Enhancement of Gaseous Iodine Emission by Aqueous Ferrous Ions

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Enhancement of Gaseous Iodine Emission by Aqueous Ferrous Ions during the Heterogeneous Reaction of Gaseous Ozone with Aqueous Iodide Yosuke Sakamoto, Shinichi Enami, and Kenichi Tonokura J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp308407j • Publication Date (Web): 13 Mar 2013 Downloaded from http://pubs.acs.org on March 17, 2013

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The Journal of Physical Chemistry

Enhancement of Gaseous Iodine Emission by Aqueous Ferrous Ions during the Heterogeneous Reaction of Gaseous Ozone with Aqueous Iodide Yosuke Sakamoto†,¶,*, Shinichi Enami‡,§,# and Kenichi Tonokura†,±

†

Department of Chemical Systems Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo 113-0033, Japan ‡

§

The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan

Research Institute for Sustainable Humanosphere, Kyoto University, Uji 611-0011, Japan #

PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan

±

Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, Chiba 277-8563, Japan *To whom correspondence should be addressed. E-mail: [email protected]. Tel: +81-11-706-4581

1 Environment ACS Paragon Plus


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PRESENT ADDRESS ¶

Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0610, Japan

KEYWORDS Marine boundary layer, halogen activation, Fenton-like reaction, ozonation, sea surface, sea-salt aerosols

ABSTRACT Gaseous I2 formation from the heterogeneous reaction of gaseous ozone with aqueous iodide in the presence of aqueous ferrous ion (Fe2+) was investigated by electron impact ionization mass spectrometry. Emission of gaseous I2 increased as a function of the aqueous FeCl2 concentration and the maximum I2 formation with Fe2+ was about 10 times than without Fe2+. This enhancement can be explained by the OH- scavenging by Fe3+ formed from Fe2+ ozonation to produce colloidal Fe(OH)3. This mechanism was confirmed by measurements of aqueous phase products using a UV-Vis spectrometer and an electrospray ionization mass spectrometer. We infer that such a pH-buffering effect may play the key role in general halogen activations.

INTRODUCTION Recent field observations, atmospheric box models, and laboratory experiments have revealed that activated halogens play vital roles in the global environment1-11. Current halogen-ozone


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chemistry, for example, corrects atmospheric box models that underestimated the extent of tropospheric ozone loss by ~50 %.11 Among the halogens, reactive iodine species most effectively deplete tropospheric ozone levels, control the HOx/NOx cycle, and possibly generate cloud condensation nuclei via OIO formation12-16. Large amounts of iodine monoxide radical, IO, have been detected in the open ocean marine boundary layer (MBL), i.e. far from biogenic sources, which indicates that iodine chemistry doubtlessly plays a global role in the lower Atmosphere.11 Gaseous inorganic iodine species IX (X = I, Br, and Cl) have much shorter lifetimes than organic iodohydrocarbons because they absorb longer wavelength visible light and are photodissociated. Consequently, they more effectively deplete O3 via I atom/IO radical cycles. It is now apparent that inorganic iodine sources are more important than organic iodohydrocarbons, which were previously considered the major iodine source in the MBL.1,5,17,18 I2 emission caused by the deposition of O3 onto the sea surface/sea-salt aerosols is one of possible sources of gaseous iodine which may explain the discrepancy between field observations and atmospheric models, where models that do not include inorganic halogen sources cannot explain persistently high concentrations of IO over the open ocean.1 Iodine molecule (I2) emission via the heterogeneous reaction of gaseous ozone (O3(g)) with aqueous iodide (I–) was first proposed by Garland and Curtis.19 Subsequently, the reaction mechanism, including formation of the IOOO– interfacial intermediate, was proposed by Sakamoto et al. after directly monitoring the concentrations of I2 and the IO radical in the gas phase18.

I- + O3(g) → IOOO-

(1)

IOOO- → IO- + O2

(2)

IOOO- →→ IO + products

(3)

-

(4)

IO + H

+

↔ HOI (pKa = 10.8) -

+

HOI + I + H ↔ I2 +H 2 O


(5)


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I 2 → I 2(g) -

I 2 + I → I3

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(6) -

(7)

Enami et al. reported that the reaction of I– with O3(g) at the air-water interface will initiate, and hence become the gateway of, the halogen activation by showing the rapid IBr2– and ICl2– formations with the use of interface-specific electrospray mass spectrometry of mixed aqueous halides microjets exposed to O3(g).5,20,21 In a related study based on interface-specific glancingangle Raman spectroscopy, Wren and Donaldson found that the OH Raman signals generated on water surfaces exposed to O3(g) were attenuated by dissolved I– with a Langmuir-Hinshelwood dependence.22 This phenomenon indicates that I3–, a product of iodide ozonation, physically inhibits OH signals from the water’s surface. Several reports show that the emission of I2 via I– + O3 heterogeneous reactions is affected by the presence of co-solutes. Hayase et al. reported reactive phenolate ions suppress I2 emission from the ozone-iodide heterogeneous reaction, while undissociated phenols have no effect.23 Reeser and Donaldson reported that octanol blocks I2 release into the gas phase, although it accelerates the reactions in the aqueous bulk phase.24 They also reported that the total production of iodine displays a Langmuir-Hinshelwood dependence on the concentrations of bulk aqueous I– and O3(g). Rouvière et al. reported that long straight chain fatty acids (≥C15) block the reaction of deliquesced potassium iodide aerosol particles with O3(g).25 By contrast, Hayase et al. reported amphiphilic organic weak acids enhanced this reaction even though these species themselves are inert against O326,27. This is because weak acids work as efficient proton donors and supplying protons that are consumed in reactions (4) and (5), which implies that co-solutes acting as efficient pH-buffer would generally enhance halogen activation. They also found the


