Desorption Kinetics of Naphthalene and Acenaphthene over Two

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Desorption Kinetics of Naphthalene and Acenaphthene over Two Activated Carbons via Thermogravimetric Analysis (TGA)

Ziyi Lia, Yingshu Liua, Xiong Yanga, *, Yi Xingb, Zhanying Wanga, Quan Yanga, Ralph T. Yangc

a

School of Mechanical Engineering, University of Science and Technology Beijing, Beijing, 100083,China b School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083,China c Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States

*Corresponding author (T) +86-10-62332730; (F) +86-10-62334210. Email: [email protected].

1

ACS Paragon Plus Environment

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ABSTRACT Activated carbon is a promising sorbent for adsorption removal of polycyclic aromatic hydrocarbons (PAHs) because of its cost effectiveness. The desorption kinetics of two-ring PAHs, naphthalene and acenaphthene, over bituminous coal-based (AC WY) and coconut shell-based (ACNT ) activated carbons were investigated. The desorption kinetics were studied over the temperature range of 400800 K at different heating rates (820 K/min)

using

thermogravimetric

analysis

techniques.

The

activation

energy,

pre-exponential factor and the kinetic model for each sorbate-sorbent pair were determined by applying analytical methods to the nonisothermal data.

The

Johnson-Mehl-Avrami (JMA) rate equation, g(α) = [ln(1-α)]n (in integral form, where α is fractional completion), following the nucleation and growth model was found to best describe the PAHs desorption from both sorbents. Strong molecular sieving effects were found to influence both adsorption capacity and desorption rates. AC WY, with less micropore (< 0.7 nm) volume and more larger pores (0.72 nm) compared to AC NT , favors PAHs adsorption and desorption rates, leading to different values of kinetic exponent (n) and other kinetic parameters. Likewise, the sieving effects favor adsorption and desorption of naphthalene (kinetic diameter: 0.62 nm) over acenaphthene (0.66 nm) for both carbons.

2


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1 INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs), as a class of important pollutants causing carcinogenic and mutagenic effects to human,1,2 have drawn worldwide attention in the past decades. PAHs are mainly emitted from combustion associated with several anthropogenic sources such as engine exhaust,3 incinerators4 and industrial processes.5 During these combustion processes, as a group of semivolatile organic compounds, PAHs can exist in the gas or particulate phase depending on their volatility.6 The main PAHs compounds released in the gas phase were those of two aromatic rings with a higher volatility such as naphthalene (Nap) and acenaphthene (Ace). In general, these low-ring PAHs cannot be efficiently removed by traditional physicochemical processes such as filtration, sedimentation, flocculation or ozonation.7 In order to reduce gaseous PAH emissions, adsorption has been considered as one of the most competitive technologies because of its high efficiency and simplicity. 8-10 A promising adsorbent in application of adsorption technology regarding availability and cheapness is the activated carbon (AC), a great portion of which is derived from natural coal and coconut shell. 11 Studies on adsorption of PAHs from ACs have been reported regarding the influence of sorbent characteristics,6,12 inlet gas composition,13 temperature14 and humidity.15 However, so far, the existing literature on the desorption of PAHs from ACs is scarce and limited. Desorption characteristics are significant in terms of selecting appropriate sorbents to remove or recycle PAHs during chemical engineering processes. Adequate understanding and description of desorption kinetics are crucially needed for predicting the fate of PAHs 3


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on sorbents and providing guidance for adsorptiondesorption processes at a low regeneration energy cost and sustainable performance. Thermal gravimetric analysis (TGA) was widely used to study the thermal desorption of compounds from AC sorbents. Through the multi- heating rate (nonisothermal) or temperature-programmed desorption (TPD) method, the key kinetic information (or, “kinetic triplet”) – the activation energy, the pre-exponent factor and the kinetic model, can be determined using combined kinetic analysis of the TPD results.16 Some authors have studied desorption activation energy for hazardous gaseous VOCs other than PAHs, such as diphenylfuran on zeolites,17 dioxin on ACs18 and formaldehyde on Al2 O3 materials.19 In the present work, we focus on the desorption of PAHs from ACs. The kinetics of nonisothermal desorption of Nap and Ace on two types of AC was examined. Values of the kinetic parameters and kinetic model for the investigated desorption processes were determined by applying most commonly used methods. Furthermore, effects of pore size distribution of the sorbents on adsorption and desorption performances over different ACs are discussed.

