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Oxygen-rich porous carbon derived from biomass for mercury removal: an experimental and theoretical study Fenghua Shen, Jing Liu, Zhen Zhang, Yuchen Dong, Yingju Yang, and Dawei Wu Langmuir, Just Accepted Manuscript • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018
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Oxygen-rich porous carbon derived from biomass for mercury removal: an experimental and theoretical study Fenghua Shen, Jing Liu, Zhen Zhang, Yuchen Dong, Yingju Yang, Dawei Wu State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ABSTRACT: A porous carbon was synthesized by the combination of freeze-drying and CO2 activation from starch. Non-thermal plasma was employed to quickly produce oxygen functional groups on porous carbon surface. The plasma treatment has negligible effect on the textural properties of the porous carbon. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses suggested that the plasma treatment significantly increased the amount and promoted the evolution of oxygen groups on surface. The unique pore structure of porous carbon was proven favorable to effective oxygen loading. The elemental mercury (Hg0) adsorption ability of the oxygen enriched porous carbon was tested. The results indicated that the oxygen-rich porous carbon constitutes an effective sorbent for Hg0 removal. The excellent textural properties, surface atomic oxygen concentration and the type of oxygen group are the three key factor for realizing high Hg0 removal performance. Density functional calculations were performed to understand the effect mechanism of oxygen groups on Hg0 adsorption. Carbonyl and ester are beneficial for Hg0 adsorption, whereas epoxy, carboxyl and hydroxyl inhibit Hg0 adsorption. Plasma treatment enhances Hg0 adsorption by increasing the amount of ester and carbonyl groups on surface. Keywords: Porous carbon; Non-thermal plasma; Biomass; Mercury removal; Density functional theory
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INTRODUCTION Anthropogenic mercury emissions from coal combustion, waste incineration and metal smelting have attracted particular attention because of its toxicity and bioaccumulation in food chains.1, 2 The new Emission Standard of Air Pollutants for Thermal Power Plants (GB1323-2011) issued by Chinese government has limited the mercury emission to air.3, 4, 5 In 2017, the Minamata Convention on Mercury has been ratified to protect human health and the environment from the dangers of mercury.6, 7, 8 Thus, it is of great importance to reduce the mercury emission. The mercury species in exhaust smoke contain three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hgp).9 Hg2+ and Hgp can be effectively removed by current air pollution control devices. However, Hg0 is difficult to be removed because of its insolubility in water and high volatility.10, 11, 12 Therefore, Hg0 is the main mercury species emitted into the atmosphere.13, 14, 15
At present, Hg0 removal by activated carbon injection is the most promising technology.16
Particularly, activated carbons impregnated with chloride and bromide have displayed high Hg0 removal ability.17, 18 However, the adsorption of Hg0 on such chemical impregnated activated carbons leads to the formations of HgCl2 and HgBr2, which are more toxic than Hg0.19, 20 In addition, the high cost of the production of chemical impregnated activated carbons still limits their large-scale practical utilization.21 To develop efficient and cost-effective sorbents for Hg0 removal, many efforts have been paid on prepare biochar from widely available inexpensive precursors.22 23 Although the biochar is low-cost, its Hg0 removal ability is poor. It has been reported that steam activation could improve the surface area of biochar, thereby enhancing the physisorption of Hg0.24 The improving in surface area is also beneficial for chlorides loading, which can provide more active sites for Hg0 adsorption.25 Antuña-Nieto et al.26 ACS Paragon Plus Environment
