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. 2024 Mar 15;146(12):8327–8334. doi: 10.1021/jacs.3c14040

Quantification and Mechanistic Investigation of the Spontaneous H2O2 Generation at the Interfaces of Salt-Containing Aqueous Droplets

Maria Angelaki 1, Yoan Carreira Mendes Da Silva 1, Sébastien Perrier 1, Christian George 1,*
PMCID: PMC10979748  PMID: 38488457

Abstract

graphic file with name ja3c14040_0006.jpg

There is now much evidence that OH radicals and H2O2 are spontaneously generated at the air–water interface of atmospheric aerosols. Here, we investigated the effect of halide anions (Cl–, Br–, I–), which are abundant in marine aerosols, on this H2O2 production. Droplets were generated via nebulization of water solutions containing Na2SO4, NaCl, NaBr, and NaI containing solutions, and H2O2 was monitored as a function of the salt concentration under atmospheric relevant conditions. The interfacial OH radical formation was also investigated by adding terephthalic acid (TA) to our salt solutions, and the product of its reaction with OH, hydroxy terephthalic acid (TAOH), was monitored. Finally, a mechanistic investigation was performed to examine the reactions participating in H2O2 production, and their respective contributions were quantified. Our results showed that only Br– contributes to the interfacial H2O2 formation, promoting the production by acting as an electron donor, while Na2SO4 and NaCl stabilized the droplets by only reducing their evaporation. TAOH was observed in the collected droplets and, for the first time, directly in the particle phase by means of online fluorescence spectroscopy, confirming the interfacial OH production. A mechanistic study suggests that H2O2 is formed by both OH and HO2 self-recombination, as well as HO2 reaction with H atoms. This work is expected to enhance our understanding of interfacial processes and assess their impact on climate, air quality, and health.

1. Introduction

Biogenic and anthropogenic atmospheric aerosols have a significant impact on climate, air quality, and health.1,2 Sea-salt aerosol particles, generated at the air–sea interface via wave-breaking,3 are considered to be a major source of biogenic atmospheric particles in marine environments. These particles can influence cloud albedo and the Earth’s radiation budget.1,4−7 Primary sea-water droplets consist mainly of inorganic matter, salts, or their ions (Na+, Mg2+, Ca2+, K+, Cl–, Br–, SO42–).8−12 Their size can range from submicron to supermicron sizes, exposing a substantial air/water interface in the atmosphere.13 Such interfaces have been proven to be unique environments for chemical processes, especially for small droplets, due to their high surface-to-volume ratio. Many studies have shown that a great variety of interfacial chemical reactions have rate coefficients up to 100 times greater than the bulk,14−17 while others have focused on the spontaneous reduction of chemical species, i.e., metal ions and organic molecules.18−22

Recently, several studies have reported the spontaneous formation of OH and H2O2 at the air–water interface23−31 in the absence of any external trigger (such as catalysts, chemical precursors, or photons). Kloss,32 while studying the thermodynamics of the OH– anion, suggested that it may partially exist, to a small extent, as an ion pair (OH···e–), which may undergo charge separation (and not oxidation) in the presence of an electric field, producing OH radicals (R1). Interestingly, it has been debated and recently shown that a strong electric field (∼109 V m–1) may exist at the air–water interface, initiating this chemistry.33−35

The spontaneous production of H2O2 in water microdroplets has given rise to a certain amount of controversy in the literature, but finally the various studies could be reconciled by taking into account the various bath gases, the detection limits, and the specific role of electron scavengers.23−31 The mechanistic scheme that leads to the H2O2 production and the fate of the solvated electrons are still under debate. Lee et al.,23,24 Mehrgardi et al.,28 Heindel et al.,25 and Nguyen et al.26 proposed that H2O2 can be formed via self-recombination of OH radicals (R2). Heindel et al.25 and Nguyen et al.26 also reported the formation of atomic hydrogen from the reaction of H+ with the electrons (R3). H atoms may react with OH radicals, adding a competitive pathway to the H2O2 formation, but they can also form H2 via self-recombination (R4 and R5). In contrast, Li et al.27 suggested that appart from OH recombination, H2O2 can be formed via a more complex scheme. Specifically, they proposed that in the presence of oxygen, the electron will be trapped by the O2 resulting in O2–. Eventually, O2– will react with H+ and the recombination of the produced HO2 will lead to H2O2 (R6–R8). While interfacial electrostatic forces are invoked in the production of oxidants, it is important to point out that the solvation dynamics, which has unique dynamical behavior at the interface, could also be influential, affecting redox potentials and especially the O2/O2– couple. Other recent calculations have confirmed that the hydroxide anion can dissociate more easily at the interface due to lower solvation.36,37

