domain: chim sde sdu

text:
Guillaume Morin
email: guillaume.morin@sorbonne-universite.fr

Frédéric Averseng

Xavier Carrier

Pierre Le Pape

Yishan Du

Yusheng Honge

Elora Bourbon

Giuseppe Sportelli

Tiago A Da Silva

Alberto Mezzetti

Pierre Le Pape

Camille Baya

Thierry Allard

Jessica Brest

Maryse Rouelle

Phosphate Boosts Nonhydroxyl Radical Species Production upon Air Oxidation of Magnetite and Iron Sulfides at Neutral pH

Keywords: Reactive oxygen species, phosphate, hydroxyl radical, ferryl ion, magnetite, mackinawite, pyrite, Fenton, ROS, DMPO, EPR, XAS, Fe(IV)

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

A growing interest has risen to use air-oxidation of iron minerals as a source for reactive species, especially in order to progress in decreasing the environmental impacts of oxidative pollutant degradation processes, particularly by limiting or avoiding the use of strong oxidants. For instance, several organic contaminants can be degraded upon air-oxidation of nanoparticulate zero-valent iron (nZVI) [1] , fine-particle Fe3O4 (magnetite) [2,3] , FeS [4] and clay minerals [5] . These degradation reactions are thought to proceed via to reactive species generated through oxidation of the Fe(II)-substrate by airborne O2, these reactive species being then able to oxidize certain organic molecules [1] . However, especially concerning air-oxidation of iron substrates, the identity of the involved reactive species is still debated. In particular, the respective roles of the hydroxyl radical [2][3][4][5][6] and the ferryl ion (i.e., Fe(IV) in the form of FeO 2+ ) [START_REF] Joo | Quantification of the oxidizing capacity of nanoparticulate zerovalent iron[END_REF][START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Kim | Polyphosphate-enhanced production of reactive oxidants by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen: Yield and nature of oxidants[END_REF][START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF] are particularly difficult to distinguish.

Nevertheless, various studies using measurements of the conversion rate of benzoic acid (BA) into parahydoxybenzoic acid (p-HBA) have proven the ability of zero-valent iron [START_REF] Joo | Quantification of the oxidizing capacity of nanoparticulate zerovalent iron[END_REF] , FeS [4,[START_REF] Cheng | Oxidizing impact induced by mackinawite (FeS) nanoparticles at oxic conditions due to production of hydroxyl radicals[END_REF] , clay-minerals [6] as well as natural sediments [START_REF] Tong | Production of abundant hydroxyl radicals from oxygenation of subsurface sediments[END_REF] to produce reactive species upon oxygenation. However, the relative efficiencies of such iron substrates have rarely been evaluated within a same study under equivalent conditions.

In the present study, we aimed at contributing to this active research field by directly comparing the ability of environmentally relevant minerals, i.e., fine particle Fe3O4 (magnetite), FeS (mackinawite) and FeS2 (pyrite/marcasite) to produce reactive species upon airborne O2-oxidation on a same basis. We chose to use Electron Paramagnetic Resonance (EPR) spin-trapping to evaluate the rate of paramagnetic adduct production upon oxidation of the iron substrates in order to provide a complementary approach to, for instance, the BA/p-HBA conversion rate measurements that have already been developed in the literature. Although EPR spintrapping has been also previously used in the context of air-oxidation of Fe-substrates for fingerprinting reactive species, especially the hydroxyl radical (•OH) [2,4,[START_REF] Cheng | Oxidizing impact induced by mackinawite (FeS) nanoparticles at oxic conditions due to production of hydroxyl radicals[END_REF] , our approach here aimed at tentatively use this method to compare the reactivity of various iron substrates on a quantitative basis. The obtained results yield straightforward information on the relative efficiency of these three substrates to produce reactive species upon short-term oxidation, i.e. within the 1-20 min range. The identity of these reactive species is discussed by comparing air-and H2O2-oxidation experiments conducted in the presence or absence of ethanol as radical scavenger and as a function of phosphate buffer concentration.

Experimental Methods

Syntheses procedures and mineralogical analyses. All syntheses and sample preparation procedures were performed in a Jacomex™ glove box under N2 atmosphere (<5 ppm O2). All solutions were prepared in the glove box with O2-free milli-Q water, i.e. degassed under N2-bubbling for 40 min at 75°C. Fe3O4 (magnetite) was synthesized according to a proven procedure [3,[START_REF] Ona-Nguema | XANES evidence for rapid arsenic(III) oxidation at magnetite, and ferrihydrite surfaces by dissolved O2, via Fe 2+ -mediated reactions[END_REF] by coprecipitating Fe 2+ and Fe 3+ at a final pH of 12. Briefly, acidic stock solutions of 1M FeCl3•6H2O and 1M FeCl2•4H2O were mixed in stoichiometric proportions (2:1) and neutralized to pH 12 by adding appropriate volume of 1M NaOH. The suspension was sealed with butyl rubber stopper, agitated for 24 h at 25°C, three times centrifuged at 6000 rpm for 20 min and washed with O2-free milli-Q water, and vacuum-dried in a dessicator in the glove box. FeS (mackinawite) was synthesized by reacting Fe 2+ with aqueous H2S followed by aging [START_REF] Noël | EXAFS analysis of iron cycling in mangrove sediments downstream of a lateritized ultramafic watershed (Vavouto Bay, New Caledonia)[END_REF][START_REF] Morin | Nickel accelerates pyrite nucleation at ambient temperature[END_REF] . Briefly, a stock solution of 0.625M FeCl2•4H2O was mixed in equal proportions with a 0.625M Na2S solution and was then 7-fold diluted in O2-free milli-Q water. The suspension was sealed with butyl rubber stopper, agitated for 65 h at 25°C, aged for 11 months, centrifuged at 6000 rpm for 20 min and washed three times with O2-free milli-Q water, and vacuum-dried in a dessicator in the glove box.

FeS2 (65% pyrite -34% nanomarcassite -1% a-sulfur) was synthesized by reacting Fe 3+ with aqueous H2S [START_REF] Noël | EXAFS analysis of iron cycling in mangrove sediments downstream of a lateritized ultramafic watershed (Vavouto Bay, New Caledonia)[END_REF][START_REF] Morin | Nickel accelerates pyrite nucleation at ambient temperature[END_REF][START_REF] Baya | Influence of trace level As or Ni on pyrite formation kinetics at low temperature[END_REF] . A stock solution of 0.625M FeCl3•6H2O was mixed in equal proportions with a 0.625M Na2S solution and then 4-fold diluted in O2-free milli-Q water. The suspension was sealed with butyl rubber stopper, agitated for 528 h at 25°C, aged for 19 months, centrifuged at 6000 rpm for 20 min and washed three times with O2-free milli-Q water, and vacuum-dried in a dessicator in the glove box.

The mineralogical composition of each substrate was checked by using Rietveld analysis of powder X-ray Diffraction patterns collected under anoxic conditions. For this purpose, a thin coating of powder was deposited on a pure silicon-wafer -zero-background -sample-holder that was mounted within an anoxic chamber (IMPMC) equipped with a Kapton window. XRD data were collected in Bragg-Brentano geometry using an X'pert pro® Panalytical TM diffractometer equipped with an X'celerator® solid state detector, counting 4h per sample in continuous mode over the 5-80° 2q-range with a 0.0334° 2q-step.

Rietveld analysis of XRD patterns collected on the synthetic samples (Figure S1; Table S1) indicated that the magnetite and mackinawite samples were mineralogically pure except for traces of NaCl in the magnetite sample, whereas the pyrite (cubic FeS2) sample contained 34 wt% of nanosized (4 nm) marcasite (orthorhombic FeS2) and 1 wt% of elemental sulfur. The average MCD sizes of magnetite (~14 nm), mackinawite (~9 nm) and pyrite (~12 nm) were of similar order of magnitude, and were consistent with Brunauer-Emmet-Teller (BET) surface areas determined as 103 and 122 m 2 /g for magnetite and mackinawite, respectively. BET measurements could not be performed on our pyrite sample because of insufficient amount of material.

Oxidation experiments.

Prior to oxidation experiments, mineral suspensions were prepared in the anoxic glove box by suspending a given weight of mineral powder in a solution consisting of 350 µL of 0.1M or 1M phosphate-buffer (pH 7, prepared from anhydrous NaH2PO4 and Na2HPO4), with or without addition of 16µL of 99%vol CH3CH2OH (ethanol), the mixture being then equilibrated overnight within a 5 mL glass vial closed by a butyl rubber stopper. A volume of 300 µL of 92 mM solution of the spin-trap 5,5-Dimethyl-1-pyrroline N-oxide (DMPO ≥ 98.0 % for ESR spectroscopy, Sigma Aldrich® #92688) was then added to the suspension in each vial within the glove box. The vials were then sealed with butyl rubber stoppers and taken out from the glove box for performing the oxidation experiments. Molarity of phosphate, ethanol and DMPO in the reaction vessel were thus 0.053 or 0.525 M, 0.407 M and 41 mM. Ethanol was used as additional radical trap ion order to decipher among possible reactive species, as discussed in the text.

Air-oxidation experiments were conducted by bubbling 100 mL air for 1 min into the mineral suspension followed by 1 min shaking. Air was bubbled using a 50 mL syringe with a needle plunging into the suspension, whereas another free needle ensured evacuation of the headspace overpressure. Immediately after air bubbling, the vials were shaken for 1 min before the suspension was sampled using a 5mL syringe and needle and readily filtered through a 0.2µm nylon filter. The filtrate was rapidly transferred into the Electron Paramagnetic Resonance (EPR) liquid flat-cell for measurements. The first EPR spectrum could be collected about 5 min after the beginning of the air bubbling that was taken as starting time (t = 0 min). The EPR spectrum of the filled liquid flat-cell was then monitored for one hour, collecting an EPR spectrum every five minutes. In order to maximize the radical species yield, complementary 20 min air-oxidation experiments were also performed following the same procedure, but using a gas ramp providing a flow rate of approximately 100 mL/min. Besides, to investigate the role of the dissolved species in free-radical production, complementary 1 min airoxidation experiments were performed on the filtrate of the mineral suspension instead of the full suspension.

