Ozonation rate constants

Ozone rate constants, 27 771 Ozone reactions activation of, 27 779 kinetics and mechanism of, 27 778-779 Ozone resistance, of ethylene-propylene polymers, 10 704, 717 Ozone synthesis, energy requirements for, 27 798... 

A variety of methods have been recently reviewed [23] and several new methods have been published since then [24]. There is one program, the atmospheric oxidation rate prediction program or AOPWIN - part of the EPI Suite software - that will calculate the hydroxyl radical and ozone rate constants and atmospheric half-lives (with selected oxidant concentrations) [23,25]. The program uses the method of Atkinson and coworkers [26]... 

The ozone rate constants for deethylatrazine and deisopropylatrazine are even slower, 0.18 and 3.1 lmol s respectively, while the ozone rate constant for didealkylatrazine is slower yet at <0.1 lmoU s (Acero et al. 2000). Thus,... 

The ozonation rate constants (mol s ) reported in the literature for some polyolefins are in order [154] ... 

Reaction 1 is the rate-controlling step. The decomposition rate of pure ozone decreases markedly as oxygen builds up due to the effect of reaction 2, which reforms ozone from oxygen atoms. Temperature-dependent equations for the three rate constants obtained by measuriag the decomposition of concentrated and dilute ozone have been given (17—19). 

The calculated half-life of 1 mol % (1.5 wt %) of pure gaseous ozone diluted with oxygen at 25, 100, and 250°C (based on rate constants from Ref. 19) is 19.3 yr, 5.2 h, and 0.1 s, respectively. Although pure ozone—oxygen mixtures are stable at ordinary temperatures ia the absence of catalysts and light, ozone produced on an iadustrial scale by silent discharge is less stable due to the presence of impurities however, ozone produced from oxygen is more stable than that from air. At 20°C, 1 mol % ozone produced from air is - 30% decomposed ia 12 h. 

The two main termination steps for neutral solutions are HO + HO — H2O2 + 2 O3 and HO + HO3 — H2O2 + O3 + O2. An alternative mechanism has been proposed that does not involve HO and HO but has a different initiation step (26). Three ozone molecules are destroyed for each primary event. In the presence of excess HO radical scavengers, ie, bicarbonate, the pseudo-first-order rate constant at 20°C for the initiation step is 175 X. This yields an ozone half-hfe of 66 min at pH 8. In distilled water = 50 mmol/L), the half-hfe is significantly lower, ie, 7 min. 

The inoigaiiic chemistiy of ozone is extensive, encompassing virtually every element except most noble metals, fluorine (qv), and the inert gases. Repotted second-order rate constants (L/(mol-s)) at 20—23°C refer to the disappearance of ozone unless otherwise stated. 

Ha.logen Compounds. Fluorine is unreactive toward ozone at ordinary temperatures. Chlorine is oxidized to Cl20 and Cl20y, bromine to Br Og, and iodine to I2O2 and I4O2. Oxidation of haUde ions by ozone increases with the atomic number of haUde. Fluoride is unreactive chloride reacts slowly, ultimately forming chlorate and bromide is readily oxidized to hypobromite (38). Oxidation of iodide is extremely rapid, initially yielding hypoiodite the estimated rate constant is 2 x 10 (39). HypohaUte ions are oxidized to haUtes hypobromite reacts faster than hypochlorite (40). 

Sulfur Compounds. Aqueous sulfide and H2S, an odiferous compound in some waters, are oxidized rapidly (initially to sulfite and sulfurous acid) the rate constants ate 3x10 and 3 X 10 , respectively. Thiocyanate is oxidized by ozone to cyanide and sulfate via the intermediate formation of sulfite (47). 

Effect of Nitric Oxide on Ozone Depletion. Nitrous oxide is injected into the atmosphere from natural sources on earth about 10% is converted to nitric oxide (N20 + 0( D) — 2 NO), which in turn can catalyze the destmction of ozone (11,32,75). The two main cycles are 1 and 2. Rate constant data are given in Reference 11. 

Halogen radicals account for about one-third of photochemical ozone loss observed in the spring in the lower stratosphere (below 21 km) at 15—60°N latitude (76). The following three cycles (4—6) are the most important. Rate constant data are given in Reference 11. 

Ozone can be destroyed thermally, by electron impact, by reaction with oxygen atoms, and by reaction with electronically and vibrationaHy excited oxygen molecules (90). Rate constants for these reactions are given ia References 11 and 93. Processes involving ions such as 0/, 0/, 0 , 0 , and 0/ are of minor importance. The reaction O3 + 0( P) — 2 O2, is exothermic and can contribute significantly to heat evolution. Efftcientiy cooled ozone generators with typical short residence times (seconds) can operate near ambient temperature where thermal decomposition is small. 