1. R1, R-1
1. R2
1. R3
1. R4
1. R5
1. R6
1. R7, R-7
1. R8

While the above reactions seem to be the key to H2O2 production in the presence of an electric field, the composition of the sea-spray aerosols cannot be overlooked. In this study, we investigated how the presence of different halides in aerosols could influence spontaneous H2O2 formation and also their potent involvement in the total mechanistic scheme. Mimicking sea-salt aerosols, we nebulized solutions of Na2SO4, NaCl, NaBr, and NaI, as their ions are the most abundant in sea-water and marine aerosols,8,9,38,39 and we measured H2O2 in the collected droplets. Apart from their atmospheric relevance, these salts are characterized by different ionic activities and also their ability to act as electron donors is expected to vary. Thus, we studied the impact of these parameters on H2O2 production at the interface of salt containing water droplets. Additionally, the effect of the bulk concentration of the salt containing solutions on the H2O2 production was also examined. Finally, a mechanistic investigation was performed to assess the contribution of reactions R2 and R6–R8 to the H2O2 formation and their branching ratio (R2/R6–8) was quantified. The molecular organization and preferential alignment of interfacial water molecules as well as the distributions at different charged interfaces are still debated and certainly beyond the scope of the following experimental approach.40 Our results aim to unravel the impact of marine aerosols on OH/H2O2 formation and provide new insights into this complex mechanism.

2. Results and Discussion

2.1. Spontaneous H2O2 Production at the Interfaces of Aqueous Salt-Containing Droplets

To investigate H2O2 production in aqueous droplets containing different salts, we monitored its concentration in collected droplets generated by nebulizing bulk solutions inside a flow tube reactor. The bath gas was always compressed air unless it is mentioned otherwise. A detailed description of the apparatus, instrumentation, and experimental conditions is provided in the Supporting Information (SI): Text S1, Figure S1. Initially, experiments were performed using Na2SO4, NaCl, and NaBr at a concentration of 2.5 mM to prevent droplet evaporation. The optical (do) and electrical mobility (de) size distributions of the generated droplets were found to be independent of the salt type, with values of 150–900 and 20–400 nm, respectively (Figure S2). In Figure 1, the H2O2 produced in the collected droplets is presented.

Figure 1.

Figure 1

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H2O2 concentration generated in the collected droplets resulting from the nebulization of 2.5 mM Na2SO4 (blue), NaCl (green), and NaBr (yellow). The H2O2 measurements of the bulk solutions are also given. Error bars represent the 2σ and include the estimated systematic uncertainties.

From Figure 1, it is evident that H2O2 is clearly produced only in droplets, suggesting that its production is an interfacial process. The potential bulk H2O2 formation was measured before and after each experiment to ensure that the interface of the bulk stock solutions (used to spray droplets) did not contribute to the observed signal. Na2SO4, NaCl, and NaBr led to a production of [H2O2]Na2SO4 = 56.92 ± 6.82 nM, [H2O2]NaCl = 65.78 ± 7.85 nM, and [H2O2]NaBr = 106.10 ± 20.12 nM, respectively, after ∼10 s (residence time in the reactor). The quoted errors are at the 2σ level and include the estimated systematic uncertainties of the H2O2 analyzer calibration. To ensure that compressed air does not contain any impurities, we performed the same experiments by controlling the flows with a mixture of UHP N2/O2 in a ratio of 80%/20%. No difference in H2O2 production was observed (Figure 2, open symbols).

Figure 2.

Figure 2

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H2O2 concentration measured in microdroplets as a function of the salt concentration, Na2SO4 (blue squares), NaCl (green circles), NaBr (yellow diamonds), NaI (violet triangles) and NaCl/NaBr mixture (black diamonds). Solid and open-colored symbols represent the results obtained in air and N2/O2 (80%/20%) environment, respectively. The bottom axis is split into two concentration regimes i.e., for clarity reasons. Error bars represent the 2σ and include the estimated systematic uncertainties. Dashed lines are just meant to guide the eye.

