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Photocatalytic Degradation of Organophosphoros Pesticides Using TiO2Supported on Fiberglass
JOBNAME: MIC 54#1 96 PAGE: 1 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
MICROCHEMICAL JOURNAL ARTICLE NO. 0076
54, 54–58 (1996)
Photocatalytic Degradation of Organophosphoros Pesticides Using TiO2 Supported on Fiberglass CHEN SHIFU,1 ZHAO MENGYUE,
AND
TAO YAOWU
Department of Chemical Engineering, Zhengzhou Institute of Technology, Zhengzhou, 450002, Henan, People’s Republic of China Received August 10, 1995; accepted November 10, 1995 This paper studies the feasibility of photocatalytic degradation of organophosphoros pesticides using supported TiO2 as a catalyst, which is prepared by thermal decomposition and calcination of colloidal solution made from hydrolysis of titanium tetraisopropoxide [Ti(iso-OC3H7)4] on fiberglass cloth. The results show that 0.65 × 10−4 mol/dm3 of dichlorvos, monocrotophos, phorate, and parathion can be completely photocatalytically degraded into PO3− 4 after 200 min illumination with a 375 W medium pressure mercury lamp; the TiO2 supported on the fiberglass is not easily removed and after 120 h illumination there is no significant loss of photocatalytic activity of TiO2. © 1996 Academic Press, Inc.
1. INTRODUCTION In recent years the photocatalytic degradation of organic pollutants at semiconductors has been studied extensively (1). Such reactions have usually been carried out using suspensions of semiconductor particles in aqueous solutions irradiated with band-gap light. Recently we published some results (2) of photocatalytic degradation of organophosphoros pesticides in aqueous titanium dioxide suspension. Harada et al. (3, 4) and Grätzel et al. (5) have also reported the results of photocatalytic degradation of organophosphoros compounds. These reports are satisfactory. But it is evident that in any waste water treatment process filtration and resuspension of semiconductor powders should be avoided if possible. Sabate et al. (6) and Lu et al. (7) have supported TiO2 on a glass external surface and a glass internal surface, respectively; both systems proved to be both stable and efficacious catalysts for photodegradation of organic pollutants. In this paper, we describe a method of preparing TiO2 supported on fiberglass cloth and discuss the feasibility of photocatalytic degradation of organophosphoros pesticides using supported TiO2. 2. EXPERIMENTAL 2.1. Materials Four organophosphoros pesticides used in the experiments were dichlorvos (C 4 H 7 Cl 2 O 4 P, 98% Pur.), monocrotophos (C 7 H 14 NO 5 P, 98% Pur.), parathion (C10H14O5NPS, 97% Pur.), and phorate (C7H17O2PS3, 97% Pur.). Ti(iso-OC3H7)4 was prepared in our laboratory. Isopropanol and other reagents were analytically pure grade. 1
To whom correspondence should be addressed. 54
0026-265X/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
JOBNAME: MIC 54#1 96 PAGE: 2 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
PHOTOCATALYTIC PESTICIDE DEGRADATION ON TiO2
55
2.2. Photoreactor and Procedure Experiments were carried out in a quartz photochemical reactor. A schematic diagram of the photochemical reactor is shown in Fig. 1. The cylindrical annular-type reactor consists of three parts. The first part is an empty chamber in which a 375 W medium pressure mercury lamp is hung. The second part is an inside thimble. Running water is passed through the thimble to cool the reaction solution and to remove the IR fraction of the light as well as any irradiation below 300 nm. Owing to the continuous cooling, the temperature of the reaction solution is maintained at approximately 30°C. The third part is an outside thimble. At the start of the experiment the fiberglass cloth supported TiO2 was wrapped between the inside thimble and the outside thimble and the reaction solution (volume, 400 ml) was put in the reaction chamber. Air was introduced into the reaction solution through the gas entry at the base of the reactor. All experiments were performed at an initial pH of 6.5; air flow rate was 0.03 m3/h at 1 atm. After illumination, samples (volume of each sample was 10 cm3) were taken intermittently for analysis. 2.3. Analysis One of the final degradation products of organophosphoros pesticides is PO3− 4 , the can thus express the rate of complete degradation of organophosformation rate of PO3− 4 phoros pesticides. The determination of PO3− 4 was performed colorimetrically by the molybdenum blue method. The photodegradation efficiency for each sample was calculated from the expression
h4
Pt 2 100%, P0
where h is photodegradation efficiency; Pt is amount of phosphate in solution after t illumination, and P0 is whole amount of organophosphate in solution before illumination.
