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Photocatalytic degradation of waste-water pollutants: titanium dioxide-mediated oxidation of methyl vinyl ketone
/.
Photochem.
Photobiol.
A:
Chem.,
63 (1992)
Photocatalytic degradation titanium dioxide-mediated ketone* M. Muneer
107
of waste-water pollutants: oxidation of methyl vinyl
and S. Dastt
Photochemistry Research),
107-114
Research
Trivundrum
Unit, Regional 695
019
Research
Laboratory
V. B. Manila1 and A. Haridas Process Engineering Division, Regional Research Laboratoq Research),
Trivandrum
(Received
May
16,
695
1991;
019
(Council
of Scientific
and Industrial
(India)
(Council
of Scientific
and Industrial
(India)
accepted
August
8, 1991)
Abstract The results of photocatalysed degradation of methyl vinyl ketone (MVK), an industrial pollutant, in aqueous solutions using both suspended titanium dioxide and titanium dioxide immobilized on glass surfaces are presented. The rate of MVK degradation as a function of MVK concentration fits a Langmuir-Hinshelwood equation. Carbon dioxide yield measurements indicate that only partial mineralization occurs in the initial phase of oxidation.
1. Introduction Redox reactions induced by electrons (e- =,,) and (Hfvb), photogenerated by band gap excitation of semiconductor particles can potentially be used for several technical applications [l]. Major efforts in this area have been directed primarily towards the photocleavage ofwater [l-6]. Recently, there have been several reports on semiconductormediated photodegradation of both organic and inorganic toxic chemicals in aqueous media [7-141. These redoxprocesses have potential application forwaste-water treatment_ Although biodegradation of several pollutants is well known, a large number of such pollutants are resistant to bacterial activity. Methyl vinyl ketone (MVK) is a known industrial pollutant which can inhibit biological waste-water treatment systems. We have observed, for example, respiration inhibition of unacclimatized activated sludge for MVK dosages greater than 10 mg 1-l. The present investigation deals with the photocatalysed degradation of MVK, using both aqueous suspensions of titanium dioxide (TiO,) and Ti02 immobilized on glass surfaces. 2. Experimental
details
Freshly distilled MVK was collected in water, to prevent made up to a stock solution of 0.1 M in water. Double-distilled
polymerization, and water was used in
+Document RRLT-PRLJ-14 from the Regional Research Laboratory (Council of Scientific and Industrial Research), Trivandrum, India. ttAuthor to whom correspondence should be addressed.
lOlO-6030/92/$5.00
0 1992 -
Elsevier Sequoia. All rights reserved
108 all experiments. Ti02 used as suspensions was Degussa P25 grade with a particle size titanate was obtained from of 30 nm and a surface area of 50 m2 g-‘. Tetraisopropyl Fluka. Irradiations were carried out using either a 500 W superhigh pressure mercury lamp on an optical bench or a 125 W medium pressure mercury lamp placed inside the well of a double-walled borosilicate glass photoreactor of 120 ml capacity. The output of the 500 W lamp was filtered through a Pyrex filter (optical cut-off, 290 nm) and focused onto a three-necked Pyrex round-bottomed flask, which was magnetically stirred. The temperature was controlled by air cooling (30-35 “C) and constant light intensity levels were maintained and monitored with a light-sensing device (Oriel68850 photofeedback system). For monochromatic irradiations, an additional 360 nm bandpass filter was used. The TiOz-immobilized reactions were carried out with the 125 W lamp reactor. TiOz was immobilized on both the outer wall of the lamp jacket and the inner wall of the reaction vessel by rinsing the clean vessel with a 10% solution of tetraisopropyl orthotitanate in isopropanol and draining off the excess solution. Hydrolysis by a stream of moist air led to the formation of an opaque titanium hydroxide film and the reactor was kept at 450 “C for 4 h to convert the hydroxide film to TiOZ. Serpone et al. [IS] have also used alkoxide coatings to immobilize TiO, on glass surfaces. O2 depletion was measured on line during irradiation on the optical bench, using This electrode is capable of detecting a selective membrane electrode (Oximeter 0X196). oxygen levels up to 0.5 mg 1-l accurately. Kinetic measurements were carried out in air or in oxygen-saturated solutions sealed under ambient pressure with the oxygen electrode. Carbon dioxide (CO,) measurements were carried out on solutions, irradiated with the 500 W lamp on the optical bench, using a CO, specific electrode (Orion), capable of measuring CO1 in the range 4-4000 mg 1-l. The irradiated solutions were adjusted to constant ionic strength and pH using sodium citrate buffer. The error involved in CO, measurements is f3%. MYK was analysed with a gas chromatograph, equipped with a flame ionization detector, using a Poropak Q column. The samples containing suspended TiOZ were centrifuged, following NaCl addition, prior to gas chromatography analysis. Control experiments were carried out in all cases, employing unirradiated blank solutions. There was no detectable removal of MVK, either by volatile loss during gas purging or under sample preparation for gas chromatography. It was observed that the dissolved CO2 concentrations in the suspensions of TiOZ prior to irradiation were less than the detection limit of our experiments (4 mg 1-l). Also, there was no observable loss of MVK, when the irradiations were carried out in the absence of TiOz.
