Electrosorption of Lead Ions by Nitrogen-Doped ... - ACS Publications

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Electrosorption of Lead Ions by Nitrogen-Doped Graphene Aerogels via One-Pot Hydrothermal Route Yong Wei,†,‡ Lan Xu,†,‡ Yongxin Tao,† Chao Yao,† Huaiguo Xue,*,§ and Yong Kong*,† †

Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School of Petrochemical Engineering, and ‡School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, China § School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China ABSTRACT: We present an efficient Pb2+ electrosorption by nitrogen-doped graphene aerogels (NGAs) prepared by one-pot hydrothermal synthesis of nitrogen-doped graphene hydrogels (NGHs) followed by freeze-drying treatment. Pb2+ can be effectively removed by the as-prepared NGAs at an applied negative potential, and the removal mechanisms include (1) electrostatic attraction derived from external electric field, (2) electrostatic attraction caused by intrinsic charges on NGAs and Pb2+, (3) large specific surface area (SBET) of NGAs, and (4) coordination between doped nitrogen atoms and Pb2+. More importantly, after a simple and convenient electrodesorption treatment, the NGAs exhibit promising performance in recyclable electrosorption, and the removal ratio (%R) of Pb2+ decreases only ∼5% after successive 100 cycles, which is significantly superior to conducting polymer and conducting polymer/reduced graphene oxide (rGO) composites-based electrosorption.

1. INTRODUCTION Among the techniques used in the treatment of wastewaters containing heavy metal ions, electrosorption has been shown to be an attractive technique because it does not require any added chemicals for oxidation or reduction reactions, nor does it involve any toxic material, only inert electrodes.1−3 For electrosorption, nanostructured materials with high specific surface area are ideal candidates on which an external electric field is applied for removing charged ionic species from aqueous solutions.4,5 Carbon-based materials of porous structure, such as activated carbon,6 carbon nanotubes,7 and carbon aerogels,8 have been used as electrode materials for electrosorption of heavy metal ions. Although carbon nanotubes9 and graphene-based nanomaterials10,11 are excellent adsorbents for the removal of heavy metal ions, introducing heteroatoms such as nitrogen atoms and sulfur atoms into solid adsorbents can enhance the adsorption capacity since the introduced heteroatoms can coordinate strongly with heavy metals.12−14 Electrosorption based on conducting polymer (poly(m-phenylenediamine)15) and its composites (poly(m-phenylenediamine)/reduced graphene oxide (rGO)16) with plenty of nitrogen atoms has been reported by our group. Recently, nitrogen-doped graphene has aroused considerable attention owing to the fact that the nitrogen atom can be regarded as the ideal dopant for graphene because of its comparable atomic size to carbon and its high electronegativity.17−20 In particular, the doped nitrogen atoms favor the reactivity of the neighborly linked carbon atoms via alteration of the electronic structure.21,22 The intriguing © 2016 American Chemical Society

properties endow nitrogen-doped graphene with a variety of potential applications including energy storage materials,23−25 electrocatalysis,26−28 environmental remediation,29,30 sensors,31,32 and so on. It is noteworthy that nitrogen-doped graphene combines the advantages of graphene (large specific surface area and excellent electrical conductivity) and nitrogen atoms (strong coordination ability with metal cations), so it should be a promising material for electrosorption of heavy metal ions. However, there is little work concerning the electrosorption based on nitrogen-doped graphene, which strongly motivates research in this aspect. Herein, we report on the electrosorption of Pb2+ with nitrogen-doped graphene aerogels (NGAs) for the first time. The NGAs were prepared via a facile one-pot hydrothermal route,33 and fully characterized in this work. The parameters affecting the electrosorption as well as the electrosorption behaviors of Pb2+ on the NGAs electrode including electrosorption model and electrosorption kinetics were also discussed in detail. Finally, the regeneration of the NGAs electrode by electrodesorption and the recyclability of the NGAs-based electrosorption were investigated and compared with previous reports on the removal of Pb2+. Received: Revised: Accepted: Published: 1912

