One-pot synthesis of nitrogen-enriched carbon spheres for hexavalent chromium removal from aqueous solution

Abstract

Nitrogen-enriched carbon spheres (NECS) were obtained after ZnCl₂ activation of high-nitrogen-content polymer spheres prepared by one-pot hydrothermal synthesis using m-phenylenediamine and hexamethylenetetramine as raw materials. The NECS were characterized by SEM, BET, FT-IR, and XPS. It was found that the NECS possessed homogeneously spherical morphology with diameters of about 0.6–1.2 μm and a high specific surface area (1237 m² g⁻¹). Elemental analysis indicated that the nitrogen content of NECS was up to 10.21 wt%. When the hexavalent chromium (Cr(VI)) in acidic solution was used to evaluate the adsorption performance, the maximum Cr(VI) removal capacity was as high as 279 mg g⁻¹ at pH 2.0. The pseudo-second-order and Freundlich model fitted the Cr(VI) adsorption behavior well. Based on zeta potential and XPS analysis, the Cr(VI) removal mechanism was proposed to be electrostatic adsorption of Cr(VI) anions on the NECS surface at low pH, followed by a reduction reaction of Cr(VI) to Cr(III) via the nitrogen functionality of carbon spheres and possible coordination reaction of the resulting Cr(III) with the lone pair electrons of nitrogen. The NECS adsorbent was shown to maintain over 90% Cr(VI) removal efficiency after six cycles. Finally, the Cr(VI) removal efficiency of NECS reached 99.9% with a dosage of 12 g L⁻¹ for real acidic electroplating wastewater (initial concentration 936.68 mg g⁻¹ and pH 1.5), clearly showing the promise of this novel adsorbent for treating Cr-containing wastewater.

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1 Introduction

Cr(VI) widely exists in industrial wastewater from electroplating, leather tanning, textiles, printing, dying, etc.¹ Cr(VI) is a non-biodegradable, carcinogenic, and mutagenic metal that is easily accumulated in living organisms.² Comparing to trivalent chromium (Cr(III)), Cr(VI) has high toxicity, super water solubility and strong oxidizing property.^3–5 Therefore, it is necessary to remove toxic Cr(VI) before industrial wastewater discharge. Currently, the disposal of Cr(VI) in wastewater mainly includes chemical precipitation, ion-exchange method, membrane treatment and adsorption.⁶ Among these methods, adsorption is a facile and economical method.⁷ At present, nanoscale adsorbent with large specific surface area has received great attention. Mg(OH)₂ and layered double hydroxides (LDHs) have been shown to be effective for Cr(VI) removal and recycling from Cr-containing wastewater.^8,9 Iron-based materials (i.e., nanoscale Fe⁰, nanoscale Fe₂O₃, and nanoscale Fe₃O₄) have also been applied extensively to remove Cr(VI).^10–12 However, these adsorbents failed in practical acidic Cr-containing wastewater, especially in strong acidic solution, as a result of their poor acid stability. Activated carbon showed promise in treating wastewater containing low concentrations of Cr because of its resistance to acid, low cost, and feasibility for industrialization. Nevertheless, activated carbon is limited by its low adsorption capacity.^6,13,14 An adsorbent with high adsorption capacity and good chemical stability is strongly demanded.

In the past decade, nitrogen-containing carbon materials have been successfully applied in gas adsorption and water treatment due to their good chemical and thermal stability.^15,16 It was reported that nitrogen functionality on the carbon surface not only enhanced the interaction between carbon surface and anions,¹⁵ but also facilitated the reduction of Cr(VI) to Cr(III).¹⁷ Consequently, the high nitrogen content on the carbon surface is supposed to improve adsorption ability as well as build a reductive environment for Cr(VI) detoxification. However, the previously reported nitrogen-containing carbon materials showed poor Cr(VI) removal ability, which could be ascribed to low nitrogen content.^17,18 In terms of morphology of adsorbents, spherical shape is considered to be ideal in view of fluid mechanics.¹⁹ Spheres can improve fluid permeability and phase separation in the real fluidized bed.²⁰ Spheres can also heighten the mechanical resistance of adsorbent due to its weak spin friction. Moreover, the exit velocity in fluidized bed can basically remain stable under the certain fluid mechanics because the dynamic stacking densities of spheres keep unchanged.²¹ Thus, a new type of carbon spheres possessing a high nitrogen content should combine the two advantages described above and prove desirable for Cr(VI) removal.

