Nitrogen-Doped Nanoporous Carbon Nanospheroids for Selective Dye Adsorption and Pb(II) Ion Removal from Waste Water.

In the presence of melamine and block copolymers, namely, F108, F127, and P123, nitrogen-doped nanoporous carbon nanospheroids (N@CNSs) were synthesized by the hydrothermal process. The F127-modified sample (CNF127) exhibits the maximum Brunauer-Emmett-Teller (BET) surface area of 773.4 m2/g with a pore volume of 0.877 cm3/g. The microstructural study reveals that nanospheroids of size 50-200 nm were aggregated together to form a chainlike structure for all triblock copolymer-modified samples. The X-ray photoelectron spectroscopy study shows the binding energies of 398.33 and 400.7 eV attributed to sp2 (C-N=C)- and sp3 (C-N)-hybridized nitrogen-bonded carbons, respectively. The synthesized N@CNS samples showed selective adsorption of organic dye methylene blue (MB) in the presence of methyl orange (MO) as well as Pb(II) ion removal from contaminated water. The adsorptions for MB and Pb(II) ions followed pseudo-first-order and pseudo-second-order kinetic models, respectively. The sample CNF127 showed the highest adsorption of 73 and 99.82 mg/g for MB and Pb(II) adsorptions, respectively. The adsorption capacity for MB of the copolymer-modified samples follows the order CNF127 > CNP123 > CNF108, which corroborated with the mesoporosity as well as nitrogen content of the corresponding samples. The maximum % adsorption of Pb(II) follows the order CNF127 (99.82%) > CNF108 (98.74%) > CNP123 (91.82%), and this trend is attributed to the BET surface area of the corresponding samples. This study demonstrates multicomponent removal of water pollutants, both organic dyes and inorganic toxic metal ions.

Introduction

With rapid industrial growth, water contamination by organic and inorganic pollutants has become an important environmental concern to living beings. Organic dyes are essential chemicals for a variety of industries like textile, pulp and paper, paints, cosmetics, pharmaceuticals, etc. The organic dyes are one of the major water pollutants causing carcinogenic and teratogenic effects on human beings.[1] There are various methods like ozone treatment, photocatalysis, adsorption, ion exchange, electrochemical oxidation, etc. for the removal of organic dyes. Among the different methods, adsorption is the most effective method for dye degradation because of total removal of the dye content without formation of any by-products.[2] Among the inorganic pollutants, lead is found to be a dreadful water pollutant causing detrimental effects on human body like anemia, encephalopathy, hepatitis, nephritic syndrome, etc.[3] Various industries like battery and chemical manufacturing, petrochemicals, agrochemicals, etc. are the main sources of Pb poisoning in waste water. Different processes like precipitation, oxidation, ion exchange, filtration, reverse osmosis, electrodialysis, adsorption, etc. are reported for the removal of lead ions from contaminated water. Among the various processes, adsorption is the promising process because of its high efficiency; effectiveness; and economical, versatile, and simple operation for removing trace amounts of metal ions. Therefore, it is important to fabricate such kind of adsorbents that have the efficacy to remove both organic dyes and lead-containing inorganic water pollutants simultaneously.

In recent time, nanoscience and nanotechnology have significant potential for remediation of environmental hazards. Different nanoadsorbents such as activated carbon,[4] layered double hydroxide,[5] zeolite,[6] iron-based nanoadsorbents,[7] etc. have been widely used for decontamination of water. Meng et al. reported a hybrid Si–C–N material for selective adsorption of an organic dye.[8] Poly(1-vinylimidazole) ionic liquid-modified graphene showed high adsorption capacity for methyl blue (MB).[9]

Porous carbon and carbonaceous materials are popular adsorbents for their large surface-to-volume ratio and high mechanical strength. Recently, new-generation N-doped carbons have been found to be interesting materials because of their superior physical and chemical properties as compared to those of their undoped counterparts. Accordingly, a wide range of applications are anticipated for these nitrogen-doped carbon materials such as anodes in high-performance lithium ion batteries,[10] high-performance supercapacitors,[11] etc. A nitrogen-doped carbon material can provide a large number of anchoring sites for adsorption of positive-charged metal ions due to electronegativity difference between N and C atoms.[12] Hence, it is necessary to develop and investigate nitrogen-doped carbon materials as an adsorbent to be exploited for removing toxic pollutants from water.

