Environ Sci Pollut Res (2015) 22:16554–16569 DOI 10.1007/s11356-015-4832-9
RESEARCH ARTICLE
Photochemical oxidation of chloride ion by ozone in acid aqueous solution Alexander V. Levanov 1 & Oksana Ya. Isaykina 2 & Nazrin K. Amirova 1 & Ewald E. Antipenko 1 & Valerii V. Lunin 1,2
Received: 2 March 2015 / Accepted: 2 June 2015 / Published online: 17 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The experimental investigation of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in the bulk of acid aqueous solution at pH 0–2 has been performed. Processes of chloride oxidation in these conditions are the same as the chemical reactions in the system O3 – OH – Cl−(aq). Despite its importance in the environment and for ozone-based water treatment, this reaction system has not been previously investigated in the bulk solution. The end products are chlorate ion ClO3− and molecular chlorine Cl2. The ions of trivalent iron have been shown to be catalysts of Cl− oxidation. The dependencies of the products formation rates on the concentrations of O3 and H+ have been studied. The chemical mechanism of Cl− oxidation and Cl2 emission and ClO3− formation has been proposed. According to the mechanism, the dominant primary process of chloride oxidation represents the complex interaction with hydroxyl radical OH with the formation of Cl2− anion-radical intermediate. OH radical is generated on ozone photolysis in aqueous solution. The key subsequent processes are the reactions Cl2− +O3 →ClO+O2 +Cl− and ClO+H2O2 →HOCl+ HO2. Until the present time, they have not been taken into Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-015-4832-9) contains supplementary material, which is available to authorized users. * Alexander V. Levanov [email protected] 1
Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskiye Gory 1, building 3, 119991 Moscow, Russia
2
A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky prospect 29, 119991 Moscow, Russia
consideration on mechanistic description and modelling of Cl− oxidation. The final products are formed via the reactions 2ClO → Cl 2 O 2 , Cl 2 O 2 + H 2 O → 2H + + Cl − + ClO 3 − and HOCl+H+ + Cl− ⇄ H2O+Cl2. Some portion of chloride is oxidized directly by O3 molecule with the formation of molecular chlorine in the end. Keywords Ozone . Chloride . Chlorate . Molecular chlorine . Mechanism of chemical reaction . Chemical kinetics
Introduction The investigation of the complex interaction between ozone and chloride-ion in aqueous solution under ultraviolet illumination is of great importance to environmental chemistry, and for the chemistry of processes of water and wastewater treatment with ozone. Ozone is ubiquitous in the Earth atmosphere, chloride ion is abundant in the nature and represents the main anion of sea water, and ultraviolet radiation with various wavelengths is present in the sunlight. That is why the photochemical reaction between O3 and Cl− can play an important role in the environment, e.g., as a material contributor of active chlorine in the troposphere, and a significant non-anthropogenic source of environmental contaminants, such as chlorate and perchlorate. Chemical processes involving ozone in aqueous media are closely related to the reactions of more active species, hydroxyl free radical OH in particular, which is an intermediate of ozone self-decomposition reaction in aqueous solution. The effective laboratory method of hydroxyl radical generation is the photolysis of ozone by UV-radiation with wavelength 254 nm in the presence of water. As this takes place, both hydroxyl radical OH and ozone O3 are present in the reaction system. Therefore, the study of the photochemical reaction
Environ Sci Pollut Res (2015) 22:16554–16569
between ozone and aqueous chloride ion is the same as the investigation of the complex reaction system O3 – OH – Cl−(aq). Besides natural conditions, chemical interactions in this system are of significance for ozone-based water treatment. Chloride ion is the main end product of mineralization of chlorinated organics with ozone. Further interactions of Cl− with O3 and OH under various conditions can lead to the generation of hazardous by-products Cl2, ClO3−, and ClO4−. Also, the formation of these by-products should be taken into account on medical use of ozonized physiological saline. However, in spite of their importance, the photochemical reaction between O3 and Cl−(aq), and the chemical interactions in the system O3 – OH – Cl−(aq) have not been yet investigated in the bulk of aqueous solution. On the contrary, the thermal (non-photochemical) reaction between ozone and chloride-ion is quite well investigated. The first published study of molecular chlorine formation on interaction of ozone with hydrogen chloride and water was carried out in the nineteenth century (Broek 1862). The kinetics of the reaction of O3 with Cl− in aqueous solution was investigated for the first time by (Yeatts and Taube 1949). Among other things, they found out that the reaction was catalyzed by H+ ions, and determined rate constants of noncatalytic and catalytic pathways at 0 and 9.5 °C. They were unable to determine rate constants at higher temperatures, due to ozone decomposition. For the same reason, only the upper limit of the non-catalytic rate constant at 23 °C was measured by (Hoigné et al. 1985). The comprehensive studies of the O3 +Cl−(aq) thermal reaction kinetics and mechanism were presented in the series of papers by Levanov et al. In particular, the main products of the reaction in acid (Levanov et al. 2012c; Levanov et al. 2003a) (molecular chlorine Cl2) and basic (Levanov et al. 2008) (chlorate-ion ClO3−) media were determined; the catalysis by H+ was investigated in more detail (Levanov et al. 2006c; Levanov et al. 2003a), and the non-catalytic and catalytic rate constants were determined at 7–60 °C Levanov et al. (2003a), see also (Levanov et al. 2012b); the inhibiting action of Cl− and acceleration effect of ion strength were discovered (Levanov et al. 2003a); the catalysis by metal ions (Fe3+, Co2+, MnO4−, and others) were explored (Levanov et al. 2006a, b, c; Levanov et al. 2005); and the mechanism of the reaction was proposed (Levanov et al. 2012a, b). The emergence of perchlorate-ion ClO4− as a by-product of the O3 +Cl−(aq) reaction in neutral and basic solutions was discovered in the pioneer works (Dasgupta et al. 2005; Kang et al. 2008; Rao et al. 2010); also, the authors (Kang et al. 2008; Rao et al. 2010) observed the formation of the main product chlorate-ion ClO3−. The extensive and many-sided investigations of B.J. Finlayson-Pitts et al. (Knipping and Dabdub 2002; Knipping et al. 2000; Laskin et al. 2006; Nissenson et al. 2008; Oum
