J . Phys. Chem. 1984,88, 5999-6004
5999
Ozone Decomposition In Water Studied by Pulse Radiolysis. 2. OH and H04 as Chain Intermediates J. Staehelin,+R. E. Biihler,*$ and J. HoignB*+ Laboratory for Physical Chemistry, Swiss Federal Institute of Technology, ETH- Zentrum, 8092 Zurich, Switzerland, and Institute for Aquatic Sciences and Water Pollution Control, E A WAG, Swiss Federal Institute of Technology, 8600 Dubendorf, Switzerland (Received: March 30, 1984)
Ozone decomposition in pure water involves a chain mechanism, initiated by the reaction OH- + O3 and propagated by 02and OH. In the present studies this chain is initiated by pulse radiolysis of aqueous solutions of ozone. The chain propagation steps were studied in two parts: (I) 0, 0, H 0 3 OH and (11) OH + O3 ... H 0 2 0,. By computer simulation of the rate curves, it is shown that from OH + O3an intermediate H 0 4must be formed, most likely a charge-transfer complex (H0.03), which eventually decays into H02. The derived rate constants are k(OH+O,) = (2.0 f 0.5) X lo9 M-’ s-l, k(H04-+H02+02)= (2.8 f 0.3) X lo4 s-I, and k(H04--+OH+03) < k(H04-H02+02). The spectrum of H 0 4 is derived. It is similar to the one of ozone, but the absorption coefficients are about 50% larger. In the presence of high ozone concentration, the dominant chain termination reactions are H 0 4 + H 0 4 and H 0 4 + HOB. The effect on chain length, dose, overall rate, and pH and of added scavengers is described. The implications for the natural ozone decay mechanism are discussed.
- - -
-- -
presented and the rate constants of the detailed reactions 5 and Introduction 6 are given (see Figure 1). The importance of H 0 4 for chain The reactions leading to the decomposition of ozone in water termination in “pure” water is discussed. are of interest, not only in water treatment,’ but also for the chemistry of ozone in atmospheric cloud droplets.2 This deExperimental Section composition occurs by a radical chain mechanism which in pure water is initiated by the reaction between OH- ions and o z ~ n e . ~ , ~ The method of pulse radiolysis and sample preparation is the same as described in the previous paper.5 N 2 0 solutions were OH- + O3-,02-+ H 0 2 (1) prepared by mixing saturated aqueous 03/02 with saturated N 2 0 solution (Sauerstoffwerke, Luzern), both of the same pH and 02-then reacts with ozone rather selectively as part of a chain phosphate buffer concentration. For computer simulations the cycle. Together with OH, it is a chain carrier. As both carriers spectra were used as given in Figure 1 of part 1.5 can be produced selectively by pulse radiolysis, this technique was applied to study details of the chain process. Results In our previous paper (part 1)5 we reported results on the first part of this chain cycle: Various chemical systems (Table I) were chosen to analyze the kinetics of the different parts of the chain process in pure water. 02- O3-,03- O2 (2) The notation of systems A-D is the same as that used in part 1. As ozone depletion is a direct measure for the extent of the chain 0,- + H+ e H O ~ (3) mechanmism, almost all spectral observations discussed in this paper (part 2) were performed in the UV range, although it is H03 OH + 0 2 (4) extremely difficult to study the individual transients in this range due to strongly overlapping bands (Figure 1 of part 15). The with results from the visible range for systems A-D have already been k2 = (1.6 f 0.2) X lo9 M-’ s-’ (see also ref 6) reported and discussed in part l.s System C, a neutral, buffered solution of ozone, without any k3f = (5.2 f 0.6) X 1OIo M-’ s-I scavengers added (Table I, four individual solutions) shows the k31 = (3.3 f 0.3)X lo2 s-I largest transient signals of all systems studied. (Phosphate buffer ions are present in too low concentration to act as O H scavengers k4 = (1.1 f 0.1) X los in our systems.) The time profile for the UV absorbance (Figure 2b, 280 nm) shows an immediate increase within the irradiation and the new transient spectrum for H 0 3 (A, = 350 f 20 nm, pulse, followed by a slower increase for about 0.5 p s , and then emax = 300 & 30 M-’ cm-l). exhibits a very strong decrease for more than 500 p s (Figure 2c). In the present paper (part 2), the second part of the cycle is The spectrum (Figure 2a) shows that this decrease is dominated studied by the ozone depletion. OH + O3 -,H 0 2 + O2 (516) followed by
+
+
S
+
0; + H+ pK, = 4.8 (7) The rate constant of reaction 516 was determined by Bahnemann and Hart’ to be 3 X lo9 M-I s-’ , a nd later corrected to be 1.1 X lo8 M-I s-l? In this paper (part 2), it is shown that the discrepancy in rate constants appears to be related to a new intermediate, H04, originally proposed by Bahnemann and Hart,’ and then rejected again by Holcman, Sehested, and Harts8 The H 0 4 spectrum is
H02
‘Institute for Aquatic Sciences and Water Pollution Control Laboratory for Physical Chemistry.
