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TiO2 process under periodic illumination
Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 299–305
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Decomposition of dimethyl phthalate in aqueous solution by UV–LED/ TiO2 process under periodic illumination Young Ku* , Shiau-Ju Shiu, Hsuan-Chih Wu Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
A R T I C L E I N F O
Article history: Received 15 July 2016 Received in revised form 23 August 2016 Accepted 11 September 2016 Available online 13 September 2016 Keywords: Photocatalysis Periodic illumination UV–LEDs Dimethyl phthalate
A B S T R A C T
The effect of controlled periodic illumination on the photonic efficiency and temporal decomposition behavior of dimethyl phthalates (DMP) in aqueous solution by UV–LED/TiO2 process was investigated in this study. For experiments conducted with the same total illumination time, the decomposition of DMP was enhanced with increasing dark period possibly ascribed to the enhancement of surface replenishment and the inhibition of electron–hole recombination. The existence of oxygen molecules might be beneficial for replenishing the TiO2 surface during the dark periods. Experiments conducted with longer illumination might not be favorable to enhance DMP decomposition, instead was more favorable for electron-hole recombination. Photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination was agreeably modeled by the proposed Langmuir–Hinshelwood kinetic equation. Under the same illumination period of 5 s, the calculated energy consumption per order was decreased by more than 40% for experiments conducted with the increasing dark period from 0.5 to 5.0 s; thus, the electric energy consumption was substantially decreased. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl phthalates (DMP) represent a family of chemicals which are widely used as plasticizers, primarily in the production of polyvinyl chloride resins. DMP is considered to be an endocrine disruptor and of particular attention because of its possible accumulation in the environment [1] and is listed as a priority pollutant by US EPA [2]. DMP is a relatively stable compound in natural environment and its hydrolysis half-life is estimated to be about 20 years [3–5]. Glowing concerns on the contamination caused by the discharge of numerous organic compounds to water environment has vigorously stimulated the development of various treatment technologies. Photocatalytic processes using semiconductor catalysts under appropriate illumination have been demonstrated to be effective in decomposing numerous organic species regarding water and wastewater treatment [6]. However, the practical employment of photocatalytic processes is mainly hindered by the development of effective photocatalysts for durable and energysaving application. Anatase form of titanium dioxide (TiO2) is a frequently studied photocatalyst for various environmental
* Corresponding author. E-mail address: [email protected] (Y. Ku). http://dx.doi.org/10.1016/j.jphotochem.2016.09.011 1010-6030/ã 2016 Elsevier B.V. All rights reserved.
applications; however, the effectiveness of TiO2 as photocatalyst is yet hampered primarily by the recombination of photogenerated electrons and holes [7,8]. The adsorption of oxygen molecules onto the photocatalyst surface and/or the transfer of the conduction band electrons to the adsorbed oxygen molecules were suggested to be the rate-limiting steps for most photocatalytic reactions [9]. Periodical illumination of UV light on the catalyst might allow more time for the ratelimiting steps to inhibit the build–up of intermediate species and to reduce the electron–hole recombination [10]. Buechler et al. [11] reported that the apparent quantum yield was enhanced up to 40% for photocatalytic oxidation of gaseous trichloroethylene under periodic illumination because dark periods allow trichloroethylene to be adsorbed on the TiO2 surface. Cornu et al. [12] stated that the quantum yields for the photocatalytic decomposition of aqueous formate were more than doubled for experiments conducted under periodic illumination. Wang et al. [13] demonstrated that periodic illumination increased the quantum yield of formaldehyde formation by 50% than the experiments conducted with continuous illumination. The photonic efficiencies for the photocatalytic decomposition of RR22 and Bisphenol A were significantly promoted for experiment conducted with periodic illumination [14,15]. In this research, the temporal decomposition behavior of aqueous DMP was studied in a batch photocatalytic reactor under controlled UV periodic illumination, and expressed using a
300
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Langmuir-Hinshelwood-type kinetic model. The effect of illumination and dark periods, solution pH, light intensity, and dissolved oxygen concentration on the photocatalytic decomposition of aqueous DMP were discussed. The duty cycle and electric energy consumption for experiments operated with periodic illumination were also discussed. 2. Experimental The chemicals used in this study were certified reagent grade without further treatment and all experimental solutions were prepared with double distilled water. TiO2 suspension was prepared by dispersing Aeroxide P-25 TiO2 particles in ethanol solution, and was then stirred vigorously for 30 min and sonicated for more than 6 h. A 4 10 cm Ti plate was dipped in the prepared TiO2 suspension to coat TiO2 films on the surface of Ti plate. The TiO2-coated Ti plate was subsequently air dried and sintered at 350 C for one hour. The thickness and surface roughness of the coated TiO2 film were determined by field emission scanning electron microscope (FESEM, JEOL, JSM-6500F). As shown schematically in Fig. 1, the experimental system regarding to the photocatalytic decomposition of DMP in aqueous solution was consisted of a photoreactor and an UV light source. The 200-ml capped rectangular batch photoreactor reactor employed in this study was made entirely of quartz and was water-jacketed to maintain the solution temperature at 20 C for all experiments. 150 ml of aqueous solution containing 10 mg/L of DMP was initially charged to the reactor. Solution pH was kept constant at desired levels by the additions of sodium hydroxide and/or sulfuric acid solutions using a Kyoto Electronic AT-400 automatic potentiometric titrator. The dissolved oxygen of the reaction solution was maintained by varying the air flow rate and was measured with a dissolved oxygen analyzer (Orion 820). The prepared TiO2-coated Ti plate was located at the centerline of the photoreactor. Short wavelength UV-LEDs were used as the light source (Diana, B5-437-CVD, maximum output 180W primarily at 395 nm) and located outside the photoreactor. Light intensity of the UV-LEDs was detected by a radiometer (International Light, IL1400A) equipped with an UV detector (International Light, SEL005/WBS320/TD). UV light intensity and pulse frequency of the UV-LEDs was controlled by a KOEO SPS150-8 programmable DC
