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Adsorption behavior of multicomponent volatile organic compounds on a citric acid residue waste-based activated carbon: Experiment and molecular simulation
Journal of Hazardous Materials 392 (2020) 122323
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Adsorption behavior of multicomponent volatile organic compounds on a citric acid residue waste-based activated carbon: Experiment and molecular simulation
T
Xiaolong Yaoa,b, Yao Liua,b, Tong Lia,b, Tingting Zhangc, Hailong Lid, Wei Wange, Xianbao Shena,b, Feng Qiana,b, Zhiliang Yaoa,b,* a
Department of Environmental Science and Engineering, Beijing Technology and Business University, Beijing 100048, China Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry, Beijing Technology and Business University, Beijing 100048, China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China d School of Energy Science and Engineering, Central South University, Changsha, 410083, China e State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xi’ning, Qinghai Province 810016, China b c
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: Danmeng Shuai
A considerable amount of volatile organic compounds (VOCs) is emitted, and a vast amount of citric acid residue (CAR) waste is simultaneously produced during citric acid production. Thus, a suitable method realizing the clean production of citric acid must be developed. This study investigated the adsorption of the multicomponent VOCs in a homemade CAR waste-based activated carbon (CAR-AC). A fixed-bed experimental setup was used to explore the adsorption and desorption of single- and multi-component VOCs. Surface adsorption and diffusion molecular models with different defects were built to study the underlying adsorption and diffusion mechanisms of multicomponent VOCs on CAR-AC. The adsorption amount of ethyl acetate in CAR-AC from multicomponent VOCs was 3.04 and 5.91 times higher than those of acetone and acetaldehyde, respectively, and the interaction energy between ethyl acetate and C surfaces was low at −13.41 kcal/mol. During desorption, the most weakly adsorbed acetaldehyde desorbed from the surface of CAR-AC first, followed by acetone and ethyl acetate. The regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate reached 88.77, 85.55, and 91.46 %, respectively, after four adsorption/desorption cycles. We aimed to provide a new strategy to realize the recycle use of CAR and the clean production of citric acid.
Keywords: Adsorption Citric acid residue waste-based activated carbon Multicomponent volatile organic compounds Molecular simulation
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Corresponding author at: Department of Environmental Science and Engineering, Beijing Technology and Business University, Beijing 100048, China. E-mail address: [email protected] (Z. Yao).
https://doi.org/10.1016/j.jhazmat.2020.122323 Received 14 December 2019; Received in revised form 12 February 2020; Accepted 14 February 2020 Available online 15 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 392 (2020) 122323
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1. Introduction
common VOCs in exhaust gas emitted from citric acid production, was adsorbed in the CAR waste-based activated carbon. Experiments and molecular dynamic (MD) simulation methods were used to study the adsorption behaviors of these VOCs during adsorption in the activated carbon. This study aimed to understand the competitive adsorption mechanisms of multicomponent VOCs in this activated carbon comprehensively and provide a new strategy to recycle CAR and realize the clean production of citric acid.
Volatile organic compounds (VOCs) are typical air pollutants (Shamskar et al., 2019; Yang et al., 2019). They are not only the main precursors of secondary aerosol (SOA) and ozone in the atmosphere but also threaten the environment and exert direct carcinogenic, teratogenic, and mutagenic effects on humans (Huang et al., 2018; Zhang et al., 2019). Until 2018, total VOC emission reached 25–30 million tons in China. Chemicals for synthesis, coating and printing industries, and fermentation industries are the most common sources of VOC emission (Cao et al., 2019; Diao et al., 2019) and account for about half of the total VOC emission (Yang et al., 2019). Citric acid production is a typical fermentation industry, and over 1.2 million tons citric acid is produced in China each year, which accounts for over 80 % the world’s total citric acid (Ao et al., 2013). Based on our preliminary test results, we found that a considerable amount of VOCs was emitted in fermentation and citric acid drying, including aldehydes, ketones, alcohols and esters. Moreover, VOCs emitted from these exhaust gas streams are mostly multicomponent gas, increasing the difficulty to purify them from waste gas (Lashaki et al., 2012; Wang et al., 2012). Therefore, controlling the release of VOCs from citric acid production is an urgent problem. Adsorption is widely used in purifying VOCs from citric acid production because it is highly effective and economical (Sui et al., 2019). Activated carbons are commonly used as absorbents for VOC removal because of their large micropore volume, high specific surface area, and excellent adsorption capacity (Ma et al., 2018; Zhou et al., 2019). Activated carbons are generally prepared using nonrenewable materials, including coal, petroleum residues, wood, and lignite (Mondal and Garg, 2017; Liu et al., 2019). However, these precursors are expensive (Brito et al., 2017). Thus, they have been replaced by several low-cost and renewable raw materials, such as agricultural residues, food wastes, and industrial by-products, in recent years (Yahya et al., 2015). Preparing activated carbons from industrial by-products can reduce the production cost, allow the recycling, and increase the additional value of products (Tan et al., 2017). Citric acid residue (CAR), another typical by-product in citric acid production (Piotrowska et al., 2015; Juliastuti et al., 2017), has been made to an activated carbon in our previous study (Liu et al., 2019). Therefore, VOCs from citric acid production may be removed using the prepared activated carbon to recycle the use of CAR and realize the clean production of citric acid. Adsorption of VOC gas mixture on activated carbons is affected by temperature and pressure and also competing adsorption occurs with the change in gas composition (Cabrera-Codony et al., 2018; Laskar et al., 2019). The complexity of the activated carbon adsorption process increases with the number of components in the mixture exhaust gas (Tefera et al., 2014). During competitive adsorption, VOCs with a strong affinity to the activated carbon are easily adsorbed on the surface of the activated carbon compared with the component with a weak affinity (Lillo-Rodenas et al., 2006). The experimental study of multicomponent adsorption is usually macroscopic, thus, it is limited to obtaining a fundamental understanding of multicomponent VOC adsorption mechanisms in activated carbons. In recent years, molecular simulation has gradually become a common method to study the interactions between VOCs and activated carbon at the molecular level (Do and Do, 2006; Klomkliang et al., 2012). The orientation of several types of VOCs in slit pores is a function of pore width (Diao et al., 2019). However, most studies focused on the adsorption behavior of single-component VOCs on activated carbon (Coasne et al., 2011). Limited works studied the adsorption behavior of multicomponent VOCs on a CAR waste-based activated carbon, and the interactions among the VOC molecules during adsorption are unclear. Thus, this investigation would help elucidate the adsorption mechanisms of multicomponent VOCs in a CAR waste-based activated carbon. In the present study, a mixture gas of acetaldehyde (CH3CHO), acetone (CH3COCH3), and ethyl acetate (CH₃CH₂OOCCH₃), which are