2. EXPERIMENTAL SECTION 2.1. Preparation of PAHs loaded AC samples Naphthalene (with a purity of ≥ 99 %) and acenaphthene (with a purity of ≥ 98.5 %) were purchased from Sigma-Aldrich Chemical Co., Ltd. of Germany. Two commercial ACs prepared from bituminous coal (A-BAC SP, Kureha Chemical Co., Ltd, Japan) and 4


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coconut shell (ST1000, Nantong Carbon Co., Ltd, China) were selected and denoted as ACWY and ACNT in this work, respectively. Both raw ACs were sieved to the U.S. mesh size 80100, washed with deionized (DI) water to eliminate ash impurities, and dried at 383 K for 12 h to remove moisture. In order to remove most of the surface functional groups, heat treatment for the ACs was carried out in a horizontal tubular furnace by raising the temperature to 923 K in a flow of high-purity N 2 (150 mL/min) at 25 K/min. After holding at this temperature for 2 h, the heat-treated samples were cooled to room temperature and stored in ambient air that was essentially free of O 2 . To prepare the PAHs loaded sorbents, 50 mg of solid Nap and Ace were dissolved into 10 mL of methanol (HPLC grade) and mixed with 0.1 g of the above prepared ACWY and ACNT , respectively. After 12 hours’ loading, the samples were filtered and kept at room temperature for 24 h in air to remove the residual methanol. 2.2. Characterizations of ACs The textural characterization of the AC sorbents was performed using a surface area and porosimetric instrument (Autosorb-1, Quantachrome Instruments, USA). Prior to the measurement, samples were heated at 393 K and outgassed at this temperature under a vacuum of 10−5 Torr for at least 12 hours. The pore size distribution (PSD) ranging from 0.2 to 1.5 nm and from 1.5 to 10 nm were obtained using the non-local density functional theory (NLDFT) method based on the adsorption isotherms of CO2 at 273 K and N2 at 77 K, respectively.

The specific surface areas, SBET , were determined by the

Brunauer-Emmett-Teller (BET) method using N 2 adsorption data in a relative pressure 5


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range from 0.1 to 0.3. The micropore volumes, Vmicro , and the average micropore width, L, were obtained from CO 2 isotherms using the Dubinin- Radushkevich (DR) equation and the Stoeckli equation, respectively. The mesopore volumes, Vmeso , were estimated using the standard t-method applied to the N 2 adsorption data. 2.3. Thermogravimetric curve measurements The thermogravimetric nonisothermal experiments were performed using a TA Q500 instrument (TA Instruments, New Castle, DE, USA) from room temperature to 850 K at four heating rates (β = 8, 12, 16 and 20 K/min). In each run, 20 ± 5 mg of the PAH- loaded sample was placed on a platinum pan and purged with a pure-helium flow (20 mL/min). The curves of mass loss versus temperature (TG) were then recorded. The degree of desorption, α, can be defined as:



m0  mt m0  m f

(1)

where, m0 is initial weight, m t is weight at time t, and mf is final weight.

3. RESULTS AND DISCUSSION 3.1. Texture properties of ACWY and ACNT The PSD ranging from ~ 0.3 nm (accessible to CO 2 ) to 10 nm for the two carbon samples are shown in Figure 1 and texture parameters are shown in Table 1. Both ACs 6


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contain well developed microporosity occupying the majority of the total pore structure with only a small portion of mesoporosity. The coconut shell derived AC (AC NT ) contained considerably more micropore volume with pore diameter < 0.7 nm, while the bituminous coal derived AC (AC WY) had more larger pores with pore diameters > 0.7 nm. This to some extent contributes to larger SBET , and smaller L for ACNT as compared to ACWY. It can be concluded that the size distribution of microporosity should be the chief difference between the texture properties of two ACs. 3.2. TPD results of ACWY and ACNT The TPD results for the two PAHs on AC WY are shown in Figure 2, and those on ACNT are shown in Figure 3. As mentioned, these “pre- loaded” samples were obtained by adsorption of the PAHs from their solutions at room temperature, equilibrated after 12 hours, i.e., via batch adsorption. The equilibrium concentration of the solution can be calculated from mass balance, because the initial concentration was fixed (at 50 mg PAH/10 ml methanol) and the sorbent mass was also fixed (at 0.1 g). Blank tests for both sorbents were conducted, showing that impurities in the pre-treated AC samples volatilizing during the TGA processes can be neglected. As can be seen from the TG curves at all investigated conditions in the figures, the temperature range of sigmoid part for the residual mass, W, increases with increasing heating rate. The desorption amounts of Nap over both sorbents were higher than those of Ace corresponding to larger mass losses. The difference in Nap and Ace desorption amounts is larger for AC NT compared to ACWY. Three significant observations are obtained from these results. 7