3. Results
and discussion
The initial step in TiO*-photocatalysed of both hydroxyl radicals and superoxide TiO,-
hu
h+ ,+H,O e-&+02-
e-&+h+, -
‘OH +H+ 0,-
oxidations is believed radical anions:
to be the formation
(1) (2) (3)
109
It has been suggested earlier [16] that, of the two oxidizing species, namely OH and 02-it is the former which is the primary reacting species in photocatalytic degradation processes:These oxidation reactions would result in the depletion of O2 and MVK and in the formation of CO2 and other semioxidized products in solution. Figure 1 shows the measured O2 concentrations, on irradiation of TiOl suspensions in absence of MVK and at extremely low concentrations of MVK. Within experimental error these values are identical. It is interesting to note that, even in these blank and low MVK concentration solutions, a certain amount of oxygen depletion was observed [17, 183. This could possibly be attributed to the following process: e-,b+&02+H+
-
B&O2
(4)
It may be mentioned in this connection that the formation of H202 in illuminated aqueous suspensions of Ti02, ZnO and desert sand has been reported earlier [19]. Figure 2 shows the rate of O2 consumption, after correcting for the rate of O2 depletion in the blank experiment. The rate of O2 depletion has been found to increase with increasing MVK concentration. The linearity of O2 depletion with time, as seen in Fig. 2, indicates that the rate of oxidation is independent of O2 concentration over the range studied (l-20 mg 1-l). Okamato et al. [18] have determined the kinetics of oxidation of phenol with respect to Oz and have shown that the rate is given by the following Langmuir-Hinshelwood form: rate =
&[OZ] 1+
01 0
(5)
KJxyPzl
I
I
I
I
I
2
4
6
8
IO
IRRADIATION
TIME
12
/Cmin)
Fig. 1. O2 concentration US. irradiation time for TiOz (1 g 1-l) containing 0 M (0), and lo-’ M (0) of MVK initially.
(+)
10e6 M
110
5
0.150
o.'25 is
0 4 ';,
E 0.100
5
0075
s 0
w 0
53 E” :
G
a
02 4
0.050 I
0.025
0 0
4
2
IRRADIATION
6
8
IO
12
TIME/(min)
Fig. 2. O2 consumption, at various initial MVK concentrations, on irradiating 1 g I-’ TiO, suspension after substracting O2 consumption in the blank Ti02 suspension, for various MVK concentrations: l , 1 X lo-’ M; +5X 10m3 M; *, 1 X 10v3 M; X, 5 x low4 M; 0, 1 x 10-4; A, 5x10-’ M.
I& =O.ll kPa_’ (at 298 K this corresponds to 1.375 mg 1-l). This suggests that for [02] B- UK,,,, i.e. for O2 concentrations greater than 0.73 mg l-l, the rate of oxidation is independent of O2 concentration. Our experimental results are in agreement with this assumption. The kinetics of the initial phase of oxidation of MVK can be adequately modelled by the Langmuir-Hinshelwood form
where
hWK[M=l
rate=
(6)
1 +K~v@4vlq
Figure 3 shows the plot of rate of O2 consumption vs. reciprocal of MVK concentration. The constants in eqn. (6) could be obtained from the slope and intercept values of Fig. 3, after incorporating the stoichiometric constant of 2.5, i.e. for every 2.5 mol of O2 consumed, 1 mol of MVK is depleted (see eqn. (7)). The values obtained were kMvK= 6.1-1O-2 mine’ and &,=3.4x 10’ M-‘. These constants are valid for the initial phase of oxidation, since the products of oxidation compete for the hydroxyl radicals. The rates of photocatalytic degradation of several organic compounds have been shown to follow the Langmuir-Hinshelwood form [13]. Figure 4 shows the degradation of MYK for an initial MVK concentration of 1.5 mM. The formation of CO, for a 1.5 mM initial concentration of MVK is shown in Fig. 5. The initial rate of O2 and MVK depletion and COz formation on irradiation of 1.5 mM solution of MVK indicates the following stoichiometry: 1 MVK + 2.502
= 0.SSCO,
+ products
(7)
111
01
I
I
I
I
-0
5
IO
15
20
25
&/(mM-‘I
Fig. 3. Reciprocal
rate of O2 consumption vs. reciprocal of initial MVK concentration.