For the 2.5 mM bulk solution, the production of H2O2 in NaBr containing droplets is 38% higher than NaCl and Na2SO4 whose difference (15%) is within our error limits. A possible explanation for this increase could be that, unlike SO42– and Cl–, the Br– ion has been discussed to be somehow surface active,33,41−43 but recent studies44,45 have refuted this, showing that Br– is evenly distributed within the aqueous phase. To explain our observations, we suggest that, similarly to OH–, Br– exists partially as an ion pair or an ion-neutral pair, which could also undergo charge separation, i.e.,

2.1. R9, R-9

in the presence of an electric field. This is consistent with Guo et al., who recently discussed such processes in the case of iodide.46 The results suggest that the addition of bromide enhances the production of free electrons. This additional production of electrons in the presence of Br– may take place near the surface area, and therefore, the generation of the products is possibly happening within the near-interfacial region.47,48 Once produced close to the interface, the electrons undergo solvation or react directly with dissolved oxygen on a time scale of 100 ns or less. During this period, it might certainly diffuse towed the bulk and react at different penetration depths as recent studies have shown that surface solvated electrons are less stable in the interfacial region than in the bulk.49−51 In such a situation, all reactions involving an electron in the mechanism discussed above (i.e., R3 and R6) will play a pivotal role.

2.2. Investigating the Effect of Concentration on H2O2 Production

NaCl is the dominant salt in both sea-water and sea-spray, with a concentration ranging from ca. 0.5 to 5 M, while Na2SO4, NaBr, and NaI are present at significantly lower levels, from a few nM to mM levels.8,9 To study the dependence of H2O2 production as a function of salt concentration, experiments were carried out in the range of 0.06 to 1300 mM, where our measurements showed the highest sensitivity. The results are presented in Figure 2 and Table S1 in the Supporting Information. Experiments were carried out at the natural pH of the solutions, as given in Figure S3. For the concentration range between 0.06 and 5 mM, the pH decreased less than 2%, which is within the measurement uncertainties. Between 660 and 1300 mM the pH decreased by 15%. To ensure the lower pH at the high salt content did not affect our measurements, one experiment at high concentration was performed by regulating the pH at 6.6 and 6.4, for NaCl and NaBr, respectively. No difference in the H2O2 production was observed.

H2O2 produced at the interface of Na2SO4 and NaCl droplets did not show any dependence on their bulk concentrations. For our experimental conditions and a residence time of ∼10 s, the mean values, derived from the linear fit of the overall data, were measured to be [H2O2]Na2SO4 = 57.71 ± 13.6 nM and [H2O2]NaCl = 65.78 ± 6.84 nM. The corresponding size distributions of the generated particles as a function of the concentration are presented in Figure S4. Only five different concentrations are presented for clarity reasons. The particles showed a gradual growth as the concentration increased. For 2.5–110 mM the population is mainly located between 150 and 900 nm, and only at high salt content (>330 mM), the size distribution tail extended up to 4 μm. The contribution of these bigger particles to the total population was only 2%, and therefore, the H2O2 generation was not affected. Furthermore, no correlation between the ionic strength of the solution and H2O2 production was found. For the concentration range of 0 to 1.2 M, the activity coefficient (γ±) of Na2SO4 is significantly lower (>50%) than that of NaCl and NaBr, the γ± of which is almost the same within a range of 20%.52−54 Also, their γ± decreases for the same concentration range, a trend that was not observed in our experiments.52−54

H2O2 production close to the interfacial region of NaBr containing droplets displayed an increase with concentration, reaching a plateau above 0.66 mM. Such behavior is typical for a so-called Langmuir–Hinshelwood process, possibly indicative of surface processes, where at low concentration a linear trend is observed followed by surface saturation.55 In this particular case, we are associating this trend with the ability of Br– to produce electrons in the near-interfacial region and not to surface properties of the produced electrons. Since Br– is not surface active, we attribute this behavior to the production of electrons close to the interfacial region, which seems to increase as a function of NaBr concentration. To ensure the accuracy of our measurements at low NaBr bulk concentrations and to verify that the observed trend is not induced by droplet evaporation, we performed experiments with mixed NaCl/NaBr solutions. NaCl concentration was kept constant at 2.5 mM, while NaBr varied between 0.4 and 2.5 mM (black symbols in Figure 2). The simultaneous nebulization of the two salts led to identical H2O2 generation as the one that results from NaBr droplets, confirming that no loss of droplets occurred by spraying low concentrations of bulk solution. The results suggest that there is no synergy or inhibition to the total process in the presence of Cl– and Br–, and only Br– is producing solvated electrons, in contrast to Cl–.

