333.3 mg/g (pH= 2) and 17.2 mg/g (pH= 5), respectively. CTNCs also showed preferential adsorption for U(VI) compared to other ions, which might be explained by the hard and soft acids and bases (HSAB) theory. Thus, the copolymer-templated nitrogen-doped mesoporous carbons developed in this study represent a new class of nanocarbon sorbents with potential for removing heavy metal contaminants in either cationic or anionic form from aqueous media and thus mitigating environmental pollution. Keywords: block copolymer, nitrogen-doped carbons, adsorption, chromium, uranium

Introduction Mesoporous carbons have been widely explored over the past few decades due to their high surface area, excellent physicochemical properties and availability of various synthetic strategies,1, 2 in applications such as electrode materials,3 catalyst supports4 as well as sorbents of gases,5 metal ions6 and organic molecules.7 Incorporation of heteroatom dopants (e.g., N, F, S, B, P) in the sp2 framework has been demonstrated to improve the properties of various graphitic carbons.8-10 Among them, nitrogen-doped mesoporous carbons are the most widely investigated due to their superior performance in electrochemical devices,11, 12 catalysis13 or CO2 adsorption.14


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A copolymer-templating method was developed by our groups for facile preparation of nitrogen-doped mesoporous carbons.15 Namely, poly(n-butyl acrylate)-block-polyacrylonitrile (PBA-b-PAN) synthesized by atom transfer radical polymerization (ATRP) was used as the block copolymer (BCP) template, which could form various morphologies through microphase separation and generate porous carbon in a subsequent thermal annealing procedure. Usually, a moderate temperature (200-300 °C) oxidative annealing step is necessary to preserve BCP nanostructures,16 followed by an anaerobic carbonization at higher temperatures (above 500 °C) to decompose the PBA block and carbonize the PAN block, generating mesoporous channels and nitrogen-doped carbon framework, respectively. The morphology or mesopore size could be easily tuned by varying the relative lengths of the PBA and PAN blocks.17, 18 Compared to hardtemplating methods, this method avoided the introduction of highly corrosive reagents for removing hard templates, e.g., SiO2 particles.19 Resulting copolymer-templated nitrogen-doped nanocarbons (CTNCs) were successfully introduced to a variety of fields including supercapacitor,20 oxygen reduction reaction (ORR),17 hydrogen evolution reaction (HER),21 CO2 adsorption22 and dye-sensitized solar cell (DSSC).23 Importantly, CTNCs exhibit great potential in adsorption area, which could efficiently and selectively adsorb CO2 due to the high surface area and abundance of nitrogen surface sites.22 However, to more effectively exploit their capability as sorbent materials, extensive work involving optimization of the structural properties and tuning the composition of nitrogen dopants while identifying their speciation is required. Hexavalent chromium and uranium are two highly toxic contaminants in polluted soil and water bodies. Generally, hexavalent chromium exists in the form of anions (e.g., HCrO4-, CrO42-, Cr2O72-), while uranium is in the form of uranyl, a di-oxo cation (UO22+). Removing these ions from contaminated solutions is of great importance for human’s health and ecosystem.


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Additionally, uranium is an important source for nuclear fuels, recycling of which is meaningful for the sustainable development of nuclear energy.24-27 Carbonaceous materials have been demonstrated to be efficient sorbents for hexavalent chromium28-31 and uranium.32, 33 Due to the absence of surface functional groups, surface modification of carbonaceous materials was typically conducted for improving adsorption performances of metal ions.29, 32-34 In comparison, nitrogen-doped carbons have intrinsic structural advantage with abundant functional sites incorporated into the carbon framework, requiring no extra modification procedures. However, unlike traditional surface-engineered sorbents, the adsorption behavior and mechanism of nitrogen-doped carbons toward hexavalent chromium and uranium have not been adequately investigated.35-38 In this work, PAN homopolymer and PAN-b-PBA copolymer were synthesized by supplemental activator reducing agent (SARA) ATRP39-41 and pyrolyzed to obtain nanoporous carbons in order to study the impact of surface area and porosity on metal ions adsorption (Scheme 1).18 Different carbonization temperatures, 500 °C and 800 °C, were used to generate nitrogen-doped nanocarbons with different nitrogen contents to evaluate the function of nitrogen active sites. Overall, four kinds of nitrogen-doped nanocarbons were obtained, namely PANCs, nanocarbons obtained by pyrolysis of PAN homopolymer, and CTNCs, obtained by pyrolysis of PAN-b-PBA block copolymer. Hexavalent chromium and uranium adsorption experiments were conducted using PANCs and CTNCs as sorbents. The impact of both porosity and nitrogen contents were systematically studied.