It was reported previously that the Stöber-like synthesis could be used for synthesizing nitrogen-containing carbon spheres.^22–24 However, the synthesized carbon spheres presented relatively low nitrogen content.^23–26 This may be attributable to the choice of nitrogen source. Herein, we report the synthesis of high nitrogen content polymer spheres using m-phenylenediamine and hexamethylenetetramine (HMT) in a one-pot synthesis process, followed ZnCl₂ activation to generate NECS adsorbent. The NECS adsorbent was evaluated using Cr(VI) as model pollution in the lab and in real acidic electroplating wastewater. Regeneration of this new adsorbent was also studied. Results showed that the NECS could be a promising adsorbent for Cr-containing wastewater.

2 Experimental

2.1 Materials and characterization

All chemicals (AR grade) were purchased from Sinopharm Chemical Reagent Co., Ltd., of China, and used without further purification.

Morphology and size of samples were characterized by scanning electron microscopy (SEM, JSM-6700-F). Elemental analysis was performed on a CHNO elemental analyzer (Vario EL-Cube). IR spectra were recorded on a VERTEX70 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher ESCALAB 250Xi with Al Kα radiation. Nitrogen adsorption–desorption isotherms were collected on a Micromeritics ASAP 2020 adsorption analyzer. Before adsorption experiment, the samples were degassed at 200 °C for at least 5 h. Specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. Total pore volumes were obtained from the amount of nitrogen adsorbed at a relative pressure P/P₀ of 0.95. Incremental pore size distribution was determined from the nitrogen adsorption isotherm by the Density Functional Theory (DFT) method. Zeta potentials of the samples were measured by a Zetasizer Nano ZS90 (Malvern Instrument).

2.2 Synthesis of NECS

In a typical procedure, 1.08 g m-phenylenediamine and 0.7 g HMT were dissolved in 52 mL distilled water, followed by the addition of 2.0 mL of ammonia solution (28%). The resulting solution was magnetically stirred at room temperature for 1 h, poured into a Teflon-lined autoclave, and incubated in an oven at 90 °C for 24 h. The orange solid polymer, denoted as HNPS, was collected by centrifugation and dried at 60 °C for 24 h.

To prepare NECS, HNPS was soaked in ZnCl₂ solution (10%, w/v) for 24 h at a ZnCl₂/HNPS mass ratio of 0.25 : 1. The sample was then activated at 800 °C for 1 h under a nitrogen flow of 50 mL min⁻¹. After cooling, the solid sample was washed with 0.5 M HCl and deionized water, and dried at 120 °C for 12 h. The resulting product was marked as NECS.

2.3 Adsorption experiments

Adsorption experiments were carried out at room temperature with an adsorbent dosage of 1 g L⁻¹. The Cr(VI) solutions after adsorption experiments were filtered by polyether sulfone (PES) membranes (0.22 μm pore size). The concentration of Cr(VI) in solution was determined by the 1,5-diphenylcarbohydrazide spectrophotometric method (GB 7467 and GB/T 15555.4) on a Shimadzu UV-2550 spectrophotometer. The total Cr concentrations were measured by a Varian AA240 atomic absorption spectrophotometer (AAS) equipped with an air-acetylene flame. Experiments were performed at least in triplicate, and average values along with one standard deviation were presented.

The Cr(VI) adsorption capacity and Cr(VI) removal efficiency can be calculated using the following equations, respectively.

where q_e (mg g⁻¹) is the Cr(VI) adsorption capacity at equilibrium; C₀ is the initial concentration of Cr(VI) in solution and C_e (mg L⁻¹) is the Cr(VI) concentration at equilibrium; m (g) is the mass of the adsorbent used; V (L) is the volume of the solution.

Adsorption isotherm. NECS were immersed in Cr(VI) solutions with different initial concentrations (50, 100, 200, 300, 400, 500 and 600 mg L⁻¹, pH 2.0). After reaching equilibrium, the concentrations of Cr(VI) solutions were measured and the corresponding adsorption capacities were calculated.

Adsorption kinetics. Aliquots of 0.5 mL were extracted from Cr(VI) solutions (100 and 200 mg L⁻¹, pH 2.0) at appropriate time intervals and then immediately filtered. The filtrate was analyzed for its Cr(VI) concentration using a UV visible spectrophotometer.

