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Influence of NO2 and dissolved iron on the S(IV) oxidation in synthetic aqueous solution
Atmospheric Environment 35 (2001) 97}104
In#uence of NO and dissolved iron on the S(IV) oxidation in synthetic aqueous solution Janja Turs\ ic\ , Irena GrgicH *, Mirko Bizjak National Institute of Chemistry, Hajdrihova 19, P.O. Box 3430, SI-1000 Ljubljana, Slovenia Received 22 October 1999; received in revised form 18 April 2000; accepted 18 May 2000
Abstract The in#uence of dissolved NO and iron on the oxidation rate of S(IV) species in the presence of dissolved oxygen is presented. To match the conditions in the real environment, the concentration of iron in the reaction solution and trace gases in the gas mixture was typical for a polluted atmosphere. The time dependence of HSO\, SO\, NO\ and NO\ and the concentration ratio between Fe(II) and total dissolved iron were monitored. Sulphate formation was the most intensive in the presence of an SO /NO /air gas mixture and Fe(III) in solution. The highest contribution to the overall oxidation was from Fe-catalysed S(IV) autoxidation. The reaction rate in the presence of both components was equal to the sum of the reaction rates when NO and Fe(III) were present separately, indicating that under selected experimental conditions there exist two systems: SO /NO /air and SO /NO /air/Fe(III), which are unlikely to interact with each other. The radical chain mechanism can be initiated via reactions Fe(III)}HSO\ and NO }SO\/HSO\. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Acid rain formation; Aqueous S(IV) oxidation; Iron; NO
1. Introduction The oxidation processes of sulphur(IV) oxides and NO species play an important role in atmospheric W chemistry as well as in industrial processes, for example in the control of emissions from coal-"red power plants (Brandt and van Eldik, 1995; Clifton et al., 1988; Coichev and van Eldik, 1994; Littlejohn et al., 1993; Shen and Rochelle, 1998). Atmospheric transformations of sulphur and nitrogen oxides, which produce sulphuric and nitric acid are mainly responsible for acid rain. These reactions occur spontaneously when SO and NO are emitted in the presence of oxidants (e.g., O , O , H O and OH ). They also exhibit a marked sensitivity towards the catalytic e!ects of metal ions and complexes, that are either in solution: rain, cloud and fog droplets, or on the surface of aerosol particles at high humidity.
* Corresponding author. Tel.: #386-1-4760200; fax: #38661-4259244. E-mail address: [email protected] (I. GrgicH ).
The transition metals that have received the most attention as catalysts are Fe(II)/Fe(III), Mn(II), Co(II), Cu(II) and Ni(II) (Brandt and van Eldik, 1995; Coichev and van Eldik, 1994). The importance of iron is that it is the most abundant transition metal in the atmosphere and besides Mn(II), the most e!ective catalytic species (GrgicH et al., 1991; Ho!mann et al., 1996; Novic\ et al., 1996). The autoxidation of S(IV) oxides catalysed by transition metal ions is the subject of a number of studies (Brandt and van Eldik, 1995; Coichev et al., 1992; Coichev and van Eldik, 1994; GrgicH et al., 1992; Martin and Good, 1991). A crucial aspect of metal-catalysed autoxidation is the nature of the reactions leading to the reoxidation of reduced metal ions and complexes to complete the catalytic cycle (Bal Reddy et al., 1991; Brandt et al., 1994; van Eldik et al., 1992). These reactions exhibit meaningful synergistic e!ects, as shown by a combination of metal ions like Fe(III)/Cu(II), Fe(III)/Mn(II), Mn(II)/Cu(II) and Co(II)/Mn(II). The increasing interest in nitrous acid is because its photolysis signi"cantly enhances photooxidation
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 2 8 3 - 1
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processes, resulting from the rapid production of OH radicals (Kle!mann et al., 1998). The reaction of NO with acidi"ed cloud water can be an important source of gaseous HNO (Bambauer et al., 1994). For NO and NO an additional complexity arises, since they form a dimer either in the gaseous or in the aqueous phase, which have di!erent activities (Brandt and van Eldik, 1995; Lee and Schwartz, 1981). The possible interaction of NO species (e.g., NO, NO>, NO\ and HNO ) W with metal ions and complexes is complex and can lead to a series of redox reactions involving both NO and W metal species. To date, such processes are yet to be studied in detail, although they could easily produce serious pollutants such as N O and HNO . Iron(II) complexes are capable of binding NO extremely rapidly and nitrous acid interacts with Fe(II) complexes to form a nitrous oxide complex, which further decomposes to NO (Coichev and van Eldik, 1994; Zang and van Eldik, 1990). The reactions occurring between SO and NO species V W in the absence or presence of metal ions are also important because these processes lead to a variety of new reactions and products that can have a signi"cant in#uence on atmospheric oxidation processes. Under acidic conditions, typical for the atmosphere, these reactions are further complicated by the production of a series of mixed N}S-oxides (Clifton et al., 1988; Coichev and van Eldik, 1994; Littlejohn et al., 1993). Since NO rapidly oxidises to NO , the reaction of NO with HSO\/SO\ results in the formation of NO\, NO\, SO\ and S O\. In general, Fe(II) ions and complexes promote the interaction between NO or HNO and HSO\/SO\ via a nitrosyl complex. This leads to the formation of HADS (HON(SO )\-hydroxylaminedisulfonate), SA (H N(SO )\-sulfamate), N O and HSO\ and is accom panied by the oxidation of Fe(II) to Fe(III). Completion of the catalytic cycle occurs spontaneously in the presence of HSO\/SO\ (Coichev and van Eldik, 1994; Gei{ler and van Eldik, 1994; Zang and van Eldik, 1993). Additionally, Fe(III) reacts with mixed N}S-oxides. During the "rst step in the reaction sequence a complex is formed, which decomposes to HSO\ and N O (Lepentsiotis et al., 1996). Our aim is to investigate the reactions between NO and SO /air in the absence and presence of dissolved iron in an aqueous solution. Our interest in these reactions concerns their possible role and importance in atmospheric oxidation processes. In this paper we present the in#uence of NO and/or iron on S(IV) oxidation at two di!erent initial pH values. We have also designed a system that enables the introduction of gaseous mixtures (SO /air, NO /air and SO / NO /air), which allows us to monitor the concen tration of SO\, HSO\, NO\ and NO\ and the concentration ratio between Fe(II) and total dissolved iron.
2. Experimental 2.1. Reagents All the reagents used were analytical grade. For the preparation of solutions Milli-Q water was used. Iron(III) and Fe(II) stock solutions were prepared from Fe(ClO ) ) 9H O and (NH ) Fe(SO ) ) 6H O, respec tively. For acidi"cation we used hydrochloric acid. Stock solutions of SO\, NO\ and NO\ were prepared from the corresponding sodium salts, while standards for determining S(IV) were made by dissolving Na S O in formaldehyde and acetate bu!er. The pH of the model solutions was adjusted by adding dropwise 1 mol l\ HCl and 0.5 mol l\ NaOH> 1,10-phenanthroline monohydrate, ammonium acetate, acetic acid and L(#)ascorbic acid were used to determine Fe(II) and total dissolved iron. The gas mixture was prepared using SO and/or NO in synthetic air (0.01 vol%) and "ltered air from a compressor.