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For each PAH-AC sample, four different heating rates were applied. The total weight loss was the same from four heating rates. This result indicates that complete weight loss was obtained for all samples below 800 K. On both carbons, the temperatures that were required for desorption were considerably higher for Ace than Nap. Thus, the binding energies for Ace on each carbon were higher than that of Nap, as expected from the larger molecular size of Ace (C 12 H10 ) than Nap (C 10 H8 ). The acenaphthene molecule is formed by adding an ethylene bridge to the two benzene rings of naphthalene. Thus, it has a larger size. The results in Figures 2 and 3 also show strong molecular sieving effects, in two ways. First, as mentioned, the binding energy for Ace on carbon is higher than that of Nap. At the same solution concentration, more Ace should have been adsorbed. On the contrary, less Ace was adsorbed on both ACs (even though mass balance showed higher equilibrium solution concentrations for Ace for both samples). Second, comparing the two ACs, the adsorbed amounts of both PAHs are higher for AC WY than ACNT (despite lower equilibrium solution concentrations for AC WY), clearly due to the larger pore sizes of ACWY, as shown in Figure 1. The close fitting in the micropores contributes significantly to strong binding energy. Thus, the desorption of the strongly bound PAHs on these carbons merit further kinetic analyses. Figure 4 shows the Nap and Ace desorption rates as a function of temperature from ACWY at different heating rates, respectively. The data for AC NT are presented in Figure 5. It can be seen that for both PAHs on either ACs, the increase in heating rate leads to the shift of the maximal desorption rate to the lower values and of the temperature for 8


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desorption rate peak, Tmax, to the higher values. Sharper desorption peaks and lower Tmax values for AC WY indicate that the desorption of PAHs from AC WY were faster and easier as compared to AC NT . These results could be also associated with the preference in adsorbing two PAHs on sorbents due to molecular sieving effects as discussed above 3.3. Identification of Kinetic Triplet The general equation for the solid state desorption reaction rate under nonisothermal conditions can be written as:20

d d   A  e  Ea / RT  f ( ) dt dT

(2)

where Ea is the activation energy; A is the pre-exponential factor of Arrhenius, and f(α) is the differential function for the reaction model; T is the absolute temperature; R is the gas constant; β is heating rate. In order to analyze the kinetics of the desorption processes, model fitting was performed in the conversion region where the activation energy was approximately constant and a single-step kinetic model could be used.21 Prior to fitting the nonisothermal desorption conversion data with potential kinetic models, initial calculations of activation energy, Ea,k, and the pre-exponential factor, Ak, are needed for references. The Kissinger method has been frequently used to determine these values as:22

9


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ln(

 2 max

T

)  ln(

Ak R Ea ,k 1 ) ( ) Ea ,k R Tmax

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(3)

where the Ea,k and the lnAk can be obtained by plotting ln(β/(Tmax )2 ) as a function of 1/Tmax . Since Kissinger method yields a reliable estimate of Ea only when the degree of the desorption reaction at the maximum desorption rate, αmax, does not practically vary with β, the latter condition must be checked by evaluating the αmax values.23 In our experiments, the αmax for all desorption processes almost kept unchanged with heating rate as shown in Table 2. Values of Ea,k and lnAk for each process are also listed. For the current nonisothermal experiments, the model fitting method involves fitting different models to αtemperature (αT) curves. One of the most popular method is based on the Coats and Redfern equation:24,25

ln(

 Ai R  Ai R  Ea ,i gi ( ) 2 RT  Ea ,i )  ln (1  )   ln     T2 Ea ,i  RT   Ea ,i   Ea ,i  RT

(4)

where the subscript “i” refers to the selected reaction model; gi(α) is the integral form for the selected kinetic model. The non-isothermal desorption data for Nap and Ace over ACWY and ACNT were fitted to each of the 16 recommended kinetic models as listed in Table 3. The dependence of ln[g(α)/T2 ] on 1/T for each investigated models should be linear. Six models, D3, AE1.5, AE2, AE3, AE4 and F2, are picked out because of their high 10


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squared correlation coefficient (r2 >0.98). Plotting the left-hand side of eq 4 versus 1/T gives Ea,i and lnAi from the slope and intercept for one model, respectively. Values of Ea,i, lnAi and r2 for each of these 6 models at each heating rate on AC WY and ACNT are given in Table S1 and S2 of the Supporting Information, respectively. As can be seen from the tables, the selected models showed great difference in the kinetic parameters, Ea,i and lnAi, although they all fit the experimental data well. Choosing the closest values of Ea,i and lnAi to the ones calculated by Kissinger method in Table 2 gives the best model for each investigated desorption case: AE2 for both Nap-on-ACWY and Ace-on-ACWY, AE4 for Nap-on-ACNT and AE3 for Ace-on-ACNT . It can be seen that all of the processes follow the Johnson-Mehl- Avrami (JMA) rate equation,26,27 g(α) = [ln(1-α)]n in integral form, which belongs to the nucleation and growth model type. The JMA equation has often been used to describe the reaction kinetics of phase transformation and decomposition, such as the rate of interlayer H2 O loss from a hydrotalcite during TPD. 28 The value of the exponent, n, in the JMA rate equation depends on the nucleation and growth mechanism.29 The average values of Ea (kJ/mol) and lnA (min-1 ) based on Coats and Redfern method are 65.14 and 13.29 for Nap-on-ACWY, 83.81 and 10.96 for Ace-on- ACWY, 101.4 and 15.90 for Nap-on-ACNT , 124.91 and 17.68 for Ace-on-ACNT , respectively. As compared to Ace, Nap could be more preferably desorbed on ACs with lower values of Ea due to its higher volatility or larger previous adsorption amount. ACNT exhibited higher values of Ea and A for desorption of PAHs indicating the stronger 11


