Interaction of ozone and carbon dioxide with polycrystalline potassium bromide and its atmospheric implication

Accepted Manuscript Interaction of ozone and carbon dioxide with polycrystalline potassium bromide and its atmospheric implication Alexander V. Levanov, Oksana Ya Isaikina, Ivan B. Maksimov, Valerii V. Lunin PII:

S1352-2310(16)31020-2

DOI:

10.1016/j.atmosenv.2016.12.044

Reference:

AEA 15106

To appear in:

Atmospheric Environment

Received Date: 22 September 2016 Revised Date:

22 December 2016

Accepted Date: 23 December 2016

Please cite this article as: Levanov, A.V., Isaikina, O.Y., Maksimov, I.B., Lunin, V.V., Interaction of ozone and carbon dioxide with polycrystalline potassium bromide and its atmospheric implication, Atmospheric Environment (2017), doi: 10.1016/j.atmosenv.2016.12.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

O3 + CO2 + H2O gas mixture

Br2(gas)

KHCO3

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KBr(cr.)

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KBrO3

ACCEPTED MANUSCRIPT INTERACTION OF OZONE AND CARBON DIOXIDE WITH POLYCRYSTALLINE POTASSIUM BROMIDE AND ITS ATMOSPHERIC IMPLICATION

1 2 3 4

8

a

Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskiye

Gory 1, building 3, 119991 Moscow (Russia)

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b

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Br–

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6 7

Alexander V. Levanov,a* Oksana Ya. Isaikina,a,b Ivan B. Maksimov,a Valerii V. Lunina,b

SC

A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky prospect 29, 119991 Moscow (Russia) * Corresponding Author. E-mail address: [email protected]; Fax: (+7) 495-939-4575; Phone: (+7) 495-939-3685

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It has been discovered for the first time that gaseous ozone in the presence of carbon dioxide and water vapor interacts with crystalline potassium bromide giving gaseous Br2 and solid salts KHCO3 and KBrO3. Molecular bromine and hydrocarbonate ion are the products of one and the same reaction described by the stoichiometric equation 2KBr(cr.) + O3(gas) + 2CO2(gas) + H2O(gas) → 2KHCO3(cr.) + Br2(gas) + O2(gas). The dependencies of Br2, KHCO3 and KBrO3 formation rates on the concentrations of O3 and CO2, humidity of initial gas mixture, and temperature have been investigated. A kinetic scheme has been proposed that explains the experimental regularities found in this work on the quantitative level. According to the scheme, the formation of molecular bromine and hydrocarbonate is due to the reaction between hypobromite BrO–, the primary product of bromide oxidation by ozone, with carbon dioxide and water; bromate results from consecutive oxidation of bromide ion by ozone BrO–

,

BrO2–

,

BrO3–.

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Keywords: ozone, bromide, molecular bromine, carbon dioxide, kinetics, heterogeneous chemical reactions.

INTRODUCTION. The composition and chemistry of the troposphere exert a major influence on life on Earth. Reactive halogens are of significance in tropospheric chemistry, and reactive bromine species such as Br2, HOBr, BrO, play a dominant role in the destruction of ozone. The detailed information on the subject and the references to the original literature can be found in the comprehensive reviews (Abbatt et al., 2012; Saiz-Lopez and von Glasow, 2012; Simpson et al., 2007; von Glasow and Crutzen, 2014). Despite the great importance of active bromine (see e.g. (von Glasow et al., 2004)), its primary sources and the mechanism of the release to the troposphere are not well understood (Hunt et al., 2004; von Glasow and Crutzen, 2014). 1

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ACCEPTED MANUSCRIPT A significant primary contributor of active bromine can result from the complex interaction between ozone and dry or deliquescent marine aerosol containing bromide ions. The reaction of ozone with bromide ion in the bulk of aqueous solution (Disselkamp et al., 1999; Garland et al., 1980; Haag and Hoigné, 1983; Haruta and Takeyama, 1981; Liu et al., 2001; Taube, 1942) and in liquid aqueous aerosol particles (Anastasio and Mozurkewich, 2002; Clifford and Donaldson, 2007; Hunt et al., 2004; Nissenson et al., 2009; Nissenson et al., 2014; Oldridge and Abbatt, 2011; Oum et al., 1998) has been comprehensively investigated. In our work (Levanov et al., 2016) the interaction of ozone with crystalline KBr has been studied, which models the reaction of O3 with Br– incorporated into dry marine aerosol. Its only product detected was bromate ion BrO3– remaining in the solid phase. At the same time, one would expect that carbon dioxide, an important atmospheric constituent, can exert an appreciable influence on the direction and rate of the complex reaction of ozone with bromide ion. However, this problem has not been investigated until the present time. The subject of this work is the research on the interaction of gaseous ozone and carbon dioxide with powdered crystalline potassium bromide, namely, the determination of the composition of the products formed in the solid phase and released into the gas phase, a quantitative study of the kinetics of their formation, and the elucidation of a plausible chemical mechanism of the process. The system KBr(cr.) + О3(gas) + СО2(gas) is a convenient laboratory model of the natural process of interaction of dry marine aerosol with the mentioned components of the troposphere.

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EXPERIMENTAL METHODS The scheme of the experimental setup is shown in Fig. 1. The interaction between crystalline KBr powder and gaseous mixture O3 – CO2 – O2 took place in a glass reactor provided with a thermostatically controlled jacket. The accuracy of temperature maintenance was within ± 0.5 °C. In single experiments, NaBr was used instead of KBr. The working part of the reactor constituted a glass cylinder with a porous glass filter plate at its bottom, through which the initial gases were fed in. 10 or 25 grams of bromide powder were placed in the reactor, and resided above the filter. To ensure an effective contact of the surface of bromide crystals with the gases, during the experiments the reactor was shaken in vertical direction with the frequency ~7 s–1 and amplitude of motion ~10 cm. It should be noted that the experiments of this work and of our study (Levanov et al., 2016) were carried out on the same setup, and the experimental procedures were by and large similar. Ozone was synthesized from molecular oxygen of very high purity grade in a selfmade barrier discharge ozonizer. Its concentration was measured by UV photometric ozonometers “Medozon” verified by means of Agilent 8453 spectrophotometer. The maximum attainable ozone concentration was 4 vol.% at oxygen flow rate 22 L/h (STP). A flow of carbon dioxide (GOST 8050-85 first-rate grade) was added through a tee to the 2