2.2. Apparatus and procedures The experimental apparatus is shown in Fig. 1 and consists of gas cylinders (SO and/or NO ), an inlet for "ltered air from the compressor, a gas-mixing station, an air bubbler and two #ow meters. The air bubbler is covered with aluminium foil to prevent photochemical reactions. The temperature of the solution is 20$13C and the #ow rate of the gas mixture through the air bubbler is 100 l h\. The pH of the model solution was "rst adjusted to pH 3 or 4 by adding HCl. When studying the e!ects of iron, we added the acidi"ed Fe(III) solution after 1 h of bubbling. The "nal concentration of total iron in solution is 3.6;10\ mol l\. We collected sample volumes (1 ml) of the reaction solution every 30 min, while the system was continuously fed with the gas mixture. A sample of the reaction solution, collected to determine HSO\, was immediately stabilised by adding formaldehyde to prevent further oxidation of dissolved S(IV). We then determined the concentration of HSO\ (as hydroxy methane sulphonate), SO\, NO\ and NO\ by ion-exchange chromatography. The system comprises of a Dionex 4000i chromatograph equipped with a Dionex IonPac AS4A-SC separation column and a conductivity and UV detector (210 nm) for the determination of NO\ and NO\. The eluent is a mixture of 1.8;10\ mol l\ Na CO and 1.7;10\ mol l\ NaHCO . We mea sured the concentration of Fe(II) using a #ow injection analyser (ASIA Ismatec) with spectrophotomeric detection at 512 nm. Measurements are based on the colour reaction between Fe(II) and 1,10-phenanthroline. Total dissolved iron was determined after the reduction of Fe(III) with ascorbic acid.
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Fig. 1. Scheme of the system for introduction of gases (1 } air bubbler, 2 } gas mixing station, 3 } #ow meters, 4 } cylinder with SO or NO , 5 } "ltered air from compressor station, 6,7 } outlets).
To check the purity of the "ltered air from the compressor station, air was bubbled through the model solution (pH 4) for 4 h under the same experimental conditions as those for studying the reactions. The concentrations of HSO\, SO\, NO\ and NO\ were below their limits of detection, proving that "ltered air is suitable for diluting the experimental gases. Since the mass transfer of SO and NO from the gaseous to the aqueous phase and reactant di!usion may limit the kinetics, the dimerisation of NO in both phases and the di!erent activities of all the species present complicate the kinetics in the system, no attempt was made to determine the rate constant for the reaction of NO with S(IV) species in the presence of O and Fe(III). For the systems NO /SO /air and NO /SO /air/Fe(III) the sul phate formation rates were determined, while for the system SO /air/Fe(III) at pH 3 the rate constant is also calculated. However, on the basis of available literature data we give the interpretation of our experimental results by describing the processes involved at the level of elementary reactions.
3. Results and discussion 3.1. Introduction of SO2 /air into the model solutions During the introduction of SO (166, 290 and 415 ppb) at two di!erent initial pH values (pH 3 and 4), almost no SO\ is formed (Fig. 2). The concentration of HSO\ } the dominant dissolved S(IV) species, increases towards the saturation level. Since the solubility of SO at pH 4 is greater than that at pH 3, saturation is reached earlier at pH 3. The saturation equilibrium concentration of
Fig. 2. Time dependence of [HSO\] and [SO\] during the non-catalytic and catalytic reaction. Initial conditions: pH"4, [SO ]"290 ppb, addition of Fe(III) after an hour of bubbling, total [Fe]"3.6;10\ mol l\.
dissolved S(IV) species at the same concentration in the gaseous phase is approximately 10 times higher at pH 4 than at pH 3, agreeing with Seinfeld and Pandis (1998). When Fe(III) is added to the model solution, SO\ forms immediately and increases linearly with time. The concentration of HSO\, however, becomes stabilised at a concentration lower (at pH 3 at about 12% and at pH 4 at about 4% of the saturation level) than the saturation concentration under experimental conditions. The formation of SO\ is more pronounced at pH 4 because SO has a higher solubility and a higher rate constant
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Table 1 Oxidation rates d[SO\]/dt during the introduction of SO /air in the presence of Fe(III) under various experimental conditions [SO ] (ppb)
pH
d[SO\]/dt (mol l\ min\)
166 290 415 166 290 415
3 3 3 4 4 4
0.07;10\ 0.13;10\ 0.2;10\ 0.11;10\ 0.2;10\ 0.31;10\
3.2. Introduction of NO2 /air into model solutions
(Table 1). Accordingly, the oxidation rate is higher at increased concentrations of SO in the gas mixture. The rate constant for SO oxidation in the presence of Fe(III) at pH 3 is calculated according to Eq. (1). When the concentration of dissolved HSO\ is stabilised, the dissolution of SO from the gaseous phase equals the consumption of HSO\ by oxidation: d[HSO\] "0NK a(cH \ !c \ )"k c \ c , * &1- &1- 0 &1- $ dt
of the experiment, which agrees with the radical chain mechanism proposed for Fe-catalysed S(IV) oxidation (Warneck et al., 1996). Iron(III) is immediately reduced by HSO\. Because of the continuous introduction of SO , both the concentration of HSO\ and the ratio between [Fe(II)] and [Fe(III)] remain constant.
(1)
where K a is the overall mass transfer coe$cient, * cH \ the saturation concentration of HSO\, c \ the &1 &1concentration of HSO\ during autoxidation, k the rate 0 constant and c the concentration of total dissolved Fe. $ According to the literature, we propose that the reaction rate follows "rst-order kinetics regarding the concentration of dissolved S(IV) and iron (Brandt and van Eldik, 1995; GrgicH et al., 1991; Seinfeld and Pandis, 1998). For calculating the rate constant the interfacial area (a) between bubbles and liquid, and mass transfer coe$cient (K ) are needed. Both depend on the properties of the * gas, the type of reactor, and on the experimental parameters. Usually these are de"ned as a single parameter called the overall mass transfer coe$cient (K a) (Perry et * al., 1984). The K a coe$cient is determined from the * saturation curves measured under the same experimental conditions in the absence of iron. From the saturation curves measured for di!erent SO /air mixtures K a coef * "cient is obtained by "tting the experimental data with the unsteady-state version of the left-hand side of Eq. (1). For data measured at pH 3 the optimal value for K a * was 0.33;10\ s\. Using this value in Eq. (1), the rate constant k is obtained from steady-state data. The cal0 culated value k "0.74;10 mol\ l s\ (pH 3 and 0 ¹"203C) is very close to that determined by GrgicH et al. (1991) for a homogeneous reaction in a semi-batch CSTR reactor. The ratio between the concentration of Fe(II) and total dissolved iron was also monitored. Immediately after the addition of Fe(III), an equilibrium between Fe(II) and Fe(III) is established, and remains constant until the end
We investigated the in#uence of Fe(III) on a disproportionate amount of NO in the aqueous phase using two di!erent concentrations of NO in the gas mixture (0.84 and 1.68 ppm) at pH 3 and 4. Since the solubility of NO in water is very low (H"1.0;10\ M atm\, ¹"298 K, Seinfeld and Pandis, 1998), we used higher concentrations of NO in the gas phase than that ex pected in the urban atmosphere. As the concentration of NO in the gas mixture in creases so does the amount of NO\ and NO\. At a con centration of 0.84 ppm of NO in the gas phase, the concentration of both anions in the aqueous solution at pH 4 is 5;10\ mol l\ after 270 min, while at 1.68 ppm the concentration is doubled. It is evident that the molar ratio between NO\ and NO\ in the aqueous phase at pH 4 is equal as expected from the following reaction:
(2) However, a slower increase in the concentration of NO\ is found at pH 3, which is ascribed to the undisociated HNO . At pH 3 there is approximately 66% less NO\ than at pH 4. HNO is a weak acid with K " 4.6;10\ M (Bambauer et al., 1994) that does not fully dissociate, especially at pH 3 where at equilibrium almost 69% of HNO is undissociated. This reaction is an important potential source of gaseous HNO in the atmosphere during the contact of NO with acidic cloud droplets (Bambauer et al., 1994). The in#uence of Fe(III) is poorly expressed. However, the formation of NO\ is slightly slower in the presence of Fe(III) compared to experiments in which only NO is introduced. Since the Fe(III) solution always contains a low concentration of Fe(II) ()5%), the reason for this observation could be in the reaction between Fe(II) and HNO /NO\, where NO is formed (Coichev and van Eldik, 1994; Zang and van Eldik, 1990), and blown out by the intensive bubbling. 3.3. Introduction of SO2 NO2 /air into model solutions 3.3.1. The SO2 /NO2 /air system The SO /NO /air mixture comprised of 415 ppb SO during all the experiments, and 0.84 and 1.68 ppm of
J. Turs\ ic\ et al. / Atmospheric Environment 35 (2001) 97}104
Fig. 3. Time dependence of [HSO\], [SO\], [NO\] and [NO\] during the introduction of gas mixture SO /NO / air. Initial conditions: pH"4, [SO ]"415 ppb, [NO ]" 1.68 ppm.
