Adsorption of anionic azo dyes using lignite coke by one-step short-time pyrolysis

Fuel 267 (2020) 117140

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Adsorption of anionic azo dyes using lignite coke by one-step short-time pyrolysis ⁎

T

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Qiongqiong Hea, , Guoqiang Wangb, Zishan Chenb, Zhenyong Miaoa,b, , Keji Wana, Shaomeng Huanga a b

National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Lignite coke One-step Pore structure Adsorption Azo dyes

Textile wastewater is among the top three major industrial wastewaters in China and efforts have been made by both the government and industry to develop a low-cost treatment process. In this study, lignite cokes (LCs) prepared by one-step low, medium, and high temperature pyrolysis were used for the removal of four anionic azo dyes. The adsorption efficiency of LCs increased when heated to a temperature higher than 700 °C for a period of time longer than 5 min; this improvement in the adsorption efficiency weakened for temperature higher than 700 °C. The heating of LCs for 5 min was commonly adopted for industrial adsorbents preparation, which considered both processing costs and the adsorption efficiency. The LC treatment at 700 °C for 5 min (LC700-5) could remove more than 95% of Congo red (CR), methyl orange (MO), and direct fast blue B2RL (DB) dye. Excellent dye removal efficiency was observed for LC-900-5, in which more than 99% of CR, MO, and DB and 89.8% of direct yellow brown D3G were removed. The oxygen-containing groups of LCs decomposed, and its micropores and macropores were substantially developed to form a honeycomb porous structure with a significantly larger surface area. The combination of hydrophobic adsorption, large surface area, and rich macropores and micropores contributed to the high adsorption performance of LCs obtained at high temperatures. Therefore, LCs obtained by one-step short-time medium or high-temperature pyrolysis in this study could be used as effective adsorbents for dyes, which demonstrated potentially significant environmental, social, and economic benefits.

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Corresponding authors at: National Engineering Research Center of Coal Preparation & Purification, China University of Mining and Technology, 1 Daxue Road, Xuzhou, Jiangsu 221008, China (Q. He). School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China (Z. Miao). E-mail addresses: [email protected] (Q. He), [email protected] (Z. Miao). https://doi.org/10.1016/j.fuel.2020.117140 Received 28 October 2019; Received in revised form 4 January 2020; Accepted 17 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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1. Introduction China produces, consumes, and exports the majority of dyes worldwide, and produces more than 2 billion tons of dye wastewater each year [1] owing to the rapid development of China's textile industry. The dye wastewater contains high concentrations of contaminants and exhibits high chroma, chemical stability, and resistance to oxidative degradation and photodegradation under natural conditions [2]. Wastewater has become the most severe problem faced by the dyeing industry. Owing to the environmental concerns caused by wastewater, 80% of the dyeing plants in Jiangsu and Zhejiang (accounting for over 75% of China’s dyeing plants) are facing production restrictions or even termination. As one of the most promising water treatment technologies, the use of adsorption has been widely favored in the industry. Although activated carbons have mainly been used for pollutant removal from wastewater, the associated cost is high, and numerous studies have attempted to find alternative adsorbents with lower cost [3–6]. The lignite resources in China are more than 130 billion tons, which is approximately 13% of the total coal reserve [7,8]. The price of lignite is relatively low (20–30 USD per ton) while its potential as an adsorbent [9,10] to remove heavy metal ions and organic matter is promising [11–13]. In addition, after adsorption, the lignite with organic pollutants can be directly combusted in boilers to eliminate the requirement for sludge treatment and resolve secondary pollution problems [14,15], which enhances its potential application as a low-cost adsorbent in the industry. The adsorption of cationic methylene blue dye by lignite was studied by Qi et al. [16] and Gurses et al. [17]. They found that the adsorption capacity of lignite was higher than that of a coconut-shellbased activated carbon, but lower than that of a coal-based activated carbon. Zheng et al. [18] found that the activated coke adsorption of lignite exhibited superior removal efficiency for total phenols, total organic carbons, and total nitrogen compared with that of powdered activated carbons. Nazari et al. [19] used base-washed brown coal for the removal of ammonium, and found that the adsorption was mainly governed by the reaction process rather than physical diffusion. In addition, coal fly ash and gasification fine slag as precursor for adsorbents also made progresses [5,20–22]. In this study, one-step low, medium, and high-temperature pyrolysis was used for the preparation of lignite cokes (LCs). Several important aspects of using LCs include characterization of the surface and porous properties of the LCs, adsorption mechanisms of dyes by LCs, and the relationship between the adsorbent characteristics and adsorption performance. As demonstrated in our study, the simple and quick preparation process of LCs provides potential industrial applications as low-cost adsorbents.

