Thomas G Müller

N Biver

D Bockelée-Morvan

S Hasegawa

I Valtchanov

M Küppers

S Fornasier

H Campins

H Fujiwara

L O'rourke
email: lorourke@esa.int

D Teyssier

T Lim

Low Water Outgassing from

INTRODUCTION

Observational and theoretical data suggest that solar system objects located in the main asteroid belt hold more water ice than originally thought. Indeed the detection of water vapour on (1) Ceres (Küppers et al. 2014;[START_REF] Combe | 47th Lunar and Planetary Science Conference[END_REF], asteroids that present cometary behaviour (active asteroids -Hsieh & Jewitt 2006), and 3.1 μm absorption features in asteroid reflectance spectra (Campins et al. 2010;Licandro et al. 2011) have reinforced this line of thinking. Hydrated minerals (any mineral containing H2O or OH) and water ice are known to have strong spectral absorptions in the ~2.5-3.5μm wavelength range (Rivkin et al. 2002).

Features at 0.7 μm and 2.7μm are attributed to the presence of hydrated minerals and those at ∼3.05μm to water ice.

Takir & Emery 2012 examined the distribution and abundance of hydrated minerals on 28 outer mainbelt asteroids and proposed four 3 μm spectral groups based on the absorption shapes observed: Cereslike, Europa-like, rounded and sharp. Of the four absorption shapes, only the rounded 3μm band shape has been linked to the presence of water ice on the surface. Besides being observed on both Themis (Fornasier et al. 1999;[START_REF] Rivkin | [END_REF]Campins et al. 2010) and 65 Cybele (Licandro et al. 2011), it has been found on numerous other asteroids e.g. ( 76) Freia (Hargrove et al. 2012), (361) Bononia (Usui et al. 2019), correlating well with laboratory experiments suggesting [START_REF] Rivkin | [END_REF]) that a fine coating of ice on pyroxene grains mixed with amorphous carbon was covering the entire surface of these asteroids.

(24) Themis is the parent body of the Themis family of asteroids located at the edge of the Main Belt consisting of about 5000 members [START_REF] Nesvorny | Nesvorny HCM Asteroid Families V2.0[END_REF], mostly belonging to primordial C and B-type (Fornasier et al. 2016a). In addition, to the 3.1 μm feature, the asteroid also has an absorption feature at approximately 2.76 μm associated with hydrated minerals (Usui et al. 2019), with no feature at 0.7 μm. (65) Cybele, a P-type asteroid, is the most prominent member of a primitive outer edge main-belt asteroid population, known as the Cybele asteroids. Although it has the 3.1 μm feature, the asteroid has no hydrated features at 0.7 μm nor at 2.76 μm (Usui et al. 2019).

Besides the search therefore for hydration in asteroids with spectral features from hydroxyl-bearing minerals (Takir & Emery 2012;Usui et al. 2019) or oxidized iron in phyllosilicates (Fornasier et al. 2014), the detection of gaseous water vapour or its disassociation products provide a more direct way to identify the presence of water. Up to now water vapour has not been detected on Themis or Cybele although upper limits have been derived for each.

Optical spectra was obtained (Jewitt & Guilbert-Lepoutre 2012) of the two asteroids from the Keck I 10 m telescope to search for outgassed CN, a photodissociation product of HCN. An average CN/H2O ratio of 0.3% was applied (as done for comets) to these asteroids, and upper limits for the water production rate of Q(H2O) < 1.3 ×10 28 mol. s -1 for Themis and Q(H2O) < 1.2 ×10 28 mol. s -1 were derived. A global coverage of water ice was discarded by the authors with these upper limits constraining the ice coverage on the Earth-facing asteroids surface to be less than 10% based upon the presence of "relatively clean" (non-mixed) ice with a bond Albedo >0.3.

Radio and UV observational searches for OH emission around Themis were performed (Lovell et al. 2010) using the National Radio Astronomy Observatory (NRAO) 100-m Green Bank Telescope (GBT) and the Lowell Observatory 1.1-m Hall Telescope. A 3σ upper-limit for Q(H2O) of 1x10 28 mol. s -1 was obtained.

