Atmospheric fates of SO₂ at the gas–solid interface of iron oxyhydroxide (FeOOH) minerals: effects of crystal structure, oxalate coating and light irradiance

Abstract

Iron oxyhydroxide (FeOOH) is one of the most abundant atmospheric iron oxides. Considering their diverse emission features and reaction processes, airborne FeOOH minerals present distinct crystal structures and coating compositions. Nevertheless, little attention has been paid to the atmospheric fates of SO₂ over the gas–solid interface of FeOOH, let alone the impacts of sunlight and its irradiance. Herein, by employing in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the heterogeneous reaction of SO₂ was investigated on nanoscale FeOOH with different crystal structures, followed by a discussion on the effects of oxalate coating and light irradiance. Results indicated that the heterogeneous uptake capacity varies with crystal structure (α-FeOOH > β-FeOOH > γ-FeOOH), probably due to the different structural properties of the exposed crystal structures. The presence of stimulated solar irradiation facilitates the conversion of SO₂ on β-FeOOH and γ-FeOOH, whereas it inhibits the heterogeneous uptake on α-FeOOH. The addition of 5 wt% oxalate was discovered to significantly increase the reactive uptake coefficients of irradiated α-FeOOH and β-FeOOH by a factor of 3 and 2, which can be explained by the complex nature of iron with oxalate. The kinetic synergism between oxalate and sunlight was, for the first time, investigated in the heterogeneous regime of sulfate formation. The heterogeneous reactivity of FeOOH is nonlinearly correlated to light intensity and the exact dependence varies with crystal structure. This study contributes to a better understanding of atmospherically relevant heterogeneous reactions and haze formation, thus promoting laboratory studies and model simulations concerning atmospheric chemistry.

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Environmental significance

SO₂ is one of the most important precursors of atmospheric fine particulate matter, thus playing a crucial role in haze formation. FeOOH, one of the most abundant atmospheric iron oxides, not only provides numerous reactive sites for the removal of airborne SO₂ but also supports the rapid formation of secondary sulfate aerosols. This study for the first time investigates the atmospheric fate of SO₂ on pristine and oxalate-containing FeOOH nanoparticles with different crystal structures. Experimental results indicate that the crystal factor affects the heterogeneous sulfate formation in the order α-FeOOH > β-FeOOH > γ-FeOOH, in relation to their inherent crystal characteristics. In addition, the synergism between oxalate coating and sunlight irradiation is for the first time discovered on airborne gas–solid interfaces. This study reveals the atmospheric relevance of FeOOH in SO₂ oxidation and is helpful for better understanding dust-related heterogeneous reactions.

1. Introduction

Sulfur dioxide (SO₂), one of the typical air pollutants emitted from the combustion of fossil and biomass fuels, contributes greatly to atmospheric sulfate aerosols.^1,2 Globally, field-based observations have suggested that nearly half of the emitted SO₂ is converted to sulfate, mainly through gas-phase oxidation by hydroxyl radicals, aqueous-phase oxidation by dissolved hydrogen peroxide, nitrogen dioxide, ozone, and the O₂ catalyzed by transition metal ions, and heterogeneous oxidation on aerosol surfaces.^1,3–5 However, traditional models cannot capture well the key feature of sulfate concentration derived from field measurements, indicating that the sulfate generated from gas- and aqueous-phase oxidation is not able to bridge the gap on a global scale.^3,6 Atmospheric observations discovered that the surfaces of atmospheric particles serve as efficient media for the rapid formation of sulfate.^7,8 However, the influential factors for heterogeneous chemistry and the exact reaction mechanism remain poorly understood.

Mineral dust,⁹ constituting ∼36 wt% of primary aerosol emissions, is the most widespread and concentrated aerosol in the troposphere.^10,11 These nanoscale and microscale particles can be lifted from the ground and transported over thousands of kilometers due to wind force, leaving a rather long lifetime of up to 1 to 2 weeks depending on the specific geographical and meteorological conditions.¹² Hence, these particles provide abundant reactive sites for the heterogeneous conversion of atmospheric trace gases within their lifespan.¹³ Hoffmann and co-workers¹⁴ found that the total iron concentrations in urban fogs and clouds were unusually high. Iron was estimated to account for 7 wt% of an urban aerosol sample.¹⁴ Given the abundance of iron oxides in aerosols,¹⁵ a deep insight into SO₂ heterogeneous oxidation on the surface of iron oxides is warranted. The mass fractions of α-Fe₂O₃, α-FeOOH, and Fe₃O₄ in the community of Fe-bearing minerals were determined to be 7.5%, 60.8%, and 9.8%, respectively.² Previous studies showed the important role of Fe₂O₃ and Fe₃O₄ in SO₂ conversion, but few of them examined the contributions of the more abundant FeOOH.^2,16–20 Previous studies suggested that FeOOH widely exists in atmospheric particulate matter, especially mineral dusts.^21–24 In addition, the abundance of FeOOH is comparable to or even higher than that of hematite.²⁵ Therefore, FeOOH can be used as a dust proxy to investigate the heterogeneous SO₂ conversion. It was shown that the heterogeneous oxidation of SO₂ depends largely on the crystalline phases of MnO₂.²⁶ Motivated by this finding, crystal structure may act as an important influential factor for the heterogeneous reaction of SO₂ on FeOOH, with various crystalline forms in nature. FeOOH comprises goethite (α-FeOOH), akageneite (β-FeOOH) and lepidocrocite (γ-FeOOH), which have different crystal system types and crystalline surface structures, and may thus present different SO₂ chemisorption processes.²⁷ In addition, considering the formation process of FeOOH,²⁸ they are widely present in aerosol particles with varied proportions.^29–31

Solar irradiation was frequently considered in laboratory research. Irradiated Fe-containing dust may present higher heterogeneous reactivity toward atmospheric trace gases due to the photocatalytic effect of the transition metal oxides therein or the photochemical effects induced by the specific dust coatings.^4,18 However, the relevant studies concerned the photocatalytic and/or photochemical impacts of the irradiated particles rather than the irradiation intensity dependence of a series of reactions. The intensity of sunlight irradiation should be emphasized by laboratory stimulations because it changes with longitude, latitude, altitude, season, and time of day.^32,33 Such a variable was recently discovered to present significant impacts on the heterogeneous SO₂ oxidation on Fe₂O₃ and ZnO,^4,16 while no study of light irradiance has been performed for FeOOH.

