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Efficient photocatalytic degradation of chloral hydrate in aqueous semiconductor suspension
Journal of Photochemistry
and Photobiology,
A: Chemistry, 48 (1989)
155 - 159
155
EFFICIENT PHOTOCATALYTIC DEGRADATION OF CHLORAL HYDRATE IN AQUEOUS SEMICONDUCTOR SUSPENSION KEIICHI
TANAKA,
TERUAKI
HISANAGA
and KENJI
National Chemical Laboratory for Industry, Wig-hi (Received
August
1,198s
l-l,
HARADA
Tsukuba (Japan)
; in revised form 3 January 1989)
Summary
Chloral hydrate was degraded in the presence of suspended TiOz by illumination with a super-high-pressure mercury lamp through the Pyrex glass wall of a cell. The addition of H,O, enhanced the degradation rate 3.4 times. The degradation rate increases with pH in the absence of H,O, and exhibits a maximum at pH 5.5 in the presence of H,O,. Of the semiconductors tested ZnO was the most efficient catalyst. The effect of H,Oz was observed only with TiOz and WOs.
1. Introduction Photocatalytic degradation of organic and inorganic materials on semiconductor powder is a new method for waste water treatment_ Several pollutants in aqueous solution have been reported to be degraded by this method [l - 121. The photochemical process for waste water treatment, in general, requires a large amount of electric power at high cost. Hence a high efficiency is required for practical applications. We found that the addition of hydrogen peroxide enhanced the photocatalytic efficiency of the semiconductor considerably [13]. The degradation of chloral hydrate on semiconductor powder in the presence of H,Oz is reported in this paper. Chloral hydrate is a toxic compound used for the synthesis of insecticides. 2. Experimental details The use of Ti02 has been described previously [ll]. The other semiconductors were reagent grade commercial products. 0.9 g and 0.08 g of semiconductor powders were stirred in a 500 ml Pyrex glass flask and a 30 ml Pyrex glass bottle containing 300 ml and 25 ml of aqueous chloral hydrate solution respectively; these were illuminated by a 500 W super-highpressure mercury lamp through an IR cut-off filter_ The degradation rate was 1010-6030/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
156
estimated by determining the remaining non-degraded chloral hydrate by a gas-chromatograph equipped with an electron capture detector. For the gaschromatographic analysis the illuminated sample was centrifuged and the supematant solution was extracted with n-hexane. The concentration of Clwas determined colourimetrically by using mercury thiocyanate and iron(I1) perchlorate as colouring agents [14]_ The total organic carbon was measured by a Shimadzu TOG500. The amount of evolved COP was determined as described previously Ill]. Metal was loaded onto TiOz by the photochemical method 1153. When investigating the effects of pH on the degradation rate and on O2 evolution, the pH of the solution was adjusted by adding appropriate amounts of HCl and KOH solutions.
3. Results and discussion The photodegradation rate of chloral hydrate and the effect of the addition of H,Oz are shown in Fig. 1. The degradation reaction follows first-order reaction kinetics. The apparent rate constants obtained from the data in Fig’. 1 are 0.039 min-l and 0.016 min-’ in the presence and absence of H,O, respectively. It was thus found that addition of H,O, accelerated the degradation 2.4 times. In the other experiment. with 10T3 M chloral hydrate solution, the degradation was 3.4 times faster in the presence of H202 (2 X lo-* M) than in its absence. These values are smaller than those obtained for the other organic ‘halide compounds studied previously [ 131. The effect of the concentration of H202 is shown in Fig. 2. The optimum concentration ranges from 3.4 X 10Y4 to 7 X 10m3 M for 1O-4 M chloral hydrate solution. The photocatalytic efficiency decreased as the concentration of H202 increased beyond the optimum. The blank experiments for either illuminated chloral hydrate solution or the suspensiop containing Ti02 and chloral hydrate in the dark showed that both the iHumination and the presence of TiOz were necessary for the degradation. :.
r”
1.0
5.5 . A
2
-0 lx l-l
5.073 . 3
z $5
I
Q5
c1 -1
45 .2Z& z 40 - 1 -
E= t% C-Z-o 0
60 Illumination
120
35
0
time(min.)