November 2, 2015 January 24, 2016 February 5, 2016 February 5, 2016 DOI: 10.1021/acs.iecr.5b04142 Ind. Eng. Chem. Res. 2016, 55, 1912−1920

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Industrial & Engineering Chemistry Research

low,15 the residual Pb2+ after electrosorption can be easily known according to a calibration curve (concentration vs conductivity). The whole process of NGAs synthesis and application in the electrosorption of Pb2+ is illustrated in Figure 1. The electrosorption efficiency can be assessed by the

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus. Natural graphite powder (99.95%, 8000 mesh), urea, and other chemicals not mentioned here were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. All solutions were prepared with ultrapure water having a resistivity of 18.2 MΩ. The morphology of NGAs was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using a Supra55 field-emission scanning electron microscope (Zeiss, Germany) and a JEM-2100 transmission electron microscope (JEOL, Japan), respectively. Fourier transform infrared (FT-IR) spectra of graphene oxide (GO), graphene aerogels (GAs), and NGAs were measured on a FTIR-8400S spectrometer (Shimadzu, Japan). The N2 adsorption/desorption isotherms were recorded at 77 K with an ASAP 2010 specific surface area and pore size analyzer (Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectrometer. The phase purity and crystallization degree of GO and NGAs were determined by a Rigaku D/max2500PC X-ray powder diffractometer (XRD). All electrosorption and electro-desorption experiments were conducted by a CHI 660D electrochemical workstation (China) in a three-electrode system. 2.2. One-Pot Hydrothermal Synthesis of NGAs. GO was prepared from natural graphite according to the method previously reported by Marcano et al.34 In a typical procedure, a GO suspension was prepared by the sonication of 200 mg of GO in 100 mL of ultrapure water, and then 6 g of urea was added into the GO dispersion of 2 mg mL−1. The mixture was stirred for 15 min, resulting in a homogeneous suspension. Next, the obtained suspension of GO and urea was transferred to a Teflon-lined autoclave and subjected to hydrothermal treatment for 12 h at 180 °C. After that, the autoclave was naturally cooled to room temperature, and the formed cylinders of nitrogen-doped graphene hydrogels (NGHs) were immersed into ultrapure water for 72 h, during which the water was renewed every 4 h to remove any impurities. Finally, the NGAs were obtained by freeze-drying the NGHs at −52 °C in a vacuum freeze-dryer. For control experiments, GA samples were prepared by the same procedure except for the addition of urea, and rGO was also prepared via chemical reduction of GO.35 2.3. Construction of NGAs Paper Electrode. A paper electrode of the NGAs was constructed for the electrosorption of Pb2+. A 90 mg aliquot of ground NGAs was ultrasonically dispersed in 1 mL of 4 wt % poly(vinyl alcohol), and then the dispersion of paste was coated onto both sides of a piece of stiff paperboard (35 mm × 8 mm) about 400 μm in thickness. The as-prepared NGAs paper electrode was freeze-dried at −52 °C for 12 h prior to use. 2.4. Electrosorption of Pb2+ and Electrode Regeneration by Electrodesorption. Electrosorption of Pb2+ was performed in a conventional three-electrode system consisting of NGAs paper electrode as working, a saturated calomel electrode (SCE) as reference, and a platinum foil as auxiliary electrode, respectively. The electrolytes were 80 mL Pb(NO3)2 solutions of different concentrations. During the electrosorption, a negative potential of −0.3 V was exerted on the NGAs working electrode for 1 min under stirring. Because the conductivity of a strong electrolyte solution is directly proportional to its concentration when the concentration is

Figure 1. Schematic illustrating the whole process for NGAs synthesis and application in the electrosorption of Pb2+.

following equation: %R = (C0 − Ce)/C0 × 100, where %R, C0, and Ce are the removal ratio of Pb2+, the initial and equilibrium concentrations of Pb2+, respectively. To estimate the recyclability of the NGAs-based electrosorption, the NGAs paper electrode was regenerated by a simple and convenient electro-desorption process, in which the applied external electric field between the working electrode and the auxiliary electrode was completely removed.

