J . Phys. Chem. 1984, 88, 2560-2564
2560
Ozone Decompositlon in Water Studied by Pulse Radiolysis. 1. H02/02- and as Intermediates
H03/03-
R. E. Buhler,* Laboratory for Physical Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum. 8092 Zurich, Switzerland
J. Staehelin? and J. HoignB* Institute for Aquatic Sciences and Water Pollution Control, EA WAG, Swiss Federal Institute of Technology, 8600 Diibendorf, Switzerland (Received: August 29, 1983)
Pulse radiolysis allows one to selectively enter the chain mechanism for ozone decomposition in water by OH or 02-and to study the elementary reactions involved. Details of the reaction mechanism of the first chain propagation step (020,) in the Weiss model have been resolved to include the two additional transients 03-(A, = 430 & 10 nm,emax = 2000 M-’ cm-’) and HOB(A, = 350 20 nm, emax = 300 i 30 M-I cm-’). The elementary steps are as follows: 0, + O3 03-+ 02,k = (1.6 0.2) X lo9 M-’ s-l; 03- H+ + H 0 3 (a, b), k, = (5.2 0.6) X 1O’O M-I s-l, kb = (3.7 i 0.3) X lo4 s-’; H 0 3 -OH 02,k = (1.1 i 0.1) X lo5 s-’. A pK(H03/03-) = 6.15 i 0.05 is derived. Phosphate buffer also takes part in the protonation/deprotonation of H03/03-: 03-+ H2P04-+ H 0 3 + HP042-(a, b), k, = (2.1 0.2) X lo8 M-I 8,kb = (2.0 & 0.3) X lo7 M-’ s-l. Consequences of these high rate constants on the possible chain termination processes and on the possibility of interactions of organic solutes with the chain reactions are discussed.
+
-
*
*
*
Introduction The pathways and kinetics of demmposition of aqueous ozone are of interest for a wide range of topics such as the behavior of ozone in cloud droplets,’ in water treatment processes: and in radiolytic effects in oxygen-cpntaining watera3 Correspondingly, numerous studies on the fate of aqueous ozone have been performed during the last 50 years (for literature reviews see ref 4-7). Nevertheless, many important features of the reactions involved remained unknown. It appears, however, that the reactions of the transient H02/02- with ozone play a key role: (i) In water treatment processes H02/02- and OH radicals act as carriers for a chain of ozone decomp~sition,~ and (ii) H 0 2 present in cloud droplets may become the main reactant for ozone in these droplets.’ So far the best reaction model to explain the decomposition of ozone in “pure water” is the chain mechanism of Weiss,8 here extended by the deprotonation equilibria of H 0 2 and further termination reactions:
+
HOz & b 02- H+ initiation
O3 + OHpropagation
+ OH + O3
--
02- O3
pK, = 4.8
H02(O 10 from 0- O2 03-17,18 (see also review by Bielski et aL3), and recently deduced by Sehested et al. from ozone reactions at pH >13.19 From our experiments in the pH range 4-7.9 all rate constants for these reactions (including pK,), together with a new = 350 nm),will be derived. Details of spectrum for H 0 3 (A, the propagation step O H + O3 (eq 4) and a discussion of the complete chain mechanism will be presented in a forthcoming paper (part 2). Experimental Section The technique of pulse radiolysis with a Febetron 705 accelerator (Hewlett-Packard) for pulses of 2-MeV electrons was basically as described earlier.20 All experiments were at room temperature (21 f 1 "C) with an effective 50-ns irradiation pulse and a 2-cm optical path length in a quartz cell. The analyzing light from a 450-W Osram Xe lamp was filtered by an OX7 band-pass for X < 320 nm and by interference filters for X < 280 nm. Doses between 10 and 80 Gy (1-8 krd) were used. Since ozone solutions are sensitive to analyzing light, a shutter was opened only 3 ms before the actual experiment, so that photolytically only about 1% of the ozone was decomposed prior to the experiment. From the photolysis by the inherently