Rate constants, ozone reactions

Pitts, J. N Jr A. M. Winer, D. R. Fitz, S. M. Aschmann, and R. Atkinson, Experimental Protocol for Determining Ozone Reaction Rate Constants, U.S. Environmental Protection Agency, Report No. EPA-600/S3-81-024, May 1981. 

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... 

The study of the detailed mechanism of free radical initiation (rate constant k ) and ozone decay (rate constant d) by the reaction with cyclohexane, cumene, and aldehydes gave the following results (7 = 298 K) ... 

We first consider the continuous flow stirred tank reactor (CFSTR). A schematic presentation of the continuous flow stirred tank reactor is given in Figure 1. It is assumed that no mass transfer limitations exist regarding the supply of ozone. The accuracy of this assumption depends on the way ozone is supplied to the system and the reaction rate constants of the components involved. 

All three terms which contribute to ozone losses are minimal in a PFR. The relative importance and the absolute values of these terms and the ozone consumption factor can be calculated from equations (7) and (16), provided the reaction rate constants and the reaction conditions are known. 

As already has been mentioned mass transfer of ozone from the gas phase to the liquid phase may be enhanced by the chemical reactions of ozone with components A and B and by the decay of ozone. The effect of this enhancement in mass transfer on the selectivity will be discussed now semi-quantitatively13. To that aim we consider a gas phase in contact with a liquid phase. The liquid phase consists of a thin stagnant film at the interface with the gas phase, and a liquid bulk phase. We assume that the ozone is completely converted in the stagnant liquid film. This is for example the case if we have to deal with a high reaction rate constant and a relatively high concentration of one of the pollutants in the liquid film. Figure 5 gives a schematically presentation of this situation. 

FIGURE 3-13 Relations between conversion of nitric oxide to nitrogen dioxide and ozone, atomic oxygen, and hydroxyl-radical reaction rate constants. Reprinted with permission from Grosjean. ... 

The major respiratory factors in the control of ozone uptake are the morphology (including the mucus layer), the respiratory flow, the physical and chemical properties of mucus, and the physical and chemical properties of ozone. The next two sections discuss models of the morphology and the air and mucus flow. The physical and chemical properties of bronchial secretions have been reviewed by Barton and Lourenco and Charman et al. The relevant physical and chemical properties of ozone, are its solubility and diffusivity in mucus and water and its reaction-rate constants in water, mucus, and tissue. 

Because oxygen-atom reaction rate constants can be orders of magnitude greater than those for ozone, an experiment done on material subject to a reduced-pressure discharge is likely to signify damage done by oxygen atoms, rather than ozone. 

Photolytic. The following rate constants were reported for the reaction of 1,3-butadiene and OH radicals in the atmosphere 6.9 x 10 " cmVmolecule-sec (Atkinson et al., 1979) and 6.7 x lO " cmVmolecule-sec (Sabljic and Glisten, 1990). Atkinson and Carter (1984) reported a rate constant of 6.7-8.4 X 10 " cmVmolecule-sec for the reaction of 1,3-butadiene and ozone in the atmosphere. Photooxidation reaction rate constants of 2.13 x 10 and 7.50 x 10cm /molecule-sec were reported for the reaction of 1,3-butadiene and NO3 (Renter and Schindler, 1988 Sabljic and Glisten, 1990). The half-life in air for the reaction of 1,3-butadiene and NO3 radicals is 15 h (Atkinson et al., 1984a). 

Photolytic. Fluorene reacts with photochemically produced OH radicals in the atmosphere. The atmospheric half-life was estimated to range from 6.81 to 68.1 h (Atkinson, 1987). Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of fluorene (25 tig/g substrate) using a rotary photoreactor. The photolytic half-lives of fluorene using silica gel, alumina, and fly ash were 110, 62, and 37 h, respectively. Gas-phase reaction rate constants for OH radicals, NO3 radicals, and ozone at 24 °C were 1.6 x lO , 3.5 x 10 and <2 x 10in cmVmolecule-sec, respectively (Kwok et al., 1997). 

Photolytic. The reported reaction rate constants for the reaction of 2-methyTl-pentene with OH radicals and ozone in the atmosphere are 1.05 x lO and 6.26 x 10 " cmVmolecule-sec, respectively (Atkinson and Carter, 1984 Atkinson, 1985). 

Photolytic. Anticipated products from the reaction of propylene oxide with ozone or OH radicals in the atmosphere are formaldehyde, pyruvic acid, CH3C(0)OCHO, and HC(0)OCHO (Cupitt, 1980). An experimentally determined reaction rate constant of 5.2 x lO cmVmolecule-sec was reported for the gas phase reaction of propylene oxide with OH radicals (Glisten et al, 1981). 