power supply. Photocatalytic decomposition of DMP in aqueous solution by UV-LED/TiO2 photocatalytic process under periodic illumination was assumed to be started when the pre-warmed light source was turned on for experiments conducted with illumination period from 0.1 to 5 s and dark period from 0.1 to 5 s. Photocurrent measurements were performed with an Autolab PGSTAT 302N potentiostat using a three electrode setup with Pt as a counter electrode and Ag/AgCl as a reference electrode. At predetermined time periods, aliquots of the reaction solution were withdrawn from the reactor, and were immediately stored in the amber glass bottle for further analysis. The concentration of DMP in the samples was measured on a Spectra Series P100 HPLC equipped with an UV detector at 254 nm. The concentration of total organic carbon (TOC) in reaction solution was analyzed by a Tekmar Dohrmann Phoenix 800 TOC analyzer. 3. Results and discussion Decomposition of dimethyl phthalate in aqueous solution by UV-LED/TiO2 photocatalytic process for experiments conducted with various controlled period illumination were examined in this study. The photonic efficiency, j, is defined as the ratio of the rate of photocatalytic reaction to the rate of incident photons, as given in the following equation [14,16]:
j¼
Rate of reactionðmols1 Þ DCA V=Dt ¼ ðIint =U l Þ Rate of incident photonsðmols1 Þ
ð1Þ
In this study, the rate of photocatalytic reaction is defined as the decomposition of DMP within irradiation time of 60 min. The rate of incident photons is equivalent to the incident light intensity, Iint, divided by the molar photon energy of the 395 nm UV light. Fig. 2 indicates the decomposition of DMP by UV-LED/TiO2 process with total irradiation time of 60 min under various illumination and dark periods. The decomposition of DMP and the photonic efficiency calculated for experiments conducted with periodic illumination were significantly higher than that with continuous illumination, and were evidently increased with short illumination periods and longer dark periods. For instance, more than 80% DMP was decomposed of for experiment conducted with illumination period of 0.1 s, and dark period of 5.0 s. Various researchers have concluded that electron–hole recombination is
Fig. 1. Schematic diagram of the capped cylindrical batch photoreactor system used in this study.
Y. Ku et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 299–305
301
100
2
Light intensity = 120 mW/cm Solution pH = 6 Dissolved oxygen = 8.5 mg/L Total irradiation time = 60 min
Decomposition of DMP
80
60
12
10 -5
Dark period = 5 sec Dark period = 3 sec Dark period = 1 sec Dark period = 0.5 sec Dark period = 0.3 sec Dark period = 0.1 sec
[Na2SO4]0= 0.1 M
8
6
40
4
Photonic efficiency*10
[DMP]0= 10 mg/L
20
19.18% of DMP decomposition by UV-LEDs/TiO2 process
2
under continuous illumination 0 0.0
0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Illumination period (sec) Fig. 2. Effect of illumination period on photocatalytic decomposition of DMP and the photonic efficiency by UV–LED/TiO2 process.
9.0
[DMP]0= 10 mg/L
Illumination period = 5 sec Illumination period = 3 sec Illumination period = 1 sec Illumination period = 0.5 sec Illumination period = 0.3 sec Illumination period = 0.1 sec
[Na2SO4]0= 0.1 M 2
Photocurrent (mA)
7.5
Light intensity = 120 mW/cm Solution pH = 6 Dissolved oxygen = 8.5 mg/L
6.0
4.5
3.0
possibly due to the photo-induced electrons and holes were considered to be consumed more rapidly by recombination than by photocatalytic reaction. Solution pH has been reported to be an important factor for the application of photocatalytic process in aqueous solutions [17,18]. Fig. 4 shows the effect of solution pH on the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination. The decomposition of DMP were similar for experiments conducted in solutions of pH 2 and 3, while the decomposition dropped for experiments conducted in solution of pH 6. The increase in DMP decomposition for experiments conducted in acidic solutions may be ascribed to the presence of excessive H+, thus more photo-induced electrons are transferred and reacted with oxygen molecules adsorbed on the TiO2 surface to generate hydroperoxyl radicals (HO2), to hinder the recombination of electrons with holes [19–21]. It has been reported that photocatalytic reactions were usually enhanced with the application of more intensive illumination [14,22,23]. Fig. 5 demonstrates the effect of light intensity on the photocatalytic decomposition of DMP by UV–LED/TiO2 process 100 [DMP]0= 10 mg/L
pH = 6 pH = 3 pH = 2
[Na2SO4]0= 0.1 M 2
80
Decomposition of DMP (%)
the major concern for the progress of photocatalytic reactions [7,8,15]. Experimental results obtained in this study suggest that the reaction between reacting species (such as oxygen and DMP deposited on the surface of photocatalyst) and photo-induced electrons and/or holes could be accomplished during fairly short illumination periods. Thus, experiments conducted with longer illumination might not be favorable to enhance DMP decomposition, instead was more favorable for electron-hole recombination. However, more time is required for the desorption of product species and the re-adsorption of reacting species occurred on the surface without light irradiation (dark period). Fig. 3 indicates that the photocurrents were increased for experiments conducted with increasing dark period and same illumination periods, further demonstrating enhanced charge transfer to impede electron–hole recombination. For experiments conducted with illumination period of 0.1 s, the photocurrent was significantly enhanced with dark period ranged from 0.1 to 5.0 s. However, as for experiments conducted with illumination period longer than 0.1 s, the inference of dark period on enhanced photocurrent was found to be significantly decreased with increasing illumination period,
Light intensity = 120 mW/cm Dissolved oxygen = 8.5 mg/L Illumination period = 5 sec Total irradiation time = 60 min
60
40
20
1.5
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dark period (sec) Fig. 3. Effect of dark period on the measured photocurrent during photocatalytic decomposition of DMP.
