Conversion characteristics and kinetic analysis of gaseous α-pinene degraded by a VUV light in various reaction media

Separation and Purification Technology 77 (2011) 26–32

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Conversion characteristics and kinetic analysis of gaseous ␣-pinene degraded by a VUV light in various reaction media Zhuo-Wei Cheng, Yi-Feng Jiang, Li-Li Zhang, Jian-Meng Chen ∗ , Ying-Ying Wei College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, China

a r t i c l e

i n f o

Article history: Received 29 November 2009 Received in revised form 11 November 2010 Accepted 12 November 2010 Keywords: ␣-Pinene VUV Kinetic model Rate constant Photodegradation

a b s t r a c t The photodegradation of gaseous ␣-pinene by a vacuum ultraviolet (VUV) light was investigated under different process parameters and reaction media. The degradation of ␣-pinene was examined at nominal concentrations ranging from 50 to 1000 ppm, and resulted from the combination of direct photolysis, OH• oxidation and O3 oxidation, in which O3 oxidation played a dominant role. A high conversion efficiency of ␣-pinene was achieved at a moderate relative humidity (RH) of 35–40% with an efficient utilization of the electrical energy. The conversion of ␣-pinene were remarkably reduced while a RH was increased to 75–80% since a higher RH inhibited the production of ozone and further affected the conversion of ␣-pinene adversely. The ␣-pinene conversion followed the first-order kinetic model at the low initial concentrations (50–200 ppm), and the second-order kinetic model provided a good fit to the data at the high concentrations (400–1000 ppm). The kinetic models including the initial target concentrations and the produced ozone amounts were developed to describe their mutual relationships. Preliminary results indicate that the VUV photooxidation was an appropriate technology for conversion of ␣-pinene not only as a single treatment but also as a pretreatment followed by the subsequent biodegradation. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Volatile organic compounds (VOCs) are common air pollutants found in industrial air emissions, such as from chemical manufacturing plants and various hazardous sites [1]. Most of these compounds are of environmental concern due to their toxicity. The conventional physical and chemical treatment technologies such as air stripping, adsorption, condensation and incineration are inherently unsuitable for complete removal of VOCs, because none of them is cost-effective or environmentally-friendly [2]. Biotreatment of VOCs offers an inexpensive alternative to conventional technologies. However, this technology could not achieve satisfactory performance for some hydrophobic and recalcitrant VOCs [3]. Therefore, the suitable pre-treatment is demanded to enhance the solubility and biodegradability of these recalcitrant compounds, and makes the subsequent biotreatment possible. Ultraviolet (UV) oxidation, including the photocatalytic oxidation and the photochemical oxidation, is a promising pre-treatment for enhancing the solubility and biodegradability of these VOCs. UV oxidation has been widely applied in many VOCs’ treatments [4,5] because of its several attractive traits such as lower and higher absorption by air and organic compounds, the presence of reactive

∗ Corresponding author. Tel.: +86 571 88320881; fax: +86 571 88320882. E-mail address: [email protected] (J.-M. Chen). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.11.014

species and the absence of free radicals scavengers. Although the photocatalytic oxidation is a cost-effective technology that can be processed at ambient temperature and pressure, the catalyst deactivation induced by moisture and some aromatic compounds often limits its industrial applications [6–8]. In contrast, the photochemical oxidation generated by commercially available low-pressure mercury lamps has been increasingly highlighted in the air treatments, and proved to degrade many organic compounds effectively [4,9–11]. Recently, some researchers employ ozone to enhance the photochemical process and obtain much higher conversion efficiency in the combined UV/O3 system [12,13]. As oxygen photolysis by 184.9 nm light could produce ozone resulting in much higher efficiencies, the vacuum-ultraviolet (VUV) as an efficient and cost-effective technology, is widely applied in many waste treatments [14–17]. Alpha-pinene (␣-pinene, C10 H16 ) as a typically hydrophobic and recalcitrant VOC is usually emitted from the forest product industries, pulp and paper industries and fragrance manufactures, etc. [18]. The gaseous ␣-pinene is poorly removed by traditional biological treatments due to its special characteristics. The reported critical elimination capacities in the bacterial-biofilters were 3.9 g m−3 h−1 to 60 g m−3 h−1 , which were much lower than that of the hydrophilic and readily biodegradable compounds (methanol with 250 g m−3 h−1 and methyl ethyl ketone with 120 g m−3 h−1 ) [19–21]. Thus, some researchers propose the UV technology as a pretreatment of the biofiltration for such hydrophobic and recalcitrant compounds (␣-pinene, toluene, o-xylene, etc.),