Duplicate experiments were performed after having filtrated the anoxic mineral suspension through a 0.2µm nylon filter in the glove box, and single experiments were performed after 3kDa ultrafiltration also in the glove box.

Filtrates were analyzed after oxidation reactions to determine total dissolved Fe concentration. Aliquots of 200 µL were acidified using 69% NORMAPUR® nitric acid addition and appropriately diluted before being analyzed by inductively-coupled plasma-optical emission spectrometry (ICP-OES) (AGILENT 5100 SVDV) at the ALIPP6 platform of Sorbonne University. Uncertainties were ±2% as measured on standard reference samples.

Hydrogen peroxide (H2O2)-oxidation experiments were also carried out for a 12 mg magnetite suspension.

For this purpose, the oxidation was performed by adding 50 µL of a 0.1M H2O2 solution and shaking for 2 min before sampling, filtrating and EPR monitoring, H2O2-spiking being taken as starting time.

For every above-mentioned experiment, suspensions were prepared with 12 mg of magnetite, or 4.6 mg of mackinawite, or 6.2 mg of pyrite. Assuming stoichiometric compositions for our mineral samples, these suspensions had a mineral concentration equivalent to a same total Fe(II) molarity of 78 mM. A blank experiment was also conducted without any solid. The 1 min air-oxidation experiments for magnetite and mackinawite were conducted in triplicate. Single experiments were conducted for pyrite because of low amount of available material.

Electron Paramagnetic Resonance (EPR) data collection and analysis. EPR spectra were collected at room temperature on the filtered solutions inserted in a 150 µL Suprasil® flat cell (JEOL ES-LC12: 10 x 0.3 x 50 mm 3 inner dimensions) using a JEOL FA-300 spectrometer operating at 9.4 GHz (X-band). All spectra were recorded using the same setup of EPR parameters. Microwave power, frequency modulation, amplitude modulation, time constant, and gain were 2 mW, 100kHz, 5 10 -5 Tesla, 30 ms, 800, respectively, resulting in ~2 min long scans of 4096 points over the 0.33204 -0.34204 Tesla (i.e., 3320.4 -3420.4 Gauss) magneticfield range. The magnetic-field offset related to the off-center position of the Hall-probe with respect to the EPR cavity was corrected to -13. 7 10 -4 Tesla, as determined by setting the g value of the Bruker® strongpitch standard to 2.0018.

For each oxidation experiment, a series of EPR spectra was collected on the filtered suspension every ~5 min during ~1 hour after the starting time corresponding to the beginning of air-bubbling in the mineral suspension. These time course measurements were designed to monitor the evolution of the EPR signal after a single 1min air-oxidation of the mineral suspension, as described in the above section. EPR spectra were least squares fit using the EPRPLP code [START_REF] Morin | Modeling EPR powder spectra using numerical diagonalization of the spin hamiltonian[END_REF] in order to extract EPR parameters of the observed free radical-DMPO adducts. This fitting procedure also yielded a scale factor proportional to the abundance of each adduct, which allowed us to monitor their evolution from sample to sample and over time. The scale factors values were converted to concentration units by comparison with the Bruker® strong-pitch EPR spectrum collected in the same conditions, equivalent to 0.11/0.0003 10 13 = 3.67 10 15 spin units per cm, which were then converted to molar concentrations of adducts in the flat-cell volume of 30µL per cm.

X-ray absorption spectroscopy data collection and analysis. X-ray absorption spectroscopy (XAS) analysis at the iron K-edge was performed on dried solid samples collected after similar air-oxidation protocol than that used for the EPR spin trapping experiments. The mineral suspensions contained 12, 4.3 and 6.2 mg of magnetite, mackinawite and pyrite, respectively, and were prepared with added DMPO, CH3CH2OH, and pH 7 phosphate buffer. After 1 min air bubbling, 1 min shaking and needle/syringe sampling, the suspensions were centrifuged for 1 min at 6000 rpm and immediately transferred into the anoxic glove box, in which they were vacuum dried in a dessicator. Pressed pellets were prepared by mixing appropriate amounts of the initial or oxidized mineral powders with cellulose. Data were collected at 10K in transmission detection mode at the SAMBA beamline of the SOLEIL synchrotron facility, Orsay (France), using a 220 double crystal monochromator with sagittal focusing of the second crystal and a liquid He cryostat. Energy was calibrated by setting to 9112 eV the first inflexion point of the absorption edge of a metallic iron foil collected in doubletransmission setup. A series of 3 to 5 fast-scans of 5 min long was merged for each sample. XAS data were merged, normalized and background subtracted using the Athena software [START_REF] Ravel | Data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF] to extract X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectra. XANES and EXAFS data were analyzed using a least-squares Linear Combination Fitting (LCF) procedure, with a homebuilt software based on a Levenberg-Marquardt minimization algorithm [START_REF] Noël | EXAFS analysis of iron cycling in mangrove sediments downstream of a lateritized ultramafic watershed (Vavouto Bay, New Caledonia)[END_REF][START_REF] Morin | Nickel accelerates pyrite nucleation at ambient temperature[END_REF][START_REF] Baya | Influence of trace level As or Ni on pyrite formation kinetics at low temperature[END_REF][START_REF] Morin | Bacterial formation of tooeleite and mixed arsenic(III) or arsenic(V)-iron(III) gels in the Carnoulès acid mine drainage, France. A XANES, XRD and SEM study[END_REF] .

Results and Discussion

Reactive species production after air-oxidation of magnetite, mackinawite or pyrite, with or without CH3CH2OH addition. Typical EPR spectra of the filtrates obtained after 1 min air-oxidation of the CH3CH2OH-added magnetite, mackinawite, pyrite and blank suspensions are displayed in Figure 1. The spectra in Figure 1a were collected on the filtrates at t = 25 min after the initial single 1 min long air-bubbling followed by 1 min shaking and immediate filtration. These spectra are largely dominated by that of an alkyl-DMPO adduct interpreted as the a-hydroxyethyl radical, CH3CH•OH-DMPO, with the following refined EPR parameters: giso =2.0054 (1), AHiso = 0.00217(1) cm -1 [23.2(1) Gauss], ANiso = 0.00150(1) cm -1 [16.0(1) Gauss] with a Gaussian line-shape and a full-width of 1.54(1) Gauss. These parameters are fully consistent with those of the CH3CH•OH-DMPO adduct, as reported by Finkelstein [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF] and Buettner [START_REF] Buettner | Spin Trapping: ESR parameters of spin adducts[END_REF] . In these air-oxidation experiments, a very minor contribution from a •OH-DMPO adduct could be added in the fit but did not exceed 1% of the most intense CH3CH•OH-DMPO signal observed for magnetite, as illustrated in Figure S2. The slight asymmetry of the CH3CH•OH-DMPO adduct signal results from two isomeric adducts with different C2-H proton coupling constants [START_REF] Kotake | Electron spin resonance spectra of diastereomeric nitroxyls produced by spin trapping hydroxyalkyl radicals[END_REF] that are present in unequal proportions [START_REF] Taniguchi | DMPO-alkyl radical spin trapping: an in situ radiolysis steady-state ESR study[END_REF] . Such asymmetry due to the superposition of different conformers has been also reported for other adducts such as the superoxide-DMPO adduct, with obviously different hyperfine parameters [START_REF] Clément | Assignment of the EPR spectrum of 5,5dimethyl-1-pyrroline N-oxide (DMPO) superoxide spin adduct[END_REF] . In our case, a single component fit was found however accurate enough to quantify the CH3CH•OH-DMPO adducts, as compared to a two-component fit (Figure S3; Table S3). 1, with fitting parameters reported in Table 1.

For each substrate, similar shapes were observed for all EPR spectra as a function of time after the single 1 min air-oxidation but the intensity of the CH3CH•OH-DMPO adduct signal significantly varied over the 60 The production of CH3CH•OH-DMPO adduct significantly differed among the mineral substrates studied (Figure 1), being ~30% higher for magnetite than for mackinawite, and being negligible for pyrite, similar to the blank. The production of CH3CH•OH-DMPO adduct appeared more variable for magnetite than for mackinawite over the triplicate experiments (Figure 1). Although yet unexplained, this difference could be due to the ability of nanomagnetite particles to aggregate, which may cause variations in the effective reactive surface among experiments, especially in our high ionic-strength conditions, i.e. 0.525 M phosphatebuffer [START_REF] Vikesland | Particle size and aggregation effects on magnetite reactivity toward carbon tetrachloride[END_REF][START_REF] Favela-Camacho | How to decrease the agglomeration of magnetite nanoparticles and increase their stability using surface properties[END_REF] .

Another important result could be derived from the EPR monitoring of the magnetite and mackinawite suspensions after 1 min air-oxidation without prior addition of CH3CH2OH. Indeed, the results displayed in Free-radical production after oxidation of magnetite by H2O2 with or without CH3CH2OH addition.