2. Materials and methods 2.1. Materials CAR waste was collected from a citric acid plant in Shandong Province, China. High-purity standard gases of acetaldehyde, acetone, and ethyl acetate used in the experiments were obtained from Zhaoge Gas Technology Co. Ltd. (China). Phosphoric acid (85 %) was obtained from Sinopharm Group (China). The chemical solutions were prepared in ultra-pure water produced by a Milli-Q system (Millipore, USA). A CAR waste-based activated carbon (CAR-AC) was prepared as follows. CAR waste was first washed with water to remove impurities and then dried at 383.15 K for 12 h. Next, 5.0 g of CAR was immersed in 40 % phosphoric acid solution with an impregnation ratio of 2.0 (i.e., the mass ration of CAR to phosphoric acid was 2.0) at room temperature for 12 h. The rest of preparation steps were the same as the method reported in our previous study (Liu et al., 2019). A scanning electron microscope (SEM) (Merlin compact, Zeiss, Germany) was used to study the surface morphology of the citric acid residue and activated carbons. The N2 adsorption–desorption isotherms and the pore size distribution of CAR-AC were characterized by an automated gas sorption analyzer (Quantachrome, Nova 2000e, USA). Fourier transform infrared (FTIR) spectra of CAR-AC were recorded using FTIR spectroscopy (Avater 370, Nicolet, USA) in the range of 400–4000 cm−1. 2.2. Dynamic adsorption measurements A fixed-bed experimental setup was used to explore the single component and multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption in CAR-AC, and the experimental apparatus is shown in Fig. S4. The prepared high-purity standard gases of single component and multicomponent VOCs were placed in gas cylinders. When the experiment started, the gas channel was opened by a valve, and the flow rate of the stream was adjusted by the mass flow controllers. The gas stream was preheated before entering the adsorption column. Then, 0.2 g of outgassed adsorbent was loaded into an adsorption column (φ 6 mm × 150 mm). A filter plate was located at the bottom of the column as a support of the adsorbent bed. During the adsorption experiments, the inlet concentration (Cin) of the VOCs was maintained constant, and the VOC vapor flowed through the fixed bed from the bottom of the adsorption column. For single-component VOC adsorption, bed saturation was achieved when the VOC vapor concentrations in the outlet of the adsorption column matched the inlet concentrations. For the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption, bed saturation was achieved when the outlet concentration (Cout) for all of them matched the inlet concentrations. The adsorption temperature was controlled by a temperature controller. After adsorption, the tail gas was purified before it escaped into the atmosphere. The inlet and outlet concentrations of each compound in the single component and multicomponent streams were measured by a gas chromatography instrument (Techcomp GC7900, China) equipped with a TM-624 quartz capillary column (length × inner diameter × thickness, 60 m × 0.32 mm ×2.0 μm) and a flame-ionization detector. Operating conditions were as follows: oven temperature, 353.15 K; detector temperature, 553.15 K; carrier gas, N2 at 20 mL/min. Single2
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component adsorption tests were carried out at the feed flow rate of 100 mL/min with the inlet concentration of 500 mg/m3 at 298.15–318.15 K. Multicomponent adsorption experiments were conducted at the same flow rate with the inlet concentration of each component at 125, 250, 500, and 750 mg/m3 at 298.15–318.15 K. The adsorption capacity was determined by the difference between the inlet and outlet VOC mass until the adsorption bed reached the saturated state, and the adsorption amount (qs) was calculated as follows (Eq. (1)):
qs =
Q m × 10
ts 6
0
(Cin
Cout )dt
temperature was 393.15 K, and the regeneration time was 6 h). Then, the adsorbent was cooled to room temperature and reused in the next adsorption cycle. The adsorption/desorption experiments were carried out for four cycles. 2.4. Simulation method During the MD simulation, the interactions between C surface and molecules of acetaldehyde, acetone and ethyl acetate were characterized. According to the pore size distribution of CAR-AC calculated by the N2 adsorption–desorption isotherms and the molecular diameters for acetaldehyde (3.2 Å), acetone (4.8 Å), and ethyl acetate (6.0 Å), three types of C(001) surface, namely, preface surface (Type I), surface with a narrow circle defect of 5 Å radius (Type II), and surface with a wide circle defect of 12 Å radius (Type III), were built with a 20 Å vacuum layer to simulate different porosities (Fig. S5). Acetaldehyde, acetone, and ethyl acetate were added at the top of optimized surface with close contact. Diffusivity was simulated by the layer structure with Type II and Type III defects, and the acetaldehyde, acetone, and ethyl acetate layers with equal mass were covered on the surface, as shown in Fig. S6. The structure and charge of MD simulation were optimized by COMPASS force field to ensure the accuracy of the calculation results (Sun, 1998; Xu and Wang, 2017). The Van der Waals interaction was calculated by the atom-based method with a cutoff radius of 18.5 Å to avoid the errors of relatively short-range non-bond interactions. The electrostatic summation was calculated using the Ewald method to avoid the errors of long-range non-bond energy in periodic systems. The initial models with 3D periodic boundary conditions were optimized with the smart minimizing method, and the control precision of the energy gradient, maximum force, and maximum displacement reached less than 2.0 × 10−5 kcal/mol, 0.001 kcal/(mol Å), and 1.0 × 10−5 Å, respectively. The optimized model structure was combined with a series of
(1)
where Q is the flow rate of gas stream, mL/min; m is the mass of the adsorbent, g; Cin and Cout are the inlet and outlet concentrations of each component, respectively, mg/m3; t is the adsorption time, min; ts is the time of break-through, min; and qs is the adsorption amount, mg/g. The adsorption kinetics of simple VOCs upon CAR-AC was determined by the Yoon-Nelson (Y-N) model (Yoon and Nelson, 1984; Sui et al., 2019), and the values were calculated as follows (Eq. (2)):
t = t50% +
1 Ci (t ) ln kYN Ci 0 Ci (t )
(2)
where t (min) is adsorption time, t50 % (min) is the time required for 50 % adsorbate breakthrough, kYN (/min) is the proportionality constant of the Y–N model, and Ci0 (mg/m3) and Ci(t) (mg/m3) are the inlet and outlet concentrations of component i at time t, respectively. 2.3. Regeneration experiments After adsorption of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate with the inlet concentrations of each component at 750 mg/m3 at 298.15 K, the activated carbon was regenerated by hot nitrogen gas at an optimized regeneration condition (i.e. flow rate of nitrogen gas was 100 mL/min, regeneration
Fig. 1. Breakthrough curves of acetaldehyde, acetone, and ethyl acetate at 298.15 (a), 308.15 (b), and 318.15 K (c); diffusion coefficients of acetaldehyde, acetone, and ethyl acetate calculated by the MD model (d). 3
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The diffusivity Dα is given by using Eq. (4) (Yeganegi and Gholampour, 2016):
Table 1 Fitting parameters of the Y-N model and the predicted penetration time at different temperatures. Single component VOCs
Temperature(K)
1/k (min)
k (×10−1 /min)
t50
acetaldehyde
298.15 308.15 318.15 298.15 308.15 318.15 298.15 308.15 318.15
8.08 7.69 7.31 5.77 4.11 4.12 15.36 15.75 11.78
1.238 1.300 1.368 1.733 2.433 2.427 0.651 0.635 0.849
47.89 45.16 42.85 150.4 127.64 107.56 236.95 188.88 171.91
acetone ethyl acetate
%
(min)
N
D =
R2
(Emolecule + Esurface)
ri (0)]2 }
(4)
where Nα is the number of diffusive atoms in the system and ri(t) and ri (0) represent the coordinates of the i-th particle at time 0 and time t, respectively.
0.970 0.995 0.990 0.995 0.995 0.999 0.989 0.996 0.888
3. Results and discussion 3.1. Characterization of CAR-AC The SEM images of CAR-AC are shown in Fig. S1. The surfaces of CAR-AC are covered with holes of different shapes and sizes, and with well homogeneity. The N2 adsorption–desorption isotherms and the pore size distribution of CAR-AC are shown in Fig. S2. The N2 adsorption–desorption isotherms of CAR-AC present Type IV in accordance with IUPAC. The micropores and mesopores in CAR-AC are mainly around 1.0, 2.4, and 3.2 nm. The porous structural parameters of CAR-AC are shown in Table S1. The BET surface area and total pore volume of CAR-ACs reach 786 m2/g and 0.71 cm3/g, respectively. Meanwhile, the microporous and mesoporous volume of CAR-ACs is 0.33 and 0.35 cm3/g, respectively, which respectively accounts for 46.47 and 49.30 % of the total pore volume. The FTIR spectra of CARAC are shown in Fig. S3. Almost no oxygen-containing function groups are observed on the CAR-AC surface. Evidently, lots of oxygen-containing function groups were decomposed from the surfaces of synthesized activated carbon samples.
environmental conditions, such as constant molecule numbers, volume, and temperature (NVT) or constant molecule numbers, volume, and energy (NVE) to execute a dynamic calculation (Wang et al., 2019). Moreover, the operating temperature was set at 298.15 K and controlled using the Nose thermostat method to match the real experiment procedure (Nosé, 1984; Hoover, 1985). The adsorption behavior was studied at the NVT ensemble with 1000 ps. Diffusion behaviors were explored and analyzed at the NVT and NVE ensembles with 1000 ps each with different temperatures (Kurisaki and Takahashi, 2011; Bazooyar et al., 2012). The interaction energy Einteraction between the activated carbon surface and the VOC molecules was calculated by using Eq. (3) (Chun et al., 2015; Hu et al., 2018):
Einteraction = Etotal
1 d lim {[ri (t ) 6N t dt i = 1
(3)
3.2. Adsorption single component of acetaldehyde, acetone, and ethyl acetate in CAR-AC
where Etotal is the total energy of the system including that of molecules adopted on the surface, Emolecule is the energy of individual molecules, and Esurface is the energy on the C (001) surface.