Fig. 1. Degradation of chloral hydrate in the presence and absence of Hz02 and the formation of Cl- (A) and H+ (a) by the degradation in the presence of Hz02. Chloral 0, in the presence of hydrate, 1O-4 M; TiOz, 3 g 1-l; H202, 3.4 X 10 --j M. Degradation: H202 ; A, in the absence of Hz02 ; X, in the presence of Hz02 and in the absence of TiO,.
157 0.8
+i
I”
1.0 -
2
0 0
0
v
“0
0.5
0
5 .-
0
z-
0
23
b ‘0 :C-ov
P
-:\::::,::,/* 0
1o-3 H20,
1o-4
ct.4Y2
lo-'
Fig. 2. Effect of the l&O2 concentration on the degradation of chloral hydrate, 1OA4 M; TiOp, 3 g 1-l. Illumination time: 0, 30 min; A, 60 min.
hydrate.
Chloral
Upon degradation, the corresponding amounts of Cl- and Hi were released simultaneously in both the presence and the absence of H,Oz (Fig. 1). Total organic carbon measured for the illuminated suspension containing 10d3 M chloral hydrate, 3 g 1-r TiOz and 6.9 X 10e3 M H,Oz approximately corresponds to the value calculated for the remaining non-degraded chloral hydrate. Thus it was suggested that no significant amounts of intermediate products were formed during the photodegradation. The amounts of Cl- and H+ measured during the degradation and that of COz formed after the completion of the degradation suggest the following reaction in the absence of H,Oz : CCl,CH(OH),
+ O2 -
2C02 + 3Cl- + 3H+
(1)
Photocatalytic degradation on semiconductor powder has been considered to be initiated by the formation of OH radical [ 16, 171. Addition of H202 seems to increase the number of OH radicals according to the following reactions [13,16,17]: TiO, +
e-- + h+
O2 + e- Oa- + HzOz -
(2)
oz-
(3) -OH+OH--+02
H,Oz + e- -.OH+OHHz0 + h+ -
(4) (5)
lOH+H+
(6)
For the oxidation of chloral hydrate by -OH, the following reaction similar to eqn. (1) is suggested from the concentrations of Cl- and H+ formed during the degradation and the total organic carbon in the solution: CCl,CH(OH),
+ 4 lOH -
2C02 + 3Cl- + 3H+ + 2H,O
(7)
The apparent rate constant is dependent on the initial pH of the suspension as shown in Fig. 3. The augmentation of the rate constant with pH in the absence of H,02 can be explained by eqn. (1) which shows the release
158
Fig. 3. Effect of pH on the apparent photodegradation rate of chloral hydrate (k,) and on the 02 evolution rate (at 24 “C). Chloral hydrate, 10e4 M; TiOz, 3.2 g 1-l; HZ02, 3.4 x 1O-3 M. k @: o, in the presence of H202, . A, in the absence of H202. O2 evolution rate: o, in the absence of chloral hydrate.
