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Article

Lignite-Based N-Doped Porous Carbon as an Efficient Adsorbent for Phenol Adsorption

by
Yanfeng Xue
1,*,
Yanyan Chen
2,
Linxia Shi
1,
Haotian Wu
1,
Chao Zhang
1,
Minghuang Cheng
1,
Hongbin Li
1,
Wanjun Li
1 and
Yulan Niu
1
1
Collaborative Innovation Center of CO2 Conversion and Utilization, Department of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China
2
Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
*
Author to whom correspondence should be addressed.
Processes 2022, 10(9), 1746; https://0-doi-org.brum.beds.ac.uk/10.3390/pr10091746
Submission received: 4 August 2022 / Revised: 27 August 2022 / Accepted: 30 August 2022 / Published: 2 September 2022
(This article belongs to the Topic Energy Efficiency, Environment and Health)
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Abstract

:
The treatment of phenolic-containing wastewater has received increased attention in recent years. In this study, the N-doped porous carbons were prepared from lignite with tripolycyanamide as the N source, and their phenol adsorption behaviors were investigated. Results clearly showed that the addition of tripolycyanamide largely improved the surface area, micropore volume, N content and thus the phenol adsorption capacity of lignite-based carbons. The N-doped sample prepared at 700 °C showed a surface area of 1630 m2/g and a phenol adsorption capacity as high as 182.4 mg/g at 20 °C, which were 2.0 and 1.6 times that of the lignite-based carbon without N-doping. Pseudo-second order and Freundlich adsorption isotherm models could better explain the phenol adsorption behaviors over lignite-based N-doped porous carbon. Theoretical calculations demonstrated that phenol adsorption energies over graphitic-N (−72 kJ/mol) and pyrrolic-N (−74 kJ/mol) groups were slightly lower than that over the N-free graphite layer (−71 kJ/mol), supporting that these N-containing groups contribute to enhance the phenol adsorption capacity. The adsorption mechanism of phenol over porous carbon might be interpreted by the π–π dispersion interactions between aromatic-ring and carbon planes, which could be enhanced by N-doping through increasing π electron densities in the carbon plane.

Graphical Abstract

1. Introduction

Phenol compounds are prevalently contained in industrial effluents from petroleum refining, coking, plastic, pharmaceutical, dyeing and many other processes, thus leading to a large amount of phenolic wastewater discharging into water streams [1,2]. According to industrial statistics, the concentration of phenol in synthetic concocted wastewater reaches 1000 mg/L [3], while wastewater from petrochemical and coal conversion industries gives a wide phenol concentration ranging from 200 up to 7000 mg/L [4]. Because of the high biotoxicity and carcinogenicity of phenols to humans and ecosystems, phenolic wastewater currently results in serious environmental and ecological problems [5]. Therefore, the removal of phenols from wastewater before discharging has become an urgent problem that needs to be solved. Several approaches have been implemented for the treatment of phenol-containing wastewater, including membrane separation [3,6], electrochemical oxidation [7,8], microbial biodegradation [9], photocatalytic degradation [10], and adsorption [11,12]. Among these methods, adsorption has aroused many concerns because of its advantages of high efficiency, low cost, simplicity, and non-pollution [13,14,15].
In the adsorption process, a cost-effective adsorbent with high adsorption performance is highly desired. Activated carbon (AC) has been vastly applied presently, especially for wastewater with low adsorbate concentration due to the high surface area, developed pore structure and the presence of varieties of surface functional groups [16,17]. For instance, activated carbon samples were prepared from oily sludge to remove phenol. The synthesized adsorbent showed a high adsorption capacity of 434 mg/g [18]. Saleh et al., for example, employed waste rubber-derived AC to extract phenol from an aqueous solution, and the produced AC showed a maximum adsorption capacity of 18.12 mg/g [19]. In another study, KOH-activated carbon derived from cattle bone showed a high adsorption capacity of 431 mg/g [20].
Generally, the pore structure of AC has important roles in physical adsorption because it provides accessible adsorption sites and diffusion channels to the adsorbate molecules, while another key factor in controlling adsorption capacity is the surface chemistry [21,22]. The complex nature of surface chemistry mainly depends on the surface functional groups, which determine the acid–base property, hydrophilia-hydrophobicity, and electron density of the carbon surface, further impacting the specific interactions between adsorbate and adsorption sites [23]. To further elucidate the effects of surface chemistry or mechanism of adsorption, the heteroatoms such as O [24], N [25], P [26], and B [27] have been introduced to the functional groups or carbon matrix of AC.
However, the introduction of oxygen atoms by oxidation was found to lead to a decrease in the adsorption capacity of phenols [22,28]. The surface oxygen groups could attract the π-electrons from the basal plane of AC, thus decreasing the interaction between the phenol aromatic-ring and carbon surface [28]. Moreover, the increase in oxygen-containing functional groups, such as carboxylic groups, improves the hydrophilia of AC, thereby enhancing water adsorption and hindering the accessibility and affinity of phenols on adsorption sites [22,28]. In contrast, the N-doped modification of carbons was found to be an effective method of facilitating the adsorption capacity of phenols. The electron-rich nitrogen groups are considered to provide π-electrons to form a π–π bond with the π-electron from the aromatic-ring, thus strengthening the interactions between the phenol and adsorbent [13]. As reported by Yang et al. [16], the AC treated with ammonia at 650 °C gives a 28.9% increase in phenol adsorption. The microporous-dominated N-doped carbons prepared by EDTA-4Na exhibit a large adsorption capacity of 521 mg/g toward bisphenol [23]. Liu et al. [13] further verified with theoretical calculations that the adsorption energy of phenol over the N-doped carbon plane is 12.37 kJ/mol lower than that over N-free graphite carbon. These previous studies demonstrate that N-doped carbon materials are fine candidates for removing the phenols from wastewater. However, the specific mechanisms of the adsorption of phenol on N-doped carbon materials are still fragmentary and not adequately understood, and the interactions between phenols and different N-containing functional groups are still especially ambiguous. Therefore, it is essential to further investigate the phenol adsorption behaviors over N-doped carbon materials.
Low-cost lignite is a kind of mineral coal with rich reserves. It has great potential to be prepared as a pollutant adsorbent for wastewater treatment because of its developed porous structure and abundant functional groups [29]. In addition, varieties of structural defects are irregularly distributed on the carbon matrix of lignite, which makes it possible for nitrogen atoms to be doped into the carbon structures or surface functional groups. Therefore, lignite-based N-doped porous carbon should be a promising adsorbent for treating phenolic wastewater. To the best of our knowledge, however, the preparation of lignite-based N-doped porous carbon and its application for wastewater treatment was rarely reported.
In this study, the lignite-derived N-doped carbons were prepared and applied as the adsorbent for phenol adsorption in simulated phenolic wastewater. The structure and surface chemistry of lignite-based N-doped carbons were systematically characterized by using various characterization techniques. Pseudo-first-order (PFO) and pseudo-second-order (PSO) models were used to investigate the adsorption kinetics, and isotherm adsorption was studied using Langmuir and Freundlich models. The adsorption mechanism of N-doped carbon towards phenol in relation to different N-containing groups was elucidated on the basis of theoretical calculations. The main objectives of this study were to: (1) investigate the modification effects of N-doping on the structural and surface chemistry of lignite-based porous carbon; (2) evaluate the adsorption performances of lignite-based porous carbons; (3) establish the relationships between the physicochemical properties of lignite-based porous carbon and its phenol adsorption capacity. The findings obtained in this work will provide a better understanding of the preparation of lignite-based N-doped carbon and its application in phenol-containing wastewater treatment.