The breakthrough curves of acetaldehyde, acetone, and ethyl
Fig. 2. Adsorption experiments of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate at concentrations of 125 (a), 250 (b), 500 (c), and 750 mg/L (d). 4
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acetate with the inlet concentration of 500 mg/m3 at 298.15, 308.15, and 318.15 K are shown in Fig. 1. At 298.15 K (Fig. 1a), the penetration times (i.e., the time at Cout/Cin = 0.05) (Wołowiec et al., 2017) for acetaldehyde, acetone, and ethyl acetate are 28, 136 and 210 min, respectively. With increasing adsorption temperature, the penetration times for acetaldehyde, acetone, and ethyl acetate appear earlier (Fig. 1b and Fig. 1c), specifically 23, 104, and 163 min at 318.15 K, respectively. Moreover, the diffusion coefficients of acetaldehyde, acetone, and ethyl acetate calculated with the MD simulation method are shown in Fig. 1d. The order of diffusion coefficients is acetaldehyde > acetone > ethyl acetate, and the diffusion coefficients increase with the temperature. These results indicate that the diffusion of acetaldehyde in CAR-AC is the fastest, followed by that of acetone, and the slowest is that of ethyl acetate. The fitting parameters of the YN model at different adsorption temperatures are given in Table 1. Results show that the Y-N model provides a good agreement for the experimental breakthrough data of single-component adsorption. At the same adsorption temperature, the value of t50 % is followed as acetaldehyde < acetone < ethyl acetate. The finding also indicates that the diffusion of acetaldehyde in CAR-AC is the fastest among the three, and that of ethyl acetate is the slowest. Moreover, for the three VOCs, increasing the adsorption temperature can decrease time t50 %, suggesting that a high temperature increases the mass transfer driving force and accelerates the adsorption bed penetration (Lemus et al., 2012; Wang et al., 2015).
compounds (Stefan and Akgerman, 1998). However, the adsorption amount of each component in the multicomponent VOCs is much less than that in the simple system. At 298.15 K, the adsorption amounts for acetaldehyde, acetone, and ethyl acetate in the multicomponent VOCs only account for 59.76, 38.77, and 71.90 % of that in the simple system, respectively. Moreover, the adsorption amount of ethyl acetate in CARAC from multicomponent VOCs is 3.04 times greater than that of acetone and 5.91 times greater than that of acetaldehyde. These results indicate that the adsorption selectivity of CAR-AC for ethyl acetate is higher than that for acetone and acetaldehyde. The interactions between molecules of acetaldehyde, acetone, and ethyl acetate and C surface at different porosities were characterized. The adsorption molecule models are shown in Fig. S7, and the calculation results are shown in Fig. 3b. On the preface surfaces of CAR-AC (i.e., Type I), the Einteraction values for acetaldehyde, acetone, and ethyl acetate are −7.05, −9.57, and −9.39 kcal/mol, respectively. These results indicate that acetone and ethyl acetate can more easily adsorb on the preface surfaces of CAR-AC than acetaldehyde, which may be partly attributed to the difference in molecular diameter for acetaldehyde (3.2 Å), acetone (4.8 Å), and ethyl acetate (6.0 Å); in addition, the larger molecular diameter of acetone and ethyl acetate may have a stronger force between adsorbent molecules and surface carbon atoms (Lee et al., 2008; Mao et al., 2016). The Einteraction values for acetaldehyde, acetone, and ethyl acetate are different when adsorbed on the surface with a circle defect of 5 Å radius (i.e., Type 2). For acetaldehyde, Einteraction is −6.93 kcal/mol and is slightly higher than that adsorbed on the preface surfaces. However, the interaction energies of
3.3. Adsorption of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate in CAR-AC The adsorption breakthrough curves of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption are shown in Fig. 2. Compared with a single component, the ternary mixture shows evidently different breakthrough curves. The outlet concentration of acetaldehyde first exceeds the inlet concentration after a certain period, and then the acetone in the outlet stream exceeds its inlet concentration. Moreover, the penetration time of ethyl acetate becomes shorter, and the penetration rate of ethyl acetate in CAR-AC increases slightly slower than before. This result may be attributed to the fact that the diffusion of acetaldehyde in CAR-AC is the fastest, and it may first adsorb on the finite adsorption active sites. When the three VOCs reach the surface of the adsorbent, the most strongly adsorbed ethyl acetate is preferentially captured on the adsorbent by displacing the weakly adsorbed acetone, and the acetone is captured on the adsorbent by displacing the most weakly adsorbed acetaldehyde (Lillo-Rodenas et al., 2006; Sui et al., 2019). Moreover, the margin of excess concentration for acetaldehyde and acetone decreases with increasing adsorption temperature. These results are mainly attributed to the decrease in acetaldehyde and acetone adsorption amounts on the CAR-AC surface at a high temperature. Thus, the amounts of acetaldehyde and acetone were replaced by ethyl acetate decrease. The adsorption amounts of acetaldehyde, acetone, and ethyl acetate in CAR-AC can reflect the adsorption capacity and selectivity of CAR-AC for these three VOCs (Shi et al., 2018; Sui et al., 2019). The adsorption amounts of single compound and their multicomponent VOCs at the equilibrium concentration for each component 500 mg/m3 are shown in Fig. 3a. The adsorption amounts of single compounds of acetaldehyde, acetone, and ethyl acetate in CAR-AC at 298.15 K are 12.5, 37.5, and 61.5 mg/g, respectively. These values decrease to 9.75 mg/g for acetaldehyde, 27.5 mg/g for acetone, and 40.5 mg/g for ethyl acetate when the adsorption temperature is increased to 318.15 K. Rising temperature can accelerate the diffusion of VOCs in activated carbon and reduce the captive probability of adsorption active sites for VOCs (Wang et al., 2017). The total adsorption amount of the mixture of acetaldehyde, acetone, and ethyl acetate in CAR-AC is higher than those of single compounds, indicating that several adsorption active sites on the CAR-AC surface may co-adsorb two or three of these organic
Fig. 3. Adsorption amounts of single compounds and multicomponent mixture of acetaldehyde, acetone, and ethyl acetate in CAR-AC at the equilibrium concentration for each component 500 mg/m3 (a) and the interaction energies between molecules of acetaldehyde, acetone, and ethyl acetate and C surface on the three types of surfaces (b). 5
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acetone and ethyl acetate are stronger than those adsorbed on preface surfaces, especially for ethyl acetate, whose interaction energy value reaches −13.41 kcal/mol. These results indicate that ethyl acetate can most easily adsorb on the defect surface of 5 Å radius, followed by acetone and acetaldehyde. This result can be attributed to the pore structure of the activated carbon and the molecular diameter of VOCs. On the circle defect surface of 5 Å radius, the pore size is 3.13, 2.08, and 1.67 times wider than the diameter of acetaldehyde, acetone, and ethyl acetate, respectively. Therefore, wider pore size of activated carbon than the diameter of VOCs (about two times), leads in the stronger Van der Waals forces between adsorbent molecules and surface carbon atoms (Long et al., 2011; Yang et al., 2011). When the radius of the defect surface is increased to 12 Å, Einteraction for acetaldehyde slightly decreases to −6.65 kcal/mol. The results indicate that the interaction between molecules of acetaldehyde and C surface is the weakest among all three VOCs, and changing the pore size of CAR-AC has minimal effect on acetaldehyde adsorption. However, the Einteraction values for acetone and ethyl acetate increase substantially to −8.78 and −10.52 kcal/mol, respectively, indicating that both can more easily adsorb in the pore with a size of around 1.0 nm than in the pore with a size of around 2.4 nm, and these three kinds of VOCs gas are mainly adsorbed in the mciropores of CAR-AC (Veksha et al., 2009; Li et al., 2011). The pores with the size larger than 2.4 nm in CAR-AC did not affect the calculation results since their pore size are much higher than the molecular diameter of VOCs gas (Long et al., 2011; Yang et al., 2011). In addition, the adsorptive selectivity of CAR-AC for ethyl acetate is better than that for acetone because the Einteraction for acetone is higher than that for ethyl acetate. These results are consistent with the experimental studies.