Z.-W. Cheng et al. / Separation and Purification Technology 77 (2011) 26–32

27

lamp on lamp off

80

2

60

-ln(1-X)

Conversion efficiency (%)

100

1

40

20

0

0 0 Fig. 1. Schematic diagram of the experimental apparatus: (1) purified air/nitrogen; (2) alpha-pinene sparger; (3) humidifier; (4) mixing chamber; (5) spiral quartz reactor; (6) off-gas collector; (7) GC-FID with gas autosampler; (8) hygrothermograph; (9) mass flow meter; (10) KI-coated denuder

since more water soluble and biodegradable intermediates are generated in the photodegradation process [22–25]. In this study, gaseous ␣-pinene was photodegraded in a selfmade spiral reactor by an ozone-producing UV lamp. The effects of various process parameters such as initial concentration, residence time, relative humidity (RH) on the conversion efficiency of ␣-pinene were investigated in detail, and the optimal photodegradation conditions were determined. Also the degradation mechanisms of VUV treatment were explored briefly. Furthermore, the kinetic models were developed to describe the conversion behaviors of ␣-pinene in the VUV reaction system. Finally, the feasibility of VUV photodegradation as an economical and effective alternative technology treating some recalcitrant and hydrophobic VOCs was evaluated through the EEO calculations. 2. Method and materials 2.1. Experimental set-up Fig. 1 is a schematic diagram of the reaction system. A 150-mL spiral quartz column (with an inner diameter of 8 mm and a total length of 300 mm) was used as the main photo-reactor. In the center of the spiral column, a special low-pressure mercury vapor lamp with a 200 mm light-emitting section (36 W, Electric Light Sources Research Institute, Beijing, China) was installed. The light intensity I0 and the main emission wavelength were 0.206 mW cm−2 and 185 nm, respectively. A cooling water sleeve was used to maintain the reaction at room temperature. 2.2. Materials Analytical grade (purity 97%) ␣-pinene obtained from J&K Chemical (China), was used without any further purification. The purified air/nitrogen (99.99%) and the standard gas of ␣-pinene (364 ppm, N2 balance) were supplied by Jinggong Gas Company, Ltd. (Hangzhou, China) and Weichuang Standard Gas Company, Ltd. (Shanghai, China), respectively. All the other chemicals were of the highest purity commercially available.

30

60

90

Residence time(s) Fig. 2. Effect of residence times on the conversion efficiencies and the plug-flowform kinetics of the conversion process.

RHs. Before each experiment, the reactor was firstly purged with purified air for 30 min to decontaminate both the quartz column and the lamp. Then, the ␣-pinene contaminated gas was generated and introduced into the reaction system. With the equality of the inlet and outlet ␣-pinene concentrations, the lamp was switched on for 20 min to reach its stable work condition. Several pre-designed experimental conditions were tested, with the initial concentrations of ␣-pinene and RHs being fixed at 50–1000 ppm and 2–80%, respectively. Each photodegradation experiment was repeated three times. 2.4. Analytical methods The concentrations of ␣-pinene were analyzed by an Agilent 6890N GC equipped with a flame ionization detector (FID). The gas samples were collected using a six-way valve with a gas sampling loop and then transferred into a silica HP-Innowax capillary column (30 m × 0.32 mm × 0.5 ␮m, J&W Scientific, USA). The operating conditions were as follows: injector, 250 ◦ C; oven, 140 ◦ C for 3.5 min and detector, 300 ◦ C. With the GC column seriously damaged by the effluent gas containing ozone, a KI-coated annular denuder was used to selectively remove ozone [26]. Ozone concentration was determined by indigo disulphonate spectrophotometry method (GB/T 15437-1995). Three bottles containing 30 mL indigo disulphonate-phosphate solution (0.050 M), respectively were connected in series to absorb the gaseous ozone, and the captured gas volume was controlled at 19.98 L (101.32 kPa and 0 ◦ C) by an automatic gas sampler (LaoYing 3072, Qingdao, China). Photooxidation of air without any ␣-pinene feed was served as the blank control to determine the total ozone amount generated by the air-photolysis. The OD610 nm of the absorbed solutions were measured by the UV spectrophotometer (UV2910, Shimadzu, Japan). The humidity and temperature were determined by a hygrometer equipped with a temperature sensor (Testo 625, Testo  AG, Germany). 3. Results and discussion