2. Experimental

2.1. Materials

The lignite was supplied by Yunnan Xiaolongtan Mining Bureau. The raw lignite was dried at 60 °C for 12 h, then crushed and passed through a 100-mesh sieve to obtain lignite powder. Phenol, potassium hydroxide (KOH) and tripolycyanamide were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation of Lignite-Based Porous Carbon

The lignite-based porous carbons were synthesized with the chemical activation methods reported in Refs. [30,31]. Typically, 7.0 g of tripolycyanamide was dissolved in 300 mL of deionized water at 80 °C. After stirring for 30 min, 50.0 g lignite powder and 50.0 g KOH were added, and the mixture was stirred at 80 °C for 2 h. After that, the slurry was dried at 100 °C overnight to evaporate water. The sample was activated in 40 mL/min N2 flow at 500–700 °C for 2 h, then washed with deionized water until the pH value of filtrate reached 7. Finally, the sample was dried at 100 °C for 4 h. The schematic of the fabrication process is described in Scheme 1. For brevity of description, the samples were denoted as L-CN-x, with x representing the activation temperature. For comparison, the lignite-based porous carbon without N-doping was prepared with a similar procedure, except that no tripolycyanamide was added. These samples were denoted as L-x, where x represents the activation temperature.

2.3. Sample Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlex II desktop X-ray diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) images were collected with a JEOL JSM-700 microscope at an accelerating voltage of 10.0 kV. The X-ray photoelectron spectra (XPS) were measured on a Krato AXIS Ultra DLD spectrometer with Al Kα resource. The elemental compositions of the sample were measured with an Elemntar Vario EL Cube microanalyzer. N2 sorption analysis was performed at −196 °C on Micromeritics ASAP2460. Before the measurement, the sample was degassed at 300 °C for 4 h. The total surface area, mesopore and micropore volume were obtained by the BET, BJH and t-plot methods, respectively. The Raman spectra were measured on a Renishaw inVia Qontor spectrometer using a laser with a wavelength of 532 nm as the excitation source.