NO . After an induction period, the formation of SO\ accelerates in the presence of NO , showing a linear increase with time (Fig. 3). The concentration of dissolved S(IV) reaches a constant value depending on the
101
pH and concentration of trace gasses in the gas mixture and is close to the saturation level. The actual mechanism of the reaction between NO and SO in an aqueous solution is complex. The absorp tion of NO in water leads to the formation of NO\ and NO\ ions (reaction (2)). In acidic conditions typical for the atmosphere, HNO /NO\ reacts with HSO\/SO\ in aqueous solution to produce a series of mixed N}Soxides (Scheme 1) (Gei{ler and van Eldik, 1994; Oblath et al., 1981, 1982), which either decompose or undergo further reactions with HSO\ to produce "nal pro ducts (Coichev and van Eldik, 1994). The reaction of HNO /NO\ with HSO\ exhibits a characteristic pH dependence in agreement with HNO being the main reactive N(III) species in aqueous solution. HNO is ca. 10 times more reactive than NO\ (Zang and van Eldik, 1990). However, Oblath et al. (1981, 1982) found that the rates of these reactions are too small for them to occur to any signi"cant extent in the boundary layer. In the presence of oxygen, the free radicals produced by the reaction of NO with HSO\/SO\ (Nash, 1979) (reaction (3)) initiate sulphite oxidation in a chain mechanism (Huie and Neta, 1984; Littlejohn et al., 1993; Shen and Rochelle, 1998): NO #HSO\/SO\PNO\#SO\#H>.
Scheme 1. Interactions between HNO /NO\ and HSO\ under acidic conditions (Coichev and van Eldik, 1994).
(3)
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Table 2 Oxidation rates d[SO\]/dt during the introduction of SO /NO /air in the absence and presence of Fe(III) after induc tion period at various experimental conditions [SO ] (ppb)
[NO ] (ppm)
415 415 415 415 415 415
1.68 1.68 1.68 1.68 0.84 0.84
[Total Fe] ;(10\ mol l\)
3.6 3.6 3.6
pH
d[SO\]/dt (mol l\ min\)
4 4 3 3 4 4
0.07;10\ 0.39;10\ 0.02;10\ 0.22;10\ 0.03;10\ 0.36;10\
These reactions are expected to occur in the mass transfer boundary layer with the net result being every mole of NO absorbed leads to the oxidation of 4 or more moles of HSO\/SO\ in the presence of oxygen. Importantly, NO is not a catalyst for S(IV) oxidation, since it is not regenerated in the same way that metal ions are (Littlejohn et al., 1993). The production of SO\ is faster at pH 4 and increases with the increasing concentration of NO in the gas mixture (Table 2). Since the distribution and reactivity of S(IV) species are in#uenced by pH, it is reasonable to obtain higher reaction rates at pH 4. The higher concentration of NO in the gas mixture results in an increase in the concentration of dissolved NO and HNO /NO\ in the model solution and consequently in the oxidation rate of HSO\. On the basis of our results and available literature data we suggest that in the presence of oxygen the important part of HSO\ oxidation with NO proceeds through a free-radical chain mechanism. With the addition of SO in the NO /air system, a signi"cantly lower concen tration of NO\, which is formed only via reaction (2), is observed. Thus, in the boundary layer a part of the dissolved NO does not dissociate but initiates the rad ical chain mechanism via reaction (3). SO\ then reacts fast with dissolved oxygen and SO\ is formed. SO\ is produced through other radical reactions (Littlejohn et al., 1993; Warneck et al., 1996). Our results show that at pH 4, [NO ]"1.68 ppm and [SO ]"415 ppb approx imately 60% and at pH 3 30% of dissolved NO react directly with HSO\. We believe that at selected experi mental conditions the formation of mixed N}S-oxides is also important (especially at pH 3), which could be the reason for the observed induction period for SO\ formation. According to Scheme 1, the reaction route starting with HNO /NO\ and HSO\ produces HON(SO )\ (HADS). One of the reasons for the slow increase in NO\ during the induction period could be the more e$cient consumption of HNO /NO\ at the
beginning. Later, only HSO\ is needed for the formation of intermediates and "nal products. 3.3.2. The SO2 /NO2 /air/Fe(III) system The above experiments were repeated in the presence of Fe(III). The time dependence of SO\, HSO\, NO\ and NO\ during the introduction of a gas mixture SO /NO /air in the presence of Fe(III) is shown in Fig. 4. The sudden formation of SO\, occurring immediately after the addition Fe(III), is almost 10 times faster than in the absence of Fe(III). The rate is greater at pH 4, but the e!ect of the NO concentration in the gas phase is insigni"cant (Table 2). The concentration of dissolved SO is stabilised at lower values and is comparable to that obtained during the introduction of SO /air in the presence of Fe(III). The production of NO\ is similar, while the formation of NO\ is slower when compared with experiments conducted in the absence of Fe(III). An equilibrium between Fe(II) and total dissolved iron, established immediately after the addition of Fe(III), slightly increases during the reaction. An increase in total dissolved iron is observed at pH 4 and is attributable to the dissolution of Fe(III) polymerised hydroxo species, formed during the lowering of the pH by the formation of acidic products (Novic\ et al., 1996). The overall mechanism for HSO\ oxidation in the presence of NO , Fe(III) and dissolved oxygen is com plicated by the many possible routes. Besides the two mechanisms mentioned above, an additional mechanism is the catalytic e!ect of Fe(III) on the S(IV) autoxidation, which appears to be the most important under the experimental conditions. Therefore, the radical chain mechanism can be initiated via reactions (3) and (4): Fe(III)#HSO\PFe(II)#SO\#H>.
(4)
Fig. 4. Time dependence of [HSO\], [SO\], [NO\] and [NO\] during the introduction of gas mixture SO /NO /air in the presence of Fe(III). Initial conditions: pH"4, [SO ]" 415 ppb, [NO ]"1.68 ppm, addition of Fe(III) after an hour of bubbling, total [Fe]"3.6;10\ mol l\.
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1993). However, SO\ radicals formed in NO }S(IV) re action can continue to oxidise S(IV) via radical chain mechanism.