of H+ upon illumination (Fig. 1). In the presence of H,02, the pH effect is more complicated. The maximum of the rate constant was observed at pH 5 -5. Weinstein et al. [ 181 showed the pH dependency of the propagation and the quenching rates of *OH by measuring O2 evolution in r-irradiated H,Oz solution. We measured O2 evolution in TiO, suspension containing H,O, (Fig. 3). Without illumination O2 did not evolve in an appreciable amount. Under illumination, however, O2 evolved rapidly and the evolution rate measured for the initial 9 min increased with pH. Assuming that the formation of *OH is not affected by the pH of the solution in the pH range studied here [19], it may be considered that the lifetimes of *OH and the subsequently formed radicals are reflected in the evolution rate of 0, which is formed in the propagation and quenching processes of OH [18]. The effect of pH on the photodegradation in the presence of HzO1 is tentatively interpreted as follows. -OH and other radicals are less stable at higher pH, and therefore are less available for the reaction with chloral hydrate. The maximum observed in the presence of H,O, (Fig. 3) can be attributed to the combined effects of pH on the stability of 9OH and on the oxidation of chloral hydrate expressed by eqn. (7). The photocatalytic efficiencies of other semiconductors are listed in Table 1, where the illumination time required for 50% degradation of chloral hydrate in 25 ml of 10Y3 M solution are presented. low3 M solution containing 2 X lo-’ M H20, was used for these experiments instead of 10e4 M solution, because the photodegradation is too rapid for 10s4 M when 25 ml of the solution is illuminated. It has been confirmed previously that 2 X lo-* M Hz02 is in the optimum concentration region for 10m3 M chloral hydrate. In the absence of H202, ZnO exhibited a higher efficiency than TiO,, while WO, was less effective than either, and SrTiO, showed only a low efficiency. The effect of the addition of H202 was slightly negative for ZnO. Photocorrosion of ZnO was confirmed in our previous paper [20] by detecting Zn*+ in the illuminated suspension. Degradation rates in W03 and SrTi03 suspensions containing H,Oz were slower than in the control solution l
159 TABLE
1
Degradation Semiconductor
rates on different
semiconductors
tl/2
(minIa
Without TiO2 ZnO wo3
SrTi02 Si02 No catalyst
H202
8 7 49 78b -
aThe illumination time for 50% degradation Ti02, 3.2 g 1-l; H202, 2 x 10e2 M.
bt
With Hz02 2 11 34 34b 36b 21
of chloral hydrate
in 25 ml of 10e3 solution.
314 -
that had no catalyst other than H,Oz. This may be explained in terms of the scattering of the incident light by semiconductor powder. Since SnOz is considered to have no photocatalytic activity, a comparison of the degradations between in the control solution and in the Sn02 suspension containing H202 provides an estimate of how much the incident light is reduced by the scattering on semiconductor particles. Loading of metals such as Pt, Rh and Pd onto TiO, had a negative effect in both the presence and the absence of H,O,. These metals may serve as catalyst for the decomposition of H202. References 1
A. L. Pruden and D. F. Ollis, J. CatuZ., 17 (1983) 404. 2 D. F. Ollis, Environ. Sci. Technol., 19 (1985) 480. 628. 3 A. L. Pruden and D. F. Ollis, Environ. Sci. Technol., 17 (1983) 4 C. Y. Hsiao, C. L. Lee and D. F. Ollis, J. Catal., 82 (1983) 418. 5 S. A. Ahmed and D. F. Ollis, Sol. Energy, 32 (1984) 597. 6 D. F. Ollis, C. Y. Hsiao, L. Budiman and C. L. Lee, J. Catal., 88 (1984) 89. 7 M. Barbeni, E. Pramauro, E. Pelizzetti, E. Borgarello and M. Gratzel, Nouu. J. Chim., 8 (1984) 547. 8 H. Hidaka, H. Kubota, M. Serpone and N. Gritzel, Nouu. J. Chim., 9 (1985) 67. 9 R. W. Matthews, Water Res., 20 (1986) 569. 10 K. Tanaka, K. Harada and S. Murata, Sol. Energy, 36 (1986) 159. 11 K. Harada, T. Hisanaga and K. Tanaka, New. J. Chem., 11 (1987) 597. 12 R. W. Matthews, Sol. Energy, 37 (1987) 405. 13 K. Tanaka, T. Hisanaga and K. Harada, New J. Chem., 13 (1989) 5. 14 A. Tomonari, Nippon Kagaku Kaishi, 83 (1962) 693. 15 B. Kraeutler and A. J. Bard, J. Am. Chem. Sot., 78 (1978) 4317. 16 I. Izumi, W. W. Dunn, K. 0. Wilbourn, F. F. Fan and A. J. Bard, J. Phys. Chem., 84 (1980) 3207. 17 M. Fujihira, Y. Satoh and T. Osa, Bull. Chem. Sot. Jpn., 55 (1982) 666. 18 J. Weinstein and B. H. J. Bielski, J. Am. Chem. Sot., 101 (1979) 58. 19 F. T. Wagner and G. A. Somorjai, J. Am. Chem. Sot., 102 (1980) 5494. 20 T. Hisanaga, K. Harada and K. Tanaka, Mizushori Gijitsu, 28 (1987) 445.