Fig. 1. Schematic diagram of the fixed bed heat treatment system.

nitrogen atmosphere. The temperatures were set at 300, 600, 700, 800, and 900 °C with times of 2, 5, and 8 min, which were denoted by LCtemperature-time (e.g., LC-300-8 represented LCs that were obtained at 300 °C for 8 min). The raw lignite and LC-300-8, LC-600-8, andLC-9008, were selected for the surface and pore distribution analysis. Ultimate and proximate analyses were conducted using a Vario MACRO cube from Elementar Co., Ltd. and a 5E-MAG6700 from Changsha Kaiyuan Instrument Co., Ltd., respectively. 2.3. Adsorption experiments

2. Experimental methods

In this study, 50 mL of dye wastewater was placed LC in 250 mL conical flasks with different LCs, and the containers were sealed with Parafilm. The initial concentration of dyes was 100 mg/L for CR, DB, and YB, and 20 mg/L for MO due to its poor solubility. The conical flasks were then placed in a water bath thermostat shaker to oscillate. The temperature of the constant temperature oscillating box was set to 25 °C with an oscillation speed of 130 rpm. After the required time, the conical flasks were taken out, and the solutions in the conical flasks were transferred to a centrifuge tube. Solid-liquid separation was achieved after 10 min with a centrifugation speed of 8000 rpm. The supernatant was tested to determine the absorbance of the solution using a spectrophotometer (Spectro UV-2550, Shimadzu, Japan) with a scanning wavelength of 340–1100 nm. The wavelengths of CR, MO, YB, and DB were 97, 462, 374, and 585 nm, respectively. The adsorption tests were repeated at least twice, and the absorbance of each group was measured three times in the experiment. The average absorbance was considered as the final results and the range of error was controlled within ± 3%. The associated R2 values of the standard operating curves corresponding to the absorbance of the four azo dye concentrations were all above 0.999. The adsorption rate γ was used to describe the adsorption of azo dyes, which can be defined as follows:

2.1. Materials

γ=

The run-of-mine Mengdong lignite originated from Inner Mongolia, China. Four azo anionic dyes from Nanjing Chemical Reagent Co., Ltd. were used in this study, which included Congo red (CR; a disazo dye), methyl orange (MO; a monoazo dye), direct yellow brown D3G (YB; a polyazo dye), and direct fast blue B2RL (DB; a polyazo dye). The molecular structures of these dyes are shown in Fig. S1. Concentrated sulfuric acid and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents used in this study were of analytical grade.

where C0 is the initial concentration of the dyes (mg/L) and Ct is the dye concentration at adsorption time t (mg/L).

C0 − Ct × 100% C0

(1)

2.4. Surface and porous properties of lignite cokes 2.4.1. Surface morphology of lignite cokes using scanning electron microscope analysis The surface morphology of LCs was measured by a scanning electron microscope (SEM) using the model SIGMA manufactured by Carl Zeiss Jena, Germany. LCs of −200 mesh were dried in a vacuum oven at 60 °C for 24 h. An accelerating voltage of 15 kV was selected, and the surface morphology of the particles was obtained using SEM.

2.2. Preparation of lignite cokes Lignites with a particle size of −0.5 mm were placed in a self-developed fixed-bed heat treatment apparatus, as shown in Fig. 1, in which the entire lignite heat treatment process was conducted in in a

2.4.2. Porous properties of lignite cokes by N2 adsorption The specific surface area (SA) and pore structure of LCs were 2