Nitrogen-doped carbon materials have been used for the removal of water contaminants. Recently, a N-doped carbon quantum dot–magnetite nanocomposite was exploited for adsorption of methylene blue from waste water.[13] The N-doped carbon synthesized from ionic liquid@ZIF 8 was used for the removal of herbicides from water.[14] Perez-Aguilar et al. reported the adsorption of Cd and Pb by N-doped multiwall carbon nanotubes prepared by the chemical vapor deposition process.[15] Nitrogen-doped magnetic carbon nanoparticles have been used for the removal of Cr3+ ions from water.[16] For the removal of toxic metal ions from water, N-doped carbon nanofibers were used.[17] A nitrogen-doped carbon xerogel was utilized for decontamination of water by removing heavy metal ions.[18]

With the above motivation, the main goal of the present research study is to synthesize highly porous nitrogen-doped carbon materials prepared using melamine and triblock copolymers (TBCs) and investigation of their performance as an adsorbent for the removal of both organic dye, methylene blue (MB), and inorganic pollutant, lead (Pb), from contaminated water. This study is significant for the fabrication of a suitable adsorbent, which could be applicable for the removal of multicomponent water pollutants, both organic dyes and inorganic toxic metal ions.

Results and Discussion

Material Characteristics

Figure shows the Raman spectra of samples CNF108, CNF127, and CNP123. The disorder structure of N-doped carbon materials is confirmed by the appearance of peak at 1346 cm–1 (D band), whereas the band at 1596 cm–1 is due to the sp2-hybridized C–C bond in-plane stretching vibration (G band).[19,20] The broad band appeared at around 2500–3000 cm–1 is considered as a G″ band, i.e., overtone of the D band induced by disorder.[21] It is to be noted that there are no significant changes in Raman bands of the three different samples, CNF108, CNF127, and CNP123.

Figure 1

Raman spectra of nitrogen-doped nanoporous carbon nanospheroid (N@CNS) samples.

Figure shows the N2 adsorption–desorption isotherms of (a) CNF108, (b) CNF127, and (c) CNP123, indicating a pseudo-type-1 isotherm. The pore size distributions of the samples determined by the Barrett–Joyner–Halenda (BJH) method are shown in Figure , whereas the pore size distributions determined by the density functional theory (DFT) method are revealed in the insets of the corresponding figures. It is noticed that the mesopores generated in the samples are in the range of 3–4 nm, whereas bi- or trimodal distributions of micropores are observed for all of the samples. Table depicts the surface area, pore volume, and pore size data of CNF108, CNF127, and CNP123. Interestingly, the sample CNF127 exhibits the highest Brunauer–Emmett–Teller (BET) surface area (773.4 m2/g) having a pore volume of 0.877 cm3/g. The total BET surface area comprises micropore surface area (545.5 m2/g) and mesopore surface area (228 m2/g). The surface area follows the order CNF127 > CNF108 > CNP123. However, the pore volume of CNP123 is found to be higher than that of CNF108, which is due to the presence of abundance mesoporosity in the former sample, contributing the higher pore volume. The pore size of all of the samples does not changed significantly.

Figure 2

N2 adsorption–desorption isotherms of N@CNS samples: (a) CNF108, (b) CNF127, and (c) CNP123.

Figure 3

BJH pore size distributions of N@CNS: (a) CNF108, (b) CNF127, and (c) CNP123; insets reveal DFT pore size distributions.

Table 1

Textural Properties of the Samples

	surface area (m2/g)a
sample ID	SBET	Smicropore	Smesopore	Vp-total (cm3/g)b	pore diameter (nm)
CNF108	624.7	497	127.7	0.482	3.50
CNF127	773.4	545.4	228	0.877	3.14
CNP123	462.6	255.6	207	0.660	3.71

BET surface area.

Pore volume.