the diffusion of these three kinds of VOCs in CAR-AC. The diffusion coefficient values of acetaldehyde, acetone, and ethyl acetate increase to 9.16 × 10−5, 3.71 × 10−5, and 2.46 × 10−5 cm2/s respectively when the adsorption temperature increases to 318.15 K. Thus, the acetaldehyde is first desorbed from CAR-AC, followed by acetone and ethyl acetate. 3.5. Adsorption mechanisms of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate on CAR-AC The multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption on CAR-AC is closely related to their diffusion in the pore of CAR-AC and the interactions between their molecules and the C surface. The adsorption processes can be divided into five stages to clearly understand the ternary adsorption behaviors in CAR-AC (Fig. 6). In Stage I, acetaldehyde, acetone, and ethyl acetate are delivered into the pores. The diffusion of acetaldehyde is the fastest and first adsorbed on the adsorption active sites. Acetone and ethyl acetate are subsequently adsorbed on the surfaces and in pores. The outlet concentrations are equal to zero because CAR-AC can adsorb almost all three VOCs. Compared with Stage I, acetaldehyde can first diffuse to the top of the adsorption column, and some pre-adsorbed acetaldehyde molecules can be replaced by acetone or ethyl acetate molecules. Hence, a certain concentration of acetaldehyde outside the pores is observed in Stage II. In Stage III, more pre-adsorbed acetaldehyde is replaced by acetone or ethyl acetate, and the concentration of acetaldehyde in the outlet stream first exceeds the inlet concentration. Acetone also diffuses to the top of the adsorption column and is partly detected at the outlet
3.4. Regeneration of CAR-AC The desorption curves of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate at 298.15 K are shown in Fig. 4a, and their regeneration efficiencies in four successive adsorption cycles are shown in Fig. 4b. Fig. 4a shows that when the desorption time is about 20 min, acetaldehyde is almost desorbed from CAR-AC, and the times for acetone and ethyl acetate are about 60 and 280 min, respectively. Acetaldehyde and acetone on CAR-AC desorb more easily than ethyl acetate, which can be attributed to their properties and the pore structure of CAR-AC. On the one hand, the diffusion of acetaldehyde and acetone in CAR-AC is faster than that of ethyl acetate. On the other hand, rich mesopores (mesoporous volume accounts for 49.30 % of the total pore volume) and well homogeneity pore structure in CAR-AC facilitate desorption of organic molecules at relatively low temperature (Huang et al., 2002; Silvestre-Albero et al., 2009). The desorption capacities of acetaldehyde, acetone, and ethyl acetate are 6.74, 13.84, and 41.23 mg/g, respectively, with desorption efficiencies of 90.20, 95.42 and 93.24 %, respectively. Fig. 4b shows that after four adsorption/ desorption cycles, the regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate are 88.77, 85.55 and 91.46 %, respectively. The results demonstrate that CAR-AC has a stable structure and good adsorption/desorption performance. During desorption, the most weakly adsorbed acetaldehyde is the first desorbed from the surface of the adsorbent, followed by acetone, and the strongest adsorbed ethyl acetate is the slowest one that is desorbed from the surface of CAR-AC. The diffusivities of acetaldehyde, acetone, and ethyl acetate in CAR-AC are simulated by the layer structure with Type II and Type III defects, and the results are shown in Fig. 5. At 298.15 K, the diffusion coefficients of acetaldehyde, acetone, and ethyl acetate are 8.19 × 10−5, 3.55 × 10−5, and 2.01 × 10−5 cm2/s, respectively. It shows that the diffusion coefficient of acetaldehyde is the largest; the large to small sequence is acetaldehyde > acetone > ethyl acetate. The larger the diffusion coefficients are, the faster diffusion is through the pore of the activated carbon. Moreover, increasing adsorption temperature is beneficial to
Fig. 4. Desorption curves of the multicomponent mixture of acetaldehyde, acetone and ethyl acetate from activated carbon (a), and regeneration efficiencies of acetaldehyde, acetone and ethyl acetate on activated carbon in four successive adsorption cycles (b). 6
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The adsorption amount of ethyl acetate in CAR-AC from the multicomponent VOCs is 3.04 and 5.91 times greater than that of acetone and acetaldehyde, respectively. Rising temperature could accelerate the diffusion of acetone, acetaldehyde, and ethyl acetate in CAR-AC and reduce the captive probability of adsorption active sites for VOC. The diffusion coefficients of acetaldehyde, acetone, and ethyl acetate are 8.19 × 10−5, 3.55 × 10−5, and 2.01 × 10−5 cm2/s, respectively. Acetaldehyde and acetone on CAR-AC desorb more easily than ethyl acetate, and desorption time for acetaldehyde, acetone and ethyl acetate is about 20, 60 and 280 min, respectively. After four adsorption/desorption cycles, the regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate are 88.77, 85.55 and 91.46 %, respectively. CAR-AC has a stable structure and good adsorption/desorption performance. The study provides a new strategy to recycle the use of CAR and realize the clean production of citric acid. Fig. 5. Diffusion coefficients of the multicomponent mixture of acetaldehyde, acetone and ethyl acetate calculated by the MD model.
CRediT authorship contribution statement Xiaolong Yao: Methodology, Writing - original draft. Yao Liu: Investigation, Data curation. Tong Li: Data curation. Tingting Zhang: Resources. Hailong Li: Resources. Wei Wang: Data curation. Xianbao Shen: Investigation. Feng Qian: Funding acquisition. Zhiliang Yao: Supervision.
of the column. In Stage IV, no adsorption sites can further adsorb acetaldehyde, and the concentration of acetaldehyde at the outlet is equal to that at the inlet of the column. However, the adsorbed acetone can also be replaced by ethyl acetate, and the acetone in the outlet stream exceeds the inlet concentration. Part of ethyl acetate can reach the top of adsorption column. Finally, the adsorption of the ternary mixture of VOCs on CAR-AC reaches a real equilibrium state at Stage V. Each adsorbate molecule maintains a dynamic balance in the pore and on the surface of CAR-AC. The concentration and composition of the VOCs at outlet of the column remain consistent with those at the inlet of the column.
Declaration of Competing Interest None. Acknowledgements
4. Conclusion
The authors are grateful for the financial support from the General Project of Beijing Educational Committee (KM202010011002), and the National Natural Science Foundation of China (No.41977142, 41601516). English editing of the whole manuscript by Dr. Mohammadtaghi Vakili is also appreciated.
A multicomponent mixture of acetaldehyde, acetone, and ethyl acetate is adsorbed in a CAR waste-based activated carbon. The interaction energy of ethyl acetate reaches −13.41 kcal/mol, and ethyl acetate could most easily adsorb on the defect surface in all three VOCs.
Fig. 6. Schematic of adsorption of the multicomponent mixture of acetaldehyde, acetone and ethyl acetate on CAR-AC.
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Appendix A. Supplementary data
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Energy Fuel 29, 877–893. Shamskar, K.R., Rashidi, A., Azar, P.A., Yousefi, M., Baniyaghoob, S., 2019. Synthesis of graphene by in situ catalytic chemical vapor deposition of reed as a carbon source for VOC adsorption. Environ. Sci. Pollut. R 26, 3643–3650. Shi, Q.Y., Yang, X.Y., Wu, L.Y., Liu, H.J., Zhang, J., Zhang, F., Long, C., 2018. Binary adsorption equilibrium and breakthrough of toluene and cyclohexane on macroporous and hypercrosslinked polymeric resins. Microporous Mesoporous Mater. 271, 73–82. Silvestre-Albero, A., Silvestre-Albero, J., Sepulveda-Escribano, A., Rodriguez-Reinoso, F., 2009. Ethanol removal using activated carbon: effect of porous structure and surface chemistry. Microporous Mesoporous Mater. 120, 62–68. Stefan, D.G., Akgerman, A., 1998. Single-component and binary-mixture adsorption of volatile organic contaminants on silica gel. Environ. Eng. Sci. 15, 191–202. Sui, H., Jiang, P., Li, X., Liu, J.J., Li, X.G., He, L., 2019. 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The influence of porosity and surface oxygen groups of peat-based activated carbons on benzene adsorption from dry and humid air. Carbon 47, 2371–2378. Wang, H.Y., Lashaki, M.J., Fayaz, M., Hashisho, Z., Philips, J.H., Anderson, J.E., Nichols, M., 2012. Adsorption and desorption of mixtures of organic vapors on beaded activated carbon. Environ. Sci. Technol. 46, 8341–8350. Wang, G., Dou, B.J., Zhang, Z.S., Wang, J.H., Liu, H.E., Hao, Z.P., 2015. Adsorption of benzene, cyclohexane and hexane on ordered mesoporous carbon. J. Environ. Sci. China 30, 65–73. Wang, J.J., Yuan, Q.W., Dong, M.Z., Cai, J.C., Yu, L., 2017. Experimental investigation of gas mass transport and diffusion coefficients in porous media with nanopores. Int. J. Heat Mass Transf. 115, 566–579. Wang, Z., Wang, T.T., Zhu, J.B., Wei, L.P., Shen, Y.Z., Li, N., Hu, J., 2019. Synergistic effect and mechanism of copper corrosion inhibition using cinnamaldehyde and vanillin in HCl solution: an experimental and theoretical approach. Colloid Surf. A 563, 246–254. Wołowiec, M., Muir, B., Zięba, K., Bajda, T., Kowalik, M., Franus, W., 2017. Experimental study on the removal of VOCs and PAHs by zeolites and surfactant-modified zeolites. Energy Fuel 31, 8803–8812. Xu, G.J., Wang, H., 2017. Molecular dynamics study of oxidative aging effect on asphalt binder properties. Fuel 188, 1–10. Yahya, M.A., Al-Qodah, Z., Ngah, C.W.Z., 2015. Agricultural bio-waste materials as potential sustainable precursors used for activated carbon production: a review. Renew. Sust. Energy Rev. 46, 218–235. Yang, K., Sun, Q., Xue, F., Lin, D.H., 2011. Adsorption of volatile organic compounds by metal-organic frameworks MIL-101: influence of molecular size and shape. J. Hazard. Mater. 195, 124–131. Yang, C., Miao, G., Pi, Y., Xia, Q., Wu, J., Li, Z., Xiao, J., 2019. Abatement of various types of VOCs by adsorption/catalytic oxidation: a review. Chem. Eng. J. 370, 1128–1153. Yeganegi, S., Gholampour, F., 2016. Simulation of methane adsorption and diffusion in a carbon nanotube channel. Chem. Eng. Sci. 140, 62–70. Yoon, Y.H., Nelson, J.H., 1984. Application of gas adsorption kinetics I. A theoretical model for respirator cartridge service life. Am. Ind. Hyg. Assoc. J. 45, 509–516. Zhang, Y.G., Wu, M.Y., Kwok, Y.H., Wang, Y.F., Zhao, W., Zhao, X.L., Huang, H.B., Leung, D.Y.C., 2019. In-situ synthesis of heterojunction TiO2/MnO2 nanostructure with excellent performance in vacuum ultraviolet photocatalytic oxidation of toluene. Appl. Catal. B Environ. 259, 118034. Zhou, K., Ma, W., Zeng, Z., Ma, X., Xu, X., Guo, Y., Li, H., Li, L., 2019. Experimental and DFT study on the adsorption of VOCs on activated carbon/metal oxides composites. Chem. Eng. J. 372, 1122–1133.