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constrains by its abundance in narrow microporosity compared to AC WY. Entrapment of Nap and Ace in intraparticle micropores of the AC sorbents associated with different steric restriction or tortuosity suggests the release into bulk gas during desorption is controlled in different diffusion-limited nucleation growth rates depending on varied exponent n in the JMA rate equation. For ACWY samples with 1.5 < n < 2.5, the PAHs desorption correspond to process like all shapes growing from small dimensions with decreasing nucleation rate, while for AC NT samples with higher n > 2.5, all shapes grow from small dimensions with increasing nucleation rate.29 3.4. Validation of the obtained kinetic triplet To evaluate the accuracy of the potential reaction models, the invariant kinetic parameters (IKP) method is applied, making use of the so-called “compensation effect”.23,30 This is a pattern of kinetic behavior recognized within groups of related kinetic models. Based on this effect, 16 pairs of the activation energy (Ea,i) and pre-exponential factor (Ai ) for each model listed in Table 4 calculated using the Coats and Redfern method tend to demonstrate a strong correlation via:31

ln Ai  a  bEa,i

(5)

where a and b are constant parameters (compensation effect parameters). The parameters of eq 5 are a = lnk iso (an artificial isokinetic rate constant) and b = 1/RTiso (R is the gas constant and Tiso is an artificial isokinetic temperature). The compensation relationships 12


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(ln Ai vs Ea,i) at different heating rates for each desorption case are shown in Figure 6, with the values of the parameters shown in Table 4. With increasing the heating rate, the values of a, k iso and Tiso increase while the value of b decreases. The calculated values for Tiso from all four desorption conditions are within the experimental ranges. The compensation parameters (a* , b* ) are determined above using the relation for each heating rate. The straight lines ln Ai vs. Ea,i for several heating rates should intersect in a point which corresponds to the true values of the invariant activation parameters, Ainv and Ea,inv ,32,33 which can be evaluated using the following relation:

ln Ainv  a*  b* Ea,inv

(6)

which leads to the supercorrelation relation:

a*  ln Ainv  b* Ea,inv

(7)

The plots of a* versus b* for each investigated process give the values of Ea,inv and Ainv . On the other hand, to obtain better values of Ea,inv and Ainv, we need to test whether the process under study can be adequately described as single-step kinetics before applying the IKP method, since only in this case a single pair of Ea,inv and Ainv produced by this method

can

be

deemed

adequate.23

This

can

13


be

done

by

applying

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Kissinger-Akahira-Sunose (KAS) method

34

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capable of giving Ea and A at any given

extent of conversion (α). This method is actually a particular example of a more exact approximation of the temperature integral that leads to equation:35

ln(

 T

2

)  ln(

A R Ea , g ( )

)

E RT

(8)

For a given degree of the PAHs desorbed from the ACs, the plot ln(β/T2 ) versus 1/T should be a straight line giving the activation energy (Ea,α) from the slope. The Ea,α as a function of α for the investigated processes are shown in Figure 7. As can be seen from the figure, the Ea,α in all the cases did not vary significantly with α, indicating the processes are single-step step kinetics processes. As would be expected, this result in turn verifies the applicability of the model fitting method based on Coats and Redfern equation. It is apparent to observe the obtained invariant parameters (Ea,inv and Ainv) shown in Table 4 from the processes of Nap-on-ACWY, Ace-on-ACWY, Nap-on-ACNT and Ace-on-ACNT , are very close to those obtained using the Coats-Redfern method from models of AE2, AE2, AE4 and AE3, respectively. This to a large extent validate the kinetic parameters obtained from the model fitting with Coats-Redfern method. The use of the so-called “masterplots” has drawn increased attention as a tool for the determination of the solid-state desorption reaction model.20 Transforming the 14


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experimental kinetic data into masterplots in comparison to the theoretical masterplots obtained by different models provides a way to check the previously obtained model. The integrated form of the kinetic rate equation is obtained from eq 2 as follows:36

g ( )  



0

d AE  p ( x) f ( ) R

(9)

where x = E/RT and p(x) is the Arrhenius temperature integral based on the approximation formula of Tang et al:34,35

e x 1 p ( x)  ( ) x 1.00198882 x  1.87391198

(10)