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ACCEPTED MANUSCRIPT flow of ozonized oxygen before entering the reactor. In all the experiments the sum of flow rates of initial gases O2 and CO2 was 22 L/h (STP). The gas communications were made of polytetrafluorethylene or medical polyvinylchloride tubes resistant to the action of ozone. The moisture content of oxygen and carbon dioxide gases was estimated in the same way as in the work (Levanov et al., 2016), by measuring the weight increase of a trap cooled to –65 - –70 °C, through which the gas was passed at a constant rate for a definite time. The absolute humidity (the content of water vapor in vol.%) of oxygen proved to be 0.15 vol.% and of carbon dioxide 1.06 vol.%. To attain the required humidity of the initial gases in the course of the experiments, before entering the reactor they were passed through the bubbler filled with distilled water and maintained at a constant temperature. The humidity was determined by the bubbler temperature, which did not exceed room temperature. Potassium bromide reagent (GOST 4160-74, chemically pure grade, 0.01 % weight loss on heating to 320 °C) was used in the experiments. Single experimental runs were performed with sodium bromide reagent (GOST 4169-76, pure grade). Its weight loss on heating was 3.8 % at 100 °C and 4.4 % at 320 °C, which is due to the hygroscopicity of NaBr and the possibility of NaBr·2H2O crystalline hydrate formation. Bromide specimens were not additionally purified or dried; their handling was carried out in usual laboratory atmosphere at temperature 20-25°C. Before treatment with O3 – CO2 – O2 gas mixture, powdered bromide salts were held in the reactor in the flow of CO2 and O2 at required temperature and humidity for 20 min. The characterization of potassium bromide specific surface area has been given in work (Levanov et al., 2016). Qualitative analysis of potassium bromide powder after the treatment with O3 – CO2 – O2 gas mixture was carried out by infrared spectroscopy. From the powder a pellet was pressed off, and its IR spectra were taken by means of Equinox 55/S IR spectrometer (Bruker) in the range 360 – 4000 cm–1. Qualitative analysis of gaseous products formed on the treatment of bromide powder with the gas mixture was performed by UV-visible spectrophotometry. 25 g of bromide were placed in the reactor, the initial gases were passed through the sample without shaking, and exit gases were promptly directed into the optical cell of Agilent 8453 spectrophotometer, where the UV-vis spectra were recorded. Quantitative determination of bromate BrO3– in the treated potassium bromide powder was carried out in the same way as in work (Levanov et al., 2016), by an indirect spectrophotometric method, measuring the concentration of tribromide ion Br3– formed on dissolution of 5 g of the sample in 50 ml of 2.5 М H2SO4. Quantitative determination of hydrocarbonate in the treated KBr samples was conducted by acid-base titration. 10 g of the sample were dissolved in sufficient amount of distilled water. Then a drop of methyl orange indicator was poured, and the solution being analyzed turned yellow. It was titrated with 0.1 HCl until the color changed to 3

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152 153 154 155 156 157 158

(

)

) ∆

and

r(HCO3–) =

(

)

,

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(

(

)

∆

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SC

where n(BrO3–) and n(HCO3–) are the quantities of the ions, µmole, formed in a potassium bromide sample with the mass m(KBr), g, for a time ∆t, min. Quantitative determination of molecular bromine in the exit gases was performed by the method of photometrical iodometry with thermal decomposition of ozone. The method was proposed in our work (Levanov et al., 2003) for determination of small quantities of molecular chlorine in the presence of ozone. The method consists in prior ozone breaking down in a tubular furnace and subsequent iodometric determination of intact halogen. The exit gas mixture flowed through a quartz tube located along the axis of the tubular furnace. The furnace temperature was ~550 °C, that assured virtually complete decomposition of ozone, while molecular bromine underwent no change. Then ozone-free gas mixture was directed into the trap filled with 100 ml of aqueous solution of 50 g/L KI and 0.01 M NaHCO3. In the trap molecular bromine was quantitatively transformed to triiodide ion according to the reaction Br2 + 3I– → I3– + 2Br–. Samples were periodically taken from the trap into glass cells with optical path length 1 or 2 cm, and absorbance of I3– at 350 or 450 nm was measured in them by KFK-3 photometer. After the measurements the samples were poured back into the trap. The absorbance readings were converted into the concentration of I3– with the help of calibration graphs. The amount of moles of I3– was found from its concentration and the volume of the solution in the trap. A typical time dependency of I3– amount is shown in Fig. S1 (line 1) of Supporting Information. From its slope, the rate of triodide accumulation in the trap was calculated, which was equal to the rate of molecular bromine entering the trap, dn(I3–)/dt = dn(Br2)/dt. The kinetics of bromine formation was characterized by the specific rate of bromine emission r(Br2) computed from experimentally measured values of dn(Br2)/dt, µmole/min, with the formula

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r(BrO3–) =

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ACCEPTED MANUSCRIPT orange. As the initial potassium bromide reagent also possesses some basicity, it was titrated in the same way. The hydrocarbonate concentration in the treated sample was calculated from the difference of HCl volumes consumed for the titration of the treated and untreated samples. Kinetics of BrO3– and HCO3– accumulation in our experiments was characterized by specific rates of formation of bromate r(BrO3–) and hydrocarbonate r(HCO3–), µmole min–1g–1, calculated according to the expressions

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r(Br2) =

( (

)

)

, µmole min–1g–1,

where m(KBr) is the weight of potassium bromide sample loaded in the reactor. In our experiments the relative error of determination of the bromine emission rate r(Br2) was less than 15%. It corresponds to 95% confidence interval, estimated on 8 measurements of the value of r(Br2) at similar values of experimental factors. The relative errors of the specific rates of formation of bromate and hydrocarbonate are of the order of 50% and 15% correspondingly. Plotted points shown in the figures of this work represent 4