Oxalate is one of the most abundant low molecular weight carboxylates in atmospheric aerosols,³⁴ which is mainly formed via atmospheric oxidation of its higher homologues of long-chain diacids and other pollution-derived organic precursors (e.g., olefins and aromatic hydrocarbons).³⁵ Oxalate can effectively form chelates with Fe(III) (Fe(III)–oxalate chelates), which is photochemically active and can initiate free radical chain reactions. Fe(III)–oxalate complexes are generally more photoactive than unchelated Fe(III) ions because the complexation produces a stronger absorbing chromophore, which comes from the transfer of electrons from the oxalate ligand to Fe(III).³⁶ The photoreactions of Fe(III)–oxalate complexes have significant impacts on the redox cycle of iron and may be an important source of oxidizing species in the atmosphere, thus contributing to a profound effect on atmospheric SO₂ photooxidation.^37,38 Thus, it is highly desirable to investigate the interaction between oxalate and iron on typical mineral surfaces.

To the best of our knowledge, only Fu et al.² discussed the heterogeneous sulfate formation on pristine FeOOH under dark conditions, while the effects of various variables were not considered. Herein, we discussed the effects of crystal structure, oxalate coating and light irradiance based on high-resolution infrared spectral data. The promotion effect of oxalate and its synergism with solar irradiation were for the first time reported on airborne gas–solid interfaces, followed by the illustration of reaction mechanisms varying with reaction conditions and particle types. This study is expected to complement the role of FeOOH in SO₂ conversion and provide a new approach for bridging the gap between field observation and model simulation.

2. Experimental

2.1 Materials

All chemicals are of analytical grade and used as received without further purification. Deionized (DI) water (specific resistance ≧18.2 MΩ cm) was used throughout the experiment. High-purity air (79% N₂ and 21% O₂, 99.999% purity) and SO₂ (100 ppm, balanced by N₂) were supplied by Shanghai Yunguang Specialty Gases Inc. Before introducing into the reaction system, the high-purity air was dried and purified using silica gel and a molecular sieve. A xenon lamp (CEL-TCX250, Beijing Ceaulight Co., Ltd., China) was used as a simulated sunlight source, and its spectrum is shown in Fig. S1.† To ensure the stability of light irradiance during the whole test process, the lamp was turned on for 30 min before irradiating the sample.

Three types of iron oxyhydroxide with different crystal structures, goethite (α-FeOOH), akageneite (β-FeOOH) and lepidocrocite (γ-FeOOH), were synthesized according to previous reports.^39–41 Sodium oxalate (Na₂C₂O₄) was used as a proxy for oxalate, representative of the abundant dicarboxylic compounds in coarse aerosol mode. The FeOOH–oxalate mixtures were prepared by mechanically mixing Na₂C₂O₄ with the as-prepared FeOOH powder (5% mass fraction of Na₂C₂O₄ for all samples), followed by manual grinding in an agate mortar for 15 min.

2.2 Sample characterization

The crystal structure of FeOOH was characterized using an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Kα radiation (λ = 1.54 Å) ranging from 10° to 80° (2θ) with a scan speed of 2° min⁻¹. Field emission scanning electron microscopy (FE-SSEM, Hitachi S-4800, Japan) was used to investigate the microscopic morphology of samples. The Brunauer–Emmett–Teller (BET) specific surface area, pore volume and pore size were analyzed by nitrogen adsorption–desorption isotherms measured at 78 K on a Micromeritics Tristar 3000 analyzer.

2.3 In situ DRIFTS experiments

In situ DRIFTS experiments were performed on an FTIR spectrometer (Tracer-100, Shimadzu, Japan) equipped with a high-sensitivity and liquid-nitrogen-cooled mercury–cadmium–telluride (MCT) detector. The reaction temperature was maintained at 25 °C using a temperature controller. More details on the experimental equipment can be found in our previous reports.^{4,16,17,42,43}

First, the prepared particles were placed in a ceramic crucible in the reaction chamber. Before reaction, the samples were purged by high-purity air with a flow rate of 150 mL min⁻¹ for 30 min to remove the surface absorbed water and impurities. Then, a single-beam spectrum was collected as the background spectrum. Next, a mixture of high-purity air and SO₂ at stable RH was introduced into the reaction chamber at a total flow rate of 110 mL min⁻¹. The SO₂ concentration of the reactant gas flowing into the DRIFTS chamber was 8 ppm (1.97 × 10²⁰ molecules per m³), which is comparable to the experimental settings in previous studies.⁴ The humidity of the gas mixture is maintained within 68 ± 5% at all times. In order to investigate the effects of sunlight and its light irradiance on the heterogeneous conversion of SO₂, light irradiation with a pre-designed power density (15, 45 and 80 mW cm⁻², denoted as P15, P45 and P80, respectively) was turned on at the same time. DRIFTS spectra ranging from 4500 to 700 cm⁻¹ were collected automatically every 2 min with 100 scans averaged for each spectrum and a resolution of 4 cm⁻¹. The entire reaction process took 60 min. Every experiment was repeated at least three times.

2.4 Ion analysis

The S(VI) species formed via a heterogeneous reaction can be quantified by ion chromatography (IC). As previously performed, the samples were extracted by a 10 min oscillation in 5 mL H₂O containing 5 vol% formaldehyde to inhibit the oxidation of S(IV) species.¹ Then, the leaching solution was filtered through a 0.22 μm polytetrafluoroethylene (PTFE) membrane filter for analysis by IC (Basic 883, Metrohm, Switzerland). The detection was performed with a weak base eluent (3.5 mM Na₂CO₃/1.0 mM NaHCO₃) at a stable flow rate of 0.7 mL min⁻¹. Sulfite was not analyzed due to the limitations of the analytical column. The quantitative analysis was guaranteed by reference to the reference materials produced by the National Research Center for Reference Materials.