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et al. 1998; Thomas et al. 2006) have been focused on the study of the photochemical interaction of chloride-ion placed in the liquid aqueous aerosol with ozone and hydroxyl radical, formed on ozone photolysis in the gas phase. Under such conditions, reactions on the gas/aerosol interface are of great importance. The enhanced formation of molecular chlorine has been found, and Cl2 concentrations were much more than those calculated on the basis of known chemical reactions. The hypothetical reaction between gas-phase hydroxyl radical and chloride ion on the aerosol surface has been proposed as an efficient additional source of Cl2. This reaction was supposed to proceed with anomalously high velocity due to the conjectured increase in reactivity on the interface (Knipping and Dabdub 2002; Knipping et al. 2000; Nissenson et al. 2008; Oum et al. 1998). In general, the formation of significant amounts of Cl2 and other chloride ion oxidation products can as well be caused by some chemical reactions, not yet known, proceeding in the bulk of the aqueous solution instead of the interface. However, to the present day, the studies of chloride ion photochemical oxidation by ozone in the bulk solution have not been described in the literature. The aim of this work is to investigate the kinetics of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in acid aqueous solution, and in particular, first, to determine the composition of products formed on photochemical interaction between ozone and acid chloride solutions; second, to study the dependencies of the products formation rates on significant experimental factors, ozone concentration in initial gas mixture, and acidity of reaction solution; third, to propose a justified chemical mechanism of photochemical chloride ion oxidation by ozone and the products formation. The acid medium has been chosen, because in these conditions, the side reaction of ozone selfdecomposition is largely suppressed, and it is much easier to follow the formation of chloride ion oxidation products. Methods of experiment and kinetic calculations The scheme of the experimental setup is shown in Fig. 1. The interaction between ozone and chloride ion was carried out in a bubble column reactor filled with the investigated solution of NaCl and 0.01–1 M HCl. In all the experiments, the solution volume was 400 ml and chloride ion concentration and ion strength were equal to 1 M. The reactor was composed of three sections connected with ground joints. The central irradiation section constituted a tube (height ~50 cm, inner diameter 2.5 cm) made of optical grade fused quartz transparent for ultraviolet radiation. The lower section represented a short glass tube with a porous glass filter plate at its bottom. Through the filter, initial gases were fed in the reactor to ensure the fragmentation of gas flow to a large number of small bubbles and efficacious gas–liquid contact. The solution was
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Environ Sci Pollut Res (2015) 22:16554–16569
Cl2+O2+O3
Cl2+O2 Tube furnace for O3 decomposion
Bubble column reactor
UV lamp
Opcal cell of spectrophotometer hν 254 nm vented to fume hood
NaCl(aq) + HCl(aq)
collecng samples for determinaon of ClO3–
O2
Ozonizer
O2+O3
О3-meter
Fig. 1 Principal scheme of the experimental setup
situated over the filter, the height of liquid column measured ~45 cm. The upper section was just a glass head with a tube for gas withdrawal. The gas communications were made of polytetrafluorethylene or medical polyvinylchloride tubes. Special tests have shown that the decomposition of the tube material by the action of ozone and formation of Cl2 is negligible under the conditions of our experiments. The following reagents were used: distilled water, hydrochloric acid (GOST 3118–77, chemically pure grade, or GOST 14261–77, very high purity grade), sodium chloride (GOST 4233–77, chemically pure grade), hydrogen peroxide unstabilized, and iron(III) chloride hexahydrate (GOST 4147–74, pure for analysis grade). Reaction solutions were prepared by mixing more concentrated stock solutions of 3.6 M HCl and 4.5 M NaCl+0.25 M HCl. The stock solutions were purified from admixtures of bromide ion and other easily oxidized substances by the technique (Levanov et al. 2012c): a stream of ozonized oxygen (ozone concentration 60 g/m3) was passed through the boiling solutions, placed in round bottom flask with reflux condenser, during 1.5 h. The test according to (Levanov et al. 2012c) showed the absence of bromide ion in the