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Scheme 1. Synthetic route of PAN-b-PBA and preparation of CTNC.

Experimental Materials. Acrylonitrile (AN, Sigma-Aldrich, >99%) and n-butyl acrylate (BA, SigmaAldrich, >99%) were purified by removing the inhibitor with a basic alumina column before use. 2-Bromopropionitrile (BPN, Sigma-Aldrich, 97%), copper(II) bromide (CuBr2, Acros Organics, >99%), dimethyl formamide (DMF, Fisher, 99.9%), dimethyl sulfoxide (DMSO, Fisher, 99.9%), methanol (Fisher, 99.9%) and potassium dichromate (K2Cr2O7, GR) were used as received. Copper wire (diameter 1.0 mm, Aldrich, 99.9%) was cleaned with HCl/MeOH solution (1:1, v/v) before adding to the experimental system. Tris(2-pyridylmethyl)amine (TPMA) was synthesized referring to the previous reported procedure.42 Characterization. Gel permeation chromatography (GPC) was used to determine the molecular weight (Mn) and molecular weight distribution (MWD, Mw/Mn). The GPC system used a Waters 515 HPLC pump and a Waters 2414 refractive index detector using Waters column (Styrogel 102, 103 and 105 Å) with 10 mM LiBr-containing DMF as the eluent at 50 °C (flow rate = 1 mL/min). Linear poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) were


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used as standards for PAN and PAN-b-PBA, respectively. Monomer conversions, PAN molecular weights and copolymer composition were determined by 1H NMR, using a Bruker Avance 300 MHz spectrometer, in DMSO-d6 or DMF-d7. Transmission electron microscopy (TEM, HT-7700, Hitachi Ltd. Tokyo, Japan) was conducted at an accelerating voltage of 120 kV. Nitrogen adsorption measurements were performed on a Surface Area and Porosity Analyzer (Nava 3200e). All samples were degassed at 300 °C for 8 h prior to measurement. Brunauer−Emmett−Teller (BET) method was used to calculate specific surface area, while Barrett−Joyner−Halenda (BJH) model with the Kruk−Jaroniec−Sayari (KJS) correction was adopted to obtain the information of adsorption isotherms and pore size distribution. The t-plot method with the KJS thickness correction was utilized to estimate the surface area of micropores. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi X-ray Photoelectron Spectrometer Microprobe, with a 900 mm spot size. Synthesis of PAN-Br Macroinitiator. Thirty milliliters of AN (4.6 × 10-1 mol) was bubbled with nitrogen for 20 min, followed by injecting into a 100 mL Schlenk flask. Copper wire (10 cm × 1mm) was placed in the Schlenk flask in advance. CuBr2 (2.3 × 10-5 mol), TPMA (6.9 × 10-5 mol), DMF (3.75 mL) and DMSO (37.5 mL) were mixed and bubbled with nitrogen for 20 min. After adding BPN (2.3 × 10-3 mol), the mixture was transferred to the Schlenk flask under nitrogen. The reaction was conducted at room temperature for 4 h. Polymerization was stopped by addition of aerated DMSO and the PAN-Br macroinitiator was precipitated in methanol. After filtering, the solid product was dried in a vacuum oven for 24 h. Synthesis of PAN-b-PBA. PAN-Br (8.2 g, Mn,GPC= 5100, Mw/Mn= 1.21) was mixed with 50 mL DMF in a 100 mL Schlenk flask under magnetic stirring . 17.5 mL BA (1.2 × 10-1 mol) was slowly added, preventing PAN-Br precipitation. Then, CuBr2 (6.1 × 10-6 mol) and TPMA (1.8 ×