Y
5z
5 20’ 0
/
/
IO
/
/g
20
n
30
IRRADIATION
40
50
60
0 70
TIME~min)
Fig. 4. MVK degradation for irradiated aqueous suspensions of TiOz (1 g 1-l) containing an initial MVK concentration of 1.5 mM. O2 was purged continuously during irradiation.
0
IO
20
30
IRRADIATION
40
50
60
1
70
TlME/
Fig. 5. CO, formation for irradiated aqueous suspension of TiOl containing an initial MVK concentration of 1.5 mM. The suspension was O2 saturated and sealed prior to irradiation. At least 1 mg I-’ O2 was remaining at the end of each run. The
stojchiometry
MVK+
50, = 4C0,
for complete + 3H,O
mineralization
is given by the expression (8)
Therefore it is evident that several intermediary products are formed before complete mineralization. The concentrations of these intermediates were too low to be detected by gas chromatography. Figure 6 shows the rate of MVK depletion on irradiation with the 125 W lamp, using TiO, suspension and TiO,-immobilized reactors. On the assumption that all the light is absorbed in both reactors, the difference in rates can be attributed to mass transfer limitations. In TiOz suspensions the concentration of hydroxyl radicals in solution is expected to be close to that of homogeneous systems [16], while in the TiOz-immobilized reactors the hydroxyl radical will be generated close to the reactor walls. Nevertheless, if we consider the advantages of practical usability, the reduction in efficiency in the immobilized reactors may be acceptable. In heterogenous systems, quantum yield measurements are not practicable [20], since a substantial amount of light is either scattered or reflected. The light intensity was measured for the 500 W lamp by ferrioxalate actinometry [21) in the reaction vessel employed for the O2 depletion measurements. Additional neutral density filters to reduce the light intensity and a 360 nm bandpass filter were used for these experiments. Under these conditions the light intensity was measured as 5.14 x lop5 Einsteins 1-l min-‘. Oz depletion rates for 1.5 mM aqueous solution of MVK under identical conditions were measured to be 3.13X 10e6 M min-l. This corresponds to a rate of MVK depletion of 1.49X 10V6 mol 1-l min”, under identical conditions. On the assumption that ail the incident light is absorbed by TiOs, the quantum yield of MVK degradation for these solutions is calculated as 0.03 mol Einstein-‘.
113
1.6
o-2 0.
/
, IO
I 20
I 30
IRRADIATION
I 40
I 50
I 60
70
TI ME/(min)
Fig. 6. MVK degradation on irradiation of aqueous solution containing an initial MVK concentration at 1.5 mM in a 125 W lamp photoreactor: El, *, TiOt suspension (1 g I-‘); +, n , immobilized TiOz. O2 was purged continuously during irradiation.
4. Conclusions Dissolved O2 measurements were used to study the heterogeneous photocatalytic oxidation of MVK. This is a convenient method for studying the efficiency of the photocatalytic processes and screening of potential photocatalysts. The photocatalytic degradation of MVK leads to partial mineralization as observed from CO2 measurements. These studies indicate the feasibility of using immobilized reactors for such processes.
Acknowledgments The authors thank the of India and the Regional Research), Trivandrum, for H. K. L. Varma, Regional coatings_
Council of Scientific and Industrial Research, Government Research Laboratory (Council of Scientific and Industrial financial support of this work. The authors also thank Mr. Research Laboratory, for his help in developing the TiQ
References 1 M. Gratzel, (ed.), Energy Resources Through Photochemishy and Cut@&, Academic Press, New York, 1983. 2 A. Harriman and M. A. West (eds), Photogeneration of Hydrogen, Academic Press, New York, 1982.