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ACCEPTED MANUSCRIPT the values of the specific rates in particular experimental runs (and not the values averaged on several experiments). The detection limit of Br2 is governed by not full decomposition of ozone in the furnace, plausible evolution of Cl2 on interaction of O3 with gas ducts made of polyvinylchloride, and formation of some additional triiodide through oxidation of iodide in KI trap with molecular oxygen O2. The contribution of these factors is small and can be quantitatively accounted for by means of blank experiments with empty reactor containing no potassium bromide. The conditions of blank experiments were analogous to real ones. It was established that the low detection limit of the bromine emission rate takes the value 1 × 10–2 / m(KBr), µmole min–1g–1, in the experiments without special addition of water vapor to the initial gas mixture, and 4 × 10–2 / m(KBr), µmole min–1g–1, when water vapor was added by the use of the bubbler. In most experiments the time dependencies of triiodide amount in the trap were rectilinear after small nonstationary starting section (as an example see line 1 in Fig. S1 of Supporting Information). It means that after the admission of gases and establishment of the steady speed of their flow through the reactor, the rate of bromine emission is constant and does not alter with time. Generally speaking, the process of interaction between O3 – CO2 – O2 gas mixture and bromide crystals with the formation of Br2(gas) is a topochemical reaction; the reasons for its rate constancy under the conditions of our experiments are discussed in (Levanov et al., 2016). In some experiments at higher humidity of the initial gases, the bromine emission rate decreases to some extent with the course of time (see line 2 in Fig. S1 of Supporting Information). In those cases the rate of bromine emission was characterized by the two values, maximal and average, estimated correspondingly on the slopes of the initial rectilinear range of the time dependency of n(Br2), and the whole dependency, with the exception of nonstationary starting section. In the figures which show the dependencies of bromine emission rate on experimental factors, such values of r(Br2) are specially designated with short horizontal segments.

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RESULTS AND DISCUSSION In the infrared spectrum of potassium bromide powder treated with O3 – CO2 – O2 gas mixture, “new” strong lines appear (Fig. 2, spectrum 1). The comparison with the spectrum of potassium hydrocarbonate (Fig. 2, spectrum 2) indicates that the majority of these lines are due to hydrocarbonate ion HCO3–. Also less intensive signals of bromate ion BrO3– with the maxima at 431 and 796 cm–1 are clearly observed (cf. spectra 1 and 3 in Fig. 2). Thus, on interaction of crystalline KBr with ozone in the presence of humidity and considerable concentration of carbon dioxide, potassium hydrocarbonate KHCO3 and bromate KBrO3 are formed in the solid phase. Other plausible products of potassium bromide oxidation, hypobromite KOBr, bromite KBrO2, and perbromate KBrO4, also possess characteristic and sufficiently 5

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ACCEPTED MANUSCRIPT intense absorption bands in the infrared region (Appelman, 1969; Brown et al., 1969; Levason et al., 1990; Nakamoto, 2008). The absence of their bands in our experimental spectra is the evidence that these compounds virtually do not accumulate in the course of the interaction between KBr(cr.) and O3 – CO2 – O2 gas mixture. The absence of hypobromite in the products is also confirmed by a sensitive test with methyl orange. This indicator is effectively oxidized by hypohalogenites in acid medium with the formation of colorless products (Bányai, 1972; Larson and Sollo, 1970; Winkler, 1915). In all our experiments, the acid titration of potassium bromide samples treated with ozone was conducted (see Experimental Methods) with the use of methyl orange as an indicator, and decoloration, which might witness the presence of hypobromite, was not observed. In the UV - visible spectra of the gases that exit from the reactor on treatment of potassium bromide powder with ozonated oxygen or O3 – CO2 – O2 gas mixture at 60 ºC, the very strong Hartley absorption band (Sander et al., 2011) of initial unreacted ozone with the maximum at 254 nm dominates (see Fig. S2 of Supporting Information)). The spectra are shown in Fig. 3 in the wavelength range 300 – 800 nm, so that possible signals of gaseous products of the interaction would be clearly visible. On treatment of KBr with ozonated oxygen, no gaseous products have been detected, and only the weaker Chappuis band (Sander et al., 2011) of unreacted O3 is observed in the range 375 – 650 nm (spectrum 1 in Fig. 3). However, if the initial gas mixture contains additionally an appreciable portion of carbon dioxide, then a new characteristic signal with the maximum at 416 nm and point of inflection at ~480 nm arises (spectra 2 and 3 in Fig. 3). According to the literature (Hubinger and Nee, 1995; Maric et al., 1994), this signal corresponds to molecular bromine Br2. Furthermore, the intensity of Br2 peak grows drastically with the increase in humidity of initial gases. As shown in Fig. S3 of Supporting Informaton, on treatment of sodium bromide powder with gaseous mixture O3 – CO2 – O2, the signal of molecular bromine is well observed even at 22 ºC. Sodium bromide much more readily gives off gaseous bromine, because of its greater moisture content and the possibility of existence of NaBr·2H2O crystalline hydrate. Thus on interaction between KBr(cr.) or NaBr (cr.) with gaseous ozone and carbon dioxide in the presence of water vapor, molecular bromine Br2 is released into the gas phase. Any other gaseous products have not been detected. The evolution of molecular bromine on treatment with ozone of substrates containing bromide ion is also reported by Hirokawa et al. (1998) for the interaction with NaBr(cr.) in the presence of water vapor, Hunt et al. (2004) and Anastasio and Mozurkewich (2002) for the reaction with liquid aerosol containing Br–, and Oum et al. (1998) for the treatment of sea ice containing Br–. Of considerable interest is the elucidation of mutual relation between the yields of the products Br2(gas), KBrO3(solid) and КНСО3(solid) formed on interaction of KBr(cr.) with O3 – CO2 – O2 gas mixture. It turns out that there is virtually no interrelation between the formation of bromine and bromate (more precisely, an antithetical correlation takes place), while the yields of bromine and hydrocarbonate are linked with