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enhancement effect by amphiphilic organic weak acid depends on their interfacial affinity implying interfacial reactions should have a contribution to I2 release from I– + O3 heterogeneous reaction.26 Note the experimental observation that acetic acid never enhances the I2(g) emission while surface-active hexanoic acid and octanoic acid do so, where these three carboxylic acids have the same pKa ~ 4.8 and the only difference is the interfacial affinity caused by the alkyl chain length.26 Iron is one of the most abundant metals on Earth and plays vital roles in atmospheric chemistry, environmental chemistry and biology. Approximately 50–90 % of soluble iron exists in the ferrous state in atmospheric aerosols28-30, and this will deposit on the seawater surface by mineral aerosol deposition.31 This Fe2+ can produce Fe3+ by reaction with O332-36.

Fe2+ + O3 → Fe3+ + O3-

(8a)

Fe2+ + O3 → FeO2+ + O2

(8b)

and/or,

The rate coefficient for reaction (8a) was reported to be (1.7 ± 0.4) × 105 36 or ≥ 5 × 105 M-1 s-1 33

. The rate coefficient for reaction (8b) was to be (8.2 ± 0.3) × 105 M-1 s-1 35. When H+ is

consumed, Fe3+ may maintain the pH by scavenging OH– and by converting it into Fe(OH)3 as follows; 37

Fe3+ + 3OH - → Fe(OH)3 3+

(9a) +

Fe + 3H 2O ↔ Fe(OH)3 + 3H .


(9b)


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Therefore, Fe2+ is expected to enhance I2 emission via the ozone-iodide heterogeneous reaction by regulating the availability of protons as proposed by Hayase et al.

26,27

. In this study, we

examined the enhancement of I2 emission from the heterogeneous reaction of O3(g) with I– in the presence of Fe2+, and its mechanism, by measuring the products in both gas and aqueous phases.

EXPERIMENTAL Reaction products in both gas and aqueous phases were measured in separate experiments. Figure 1 shows a schematic diagram of the present experimental setup for measurement of gas phase products with an electron ionization mass spectrometer (EI-MS) at The University of Tokyo. The gas/liquid interaction cell consisted of a Pyrex glass container (length × width × height; 40 × 140 × 30 mm), which was evacuated by a combination of a liquid N2 trap and an oil rotary pump to maintain a total pressure of 12 kPa (= 90 Torr). The pressure was monitored with an absolute pressure gauge. All experiments were conducted at room temperature. The reaction cell was filled with sample solution to 5 mm above the bottom of the cell. The sample inlet was 2 mm above the solution surface. To minimize possible secondary reactions, a freshly prepared solution was always used for measurement of each data point. O3(g) was produced from O2 (purity >99.995 %) flowing at 1.0 standard liters per minute through a high pressure discharge ozonizer, which was constructed in our laboratory. O3(g) production was monitored by UV absorption by a 253.7 nm Hg lamp before entry into the gas/liquid interaction cell. The gas flow rate was regulated by mass flow controllers. The gaseous reaction products were introduced into a quadrupole mass spectrometer (M-400QA-M, mass resolution of ∆m/z = 1, Canon Anelva Kawasaki, Japan) through a sample inlet consisting of tandem quartz tubing (ø < 0.1 mm) and a


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stainless steel skimmer (ø 0.3 mm). The vacuum chamber was differentially pumped by a rotary pump, and two secondary turbo molecular pumps of 500 and 50 L s-1 (see Fig. 1). The electron impact energy for the electron ionization source was 70 eV, and it was installed 18 cm downstream from the sample inlet. Aqueous phase measurements were performed separately from gas phase measurements using a UV-Vis spectrometer (Agilent 8453, Agilent Technologies, Santa Clara, CA) with a measurement cell of 1 cm optical length and an electrospray ionization mass spectrometer (ESIMS) (Agilent 6130 Quadrupole LC/MS Electrospray System). These measurements were performed at Kyoto University. Ozone was produced from O2 (purity >99.995 %) flowing at 1.0 standard liters per minute through a high pressure discharge ozonizer (KSQ-050, Kotohira, Nagano, Japan). For the UV-Vis and ESI-MS measurements, the sample solution (5 mL) was placed in a glass beaker and ozone was bubbled through (for UV-Vis) or flowed over (for ESIMS) the solution. After a set time, the sample solutions were then analyzed by UV-Vis or ESIMS. The pH of a sample solution before and after ozonation was also measured with a calibrated pH meter (F-74S, Horiba, Kyoto, Japan). All experiments were repeated to confirm the reproducibility. Sodium iodide (NaI, Merck KGaA, Darmstadt, Germany), NaCl (Wako Pure Chemical Industries, Osaka, Japan), FeCl2·4H2O (Wako Pure Chemical Industries), FeCl3·6H2O (Wako Pure Chemical Industries), HCl (Wako Pure Chemical Industries) and ultra-pure deionized water were used for sample solution preparation.