Effect of pH. Evaluation of Cr(VI) removal efficiency was performed at different pH values. The pH values of the Cr(VI) solutions were measured by a pH meter and adjusted with 1 M NaOH or 1 M HCl.

2.4 Regeneration and recycling

In a typical process, 20 mg NECS and 20 mL 100 mg L⁻¹ Cr(VI) solution (pH 2.0) were mixed and allowed to adsorption equilibrium, and the corresponding Cr(VI) removal efficiency was calculated. For regeneration, the exhausted NECS were immersed in 5 mL of 1 M NaOH solution and allowed to reach desorption equilibrium. Then the NECS were washed with 0.5 M HCl and deionized water till neutral pH for the next round of recycling experiment.

3 Results and discussion

3.1 Synthesis and characterization

The synthesis procedure of NECS is illustrated in Scheme 1. During the hydrothermal process, HMT slowly generates formaldehyde and ammonia.²⁵ The resulting ammonium ions (NH₄⁺) act as the catalyst for the formation of Schiff base units by the reaction between the amino (–NH₂) groups of m-phenylenediamine and the aldehyde groups of formaldehyde. HNPS is subsequently formed via a polycondensation process of these Schiff base units.^27,28 Interestingly, NH₄⁺ may also act as a nitrogen source,²⁹ and adjust the spherical morphology and sizes for HNPS.²⁸

Scheme 1 Schematic illustration of the synthesis process for NECS.

Fig. 1a and b show the SEM images of the HNPS and NECS, respectively. HNPS appear to be homogeneous spheres with diameters of about 0.6–1.2 μm. After activation, the NECS still maintain spherical with approximately the same dimensions. It is worth noting that the type of activator can influence the final morphology of the carbon spheres.^30–32 Both our experimental results and the literature³² show that KOH activation destroyed seriously the spherical morphology of carbon precursors. Therefore, we used ZnCl₂ to activate the HNPS at 800 °C for 1 h, and the results indicated that ZnCl₂ activation induced minor erosion without disrupting the morphology of the carbon precursors. Moreover, since the activation process was performed at 800 °C, ZnCl₂ completely volatilized beyond its boiling point (756 °C), leaving no residues on the carbon sample.³³

Fig. 1 SEM image of synthesized (a) HNPS, (b) NECS. (c) Nitrogen adsorption-desorption isotherm and (d) the corresponding incremental pore size distribution curves for NECS. (e) FI-IR of HNPS and NECS.

The porosity of the NECS was characterized by nitrogen adsorption–desorption measurement. The resulting isotherm is shown in Fig. 1c and the physical parameters are listed in Table 1. The isotherm corresponds to a typical type I response, suggesting the sample is microporous. In Table 1, increased porosity is evident in NECS as compared to HNPS as indicated by a significantly enlarged specific surface area of 1237 m² g⁻¹, and a total pore volume of 0.5790 cm³ g⁻¹. Besides, it can be seen in Fig. 1d that the pore size of NECS distributed primarily in the micropore range of 0.6–2.0 nm.

Table 1 Physical properties and chemical compositions of the HNPS and NECS

Samples	Physical properties			Chemical composition
	SBET^a (m^2 g^−1)	Vtotal (cm^3 g^−1)	Vmicro^b (cm^3 g^−1)	N (wt%)	C (wt%)	H (wt%)
a SBET: the specific surface area was calculated by the BET method.b Vmicro: micropore volume was calculated by a t-plot analysis.
HNPS	2	—	—	23.00	67.03	6.66
NECS	1237	0.58	0.35	10.21	81.90	3.54

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The nitrogen content is 23.00 wt% for HNPS and 10.21 wt% for NECS (Table 1). The significant decrease in nitrogen content can be attributed to the condensation of polymeric framework and the release of ammonia.²⁵ The nitrogen content of NECS obtained in this study is relatively high as compared to the literature values,^{23–26,34,35} which can be ascribed to the high-nitrogen source m-phenylenediamine.