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ACCEPTED MANUSCRIPT each other by a clearly defined directly proportional dependency (see Fig. S4 of Supporting Information). Moreover, the evolution of one mole of Br2 into the gas phase is accompanied by the formation of 2.1 ± 0.1moles of HCO3– in the solid phase. On the basis of these data we conclude that in our experiments molecular bromine and hydrocarbonate ion are the products of one and the same reaction, described by the following stoichiometric equation 2Br–(cr.) + 2CO2(gas) + H2O(gas) + O3(gas) → (1) – → 2HCO3 (cr.) + Br2(gas) + O2(gas). Water, which is involved in the reaction, is contained in the initial gases and also present in adsorbed form on KBr crystals. The succeeding sections of this work are devoted to the investigation of the kinetics of molecular bromine and bromate ion formation. As can be seen from Fig. 4, the rate of bromine emission depends substantially on the humidity of initial gases. This agrees with the fact that water is an initial substance of the reaction of Br2 formation. At zero humidity of the gases, Br2 is still generated with a constant rate at the expense of surface-adsorbed water. With an increase in humidity, the rate of Br2 emission at first grows, but then reaches a constant magnitude at absolute humidity of 2 vol.% and higher. Apparently at low humidity the processes with the participation of water determine the rate of the whole complex reaction of Br2 formation. At higher humidity the rate limiting step changes, and water no longer influences the reaction rate but still remains its necessary initial substance. As ozone concentration rises, both the rates of bromine and bromate formation increase, but the behavior of the dependencies of r(Br2) and r(BrO3–) on O3 concentration is different (Fig. 5). On addition of carbon dioxide to initial gases, the rate of bromine emission rises markedly, while that of bromate formation falls (Fig. 6). This may evidence that the formation of both Br2 and BrO3– proceeds through one and the same intermediate, which can interact with both CO2 and O3. In doing so, its reaction with carbon dioxide leads to the formation of final product Br2, and with ozone – to BrO3–. The temperature dependencies of bromine emission rates are of complex character (pass through maxima) and do not obey to Arrhenius law (Fig. 7). At the same time the rate of bromate formation is well described by an empirical Arrhenius-type formula r(BrO3–) = 8.65 × 1013 exp(–11900/T), µmole min–1g–1, with apparent activation energy of 100 kJ/mole. It is noteworthy that perceptible amounts of Br2(gas) are generated only if O3 and CO2 are both present in the initial gas mixture. If the treatment of potassium bromide is performed separately, first with ozone and next with carbon dioxide, then Br2 formation does not practically take place. During the treatment of KBr with ozone only, Br2 is released with a very small rate: r(Br2) is greater than the detection limit by 3 – 5 times at the beginning of the treatment, and decreases as time goes by. After stopping ozone delivery and turning on CO2 flow, bromine evolution ceases at all, and the measured 7

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ACCEPTED MANUSCRIPT value of r(Br2) does not exceed the detection limit. These results mean that the end product of bromide oxidation with ozone (bromate ion BrO3–) does not react with CO2, and the release of Br2 is due to the interaction of carbon dioxide with an intermediate of the complex reaction between Br–(cr.) and O3. If bromate is formed through consecutive oxidation of bromide ion with ozone by the scheme Br– → BrO– → BrO2– → BrO3– (see (Levanov et al., 2016)), then the intermediates are bromite BrO2– and hypobromite BrO–. In the literature there is no information on reaction of bromite with CO2. On the other hand, it is known that hypobromite ion BrO– is an effective catalyst of carbon dioxide hydratation reaction, and in this process it interacts chemically with CO2 (Caplow, 1971; Kiese and Hastings, 1940; Sharma and Danckwerts, 1963; Tavares da Silva and Danckwerts, 1973). Therefore it would appear reasonable that the release of Br2 into the gas phase in our experiments is due to the interaction between hypobromite BrO–, the primary product of bromide oxidation by ozone, with carbon dioxide in the presence of water vapor according to the stoichiometric equation BrO– + 2CO2 + Н2О + Br– → Br2 + 2HCO3–. Additionally it should be recognized that the acidity provided by the dissociation of carbonic acid (Ka1,298(H2CO3) = 4.5 × 10–7 M) (Lide, 2010) can be sufficient to shift the equilibrium in aqueous solution between hypobromite ion and molecular bromine to the side of the latter. Indeed HOBr is a very weak acid, Ka,298(HOBr) = 2.8 × 10–9 M (Lide, 2010); and the equilibrium constant of bromine hydrolysis Br2 + H2O ⇄ HOBr + H+ + Br– K298 = 3.5 × 10–9 M2 (Beckwith et al., 1996) is also very small. Thus the following scheme can be proposed for the complex heterogeneous chemical reaction between bromide ion on the surface of the bromide crystals and gaseous O3 and CO2 under the conditions of our experiments, Br– + O3 → BrO– + O2 (2) – – BrO + O3 → Br + 2O2 (3) – – BrO + O3 → BrO2 + O2 (4) – – BrO2 + O3 → BrO3 + O2 (5) – – – BrO + 2CO2 + Н2О + Br → Br2 + 2HCO3 . (6) Reaction (2-5) are similar to analogous well examined reactions in aqueous solution (Haag and Hoigné, 1983; Haruta and Takeyama, 1981; Liu et al., 2001; von Gunten and Hoigné, 1994; von Gunten and Oliveras, 1998). The suggested reaction (6) is a complex process, and its detailed mechanism is unknown. We consider its rate to be in direct proportion to concentrations of BrO– on the crystal surface and CO2 in the gas phase and not to depend on bromide ion concentration, because it is present in a substantial excess in the reaction system. The complex reaction (2-6) proceeds on the surface of potassium bromide crystals and in principle can be slowed down owing to the formation of the layer of reaction products KHCO3 and KBrO3. However, in the most of our experiments the rate of bromine emission was constant, which constitutes evidence that the slowing down by the 8

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SC

(

)С(



M AN U

Ө

– С(

)

.