2.5 Uptake coefficient estimations

The reactive uptake coefficient γ is defined as the number of reactive collisions between reactant gas molecules and particle surfaces per unit time (−d[SO₂]/dt) divided by the theoretical total number of collisions per unit time (Z). The heterogeneous reaction on the surface of FeOOH is most probably irreversible as the generated products ((bi)sulfate and (bi)sulfite) are in the solid phase while the reactants (SO₂ and O₂) are gaseous. Since the concentration of O₂ is very abundant compared to that of SO₂, it can be assumed that the concentration of O₂ remains constant. Furthermore, the concentration of active sites on FeOOH for SO₂ adsorption in the initial period is also constant. In addition, it is reported that the correlation coefficients of ln[C/C₀] versus time (t) (where C₀ is the initial concentration of SO₂ and C is the concentration of SO₂) for metal oxides under different initial concentrations of SO₂ were greater than 0.95 from some previous studies.^1,44,45 Thus, the heterogeneous conversion of SO₂ was consistent with pseudo-first-order reaction kinetics, i.e. −d[SO₂]/dt = d[SO₄²⁻]/dt = KC_SO2, where K is the rate constant and C_SO2 is the concentration of SO₂ at certain reaction times. The reactive uptake coefficient (γ) based on pseudo-first-order kinetics is calculated by the following equations (eqn (1)–(4)):^45,46

γ_BET = A_geo × γ_geo/A_BET (4)

where γ_geo represents the geometric uptake coefficient, γ_BET is the BET uptake coefficient, (d[SO₄²⁻])/dt represents the formation rate of sulfates (ions per second), v_SO2 is the mean molecular velocity of SO₂ (m s⁻¹), A_geo is the geometric surface area of FeOOH (m²), C_SO2 is the concentration of SO₂ (molecules per m³), R is the gas constant (J mol⁻¹ K⁻¹), T is the temperature (K), M_SO2 is the molecular weight of SO₂ (kg mol⁻¹), and A_BET is the BET surface area (m²). Detailed information on these parameters is given in Table S1.† The upper and lower limits of the uptake coefficients are γ_geo and γ_BET depending on the reaction probability, respectively.¹

3. Results and discussion

3.1 Particle characterization

According to Fig. 1a–c displaying the crystal structure results, all the diffraction peaks of the samples matched well with the corresponding standard cards for α-FeOOH (JCPDS29-0713), β-FeOOH (JCPDS34-1266), and γ-FeOOH (JCPDS44-1415). In addition, there was no other peak observed in the XRD pattern, suggesting that the prepared samples were of high purity. The XRD characteristic peaks are narrow and sharp, indicating the high crystallinity of the prepared FeOOH nanoparticles. The as-prepared FeOOH samples showed obviously different crystal forms, which would be appropriate for studying the effect of crystal structure on the heterogeneous conversion of SO₂.

Fig. 1 XRD patterns, N₂ adsorption–desorption isotherms, and SEM images for the three FeOOH samples: (a and d), α-FeOOH, (b and e), β-FeOOH, (c and f), γ-FeOOH.

In order to investigate the morphology structure of FeOOH, the FE-SEM images and BET data were obtained, shown in Fig. 1d–f. The FE-SEM images show that α-FeOOH (Fig. 1d) and β-FeOOH (Fig. 1e) were mainly composed of nanorod particles, but the length of α-FeOOH nanorods (∼100 nm) was about one-half that of β-FeOOH (∼210 nm). γ-FeOOH (Fig. 1f) presented a nanosheet structure. The close morphology and size of nano-FeOOH could minimize the effects of morphology and particle size on the heterogeneous SO₂ uptake.

The N₂ adsorption–desorption isotherms of the three samples (Fig. 1d–f) could be classified as type IV in the IUPAC classification with H3 hysteresis loops in the relative pressure range of 0.8 < p/p₀ < 1.0, indicating the presence of large secondary mesoporous architectures (pore sizes are in the range of 2 to 50 nm). The BET specific surface areas of γ-FeOOH, α-FeOOH, and β-FeOOH were 96.20, 38.56 and 28.96 m² g⁻¹, respectively.

3.2 Effects of crystal structure and oxalate

3.2.1. Surface sulfur-containing species. DRIFTS spectra are able to provide valuable information on adsorbed surface species by the vibrational modes. The assignments of the vibrational bands of adsorbed sulfur-containing species were analyzed in previous studies (Table S2†).^2,5,46 In detail, the vibrational bands in the range of 1000–900 cm⁻¹ correspond to chemisorbed (bi)sulfite, whereas the peaks at 1050 and 1010 cm⁻¹ are assigned to bridging (bi)sulfate. In addition, the peaks at 1150 and 1250 cm⁻¹ are attributed to adsorbed bidentate sulfate and water-soluble sulfate, respectively. As shown in Fig. 2, the spectra (1300–800 cm⁻¹) of surface-bound products were recorded as a function of time. It is evident that new vibrational bands could be observed after SO₂ exposure, and the intensities of these peaks rapidly increased in the first 10 min and then almost reached saturation with increasing time probably due to the complete consumption of surface active sites. Clearly, the DRIFTS spectra were distinct by the nanoscale FeOOH samples with different crystal structures, as discussed below.

Fig. 2 In situ DRIFTS spectra collected for the three (a–c) pristine and (d–f) oxalate-coated samples during dark reactions: (a and d) α-FeOOH, (b and e) β-FeOOH, and (c and f) γ-FeOOH.