Finally, experiments using NaI were carried out (Figure 2). According to Guo et al.,46 iodide can spontaneously produce solvated electrons and atomic-iodine that recombine forming I2 and, finally, through sequential reactions, I2– and I3–. Hence, one would expect enhanced H2O2 production on NaI containing droplets. However, no significant H2O2 formation was observed, [H2O2]NaI = 11.51 ± 1.29 nM when nebulizing 2.5 mM NaI. Furthermore, unlike the other salts, H2O2 decreased with the increase in bulk concentration and was finally suppressed at 330 mM NaI. We attribute the observed decrease in H2O2 concentration to the reaction between I– and H2O2.56 This conclusion is supported by experiments involving bulk H2O2 measurements in H2O2 solutions. The addition of NaI resulted in a noticeable reduction in the initial H2O2 concentration, with the decrease becoming more pronounced as the NaI concentration increased (SI: Text S5, Figure S5). Mulazzani et al.57 and Xing et al.58 also proposed that I– can react with OH radicals rapidly, forming HOI– (with a rate coefficient of k = 1.6 × 1010 M–1 s–1) and atomic-iodine, respectively. This suggests a mechanism where I– scavenges OH radicals, providing an explanation for our results. However, it is worth noting that Br– can also react with OH radicals, at a rate up to four times lower than that of I– (k̅ = (0.4–1.1) × 1010 M–1 s–1).59,60 Despite that this reaction is also fast, the H2O2 production as a function of NaBr did not show a similar decrease, indicating that the loss of H2O2 in NaI droplets is mainly due to the reaction of H2O2 with iodide anions. In contrast, NaBr, as well as Na2SO4 and NaCl, did not show any significant reaction in the bulk with H2O2 (Figure S5).

2.3. Mechanistic Investigation

Our experimental results, along with those of other studies,23−28 provide strong evidence that H2O2 is produced at the air–water interface of aqueous droplets. A series of experiments aimed at enhancing our understanding of the pathways participating in this production, i.e., (pathway A) recombination of OH radicals and (pathway B) HO2 initiated reactions (resulting from electron scavenging by the O2). These results are summarized in the chemical scheme given in Figure 3.

Figure 3.

Figure 3

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Proposed mechanistic scheme for H2O2 production in the interface of aqueous droplets.

We first deactivated pathway B by performing experiments in pure N2, i.e., in the absence of O2. The results are listed in Figure 4. A 91% decrease in the produced H2O2 was measured, for both NaBr and NaCl, indicating that the electron mainly reacts back with the OH radical to produce OH– anions under pure nitrogen (R-1). These experiments confirmed that O2, which is enriched on the droplet surface,61 shifts the equilibrium R1 to the right, via its reaction with the solvated electrons, promoting the OH radical and HO2 formation, and thus of H2O2 via both pathways. Our results align with those of Mehrgardi et al.,28 who noticed a significant increase in the measured H2O2 in the presence of O2, attributing also this behavior to the O2 adsorption at the air–water interface of the droplets. Furthermore, Li et al.27 also reported an enhancement in OH radical production in the presence of O2.

Figure 4.

Figure 4

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H2O2 generated at the interfaces of NaCl (green) and NaBr (yellow) droplets in the presence (synthetic air) and in the absence (N2) of O2.