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for nitrogen adsorption are shown in Table 2. The treatment temperature significantly influenced the SA, total pore volume, and average pore size of lignite, which was consistent with the results of previous studies [24]. The SA of LC-300-8 was reduced to 11.20 m2/g compared with 17.60 m2/g for raw lignite. When the treatment temperature was raised to 600 °C, a large amount of volatile matter in the coal was released, as shown in Table 1. SA and the total pore volume of LC increased to 30.36 m2/g and 0.0713 cm3/g, respectively. When heated to 900 °C, most of the volatile matter in the lignites was released, SA of LCs of 900 °C 8 min increased to 117.74 m2/g, and the total pore volume rose to 0.1337 cm3/g. The coal-based adsorbents have a wide range of SA in the literature [25–27] on several orders of magnitude, but the coal-based adsorbents with high SA were usually obtained by several steps, including leaching by strong acid, pyrolysis, activation, catalysis (metal loading), and so on. This study aimed to obtain adsorbents for industry, so one-step short time pyrolysis processes were explored. The pore size distribution of raw lignite and LCs was obtained according to the Barrett, Joyner, and Halenda (mesopore) and HorvathKawazoe (micropore) models, as shown in Fig. 3a and Fig. 3b, respectively. rp is the pore radius, and w is the pore diameter (w = 2rp). From Fig. 3a, a clear increase in the pore volume of the micropores (w < 2 nm) of LC-600-8 and LC-900-8 could be observed, especially for LC-900-8 when w was in the range of 0.5–1.0 nm. As shown in Fig. 3b, the increase in the pore volume of LC-600-8 and LC-900-8 occurred mainly when rp < 3 nm; a small increase in the mesopore volume could also be found for rp > 9 nm. The difference between the mesopore volume of raw lignite and LCs was not as large as that for micropores. Therefore, the significant increase in the SA [28] and the total pore volume of LC-900-8 was due to the sharp increase in micropores (w in the range of 0.5–1.0 nm). Correspondingly, the average pore size of LC-900-8 decreased to 4.54 nm. The mean pore diameter decreased with increasing treatment temperature because the increase of micropore volume (Fig. 3) with the release of volatile matter. Combined with the SEM results, although a large amount of volatile matter was released in the 600 °C pyrolysis process, the increases in SA and total pore volume were not as apparent. Moreover, in the 8 min 900 °C pyrolysis process, almost all the moisture and volatile matter were released in a short period; there were substantial increases in both micropores (from N2 adsorption results in Fig. 3) and macropores (from SEM results in Fig. 2) for LC-900-8, which formed a honeycomb porous structure.

Table 1 Proximate and ultimate analysis of raw lignite and LCs.

Ad Vdaf FCdaf Cdaf Hdaf Ndaf Sdaf ODiff O/C

Raw lignite

LC-300-8

LC-600-8

LC-900-8

32.11 47.12 52.88 71.51 5.67 0.93 0.78 21.11 0.22

34.51 42.28 57.72 72.99 5.24 0.97 0.66 20.14 0.21

37.22 14.58 85.42 84.55 3.21 1.23 0.68 10.33 0.09

42.75 3.62 96.38 91.35 0.39 1.07 0.64 6.55 0.05

d: ash content on dry basis; daf: on dry and ash-free basis; Diff: By difference

analyzed by a BEL-MAX nitrogen adsorption instrument (BEL Company, Japan). The LCs were treated in the vacuum oven at 80 °C for 24 h before degassing for 12 h under the same temperature in the BEL-MAX instrument. Then, N2 was used as the gas adsorption medium to complete the adsorption and desorption in liquid nitrogen at −196 °C. 2.4.3. Surface properties of lignite cokes by Fourier-transform infrared spectroscopy analysis A VERTEX 80v from Bruker, Germany was used for Fourier-transform infrared spectroscopy (FT-IR) analysis in this study using the KBr tablet method. The ratio of LCs to KBr was 1:100 with the measured spectra in the range of 400–4000 cm−1; the resolution was 4 cm−1 and the spectra of each sample were scanned 16 times cumulatively. 3. Results and discussion 3.1. Proximate and ultimate analyses of raw lignite and lignite cokes The results of the proximate and ultimate analyses of raw lignite and LCs are shown in Table 1. With the increase in pyrolysis temperature, the volatile matter on a dry and ash-free basis (Vdaf) decreased from 47.12% (raw lignite) to 3.62% (LC-900-8) and the ash content increased as the volatile matter was progressively released. The oxygen content decreased from 21.11% (raw lignite) to 6.55% (LC-900-8) while the carbon content was increased to 91.35% (LC-900-8). All the results implied that LCs are prone to high aromaticity and strong hydrophobicity. 3.2. Microscopic observations of raw lignite and lignite cokes by scanning electron microscope analysis

3.4. Surface properties determined by Fourier-transform infrared spectroscopy analysis

The SEM images of raw lignite and LC-300-8, LC-600-8, and LC-9008 are shown in Fig. 2. The surface of the raw lignite was relatively smooth with a lamellar structure and small pore structure. The surface morphology of the lignite produced a series of significant changes with the increase in treatment temperature. The surface of LC-300-8 became rough as a result of the release of water and a small portion of volatile matter [23]. As shown in Table 1, the Vdaf decreased from 47.12% (raw lignite) to 42.28% (LC-300-8). The surface of LC-600-8 was much rougher and pores could be found owing to the increased volatile release, and the lignite structure was decomposed during pyrolysis at 600 °C [23] with a Vdaf of 14.58%. When heated to 900 °C, almost all the volatile matter was released and Vdaf was reduced to 3.62%. Additionally, the macropore structure of LC-900-8 was found to develop substantially, and exhibited a honeycomb structure. Therefore, the treatment temperature demonstrated significant impacts on the surface and pore structure of the LCs.