A more recent study (McKay et al. 2017) presented the result of a non-detection following a search for gaseous H2O at optical wavelengths using the forbidden oxygen line at 6300.3 A˚ in line with similar observations performed on comets (McKay et al. 2014(McKay et al. , 2015)). While no detection was made, an upper limit of Q(H2O) < 4.5 ×10 27 mol. s -1 was provided for Themis and conclusions equivalent to Jewitt & Guilbert-Lepoutre (2012) were reached as regards the localized nature and purity of the ice needed to match the upper limit obtained.

The Heterodyne Instrument for the Far Infrared (HIFI; de Graauw et al. 2010) on board the ESA Herschel Space Observatory (Pilbratt et al. 2010) has proven to be the most sensitive instrument ever built for directly observing water vapour on comets (Bockelée-Morvan et al. 2010) as well as in the asteroid belt. Indeed, it was through use of the HIFI instrument that H2O gas was successfully detected around (1) Ceres (Küppers et al. 2014).

In section 2 the observations obtained by the HIFI instrument of Cybele and Themis as well as those from Subaru/COMIC (Cooled Mid-Infrared Camera and Spectrometer) and the Herschel/SPIRE (Spectral and Photometric Imaging Receiver) instrument are presented and the data analysed. In section 3 we present new radiometric properties derived from our Thermophysical Model (TPM). In section 4 we feed in our radiometric properties to derive upper limits for the water production rate of both asteroids. In section 5, we calculate the fraction of ice coverage required to achieve this upper limit, discuss our results and present our conclusions.

OBSERVATIONS & DATA ANALYSIS

Herschel HIFI observations

Themis was observed with the HIFI instrument on Herschel on 2012 January 30 (see Table 1). The asteroid was travelling in-bound (2.92 au) and close to perihelion (2.75 au) when the observation was carried out. Cybele was observed with the HIFI instrument on Herschel on 2012 December 21 (see Table 1) when the asteroid was 3.52 au from the sun and was outbound having already passed aphelion (3.8 au).

It's important to highlight that with the low escape velocity from these asteroids, any escaping water vapour would have been visible to Herschel independent of its source location as the footprint of the telescope was much larger than the asteroids themselves.

The line emission from the fundamental ortho-H2O 110-101 line at 556.936 GHz was searched for in the lower sideband of the HIFI band 1b mixer. The observations were performed in the frequency-switching observing mode, using both the wide-band spectrometer (WBS) and the high-resolution spectrometer (HRS). The spectral resolution of the WBS is 1 MHz (0.54 km s -1 at the frequency of the observed line), while the HRS was used in its high-resolution mode with a resolution of 120 kHz (0.065 km s -1 ). Initial processing of the HIFI data set was carried out using the standard HIFI processing pipeline of HIPE v15.0.1 (Herschel interactive processing environment; Ott 2010), followed by processing with the GILDAS CLASS Software.

In CLASS, vertical and horizontal polarizations were folded and averaged. The frequency switching observing mode introduces quite a strong baseline ripple, which was removed through a polynomial baseline correction performed on the final spectra. The averaged and baseline corrected HRS spectra of the two asteroids are shown in Fig. 1(a) and (b). The pointing offset of both the vertical and the horizontal polarization spectra is 3.5" in band 1b with respect to the target position. The Half Power Beam Width (HPBW) is 38.1'' at 557 GHz. The main beam brightness temperature scale was computed using a forward efficiency of 0.96 and a beam efficiency of 0.62. The output line area r.m.s. values are provided in Table 2.

Although the H2O line is not detected in the resultant spectra for either of the two asteroids, sensitive upper limits could be derived. Table 2 provides a summary of the 1σ ʃTmBdv line area upper limits calculated for both asteroids. The 1σ upper limits were computed on a [-1,1 km s -1 ] window. The mean of the HRS and WBS derived values have been used to derive the 3σ upper limits of 8.7 mK km s -1 ʃTmBdv for Themis and 8.5 mK km s -1 ʃTmBdv for Cybele.

Observations used for the radiometric analysis

Database of existing observations

For the radiometric analysis of these two asteroids we used a wide range of thermal IR and submm/mm measurements. Contrary to Cybele, Themis has not been well observed at thermal IR wavelengths. Besides using a Spitzer-IRS spectrum (Landsman et al. 2016), our study was limited to AKARI and WISE measurements. We used two different AKARI datasets; the first was taken from the SBNAF public database10 (Szakáts et al. 2020) and originated from the IRC instrument using the MIR-S (5.8-14.1 μm) and the MIR-L (12.4-26.6 μm) cameras. For the second, produced by the IRC-NIR camera, we used the spectral data in the range 4.5 to 5.0 μm only (Usui et al. 2019) where the fluxes are pure thermal emission. We note that in the case of the WISE data, the W3 data (11.10μm) was strongly saturated and the saturation-corrected fluxes problematic while the W4 (22.64μm) data was only partly saturated and the saturation-corrected fluxes of good quality. We added to these observational datasets the Subaru/COMICS dataset as presented in section 2.2.2.