) ! С(

)

For the rates of bromine emission and bromate formation the following expressions are obtained

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339 340

ӨBrO– =

r(Br2) = k6ӨBrO–C(CO2) = –

(

! С(

)С(



)С(

)Ө"#–

,

) ! С(

r(BrO3 ) = k5ӨBrO2–C(O3) = k4ӨBrO–C(O3) =

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338

ACCEPTED MANUSCRIPT products does not take place. A plausible explanation of this phenomenon is that the products form separate phases and are removed from the surface because of shaking of the reaction system. In more detail this issue is analyzed in our preceding work (Levanov et al., 2016). Let us perform the kinetic analysis of the scheme (2-6). In doing so we assume that the rate of the overall reaction (2-6) is not affected by mass transfer of gaseous reagents and products (i.e. the reaction proceeds in the kinetic region in terms of macrokinetics). Under the conditions of our experiments BrO– and BrO2– are reactive intermediates, and hence the steady state approximation can be applied for them. The expressions of equality of BrO– and BrO2– generation and consumption rates are as follows, k2ӨBr–C(O3) = k3ӨBrO–C(O3) + k4ӨBrO–C(O3) + k6ӨBrO–C(CO2), k4ӨBrO–C(O3) = k5ӨBrO2–C(O3), where C(O3) and C(CO2) are the concentrations in the gas phase, ӨBr–,ӨBrO– and ӨBrO2– are the surface concentrations. We will assume that ӨBr– >> ӨBrO–, ӨBr– >> ӨBrO2–, and the value of ӨBr– virtually coincide with the number of bromide ions on the unit area of the bromide crystal in the absence of chemical interactions. The steady state relationship for BrO– gives



(



(7)

)

Ө )С(

–

(

)

) ! С(

. (8)

)

It should be noted that although water concentration does not appear in formulas (7-8), its effect is accounted for implicitly, by taking into consideration that the values of the apparent rate constants of stages (2-6) may essentially depend on humidity. By introducing the parameters R2 = k2ӨBr–, K3 = (k3 + k4)/k6 and K4 = k4/k6, the expressions (6-7) are rearranged in the forms

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r(Br2) =

$ С(

r(BrO3–) =

С(

)С(% ) С(

)

,

)

$ ( ) . С( ) С( )

(9) (10)

The parameters R2, K3 and K4 can be unequivocally (within the limits of experimental error) determined from the experimental dependencies of Br2 emission and BrO3– formation rates found in this work. The parameters K3 and K4 are dimensionless; R2 can

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Table 1. Fitted values of the parameters R2, K3 and K4 at 60 °C. vol.% H2O R2 K3 K4 0.6 60 12 0.2 µmole min–1g–1 (vol.%)–1 = 5.1 × 10–4 L min–1g–1 2.6 8 – The expressions (9-10) are applicable for the description of temperature dependencies of bromine emission and bromate formation rates (Fig. 7), if an evident assumption is taken that parameters R2, K3 and K4 depend on temperature according to the Arrhenius law. The lines in Fig. 7 have been calculated by formulas (9-10) with the use of Arrhenius temperature dependencies of the parameters presented in Table 2. It should be stressed that an unusual behavior of bromine emission rate with temperature shown in Fig. 7 (r(Br2) first increases but then passes through a maximum and next falls) is well reproduced by theoretical formula (9) with the fitted Arrhenius temperature functions of R2 and K3. The agreement between theoretical expressions (7-10) and experimental data of Fig.5-7 (making allowance to their errors) supports the validity of the kinetic scheme (2-6) and corroborates that it is adequate to the available experimental information.

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ACCEPTED MANUSCRIPT be expressed in µmole min–1g–1 (vol.%)–1 or L min–1g–1, which corresponds to the units of gaseous concentration vol.% or µmole L–1. The qualitative characteristics of dependencies of Br2 and BrO3– formation rates on experimental factors, predicted by theoretical formulas (6-9), are consistent with the experimental data within the investigated range of factor changing. Moreover, with appropriately chosen values of parameters R2, K3 and K4, theoretical expressions of the rates of Br2 and BrO3– formation (9-10) agree with the experiment on the quantitative level. As can be seen from Fig. 5-6 the lines calculated with formulas (9-10) using the parameters values given in Table 1, reproduce the experimental dependencies quite satisfactorily. The magnitude of R2 appears to be the same at different humidities, while K3 decreases with the increase in moisture content of initial gases. It is noteworthy that such behavior of parameters R2 and K3 with respect to humidity is consistent with the character of the dependency of Br2 emission rate on moisture content of initial gases depicted in Fig. 4. However, the elucidation of the reliable dependencies of parameters R2, K3 and K4 on humidity on the quantitative level requires additional experimental investigations. The values of K3 = (k3 + k4)/k6 > 1 mean that hypobromite ion on the surface of KBr(cr.) is more reactive towards O3(gas) than to CO2(gas) under our experimental conditions.

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Table 2. Fitted temperature dependencies of the parameters R2, K3 and K4. vol.% H2O R2 K3 K4 '(( * '( 0.6 5.28 × 1016 · & 1.47 × 105 · & ) 1.25 × 1014 · & ) 10



+ ((( )

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As is well known, reactive bromine chemistry is of major importance in the troposphere. Most notable are the regular springtime ozone depletion events in the arctic boundary layer (Barrie et al., 1988; Hausmann and Platt, 1994; Tuckermann et al., 1997). They are due to a complex autocatalytic reaction, called “bromine explosion”, involving bromine compounds Br2, Br, BrO, HOBr and others (Fan and Jacob, 1992; Tang and McConnell, 1996; Vogt et al., 1996). However, to initiate the explosion, a primary source of reactive bromine is required. Reaction (1) of bromide ion with O3 and CO2 is one of such sources in the marine troposphere. Of great interest is the question whether can an increase in atmospheric carbon dioxide concentration lead to an increase in bromine emission rate from the aerosol due to this reaction? If our results are extrapolated to tropospheric conditions (C(O3) = (1 – 4) × 10–6 vol.%, C(CO2) ~ 0.04 vol.%), then the equation (9) for bromine emission rate reduces to the form r(Br2) ≃ R2C(O3) = k2ӨBr–C(O3), i.e., the bromine emission rate turns out to not virtually depend on CO2 concentration (but to be in direct proportion to concentration of O3). This is because the atmospheric CO2 concentration is much greater than that of O3, and the rate of the complex reaction (2-6) is limited by O3 concentration. Thus, although carbon dioxide is the necessary initial substance of the bromine evolution reaction (1) discovered in this work, under tropospheric conditions any feasible variation of CO2 concentration does not exert any influence on the rate of bromine release due to this process. If we take for KBr crystals in our experiments the value of specific area sKBr = 50 100 cm2/g and the number of bromide ions on 1 cm2 of crystal surface ӨBr–,KBr = 5.8 × 1014 ion/cm2 (see (Levanov et al., 2016)), then the rate constant of reaction (2), related to one surface bromide ion, is expressed as follows:

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k͂ 2 =

...