The oxygen-containing functional groups (OFGs) of LCs were obtained from FT-IR, as shown in Fig. 4. The results of the ultimate analysis of LCs, as shown in Table 1, showed that the oxygen content in the LCs decreased as the treatment temperature increased. The O–H vibration peak (3700–3200 cm−1), aliphatic hydrocarbon vibration peak (3000–2800 cm−1), C]C double bond stretching vibration peak (1610–1580 cm−1), C]O expansion and contraction vibration peak (1697 cm−1), and C–O stretching vibration peak (1100–1000 cm−1) could be found in the FT-IR spectrum [29,30]. Small differences between the FT-IR results of raw lignite and LC300-8 were observed as the oxygen content decreased by only 1%, which meant that only a few surface functional groups of lignite were destroyed at 300 °C. When the heat treatment temperature was increased to 600 °C and 900 °C, peaks of O–H (3700–3200 cm−1), C–H (3000–2800 cm−1), and C]O (1697 cm−1) were largely weakened owing to the nearly complete decomposition and release of aliphatic hydrocarbon structures at temperatures higher than 600 °C. The C]C bond (1610–1580 cm−1) could still be found in the spectrum of LC-6008, although it was much smaller compared with that of LC-300-8 and it disappeared completely for LC-900-8. This was because the C]C

3.3. Pore distribution by N2 adsorption The changes in SA, total pore volume, and average pore size of LCs 3

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Fig. 2. SEM analysis of (a) raw lignite, (b) LC of 300 °C 8 min, (c) LC of 600 °C 8 min and (d) LC of 900 °C 8 min.

900 °C), adjacent C]O and C–O species were severely destroyed. In this case, the C]C groups became symmetrical and IR-inactive and then disappeared [31]. Only C–O (1100–1000 cm−1) was found in LC-900-8, which corresponded to an oxygen content of 6.55%, as listed in Table 1.

Table 2 Pore structure analysis of raw lignite and lignite cokes. Samples

Surface area/ m2 g−1

Total pore volume/cm3 g−1

Mean pore diameter/ nm

Raw LC-300-8 LC-600-8 LC-900-8

17.60 11.20 30.36 117.74

0.061 0.045 0.071 0.134

14.04 15.94 9.39 4.54

3.5. Azo dye adsorption experiment using lignite cokes The removal of four dyes by raw lignite and LCs at 300, 600, 700, 800, and 900 °C is shown in Fig. 5. As shown in Fig. 5a, the LCs exhibited good adsorption properties for CR, and the removal of CR with all tested LCs was greater than 94%. For LC-900-8, the removal of CR was 99.99%, thereby indicating that CR in the solution was completely absorbed. LC-600-8 could already provide a comparable removal rate of

groups in the raw lignite samples (or samples pyrolyzed at 300 °C) were IR-active as their symmetry was distorted by adjacent C]O and C–O species. When samples were pyrolyzed at high temperature (600 °C and 4

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Fig. 3. Micropore and mesopore distributions of raw lignite and lignite cokes (rp is the pore radius and w = 2rp), obtained according to (a) HK, and (b) BJH.

Both hydrophilic and hydrophobic groups were found in the dyes and LCs (Fig. S1). Many molecular models of lignite and char were proposed in previous studies [32–34]. In this study, the Wender model of lignite [35] was used to explain the adsorption interactions between dyes and LCs, which included hydrophobic interactions, π⋅⋅⋅π bonding, hydrogen bonding, and covalent and electrostatic interactions. The molecular structures of dyes and the Wender model are shown in Fig. S1, and their interactions are described in Table S1. OFGs such as hydroxyl groups of LCs and the amino group of dyes were the main groups involved in the formation of hydrogen bonds between dyes and LCs. π⋅⋅⋅π and π-hydrogen bonding generally occurred between the π system of LCs and dye molecules with benzene rings containing C]C or naphthyl groups. The zeta potentials of the dyes [36] and LCs [37,38] were negative in environments with a pH higher than 3; thus, the dyes and LC molecules were electronegative overall, thereby indicating that electrostatic repulsions were present between the dyes and LCs. The carbon chain and aromatic structures were important for LCs [23]; thus, we inferred that hydrophobic forces existed between them. Hu et al. [39] also confirmed the importance of hydrophobic attraction for the adsorption of dyes on resins. As observed in this study, a large number of hydrophilic groups, such as carboxyl and hydroxyl groups, were originally distributed on the surface of raw lignite, which was advantageous for the adsorption of hydrophilic groups of dyes. In addition, most of the surface OFGs of LC900-8 were decomposed, thereby resulting in an increase in the hydrophobicity of lignites, which was conducive to the adsorption of hydrophobic groups of azo dye molecules. The molecular structures of the azo dyes were amphiphilic, and the heterogeneity of LCs made it difficult to quantify the LC-dye interaction during the adsorption process. However, the combination of hydrophobic adsorption, a large SA, and rich macropores and micropores contributed to the adequate adsorption performance of LC-900-8. Therefore, LCs obtained by one-step short-time medium or high-temperature pyrolysis could be used as effective adsorbents for dye adsorption.