To investigate the involvement of pathway A, we nebulized salt solutions containing an organic OH radical scavenger, i.e., terephthalic acid (TA), kOH+TA = (4.1 ± 0.1) × 109 M–1 s–1.62 If H2O2 is also formed from OH radical recombination, its production should be minimized in the presence of TA (assuming that both OH and TA are collocated at the interface). The results obtained for 2.5 mM NaCl and NaBr solutions, containing different concentrations of TA in the range of 0.027–0.3 mM are summarized in Figure 5 and Table S2. For this concentration range, the bulk pH showed a significant decrease when the concentration of TA was increased (Figure S6). Li et al. have reported an increase in TAOH production under acidic conditions.27 To avoid the potential impact of the pH on the H2O2 production, the pH of all bulk solutions was kept constant at the value corresponding to 2.5 mM of the salt, i.e., pHNaCl = 6.6 and pHNaBr = 6.4.

Figure 5.

Figure 5

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Mechanistic investigation by deactivating OH recombination. Top: H2O2 as a function of TA concentration observed in the collected droplets of NaCl/TA (circles) and NaBr/TA (diamonds). Solid lines represent the exponential fit used to describe the decrease of H2O2. Middle: % decrease of H2O2 as a function of [TA] for both salts. The decrease observed in the plateau resulted from the linear fit of the data and is given as an inset. Bottom: TAOH production as a function of the TA concentration. Open symbols are the results obtained in the absence of O2 and are not included in the fit of the data.

For both NaCl/TA and NaBr/TA, H2O2 production was decreased as a function of the TA concentration, reaching a plateau (after 0.12 mM TA), denoting that OH is sufficiently scavenged and further increase in the organic does not affect its formation. 2-Hydroxy terephthalic acid (TAOH), a product of the reaction of OH radicals with TA, was monitored by analyzing the collected droplets via fluorescent spectroscopy (Figure S7). As shown in Figure 5, H2O2 and TAOH displayed opposite behaviors, confirming the reaction between the OH radicals and the organic scavenger. TAOH production increased with TA for both salts until 0.12 mM TA, while between 0.12 and 0.17 mM no significant difference was observed. Due to the high interference of the TA fluorescent peak (ITA > ITAOH), accurate quantification of TAOH for higher TA content was not possible. The procedure for the TAOH quantification is described in Text S7 and Figure S8, and the values are given in Table S2. For the first time, TAOH was also monitored in the particle phase by performing online particle fluorescent measurements. These online measurements support our findings in collected droplets. The obtained fluorescent spectra of eight particles of different sizes that were analyzed during one single experiment are presented in Figure S9. The identification of TAOH was made by comparing actual spectra to those from droplets containing TAOH standards. The intensity of the fluorescence varies, implying that parameters such as size and shape affect our measurements, and therefore, these data are only qualitative (Text S7). The intensity of the spectra is almost the same for particles with a diameter between 1.9 and 5.6 μm. It is interesting that the larger particle, d = 8.0 nm, resulted in less TAOH production, which may be attributed to the smaller surface-to-volume ratio.

At the plateau, the decrease in H2O2 was measured, (34 ± 12) and (41 ± 10) % for NaCl and NaBr, respectively. The discrepancies are inside the uncertainties of the measurements. This averaged ∼37% reduction and the fact that H2O2 formation was not switched off suggest that both pathways A and B contribute to the H2O2 production by ∼37% and ∼63%, respectively, considering that in the presence of the organic OH radicals are sufficiently scavenged. By titration of the OH radicals, equilibrium R1 is shifted to the right promoting pathway B. If solely the sequence of reactions (R7 and R8) would be operating, this should lead to an increase in the H2O2 production at high concentrations of TA, which was not observed. Hence, the contribution of each pathway strongly depends on the actual chemical competition induced in a given set of experiments. To establish the involvement of Path B in the overall process, experiments were conducted with TA/salt solutions (0.17/2.5 mM) in the absence of O2. The results are presented in Figure 5 with open symbols. H2O2 production decreased in the presence of TA, reaching values equal to those obtained in the experiments without TA. Additionally, no TAOH was produced. These findings confirm the significance of path B in interfacial H2O2 production.