RESULTS AND DISCUSSION



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Gaseous reaction products from the heterogeneous reaction of O3(g) with aqueous NaI were measured as a function of the ferrous chloride (FeCl2) concentration by EI-MS. Figure 2 shows typical differential spectra for the products obtained with and without FeCl2. Each spectrum is the result of subtracting the spectrum recorded before the reaction began from that recorded 40 s later. For comparison, two sample solutions with Fe2+ ([NaI] = 30 mM and [FeCl2] = 15 m M) and without Fe2+ ([NaI] = 30 mM and [NaCl] = 30 mM) were prepared and measured in the presence of O3(g). An ion signal at m/z = 48 was observed for O3+ assigned to O3(g). Ion signals at

m/z = 64, 127 and 254 were observed for I2+, I+ and I2+, respectively, and these species originated from I2(g) released from the rapid O3(g)-I– interactions. The signals at m/z = 64, 127 and 254 behaved the same with changes in pH, [O3(g)] and [I–], which confirmed their source as described above. No signals were detected for HOI, ICl, IO radical, or other iodine containing compounds under the present conditions even with low electron impact energy (20 eV). This excluded the possibility that the observed I2+, I+ and I2+ signals were generated from the fragments of such species. The IO radical has previously been observed by cavity ring-down absorption spectroscopy as a minor product; the observed [IO(g)] was lower than 0.01 × [I2(g)].18 However in the present study, the concentration of the IO radical would be below the detection limit. Figure 3 shows the I+ and I2+ signals at the 40 s after reaction began as a function of [FeCl2], where these ions are from the ionization of I2(g) emitted from the heterogeneous reactions. These positive ion signals increased as a function of [Fe2+] by up to ~ 10 times compared to of the signals without Fe2+. The [Cl–] was kept constant by adding NaCl so the effect of Cl– on the reactions could be ignored. We confirmed the effect of Na+ and Cl– on I2 emission was minimal in a separate experiment with NaCl addition to the sample. These results clearly show that Fe2+ is a dominant contributor to enhancement of the I2 emission during the heterogeneous reaction of


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O3(g) with I–. The dependency on [Fe2+] shown in Fig. 3 indicates that Fe3+ formed by the reaction (8) scavenges OH–, which acts as a buffer to maintain the pH and sustain I2 formation. We observed that a sample containing both I– and Fe2+ formed Fe(OH)3 colloids, and the pH of the solution was unchanged during the ozonation in the separate experiments (see Table 1). The schematic diagram of this enhancement mechanism is shown in Scheme 1. The O3(g) signal (m/z = 48) was unchanged by adding Fe2+ compared to those of I2+, I+ and I2+ as shown in Fig. 2 implying the increase of I2 emission is not caused by an increase in O3 uptake. This observation is consistent with the proposed mechanism. Enhancement of I2 formation was also observed in the aqueous phase measurements by UV-Vis spectrometry. Figure 4 shows the typical UV-Vis spectra of I2 (centered at 460 nm) and I3– (centered at 290 and 350 nm) from samples with Fe2+ ([NaI] = 0.5 mM, [FeCl2] = 0.5 mM) and without Fe2+ ([NaI] = 0.5 mM, [NaCl] = 1.0 mM) after the reaction with O3(g). The spectra of I3– and I2 were recorded 5 s and 10 s after the reaction began, respectively. The concentrations of I3– (2.6 µM, at 5 s) and I2 (8.1 µM, at 10 s) in the absence of Fe2+ and of I3– (25 µM, at 5 s) and I2 (270 µM, at 10 s) in the presence of Fe2+ were estimated by subtracting the UV-Vis spectra of a sample solution of FeCl2 (0.5 mM) reacted with O3(g) for the same reaction time, and using the following equations proposed by Morrison et al. 38;

A350 -0.035 × [I 2 ] A ≈ 350 21.9 21.9 A460 - 0.0397 × A350 [I 2 ] = 0.699 .

[I3- ] =

(10) (11)

Here, the concentrations are in mM, A is the absorbance at each wavelength, and the optical length is 1 cm. I2 formation was enhanced approximately 30 times in the presence of Fe2+ in the



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aqueous phase. This led to enhancement of I2 release into the gas phase. The temporal decrease of I3– and the increase of I2 in the aqueous phase is explained by the decrease of I– after subsequent reactions occurred because I3– is in equilibrium with I– and I2 (Eq. (7)). Aqueous phase measurements using an ESI-MS provided more detail on the enhancement mechanism. Figure 5 shows an ESI-MS spectrum of a solution ([NaI] = 0.05 mM and [FeCl2] = 0.05 mM) with O3(g) at 40 s after the reaction initiation. In the ESI-MS measurements, signals originating from I– (m/z = 127), IO3– (m/z =175), I3– (m/z = 381), Fe2+ (m/z = 161 as FeCl3-) and Fe3+ (m/z = 196 as FeCl4-) were observed. Shoulder peaks observed for FeCl3– and FeCl4– displayed the multiplets expected from

35, 37

Cl isotopic composition. The observation of FeCl4–

signals confirms Fe3+ formation by reaction (8a). Relative interfacial affinities, that contribute to signal intensities in the ESI mass spectra, of I3- and IO3- compared to I- were previously determined to be I3-/I- = 0.76 and IO3-/I- = 0.80, respectively.5 Figure 6 shows time profiles of changes in the I–, IO3– and I3– concentrations for sample solutions with Fe2+ ([NaI] = 0.05 mM and [FeCl2] = 0.05 m M) and without Fe2+ ([NaI] = 0.05 mM and [NaCl] = 0.10 mM) as a function of the ozonation time. We observed a strong enhancement of I3– formation that was consistent with the results from EI-MS for the gaseous products and also consistent with the results obtained by UV-Vis spectrometry. Under the present conditions with a high [O3(g)]/[I–] ratio, the reaction path to produce IO3– from HOI/IO– 39

is dominant over the reaction (5).