The FI-IR (Fig. 1e) spectra of HNPS and NECS reveal information regarding the surface functional groups of both. The peak at around 3435 cm⁻¹ may be assigned to N–H or O–H stretching vibrations.³⁶ The peaks around 1625 cm⁻¹ and 1387 cm⁻¹ correspond to N–H in-plane deformation vibration and the C–N stretching vibration, respectively.³⁷ The broad peaks between 950 and 650 cm⁻¹ are related to the out-of-plane N–H deformation vibration.³⁸ In conclusion, the FT-IR analysis confirms the existence of N–H and C–N bonds for HNPS and NECS. The changes of nitrogen species on the HNPS surfaces were analyzed using the XPS spectra of the N 1s signal. In the case of the HNPS (Fig. 2a), there is only one obvious fitted peak at ∼399.4 eV (amide nitrogen).³⁹ After activation, the N 1s spectra of NECS (Fig. 2b) could be deconvoluted to four peaks with pyridinic-N (∼398.3 eV), pyrrolic-N (∼400.0 eV), graphitic-N (∼401.1 eV) and nitrogen-oxides (∼402.6 eV),^40–42 suggesting that the nitrogen types in HNPS rearranged and further transformed into different nitrogen species with elevated temperature. As a result, NECS contain a large amount of nitrogen-containing functionality and the high specific surface, both of which are conducive to Cr(VI) adsorption.

Fig. 2 XPS N 1s spectra of (a) HNPS, (b) NECS.

3.2 Adsorption isotherms

Adsorption isotherms of NECS fitted by Langmuir and Freundlich models are shown in Fig. 3. The equations are expressed as follows.where q_e (mg g⁻¹) is the Cr(VI) adsorption capacity at equilibrium; C_e (mg L⁻¹) is the equilibrium concentration; q_max (mg g⁻¹) is the maximum adsorption capacity; b (L mg⁻¹) is a constant related to the absorbing energy; k is the Freundlich constant and 1/n is the heterogeneity factor. The corresponding fitting parameters are listed in Table 2. It can be seen that the Freundlich model has a better fit than the Langmuir model. The value of the heterogeneity factor 1/n equals 0.218, which lies between 0.1 and 1, suggesting the adsorption of Cr(VI) by NECS is favored. It should be noted that the adsorption capacity of NECS can reach as high as 279 mg g⁻¹ (Fig. 3). This capacity is much higher than that of the other nitrogen-containing carbon materials (Table 3), indicating that NECS are excellent adsorbents for Cr(VI) removal from aqueous solution. In addition, as shown in Table 3, although some nitrogen-containing carbon materials own high surface areas,^43,46,48 they display low Cr(VI) adsorption capacities. We consider that can be ascribed to the surface functionality of these carbon materials obtained by different synthesis method.

Fig. 3 Adsorption isotherms of Cr(VI) by NECS fitted by Langmuir and Freundlich models.

Table 2 Constants of Langmuir and Freundlich models for NECS

Isotherm model	Constant	NECS
Langmuir	qmax (mg g^−1)	285
	b (L mg^−1)	0.039
	R^2	0.7676
Freundlich	k	78.51
	1/n	0.218
	R^2	0.9785

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Table 3 Comparison of Cr(VI) adsorption capacities with other nitrogen-containing carbon materials

Chemical compositions of the different nitrogen-containing carbon materials	Physical properties	Adsorption capacity (mg g^−1)	Reference
Fe3O4@N-doped porous carbon	SBET: 1136 m^2 g^−1	30	43
Activated carbon derived from acrylonitrile–divinylbenzene	SBET: 579 m^2 g^−1, Vtotal: 0.30 cm^3 g^−1	101	44
Amine-group on the Arundo donax Linn activated carbon	SBET: 9 m^2 g^−1, Vtotal: 0.03 cm^3 g^−1	103	45
Melamine or urea modified bamboo activated carbon	SBET: 1263 m^2 g^−1	185	46
Magnetic mesoporous carbon incorporated with polyaniline	SBET: 56 m^2 g^−1, Vtotal: 0.11 cm^3 g^−1	172	47
Melamine resins porous carbon	SBET: 2699 m^2 g^−1, Vtotal: 1.82 cm^3 g^−1	200	48
Polyacrylonitrile-based activated carbon	SBET: 12 m^2 g^−1	188	49
Polymer based activated carbon	—	143	50
Amino-functionalized carbon spheres	—	240	51
Urea and melamine-bamboo activated carbon	SBET: 1511 m^2 g^−1, Vtotal: 0.69 cm^3 g^−1	143	18
m-Phenylenediamine formaldehyde polymer porous carbon	SBET: 1237 m^2 g^−1, Vtotal: 0.58 cm^3 g^−1	279	This study