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Cao, H., Jiao, Y., Yin, N., Li, Y., Ling, J., Mao, Z., Yang, Y., Xie, B., 2019. Analysis of the activity and biological control efficacy of the Bacillus subtilis strain Bs-1 against Meloidogyne incognita. Crop Prot. 122, 125–135. Chun, B.J., Lee, S.G., Choi, J.I., Jang, S.S., 2015. Adsorption of carboxylate on calcium carbonate (10 (1)over-bar 4) surface: molecular simulation approach. Colloid Surf. A 474, 9–17. Coasne, B., Alba-Simionesco, C., Audonnet, F., Dosseh, G., Gubbins, K.E., 2011. Adsorption, structure and dynamics of benzene in ordered and disordered porous carbons. Phys. Chem. Chem. Phys. 13, 3748–3757. Diao, R., Zhang, H.Y., Zhao, D., Li, S., 2019. Adsorption and structure of benzene, toluene, and p-xylene in carbon slit pores: a Monte Carlo simulation study. Chem. Eng. Sci. 197, 120–134. Do, D.D., Do, H.D., 2006. Adsorption of benzene on graphitized thermal carbon black: reduction of the quadrupole moment in the adsorbed phase. Langmuir 22, 1121–1128. Hoover, W.G., 1985. 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Affinity and packing of benzene, toluene, and p-xylene adsorption on a graphitic surface and in pores. Ind. Eng. Chem. Res. 51, 5320–5329. Kurisaki, I., Takahashi, T., 2011. Assessment of dynamic properties of water around a monovalent ion: a classical molecular dynamics simulation study. Comput. Theor. Chem. 966, 26–30. Lashaki, M.J., Fayaz, M., Wang, H.Y., Hashisho, Z., Philips, J.H., Anderson, J.E., Nichols, M., 2012. Effect of adsorption and regeneration temperature on irreversible adsorption of organic vapors on beaded activated carbon. Environ. Sci. Technol. 46, 4083–4090. Laskar, I.I., Hashisho, Z., Phillips, J.H., Anderson, J.E., Nichols, M., 2019. Competitive adsorption equilibrium modeling of volatile organic compound (VOC) and water vapor onto activated carbon. Sep. Purif. Technol. 212, 632–640. Lee, S.W., Cheon, J.K., Park, H.J., Lee, M.G., 2008. Adsorption characteristics of binary vapors among acetone, MEK, benzene, and toluene. Korean J. Chem. Eng. 25, 1154–1159. Lemus, J., Martin-Martinez, M., Palomar, J., Gomez-Sainero, L., Gilarranz, M.A., Rodriguez, J.J., 2012. Removal of chlorinated organic volatile compounds by gas phase adsorption with activated carbon. Chem. Eng. J. 211, 246–254. Li, L., Liu, S.Q., Liu, J.X., 2011. Surface modification of coconut shell based activated carbon for the improvement of hydrophobic VOC removal. J. Hazard. Mater. 192, 683–690. Lillo-Rodenas, M.A., Fletcher, A.J., Thomas, K.M., Cazorla-Amoros, D., Linares-Solano, A., 2006. Competitive adsorption of a benzene-toluene mixture on activated carbons at low concentration. Carbon 44, 1455–1463. Liu, Y., Yao, X.L., Wang, Z.B., Li, H.L., Shen, X.B., Yao, Z.L., Qian, F., 2019. Synthesis of activated carbon from citric acid residue by phosphoric acid activation for the removal of chemical oxygen demand from sugar-containing wastewater. Environ. Eng. Sci. 36, 656–666. Long, C., Liu, P., Li, Y., Li, A.M., Zhang, Q.X., 2011. 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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Adsorption behavior of multicomponent volatile organic compounds on a citric acid residue waste-based activated carbon: Experiment and molecular simulation
T
Xiaolong Yaoa,b, Yao Liua,b, Tong Lia,b, Tingting Zhangc, Hailong Lid, Wei Wange, Xianbao Shena,b, Feng Qiana,b, Zhiliang Yaoa,b,* a
Department of Environmental Science and Engineering, Beijing Technology and Business University, Beijing 100048, China Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry, Beijing Technology and Business University, Beijing 100048, China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China d School of Energy Science and Engineering, Central South University, Changsha, 410083, China e State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xi’ning, Qinghai Province 810016, China b c
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: Danmeng Shuai
A considerable amount of volatile organic compounds (VOCs) is emitted, and a vast amount of citric acid residue (CAR) waste is simultaneously produced during citric acid production. Thus, a suitable method realizing the clean production of citric acid must be developed. This study investigated the adsorption of the multicomponent VOCs in a homemade CAR waste-based activated carbon (CAR-AC). A fixed-bed experimental setup was used to explore the adsorption and desorption of single- and multi-component VOCs. Surface adsorption and diffusion molecular models with different defects were built to study the underlying adsorption and diffusion mechanisms of multicomponent VOCs on CAR-AC. The adsorption amount of ethyl acetate in CAR-AC from multicomponent VOCs was 3.04 and 5.91 times higher than those of acetone and acetaldehyde, respectively, and the interaction energy between ethyl acetate and C surfaces was low at −13.41 kcal/mol. During desorption, the most weakly adsorbed acetaldehyde desorbed from the surface of CAR-AC first, followed by acetone and ethyl acetate. The regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate reached 88.77, 85.55, and 91.46 %, respectively, after four adsorption/desorption cycles. We aimed to provide a new strategy to realize the recycle use of CAR and the clean production of citric acid.
Keywords: Adsorption Citric acid residue waste-based activated carbon Multicomponent volatile organic compounds Molecular simulation
⁎
Corresponding author at: Department of Environmental Science and Engineering, Beijing Technology and Business University, Beijing 100048, China. E-mail address: [email protected] (Z. Yao).
https://doi.org/10.1016/j.jhazmat.2020.122323 Received 14 December 2019; Received in revised form 12 February 2020; Accepted 14 February 2020 Available online 15 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
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1. Introduction
common VOCs in exhaust gas emitted from citric acid production, was adsorbed in the CAR waste-based activated carbon. Experiments and molecular dynamic (MD) simulation methods were used to study the adsorption behaviors of these VOCs during adsorption in the activated carbon. This study aimed to understand the competitive adsorption mechanisms of multicomponent VOCs in this activated carbon comprehensively and provide a new strategy to recycle CAR and realize the clean production of citric acid.