3. RESULTS AND DISCUSSION 3.1. Characterization of NGAs. Morphologies of the obtained NGAs as well as GAs are characterized by SEM and TEM. The SEM images of GAs and NGAs are shown in Figure 2 panels A and B, respectively. As can be seen, both of the two

Figure 2. SEM and low-resolution TEM images of GAs (A, C) and NGAs (B, D).

samples exhibit a porous and multilayer structure with extensive stacking and folding. The GAs nanosheets are randomly compact and stacked together (Figure 2C), exhibiting a morphology of crumpled silk veil waves similar to that of graphene. As shown in Figure 2D, defective structures are clearly observed for NGAs, and this morphology is attributed to the presence of doped nitrogen atoms.36 Zhao et al.33also 1913

DOI: 10.1021/acs.iecr.5b04142 Ind. Eng. Chem. Res. 2016, 55, 1912−1920

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Halenda (BJH) method is 4.86 nm (Figure 4B), which is larger than that of GAs (1.26 nm) (inset of Figure 4B). The formation of mesoporous structure of NGAs can be attributed to the fact that ammonia gas generated from the decomposition of urea diffuses into the cavities of graphene hydrogels and combines with carbon atoms, resulting in the increase in the pore-size distribution in the NGAs. Also, the Brunauer− Emmett−Teller (BET) specific surface areas (SBET) of rGO, GAs, and NGAs are measured to be 16.9, 393.4, and 434.4 m2 g−1, respectively. It is amazing that the SBET of rGO is significantly lower than the theoretical SBET of 2630 m2 g−1 for individual isolated graphene nanosheets,41 the SBET of 2607 m2 g−1 for graphene-like carbon nanosheets,42 and 3633 m2 g−1 for hydrothermal carbon.43 Both GAs and NGAs have a greatly increased SBET compared with rGO, and the doping of nitrogen results in a larger SBET of NGAs (434.4 m2 g−1) than that of GAs (393.4 m2 g−1). Obviously, the larger SBET of NGAs is beneficial to the adsorption of electrolyte ions from the solutions. To determine the exact chemical composition and nitrogen bonding configurations of NGAs, XPS measurements are carried out. As shown in Figure 5A, the XPS spectrum of GO shows only the presence of C 1s (284.4 eV) and O 1s (532.2 eV). After nitrogen doping using urea, the intensity of O 1s decreases significantly, and a new N 1s peak at 400.0 eV appears at the XPS spectrum of NGAs, demonstrating the removal of most oxygenated functionalities and incorporation of nitrogen during the hydrothermal process. High resolution XPS measurements of C 1s deconvolution spectra of GO (Figure 5B) and NGAs (Figure 5C) are further investigated for a better comparison. For GO, the four peaks at 284.6, 285.9, 287.3, and 289.2 eV are attributed to C−C, C−O, CO, and OC−O, respectively.44 However, the peaks corresponding to the oxygen-containing groups decrease remarkably (CO and OC−O) or disappear completely (C−O) for NGAs, suggesting the efficient removal of oxygenated functionalities, and a new peak at 285.8 eV implies the doping of nitrogen and the formation of C−N bonds.24 Similarly, the nitrogen bonding configurations in NGAs are also characterized by high resolution N 1s deconvolution spectra (Figure 5D). It is shown that the N 1s spectra can be fitted into three peaks at 398.1, 399.5, and 401.4 eV, which correspond to pyridinic N, pyrrolic N, and graphitic N, respectively, as shown in Figure 5E. It is noteworthy that the intensity of pyrrolic N is significantly higher than that of pyridinic N and graphitic N, so it can be concluded that it is the favorable N-type in the NGAs.

reported that in the hydrothermal synthesis of nitrogen-doped graphene, urea functions as a reducing-doping agent to continuously release NH3, and N element of which is doped into graphene, resulting in defective structures containing numerous interconnected three-dimensional porous framework. The reduction of oxygen-containing groups in GO and doping of nitrogen atoms into NGAs during the hydrothermal process is further confirmed by the FT-IR spectra (Figure 3).

Figure 3. FT-IR spectra of GO, GAs, and NGAs.