FIG. 1. Schematic diagram of photochemical reactor. (1) Lamp; (2) water-cooling inlet; (3) water-cooling outlet; (4) vent; (5) in–out sampling port; (6) gas entry; (7) reaction chamber.
JOBNAME: MIC 54#1 96 PAGE: 3 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
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CHEN, ZHAO, AND TAO
3. RESULTS AND DISCUSSION 3.1. Preparation of the Colloidal Solution The colloidal solution was prepared according to Ref. (6). A solution of titanium tetraisopropoxide [Ti(iso-OC3H7)4] in isopropanol was rapidly mixed with water at room temperature in a well stirred vessel. The hydrolysis reaction was allowed to procede for 0.5 h: Ti~iso-OC3H7!4 + 4H2O → Ti~OH!4 + 4C3H7OH. The above hydroxide precipitate was peptized with appropriate amounts of HNO3 to form a highly dispersed stable colloidal solution. The final colloidal solution was aged for 12 h before being deposited on a fiberglass cloth. The fiberglass cloth was coated with the colloidal solution using the following procedure: the fiberglass cloth was dipped into the colloidal solution for approximately 2 min and then it was withdrawn almost vertically at rates ranging from 10 to 20 cm/min. Finally the coated material was dried at room temperature for 2 h in a dust-free environment before heat treatment. 3.2. Heat Treatment The colloidal solution was dried at room temperature in a dust-free environment. The resulting solid was calcinated in air at 500, 550, 580, 600, 620, and 650°C for 1.5, 3, and 5 h to form different TiO2 powder samples. The photocatalytic activity of each TiO2 sample was determined by degradation of monocrotophos. For each temperature investigated, 400 ml of 0.65 × 10−4 mol/dm3 of monocrotophos was introduced into the photoreactor with 1.2 g/dm3 TiO2 powders. The experimental methods were described previously (2). After 20 min illumination by a 375 W medium pressure mercury lamp the reaction solution was immediately centrifuged for analysis. The results are shown in Table 1. From the Table 1 it can be seen that the coated material on the fiberglass cloth should be treated at 580°C for 3 h. The procedure of thermal treatment was as follows: the temperature was first raised from room temperature to 580°C in 4 h, maintained at 580°C for 2 h, and allowed to come to room temperature in 4 h or more. It is known from weighing that the amount of TiO2 supported on the fiberglass cloth is 0.8 g. From analysis of x-ray powder diffraction patterns, it can be seen that the anatase–rutile ratio is about 2:1, and from SEM observation, the average diameter of the particles is about 5 mm. TABLE 1 The Experimental Results of the Photocatalytic Activity of TiO2 T (°C):a b
t (h): h (%)c a
500 °C 1.5 12
3 16
550 °C 5 23
1.5 31
3 40
580 °C 5 45
1.5 49
Calcination temperature. Thermal treatment time. c Photodegradation efficiency of monocrotophos. b
3 58
600 °C 5 44
1.5 51
3 55
620 °C 5 41
1.5 50
3 48
650 °C 5 37
1.5 45
3 38
5 26
JOBNAME: MIC 54#1 96 PAGE: 4 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
57
PHOTOCATALYTIC PESTICIDE DEGRADATION ON TiO2