2.3. Experimental procedures

3.1. Effect of the residence times

Contaminated gas stream was generated by passing air/nitrogen through a sparger containing pure ␣-pinene liquid and then mixed with the other flow of humid/dry air/nitrogen. Different ratios of these streams determined the inlet concentrations of ␣-pinene and

Fig. 2 shows the effect of residence times on the conversion characteristics of ␣-pinene. When 175 ppm of ␣-pinene with 35% RH was introduced, the conversion efficiency gradually decreased with the increasing flow rates (corresponding residence times var-

Z.-W. Cheng et al. / Separation and Purification Technology 77 (2011) 26–32

Ce

k denotes the proportionality constant

Ln C

1

The quantum yield ˚ for ␣-pinene in the spiral reactor was 0.217, which was much higher than that in the conventional column reactor, whereas the values of other parameters including the intensity I0 and the molar coefficient ε0 , etc. were almost equal. Accordingly, such a spiral-flow mode enhanced the utilization efficiency of the VUV light and provided more reaction chances for the reactant molecules and free radicals, achieving a high conversion efficiency and quantum yield. Therefore, regardless of the cost and manufacture difficulties, this study showed that a spiral reactor with small cross-sectional area and long “effective” length would be helpful for achieving a much higher conversion efficiency, and the rate constant in the spiral reactor was 1.4 times more than that obtained in the traditional column ones. 3.2. Effect of initial concentrations The initial ␣-pinene concentrations were varied from 50 to 1000 ppm under the same RH of 35–40%. Fig. 3 shows the effects of concentration variance on the ␣-pinene conversion with different residence times. Through fitting the experimental data, it was found that the conversion followed the first- and second-order kinetic models in the range of 50–200 ppm and 400–1000 ppm, respectively. The corresponding rate constants are listed in Table 1. Photolysis of oxygen and water molecule by 185 nm light would generate ozone and hydroxyl radical, which reacted with organic molecules competitively [29,30]. Hatakeyama et al. [31] reported that the rate constants of ␣-pinene with hydroxyl radical and ozone were, respectively 5.3 × 10−11 cm3 mol−1 s−1 and 8.4 × 10−17 cm3 mol−1 s−1 , indicating that the former would dominate in the competition reaction. With low initial concentration, hydroxyl radical produced by photolysis could sufficiently oxidize

0

30 60 Residence time (s)

90

50ppm 100ppm 200 ppm

84

42

0 30

60

90

Residence time (s)

(1)

(2)

3 2

126

0

1 = − ln(1 − X) k

where Ci represents the influent concentration, Ce denotes the effluent concentration, X is the conversion efficiency, and  is the residence time. By plotting ln(1 − X) vs. t, the apparent rate constant (k) can be determined from the slope of the curve. In the spiral reactor, the rate constant k for ␣-pinene was 0.082 s−1 , which was much higher than that of photodegradation in the traditional column reactor with a rate constant of 0.059 s−1 . The rate constant k is generally dependent on several parameters such as light intensity I0 , quantum yield ˚, absorption coefficient ε0 , UV path length L, reactor cross-sectional area A, and reactor volume V. Bhowmick and Semmens [28] indicated that the following equation could depict the relationships between these parameters (Eq. (2)). LA k = k I0 ε0 , V

4

168

b

400ppm 600ppm 800ppm 1000ppm

1200

1000

8 6

-

i

210 5

Concentration (ppm)

1  = ln k

C 

a

1/C (*10 3)

ied from 90 s to 4.5 s). As is known, ␣-pinene molecule contains C C, C–C and C–H bonds, energies of which are 596.4 kJ mol−1 , 336 kJ mol−1 and 411.6 kJ mol−1 , respectively, and the energies are lower than those of the generated wavelengths from 185 nm (647.23 kJ mol−1 ) to 200 nm (471.41 kJ mol−1 ). When the residence time was long enough, some of ␣-pinene was decomposed into excited states or intermediates through absorbing VUV light. Simultaneously, the longer irradiations of air and water molecule would generate more ozone and hydroxyl radical [4,14], which could easily oxidize these compounds formed in the photodegradation process. Therefore, the residence time was an important parameter to the conversion process, and the gradual reduction of the conversion efficiency was attributed to the shorter contact time between the reactants and the radicals. The gas stream in the reactor was a plug-flow form, and the experimental data might obey the first order kinetics (Eq. (1)) [27]:

Concentration (ppm)

28

4 2 0 0

30

800

60 Residence time (s)

90

600

400

200 0

20

40

60

80

100

Residence time (s) Fig. 3. Variation of the initial concentrations as a function of the residence times: (a) low initial concentrations and (b) high initial concentrations.

most of the initial ␣-pinene. With higher initial concentration, the generated hydroxyl radical was possibly not enough for ␣-pinene decomposition, and the surplus organic molecules would be oxidized by ozone and other radicals. Therefore, the variety of reaction order caused by concentrations would contribute to the dominant reactions and oxidation mechanisms induced by different types of radicals [12]. 3.3. Effect of reaction media The conversions and reaction mechanisms are strongly influenced by the types and amounts of reactive species [4,27]. Different reaction media, such as nitrogen and oxygen with various RH values, might generate different reactive species by the UV light. Generally, three decomposition modes are included in the photodegradation system: the direct photolysis, the indirect oxidation by active radicals such as ozone and hydroxyl radical, and the chain reaction by some atoms (chlorine atom, etc.)[4]. 3.3.1. Direct photolysis of ˛-pinene in N2 ␣-Pinene was degraded by UV light in the spiral reactor under the different relative humidities with nitrogen as the reaction media, and the results are shown in Fig. 4. The initial ␣-pinene concentration was 100 ppm, and the residence time was controlled at 18 s or 45 s, respectively. In the VUV reaction system with nitrogen as the medium, the conversion efficiency was enhanced with the increase of RH values. Nitrogen could not absorb the light

Z.-W. Cheng et al. / Separation and Purification Technology 77 (2011) 26–32

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Table 1 Photodegradation rate constants of ␣-pinene with different initial concentrations. Initial concentration (ppm)

Rate constants

50 100 200 400 600 800 1000

1st order rate constant (s−1 )

R2

2nd order rate constant (×10−6 ppm−1 s−1 )

R2

0.117 0.082 0.056 – – – –

0.9914 0.9977 0.9619 – – – –

– – – 67.7 19.3 11.4 5.87

– – – 0.9948 0.9819 0.9925 0.9864

until below 125 nm, thus only the direct photolysis occurred in the dry nitrogen medium [24]. In such a dry system, ␣-pinene was converted into the electronic excitation only through absorbing photons, and was subsequently dissociated to form other intermediates, resulting in a low conversion efficiency. When water molecule was introduced into the system, the conversion efficiency increased dramatically because much more hydroxyl radicals were generated (Eq. (3)) [4] to oxidize ␣-pinene molecules. H2 O + hv → H• + OH•

(3)

3.3.2. Photooxidation of ˛-pinene by ozone produced from the air-photolysis Fig. 5 shows the ozone generation and the conversion efficiency of ␣-pinene in air media under the different RHs. The highest ozone concentrations of 6.406 and 14.088 mg m−3 were detected in the dry air without any ␣-pinene feed at 18 s and 45 s, respectively. It was found that water vapor could inhibit the ozone generation, and Jeong et al. [14] also reported the similar phenomena. The hydroxyl radicals generated by water vapor photolysis also reacted with the generated ozone, further reducing the amounts of ozone in the reaction system. Therefore, the RH must be strictly controlled in the photodegradation system to obtain the maximum conversion efficiency by both ozone and hydroxyl radical oxidation. Oxygen absorbed 185 nm light to form ozone through the following process (Eqs. (4)–(6)) [14] (<242 nm)

O2 + hv

−→

O(1 D) + O(3 P)

O(1 D) + O2 + M → O3 + M,

(4)

M : N2 or O2

(5)

O(1 D) + O(1 D) + M → O2 + M

(6)

30

For that the electron affinity of ozone was higher than that of oxygen (2.1 eV vs. 0.44 eV) [32], the ozone-oxidation predominated in less humid reaction media (approximately 75% at 68 s). Owing to the competitive absorption by oxygen and water molecule (Eqs. (3) and (4)) [4], the formation of ozone was probably inhibited in the higher RH system. Besides, there were other two ways that indirectly affected the produced amounts of ozone (Eqs. (7) and (8)): (1) the precursor of ozone, which was electronically excited oxygen atoms formed by photolysis of oxygen, could react with water molecule [33], and (2) H• generated by photolysis of water molecule could also consume ozone [4]: O(1 D) + H2 O → 2OH•