/.



$ 01"# 2"# ,1"#

= (1.1 - 2.1) × 10–13 · &



'(( )

cm3 ion–1 s–1.

It is interesting to note, that the rate constant of the analogous reaction Br– + O3 → BrO– + O2 in the bulk of aqueous solution is calculated with the formulas k (cm3 ion–1 s–1) =

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417 418 419 420 421 422

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'( ( )

(Haruta and Takeyama, 1981), 1.05 × 10–12 · & '



'( )

(Haag and

(

Hoigné, 1983), 1.70 × 10–11 · & ) (Liu et al., 2001), and it is 6 – 14 times greater than k͂ 2 at the same temperature. In principle, the intensity r͂ (Br2), mole cm–3s–1, of reactive bromine source in the troposphere due to reaction (1) of O3 and CO2 with Br– incorporated into dry marine aerosol can be estimated by the expression 0 r͂ (Br2) = k͂ 2ӨBr–C(O3) , 3

423 424

where C(O3) is gaseous ozone concentration, mole cm–3, ӨBr– is number of bromide ions 0 on 1 cm2 of dry aerosol surface, ion/cm2, and is collective surface area of aerosol 3

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CONCLUSIONS Thus, in this work the interaction between crystalline potassium bromide and gaseous mixture of ozone and carbon dioxide containing water vapor has been investigated for the first time. It has been found that the products are gaseous Br2 and solid salts KHCO3 and KBrO3. Molecular bromine and hydrocarbonate ion are the products of one and the same complex reaction described by the stoichiometric equation 2KBr(cr.) + O3(gas) + 2CO2(gas) + H2O(gas) → 2KHCO3(cr.) + Br2(gas) + O2(gas). Bromate arises from consecutive oxidation of bromide ion by ozone. Kinetic laws of Br2, KHCO3 and KBrO3 generation have been established, and a kinetic scheme has been proposed that explains satisfactorily the experimental regularities found in this work. Formulas derived from the scheme can be used for quantitative estimation of Br2, HCO3– and BrO3– formation rates at various conditions. ACKNOWLEDGEMENT This work was supported by the Russian Foundation for Basic Research, grant # 10-0501136-a.

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REFERENCES Abbatt, J.P.D., Thomas, J.L., Abrahamsson, K., Boxe, C., Granfors, A., Jones, A.E., King, M.D., Saiz-Lopez, A., Shepson, P.B., Sodeau, J., Toohey, D.W., Toubin, C., von Glasow, R., Wren, S.N., Yang, X., 2012. Halogen activation via interactions with environmental ice and snow in the polar lower troposphere and other regions. Atmos. Chem. Phys. 12, 6237-6271. DOI: 10.5194/acp-12-6237-2012 Anastasio, C., Mozurkewich, M., 2002. Laboratory Studies of Bromide Oxidation in the Presence of Ozone: Evidence for a Glass-Surface Mediated Reaction. J. Atm. Chem. 41, 135-162. DOI: 10.1023/A:1014286326984 Appelman, E.H., 1969. Perbromic acid and perbromates: synthesis and some properties. Inorg. Chem. 8, 223-227. DOI: 10.1021/ic50072a008 Bányai, É., 1972. Chapter 3. Acid-Base Indicators, in: Bishop, E. (Ed.), Indicators. Pergamon Press, Oxford, pp. 65-176. Barrie, L.A., Bottenheim, J.W., Schnell, R.C., Crutzen, P.J., Rasmussen, R.A., 1988. Ozone destruction and photochemical reactions at polar sunrise in the lower Arctic atmosphere. Nature 334, 138-141. DOI: 10.1038/334138a0 Beckwith, R.C., Wang, T.X., Margerum, D.W., 1996. Equilibrium and Kinetics of Bromine Hydrolysis. Inorganic Chemistry 35, 995-1000. DOI: 10.1021/ic950909w