produced Cerenkov radiation an increased light level by not more than 1% due to ozone photolysis during the irradiation pulse was estimated. Both photolytic effects are considered to be of no influence on the conclusions in this paper. Any H 2 0 2produced by photolysis or by ozone decomposition reacts too slowly with 039 to interfere within the time scale of the present experiments. The light signals were recorded by two photomultipliers and digitized by the Tektronix 7912 AD transient digitizer (bandwidth 100 M H z by the plug-in) and simultaneously by a slow twochannel transient recorder VUKO 22-2 with 1-MHz sampling rate. A PDP 11/23 computer with TEK-SPS-BASIC software by Tektronix was used to work up the data, analyze them for kinetic order, and derive the transient spectra. The linearity test for first-order kinetics always covered at least three half-lives. NaOH, H3PO4, and HC104 (all suprapur) and tert-butyl alcohol (puriss) from Fluka were used without further purification. Water was triply distilled. An ozone stock solution was obtained by continuously saturating water containing the buffer and scavenger with a fixed 03/02 ratio gas stream produced from oxygen in a Fischer ozonizer type 502. Shortly before the experiment, the cell was filled and the concentration of ozone determined spectroscopically with ~ ~ ( = 02900 , )M-' s-l (ref 21). Further ozone decay until the time of the actual experiment (about 1 min) was corrected for by the known decay rate. The pH was controlled by phosphate buffers, which were adjusted with NaOH or HC104. It was measured at the beginning and end of each series of experiments by a glass electrode calibrated with Merck buffer solutions. Results and Discussion Survey of Experiments. Starting from the primary species of water radiolysis, G(e, -) = G(OH) = 2.7 and G(H) = 0.5, the competing reactions o? interest can be influenced by varying the ozone concentrations and the pH and by selectively adding scavengers. The 11 chemical systems studied in this paper can be classified into four types (Table I). The initial transient yields, (17) Czapski, G.; Dorfman, L. M. J . Phys. Chem. 1964, 68, 1169. (18) Adams, G. E.; Boag, J. W.; Michael, B. D. Nature (London) 1965, 205,898. Adams, G. E.; Boag, J. W.; Michael, B. D. Proc. R. SOC.London Ser. A 1966, 289, 321. (19) Sehested, K.; Holcman, J.; Bjergbakke, E.; Hart, E. J. J. Phys. Chem. 1982,86, 2066. (20) Hurni, B.; Briihlmann, U.; Biihler, R. E. Radial. Phys. Chem. 1975, 7, 499. (21) Hoignt, J.; Bader, H. Water Res. 1976, 10,377. (22) (a) Pagsberg, P.; Christensen, H.; Robani, J.; Nilsson, G.; Fender, J.; Nielson, S . 0. J . Phys. Chem. 1969, 73, 1029. (b) Behar, D.; Czapski, G.; Robani, J.; Dorfman, L. M.; Schwarz, H. A. Zbid. 1970, 74, 3209. (c) Bielski, B. H. J.; Schwarz, H. A. Zbid. 1968, 72, 3836. (23) Felix, W. D.; Gall, B. L.; Dorfman, L. M. J . Phys. Chem. 1967, 71, 384. (24) Simic, M.; Neta, P.; Hayon, E. J . Phys. Chem. 1969, 73, 3794.
EX
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10'3
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300
250
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I
400
350
500
450
X,nm Figure 1. Known absorption spectra of OH,ZZa HO zzc 03- 17-19,23 eaq-,15and HO-C(CH,)2-CHz0-,24 together with H03. derived in this 23
9
paper.
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Figure 2. System A (pH 4): Spectrum of H03 as derived from the time profiles after complete disappearance of the 03absorption (see inset and text). Dose: 44 Gy. The start and end of the irradiation pulse (blanking of Cerenkov signal) is marked by negative-going vertical bars.
Le., yields of products from the initial reaction of the primary species of water radiolysis (ea