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dark period (sec) Fig. 4. Effect of solution pH on photocatalytic decomposition of DMP by UV–LED/ TiO2 process under periodic illumination.
302
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100
50 2
[DMP]0= 10 mg/L [Na2SO4]0= 0.1 M Solution pH = 6 Dissolved oxygen = 8.5 mg/L Illumination period = 5 sec Total irradiation time = 60 min
2
40
60
40
20
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Dissolved oxygen = 32 mg/L Dissolved oxygen = 8.5 mg/L Dissolved oxygen = 0.4 mg/L
[Na2SO4]0= 0.1 M
Decomposition of DMP (%)
Decomposition (%)
80
[DMP]0= 10 mg/L
Light intensity = 120 mW/cm 2 Light intensity = 60 mW/cm 2 Light intensity = 10 mW/cm
30
20
10
0 0.0
5.0
Dark period (sec)
Light intensity = 120 mW/cm Solution pH = 6 Illumination period = 5 sec Total irradiation time = 60 min
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dark period (sec)
Fig. 5. Effect of light intensity on photocatalytic decomposition of DMP by UV–LED/ TiO2 process under periodic illumination.
Fig. 7. Effect of dissolved oxygen concentration on photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination.
under periodic illumination. The decomposition of DMP was enhanced for experiments conducted with higher light intensities. However, prolonged dark periods apparently improved DMP decomposition for experiments conducted with higher light intensities, possibly because more electron–hole pairs were generated on TiO2 surface with more intense illumination. Thus, longer dark periods were required to promote the replenishment of TiO2 surface. Fig. 6 exhibits the effect of initial DMP concentration on the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination. The decomposition of DMP was decreased for experiments conducted with higher initial DMP concentrations possibly because of the occurrence of abundant reactant and product species on the surface, consequently requiring longer dark period for surface replenishment. It has been reported that the presence of dissolved oxygen might pose significant impact for the application of photocatalytic process in aqueous solutions [24,25]. Fig. 7 demonstrates the effect of dissolved oxygen concentration on the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination. The decomposition of DMP was increased for experiments conducted with higher dissolved oxygen concentrations, possibly
because the existence of additional oxygen molecules. Dissolved oxygen molecules are more readily to react with photo-induced electrons on the TiO2 surface to generate hydroperoxyl radicals (HO2), which is favorable for replenishing the TiO2 surface during the dark period. In this study, the temporal behavior of DMP decomposition in aqueous solution by UV-LED/TiO2 process under periodic illumination was described by a modified Langmuir–Hinshelwood kinetic equation, as exercised by most researchers for the application of photocatalytic processes on various organic compounds [7,26]:
[DMP]0= 5 mg/L 2
Decomposition of DMP (%)
80
[DMP]0= 10 mg/L
Light intensity = 120 mW/cm Solution pH = 6 Dissolved oxygen = 8.5 mg/L Illumination period = 5 sec Total irradiation time = 60 min
[DMP]0= 20 mg/L
60
40
20
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
@C A K ads C A ¼ rA ¼ kg n Im ð Þ @t 1 þ K ads C A
ð2Þ
where CA is the concentration of DMP; rA is the decomposition rate of DMP; t is the reaction time; m is the order with respect to UV light intensity; k is the intrinsic decomposition rate constant of DMP; Kads is the adsorption rate coefficient of DMP on photocatalyst; g is the duty cycle defined by the following Eq. (3); and n is the order with respect to duty cycle.
g¼
Illumination period ðIllumination period þ Dark periodÞ
ð3Þ
The experimental results conducted at various operating condition discussed in previous sections were correlated and described by the following equation: ! 6:99 103 C A ð4Þ rA ¼ 2:11 107 ðg Þ0:51 ðIÞ0:27 1 þ 6:99 103 C A
100 [Na2SO4]0= 0.1 M
5.0
Dark period (sec) Fig. 6. Effect of initial DMP concentration on photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination.
As depicted in Fig. 8, the experimental results and calculated values based on Eq. (4) for the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination are fairly consistent. Based on the kinetic analysis, the decomposition rate of DMP in aqueous solution for UV-LED/TiO2 process under periodic illumination was increased with increasing duty cycle (g). Hence, the decomposition rate of DMP was predictably decreased for experiments conducted with illumination period of 0.1 s and dark period longer than 5.0 s. As shown in Table 1, the order (n) with respect to duty cycle was found to be increased for experiments conducted with longer dark periods, explicitly, the photo-induced electrons and holes were considered to be consumed more rapidly by photocatalytic reaction than by recombination. Several previous researchers
Y. Ku et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 299–305
303
experimental results from this study. DMP molecules were assumed to be adsorbed on TiO2 surface, while the electron-hole pair generated during the illumination period, and then DMP was oxidized into some small molecule intermediates by OH, O2 and h+, intermediates were then desorbed from TiO2 surface. On the other hand, the steps during the dark period include adsorption/ desorption of reacting species and products. Nevertheless, the steps carried out during dark period also occurred during illumination period [26]. It was assumed that the photogenerated electrons and holes were mainly consumed by four main pathways: DMP oxidation, OH oxidation, oxygen reduction and electron–hole recombination. Therefore, transient balances of hole and electron are established and demonstrated by the following equation [29]: þ þ þ d h ¼ kg I kr h ðe Þ ko h nA VA ð5Þ dt
Fig. 8. Effect of duty cycle (g) on photocatalytic degradation of DMP by UV–LED/ TiO2 process under periodic illumination.