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ACCEPTED MANUSCRIPT Brown, L.C., Begun, G.M., Boyd, G.E., 1969. Production of bromite and perbromate ions in crystalline CsBrO3 irradiated with cobalt-60 g - rays and the vibrational spectrum of the BrO4- ion. J. Am. Chem. Soc. 91, 2250-2254. DOI: 10.1021/ja01037a012 Caplow, M., 1971. Bromine catalysis for carbon dioxide hydration and dehydration and some observations concerning the mechanism of carbonic anhydrase. J. Am. Chem. Soc. 93, 230-235. DOI: 10.1021/ja00730a038 Clifford, D., Donaldson, D.J., 2007. Direct Experimental Evidence for a Heterogeneous Reaction of Ozone with Bromide at the Air−Aqueous Interface. J. Phys. Chem. A 111, 9809-9814. DOI: 10.1021/jp074315d Disselkamp, R.S., Chapman, E.G., Barchet, W.R., Colson, S.D., Howd, C.D., 1999. BrCl production in NaBr/NaCl/HNO3/O3 solutions representative of sea-salt aerosols in the marine boundary layer. Geophys. Res. Lett. 26, 2183-2186. DOI: 10.1029/1999GL900251 Fan, S.-M., Jacob, D.J., 1992. Surface ozone depletion in Arctic spring sustained by bromine reactions on aerosols. Nature 359, 522-524. DOI: 10.1038/359522a0 Garland, J.A., Elzerman, A.W., Penkett, S.A., 1980. The mechanism for dry deposition of ozone to seawater surfaces. J. Geophys. Res. Oceans 85, 7488-7492. DOI: 10.1029/JC085iC12p07488 Haag, W.R., Hoigné, J., 1983. Ozonation of Bromide-Containing Waters: Kinetics of Formation of Hypobromous Acid and Bromate. Env. Sci. Tech. 17, 261-267. DOI: 10.1021/es00111a004 Haruta, K., Takeyama, T., 1981. Kinetics of Oxidation of Aqueous Bromide Ion by Ozone. J. Phys. Chem. 85, 2383-2388. DOI: 10.1021/j150616a018 Hausmann, M., Platt, U., 1994. Spectroscopic measurement of bromine oxide and ozone in the high Arctic during Polar Sunrise Experiment 1992. J. Geophys. Res. Atmospheres 99, 25399-25413. DOI: 10.1029/94JD01314 Hirokawa, J., Onaka, K., Kajii, Y., Akimoto, H., 1998. Heterogeneous processes involving sodium halide particles and ozone: Molecular bromine release in the marine boundary layer in the absence of nitrogen oxides. Geophys. Res. Lett. 25, 2449-2452. DOI: 10.1029/98GL01815 Hubinger, S., Nee, J.B., 1995. Absorption spectra of Cl2, Br2 and BrCl between 190 and 600 nm. J. Photochem. Photobiol. A 86, 1-7. DOI: 10.1016/1010-6030(94)03949-U Hunt, S.W., Roeselová, M., Wang, W., Wingen, L.M., Knipping, E.M., Tobias, D.J., Dabdub, D., Finlayson-Pitts, B.J., 2004. Formation of Molecular Bromine from the Reaction of Ozone with Deliquesced NaBr Aerosol:  Evidence for Interface Chemistry. J. Phys. Chem. A 108, 11559-11572. DOI: 10.1021/jp0467346 Kiese, M., Hastings, A.B., 1940. The Catalytic Hydration of Carbon Dioxide. J. Biol. Chem. 132, 267-280.

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ACCEPTED MANUSCRIPT Larson, T.E., Sollo, F.W.J., 1970. Determination of Free Bromine in Water (U): Final Report (ISWS Contract Report CR-117. http://hdl.handle.net/2142/55563). Illinois State Water Survey. Levanov, A.V., Kuskov, I.V., Zosimov, A.V., Antipenko, E.E., Lunin, V.V., 2003. Photometric Determination of Chlorine in a Gas Flow in the Presence of Ozone. J. Anal. Chem. 58, 439-441. DOI: 10.1023/A:1024069912606 Levanov, A.V., Maksimov, I.B., Isaikina, O.Y., Antipenko, E.E., Lunin, V.V., 2016. Interaction between Gaseous Ozone and Crystalline Potassium Bromide. Russian Journal of Physical Chemistry A 90, 1312–1318. DOI: 10.1134/S0036024416070189 Levason, W., Ogden, J.S., Spicer, M.D., Young, N.A., 1990. Characterisation of the oxoanions of bromine BrOx-(x= 1-4) by infrared, Raman, nuclear magnetic resonance, and bromine K-edge extended X-ray absorption fine structure techniques. J. Chem. Soc. Dalton Trans., 349-353. DOI: 10.1039/DT9900000349 Lide, D.R., 2010. CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), 90 ed. CRC Press / Taylor and Francis, Boca Raton, FL. Liu, Q., Schurter, L.M., Muller, C.E., Aloisio, S., Francisco, J.S., Margerum, D.W., 2001. Kinetics and mechanisms of aqueous ozone reactions with bromide, sulfite, hydrogen sulfite, iodide, and nitrite ions. Inorg. Chem. 40, 4436-4442. DOI: 10.1021/ic000919j Maric, D., Burrows, J.P., Moortgat, G.K., 1994. A study of the UV-visible absorption spectra of Br2 and BrCl. J. Photochem. Photobiol. A 83, 179-192. DOI: 10.1016/10106030(94)03823-6 Nakamoto, K., 2008. Ch.2. Applications in Inorganic Chemistry, Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part A. Theory and Applications in Inorganic Chemistry. Sixth Edition. John Wiley & Sons, Inc., Hoboken (New Jersey), pp. 149-354. Nissenson, P., Packwood, D.M., Hunt, S.W., Finlayson-Pitts, B.J., Dabdub, D., 2009. Probing the sensitivity of gaseous Br2 production from the oxidation of aqueous bromide-containing aerosols and atmospheric implications. Atm. Env. 43, 3951-3962. DOI: 10.1016/j.atmosenv.2009.04.006 Nissenson, P., Wingen, L.M., Hunt, S.W., Finlayson-Pitts, B.J., Dabdub, D., 2014. Rapid formation of molecular bromine from deliquesced NaBr aerosol in the presence of ozone and UV light. Atm. Env. 89, 491-506. DOI: 10.1016/j.atmosenv.2014.02.056 Oldridge, N.W., Abbatt, J.P.D., 2011. Formation of Gas-Phase Bromine from Interaction of Ozone with Frozen and Liquid NaCl/NaBr Solutions: Quantitative Separation of Surficial Chemistry from Bulk-Phase Reaction. J. Phys. Chem. A 115, 2590-2598. DOI: 10.1021/jp200074u Oum, K.W., Lakin, M.J., Finlayson-Pitts, B.J., 1998. Bromine activation in the troposphere by the dark reaction of O3 with seawater ice. Geophys. Res. Lett. 25, 39233926. DOI: 10.1029/1998GL900078

