J. Phys. Chem. 1987, 91, 2359-2361
2359
Ozone Decomposition in Aqueous Acetate Solutions K. Sehested,* J. Holcman, E. Bjergbakke, and Edwin J. Hart+ Accelerator Department, R i m National Laboratory, DK 4000 Roskilde, Denmark (Received: August 26, 1986)
The acetate radical ion reacts with ozone with a rate constant of k = (1.5 f 0.5) X lo9 dm3 mol-' s-'. The products from this reaction are C02, HCHO, and 02-.By subsequent reaction of the peroxy radical with ozone the acetate radical ion is regenerated through the OH radical. A chain decomposition of ozone takes place. It terminates when the acetate radical ion reacts with oxygen forming the unreactive peroxy acetate radical. The chain is rather short as oxygen is developed, as a result of the ozone consumption. The inhibiting effect of acetate on the ozone decay is rationalized by OH scavenging by acetate and successive reaction of the acetate radical ion with oxygen. Some products from the bimolecular disappearance of the peroxy acetate radicals, however, react further with ozone, reducing the effectiveness of the stabilization.
Introduction Acetic acid and acetate are known to stabilize ozone in aqueous solutions.'-* The hydroxide ion catalyzed decomposition of ozone and the mechanism of the stabilizing effect of acetate are not fully understood. However, as the OH radical was shown to be an important transient species formed during the OH-catalyzed ozone decomposition' and as it is the important chain-propagating r a d i ~ a l , ~the - ~ inhibiting effect of the acetate ion has been rationalized in terms of its ability to scavenge OH radicalse2 The acetate ion was used in the determination of the rate constant of the reaction OH O3 by competition kinetics,6 but because of the complexity of the reactions involved, the interpretation of the system has led to an erroneous result.' In our previous paper' we indicated that the experimental findings in the 03/acetate system can be explained by taking into account the reactions of the acetate radicals with ozone and oxygen. A pulse radiolysis study and product determination of y-radiolysis has therefore been undertaken to provide further details concerning the mechanism in the 03/acetate system.
+
Experimental Section Irradiations. For pulse radiolysis the 10-MeV H R C linear electron accelerator at Ris0 was used with a pulse width of 0.2-2 ps and doses of 1-10 krd. The optical detection system and data recording and treatment has been described previously.8 The @Co y-irradiations were performed in a 300043 y-cel19 at a dose rate of 250 krd/h. Materials. The method of preparation of concentrated ozone solutions with or without oxygen is described elsewhere.' Equal volumes of acidic ozone solutions and appropriate alkaline solutions were rapidly mixed and flowed through the radiolytic cell. Immediately after the flow was stopped, the electron pulse was delivered. All chemicals were p.a. and were used without further purification; the water was triply distilled. The dosimetry was performed with a hexacyanoferrate dosimeterL2using G = 5.9 and c~~~ = lo3 dm3 mol-' cm-'. Product Analysis. Irradiated samples were rapidly degassed upon entering the evacuated van Slyke pipet and the carbon dioxide was measured by gas chromatography.1° The presence of formaldehyde was shown by a spot-test using condensation with chromotropic acid in concentrated sulfuric acid." The amount of formaldehyde was determined by diluting the reaction mixture (as in ref. 11) to 3 mL with water and measuring the absorbance at 570 nm where the reaction product has an absorption maximum. These measurements have been used to determine the ratio of the formaldehyde yields formed in "oxygen-free" and oxygen-saturated solutions. A blind test has shown that this procedure is fully adequate for semiquantitative measurements. Results and Discussion Pulse Radiolysis. When a N20-saturated solution (pH 9-10) mol containing (1-2) X IO4 mol dm-3 ozone and (1-10) X Port Angeles, WA, 98362.