4. Conclusions
Fig. 5. In#uence of Fe(III) and/or NO on the rate of SO\ formation. Initial conditions: pH"4, [SO ]"415 ppb, [NO ]" 1.68 ppm, addition of Fe(III) after an hour of bubbling, total [Fe]"3.6;10\ mol l\.
Alternatively, Fe(II) can promote an interaction between HNO /NO\ and HSO\. This is ascribed to the interac tion of Fe(II) with HNO to produce a nitrosyl complex that can react with HSO\ according to the Boedeker reaction, where mixed N}S-oxides are formed (Zang and van Eldik, 1993; Coichev and van Eldik, 1994; Gei{ler and van Eldik, 1994). Additionally, Fe(III) and mixed N}S-oxides can react. This can lead to a further complication, because this redox reactions can in principle account for the inhibiting e!ect of S}N-oxides on the Fe-catalysed S(IV) autoxidation (Lepentsiotis et al., 1996). From a comparison of the reaction rates in Table 2, it is obvious that the contribution from the oxidation by NO is low (about 10%). The reason is most likely the low concentration of dissolved NO (+10\ mol l\), which initiates the radical chain mechanism in the boundary layer. Alternatively, reactions between N(III), which is present at a concentration level of 10\ mol l\ in aqueous solution, and S(IV) are slow. Fig. 5 represents a summary of our investigations } the in#uence of Fe(III), NO and combination of both on S(IV) oxidation at pH 4 in the presence of oxygen in aqueous solution. It is evident that the reaction rate in the presence of NO and Fe(III) is equal to the sum of the reaction rates when both components are present separately. The sum of the reaction rates can also be seen at pH 3 (Table 2). From these results it seems that under selected experimental conditions the SO /NO /air/Fe(III) system behaves as two separate systems: SO /NO /air and SO /Fe(III)/air. It should be pointed out that our experimental conditions are similar to those in a polluted atmosphere. It is calculated that the contribution made by the direct reaction of NO with S(IV) is comparable to the contribution made by the H O }S(IV) reaction only at pH 6, which is not typical for atmospheric conditions (Littlejohn et al.,
Our experiments of SO oxidation in the presence of dissolved oxygen in an aqueous solution performed at pH 3 and 4 and under conditions typical for polluted atmosphere showed that Fe(III) and NO accelerate the reaction. The formation of SO\ is the most pronounced in the presence of a SO /NO /air gas mixture and Fe(III) in solution. However, the most important is the catalytic e!ect of Fe(III). The rate constant for the system SO /air/Fe(III) at pH 3 is similar to the rate constant for the homogenous reaction in a semi-batch CSTR reactor (GrgicH et al., 1991). Moreover, the reaction rate in the presence of both components is equal to the sum of the reaction rates when NO and Fe(III) are present separately, indicating that under selected experimental conditions the SO / NO /air/Fe(III) system behaves as two separate systems: SO /NO /air and SO /Fe(III)/air, which are unlikely to interact with each other. Thus, the radical chain mechanism can be initiated via a reaction between Fe(III) and HSO\, and by an NO reaction with SO\/ HSO\. Alternatively, Fe(II) can promote an interaction between HNO /NO\ and HSO\. Under nighttime conditions with low H O and O , the aqueous-phase interaction of NO with S(IV) may considerably contribute to sulphate formation. Further, the presence of dissolved iron (as Fe(II) or Fe(III)) can additionally contribute to the formation of sulphuric acid. Our preliminary experiments with atmospheric aerosol particles of industrial and urban origin show that the SO\ formation, due to the reaction of NO with S(IV) in the presence of oxygen, is important. It is known that organic compounds (especially oxalate) inhibit Fecatalysed S(IV) autoxidation (GrgicH et al., 1998, 1999). The concentration of oxalate determined in aerosol particles is comparable to that of dissolved Fe. Therefore, we suppose that the main contribution to the sulphate formation is NO }S(IV) reaction, where SO\ radicals are generated.
Acknowledgements The authors wish to thank Gorazd Berc\ ic\ from National Institute of Chemistry, Ljubljana for his helpful discussions. The "nancial support of the Ministry for Science and Technology of Republic of Slovenia (contract no. P1-0511) is gratefully acknowledged. A part of this work has been done in the frame of AEROSOL/ EUROTRAC-2 project.
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References Bal Reddy, K., Coichev, N., van Eldik, R., 1991. Redox cycling of iron in atmospheric water: the important role of sulphite. Journal of the Chemical Society-Chemical Communications, 481}483. Bambauer, A., Brantner, B., Paige, M., Novakov, T., 1994. Laboratory study of NO reaction with dispersed and bulk liquid water. Atmospheric Environment 28, 3225}3232. Brandt, C., van Eldik, R., 1995. Transitional metal-catalyzed oxidation of sulfur(IV) oxides. Atmospheric-relevant processes and mechanisms. Chemical Reviews 95, 119}190. Brandt, C., FaH biaH n, I., van Eldik, R., 1994. Kinetics and mechanism of iron(III)-catalyzed autoxidation of sulfur(IV) oxides in aqueous solution. Evidence for the redox cycling of iron in the presence of oxygen and modeling of the overall reaction mechanism. Inorganic Chemistry 33, 687}701. Clifton, C.L., Altstein, N., Huie, R.E., 1988. Rate constant for the reaction of NO with sulfur(IV) over the pH range 5.3}13. Environmental Science and Technology 22, 586}589. Coichev, N., Bal Reddy, K., van Eldik, R., 1992. The synergistic e!ect of manganese(II) in the sul"te-induced autoxidation of metal ions and complexes in aqueous solution. Atmospheric Environment 26A, 2295}2300. Coichev, N., van Eldik, R., 1994. Metal catalyzed atmospheric reactions. A challenge to coordination chemists. New Journal of Chemistry 18, 123}131. van Eldik, R., Coichev, N., Bal Reddy, K., Gerhard, A., 1992. Metal ion catalyzed autoxidation of sulfur(IV)-oxides: redox cycling of metal ions induced by sul"te. Berichte der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 96, 478}481. Gei{ler, M., van Eldik, R., 1994. Product identi"cation and kinetic data for the reaction of S(IV) with N(III) oxoacids in aqueous solution in the absence and presence of metal ions. Polyhedron 13, 2983}2991. GrgicH , I., Hudnik, V., Bizjak, M., Levec, J., 1991. Aqueous S(IV) oxidation } I. Catalytic e!ect of some metals. Atmospheric Environment 25A, 1591}1597. GrgicH , I., Hudnik, V., Bizjak, M., Levec, J., 1992. Aqueous S(IV) oxidation } II. Synergistic e!ect of some metal ions. Atmospheric Environment 26A, 571}577. GrgicH , I., Dovzan, A., Berc\ ic\ , G., Hudnik, V., 1998. The e!ect of atmospheric organic compounds on the Fe-catalyzed S(IV) autoxidation in aqueous solution. Journal of Atmospheric Chemistry 29, 315}337. GrgicH , I., Poznic\ , M., Bizjak, M., 1999. S(IV) autoxidation in atmospheric liquid water: the role of Fe(II) and the e!ect of oxalate. Journal of Atmospheric Chemistry 33, 89}102. Ho!mann, P., Dedik, A.N., Ensling, J., Weinbruch, S., Weber, S., Sinner, T., GuK tlich, P., Ortner, H.M., 1996. Speciation of iron in atmospheric aerosol samples. Journal of Aerosol Science 27, 325}337. Huie, R.E., Neta, P., 1984. Chemical behavior of SO\ and SO\ radicals in aqueous solutions. Journal of Physical Chemistry 88, 5665}5669.
