J . Phys. Chem. 1986, 90, 3061-3062
3061
Sonolysis of Ozone in Aqueous Solution Edwin J. Hart and Arnim Henglein* Hahn-Meitner-Institut fur Kernforschung, Bereich Strahlenchemie, 0-1000 Berlin 39, F.R.G. (Received: April 22, 1985)
When water is irradiated with ultrasound under an atmosphere containing oxygen and ozone, an extremely rapid decomposition of 0,takes place. The rate of decomposition increases with the O3concentration in the liquid phase; at [O,] = 1 mM, the rate is about 3 mM m i d . Argon in the 02-O3mixture acts merely as a diluant. However, the accompanying formation of H 2 0 2occurs with maximum yield at 80 vol 76 argon. These effects are explained by the thermal instability of ozone. Complete decomposition of the O3content of a pulsating gas bubble occurs regardless of the composition of the bubble, while H202formation takes place with greater efficiency in 0,-Ar bubbles where higher temperatures are produced in the acoustic compression phase.
Introduction Gas reactions that resemble the reactions in flames can be initiated when ultrasound passes through water containing a gas or a mixture of gases. We have recently reported isotopic exchange in the systems D2-H20,' I8,I8O2-H2l60: and 14,14N2-15J5N2-H20: and the decomposition of nitrous ~ x i d e . ~ These J chemical effects are brought about by temperatures of several thousand Kelvin which exist in the compression phase of oscillating or collapsing gas bubbles. The yields of such gas-phase reactions often are substantially higher than for the reactions which occur in the liquid phase such as the formation of H202and the oxidation of solutes. In the present paper, we describe the decomposition of ozone in cavitation bubbles of oxygen and oxygen-argon mixtures. Ozone is thermally very unstable. In a previous study we have briefly reported its instability in an ultrasonic field.(' The decomposition of ozone in sonolized water is so fast that irradiation times of seconds have to be used to measure the rate of reaction. Experimental Section The irradiation vessel, the 300-kI-I~generator, and the gas-fiiling procedure using Van Slyke and syringe techniques have been described previously.',6 The gas was oxygen containing ozone in varying concentrations. In some of the experiments, the 0,-O3 mixture was diluted by argon. Ozone was prepared and analyzed by its optical absorption ( E = 3300 M-' cm-' at 260 nm) as described previo~sly.~The irradiation vessel contained 37.5 cm3 of 0.1 N HC104 and 20 cm3 gas phase. Equilibration between the two phases always occurred a t 25 O C . At this temperature, the pressure of ozone in mbar is 7.6 X 104M, where M is the concentration of ozone in the solution.8 Three milliliters of the sonolized solution was withdrawn from the equilibrated liquid, the absorbance at 260 nm determined, and the solution returned to the cell for further irradiation. Hydrogen peroxide analysis was carried out on the 02-purged irradiated solution by reaction with FeS04. Ten milliliters of the purged solution was slowly run into 10 mL of 2 mM FeS04 in 1.6 N HzSO4. The Fe3+content was then determined from the absorbance at 305 nm ( E = 2201 M-' cm-I). During the first few minutes of irradiation, the temperature of the liquid rose to a stationary value of 33 O C . In a control experiment without sonolysis, the solution was exposed to 3 3 OC
TABLE I: Ozone Concentration in Sonolized Solutions (0.1 N HCIO,) ozone concn. U M
irradiation time, s
unequilibrated
equilibrated
0 30
628 242 175
628
60 120 180
70 40
for 1 min. No detectable change in the total O3 concentration in solution could be found.
Results and Discussion Figure 1 shows the rate of total O3consumption as a function of the O3 concentration in the liquid. The measurements were made at 10,30, and 60 s of irradiation. Higher rates were observed in the experiments with the shorter irradiation time. ' This dependence of the measured rate on irradiation time is explained by the rapid disappearance of ozone due to its decomposition, the ozone in the gas phase not being able to migrate into the solution at a rate sufficiently high to maintain a constant O3concentration. The nonequilibrium conditions existing during irradiation are also recognized from the data of Table I. In these experiments various irradiation times were applied and the O3 content of the liquid was determined immediately after irradiation without equilibration and then with equilibration by shaking the cell for 1 min. It is seen that the 0,concentration of the unequilibrated solution was substantially lower than that of the equilibrated one. The effect of argon added to the 0 2 - 0 3 mixture on O3 decomposition and HzO2formation is shown in Figure 2. In these experiments an initial mixture of 353 MMO3 and associated O2 was diluted with argon so that proportionally lower initial O3 concentrations were present. It is seen that the yield of O3 consumption changes nearly linearly with the concentration of O3 while that of H202passes through a maximum at 80 vol 7' 6 argon. The rates of O3decomposition at the higher O3concentrations in Figure 1 are of the order of several mM min-I. Since the H202 yield in oxygenated water is only 0.02 m M m i d , it is recognized that the decomposition of O3proceeds about hundred times faster than the more conventional sonolytic reactions. Ozone probably is not decomposed in a chain reaction, but in two main stepsg O3
(1) Fischer, Ch.-H.; Hart, E. J.; Henglein, A. J . Phys. Chem. 1986, 90, 222. (2) Fischer, Ch.-H.; Hart, E. J.; Henglein, A. J . Phys. Chem. 1986, 90, 1954. (3) Hart, E. J.; Fischer, Ch.-H.; Henglein, A. J . Phys. Chem., in press. (4) Henglein, A. 2.Naturforsch.B 1985, 40, 100. (5) Hart, E. J.; Henglein, A. J . Phys. Chem., submitted for publication. ( 6 ) Hart. E. J.: Henelein. A. J. Phvs. Chem. 1985. 89. 4342. (7) Hart; E. J..; Sehksted; K.; Holfman; J. Anal. Chem. 1983, 55, 46. (8) Kilpatrick, M. L.; Herrick, C. C.; Kilpatrick, M. J. Am. Chem. SOC. 1956, 78, 1784.