3.3. Photocatalytic Degradation of Organophosphoros Pesticides The relationship between the photodegradation efficiency of organophosphoros pesticides and the illumination time is shown in Table 2. It is clear that the photodegradation efficiency increases with increasing illumination time. For 0.65 × 10−4 mol/dm3 of dichlorvos and monocrotophos the photodegradation efficiency are above 90% after 90 min illumination; and parathion and phorate can be completely photocatalytically degraded into PO−3 4 after 200 min illumination. The process and mechanism of photocatalytic degradation of organophosphoros pesticides have been described (2, 5). The photocatalytic degradation of the four organophosphoros pesticides is shown by Eq. (1)–(4): 9 (CH3O)2POOCHCCI2 + O2 2 − + → PO3− 4 + 2CI + 4CO2 + 5H + H2O
(1)
21 O 2 2 + → NO−3 + PO3− 4 + 7CO2 + 5H2O + 4H
(CH3O)2POOCCH3CHCONHCH3 +
(2)
(C2H5O)2PSOC6H4NO2 + 15O2 3− − + → SO2− 4 + PO4 + NO3 + 10CO2 + 4H2O + 6H
(3)
(C2H5O)2PSSCH2SC2H5 + 16O2 3− + → 3SO2− 4 + PO4 + 7CO2 + 4H2O + 9H .
(4)
The formation of substances such as Cl−, CO2, SO2− 4 has been confirmed by experiments in our laboratory (2). The data in Table 2 also show that with the same concentration of the organophosphoros pesticides, the photodegradation efficiency decreases as follows: Dichlorvos 4 Monocrotophos > Phorate > Parathion, From this series it is clear that the photodegradation efficiency is related to the structure of the organophosphoros pesticides.
TABLE 2 Relationship Between the Photodegradation Efficiency of Organophosphoros Pesticides and the Illumination Time Illumination time (min)
10
30
50
70
90
120
150
180
200
Dichlorvos h (%) Monocrotophos h (%) Phorate h (%) Parathion h (%)
27.3 24.1 15.0 9.8
52.0 48.6 32.0 22.3
72.1 69.8 42.6 32.8
85.6 83.9 55.7 42.8
93.0 90.5 60.0 45.0
100 100 72.0 59.3
89.1 74.5
100 91.0
100
Note. Initial concentration: 0.65 × 10−4 mol/dm3.
JOBNAME: MIC 54#1 96 PAGE: 5 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
58
CHEN, ZHAO, AND TAO
3.4. The Photocatalytic Activity Life of TiO2 The results show no significant loss of the photocatalytic activity of TiO2 after 120 h illumination by a 375 W medium pressure lamp. The TiO2 supported on the fiberglass cloth is not easily removed. 4. CONCLUSION TiO2 which is made from [Ti(iso-OC3H7)4] supported on fiberglass cloth is able to function as a catalyst for the photodegradation of organophosphoros pesticides. Although the illumination time is longer than in a suspension of TiO2 when the amount of TiO2 is within a weight limit (2) it is not necessary to recover TiO2 powders. REFERENCES 1. 2. 3. 4. 5. 6. 7.
Ollis, D. F.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol., 1991, 25, 1523–1529. Zhao Mengyue; Luo Jufen Huagong Huanbao, 1993, 13, 74–79. Harada, K.; Hisanaga, T.; Tanaka, K. New J. Chem., 1987, 11, 597–600. Harada, K.; Hisanaga, T.; Tanaka, K. Water Res., 1990, 24, 1415–1417. Grätzel, C. K.; Jirousek, M.; Grätzel, M. J. Mol. Catal., 1990, 60, 375–387. Sabate, J.; Anderson, M. A.; Kikkawa, H. A. J. Catal., 1991, 127, 167–177. Lu, M. C.; Roam, G. D.; Chen, J. N.; Huang, C. P. J. Photochem. Photobiol. A. Chem., 1993, 76, 103–110.