Although OH self-recombination (R2 - k2 = 5.5 × 109 M–1 s–1)33 is a much faster process than the one of HO2 (k8 between 0.83 and 34.0 × 106 M–1 s–1),15,16 the reaction of oxygen with the free electron leading to HO2 (k6 = 1.8 × 1010 M–1 s–1 and k7 = (5.0–7.2) × 1010 M–1 s–1)63−66 promotes pathway B. Since H atoms can be formed from the reaction of H+ with the solvated electrons, subsequent reactions have to be considered. In fact, H atoms react fast with O2 producing HO2 (R10, k10 = 1.2 × 1010 M–1 s–1).63,67 Furthermore, it can also react with HO2 creating directly H2O2 (R11, k11 = 1.0 × 1010 M–1 s–1).68 These two reactions, with rate coefficients close to the diffusion limit, cannot be excluded from the total mechanistic scheme (Figure 3). In contrast with the majority of the previous studies, the findings reported here emphasize that H2O2 production does not result solely from OH recombination, as initially believed. The complexity of the mechanism underscores the need to study all the potential reactions occurring at the air–water interface, both kinetically and thermodynamically, to gain a better understanding of OH/H2O2 formation. In the Supporting Information, we propose a series of reactions that may occur at the interfaces of the aqueous droplets (RS1–RS5). Further investigation through theoretical simulations is needed to explore and validate these proposed reactions.

2.3. R10
2.3. R11

Finally, TA experiments were also employed to estimate the OH radicals that can be produced at the air–water interface. At the highest concentration of TA used here, the mean value of TAOH measured was 2.85 nM. Considering that the residence time in our experimental setup is ∼10 s and the TAOH production yield is 0.315, the interfacial OH radical production can be described in terms of production rate, 0.09 nM s–1. Li et al. reported the OH production rate at the interface of bulk water to be 0.57–0.63 nM day–1.27 The higher production rate that we report here is due to the higher surface-to-volume ratio of the particles compared to the bulk interfaces, suggesting that interfacial OH/H2O2 production may be important in small sea-spray particles. These production rates should be included in the atmospheric models to assess the contribution of the aerosols to oxidant production.

3. Conclusions

The production of H2O2 at the air–water interface of salt containing aqueous droplets (Na2SO4, NaCl, NaBr, and NaI) was investigated. For salt concentrations above 0.66 mM, NaBr droplets were found to produce almost two times more H2O2 than Na2SO4 and NaCl due to the ability of Br– to generate solvated electrons and Br radicals in the presence of a strong electric field. However, at lower salt concentrations, the trend is subject to caution due to experimental uncertainties (with droplets potentially subject to easy evaporation). O2 adsorption at the interface enhances the OH and electron formation by shifting the equilibrium of the reaction OH– ⇌ OH• + e– to the right, thereby promoting H2O2 production via both pathways. Experiments with terephthalic acid (TA) confirmed the OH radical production at the interface via 2-hydroxy terephthalic acid (TAOH) detection in the collected droplets and, for the first time, in the particle phase. In the presence of TA, H2O2 production was reduced, indicating that OH recombination is not the sole pathway involved in the total mechanism. HO2 initiated reactions including recombination and/or those involving H atoms contribute to H2O2 formation and should be further investigated. Overall, the findings suggest that, among the halides that are present in sea-spray aerosols, Br and I ions can lead to enhancement or suppressing of H2O2, respectively, due to the secondary reactions that can initiate, and thus, their chemistry should be thoroughly examined. Factors such as size and composition need careful consideration to assess the impact of sea aerosols on the OH/H2O2 atmospheric budget and on the atmospheric multiphase oxidation chemistry.

Acknowledgments

The authors wish to thank the European Research Council under the Horizon 2020 research and innovation program/ERC Grant Agreement 101052601 — SOFA and the CNRS, University Lyon 1, for the financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c14040.

  • Text S1: Description of the apparatus, instrumentation, chemicals, and experimental conditions that the experiments carried out. Figure S1: Experimental apparatus and instrumentation used. Figure S2: The optical and electrical mobility size distributions of salt droplets (2.5 mM). Figure S3: Bulk pH as a function of salt concentration for NaCl and NaBr. Figure S4: Optical size distributions for five different concentrations of each salt. Text S5 and Figure S5: Control experiments for the potent reaction of H2O2 with the salts, in the bulk. Figure S6: Bulk pH measurements for salt/TA solutions. Text S7 and Figures S7–S9: TAOH detection and quantification. Table S1: H2O2 production as a function of salt c concentration. Table S2: H2O2 and TAOH production as a function of TA concentration in bulk solutions of NaCl and NaBr. (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja3c14040_si_001.pdf (971.9KB, pdf)

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