(7)

H• + O3 → O2 + OH•

(8)

With the higher RH, the phenomena that affected ozone formation were more obvious, and it finally resulted in lower conversion efficiencies. Therefore, in this study the optimal RH range of the air should be controlled at 35–40%. 3.3.3. Photooxidation of ˛-pinene by hydroxyl radical originated from the water-photolysis As previously reported in the literatures, RH values could significantly affect UV oxidation process [34,35]. Fig. 6 shows the variable trend of the conversion efficiency with RHs ranging from 12 to 75%. The conversion efficiency of ␣-pinene with high initial concentrations were increased initially, and then decreased with the further increasing of RHs. The most efficient conversion was obtained under the moderate humidity condition. Whereas, an almost stable conversion efficiency of ␣-pinene was obtained with low initial concentrations. It was found that the conversion efficiency of ␣-pinene was very low (about 1–4%) when the light was switched off, which indicated that the amounts converted by other physical processes (absorption, etc.) might be neglected. In theory,

Ozone concentration (log C (mg m-3))

20

27 s 68 s

10

80 70 0 60 50 -1

-2

Ozone 27s 68s Removal efficiency 27s 68s

40 30

After reaction

Conversion efficiency (%)

Conversion efficiency (%)

90 1

20 10

0 < 3%

35-40%

75-80%

Relative humidity (%) Fig. 4. Conversion efficiencies of ␣-pinene photodegraded in the nitrogen with different RHs.

-3 < 3%

35-40%

75-80%

Relative humidity (%) Fig. 5. Conversion efficiencies of ␣-pinene and generation characteristics of ozone in the air with different RHs.

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100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20

3

Conversion efficiency (%)

650 ppm 650 ppm 200 ppm 200 ppm

Experiment, Relative humidity 2-3% Model, Relative humidity 2-3% Experiment, Relative humidity 35-40% Model, Relative humidity 35-40% Experiment, Relative humidity 75-80% Model, Relative humidity 75-80%

68s 27s 68s 27s

Ln(C0 /C)

2

1

0 10

20

30

40

50

60

70

0

80

30

60

Residence time (s)

Relative humidity (%) Fig. 6. Effect of different RHs on the conversion efficiencies of ␣-pinene photodegraded in the air.

Fig. 7. Plots of Ln(C0 /C) vs. residence times at the different ranges of RHs, where the points are the experimental date and the lines are the model predicted values.

the VUV light. the water solubility and the Henry’s constant of ␣-pinene were 2.49 mg L−1 and 2.94 × 104 Pa m3 mol−1 , respectively, suggesting that it was extremely difficult for water absorbing many ␣-pinene molecules within a limited time. Therefore, the photodegradation might dominate in the conversion process. As hydroxyl radical was highly reactive with a shorter life span (about 1 ␮s) [36], it was difficult to be analyzed qualitatively or quantitatively. The introduction of hydroxyl radical scavenger could indirectly test the presence of hydroxyl radical in the system. In this study, isoprene was chosen as the scavenger for that its rate constant with hydroxyl radical was higher than that of ␣pinene (1.0 × 10−10 cm3 mol−1 s−1 vs. 5.3 × 10−11 cm3 mol−1 s−1 ) [37]. Compared with the reaction system without the addition of isoprene, the conversion efficiencies were much lower and almost kept constant irrespective with the RHs (Table 2). These results indirectly demonstrated that hydroxyl radical was indeed present in the photodegradation system and its amounts would affect the conversion efficiencies. From Table 2, the RHs might exhibit positive or negative effects on the ␣-pinene conversion. When the RH was below 35–40%, most ␣-pinene reacted with hydroxyl radical and the surplus were consumed by ozone, and a much higher conversion efficiency was obtained than that in the dry reaction media. However, under the high RH of about 75–80%, more water molecules were introduced into the reaction system, which reduced the contact of ␣-pinene with light, ozone and hydroxyl radical, thus affecting the ␣-pinene conversion adversely. Simultaneously, the reactive compounds such as ozone and HO2 could also react with hydroxyl radical firstly (Eqs. (9) and (10)), further leading to much lower conversion efficiencies. Therefore, these results suggested that RH of 35–40% was appropriate for the photodegradation of ␣-pinene by