MICROCHEMICAL JOURNAL ARTICLE NO. 0076
54, 54–58 (1996)
Photocatalytic Degradation of Organophosphoros Pesticides Using TiO2 Supported on Fiberglass CHEN SHIFU,1 ZHAO MENGYUE,
AND
TAO YAOWU
Department of Chemical Engineering, Zhengzhou Institute of Technology, Zhengzhou, 450002, Henan, People’s Republic of China Received August 10, 1995; accepted November 10, 1995 This paper studies the feasibility of photocatalytic degradation of organophosphoros pesticides using supported TiO2 as a catalyst, which is prepared by thermal decomposition and calcination of colloidal solution made from hydrolysis of titanium tetraisopropoxide [Ti(iso-OC3H7)4] on fiberglass cloth. The results show that 0.65 × 10−4 mol/dm3 of dichlorvos, monocrotophos, phorate, and parathion can be completely photocatalytically degraded into PO3− 4 after 200 min illumination with a 375 W medium pressure mercury lamp; the TiO2 supported on the fiberglass is not easily removed and after 120 h illumination there is no significant loss of photocatalytic activity of TiO2. © 1996 Academic Press, Inc.
1. INTRODUCTION In recent years the photocatalytic degradation of organic pollutants at semiconductors has been studied extensively (1). Such reactions have usually been carried out using suspensions of semiconductor particles in aqueous solutions irradiated with band-gap light. Recently we published some results (2) of photocatalytic degradation of organophosphoros pesticides in aqueous titanium dioxide suspension. Harada et al. (3, 4) and Grätzel et al. (5) have also reported the results of photocatalytic degradation of organophosphoros compounds. These reports are satisfactory. But it is evident that in any waste water treatment process filtration and resuspension of semiconductor powders should be avoided if possible. Sabate et al. (6) and Lu et al. (7) have supported TiO2 on a glass external surface and a glass internal surface, respectively; both systems proved to be both stable and efficacious catalysts for photodegradation of organic pollutants. In this paper, we describe a method of preparing TiO2 supported on fiberglass cloth and discuss the feasibility of photocatalytic degradation of organophosphoros pesticides using supported TiO2. 2. EXPERIMENTAL 2.1. Materials Four organophosphoros pesticides used in the experiments were dichlorvos (C 4 H 7 Cl 2 O 4 P, 98% Pur.), monocrotophos (C 7 H 14 NO 5 P, 98% Pur.), parathion (C10H14O5NPS, 97% Pur.), and phorate (C7H17O2PS3, 97% Pur.). Ti(iso-OC3H7)4 was prepared in our laboratory. Isopropanol and other reagents were analytically pure grade. 1
To whom correspondence should be addressed. 54
0026-265X/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
JOBNAME: MIC 54#1 96 PAGE: 2 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
PHOTOCATALYTIC PESTICIDE DEGRADATION ON TiO2
55
2.2. Photoreactor and Procedure Experiments were carried out in a quartz photochemical reactor. A schematic diagram of the photochemical reactor is shown in Fig. 1. The cylindrical annular-type reactor consists of three parts. The first part is an empty chamber in which a 375 W medium pressure mercury lamp is hung. The second part is an inside thimble. Running water is passed through the thimble to cool the reaction solution and to remove the IR fraction of the light as well as any irradiation below 300 nm. Owing to the continuous cooling, the temperature of the reaction solution is maintained at approximately 30°C. The third part is an outside thimble. At the start of the experiment the fiberglass cloth supported TiO2 was wrapped between the inside thimble and the outside thimble and the reaction solution (volume, 400 ml) was put in the reaction chamber. Air was introduced into the reaction solution through the gas entry at the base of the reactor. All experiments were performed at an initial pH of 6.5; air flow rate was 0.03 m3/h at 1 atm. After illumination, samples (volume of each sample was 10 cm3) were taken intermittently for analysis. 2.3. Analysis One of the final degradation products of organophosphoros pesticides is PO3− 4 , the can thus express the rate of complete degradation of organophosformation rate of PO3− 4 phoros pesticides. The determination of PO3− 4 was performed colorimetrically by the molybdenum blue method. The photodegradation efficiency for each sample was calculated from the expression
h4
Pt 2 100%, P0
where h is photodegradation efficiency; Pt is amount of phosphate in solution after t illumination, and P0 is whole amount of organophosphate in solution before illumination.