0022-3654/84/2088-5999$01.50/0
(1) Hoig6, J. In “Handbook of Ozone Technology and Application”; Rice, R. G. Netzer, A., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1982; Vol. I, pp 341-79. (2) Chameides, W. L.; Davis, D. D. J . Geophys. Res. 1982, 87C7, 4863. Graedel, T. E.; Weschler, C. J. Rev. Geophys. Space Phys. 1981, 19, 505. (3) Staehelin, J.; HoignC, J. Enuiron. Sci. Technol. 1982, 16, 676. (4) Forni, L.; Bahnemann, D.; Hart, E. J. J . Phys. Chem. 1982,86, 2 5 5 . (5) Bilhler, R. E.; Staehelin, J.; HoignC, J. J . Phys. Chem. 1984, 88,2560 (paper 1 of this series); J . Phys. Chem. 1984, 88, 5450 (erratum). Staehelin, J. Dissertation ETH Nr. 7342, Swiss Federal Institute of Technology, Zurich, Switzerland, 1983. (6) Sehested, K.; Holcman, J.; Hart, E. J. J . Phys. Chem. 1983.87, 1951. (7) Bahnemann, D.; Hart, E. J. J . Phys. Chem. 1982,86, 2 5 2 . (8) Holcman, J.; Sehested, K.; Hart, E. J. Radial. Res., Proc. Inr. Congr., 7th 1983, paper A2-12.
0 1984 American Chemical Society
6000 The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
Staehelin et al.
TABLE I: Survey of the Chemical Systems Studied, with Yields of Transients and Ozone Depletion' systems [os],M
LO,], M PH
A
B
4 x 10-4 1.4 x 10-3 4
4x 1.4 x 10-3 1.8
C
[PO,ltot,b M [scavenger]
02-
HO,
03HO3 OH - 0,
1.7
0.2
0.4
1.9
0.9
0.9
0.2 2.1 1.1
0.2 2.7
0.1 1 .o 2.1 1.1
3.8
Gt0t(-03)
0.1
1.o 2.7 3.8
1 x 10-4 1.4 x 10-3 6.3 or 1.3 1 X 1OW4-5 X
2.4 0.44 0.3 0.06 2.7 0.36
4.8
0.9 0.6 0.1 5.4
D
E
1 x 10-4 1.4 x 10-3 6.3, 1.3, or 7.9 1 X 10-4-5 X 1 X lo-' M t-BuOH
6 X lo-' 7 x 10-4 7.3 5 X 10-4-5 X 1.35 X M N,O
2.4 0.44 0.3 0.06 2.7 0.36
0.27
2.4 0.44
0.43 0.05
0.07 5.08
0.54 0.86
0.1 0.14 10.16
0.12 2.84
11.8
11.8
The yield is expressed as G value = number of species per 100-eV absorbed energy. Total phosphate buffer concentration. Calculation is based on the yields of primary species of water radiolysis (G(0H) = G(eaq-) = 2.7 and G(H) = 0.5 for all pH values involved) and on the rate constants as given in Table I of part 1 . 5 Theoretical ozone decomposition for one complete chain cycle (systems C and E). G, = G t o t e Theoretical ozone decomposition with the chain being stopped at HO (by protonation of Oz-), without termination reactions (systems A and B). For system A this is only partly true: pH is too close to pK. Theoretical ozone decomposition with the chain being stopped at OH, assuming complete scavenging of the OH by t-BuOH (system D). TABLE II: Apparent First-Order Rate Constants for the 0, Decay in Systems with Different Chain Contributionsfrom the OH Radicals' chain apparent rate constant, system characteristics 104 s-1 Db OH suppressed 3.1 f 0.3 CC "free running" 1.6 f 0.2 Ed OH boosted 1.2 f 0.4 "All systems pH 7.3 and buffer concentration 5 X M. bWith M r-BuOH. CNeutralsolution. dWith 1 2 5 X lom2M N20.