Table 2 Comparison of the conversion efficiencies with and without isoprene as the scavenger. RH (%)

Conversion (%) Nitrogen

2–3 35–40 75–80

Air

Isoprene

No isoprene

Isoprene

No isoprene

13.98 ± 0.58 16.12 ± 0.69 16.68 ± 1.29

15.38 ± 1.94 19.38 ± 1.56 27.64 ± 1.29

59.88 ± 1.40 60.89 ± 1.18 61.25 ± 1.23

62.99 ± 1.53 86.65 ± 1.10 60.10 ± 1.94

OH• + O3 → HO2 + O2

(9)

OH• + HO2 → H2 O + O2

(10)

3.3.4. Kinetic and mechanism analysis Studies were conducted in the spiral photoreactor to quantitatively describe the influence of different decomposition on the kinetics of ␣-pinene degradation. Such factors including initial concentration C0 , ozone concentration M and hydroxyl radical concentration N would affect the kinetic constant k, which could be calculated from the following equation (Eq. (11)): k = f (C0 , M, N) = aC0b M c N d

(11)

where a, b, c and d are constant. As the concentration of hydroxyl radical was difficult to be measured, the quantitative description of this parameter was not available. Thus, a kinetic model involving C0 and M was developed, and the factor hydroxyl radical was replaced by three RH ranges (2–3%, 35–40% and 75–80%). To establish the kinetic model, the initial concentration range was set at 50–200 ppm, and the experimental data could be fitted by the first-order kinetic model given as (Eq. (12)): ln

C0 = kt C

(12)

With single-factor variance and linear regression, three groups of constants (a, b and c) could be calculated under the different RHs. Therefore, the remaining concentration of ␣-pinene (Ct ) could be expressed as follows (Eqs. (13)–(15)): Ct = C0 exp(−13.89845C0−0.9446 M −1.1764 t) Ct =

C0 exp(−0.43896C0−0.4359 M −0.2613 t)

Ct = C0 exp(−0.171435C0−0.4559 M −0.3073 t)

(RH 2–3%)

(13)

(RH 35–40%)

(14)

(RH 75–80%)

(15)

To test the accuracies of these kinetic models, Fig. 7 shows the deviation between the experimental and theoretical data. The relative errors of Ct were 0.27 to 6.89%, −14.10 to 7.16% and −2.04 to 17.38% for the RH ranges of 2–3%, 35–40% and 75–80%, respectively. The data showed that the experimental results could fit the developed model better with the lower RH, and the deviation was more obvious with the higher RH. These phenomena further suggested that a complex interplay could be present between ozone and hydroxyl radical, which not only caused the difference between the actual

Formed ozone amount (mg m-3 )

Removal amount (mg m-3 )

Z.-W. Cheng et al. / Separation and Purification Technology 77 (2011) 26–32

500

OH oxidation O3 oxidation

400

photolysis actual value

Table 3 Comparison of the EEO values for ␣-pinene conversion at different residence times and RHs.

300 200 100 0 15 10

5

Flow rate (m3 h−1 )

Residence time (s)

RH (%)

EEO (kWh order−1 )

0.09 0.06 0.03 0.012 0.006 0.03 0.03 0.03 0.03 0.03

6 9 18 45 90 18 18 18 18 18

35–40 35–40 35–40 35–40 35–40 12 30 40 50 80

4.137 4.184 3.973* 3.456 3.263 9.818 6.161 4.868* 5.388 10.427

*

0 RH 2-3%

31

RH 35-40%

RH 75-80%

Fig. 8. Relationships between the conversion amounts of the respective and combined decomposition under different RHs. The actual values represent the amounts converted by the combination of photolysis, OH• photooxidation and O3 photooxidation, while the respective values represent the actual amounts converted by each decomposition only.