purified stock solutions. The exact concentration of HCl in the stock solutions was determined by acid–base titration with borax using methyl orange indicator, and NaCl was measured by weight of dry residue after evaporation. Ozone was synthesized in a self-made barrier discharge ozonizer from gaseous oxygen (very high purity grade). Oxygen flow rate was 21 L/h (STP) in all the experiments. Ozone concentration was controlled at entry in the reactor by photometric ozonometer Medozon 254/5 and took values 3–30 g/m3. The reactor was irradiated by ozoneless amalgam low pressure germicidal lamp ANC 100/32 (LIT Company), consumed power 100 W, and nominal UV radiation power 23 W. The main radiated wavelength was 253.7 nm, the shorter wavelengths being absent. The lamp was 40 cm in length and was arranged vertically parallel to the reactor irradiation section at the distance 3–3.5 cm. The lamp and the reactor were placed inside a special cardboard box for protection against UV radiation. During the experiments, the reaction solution was heated by the lamp from the room temperature (~20 °C) to ~25 °C. The number of photons absorbed into the reactor was determined by the method of chemical actinometry. As an actinometer was employed, aqueous hydrogen peroxide solution with the initial concentration of 0.3 M, which was placed in the reactor and irradiated in the same conditions as in the regular experiments. The decrease in H2O2 concentration was determined by titration with potassium permanganate according to a standard procedure (Alekseev 1969). The decay of hydrogen peroxide was a zero-order reaction with the rate constant (7.2±0.1)×10−4 mol L−1 min−1. Since one absorbed UV photon leads to the decay of one hydrogen peroxide molecule in aqueous solution (Baxendale and Wilson 1957; Goldstein et al. 2007; Hunt and Taube 1952), the number of photons absorbed per unit volume of reaction solution, NΦ, is the same as the rate constant: NΦ =(7.2±0.1)×10−4 Einstein L−1 min−1 =(4.34±0.06)×1020 photons L−1 min−1 in all the experiments of the present work. Qualitative analysis of the exit gases was carried out by Raman spectroscopy of their low temperature condensate. The gas sample with a volume of ~1.5 L (STP) was admitted in vacuum flow setup (evacuated by a forvacuum pump) and condensed on the Bfinger^ of a low-temperature reactor-cryostat filled with liquid nitrogen. The Raman spectra of this condensate were taken by means of Horiba Jobin Yvon LabRam HR 800 UV spectrometer (diffraction grating 1800 lines/ mm; laser radiation wavelength 534.532 nm). Mainly, those gases were present in the condensate whose pressure at 77 K is the same as of ozone (2×10−3 mm Hg) or less; the most part of molecular oxygen, the principal component of the gas mixture, did not condense and
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was evacuated by pumping. The description of the vacuum setup, the scheme of the low temperature reactorcryostat and the procedure of taking the Raman spectra are given in (Levanov et al. 2011). Qualitative composition of nonvolatile reaction products present in the reaction solution was determined by infrared spectroscopy. Reaction solution treated with ozone and UV radiation was evaporated to dryness by a rotary evaporator at 90 °C under a vacuum of water-jet pump. From the powder obtained, a pellet was pressed off, and its IR spectra were taken by means of Equinox 55/S IR spectrometer (Bruker). Quantitative determination of molecular chlorine in the exit gases was performed by the method of direct UV spectrophotometry. In the UV–vis range, molecular chlorine has a single absorption band with the maximum at 329.5 nm (ε329.5 = 66.82 L mol−1 cm−1) (Maric et al. 1993). In our experiments, an intense Hartley band of unreacted ozone (λmax = 255 nm, ε255 = 3002.5 L mol − 1 cm − 1 ) (Sander et al. 2011b) is superimposed on the relatively weak chlorine band. For this reason, the gases were initially passed through the tube furnace. Its temperature was specially determined (see (Levanov et al. 2003b)) and took values 530–540 °C so that ozone was fully decomposed and Cl2 underwent no change. Then, the gases were directed into the optical cell of Agilent 8453 spectrophotometer, where the UV–vis spectra were recorded. After the treatment, the experimental spectra showed the only peak—the signal of molecular chlorine with the maximum at 330 nm. Cl2 concentration, C (Cl2), mol/L, was calculated from the net absorbance (optical density) at 330 nm, A330, with the formula CðCl2 Þ ¼ A330 =ðε329:5 ⋅ℓ Þ ¼ A330 =668:2 where ℓ=10.0 cm is the optical cell path length. During the experiments, time dependencies C(Cl2)(t) were recorded, and from them, chlorine concentrations in the exit gases in the steady state C(Cl2)∞ were found. Chlorine emission rate (dnCl2/dt)/Vliq, μmol L−1 min−1, was calculated by the formula ðdnCl2 =dtÞ=V liq ¼ υ=V liq CðCl2 Þ∞ ;
ð1Þ
where C(Cl2)∞, μmol/L, is Cl2 concentration in the exit gases in the steady state, υ=21 L/h=0.35 L/min is gas mixture flow rate, and Vliq =0.4 L is the volume of reaction solution. The uncertainty of experimental determination of chlorine emission rate was estimated to be less than 4.2 μmol L−1 min−1. Quantitative determination of chlorate ion in the reaction solution was performed using the indirect spectrophotometric method proposed in (Levanov et al. 2008). On completion of the treatment with ozone and UV light, molecular oxygen was bubbled through the solution during 6 min,
which ensured withdrawal of ozone and molecular chlorine. Then, 2.5 ml of the solution sample was poured in a 25-ml graduated flask, 20 ml of the solution 10 M HCl+1 mM KBr was added, and the rest of the flask was filled with distilled water. In thus prepared analytic solution, chlorate ion was largely transformed to stable complex ion BrCl2− according to the reaction ClO3− + 6H+ + 5Cl− + 3Br− → 3BrCl2− +3H2O (the equilibrium constants of BrCl2− formation reactions are as follows: BrCl+Cl−⇄ BrCl2−, K=6 М−1 (Wang et al. 1994); Br− +Cl2 ⇄ BrCl2−, K=4.2×106 М−1 (Liu and Margerum 2001)). BrCl2− ion possesses an intense and well-pronounced absorption line with the maximum at 232 nm, ε232 =32700 L mol−1 cm−1 (Wang et al. 1994). Spectra of the analytic solution were recorded by Agilent 8453 spectrophotometer, blank sample being water. The maximum absorbance due to the presence of chlorate occurred at 237 nm. Chlorate ion concentration was determined from absorbance at 237 nm with the use of calibration curve. The uncertainty of chlorate determination in model solutions of 1 M NaCl and 10–300 μM ClO3− was less than 5 μM. This corresponds to relative error less than 1.7 % at chlorate concentration 100–300 μM. The considerable advantage of this method