2.4. Adsorption Experiments

The adsorption experiments were performed in a conical flask by immersing 20 mg of carbon materials into a 50 mL phenol solution. The flasks were placed on a shaker and then shaken at 200 rpm at room temperature (20 °C). For the adsorption kinetic experiments, a 133 mg/L phenol solution was used, and the adsorption was performed at set times (10, 15, 20, 30, 50, 70, 90, and 120 min). The adsorption isotherm studies were conducted at 20 °C for 120 min with a series of different initial phenol concentrations ranging from 50 to 275 mg/L. After adsorption, the mixed solution was filtered with a 0.45 μm membrane filter to remove the absorbent, and then the filtrate was analyzed for the concentration of phenol by an Agilent 1100 series high-performance liquid chromatography (HPLC) system equipped with a C18 column (250 mm × 4.6 mm, 2.5 μm). The mobile phase of HPLC was acetonitrile-water (20/80 in mass ratio), and the flow rate was maintained at 0.8 mL/min. The detection wavelength was 270 nm, and the column temperature was set at 25 °C. The retention time of phenol was 14 min.
The reusability performance of L-CN-700 was investigated at the concentration of 133 mg/L at 20 °C. After adsorption, the used L-CN-700 sample was placed in 50 mL of methanol solvent and stirred for 5 h to desorb phenol. Then, the washed sample was used for six adsorption–desorption cycles to investigate its regeneration performance.
The phenol uptake Qt (mg/g) of the sample was calculated from the following equation:
Qt = V × (C0 − Ct)/m
where V (L) is the volume of the phenol solution, C0 and Ct (mg/L) represent the initial and final phenol concentrations, respectively, m (g) is the mass of the adsorbent.
The PFO and PSO models are widely used for analyzing the adsorption kinetic behaviors of adsorbents [32,33]. The PFO and PSO models are expressed as Equations (2) and (3), as follows:
Qt = Qe × (1 − exp(−k1 × t))
Qt = Qe2 × k2 × t/(1 + Qe × k2 × t)
where Qt (mg/g) represents the phenol uptake of the sample, Qe (mg/g) is the calculated equilibrium adsorption amount, k1 (min−1) and k2 (g/mg·min) represent the rate constants of PFO and PSO models, respectively, and t is the adsorption time (min).
The adsorption isotherm data were simulated by the Langmuir and Freundlich models to disclose more information about adsorption behaviors [34,35]. The Langmuir and Freundlich models are usually described by Equations (4) and (5) as follows:
Qe = Qm × kL × Ce/(1 + kL × Ce)
Qe = kF × Ce1/n
where Qm (mg/g) and Ce (mg/L) are the theoretical maximum adsorption capacity and equilibrium concentration of adsorbate, respectively, kL (L/mg) is the Langmuir constant, and kF (mg1−1/n·L1/n·g) and n are the Freundlich constants.

2.5. Models and Method

To investigate the phenol adsorption mechanism on N-doped carbons, a theoretical calculation was used to simulate phenol adsorption over different N-containing functional groups. Herein, four cluster models of graphene, including modified graphitic-N, pyrrolic-N, pyridinic-N, and without modification (N-free graphite layer), are built, respectively. The peripheral carbon atoms are saturated by H atoms. All calculations were carried out using the Gaussian 09 package [36]. All atoms are active and relaxed, and the ωB97X-D functional, including dispersion corrections and the 6-31G(d,p) basis set, was used in all geometry optimizations. The adsorption energy (Eads) of phenol is defined as Equation (6), where E(AB), E(A) and E(B) are the complex, adsorbent, and adsorbate single-point energy, respectively.
Eads = E(AB) − E(A) − E(B)