The X-ray photoelectron spectroscopy (XPS) spectra of the products in full spectrum are shown in Figure a. This indicates the presence of C 1s, N 1s, and O 1s. The C 1s core level is deconvoluted into three subpeaks positioned around the binding energies of 284.6, 285.1, and 287.8 eV (Figure b). It is found that the position of the C 1s line assigning to the C–C bond is shifted from 284.4 to 284.6 eV due to nitrogen doping. After deconvolution, two additional peaks appear at around 285.1 and 287.87 eV. The sharp peak with the binding energy of 285.1 eV is attributed to the sp2 C=N bonds, and the broad peak at 287.87 eV is attributed to sp3 C–N bonds.[22] The high-resolution deconvoluted spectrum of N 1s is shown in Figure c–e for the samples CNF108, CNF127, and CNP123, respectively. The deconvoluted N 1s spectra render two distinguishable peaks with binding energies of 398.33 and 400.7 eV. The peaks at 398.33 and 400.7 eV are attributed to the pyridinic nitrogen or sp2-hybridized nitrogen (C–N=C) and sp3-hybridized nitrogen (C–N), respectively.[23−25] The above results confirm that in the nitrogen-doped carbon sample (N@CNS), nitrogen interacts with carbon via double and single bonds with sp2- and sp3-hybridized states, respectively. It is worth noting that for different co-polymer-modified N@CNS samples, the C 1s and N 1s peak positions remain unchanged. However, the atomic % values of doped nitrogen obtained from XPS results were found to be 4.05, 5.40, and 4.50 for the samples CNF108, CNF127, and CNP123, respectively.

Figure 4

XPS spectra of the N@CNS samples (a) in full spectrum; and (b) deconvoluted C 1s spectra and deconvoluted N 1s spectra of (c) CNF108, (d) CNF127, and (e) CNP123.

Microstructural features of the samples were studied by field emission scanning electron microscopy (FESEM) (Figure ) and transmission electron microscopy (TEM) (Figure ). Figure reveals the FESEM microstructures of the products CNF108, CNF127, and CNP123, and the arrow marks represent the images with higher magnifications. The carbon nanospheroids of size 50–200 nm were highly aggregated. The high surface charge of the particles due to the polarity difference between carbon and nitrogen atoms in N@CNS facilitates agglomeration of the particles. The TEM images (Figure ) show that the particles are arranged in a chainlike fashion. The TEM images with a higher magnification reveal the pores in the products. The energy dispersive X-ray spectroscopy (EDS) analysis of the three samples reveals the presence of nitrogen in nitrogen-doped carbon samples (N@CNS).

Figure 5

FESEM microstructures of N@CNS: (a) CNF108, (b) CNF127, and (c) CNP123.

Figure 6

TEM images and EDS of N@CNS samples: (a) CNF108, (b) CNF127, and (c) CNP123.

Dye Adsorption Study

Selective Adsorption of Dye

To study the selective adsorption capacity of the synthesized products, the cationic dye MB and anionic dye methyl orange (MO) were used with an equal concentration of 10 × 10–2 mM. Figure shows the UV–vis spectral analysis for the selective dye adsorption of MB and MO onto the samples. For comparison, Figure S1, Supporting Information (SI) shows the UV–vis spectrum of each dye MO and MB separately for sample CNF127. To further investigate the selective removal of the dyes having the same charge, the cationic dyes methylene blue (MB) and methyl violet (MV) with a concentration of 5 × 10–2 mM each were used for sample CNF127. Figure S2, SI, shows (a) UV–vis spectral analysis and (b) bar diagram (time vs adsorption %) of the adsorption of two cationic dyes MB and MV for sample CNF127. This reveals that adsorption % for MV is slightly higher than that of MB up to 40 min, followed by their comparable adsorption for 50 min. However, after 60 min, adsorption % values for MB and MV were found to be 84 and 83.5%, respectively. It is worth noting that adsorption % for MB in the presence of MV is slightly less than that in the presence of MO because of competitive adsorption of both the cationic dyes in the former dyes (MB and MV).

Figure 7

UV–vis spectral analysis for selective dye adsorption for N@CNS samples: (a) CNF108, (b) CNF127, and (c) CNP123.

The adsorption of MB onto the nitrogen-doped carbon nanospheroids (N@CNS) is due to electrostatic interaction between nitrogen lone pairs in N@CNS and the cationic dye, MB. On the other hand, due to the presence of electron cloud in both the anionic MO dye molecules and N@CNS having sp2-hybridized nitrogen, the adsorption of MO seldom occurs onto N@CNS. The molecular sizes of hydrated MB and MO in aqueous solution are 11.2[26] and 26.14 Å,[27] respectively. Therefore, it is evident that the adsorption of MB is more facile than that of MO due to the smaller size of the former dye. The adsorption capacity for MB of the three samples follows the order CNF127 > CNP123 > CNF108, which corroborated with the mesoporosity of the corresponding samples (Table ). The maximum nitrogen content in the CNF127 sample also contributed to the faster adsorption of MB.