Kle!mann, J., Becker, K.H., Wiesen, P., 1998. Heterogeneous NO conversion process on acid surfaces: possible atmo spheric implications. Atmospheric Environment 32, 2721}2729. Lee, Y.-N., Schwartz, S.E., 1981. Reaction kinetics of nitrogen dioxide with liquid water at low partial pressure. Journal of Physical Chemistry 85, 840}848. Lepentsiotis, V., Prinsloo, F.F., van Eldik, R., Gutberlet, H., 1996. In#uence of mixed sulfur}nitrogen oxides on the redox kinetics of iron ions in aqueous solution. Journal of the Chemical Society-Dalton Transactions, 2135}2141. Littlejohn, D., Wang, Y., Chang, S.-G., 1993. Oxidation of aqueous sul"te by nitrogen dioxide. Environmental Science and Technology 27, 2162}2167. Martin, L.R., Good, T.W., 1991. Catalyzed oxidation of sulfur dioxide in solution: iron}manganese synergism. Atmospheric Environment 25A, 2395}2399. Nash, T., 1979. The e!ect of nitrogen dioxide and of some transition metals on the oxidation of dilute bisulphite solutions. Atmospheric Environment 13, 1149}1154. Novic\ , M., GrgicH , I., Poje, M., Hudnik, V., 1996. Iron-catalyzed oxidation of S(IV) species by oxygen in aqueous solution: in#uence of pH and on the redox cycling. Atmospheric Environment 30, 4191}4196. Oblath, S.B., Markowitz, S.S., Novakov, T., Chang, S.G., 1981. Kinetics of the formation of hydroxylamine disulfonate by reaction of nitrite with sul"tes. Journal of Physical Chemistry 85, 1017}1021. Oblath, S.B., Markowitz, S.S., Novakov, T., Chang, S.G., 1982. Kinetics of the initial reaction of nitrite ion in bisul"te solutions. Journal of Physical Chemistry 86, 4853}4857. Perry, R.H., Green, D.V., Maloney, J.O., 1984. Perry's Chemical Engineers' Handbook, 6th Edition, Section 14. McGraw-Hill Book Company, Tokyo, pp. 1}40. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Wiley, New York, pp. 369}372. Shen, C.H., Rochelle, G.T., 1998. Nitrogen dioxide absorption and sul"te oxidation in aqueous sul"te. Environmental Science and Technology 32, 1994}2004. Warneck, P., Mirabel, P., Salmon, G.A., van Eldik, R., Vinckier, C., Wannowius, K.J., Zetzsch, C., 1996. Review of the activities and achievements of the EUROTRAC subproject HALIPP, Transition metal ions and their role in atmospheric waters. In: Warneck, P. (Ed.), Heterogeneous and Liquid Phase Processes, Laboratory Studies Related to Aerosols and Clouds. Springer, Berlin, pp. 20}26. Zang, V., van Eldik, R., 1990. In#uence of the polyamino carboxylate chelating ligand (L) on the kinetics and mechanism of the formation of Fe''(L)NO in the system Fe''(L)/NO HONO/NO\ in aqueous solution. Inorganic Chemistry 29, 4462}4468. Zang, V., van Eldik, R., 1993. Reaction of nitric oxide with sulfur(IV) oxides in the presence of iron(II) complexes in aqueous solution. Journal of the Chemical Society-Dalton Transactions, 111}118.
In#uence of NO and dissolved iron on the S(IV) oxidation in synthetic aqueous solution Janja Turs\ ic\ , Irena GrgicH *, Mirko Bizjak National Institute of Chemistry, Hajdrihova 19, P.O. Box 3430, SI-1000 Ljubljana, Slovenia Received 22 October 1999; received in revised form 18 April 2000; accepted 18 May 2000
Abstract The in#uence of dissolved NO and iron on the oxidation rate of S(IV) species in the presence of dissolved oxygen is presented. To match the conditions in the real environment, the concentration of iron in the reaction solution and trace gases in the gas mixture was typical for a polluted atmosphere. The time dependence of HSO\, SO\, NO\ and NO\ and the concentration ratio between Fe(II) and total dissolved iron were monitored. Sulphate formation was the most intensive in the presence of an SO /NO /air gas mixture and Fe(III) in solution. The highest contribution to the overall oxidation was from Fe-catalysed S(IV) autoxidation. The reaction rate in the presence of both components was equal to the sum of the reaction rates when NO and Fe(III) were present separately, indicating that under selected experimental conditions there exist two systems: SO /NO /air and SO /NO /air/Fe(III), which are unlikely to interact with each other. The radical chain mechanism can be initiated via reactions Fe(III)}HSO\ and NO }SO\/HSO\. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Acid rain formation; Aqueous S(IV) oxidation; Iron; NO
1. Introduction The oxidation processes of sulphur(IV) oxides and NO species play an important role in atmospheric W chemistry as well as in industrial processes, for example in the control of emissions from coal-"red power plants (Brandt and van Eldik, 1995; Clifton et al., 1988; Coichev and van Eldik, 1994; Littlejohn et al., 1993; Shen and Rochelle, 1998). Atmospheric transformations of sulphur and nitrogen oxides, which produce sulphuric and nitric acid are mainly responsible for acid rain. These reactions occur spontaneously when SO and NO are emitted in the presence of oxidants (e.g., O , O , H O and OH ). They also exhibit a marked sensitivity towards the catalytic e!ects of metal ions and complexes, that are either in solution: rain, cloud and fog droplets, or on the surface of aerosol particles at high humidity.
* Corresponding author. Tel.: #386-1-4760200; fax: #38661-4259244. E-mail address: [email protected] (I. GrgicH ).