3. Results and Discussions

3.1. Characteristics of Lignite-Based Porous Carbon

The micro-scale morphology of lignite-based N-doped carbon materials was characterized by SEM measurement. As shown in Figure 1, L-600 and L-CN-600 are irregular blocky solids with a rough and etched surface. A large number of macropores or cavities are formed over the surface of samples due to the harsh activation by KOH at 600 °C [30]. However, the L-CN-600 presents an interconnected three-dimensional hollow morphology, with a larger macropore diameter than that of L-600. By contrast, it is observed that the destruction by KOH etching over L-600 mostly occurs on the surface of lignite. These results indicate that the etching process by KOH is intensified by the introduction of tripolycyanamide, probably due to the fact that the decomposition products of tripolycyanamide, such as N2 and NH3, contribute to the formation, expansion and connection of pore systems during activation at high temperature (500–700 °C) [37].
The textural properties of samples were characterized by an N2 adsorption–desorption isotherms method, and the results are shown in Figure 2 and Table 1. All the samples show the typical Type-IV isotherms, indicative of their micro-mesoporous structures. The adsorption volume of N2 rapidly lifts at relative pressures of P/P0 < 0.1 in all the samples, indicating that abundant micropores are formed. In addition, all samples present the hysteresis loop in the relative pressure P/P0 of 0.45–1.0, suggesting a distinct formation of mesopores. The BJH pore size distributions of samples are shown in Figure 2c,d, which demonstrates that the pore size of mesopores for all samples is centered at around 4 nm.
Table 1 shows that L-500 has a surface area of 424 m2/g and a total pore volume of 0.25 cm3/g. A further increase in activation temperature increases the surface area and pore volume, and L-700 gives a surface area of 814 m2/g and a pore volume of 0.43 cm3/g. However, with increasing the activation temperature, the average pore size is slightly decreased. The L-CN-500, 600 and 700 samples show a similar trend. Interestingly, the introduction of tripolycyanamide largely improves the surface area and micropore volume (Vmicro) of lignite-based carbon materials. The L-CN-500, 600 and 700 samples have a surface area of 762, 1041 and 1630 m2/g, which are 1.8, 1.9 and 2.0 times that of L-500, 600 and 700, respectively. The total pore volume of L-CN-700 is 0.66 cm3/g, which is 0.35 cm3/g larger than L-700. However, Table 1 discloses that the external surface areas (Smeso) of L-CN-500, 600 and 700 samples are quite comparable with L-500, 600 and 700, respectively, suggesting that the introduction of tripolycyanamide mainly improves the amounts of micropores. The SEM analysis shows that introduction of tripolycyanamide results in a larger macropore diameter over samples, while the N2 sorption results reveal that the surface area and micropore volume are largely improved with the assistance of tripolycyanamide. Obviously, this micro-meso-macro hierarchical structure of lignite-based N-doped carbon is facilitated to reduce the diffusion resistance of adsorbate, thus enhancing the accessibility of phenol molecules to adsorption sites.
The XRD patterns of L-600 and L-CN-600 are shown in Figure 3. One broad diffraction peak located at 2θ of ca. 22.9° corresponding to the (002) plane is observed for both samples, indicating that they are amorphous carbon materials [31,38]. In addition, the (100) diffraction peak (2θ of ca. 42.5°) is also distinct for these two samples, implying that they have a high graphitization degree [39].
In order to obtain further confirmation on the structure of lignite-based carbon, Raman spectra of all samples were collected, as shown in Figure 4. All samples show two distinct bands at 1588 (G-band, highly ordered graphitic structure) and 1340 cm−1 (D-band, disordered structure), indicating that the lignite-based carbon materials are composed of highly ordered graphite and disordered structures, consistent with the XRD results. The intensity ratio of D-band to G-band (ID/IG) is often used to estimate the degree of graphitization for carbon materials [40,41]. A higher value of ID/IG represents a lower graphitization degree with more disordered structures or topological defects. Figure 4 shows that the ID/IG value increases from 0.87 for L-500 to 1.02 for L-700, suggesting that the increase in activation temperature leads to a decline in graphitization degree. This phenomenon can be explained by the fact that a higher activation temperature causes a more serious decomposition of functional groups over the lignite-based carbon materials, thus generating more defects or disordered structures over the carbon matrix.
The chemical compositions of lignite-based porous carbons prepared at different activation temperatures are listed in Table 2. The samples with no addition of tripolycyanamide have a nitrogen content of 0.90–1.75% because the raw lignite itself contains a certain amount of nitrogen. In addition, sulfur is also included in all samples as an impurity of lignite. With increasing the activation temperature, the N, H and S contents distinctly decrease owing to the decompositions of functional groups over the lignite-based carbons. In addition, the introduction of tripolycyanamide strikingly improves the N content in carbon materials, which increases from 1.75%, 1.64% and 0.90% for L-500, 600, and 700 to 3.49%, 3.50% and 1.53% for L-CN-500, 600 and 700, respectively, indicating that N atoms are successfully doped into lignite-based carbons with tripolycyanamide as the N source.
The presence of surface functional groups over lignite-based porous carbon was evidenced by XPS measurement. Figure 5 shows the wide scan of the L-CN-600 sample, and it can be seen that some impurity peaks assigned to Ca and K are observed. Figure 6 shows the N 1s XPS spectra in narrow scans, which can be deconvolved into three groups depending on the binding energies. The N 1s peaks located at 398.6, 400 and 401 eV are assigned to pyridinic-N, pyrrolic-N and graphitic-N groups, respectively [13,16,42]. Table 3 shows that the relative amounts of graphitic-N and pyrrolic-N increase with the activation temperature, implying that the high activation temperature was favorable to the formation of graphitic-N and pyrrolic-N groups, although the total amount of different types of nitrogen species decreased. In contrast, the relative content of the pyridinic-N group steadily decreases from 29.2% for L-500 to 22.0% for L-700, with activation temperature increasing from 500 to 700 °C, while the relative content of pyridinic-N reaches the maximum value of 41.2% over the L-CN-500 sample, then rapidly declines to 14.1% when activation temperature increases to 700 °C. These results suggest that some of the pyridinic-N groups are unstable at high activation temperatures, probably convert into graphitic-N and pyrrolic-N groups or are decomposed during the activation process. As reported previously [13], these N-containing groups are important functional groups for phenol adsorption despite the fact that their adsorption mechanisms are not fully elucidated.