It was noticed that CNF127 shows a higher adsorption capacity for MB. The adsorption parameters like contact time, adsorbent (MB) concentration, and temperature were varied for determining the adsorption capacity of CNF127. Figure a,b shows the UV–vis spectral study and C/C0 vs time plot for the adsorption of MB on CNF127, respectively. The adsorption of MB is high at the initial stage of adsorption, and after that, the adsorption capacity decreases with time. At the initial stage of adsorption, a large number of adsorption sites are available in the adsorbent (CNF127), causing the rapid adsorption of MB. The adsorption capability (q) of the sample was calculated using eq , and the q vs time plot is shown in Figure c. It shows that after 40 min of adsorption equilibrium is reached, rendering about 98% adsorption.

Figure 8

(a) UV–vis spectral study for MB adsorption with time for CNF127, (b) C/C0 vs time plot, and (c) adsorption capacity vs time plot.

To understand the adsorption mechanisms, the adsorption kinetics of MB on CNF127 revealing the highest adsorption capacity was carried out by employing three kinetic models. The experimental data were fitted with pseudo-first-order (eq ) and pseudo-second-order (eq ) models.where qe and qt (mg/g) are the amounts of pollutants adsorbed at equilibrium and at time t (min), respectively, k1 (min–1) is the Lagergren pseudo-first-order and k2 (g/mg/min) is the corresponding second-order rate constant, and t (min) is the contact time between the adsorbent and adsorbate. The fitted plots for pseudo-first-order and pseudo-second-order models are shown in Figures S3 and S4 (SI), respectively. All of the experimental data obtained from fitted first-order and second-order models is summarized in Table . The R2 (correlation coefficient) values of 0.999 confirm that MB adsorption on CNF127 occurs with the pseudo-first-order kinetics model.

Table 2

Parameters of the Adsorption Isotherm and Kinetic Rate Models for the Adsorption of MB

pseudo-first-order kinetics	ln(qe – qt) = −k1t + ln qe	k1 = 0.097 min–1
		qe = 31.28 mg/g
		R2 = 0.999
pseudo-second-order kinetics	t/qt = 1/k2qe2 + (1/qe)t	k2 = 0.0095 (g/mg/min)
		qe = 32.62 mg/g
		R2 = 0.9873
intraparticle diffusion kinetic	qt = kdi√t + Ci	kd1 = 6.2081	C1 = 2.512	(R1)2 = 1
		kd2 = 2.552	C2 = 15.272	(R2)2 = 0.972
		kd3 = 0.151	C3 = 30.608	(R3)2 = 0.764
Langmuir isotherm	Ce/qe = Ce/qm + 1/KL × qm	KL = 0.201
		qm = 73 mg/g
		R2 = 0.999
Freundlich isotherm	log qe = log KF + 1/nF log Ce	KF = 26.29
		nF = 5.426
		R2 = 0.959

For a better perceptive of the adsorption mechanism with diffusion, the intraparticle diffusion model was examined by applying the Weber and Morris equationwhere kd is the intraparticle rate constant (mg/g min1/2), t is the time (min), and C (mg/g) is a constant related to the thickness of the boundary layer. Figure S3 (SI) shows q vs √t plots representing trilinear regimes of three successive adsorption steps for MB. The sharp rise in the 1st linear region of adsorption involves the instantaneous adsorption where a high concentration of MB is the driving force of diffusion. The second linear region relates relatively slow and steady which is the rate-determining step attributing intraparticle diffusion of MB onto the adsorbent pores.[28] Finally, due to the presence of low concentration of dye molecules, the intraparticle diffusion rate decreases and an equilibrium stage is reached. All of the experimental data obtained from the trilinear plot (Figure S5, SI) is summarized in Table .

To evaluate the adsorption capacity, the adsorption isotherms are obtained by varying the MB concentrations in solution. It was observed that the amount of equilibrium adsorption (qe) increases with increasing equilibrium concentration (Ce) of MB up to 25 × 10–2 mM (Figure ). The qe and Ce are fitted with the two isotherm models (Langmuir and Freundlich) for understanding the interaction between CNF127 and MB. Linear plots of Langmuir and Freundlich isotherm models are revealed in Figure S6a,b, SI, respectively. The correlation coefficient (R2) obtained for the fitted plot of the Langmuir isotherm (R2 = 0.999) is higher than that for the Freundlich isotherm (R2 = 0.959). This demonstrates that the Langmuir model is fitted satisfactorily, implying the monolayer adsorption of dye molecules onto CNF127.[29] According to the Langmuir model, the maximum adsorption capacity of CNF127 for MB adsorption was found to be 73 mg/g.