From the integral kinetic equation at infinite temperature in integral form, eq 9, we can obtain the following equation using a reference point at α = 0.5:

g ( ) p( x)  g (0.5) p(0.5)

(11)

where x 0.5 = E/RT0.5 and T0.5 is the temperature corresponding to 50 % conversion. With knowledge of T as a function of α and the predetermined value of Einv, the master plots of p(x)/p(x 0.5 ) versus α from experimental data under different heating rates can be obtained. 15


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Comparisons of the experimental master plots for Nap-on-ACWY, Ace-on-ACWY, Nap-on-ACNT and Ace-on-ACNT with the theoretical plots of g(α)/g(0.5) against α corresponding to the obtained models (AE2, AE2, AE3 and AE4) are presented in Figure 6. Good agreements between the experimental data for both PAHs over ACWY and the calculated curve from the AE2 model could be found. For the cases over ACNT , masterplots of Nap and Ace agree well with the curves from the AE4 and AE3 models, respectively. Therefore, results of the masterplot graphic method can be a direct proof of the goodness of the model fitting results based on Coats and Redfern method. 3.5. Effect of PSD on PAHs desorption From the kinetic results obtained, the kinetic models for Nap and Ace are same on ACWY (AE2) and different on ACNT (AE4 and AE3, respectively). This to some extent indicates the desorption behaviors of Nap and Ace over ACWT were similar, in contrast to the situation on AC NT . The main reason for that is believed to be the apparent different PSD of two ACs, as the chemical properties can be ruled out here based on the fact that the heat-treated ACs had very small amount of surface groups, and that Nap and Ace have no strongly interacting functional groups. As can be seen from Figure 1, volume of narrow micropore is larger and of micropore from 0.7 nm to 2 nm is lower for AC NT compared to AC WY. As reported in the previous literature, the Nap adsorption is directly related to the narrow micropore volume where micropore filling occurs.10 Trapping by narrow micropores leads to tighter sorption of the two-ring PAHs which would lead to difficulty in desorption. This explains the harder 16


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desorption for both Nap and Ace on ACNT with higher values of Ea , Tmax and n than those on ACWY. On the other hand, compared to the AC WY, the ACNT with smaller amount of micropore (0.72 nm) tended to exhibit greater resistance in intrapore diffusion for Ace that is larger than Nap in kinetic diameter.10 Nap of 0.62 nm diffuses more quickly and smoothly through the scarce micropores (0.72 nm) of ACNT than Ace of 0.66 nm16 does, which leads to the sieving phenomenon as discussed earlier, or an interplay between molecular size and pore size.39 In this respect, Nap and Ace were observed to show different desorption kinetics over ACNT with different values of n in JMA kinetic equations. As would be expected, the ACWY with larger amount of micropores from 0.7 to 2 nm facilitates the desorption of both PAHs from the inner narrow micropores to the outer surface with weaker diffusional sieving effects on Ace, corresponding to the same kinetic model (AE2).

4. CONCLUSIONS Desorption kinetics of two-ring PAHs, Nap and Ace, on bituminous coal-based ACWY and coconut shell-based ACNT were studied on the basis of experimental desorption rates via nonisothermal TGA, respectively. By applying model fitting method, the basic kinetic information (or kinetic triplet) for each investigated process was obtained. The desorptions of Nap-on-ACWY, Ace-on-ACWY, Nap-on-ACNT and Ace-on-ACNT followed the nucleation and growth model regarding the JMA rate equation, g(α) = [ln(1-α)]n in integral form, with n = 2, 2, 4 and 3 (AE2, AE2, AE4 and AE3), respectively, and the corresponding Ea (kJ/mol) and lnA (min-1 ) were estimated to be 65.14 and 13.29, 83.81 17


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and 10.96, 101.4 and 15.90, 124.91 and 17.68, respectively. The dependence of activation energy on the degree of desorption using the KAS method suggested the processes under study were single-step desorption kinetics. The accuracy of the obtained kinetic parameters and kinetic models was further tested and validated using the IKP method and masterplot graphic method, respectively. By JMA rate equation, PAHs desorption kinetics is found to be dependent on the PSD of sorbents. The difference in the exponent value (n) indicates different desorption behavior for Nap (AE4) and Ace (AE3) on ACNT . ACNT of smaller amount of pores from 0.7 to 2 nm tended to exhibit greater diffusion resistance on Ace that is larger than Nap in kinetic diameter. On the other hand, larger amount of the narrow micropore (pore size < 0.7 nm) on ACNT offered tighter sorption of the PAHs corresponding to a higher Ea , as compared to AC WY. More detailed elucidation of this effect by using other types of sorbents is greatly desired in future work. The current obtained kinetic triplet capable of describing the two-ring PAHs desorption over commonly used ACs are expected to provide references for the selection of appropriate sorbents and the application of PAHs removal facilities in terms of operation, sorption capacity, and regeneration ability.