Experimental studies show that the ozone concentration iacreases with specific energy (eV/O2) before reaching a steady state. The steady-state ozone concentration varies iaversely with temperature but directiy with pressure, reaching a maximum at about 101.3 kPa (1 atm). Above atmospheric pressure the steady-state ozone concentration decreases with pressure, apparentiy due to the pressure dependence of the rate constant ratio for the... 

A single chlorine atom can bring about the decomposition of tens of thousands of ozone molecules. Bromine atoms can substitute for chlorine indeed the rate constant for the Br-catalyzed reaction is larger than that tor the reaction just cited. 

As a final example of numerical simulations, consider the base-catalyzed decomposition of ozone in aqueous solution. This multistep reaction is controversial in that contradictory mechanisms have been suggested.33 34 The set of reactions that appears to be the most consistent with the experimental data is shown in Table 5-1, with a set of rate constants. Most of these values were reported in the literature, but several were refined to give agreement with experiments that measured the decline in concentration O3. 

Reactions" and rate constants"6 for the decomposition of ozone in alkaline solutions... 

Table I. Trace gas rate constants and lifetimes for reaction with ozone, hydroxyl radical, and nitrate radical. Lifetimes are based upon [O3]=40ppb [HO ]=1.0x10 molecules cm (daytime) [NO3 ]=10ppt (nighttime).

HO-oxidation of an individual NMHCj produces H02 radicals with a yield aj, and oxidation of the NMHC oxidation product produces H02 in stoichiometric amount The lumped coefficients or yields a and p need not be integers, and represent the effectiveness of a particular NMHCj in producing RO2. and H02 radicals (lumped together as HO2) that will then oxidize NO. to N02 in processes such as R6 and R13, producing one net ozone molecule each. Alternatively, when the NO. concentration is low, peroxyl radicals may form PAN (as in R22) or hydrogen peroxide (as in R33) which are other oxidant species. In this formulation, transport is expressed by an overall dilution rate of the polluted air mass into unpolluted air with a rate constant (units = reciprocal time dilution lifetime=1// ). This rate constant includes scavenging processes such as precipitation removal as well as mixing with clean air. 

Here the rate constants k refer to the rates of the numbered reactions above the value ho2/ro2 an average for different R02 entities. The A term accounts for HOjj production via ozone photolysis R1-R3, the Bj term accounts approximately for the source from aldehyde photolysis (R12 plus higher aldehydes), and the B2 term is a composite source from formaldehyde (RIO) and dicarbonyls (Cj) less the HOjj sink from PAN formation (R22) B2=Ci-C2). Values for Bj,... 

It is interesting to compare the rate constants of the oxygen-only ozone destruction reaction with those of the catalytic ozone destruction cycle. The rate constants for reactions 4-6 at 30 km are given below in units of cm molecules s . 

Relative Second-Order Rate Constants for Ozonations of Selected Olefins in CCI4 at Room Temperature... 

The length and amount of cracks is assessed according to the Bayer method [72,73]. The ISO standard ozone test conditions involve a test temperature of 40°C zE 1°C and an ozone level of 50 5 pphm, with a test duration of 72 h. Testing is done under static [72] and/or dynamic strain [73]. These are accelerated tests and should be used for the relative comparison of compounds, rather than for the prediction of long-term service life. The method is rather complicated and demands a long duration of ozone exposure. Therefore, in some cases the rate constants of the antiozonants reaction with ozone in solution are used instead to evaluate the efficiency of different antiozonants [74]. 

In other words, the reaction is flooded with a large excess of ozone. Because the concentration of ozone does not change appreciably, the near-constant concentration of ozone can be grouped with the rate constant ... 

Before we can calculate the true rate constant (.4"), we must determine y, the order with respect to ozone. One way to determine y is to repeat the isolation experiment with a different initial concentration of ozone. For example, the experiment could be repeated with the same initial isoprene concentration but an initial ozone concentration of 2.70X half as large as the concentration in the first experiment. When all the isoprene has been... 

In the presence of oxygen, NO is mainly oxidized to NOz, by reactions with O atoms and with ozone [33,46,76-78], The rate constant of molecular dissociation of 02 by electron collisions is almost two orders of magnitude higher than the dissociation of N2 in... 

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kNQ3 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures see reference kQ3 = 1.08 x 10-17 cm3 molecule-1 s-1 for reaction with ozone in the gas phase (Atkinson Carter 1984) kQ3 = 9.3 x 10-1S cm3 molecule-1 s-1 at 298 K (recommended, Atkinson 1997)... 

Adeniji, S.A., Kerr, J.A., Williams, M.R. (1981) Rate constants for ozone-alkene reactions under atmospheric conditions. Int. J. Chem. Kinet. 13, 209-217. 

Atkinson, R., Aschmann, S.M., Carter, W.L., Pitts, Jr., J.N. (1983) Effects of ring strain on gas-phase rate constants. 1. Ozone reactions with cycloalkenes. Int. J. Chem. Kinet. 15, 721-731. 

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