The transition metals that have received the most attention as catalysts are Fe(II)/Fe(III), Mn(II), Co(II), Cu(II) and Ni(II) (Brandt and van Eldik, 1995; Coichev and van Eldik, 1994). The importance of iron is that it is the most abundant transition metal in the atmosphere and besides Mn(II), the most e!ective catalytic species (GrgicH et al., 1991; Ho!mann et al., 1996; Novic\ et al., 1996). The autoxidation of S(IV) oxides catalysed by transition metal ions is the subject of a number of studies (Brandt and van Eldik, 1995; Coichev et al., 1992; Coichev and van Eldik, 1994; GrgicH et al., 1992; Martin and Good, 1991). A crucial aspect of metal-catalysed autoxidation is the nature of the reactions leading to the reoxidation of reduced metal ions and complexes to complete the catalytic cycle (Bal Reddy et al., 1991; Brandt et al., 1994; van Eldik et al., 1992). These reactions exhibit meaningful synergistic e!ects, as shown by a combination of metal ions like Fe(III)/Cu(II), Fe(III)/Mn(II), Mn(II)/Cu(II) and Co(II)/Mn(II). The increasing interest in nitrous acid is because its photolysis signi"cantly enhances photooxidation
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 2 8 3 - 1
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processes, resulting from the rapid production of OH radicals (Kle!mann et al., 1998). The reaction of NO with acidi"ed cloud water can be an important source of gaseous HNO (Bambauer et al., 1994). For NO and NO an additional complexity arises, since they form a dimer either in the gaseous or in the aqueous phase, which have di!erent activities (Brandt and van Eldik, 1995; Lee and Schwartz, 1981). The possible interaction of NO species (e.g., NO, NO>, NO\ and HNO ) W with metal ions and complexes is complex and can lead to a series of redox reactions involving both NO and W metal species. To date, such processes are yet to be studied in detail, although they could easily produce serious pollutants such as N O and HNO . Iron(II) complexes are capable of binding NO extremely rapidly and nitrous acid interacts with Fe(II) complexes to form a nitrous oxide complex, which further decomposes to NO (Coichev and van Eldik, 1994; Zang and van Eldik, 1990). The reactions occurring between SO and NO species V W in the absence or presence of metal ions are also important because these processes lead to a variety of new reactions and products that can have a signi"cant in#uence on atmospheric oxidation processes. Under acidic conditions, typical for the atmosphere, these reactions are further complicated by the production of a series of mixed N}S-oxides (Clifton et al., 1988; Coichev and van Eldik, 1994; Littlejohn et al., 1993). Since NO rapidly oxidises to NO , the reaction of NO with HSO\/SO\ results in the formation of NO\, NO\, SO\ and S O\. In general, Fe(II) ions and complexes promote the interaction between NO or HNO and HSO\/SO\ via a nitrosyl complex. This leads to the formation of HADS (HON(SO )\-hydroxylaminedisulfonate), SA (H N(SO )\-sulfamate), N O and HSO\ and is accom panied by the oxidation of Fe(II) to Fe(III). Completion of the catalytic cycle occurs spontaneously in the presence of HSO\/SO\ (Coichev and van Eldik, 1994; Gei{ler and van Eldik, 1994; Zang and van Eldik, 1993). Additionally, Fe(III) reacts with mixed N}S-oxides. During the "rst step in the reaction sequence a complex is formed, which decomposes to HSO\ and N O (Lepentsiotis et al., 1996). Our aim is to investigate the reactions between NO and SO /air in the absence and presence of dissolved iron in an aqueous solution. Our interest in these reactions concerns their possible role and importance in atmospheric oxidation processes. In this paper we present the in#uence of NO and/or iron on S(IV) oxidation at two di!erent initial pH values. We have also designed a system that enables the introduction of gaseous mixtures (SO /air, NO /air and SO / NO /air), which allows us to monitor the concen tration of SO\, HSO\, NO\ and NO\ and the concentration ratio between Fe(II) and total dissolved iron.
2. Experimental 2.1. Reagents All the reagents used were analytical grade. For the preparation of solutions Milli-Q water was used. Iron(III) and Fe(II) stock solutions were prepared from Fe(ClO ) ) 9H O and (NH ) Fe(SO ) ) 6H O, respec tively. For acidi"cation we used hydrochloric acid. Stock solutions of SO\, NO\ and NO\ were prepared from the corresponding sodium salts, while standards for determining S(IV) were made by dissolving Na S O in formaldehyde and acetate bu!er. The pH of the model solutions was adjusted by adding dropwise 1 mol l\ HCl and 0.5 mol l\ NaOH> 1,10-phenanthroline monohydrate, ammonium acetate, acetic acid and L(#)ascorbic acid were used to determine Fe(II) and total dissolved iron. The gas mixture was prepared using SO and/or NO in synthetic air (0.01 vol%) and "ltered air from a compressor.
2.2. Apparatus and procedures The experimental apparatus is shown in Fig. 1 and consists of gas cylinders (SO and/or NO ), an inlet for "ltered air from the compressor, a gas-mixing station, an air bubbler and two #ow meters. The air bubbler is covered with aluminium foil to prevent photochemical reactions. The temperature of the solution is 20$13C and the #ow rate of the gas mixture through the air bubbler is 100 l h\. The pH of the model solution was "rst adjusted to pH 3 or 4 by adding HCl. When studying the e!ects of iron, we added the acidi"ed Fe(III) solution after 1 h of bubbling. The "nal concentration of total iron in solution is 3.6;10\ mol l\. We collected sample volumes (1 ml) of the reaction solution every 30 min, while the system was continuously fed with the gas mixture. A sample of the reaction solution, collected to determine HSO\, was immediately stabilised by adding formaldehyde to prevent further oxidation of dissolved S(IV). We then determined the concentration of HSO\ (as hydroxy methane sulphonate), SO\, NO\ and NO\ by ion-exchange chromatography. The system comprises of a Dionex 4000i chromatograph equipped with a Dionex IonPac AS4A-SC separation column and a conductivity and UV detector (210 nm) for the determination of NO\ and NO\. The eluent is a mixture of 1.8;10\ mol l\ Na CO and 1.7;10\ mol l\ NaHCO . We mea sured the concentration of Fe(II) using a #ow injection analyser (ASIA Ismatec) with spectrophotomeric detection at 512 nm. Measurements are based on the colour reaction between Fe(II) and 1,10-phenanthroline. Total dissolved iron was determined after the reduction of Fe(III) with ascorbic acid.
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Fig. 1. Scheme of the system for introduction of gases (1 } air bubbler, 2 } gas mixing station, 3 } #ow meters, 4 } cylinder with SO or NO , 5 } "ltered air from compressor station, 6,7 } outlets).
To check the purity of the "ltered air from the compressor station, air was bubbled through the model solution (pH 4) for 4 h under the same experimental conditions as those for studying the reactions. The concentrations of HSO\, SO\, NO\ and NO\ were below their limits of detection, proving that "ltered air is suitable for diluting the experimental gases. Since the mass transfer of SO and NO from the gaseous to the aqueous phase and reactant di!usion may limit the kinetics, the dimerisation of NO in both phases and the di!erent activities of all the species present complicate the kinetics in the system, no attempt was made to determine the rate constant for the reaction of NO with S(IV) species in the presence of O and Fe(III). For the systems NO /SO /air and NO /SO /air/Fe(III) the sul phate formation rates were determined, while for the system SO /air/Fe(III) at pH 3 the rate constant is also calculated. However, on the basis of available literature data we give the interpretation of our experimental results by describing the processes involved at the level of elementary reactions.
3. Results and discussion 3.1. Introduction of SO2 /air into the model solutions During the introduction of SO (166, 290 and 415 ppb) at two di!erent initial pH values (pH 3 and 4), almost no SO\ is formed (Fig. 2). The concentration of HSO\ } the dominant dissolved S(IV) species, increases towards the saturation level. Since the solubility of SO at pH 4 is greater than that at pH 3, saturation is reached earlier at pH 3. The saturation equilibrium concentration of
Fig. 2. Time dependence of [HSO\] and [SO\] during the non-catalytic and catalytic reaction. Initial conditions: pH"4, [SO ]"290 ppb, addition of Fe(III) after an hour of bubbling, total [Fe]"3.6;10\ mol l\.
dissolved S(IV) species at the same concentration in the gaseous phase is approximately 10 times higher at pH 4 than at pH 3, agreeing with Seinfeld and Pandis (1998). When Fe(III) is added to the model solution, SO\ forms immediately and increases linearly with time. The concentration of HSO\, however, becomes stabilised at a concentration lower (at pH 3 at about 12% and at pH 4 at about 4% of the saturation level) than the saturation concentration under experimental conditions. The formation of SO\ is more pronounced at pH 4 because SO has a higher solubility and a higher rate constant
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100
Table 1 Oxidation rates d[SO\]/dt during the introduction of SO /air in the presence of Fe(III) under various experimental conditions [SO ] (ppb)
pH
d[SO\]/dt (mol l\ min\)
166 290 415 166 290 415
3 3 3 4 4 4
0.07;10\ 0.13;10\ 0.2;10\ 0.11;10\ 0.2;10\ 0.31;10\
3.2. Introduction of NO2 /air into model solutions
(Table 1). Accordingly, the oxidation rate is higher at increased concentrations of SO in the gas mixture. The rate constant for SO oxidation in the presence of Fe(III) at pH 3 is calculated according to Eq. (1). When the concentration of dissolved HSO\ is stabilised, the dissolution of SO from the gaseous phase equals the consumption of HSO\ by oxidation: d[HSO\] "0NK a(cH \ !c \ )"k c \ c , * &1- &1- 0 &1- $ dt
of the experiment, which agrees with the radical chain mechanism proposed for Fe-catalysed S(IV) oxidation (Warneck et al., 1996). Iron(III) is immediately reduced by HSO\. Because of the continuous introduction of SO , both the concentration of HSO\ and the ratio between [Fe(II)] and [Fe(III)] remain constant.