Unlike the results obtained for air-oxidation, magnetite oxidation by H2O2 produced high amounts of the •OH-DMPO adduct in the experiment conducted without added CH3CH2OH, as shown in Figure 2. Indeed, the

•OH-DMPO adduct signal largely dominated the EPR spectrum, with the following fitting EPR parameters: giso =2.0054(1), AHiso = 0.00140(1) cm -1 [15.0(1) Gauss], ANiso = 0.00140(1) cm -1 [15.0(1) Gauss] with a Gaussian line-shape and a full-width of 1.28(1) Gauss. These parameters are fully consistent with those reported in the literature for this adduct [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF][START_REF] Buettner | Spin Trapping: ESR parameters of spin adducts[END_REF][START_REF] Finkelstein | Spin trapping of superoxide and hydroxyl radical: Practical aspects[END_REF] . A weak signal from the CH3CH•OH-DMPO adduct was also observed, which is again attributed to the presence of residual CH3CH2OH vapor in the glove box atmosphere during our sample preparations. By contrast, oxidation by H2O2 of the CH3CH2OH added magnetite suspension, produced a strong signal of the CH3CH•OH-DMPO adduct (Figure 2). These results suggest that oxidation of magnetite by H2O2 in our buffered pH 7 conditions lead to •OH radical production. The latter likely quickly reacted with CH3CH2OH to form CH3CH•OH that is then captured by DMPO to form the CH3CH•OH-DMPO adduct, or was directly captured by DMPO to form the •OH-DMPO adduct in the absence of CH3CH2OH [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF][START_REF] Finkelstein | Spin trapping of superoxide and hydroxyl radical: Practical aspects[END_REF] . It is noticeable that H2O2 oxidation generating •OH radicals lead to rapid production of CH3CH•OH-DMPO or •OH-DMPO adducts, followed by their decay (Figure 2). In contrast, air-oxidation did not involve •OH radicals in our pH 7 experiments, yielded either no significant •OH-DMPO adducts or a buildthen-decay of CH3CH•OH-DMPO adduct (Figures 1 and2), in the absence or presence of added CH3CH•OH, respectively. S2. In the absence of ethanol, note the dominant signal of the •OH-DMPO adduct after H2O2 addition and the absence of this signal after air-bubbling. Center panel: EPR-monitoring of the CH3CH•OH-DMPO adduct scale-factor from 5 to 60 min after having started the 1 min air-oxidation or H2O2 oxidation. Right panel: EPR-monitoring of the •OH-DMPO adduct scale-factor from 5 to 60 min after having started the 1 min air-oxidation or H2O2-oxidation (Mt stands for magnetite). The kinetics fitting curves displayed as plain lines are obtained with Eq. 1 or 1', with fitting parameters reported in Table 1.

Effective kinetics of reactive species production. The time-course evolution of the CH3CH•OH-DMPO adduct scale-factor (A) obtained from our EPR monitoring data of air-oxidation (Figure 1) was satisfactorily fitted by a combination of two first-order kinetics model, with Amax being the maximum quantity of adduct and, kprod and kdecay, the effective production and decay rate constants, respectively (Table 1):

! CH3CH•OH-DMPO (,) = ! 012 3 4567
(3 789:; <3 4567 ) (= <3 4567 .? -= <3 789:; .? ) (Eq. 1)

A single first-order decay kinetic model was used to fit the time-course evolution of the •OH-DMPO and CH3CH•OH-DMPO adducts in the H2O2-oxidation experiments (Figure 2; Table 1): .? (Eq. 1')

When converting Amax values into molarity unit by comparison with a strong-pitch EPR-standard, the maximum CH3CH•OH-DMPO adduct concentrations detected in our experiments were found to reach the micro-molar range (Table 1), with about four-fold higher values after H2O2-oxidation of magnetite (~ 12.2 µM) than after air-oxidation (~ 3.0 µM). Note: * fixed value ; n.a.: Eq.1 was not applicable and Eq. 1' was used to fit a single first-order decay.

Interestingly, the maximum •OH-DMPO adduct concentration (~ 12.2 µM) detected upon H2O2-oxidation of magnetite without CH3CH2OH addition was similar to the CH3CH•OH-DMPO adduct concentration (~12.8 µM) detected with CH3CH2OH addition (Figure 2; Table 1), which confirmed the ability of CH3CH2OH to quench almost the whole •OH pool produced in the presence of H2O2 [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF][START_REF] Finkelstein | Spin trapping of superoxide and hydroxyl radical: Practical aspects[END_REF] . In contrast, the maximum concentration of •OH-DMPO adduct (0.01 µM) detected upon air-oxidation of magnetite without CH3CH2OH addition was 300-fold lower than the CH3CH•OH-DMPO adduct concentration detected with CH3CH2OH addition (~3.0 µM) (Figure 2; Table 1), which indicated that •OH was not the main reactive species responsible for CH3CH2OH oxidation upon air-oxidation of magnetite. The origin of the CH3CH•OH-DMPO adduct in this case is further addressed in the next sections.

The effective production rate (kprod) of the CH3CH•OH-DMPO adduct determined from the rise of this signal after filtration of the solids in the air-oxidation experiments (Figure 1b,d) exhibited comparable values for the three mineral substrates studied as well as for the blank experiment (Table 1), yielding an average value of kprod = 0.25±0.03 min -1 , i.e. a half-life of t1/2 = 2.8±3 min. (Table 1). This production rate (kprod) is close to the typical oxidation rate of an oxygen saturated ferrous solution at pH 7 (0.16 min -1 ; t1/2 = 4.3 min [START_REF] Singer | Acidic Dine Drainage: the rate-determining step[END_REF] . This rise of the CH3CH•OH-DMPO adduct signal after filtration thus suggested a contribution from dissolved Fe 2+ species that could further oxidize after filtration, as confirmed by complementary oxidation experiments (Figure 3) and ICP-OES analysis of Fe concentration in the filtrates (Figure 4). Indeed the CH3CH•OH-DMPO adduct was produced by oxidizing the anoxic filtrates of the magnetite or mackinawite suspensions instead of the full-suspension, which resulted in similar kinetics but lower adduct production (Figure 3; Table 1).

Figure 3 EPR monitoring of the DMPO-and CH3CH2OH-added mineral suspensions of magnetite (blue), mackinawite (green), pyrite (brown), after 20 min air bubbling and immediate filtration (plain circles), after 1 min air bubbling, 1 min shaking and immediate filtration (plain squares), and after filtration in anaerobic glove box, 1 min air bubbling, 1 min shaking (0.2 µm : open squares; 3kDa : crosses). The fitting curves displayed as plain lines were obtained with Eq. 1, with fitting parameters reported in Table 1. Experiments conducted in 0.525M phosphate buffer at pH = 7.

For magnetite and mackinawite, respectively, the same amounts of CH3CH•OH-DMPO adducts were obtained from the 1 min air-oxidation of ultra-filtered (3 kDa) and filtered (0.2 µm) suspensions (Figure 3; Table 1), which was consistent with the similar amount of aqueous Fe detected by ICP-OES in the corresponding filtrates (Figure 4). These similar results obtained using 3 kDa and 0.2µm filters indicated that the contribution of colloidal material to CH3CH•OH-DMPO adduct production after filtration is negligible in our protocol. Nevertheless, the amount of CH3CH•OH-DMPO adduct produced by oxidizing the filtrates represented however only ~40% of the amount of adduct produced by oxidizing the full mineral suspension for 1 min (Figure 3; Table 1). This result indicated that Fe(II) from the mineral surfaces (Figure 4; Table S4 contributed to more than half of the amount of CH3CH•OH-DMPO adduct produced upon oxidation of the full mineral suspensions (Figure 3; Table1). Partial oxidation of Fe(II) to Fe(III) in the solid phase is also observed directly using XANES spectroscopy upon 1 min air-oxidation of the full mineral suspensions (Figure S4 ; Table S4). The amount of oxidized Fe(II) is however found to be lower than the available amount of surface Fe(II) (Table S4). The remaining available solid Fe(II) pool may then explain the higher CH3CH•OH-DMPO adduct production observed after 20 min air-oxidation (Figure 3; Table 1). 3 for magnetite (blue) and mackinawite (green). Reported dissolved concentrations correspond to total iron determined by ICP-OES. Reported solid concentrations correspond to Fe(II) in the added mineral particles (78 mM) split into a surface Fe(II) atoms pool calculated for a surface shell of 3 Å thickness, and a remaining bulk Fe(II) pool, assuming average particle diameters of 140 and 90 Å for our magnetite and mackinawite, respectively (Table S2).

Owing to the long half-life of the CH3CH•OH-DMPO adduct in our air-oxidation experiments (t1/2 = 360 min), we also investigated the production of this adduct by EPR monitoring of the filtrate obtained after 20 min airoxidation of the mineral suspensions (Figure 3; Table 1). These longer oxidation experiments produced 30-40% more CH3CH•OH-DMPO adduct than the 1 min air-oxidation experiments and the shape of the kinetic curves approached a simple decay curve (Figure 3), which suggested that the major fraction of both the aqueous and mineral-surface pool of readily oxidizable Fe 2+ would be likely reached within the 30 min of air-oxidation.

The results also confirmed that magnetite was more reactive than mackinawite, and that pyrite was not reactive under our reaction conditions, for a similar equivalent total Fe 2+ solid concentration (78 mmol.L -1 ). Finally, the effective decay rate (kdecay) values of the CH3CH•OH-DMPO adduct were scattered among the various experiments, with half-lives in the 40 -400 min range. It is noticeable that the decay rate of this adduct in the magnetite oxidation experiments was about 10-fold higher with H2O2 addition (kdecay = 0.0179 ± 0.0002 min -1 ; t1/2 = 38 ± 2 min) than without (kdecay = 0.0019 ± 0.0002 min -1 ; t1/2 = 360 ± 40 min) (Table 1), which may be related to the complexity of decay processes for this adduct [START_REF] Kirino | Spin trapping with 5,5-Dimethylpyrroline-N-oxide in aqueous solution[END_REF] .