Fig. 4. Fourier-transform infrared spectroscopy of raw lignite and lignite cokes.

CR at 99.23%. According to Fig. 5b, the concentration of MO was only 20 mg/L owing to its poor solubility. The adsorption efficiency of LCs at 300 and 600 °C was not satisfied. And the effect of heating time on the adsorption of MO was more significant than that for CR, and the removal rate of LCs pyrolyzed for 2 min at all the temperatures were not high. For example, LC-900-8 could remove 99.95% of MO, while the removal rate was only 72.81% for LC-900-2. As shown in Fig. 5c, the removal rate of YB by LCs was lower compared with that of CR and MO. The removal rate of YB by LCs when heated to 300, 600, and 700 °C was lower than 65%. For LC-800-5 and LC-800-8, the removal rate of YB increased to more than 76%, while this removal rate was 89.8% and 91.8% for LC-900-5 and LC-900-8, respectively. According to Fig. 5d, temperature produced the most significant improvement in the adsorption of DB when increasing the removal rate from 24.03% (LC-300-8) to 99.84% (LC-900-8), thereby indicating that the adsorption of DB may have been more sensitive to SA compared with other dyes. When heated at a higher temperature for a longer period, improved adsorption efficiency could be observed for LCs, although the improvement became less significant at temperatures higher than 700 °C. The pyrolysis time was also found to be important for adsorption performance (LC-300), and LCs heated for 5 min were much better than LCs heated for 2 min and only slightly inferior to LCs heated for 8 min. Generally, 5 min of pyrolysis was considered the optimal choice for the preparation of industrial adsorbents. LC-700-5 could remove more than 95% of CR, MO, and DB. For LC-900-5, more than 99% of CR, MO, and DB and 89.8% of YB could be removed.

4. Conclusions LCs obtained by one-step low, medium, and high-temperature pyrolysis were used for the removal of four anionic azo dyes. In our experiments, it was found that the OFGs and pore structure of LC-300-8 were slightly damaged; the OFGs of LC-300-8 were decomposed and the pore structure of LC-600-8 was developed to a certain degree; and the OFGs of LC-900-8 were decomposed with only a small amount of C-O. The pore structure of LC-900-8 was substantially developed with a sharp increase in the SA to 117.740 m2/g, which could be used as rich adsorption sites. When heated at a higher temperature for a longer period, LCs were found to have high adsorption efficiency, but the 5

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Fig. 5. The removal rate of (a) Congo red (CR), (b) methyl orange (MO), (c) direct yellow brown D3G (YB), and (d) direct fast blue B2RL (DB) by lignite cokes (LCs) obtained at 300, 600, 700, 800, and 900 °C for 2, 5, and 8 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Appendix A. Supplementary data

improvement became less significant as the temperature increased to higher than 700 °C. Considering both the processing costs and adsorption efficiency, heating LCs for 5 min was the optimal choice for industrial absorbent preparation. LC-700-5 could remove more than 95% of CR, MO, and DB. LC-900-5 maintained excellent removal efficiency for all dyes, with removal efficiencies of greater than 99% for CR, MO, and DB and 89.8% for YB. In general, the combination of hydrophobic adsorption, large SA, and rich macropores and micropores contributed to the adequate adsorption performance of LCs at high temperature. LCs obtained by one-step short-time medium or hightemperature pyrolysis could be used as effective absorbents for dye adsorption, and can be potentially used for industrial applications.

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CRediT authorship contribution statement Qiongqiong He: Conceptualization, Funding acquisition, Project administration, Writing - original draft. Guoqiang Wang: Investigation, Writing - review & editing. Zishan Chen: Data curation, Formal analysis. Zhenyong Miao: Resources, Supervision. Keji Wan: Writing - review & editing. Shaomeng Huang: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Natural Science Foundation of Jiangsu Province of China [grant number BK20170284], National Natural Science Foundation of China [grant number 51704291], and Qing Lan Project. 6

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