Fig. 2a displays the DRIFTS spectra of surface species on α-FeOOH as the reaction proceeded. There is a weak peak at 990 cm⁻¹ and two main peaks at 1062 and 1170 cm⁻¹, which could be attributed to (bi)sulfite, bridging (bi)sulfate and bidentate sulfate, respectively. Such spectral characteristics indicate that the prominent surface species were sulfates produced from SO₂ conversion on α-FeOOH. The peak at 1260 cm⁻¹ is not a real peak of surface generated species because it is obtained from the negative peak through the Kubelka–Munk transformation (Fig. S4†). The spectra collected for the SO₂ conversion on β-FeOOH (Fig. 2b) indicate four surface products: (bi)sulfite (980 cm⁻¹), bridging (bi)sulfate (1012 and 1060 cm⁻¹), bidentate sulfate (1125 cm⁻¹) and water-soluble sulfate (1235 cm⁻¹). Similarly, the dominant generated surface species on β-FeOOH were sulfates with negligible production, suggesting that the pristine β-FeOOH is not conducive to the heterogeneous oxidation of SO₂. Fig. 2c shows the spectra collected on γ-FeOOH, and the assignments of the peaks at 940, 1070 and 1175 cm⁻¹ could be referenced to that of β-FeOOH. It is not an accurate vibrational band of surface species at 1250 cm⁻¹ like that mentioned in α-FeOOH (Fig. S4†). Notably, on γ-FeOOH, the dominant sulfur-containing species are (bi)sulfite rather than the (bi)sulfate although it has a higher BET specific surface area, implicating the relatively weak capacity of γ-FeOOH for SO₂.

Analogous to the previously studied Fe₂O₃ and Fe₃O₄, FeOOH may also serve as an important sink of airborne SO₂ and source of sulfate aerosols. The nano-FeOOH samples displayed different DRIFTS spectra (vibrational bands and intensity), indicating that the crystal structure of mineral dusts could influence their heterogeneous reaction activities and product coordination structures. Considering the proportion of S(VI) species, α-FeOOH could be more efficient than β-FeOOH and γ-FeOOH in the oxidative conversion of S(IV) species.

The effects of oxalate on the heterogeneous uptake of SO₂ under dark conditions were further investigated (Fig. 2d–f). There are obvious enhancements of three samples at the peaks ranging from 1100 to 1000 cm⁻¹ and an extra enhancement of γ-FeOOH at 1000–900 cm⁻¹. In other words, the oxalate facilitated the formation of bridging (bi)sulfate for all the three particles as well as the SO₂ adsorption on the surface of γ-FeOOH. Furthermore, surface sulfur-containing species on pristine FeOOH under light irradiation were also investigated (Fig. S5†). Compared to the dark process, the dominant species of sulfur-containing products have changed under light irradiation. For α-FeOOH (Fig. 2a and S5a†), although the dominant species are still bridging sulfate and bidentate sulfate, there is a tendency for the peak at 1170 cm⁻¹ to shift to higher wavenumbers, indicating the formation of water-soluble sulfate. β-FeOOH has a more pronounced response to light (Fig. 2b and S5b†), with bidentate sulfate as the new dominant product under light irradiation, and there is an overall increase in peak intensity, representing higher sulfate production. The prominent species on γ-FeOOH (Fig. 2c and S5c†) has converted from bridged sulfate in the dark reaction to water-soluble sulfate in the photoreaction, indicating that light greatly enhances the oxidation capacity of γ-FeOOH for SO₂. Moreover, comparison of the spectra before and after the addition of oxalate under the dark and light conditions shows that the promotion effect of oxalate is more drastic in the presence of solar irradiation.

3.2.2. Kinetics assessment. The different crystal forms and the presence or absence of oxalate result in not only different surface product types but also kinetic differences. To further discuss how crystal structure and oxalate coating affect the heterogeneous reactivity, we also calculated the uptake coefficient for sulfate formation on different FeOOH samples. It has been reported that the concentration of surface products has a positive linear relationship with the integrated absorbance (the integrated area of the characteristic infrared peaks of sulfur-containing species) based on DRIFTS spectra.^1,3 Therefore, the amount of surface sulfate species, including water-soluble sulfate, bidentate sulfate, bridging sulfate, and bisulfate, was considered for quantitative analysis. The calibration curves of sulfate ions versus the integrated absorbance were obtained from IC analysis of a series of DRIFTS experiments with different reaction times. The calibration plots of conversion factors for α-FeOOH and β-FeOOH under light irradiation are shown in Fig. S2 and S3,† respectively. γ-FeOOH produced too little sulfate to calculate its conversion factor. In addition, according to a previous study,¹ the addition of 5 wt% oxalate has a negligible effect on the conversion factors; thus we use the conversion factor of pure FeOOH particles as an alternative to the conversion factor of mixed particles. The conversion factors (f) for α-FeOOH (f_α) and β-FeOOH (f_β) are 4.07 × 10¹⁵ and 1.89 × 10¹⁴ ions per absorbance unit (ABU), respectively. As a result, the number of sulfate ([SO₄²⁻]) can be calculated according to the following formula (eqn (5)):

[SO₄²⁻] = (integrated absorbance) × f (5)

Fig. 3 displays the S(VI) species produced on different FeOOH samples as a function of time. The S(VI) species increased rapidly at first (initial stage) and then kept steady (stable stage) as the reaction proceeded, since the integrated area of the infrared peak is proportional to the species content. This was mainly because more surface reactive sites existed in the initial stage for the adsorption and oxidation of SO₂. With increasing reaction time, the number of active sites decreased as the generated species covered the active sites, thus inhibiting further SO₂ conversion.^2,47 In addition, it was clear that the sulfates generated on different FeOOH samples were obviously distinct. The number of total sulfate species decreased following the order α-FeOOH > β-FeOOH > γ-FeOOH. It is worth mentioning that although the BET specific surface area of γ-FeOOH (96.20 m² g⁻¹) was much higher than that of α-FeOOH (38.56 m² g⁻¹), the reactivity of γ-FeOOH was much lower than that of α-FeOOH. It is possible that the density of active sites on the flake particles is very low, thus resulting in fewer active sites on γ-FeOOH.⁴⁸ Such a result suggested that the particle crystal structure is a significant impact factor for the heterogeneous conversion of SO₂ and will inevitably affect the atmospheric heterogeneous reaction, which should be paid more attention in further studies.