compared to conventional iodometry lies in the fact that in acid media, bromide ion is not oxidized by oxygen present in the solution, as opposed to iodide ion. The method is not specific for chlorate ion, and various oxidizers will interfere. However, formation of other nonvolatile oxychlorine oxidizing compounds (hypochlorite ion ClO−, hypochlorous acid HOCl, chlorite ion ClO2−, and chlorine dioxide ClO2) as end products is excluded in our experiments. Indeed, in acid media, ClO− and HOCl undergoes fast protonation with the formation of Cl2; ClO−, ClO2−, and ClO2 interact rapidly with ozone yielding chlorate ion ClO3− in the end (Hoigné et al. 1985). As was noted previously, perchlorate ion ClO4− was shown to be formed in small quantities as a byproduct of ozone interaction with aqueous chloride ion (Dasgupta et al. 2005; Kang et al. 2008; Rao et al. 2010). However, perchlorate cannot interfere with the chlorate determination if for no other reason than its great chemical inertness (Greenwood and Earnshaw 1997). Thus, the method described ensures exclusive determination of chlorate ion under our experimental conditions.Chlorate formation rate (dnClO3−/dt)/Vliq, μmol L−1 min−1 was estimated by the formula h ‐i ðdnClO3 ‐=dtÞ=V liq ¼ Δ ClO3 =Δt; ð2Þ where Δt=30 min is the time of treatment of reaction solution with ozone and UV light, Δ[ClO3−], μmole/L, is the increase in chlorate ion concentration in reaction solution during the time Δt. The uncertainty of experimental determination of chlorate formation rate was 13 %. It should be
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noted that this uncertainty is significantly greater than that of chlorate ion determination in the model solutions. With the purpose to explain the experimental kinetic regularities and to elucidate the chemical mechanism of photochemical interaction between ozone and chloride ion in aqueous solution, the mathematical modelling of the kinetics of this process has been performed. The reactor was represented in the model as a continuous stirring tank reactor with gas flow passing through. Chemical transformations were considered to proceed only in the liquid phase. Differential equations for the concentration of a substance A were written in the following way V liq dC A υ υ ¼ C ðAÞ − C ðAÞ− kL aðH A C ðAÞ−½AÞ; dt V gas V gas V gas d ½A ¼ kL aðH A C ðAÞ−½AÞ−wA dt
where wAis the rate of chemical reactions of the substance A in the liquid phase, mol L−1 s−1, C(A) and C(A)° are concentrations, mol/L, of A in the gas phase inside and at the entrance to the reactor (if A is a gaseous substance, O3 or Cl2), [A] is concentration of A in the liquid solution, mol/L, kLa is volumetric mass transfer coefficient, s−1, HA is dimensionless Henry’s law constant of A, equal to the ratio of molar concentration of A in the liquid solution to that in the gas phase in the equilibrium conditions, HA =[A]/C(A), υ=21 L/h=8.6×10−3 L/s is the volumetric rate of gas mixture flow through the reactor, Vliq =0.4 L is the volume of the liquid solution in the reactor, and Vgas is the total volume of gas phase (gas bubbles) in the reactor, L, in our experiments Vgas ≈0.1 Vliq. The set of chemical reactions included in the model was made up on the basis of general chemical knowledge about plausible chloride ion oxidation pathways, literature data on ozone photolysis and possible formation pathways of molecular chlorine and chlorate ion in aqueous solution, taking into account the values of rate constants of relevant reactions. Whenever possible, the rate and equilibrium constants of the model were taken for the temperature 23 °C; otherwise, the literature values related to the temperature 20–25 °C were used without recalculation. The influence of ionic strength on the rate constants of ion-ion reactions was neglected. The equilibrium acid dissociation constants was recomputed to the ion strength of 1 M. Mass transfer between gaseous and liquid phases was considered for ozone O3 and molecular chlorine Cl2. Henry’s law constant for chlorine was taken from (Bartlett and Margerum 1999), HCl2 =2.4. The literature values of ozone Henry’s law constant in chloride solutions exhibit a large spread, see for example (Levanov et al. 2008); and hence, the two characteristic values HO3 =0.24 (Levanov et al. 2003a) and HO3 =0.16 (Levanov et al. 2008) were used in the model. It can be
Environ Sci Pollut Res (2015) 22:16554–16569
evaluated that under the conditions of our experiments, the Hatta number (Hatta 1932) for ozone is less than 10−3.1 In this case, the rate of ozone dissolution is given by the term kLa (HO3C (O3)–[O3]) (Charpentier 1981; Sotelo et al. 1989). The volumetric mass transfer coefficient kLa was assumed to be the same for all gaseous substances; the rough estimate is that in our reactor kLa~0.2 s−1. The effect of kLa variation on the rates of chlorine emission and chlorate formation has been considered in the kinetic calculations. The direct kinetics problem has been solved employing Kintecus computer program (Ianni 2006). The calculation results represented time dependencies of the concentrations of the reacting species. The experimental time dependencies C(Cl2)(t) showed that steady state was established in our reaction system. The steady state was due to the facts that the reactor was of continuous-flow type, the experimental parameters, in particular the gas mixture flow rate, ozone supply rate, and UV light intensity were kept constant and did not change with time, and decrease in chloride and hydrogen ion concentration during the experiments was negligible. In the steady state the calculated Cl2 concentration did not depend on time and was equal to the constant quantity C(Cl2)∞calc, while ClO3− concentration increased linearly as time passes. The calculated molecular chlorine emission rate was determined by substituting C(Cl2)∞calc into relation (1), chlorate formation rate—according to relation (2), as a slope of the time dependence of ClO3− concentration in the steady state.