114 3 4 5 6
A. A. A. M.
Harriman, J. Phys. Chem.. 25 (1984) 33. J. Bard, J. Phys. Chem., 88 (1982) 2. Mills and G. Porter, J. C/rem. Sec., Faraday Trans. I, 78 (1982) 3659. Scbiavello (ed.), Photocataiysis and Environment. Trends and Applications,
Study Inst. Ser., 237 7
E. Pelezzetti Adv.
in NATO
Adv.
(1988).
and N. Serpone
Study Inst. Ser., 174
(eds.), Homogenous
and Heterogenous
Photocatulysis,
in NATO
(1986).
L. Palmisano and M. Schiavello, J. Phys. Chem., 94 (1990) 829_ T. Hisanaga and K. Harada, .I. Photochem. Photobiol. A: Chem., 48 (1989) 155. 10 K. Tennakone, S. Punchihewa, S. Wickremanayaka and R. U. Tantrigoda, J. Photochem. 8
A. Sciafani,
9 K. Tanaka, Photobiol.
A:
Chem.,
46 (1989)
247.
11 J.-F. Rantani and G. Giusti, J. Photochem. Photobiol. A: Chem., 46 (1989) 357. 12 C. Kormann, D. W. Bahnemann and M. R. Hoffman, .I. Photochem. Photobiol. A: Chem., 48 (1989) 161. 13 R. W. Mathews, J. CataL, 111 (1988) 264. 14 K. Harada, T. Hisanaga and K. Tanaka, New J. Chem., 12 (1987) 597. 15 N. Serpone, E. Borgarello, R. Harris, P. Cahill, M. Borgarello and E. Pelizzetti, Sol. Energy Mater., 24 (1986) 121. 16 C. S. Turchi and D. F. Ollis, .J. CaraL, 122 (1990) 178. 17 K. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Sot. Jpn., 58 (1985)
2015.
18 K. Okamoto, Y. Yamamoto, H. Tanaka, A. Itaya, Bull. Chem. Sot. Jpn., 58 (1985) 2023. 19 C. Kormann, D. W. Bahnemann and M. R. Hoffmann, Environ. Sci. Technol., 22 (1988) 798. 20 M. A. Fox, in M. SchiavelIo (ed.) Photocatalysis and Environments. Trends and Applications, in NATO Adv. Study. Inst. Ser., 237 (1988) 445467. 21 S. L_ Murov, Handbook of Photochemistry, Marcel Dekker, New York, 1973.
Photochem.
Photobiol.
A:
Chem.,
63 (1992)
Photocatalytic degradation titanium dioxide-mediated ketone* M. Muneer
107
of waste-water pollutants: oxidation of methyl vinyl
and S. Dastt
Photochemistry Research),
107-114
Research
Trivundrum
Unit, Regional 695
019
Research
Laboratory
V. B. Manila1 and A. Haridas Process Engineering Division, Regional Research Laboratoq Research),
Trivandrum
(Received
May
16,
695
1991;
019
(Council
of Scientific
and Industrial
(India)
(Council
of Scientific
and Industrial
(India)
accepted
August
8, 1991)
Abstract The results of photocatalysed degradation of methyl vinyl ketone (MVK), an industrial pollutant, in aqueous solutions using both suspended titanium dioxide and titanium dioxide immobilized on glass surfaces are presented. The rate of MVK degradation as a function of MVK concentration fits a Langmuir-Hinshelwood equation. Carbon dioxide yield measurements indicate that only partial mineralization occurs in the initial phase of oxidation.
1. Introduction Redox reactions induced by electrons (e- =,,) and (Hfvb), photogenerated by band gap excitation of semiconductor particles can potentially be used for several technical applications [l]. Major efforts in this area have been directed primarily towards the photocleavage ofwater [l-6]. Recently, there have been several reports on semiconductormediated photodegradation of both organic and inorganic toxic chemicals in aqueous media [7-141. These redoxprocesses have potential application forwaste-water treatment_ Although biodegradation of several pollutants is well known, a large number of such pollutants are resistant to bacterial activity. Methyl vinyl ketone (MVK) is a known industrial pollutant which can inhibit biological waste-water treatment systems. We have observed, for example, respiration inhibition of unacclimatized activated sludge for MVK dosages greater than 10 mg 1-l. The present investigation deals with the photocatalysed degradation of MVK, using both aqueous suspensions of titanium dioxide (TiO,) and Ti02 immobilized on glass surfaces. 2. Experimental
details
Freshly distilled MVK was collected in water, to prevent made up to a stock solution of 0.1 M in water. Double-distilled
polymerization, and water was used in
+Document RRLT-PRLJ-14 from the Regional Research Laboratory (Council of Scientific and Industrial Research), Trivandrum, India. ttAuthor to whom correspondence should be addressed.