Table 1 Effect of dark period on the order of duty cycle (g) for the decomposition of DMP by UV–LED/TiO2 process under periodic illumination. Effect of dark period Dark period (sec) The order of duty cycle R-square
0.1 0.28 0.85
0.3 0.33 0.98
0.5 0.41 0.91
1 0.44 0.95
3 0.53 0.99
5 0.55 0.99
Reaction conditions: Initial DMP concentration = 10 mg/L; initial Na2SO4 concentration = 0.1 M. Light intensity = 120 mW/cm2; solution pH = 6; dissolved oxygen = 8.5 mg/L. Illumination period controlled at 0.1, 0.3, 0.5, 1, 3 and 5 s for experiment conducted at each dark period.
[11,27,28] reported that the apparent quantum yield for periodic illumination was enhanced because both the adsorption for reactants and the desorption of the products on TiO2 surface was improved during the dark period. Preliminarily elucidated reaction steps for photocatalytic decomposition of DMP under periodic illumination are proposed as Fig. 9 based on the
o
þ dðe Þ ¼ kg I kr h ðe Þ kR ðe ÞnB VB dt
where I is the light intensity; kg is the light absorption rate constant; kr is the recombination rate constant; ko is the oxidation rate constant; kR is the electron transfer rate constant; nA and nB are the number of surface sites for hydroxyl or DMP and for O2, respectively; and VA and VB are the surface fractional coverage by hydroxyl or DMP and by O2, respectively. Assuming at steady state, Eqs. (5) and (6) can be further simplified and expressed as below: þ þ ð7Þ kg I ¼ kr h ðe Þ þ ko h nA VA : þ kg I ¼ kr h ðe Þ þ kR ðe ÞnB VB
ð8Þ
The profiles of surface coverage of OH, DMP, O2, O2 and carrier recombination during the photocatalytic process under periodic illumination can be proposed in Fig. 10. Therefore, as described by Eqs. (7) and (8), the recombination of photo-induced electrons and holes is enhanced with decreasing surface coverage of OH or DMP species for experiments conducted with increased illumination period. The dark period allows the replenishment of the adsorbed DMP and O2, while O2 can react with DMP which could advance the replenished surface coverage of DMP. Therefore, the renewal of fractional coverage of DMP and O2 on active sites of photocatalyst surface and O2 react with DMP enhance the utilization efficiency of photons. It is expected that electricity consumption for photocatalytic systems may be markedly improved through periodic illumination. Electric energy per order (EEO), defined as the electric energy required to degrade the initial concentration of contaminant by one order of magnitude in a unit volume of aqueous solution, is frequently used to describe the electrical efficiency of advanced oxidation processes [30]. EEO can be calculated using the following equation for a photocatalytic system operated in an idealized batch reactor: EEO ¼
Fig. 9. Proposed reaction steps for DMP decomposition by photocatalytic process under periodic illumination.
ð6Þ
P Dt V:logðCCAOA Þ
ð9Þ
where P is the rated power in kW, Dt is the total irradiation time in hour and V is the batch reactor volume in m3. As shown in Table 2, the actual power output for UV-LEDs was fairly consistent at around 0.144 kW under continuous or periodic illumination, demonstrating that UV-LEDs exhibited fast response time and stable current even under periodic illumination. Electric energy savings for experiments conducted with illumination period of 5 s and the dark period from 0.5 to 5 s were 9 to 50%.
304
Y. Ku et al. / Journal of Photochemistry and Photobiology A: Chemistry 332 (2017) 299–305
Fig. 10. (a) Proposed profiles of surface coverage by OH, DMP or O2, (b) proposed profiles of surface coverage by O2 and (c) proposed profiles of carrier recombination in the photocatalytic process under periodic illumination.
It was observed that the electric energy saving increased significantly for experiments conducted with increasing dark period. 4. Conclusions For experiments conducted with the same total illumination time, the decomposition of DMP by UV–LED/TiO2 process and the calculated photonic efficiencies were increased with increasing dark period possibly ascribed to the enhancement of surface replenishment and the inhibition of electron–hole recombination. The measurements of photocurrent further confirmed the photogenerated electron–hole recombination was hindered for experiments conducted with longer dark periods. Prolonged dark period
apparently improved DMP decomposition for experiments conducted with higher light intensities. The effect of dark period on the decomposition of DMP in aqueous solution by UV–LED/TiO2 process was greatly affected by the presence of dissolved oxygen. The existence of more oxygen molecules might be beneficial for replenishing the TiO2 surface during the dark periods. The photocatalytic decomposition of DMP was enhanced for experiments conducted in acidic solutions. Photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination was agreeably modeled by the proposed Langmuir–Hinshelwood kinetic equation. Under the same illumination period of 5 s, the calculated electric energy per order was decreased by almost 40% for experiments conducted with the increasing dark period from 0.5 to 5.0 s; thus, the electric energy consumption was substantially decreased.