The peaks at 3450 (O−H), 1745 (CO), 1216 (epoxy C−O stretching), and 1054 cm−1 (hydroxyl C−O stretching) are associated with the oxygenated functionalities in GO,37,38 and these characteristic peaks decrease remarkably or disappear completely for NGAs and GAs, suggesting that the amount of oxygen-containing groups are removed during the hydrothermal reaction. Meanwhile, two obvious new peaks appear at 1568 and 1189 cm−1 on the spectra of NGAs, corresponding to the functionalities of CN and C−N, respectively.39 This result implies that nitrogen atoms are doped into GAs successfully in the hydrothermal process in the presence of urea. The exposed surface area and pore-size distribution of the obtained NGAs are investigated from the nitrogen adsorption− desorption experiments at 77 K. As shown in Figure 4A, the profiles are characterized by a type V isotherm with an obvious hysteresis loop, suggesting the presence of mesopores along with the capillary condensation in these mesopores.40 The total pore volume of NGAs is 0.19 cm3 g−1, and the corresponding pore-size distribution determined by the Barrett−Joyner−

Figure 4. (A) Nitrogen adsorption−desorption isotherms at 77 K and (B) pore-size distribution of NGAs and GAs. 1914


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Figure 5. (A) General XPS spectra of GO and NGAs. (B) C 1s XPS spectra of GO. (C) C 1s XPS spectra of NGAs. (D) N 1s XPS spectra of NGAs. (E) Schematic structure of NGAs with three types of doped nitrogen.

Figure 6 shows the XRD patterns of GO and NGAs. The diffraction peak centered at 2θ = 9.8° is assigned to the (002) crystalline plane of GO, corresponding to a layer structure with

a basal spacing of 0.90 nm, and the peak disappears completely at the NGAs due to the removal of most oxygen-containing groups during the hydrothermal process. On the other hand, the XRD pattern of NGAs exhibits a broad peak indexed to (002) at 2θ = 25.2° with an interlayer spacing of 0.35 nm, which is a little larger than that of single-crystal graphite (0.335 nm) due to the presence of subtle oxygen-containing groups existed in the NGAs.45 It is also noted that there is a small peak at around 2θ = 43.4° (100), implying that not all the six carbon atoms of a closed ring in the honeycomb structure are sp2 hybridized,46 which probably occurs because the doped nitrogen atoms exist in the three-dimensional lattice of NGAs. 3.2. Electrosorption of Pb2+ onto NGAs Paper Electrode. The as-prepared NGAs paper electrode was used for the electrosorption of Pb2+, and GAs and rGO were also used in the electrosorption as control experiments. As a result of ionization of the residual oxygenated functionalities on the graphene sheets, NGAs, GAs, and rGO are all highly negatively charged when dispersed in water; meanwhile, the applied

Figure 6. XRD patterns of GO and NGAs. 1915


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of the applied potential below −0.3 V (from −0.4 to −0.7 V) will lead to a significant decrease in the %R probably due to undesired electrochemical reduction occurring at the electrode−solution interfaces, for example, hydrogen evolution, and this hypothesis is also verified by the phenomena that large amounts of bubbles are generated at the electrode−solution interfaces at the applied potential below −0.3 V. The effect of initial concentration on the removal of Pb2+ is shown in Figure 8B. It is obvious that the %R declines continuously from 75% to 40% with an increase in the initial concentration of Pb2+ from 0.2 to 6 mM, indicating that the NGAs are promising electrode materials used for the electrosorption of Pb2+ of low concentration. Similar influence of initial concentration on the %R has been reported in our group by using nitrogencontaining conducting polymers, poly(m-phenylenediamine) and polyaniline, as the electro-adsorbents for the electrosorption of heavy metal ions,15,16,47 implying that the coordination between doped nitrogen in the NGAs and the target cations plays an important role in the removal of Pb2+ from the solutions. 3.3. Electrosorption Models and Electrosorption Kinetics. The electrosorption data at equilibrium were fitted C C 1 by the Langmuir ( q e = bq + q e ) model48 and the Freundlich

negative potential of −0.3 V is also beneficial to the electrosorption of positively charged Pb2+. The above two factors lead to an efficient removal of Pb2+ onto the three electrode materials. As shown in Figure 7, compared with GAs