3.2. Phenol Adsorption Performance of Lignite-Based Porous Carbons

3.2.1. Adsorption Capacity of Different Lignite-Based Porous Carbons

The increasing concern about the inappropriate discharge of phenolic-containing wastewater in recent years has necessitated a more efficient and viable way for the treatment of phenolic wastewater. Herein, the lignite-based porous carbons prepared in this work were applied as adsorbents for the removal of phenol from wastewater. According to the results shown in Figure 7, all lignite-based carbons can adsorb a certain amount of phenol due to their high surface area and abundant functional groups. Specifically, with an increase in activation temperature, L-700 exhibits considerably higher adsorption performance than L-500, and the phenol adsorption capacity of the former (114.9 mg/g) is 1.9 times that of the latter (61.3 mg/g). This trend also holds true for L-CN-500, 600 and 700 samples. Meanwhile, it is worth noting that the addition of tripolycyanamide largely improved the phenol adsorption capacity, L-CN-700 showed the highest adsorption capacity (182.4 mg/g) among all the lignite-based porous carbons, which was also at a high level in the reported porous carbon adsorbents (Table 4).
The regeneration performance of adsorption materials is an important factor in the treatment of wastewater. To verify this, the regeneration performance of L-CN-700 was studied for six cycles at the adsorption temperature of 20 °C. Figure 8 shows that L-CN-700 maintains a stable adsorption capacity after six cycles, indicating that the lignite-based N-doped carbon has good regeneration performance in phenol adsorption.
It has been reported that the phenol adsorption capacity of carbon materials mainly depends on these factors, such as adsorbent–adsorbate interactions (physical and chemical effects) and pore structures (surface area and pore volume) [23]. The N2 sorption measurements verified that the addition of tripolycyanamide significantly lifted the surface area of lignite-based porous carbons, which could evidently contribute to the phenol adsorption capacity. Furthermore, the contributions of surface chemical groups, especially the N-containing groups, are considered to be another important factor in determining phenol adsorption performance. At present, there are four adsorption mechanisms, i.e., the π–π dispersion interactions, electron donor–acceptor complex, hydrophobic interactions, and the hydrogen bond effect, which have been proposed to interpret the adsorption process of phenols [21,23,43]. To further understand a deep insight into the adsorption mechanism, the adsorption behaviors, including kinetics and isotherm over lignite-based porous carbons were elaborated on in detail.

3.2.2. Adsorption Kinetics

Figure 9 compares the phenol adsorption capacity of L-CN-600 and L-600 at 20 °C at different contact times. It is found that over 50% of the equilibrium adsorption amount is achieved within the first 10–15 min of the adsorption process for L-600 and L-CN-600. The phenol adsorption capacity of L-CN-600 rapidly increases to 120.7 mg/g with increasing adsorption time to 30 min, while a further increase to 50 min slowly increases the phenol uptake to 132.4 mg/g, then reaches the equilibrium within 70 min. The uptake of phenol over L-600 monotonically rises to 62.3 mg/g with increasing adsorption time to 50 min, which then slowly achieves equilibrium at the contact time of 90 min. Figure 9 reveals the fact that the time to reach the adsorption equilibrium for L-CN-600 is shorter than that of L-600. This is expected because the micropore volume of L-CN-600 is much larger than L-600, which leads to a lower mass transfer resistance when phenol diffuses into the adsorption sites [44,45].
With the purpose of understanding the rate-controlling steps in the adsorption process, the phenol adsorption experiment data collected over L-600 and L-CN-600 were fitted by the PFO and PSO models. The kinetics fitting plots and parameters are shown in Figure 9 and Table 5. As listed in Table 5, the correlation coefficients (R2) for the PSO model of both L-600 and L-CN-600 are higher than that of the PFO model, indicating the sorption process follows the PSO adsorption rate expression. However, it should be mentioned that the calculated values of Qe,cal for the PFO model are closer to the experimental data (Qe,exp), while the Qe,cal for the PSO model are higher compared with experimental values, and this gap between the calculated and experimental values might be caused by experimental errors. The results that phenol adsorption obeying the PSO kinetics model over lignite-based carbons are in line with the previous conclusions obtained over the N-doped magnetic mesoporous hollow carbon [13], the activated carbons by thermal modification [21] and biomass-based carbons [43]. As previously mentioned [32], the PSO model is based on the assumption that the rate-limiting step is chemisorption, which is controlled by valency forces via electronic sharing or exchanging between adsorbent and adsorbate. This may support the fact that phenol adsorption on lignite-based porous carbon is mainly caused by the π–π interaction between the aromatic-ring of phenol and the basal plane of porous carbons.