Figure 9

Adsorption isotherm for different concentrations of MB for CNF127.

The adsorption capacity of MB (5 × 10–2 mM) in the presence of two electrolytes, namely, calcium nitrate and magnesium nitrate with a concentration of 10 mg/L each, was studied (Figure S7, SI). This showed that about 1 and 10% adsorption occurred for calcium nitrate and magnesium nitrate, respectively, whereas about 90% of MB was adsorbed on CNF127. The electrolytes could not effect significantly to change the adsorption of MB.

To evaluate the temperature effect on the adsorption of MB, the adsorption experiment was carried out with varying temperature keeping the initial concentration of MB as 15 × 10–2 mM. Figure shows the bar diagram of MB adsorption, q, vs temperature. With an increase in temperature, MB adsorption increased. This suggests that with an increase in temperature, diffusion of MB through the external laminar layer into the pores of the sample increased, resulting in an increase in the adsorption rate.[30] The CNF127 sample shows the maximum adsorption for MB (94.93 mg/g) at 338 K.

Figure 10

MB adsorption with temperature for CNF127.

The thermodynamic parameters such as Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated using the following equationwhere K is the thermodynamic equilibrium constant, R is the universal gas constant (8.314 J/mol/K), and T is the temperature (K). The values of ΔH° and ΔS° are obtained from the slope and intercept of the plot of ln Kd vs 1/T, respectively (Figure S8, SI). The values of ΔG°, ΔH°, and ΔS° for the adsorption process at 25 °C (298 K) are shown in Table . The negative ΔG° value signifies that MB adsorption by CNF127 is spontaneous. The adsorption process is exothermic as indicated by the negative ΔH° value, and positive ΔS° value signifies the increases in randomness at the solid–liquid interface during adsorption.[31]

Table 3

ΔH°, ΔS°, and ΔG° Values for MB Adsorption by CNF127

ΔH° (J/mol)	ΔS° (J/mol/K)	ΔG° (kJ/mol)
–74485.55	300.10	–15.678 at 298 K
		–17.153 at 308 K
		–20.632 at 318 K
		–23.751 at 328 K
		–26.705 at 338 K

To examine the suitability of the sample CNF127 for the removal of MB in municipality water (real matrix), the adsorption experiment is further carried out in municipality water samples. The simulation experiments were performed in municipality water system with different initial concentrations of MB (10 × 10–2, 15 × 10–2 and 25 × 10–2 mM). The adsorption capacity (mg/g) of CNF127 for MB in municipality water and deionized water was found to be comparable (Figure S9, SI).

Adsorption of Pb(II) Ions

The synthesized samples (CNF108, CNF127, and CNP123) also have the significant potential for removing Pb(II) ions (hydrated radius of Pb(II): 2.61 Å[32]) from waste water. To study the adsorption capacity toward Pb(II) ions removal, 5 mg of sample was utilized for 50 ppm Pb(II) ion adsorption at different time intervals. Figure shows that within 5 min the adsorption of Pb(II) ions took place rapidly, showing the adsorption capacities of 62.5, 91.4, and 71.7 mg/g for samples CNF108, CNF127, and CNP123, respectively. This indicates that a large number of active adsorption sites are available at the first stage. After that, the adsorption of Pb(II) ions continues slowly, achieving maximum adsorption at 120 min. The maximum % adsorption of Pb(II) followed in the order of CNF127 (99.82%) > CNF108 (98.74%) > CNP123 (91.82%); this trend corroborates with the BET surface area of the corresponding samples. It is clear that with a higher BET surface area of CNF127, the available active adsorption sites are abundant, leading to the higher rate of Pb(II) ion adsorption. Figures S10 and S11, SI, show pseudo-first-order and pseudo-second-order kinetic models. The parameters q, k1, k2, and R2 are summarized in Table S1, SI. For all of the samples, pseudo-second-order kinetics is a well-fitted model, as shown by the R2 values (Table S1, SI). This indicates that the chemisorption process is the rate-determining step.