I
H20
chain terminations
P
Figure 1. Complete chain mechanism (bold: natural ozone decay, thin: entering points in pulse radiolysis). System E contains N20 to convert ea; into OH, which become the prevalent radicals at the end of the pulse. The kinetics at 280 nm are similar to those of system C, except that the initial peak is larger and the O3 depletion substantially smaller. System B (pH 1.8) was studied because at this pH all 0,- are immediately converted to H02 (pK(H02/02-) = 4.8). Judging from gas-phase data, H 0 2 is very unreactive toward ozone;9 therefore the chain propagation is blocked and the O3depletion becomes much smaller. A typical result is shown in Figure 3, which reveals a succession of at least three processes. The initial negative spike was confirmed by a fast recording with 2-ps full sweep. In system A (pH 4), the chain is again suppressed by dominantly producing the unreactive HOz. This system was used to selectively enter the chain mechanism by 0,- and 03-(see part 1). The ozone consumption is shown later in Figure 7. System D contained tert-butyl alcohol (t-BuOH), which scavenges the OH and thereby suppresses the chain propagation to some extent. As the resulting peroxy radical of the alcohol strongly absorbs in the UV range, details of the initial kinetics are inaccessible. However, after about 200 ps, ozone depletion strongly dominates. A typical result for one of the five different systems (9) Orgen, P. J.; Sworski, T. J.; Hochanadel,C. J.; Cassel, C. M. J. Phys. Chem. 1982,86, 238.
studied (Table I) is shown later in Figure 7 . In systems C-E the absorption peaking at 430 nm was shown The apparent decay rate of this absorber turns to be due to 03-.$ out to depend on the amount of OH produced (Table 11). With the concentration of OH boosted by NzO (system E), the decay becomes slower, and with its concentration reduced by scavenging with t-BuOH the opposite effect occurs. This is further support for the chain mechanism, since when the O H yield is increased, more 0,- are re-formed by the chain (reactions 5-7 and 2).
Discussion The Need for a New Transient. When the experimental time profiles of the absorbances (A = 240-310 nm) are compared with those calculated from the original chain model with OH + O3 HOz + 0, (5/6)
-
assuming H 0 4 to be nonexistent, various systematic deviations became obvious: In a neutral, buffered solution of ozone (system C), there was always an unexpected growth of UV absorbance for about 0.5 p s (Figure 2b). That increase was slower than the buildup of the initial transients OH, OF, HO,, 0,-,and HO,. Of the secondary reactions only OH O3 had a rate comparable to that of the absorber formation, if the original value by Bahnemann and Hart' of 3 x 109 M-' s-1 w as applied. If the yield of OH was boosted by saturating with N,O (system E), then the initial excess rise of the UV absorbance was even more pronounced. The rate curve at 280 nm for system B (pH 1.8) similarly indicates that there must be an additional absorber on a time scale of about 1 p s . As seen in Figure 3 and confirmed by faster recordings, the initial decrease of absorbance at pulse end (arrow), mainly due to O3 consumption by the H O3reaction (see part l ) , is followed by a rapid unexplained increase within a few microseconds, thereby forming the negative spike (arrow) at pulse end. Therefore, all three systems (C, E, and B) display an absorbance increase which roughly correlates to the OH yields in the system, and to the kinetics of the reaction OH + 03.
+
+
ABS
*
1000
ABS
I
I -
-
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 6001
Ozone Decomposition in Water
a
4a
-120
*
1880
c
I0
280
1
5aa
258 ,aBO
ABS
nrn
MICROSECONDS
l:"i -80
I
-100
. I I
I I
-4
CI I
PULSE .
.
.
0 2 290 3 NANOMETER Figure 2. Typical spectra and time profiles for system C, with p H 7.3 and buffer concentration 5 X lo4 M. Time indicated in spectra is measured
230
260
from pulse start; the dose is 2.1 krd. ABS
*
ABS
1000
+--.