Fig. 3 Calculated ions of the sulfates produced on different FeOOH samples under (a) dark and (b) light conditions. Inset: local magnified image of pristine and oxalate-coating β-FeOOH.

The calculated γ_geo and γ_BET for the nano-FeOOH samples are shown in Table 1. The number of sulfate ions produced by γ-FeOOH is below the detection limit, reflecting its weakest reactivity. It is evident that the uptake coefficients were distinct, which could be attributed to the differences in surface and structure properties. Taking the BET surface area as the reaction surface area, the activities of these oxides towards SO₂ were in the order α-FeOOH > β-FeOOH > γ-FeOOH under all conditions of this experiment. The γ_BET values of α-FeOOH and β-FeOOH under light irradiation were 3.65 × 10⁻¹¹ and 0.70 × 10⁻¹¹, respectively, which were comparable with previously documented results.^2,49 Overall, it was obvious that the uptake capacity of SO₂ varies with the nanoscale FeOOH samples with different crystal structures, probably due to their various BET surface areas, particle sizes, densities of surface reactive sites and structure properties.

Table 1 Reactive surface areas and uptake coefficients of SO₂ on different FeOOH particle samples^a

Sample	A BET (m^2 g^−1)	A geo (×10^−5 m^2)	γ BET (×10^−11)	γ geo (×10^−6)
a The amounts of products are too small to be quantified by IC, so the results of γ-FeOOH are not displayed. b The oxalate addition was 5 wt% for all samples.
α-FeOOH	1.05	1.96	2.03	1.09
α-FeOOH + Ox^b	1.05	1.96	4.84	2.59
α-FeOOH + P45	1.05	1.96	3.65	1.95
α-FeOOH + Ox + P45	1.05	1.96	11.20	5.99
β-FeOOH	0.78	1.96	0.12	0.05
β-FeOOH + Ox	0.78	1.96	0.14	0.06
β-FeOOH + P45	0.78	1.96	0.70	0.28
β-FeOOH + Ox + P45	0.78	1.96	1.21	0.48

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As shown in Table 1, the presence of 5 wt% oxalate slightly increased the γ_BET of α-FeOOH under dark conditions, while there was no significant enhancement for β-FeOOH. By the presence of solar irradiation, the γ_BET of α-FeOOH and β-FeOOH were enhanced by a factor of 3.0 and 1.7 in the presence of oxalate. Thus, a more pronounced promotion effect under light irradiation can be concluded. Such a conclusion is related to the chelation of oxalate with iron, forming a chelate with a strong photoactivity, which would be expounded later in the reaction mechanism section. Moreover, compared with the slight decrease in γ_BET of α-FeOOH, the γ_BET of β-FeOOH during the photoreaction increased by 6-fold, indicating that β-FeOOH has a more pronounced response to solar irradiation.

3.3 Effect of light irradiance

In the atmosphere, solar irradiation will inevitably influence many complex physiochemical reactions. Nevertheless, previous experimental studies of heterogeneous reactions have to date largely been carried out under a single light intensity.^26,50 In this study, we investigated the effect of multiple levels of light irradiance on the heterogeneous uptakes of SO₂ on the samples. Considering that the annual averaged solar irradiation intensity reaching the mid-latitude region is about 100 mW cm⁻²,^51,52 three different light intensities of 15, 45 and 80 mW cm⁻² were selected in this study.

The final spectra (1300–800 cm⁻¹) collected for pristine α-FeOOH under the four light intensities are shown in Fig. 4a. With irradiance increasing, (bi)sulfite (971, 923, 886 cm⁻¹) and bridging sulfate (1062 cm⁻¹) decreased continuously and the predominant species gradually changed from bidentate sulfate (1158, 1125 cm⁻¹) to water-soluble sulfate (1238, 1220 cm⁻¹), implying the oxidation of S(IV) species. To further investigate the effect of light intensity on pristine α-FeOOH, the integrated area of the sulfate peaks was calculated as a function of reaction time (Fig. 4c). The integrated area decreased at first and then increased with increasing light intensity. However, within the light intensity range we studied, the integrated area of sulfate under light irradiation was generally lower than that under dark conditions. Such results indicated that the light irradiation may inhibit the heterogeneous oxidation of SO₂, which may be caused by the saturated iron atoms^42,53,54 and/or the photoinduced reductive dissolution of α-FeOOH.^4,55

Fig. 4 (a and b) Final DRIFTS spectra after 60 min of SO₂ exposure and (c and d) the temporal variations of the integrated area of sulfate peaks under four irradiance power densities (P0, P15, P45, P80). Both (a and c) pristine (light cyan) and (b and d) oxalate-containing (light LT red) α-FeOOH nanoparticles are considered.

The effect of light intensity on oxalate-coated α-FeOOH was further investigated. The spectra collected after 60 min of SO₂ exposure and the temporal variations of the integrated area of sulfate peaks are displayed in Fig. 4b and d. Like the process on α-FeOOH, the dominant species gradually transformed from bidentate sulfate to water-soluble sulfate. However, in the presence of oxalate, there was only a slight decrease of sulfite and bridging sulfate with increasing light intensity, along with a continuous increase of bidentate and water-soluble sulfates. Such results indicated that sunlight could enhance the formation and solvation of sulfate on oxalate-coated α-FeOOH (Fig. 4d).

Noticeably, the presence of oxalate could greatly accelerate the conversion of SO₂ under light conditions as mentioned above. Further calculations revealed that the integrated area of sulfate under the combined effect of oxalate and light was higher than the sum of that under the effect of oxalate and light irradiation alone (Fig. S6a and d†), indicating that there was a synergistic effect of light and oxalate in the heterogeneous conversion of SO₂ on α-FeOOH. Considering the wide availability of oxalate and FeOOH in the atmosphere, such synergistic effects may exist in the atmospheric SO₂ photooxidation and explain the stronger promotion effect of oxalate under sunlight.