Figure 11

C/C0 vs time plot for Pb(II) adsorption by N@CNS.

To evaluate the adsorption capacity for Pb(II) ions in the presence of different electrolytes, the experiment was carried out in the presence of Ca(NO3)2 and Mg(NO3)2. Figure S12, SI, shows (a) adsorption of Pb(II) ions in the absence and presence of Ca(II) and Mg(II) ions and (b) relative adsorption of Pb(II) ions in the presence of Ca(II) and Mg(II) ions. This shows that in the presence of Ca(II) and Mg(II) ions the adsorption % of Pb(II) ions decreased as about 8, 10, and 11% for the samples CNF127, CNF108, and CNP123, respectively. However, in the presence of Ca(II) and Mg(II) ions, the adsorption of Pb(II) ions was 92, 88.9, and 80.3% for CNF127, CNF108 and CNP123, respectively. The adsorption capacity of the multicomponent metal ions followed the trend Pb(II) > Mg(II) > Ca(II), which is related to their electronegativity values[33] (Pauling’s scale) (Pb(II) (2.33) > Mg(II) (1.31) > Ca(II) (1.00)). In N@CNS, the sp2-hybridized nitrogen having a lone pair electron as well as delocalized π electron facilitates more Pb(II) ion adsorption due to its higher electronegativity compared to that of Mg(II) and Ca(II) ions.

The proposed adsorption mechanism is shown in Figure . The adsorption of pollutants (MB dye and Pb(II) ion) onto the surface of N@CNS samples could be governed by the electrostatic interactions. On the basis of the XPS study, it is clear that nitrogen in N@CNS is in sp2-hybridized as well as sp3-hybridized states. In the sp2-hybridized state of nitrogen in N@CNS, both the lone pair electron in nitrogen and delocalized π electrons in the N=C bond electrostatically interact with cationic dye MB. It is worth noting that the presence of sp2-hybridized carbon in hybrid Si–C–N[8] and poly(1-vinylimidazole) ionic liquid-modified graphene[9] exhibit a higher adsorption capacity for MB. For adsorption of Pb(II) ions, it is understood that in N@CNS the sp2-hybridized nitrogen having a lone pair electron as well as delocalized π electrons could be available to donate the vacant 6p orbital of Pb(II) ions. Thus, the positively charged dye, MB, and Pb(II) ions were readily adsorbed onto N@CNS via electrostatic attractions. With increasing N sites of N@CNS samples, the adsorption capacity increases. Accordingly, the sample CNF127 having a higher amount of N content (5.4 atom %) renders maximum adsorption for both MB dye and Pb(II) ions. On the other hand, the anionic dye, MO, could not be adsorbed onto the N@CNS samples due to the presence of electron cloud in both the anionic MO dye molecules and N@CNS. It is inferred that the synthesized nitrogen-doped carbon nanospheroids (N@CNS) could be used for selective adsorption of organic dye and positively charged cations from contaminated water.

Figure 12

Adsorption mechanism for MB and Pb(II) by N@CNS samples.

Conclusions

In summary, nitrogen-doped nanoporous carbon nanospheroids were prepared using various block copolymers and melamine. The synthesized products contain both micropores and mesopores, and the BET surface area follows the order CNF127 > CNF108 > CNP123 for F127-, F108-, and P123-modified samples, respectively. The microstructural study confirms agglomerated nanospheroids due to the presence of their high surface charge. The XPS study confirms that N atoms in nitrogen-doped carbon are in sp2-hybridized as well as sp3-hybridized states. In the sp2-hybridized state of nitrogen in the product, both the lone pair electron in nitrogen and delocalized π electrons in C–N=C electrostatically interact with cationic dye MB, rendering its selective adsorption in the presence of anionic dye MO. For MB adsorption, the adsorption capacity follows the order CNF127 > CNP123 > CNF108, which corroborates with the N content in the respective samples. Similarly, Pb(II) adsorption is facilitated because the sp2-hybridized nitrogen having a lone pair electron as well as delocalized π electrons in nitrogen-doped carbon could be available to donate the vacant 6p orbital of Pb(II) ions. The maximum % of Pb(II) adsorption followed in the order CNF127 (99.82%) > CNF108 (98.74%) > CNP123 (91.82%); this trend corroborates with the BET surface area of the corresponding samples. The present work is significant for designing the synthesis of nitrogen-doped carbon nanospheroids for the removal of different cationic organic dyes and toxic metals from waste water via an environmentally friendly process.