In the case of Cybele, there exists a significant set of observational data available for our radiometric study. In particular, we used Herschel-PACS data (Alí-Lagoa et al. 2020), IRAS (Tedesco et al. 2002) and AKARI measurements (Usui et al. 2011) both extracted from the SBNAF public database (Szakáts et al. 2020); UKIRT-MAX and ISO-ISOPHOT data (Müller & Lagerros 1998, 2002); AKARI data (Usui et al. 2019); multiple ground-based data from literature and archives, as well as additional spectroscopic and photometric data from ISO-ISOPHOT and ISO-ISOCAM (Müller & Blommaert 2004b) and finally Spitzer-IRS data (Licandro et al. 2011). We added to this the Herschel SPIRE as explained in section 2.2.3

Subaru/COMICS datasets for Themis

The Mid-infrared photometric observations for Themis were conducted using the Cooled Mid-Infrared Camera and Spectrometer (COMICS; [START_REF] Kataza | Proc. SPIE[END_REF] installed at the f/12 Cassegrain focus of the 8.2 m Subaru Telescope. The 320 x 240 pixel Si:As BIB detector for COMICS has 50 μm square pixels, giving a 42" x 32" field of view with a pixel scale of 0.135". The N-band filters were centered on 8.8 μm and 11.7 μm, and 12.4 μm with effective bandwidths of 0.8 and 1.0 μm were utilized. See Table 1.

To reduce the atmospheric and telescopic background radiation, a standard chopping and nodding method was used. HD108985 was selected as an absolute calibration star (Cohen et al. 1999) as it was positioned near to Themis in the sky. The fluxes of Themis and HD108985 as provided in Table 2 were measured through circular apertures with a diameter of more than four times that of the full-width at half maximum size, using the APPHOT function in the IRAF software. Since the colour difference between the spectral energy distributions of Themis and that of HD108985 in the N88 and 11.7 filter were found to be about a few percent, colour corrections for observations with COMICS were not performed.

SPIRE datasets for Cybele

The Herschel SPIRE instrument covering 3 far-infrared bands -250 μm, 350 μm and 500 μmobserved Cybele nine times during the period 2009-2012. See Table 1. The observations were taken directly from the Herschel Science Archive11 . Photometry was carried out using the Timeline Fitter method available in HIPE. The derived total monochromatic flux densities were colour corrected to account for the assumed spectral index of α=2. A flux calibration uncertainty of 5.5% was added in quadrature for the total error budget. The output fluxes corresponding to the three SPIRE wavelength bands and associated errors are provided in Table 2.

THERMOPHYSICAL MODELLING OF THE TWO ASTEROIDS

A thermophysical model (Lagerros 1996(Lagerros ,1998;;Müller & Lagerros 1998, 2002) was used to derive the radiometric properties of the two asteroids. This model has been successfully used in the study of near-Earth asteroids (Müller et al. 2004a(Müller et al. , 2011(Müller et al. , 2012)), of main-belt asteroids (Müller & Blommaert 2004b;O'Rourke et al. 2012), and also of more distant objects (Lim et al. 2010, Müller et al. 2019).

For both asteroids, we performed a radiometric analysis combining all available mid-to-far-IR measurements with the Subaru/COMICS and Herschel/SPIRE values as described in section 2.2 of this paper and presented in Tables 1 and2.

For our work we followed a similar strategy as in Alí-Lagoa et al. ( 2020) and references therein. In the case of Themis, non-convex spin-shape solutions (Viikinkoski et al. 2017; based on two spin solutions from Hanuš et al. 2016) as well as two simpler convex solutions (DAMIT database) were tested against the available thermal measurements presented in section 2.2 in our model. Since the data coverage in rotational phase, phase angles, aspect angles or wavelength was very limited, it was not possible to identify the object's true spin and shape properties via our χ 2 approach.