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10-5 mol) were added in sequence, followed by bubbling with nitrogen for 30 min. After adding copper wire (14 cm × 1mm) under nitrogen, the reaction started. The polymerization was kept for 33.5 h. DMF was used to stop the polymerization, while methanol/water (1:1, v/v) was used to precipitate PAN-b-PBA. The resulting block copolymer was filtered and dried in a vacuum oven for 24 h. Preparation of PANCs and CTNCs. PAN-Br or PAN-b-PBA powder was placed into a tubular atmosphere furnace, which was heated to 280 °C at a rate of 1 °C/min under air flow (150 mL/min) and kept at this temperature for 1 h. After stabilizing, the furnace was naturally cooled to room temperature. Then, the stabilized samples were further carbonized at 500 or 800 °C for 30 min with a heating rate of 10 °C/min under nitrogen gas flow (150 mL/min). The resulting nanocarbons were named as PANC-500, PANC-800, CTNC-500 and CTNC-800. (“500” and “800” represent pyrolysis temperatures.) Adsorption Experiments. Cr(VI) adsorption experiments at different pH were conducted using Cr(VI) solution with the initial concentration of 200 mg/L, while 100 mg/L of U(VI) solution was utilized for studying effect of pH. The pH of Cr(VI) solution and U(VI) solution were adjusted by 0.5 mol/L HCl or 0.5 mol/L NaOH solution and calibrated by a PHS-3C model meter. Cr(VI) and U(VI) adsorption rate by CTNC-500 were respectively studied with 100 mg/L Cr(VI) solution and 50 mg/L U(VI) solution. Different concentrations of Cr(VI) and U(VI) solutions were used to obtain adsorption capacity and adsorption isotherms. Adsorption experiments were all conducted at 22 °C. After adsorption, the sorbent was separated from the aqueous phase by 0.45 µm micropore filters. The residual concentration of Cr(VI) and U(VI) was respectively determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES)


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and 721 type spectrophotometer at 650 nm (arsenazo III method43). The removal efficiency R and the adsorption capacity Q of Cr(VI) or U(VI) were defined as: Q =

(C 0 − C t ) × V M

(1) R(%) =

C0 − Ct ×100% C0

(2) where C0 and Ct are the initial and the residual concentration of Cr(VI) or U(VI) in aqueous solution. V represents the volume of the initial solution, while M is the dosage of the sorbent. Competitive adsorption experiments were conducted using the simulated U(VI)-containing waste water with various interfering ions. The concentrations of different coexisting ions were determined by ICP-AES and listed in Table S1.

Results and discussion Synthesis of PAN-b-PBA Block Copolymer. PAN-Br macroinitiator with degree of polymerization (DP) of 86 was first synthesized by SARA ATRP. Table S2 shows that the polymerization was well controlled since the theoretical molecular weight (Mn,theory) was close to Mn,GPC. This is also supported by narrow MWD (Mw/Mn = ~1.24). PAN-Br (DP= 86) was then used as the macroinitiator for synthesis of the PAN-b-PBA copolymer. To assure ~40 wt% of PAN in the copolymer, which was verified to yield well-defined porous carbon morphologies,17, 20

80% of the conversion of BA was targeted when the initial mole ratio of BA/PAN-Br was

70/1. The excellent consistency between Mn,theory and Mn,NMR (11,600 determined by NMR) and


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narrow MWD (Mw/Mn = 1.24) were observed for the resulting BCP. Composition of the block copolymer was determined by NMR to be PAN86-b-PBA54 (i.e. ~40 wt% of PAN). Structural Properties of PANCs and CTNCs. To study the effect of surface area and porosity on metal ions adsorption, both PAN homopolymer and PAN-b-PBA block copolymer were pyrolyzed at two different temperatures (500 and 800 ºC) to prepare four kinds of nitrogendoped nanocarbons, denoted as PANC-500, PANC-800, CTNC-500 and CTNC-800. Transmission electron microscopy (TEM) was used to characterize morphologies and porosities of obtained nanocarbons (Figure 1) and revealed no distinct pore channels for PANC-500 and PANC-800, while CTNC-500 and CTNC-800 presented visible mesopores and bicontinuous pore structure typical for CTNCs.18

Figure 1. TEM images: (a) PANC-500; (b) PANC-800; (c) CTNC-500; (d) CTNC-800. N2 adsorption measurements were further used to study porosity of PANCs and CTNCs. Figure 2(a) shows that N2 adsorption isotherms of CTNC-500 and CTNC-800 are typical typeIV curves with clear hysteresis loops at the relative pressure (P/P0) range of 0.4-0.9, suggesting the existence of relatively uniform mesopores for CTNCs. Comparatively, N2 adsorption


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isotherms of PANC-500 and PANC-800 reflect type-II curves, which often correspond to disordered materials with ill-defined distribution of pore size and shape.44 No mesopores were visible for PANCs, while distinct mesopores derived from the decomposition of the PBA block in the range of 4-25 nm can be seen in CTNCs (Figure 2(b)). Structure parameters of PANCs and CTNCs including surface area, pore volume and mesopore size distribution maxima are shown in Table 1.