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prepared activated carbon foam loaded with gold for Hg0 removal, and they found that the activated carbon foam possessing excellent textural properties was a fine support for gold loading, which was favorable to Hg0 adsorption. Yang et al.27 suggested that activated carbon fiber with large surface area was beneficial for the well-dispersion of iron-manganese binary oxide particles, which favored the Hg0 adsorption. The large surface area and pore volume of a material can improve the dispersion of active agent on surface, and facilitate the effective collision of Hg0 with such active agent. Therefore, it is meaningful to develop biomass-derived carbon materials with excellent textural properties for Hg0 removal. In this aspect, biomass-derived porous carbon provides a promising candidate.28 The large surface area and developed pore structure of porous carbon are very attractive for active agent loading and the following effective Hg0 removal. In addition, the heteroatoms (O, S, etc.) or functional groups on carbon surface can improve the Hg0 adsorption.29 Ideally, the oxygen enriched porous carbon from biomass is desirable for effective and low-cost Hg0 removal. Nevertheless, the conventional chemical impregnation for functionalization is generally costly and time-consuming. A strong alternative approach is to employ the non-thermal plasma technique directly producing oxygen groups on carbon surface.30, 31 However, few studies have comprehensively considered the advantages of porous carbon possessing excellent textural properties in plasma functionalization and Hg0 removal. The generation and evolution processes of oxygen groups on porous carbon surface during plasma treatment, as well as the roles of different oxygen groups in Hg0 adsorption have not been investigated in detail. In this work, a convenient method based on the freeze-drying coupled with CO2 activation was developed for porous carbon preparation from starch. Non-thermal plasma technology was utilized to ACS Paragon Plus Environment
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produce oxygen groups on carbon surface. Fourier transform infrared spectra and X-ray photoelectron spectroscopy were employed to characterize the oxygen groups on surface. The Hg0 adsorption abilities of the prepared oxygen-rich porous carbons were tested, and the roles of different oxygen groups in Hg0 adsorption were investigated. Moreover, density functional theory were employed to provide a better molecular-level understanding of Hg0 adsorption on the oxygen-rich porous carbon. EXPERIMENTAL AND COMPUTATIONAL METHODS Preparation of porous carbon The porous carbon was prepared by the combination of freeze-drying and CO2 activation.32 Typically, starch (20 g) was dissolved in deionized water (100 ml) under stirring in a water bath at 90 o
C for 0.5 h. The resulting gelatinized product was cooled down and kept at room temperature for 6 h.
Then, the gel was freeze-dried for 72 h to obtain a porous scaffold. Subsequently, the porous scaffold was carbonized at 600 oC for 3 h (5 oC/min heating rate) under N2 flow. To further improve the porosity, the obtained carbon was activated at 900 oC for 3 h under CO2 flow (100 ml/min). Finally, the original porous carbon (denoted as PC) was obtained. Modification of sorbents by non-thermal plasma The experimental setup of non-thermal plasma treatment is showed in Figure 1(a). The porous carbon were treated by non-thermal plasma in a dielectric barrier discharge reactor. This reactor was consisted of two quartz parallel plates and two stainless steel electrodes. 0.2 g sample was loaded in the reactor during the plasma treatment process. The original PC was treated by non-thermal plasma in air for 30 s (PC-30), 45 s (PC-45) and 60 s (PC-60), respectively. In addition, to demonstrate the advantage
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of porous carbon with excellent textural properties in functionalization for Hg0 removal, a commercial activated carbon (AC) from coconut shell was utilized and treated by non-thermal for 30 min (AC-30m).