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Table 2 Energy consumption and energy saving for experiments conducted with various illumination and dark periods. Illumination period (s)
Dark period (s)
Theoretical energy consumption (kW)
Actual energy consumption (kW)
Actual EEO (kWh/m3/order)
Actual energy saving (%)
1 5 5 5
0 5 3 0.5
0.1440
0.1420 0.1450 0.1461 0.1435
10,236 5078 6560 9313
– 50.39 35.91 9.02
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Decomposition of dimethyl phthalate in aqueous solution by UV–LED/ TiO2 process under periodic illumination Young Ku* , Shiau-Ju Shiu, Hsuan-Chih Wu Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
A R T I C L E I N F O
Article history: Received 15 July 2016 Received in revised form 23 August 2016 Accepted 11 September 2016 Available online 13 September 2016 Keywords: Photocatalysis Periodic illumination UV–LEDs Dimethyl phthalate
A B S T R A C T
The effect of controlled periodic illumination on the photonic efficiency and temporal decomposition behavior of dimethyl phthalates (DMP) in aqueous solution by UV–LED/TiO2 process was investigated in this study. For experiments conducted with the same total illumination time, the decomposition of DMP was enhanced with increasing dark period possibly ascribed to the enhancement of surface replenishment and the inhibition of electron–hole recombination. The existence of oxygen molecules might be beneficial for replenishing the TiO2 surface during the dark periods. Experiments conducted with longer illumination might not be favorable to enhance DMP decomposition, instead was more favorable for electron-hole recombination. Photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination was agreeably modeled by the proposed Langmuir–Hinshelwood kinetic equation. Under the same illumination period of 5 s, the calculated energy consumption per order was decreased by more than 40% for experiments conducted with the increasing dark period from 0.5 to 5.0 s; thus, the electric energy consumption was substantially decreased. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl phthalates (DMP) represent a family of chemicals which are widely used as plasticizers, primarily in the production of polyvinyl chloride resins. DMP is considered to be an endocrine disruptor and of particular attention because of its possible accumulation in the environment [1] and is listed as a priority pollutant by US EPA [2]. DMP is a relatively stable compound in natural environment and its hydrolysis half-life is estimated to be about 20 years [3–5]. Glowing concerns on the contamination caused by the discharge of numerous organic compounds to water environment has vigorously stimulated the development of various treatment technologies. Photocatalytic processes using semiconductor catalysts under appropriate illumination have been demonstrated to be effective in decomposing numerous organic species regarding water and wastewater treatment [6]. However, the practical employment of photocatalytic processes is mainly hindered by the development of effective photocatalysts for durable and energysaving application. Anatase form of titanium dioxide (TiO2) is a frequently studied photocatalyst for various environmental
* Corresponding author. E-mail address: [email protected] (Y. Ku). http://dx.doi.org/10.1016/j.jphotochem.2016.09.011 1010-6030/ã 2016 Elsevier B.V. All rights reserved.
applications; however, the effectiveness of TiO2 as photocatalyst is yet hampered primarily by the recombination of photogenerated electrons and holes [7,8]. The adsorption of oxygen molecules onto the photocatalyst surface and/or the transfer of the conduction band electrons to the adsorbed oxygen molecules were suggested to be the rate-limiting steps for most photocatalytic reactions [9]. Periodical illumination of UV light on the catalyst might allow more time for the ratelimiting steps to inhibit the build–up of intermediate species and to reduce the electron–hole recombination [10]. Buechler et al. [11] reported that the apparent quantum yield was enhanced up to 40% for photocatalytic oxidation of gaseous trichloroethylene under periodic illumination because dark periods allow trichloroethylene to be adsorbed on the TiO2 surface. Cornu et al. [12] stated that the quantum yields for the photocatalytic decomposition of aqueous formate were more than doubled for experiments conducted under periodic illumination. Wang et al. [13] demonstrated that periodic illumination increased the quantum yield of formaldehyde formation by 50% than the experiments conducted with continuous illumination. The photonic efficiencies for the photocatalytic decomposition of RR22 and Bisphenol A were significantly promoted for experiment conducted with periodic illumination [14,15]. In this research, the temporal decomposition behavior of aqueous DMP was studied in a batch photocatalytic reactor under controlled UV periodic illumination, and expressed using a
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Langmuir-Hinshelwood-type kinetic model. The effect of illumination and dark periods, solution pH, light intensity, and dissolved oxygen concentration on the photocatalytic decomposition of aqueous DMP were discussed. The duty cycle and electric energy consumption for experiments operated with periodic illumination were also discussed. 2. Experimental The chemicals used in this study were certified reagent grade without further treatment and all experimental solutions were prepared with double distilled water. TiO2 suspension was prepared by dispersing Aeroxide P-25 TiO2 particles in ethanol solution, and was then stirred vigorously for 30 min and sonicated for more than 6 h. A 4 10 cm Ti plate was dipped in the prepared TiO2 suspension to coat TiO2 films on the surface of Ti plate. The TiO2-coated Ti plate was subsequently air dried and sintered at 350 C for one hour. The thickness and surface roughness of the coated TiO2 film were determined by field emission scanning electron microscope (FESEM, JEOL, JSM-6500F). As shown schematically in Fig. 1, the experimental system regarding to the photocatalytic decomposition of DMP in aqueous solution was consisted of a photoreactor and an UV light source. The 200-ml capped rectangular batch photoreactor reactor employed in this study was made entirely of quartz and was water-jacketed to maintain the solution temperature at 20 C for all experiments. 150 ml of aqueous solution containing 10 mg/L of DMP was initially charged to the reactor. Solution pH was kept constant at desired levels by the additions of sodium hydroxide and/or sulfuric acid solutions using a Kyoto Electronic AT-400 automatic potentiometric titrator. The dissolved oxygen of the reaction solution was maintained by varying the air flow rate and was measured with a dissolved oxygen analyzer (Orion 820). The prepared TiO2-coated Ti plate was located at the centerline of the photoreactor. Short wavelength UV-LEDs were used as the light source (Diana, B5-437-CVD, maximum output 180W primarily at 395 nm) and located outside the photoreactor. Light intensity of the UV-LEDs was detected by a radiometer (International Light, IL1400A) equipped with an UV detector (International Light, SEL005/WBS320/TD). UV light intensity and pulse frequency of the UV-LEDs was controlled by a KOEO SPS150-8 programmable DC