We obtained a best fit however using the second convex spin-shape solution with (λecl, βecl) = 2017)) with a spin-pole at (λecl, βecl) = 208° ± 1°,-3° ± 3°) and a rotation period of 6.08 h. They found that a best-fitting radiometric diameter of 277 +4 -2 km and a thermal inertia value of 30 +10 -15 J m -2 s -1/2 K -1 could explain these thermal fluxes very well based on a reduced χ 2 minimum <1. Therefore, we used, as in the case of Themis, the shape model with the size as a free parameter.

In our work we used the same non-convex shape model of Cybele as in Ali-Lagoa et al. (2020) which has 2040 surface elements and 1022 vertices13 (DAMIT Database ID1843) with a spin pole at (λecl, βecl) = (207,-6) and Psid = 6.081435 h. The reduced χ 2 minimum was at 0.75 for an intermediate surface roughness (rms of surface slopes of 0.4). We find an effective diameter Deff = 282 ± 9 km, a best-fit thermal inertia of Γ =25 +28 -19 J m -2 s -1/2 K -1 , and a geometric V-band albedo of pV = 0.042 ± 0.005 (connected to an absolute V-band magnitude of H=6.82 mag from Oszkiewicz et al. ( 2011)) (Table 4). Our solution was found to match all photometric IR data points as well as the Spitzer-IRS spectrum and the ISO-ISOCAM CVF spectrum (except in the range of the emission feature discussed in Müller & Blommaert (2004b)). See

OUTGASSING RATE UPPER LIMITS

To derive the upper limits on the water production rate for the two asteroids, we used a molecular excitation model that calculates the population of the rotational levels of water as a function of distance from the asteroid. Our excitation model takes into account collisions of water and electrons, infrared pumping of the vibrational bands, and treats self-absorption using the escape probability formalism (Biver et al. 2015(Biver et al. , 2012;;O'Rourke et al. 2013;Zakharov et al. 2007). Further to this the model takes into account the thermal background emission from the asteroid and non-constant gas kinetic temperature radial profiles (Biver et al. 2019).

Input parameters to the model include; the asteroid radius (from our TPM), electron density, the gas expansion velocity, the derived surface temperature (from our TPM which includes surface roughness effects that enhance the temperatures significantly), and finally the gas temperature. Synthetic spectra are computed taking into account the transfer of line radiation of ortho-water in the asteroidal "atmosphere".

Since the electron density is not well constrained, a scaling factor of xne = 0.2 with respect to the standard profile derived from observations of comet 1P/Halley has been used (Hartogh et al. 2010). For the radial gas density profile, we adopted the standard Haser law (Haser 1957). We assumed a nominal expansion velocity vexp of 0.5 km s -1 (characteristic of weak comets)..

Assuming a rarefied free-expanding exosphere for the asteroids and based on these "nominal" xne and vexp values we calculated the water production rates (see Table 3) based upon gas kinetic temperature values of 10 K, 20 K and 50 K as well as a set of variable temperature profiles. 50K was included as temperatures of approximately 50-60 K are expected for Ceres at 3000 km from its body center (Küppers et al. 2014). We obtained similar results for the two asteroids irrespective of the temperature profile applied.

This suggests that we are more likely sensitive to the gas temperature at some distance from the surface where collisions become too weak.

We find that for the low production rates we estimate, water excitation is not affected significantly by electron collisions because of low electron densities. Applying a value of xne = 0.2 or 0.0 does not change our results. Using xne = 1.0 decreases the upper limits on the water production rate by 10%, and doubling the collision rate with neutrals decreases the values by 8%.

Taking into account the above findings, we added a column in Table 3 defining highly conservative 3σ upper limits for the water production rate of Q(H2O) < 4.1 x 10 26 mol. s -1 for Themis and Q(H2O) < 7.6 x 10 26 mol.s -1 for Cybele. These values were obtained by assuming a kinetic temperature of 10 K, a high expansion velocity vexp of 0.7 km s -1 and no excitation by photoelectrons i.e. xne = 0.0. These upper limits provide more than an order of magnitude improvement on previously published Q(H2O) values which are presented in the final column of Table 3 for comparison purposes. We also have derived sensitive 3σ upper limits for the column density of Ncol < 0.58 x 10 12 cm s -1 for Themis and Ncol < 0.82 x 10 12 cm s -1 for Cybele (Table 3 andTable 4).