3.2.3. Adsorption Isotherms

The isotherm adsorption model is widely used as an effective tool to describe the interaction between adsorbent and adsorbate. Herein, two classical isotherm models, i.e., the Langmuir and Freundlich models, were adopted to analyze the equilibrium adsorption isotherms of phenol over L-CN-600 and L-600 samples at 20 °C, and the fitting plots and parameters are shown in Figure 10 and Table 6. As shown in Figure 10, the adsorption equilibrium curves of both L-CN-600 and L-600 samples show the typical-I isotherm, where the equilibrium adsorption capacity of samples steadily increases with the phenol concentrations. The equilibrium adsorption capacity of L-CN-600 reaches 171.6 mg/g when the phenol concentration is 203.9 mg/L.
Table 6 discloses that Langmuir and Freundlich models both fit well with the experimental results of the L-600 sample because the correlation coefficients of the Langmuir (0.9960) and Freundlich (0.9938) models are very close to 1. The Langmuir model is more applicable for describing the adsorption behavior of phenol because of its higher value of R2. This indicates that phenol adsorption over L-600 should obey the monolayer adsorption mechanism, which assumes that adsorbate molecules form a localized and uniform monolayer coverage on the adsorbent [46,47]. On the contrary, a comparison of the correlation coefficient reveals that the Freundlich model is superior to the Langmuir model over L-CN-600, suggesting the occurrence of multi-molecular layers adsorption over L-CN-600, most likely due to its heterogeneous surface and abundant N-containing functional groups as adsorption sites for phenol [23,48,49]. As listed in Table 6, the n value obtained in this work is 3.2859 for L-CN-600, which means that phenol is easily adsorbed on the surface of lignite-based N-doped porous carbon [26].

3.3. Theoretical Calculations and Adsorption Mechanism

With the aim of providing a deep understanding of the adsorption mechanism over lignite-based porous carbon, four theoretical models with different functional groups were established, by which the phenol adsorption energies (Eads) over different adsorption sites were calculated. The graphene was used to simulate the base plane of carbon materials in this work, while three N-containing groups (the graphitic-N, pyrrolic-N, and pyridinic-N) were adopted according to the XPS measurements.
Figure 11 shows the Eads and the optimized geometric structures of phenol adsorbed on these adsorption sites. It can be seen that the adsorption energies between phenol and the N-free graphite layer are as low as −71 kJ/mol (Figure 11a), suggesting that the phenol molecule can be stably adsorbed on the carbon materials. The value of Eads over the N-free layer is higher than those of graphitic-N and pyrrolic-N groups, supporting the fact that the interactions between phenol and these N-containing groups are stronger. Nevertheless, it should be mentioned that despite the fact that the Eads over pyrrolic-N shows the lowest value, the gap of Eads between pyrrolic-N and N-free graphite layer is only 3 kJ/mol. This means that the adsorption capacity of pyrrolic-N groups is only slightly stronger than the N-free graphite layer. However, the weak advantage of N-containing functional groups should still not be ignored because it could be effectually enlarged by increasing the amount of N-containing groups during application. On the other hand, it should be noted that the value of Eads over pyridinic-N groups is 1 kJ/mol higher than the N-free graphite layer, which indicates that the contribution of pyridinic-N to phenol adsorption should be not obvious or even negative in contrast to the N-free graphite layer.
It is interesting and worth noting that phenol exhibits different adsorption energies on different N-containing functional groups. The value of Eads for phenol is in the order of pyridinic-N > N-free graphite layer > graphitic-N > pyrrolic-N groups. The nature of phenol adsorption is primarily caused by the π–π dispersion interactions between the π electrons in aromatic rings and those in graphite layers, as supported by the kinetics results and previous studies [13,23]. Thus, it can be inferred that the gaps in the adsorption capacity could be interpreted by the different π electron densities over different functional groups. The pyrrolic-N group should provide the highest π electron density due to its smaller five-membered ring structure. Meanwhile, in comparison to the N-free graphite, the enhanced electron density could be obtained over the graphitic-N group owing to the doping of the electron-rich nitrogen atom. In the case of pyridinic-N groups, the N atom is located at the margin of the carbon plane, where the asymmetrical distribution of the π electron might weaken the π–π conjugated interactions.
In this work, because the specific surface areas and pore volumes of lignite-based porous carbon exhibit wide variation due to the addition of tripolycyanamide, it is inappropriate to only attribute the enhancement of phenol uptakes to the carbon surface chemistry or the presence of N-containing functional groups. In particular, Figure 12 shows the good linear relationships between phenol adsorption capacity and the surface area (R2 = 0.9588) or micropore volume (R2 = 0.9523), suggesting that the micropore properties of lignite-based porous carbon strongly influence the phenol adsorption. In contrast, the correlation coefficients between the N contents and phenol adsorption capacity were only 0.0009–0.1015. Such low correlation coefficients reveal that N-containing groups in the lignite-based porous carbon may not be the dominant factor in the experimental range, despite the fact that it is usually considered the principal factor influencing phenol adsorption [13]. Similar results were reported by Cansado and co-workers related to the removal of 4-chloro-2-methyl-phenoxyacetic acid by ACs containing different N contents [50].