(b) Hg0 adsorption Sorbent bed Computer MFC Mercury analyzer
N2 Reactor MFC Mercury generator
NaOH solution
Silica-gel desiccant
Figure 1. (a) Non-thermal plasma treatment setup; (b) Experimental setup for Hg0 adsorption. Characterization of porous carbon The surface morphology and microstructure of the prepared porous carbons were examined by scanning electron microscope (SEM, Zeiss sigma 300) and transmission electron microscopy (TEM, Talos F200X). The specific surface area and pore size distribution were measured at 77 K by using a Micromeritics ASAP 2020 analyzer. Fourier transform infrared (FT-IR) spectroscopy (500-4000 cm-1) was recorded using a Nicolet FT-IR 6700 spectrometer. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) ACS Paragon Plus Environment
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was employed to analysis the surface chemistry of the prepared porous carbons. The binding energy was corrected by setting C1s to 284.8 eV. Mercury adsorption experiments The Hg0 adsorption abilities of porous carbons were tested under N2 atmosphere in a fixed-bed reactor, as shown in Figure 1(b). The experiments aimed to evaluate the Hg0 adsorption ability of oxygen-rich porous carbon, and clarify the roles of oxygen groups in Hg0 adsorption. An Hg0 permeation tube within a U-type glass tube, which was constantly heated by a thermostat water bath, was employed to generate Hg0 vapor. The inlet Hg0 concentration was 50 µg/m3, which was carried by N2. In each test, 0.1 g sorbent was used and the flow rate of N2 fed into the reactor was 1 L/min, which corresponded to a gas hourly space velocity (GHSV) of approximately 76000 h-1. The inlet Hg0 concentration (Hg0in ) and outlet Hg0 concentration (Hg0out ) were measured continuously by an on-line mercury analyzer (Lumex RA-915М). The Hg0 removal efficiency (ηi) is expressed as: ηi =
Hg0in -Hg0out Hg0in
×100%
(1)
Computational methods The geometry optimizations and energy calculations were performed by Gaussian 09.33 Density functional theory (DFT) has been employed, and full geometric optimizations and energy calculations were performed at B3PW91 level of theory.34 Effective core potential was used to replace the inner electrons of Hg, as a result of the large amount of electrons in heavy elements.35 RCEP60VDZ was employed for Hg atom, and the 6-31G(d) basis set was used for C, O and H. In addition, a five-ring cluster model was used to simulate the structure of porous carbon. The upper side carbon atoms in this ACS Paragon Plus Environment
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model were unsaturated to simulate the active sites and the carbon atoms on the other sides were terminated with hydrogen atoms.32 Five oxygen functional groups, including epoxy, carboxyl, hydroxyl, carbonyl and ester, were considered in this study.
RESULT AND DISCUSSION Structural and chemical properties of the prepared porous carbons SEM and TEM reveal the pore morphology of the prepared porous carbons, as shown in Figure 2. The original PC exhibits morphology of irregular shaped particle with smooth surface (Figure 2a). After the non-thermal plasma treatment, the surface of PC-60 becomes rough (Figure 2c). This reveals that non-thermal plasma treatment can sculpture the surface of porous carbon, which is owing to the reaction of oxygen radicals with surface carbon atoms. The TEM images (Figure 2b and 2d) reveal the presence of abundant nanopores in PC and PC-60. It seems that non-thermal plasma has negligible effect on the porosity of porous carbon.
Figure 2. SEM and TEM images for porous carbons: (a, b) PC; (c, d) PC-60. ACS Paragon Plus Environment
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The microstructure of PC and PC-60 were analyzed by N2 adsorption/desorption isotherms as shown in Figure 3. According to IUPAC classification, PC and PC-60 exhibit the type-IV isotherm that is characteristic for mesoporous materials.36 The pore size distribution is mainly centralized at about 5 nm. This reveals that the prepared porous carbon mainly contains mesopores. The N2 adsorption/desorption isotherm and pore size distribution of PC-60 is very close to that of PC. This indicates that the nonthermal plasma treatment has negligible effect on the microstructure of porous carbon. The specific surface area of original PC is 1143 m2/g. The total pore volume of PC is 0.91 cm3/g. The mean pore size of PC is 4.97 nm. After non-thermal plasma treatment, the specific surface area of PC-60 is 1168 m2/g, the total pore volume is 0.97 cm3/g, and the mean pore size is 4.86 nm. The combination of freezedrying and CO2 activation provides a promising method for porous carbon preparation without any hard template. The large surface area and developed pore structure of the prepared porous carbon are beneficial for the quick formation of oxygen groups during the plasma treatment, and the following Hg0 adsorption. 800