lOlO-6030/92/$5.00
0 1992 -
Elsevier Sequoia. All rights reserved
108 all experiments. Ti02 used as suspensions was Degussa P25 grade with a particle size titanate was obtained from of 30 nm and a surface area of 50 m2 g-‘. Tetraisopropyl Fluka. Irradiations were carried out using either a 500 W superhigh pressure mercury lamp on an optical bench or a 125 W medium pressure mercury lamp placed inside the well of a double-walled borosilicate glass photoreactor of 120 ml capacity. The output of the 500 W lamp was filtered through a Pyrex filter (optical cut-off, 290 nm) and focused onto a three-necked Pyrex round-bottomed flask, which was magnetically stirred. The temperature was controlled by air cooling (30-35 “C) and constant light intensity levels were maintained and monitored with a light-sensing device (Oriel68850 photofeedback system). For monochromatic irradiations, an additional 360 nm bandpass filter was used. The TiOz-immobilized reactions were carried out with the 125 W lamp reactor. TiOz was immobilized on both the outer wall of the lamp jacket and the inner wall of the reaction vessel by rinsing the clean vessel with a 10% solution of tetraisopropyl orthotitanate in isopropanol and draining off the excess solution. Hydrolysis by a stream of moist air led to the formation of an opaque titanium hydroxide film and the reactor was kept at 450 “C for 4 h to convert the hydroxide film to TiOZ. Serpone et al. [IS] have also used alkoxide coatings to immobilize TiO, on glass surfaces. O2 depletion was measured on line during irradiation on the optical bench, using This electrode is capable of detecting a selective membrane electrode (Oximeter 0X196). oxygen levels up to 0.5 mg 1-l accurately. Kinetic measurements were carried out in air or in oxygen-saturated solutions sealed under ambient pressure with the oxygen electrode. Carbon dioxide (CO,) measurements were carried out on solutions, irradiated with the 500 W lamp on the optical bench, using a CO, specific electrode (Orion), capable of measuring CO1 in the range 4-4000 mg 1-l. The irradiated solutions were adjusted to constant ionic strength and pH using sodium citrate buffer. The error involved in CO, measurements is f3%. MYK was analysed with a gas chromatograph, equipped with a flame ionization detector, using a Poropak Q column. The samples containing suspended TiOZ were centrifuged, following NaCl addition, prior to gas chromatography analysis. Control experiments were carried out in all cases, employing unirradiated blank solutions. There was no detectable removal of MVK, either by volatile loss during gas purging or under sample preparation for gas chromatography. It was observed that the dissolved CO2 concentrations in the suspensions of TiOZ prior to irradiation were less than the detection limit of our experiments (4 mg 1-l). Also, there was no observable loss of MVK, when the irradiations were carried out in the absence of TiOz.
3. Results
and discussion
The initial step in TiO*-photocatalysed of both hydroxyl radicals and superoxide TiO,-
hu
h+ ,+H,O e-&+02-
e-&+h+, -
‘OH +H+ 0,-
oxidations is believed radical anions:
to be the formation
(1) (2) (3)
109
It has been suggested earlier [16] that, of the two oxidizing species, namely OH and 02-it is the former which is the primary reacting species in photocatalytic degradation processes:These oxidation reactions would result in the depletion of O2 and MVK and in the formation of CO2 and other semioxidized products in solution. Figure 1 shows the measured O2 concentrations, on irradiation of TiOl suspensions in absence of MVK and at extremely low concentrations of MVK. Within experimental error these values are identical. It is interesting to note that, even in these blank and low MVK concentration solutions, a certain amount of oxygen depletion was observed [17, 183. This could possibly be attributed to the following process: e-,b+&02+H+
-
B&O2
(4)
It may be mentioned in this connection that the formation of H202 in illuminated aqueous suspensions of Ti02, ZnO and desert sand has been reported earlier [19]. Figure 2 shows the rate of O2 consumption, after correcting for the rate of O2 depletion in the blank experiment. The rate of O2 depletion has been found to increase with increasing MVK concentration. The linearity of O2 depletion with time, as seen in Fig. 2, indicates that the rate of oxidation is independent of O2 concentration over the range studied (l-20 mg 1-l). Okamato et al. [18] have determined the kinetics of oxidation of phenol with respect to Oz and have shown that the rate is given by the following Langmuir-Hinshelwood form: rate =
&[OZ] 1+
01 0
(5)
KJxyPzl
I
I
I
I
I
2
4
6
8
IO
IRRADIATION
TIME
12
/Cmin)
Fig. 1. O2 concentration US. irradiation time for TiOz (1 g 1-l) containing 0 M (0), and lo-’ M (0) of MVK initially.