Table 3 : H2O Production Rate (Q(H2O)) and Column Density (Ncol) derived for different gas kinetic temperatures, either constant throughout the coma (Tkin), or variable (Tvar = (T0-Tf) rN/r +Tf, with T0 = 155 K for Themis and T0 = 138 K for Cybele corresponding to 86% the surface temperature (Crifo et al. 2002); rN is the radius of the asteroid derived from our TPM and r is the distance to the body centre. The "Upper Limits" column corresponds to the worst case estimate of the upper limitwhereby (Tkin), = 10K, vexp = 0. Ncol (cm -2 ) < 0.82 x 10 12 < 0.84 x 10 12 < 0.73 x 10 12 < 0.65 x 10 12 < 0.69 x 10 12 < 0.67 x 10 12 < 0.66 x 10 12

DISCUSSION AND CONCLUSIONS

We use the upper limits for the water loss rate presented in section 4 to produce, using a sublimation model, equivalent upper limits for the percentage of exposed ice that is present on the surface of the two asteroids. In our analysis, we study the implications of mixing both at a fine scale (intimate mixing)

and also at a more macroscopic level (areal mixing) the ubiquitous dark dust and the ice present on the asteroid surfaces whereby the level of mixing can affect the contribution of the ice albedo to the areaaveraged albedo. For intimate mixing, laboratory experiments show that only a few weight percent of dust is sufficient to reduce the albedo of the ice well below 10% (Pommerol et al. 2015). In the areal mixing case, the cleaner (less mixed) the ice is with the dust, the brighter (higher bond albedo) and colder it will be, resulting in less sublimation as a result. This leads to a higher area percentage coverage deduced from the water production rate for the areal mixing case than for the case of intimately mixed ice (Jewitt & Guilbert-Lepoutre 2012).

We used a sublimation model14 (Cowan & A'Hearn 1979 -slow rotator version) where the rotational pole is pointed towards the Sun. This is appropriate for bodies with a small thermal inertia, as is the case (from our TPM) for Themis and Cybele. This sublimation model provides the average water sublimation rate per surface unit, Z, at a given heliocentric distance. The upper limit on the area of exposed ice Aexp is obtained by dividing the upper limit on Q(H2O) by Z, and the upper limit on the fraction of the surface occupied by ice (fice) is obtained by dividing Aexp by the asteroid surface area (4 𝜋𝜋rN 2 , where rN is the effective radius). In addition to the heliocentric distance, the input parameters of the model include the Bond albedo of the icy material and the infrared emissivity equal to 0.9.

Figure 2 shows derived upper limits for the ice coverage of the two asteroids. We plot our water production upper limits against the Bond albedo and the percentage ice coverage; acceptable solutions must lie on or below the water production curves as otherwise gas production through sublimation would have been large enough for detection by HIFI. In this figure, we study two scenarii for the water ice interaction with the ubiquitous dark dust of the asteroid. In the first scenario, we gradually mix a low geometric albedo (0.02 to 0.04) surface dust with ice of an increasing bond albedo value (brightness) resulting in an "areal" type mixture. In the second scenario, we study the "intimate" mixture case on the basis that the bond albedo of the asteroid represents the combined albedos of the icy dust mixture.

Applying scenario 1 and in line with the approach of Jewitt & Guilbert-Lepoutre (2012), we plot two dotted lines in Figure 2 matching with geometric albedo values for pv(dust) = 0.02-0.04 for Themis and 0.02-0.03 for Cybele corresponding to the least reflective asteroid surfaces known. 0.03 is set as a conservative value for the Cybele upper range due to its already very low albedo (from our TPM) of 0.042. The two constraints (water production rate limit and albedo) are found to be acceptably achieved only if the water ice on the two asteroids is very bright (Bond Albedo of 0.8 for Themis, 0.7 for Cybele) and if the water ice covers < 2.2% of the surface for Themis and < 1.5% for Cybele. Upper limits of 2.2% and 1.5% ice respectively on the surface of the Themis and Cybele asteroids will lead to similar upper limits in the depth of water ice in the 3.1μm absorption feature as pure water ice is fully absorbing at this wavelength (e.g.

Protopapa et al. 2014).

As the 3.1μm band depth of both asteroids is ∼10% of the local continuum [START_REF] Rivkin | [END_REF]Licandro et al. 2011), we conclude that for areal mixing, the 3.1 μm absorption feature cannot be explained by surface water ice on these asteroids.