FIG. 1. Schematic diagram of photochemical reactor. (1) Lamp; (2) water-cooling inlet; (3) water-cooling outlet; (4) vent; (5) in–out sampling port; (6) gas entry; (7) reaction chamber.
JOBNAME: MIC 54#1 96 PAGE: 3 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
56
CHEN, ZHAO, AND TAO
3. RESULTS AND DISCUSSION 3.1. Preparation of the Colloidal Solution The colloidal solution was prepared according to Ref. (6). A solution of titanium tetraisopropoxide [Ti(iso-OC3H7)4] in isopropanol was rapidly mixed with water at room temperature in a well stirred vessel. The hydrolysis reaction was allowed to procede for 0.5 h: Ti~iso-OC3H7!4 + 4H2O → Ti~OH!4 + 4C3H7OH. The above hydroxide precipitate was peptized with appropriate amounts of HNO3 to form a highly dispersed stable colloidal solution. The final colloidal solution was aged for 12 h before being deposited on a fiberglass cloth. The fiberglass cloth was coated with the colloidal solution using the following procedure: the fiberglass cloth was dipped into the colloidal solution for approximately 2 min and then it was withdrawn almost vertically at rates ranging from 10 to 20 cm/min. Finally the coated material was dried at room temperature for 2 h in a dust-free environment before heat treatment. 3.2. Heat Treatment The colloidal solution was dried at room temperature in a dust-free environment. The resulting solid was calcinated in air at 500, 550, 580, 600, 620, and 650°C for 1.5, 3, and 5 h to form different TiO2 powder samples. The photocatalytic activity of each TiO2 sample was determined by degradation of monocrotophos. For each temperature investigated, 400 ml of 0.65 × 10−4 mol/dm3 of monocrotophos was introduced into the photoreactor with 1.2 g/dm3 TiO2 powders. The experimental methods were described previously (2). After 20 min illumination by a 375 W medium pressure mercury lamp the reaction solution was immediately centrifuged for analysis. The results are shown in Table 1. From the Table 1 it can be seen that the coated material on the fiberglass cloth should be treated at 580°C for 3 h. The procedure of thermal treatment was as follows: the temperature was first raised from room temperature to 580°C in 4 h, maintained at 580°C for 2 h, and allowed to come to room temperature in 4 h or more. It is known from weighing that the amount of TiO2 supported on the fiberglass cloth is 0.8 g. From analysis of x-ray powder diffraction patterns, it can be seen that the anatase–rutile ratio is about 2:1, and from SEM observation, the average diameter of the particles is about 5 mm. TABLE 1 The Experimental Results of the Photocatalytic Activity of TiO2 T (°C):a b
t (h): h (%)c a
500 °C 1.5 12
3 16
550 °C 5 23
1.5 31
3 40
580 °C 5 45
1.5 49
Calcination temperature. Thermal treatment time. c Photodegradation efficiency of monocrotophos. b
3 58
600 °C 5 44
1.5 51
3 55
620 °C 5 41
1.5 50
3 48
650 °C 5 37
1.5 45
3 38
5 26
JOBNAME: MIC 54#1 96 PAGE: 4 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
57
PHOTOCATALYTIC PESTICIDE DEGRADATION ON TiO2
3.3. Photocatalytic Degradation of Organophosphoros Pesticides The relationship between the photodegradation efficiency of organophosphoros pesticides and the illumination time is shown in Table 2. It is clear that the photodegradation efficiency increases with increasing illumination time. For 0.65 × 10−4 mol/dm3 of dichlorvos and monocrotophos the photodegradation efficiency are above 90% after 90 min illumination; and parathion and phorate can be completely photocatalytically degraded into PO−3 4 after 200 min illumination. The process and mechanism of photocatalytic degradation of organophosphoros pesticides have been described (2, 5). The photocatalytic degradation of the four organophosphoros pesticides is shown by Eq. (1)–(4): 9 (CH3O)2POOCHCCI2 + O2 2 − + → PO3− 4 + 2CI + 4CO2 + 5H + H2O
(1)
21 O 2 2 + → NO−3 + PO3− 4 + 7CO2 + 5H2O + 4H
(CH3O)2POOCCH3CHCONHCH3 +
(2)
(C2H5O)2PSOC6H4NO2 + 15O2 3− − + → SO2− 4 + PO4 + NO3 + 10CO2 + 4H2O + 6H
(3)
(C2H5O)2PSSCH2SC2H5 + 16O2 3− + → 3SO2− 4 + PO4 + 7CO2 + 4H2O + 9H .