(+)
10e6 M
110
5
0.150
o.'25 is
0 4 ';,
E 0.100
5
0075
s 0
w 0
53 E” :
G
a
02 4
0.050 I
0.025
0 0
4
2
IRRADIATION
6
8
IO
12
TIME/(min)
Fig. 2. O2 consumption, at various initial MVK concentrations, on irradiating 1 g I-’ TiO, suspension after substracting O2 consumption in the blank Ti02 suspension, for various MVK concentrations: l , 1 X lo-’ M; +5X 10m3 M; *, 1 X 10v3 M; X, 5 x low4 M; 0, 1 x 10-4; A, 5x10-’ M.
I& =O.ll kPa_’ (at 298 K this corresponds to 1.375 mg 1-l). This suggests that for [02] B- UK,,,, i.e. for O2 concentrations greater than 0.73 mg l-l, the rate of oxidation is independent of O2 concentration. Our experimental results are in agreement with this assumption. The kinetics of the initial phase of oxidation of MVK can be adequately modelled by the Langmuir-Hinshelwood form
where
hWK[M=l
rate=
(6)
1 +K~v@4vlq
Figure 3 shows the plot of rate of O2 consumption vs. reciprocal of MVK concentration. The constants in eqn. (6) could be obtained from the slope and intercept values of Fig. 3, after incorporating the stoichiometric constant of 2.5, i.e. for every 2.5 mol of O2 consumed, 1 mol of MVK is depleted (see eqn. (7)). The values obtained were kMvK= 6.1-1O-2 mine’ and &,=3.4x 10’ M-‘. These constants are valid for the initial phase of oxidation, since the products of oxidation compete for the hydroxyl radicals. The rates of photocatalytic degradation of several organic compounds have been shown to follow the Langmuir-Hinshelwood form [13]. Figure 4 shows the degradation of MYK for an initial MVK concentration of 1.5 mM. The formation of CO, for a 1.5 mM initial concentration of MVK is shown in Fig. 5. The initial rate of O2 and MVK depletion and COz formation on irradiation of 1.5 mM solution of MVK indicates the following stoichiometry: 1 MVK + 2.502
= 0.SSCO,
+ products
(7)
111
01
I
I
I
I
-0
5
IO
15
20
25
&/(mM-‘I
Fig. 3. Reciprocal
rate of O2 consumption vs. reciprocal of initial MVK concentration.
Y
5z
5 20’ 0
/
/
IO
/
/g
20
n
30
IRRADIATION
40
50
60
0 70
TIME~min)
Fig. 4. MVK degradation for irradiated aqueous suspensions of TiOz (1 g 1-l) containing an initial MVK concentration of 1.5 mM. O2 was purged continuously during irradiation.