Figure 2. (a) N2 adsorption isotherms and (b) pore size distribution of PANCs and CTNCs. Table 1. Structure parameters of PANCs and CTNCs. specific surface area (m2/g) samples

microporesa

mesoporesb

total

pore volume (cm3/g)

PANC-500 0 15 15 0.08 PANC-800 128 63 191 0.17 CTNC-500 116 266 382 0.48 CTNC-800 207 214 421 0.45 a b determined by the t-plot method. determined by the BJH method.

mesopore size distribution maxima (nm)b / / 11.5 10.3

As expected, CTNCs possessed significantly higher surface area and larger pore volume than PANCs. Decomposition of the PBA block not only generated large mesoporous space, but also resulted in the formation of additional micropores, since CTNCs possess higher microporous surface area than PANCs at the same pyrolysis temperature. Higher temperature was beneficial


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to the increase of micropores, attributed to extended crosslinking and carbonization of the PAN block. Meanwhile, higher temperature resulted in the shrinkage of carbon framework, reducing the mesopore surface area and size. Overall, PANCs and CTNCs with different specific surface areas and porosities were obtained to study the adsorption behavior of nitrogen-doped mesoporous carbons toward metal ions. Nitrogen Dopants in PANCs and CTNCs. PANCs and CTNCs prepared via pyrolysis under different temperatures (500 and 800 °C) may contain different nitrogen species, which would serve as ligands for binding metal ions as recently shown for Pt.21 XPS was employed to investigate the composition of PANCs and CTNCs. Table 2 shows that both PANCs and CTNCs were mainly comprised of C, O and N. As expected, with the increase of pyrolysis temperature, nitrogen contents underwent significant decrease. Notably, samples carbonized at higher temperature retained still relatively high nitrogen contents (10-11 at%). PANC-500 and CTNC500 possessed higher nitrogen contents up to 18.6 at% and 15.1 at%, respectively. Table 2. XPS results of PANCs and CTNCs.

samples

element composition (at%)

peak position (eV)

ratio of peak area (%)

C

O

N

N-P

N-X

N-O

N-P

N-X

N-O

PANC-500

76.9

4.5

18.6

398.2

399.9

401.8

41

37

22

PANC-800

85.5

3.4

11.1

398.1

400.5

402.6

38

39

23

CTNC-500

79.2

5.7

15.1

398.5

400.3

402.6

41

35

24

CTNC-800

86.5

3.1

10.4

398.2

400.6

402.3

36

39

25


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Figure 3. High resolution XPS N1s spectra: (a) PANC-500; (b) PANC-800; (c) CTNC-500; (d) CTNC-800. In addition, deconvolution of high resolution XPS N1s spectra was carried out to quantify the nitrogen species. From Figure 3, it can be concluded that pyridinic-N (N-P), pyrrolic- or pyridonic-N (N-X) and pyridine oxide-N (N-O) were the main nitrogen types existing in the carbon framework of PANCs and CTNCs. The peak position and the content of different nitrogen-containing groups are showed in Table 2. At higher carbonization temperature, the ratio of N-P decreased, while the ratio of N-X increased, with little difference in N-O species. This result was consistent with other PAN-based carbonaceous materials.45 PANCs and CTNCs as Adsorbents for Hexavalent Chromium. To test the performance of PAN-derived N-doped carbons as heavy metal sorbents, CTNC-500 was first used to adsorb