and Photobiology,
A: Chemistry, 48 (1989)
155 - 159
155
EFFICIENT PHOTOCATALYTIC DEGRADATION OF CHLORAL HYDRATE IN AQUEOUS SEMICONDUCTOR SUSPENSION KEIICHI
TANAKA,
TERUAKI
HISANAGA
and KENJI
National Chemical Laboratory for Industry, Wig-hi (Received
August
1,198s
l-l,
HARADA
Tsukuba (Japan)
; in revised form 3 January 1989)
Summary
Chloral hydrate was degraded in the presence of suspended TiOz by illumination with a super-high-pressure mercury lamp through the Pyrex glass wall of a cell. The addition of H,O, enhanced the degradation rate 3.4 times. The degradation rate increases with pH in the absence of H,O, and exhibits a maximum at pH 5.5 in the presence of H,O,. Of the semiconductors tested ZnO was the most efficient catalyst. The effect of H,Oz was observed only with TiOz and WOs.
1. Introduction Photocatalytic degradation of organic and inorganic materials on semiconductor powder is a new method for waste water treatment_ Several pollutants in aqueous solution have been reported to be degraded by this method [l - 121. The photochemical process for waste water treatment, in general, requires a large amount of electric power at high cost. Hence a high efficiency is required for practical applications. We found that the addition of hydrogen peroxide enhanced the photocatalytic efficiency of the semiconductor considerably [13]. The degradation of chloral hydrate on semiconductor powder in the presence of H,Oz is reported in this paper. Chloral hydrate is a toxic compound used for the synthesis of insecticides. 2. Experimental details The use of Ti02 has been described previously [ll]. The other semiconductors were reagent grade commercial products. 0.9 g and 0.08 g of semiconductor powders were stirred in a 500 ml Pyrex glass flask and a 30 ml Pyrex glass bottle containing 300 ml and 25 ml of aqueous chloral hydrate solution respectively; these were illuminated by a 500 W super-highpressure mercury lamp through an IR cut-off filter_ The degradation rate was 1010-6030/89/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
156
estimated by determining the remaining non-degraded chloral hydrate by a gas-chromatograph equipped with an electron capture detector. For the gaschromatographic analysis the illuminated sample was centrifuged and the supematant solution was extracted with n-hexane. The concentration of Clwas determined colourimetrically by using mercury thiocyanate and iron(I1) perchlorate as colouring agents [14]_ The total organic carbon was measured by a Shimadzu TOG500. The amount of evolved COP was determined as described previously Ill]. Metal was loaded onto TiOz by the photochemical method 1153. When investigating the effects of pH on the degradation rate and on O2 evolution, the pH of the solution was adjusted by adding appropriate amounts of HCl and KOH solutions.
3. Results and discussion The photodegradation rate of chloral hydrate and the effect of the addition of H,Oz are shown in Fig. 1. The degradation reaction follows first-order reaction kinetics. The apparent rate constants obtained from the data in Fig’. 1 are 0.039 min-l and 0.016 min-’ in the presence and absence of H,O, respectively. It was thus found that addition of H,O, accelerated the degradation 2.4 times. In the other experiment. with 10T3 M chloral hydrate solution, the degradation was 3.4 times faster in the presence of H202 (2 X lo-* M) than in its absence. These values are smaller than those obtained for the other organic ‘halide compounds studied previously [ 131. The effect of the concentration of H202 is shown in Fig. 2. The optimum concentration ranges from 3.4 X 10Y4 to 7 X 10m3 M for 1O-4 M chloral hydrate solution. The photocatalytic efficiency decreased as the concentration of H202 increased beyond the optimum. The blank experiments for either illuminated chloral hydrate solution or the suspensiop containing Ti02 and chloral hydrate in the dark showed that both the iHumination and the presence of TiOz were necessary for the degradation. :.
r”
1.0
5.5 . A
2
-0 lx l-l
5.073 . 3
z $5
I
Q5
c1 -1
45 .2Z& z 40 - 1 -
E= t% C-Z-o 0
60 Illumination
120
35
0
time(min.)