Results and discussion For the qualitative analysis, the exit gases were condensed on the cold Bfinger^ at ~80 K, as described above. The condensate was non uniform along its height. Its upper part was a liquid of dark blue color, consisting mainly of unreacted ozone and carrier gas molecular oxygen. The lower part was a white solid. Its Raman spectra (Fig. 2) exhibit very strong and wellpronounced peaks with the maxima at 95.5, 112, 525, 532.5, and 540 cm−1. Comparison with the literature (Anderson and Sun 1970; Cahill and Leroi 1969; Suzuki et al. 1969) discloses that these signals constitute the spectrum of molecular chlorine in the crystalline state. The lines at 540, 532.5, and 525 cm−1 correspond to the fundamental vibrations 35Cl −35Cl, 37Cl −35Cl, and 37Cl −37Cl, while the peaks at 95.5 1
The Hatta number has been calculated with the formula pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ha= DO3 k 1;O3 =k L (Beltran 2004), where DO3 is ozone diffusion co-
efficient in aqueous solution, DO3 =2×10−9 m2 s−1 (Beltran 2004), kL is the mass transfer coefficient, in the analogous reactors k L = 3 × 10−4 m s−1 (Benbelkacem et al. 2003; Khudoshin et al. 2008), k1, O3 is the apparent pseudo-first-order rate constant of ozone reactions, in our experimental conditions k1, O3
RESEARCH ARTICLE
Photochemical oxidation of chloride ion by ozone in acid aqueous solution Alexander V. Levanov 1 & Oksana Ya. Isaykina 2 & Nazrin K. Amirova 1 & Ewald E. Antipenko 1 & Valerii V. Lunin 1,2
Received: 2 March 2015 / Accepted: 2 June 2015 / Published online: 17 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The experimental investigation of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in the bulk of acid aqueous solution at pH 0–2 has been performed. Processes of chloride oxidation in these conditions are the same as the chemical reactions in the system O3 – OH – Cl−(aq). Despite its importance in the environment and for ozone-based water treatment, this reaction system has not been previously investigated in the bulk solution. The end products are chlorate ion ClO3− and molecular chlorine Cl2. The ions of trivalent iron have been shown to be catalysts of Cl− oxidation. The dependencies of the products formation rates on the concentrations of O3 and H+ have been studied. The chemical mechanism of Cl− oxidation and Cl2 emission and ClO3− formation has been proposed. According to the mechanism, the dominant primary process of chloride oxidation represents the complex interaction with hydroxyl radical OH with the formation of Cl2− anion-radical intermediate. OH radical is generated on ozone photolysis in aqueous solution. The key subsequent processes are the reactions Cl2− +O3 →ClO+O2 +Cl− and ClO+H2O2 →HOCl+ HO2. Until the present time, they have not been taken into Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-015-4832-9) contains supplementary material, which is available to authorized users. * Alexander V. Levanov [email protected] 1
Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskiye Gory 1, building 3, 119991 Moscow, Russia
2
A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky prospect 29, 119991 Moscow, Russia
consideration on mechanistic description and modelling of Cl− oxidation. The final products are formed via the reactions 2ClO → Cl 2 O 2 , Cl 2 O 2 + H 2 O → 2H + + Cl − + ClO 3 − and HOCl+H+ + Cl− ⇄ H2O+Cl2. Some portion of chloride is oxidized directly by O3 molecule with the formation of molecular chlorine in the end. Keywords Ozone . Chloride . Chlorate . Molecular chlorine . Mechanism of chemical reaction . Chemical kinetics
Introduction The investigation of the complex interaction between ozone and chloride-ion in aqueous solution under ultraviolet illumination is of great importance to environmental chemistry, and for the chemistry of processes of water and wastewater treatment with ozone. Ozone is ubiquitous in the Earth atmosphere, chloride ion is abundant in the nature and represents the main anion of sea water, and ultraviolet radiation with various wavelengths is present in the sunlight. That is why the photochemical reaction between O3 and Cl− can play an important role in the environment, e.g., as a material contributor of active chlorine in the troposphere, and a significant non-anthropogenic source of environmental contaminants, such as chlorate and perchlorate. Chemical processes involving ozone in aqueous media are closely related to the reactions of more active species, hydroxyl free radical OH in particular, which is an intermediate of ozone self-decomposition reaction in aqueous solution. The effective laboratory method of hydroxyl radical generation is the photolysis of ozone by UV-radiation with wavelength 254 nm in the presence of water. As this takes place, both hydroxyl radical OH and ozone O3 are present in the reaction system. Therefore, the study of the photochemical reaction
Environ Sci Pollut Res (2015) 22:16554–16569
between ozone and aqueous chloride ion is the same as the investigation of the complex reaction system O3 – OH – Cl−(aq). Besides natural conditions, chemical interactions in this system are of significance for ozone-based water treatment. Chloride ion is the main end product of mineralization of chlorinated organics with ozone. Further interactions of Cl− with O3 and OH under various conditions can lead to the generation of hazardous by-products Cl2, ClO3−, and ClO4−. Also, the formation of these by-products should be taken into account on medical use of ozonized physiological saline. However, in spite of their importance, the photochemical reaction between O3 and Cl−(aq), and the chemical interactions in the system O3 – OH – Cl−(aq) have not been yet investigated in the bulk of aqueous solution. On the contrary, the thermal (non-photochemical) reaction between ozone and chloride-ion is quite well investigated. The first published study of molecular chlorine formation on interaction of ozone with hydrogen chloride and water was carried out in the nineteenth century (Broek 1862). The kinetics of the reaction of O3 with Cl− in aqueous solution was investigated for the first time by (Yeatts and Taube 1949). Among other things, they found out that the reaction was catalyzed by H+ ions, and determined rate constants of noncatalytic and catalytic pathways at 0 and 9.5 °C. They were unable to determine rate constants at higher temperatures, due to ozone decomposition. For the same reason, only the upper limit of the non-catalytic rate constant at 23 °C was measured by (Hoigné et al. 1985). The comprehensive studies of the O3 +Cl−(aq) thermal reaction kinetics and mechanism