(1)
where K a is the overall mass transfer coe$cient, * cH \ the saturation concentration of HSO\, c \ the &1 &1concentration of HSO\ during autoxidation, k the rate 0 constant and c the concentration of total dissolved Fe. $ According to the literature, we propose that the reaction rate follows "rst-order kinetics regarding the concentration of dissolved S(IV) and iron (Brandt and van Eldik, 1995; GrgicH et al., 1991; Seinfeld and Pandis, 1998). For calculating the rate constant the interfacial area (a) between bubbles and liquid, and mass transfer coe$cient (K ) are needed. Both depend on the properties of the * gas, the type of reactor, and on the experimental parameters. Usually these are de"ned as a single parameter called the overall mass transfer coe$cient (K a) (Perry et * al., 1984). The K a coe$cient is determined from the * saturation curves measured under the same experimental conditions in the absence of iron. From the saturation curves measured for di!erent SO /air mixtures K a coef * "cient is obtained by "tting the experimental data with the unsteady-state version of the left-hand side of Eq. (1). For data measured at pH 3 the optimal value for K a * was 0.33;10\ s\. Using this value in Eq. (1), the rate constant k is obtained from steady-state data. The cal0 culated value k "0.74;10 mol\ l s\ (pH 3 and 0 ¹"203C) is very close to that determined by GrgicH et al. (1991) for a homogeneous reaction in a semi-batch CSTR reactor. The ratio between the concentration of Fe(II) and total dissolved iron was also monitored. Immediately after the addition of Fe(III), an equilibrium between Fe(II) and Fe(III) is established, and remains constant until the end
We investigated the in#uence of Fe(III) on a disproportionate amount of NO in the aqueous phase using two di!erent concentrations of NO in the gas mixture (0.84 and 1.68 ppm) at pH 3 and 4. Since the solubility of NO in water is very low (H"1.0;10\ M atm\, ¹"298 K, Seinfeld and Pandis, 1998), we used higher concentrations of NO in the gas phase than that ex pected in the urban atmosphere. As the concentration of NO in the gas mixture in creases so does the amount of NO\ and NO\. At a con centration of 0.84 ppm of NO in the gas phase, the concentration of both anions in the aqueous solution at pH 4 is 5;10\ mol l\ after 270 min, while at 1.68 ppm the concentration is doubled. It is evident that the molar ratio between NO\ and NO\ in the aqueous phase at pH 4 is equal as expected from the following reaction:
(2) However, a slower increase in the concentration of NO\ is found at pH 3, which is ascribed to the undisociated HNO . At pH 3 there is approximately 66% less NO\ than at pH 4. HNO is a weak acid with K " 4.6;10\ M (Bambauer et al., 1994) that does not fully dissociate, especially at pH 3 where at equilibrium almost 69% of HNO is undissociated. This reaction is an important potential source of gaseous HNO in the atmosphere during the contact of NO with acidic cloud droplets (Bambauer et al., 1994). The in#uence of Fe(III) is poorly expressed. However, the formation of NO\ is slightly slower in the presence of Fe(III) compared to experiments in which only NO is introduced. Since the Fe(III) solution always contains a low concentration of Fe(II) ()5%), the reason for this observation could be in the reaction between Fe(II) and HNO /NO\, where NO is formed (Coichev and van Eldik, 1994; Zang and van Eldik, 1990), and blown out by the intensive bubbling. 3.3. Introduction of SO2 NO2 /air into model solutions 3.3.1. The SO2 /NO2 /air system The SO /NO /air mixture comprised of 415 ppb SO during all the experiments, and 0.84 and 1.68 ppm of
J. Turs\ ic\ et al. / Atmospheric Environment 35 (2001) 97}104
Fig. 3. Time dependence of [HSO\], [SO\], [NO\] and [NO\] during the introduction of gas mixture SO /NO / air. Initial conditions: pH"4, [SO ]"415 ppb, [NO ]" 1.68 ppm.
NO . After an induction period, the formation of SO\ accelerates in the presence of NO , showing a linear increase with time (Fig. 3). The concentration of dissolved S(IV) reaches a constant value depending on the
101
pH and concentration of trace gasses in the gas mixture and is close to the saturation level. The actual mechanism of the reaction between NO and SO in an aqueous solution is complex. The absorp tion of NO in water leads to the formation of NO\ and NO\ ions (reaction (2)). In acidic conditions typical for the atmosphere, HNO /NO\ reacts with HSO\/SO\ in aqueous solution to produce a series of mixed N}Soxides (Scheme 1) (Gei{ler and van Eldik, 1994; Oblath et al., 1981, 1982), which either decompose or undergo further reactions with HSO\ to produce "nal pro ducts (Coichev and van Eldik, 1994). The reaction of HNO /NO\ with HSO\ exhibits a characteristic pH dependence in agreement with HNO being the main reactive N(III) species in aqueous solution. HNO is ca. 10 times more reactive than NO\ (Zang and van Eldik, 1990). However, Oblath et al. (1981, 1982) found that the rates of these reactions are too small for them to occur to any signi"cant extent in the boundary layer. In the presence of oxygen, the free radicals produced by the reaction of NO with HSO\/SO\ (Nash, 1979) (reaction (3)) initiate sulphite oxidation in a chain mechanism (Huie and Neta, 1984; Littlejohn et al., 1993; Shen and Rochelle, 1998): NO #HSO\/SO\PNO\#SO\#H>.
Scheme 1. Interactions between HNO /NO\ and HSO\ under acidic conditions (Coichev and van Eldik, 1994).