Fig. 1. Degradation of chloral hydrate in the presence and absence of Hz02 and the formation of Cl- (A) and H+ (a) by the degradation in the presence of Hz02. Chloral 0, in the presence of hydrate, 1O-4 M; TiOz, 3 g 1-l; H202, 3.4 X 10 --j M. Degradation: H202 ; A, in the absence of Hz02 ; X, in the presence of Hz02 and in the absence of TiO,.
157 0.8
+i
I”
1.0 -
2
0 0
0
v
“0
0.5
0
5 .-
0
z-
0
23
b ‘0 :C-ov
P
-:\::::,::,/* 0
1o-3 H20,
1o-4
ct.4Y2
lo-'
Fig. 2. Effect of the l&O2 concentration on the degradation of chloral hydrate, 1OA4 M; TiOp, 3 g 1-l. Illumination time: 0, 30 min; A, 60 min.
hydrate.
Chloral
Upon degradation, the corresponding amounts of Cl- and Hi were released simultaneously in both the presence and the absence of H,Oz (Fig. 1). Total organic carbon measured for the illuminated suspension containing 10d3 M chloral hydrate, 3 g 1-r TiOz and 6.9 X 10e3 M H,Oz approximately corresponds to the value calculated for the remaining non-degraded chloral hydrate. Thus it was suggested that no significant amounts of intermediate products were formed during the photodegradation. The amounts of Cl- and H+ measured during the degradation and that of COz formed after the completion of the degradation suggest the following reaction in the absence of H,Oz : CCl,CH(OH),
+ O2 -
2C02 + 3Cl- + 3H+
(1)
Photocatalytic degradation on semiconductor powder has been considered to be initiated by the formation of OH radical [ 16, 171. Addition of H202 seems to increase the number of OH radicals according to the following reactions [13,16,17]: TiO, +
e-- + h+
O2 + e- Oa- + HzOz -
(2)
oz-
(3) -OH+OH--+02
H,Oz + e- -.OH+OHHz0 + h+ -
(4) (5)
lOH+H+
(6)
For the oxidation of chloral hydrate by -OH, the following reaction similar to eqn. (1) is suggested from the concentrations of Cl- and H+ formed during the degradation and the total organic carbon in the solution: CCl,CH(OH),
+ 4 lOH -
2C02 + 3Cl- + 3H+ + 2H,O
(7)
The apparent rate constant is dependent on the initial pH of the suspension as shown in Fig. 3. The augmentation of the rate constant with pH in the absence of H,02 can be explained by eqn. (1) which shows the release
158
Fig. 3. Effect of pH on the apparent photodegradation rate of chloral hydrate (k,) and on the 02 evolution rate (at 24 “C). Chloral hydrate, 10e4 M; TiOz, 3.2 g 1-l; HZ02, 3.4 x 1O-3 M. k @: o, in the presence of H202, . A, in the absence of H202. O2 evolution rate: o, in the absence of chloral hydrate.