Figure 7. Electrosorption of 1 mM Pb2+ (pH 4.2) on NGAs, GAs, and rGO paper electrodes at an applied potential of −0.3 V.

e

and rGO, the NGAs exhibit the highest removal ratio (%R) of Pb2+ and the shortest adsorption equilibrium time (within 5 s) for the treatment of 1 mM Pb2+, and the superior electrosorption of Pb2+ on the NGAs can be attributed to the largest SBET of NGAs (434.4 m2 g−1) and the introduction of nitrogen atoms. The former contributes to the physical adsorption of Pb2+ onto NGAs, and the latter provides an effective coordination between doped nitrogen atoms and Pb2+. On the basis of the above phenomena, the removal mechanisms on the NGAs paper electrode can be concluded as follows: (1) electrostatic attraction derived from external electric field, (2) electrostatic attraction caused by intrinsic charges on NGAs and Pb2+, (3) large SBET of NGAs, and (4) coordination between doped nitrogen atoms and Pb2+. Applied potential and initial concentration of Pb2+ are two important parameters affecting the removal of Pb2+, and thus both of them are fully addressed in this work. Figure 8A shows that the %R increases greatly with increasing exerted potential from −0.1 to −0.3 V, suggesting that electrostatic attraction between NGAs and Pb2+ occurs more easily at a higher negative potential. However, it is interesting to find that further increase

m

m

1 C ),49,50 n e

model (ln qe = ln KF + respectively, where qe and qm represent the equilibrium and maximum adsorption capacity (mg g−1), Ce is the equilibrium concentration of Pb2+, and b, KF, and n stand for the adsorption equilibrium constants corresponding to the adsorption capacity and intensity. The fitted plots and related parameters are shown in Figure 9 and Table 1, respectively. The correlation coefficient (R2) for the Freundlich model is closer to 1 compared with that for the Langmuir model (0.9778 vs 0.8651), suggesting that the electrosorption of Pb2+ by the NGAs electrode can be described more accurately by the Freundlich model. Next, the electrosorption kinetics of Pb2+ on the NGAs electrode were investigated by analyzing the obtained equilibrium data using the pseudo-first-order kinetic model (ln (qe − qt) = ln qe − k1t) and the pseudo-second-order kinetic t 1 t model ( q = 2 + q ), respectively, where qe and qt are the t

k 2qe

e

equilibrium adsorption capacity and the adsorption capacity at time t (mg g−1), and k1 (s−1) and k2 (g mg−1 s−1) represent the rate constants. The fitted linear plots are shown in Figure 10,

Figure 8. (A) Effect of applied potential on the electrosorption of 1 mM Pb2+. (B) Effect of initial concentration of Pb2+ on the electrosorption at −0.3 V. Errors bars represent the standard deviation for three independent measurements. 1916


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Figure 9. Fit of electrosorption data at equilibrium to the Langmuir model (A) and the Freundlich model (B).

Table 1. Related Parameters for the Langmuir Model and the Freundlich Model Langmuir model −1

−1

Freundlich model −1

2

qm (mg g )

b (L mg )

R

67.5

7.7393

0.8651

−1 1/n

KF (mg g (L mg ) )

n

R2

10.4855

1.2195

0.9778

Figure 10. Pseudo-first-order kinetic model (A) and pseudo-second-order kinetic model (B) for the electrosorption of Pb2+ on the NGAs electrode.