Quantity Adsorbed (cm3/g)
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
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700
(a) N2 adsorption/desorption isotherm PC PC-60
2.0
(b) Pore size distribution PC PC-60
1.5 600 500
1.0
400
0.5 300 200 0.0
0.2
0.4
0.6
Relative Pressure (P/Po)
0.8
1.0
0.0
0
20
40
60
80
100 120 140
Pore Diameter (nm)
Figure 3. (a) N2 adsorption/desorption isotherms for PC and PC-60; (b) pore size distributions of PC and PC-60. ACS Paragon Plus Environment
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The Hg0 adsorption ability of sorbents is closely related with their surface chemistry.37,
38
Understanding the variation of surface oxygen groups is thus essential for clarifying the Hg0 adsorption on oxygen enriched porous carbon. The FT-IR analyses were conducted to identify the oxygen groups on porous carbon surface. The FT-IR spectra of the prepared porous carbons is showed in Figure 4a. The spectral peaks are weak for the spectrum of raw PC. In contrast, the plasma functionalized samples exhibit broader and overlapping bands. The characteristic peak at about 833 cm-1 is ascribable to the CO-C stretching vibration, which belongs to ester and epoxy groups. Bands at 1023-1745 cm-1 are associated with O=C-O, C=O, C-O and C=C bonds.39, 40 The peaks at 2854 and 2921cm-1 are ascribed to C-H stretching vibration, which associate with hydrocarbon.41 The broad band around 3431 cm-1 is attributed to the presence of O-H stretching vibration in hydroxyl and carboxyl groups.42 The relative intensity of these bands in plasma treated samples are higher than those of the raw PC, indicating that more oxygen groups are introduced when the plasma treatment have been done. The appearances of these spectral peaks are clearly attributed to the reactions of oxygen radicals, which are generated by non-thermal plasma, with the carbon atoms on PC surface. It thus can be concluded that porous carbon can gain more oxygen groups by the oxidizing effects of non-thermal plasma. The increment of oxygen groups on PC surface is attractive because of the generation of new active sites, which is beneficial for Hg0 adsorption. The XPS analyses were further conducted to identify the oxygen groups on porous carbons surface. The XPS survey spectra of porous carbons displays two distinct carbon (C1s) and oxygen (O1s) peaks, as shown in Figure 4b. The peak intensity of O1s is enhanced obviously after the non-thermal plasma treatment, thereby implying a significant increment of the oxygen groups. The atomic concentration of ACS Paragon Plus Environment
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oxygen on PC surface increases with the increasing of treatment time. The active oxygen radicals can effectively react with carbon atoms on PC surface during the process of non-thermal plasma treatment, which will generate abundant oxygen groups. Therefore, the percentage of O1s increases substantially upon non-thermal plasma treatment. (b) XPS survey spectra
(a) FT-IR spectra 1023
1745
833
2854
2921
3431
PC-45
Counts (s)
PC-60
Transmittance (%)
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PC-30
Atom (%) C1s 77.65 O1s 22.35
C1s
O1s
Atom (%) C1s 80.22 O1s 19.78
C1s
O1s
Atom (%) C1s 81.80 O1s 18.20
C1s
Atom (%) C1s 95.55 O1s 4.45
C1s
PC-60 PC-45
O1s
PC-30
PC
O1s
PC 500
1000
1500
2000
2500
3000
3500
0
4000
Wavenumber (cm-1)
200
400
600
800
1000
1200
Binding energy (eV)
Figure 4. (a) FT-IR spectra of porous carbons; (b) XPS survey spectra of porous carbons. The C1s XPS spectrums of PC, PC-30, PC-45 and PC-60 are showed in Figure 5. Four different peaks centering at 284.80, 285.79, 286.64 and 288.81 eV are corresponded to C-C in aromatic rings, CO in hydroxyl and epoxy groups, C=O in carbonyl group, and O=C-O in carboxyl and ester groups.43, 44 The relative content of each groups is listed in Table 1. The amount and ratio of each oxygen groups vary widely. The content of O=C-O group increases after plasma treatment, whereas the contents of CO and C=O groups may decline. The results suggest that C-O and C=O groups can be converted into the other oxygen groups by the oxidization effect of plasma. The plasma treatment plays double roles in both increasing the total amount of oxygen groups on surface and adjusting the proportion of different oxygen groups.