0022-3654/86/2090-3061$01.50/0
465 3 29 225 141
k = 4.1
X
M
O2
+0
-
10-10e-'1430/T molecule-l cm3 s-l 0 + O3 2 0 2
(1) (2)
k = 1.9 X 10-11e-23m/T molecule-' cm3 s-' (9) Hampson, R. F.; Gamin, D.Natl. Bur. Stand. Spec.Publ. 1978, No. 51 3.
0 1986 American Chemical Society
Letters
3062 The Journal of Physical Chemistry, Vol. 90, No. 14, 1986
We conclude that the decomposition of ozone is so fast that all O3molecules in a compressed gas bubble are decomposed regardless of the composition of the bubble. The observed rate of O3decomposition therefore is not dependent on the specific rate but only on the amount of O3available in the gas bubble. The hydrogen peroxide yield in the maximum of the curve in Figure 2 is higher by a factor of six than in water containing pure yield vs. O2concentration curve oxygen. A maximum in the H202 in the irradiation of water under argon-oxygen mixtures has been observed previously.6.'0 In this maximum, the H202yield was only three times higher than under pure argon. Water is decomposed according to
3.0
-.-I
c
E
2.0
r
E
L
I
H20
0.5
0
1.0
1.5
[03l[rnMI Figure 1. Rate of 0,disappearance as a function of the concentration of 0,in the liquid phase. At the highest O3concentration of 1.6 m M , the O,-O, gas mixture under which the 0.1 N HC104 solution was irradiated had a composition of 85 vol % 02-15 vol % 0,.
M
+ OH
H
(3)
Both radicals have been detected by ESR techniques." Most of the radicals recombine to re-form water molecules, if no radical scavenger is present in the gas phase.l In the presence of 02,more oxidizing species are formed:
H + 02
+
HOz
+
OH + 0
(4)
and these species produce H202via the reactions
0 + H2O
-
+
20H
(5)
M
0.6
20H
2H02 0.4
,c ._
7
E x
- 0.2 E
"0 100
20 80
40 60 60 40 vol. O/O
80 20
100 02/03 0 Ar
Figure a. Rate of formation of H202 and rate of O3 disappearance at various compositions of the mixture of argon with the 02-03 mixture. The latter had a composition of 96.6 vol % 02-3.4 vol % 0,. The large yields are due to the low activation energies of the elementary reactions involved. In the presence of argon no acceleration of O3decomposition was observed although one could expect that in a gas bubble Containing a large percentage of argon higher temperatures are produced. In fact, the accelerating action of added argon has been observed for many other reaction^.^^^^'^
+
H202
(6)
H202 + 02
(7)
The maximum in the yield vs. concentration curve is brought about by two opposing effects. With increasing O2concentration in the argon atmosphere more H atoms are scavenged (eq 4). On the other hand, the temperature in the gas bubbles becomes lower a t higher O2concentrations, i.e. reaction 3 takes place less frequently: When ozone is also present in the argon-oxygen mixture, additional H202may be formed via reactions 1, 5 , and 6, i.e. O3 decomposition and H202 formation are coupled together to a certain degree. However, most oxygen atoms probably undergo reaction 2 in view of its low activation energy and high rate constant. Ozone was reported to be formed in the ultrasonic irradiation of oxygenated water.I2 We have recently pointed out that we could not confirm this finding.6 Our present results show that O3decomposes even faster in an ultrasonic field than was anticipated in ref 6. Therefore it seems highly improbable that ozone could be formed in amounts worth mentioning, although it may occur as a short-lived intermediate in the irradiation of oxygenated water. (10) Henglein, A. Natunvissenrchaften 1957, 44, 179. (11) Makino, K.; Massoba, M. M.; Riesz, P. J . Phys. Chem. 1983, 87, 1369.
(12) Haissinsky, M.; Mangeot, A. Nuouo Cimenro 1956,4, 1086.