and the obtained R2 for the pseudo-second-order model (0.9984) is closer to 1 than that for the pseudo-first-order (0.8465), revealing that the electrosorption kinetics of Pb2+ on the NGAs electrode fits the pseudo-second-order model better. 3.4. Recyclable Electrosorption via Electrodesorption Regeneration. One of the most promising features of electrosorption is that electrode materials can be effectively regenerated by electrodesorption, and thus recyclable electrosorption can be achieved via reversible electrosorption/ electrodesorption cycles. For a complete electrosorption/ electrodesorption cycle, the NGAs electrode is first polarized at −0.3 V for 1 min for the electrosorption of Pb2+, and then the external electric field is removed completely for the electrode regeneration. As shown in Figure 11, the %R still retains 70% after successive electrosorption/electrodesorption treatment of 80 mL 0.2 mM Pb2+ for 100 cycles, which is slightly lower than the initial %R (75%). Compared with poly(m-phenylenediamine)15 (only used for 10 cycles) and poly(m-phenylenediamine)/rGO nanocomposites16 (used for 31 successive cycles), the operational stability of the NGAs electrode is greatly enhanced to 100 cycles with a high %R retention (70%) for the electrosorption of Pb2+. The amazing enhancement in the stability of the NGAs electrode is

Figure 11. Successive electrosorption/electrodesorption cycle for the treatment of 80 mL of 0.2 mM Pb2+ at the same NGAs paper electrode at an applied potential of −0.3 V.

attributed to the excellent mechanical strength of NGAs possessing interconnected three-dimensional porous networks generated by nitrogen doping,17,24,33 which overcomes the cycling degradation problems caused by volumetric changes or mechanical forces for most electrode materials used in electrosorption/electro-desorption. 1917



■

3.5. Comparison of Equilibrium Adsorption Capacity (qe) with Previous Reports. To evaluate the removal efficiency of the proposed NGAs electrode for Pb2+, the qe in this work is compared with several previous reports regarding the removal of Pb2+ via electrosorption47 or adsorption.51−54 In this work, qe is calculated by the following equation: qe = V(C0 − Ce)/m, where C0 and Ce are the initial and equilibrium concentrations of Pb2+ (mg L−1), m is the mass of NGAs (g), and V is the solution volume (L). The results are listed in Table 2. As can be seen, the removal efficiency of the as-prepared

technique

equilibrium adsorption capacity (mg g−1)

N-doped graphene aerogels (urea) polyaniline/ attapulgite composites graphene aerogels (NaHCO3) graphene aerogels (mercaptoacetic acid) graphene aerogels (carbon nanotubes) graphene hydrogels (polydopamine)

electrosorption

650.4

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

electrosorption

15.4

adsorption

301.9

51

adsorption

87

52

adsorption

104.9

53

adsorption

340

54

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors are grateful to the financial supports from National Natural Science Foundation of China (21275023, 21173183, 51208068), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Science & Technology Program of Huaian city (HG201303).

ref present work 47

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Corresponding Authors

Table 2. Comparison of Equilibrium Adsorption Capacity between the Present Work and Other Previous Reports electrode materials (nitrogen source)