power supply. Photocatalytic decomposition of DMP in aqueous solution by UV-LED/TiO2 photocatalytic process under periodic illumination was assumed to be started when the pre-warmed light source was turned on for experiments conducted with illumination period from 0.1 to 5 s and dark period from 0.1 to 5 s. Photocurrent measurements were performed with an Autolab PGSTAT 302N potentiostat using a three electrode setup with Pt as a counter electrode and Ag/AgCl as a reference electrode. At predetermined time periods, aliquots of the reaction solution were withdrawn from the reactor, and were immediately stored in the amber glass bottle for further analysis. The concentration of DMP in the samples was measured on a Spectra Series P100 HPLC equipped with an UV detector at 254 nm. The concentration of total organic carbon (TOC) in reaction solution was analyzed by a Tekmar Dohrmann Phoenix 800 TOC analyzer. 3. Results and discussion Decomposition of dimethyl phthalate in aqueous solution by UV-LED/TiO2 photocatalytic process for experiments conducted with various controlled period illumination were examined in this study. The photonic efficiency, j, is defined as the ratio of the rate of photocatalytic reaction to the rate of incident photons, as given in the following equation [14,16]:
j¼
Rate of reactionðmols1 Þ DCA V=Dt ¼ ðIint =U l Þ Rate of incident photonsðmols1 Þ
ð1Þ
In this study, the rate of photocatalytic reaction is defined as the decomposition of DMP within irradiation time of 60 min. The rate of incident photons is equivalent to the incident light intensity, Iint, divided by the molar photon energy of the 395 nm UV light. Fig. 2 indicates the decomposition of DMP by UV-LED/TiO2 process with total irradiation time of 60 min under various illumination and dark periods. The decomposition of DMP and the photonic efficiency calculated for experiments conducted with periodic illumination were significantly higher than that with continuous illumination, and were evidently increased with short illumination periods and longer dark periods. For instance, more than 80% DMP was decomposed of for experiment conducted with illumination period of 0.1 s, and dark period of 5.0 s. Various researchers have concluded that electron–hole recombination is
Fig. 1. Schematic diagram of the capped cylindrical batch photoreactor system used in this study.
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301
100
2
Light intensity = 120 mW/cm Solution pH = 6 Dissolved oxygen = 8.5 mg/L Total irradiation time = 60 min
Decomposition of DMP
80
60
12
10 -5
Dark period = 5 sec Dark period = 3 sec Dark period = 1 sec Dark period = 0.5 sec Dark period = 0.3 sec Dark period = 0.1 sec
[Na2SO4]0= 0.1 M
8
6
40
4
Photonic efficiency*10
[DMP]0= 10 mg/L
20
19.18% of DMP decomposition by UV-LEDs/TiO2 process
2
under continuous illumination 0 0.0
0 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Illumination period (sec) Fig. 2. Effect of illumination period on photocatalytic decomposition of DMP and the photonic efficiency by UV–LED/TiO2 process.
9.0
[DMP]0= 10 mg/L
Illumination period = 5 sec Illumination period = 3 sec Illumination period = 1 sec Illumination period = 0.5 sec Illumination period = 0.3 sec Illumination period = 0.1 sec
[Na2SO4]0= 0.1 M 2
Photocurrent (mA)
7.5
Light intensity = 120 mW/cm Solution pH = 6 Dissolved oxygen = 8.5 mg/L
6.0
4.5
3.0
possibly due to the photo-induced electrons and holes were considered to be consumed more rapidly by recombination than by photocatalytic reaction. Solution pH has been reported to be an important factor for the application of photocatalytic process in aqueous solutions [17,18]. Fig. 4 shows the effect of solution pH on the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination. The decomposition of DMP were similar for experiments conducted in solutions of pH 2 and 3, while the decomposition dropped for experiments conducted in solution of pH 6. The increase in DMP decomposition for experiments conducted in acidic solutions may be ascribed to the presence of excessive H+, thus more photo-induced electrons are transferred and reacted with oxygen molecules adsorbed on the TiO2 surface to generate hydroperoxyl radicals (HO2), to hinder the recombination of electrons with holes [19–21]. It has been reported that photocatalytic reactions were usually enhanced with the application of more intensive illumination [14,22,23]. Fig. 5 demonstrates the effect of light intensity on the photocatalytic decomposition of DMP by UV–LED/TiO2 process 100 [DMP]0= 10 mg/L
pH = 6 pH = 3 pH = 2
[Na2SO4]0= 0.1 M 2
80
Decomposition of DMP (%)
the major concern for the progress of photocatalytic reactions [7,8,15]. Experimental results obtained in this study suggest that the reaction between reacting species (such as oxygen and DMP deposited on the surface of photocatalyst) and photo-induced electrons and/or holes could be accomplished during fairly short illumination periods. Thus, experiments conducted with longer illumination might not be favorable to enhance DMP decomposition, instead was more favorable for electron-hole recombination. However, more time is required for the desorption of product species and the re-adsorption of reacting species occurred on the surface without light irradiation (dark period). Fig. 3 indicates that the photocurrents were increased for experiments conducted with increasing dark period and same illumination periods, further demonstrating enhanced charge transfer to impede electron–hole recombination. For experiments conducted with illumination period of 0.1 s, the photocurrent was significantly enhanced with dark period ranged from 0.1 to 5.0 s. However, as for experiments conducted with illumination period longer than 0.1 s, the inference of dark period on enhanced photocurrent was found to be significantly decreased with increasing illumination period,
Light intensity = 120 mW/cm Dissolved oxygen = 8.5 mg/L Illumination period = 5 sec Total irradiation time = 60 min
60
40
20
1.5
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dark period (sec) Fig. 3. Effect of dark period on the measured photocurrent during photocatalytic decomposition of DMP.