of H+ upon illumination (Fig. 1). In the presence of H,02, the pH effect is more complicated. The maximum of the rate constant was observed at pH 5 -5. Weinstein et al. [ 181 showed the pH dependency of the propagation and the quenching rates of *OH by measuring O2 evolution in r-irradiated H,Oz solution. We measured O2 evolution in TiO, suspension containing H,O, (Fig. 3). Without illumination O2 did not evolve in an appreciable amount. Under illumination, however, O2 evolved rapidly and the evolution rate measured for the initial 9 min increased with pH. Assuming that the formation of *OH is not affected by the pH of the solution in the pH range studied here [19], it may be considered that the lifetimes of *OH and the subsequently formed radicals are reflected in the evolution rate of 0, which is formed in the propagation and quenching processes of OH [18]. The effect of pH on the photodegradation in the presence of HzO1 is tentatively interpreted as follows. -OH and other radicals are less stable at higher pH, and therefore are less available for the reaction with chloral hydrate. The maximum observed in the presence of H,O, (Fig. 3) can be attributed to the combined effects of pH on the stability of 9OH and on the oxidation of chloral hydrate expressed by eqn. (7). The photocatalytic efficiencies of other semiconductors are listed in Table 1, where the illumination time required for 50% degradation of chloral hydrate in 25 ml of 10Y3 M solution are presented. low3 M solution containing 2 X lo-’ M H20, was used for these experiments instead of 10e4 M solution, because the photodegradation is too rapid for 10s4 M when 25 ml of the solution is illuminated. It has been confirmed previously that 2 X lo-* M Hz02 is in the optimum concentration region for 10m3 M chloral hydrate. In the absence of H202, ZnO exhibited a higher efficiency than TiO,, while WO, was less effective than either, and SrTiO, showed only a low efficiency. The effect of the addition of H202 was slightly negative for ZnO. Photocorrosion of ZnO was confirmed in our previous paper [20] by detecting Zn*+ in the illuminated suspension. Degradation rates in W03 and SrTi03 suspensions containing H,Oz were slower than in the control solution l
159 TABLE
1
Degradation Semiconductor
rates on different
semiconductors
tl/2
(minIa
Without TiO2 ZnO wo3
SrTi02 Si02 No catalyst
H202
8 7 49 78b -
aThe illumination time for 50% degradation Ti02, 3.2 g 1-l; H202, 2 x 10e2 M.
bt
With Hz02 2 11 34 34b 36b 21
of chloral hydrate
in 25 ml of 10e3 solution.
314 -
that had no catalyst other than H,Oz. This may be explained in terms of the scattering of the incident light by semiconductor powder. Since SnOz is considered to have no photocatalytic activity, a comparison of the degradations between in the control solution and in the Sn02 suspension containing H202 provides an estimate of how much the incident light is reduced by the scattering on semiconductor particles. Loading of metals such as Pt, Rh and Pd onto TiO, had a negative effect in both the presence and the absence of H,O,. These metals may serve as catalyst for the decomposition of H202. References 1
A. L. Pruden and D. F. Ollis, J. CatuZ., 17 (1983) 404. 2 D. F. Ollis, Environ. Sci. Technol., 19 (1985) 480. 628. 3 A. L. Pruden and D. F. Ollis, Environ. Sci. Technol., 17 (1983) 4 C. Y. Hsiao, C. L. Lee and D. F. Ollis, J. Catal., 82 (1983) 418. 5 S. A. Ahmed and D. F. Ollis, Sol. Energy, 32 (1984) 597. 6 D. F. Ollis, C. Y. Hsiao, L. Budiman and C. L. Lee, J. Catal., 88 (1984) 89. 7 M. Barbeni, E. Pramauro, E. Pelizzetti, E. Borgarello and M. Gratzel, Nouu. J. Chim., 8 (1984) 547. 8 H. Hidaka, H. Kubota, M. Serpone and N. Gritzel, Nouu. J. Chim., 9 (1985) 67. 9 R. W. Matthews, Water Res., 20 (1986) 569. 10 K. Tanaka, K. Harada and S. Murata, Sol. Energy, 36 (1986) 159. 11 K. Harada, T. Hisanaga and K. Tanaka, New. J. Chem., 11 (1987) 597. 12 R. W. Matthews, Sol. Energy, 37 (1987) 405. 13 K. Tanaka, T. Hisanaga and K. Harada, New J. Chem., 13 (1989) 5. 14 A. Tomonari, Nippon Kagaku Kaishi, 83 (1962) 693. 15 B. Kraeutler and A. J. Bard, J. Am. Chem. Sot., 78 (1978) 4317. 16 I. Izumi, W. W. Dunn, K. 0. Wilbourn, F. F. Fan and A. J. Bard, J. Phys. Chem., 84 (1980) 3207. 17 M. Fujihira, Y. Satoh and T. Osa, Bull. Chem. Sot. Jpn., 55 (1982) 666. 18 J. Weinstein and B. H. J. Bielski, J. Am. Chem. Sot., 101 (1979) 58. 19 F. T. Wagner and G. A. Somorjai, J. Am. Chem. Sot., 102 (1980) 5494. 20 T. Hisanaga, K. Harada and K. Tanaka, Mizushori Gijitsu, 28 (1987) 445.