Identity of the reactive species produced after air-or H2O2-oxidation of magnetite and mackinawite.

The results of our air-oxidation experiments in the presence of CH3CH2OH indicate that the reaction of •OH captured by DMPO, from the same adduct deriving from the decomposition of O2• --DMPO [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF][START_REF] Finkelstein | Spin trapping of superoxide and hydroxyl radical: Practical aspects[END_REF] . This latter decomposition is however very limited, typically below a few percent of the O2• --DMPO pool [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF][START_REF] Buettner | The spin trapping of superoxide and hydroxyl free radicals with DMPO (5,5-dimethylpyrroline -Noxide): more about iron[END_REF] .

However, even though no O2• --DMPO adduct signal is observed in our EPR spectra, we cannot exclude the formation of the superoxide radical in our experiments since the half-life of the O2• --DMPO adduct is typically shorter (~ 1 min) [START_REF] Buettner | The spin trapping of superoxide and hydroxyl free radicals with DMPO (5,5-dimethylpyrroline -Noxide): more about iron[END_REF][START_REF] Buettner | Consideration in the spin trapping of superoxide and hydroxyl radical in aqueous systems using 5,5-Dimethyl-1-oxide[END_REF] than the delay between the air-oxidation of our mineral suspension and the first EPR spectrum (5 min) of our time-series (Figures 1 and2). The decay of the O2• --DMPO adduct have been reported to be even faster in the presence of Fe 2+ [START_REF] Buettner | The spin trapping of superoxide and hydroxyl free radicals with DMPO (5,5-dimethylpyrroline -Noxide): more about iron[END_REF][START_REF] Buettner | Consideration in the spin trapping of superoxide and hydroxyl radical in aqueous systems using 5,5-Dimethyl-1-oxide[END_REF] , thus requiring more specific spin traps to better stabilize superoxide ion adducts [START_REF] Frejaville | 5-(Diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide: a new efficient phosphorylated nitrone for the in vitro and in vivo spin trapping of oxygen-centered radicals[END_REF][START_REF] Zhao | Synthesis and biochemical applications of a cyclic nitrone spin trap: a relaticely superior trap for detecting superoxide anions and glutathiyl radicals[END_REF] . Nevertheless, even if we cannot exclude the formation of O2• -, this species cannot be responsible for the H-atom abstraction from CH3CH2OH observed in our air-oxidation experiments. Indeed, the inability of O2• -to directly react with CH3CH2OH to form the CH3CH•OH radical is classically used as for distinguishing it from •OH [START_REF] Finkelstein | Production of hydroxyl radical by decomposition of superoxide spintrapped adducts[END_REF][START_REF] Finkelstein | Spin trapping of superoxide and hydroxyl radical: Practical aspects[END_REF] .

Another intermediate species has then to be invoked to explain CH3CH2OH oxidation in our air-oxidation experiments at pH 7. Such species could be Fe(IV), in the form of the ferryl ion, Fe IV O 2+ , as suggested by several hints. The ferryl ion has been proposed as an additional product of the Fenton reaction, based on experimental [START_REF] Bray | Ferryl ion, a compound of tetravalent iron[END_REF][START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][37][START_REF] Pignatello | Evidence for an additional oxidant in the photoassisted Fenton reaction[END_REF][START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF][START_REF] Lee | pH-Dependent reactivity of oxidants formed by iron and copper-catalyzed decomposition of hydrogen peroxide[END_REF] and theoretical investigations [START_REF] Ensing | Chemical involvement of solvent water molecules in elementary steps of the Fenton oxidation reaction[END_REF][START_REF] Ensing | A Car-Parrinello study of the formation of oxidizing intermediates from Fenton's reagent in aqueous solution[END_REF][START_REF] Ensing | Fenton-like chemistry in water: oxidation catalysis by Fe(III) and H2O2[END_REF] . The ferryl ion was especially proposed to explain the decrease in •OH production rate observed in the Fenton reaction when Fe 2+ is chelated by phosphate, adenosine diphosphate (ADP) and ethylenediaminetetraacetate (EDTA) and to be responsible for oxidizing ethanol when Fe 2+ is chelated by EDTA [START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF] . Unlike suggested by Walling and Amarnath [START_REF] Walling | Oxidation of Mandelic Acid by Fenton's Reagent[END_REF] , the ferryl ion is not considered as an intermediate in the •OH production process, which was rather demonstrated to proceed by direct H2O2 homolytic cleavage in the Fenton reaction [START_REF] Lloyd | The origin of the hydroxyl radical oxygen in the Fenton reaction[END_REF] . However, the ferryl ion could form as an additional species from the reaction of Fe 2+ with H2O2 [START_REF] Bray | Ferryl ion, a compound of tetravalent iron[END_REF][START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF][START_REF] Ensing | Chemical involvement of solvent water molecules in elementary steps of the Fenton oxidation reaction[END_REF][START_REF] Ensing | A Car-Parrinello study of the formation of oxidizing intermediates from Fenton's reagent in aqueous solution[END_REF][START_REF] Walling | Oxidation of Mandelic Acid by Fenton's Reagent[END_REF][START_REF] Lloyd | The origin of the hydroxyl radical oxygen in the Fenton reaction[END_REF] and would be stabilized when iron is chelated by appropriate ligands [START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][37][START_REF] Price | The first direct characterization of a high-valent iron intermediate in the reaction of an alpha-ketoglutarate-dependent dioxygenase: a high-spin Fe(IV) complex in taurine/alpha-ketoglutarate dioxygenase (TauD) from Escherichia coli[END_REF][START_REF] Lacy | Structure, and EPR detection of a High Spin Fe IV -oxo species derived from either an Fe III -oxo or Fe III -OH complex[END_REF] under near-neutral to alkaline pH conditions [START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF][START_REF] Halliwell | Biologically relevant metal ion-dependent hydroxyl radical generation, an update[END_REF][START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF] . More recently, the ferryl ion has been shown to be produced from the reaction of Fe 2+ with other strong oxidants such as ozone [START_REF] Jacobsen | Reactions of the ferryl ion with some compounds found in cloud water[END_REF][START_REF] Pestovsky | Reactivity of aqueous Fe(IV) in hydride and hydrogen atom transfer reactions[END_REF][START_REF] Pestovsky | Aqueous FeIV=O: spectroscopic identification and oxo-Group exchange[END_REF] , peroxysulfates [START_REF] Dong | Both Fe(IV) and radicals are active oxidants in the Fe(II)/peroxydisulfate process[END_REF][START_REF] Wang | Relative contribution of ferryl ion species (Fe(IV)) and sulfate radical formed in nanoscale zero valent iron activated peroxydisulfate and peroxymonosulfate processes[END_REF] and periodate [START_REF] Zong | Enhanced oxidation of organic contaminants by iron(II)-activated periodate: The significance of high-valent iron-oxo species[END_REF] .

The formation of the ferryl ion under sole airborne O2-oxidation of Fe 2+ is debated, since •OH production is thought to be generated at acidic pH [2,[START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF] and is proposed by some authors for near-neutral pH conditions [3,4,[START_REF] Cheng | Oxidizing impact induced by mackinawite (FeS) nanoparticles at oxic conditions due to production of hydroxyl radicals[END_REF][START_REF] Tong | Production of abundant hydroxyl radicals from oxygenation of subsurface sediments[END_REF][START_REF] Pryor | Oxy-radicals and related species: Their formation, lifetimes and reactions[END_REF][START_REF] Zalma | Formation of oxy radicals by oxygen reduction arising from the surface activity of asbestos[END_REF] . Nevertheless, the ferryl ion has been proposed to form upon air-oxidation of Fe 2+ [START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF] or zero-valent iron [START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Kim | Polyphosphate-enhanced production of reactive oxidants by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen: Yield and nature of oxidants[END_REF] at near-neutral pH. The EPR spin trapping studies by Reinke et al. [START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF] and Welch et al. [START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] are particularly relevant to the interpretation of our results. Indeed they show that the CH3CH•OH-DMPO adduct is formed after air-oxidation of dissolved Fe 2+ in the presence of CH3CH2OH, while no •OH-DMPO is detected in the absence of CH3CH2OH. Reinke et al. [START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF] proposed that the ferryl ion may be formed by airoxidation of Fe 2+ and would then oxidize CH3CH2OH into CH3CH•OH, consistently with the known ability of the ferryl ion for this hydrogen atom abstraction reaction [START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Jacobsen | Reactions of the ferryl ion with some compounds found in cloud water[END_REF][START_REF] Pestovsky | Reactivity of aqueous Fe(IV) in hydride and hydrogen atom transfer reactions[END_REF] . In addition, Reinke et al. [START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF] noticed that the intensity of the CH3CH•OH-DMPO adduct increased with increasing the concentration of phosphate buffer upon homogeneous oxidation of aqueous Fe(II) species, which they explained by the ability of appropriate Fe(II)-ligands to promote Fe(IV) over the hydroxyl radical in the Fenton reaction [START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] . S2. Right panel: EPR-monitoring of the CH3CH•OH-DMPO adduct scale-factor from 5 to 60 min after having started the 1 min air-oxidation. The kinetics fitting curves displayed as plain lines are obtained with Eq. 1 or 1', with fitting parameters reported in Table 1.

Here, we show that this effect is also observed in the case of heterogeneous air-oxidation of Fe 2+ from mineral substrates. This result is consistent with a surface-localized oxidant, distinct from the aqueous hydroxyl radical, and recently reported as forming upon oxygenation of mackinawite [START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF] .