HOI + 2O3 → IO3- + 2O2 + H + -

-

IO + 2O3 → IO3 + 2O2


(12) (13)

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As seen in reactions (12) and (13), IO3– formed by the reaction of I– + O3 does not consume H+. Figure 6 shows that the consumption rate of I– is almost the same with and without Fe2+, and a small discrepancy between the two conditions may occur because of complex formation of Iwith Fe2+/Fe3+. By contrast, I3– production was dramatically enhanced by the presence of Fe2+. The production of IO3– was almost the same with and without Fe2+. These results agree with the reaction mechanism proposed here (Scheme 1) because both the initial reaction rate for I– + O3(g) and the production rate of IO3– are expected to be independent of the pH, and only I3– formation needs protons (pH dependent). Figure 7 shows the time profile of Fe3+ (detected as FeCl4–) concentration changes for sample solutions with I– ([NaI] = 0.05 mM, and [FeCl2] = 0.05 mM) and without I– ([FeCl2] = 0.05 mM) in the presence of O3(g). The Fe3+ slowly increased in the presence of I–, since O3(g) preferentially reacted with I- than Fe2+ , and also Fe3+ was rapidly converted into Fe(OH)3. This result is also consistent with the proposed mechanism. Finally, enhancement of I2 emission in the presence of Fe3+ was examined. Figure 8 shows the EI-MS time profile of I2 emission into the gas phase from sample solutions with Fe3+ ([NaI] = 30 mM and [FeCl3] = 15 mM, pH = 2.6) and without Fe3+ ([NaI] = 30 mM, pH = 2.6 adjusted with HCl) in the reaction with O3(g). I2 emission was enhanced by about 10 times by addition of inert Fe3+, which strongly supports the present mechanism. The reaction of Fe3+ with I– 40-42 may enhance I2 formation.

Fe3+ + I- → Fe 2+ +

1 I2 2 .

(14)

This reaction has first order dependence on Fe3+ and second order dependence on I–, thus has a third order rate coefficient of 16–30 M-2s-1.40,42 By comparison, the rate coefficient in bulk for 11 Environment ACS Paragon Plus


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the reaction of O3(g) with I– is 109 M-1 s-1.43,44 Therefore, under our experimental conditions where [I–] = 0.05–30 mM, [Fe3+] ≤ [Fe2+] = (0.05–15 mM), and pH = 4–6, reaction (14) cannot compete with the reaction of I– with O3 and hence is negligible. Another possible mechanism to enhance I2 emission includes the OH radical, which should be produced in reaction (8).32-36 This secondary I2 production mechanism is detailed below.45-47

O3 - ↔ O 2 + O -

(15)

O - + H + → OH -

(16)

OH + I → I + OH

-

(17)

I + I- → I2-

(18)

2I2- → I2 + 2I-

(19)

The effect of the OH radical was examined by adding excess tert-butanol as an effective OH scavenger48. Figure 9 shows the ESI-MS spectra of sample solutions with tert-butanol ([NaI] = 0.05 mM, [FeCl2] = 0.05 mM and [tert-butanol] = 4 mM) and without tert-butanol ([NaI] = 0.05 mM , [FeCl2] = 0.05 mM ) 20 s after reaction with O3. Compared to without tert-butanol, the observed loss of I– and the production of I3– were almost unchanged with tert-butanol. This indicates that OH has a minor (less than 10 %) contribution to I2 formation under these conditions.

SUMMARY In this study, enhancement of I2 production by aqueous Fe2+ during the heterogeneous reaction of gaseous ozone with aqueous iodide was observed both in gas and aqueous phases. A mechanism


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for pH-buffering of Fe3+ formed by Fe2+ ozonation is proposed (Scheme 1). The OH–-scavenging effect by Fe3+ was found to be the most important factor for the enhancement. This enhancement mechanism is similar to that reported by Hayase et al. where weak undissociated organic acids buffer the pH and sustain persistent I2 production26,27. Our enhancement mechanism is likely to be applicable to activation of other halogens since reactions of ozone with bromide (Br–) and chloride (Cl–) proceed via similar mechanisms to produce bromine (Br2) and chlorine (Cl2), respectively.6,44,49,50 In fact, enhancement of Br2 formation from the heterogeneous reaction of O3(g) with synthetic and natural sea-salt was attributed to unknown minor components, that may include ferrous and/or ferric ion.51 More directly, the similar enhancements of Br2 and Cl2 formation from the heterogeneous reaction of O3(g) with NaCl powder and synthetic sea-salt by adding Fe3+ have been reported.52 We infer that pH-buffering effect is the key for general enhancement mechanism in halogen activation. We note whether the same enhancement of halogen activation by pH buffering species occurs under atmospherically relevant condition (i.e., much lower [I–] and presence of other halide ions (Br– and Cl–)) remains to be tested. Under a lower [I–] (i.e., below µM), not only I2 but also HOI may be emitted to gas phase during the heterogeneous I--O3(g) reaction, as proposed by Carpenter, Plane and co-workers.53 Actual seawater contains halide ions X– (= I–, Br– and Cl–) in very different concentrations, e.g., [Cl–] : [Br–] : [I–] = 550 : 0.85 : 0.001,4 and their reactions are linked each other.5,6 Even in such a condition, halogen activation will be initiated by the reaction of I– with O3(g) due to >107 times larger rate coefficient than that of Br–/Cl– + O3. Then, reactive halogen release will be catalytically assisted by I– through the reaction cycle.5 HOI + 2X - + H + ↔ IX 2 -