4. Conclusions

In this work, using lignite as the carbon precursor, a series of lignite-based porous carbons were prepared for phenol adsorption in an aqueous solution. It was found that the N-doping with tripolycyanamide as an N source significantly enhanced the etching of lignite by KOH during activation, resulting in an increase in the specific surface area, pore volume, N content, and hence, the phenol adsorption capacity. The equilibrium adsorption capacity over the lignite-based N-doped porous carbon achieves 171.6 mg/g for 203.9 mg/L of phenol at 20 °C. The kinetics adsorption results disclosed that phenol adsorption over lignite-based porous carbon matches well with the PSO model, indicative of the chemical interactions between phenol and adsorbent. The adsorption isotherm over lignite-based N-doped porous carbon is fitted well with the Freundlich model, suggesting that phenol molecules are adsorbed in multiple layers on the surface of carbons. Furthermore, theoretical calculations demonstrate that graphitic-N and pyrrolic-N groups could play positive roles in phenol adsorption, probably attributed to the enhancement of the π–π interaction between the aromatic-ring of phenol and the carbon plane, while the π–π interactions are essentially influenced by the π electron densities of the functional groups over carbon planes. Finally, the work presented here could provide a valuable reference for fabricating a lignite-based adsorbent to remove phenolic compounds from wastewater.