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Cr(VI) from aqueous solutions with different pH (2-7). The adsorption result is shown in Figure 4(a). The highest adsorption capacity of Cr(VI) was acquired at pH= 2 with the value of 59.1 mg Cr(VI)/g sorbent. Increasing pH values resulted in gradually decreased adsorption capacities due to stronger protonation of carbon surface and nitrogen active sites at lower pH and deprotonation at higher pH. HCrO4- and CrO42- are two main existing states for Cr(VI) in aqueous solution at pH < 7. At lower pH, the positively charged carbon surfaces and nitrogen active sites could strongly interact with HCrO4- and CrO42- by electrostatic attraction. With the increase of pH, the deprotonation altered the surface charge of the sorbents, resulting in weak interaction, or even electrostatic repulsion between the nitrogen active sites and HCrO4- or CrO42-. Besides, strong adsorption ability of CTNC-500 for Cr(VI) also attributes to the reduction of Cr(VI) to Cr(III) in the presence of nitrogen sites.46 That means chemisorption and physical adsorption were coexisted in the process of Cr(VI) adsorption by CTNC-500. Subsequently, four kinds of nanocarbons were used to adsorb Cr(VI) at pH = 1 and pH = 7. Similarly, all the samples showed weaker adsorption abilities at higher pH condition (Figure 4(b)). At pH = 7, PANCs showed no adsorption of Cr(VI), while CTNCs with high surface area presented detectable adsorption capacity which might be associated with the non-specific physical adsorption. In comparison, slightly more Cr(VI) was adsorbed by CTNC-800 because of the relatively higher BET specific surface area than that of CTNC-500. At pH = 1, PANCs possessed significantly lower adsorption capacities (5.1 mg/g for PANC-500 and 3.8 mg/g PANC-800) compared to CTNCs even though they had similar nitrogen contents. However, for CTNC-500 and CTNC-800, more nitrogen active sites incorporated in the carbon framework resulted in stronger Cr(VI) adsorption capacities at pH = 1. CTNC-500 presented much higher adsorption capacities (39.8 mg/g) than that of CTNC-800 (24.9 mg/g), due to more nitrogen


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dopants which could increase negative charge density of carbon surface and lower adsorption energy between CTNCs and Cr(VI) more significantly.38

Figure 4. Adsorption experiments: (a) Cr(VI) adsorption by CTNC-500 at pH = 2-7 (M/V = 1 g/L); (b) Cr(VI) adsorption by PANCs and CTNCs at pH = 1 (M/V = 2 g/L) and pH = 7 (M/V = 5 g/L); (c) Cr(VI) adsorption rate by CTNC-500 at pH = 2 (M/V = 1 g/L); (d) Cr(VI) adsorption isotherms by CTNC-500 at pH = 2 (M/V = 1 g/L). Adsorption rate and adsorption isotherms were also studied. Figure 4(c) shows that adsorption equilibrium was reached at ~3h, suggesting relatively rapid adsorption rate. Adsorption isotherms were presented in Figure 4(d). It shows that Cr(VI) adsorption by CTNC-500 is closer to Freundlich model, implying a heterogeneous adsorption behavior.6 Theoretical maximum adsorption capacity was calculated to be 333.3 mg/g at pH=2, much higher than previously


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reported nitrogen-doped carbonaceous submicron spheres37 because of better structural properties and higher nitrogen contents (10.4-18.6 at%). However, this adsorption capacity was lower than novel nitrogen-doped magnetic carbons, which consisted of positively charged metal oxide.38 In general, the adsorption ability of CTNCs on Cr(VI) was significantly higher than most of porous carbons.47, 48 PANCs and CTNCs as Adsorbents for Uranium. PANCs and CTNCs were also used as sorbents of U(VI). The U(VI) solutions with the pH from ~2 to ~7 were tested. Figure 5(a) shows that CTNC-500 and CTNC-800 realized almost 100% removal of U(VI) from aqueous solution at pH = ~ 7. Comparatively, PANCs showed lower removal efficiencies than CTNCs at pH = ~ 5-7, attributed to lower surface area and porosity since PANCs and CTNCs had similar nitrogen contents. By comparing adsorption capacities of PANC-500 and PANC-800 at pH = ~ 5-7, it was shown that PANC-500 had stronger removal abilities than PANC-800 even if the surface area of PANC-500 was much lower than PANC-800. Hence, nitrogen dopants were crucial for efficient U(VI) adsorption. Furthermore, due to larger surface area but lower nitrogen contents, CTNC-800 presented similar adsorption curves with CTNC-500. When higher concentration of U(VI) solution (200 mg/L, M/V= 1 g/L) was used, CTNC-500 (75.3%) realized higher U(VI) removal efficiency than CTNC-800 (70.7 %), which might be also attributed to higher nitrogen contents. In contrary to Cr(VI) adsorption, PANCs and CTNCs showed weaker adsorption abilities at lower pH due to U(VI) was in the form of (hydrated) uranyl cations in aqueous solution.49 Stronger protonation of carbon surface and nitrogen active sites at lower pH caused strengthened electrostatic repulsion for UO22+. Because of strong protonation at pH = ~ 24, PANCs and CTNCs showed non-significant difference in adsorption ability to U(VI). Adsorption rate curve implies that 3 h was enough for reaching adsorption equilibrium (Figure