(20)

IX 2 − ↔ I- + X 2 (X = Cl, Br, I)

(21)



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Considering that the similar rate coefficients of the reactions of HOI with halide ions in bulk solution, where k(HOI + I– + H+) = 4.4 × 1012 M-2 s-1, k(HOI + Br– + H+) = 3.3× 1012 M-2 s-1 and

k(HOI + Cl– + H+) = 3.3× 1010 M-2 s-1,14 and the large concentration difference4, release of chlorine and bromine would be also important. Finally, natural pH-buffering species such as carbonate/bicarbonate and phosphates that are observed in seawater/aerosols should take part in the similar enhancement mechanism. Therefore, further work in this regard is required.

ACKNOWLEDGMENT Y.S. was supported by a research fellowship from the JSPS for young scientists. This work was partly supported by the Japan Science and Technology Agency (JST) PRESTO program.


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REFERENCES (1) Mahajan, A. S.; Plane, J. M. C.; Oetjen, H.; Mendes, L.; Saunders, R. W.; SaizLopez, A.; Jones, C. E.; Carpenter, L. J.; McFiggans, G. B. Measurement and Modelling of Tropospheric Reactive Halogen Species over the Tropical Atlantic Ocean. Atmos. Chem. Phys. 2010, 10, 4611-4624. (2) Lee, J. D.; McFiggans, G.; Allan, J. D.; Baker, A. R.; Ball, S. M.; Benton, A. K.; Carpenter, L. J.; Commane, R.; Finley, B. D.; Evans, M.et al. Reactive Halogens in the Marine Boundary Layer (RHaMBLe): the Tropical North Atlantic Experiments. Atmos. Chem. Phys. 2010, 10, 1031-1055. (3) Saiz-Lopez, A.; Mahajan, A. S.; Salmon, R. A.; Bauguitte, S. J. B.; Jones, A. E.; Roscoe, H. K.; Plane, J. M. C. Boundary Layer Halogens in Coastal Antarctica. Science 2007, 317, 348-351. (4) von Glasow, R.; Crutzen, P. J. 4.02 - Tropospheric Halogen Chemistry. In Treatise on Geochemistry; Editors-in-Chief: Heinrich, D. H., Karl, K. T., Eds.; Pergamon: Oxford, 2003. (5) Enami, S.; Vecitis, C. D.; Cheng, J.; Hoffmann, M. R.; Colussi, A. J. Global Inorganic Source of Atmospheric Bromine. J. Phys. Chem. A 2007, 111, 8749-8752. (6) Finlayson-Pitts, B. J. The Tropospheric Chemistry of Sea Salt: A Molecular-Level View of the Chemistry of NaCl and NaBr. Chem. Rev. 2003, 103, 4801-4822. (7) Brown, M. A.; Liu, Z.; Ashby, P. D.; Mehta, A.; Grimm, R. L.; Hemminger, J. C. Surface Structure of KIO3 Grown by Heterogeneous Reaction of Ozone with KI (001). J. Phys. Chem. C 2008, 112, 18287-18290. (8) Rouvière, A.; Sosedova, Y.; Ammann, M. Uptake of Ozone to Deliquesced KI and Mixed KI/NaCl Aerosol Particles. J. Phys. Chem. A 2010, 114, 7085-7093. (9) Oldridge, N. W.; Abbatt, J. P. D. Formation of Gas-Phase Bromine from Interaction of Ozone with Frozen and Liquid NaCl/NaBr Solutions: Quantitative Separation of Surficial Chemistry from Bulk-Phase Reaction. J. Phys. Chem. A 2011, 115, 2590-2598. (10) Clifford, D.; Donaldson, D. J. Direct Experimental Evidence for a Heterogeneous Reaction of Ozone with Bromide at the Air-Aqueous Interface. J. Phys. Chem. A 2007, 111, 9809-9814. (11) Read, K. A.; Mahajan, A. S.; Carpenter, L. J.; Evans, M. J.; Faria, B. V. E.; Heard, D. E.; Hopkins, J. R.; Lee, J. D.; Moller, S. J.; Lewis, A. C.et al. Extensive HalogenMediated Ozone Destruction over the Tropical Atlantic Ocean. Nature 2008, 453, 1232-1235. (12) Saiz-Lopez, A.; Plane, J. M. C.; Baker, A. R.; Carpenter, L. J.; von Glasow, R.; Martin, J. C. G.; McFiggans, G.; Saunders, R. W. Atmospheric Chemistry of Iodine. Chem. Rev. 2012, 112, 1773-1804. (13) Saiz-Lopez, A.; Plane, J. M. C. Novel Iodine Chemistry in the Marine Boundary Layer. Geophys. Res. Lett. 2004, 31, L04112. (14) Vogt, R.; Sander, R.; von Glasow, R.; Crutzen, P. J. Iodine Chemistry and Its Role in Halogen Activation and Ozone Loss in the Marine Boundary Layer: A Model Study. J. Atmos. Chem. 1999, 32, 375-395. (15) Burkholder, J. B.; Curtius, J.; Ravishankara, A. R.; Lovejoy, E. R. Laboratory Studies of the Homogeneous Nucleation of Iodine Oxides. Atmos. Chem. Phys. 2004, 4, 19-34.