Moreover, the results presented in Figure 5 indicate that decreasing the phosphate buffer concentration by a 10-fold factor, from 0.525 M to 0.053 M, decreases the CH3CH•OH signal by a factor of about 2.5 to 3.5 for magnetite and mackinawite, respectively (Table 1). The mechanisms, by which phosphate and other specific ligands may promote the ferryl ion upon H2O2 or air-oxidation of ferrous iron in aqueous media, have not been yet elucidated [START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] . Nevertheless, the net boosting of CH3CHOH oxidation when increasing the concentration of the phosphate buffer supports the formation of the ferryl ion in our experiments.

Based on these findings, and without considering yet the putative role of phosphate, we might hypothesize the following overall reactions for explaining the formation and decay of the observed CH3CH•OH-DMPO adduct in our air-oxidation experiments of magnetite and mackinawite (Reactions 1a-c to 4): Fe II Fe III 2O4(magnetite) + ½ O2 + 2H + ó Fe IV O 2+ + g-Fe III 2O3(maghemite) + H2O (1a)

Fe II S(mackinawite) + 5 2 B O2 ó Fe IV O 2+ + SO4 2- (1b) Fe 2+ + ½ O2 ó Fe IV O 2+ (1c)
Fe IV O 2+ + CH3CH2OH ó Fe III (OH) 2+ + CH3CH•OH ; k = 2.5 10 3 M -1 .s -1 [START_REF] Jacobsen | Reactions of the ferryl ion with some compounds found in cloud water[END_REF] (2) CH3CH•OH + DMPO ó CH3CH•OH-DMPO; k = 4 10 7 M -1 .s -1 [START_REF] Taniguchi | An in situ radiolysis time-resolved ESR study of the kinetics of spin trapping by 5,5dimethyl-1-pyrroline-N-oxide[END_REF] (3)

CH3CH•OH-DMPO ó non-paramagnetic species (4)
In reaction 1a, hypothetical production of the ferryl ion is proposed to occur via Fe 2+ oxidation at the surface of magnetite that is known to transform into maghemite under similar air-oxidation experiments [3] (Figure S4).

Reaction 1b represents hypothetical production of the ferryl ion via surface oxidation of FeS, whereas the ultimate solid product was ferrihydrite in our oxidation conditions, as indicated by our EXAFS results (Figure S4). The actual FeS surface oxidation mechanisms may likely be more complex since multiple other reactive species, albeit distinct from aqueous •OH, have been proposed to form, especially surface-localized entity such as high-valent iron or surface-associated hydroxyl or sulfur-based radical [START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF] . Reaction 1c represents the hypothetical formation of the ferryl ion via air-oxidation of aqueous Fe 2+ , as proposed in previous studies, though it must be recalled here that Fe 2+ complexation by appropriate ligands are thought to actually favor the ferryl ion formation upon oxidation [START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] .

Once the ferryl ion would have been produced, it would be expected to exert a very fast H-atom abstraction from ethanol [START_REF] Jacobsen | Reactions of the ferryl ion with some compounds found in cloud water[END_REF] (reaction 2), with an effective first-order kinetic constant of ~1000 s -1 , i.e. a half-life of 0.7 ms, for the ethanol molarity in our experiments (0.407 M). Since then, the formation of the CH3CH•OH-DMPO adduct [START_REF] Taniguchi | An in situ radiolysis time-resolved ESR study of the kinetics of spin trapping by 5,5dimethyl-1-pyrroline-N-oxide[END_REF] (reaction 3) is expected to be even faster with an effective kinetics constant of 1.6 10 6 s -1 , i.e. a halflife of 0.4 µs, considering the DMPO molarity in our experiments (0.041 M). Reactions 2 and 3 are thus extremely fast compared to the minute/hour time-scale of our time-course EPR monitoring (Figures 1 and2), which leads to attribute the observed build and decay of the CH3CH•OH-DMPO adduct signal, to reactions 1a-c and 4, respectively. The observed kinetic constants for the build of the CH3CH•OH-DMPO adduct (Figure 1; Table 1) are close to that of Fe 2+ air-oxidation at pH = 7, i.e. a half-life of a few minutes, which supports the role of Fe 2+ air-oxidation in the building of the CH3CH•OH-DMPO adduct signal.

Hypothetical mechanisms for ferryl formation upon aqueous air-oxidation of ferrous iron. According to literature findings discussed above and based on our comparative EPR spin-trapping results obtained with or without CH3CH2OH addition, we may hypothesize that Fe IV O 2+ could form after reaction of dissolved O2 with the magnetite and mackinawite surface together with aqueous Fe 2+ species (reactions 1a-c). However, despite this reaction has been proposed previously in the literature [START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] , its mechanism is actually unclear for aqueous solution. Indeed, most authors refer to a reaction between Fe 2+ and H2O2 to form Fe IV O 2+ [START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] based on the Fenton reaction mechanisms that intrinsically involve H2O2 [START_REF] Bray | Ferryl ion, a compound of tetravalent iron[END_REF][START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF][START_REF] Ensing | Chemical involvement of solvent water molecules in elementary steps of the Fenton oxidation reaction[END_REF][START_REF] Ensing | A Car-Parrinello study of the formation of oxidizing intermediates from Fenton's reagent in aqueous solution[END_REF][START_REF] Walling | Oxidation of Mandelic Acid by Fenton's Reagent[END_REF][START_REF] Lloyd | The origin of the hydroxyl radical oxygen in the Fenton reaction[END_REF] :

Fe 2+ + H2O2 ó Fe IV O 2+ + H2O (5)
Reaction 5 is reported to actually compete with the classical Haber-Weiss reaction [START_REF] Buettner | Consideration in the spin trapping of superoxide and hydroxyl radical in aqueous systems using 5,5-Dimethyl-1-oxide[END_REF][START_REF] Ensing | Fenton-like chemistry in water: oxidation catalysis by Fe(III) and H2O2[END_REF][START_REF] Haber | Über die katalyse des hydroperoxydes[END_REF] , the latter, reaction 6, being thought to be rather favored at low pH [2,[START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF][START_REF] Lloyd | The origin of the hydroxyl radical oxygen in the Fenton reaction[END_REF][START_REF] Halliwell | Biologically relevant metal ion-dependent hydroxyl radical generation, an update[END_REF][START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF][START_REF] Halliwell | Oxygen Free Radicals and Iron in Relation to Biology and Medicine: Some Problems and Concepts[END_REF] :

Fe 2+ + H2O2 ó Fe 3+ + •OH + OH - (6)
Indeed, in near neutral pH conditions, reaction 5 is thought to proceed via an iron peroxide inner-sphere complex [START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Ensing | Chemical involvement of solvent water molecules in elementary steps of the Fenton oxidation reaction[END_REF][START_REF] Ensing | A Car-Parrinello study of the formation of oxidizing intermediates from Fenton's reagent in aqueous solution[END_REF][START_REF] Bossmann | New evidence against hydroxyl radicals as reactive intermediates in the thermal and photochemically enhanced Fenton reactions[END_REF] , that might convert into an hydroxo ferryl ion, via heterolytic O-O cleavage [START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF] :

Fe II (OH) + + H2O2 ó Fe II (OH)(H2O2) + ó Fe IV O(OH) + + H2O (5b)

In contrast, in acidic conditions, reaction 6 is thought to proceed via a Fe II (H2O2) 2+ complex that might further form the hydroxyl radical via homolytic O-O cleavage [START_REF] Bataineh | pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction[END_REF][START_REF] Lloyd | The origin of the hydroxyl radical oxygen in the Fenton reaction[END_REF] :

Fe 2+ + H2O2 ó Fe II (H2O2) 2+ ó Fe III (OH) 2+ + •OH (6b)
In the case of airborne O2 oxidation of Fe 2+ , the production of H2O2 is actually uncertain but has been proposed to proceed via the superoxide radical [2,3,[START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF][START_REF] Pryor | Oxy-radicals and related species: Their formation, lifetimes and reactions[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] :

Fe 2+ + O2 ó Fe 3+ + O2• - (7)
which might then react with Fe 2+ [2,3,[START_REF] Keenan | Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen[END_REF][START_REF] Buettner | Consideration in the spin trapping of superoxide and hydroxyl radical in aqueous systems using 5,5-Dimethyl-1-oxide[END_REF][START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF][START_REF] Rush | Pulse radiolytic studies of the reactions of HO2/O2with Fe(II)/Fe(III) ions. The reactivity of HO2/O2with ferric ions and its implication on the occurrence of the Haber-Weiss reaction[END_REF] :

Fe 2+ + O2• -+ 2H + ó Fe 3+ + H2O2 (8)
or might dismutate [2,[START_REF] Hug | Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction[END_REF][START_REF] Pryor | Oxy-radicals and related species: Their formation, lifetimes and reactions[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF] :

2O2• -+ 2H + ó H2O2 + O2 (9)
We cannot exclude initial production of the superoxide radical in our experiments, though we were unable to detect the short living O2• --DMPO adduct [START_REF] Buettner | The spin trapping of superoxide and hydroxyl free radicals with DMPO (5,5-dimethylpyrroline -Noxide): more about iron[END_REF][START_REF] Buettner | Consideration in the spin trapping of superoxide and hydroxyl radical in aqueous systems using 5,5-Dimethyl-1-oxide[END_REF] in the present study.

Because phosphate is shown to enhance non-hydroxyl reactive species production in our experiments, one could derive from reaction 5b the following reaction to describe ferryl formation from a ferrous phosphate complex in the presence of phosphate at neutral pH:

Fe II (HPO4) + H2O2 ó Fe II (HPO4)(H2O2) ó Fe IV O(HPO4) + H2O (10) Eventually, based on reactions mechanisms reported for O2 activation by ligand-stabilized Fe(IV)-species in non-aqueous solvents via µ-peroxo-bridged diiron(III) species [START_REF] Kim | Dioxygen activation and catalytic aerobic oxidation by a mononuclear nonheme Iron(II) Complex[END_REF] , one might also hypothesize that Fe IVphosphate species could directly derive from the O2-oxidation of Fe II -phosphate species in aqueous systems.