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Table 1. Surface functional groups from XPS C1s. Electron binding Functional groups
PC
PC-30
PC-45
PC-60
energy (eV) C-C
284.80
77.69
75.02
68.99
64.96
C-O
285.79
11.07
7.66
10.86
12.09
C=O
286.64
6.89
6.66
9.99
11.33
O=C-O
288.81
4.34
10.65
10.14
11.61
(b) PC-30
(a) PC
O-C=O
292
290
C-C Intensity
Intensity
C-C
C-O C=O
288
286
284
282
C-O O-C=O C=O
280 292
290
Binding Energy (eV)
288
286
284
C-C
Intensity
C-O O-C=O C=O
290
280
(d) PC-60 C-C
292
282
Binding Energy (eV)
(c) PC-45
Intensity
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
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288
286
284
282
C-O O-C=O C=O
280 292
Binding Energy (eV)
290
288
286
284
282
280
Binding Energy (eV)
Figure 5. The XPS spectra of C1s of porous carbons: (a) PC; (b) PC-30; (c) PC-45; (d) PC-60. ACS Paragon Plus Environment
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Based on the results of FT-IR and XPS analyses, the generation and evolution of oxygen groups during the processes of non-thermal plasma treatment can be explained by reactions R1-R7: C-C + O* → C-O-C
(R1)
C + O* → C=O
(R2)
C-H + O* → C-OH
(R3)
O=C-C + O* → O=C-O-C
(R4)
C-O-C + O* → O=C-O-C
(R5)
O=C-H + O* → O=C-OH
(R6)
C-OH + O* → O=C-OH
(R7)
Oxygen radicals (O*) are formed during the process of non-thermal plasma treatment. The C-C and C-H species can react with O* by forming C-O-C, C=O and C-OH groups (Reactions R1-R3). The increment in the amount of oxygen groups is owing to R1-R3. Along with the formation of new oxygen groups, the C-O-C, C=O and C-OH species can be converted into O=C-O-C and O=C-OH groups (Reactions R4-R7). As a results, the proportion of different oxygen groups on surface changes by nonthermal plasma treatment. Moreover, porous carbon can be quickly functionalized by the non-thermal plasma within tens of seconds, which is obviously shorter than that of activated carbon.45, 46, 47
Hg0 removal performance Figure 6 shows the Hg0 adsorption efficiency of the prepared porous carbons at 50 and 120 oC. The plasma treated porous carbons show significant Hg0 removal efficiency, which is obviously higher than the raw PC. The structural analyses suggest that plasma treatment has negligible effect on the textural properties of porous carbon. Therefore, the better Hg0 removal performance of plasma functionalized ACS Paragon Plus Environment
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porous carbon is due to the presence of oxygen groups. The Hg0 adsorption ability of PC-60 is better than PC-30 and PC-45, which is owing to the higher surface oxygen content. The plasma treatment in air with a very short time significantly increased the Hg0 adsorption ability of porous carbon. Sixty seconds is the optimal time for plasma treatment. The above observations reveal that the oxygen enriched porous carbon constitute an effective sorbent for Hg0 removal. The Hg0 removal efficiencies of the plasma treated porous carbons decrease with the increasing of temperature. This means that some oxygen groups are useless for Hg0 adsorption at higher temperature. It has been reported that the Hg0 adsorption on activated carbon depended on the co-effects of different types of active sites, and the active sites with higher adsorption energy were accounting for the Hg0 adsorption at higher temperature.48 Thus, the activity of the oxygen groups for Hg0 adsorption is closely related with their type. PC-30, which has higher Hg0 removal efficiency than the raw PC, significantly has more O=C-O group and less C-O group. This reveals that O=C-O group is able to improve Hg0 adsorption, whereas C-O group does just the opposite. Upon increasing the treatment time, PC-45 gains more C=O and C-O group. The Hg0 removal efficiency of PC-45 is higher than that of PC-30. This indicates that C=O group can also enhance Hg0 adsorption, which in turn counteracts the negative effect of C-O group. Moreover, PC-60 possesses more effective O=C-O and C=O groups than that of PC-30 and PC-45, thereby resulting the better Hg0 removal performance. In addition, the Hg0 removal ability of the commercial activated carbon before and after plasma treatment were tested, as illustrated in Figure 6. Although the atomic concentration of oxygen on raw AC surface (19.22 %) is higher than that on raw PC surface (4.45 %), the Hg0 adsorption ability of AC is poor than that of PC. This is due to the surface area (163 m2/g) and pore volume (0.089 cm3/g) of AC ACS Paragon Plus Environment