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(16) Cotter, E. S. N.; Booth, N. J.; Canosa-Mas, C. E.; Wayne, R. P. Release of Iodine in the Atmospheric Oxidation of Alkyl Iodides and the Fates of Iodinated Alkoxy Radicals. Atmos. Environ. 2001, 35, 2169-2178. (17) Carpenter, L. J. Iodine in the Marine Boundary Layer. Chem. Rev. 2003, 103, 4953-4962. (18) Sakamoto, Y.; Yabushita, A.; Kawasaki, M.; Enami, S. Direct Emission of I2 Molecule and IO Radical from the Heterogeneous Reactions of Gaseous Ozone with Aqueous Potassium Iodide Solution. J. Phys. Chem. A 2009, 113, 7707-7713. (19) Garland, J. A.; Curtis, H. Emission of Iodine from the Sea Surface in the Presence of Ozone. J. Geophys. Res. 1981, 86, 3183-3186. (20) Enami, S.; Vecitis, C. D.; Cheng, J.; Hoffmann, M. R.; Colussi, A. J. Mass Spectrometry of Interfacial Layers during Fast Aqueous Aerosol/Ozone Gas Reactions of Atmospheric Interest. Chem. Phys. Lett. 2008, 455, 316-320. (21) Enami, S.; Vecitis, C. D.; Cheng, J.; Hoffmann, M. R.; Colussi, A. J. Electrospray Mass Spectrometric Detection of Products and Short-Lived Intermediates in Aqueous Aerosol Microdroplets Exposed to a Reactive Gas. J. Phys. Chem. A 2007, 111, 13032-13037. (22) Wren, S. N.; Donaldson, D. J. Glancing-Angle Raman Spectroscopic Probe for Reaction Kinetics at Water Surfaces. Phys. Chem. Chem. Phys. 2010, 12, 2648-2654. (23) Hayase, S.; Yabushita, A.; Kawasaki, M.; Enami, S.; Hoffmann, M. R.; Colussi, A. J. Heterogeneous Reaction of Gaseous Ozone with Aqueous Iodide in the Presence of Aqueous Organic Species. J. Phys. Chem. A 2010, 114, 6016-6021. (24) Reeser, D. I.; Donaldson, D. J. Influence of Water Surface Properties on the Heterogeneous Reaction between O3(g) and I(aq)−. Atmos. Environ. 2011, 45, 6116-6120. (25) Rouvière, A.; Ammann, M. The Effect of Fatty Acid Surfactants on the Uptake of Ozone to Aqueous Halogenide Particles. Atmos. Chem. Phys. 2010, 10, 11489-11500. (26) Hayase, S.; Yabushita, A.; Kawasaki, M.; Enami, S.; Hoffmann, M. R.; Colussi, A. J. Weak Acids Enhance Halogen Activation on Atmospheric Water’s Surfaces. J. Phys. Chem. A 2011, 115, 4935-4940. (27) Hayase, S.; Yabushita, A.; Kawasaki, M. Iodine Emission in the Presence of Humic Substances at the Water’s Surface. J. Phys. Cham. A 2012, 116, 5779-5783. (28) Upadhyay, N.; Majestic, B. J.; Herckes, P. Solubility and Speciation of Atmospheric Iron in Buffer Systems Simulating Cloud Conditions. Atmos. Environ. 2011, 45, 1858-1866. (29) Zhuang, G.; Yi, Z.; Duce, R. A.; Brown, P. R. Link between Iron and Sulphur Cycles Suggested by Detection of Fe(n) in Remote Marine Aerosols. Nature 1992, 355, 537-539. (30) Hand, J. L.; Mahowald, N. M.; Chen, Y.; Siefert, R. L.; Luo, C.; Subramaniam, A.; Fung, I. Estimates of Atmospheric-Processed Soluble Iron from Observations and a Global Mineral Aerosol Model: Biogeochemical Implications. J. Geophys. Res. 2004, 109, D17205. (31) Bruland, K. W.; Orians, K. J.; Cowen, J. P. Reactive Trace Metals in the Stratified Central North Pacific. Geochim. Cosmochim. Acta 1994, 58, 3171-3182. (32) Yang, T. C.; Neely, W. C. Relative Stoichiometry of the Oxidation of Ferrous Ion by Ozone in Aqueous Solution. Anal. Chem. 1986, 58, 1551-1555. (33) Hoigné, J.; Bader, H.; Haag, W. R.; Staehelin, J. Rate Constants of Reactions of Ozone with Organic and Inorganic Compounds in Water—III. Inorganic Compounds and Radicals. Water Res. 1985, 19, 993-1004.