were presented in the series of papers by Levanov et al. In particular, the main products of the reaction in acid (Levanov et al. 2012c; Levanov et al. 2003a) (molecular chlorine Cl2) and basic (Levanov et al. 2008) (chlorate-ion ClO3−) media were determined; the catalysis by H+ was investigated in more detail (Levanov et al. 2006c; Levanov et al. 2003a), and the non-catalytic and catalytic rate constants were determined at 7–60 °C Levanov et al. (2003a), see also (Levanov et al. 2012b); the inhibiting action of Cl− and acceleration effect of ion strength were discovered (Levanov et al. 2003a); the catalysis by metal ions (Fe3+, Co2+, MnO4−, and others) were explored (Levanov et al. 2006a, b, c; Levanov et al. 2005); and the mechanism of the reaction was proposed (Levanov et al. 2012a, b). The emergence of perchlorate-ion ClO4− as a by-product of the O3 +Cl−(aq) reaction in neutral and basic solutions was discovered in the pioneer works (Dasgupta et al. 2005; Kang et al. 2008; Rao et al. 2010); also, the authors (Kang et al. 2008; Rao et al. 2010) observed the formation of the main product chlorate-ion ClO3−. The extensive and many-sided investigations of B.J. Finlayson-Pitts et al. (Knipping and Dabdub 2002; Knipping et al. 2000; Laskin et al. 2006; Nissenson et al. 2008; Oum
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et al. 1998; Thomas et al. 2006) have been focused on the study of the photochemical interaction of chloride-ion placed in the liquid aqueous aerosol with ozone and hydroxyl radical, formed on ozone photolysis in the gas phase. Under such conditions, reactions on the gas/aerosol interface are of great importance. The enhanced formation of molecular chlorine has been found, and Cl2 concentrations were much more than those calculated on the basis of known chemical reactions. The hypothetical reaction between gas-phase hydroxyl radical and chloride ion on the aerosol surface has been proposed as an efficient additional source of Cl2. This reaction was supposed to proceed with anomalously high velocity due to the conjectured increase in reactivity on the interface (Knipping and Dabdub 2002; Knipping et al. 2000; Nissenson et al. 2008; Oum et al. 1998). In general, the formation of significant amounts of Cl2 and other chloride ion oxidation products can as well be caused by some chemical reactions, not yet known, proceeding in the bulk of the aqueous solution instead of the interface. However, to the present day, the studies of chloride ion photochemical oxidation by ozone in the bulk solution have not been described in the literature. The aim of this work is to investigate the kinetics of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in acid aqueous solution, and in particular, first, to determine the composition of products formed on photochemical interaction between ozone and acid chloride solutions; second, to study the dependencies of the products formation rates on significant experimental factors, ozone concentration in initial gas mixture, and acidity of reaction solution; third, to propose a justified chemical mechanism of photochemical chloride ion oxidation by ozone and the products formation. The acid medium has been chosen, because in these conditions, the side reaction of ozone selfdecomposition is largely suppressed, and it is much easier to follow the formation of chloride ion oxidation products. Methods of experiment and kinetic calculations The scheme of the experimental setup is shown in Fig. 1. The interaction between ozone and chloride ion was carried out in a bubble column reactor filled with the investigated solution of NaCl and 0.01–1 M HCl. In all the experiments, the solution volume was 400 ml and chloride ion concentration and ion strength were equal to 1 M. The reactor was composed of three sections connected with ground joints. The central irradiation section constituted a tube (height ~50 cm, inner diameter 2.5 cm) made of optical grade fused quartz transparent for ultraviolet radiation. The lower section represented a short glass tube with a porous glass filter plate at its bottom. Through the filter, initial gases were fed in the reactor to ensure the fragmentation of gas flow to a large number of small bubbles and efficacious gas–liquid contact. The solution was
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Cl2+O2+O3
Cl2+O2 Tube furnace for O3 decomposion
Bubble column reactor
UV lamp
Opcal cell of spectrophotometer hν 254 nm vented to fume hood
NaCl(aq) + HCl(aq)
collecng samples for determinaon of ClO3–
O2
Ozonizer
O2+O3
О3-meter
Fig. 1 Principal scheme of the experimental setup
situated over the filter, the height of liquid column measured ~45 cm. The upper section was just a glass head with a tube for gas withdrawal. The gas communications were made of polytetrafluorethylene or medical polyvinylchloride tubes. Special tests have shown that the decomposition of the tube material by the action of ozone and formation of Cl2 is negligible under the conditions of our experiments. The following reagents were used: distilled water, hydrochloric acid (GOST 3118–77, chemically pure grade, or GOST 14261–77, very high purity grade), sodium chloride (GOST 4233–77, chemically pure grade), hydrogen peroxide unstabilized, and iron(III) chloride hexahydrate (GOST 4147–74, pure for analysis grade). Reaction solutions were prepared by mixing more concentrated stock solutions of 3.6 M HCl and 4.5 M NaCl+0.25 M HCl. The stock solutions were purified from admixtures of bromide ion and other easily oxidized substances by the technique (Levanov et al. 2012c): a stream of ozonized oxygen (ozone concentration 60 g/m3) was passed through the boiling solutions, placed in round bottom flask with reflux condenser, during 1.5 h. The test according to (Levanov et al. 2012c) showed the absence of bromide ion in the