ACKNOWLEDGMENTS The financial support of the Natural Science Foundation of China (Grant 51478038) is gratefully acknowledged.

18


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References (1) Polynuclear Aromatic Compounds, Part 1. International Agency for Research on Cancer (IARC); Chem., Environm. and Experimental Data: Lyon, France 1983. (2) Lee, M. L.; Novotny, M.; Bartle, K. D. Academic Press: New York. Anal. Chem. Polycyclic Aromatic Compounds, 1981. (3) Marr, L. C.; Kirchstetter, T. W.; Harley, R. A.; Miguel, A. H.; Hering, S. V.; Hammond, S. K. Environ. Sci. Technol. 1999, 33, 3091. (4) Zimmermann, R.; Heger, H. J.; Kettrup, A. Fresenius J. Anal. Chem. 1999, 363, 720. (5) Kirton, P. J.; Crisp, P. T. Fuel 1990, 69, 633. (6) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; López, J. M.; Navarro, M. V. Energy Fuels 2004,18, 202-208. (7) Zhou, J.; Yang, B.; Li, Z.; Lei, L.; Zhang, X. Ind. Eng. Chem. Res. 2015, 54, 2329. (8) Cudahy, J. J.; Helsel, R. W. Waste Manage. 2000, 20, 339. (9) Mastral, A. M.; Garcıá, T.; Calleń, M. S.; Navarro, M. V. Energy Fuels 2001, 15, 1. (10) Mastral, A. M.; Garcıá, T.; Calleń,M. S.; Navarro, M. V.; Galvan J. Environ. Sci. Technol. 2001, 35, 2395. (11) Hsieh, C. T.; Teng, H. Carbon 2000, 38, 863-869. (12) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V. Energy Fuels 2002, 16, 510-516. (13) Mastral, A. M.; Garcia, T.; Murillo, R.; Callén, M. S.; Lopez, J. M.; Navarro, M. V.; Galbán, J. Energy Fuels 2003, 17, 669-676. (14) Mastral, A. M.; Garcia, T.; Callen, M. S.; Lopez, J. M.; Navarro, M. V.; Murillo, R.; Galban, J. Environ. Sci. Technol. 2002, 36, 1821-1826. (15) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V. Energy Fuels 2002, 16, 205-210. (16) Perez-Maqueda, L. A.; Criado, J. M.; Gotor, F. J.; Malek, J. J. Phys. Chem. A 2002, 106, 2862-2868. (17) Xi, H. X.; Li, Z.; Zhang, H. B.; Li X.; Hu X. J. Sep. Purif. Technol. 2003, 31, 41-45. (18) Yang, R.T.; Long, R.; Joel, P.; Akira, T. Ind. Eng. Chem. Res. 1999, 38, 2726. (19) Chen, D.; Qu, Z.; Sun, Y.; Wang, Y. Colloids Surf. A 2014, 441, 433-440. (20) Gotor, F. J.; Criado, J. M.; Malek, J.; Koga, N. J. Phys. Chem. A 2002, 104, 19


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10777-10782. (21) Monazam, E. R.; Spenik, J.; Shadle, L. J. Energy Fuels 2013, 28, 650-656. (22) Kissinger, H. E. Anal. Chem. 1957, 29, 1702–1706. (23) Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Pérez-Maqueda, L. A.; Popescu, C.; Sbirrazzuoli, N. Thermochim. Acta 2011, 520, 1-19. (24) Coats, A. W.; Redfern, J. P. Nature 1964, 201, 68. (25) Coats, A. W.; Redfern, J. P. J. Polym. Sci. Part B: Polym. Lett. 1965, 3, 917. (26) Johnson, W.A.; Mehl, R.F., Trans. Am. Inst. Min. Met. Eng. 1939, 135, 416. (27) Avrami, M., J. Chem. Phys. 1941, 9, 177. (28) Drewien, C. A.; Tallant, D. R.; Eatough, M.O., J. Mater. Sci. 1996, 31, 4321. (29) Christian, J. W. The Theory of Transformations in Metals and Alloys, 3rd ed.; Pergamon: Oxford, UK, 2002; pp 546. (30) Janković, B.; Adnađević, B.; Jovanović, J. Thermochim. Acta 2007, 452, 106-115. (31) Budrugeac, P. J. Therm. Anal. Calorim. 2007, 89, 143-151. (32) Budrugeac, P.; Criado, J. M.; Gotor, F. J.; Malek, J.; Pérez‐Maqueda, L. A.; Segal, E. Int. J. Chem. Kinet. 2004, 36, 309-315. (33) Lesnikovich, A. I.; Levchik, S. V. J. Therm. Anal. 1983, 27, 83. (34) Akahira, T.; Sunose, T. Res. Rep. Chiba Inst. Technol. 1971, 16, 22. (35) Agrawal, P. K. Thermochim. Acta 1992, 203, 93. (36) Ozawa, T. Bull. Chem. Soc. Jpn. 1965, 38, 1881. (37) Tang, W. J.; Liu, Y. W.; Zhang, H.; Wang, C. X. Thermochim. Acta 2003, 408, 39. (38) Tang, W. J..; Chen, D. H.; Wang, C. X. AIChE J. 2006, 52, 2211. (39) Su, Y. C.; Kao, H. M.; Wang, J. L. J. Chromatogr. A 2010, 1217, 5643-5651.