Based on the reported O2-activation pathway [START_REF] Kim | Dioxygen activation and catalytic aerobic oxidation by a mononuclear nonheme Iron(II) Complex[END_REF] , one may speculate, for instance, a process as follows at neutral pH:

[Fe II (HPO4)]2 + O2 ó [(HPO4)Fe III O-OFe III (HPO4)] ó 2[Fe IV O(HPO4)] (11)
Our results (Figures 3 and4) indicate that both aqueous and surface-bound Fe(II)-phosphate complexes may contribute to putative ferryl formation. Reactions 10 and 11 are yet undemonstrated but might be of interest to investigate for better understanding the formation mechanisms of the ferryl ion in aqueous systems.

However, direct detection of Fe(IV) in oxygenated aqueous mineral suspensions would be particularly challenging since only a few occurrence of aqueous Fe(IV) has been directly reported and, yet solely in the presence of strong oxidants as O3 [START_REF] Pestovsky | Aqueous FeIV=O: spectroscopic identification and oxo-Group exchange[END_REF][START_REF] Gu | Role of ferryl ion intermediates in fast Fenton chemistry on aqueous microdroplets[END_REF][START_REF] Enami | Fenton chemistry at aqueous interfaces[END_REF] . The first study reported the formation of [Fe IV (O)-(H2O)5] 2+ with a t1/2 value of 7s at 25°C from O3-oxidation or Fe 2+ at acidic pH [START_REF] Pestovsky | Aqueous FeIV=O: spectroscopic identification and oxo-Group exchange[END_REF] . More recent studies using mass-spectroscopy have demonstrated the formation of Fe(IV) from O3-oxidation of Fe(II)-chloride bulk solution [START_REF] Gu | Role of ferryl ion intermediates in fast Fenton chemistry on aqueous microdroplets[END_REF] and droplets [START_REF] Enami | Fenton chemistry at aqueous interfaces[END_REF] . Hence, further work will be needed to investigate the actual roles of O2• -, H2O2 and of phosphate or other ligands in the putative formation of the ferryl ion during airborne O2 oxidation of Fe(II)-mineral substrates in aqueous media.

Environmental Implications. Our results show that fine-particle Fe3O4 (magnetite) is slightly more efficient than fine-particle FeS (mackinawite) for producing reactive species during a short (1 -20 min long) airoxidation in our reaction medium, whereas FeS2 (pyrite + nanomarcasite) appeared unable to produce similar reactive species within the same amount of time, although with comparable surface areas and similar Fe 2+ molarity. Although the production of reactive species upon air-oxidation has been scarcely compared for various Fe phases within a same study, it has been reported for specific phases via their ability to convert benzoic acid (BA) into para-hydroxybenzoic acid (p-HBA) [START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF][START_REF] Cheng | Oxidizing impact induced by mackinawite (FeS) nanoparticles at oxic conditions due to production of hydroxyl radicals[END_REF] . Especially, FeS has been reported to be effective at generating reactive species upon oxygenation [START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF][START_REF] Cheng | Oxidizing impact induced by mackinawite (FeS) nanoparticles at oxic conditions due to production of hydroxyl radicals[END_REF] , with the most recent study suggesting that surface reactive species, including hypothetical surface Fe(IV), mainly contribute to BA conversion into to dihydroxybenzoic acid (DHBA) rather than HBA [START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF] .

In this context, our EPR-spin trapping results, are complementary to previously reported findings based on BA-conversion measurements and could then also contribute to help selecting reactive substrates for airborne O2-based oxidation processes. An EPR-based approach has been for instance recently used to elucidate the radicals involved in HAP photodegradation by Fe-bearing clays [START_REF] Zhao | Humidity induces the formation of radicals and enhances photodegradation of chlorinated-PAHs on Fe(III)-montmorillonite[END_REF] . Further work could be envisaged to compare the efficiency of mackinawite and magnetite with that of Fe-bearing clays that have been also recognized as able to produce reactive species [6] and to degrade pollutants [5] upon sole air-oxidation. In contrast, our results suggest that pyrite might not be an efficient candidate for the production of reactive species upon air-oxidation at neutral pH, though it has been proven as an efficient substrate for the heterogeneous Fenton reaction using strong oxidants [START_REF] Liu | Fenton/Fenton-like processes with in-situ production of hydrogen peroxide/hydroxyl radical for degradation of emerging contaminants: Advances and prospects[END_REF] .

Conclusion

Our results show that the hydroxyl radical may not be the major reactive species generated after short airoxidation (1 -20 min) of magnetite and mackinawite, which differs with previous results reported using para-hydroxybenzoic acid (p-HBA) measurements on mackinawite [START_REF] Cheng | Oxidizing impact induced by mackinawite (FeS) nanoparticles at oxic conditions due to production of hydroxyl radicals[END_REF] . This difference may in part arise from the use of a phosphate buffer in our experiments instead of the PIPES buffer used in ref [11]. However, DMPO spin-trapping using ethanol as intermediate radical trap in our EPR experiments performed at high phosphate buffer concentration helps to reveal non-hydroxyl radical species that may actually be present in lower amount in the absence of phosphate. Indeed, a careful analysis by He et al. [START_REF] He | Production of a surface-localized oxidant during oxygenation of mackinawite (FeS)[END_REF] of the BA conversion products during oxygenation of mackinawite has revealed that a surface species, possibly attributed to Fe(IV), is dominant at pH 7 in HEPES buffer even in the absence of phosphate. Our results on magnetite and mackinawite confirm this latter study in showing that a reactive species distinct from the aqueous hydroxyl radical is produced upon oxygenation of magnetite and mackinawite. It is interpreted as Fe(IV) species since it is favored when increasing phosphate buffer concentration [START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF] . This species oxidizes ethanol and produces a strong signal from DMPO-alkyl adducts that is observed with a much lower intensity in the absence of phosphate [4] . Our results also suggest that, in the presence of O2 instead of strong oxidants, the ferryl ion would be unable to produce •OH, which gives nuance to previous findings from Fenton chemistry [START_REF] Wang | Aqueous iron(IV)-oxo complex: an emerging powerful reactive oxidant formed by iron(II)-based advanced oxidation processes for oxidative water treatment[END_REF] . Consequently, we show that ligands such as orthophosphate, that have been proposed to favor the hypothetical formation of the ferryl ion from the oxidation of aqueous Fe 2+ species [START_REF] Yamazaki | ESR spin-trapping studies on the reaction of Fe 2+ ions with H2O2-reactive species in oxygen toxicity in biology[END_REF][START_REF] Yamazaki | EPR spin-trapping study on the oxidizing species formed in the reaction of the ferrous ion with hydrogen peroxide[END_REF][START_REF] Reinke | Characteristics of an oxidant formed during iron(II) autoxidation[END_REF][START_REF] Welch | Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators[END_REF][START_REF] Wang | Effects of chelators on the on the production and nature of the reactive intermediates formed in Fe(II) activated peroxydisulfate and hydrogen peroxide processes[END_REF] can dramatically enhance the efficiency of nanocrystalline Fe 2+ -bearing substrates to produce such a reactive species. This finding raises questions on the putative role that such ligands could have on the identity and proportions of reactive species produced by airoxidation of natural soils and sediment pore-waters. Further research might then be pursued to actually evaluate the ability of Fe(IV) to degrade organic pollutants upon air-oxidation of Fe 2+ -bearing substrates, though the role of this species has been recently invoked in organic contaminant degradation processes that use peroxysulfates for oxidizing the iron substrate [START_REF] Wang | Is sulfate radical really generated from peroxydisulfate activated by Iron(II) for environmental decontamination ?[END_REF][START_REF] Wang | Relative contribution of ferryl ion species (Fe(IV)) and sulfate radical formed in nanoscale zero valent iron activated peroxydisulfate and peroxymonosulfate processes[END_REF] .

Supporting Information

Rietveld refinement of XRD powder patterns of the mineral substrates in Figure S1.

Mineralogical composition, unit cell parameters and MCD sizes in Table S1.

Fitting of the CH3CH•OH-DMPO and •OH-DMPO adducts EPR spectra in Figures S2 andS3.

EPR parameters of the CH3CH•OH-DMPO adduct in Tables S2 andS3.

Fitting of the Fe K-edge XANES and EXAFS spectra for the mineral substrates before and after airoxidation, in Figure S4.

Dissolved and solid Fe concentrations in air-oxidation experiments in Table S4. Table S1. Mineralogical composition (wt%), unit-cell parameters and mean coherent domain (MCD) sizes of the synthetic samples used in this study, as determined by Rietveld analysis of XRD powder patterns displayed in Figure S1. Uncertainties on the last digit are reported under brackets. Rietveld refinement was performed using the XND code [1] with starting parameters taken from crystal structure data by Fleet [2] for magnetite, Wyckoff [3] for halite and pyrite, Lennie et al. [4] for mackinawite, Buerger [5] for marcasite, Rettig and Trotter [6] for α-sulfur. Scale factors were refined for all phases. Unit-cell and line-shape parameters were refined for all phases except halite. Atomic parameters were fixed, except for mackinawite, for which the z position of the S atom and the occupancy factor of the Fe atom were refined to 0.221(4) and 106%, respectively. S2). The asymmetry of the experimental spectrum of the CH3CH•OH-DMPO adduct is explained by the coexistence of at least two isomers, as shown by a detailed analysis of the more intense spectrum obtained after H2O2-oxidation, see Figure S3 and Table S3. Table S2. EPR parameters of the CH3CH•OH-DMPO and •OH-DMPO adducts, as determined by fitting the spectrum obtained 25 min after 1 min air-oxidation of magnetite. Uncertainty on the fitting parameters is ±1 on the last digit. See Figure S2.