0
IO
20
30
IRRADIATION
40
50
60
1
70
TlME/
Fig. 5. CO, formation for irradiated aqueous suspension of TiOl containing an initial MVK concentration of 1.5 mM. The suspension was O2 saturated and sealed prior to irradiation. At least 1 mg I-’ O2 was remaining at the end of each run. The
stojchiometry
MVK+
50, = 4C0,
for complete + 3H,O
mineralization
is given by the expression (8)
Therefore it is evident that several intermediary products are formed before complete mineralization. The concentrations of these intermediates were too low to be detected by gas chromatography. Figure 6 shows the rate of MVK depletion on irradiation with the 125 W lamp, using TiO, suspension and TiO,-immobilized reactors. On the assumption that all the light is absorbed in both reactors, the difference in rates can be attributed to mass transfer limitations. In TiOz suspensions the concentration of hydroxyl radicals in solution is expected to be close to that of homogeneous systems [16], while in the TiOz-immobilized reactors the hydroxyl radical will be generated close to the reactor walls. Nevertheless, if we consider the advantages of practical usability, the reduction in efficiency in the immobilized reactors may be acceptable. In heterogenous systems, quantum yield measurements are not practicable [20], since a substantial amount of light is either scattered or reflected. The light intensity was measured for the 500 W lamp by ferrioxalate actinometry [21) in the reaction vessel employed for the O2 depletion measurements. Additional neutral density filters to reduce the light intensity and a 360 nm bandpass filter were used for these experiments. Under these conditions the light intensity was measured as 5.14 x lop5 Einsteins 1-l min-‘. Oz depletion rates for 1.5 mM aqueous solution of MVK under identical conditions were measured to be 3.13X 10e6 M min-l. This corresponds to a rate of MVK depletion of 1.49X 10V6 mol 1-l min”, under identical conditions. On the assumption that ail the incident light is absorbed by TiOs, the quantum yield of MVK degradation for these solutions is calculated as 0.03 mol Einstein-‘.
113
1.6
o-2 0.
/
, IO
I 20
I 30
IRRADIATION
I 40
I 50
I 60
70
TI ME/(min)
Fig. 6. MVK degradation on irradiation of aqueous solution containing an initial MVK concentration at 1.5 mM in a 125 W lamp photoreactor: El, *, TiOt suspension (1 g I-‘); +, n , immobilized TiOz. O2 was purged continuously during irradiation.
4. Conclusions Dissolved O2 measurements were used to study the heterogeneous photocatalytic oxidation of MVK. This is a convenient method for studying the efficiency of the photocatalytic processes and screening of potential photocatalysts. The photocatalytic degradation of MVK leads to partial mineralization as observed from CO2 measurements. These studies indicate the feasibility of using immobilized reactors for such processes.
Acknowledgments The authors thank the of India and the Regional Research), Trivandrum, for H. K. L. Varma, Regional coatings_
Council of Scientific and Industrial Research, Government Research Laboratory (Council of Scientific and Industrial financial support of this work. The authors also thank Mr. Research Laboratory, for his help in developing the TiQ
References 1 M. Gratzel, (ed.), Energy Resources Through Photochemishy and Cut@&, Academic Press, New York, 1983. 2 A. Harriman and M. A. West (eds), Photogeneration of Hydrogen, Academic Press, New York, 1982.
114 3 4 5 6
A. A. A. M.
Harriman, J. Phys. Chem.. 25 (1984) 33. J. Bard, J. Phys. Chem., 88 (1982) 2. Mills and G. Porter, J. C/rem. Sec., Faraday Trans. I, 78 (1982) 3659. Scbiavello (ed.), Photocataiysis and Environment. Trends and Applications,
Study Inst. Ser., 237 7
E. Pelezzetti Adv.
in NATO
Adv.
(1988).
and N. Serpone
Study Inst. Ser., 174
(eds.), Homogenous
and Heterogenous
Photocatulysis,
in NATO
(1986).
L. Palmisano and M. Schiavello, J. Phys. Chem., 94 (1990) 829_ T. Hisanaga and K. Harada, .I. Photochem. Photobiol. A: Chem., 48 (1989) 155. 10 K. Tennakone, S. Punchihewa, S. Wickremanayaka and R. U. Tantrigoda, J. Photochem. 8
A. Sciafani,
9 K. Tanaka, Photobiol.
A:
Chem.,
46 (1989)
247.
11 J.-F. Rantani and G. Giusti, J. Photochem. Photobiol. A: Chem., 46 (1989) 357. 12 C. Kormann, D. W. Bahnemann and M. R. Hoffman, .I. Photochem. Photobiol. A: Chem., 48 (1989) 161. 13 R. W. Mathews, J. CataL, 111 (1988) 264. 14 K. Harada, T. Hisanaga and K. Tanaka, New J. Chem., 12 (1987) 597. 15 N. Serpone, E. Borgarello, R. Harris, P. Cahill, M. Borgarello and E. Pelizzetti, Sol. Energy Mater., 24 (1986) 121. 16 C. S. Turchi and D. F. Ollis, .J. CaraL, 122 (1990) 178. 17 K. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka and A. Itaya, Bull. Chem. Sot. Jpn., 58 (1985)
2015.
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