0022-3654/87/2091-2359$01.50/0
dm-3 acetate was irradiated with a 2-5-krd pulse, the ozonide radical ion forms, and it is identified by its absorption spectrum with a band at 430 nm.I3 In this system hydrated electrons are converted into O H radicals by N20 and H atoms into OH radicals by ozone. As the acetate concentration exceeds the ozone concentration by a factor of more than 100 the O H radicals react exclusively with the acetate ions to form the acetate radical ion absorbing at 350 nm.I5
-
OH + CH3COO- CH2COO- + H 2 0 kl = 8.5 X lo7 dm3 mol-] s-] (ref 14)
(1)
O H + O3 H 0 2 + 0, k2 = 1 . 1 X lo8 dm3 mol-' s-I (ref 7)
(2)
-
The absorption band at 350 nm develops within a few hundred nanoseconds and thereafter decays in 5-10 ps depending on the O3concentration. The absorption at 350 nm reaches a minimum which is followed by an increase concomitant with the increase of the 03-absorption at 430 nm (Figure 1). After 50 to 100 ps the absorption band observed in the region 300-700 nm is identical with that of the 03-radical ion with a yield of G ( 0 3 - ) = G(e,; + O H + H). In the region below 300 nm, where the ozone absorbs, a decrease of absorbance concomitant with the development of the 03-spectrum is observed. From the change in absorbance at 430 and 260 nm, a stoichiometry of two ozone molecules per one ozonide radical ion formed is derived. 500 dm3 mol-' cm-' (ref 13), C ( C H ~ C O O - ) ~ ~ ~ As € ( 0 3 - ) 3 5 0 = N 800 dm3 mol-' cm-', and C ( O ~ C H , C O O - )N~ ~250 ~ dm3 mol-' cm-' (ref 16 and 17) the minimum in absorption at 350 nm after 5-10 p s (Figure 1) indicates a two-step formation of 03with an intermediate species nonabsorbing at 350 nm. Accordingly, an addition of O3 to the acetate radical ion is therefore suggested as the first step
O3 + CH2COO-
-
03CH2COO-
(3)
(1) Hoignt, J.; Bader, H. Water Res. 1976, I O , 377. (2) Forni, L.; Bahnemann, D.; Hart, E. J. J . Phys. Chem. 1982,86,255. (3) Staehelin, J.; Hoignt, J. 5th Ozon- Weltkongress, Wasser, Berlin; Colloquium Verlag: West Berlin, 1981; p 623. (4) Staehelin, J.; Btihler, R. E.; Hoignt, J. J . Phys. Chem. 1984,88, 5999. ( 5 ) Tomiyasu, H.; Fukutomi, H.; Gordon, G. Inorg. Chem. 1985,24,2962. (6) Bahnemann, D.; Hart, E. J. J . Phys. Chem. 1982,86, 252. (7) Sehested, K.; Holcman, J.; Bjerbakke, E.; Hart, E. J. J . Phys. Chem. 1984,88, 4144. (8) Sehested, K.; Holcman, J.; Hart, E. J. J . Phys. Chem. 1983,87, 1951. (9) Bjergbakke, E.; Larsen, E. E. Rim-M-1651, 1973. (10) Bjergbakke, E. In Measurement of Oxygen, Degn, H., Balslev, I., Brook, R., Eds.; Elsevier: Amsterdam, 1976; pp 1-10, ( 1 1 ) Feigl, F. Sbot Tests in Organic Analysis; Elsevier, Amsterdam, 1960; 6th ed, pp 349-351. (12) Rabani, J.; Matheson, M. S.J . Phys. Chem. 1966, 70, 761. (13) Felix, W. D.; Gall, B. L.; Dorfman, L.J . Phys. Chem. 1967, 71, 384. (14) Wilson, R. L.; Greenstock, C. L.; Adams, G. E.; Wageman, R.; Dorfman, L. M. Int. J . Radiat. Phys. Chem. 1971, 3, 259. (15) Simic, M.; Neta, P.; Hayon, E. J . Phys. Chem. 1969, 73, 3794. (16) JosimoviE, L. R.; DraganiE, I. G.; Markov%,V. M. Bull. SOC.Chim., Beograd 1976, 41, 75. ( 1 7 ) Abramovitch, D. (D.); Rabani, J. J . Phys. Chem. 1976, 80, 1562.
0 1987 American Chemical Society
2360 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
Sehested et ai.
o A
0.050
:
0.02s
0
I
0
I
I
I
I
I
I
I
I
I
0
100 200 300 400
x10-6s
4 20 30 50 10
O O
LO
x10-6s
Figure 1. Calculated optical density traces with experimental points at 350 and 430 nm in a 0.1 mol dm-3 acetate solution, N 2 0 saturated, containing 1 . 5 X IO4 ozone, pH 9.5. Dose rate = 2 krd/ps.
This reaction is analogous to the formation of the peroxy acetate radical
O2 + CH2COO-
k4 = 3 k4 = 2.1
X X
-
02CH2COO-
(4)
lo9 dm3 mol-' s-I (ref 16) lo9 dm3 mol-' s-I (ref 17)
k4 = 1.7 X lo9 dm3 mol-] s-I (ref 18) The second step of the 03-formation is tentatively explained by reaction 5 followed by reaction 6. Reaction 5 is considered
-
03CH2COO-
02-+ C02 + C H 2 0
-
O3 + 02- 0,- + 0, k6 = 1.5 X lo9 dm3 mol-' s-' (ref 8)
(5) (6)
analogous to the decomposition of the tetraoxide intermediate formed in the bimolecular reaction of peroxy acetate radicals.'* Thus the 0 3 C H 2 C 0 0 - radical may undergo internal electron transfer in its six-membered ring transition state and split into more reactive species according to eq 5.
Indeed, C 0 2 and formaldehyde are found as products in yields corresponding to reaction 5 (see later). In the O3concentration range 5 X 10-5-2 X mol dm-3 the rate of 03-formation was found to be proportional to the ozone concentration. It is therefore assumed that reaction 5 is instantaneous under our experimental conditions. Reactions 3 and 5 may then be combined to an overall reaction
O3 + CH2COO-
-
02-+ C 0 2 + CH,O
(3')
and reactions 3' and 6 treated as kinetically coupled pseudo(18) Schuchmann, M. N . ; Zegota, H.; Sonntag von, C. Z.Naiurforsch. 1985, 40b, 215.
Figure 2. Optical density traces in a 0.1 mol dm-' acetate solution, N20 saturated, pH -7, containing 1.60 X lo4 ozone. Dose rate = 2 krd/ps. A: 350 nm, absorption of CH2COO-, 03-, and O 2 e H 2 C 0 0 - . B: 430 nm, absorption of O