Sample

. Fitting results for a typical EPR spectrum of the CH3CH2OH-and DMPO-added magnetite (12 mg) suspension filtered after H2O2-oxidation. The asymmetry of the CH3CH•OH-DMPO adduct spectrum can be interpreted as a mixture of two isomers spectra (see text). Experimental and fit curves are displayed in black and orange colors, respectively. Weighted fitting components are displayed in light-gray color. Fitting parameters are reported in Table S3.

Table S3. EPR parameters of the CH3CH•OH-DMPO and •OH-DMPO adducts, as determined by fitting the spectrum obtained 25 min after H2O2 oxidation of magnetite. Uncertainty on the fitting parameters is ±1 on the last digit. See Figure S3. Results of LCF analysis of Fe K-edge XAS data collected on the solid substrate samples before and after 1 min air-bubbling and 1 min shaking of the suspensions added with CH 3 CH 2 OH, DMPO and a 0.525M phosphate buffer at pH 7. Experimental data for magnetite (12 mg), mackinawite (9.2 mg) and pyrite (6.2 mg) are displayed in blue, green and brown, respectively. Fitted curves are displayed in light-gray color and weighted fitting components are displayed below each spectrum. Left: XANES spectra; Center : k 3 -weighted EXAFS spectra ; Right : FFT modulus and imaginary part of the experimental and fitted k 3 -EXAFS spectra. The slight mismatch observed in the LCF of oxidized mackinawite could be due to minor contribution from greigite, not included in the fit. XANES LCF results show that our air-oxidation procedure was sufficient to produce detectable change in iron oxidation state in magnetite and mackinawite, likely at the surface of the fine particles. Although less accurate for evaluating iron redox state, EXAFS LCF indicated that maghemite and ferrihydrite were the major oxidation products of magnetite and mackinawite, respectively. Such Fe(II) oxidation is consistent with its possible role in the production of reactive species observed by EPR spin-trapping analysis. In contrast, no significant oxidation of Fe(II) was observed for pyrite, which was found to be unable to produce detectable reactive species in our experimental conditions.

Figure 1 .

 1 Figure 1. EPR monitoring of the DMPO-added mineral suspension (0.525 M phosphate buffer pH 7) after 1 min air-bubbling, 1 min shaking and immediate filtration. (a) EPR spectra taken 25 min after the start of 1 min air-bubbling of the magnetite (blue), mackinawite (green), pyrite (brown) and blank (gray) suspensions with prior addition of CH3CH2OH, and of the magnetite (blue) or mackinawite (green) experiments conducted with or without prior addition of CH3CH2OH. Fitting curves are displayed as light-gray lines, with fitting parameters reported in Table S2. (b) time-course evolution of the fitted CH3CH•OH-DMPO adduct scale factor during 5 to 60 min EPR monitoring of the filtrates. Error bars correspond to the standard deviation on triplicate experiments for magnetite and mackinawite. The kinetics fit curves displayed as plain lines were obtained with Equation1, with fitting parameters reported in Table1.

  monitoring. These variations are displayed on Figure1b, which represents the time-evolution of the scale factor value for the CH3CH•OH-DMPO adduct, determined by fitting each EPR spectrum collected every 5 min after the single oxidation. Interestingly, these time-course variations suggest that the CH3CH•OH-DMPO adduct continues to be produced in the filtrate after the solution has been separated from the reactive mineral substrate. The origin of this partly delayed production of CH3CH•OH-DMPO adduct is examined thereafter in the kinetic section.

Figure

  Figure1a,b show that no significant signal is detected without CH3CH2OH addition, unless a weak CH3CH•OH-DMPO adduct signal similar in height to that in the blank experiment (Figure1a,b). This small signal is attributed to the presence of residual CH3CH2OH vapor in the glove box atmosphere during our sample preparations. It is especially remarkable that virtually no •OH-DMPO adduct signal is detected without prior addition of CH3CH2OH, indicating that •OH radical is not significantly produced after 1 min air-oxidation of magnetite or mackinawite in our 0.525 M phosphate buffered pH 7 conditions. This result indicates that the formation of the CH3CH•OH-DMPO adduct observed for the CH3CH2OH added magnetite or mackinawite suspensions (Figure1a,b) cannot be due to the reaction of CH3CH2OH with •OH radical.

Figure 2 .

 2 Figure 2. EPR spin trapping analysis of the DMPO-added magnetite suspension (0.525 M phosphate buffer pH 7) filtered after either H2O2 addition (orange) or 1 min air bubbling (blue) in the presence or absence of CH3CH2OH. Left panel: EPR spectrum of each experiment collected 10 min after having started the 1min air-oxidation. Fitting curves are displayed as light-gray lines, with fitting parameters reported in TableS2. In the absence of ethanol, note the dominant signal of the •OH-DMPO adduct after H2O2 addition and the absence of this signal after air-bubbling. Center panel: EPR-monitoring of the CH3CH•OH-DMPO adduct scale-factor from 5 to 60 min after having started the 1 min air-oxidation or H2O2 oxidation. Right panel: EPR-monitoring of the •OH-DMPO adduct scale-factor from 5 to 60 min after having started the 1 min air-oxidation or H2O2-oxidation (Mt stands for magnetite). The kinetics fitting curves displayed as plain lines are obtained with Eq. 1 or 1', with fitting parameters reported in Table1.

10 !

 10 •OH-DMPO (,) = ! 012 = <3 789:;

Figure 4 .

 4 Figure 4. Dissolved and solid iron concentrations in the experiments reported in Figure3for magnetite (blue) and mackinawite (green). Reported dissolved concentrations correspond to total iron determined by ICP-OES. Reported solid concentrations correspond to Fe(II) in the added mineral particles (78 mM) split into a surface Fe(II) atoms pool calculated for a surface shell of 3 Å thickness, and a remaining bulk Fe(II) pool, assuming average particle diameters of 140 and 90 Å for our magnetite and mackinawite, respectively (TableS2).

  filtrate + 1min Air 1 min Air + 0.2µm filtration surface Fe(II) atoms bulk Fe(II) [Fe] mM dissolved O2 with magnetite or mackinawite has produced reactive species able to oxidize CH3CH2OH into the hydroxyethyl radical CH3CH•OH. However, the lack of significant hydroxyl radical •OH signal in the airoxidation experiments conducted in the absence of CH3CH2OH shows that •OH is not responsible for CH3CH2OH oxidation in our air-oxidation experiments. The use of CH3CH2OH as intermediate radical trap for detecting •OH via the formation of the CH3CH•OH-DMPO adduct is classical, especially to circumvent the ambiguity between genuine •OH-DMPO adduct, i.e.

Figure 5 .

 5 Figure 5. Effect of phosphate concentration (0.053 M or 0.525 M phosphate buffer pH 7) on the EPR spectra for DMPO-added magnetite (blue) or mackinawite (green) suspensions filtered after 1 min air bubbling in the presence of CH3CH2OH. Left panel: EPR spectrum of each experiment collected 25 min after having started the 1min air-oxidation. Fitting curves are displayed as light-gray lines, with fitting parameters reported in TableS2. Right panel: EPR-monitoring of the CH3CH•OH-DMPO adduct scale-factor from 5 to 60 min after having started the 1 min air-oxidation. The kinetics fitting curves displayed as plain lines are obtained with Eq. 1 or 1', with fitting parameters reported in Table1.

Figure S1 .

 S1 Figure S1. Rietveld refinement of XRD powder patterns of the magnetite, mackinawite and pyrite synthetic substrates prepared and used this study. Refined parameters values are reported in TableS1. Mag : magnetite ; Hl : halite ; Mck : mackinawite ; Py : pyrite ; Mrc : marcasite ; α-S, orthorhombic elemental sulfur; k : Kapton® window of the anoxic sample chamber.

Figure S2 .

 S2 Figure S2.Fitting results for a typical EPR spectrum of the CH3CH2OH-and DMPO-added magnetite (12 mg) suspension filtered after air-oxidation. The EPR spectrum (blue line) was collected 25 min after the start of the operation consisting in 1 min air-bubbling, 1 min shaking and immediate filtration. The fitted curve is displayed as light-gray line, and corresponds to 99% of a CH3CH•OH-DMPO adduct component and 1% of a •OH-DMPO adduct component (TableS2). The asymmetry of the experimental spectrum of the CH3CH•OH-DMPO adduct is explained by the coexistence of at least two isomers, as shown by a detailed analysis of the more intense spectrum obtained after H2O2-oxidation, see FigureS3and TableS3.

Table 1 .

 1 Effective kinetic parameters Amax, kprod and kdecay obtained by fitting the EPR spectra of the CH3CH•OH-DMPO and •OH-DMPO adducts as a function of time, according to first-order reaction kinetics (Eqs. 1 and 1'). Corresponding curves are displayed in Figures1, 2, 3 and 5. The maximum adduct concentration was estimated from the Amax value by comparison with a strong-pitch EPR-standard (see Materials and Methods section). All experiments were performed in 0.525 M pH7 phosphate buffer except those labeled by # that were performed in a 0.053 M pH7 phosphate buffer.