Volatile organic compounds (VOCs) are typical air pollutants (Shamskar et al., 2019; Yang et al., 2019). They are not only the main precursors of secondary aerosol (SOA) and ozone in the atmosphere but also threaten the environment and exert direct carcinogenic, teratogenic, and mutagenic effects on humans (Huang et al., 2018; Zhang et al., 2019). Until 2018, total VOC emission reached 25–30 million tons in China. Chemicals for synthesis, coating and printing industries, and fermentation industries are the most common sources of VOC emission (Cao et al., 2019; Diao et al., 2019) and account for about half of the total VOC emission (Yang et al., 2019). Citric acid production is a typical fermentation industry, and over 1.2 million tons citric acid is produced in China each year, which accounts for over 80 % the world’s total citric acid (Ao et al., 2013). Based on our preliminary test results, we found that a considerable amount of VOCs was emitted in fermentation and citric acid drying, including aldehydes, ketones, alcohols and esters. Moreover, VOCs emitted from these exhaust gas streams are mostly multicomponent gas, increasing the difficulty to purify them from waste gas (Lashaki et al., 2012; Wang et al., 2012). Therefore, controlling the release of VOCs from citric acid production is an urgent problem. Adsorption is widely used in purifying VOCs from citric acid production because it is highly effective and economical (Sui et al., 2019). Activated carbons are commonly used as absorbents for VOC removal because of their large micropore volume, high specific surface area, and excellent adsorption capacity (Ma et al., 2018; Zhou et al., 2019). Activated carbons are generally prepared using nonrenewable materials, including coal, petroleum residues, wood, and lignite (Mondal and Garg, 2017; Liu et al., 2019). However, these precursors are expensive (Brito et al., 2017). Thus, they have been replaced by several low-cost and renewable raw materials, such as agricultural residues, food wastes, and industrial by-products, in recent years (Yahya et al., 2015). Preparing activated carbons from industrial by-products can reduce the production cost, allow the recycling, and increase the additional value of products (Tan et al., 2017). Citric acid residue (CAR), another typical by-product in citric acid production (Piotrowska et al., 2015; Juliastuti et al., 2017), has been made to an activated carbon in our previous study (Liu et al., 2019). Therefore, VOCs from citric acid production may be removed using the prepared activated carbon to recycle the use of CAR and realize the clean production of citric acid. Adsorption of VOC gas mixture on activated carbons is affected by temperature and pressure and also competing adsorption occurs with the change in gas composition (Cabrera-Codony et al., 2018; Laskar et al., 2019). The complexity of the activated carbon adsorption process increases with the number of components in the mixture exhaust gas (Tefera et al., 2014). During competitive adsorption, VOCs with a strong affinity to the activated carbon are easily adsorbed on the surface of the activated carbon compared with the component with a weak affinity (Lillo-Rodenas et al., 2006). The experimental study of multicomponent adsorption is usually macroscopic, thus, it is limited to obtaining a fundamental understanding of multicomponent VOC adsorption mechanisms in activated carbons. In recent years, molecular simulation has gradually become a common method to study the interactions between VOCs and activated carbon at the molecular level (Do and Do, 2006; Klomkliang et al., 2012). The orientation of several types of VOCs in slit pores is a function of pore width (Diao et al., 2019). However, most studies focused on the adsorption behavior of single-component VOCs on activated carbon (Coasne et al., 2011). Limited works studied the adsorption behavior of multicomponent VOCs on a CAR waste-based activated carbon, and the interactions among the VOC molecules during adsorption are unclear. Thus, this investigation would help elucidate the adsorption mechanisms of multicomponent VOCs in a CAR waste-based activated carbon. In the present study, a mixture gas of acetaldehyde (CH3CHO), acetone (CH3COCH3), and ethyl acetate (CH₃CH₂OOCCH₃), which are
2. Materials and methods 2.1. Materials CAR waste was collected from a citric acid plant in Shandong Province, China. High-purity standard gases of acetaldehyde, acetone, and ethyl acetate used in the experiments were obtained from Zhaoge Gas Technology Co. Ltd. (China). Phosphoric acid (85 %) was obtained from Sinopharm Group (China). The chemical solutions were prepared in ultra-pure water produced by a Milli-Q system (Millipore, USA). A CAR waste-based activated carbon (CAR-AC) was prepared as follows. CAR waste was first washed with water to remove impurities and then dried at 383.15 K for 12 h. Next, 5.0 g of CAR was immersed in 40 % phosphoric acid solution with an impregnation ratio of 2.0 (i.e., the mass ration of CAR to phosphoric acid was 2.0) at room temperature for 12 h. The rest of preparation steps were the same as the method reported in our previous study (Liu et al., 2019). A scanning electron microscope (SEM) (Merlin compact, Zeiss, Germany) was used to study the surface morphology of the citric acid residue and activated carbons. The N2 adsorption–desorption isotherms and the pore size distribution of CAR-AC were characterized by an automated gas sorption analyzer (Quantachrome, Nova 2000e, USA). Fourier transform infrared (FTIR) spectra of CAR-AC were recorded using FTIR spectroscopy (Avater 370, Nicolet, USA) in the range of 400–4000 cm−1. 2.2. Dynamic adsorption measurements A fixed-bed experimental setup was used to explore the single component and multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption in CAR-AC, and the experimental apparatus is shown in Fig. S4. The prepared high-purity standard gases of single component and multicomponent VOCs were placed in gas cylinders. When the experiment started, the gas channel was opened by a valve, and the flow rate of the stream was adjusted by the mass flow controllers. The gas stream was preheated before entering the adsorption column. Then, 0.2 g of outgassed adsorbent was loaded into an adsorption column (φ 6 mm × 150 mm). A filter plate was located at the bottom of the column as a support of the adsorbent bed. During the adsorption experiments, the inlet concentration (Cin) of the VOCs was maintained constant, and the VOC vapor flowed through the fixed bed from the bottom of the adsorption column. For single-component VOC adsorption, bed saturation was achieved when the VOC vapor concentrations in the outlet of the adsorption column matched the inlet concentrations. For the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption, bed saturation was achieved when the outlet concentration (Cout) for all of them matched the inlet concentrations. The adsorption temperature was controlled by a temperature controller. After adsorption, the tail gas was purified before it escaped into the atmosphere. The inlet and outlet concentrations of each compound in the single component and multicomponent streams were measured by a gas chromatography instrument (Techcomp GC7900, China) equipped with a TM-624 quartz capillary column (length × inner diameter × thickness, 60 m × 0.32 mm ×2.0 μm) and a flame-ionization detector. Operating conditions were as follows: oven temperature, 353.15 K; detector temperature, 553.15 K; carrier gas, N2 at 20 mL/min. Single2
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component adsorption tests were carried out at the feed flow rate of 100 mL/min with the inlet concentration of 500 mg/m3 at 298.15–318.15 K. Multicomponent adsorption experiments were conducted at the same flow rate with the inlet concentration of each component at 125, 250, 500, and 750 mg/m3 at 298.15–318.15 K. The adsorption capacity was determined by the difference between the inlet and outlet VOC mass until the adsorption bed reached the saturated state, and the adsorption amount (qs) was calculated as follows (Eq. (1)):
qs =
Q m × 10
ts 6
0
(Cin
Cout )dt
temperature was 393.15 K, and the regeneration time was 6 h). Then, the adsorbent was cooled to room temperature and reused in the next adsorption cycle. The adsorption/desorption experiments were carried out for four cycles. 2.4. Simulation method During the MD simulation, the interactions between C surface and molecules of acetaldehyde, acetone and ethyl acetate were characterized. According to the pore size distribution of CAR-AC calculated by the N2 adsorption–desorption isotherms and the molecular diameters for acetaldehyde (3.2 Å), acetone (4.8 Å), and ethyl acetate (6.0 Å), three types of C(001) surface, namely, preface surface (Type I), surface with a narrow circle defect of 5 Å radius (Type II), and surface with a wide circle defect of 12 Å radius (Type III), were built with a 20 Å vacuum layer to simulate different porosities (Fig. S5). Acetaldehyde, acetone, and ethyl acetate were added at the top of optimized surface with close contact. Diffusivity was simulated by the layer structure with Type II and Type III defects, and the acetaldehyde, acetone, and ethyl acetate layers with equal mass were covered on the surface, as shown in Fig. S6. The structure and charge of MD simulation were optimized by COMPASS force field to ensure the accuracy of the calculation results (Sun, 1998; Xu and Wang, 2017). The Van der Waals interaction was calculated by the atom-based method with a cutoff radius of 18.5 Å to avoid the errors of relatively short-range non-bond interactions. The electrostatic summation was calculated using the Ewald method to avoid the errors of long-range non-bond energy in periodic systems. The initial models with 3D periodic boundary conditions were optimized with the smart minimizing method, and the control precision of the energy gradient, maximum force, and maximum displacement reached less than 2.0 × 10−5 kcal/mol, 0.001 kcal/(mol Å), and 1.0 × 10−5 Å, respectively. The optimized model structure was combined with a series of
(1)
where Q is the flow rate of gas stream, mL/min; m is the mass of the adsorbent, g; Cin and Cout are the inlet and outlet concentrations of each component, respectively, mg/m3; t is the adsorption time, min; ts is the time of break-through, min; and qs is the adsorption amount, mg/g. The adsorption kinetics of simple VOCs upon CAR-AC was determined by the Yoon-Nelson (Y-N) model (Yoon and Nelson, 1984; Sui et al., 2019), and the values were calculated as follows (Eq. (2)):
t = t50% +
1 Ci (t ) ln kYN Ci 0 Ci (t )
(2)
where t (min) is adsorption time, t50 % (min) is the time required for 50 % adsorbate breakthrough, kYN (/min) is the proportionality constant of the Y–N model, and Ci0 (mg/m3) and Ci(t) (mg/m3) are the inlet and outlet concentrations of component i at time t, respectively. 2.3. Regeneration experiments After adsorption of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate with the inlet concentrations of each component at 750 mg/m3 at 298.15 K, the activated carbon was regenerated by hot nitrogen gas at an optimized regeneration condition (i.e. flow rate of nitrogen gas was 100 mL/min, regeneration
Fig. 1. Breakthrough curves of acetaldehyde, acetone, and ethyl acetate at 298.15 (a), 308.15 (b), and 318.15 K (c); diffusion coefficients of acetaldehyde, acetone, and ethyl acetate calculated by the MD model (d). 3
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The diffusivity Dα is given by using Eq. (4) (Yeganegi and Gholampour, 2016):
Table 1 Fitting parameters of the Y-N model and the predicted penetration time at different temperatures. Single component VOCs
Temperature(K)
1/k (min)
k (×10−1 /min)
t50
acetaldehyde
298.15 308.15 318.15 298.15 308.15 318.15 298.15 308.15 318.15
8.08 7.69 7.31 5.77 4.11 4.12 15.36 15.75 11.78
1.238 1.300 1.368 1.733 2.433 2.427 0.651 0.635 0.849
47.89 45.16 42.85 150.4 127.64 107.56 236.95 188.88 171.91
acetone ethyl acetate
%
(min)
N
D =
R2
(Emolecule + Esurface)
ri (0)]2 }
(4)
where Nα is the number of diffusive atoms in the system and ri(t) and ri (0) represent the coordinates of the i-th particle at time 0 and time t, respectively.