Fig. 5 shows the DRIFTS spectra and the sulfate integrated areas of β-FeOOH under the same conditions of α-FeOOH. On pristine β-FeOOH, (bi)sulfite, bridging sulfate and bidentate sulfate increased at first and then decreased with the increased light intensity, which could be explained by the photoinduced reductive dissolution of β-FeOOH under strong sunlight.^4,55 However, the water-soluble sulfate kept increasing within the range of light irradiance in the experiments. The total sulfate production on pristine β-FeOOH increased initially in a low light intensity range and then decreased when the light irradiance exceeded the range, and the amount of sulfates in the presence of light irradiation was always higher than that under dark conditions. This conclusion was validated in Fig. 5c, showing the integrated areas of the characteristic peaks of sulfates. Nevertheless, on the oxalate-coated β-FeOOH nanoparticles, the peak intensities of all kinds of sulfates increased with increasing light intensity, and the corresponding integrated area increased as well, as shown in Fig. 5b and d, indicating that the oxalate increased the saturation light intensity of this reaction. It is worth noting that the dominant species was bidentate sulfate in the presence of light irradiation, which was different from the water-soluble sulfate under dark conditions (Fig. 2e). This is one point where the conversion of SO₂ on the surface of β-FeOOH was not well understood.²

Fig. 5 (a and b) Final DRIFTS spectra after 60 min of SO₂ exposure and (c and d) the temporal variations of the integrated area of sulfate peaks under four irradiance power densities (P0, P15, P45, P80). Both (a and c) pristine (light cyan) and (b and d) oxalate-containing (light LT red) β-FeOOH nanoparticles are considered.

In addition, according to the integrated area of sulfate in the absence or presence of oxalate shown in Fig. 5c and d, it could be concluded that the promotion effect of light on SO₂ oxidation was remarkable on β-FeOOH whether the oxalate coating was present or absent. In addition, based on the calculated uptake coefficients shown in Table 1, under P45 irradiance, the γ_BET of β-FeOOH was enhanced by a factor of 5.8 and 8.6 in the absence and presence of oxalate, respectively, further demonstrating the remarkable promotion of light irradiation on β-FeOOH. The weak enhancement of oxalate on γ_BET suggested that the reaction is more susceptible to solar irradiation than to oxalate on β-FeOOH. Likewise, there is also a synergism between light irradiation and oxalate for the SO₂ conversion on β-FeOOH, since the integrated area of sulfate on oxalate-coated β-FeOOH with light is higher than the sum of that with oxalate addition and light irradiation alone (Fig. S6b and e†).

For γ-FeOOH, Fig. 6(a and d) show that the light intensity promotes the conversion of SO₂ on pristine and oxalate-coated samples, including bidentate sulfate and water-soluble sulfate. Moreover, in the experimental light intensity range, the generated sulfate kept increasing with increasing light intensity (Fig. 6b and e). In contrast to α-FeOOH and β-FeOOH, no saturation light effect was observed on γ-FeOOH. This may be due to its huge specific surface area or lower activity resulting in a greater dependence on light irradiation. In addition, to account for the higher SO₂ adsorption on γ-FeOOH (Fig. 2c and f), we analyzed the (bi)sulfite chemisorbed on γ-FeOOH separately (Fig. 6c and f). It could be seen that the addition of oxalate increases the (bi)sulfite content significantly at any light intensity. As shown in Fig. 6c, the content of (bi)sulfite under different light intensities tended to be the same (P15 was an exception), indicating that light irradiation may not change the adsorption capacity of SO₂ on the surface of pristine γ-FeOOH. In the presence of oxalate, the sulfite content under light irradiation was always lower than that under dark conditions but increased with increasing light irradiance (Fig. 6f). Such results may be mainly due to the conversion of S(IV) to S(VI) and more dissolved iron under stronger light irradiation.

Fig. 6 (a and d) Final DRIFTS spectra after 60 min of SO₂ exposure and (b, c, e and f) the temporal variations of the integrated area of sulfate and sulfite peaks under four irradiance power densities (P0, P15, P45, P80). Both (a–c) pristine (light cyan) and (d–f) oxalate-containing (light LT red) γ-FeOOH nanoparticles are considered.

3.4 Proposed mechanism for SO₂ uptake on FeOOH

As discussed above, influenced by crystal structure, light irradiance and oxalate coating, there are diverse atmospheric fates for airborne SO₂ over the gas–solid interface of FeOOH. Combining the documented information and the current experimental results, the following sections are expected to illustrate the primary physical–chemical processes on the studied nanoscale minerals.

3.4.1. Pristine FeOOH. The first step of the studied heterogeneous event is the adsorption of gas-phase SO₂ onto the absorbed state (reaction (1)). On the one hand, the interaction between SO₂ and H₂O, followed by the ionization procedures, leads to the formation of S(IV) species: H₂SO₃, HSO₃⁻ and SO₃²⁻.⁵⁶ On the other hand, SO₂ adsorbed on Lewis acid sites (coordinately unsaturated iron atoms) or Lewis basic sites (exposed oxygen atoms) could form physisorbed SO₂ or chemisorbed sulfite, respectively.^42,53 The reactions are shown as reaction (2) below, where O²⁻ (lattice) refers to lattice oxygen on particle surfaces.