Chemical/Physical. In the gas phase, cycloate reacts with hydroxyl and NO3 radicals but not with ozone. With hydroxy radicals, cleavage of the cyclohexyl ring was suggested leading to the formation of a compound tentatively identified as C2H5(Cff0)NC(0)SC2H5. The calculated photolysis lifetimes of cycloate in the troposphere with hydroxyl and NO3 radicals are 5.2 h and 1.4 d, respectively. The relative reaction rate constants for the reaction of cycloate with OH and nitrate radials are 3.54 x lO " and 3.29 x 10 cm /molecule-sec, respectively (Kwok et al., 1992). 

The relationship between the peroxy radical concentration and the ozone photolysis rate constant for these higher NO conditions can be again approximated using steady-state analysis (Penkett et al., 1997 Carpenter et al., 1997). While OH is recycled in its reactions with CO and CH4 via H02, it is permanently removed at higher NOx concentrations by the reaction of OH with N02, forming nitric acid ... 

The direct oxidation (M + O-,) of organic compounds by ozone is a selective reaction with slow reaction rate constants, typically being in the range of kD = 1.0 - 103 M 1 s. The ozone molecule reacts with the unsaturated bond due to its dipolar structure and leads to a splitting of the bond, which is based on the so-called Criegee mechanism (see Figure 2-2). The Criegee mechanism itself was developed for non-aqueous solutions. 

Bubble columns and various modifications such as airlift reactors, impinging-jet-reactors, downflow bubble columns are frequently used in lab-scale ozonation experiments. Moderate /qa-values in the range of 0.005-0.01 s l can be achieved in simple bubble columns (Martin et al. 1994 Table 2-4 ). Due to the ease of operation they are mostly operated in a cocurrent mode. Countercurrent mode of operation, up-flow gas and down-flow liquid, has seldom been reported for lab-scale studies, but can easily be achieved by means of applying an internal recycle-flow of the liquid, pumping it from the bottom to the top of the reactor. The advantage is an increased level of the dissolved ozone concentration cL in the reactor (effluent), which is especially important in the case of low contaminant concentrations (c(M)) and/or low reaction rate constants, i. e. typical drinking water applications... 

Using it in the semi-batch mode of operation this reactor type has been used in the determination of the reaction rate constants of fast direct reactions of ozone with certain waste water pollutants, e. g. phenol or azo-dyes (Beltran and Alvarez, 1996). Beltran and coworkers have also successfully studied the reaction kinetics of various fast reacting substances using semi-batch mode STRs (see further references of Beltran, Benitez or Sotelo et al. in Chapters B 3 and B 4). 

The situation is characterized by the fact that both reactants are entirely consumed, so that cL = c(M) = 0 holds in the plane. Only in the latter case can kLa be determined. In the former case there is no transport of ozone into the liquid film, so that the mass transfer rate is only determined by kaa (Charpentier, 1981). The reaction rate depends on the mass transfer rate of ozone and pollutant to the reaction plane in the liquid film, but not on the reaction rate constant. Whether the reaction develops instantaneously in the liquid film depends on the experimental conditions, especially on the values of the applied ozone partial pressure p 03) and the initial concentration of M c(M)0. For example, the reaction tends toward instantaneous for low p(03) and high c(M)0. 

Reaction kinetics describes what influences the reaction and how fast it takes place. Knowledge of kinetic parameters, such as reaction order n and reaction rate constant k, helps us to assess the feasibility of using ozonation to treat waters and to design an appropriate reactor system. It can help us to understand how a reaction can be influenced, so that a treatment process can be optimized. Kinetic parameters are also necessary for use in scientific models, with which we further improve our understanding of the chemical processes we are studying. 

Knowing the reaction rate constants of the direct and indirect reactions and the concentrations, the total reaction rate can be calculated. Unfortunately some data continue to be generated that fail to distinguish between the direct ozone reaction and hydroxyl radical chain reaction. Knowledge of independent rate constants for each pathway is useful to predict competition effects. In drinking water the direct oxidation kinetic is often negligible compared with the indirect, in waste water there is often no clear preference and both pathways can develop simultaneously. This was found for example in the ozonation of 4-nitroaniline at pH = 2, 7 and 11 (T = 20 °C) (Saupe, 1997 Saupe and Wiesmann, 1998). 

Ozone is one of the strongest oxidants in drinking and waste water treatment. Due to the slow reaction rate constants and mostly incomplete mineralization with the direct reaction of ozone, treatment methods with an even stronger oxidant, the OH-radical, were developed such as ... 

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