	CH3CH•OH-DMPO adduct	Amax (a.u.)	kprod (min -1 )	kdecay (min -1 )	Chi 2	R	[CH3CH•OH-DMPO]max (µM)
	magnetite 12mg + CH3CH2OH + H2O2	1551(9)	n.a.	0.0179(2)	1183	0.999	~ 12.21(7)
	magnetite 12mg + CH3CH2OH + 20 min Air	501(7)	0.133(15)	0.0042(3)	111	0.991	~ 3.95 (5)
	magnetite 12mg + CH3CH2OH + 1 min Air	389(40)	0.248(70)	0.0019(20)	81	0.992	~ 3.0(4)
	magnetite 12mg filtered 0.2 µm + CH3CH2OH + 1 min Air	169(3)	0.253(17)	0.0015(5)	98	0.966	~ 1.33(2)
	magnetite 12mg filtered 3 kDa + CH3CH2OH + 1 min Air	185(2)	0.298(11)	0.0040(3)	27	0.989	~ 1.46(1)
	# magnetite 12mg + CH3CH2OH + 1 min Air	179(13)	n.a.	00012(7)	175	0.990	~ 1.41(10)
	mackinawite 4.6mg + CH3CH2OH + 20 min Air	409(4)	0.137(15)	0.0046(2)	55	0.996	~ 3.22(3)
	mackinawite 4.6mg + CH3CH2OH + 1 min Air	300(1)	0.308(4)	0.0045(1)	7	0.999	~ 2.36(1)
	mackinawite 4.6mg filtered 0.2 µm + CH3CH2OH + 1 min Air	123(3)	0.256(28)	0*	139	0.943	~ 0.97(2)
	mackinawite 4.6mg filtered 3 kDa + CH3CH2OH + 1 min Air	110(5)	0.237(14)	0*	48	0.979	~ 0.87(4)
	# mackinawite 4.6mg + CH3CH2OH + 1 min Air	84(1)	0.135(4)	0*	3	0.997	~ 0.66(1)
	pyrite 6.2mg + CH3CH2OH + 20 min Air	11(6)	0.05(3)	0*	11	0.782	~ 0.09(1)
	pyrite 6.2mg + CH3CH2OH + 1 min Air	10(1)	0.24(8)	0*	6	0.876	~ 0.08(1)
	blank + CH3CH2OH + 1 min Air	14(1)	0.19(3)	0.006(2)	5	0.856	~ 0.09(1)
	magnetite 12mg + H2O2	213(28)	n.a.	0.074(10)	1092	0.966	~ 1.7(2)
	magnetite 12mg + 1 min Air	22(1)	0.144(9)	0.0035(7)	2	0.991	~ 0.17(1)
	mackinawite 4.6mg + 1 min Air	20(1)	0.277(18)	0.0146(6)	5	0.999	~ 0.16(1)
	•

OH-DMPO adduct Amax (a.u.) kprod (min -1 ) kdecay (min -1 )

					Chi 2	R	[•OH-DMPO]max
							(µM)
	magnetite 12mg + CH3CH2OH + H2O2	21(7)	n.a.	0.0210(12)	6	0.986	~ 0.16(1)
	magnetite 12mg + CH3CH2OH + 1 min Air	4(1)	n.a.	0*	n.a.	n.a.	~ 0.03(1)
	magnetite 12mg + H2O2	1622(94)	n.a.	0.0545(36)	26196	0.989	~ 12.8(7)
	magnetite 12mg + 1 min Air	1(1)	n.a.	0*	2	0.991	~ 0.008(8)

  )

		500									500										500
								magnetite
								+ CH3CH2OH
								+ Air 20mn
		400									400										400
																	mackinawite
	CH3CH°OH-DMPO (a.u.)	200 300				magnetite + CH3CH2OH + Air 1mn		CH3CH°OH-DMPO (a.u.)	200 300					mackinawite + CH3CH2OH + Air 1 mn + CH3CH2OH + Air 20 mn	CH3CH°OH-DMPO (a.u.)	200 300
						magnetite filtrate 0.2µm
		100				+ CH3CH2OH + Air 1mn			100				+ CH3CH2OH mackinawite filtrate 0.2µm		100
						magnetite filtrate 3 kDa + CH3CH2OH + Air 1mn						+ Air 1 mn mackinawite filtrate 3kDa + CH3CH2OH			pyrite + CH3CH2OH + Air 1mn		pyrite + CH3CH2OH + Air 20mn
															+ Air 1 mn
		0									0										0
		0	10	20	30	40	50	60	70	80	0	10	20	30	40	50	60	70	80		0	10	20	30	40	50	60	70	80
					Time (min)							Time (min)								Time (min)

Acknowledgements

The authors thank L. Delbes for his help on XRD platform of IMPMC and F. Skouri and C. Travers for their help in the IMPMC chemistry laboratories. We also thank J. Blanchard and S. Guira for BET surface area measurements at LRS. The SOLEIL synchrotron (Orsay) is acknowledged for having provided beamtime on the SAMBA beamline and the authors are especially indebted to G. Landrot, as well as to A. Zitolo and E. Fonda for the beamline tuning and their help during data collection. This study has been granted by MATISSE labex, ANR DEPOLECO and PIREN Seine research programs.

Supporting Information

Phosphate Boosts Nonhydroxyl Radical Species Production upon Air-Oxidation of 3 and4 and Table 1. Aqueous Fe concentrations were determined by ICP-OES (see Experimental Method). Solid Concentrations correspond to 78 mM Fe(II) added as mineral component, split into surface (3Å thickness) and bulk pools, assuming mean particle sizes of 14 and 9 nm for magnetite and mackinawite, respectively (Table S1). The proportion of oxidized Fe(II) in the solid phase is obtained from XANES LCF analysis results (Figure S4). n.m. : not measured.

Magnetite and Iron Sulfides at Neutral pH


Thomas Bunel

Matteo Conforti

Julien Lumeau

Antonin Moreau

Arnaud Fernandez

Olivier Llopis

Julien Roul

Auro M Perego

Arnaud Mussot

Unexpected phase-locked Brillouin Kerr Frequency comb in fiber Fabry Perot resonators

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Stimulated Brillouin scattering (SBS) is the first nonlinear effect to be triggered when a powerful continuous wave laser interacts with a nonlinear waveguide. This effect is further exacerbated in resonators due to the multiple round trips in the cavity. SBS has proven to be extremely interesting for building ultra-stable microwave photonic synthesizers [1], to trigger broadband cavity solitons [START_REF] Bai | Brillouin-Kerr Soliton Frequency Combs in an Optical Microresonator[END_REF], or to balance thermo-optical effects to obtain ultra-stable sources [START_REF] Jia | Photonic Flywheel in a Monolithic Fiber Resonator[END_REF]. Whatever the cavity architecture, the only requirement is to obtain a spectral overlap between the SBS gain (ν B = 10 GHz, ∆ν = 50 MHz), and the cavity resonances. This condition is automatically verified in fiber ring cavities, which have free spectral ranges (FSRs) typically between 1 and 100 MHz, or can be met by fine-tuning the cavity lengths in microresonators with FSRs in the GHz range [START_REF] Bai | Brillouin-Kerr Soliton Frequency Combs in an Optical Microresonator[END_REF][START_REF] Jia | Photonic Flywheel in a Monolithic Fiber Resonator[END_REF]. In this paper we evidence an original mode-locking phenomena to generate a stable Brillouin Kerr frequency comb in fiber Fabry Perot resonators. The surprising feature of our work is illustrated in Fig. 1 (a). While there is a spectral overlap between the 8 th cavity resonance (dashed lines) and the SBS gain curve (red curve) at 9.416 GHz, we observed a Brillouin Kerr frequency comb with a line-to-line spacing equals to 10.593 GHz (exactly 9 times the cavity FSR, blue line in Fig. 1 (a)) where there is no spectral overlap. The cavity is pumped by an ultra-narrow CW laser of 1.3 W. It is swept from blue to red to reach a cavity detuning of ∆ = -0.0891 rad, in order to jump onto the upper branch of the cavity CW response. In the early steps of the process, a Stokes band at 10.593 GHz (9 times the cavity FSR) is generated, and many others through four-wave mixing processes cascade (Fig. 1 (c)). Finally, a broadband frequency comb of 2 THz (Fig. 1 (b) and (c)) is obtained corresponding to a stable pulse train (Fig. 1 (d)). Numerical simulations, including forward and backward fields [START_REF] Dong | Unified approach to cascaded stimulated Brillouin scattering and frequency-comb generation[END_REF], without any fitting parameters reproduce perfectly these experimental observations (Fig. 1 (e)-(g)). We explain this unexpected feature by performing a linear stability analysis of a meanfield reduction of the governing equations. We found a maximum of the parametric gain exactly at 10.593 GHz, revealing the parametric origin of the Stokes band at this unexpected position.

To conclude, we discovered and explained a new nonlinear mechanism in fiber Fabry Perot cavities. It enables the generation of phase-locked stable Brillouin Kerr frequency combs which could find great interest in microwave photonic synthesizers.

Figure 1 :

 1 Figure 1 : (a) Scheme representing the SBS gain curve (red curve, SBS shift is 9.655 GHz), the cavity resonance positions (vertical dashed lines) and the first Stokes band (blue curve). (b) and (e) output spectra, (c) and (f) zoom on the central part, and (d) and (g) temporal traces. (b)-(d) Experiments and (e)-(g) numerics.Parameters : length 8.78 cm, FSR=1.177 GHz, cavity detuning ∆ = -0.0891 rad, highly nonlinear fiber with group velocity dispersion β 2 = +0.38 ps 2 km -1 and nonlinearity γ = 10.8 W -1 .km -1 , and a Q factor of 65 millions. Spectra had been recorded with a high resolution (20 MHz) optical spectrum analyser and time traces with a high bandpass (700 GHz) optical sampling oscilloscope.