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Energy & Fuels

Table Table 1 Texture Properties of ACWY and ACNT sample ACWY ACNT a

S BET (m2 /g) 1175

Vmicro (cm3 /g) 0.337

Vmeso (cm3 /g) 0.102

L (nm) 1.38

1331

0.361

0.085

1.15

SBET, BET surface area; V micro , micropore volume; V meso , mesopore volume; L, average micropore

width.

Table 2 The changes of the Tmax and αmax with heating rate and kinetic parameters obtained by Kissinger method. samples Nap on ACWY

Ace on ACWY

Nap on ACNT

Ace on ACNT

β (K/min)

Tmax (K)

αmax

8

505.2

0.368

12

516.3

0.370

16

527.1

0.377

20 8

534.4 569.9

0.379 0.386

12

582.2

0.389

16

589.7

0.388

20

599.1

0.390

8

515.3

0.428

12

522.4

0.426

16

529.3

0.424

20 8

534.8 613.6

0.422 0.471

12 16

623.4 630.4

0.479 0.475

20

636.4

0.478

E a (kJ/mol)

lnA (min-1 )

60.49

12.95

80.97

11.58

97.99

17.77

120.35

18.32

21


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

Table 3 Algebraic expressions of f(α) and g(α) for the reaction models considered in the present work. symbol

model

f(α)

g(α) α2

diffusion models D1

1-D diffusion

(1/2)α-1

D2 D3

2-D diffusion 3-D diffusion

[-ln(1-α)] (3/2) (1-α)4/3 [(1-α)-1/3 -1]-1

-1

α+(1-α)ln(1-α) [(1-α)-1/3 -1]2

(Zhuralev-Lesokin-Tempelman equation) Am

nucleation models 1-1/m random nucleation and growth of nuclei m(1-α)[-ln(1-α)]

AEn

(JMA equation; m = 2, 1.5 and 4/3) random nucleation and growth of nuclei

(1/n)(1-α)[-ln(1-α)]1-n

P4

(JMA equation; n = 1.5, 2, 3 and 4) Power law (2/3)α-1/2 geometrical contraction models

R2

Phase-boundary controlled reaction

2(1-α)

1/2

[-ln(1-α)]

1/m

[-ln(1-α)]n α3/2 1-(1-α)

1/2

(contracting area, i.e., bidimensional shape) R3

Phase-boundary controlled reaction

3(1-α)2/3

1-(1-α)1/3

(contracting area, i.e., tridimensional shape) reaction-order models F0 (R1) F1 (A1)

zeroorder firstorder

1 1-α

α -ln(1-α)

F2

secondorder

(1-α)2

(1-α)-1 -1

22


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Energy & Fuels

Table 4 The values of the compensation relationships parameters for different heating rates. Samples Nap on ACWY

Ace on ACWY

Nap on ACNT

Ace on ACNT

β (K/min)

a

b

Tiso (K)

103 k iso

8

-7.636

0.2346

512.7

0.483

12

-7.260

0.2298

523.4

0.703

16

-7.022

0.2240

537.0

0.892

20

-6.825

0.2220

541.7

1.088

8

-6.620

0.2025

593.8

0.499

12

-6.259

0.1982

606.9

0.716

16

-5.996

0.1942

619.2

0.932

20 8

-5.795 -8.189

0.1917 0.2389

627.4 503.4

1.140 0. 278

12

-7.844

0.2361

509.4

0.392

16

-7.541

0.2333

515.4

0. 531

20

-7.331

0.2309

521.0

0. 655

8

-7.793

0.2012

597.8

0. 413

12

-7.418

0.1989

604.7

0. 601

16

-7.141

0.1948

617.5

0. 792

20

-6.911

0.1949

617.2

0. 997

23


E inv,a (kJ/mol)

lnA inv (min-1 )

60.48

10.67

75.18

12.71

106.78

21.44

115.24

19.53

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nomenclature A pre-exponential factor of Arrhenius calculated by Coats and Redfern equation (min-1 ) Ak pre-exponential factor of Arrhenius calculated by Kissinger method (min -1 ) A inv α

pre-exponential factor of Arrhenius calculated by IKP method (min -1 )