(3)
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Table 2 Oxidation rates d[SO\]/dt during the introduction of SO /NO /air in the absence and presence of Fe(III) after induc tion period at various experimental conditions [SO ] (ppb)
[NO ] (ppm)
415 415 415 415 415 415
1.68 1.68 1.68 1.68 0.84 0.84
[Total Fe] ;(10\ mol l\)
3.6 3.6 3.6
pH
d[SO\]/dt (mol l\ min\)
4 4 3 3 4 4
0.07;10\ 0.39;10\ 0.02;10\ 0.22;10\ 0.03;10\ 0.36;10\
These reactions are expected to occur in the mass transfer boundary layer with the net result being every mole of NO absorbed leads to the oxidation of 4 or more moles of HSO\/SO\ in the presence of oxygen. Importantly, NO is not a catalyst for S(IV) oxidation, since it is not regenerated in the same way that metal ions are (Littlejohn et al., 1993). The production of SO\ is faster at pH 4 and increases with the increasing concentration of NO in the gas mixture (Table 2). Since the distribution and reactivity of S(IV) species are in#uenced by pH, it is reasonable to obtain higher reaction rates at pH 4. The higher concentration of NO in the gas mixture results in an increase in the concentration of dissolved NO and HNO /NO\ in the model solution and consequently in the oxidation rate of HSO\. On the basis of our results and available literature data we suggest that in the presence of oxygen the important part of HSO\ oxidation with NO proceeds through a free-radical chain mechanism. With the addition of SO in the NO /air system, a signi"cantly lower concen tration of NO\, which is formed only via reaction (2), is observed. Thus, in the boundary layer a part of the dissolved NO does not dissociate but initiates the rad ical chain mechanism via reaction (3). SO\ then reacts fast with dissolved oxygen and SO\ is formed. SO\ is produced through other radical reactions (Littlejohn et al., 1993; Warneck et al., 1996). Our results show that at pH 4, [NO ]"1.68 ppm and [SO ]"415 ppb approx imately 60% and at pH 3 30% of dissolved NO react directly with HSO\. We believe that at selected experi mental conditions the formation of mixed N}S-oxides is also important (especially at pH 3), which could be the reason for the observed induction period for SO\ formation. According to Scheme 1, the reaction route starting with HNO /NO\ and HSO\ produces HON(SO )\ (HADS). One of the reasons for the slow increase in NO\ during the induction period could be the more e$cient consumption of HNO /NO\ at the
beginning. Later, only HSO\ is needed for the formation of intermediates and "nal products. 3.3.2. The SO2 /NO2 /air/Fe(III) system The above experiments were repeated in the presence of Fe(III). The time dependence of SO\, HSO\, NO\ and NO\ during the introduction of a gas mixture SO /NO /air in the presence of Fe(III) is shown in Fig. 4. The sudden formation of SO\, occurring immediately after the addition Fe(III), is almost 10 times faster than in the absence of Fe(III). The rate is greater at pH 4, but the e!ect of the NO concentration in the gas phase is insigni"cant (Table 2). The concentration of dissolved SO is stabilised at lower values and is comparable to that obtained during the introduction of SO /air in the presence of Fe(III). The production of NO\ is similar, while the formation of NO\ is slower when compared with experiments conducted in the absence of Fe(III). An equilibrium between Fe(II) and total dissolved iron, established immediately after the addition of Fe(III), slightly increases during the reaction. An increase in total dissolved iron is observed at pH 4 and is attributable to the dissolution of Fe(III) polymerised hydroxo species, formed during the lowering of the pH by the formation of acidic products (Novic\ et al., 1996). The overall mechanism for HSO\ oxidation in the presence of NO , Fe(III) and dissolved oxygen is com plicated by the many possible routes. Besides the two mechanisms mentioned above, an additional mechanism is the catalytic e!ect of Fe(III) on the S(IV) autoxidation, which appears to be the most important under the experimental conditions. Therefore, the radical chain mechanism can be initiated via reactions (3) and (4): Fe(III)#HSO\PFe(II)#SO\#H>.
(4)
Fig. 4. Time dependence of [HSO\], [SO\], [NO\] and [NO\] during the introduction of gas mixture SO /NO /air in the presence of Fe(III). Initial conditions: pH"4, [SO ]" 415 ppb, [NO ]"1.68 ppm, addition of Fe(III) after an hour of bubbling, total [Fe]"3.6;10\ mol l\.
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1993). However, SO\ radicals formed in NO }S(IV) re action can continue to oxidise S(IV) via radical chain mechanism.
4. Conclusions
Fig. 5. In#uence of Fe(III) and/or NO on the rate of SO\ formation. Initial conditions: pH"4, [SO ]"415 ppb, [NO ]" 1.68 ppm, addition of Fe(III) after an hour of bubbling, total [Fe]"3.6;10\ mol l\.
Alternatively, Fe(II) can promote an interaction between HNO /NO\ and HSO\. This is ascribed to the interac tion of Fe(II) with HNO to produce a nitrosyl complex that can react with HSO\ according to the Boedeker reaction, where mixed N}S-oxides are formed (Zang and van Eldik, 1993; Coichev and van Eldik, 1994; Gei{ler and van Eldik, 1994). Additionally, Fe(III) and mixed N}S-oxides can react. This can lead to a further complication, because this redox reactions can in principle account for the inhibiting e!ect of S}N-oxides on the Fe-catalysed S(IV) autoxidation (Lepentsiotis et al., 1996). From a comparison of the reaction rates in Table 2, it is obvious that the contribution from the oxidation by NO is low (about 10%). The reason is most likely the low concentration of dissolved NO (+10\ mol l\), which initiates the radical chain mechanism in the boundary layer. Alternatively, reactions between N(III), which is present at a concentration level of 10\ mol l\ in aqueous solution, and S(IV) are slow. Fig. 5 represents a summary of our investigations } the in#uence of Fe(III), NO and combination of both on S(IV) oxidation at pH 4 in the presence of oxygen in aqueous solution. It is evident that the reaction rate in the presence of NO and Fe(III) is equal to the sum of the reaction rates when both components are present separately. The sum of the reaction rates can also be seen at pH 3 (Table 2). From these results it seems that under selected experimental conditions the SO /NO /air/Fe(III) system behaves as two separate systems: SO /NO /air and SO /Fe(III)/air. It should be pointed out that our experimental conditions are similar to those in a polluted atmosphere. It is calculated that the contribution made by the direct reaction of NO with S(IV) is comparable to the contribution made by the H O }S(IV) reaction only at pH 6, which is not typical for atmospheric conditions (Littlejohn et al.,
Our experiments of SO oxidation in the presence of dissolved oxygen in an aqueous solution performed at pH 3 and 4 and under conditions typical for polluted atmosphere showed that Fe(III) and NO accelerate the reaction. The formation of SO\ is the most pronounced in the presence of a SO /NO /air gas mixture and Fe(III) in solution. However, the most important is the catalytic e!ect of Fe(III). The rate constant for the system SO /air/Fe(III) at pH 3 is similar to the rate constant for the homogenous reaction in a semi-batch CSTR reactor (GrgicH et al., 1991). Moreover, the reaction rate in the presence of both components is equal to the sum of the reaction rates when NO and Fe(III) are present separately, indicating that under selected experimental conditions the SO / NO /air/Fe(III) system behaves as two separate systems: SO /NO /air and SO /Fe(III)/air, which are unlikely to interact with each other. Thus, the radical chain mechanism can be initiated via a reaction between Fe(III) and HSO\, and by an NO reaction with SO\/ HSO\. Alternatively, Fe(II) can promote an interaction between HNO /NO\ and HSO\. Under nighttime conditions with low H O and O , the aqueous-phase interaction of NO with S(IV) may considerably contribute to sulphate formation. Further, the presence of dissolved iron (as Fe(II) or Fe(III)) can additionally contribute to the formation of sulphuric acid. Our preliminary experiments with atmospheric aerosol particles of industrial and urban origin show that the SO\ formation, due to the reaction of NO with S(IV) in the presence of oxygen, is important. It is known that organic compounds (especially oxalate) inhibit Fecatalysed S(IV) autoxidation (GrgicH et al., 1998, 1999). The concentration of oxalate determined in aerosol particles is comparable to that of dissolved Fe. Therefore, we suppose that the main contribution to the sulphate formation is NO }S(IV) reaction, where SO\ radicals are generated.
Acknowledgements The authors wish to thank Gorazd Berc\ ic\ from National Institute of Chemistry, Ljubljana for his helpful discussions. The "nancial support of the Ministry for Science and Technology of Republic of Slovenia (contract no. P1-0511) is gratefully acknowledged. A part of this work has been done in the frame of AEROSOL/ EUROTRAC-2 project.
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