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dark period (sec) Fig. 4. Effect of solution pH on photocatalytic decomposition of DMP by UV–LED/ TiO2 process under periodic illumination.
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100
50 2
[DMP]0= 10 mg/L [Na2SO4]0= 0.1 M Solution pH = 6 Dissolved oxygen = 8.5 mg/L Illumination period = 5 sec Total irradiation time = 60 min
2
40
60
40
20
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Dissolved oxygen = 32 mg/L Dissolved oxygen = 8.5 mg/L Dissolved oxygen = 0.4 mg/L
[Na2SO4]0= 0.1 M
Decomposition of DMP (%)
Decomposition (%)
80
[DMP]0= 10 mg/L
Light intensity = 120 mW/cm 2 Light intensity = 60 mW/cm 2 Light intensity = 10 mW/cm
30
20
10
0 0.0
5.0
Dark period (sec)
Light intensity = 120 mW/cm Solution pH = 6 Illumination period = 5 sec Total irradiation time = 60 min
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Dark period (sec)
Fig. 5. Effect of light intensity on photocatalytic decomposition of DMP by UV–LED/ TiO2 process under periodic illumination.
Fig. 7. Effect of dissolved oxygen concentration on photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination.
under periodic illumination. The decomposition of DMP was enhanced for experiments conducted with higher light intensities. However, prolonged dark periods apparently improved DMP decomposition for experiments conducted with higher light intensities, possibly because more electron–hole pairs were generated on TiO2 surface with more intense illumination. Thus, longer dark periods were required to promote the replenishment of TiO2 surface. Fig. 6 exhibits the effect of initial DMP concentration on the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination. The decomposition of DMP was decreased for experiments conducted with higher initial DMP concentrations possibly because of the occurrence of abundant reactant and product species on the surface, consequently requiring longer dark period for surface replenishment. It has been reported that the presence of dissolved oxygen might pose significant impact for the application of photocatalytic process in aqueous solutions [24,25]. Fig. 7 demonstrates the effect of dissolved oxygen concentration on the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination. The decomposition of DMP was increased for experiments conducted with higher dissolved oxygen concentrations, possibly
because the existence of additional oxygen molecules. Dissolved oxygen molecules are more readily to react with photo-induced electrons on the TiO2 surface to generate hydroperoxyl radicals (HO2), which is favorable for replenishing the TiO2 surface during the dark period. In this study, the temporal behavior of DMP decomposition in aqueous solution by UV-LED/TiO2 process under periodic illumination was described by a modified Langmuir–Hinshelwood kinetic equation, as exercised by most researchers for the application of photocatalytic processes on various organic compounds [7,26]:
[DMP]0= 5 mg/L 2
Decomposition of DMP (%)
80
[DMP]0= 10 mg/L
Light intensity = 120 mW/cm Solution pH = 6 Dissolved oxygen = 8.5 mg/L Illumination period = 5 sec Total irradiation time = 60 min
[DMP]0= 20 mg/L
60
40
20
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
@C A K ads C A ¼ rA ¼ kg n Im ð Þ @t 1 þ K ads C A
ð2Þ
where CA is the concentration of DMP; rA is the decomposition rate of DMP; t is the reaction time; m is the order with respect to UV light intensity; k is the intrinsic decomposition rate constant of DMP; Kads is the adsorption rate coefficient of DMP on photocatalyst; g is the duty cycle defined by the following Eq. (3); and n is the order with respect to duty cycle.
g¼
Illumination period ðIllumination period þ Dark periodÞ
ð3Þ
The experimental results conducted at various operating condition discussed in previous sections were correlated and described by the following equation: ! 6:99 103 C A ð4Þ rA ¼ 2:11 107 ðg Þ0:51 ðIÞ0:27 1 þ 6:99 103 C A
100 [Na2SO4]0= 0.1 M
5.0
Dark period (sec) Fig. 6. Effect of initial DMP concentration on photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination.
As depicted in Fig. 8, the experimental results and calculated values based on Eq. (4) for the photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination are fairly consistent. Based on the kinetic analysis, the decomposition rate of DMP in aqueous solution for UV-LED/TiO2 process under periodic illumination was increased with increasing duty cycle (g). Hence, the decomposition rate of DMP was predictably decreased for experiments conducted with illumination period of 0.1 s and dark period longer than 5.0 s. As shown in Table 1, the order (n) with respect to duty cycle was found to be increased for experiments conducted with longer dark periods, explicitly, the photo-induced electrons and holes were considered to be consumed more rapidly by photocatalytic reaction than by recombination. Several previous researchers
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experimental results from this study. DMP molecules were assumed to be adsorbed on TiO2 surface, while the electron-hole pair generated during the illumination period, and then DMP was oxidized into some small molecule intermediates by OH, O2 and h+, intermediates were then desorbed from TiO2 surface. On the other hand, the steps during the dark period include adsorption/ desorption of reacting species and products. Nevertheless, the steps carried out during dark period also occurred during illumination period [26]. It was assumed that the photogenerated electrons and holes were mainly consumed by four main pathways: DMP oxidation, OH oxidation, oxygen reduction and electron–hole recombination. Therefore, transient balances of hole and electron are established and demonstrated by the following equation [29]: þ þ þ d h ¼ kg I kr h ðe Þ ko h nA VA ð5Þ dt
Fig. 8. Effect of duty cycle (g) on photocatalytic degradation of DMP by UV–LED/ TiO2 process under periodic illumination.