SO₂(g) ⇌ SO₂ (ads) (R1)

SO₂(g) + O²⁻ (lattice) → SO₃²⁻ (R2)

Meanwhile, surface active oxygen (O⁻) is derived from the combination of molecular oxygen with the vacant oxygen sites viareactions (3) and (4). Subsequently, the chemisorbed sulfites are further oxidized to sulfate by O⁻, as described by reaction (5).^17,57,58

O(vacancy) + e⁻ + O₂ → O₂⁻ (ads) (R3)

O₂⁻ (ads) + e⁻ → 2O⁻ (R4)

SO₃²⁻ + O⁻ → SO₄²⁻ + e⁻ (R5)

where e⁻ represents a conductive electron trapped in the vacant oxygen site.⁵⁷

Surface-adsorbed hydroxyl groups participate in the heterogeneous reactions.⁴ SO₂ could react with one or two surface hydroxyl groups to form bisulfite or sulfite as described by reaction (6) or (8), respectively.^42,59,60 The consumption of surface hydroxyl groups seems to be fast at the initial stage of the reaction and drops to a slower trend later (Fig. 3). Such variations indicate that the relevant process would be controlled by reaction (8) at the beginning and then by reaction (6). Subsequently, the generated (bi)sulfite would be oxidized to (bi)sulfate by reactive oxygen species (ROS) and hydroxyls. In addition, a Fe(III)–Fe(II) redox cycle could participate in the formation of (bi)sulfate as well. As shown in reaction (10), Fe(III) receives an electron from HSO₃⁻ and is deoxidized to Fe(II). According to previous studies,^2,42,57 Fe(II) could be oxidized to Fe(III) by SO₅˙⁻, HSO₅˙⁻, and SO₄˙⁻ and would in turn catalyze the conversion of S(IV) species to S(VI) products. As shown in Fig. 2, under dark conditions, α-FeOOH presented the greatest sulfate products, followed by γ-FeOOH, with β-FeOOH producing the least. In addition, bridging and bidentate sulfates were mainly formed on the former two nanoscale samples, while water-soluble and bidentate sulfates were the predominant products on β-FeOOH.

◇ − Fe^III(OH) + ◇ − H₂O + SO₂ → ◇ − Fe^III(OH)·HSO₃⁻ + H⁺ (R6)

◇ − Fe^III(OH)·HSO₃⁻ → ◇ − Fe^IIIOSO₂ + H₂O (R7)

2 ◇ − OH + SO₂ → ◇ − HSO₃ + H₂O (R8)

◇ − HSO₃ + O⁻ → ◇ − HSO₄⁻ → ◇ − SO₄²⁻ + H⁺ (R9)

The effect of solar irradiation can be proved by the photoinduced variation of the infrared signals (Fig. S5a–c†), which is attributed to ROS formation in the presence of sunlight irradiation. Irradiated semiconductor FeOOH can be photo-excited to produce electron–hole pairs. As well acknowledged, h⁺ can oxidize OH⁻ and H₂O to ˙OH (h⁺ + OH⁻/H₂O → ˙OH) and e⁻ can reduce O₂ to O₂˙⁻ (e⁻ + O₂ → O₂˙⁻). In addition, the iron hydroxy complexes can absorb in the near-UV region (290–400 nm), undergoing efficient photo-dissociation to form Fe(II) and ˙OH (reactions (11) and (12)).^38,61 The generated reactive species can participate in the Fe(III)–Fe(II) redox cycle and the oxidation of S(IV) species to S(VI) products, resulting in a facilitation of SO₂ conversion relative to the dark reaction. It is worthwhile to mention that the negative impact of solar irradiation on the conversion of SO₂ can be explained by the saturation of iron atoms and photoinduced reductive dissolution, thus hindering the process on α-FeOOH described in Fig. 4a.

Fe(III) + H₂O → [Fe(III)(OH)]²⁺ + H⁺ (R11)

[Fe(III)(OH)]²⁺ + hν → Fe(II) + ˙OH (R12)

The proposed mechanism (Scheme 1) summarizes the main pathway involved in the heterogeneous reactions of SO₂ on pristine nano-FeOOH. Briefly, surface-adsorbed hydroxyl groups and ROS are involved in the formation of sulfate species, and the Fe(III)–Fe(II) redox cycle also contributes to the heterogeneous oxidation. The results from this study indicated that the heterogeneous uptake capacities are different between FeOOH samples with significantly different crystal structures, suggesting that crystal structure is an important influential factor for the heterogeneous conversion of SO₂. The different crystal planes, resulting from various crystal structures, possess diverse surface active sites and disparate distribution properties, including the density of surface active sites and SO₂ adsorption capacity. This may also explain the differences in heterogeneous reactivity between the particles with different morphologies.⁴

Scheme 1 Proposed mechanism illustrating the heterogeneous reaction of SO₂ on FeOOH surfaces. The blue arrows represent the primary pathways under dark conditions, and the red ones highlight the processes occurring merely in the presence of solar irradiation.

3.4.2. Oxalate-containing FeOOH. As discussed in section 3.2.1, the promotion effect of oxalate coating is much more significant in the presence of solar irradiation relative to that under dark conditions due mainly to the fact that the reaction process and mechanism are basically consistent in the absence of solar irradiation regardless of the oxalate addition. Under dark conditions, the oxalate coating greatly enhanced the adsorption and oxidation of SO₂ on FeOOH (Fig. 2), and the weak promoting effect of oxalate on sulfate formation could be mainly explained by the greatly enhanced dissolution of FeOOH resulting from the complexation of oxalate with iron,^23,62,63 hindering the transformation of bridging sulfate to bidentate and water-soluble sulfates. In the presence of solar irradiation, more pathways are probable in the course of the SO₂ heterogeneous reactions than those discussed in section 3.4.1. The proposed photochemical mechanism for the production of oxidizing species from Fe(III)–oxalate complexes is shown in the following reactions.^38,64,65

Fe(III) + 3C₂O₄²⁻ → [Fe(III)(C₂O₄)₃]³⁻ (R13)

[Fe(III)(C₂O₄)₃]³⁻ + hν → [Fe(II)(C₂O₄)₂]²⁻ + C₂O₄^·− (R14)

C₂O₄˙⁻ → CO₂˙⁻ + CO₂ (R15)

CO₂˙⁻ + O₂ → O₂˙⁻ + CO₂ (R16)

O₂˙⁻ + H⁺ ↔ HO₂˙ (R17)

Fe(II) + O₂˙⁻/HO₂˙ + H⁺ → Fe(III) + H₂O₂ (R18)