(4)
The formation of substances such as Cl−, CO2, SO2− 4 has been confirmed by experiments in our laboratory (2). The data in Table 2 also show that with the same concentration of the organophosphoros pesticides, the photodegradation efficiency decreases as follows: Dichlorvos 4 Monocrotophos > Phorate > Parathion, From this series it is clear that the photodegradation efficiency is related to the structure of the organophosphoros pesticides.
TABLE 2 Relationship Between the Photodegradation Efficiency of Organophosphoros Pesticides and the Illumination Time Illumination time (min)
10
30
50
70
90
120
150
180
200
Dichlorvos h (%) Monocrotophos h (%) Phorate h (%) Parathion h (%)
27.3 24.1 15.0 9.8
52.0 48.6 32.0 22.3
72.1 69.8 42.6 32.8
85.6 83.9 55.7 42.8
93.0 90.5 60.0 45.0
100 100 72.0 59.3
89.1 74.5
100 91.0
100
Note. Initial concentration: 0.65 × 10−4 mol/dm3.
JOBNAME: MIC 54#1 96 PAGE: 5 SESS: 2 OUTPUT: Wed Aug 7 11:41:42 1996 /xypage/worksmart/tsp000/72869f/14pu
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CHEN, ZHAO, AND TAO
3.4. The Photocatalytic Activity Life of TiO2 The results show no significant loss of the photocatalytic activity of TiO2 after 120 h illumination by a 375 W medium pressure lamp. The TiO2 supported on the fiberglass cloth is not easily removed. 4. CONCLUSION TiO2 which is made from [Ti(iso-OC3H7)4] supported on fiberglass cloth is able to function as a catalyst for the photodegradation of organophosphoros pesticides. Although the illumination time is longer than in a suspension of TiO2 when the amount of TiO2 is within a weight limit (2) it is not necessary to recover TiO2 powders. REFERENCES 1. 2. 3. 4. 5. 6. 7.
Ollis, D. F.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol., 1991, 25, 1523–1529. Zhao Mengyue; Luo Jufen Huagong Huanbao, 1993, 13, 74–79. Harada, K.; Hisanaga, T.; Tanaka, K. New J. Chem., 1987, 11, 597–600. Harada, K.; Hisanaga, T.; Tanaka, K. Water Res., 1990, 24, 1415–1417. Grätzel, C. K.; Jirousek, M.; Grätzel, M. J. Mol. Catal., 1990, 60, 375–387. Sabate, J.; Anderson, M. A.; Kikkawa, H. A. J. Catal., 1991, 127, 167–177. Lu, M. C.; Roam, G. D.; Chen, J. N.; Huang, C. P. J. Photochem. Photobiol. A. Chem., 1993, 76, 103–110.