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3.3 Adsorption kinetics

As presented in Fig. 4a, the adsorption capacity of Cr(VI) on NECS sees a rapid increase in the first 20 min and a plateau after. It took shorter time to reach equilibrium at an initial concentration of 100 mg L⁻¹ as compared to 200 mg L⁻¹. Table 4 shows the adsorption kinetic data fitted by pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order and pseudo-second-order kinetic models are expressed as follows.

ln(q_e − q_t) = ln q_e − k₁t (5)

where q_e (mg g⁻¹) and q_t (mg g⁻¹) are the Cr(VI) adsorption capacity at equilibrium and time t, respectively; k₁ (min⁻¹) is the rate constant of pseudo-first-order adsorption; k₂ (g mg⁻¹ min⁻¹) is the rate constant of pseudo-second-order adsorption model. As listed in Table 4, the coefficients of determination (R²) for the pseudo-second-order model all exceed 0.99. The high R² values and the consistency between calculated and experimental q_e values indicated that the adsorption process of Cr(VI) can be well fitted by the pseudo-second-order model (Fig. 4b), implying that chemisorption may be the rate-controlling step.⁵²

Fig. 4 (a) Adsorption of Cr(VI) on NECS and Cr(III) cations concentration in initial concentration of 200 mg L⁻¹ solution as a function of contact time. (b) The pseudo-second-order kinetic model for the adsorption of Cr(VI) on NECS.

Table 4 Parameters of different kinetic models for the adsorption of Cr(VI) on NECS

C0 (mg L^−1)	Pseudo-first-order			Pseudo-second-order
	k1 (min^−1)	qe (mg g^−1)	R^2	k2 (g mg^−1 min^−1)	qe (mg g^−1)	R^2
100	0.2673	94	0.9802	0.0051	101	0.9995
200	0.1965	154	0.9760	0.0046	164	0.9992

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3.4 Removal mechanism

Fig. 5a shows the Cr(VI) removal efficiency by NECS at different pH values. It can be seen that the Cr(VI) removal efficiency decreases with the increase of pH. The optimum pH is between 1.0 and 3.0, where the Cr(VI) removal efficiencies exceed 95%. In this pH range, HCrO₄⁻ and Cr₂O₇²⁻ are known to be the dominant Cr(VI) ionic state in aqueous solution.⁵³ To study the surface charge effect on adsorption, zeta potential measurement was performed as a function of pH. As shown in Fig. 5b, the pH at the point of zero charge (pH_pzc) is ∼3.6, which is relatively low comparing with those of granular activated carbon (6.3) and natural corncob (6.2) as a result of nitrogen introduction.^54,55 Under pH_pzc, the positively charged NECS surface is able to effectively adsorb Cr(VI) anions via electrostatic interaction. The positive surface charge increases as the pH decreases, resulting in higher adsorption.

Fig. 5 (a) The Cr(VI) removal efficiency of NECS at different initial pH values. (b) Zeta potential values as a function of pH for NECS. XPS spectra of Cr 2p (c) and N 1s (d) for NECS after Cr(VI) adsorption experiments.

We measured the Cr(III) cations concentration in initial Cr(VI) concentration of 200 mg L⁻¹ solution (pH 2.0) as a function of contact time (Fig. 4a). It found that a rapid increase of Cr(III) cations in solution in the first 20 min followed by a gradual increase and final Cr(III) cations concentration accounted for ∼41% of the total Cr ions, indicating partial Cr(VI) was reduced to Cr(III).⁴³ To further elucidate the removal mechanism, XPS spectra of the Cr 2p and N 1s signals were collected for the analysis of the changes of Cr(VI) and nitrogen species on the NECS surface, respectively. Fig. 5c shows two Cr 2p_3/2 and Cr 2p_1/2 asymmetric peaks of NECS after Cr(VI) adsorption. Among four fitted symmetric peaks, the peaks centered at 576.8 and 586.6 eV can be attributed to Cr(III), and the peaks at 578.6 and 588.3 eV to Cr(VI).⁴³ Therefore, Cr(VI) and Cr(III) coexist on the NECS surface, which indicates that part of the adsorbed Cr(VI) anions could be reduced to Cr(III). As shown in Fig. 5d, the N 1s spectra of NECS after Cr(VI) adsorption could be deconvoluted to four peaks with pyridinic-N (∼398.3 eV), pyrrolic-N (∼400.0 eV), graphitic-N (∼401.1 eV) and nitrogen-oxides (∼402.6 eV).^40–42 Compared with the N 1s signal peaks on the NECS surface before adsorption (Fig. 2b), the contents of pyridinic-N and pyrrolic-N decreased, while that of graphitic-N increased, indicating that the redox reaction occurred between Cr(VI) and the nitrogen functionality of carbon spheres.⁴⁷