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are significantly lower than PC, demonstrating the important role of textural properties in Hg0 removal. The atomic concentration of oxygen on AC-30m surface is 28.79 %, which is higher than that on the plasma treated porous carbon surface (Figure 4b). However, the Hg0 adsorption ability of AC-30m is poor than that of oxygen-rich porous carbons at both 50 and 120 oC. The above results suggest that the amount of surface oxygen groups is not the only variable that improves Hg0 adsorption. The better textural properties increase the accessibility of Hg0 to the inner pores of porous carbon, which is important for the sufficient utilization of the active oxygen sites on surface. In addition, the atomic concentration of oxygen on PC surface is added 13.75 % after the 30 s treatment, which is higher than AC after 30 min treatment (9.57 %). This observation demonstrates that the excellent textural properties of porous carbon facilitate the fast transfer of oxygen radicals among the pores, and are beneficial for the adsorption of oxygen radicals generated by plasma. This makes the porous carbon can be functionalized with shorter time than AC. Therefore, the development of carbon material possessing excellent textural properties is helpful for developing high-performance Hg0 removal sorbent. (a) 50 oC
80 60 AC PC PC-45
40
AC-30m PC-30 PC-60
20 0
0
10
20
30
40
50
Hg0 Removal Effeciency %
100
100
Hg0 Removal Effeciency %
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
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80 60
PC-30 PC-45 PC-60 AC-30m
40 20 0
60
(b) 120 oC
0
10
20
30
40
50
60
Time (min)
Time (min)
Figure 6. Hg0 removal performance of porous carbon and activated carbon at: (a) 50 oC; (b) 120 oC.
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Identification the mechanism of Hg0 adsorption Temperature Programmed Desorption (TPD) method was used to determine the occurrence modes of mercury.49 The Hg0-exposed sorbents (PC and PC-60) were employed for desorption experiments. The Hg0-exposed sorbents were heated from room temperature to 600 oC at 10 oC/min in 1 L/min N2. Figure 7 shows the TPD curves of Hg0-exposed raw PC and PC-60. It can be seen that Hg0 desorbs over a wide temperature range, and the desorption peak of raw PC differs from that of PC-60. This implies that different active sites are involved in the process of Hg0 adsorption. The raw PC shows only one desorption peak at around 190 oC. The results of FT-IR and XPS analyses reveal that the number of oxygen groups on PC surface is less. Therefore, Hg0 adsorption on PC is mainly though physisorption. Two obvious desorption peaks at around 240 oC and 320 oC are obtained for PC-60. This implies that chemisorption dominates the pathway of Hg0 adsorption on PC-60, and different oxygen groups contribute to Hg0 adsorption. 1.0 PC PC-60 0.8
Relative intensity
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
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0.6 0.4 0.2 0.0
0
100
200
300
400
500
600
o
Temperature ( C)
Figure 7. Temperature programmed desorption of Hg0-exposed PC and PC-60.
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The pseudo-second order kinetic model was used to further describe the Hg0 adsorption on PC-60, which was expressed as:50
=
+
(2)
where qt and qe represent the amount of Hg0 adsorption (µg/g) at time t and equilibrium, respectively. k is the rate constant (g/(µg·min)) of the kinetic model. Table 2 lists the results of kinetic model. The experimental results are fitted well with the pseudosecond order kinetic model, indicating that the Hg0 adsorption is controlled by chemisorption.