[Fe(II)C₂O₄] + H₂O₂ → [Fe(III)C₂O₄]⁺ + ˙OH + OH⁻ (R19)

After the excitation of [Fe(III) (C₂O₄)₃]³⁻ (reaction (13)), the major photochemical process is the intramolecular electron transfer from oxalate to Fe(III), forming C₂O₄˙⁻ as the primary intermediate (reaction (14)). The formed C₂O₄˙⁻ radical would further dissociate into carbon dioxide anion radical (CO₂˙⁻) and CO₂ (reaction (15)).^66–68 Although the photolysis mechanism for [Fe(III)(C₂O₄)₂]⁻ and [Fe(III)(C₂O₄)]⁺ has not been extensively studied, it is assumed that the reactions produce Fe(II) and CO₂˙⁻ eventually.⁶⁹ The subsequent reaction of CO₂˙⁻ with O₂ leads to the formation of intermediate superoxide ions (O₂˙⁻) and hydroperoxyl radicals (HO₂˙), which react with Fe(II) to produce H₂O₂ in acidic medium (reactions (16)–(18)). Furthermore, H₂O₂ will interact with Fe(II) (reaction (19)) to form ˙OH. These ROS (O₂˙⁻, HO₂˙ and ˙OH) greatly accelerate the oxidation of S(IV) (reaction (20)) along with accelerated iron dissolution.

As discussed above, the presence of illumination enhanced the formation of sulfate species on the oxalate-coated FeOOH, as proved by Fig. S5 and S6.† On α-FeOOH and γ-FeOOH, the proportions of bidentate and water-soluble sulfates further increased in the presence of oxalate coating, highlighting the crucial effects of the photoinduced ROS. On β-FeOOH, the predominant species was still bidentate sulfate and possessed a higher proportion. Sulfate weakly bound as outer sphere complexes by electrostatic attraction may serve as the main reason.²

The proposed mechanisms are summarized in Scheme 2. In summary, the addition of oxalate greatly facilitated the heterogeneous transformation of SO₂ to sulfate on FeOOH during photoreaction. However, there are different enhancements of the irradiated oxalate coating on the FeOOH samples (Fig. S5†), which can be explained by the different affinities proposed in previous studies.^55,70 In other words, the different affinities of oxalate and various crystalline types of FeOOH lead to its diverse promoting effects on the three samples, whose mechanisms remain to be further investigated.

Scheme 2 The proposed mechanism of SO₂ heterogeneous oxidation on oxalate-containing nano-FeOOH under solar irradiation.

4. Conclusions and implications

This study investigated the effects of crystal structure, oxalate coating, and light irradiance on the heterogeneous conversion of SO₂ on nano-FeOOH minerals. Experimental results indicated that the studied nano-FeOOH samples displayed different reactivities (α-FeOOH > β-FeOOH > γ-FeOOH) in relation to their different specific surface areas, surface reactive sites and intrinsic structure properties. Simulated sunlight irradiation inhibited the heterogeneous uptake on α-FeOOH and strong light intensity also hindered the reaction on β-FeOOH, mainly limited by the reactive sites and the photoinduced reductive dissolution. The light intensity within the experimental range was conducive to the heterogeneous reaction on γ-FeOOH. The addition of oxalate could facilitate the heterogeneous oxidation of SO₂ on nano-FeOOH especially under light irradiation, largely caused by the photoinduced ROS formation by iron–oxalate complexes.

Furthermore, to assess the importance of the heterogeneous reaction on FeOOH, the lifetime for removal of SO₂ was estimated according to the formula given in previous reports:^71,72

where A is the surface area density of FeOOH (cm² cm⁻³), ω is the mean molecular velocity (m s⁻¹), and γ is the uptake coefficient. The mass loading of mineral dust is set to be 55 μg m⁻³, representative of the averaged dust concentration in North China.^73,74 The atmospheric loading of goethite can be estimated by the hematite/goethite ratio ranging between 0.5 and 2.0,⁷⁵ along with the mass proportion of hematite (6.5%) in dust.^25,76 Hence, the A of goethite ranges from 6.89 × 10⁻⁷ cm² cm⁻³ to 2.76 × 10⁻⁶ cm² cm⁻³. Based on the γ of 1.95 × 10⁻⁶ for pristine irradiated goethite (see Table 1), the SO₂ lifetime with respect to the studied heterogeneous process ranges from 0.75 to 3.0 years. In comparison, the lifetime of SO₂ with respect to the gas-phase reaction with OH is 20 days if [OH] = 1.0 × 10⁶ molecular cm⁻³.⁷¹ In comparison, the heterogeneous uptake of SO₂ on goethite is negligible under common atmospheric conditions. However, during dust storms, the concentration of dust can be up to 10³–10⁴ μg m⁻³,^77,78 thus leading to a shorter SO₂ lifetime of 2.5–15 days. Considering the presence of oxalate coating, such a lifetime range may decrease to 12 h to 4.9 days. Accordingly, the heterogeneous reaction of SO₂ on FeOOH minerals plays a crucial role in the removal of SO₂ in the atmosphere.

As well acknowledged, FeOOH acts as the predominant Fe-containing airborne mineral in the atmosphere, thus making the kinetic results from this work more appropriate than those relevant to Fe₂O₃ and Fe₃O₄ for the modeling studies on the rapid formation of sulfate aerosols. In future laboratory research, FeOOH is recommended as a suitable mineral dust proxy for exploring the varied fates of airborne SO₂ through dust heterogeneous chemistry. On the other hand, the various variables, including the crystal structure, oxalate coating and light intensity considered discussed above, present great influences on the heterogeneous sulfate formation, coupled with the distinct heterogeneous chemical mechanisms. Extensive discussions on the variables are needed in the following research on dust-related sulfate formation, thus providing alternative kinetic parameter selections for chemical transport models.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The research was financially supported by the National Natural Science Foundation of China (22176036, 21976030, 22006020, 42205099) and the Natural Science Foundation of Shanghai (19ZR1471200).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2en00874b

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