0.970 0.995 0.990 0.995 0.995 0.999 0.989 0.996 0.888
3. Results and discussion 3.1. Characterization of CAR-AC The SEM images of CAR-AC are shown in Fig. S1. The surfaces of CAR-AC are covered with holes of different shapes and sizes, and with well homogeneity. The N2 adsorption–desorption isotherms and the pore size distribution of CAR-AC are shown in Fig. S2. The N2 adsorption–desorption isotherms of CAR-AC present Type IV in accordance with IUPAC. The micropores and mesopores in CAR-AC are mainly around 1.0, 2.4, and 3.2 nm. The porous structural parameters of CAR-AC are shown in Table S1. The BET surface area and total pore volume of CAR-ACs reach 786 m2/g and 0.71 cm3/g, respectively. Meanwhile, the microporous and mesoporous volume of CAR-ACs is 0.33 and 0.35 cm3/g, respectively, which respectively accounts for 46.47 and 49.30 % of the total pore volume. The FTIR spectra of CARAC are shown in Fig. S3. Almost no oxygen-containing function groups are observed on the CAR-AC surface. Evidently, lots of oxygen-containing function groups were decomposed from the surfaces of synthesized activated carbon samples.
environmental conditions, such as constant molecule numbers, volume, and temperature (NVT) or constant molecule numbers, volume, and energy (NVE) to execute a dynamic calculation (Wang et al., 2019). Moreover, the operating temperature was set at 298.15 K and controlled using the Nose thermostat method to match the real experiment procedure (Nosé, 1984; Hoover, 1985). The adsorption behavior was studied at the NVT ensemble with 1000 ps. Diffusion behaviors were explored and analyzed at the NVT and NVE ensembles with 1000 ps each with different temperatures (Kurisaki and Takahashi, 2011; Bazooyar et al., 2012). The interaction energy Einteraction between the activated carbon surface and the VOC molecules was calculated by using Eq. (3) (Chun et al., 2015; Hu et al., 2018):
Einteraction = Etotal
1 d lim {[ri (t ) 6N t dt i = 1
(3)
3.2. Adsorption single component of acetaldehyde, acetone, and ethyl acetate in CAR-AC
where Etotal is the total energy of the system including that of molecules adopted on the surface, Emolecule is the energy of individual molecules, and Esurface is the energy on the C (001) surface.
The breakthrough curves of acetaldehyde, acetone, and ethyl
Fig. 2. Adsorption experiments of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate at concentrations of 125 (a), 250 (b), 500 (c), and 750 mg/L (d). 4
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acetate with the inlet concentration of 500 mg/m3 at 298.15, 308.15, and 318.15 K are shown in Fig. 1. At 298.15 K (Fig. 1a), the penetration times (i.e., the time at Cout/Cin = 0.05) (Wołowiec et al., 2017) for acetaldehyde, acetone, and ethyl acetate are 28, 136 and 210 min, respectively. With increasing adsorption temperature, the penetration times for acetaldehyde, acetone, and ethyl acetate appear earlier (Fig. 1b and Fig. 1c), specifically 23, 104, and 163 min at 318.15 K, respectively. Moreover, the diffusion coefficients of acetaldehyde, acetone, and ethyl acetate calculated with the MD simulation method are shown in Fig. 1d. The order of diffusion coefficients is acetaldehyde > acetone > ethyl acetate, and the diffusion coefficients increase with the temperature. These results indicate that the diffusion of acetaldehyde in CAR-AC is the fastest, followed by that of acetone, and the slowest is that of ethyl acetate. The fitting parameters of the YN model at different adsorption temperatures are given in Table 1. Results show that the Y-N model provides a good agreement for the experimental breakthrough data of single-component adsorption. At the same adsorption temperature, the value of t50 % is followed as acetaldehyde < acetone < ethyl acetate. The finding also indicates that the diffusion of acetaldehyde in CAR-AC is the fastest among the three, and that of ethyl acetate is the slowest. Moreover, for the three VOCs, increasing the adsorption temperature can decrease time t50 %, suggesting that a high temperature increases the mass transfer driving force and accelerates the adsorption bed penetration (Lemus et al., 2012; Wang et al., 2015).
compounds (Stefan and Akgerman, 1998). However, the adsorption amount of each component in the multicomponent VOCs is much less than that in the simple system. At 298.15 K, the adsorption amounts for acetaldehyde, acetone, and ethyl acetate in the multicomponent VOCs only account for 59.76, 38.77, and 71.90 % of that in the simple system, respectively. Moreover, the adsorption amount of ethyl acetate in CARAC from multicomponent VOCs is 3.04 times greater than that of acetone and 5.91 times greater than that of acetaldehyde. These results indicate that the adsorption selectivity of CAR-AC for ethyl acetate is higher than that for acetone and acetaldehyde. The interactions between molecules of acetaldehyde, acetone, and ethyl acetate and C surface at different porosities were characterized. The adsorption molecule models are shown in Fig. S7, and the calculation results are shown in Fig. 3b. On the preface surfaces of CAR-AC (i.e., Type I), the Einteraction values for acetaldehyde, acetone, and ethyl acetate are −7.05, −9.57, and −9.39 kcal/mol, respectively. These results indicate that acetone and ethyl acetate can more easily adsorb on the preface surfaces of CAR-AC than acetaldehyde, which may be partly attributed to the difference in molecular diameter for acetaldehyde (3.2 Å), acetone (4.8 Å), and ethyl acetate (6.0 Å); in addition, the larger molecular diameter of acetone and ethyl acetate may have a stronger force between adsorbent molecules and surface carbon atoms (Lee et al., 2008; Mao et al., 2016). The Einteraction values for acetaldehyde, acetone, and ethyl acetate are different when adsorbed on the surface with a circle defect of 5 Å radius (i.e., Type 2). For acetaldehyde, Einteraction is −6.93 kcal/mol and is slightly higher than that adsorbed on the preface surfaces. However, the interaction energies of
3.3. Adsorption of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate in CAR-AC The adsorption breakthrough curves of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption are shown in Fig. 2. Compared with a single component, the ternary mixture shows evidently different breakthrough curves. The outlet concentration of acetaldehyde first exceeds the inlet concentration after a certain period, and then the acetone in the outlet stream exceeds its inlet concentration. Moreover, the penetration time of ethyl acetate becomes shorter, and the penetration rate of ethyl acetate in CAR-AC increases slightly slower than before. This result may be attributed to the fact that the diffusion of acetaldehyde in CAR-AC is the fastest, and it may first adsorb on the finite adsorption active sites. When the three VOCs reach the surface of the adsorbent, the most strongly adsorbed ethyl acetate is preferentially captured on the adsorbent by displacing the weakly adsorbed acetone, and the acetone is captured on the adsorbent by displacing the most weakly adsorbed acetaldehyde (Lillo-Rodenas et al., 2006; Sui et al., 2019). Moreover, the margin of excess concentration for acetaldehyde and acetone decreases with increasing adsorption temperature. These results are mainly attributed to the decrease in acetaldehyde and acetone adsorption amounts on the CAR-AC surface at a high temperature. Thus, the amounts of acetaldehyde and acetone were replaced by ethyl acetate decrease. The adsorption amounts of acetaldehyde, acetone, and ethyl acetate in CAR-AC can reflect the adsorption capacity and selectivity of CAR-AC for these three VOCs (Shi et al., 2018; Sui et al., 2019). The adsorption amounts of single compound and their multicomponent VOCs at the equilibrium concentration for each component 500 mg/m3 are shown in Fig. 3a. The adsorption amounts of single compounds of acetaldehyde, acetone, and ethyl acetate in CAR-AC at 298.15 K are 12.5, 37.5, and 61.5 mg/g, respectively. These values decrease to 9.75 mg/g for acetaldehyde, 27.5 mg/g for acetone, and 40.5 mg/g for ethyl acetate when the adsorption temperature is increased to 318.15 K. Rising temperature can accelerate the diffusion of VOCs in activated carbon and reduce the captive probability of adsorption active sites for VOCs (Wang et al., 2017). The total adsorption amount of the mixture of acetaldehyde, acetone, and ethyl acetate in CAR-AC is higher than those of single compounds, indicating that several adsorption active sites on the CAR-AC surface may co-adsorb two or three of these organic
Fig. 3. Adsorption amounts of single compounds and multicomponent mixture of acetaldehyde, acetone, and ethyl acetate in CAR-AC at the equilibrium concentration for each component 500 mg/m3 (a) and the interaction energies between molecules of acetaldehyde, acetone, and ethyl acetate and C surface on the three types of surfaces (b). 5
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acetone and ethyl acetate are stronger than those adsorbed on preface surfaces, especially for ethyl acetate, whose interaction energy value reaches −13.41 kcal/mol. These results indicate that ethyl acetate can most easily adsorb on the defect surface of 5 Å radius, followed by acetone and acetaldehyde. This result can be attributed to the pore structure of the activated carbon and the molecular diameter of VOCs. On the circle defect surface of 5 Å radius, the pore size is 3.13, 2.08, and 1.67 times wider than the diameter of acetaldehyde, acetone, and ethyl acetate, respectively. Therefore, wider pore size of activated carbon than the diameter of VOCs (about two times), leads in the stronger Van der Waals forces between adsorbent molecules and surface carbon atoms (Long et al., 2011; Yang et al., 2011). When the radius of the defect surface is increased to 12 Å, Einteraction for acetaldehyde slightly decreases to −6.65 kcal/mol. The results indicate that the interaction between molecules of acetaldehyde and C surface is the weakest among all three VOCs, and changing the pore size of CAR-AC has minimal effect on acetaldehyde adsorption. However, the Einteraction values for acetone and ethyl acetate increase substantially to −8.78 and −10.52 kcal/mol, respectively, indicating that both can more easily adsorb in the pore with a size of around 1.0 nm than in the pore with a size of around 2.4 nm, and these three kinds of VOCs gas are mainly adsorbed in the mciropores of CAR-AC (Veksha et al., 2009; Li et al., 2011). The pores with the size larger than 2.4 nm in CAR-AC did not affect the calculation results since their pore size are much higher than the molecular diameter of VOCs gas (Long et al., 2011; Yang et al., 2011). In addition, the adsorptive selectivity of CAR-AC for ethyl acetate is better than that for acetone because the Einteraction for acetone is higher than that for ethyl acetate. These results are consistent with the experimental studies.