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(34) Hart, E. J.; Sehested, K.; Holoman, J. Molar Absorptivities of Ultraviolet and Visible Bands of Ozone in Aqueous Solutions. Anal. Chem. 1983, 55, 46-49. (35) Loegager, T.; Holcman, J.; Sehested, K.; Pedersen, T. Oxidation of Ferrous Ions by Ozone in Acidic Solutions. Inorg. Chem. 1992, 31, 3523-3529. (36) Conocchioli, T. J.; Hamilton, E. J.; Sutin, N. The Formation of Iron(IV) in the Oxidation of Iron(II)1. J. Am. Chem. Soc. 1965, 87, 926-927. (37) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics - From Air Pollution to Climate Change (2nd Edition); John Wiley & Sons, 2006. (38) Morrison, M.; Bayse, G. S.; Michaels, A. W. Determination of Spectral Properties of Aqueous I2 and I3− and the Equilibrium Constant. Anal. Biochem. 1971, 42, 195-201. (39) Bichsel, Y.; von Gunten, U. Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters. Environ. Sci. Technol. 1999, 33, 4040-4045. (40) Vrkljan, P. B. A.; Bauer, J.; Tomisic, V. Kinetics and Mechanism of Iodide Oxidation by Iron(III): A Clock Reaction Approach. J. Chem. Educ. 2008, 85, 1123-1125. (41) Amankwa, L.; Cantwell, F. F. Measurement of the Rate of Oxidation of Iodide by Iron(III) Using Solvent Extraction. Anal. Chem. 1989, 61, 2562-2566. (42) Laurence, G. S.; Ellis, K. J. Oxidation of Iodide Ion by Iron(III) Ion in Aqueous Solution. J.Chem. Soc. Dalton Trans. 1972, 2229-2233. (43) Garland, J. A.; Elzerman, A. W.; Penkett, S. A. The Mechanism for Dry Deposition of Ozone to Seawater Surfaces. J. Geophys. Res. 1980, 85, 7488-7492. (44) Liu, Q.; Schurter, L. M.; Muller, C. E.; Aloisio, S.; Francisco, J. S.; Margerum, D. W. Kinetics and Mechanisms of Aqueous Ozone Reactions with Bromide, Sulfite, Hydrogen Sulfite, Iodide, and Nitrite Ions. Inorg. Chem. 2001, 40, 4436-4442. (45) Hart, E. J.; Henglein, A. Free Radical and Free Atom Reactions in the Sonolysis of Aqueous Iodide and Formate Solutions. J. Phsy. Chem. 1985, 89, 4342-4347. (46) Merouani, S.; Hamdaoui, O.; Saoudi, F.; Chiha, M. Influence of Experimental Parameters on Sonochemistry Dosimetries: KI Oxidation, Fricke Reaction and H2O2 Production. J. Hazard. Mater. 2010, 178, 1007-1014. (47) Gutierrez, M.; Henglein, A.; Ibanez, F. Radical Scavenging in the Sonolysis of Aqueous Solutions of Iodide, Bromide, and Azide. J. Phys. Chem. 1991, 95, 6044-6047. (48) Tizaoui, C.; Grima, N. M.; Derdar, M. Z. Effect of the Radical Scavenger tButanol on Gas–Liquid Mass Transfer. Chem. Eng. Sci. 2009, 64, 4375-4382. (49) Levanov, A. V.; Kuskov, I. V.; Zosimov, A. V.; Antipenko, E. E.; Lunin, V. V. Acid Catalysis in Reaction of Ozone with Chloride Ions. Kinet. Catal. 2003, 44, 740-746. (50) Levanov, A.; Antipenko, E.; Lunin, V. Primary Stage of the Reaction between Ozone and Chloride Ions in Aqueous Solution: Oxidation of Chloride Ions with Ozone through the Mechanism of Oxygen Atom Transfer. Russ. J. Phys.Chem. A 2012, 86, 519-522. (51) Mochida, M.; Hirokawa, J.; Akimoto, H. Unexpected Large Uptake of O3 on Sea Salts and the Observed Br2 Formation. Geophys. Res. Lett. 2000, 27, 2629-2632. (52) Sadanaga, Y.; Hirokawa, J.; Akimoto, H. Formation of Molecular Chlorine in Dark Condition: Heterogeneous Reaction of Ozone with Sea Salt in the Presence of Ferric Ion. Geophys. Res. Lett. 2001, 28, 4433-4436. (53) Carpenter, L. J.; MacDonald, S. M.; Shaw, M. D.; Kumar, R.; Saunders, R. W.; Parthipan, R.; Wilson, J.; Plane, J. M. C. Atmospheric Iodine Levels Influenced by Sea Surface Emissions of Inorganic Iodine. Nature Geosci. 2013, 6, 108-111.



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Page 18 of 29

FIGURES

Ion source (EI)

500 l s-1 TMP

Q-MS

to RP Ozonizer

to RP

50 l s-1 TMP

MFC Sample solution O2

Figure 1. A schematic diagram of the experimental setup. MFC: mass flow controller, RP: rotary pump, TMP: turbo molecular pump, Q-MS: quadrupole mass spectrometer.


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Signal intensity / arbitrary unit

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2+

with Fe 2+ without Fe

m/z = 127

1.0

+

I 0.5

m/z = 64

m/z = 254 + I2

2+

I 0.0

-0.5

m/z = 48 + O3 50

100

150

m/z

200

250

300

Figure 2. EI-MS differential spectra obtained by subtracting the initial spectra from those recorded 40 s after the reaction began. Blue bars: with Fe2+ ([NaI] = 30 mM, [FeCl2] = 15 mM, [O3(g)] = 3 × 1016 molecule cm-3), Red bars: without Fe2+ ([NaI] = 30 mM, [NaCl] = 30 mM, [O3(g)] = 3 × 1016 molecule cm-3).