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(1) Bayram, E.; Ayranci, E. Electrochemically Enhanced Removal of Polycyclic Aromatic Basic Dyes from Dilute Aqueous Solutions by Activated Carbon Cloth Electrodes. Environ. Sci. Technol. 2010, 44, 6331. (2) Wang, X. Q.; Lee, J. S.; Tsouris, C.; DePaoli, D. W.; Dai, S. Preparation of Activated Mesoporous Carbons for Electrosorption of Ions from Aqueous Solutions. J. Mater. Chem. 2010, 20, 4602. (3) Ayranci, E.; Conway, B. E. Removal of Phenol, Phenoxide and Chlorophenols from Waste-Waters by Adsorption and Electrosorption at High-Area Carbon Felt Electrodes. J. Electroanal. Chem. 2001, 513, 100. (4) Ban, A.; Schafer, A.; Wendt, H. Fundamentals of Electrosorption on Activated Carbon for Wastewater Treatment of Industrial Effluents. J. Appl. Electrochem. 1998, 28, 227. (5) Hou, C. H.; Liang, C. D.; Yiacoumi, S.; Dai, S.; Tsouris, C. Electrosorption Capacitance of Nanostructured Carbon-Based Materials. J. Colloid Interface Sci. 2006, 302, 54. (6) Bain, E. J.; Calo, J. M.; Spitz-Steinberg, R.; Kirchner, J.; Axen, J. Electrosorption/Electrodesorption of Arsenic on a Granular Activated Carbon in the Presence of Other Heavy Metals. Energy Fuels 2010, 24, 3415. (7) Liu, Y. X.; Yan, J. M.; Yuan, D. X.; Li, Q. L.; Wu, X. Y. The Study of Lead Removal from Aqueous Solution Using an Electrochemical Method with a Stainless Steel Net Electrode Coated with Single Wall Carbon Nanotubes. Chem. Eng. J. 2013, 218, 81. (8) Gabelich, C. J.; Tran, T. D.; Suffet, I. H. Electrosorption of Inorganic Salts from Aqueous Solution Using Carbon Aerogels. Environ. Sci. Technol. 2002, 36, 3010. (9) Yang, S. B.; Hu, J.; Chen, C. L.; Shao, D. D.; Wang, X. K. Mutual Effects of Pb(II) and Humic Acid Adsorption on Multiwalled Carbon Nanotubes/Polyacrylamide Composites from Aqueous Solutions. Environ. Sci. Technol. 2011, 45, 3621. (10) Zhao, G. X.; Ren, X. M.; Gao, X.; Tan, X. L.; Li, J. X.; Chen, C. L.; Huang, Y. Y.; Wang, X. K. Removal of Pb(II) Ions from Aqueous Solutions on Few-Layered Graphene Oxide Nanosheets. Dalton. Trans. 2011, 40, 10945. (11) Yang, S. B.; Chen, C. L.; Chen, Y.; Li, J. X.; Wang, D. Q.; Wang, X. K.; Hu, W. P. Competitive Adsorption of Pb-II, Ni-II, and Sr-II Ions on Graphene Oxides: A Combined Experimental and Theoretical Study. ChemPlusChem 2015, 80, 480. (12) Xie, A. J.; Ji, L.; Luo, S. P.; Wang, Z. L.; Xu, Y. Y.; Kong, Y. Synthesis, Characterization of Poly(m-phenylenediamine)/Palygorskite and Its Unusual and Reactive Adsorbability to Chromium(VI). New J. Chem. 2014, 38, 777. (13) Guo, X.; Fei, G. T.; Su, H.; Zhang, L. D. High-Performance and Reproducible Polyaniline Nanowire/Tubes for Removal of Cr(VI) in Aqueous Solution. J. Phys. Chem. C 2011, 115, 1608. (14) Li, X. G.; Ma, X. L.; Sun, J.; Huang, M. R. Powerful Reactive Sorption of Silver(I) and Mercury(II) onto Poly(o-phenylenediamine) Microparticles. Langmuir 2009, 25, 1675.

NGAs electrode via electrosorption is significantly superior to those obtained at other electrodes via electrosorption or adsorption. The excellent qe can be attributed to the synergistic effect of three-dimensional NGAs and doped nitrogen atoms combined with external electric field. In other words, the efficient removal of Pb2+ by the NGAs electrode can be due to (1) electrostatic attraction derived from external electric field, (2) electrostatic attraction caused by intrinsic charges on NGAs and Pb2+, (3) large SBET of NGAs, and (4) coordination between doped nitrogen atoms and Pb2+.

4. CONCLUSIONS In summary, NGAs samples were synthesized via a facile onepot hydrothermal synthesis of NGHs followed by freeze-drying treatment, and the resultant NGAs were used as the electrode material to construct a paper electrode, which was applied for the electrosorption of Pb2+ for the first time. A quick equilibrium can be achieved within 5 s, and the electrosorption capability of the NGAs electrode is superior to that of GAs and rGO, which is attributed to the largest SBET of NGAs and the doping of nitrogen atoms. The electrosorption of Pb2+ can be well described by the Freundlich model and the pseudosecond-order model. More importantly, the NGAs have a significantly enhanced operational stability until 100 electrosorption/electro-desorption cycles, which is due to the excellent mechanical strength of NGAs. Compared with other materials and techniques, the NGAs-based electrosorption exhibits an excellent equilibrium adsorption capacity, and thus the NGAs should be of great importance for the development of promising electro-adsorbents, even practical applications in the treatment of wastewaters containing heavy metal ions and environmental remediation. 1918


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