Based on the above analysis, Cr(VI) removal mechanism by NECS is proposed as follows. Cr(VI) anions are first adsorbed on the positively charged NECS surface by electrostatic interaction at low pH. The adsorbed Cr(VI) anions are then partially reduced to Cr(III). Finally, the Cr(III) ions are possibly immobilized on the NECS surface through a coordination effect with the lone-pair electrons of nitrogen.^43,47 It is worth noting that nitrogen functionality plays a key role in the adsorption and redox processes. On one hand, nitrogen functionality enhances the hydrophilicity of carbon sphere surface. On the other hand, nitrogen functionality may increase charge density of carbon sphere surface, which is beneficial for the adsorption of Cr(VI) anions. Moreover, according to the concept of Lewis base, nitrogen functionality can provide many basic sites that can offer a more delocalized electron concentration for promoting a reductive environment.¹⁷ Therefore, NECS showed a remarkable adsorption capacity and strong reduction ability.

3.5 Regeneration and recycling

The negligible Cr(VI) removal efficiency at high pH indicated that the NECS could be regenerated by base solution. Therefore, regeneration and recycling experiments of the exhausted NECS were performed using the NaOH (1 M) eluent. As shown Fig. 6, the NECS maintained high Cr(VI) removal efficiency of over 90% even after six cycles, clearly proving the effectiveness of regeneration and recycling.

Fig. 6 Recycling studies of NECS in the removal of Cr(VI) for six cycles.

3.6 Removal of Cr(VI) from acidic electroplating wastewater

Applicability of NECS was demonstrated by removing Cr(VI) from real acidic electroplating wastewater. Since the concentration of Cr(VI) in industrial discharge standard is lower than 0.5 mg L⁻¹, an increased dosage of NECS from 1 g L⁻¹ to 15 g L⁻¹ was used to ensure that the ultimate Cr(VI) concentration meets this standard. At the dosage of 12 g L⁻¹, the ultimate Cr(VI) concentration and removal efficiency were 0.19 mg L⁻¹ and 99.9%, respectively (Table 5). Moreover, the pH after treatment was elevated to 5.6 from 1.5. These results further confirm NECS to be a promising material for the removal of Cr(VI) from real acidic Cr-containing wastewater, especially in strong acidic solution.

Table 5 Removal results for acidic electroplating wastewater by NECS^a

Items	Cr(VI)	pH
a NECS: the dosage: 12 g L^−1, time: 5 h, T = 298 K.
Initial concentration (mg L^−1)	936.98	1.5
Ultimate concentration (mg L^−1)	0.19	5.6
Removal capacity (mg g^−1)	78.06	—
Removal efficiency (%)	99.9	—

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4 Conclusions

In this study, spherical NECS with diameters of about 0.6–1.2 μm were synthesized and found to possess high specific surface area (1237 m² g⁻¹) and high nitrogen content (10.21 wt%). During the synthesis, ZnCl₂ activation did not damage the morphology of HNPS precursor. As the main nitrogen source, m-phenylenediamine may be responsible for the high nitrogen content. The NECS showed strong resistance to acid and high removal capacity of Cr(VI) up to 279 mg g⁻¹. Moreover, the NECS maintained over 90% removal efficiency of Cr(VI) after six cycles. The NECS were successfully applied to real acidic electroplating wastewater samples for the removal of Cr(VI). In sum, the NECS prepared in this work would be an effective adsorbent for treating Cr-containing wastewater.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (2014CB932101, 2013CB934302), the Outstanding Youth Fund (21125730), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA09030203), the National Science Foundation Grant (21477128).

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