0 -7 -8
*
I000
I
I
20
40
I
I
60
80
-10 -20
-16
-30
t
-40 -50 0
100
MICROSECONDS
Figure 4. Computer simulation for the old reaction model (without the new transient). Experimental curve is for system C: pH 7.3 and buffer concentration 5 X lo4 M at 280 nm. Calculated curve is for the literature rate constants k5I6= 3 X lo9 M-l s-] (curve l),' ksja = 1.1 X lo8 M-I s-I (curve 3)8 and the present best fit with k S j 6= 2.6 X 10' M-' s-' (curve 2). There is no parameter available to fit the curve in the time range 15 ps (with no or negligible effect on the earlier time range). From each experimental rate curve the three parameters k5! k6, and e(HO,) could be determined. A typical curve fit is shown in Figure 5. The fit for the times up to about 70 ps is excellent. However, some deviation remained for longer times. There is no parameter available to correct this. From averaging the results of 28 experiments one obtains
k5f = (2.0 f 0.5)
X lo9 M-' s-l
k6 = (2.8 f 0.3) X lo4 S-l The spectrum for H 0 4 is given in Figure 6 . The most accurate absorption coefficient, from 17 independent experiments, is derived for 280 nm: E(HO&~~,,, = 3200 f 150 M-' cm-'. From the comment above one also concludes that k5' < k6. As the apparent first-order rate constant for OH O3is kjfi = k5f[03]= 2 X IO5 s-l, equilibrium 5 appears to lie greatly on the side of H04. In the present systems with about lo4 M ozone the OH radicals initially produced are rapidly converted into H 0 4 (half-life ca. 3 fis, [O,] dependent), which then decay much more slowly (half-life ca. 25 ps, unimolecular). H04 acts like a chain-carrier reservoir and therefore slows down the overall chain rate. Details of the concentrations of each transient are shown in Figure 5B. As the spectrum for H04is very similar to the ozone band, except for a larger absorption coefficient (ca. 60% larger than the sum of ozone and OH), it is tempting to propose that H 0 4 might correspond to an OH radical charge-transfer complex: (H0.03). Chain Termination. Termination reactions strongly affect the ozone depletion. Therefore, by checking the influence of the individual termination rate constants on the absorbance time profile, particularly for long times, one can derive a measure for the importance of the particular reaction. By setting the particular rate constant to zero, one can derive the sequence of importance for a neutral solution of ozone, as shown in Table 111. The number (arbitrary scale) for the importance is classifying the reactions by the overall effect within the observation time. The differential effect at 400 ps (end of observation) however informs about the persistent importance. All reactions with a small differential effect
+
Staehelin et al. ABS
*
1000
10 0 -I0 -20
-30 -40 -50 0
20
40
80
60
100
MICROSECONDS
M
1E-6
I 0
0
20
40
60
I00
80
MICROSECONDS Figure 5. Computer simulation for the extended reaction model with H04. Experimental curve is the same as in Figure 4. The calculated curves are for the best fit with k: = (2.0 p 0.5) X lo9 M-I s-I , k6 - 28000 k 3000 s-l, and czsonm(H04) = 3200 & 150 M-I cm-I. Furthermore kSr < k6. (A) Absorbance- and (B) concentration-time profiles of all transients involved (scale for ozone depletion 4 times reduced!).
-
IC
6I
5
I
I
1
I
I
I
I
I
T
1 I
Li
LL
; 2 yi
m
a
0-
230
Figure 6.
250
270
290
310
NANOMETER Spectrum for H 0 4 (ozone and OH for comparison).
at 400 pus are only active during the initial period (typically up to 20 ps) if a t all. This corresponds to the concentration time
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 6003
Ozone Decomposition in Water TABLE III: Relative Importance of the Termination Reactions from the Computer Simulation
reaction (14) HO, + H 0 4 (16) HO, + 02(17) HO, + H 0 3 (10) OH + 0 2 (15) HO, + OH (1 3) H 0 3 + 02(1 2) HOB+ H03 (9) OH + OH (11) OH + H 0 3
importance,” arbitrary scale
G(-03)
N
1 I .8
diff effect
SYSTEM
at 400 psb
1
DOSE KRD
large small medium
55
18 10 9 6
small .5
small small small small small
4 2.5
2 1.5
“This value represents the total deviation at 400 ps by setting the particular rate constant to zero. For the differential effect the slopes at 400 ps are compared. GC-03 > N
0
5 0
250 MICROSECONDS
0
Figure 8. Dose dependence of the ozone depletion, both systems with pH 7.3 and [PO,],,, = 5 X M. Half-life of 0,- is rl,2(O