the diffusion of these three kinds of VOCs in CAR-AC. The diffusion coefficient values of acetaldehyde, acetone, and ethyl acetate increase to 9.16 × 10−5, 3.71 × 10−5, and 2.46 × 10−5 cm2/s respectively when the adsorption temperature increases to 318.15 K. Thus, the acetaldehyde is first desorbed from CAR-AC, followed by acetone and ethyl acetate. 3.5. Adsorption mechanisms of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate on CAR-AC The multicomponent mixture of acetaldehyde, acetone, and ethyl acetate adsorption on CAR-AC is closely related to their diffusion in the pore of CAR-AC and the interactions between their molecules and the C surface. The adsorption processes can be divided into five stages to clearly understand the ternary adsorption behaviors in CAR-AC (Fig. 6). In Stage I, acetaldehyde, acetone, and ethyl acetate are delivered into the pores. The diffusion of acetaldehyde is the fastest and first adsorbed on the adsorption active sites. Acetone and ethyl acetate are subsequently adsorbed on the surfaces and in pores. The outlet concentrations are equal to zero because CAR-AC can adsorb almost all three VOCs. Compared with Stage I, acetaldehyde can first diffuse to the top of the adsorption column, and some pre-adsorbed acetaldehyde molecules can be replaced by acetone or ethyl acetate molecules. Hence, a certain concentration of acetaldehyde outside the pores is observed in Stage II. In Stage III, more pre-adsorbed acetaldehyde is replaced by acetone or ethyl acetate, and the concentration of acetaldehyde in the outlet stream first exceeds the inlet concentration. Acetone also diffuses to the top of the adsorption column and is partly detected at the outlet
3.4. Regeneration of CAR-AC The desorption curves of the multicomponent mixture of acetaldehyde, acetone, and ethyl acetate at 298.15 K are shown in Fig. 4a, and their regeneration efficiencies in four successive adsorption cycles are shown in Fig. 4b. Fig. 4a shows that when the desorption time is about 20 min, acetaldehyde is almost desorbed from CAR-AC, and the times for acetone and ethyl acetate are about 60 and 280 min, respectively. Acetaldehyde and acetone on CAR-AC desorb more easily than ethyl acetate, which can be attributed to their properties and the pore structure of CAR-AC. On the one hand, the diffusion of acetaldehyde and acetone in CAR-AC is faster than that of ethyl acetate. On the other hand, rich mesopores (mesoporous volume accounts for 49.30 % of the total pore volume) and well homogeneity pore structure in CAR-AC facilitate desorption of organic molecules at relatively low temperature (Huang et al., 2002; Silvestre-Albero et al., 2009). The desorption capacities of acetaldehyde, acetone, and ethyl acetate are 6.74, 13.84, and 41.23 mg/g, respectively, with desorption efficiencies of 90.20, 95.42 and 93.24 %, respectively. Fig. 4b shows that after four adsorption/ desorption cycles, the regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate are 88.77, 85.55 and 91.46 %, respectively. The results demonstrate that CAR-AC has a stable structure and good adsorption/desorption performance. During desorption, the most weakly adsorbed acetaldehyde is the first desorbed from the surface of the adsorbent, followed by acetone, and the strongest adsorbed ethyl acetate is the slowest one that is desorbed from the surface of CAR-AC. The diffusivities of acetaldehyde, acetone, and ethyl acetate in CAR-AC are simulated by the layer structure with Type II and Type III defects, and the results are shown in Fig. 5. At 298.15 K, the diffusion coefficients of acetaldehyde, acetone, and ethyl acetate are 8.19 × 10−5, 3.55 × 10−5, and 2.01 × 10−5 cm2/s, respectively. It shows that the diffusion coefficient of acetaldehyde is the largest; the large to small sequence is acetaldehyde > acetone > ethyl acetate. The larger the diffusion coefficients are, the faster diffusion is through the pore of the activated carbon. Moreover, increasing adsorption temperature is beneficial to
Fig. 4. Desorption curves of the multicomponent mixture of acetaldehyde, acetone and ethyl acetate from activated carbon (a), and regeneration efficiencies of acetaldehyde, acetone and ethyl acetate on activated carbon in four successive adsorption cycles (b). 6
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The adsorption amount of ethyl acetate in CAR-AC from the multicomponent VOCs is 3.04 and 5.91 times greater than that of acetone and acetaldehyde, respectively. Rising temperature could accelerate the diffusion of acetone, acetaldehyde, and ethyl acetate in CAR-AC and reduce the captive probability of adsorption active sites for VOC. The diffusion coefficients of acetaldehyde, acetone, and ethyl acetate are 8.19 × 10−5, 3.55 × 10−5, and 2.01 × 10−5 cm2/s, respectively. Acetaldehyde and acetone on CAR-AC desorb more easily than ethyl acetate, and desorption time for acetaldehyde, acetone and ethyl acetate is about 20, 60 and 280 min, respectively. After four adsorption/desorption cycles, the regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate are 88.77, 85.55 and 91.46 %, respectively. CAR-AC has a stable structure and good adsorption/desorption performance. The study provides a new strategy to recycle the use of CAR and realize the clean production of citric acid. Fig. 5. Diffusion coefficients of the multicomponent mixture of acetaldehyde, acetone and ethyl acetate calculated by the MD model.
CRediT authorship contribution statement Xiaolong Yao: Methodology, Writing - original draft. Yao Liu: Investigation, Data curation. Tong Li: Data curation. Tingting Zhang: Resources. Hailong Li: Resources. Wei Wang: Data curation. Xianbao Shen: Investigation. Feng Qian: Funding acquisition. Zhiliang Yao: Supervision.
of the column. In Stage IV, no adsorption sites can further adsorb acetaldehyde, and the concentration of acetaldehyde at the outlet is equal to that at the inlet of the column. However, the adsorbed acetone can also be replaced by ethyl acetate, and the acetone in the outlet stream exceeds the inlet concentration. Part of ethyl acetate can reach the top of adsorption column. Finally, the adsorption of the ternary mixture of VOCs on CAR-AC reaches a real equilibrium state at Stage V. Each adsorbate molecule maintains a dynamic balance in the pore and on the surface of CAR-AC. The concentration and composition of the VOCs at outlet of the column remain consistent with those at the inlet of the column.
Declaration of Competing Interest None. Acknowledgements
4. Conclusion
The authors are grateful for the financial support from the General Project of Beijing Educational Committee (KM202010011002), and the National Natural Science Foundation of China (No.41977142, 41601516). English editing of the whole manuscript by Dr. Mohammadtaghi Vakili is also appreciated.
A multicomponent mixture of acetaldehyde, acetone, and ethyl acetate is adsorbed in a CAR waste-based activated carbon. The interaction energy of ethyl acetate reaches −13.41 kcal/mol, and ethyl acetate could most easily adsorb on the defect surface in all three VOCs.
Fig. 6. Schematic of adsorption of the multicomponent mixture of acetaldehyde, acetone and ethyl acetate on CAR-AC.
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Appendix A. Supplementary data
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