Table 1 Effect of dark period on the order of duty cycle (g) for the decomposition of DMP by UV–LED/TiO2 process under periodic illumination. Effect of dark period Dark period (sec) The order of duty cycle R-square
0.1 0.28 0.85
0.3 0.33 0.98
0.5 0.41 0.91
1 0.44 0.95
3 0.53 0.99
5 0.55 0.99
Reaction conditions: Initial DMP concentration = 10 mg/L; initial Na2SO4 concentration = 0.1 M. Light intensity = 120 mW/cm2; solution pH = 6; dissolved oxygen = 8.5 mg/L. Illumination period controlled at 0.1, 0.3, 0.5, 1, 3 and 5 s for experiment conducted at each dark period.
[11,27,28] reported that the apparent quantum yield for periodic illumination was enhanced because both the adsorption for reactants and the desorption of the products on TiO2 surface was improved during the dark period. Preliminarily elucidated reaction steps for photocatalytic decomposition of DMP under periodic illumination are proposed as Fig. 9 based on the
o
þ dðe Þ ¼ kg I kr h ðe Þ kR ðe ÞnB VB dt
where I is the light intensity; kg is the light absorption rate constant; kr is the recombination rate constant; ko is the oxidation rate constant; kR is the electron transfer rate constant; nA and nB are the number of surface sites for hydroxyl or DMP and for O2, respectively; and VA and VB are the surface fractional coverage by hydroxyl or DMP and by O2, respectively. Assuming at steady state, Eqs. (5) and (6) can be further simplified and expressed as below: þ þ ð7Þ kg I ¼ kr h ðe Þ þ ko h nA VA : þ kg I ¼ kr h ðe Þ þ kR ðe ÞnB VB
ð8Þ
The profiles of surface coverage of OH, DMP, O2, O2 and carrier recombination during the photocatalytic process under periodic illumination can be proposed in Fig. 10. Therefore, as described by Eqs. (7) and (8), the recombination of photo-induced electrons and holes is enhanced with decreasing surface coverage of OH or DMP species for experiments conducted with increased illumination period. The dark period allows the replenishment of the adsorbed DMP and O2, while O2 can react with DMP which could advance the replenished surface coverage of DMP. Therefore, the renewal of fractional coverage of DMP and O2 on active sites of photocatalyst surface and O2 react with DMP enhance the utilization efficiency of photons. It is expected that electricity consumption for photocatalytic systems may be markedly improved through periodic illumination. Electric energy per order (EEO), defined as the electric energy required to degrade the initial concentration of contaminant by one order of magnitude in a unit volume of aqueous solution, is frequently used to describe the electrical efficiency of advanced oxidation processes [30]. EEO can be calculated using the following equation for a photocatalytic system operated in an idealized batch reactor: EEO ¼
Fig. 9. Proposed reaction steps for DMP decomposition by photocatalytic process under periodic illumination.
ð6Þ
P Dt V:logðCCAOA Þ
ð9Þ
where P is the rated power in kW, Dt is the total irradiation time in hour and V is the batch reactor volume in m3. As shown in Table 2, the actual power output for UV-LEDs was fairly consistent at around 0.144 kW under continuous or periodic illumination, demonstrating that UV-LEDs exhibited fast response time and stable current even under periodic illumination. Electric energy savings for experiments conducted with illumination period of 5 s and the dark period from 0.5 to 5 s were 9 to 50%.
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Fig. 10. (a) Proposed profiles of surface coverage by OH, DMP or O2, (b) proposed profiles of surface coverage by O2 and (c) proposed profiles of carrier recombination in the photocatalytic process under periodic illumination.
It was observed that the electric energy saving increased significantly for experiments conducted with increasing dark period. 4. Conclusions For experiments conducted with the same total illumination time, the decomposition of DMP by UV–LED/TiO2 process and the calculated photonic efficiencies were increased with increasing dark period possibly ascribed to the enhancement of surface replenishment and the inhibition of electron–hole recombination. The measurements of photocurrent further confirmed the photogenerated electron–hole recombination was hindered for experiments conducted with longer dark periods. Prolonged dark period
apparently improved DMP decomposition for experiments conducted with higher light intensities. The effect of dark period on the decomposition of DMP in aqueous solution by UV–LED/TiO2 process was greatly affected by the presence of dissolved oxygen. The existence of more oxygen molecules might be beneficial for replenishing the TiO2 surface during the dark periods. The photocatalytic decomposition of DMP was enhanced for experiments conducted in acidic solutions. Photocatalytic decomposition of DMP by UV–LED/TiO2 process under periodic illumination was agreeably modeled by the proposed Langmuir–Hinshelwood kinetic equation. Under the same illumination period of 5 s, the calculated electric energy per order was decreased by almost 40% for experiments conducted with the increasing dark period from 0.5 to 5.0 s; thus, the electric energy consumption was substantially decreased.
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Table 2 Energy consumption and energy saving for experiments conducted with various illumination and dark periods. Illumination period (s)
Dark period (s)
Theoretical energy consumption (kW)
Actual energy consumption (kW)
Actual EEO (kWh/m3/order)
Actual energy saving (%)
1 5 5 5
0 5 3 0.5
0.1440
0.1420 0.1450 0.1461 0.1435
10,236 5078 6560 9313
– 50.39 35.91 9.02
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