and theoretical values, but also inhibited the generation of ozone and the conversion of ␣-pinene. Three main decomposition modes (direct photolysis, photooxidation by ozone and hydroxyl radical) coexisted in the humid air system. With different reaction media, the conversion amount by each mode could be calculated from the experimental results (Fig. 8). The average conversion amount of ␣-pinene by 1 mol O3 increased with the extension of the residence time (5.36, 5.66 and 7.68 mol ␣-pinene/mol O3 at 18, 27 and 68 s, respectively). It was also noted that most ␣-pinene was converted to organic intermediates instead of the mineralization products (CO2 and H2 O), for that 1 mol O3 could only completely mineralized 0.11 mol ␣pinene. As the conversion amounts caused by photolysis were almost equal, the light utilization efficiency by ␣-pinene itself was equal and irrelative with other factors. Consistent with the previous studies, the conversion amounts caused by hydroxyl radical were increased with more water molecules introduced into the reaction system. Through the calculation of the conversion percentage, it was found that more than half of the amount were converted by ozone, and the remaining was by photolysis and hydroxyl radical, which was similar to the results observed by Hatakeyama et al. [31] and Jang and Kamens [37]. It was also worth noting that the higher RH, the larger difference between the actual converted amount by the combined mode and the sum of converted amounts by every mode (401.36 mg L−1 and 549.05 mg L−1 , respectively). Such deviations further confirmed that there was an inhibition among the active radicals, which finally affected the conversion efficiency of ␣-pinene. The actual mole ratio of ␣-pinene and ozone was larger than the theoretical one, illustrating that most ozone reacted with ␣-pinene for the first step, and most of the formed Criegee intermediates were decomposed by other oxidants (hydroxyl radical, oxygen, etc.) or direct photolysis. The increased TOC values in the absorption solutions (data not shown) also implied that some hydrophilic intermediates were possibly formed during the photodegradation process. In the effluent and absorption solution, pinocamphone, 3hydroxy-␣-pinene, myrtenol, small organic aldehydes, ketones and carboxylic acids were detected as the major intermediates. Therefore, we deduced that ozone, oxygen and hydroxyl radical firstly converted ␣-pinene to the Criegee intermediates, pinocamphone and OH-pinene adduct radical, respectively. Then, these formed intermediates were further oxidized to some small organic compounds (aldehydes, ketones, carboxylic acids, etc.) and mineralized

Different initial concentration of ␣-pinene.

products. A detailed explanation on the intermediates and degradation pathway of ␣-pinene was illustrated in another study [38]. 3.4. Photodegradation treatment efficiency index The International Union of Purified and Applied Chemistry (IUPAC) proposed a Figure of Merit for evaluating the AOPs (Advanced Oxidation Processes) [39], which was often used by researchers for the comparisons between different technologies and operational conditions. EEO (an electric energy per order) indirectly represented the relationship between the operational cost and the utilization ratio of electrical energy. As electrical energy was required for degradation in a given volume, EEO could be calculated as follows for a flow-through form (Eq. (16)): EEO =

P F × lg(Ci /Cf )

(16)

where P denotes the total power input (kW), F represents the flow rate (m3 h−1 ), Ci and Cf are the initial and final concentrations (mg L−1 or mg m−3 ), respectively. Table 3 lists the EEO values calculated under various photodegradation conditions. With different residence times and RHs, the EEO values varied from 3.26 to 4.18 and 4.87 to 10.43, respectively. It was found that RH affected EEO much more and must be strictly controlled in the systems for the purpose of higher electricalenergy-utilization ratio. When the RH was moderate (35–40%), the residence time had insignificant effect on the EEO values. It meant that electrical energy could be utilized effectively at shorter residence time irrespective with the conversion efficiency, providing a possibility for its utilization as a pretreatment before the subsequent biological processes. Therefore, the VUV technology could be used as a promising method for treating such recalcitrant VOCs as ␣-pinene in a sole or an integrated disposal system. 4. Conclusion Gaseous ␣-pinene was photodegraded by a VUV light in a spiral quartz reactor. Several important process parameters were tested, and the main photodegradation mechanism was briefly analyzed. The ␣-pinene was converted by the combination of ozone, hydroxyl radical and direct photolysis, in which ozone played a dominant role. A high RH of 75–80% in the reaction system inhibited the production of ozone, and thus adversely affected the conversion of ␣-pinene. A moderate RH of 35–40% was beneficial to both the conversion and the electrical energy utility. The conversion of ␣-pinene followed the first-order kinetic model at low initial concentrations (50–200 ppm) and the second-order kinetic model at high initial concentrations (200–1000 ppm). Through linear fitting, three kinetic models were established to characterize the conversion, and they could provide an excellent predication of the photodegradation behaviors of ␣-pinene.

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