Experimental Section

Materials

Merck, India (formaldehyde, phenol, methylene blue (MB), methyl orange (MO), methyl violet (MV), magnesium nitrate, calcium nitrate, lead nitrate, and sodium hydroxide) and Sigma-Aldrich (melamine and triblock copolymers) products were used.

Procedure

Phenol (600 mg) was added into a mix solution of formaldehyde (2.1 mL) and NaOH (15 mL 0.1 M) under stirring at 70 °C/30 min. F108, F127, and P123 (0.07 mmol, 15 mL) each was poured separately to the former solution. After stirring for 2 h, 2.66 g of melamine in water (50 mL) was added. Stirring was continued for 15–18 h. Deionized water (56 mL) was added into 18 mL of prepared solution each, and it was treated hydrothermally at 130 °C/24 h. The hydrothermally synthesized products were heated in a tube furnace by flowing N2 (40 mL/min) at 700 °C/3 h. The sample IDs of synthesized nitrogen-doped carbon nanospheroids (N@CNS) were CNF108, CNF127, and CNP123 for the different copolymers F108, F127, and P123, respectively. Scheme shows a tentative formation mechanism for the preparation of N-doped porous carbon nanospheroids. During synthesis, formaldehyde reacts with phenol in alkaline medium to form polymeric resol (water-soluble phenolic resin). In the presence of triblock copolymers (TBC, F127/F108/P123), resol and melamine form micelles in which the TBC interacts with the hydroxyl groups of resol and amino groups of melamine through hydrogen bonding. Under hydrothermal conditions at 130 °C/24 h, resol-TBC-melamine micelles self-assembled to form a yellowish product. After calcinations at 700 °C/3 h in a N2 atmosphere, templated TBC and organic moieties are removed from the hydrothermally synthesized product, forming porous N@CNS particles.

Scheme 1

Schematic Representation for the Formation of N@CNS

The synthesized products were characterized by Raman spectroscopy (Renishaw spectrometer, 514 nm Ar laser), N2 adsorption–desorption measurements (Quantachrome ASIQ MP), X-ray photoelectron spectroscopy (ULVAC-PHI), FESEM (Zeiss, Supra 35VP, Oberkochen, Germany), and TEM (Tecnai G2 30ST, FEI). UV–visible spectroscopy was performed by a UV–vis spectrophotometer (Jasco V-730).

Adsorption Experiment

The prepared sample (5 mg, N@CNS) was added into 10 mL solutions of MB with different initial concentrations in the range of 5 × 10–2 to 25 × 10–2 mM. After the specified time, the adsorbent was filtered and MB concentration in the solution was measured by a UV–vis spectrophotometer. The amount of MB removed by the samples, q (mg/g), was calculated using the following equationwhere q (mg) is the amount of MB adsorbed per gm of adsorbent at time t, C0 (at initial time) and C (at time t) are the MB concentrations, and V (mL) and m (gm) are the adsorbent volume and adsorbent mass, respectively.

For kinetic study, 5 × 10–2 mM MB (10 mL) was mixed with 5 mg of N@CNS samples under stirring for different times. At a desired time interval, the concentration of MB in the solution was measured by a UV–vis spectrophotometer. Pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were used to fit the kinetic data.

To determine the surface properties of the adsorbent and interactions between the adsorbent and adsorbate, adsorption data were fitted with Langmuir and Freundlich models. The Langmuir equation is expressed asThe linear form of the Langmuir isotherm is represented asHere, qe (mg/g) represents the amount of adsorbed MB (mg) per gm of N@CNS samples, qm (mg/g) is the equilibrium adsorption capacity (maximum), Ce (mg/L) is the MB left unadsorbed at equilibrium, and KL is the adsorption constant. By plotting Ce/qe vs Ce, qm and KL can be known from the slope and intercept, respectively.

Freundlich isotherm is expressed aswhere KF and nF are the measures of adsorption capacity and adsorption intensity, respectively, and KF and 1/nF are known from the intercept and slope of the log Ce vs log qe graph, respectively.

The adsorption experiment for Pb(II) was studied by adding 5 mg of samples into 10 mL of 50 ppm Pb(II) ion solution (Pb(II) concentrations were measured by inductively coupled plasma atomic emission spectroscopy at different time intervals).