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5(b)). Adsorption isotherms showed in Figure 5(c) present a homogeneous adsorption behavior since U(VI) adsorption by CTNC-500 corresponds to Langmuir model.6 17.2 mg/g of theoretical maximum adsorption capacity at pH=5 was obtained, higher than some other adsorbents, e.g., hematite,50 rice straw,51 carbon nanotube,52 etc. Compared to other mesoporous carbons, CTNC500 possessed higher adsorption ability at pH=6 (~ 60 mg/g) than ordered mesoporous carbons CMK-3, which has almost 3 times larger surface area than CTNC-500.53

Figure 5. Adsorption experiments: (a) U(VI) adsorption by PANCs and CTNCs at pH = ~ 2-7 (M/V = 1 g/L); (b) U(VI) adsorption rate by CTNC-500 at pH = 5 (M/V = 1 g/L); (c) U(VI) adsorption isotherms by CTNC-500 at pH = 5 (M/V = 1 g/L); (d) Adsorption selectivity of CTNC-800 on U(VI) (M/V = 2 g/L).


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Generally, U(VI)-containing waste water always includes various metal ions. Thus, adsorption selectivity of CTNC-800 to U(VI) was also conducted. Figure 5(d) shows that CTNC-800 selectively adsorbed U(VI) and Cr(III), with no significant adsorption of other metal ions, namely K(I), Co(II), Ni(II), Zn(II), Sr(II) and La(III). That means CTNC-800 can be used to effectively treat U(VI)-containing waste water. According to previous reports,35,

54

nitrogen-

doped nanocarbons tended to present stronger adsorption abilities to relatively harder metal ions such as Cd(II), Ni(II), Cu(II) and Cr(III), due to the hard nature of nitrogen donor atom. Comparatively, U(VI) also belongs to harder metal ions, which may complex with the nitrogen active sites in the carbon framework, leading to enhanced adsorption capacities and selectivity. Furthermore, enhanced adsorption of CTNC-800 to U(VI) may be associated with the coordination between UO22+ and pyridinic-N, since pyridinic groups are much more basic, which can strongly interact with UO22+ at high pH.35

Conclusion In this study, four kinds of nanocarbons were obtained by pyrolysis of either PAN homopolymers or PAN-b-PBA block copolymers synthesized by SARA ATRP. Structural properties of PANCs and CTNCs were comparatively studied by TEM and N2 adsorption measurements. CTNCs presented bicontinuous morphologies, high surface area and mesoporosity, attributed to self-assembly of PAN-b-PBA and the decomposition of the PBA block during the pyrolysis process, while PANCs showed no obvious porosity. Nitrogen contents and speciation were analyzed by XPS. Compared with PANCs, CTNCs presented excellent adsorption ability for Cr(VI) anions at low pH and for U(VI) cations at high pH conditions. Both large surface area and high nitrogen content (10.4-18.6 at%) were necessary to obtain improved adsorption capacity. Nitrogen doping enhanced the interaction between sorbents and Cr(VI) by


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increasing negative charge density and lowering adsorption energy, while U(VI) adsorption was likely promoted by a coordination mechanism. From adsorption experiments of Cr(VI) and U(VI), it can be concluded that CTNCs have a great potential to remove both anions and cations from aqueous solution. ASSOCIATED CONTENT Supporting Information Available: Composition of the simulated U(VI)-containing waste water; NMR and GPC characterization results for synthesis pf PAN-Br. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (G. Ye) *E-mail: [email protected]. (K. Matyjaszewski) Author Contributions #

These authors contributed equally.

ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China under Project 51473087, 51673109 and U1430234 as well as the National Science Foundation (DMR 1501324). MK thanks Polish Ministry of Science and Higher Education (“Mobilnosc Plus” grant no. 1055/MOB/2013/0) for financial support. ZZ and LR were supported by the Director, Office of Science, Office of Basic Energy Sciences under U.S. Department of Energy Contract No. DEAC02-05CH11231 at Lawrence Berkeley National Laboratory. REFERENCES (1) Liang, C.; Li, Z.; Dai, S., Mesoporous Carbon Materials: Synthesis and Modification. Angew.


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