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ACCEPTED MANUSCRIPT Saiz-Lopez, A., von Glasow, R., 2012. Reactive halogen chemistry in the troposphere. Chem. Soc. Rev. 41, 6448-6472. DOI: 10.1039/C2CS35208G Sander, S.P., Abbatt, J., Barker, J.R., Burkholder, J.B., Friedl, R.R., Golden, D.M., Huie, R.E., Kolb, C.E., Kurylo, M.J., Moortgat, G.K., Orkin, V.L., Wine, P.H., 2011. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17. JPL Publication 10-6, Jet Propulsion Laboratory, Pasadena. Sharma, M.M., Danckwerts, P.V., 1963. Catalysis by Bronsted bases of the reaction between CO2 and water. Trans. Faraday Soc. 59, 386-395. DOI: 10.1039/TF9635900386 Simpson, W.R., von Glasow, R., Riedel, K., Anderson, P., Ariya, P., Bottenheim, J., Burrows, J., Carpenter, L.J., Frieß, U., Goodsite, M.E., Heard, D., Hutterli, M., Jacobi, H.W., Kaleschke, L., Neff, B., Plane, J., Platt, U., Richter, A., Roscoe, H., Sander, R., Shepson, P., Sodeau, J., Steffen, A., Wagner, T., Wolff, E., 2007. Halogens and their role in polar boundary-layer ozone depletion. Atmos. Chem. Phys. 7, 4375-4418. DOI: 10.5194/acp-7-4375-2007 Tang, T., McConnell, J.C., 1996. Autocatalytic release of bromine from Arctic snow pack during polar sunrise. Geophys. Res. Lett. 23, 2633-2636. DOI: 10.1029/96GL02572 Taube, H., 1942. Reactions in Solutions Containing O3, H2O2, H+ and Br-. The Specific Rate of the Reaction O3 + Br- →. J. Am. Chem. Soc. 64, 2468-2474. DOI: 10.1021/ja01262a072 Tavares da Silva, A., Danckwerts, P.V., 1973. The effect of halogens on the rate of absorption of carbon dioxide. Chem. Eng. Sci. 28, 847-854. DOI: 0.1016/00092509(77)80019-5 Tuckermann, M., Ackermann, R., GÖLz, C., Lorenzen-Schmidt, H., Senne, T., Stutz, J., Trost, B., Unold, W., Platt, U., 1997. DOAS-observation of halogen radical-catalysed arctic boundary layer ozone destruction during the ARCTOC-campaigns 1995 and 1996 in Ny-Ålesund, Spitsbergen. Tellus B 49, 533-555. DOI: 10.1034/j.16000889.49.issue5.9.x Vogt, R., Crutzen, P.J., Sander, R., 1996. A mechanism for halogen release from sea-salt aerosol in the remote marine boundary layer. Nature 383, 327-330. DOI: 10.1038/383327a0 von Glasow, R., Crutzen, P.J., 2014. 5.2. Tropospheric Halogen Chemistry, in: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry (Second Edition). Elsevier, Oxford, pp. 19-69. von Glasow, R., von Kuhlmann, R., Lawrence, M.G., Platt, U., Crutzen, P.J., 2004. Impact of reactive bromine chemistry in the troposphere. Atmos. Chem. Phys. 4, 24812497. DOI: 10.5194/acp-4-2481-2004 von Gunten, U., Hoigné, J., 1994. Bromate Formation during Ozonization of BromideContaining Waters: Interaction of Ozone and Hydroxyl Radical Reactions. Env. Sci. Tech. 28, 1234-1242. DOI: 10.1021/es00056a009

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ACCEPTED MANUSCRIPT von Gunten, U., Oliveras, Y., 1998. Advanced Oxidation of Bromide-Containing Waters:  Bromate Formation Mechanisms. Env. Sci. Tech. 32, 63-70. DOI: 10.1021/es970477j Winkler, L.W., 1915. Beiträge zur Wasseranalyse. Angew. Chem. 28, 22-23. DOI: 10.1002/ange.19150280604

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Fig. 1. Scheme of the experimental setup.

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Fig. 2. Infrared absorption spectra of powdered crystalline KBr treated with ozone in the

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presence of carbon dioxide (1); KHCO3 in KBr pellet (2); KBrO3 in KBr pellet (3); pure KBr (4). Experimental conditions: composition of initial gas mixture 1.6 vol.% O3, 50 vol.% CO2, 0.6 vol.% H2O, O2 the rest; temperature 60 °C; duration of treatment 60 min.

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Fig. 3. UV-visible absorption spectra of exit gases on treatment of 25 g of KBr with ozonated oxygen (1) and O3 – CO2 – O2 gas mixture (2, 3) at 60 °C. Composition of initial gas mixture 1.6 vol.% O3, 0.15 vol.% H2O, O2 the rest (1); 1.6 vol.% O3, 50 vol.% CO2, 0.6 vol.% H2O, O2 the rest (2); 1.6 vol.% O3, 50 vol.% CO2, 2.6 vol.% H2O, O2 the

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rest (3). Optical path length 10 cm.

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r(Br2), μmole min–1g–1

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Fig. 4. Effect of absolute humidity of initial gases on Br2 emission rate. Experimental conditions: composition of initial gas mixture 1.6 vol.% O3, 50 vol.% CO2, H2O and O2 the rest; temperature 60 °C.

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Рис. 5. Effect of ozone concentration in initial gases on the rates of Br2 emission and BrO3– formation, 1 – r(Br2) at 0.6 vol.% H2O; 2 – r(Br2) at 2.6 vol.% H2O; 3 – r(BrO3–) at 0.6 vol.% H2O. Points are the experimental data. Lines represent the results of calculations by formulas (9-10). Experimental conditions: composition of initial gas mixture 50 vol.% CO2, H2O, O3 and O2 the rest; temperature 60 °C.

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Fig. 6. Effect of carbon dioxide concentration in initial gases on the rates of Br2 emission and BrO3– formation, 1 – r(Br2); 2 – r(BrO3–). Points are the experimental data. Lines represent the results of calculations by formulas (9-10). Experimental conditions: composition of initial gas mixture 1.6 vol.% O3, 0.6 vol.% H2O, CO2 and O2 the rest;

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Fig. 7. Effect of temperature on the rates of Br2 emission and BrO3– formation, 1 – r(Br2) at 0.6 vol.% H2O; 2 – r(Br2) at 2.1 vol.% H2O; 3 – r(BrO3–) at 0.6 vol.% H2O. Points are the experimental data. Lines represent the results of calculations by formulas (9-10). Composition of initial gas mixture 1.6 vol.% O3, 50 vol.% CO2, H2O and O2 the rest.