αmax β

The degree of desorption the degree of the desorption for the desorption rate peak heating rate (K/min)

Ea

activation energy calculated by Kissinger method (kJ/mol)

E a,k E a,inv

activation energy calculated by Coats and Redfern equation (kJ/mol) activation energy calculated by IKP method (kJ/mol)

E a,α Ak

activation energy at any given extent of conversion (α) calculated by KAS method (kJ/mol) micropore volumes (cm3 /g)

k iso L

artificial isokinetic rate constant average micropore width (nm)

m0 mt mf

initial weight of the sorbent (mg) weight of the sorbent at time t (mg) final weight of the sorbent (mg)

n p(x)

Exponent of the JMA rate equation Arrhenius temperature integral (K)

SBET t T

BET surface area (m2 /g) desorption time (min) Desorption temperature (K)

Tmax Tiso

temperature for desorption rate peak (K) artificial isokinetic temperature (K)

T0.5

temperature corresponding to 50% conversion (K)

Vmeso , Vmicro , W

mesopore volume (cm3 /g) micropore volume (cm3 /g) the residual mass of the sorbent (%)

24


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Energy & Fuels

Figure captions Figure 1. Pore size distribution for the sorbents of AC WY and ACNT . Figure 2. The TG curves of Nap and Ace sorbent obtained at different heating rates, 8, 12, 16 and 20 K/min for ACWY. Figure 3. The TG curves of Nap and Ace sorbent obtained at different heating rates 8, 12, 16 and 20 K/min for ACNT . Figure 4. The Nap and Ace desorption rates as a function of temperature on ACWY Figure 5. The Nap and Ace desorption rates as a function of temperature on ACNT Figure 6. The compensation relationships (ln Ai vs Ea,i) at different heating rates for (a) Nap-on-ACWY; (b) Ace-on-ACWY; (c) Nap-on-ACNT ; (d) Ace-on-ACNT . Figure 7. The dependence of Ea ,α and ln Aα versus degree of PAHs desorptions from ACs using KAS method. Figure 8 Master plots of g(α)/g(0.5) on versus α, for the JMA models (AE1.5, AE2, AE3 and AE4) and experimental masterplots at each condition.

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Incremental pore volume (cm3/g/nm)

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Page 26 of 33

1.2 ACWY ACNT

0.8

0.4

0 0.1

1

10

Pore width (nm) Figure 1

2


Page 27 of 33

100

Residual mass, W (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

90 Blank-ACWY 12 K/min

Nap-on-ACWY 8 K/min 12 K/min 16 K/min 20 K/min

80

Ace-on-ACWY 8 K/min 12 K/min 16 K/min 20 K/min

70 300

400

500

600

700

800

900

T (K) Figure 2

3


Energy & Fuels

100

96

Residual mass, W (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

92

Blank-ACNT 12 K/min

Nap-on-ACNT 8 K/min 12 K/min 16 K/min 20 K/min

88

Ace-on-ACNT

84

8 K/min 12 K/min 16 K/min 20 K/min

80 300

400

500

600

700

800

900

T (K) Figure 3

4


Page 29 of 33

0.2 Nap on ACWY

Ace on ACWY


0.16

dWdT (%K-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


0.12

0.08

0.04

0 300

400

500

600

700

800

900

T (K) Figure 4

5


Energy & Fuels

0.1 Nap on ACNT

Ace on ACNT 8 K/min 12 K/min 16 K/min 20 K/min


0.08

dWdT (%K-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

0.06

0.04

0.02

0 300

400

500

600

700

800

900

T (K) Figure 5

6


Page 31 of 33

30

40

Nap on ACWY


30

lnA (min-1)

lnA (min-1)

Ace on ACWY


20

10

20

10

0 0

-10

-10 0

40

80

120

0

160

40

80

EkJ/mol)

120

160

200

160

200

EkJ/mol)

(a)

(b) 30

20 Nap on ACNT

Ace on ACNT


15


20

lnA (min-1)

10

lnA (min-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5

10

0

0 -5

-10

-10 0

40

80

120

0

40

EkJ/mol)

80

120

EkJ/mol)

(c)

(d) Figure 6

7


Energy & Fuels

200 Nap on ACWY Ace on ACWY Nap on ACNT

160

Ace on ACNT

Ea,kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

120

80

40

0 0

0.2

0.4



0.6

0.8

1 Figure 7

8


Page 33 of 33

16 AE4

ACWY: Naphthalene

Acenaphthalene 8 K/min 12 K/min 16 K/min 20 K/min


12

g()/g(0.5)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

AE3

AE2 ACNT: Naphthalene

8

Acenaphthalene



AE1.5

4

0 0

0.2

0.4



0.6

0.8

1 Figure 8

9


Desorption Kinetics of Naphthalene and Acenaphthene over Two

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