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Page 20 of 29

1.2 +

I + I2

1.0 0.8 0.6 0.4 0.2 0.0 0

5

10

15

2+

[Fe ] / mM

Figure 3. I+ and I2+ signals (from gaseous I2 emission) measured by EI-MS as a function of the Fe2+ concentration 60 s after the reaction began. Filled circles: signal intensity at m/z = 127 (I+), filled triangles: signal intensity at m/z = 254 (I2+). [NaI] = 30 mM, [NaCl] = 30 - (2 × [FeCl2]) mM, [O3(g)] = 3 × 1016 molecule cm-3. The plots were fitted by Hill’s equations as a guide.


Page 21 of 29

with Fe2+ without Fe2+

1.0

0.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 250 0.3

300

350

400

with Fe2+ without Fe2+ 0.2

0.1

0.0

400

450

500

550

600

Wavelength / nm

Figure 4. UV-Vis spectra of sample solutions with Fe2+ (solid line, [NaI] = 0.5 mM, [FeCl2] = 0.5 mM and [O3(g)] = 9 × 1016 molecule cm-3) and without Fe2+ (dashed line, [NaI] = 0.5 mM, [NaCl] = 1.0 mM and [O3(g)] = 9 × 1016 molecule cm-3). Upper panel: I3– formation 5 s after the reaction began. Lower panel: I2 formation 10 s after the reaction began.


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3000

2000

m/z = 381 I3-

m/z = 175 IO3m/z = 161 FeCl3-

m/z = 196 FeCl4-

1000 m/z = 127 I0 100

150

200

350

400

m/z

Figure 5. ESI-MS spectrum of a mixture ([NaI] = 0.05 mM and [FeCl2] = 0.05 mM) recorded 40 s after the reaction with O3(g) ([O3(g)] = 9 × 1016 molecule cm-3) began.


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Page 23 of 29

10000

I- with Fe2+ IO3- with Fe2+ I3- with Fe2+ I- without Fe2+ IO3- without Fe2+ I3- without Fe2+

5000

0 0

20

40

60

Time / s

Figure 6. Time profiles for I– (squares), IO3– (circles) and I3– (triangles) measured by ESI-MS in sample solutions with Fe2+ (red, [NaI] = 0.05 mM, [FeCl2] = 0.05 mM and [O3(g)] = 9× 1016 molecule cm-3) and without Fe2+ (black, [NaI] = 0.05 mM, [NaCl] = 1.0 mM and [O3(g)] = 9 × 1016 molecule cm-3).


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Page 24 of 29

1000

500

NaI + FeCl2 FeCl2

0 0

20

40

60

Time /s

Figure 7. Time profile of Fe3+ detected as FeCl4– (m/z = 196) by ESI-MS in sample solutions with I– ([NaI] = 0.05 mM, and [FeCl2] = 0.05 mM) and without I– ([FeCl2] = 0.05 mM) after reaction with O3(g) ([O3(g)] = 9 × 1016 molecule cm-3).


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Page 25 of 29

1.0 Reaction start

0.5

0.0

+Fe

0

10

20

30

40

3+

50

Time / s

Figure 8. Time profile of gaseous I2 emission (I+ signal) measured by EI-MS in sample solutions with Fe3+ (solid line, [NaI] = 30 mM and [FeCl3] = 15 mM, pH = 2.6) and without Fe3+ (dashed line, [NaI] = 30 mM, pH = 2.6 (adjusted by HCl)) in the reaction with O3 ([O3(g)] = 3 × 1016 molecule cm-3).


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Page 26 of 29

10000

with t-butanol without t-butanol

8000 6000 4000 2000 0

100

200

300

400

500

m/z

Figure 9. ESI-MS spectra of sample solutions with tert-butanol ([NaI] = 0.05 mM, [FeCl2] = 0.05 mM and [tert-butanol] = 4 mM) and without tert-butanol (([NaI] = 0.05 mM and [FeCl2] = 0.05 mM) measured 20 s after the reaction with O3 ([O3(g)] = 9 × 1016 molecule cm-3) began.


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TABLES Table 1. Summary of sample solutions before and after ozonation* sample NaI+NaCl FeCl2 NaI+FeCl2

Fe(OH)3 colloids not found not found found

pH before after (60s) 11.0 ± 0.1 5.3 ± 0.4 5.2 ± 0.3 3.5 ± 0.1 4.1 ± 0.1 5.1 ± 0.3

*Ozone was bubbled through samples (50 ml) in a glass beaker under the following conditions: [NaI] = 30 mM, [FeCl2] = 15 mM, [NaCl] = 30 mM and [O3(g)] = 2 × 1016 molecule cm-3



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SCHEMES

Scheme 1. Schematic diagram of the I2(g) enhancement mechanism


Page 28 of 29

Page 29 of 29

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TOC IMAGE

O3(g)

I-

I2(g) +H+, I-

I2

OH-

Fe2+

Fe(OH)3


Enhancement of Gaseous Iodine Emission by Aqueous Ferrous Ions

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