Author Contributions

Conceptualization, Y.X.; investigation, Y.X. and L.S., validation, H.W. and C.Z.; formal analysis, M.C. and H.L.; resources, W.L. and Y.N.; writing—original draft preparation, writing—review and editing, Y.X. and Y.C.; project administration, Y.N.; funding acquisition, Y.X. and Y.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Natural Science Foundation of Shanxi Province of China under Grant (No. 201901D111321), the Taiyuan Institute of Technology Scientific Research Initial Funding (No. 2022KJ062), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi under Grant (No. 2020L0655) and the Fund for Shanxi “1331” Project (Collaborative Innovation Center of CO2 Conversion and Utilization).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All of the data are given in the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 10 01746 sch001 550
Scheme 1. Schematic of the preparation of the lignite-based N-doped porous carbon.
Scheme 1. Schematic of the preparation of the lignite-based N-doped porous carbon.
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Figure 1. SEM images of L-600 (a,b) and L-CN-600 (c,d) samples.
Figure 1. SEM images of L-600 (a,b) and L-CN-600 (c,d) samples.
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Figure 2. N2 sorption isotherms (a,b), and pore size distribution curves (c,d) of lignite-based porous carbons.
Figure 2. N2 sorption isotherms (a,b), and pore size distribution curves (c,d) of lignite-based porous carbons.
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Figure 3. XRD patterns of L-600 and L-CN-600 samples.
Figure 3. XRD patterns of L-600 and L-CN-600 samples.
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Figure 4. Raman spectra of the lignite-based porous carbons.
Figure 4. Raman spectra of the lignite-based porous carbons.
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Figure 5. XPS wide scan of the L-CN-600 sample.
Figure 5. XPS wide scan of the L-CN-600 sample.
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Figure 6. N 1s XPS spectra of L-500 (a), L-600 (b), L-700 (c), L-CN-500 (d), L-CN-600 (e), and L-CN-700 (f) samples.
Figure 6. N 1s XPS spectra of L-500 (a), L-600 (b), L-700 (c), L-CN-500 (d), L-CN-600 (e), and L-CN-700 (f) samples.
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Figure 7. Effect of activation temperature on the adsorption capacities of L-x (a) and L-CN-x (b) samples. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C, t = 90 min.
Figure 7. Effect of activation temperature on the adsorption capacities of L-x (a) and L-CN-x (b) samples. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C, t = 90 min.
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Figure 8. The regeneration cycles for phenol adsorption over the L-CN-700 sample. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C, t = 90 min.
Figure 8. The regeneration cycles for phenol adsorption over the L-CN-700 sample. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C, t = 90 min.
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Figure 9. Adsorption kinetic curves of lignite-based porous carbons. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C.
Figure 9. Adsorption kinetic curves of lignite-based porous carbons. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C.
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Figure 10. Adsorption isotherm curves of lignite-based porous carbons. Adsorption conditions: m/v = 0.4 g/L, 20 °C, t = 120 min.
Figure 10. Adsorption isotherm curves of lignite-based porous carbons. Adsorption conditions: m/v = 0.4 g/L, 20 °C, t = 120 min.
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Figure 11. The optimized geometric structures of phenol adsorbed on N-free carbon (a), graphitic-N (b), pyridinic-N (c) and pyrrolic-N (d) functional groups.
Figure 11. The optimized geometric structures of phenol adsorbed on N-free carbon (a), graphitic-N (b), pyridinic-N (c) and pyrrolic-N (d) functional groups.
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Figure 12. Dependence of phenol adsorption capacity on the specific surface area (a), micropore volume (b), total N content (c), graphitic-N content (d), pyrrolic-N content (e), and pyridinic-N content (f) of the lignite-based porous carbons. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C, t = 90 min.
Figure 12. Dependence of phenol adsorption capacity on the specific surface area (a), micropore volume (b), total N content (c), graphitic-N content (d), pyrrolic-N content (e), and pyridinic-N content (f) of the lignite-based porous carbons. Adsorption conditions: C0 = 133 mg/L, m/v = 0.4 g/L, 20 °C, t = 90 min.
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Table
Table 1. Textural properties of lignite-based porous carbons prepared at different activation temperatures.
Table 1. Textural properties of lignite-based porous carbons prepared at different activation temperatures.
SampleSurface Area (m2/g)Pore Volume (cm3/g)Pore Size (nm)
SBETSmesoVmicroVmeso
L-500424590.160.092.37
L-600561550.220.092.22
L-700814820.310.122.14
L-CN-500762580.300.071.93
L-CN-6001041590.420.061.85
L-CN-7001630890.660.091.83
Table
Table 2. Chemical compositions of lignite-based porous carbons prepared at different activation temperatures.
Table 2. Chemical compositions of lignite-based porous carbons prepared at different activation temperatures.
SamplesChemical Compositions (wt%)N/C aH/C a
NCHSOther
L-5001.7567.532.240.3828.110.020.40
L-6001.6466.621.590.2729.880.020.29
L-7000.9065.581.410.3031.810.010.26
L-CN-5003.4964.772.080.2929.370.050.39
L-CN-6003.5068.371.580.1626.390.040.28
L-CN-7001.5375.600.850.2221.800.020.14
a Molar ratios.
Table
Table 3. Relative peak areas determined from the N 1s XPS spectra of lignite-based porous carbons.
Table 3. Relative peak areas determined from the N 1s XPS spectra of lignite-based porous carbons.
SampleGraphitic-N (%)Pyrrolic-N (%)Pyridinic-N (%)
L-50027.643.229.2
L-60029.149.821.1
L-70036.841.222.0
L-CN-50022.836.041.2
L-CN-60024.641.833.6
L-CN-70042.343.614.1
Table
Table 4. Phenol adsorption capacity of adsorbents in some recent reports.
Table 4. Phenol adsorption capacity of adsorbents in some recent reports.
AdsorbentPhenol Adsorption Capacity, (mg/g)Reference
Molecularly imprinted composite membrane51.40[6]
Nitrogen-doped magnetic mesoporous hollow carbon55.86[13]
Oily sludge-based AC434[18]
Diethylenetriamine-modified activated carbon18.12[19]
Nitrogen-doped hierarchically porous carbon431[20]
Activated carbon96.92[26]
Graphene28.26[34]
Porous carbon from Toona sinensis leaves325[43]
Lignite-based N-doped porous carbon182.4This work
Table
Table 5. The kinetic parameters of phenol adsorption over L-600 and L-CN-600 samples.
Table 5. The kinetic parameters of phenol adsorption over L-600 and L-CN-600 samples.
AdsorbentQe,exp aPseudo-First-OrderPseudo-Second-Order
Qe,cal bk1R2Qe,cal bk2R2
L-60070.4468.490.05410.984980.440.00080.9953
L-CN-600136.64133.540.11120.9898144.190.00140.9988
aQe,exp: the equilibrium adsorption amount obtained by experiments. b Qe,cal: the calculated equilibrium adsorption amount.
Table
Table 6. Langmuir and Freundlich adsorption isotherm parameters for L-600 and L-CN-600 at 20 °C.
Table 6. Langmuir and Freundlich adsorption isotherm parameters for L-600 and L-CN-600 at 20 °C.
AdsorbentLangmuirFreundlich
QmkLR2kFnR2
L-60098.650.02380.996015.88613.20880.9938
L-CN-600187.730.03570.987034.78233.28590.9971
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Xue, Y.; Chen, Y.; Shi, L.; Wu, H.; Zhang, C.; Cheng, M.; Li, H.; Li, W.; Niu, Y. Lignite-Based N-Doped Porous Carbon as an Efficient Adsorbent for Phenol Adsorption. Processes 2022, 10, 1746. https://0-doi-org.brum.beds.ac.uk/10.3390/pr10091746

AMA Style

Xue Y, Chen Y, Shi L, Wu H, Zhang C, Cheng M, Li H, Li W, Niu Y. Lignite-Based N-Doped Porous Carbon as an Efficient Adsorbent for Phenol Adsorption. Processes. 2022; 10(9):1746. https://0-doi-org.brum.beds.ac.uk/10.3390/pr10091746

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Xue, Yanfeng, Yanyan Chen, Linxia Shi, Haotian Wu, Chao Zhang, Minghuang Cheng, Hongbin Li, Wanjun Li, and Yulan Niu. 2022. "Lignite-Based N-Doped Porous Carbon as an Efficient Adsorbent for Phenol Adsorption" Processes 10, no. 9: 1746. https://0-doi-org.brum.beds.ac.uk/10.3390/pr10091746

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