Table 2. Kinetic parameters of Hg0 on PC-60. Temperature (oC)
qe (µg/g)
k g/(µg·min)
R2
50
6293
1.24E-08
0.994
120
210
1.13E-05
0.999
The apparent activation energy of Hg0 on PC-60 was determined by the Arrhenius equation:51, 52, 53
ln = − + ln
(3)
where k1 is the Arrhenius equation factor. T is reaction temperature (K); R is the molar gas constant, 8.314 J/(mol·K). Ea is the activation energy (kJ/mol). As listed in Table 3, the activation energy is determined to be -102.7 kJ/mol, demonstrating that Hg0 adsorption on PC-60 belongs to chemisorption. Thermodynamic parameters of enthalpy change (∆H (kJ/mol)), entropy change (∆S (J/mol·K)) and Gibbs energy (∆G (kJ/mol)) were obtained based on the Van't Hoff Equation: ∆G = -RTlnk2
(4)
∆G = ∆H - T∆S
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where k2 is the distribution coefficient, k2 is equal to qe/Ce. Ce represents the initial Hg0 concentration (µg/m3). The calculated results are listed in Table 3. The negative ∆G values suggests that the Hg0 adsorption on PC-60 is spontaneous. The negative enthalpy ∆H means that the Hg0 adsorption is exothermic.
Table 3. Thermodynamic parameters of Hg0 on PC-60. Arrhenius equation
Ea (kJ/mol)
-102.7
Lnk1
20.06
Van't Hoff Equation:
R2
0.999
∆H (kJ/mol) ∆S (J/mol·K)
-51.23
∆G (kJ/mol)) 50 oC
120 oC
-12.98
-4.69
R2
0.999
-118.43
The above experimental results suggest that non-thermal plasma can quickly produce oxygen groups on porous carbon surface. The role of each group in Hg0 adsorption is closely related with its type. However, the detailed roles of oxygen groups in Hg0 adsorption are difficult to be accurately determined by using experimental investigation. Density functional theory provides a powerful tool to overcome the limitation of experimental analyses. Therefore, in this study, density functional calculations were performed to clarify the adsorption mechanism of Hg0 on oxygen-rich porous carbon. The relative energy of Hg0 adsorption on PC and different oxygen groups is showed in Figure 8. The suppression or promotion effects of oxygen groups on Hg0 adsorption are verified by comparing their Hg0 adsorption energies with that of PC. The adsorption energies of Hg0 on carbonyl and ester are higher than that of PC, indicating a positive effect of them on Hg0 adsorption. The adsorption of Hg0 on ester is more exothermic than that on carbonyl. This means that ester is more active for Hg0 adsorption. However, the adsorption energies of Hg0 on epoxy, carboxyl and hydroxyl are lower than that of PC. ACS Paragon Plus Environment
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Therefore, the presences of epoxy, carboxyl and hydroxyl groups are negative for Hg0 adsorption. The type of oxygen groups plays an important role in Hg0 adsorption, which is consistent with above experimental results. The improvement in Hg0 removal efficiency of plasma functionalized porous carbon is owing to the increase of the amount of ester and carbonyl groups.
Figure 8. Relative energy of Hg0 adsorption on different oxygen groups.
CONCLUSION A porous carbon with high surface area and developed porosity has been successfully synthesized through freeze-drying coupled with CO2 activation. The as-synthesized porous carbon modified by nonthermal plasma display huge potentials as an efficient sorbent for Hg0 removal. The porous carbon can be quickly modified by plasma within one minute. The non-thermal plasma can not only increase the total amount of oxygen groups on porous carbon surface, but also can adjust the proportion of different oxygen groups. C-O and C=O groups can be further oxidized into O=C-O group. The excellent textural
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properties of porous carbon are helpful for the fast reactions of oxygen radicals with carbon atoms, and thus can offer more active sites for Hg0 adsorption. Moreover, the favorable porous structure facilitates Hg0 fast transfer, which is helpful for efficient utilization of these active oxygen sites. In addition, the effects of plasma treatment on the porosity, structure, and surface chemistry of porous carbon have been systematically analyzed in this paper. The roles of different oxygen groups on Hg0 adsorption are clarified at the molecular level. This fundamental concept of oxygen-rich porous carbon is expected to provide opportunities for the development of effective and low-cost sorbent for Hg0 removal.
■ AUTHOR INFORMATION Corresponding Author *Tel: +86 27 87545526; fax: +86 27 87545526; e-mail address:
[email protected].
ORCID
Jing Liu: 0000-0001-6520-9612
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGEMENTS This work was supported by National Key R&D Program of China (2016YFB0600604) and National Natural Science Foundation of China (51606078).
■ REFERENCES ACS Paragon Plus Environment
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TOC
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