purified stock solutions. The exact concentration of HCl in the stock solutions was determined by acid–base titration with borax using methyl orange indicator, and NaCl was measured by weight of dry residue after evaporation. Ozone was synthesized in a self-made barrier discharge ozonizer from gaseous oxygen (very high purity grade). Oxygen flow rate was 21 L/h (STP) in all the experiments. Ozone concentration was controlled at entry in the reactor by photometric ozonometer Medozon 254/5 and took values 3–30 g/m3. The reactor was irradiated by ozoneless amalgam low pressure germicidal lamp ANC 100/32 (LIT Company), consumed power 100 W, and nominal UV radiation power 23 W. The main radiated wavelength was 253.7 nm, the shorter wavelengths being absent. The lamp was 40 cm in length and was arranged vertically parallel to the reactor irradiation section at the distance 3–3.5 cm. The lamp and the reactor were placed inside a special cardboard box for protection against UV radiation. During the experiments, the reaction solution was heated by the lamp from the room temperature (~20 °C) to ~25 °C. The number of photons absorbed into the reactor was determined by the method of chemical actinometry. As an actinometer was employed, aqueous hydrogen peroxide solution with the initial concentration of 0.3 M, which was placed in the reactor and irradiated in the same conditions as in the regular experiments. The decrease in H2O2 concentration was determined by titration with potassium permanganate according to a standard procedure (Alekseev 1969). The decay of hydrogen peroxide was a zero-order reaction with the rate constant (7.2±0.1)×10−4 mol L−1 min−1. Since one absorbed UV photon leads to the decay of one hydrogen peroxide molecule in aqueous solution (Baxendale and Wilson 1957; Goldstein et al. 2007; Hunt and Taube 1952), the number of photons absorbed per unit volume of reaction solution, NΦ, is the same as the rate constant: NΦ =(7.2±0.1)×10−4 Einstein L−1 min−1 =(4.34±0.06)×1020 photons L−1 min−1 in all the experiments of the present work. Qualitative analysis of the exit gases was carried out by Raman spectroscopy of their low temperature condensate. The gas sample with a volume of ~1.5 L (STP) was admitted in vacuum flow setup (evacuated by a forvacuum pump) and condensed on the Bfinger^ of a low-temperature reactor-cryostat filled with liquid nitrogen. The Raman spectra of this condensate were taken by means of Horiba Jobin Yvon LabRam HR 800 UV spectrometer (diffraction grating 1800 lines/ mm; laser radiation wavelength 534.532 nm). Mainly, those gases were present in the condensate whose pressure at 77 K is the same as of ozone (2×10−3 mm Hg) or less; the most part of molecular oxygen, the principal component of the gas mixture, did not condense and
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was evacuated by pumping. The description of the vacuum setup, the scheme of the low temperature reactorcryostat and the procedure of taking the Raman spectra are given in (Levanov et al. 2011). Qualitative composition of nonvolatile reaction products present in the reaction solution was determined by infrared spectroscopy. Reaction solution treated with ozone and UV radiation was evaporated to dryness by a rotary evaporator at 90 °C under a vacuum of water-jet pump. From the powder obtained, a pellet was pressed off, and its IR spectra were taken by means of Equinox 55/S IR spectrometer (Bruker). Quantitative determination of molecular chlorine in the exit gases was performed by the method of direct UV spectrophotometry. In the UV–vis range, molecular chlorine has a single absorption band with the maximum at 329.5 nm (ε329.5 = 66.82 L mol−1 cm−1) (Maric et al. 1993). In our experiments, an intense Hartley band of unreacted ozone (λmax = 255 nm, ε255 = 3002.5 L mol − 1 cm − 1 ) (Sander et al. 2011b) is superimposed on the relatively weak chlorine band. For this reason, the gases were initially passed through the tube furnace. Its temperature was specially determined (see (Levanov et al. 2003b)) and took values 530–540 °C so that ozone was fully decomposed and Cl2 underwent no change. Then, the gases were directed into the optical cell of Agilent 8453 spectrophotometer, where the UV–vis spectra were recorded. After the treatment, the experimental spectra showed the only peak—the signal of molecular chlorine with the maximum at 330 nm. Cl2 concentration, C (Cl2), mol/L, was calculated from the net absorbance (optical density) at 330 nm, A330, with the formula CðCl2 Þ ¼ A330 =ðε329:5 ⋅ℓ Þ ¼ A330 =668:2 where ℓ=10.0 cm is the optical cell path length. During the experiments, time dependencies C(Cl2)(t) were recorded, and from them, chlorine concentrations in the exit gases in the steady state C(Cl2)∞ were found. Chlorine emission rate (dnCl2/dt)/Vliq, μmol L−1 min−1, was calculated by the formula ðdnCl2 =dtÞ=V liq ¼ υ=V liq CðCl2 Þ∞ ;
ð1Þ
where C(Cl2)∞, μmol/L, is Cl2 concentration in the exit gases in the steady state, υ=21 L/h=0.35 L/min is gas mixture flow rate, and Vliq =0.4 L is the volume of reaction solution. The uncertainty of experimental determination of chlorine emission rate was estimated to be less than 4.2 μmol L−1 min−1. Quantitative determination of chlorate ion in the reaction solution was performed using the indirect spectrophotometric method proposed in (Levanov et al. 2008). On completion of the treatment with ozone and UV light, molecular oxygen was bubbled through the solution during 6 min,
which ensured withdrawal of ozone and molecular chlorine. Then, 2.5 ml of the solution sample was poured in a 25-ml graduated flask, 20 ml of the solution 10 M HCl+1 mM KBr was added, and the rest of the flask was filled with distilled water. In thus prepared analytic solution, chlorate ion was largely transformed to stable complex ion BrCl2− according to the reaction ClO3− + 6H+ + 5Cl− + 3Br− → 3BrCl2− +3H2O (the equilibrium constants of BrCl2− formation reactions are as follows: BrCl+Cl−⇄ BrCl2−, K=6 М−1 (Wang et al. 1994); Br− +Cl2 ⇄ BrCl2−, K=4.2×106 М−1 (Liu and Margerum 2001)). BrCl2− ion possesses an intense and well-pronounced absorption line with the maximum at 232 nm, ε232 =32700 L mol−1 cm−1 (Wang et al. 1994). Spectra of the analytic solution were recorded by Agilent 8453 spectrophotometer, blank sample being water. The maximum absorbance due to the presence of chlorate occurred at 237 nm. Chlorate ion concentration was determined from absorbance at 237 nm with the use of calibration curve. The uncertainty of chlorate determination in model solutions of 1 M NaCl and 10–300 μM ClO3− was less than 5 μM. This corresponds to relative error less than 1.7 % at chlorate concentration 100–300 μM. The considerable advantage of this method compared to conventional iodometry lies in the fact that in acid media, bromide ion is not oxidized by oxygen present in the solution, as opposed to iodide ion. The method is not specific for chlorate ion, and various oxidizers will interfere. However, formation of other nonvolatile oxychlorine oxidizing compounds (hypochlorite ion ClO−, hypochlorous acid HOCl, chlorite ion ClO2−, and chlorine dioxide ClO2) as end products is excluded in our experiments. Indeed, in acid media, ClO− and HOCl undergoes fast protonation with the formation of Cl2; ClO−, ClO2−, and ClO2 interact rapidly with ozone yielding chlorate ion ClO3− in the end (Hoigné et al. 1985). As was noted previously, perchlorate ion ClO4− was shown to be formed in small quantities as a byproduct of ozone interaction with aqueous chloride ion (Dasgupta et al. 2005; Kang et al. 2008; Rao et al. 2010). However, perchlorate cannot interfere with the chlorate determination if for no other reason than its great chemical inertness (Greenwood and Earnshaw 1997). Thus, the method described ensures exclusive determination of chlorate ion under our experimental conditions.Chlorate formation rate (dnClO3−/dt)/Vliq, μmol L−1 min−1 was estimated by the formula h ‐i ðdnClO3 ‐=dtÞ=V liq ¼ Δ ClO3 =Δt; ð2Þ where Δt=30 min is the time of treatment of reaction solution with ozone and UV light, Δ[ClO3−], μmole/L, is the increase in chlorate ion concentration in reaction solution during the time Δt. The uncertainty of experimental determination of chlorate formation rate was 13 %. It should be
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noted that this uncertainty is significantly greater than that of chlorate ion determination in the model solutions. With the purpose to explain the experimental kinetic regularities and to elucidate the chemical mechanism of photochemical interaction between ozone and chloride ion in aqueous solution, the mathematical modelling of the kinetics of this process has been performed. The reactor was represented in the model as a continuous stirring tank reactor with gas flow passing through. Chemical transformations were considered to proceed only in the liquid phase. Differential equations for the concentration of a substance A were written in the following way V liq dC A υ υ ¼ C ðAÞ − C ðAÞ− kL aðH A C ðAÞ−½AÞ; dt V gas V gas V gas d ½A ¼ kL aðH A C ðAÞ−½AÞ−wA dt
where wAis the rate of chemical reactions of the substance A in the liquid phase, mol L−1 s−1, C(A) and C(A)° are concentrations, mol/L, of A in the gas phase inside and at the entrance to the reactor (if A is a gaseous substance, O3 or Cl2), [A] is concentration of A in the liquid solution, mol/L, kLa is volumetric mass transfer coefficient, s−1, HA is dimensionless Henry’s law constant of A, equal to the ratio of molar concentration of A in the liquid solution to that in the gas phase in the equilibrium conditions, HA =[A]/C(A), υ=21 L/h=8.6×10−3 L/s is the volumetric rate of gas mixture flow through the reactor, Vliq =0.4 L is the volume of the liquid solution in the reactor, and Vgas is the total volume of gas phase (gas bubbles) in the reactor, L, in our experiments Vgas ≈0.1 Vliq. The set of chemical reactions included in the model was made up on the basis of general chemical knowledge about plausible chloride ion oxidation pathways, literature data on ozone photolysis and possible formation pathways of molecular chlorine and chlorate ion in aqueous solution, taking into account the values of rate constants of relevant reactions. Whenever possible, the rate and equilibrium constants of the model were taken for the temperature 23 °C; otherwise, the literature values related to the temperature 20–25 °C were used without recalculation. The influence of ionic strength on the rate constants of ion-ion reactions was neglected. The equilibrium acid dissociation constants was recomputed to the ion strength of 1 M. Mass transfer between gaseous and liquid phases was considered for ozone O3 and molecular chlorine Cl2. Henry’s law constant for chlorine was taken from (Bartlett and Margerum 1999), HCl2 =2.4. The literature values of ozone Henry’s law constant in chloride solutions exhibit a large spread, see for example (Levanov et al. 2008); and hence, the two characteristic values HO3 =0.24 (Levanov et al. 2003a) and HO3 =0.16 (Levanov et al. 2008) were used in the model. It can be
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evaluated that under the conditions of our experiments, the Hatta number (Hatta 1932) for ozone is less than 10−3.1 In this case, the rate of ozone dissolution is given by the term kLa (HO3C (O3)–[O3]) (Charpentier 1981; Sotelo et al. 1989). The volumetric mass transfer coefficient kLa was assumed to be the same for all gaseous substances; the rough estimate is that in our reactor kLa~0.2 s−1. The effect of kLa variation on the rates of chlorine emission and chlorate formation has been considered in the kinetic calculations. The direct kinetics problem has been solved employing Kintecus computer program (Ianni 2006). The calculation results represented time dependencies of the concentrations of the reacting species. The experimental time dependencies C(Cl2)(t) showed that steady state was established in our reaction system. The steady state was due to the facts that the reactor was of continuous-flow type, the experimental parameters, in particular the gas mixture flow rate, ozone supply rate, and UV light intensity were kept constant and did not change with time, and decrease in chloride and hydrogen ion concentration during the experiments was negligible. In the steady state the calculated Cl2 concentration did not depend on time and was equal to the constant quantity C(Cl2)∞calc, while ClO3− concentration increased linearly as time passes. The calculated molecular chlorine emission rate was determined by substituting C(Cl2)∞calc into relation (1), chlorate formation rate—according to relation (2), as a slope of the time dependence of ClO3− concentration in the steady state.
Results and discussion For the qualitative analysis, the exit gases were condensed on the cold Bfinger^ at ~80 K, as described above. The condensate was non uniform along its height. Its upper part was a liquid of dark blue color, consisting mainly of unreacted ozone and carrier gas molecular oxygen. The lower part was a white solid. Its Raman spectra (Fig. 2) exhibit very strong and wellpronounced peaks with the maxima at 95.5, 112, 525, 532.5, and 540 cm−1. Comparison with the literature (Anderson and Sun 1970; Cahill and Leroi 1969; Suzuki et al. 1969) discloses that these signals constitute the spectrum of molecular chlorine in the crystalline state. The lines at 540, 532.5, and 525 cm−1 correspond to the fundamental vibrations 35Cl −35Cl, 37Cl −35Cl, and 37Cl −37Cl, while the peaks at 95.5 1
The Hatta number has been calculated with the formula pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ha= DO3 k 1;O3 =k L (Beltran 2004), where DO3 is ozone diffusion co-
efficient in aqueous solution, DO3 =2×10−9 m2 s−1 (Beltran 2004), kL is the mass transfer coefficient, in the analogous reactors k L = 3 × 10−4 m s−1 (Benbelkacem et al. 2003; Khudoshin et al. 2008), k1, O3 is the apparent pseudo-first-order rate constant of ozone reactions, in our experimental conditions k1, O3