In scenario 2, we assume that the water ice is intimately mixed with the dark surface material of the asteroids (as observed on other solar system objects including 67P) and as such corresponds to the area-averaged bond albedo of the asteroid. A value for the bolometric bond albedo (A) of 0.027 was calculated for Themis and 0.019 for Cybele based on the formula A = p*q where p is the bolometric geometric albedo assumed to be equal to the V-band albedo pV and q = 0.290 + 0.684*G. G is the slope parameter in the H-G system [START_REF] Bowell | Asteroids II[END_REF]) and it was determined to be 0.16 for Themis and 0.23 for Cybele (Oszkiewicz et al. 2011). From Figure 2 (purple stars) and based on these bond albedo values, we obtain upper limits for the fraction of the surface covered by ice to be 0.0017% for Themis and 0.0033% for Cybele, meaning that only a tiny fraction of intimately mixed ice is present on the surface of these asteroids.

These values are an order of magnitude lower than those obtained by Jewitt & Guilbert-Lepoutre (2012) and McKay et al. (2017). Like them, we conclude that such an ice-dust mixture percentage is entirely insufficient to explain the 3.1μm absorption.

To appreciate the possible percentage of intimately mixed ice below the surface, we compare it with values obtained from 67P/Churyumov-Gerasimenko. On 67P the ice was observed to be intimately mixed with the dust, was visible in localised regions and considered to be just below the surface (Fornasier et al. 2016b). We applied the above model using Q(H2O) measured values of 67P at 3au based on a Bond albedo 0.012 at 3au (Fornasier et al. 2015), and emissivity of 0.97 (Spohn et al. 2015). We find an ice coverage of 0.6% at 3 au pre-perihelion and 0.2% at 3 au post perihelion, percentages more than 2 orders of magnitude higher than those derived for the asteroids. In this comparison, if ice is indeed present just below the asteroid's surface and intimately mixed with dust as in comets, then there is a very low amount present.

While surface water ice may not be the reason for the 3.1 μm absorption feature, the 3-μm region is known to host other volatile species that do show similar absorption curves. The ammonium (NH 4+ ) ion has a band centre near 3.1 μm and was interpreted to be present in the (1) Ceres spectrum [START_REF] King | [END_REF]. Poch et al (2020) have recently compared the 3.2 μm spectra of ammonium salts found on comet 67P (Altwegg et al. 2020) with the 3.1 μm feature on Themis and Cybele finding a distinct correlation between the two. Similar features have been observed on the Jupiter Trojan asteroids and the small moon Himalia matching with ammoniated minerals on their surface (Poch et al. 2020;[START_REF] Brown | [END_REF]) at this wavelength. Further to the above, minerals such as Brucite (Beck et al. 2011), magnetite (Yang & Jewitt 2010) and adsorbed water molecules in lunar regolith (Clark 2009) all represent other possible alternative explanations to the presence of surface water ice.

In conclusion, we present highly conservative sensitive 3σ upper limits for the water production rate on (24) Themis and (65) Cybele. Using these limits to model the maximum surface ice fraction, we find that the 3.1 μm absorption feature cannot be explained by water ice, be it areal or intimately mixed. Recognising the numerous spacecraft that have visited asteroids and comets where far too little surface ice has been detected to be visible in disk-integrated spectra, our results serve to confirm the continued difficulty in observing ice from the Earth whether via ground or space based observatories. between these dotted curves to guarantee the area-averaged albedo matches our modelled TPM value.

The purple stars represent the positions of the corresponding intimate mixing results.

(

  137°,59°) and Psid = 8.374187 h. This model has 2040 surface elements and 1022 vertices 12 (model ID1006;Hanuš et al. 2016). Our resulting derived radiometric properties are: thermal inertia Γ = 20 +25 -10 J m -2 s -1/2 K -1 , effective diameter Deff = diameter 192 +10 -7 km and a geometric V-band albedo pV=0.07 ± 0.01, (connected to an absolute V-band magnitude of H=7.11 mag fromOszkiewicz et al. (2011)) for an intermediate to low level of surface roughness (0.1 < r.m.s < 0.5) (Table4). See Figure1c(left-image) and 1d (left plot) for the Themis TPM output. While this multi-observatory dataset output provides a diameter consistent with previously published values it is lower than the NEATM derived measurements based on Spitzer of 218 ±1 km(Hargrove et al. 2015).. For Cybele,Viikinkoski et al. (2017) were able to eliminate the pole ambiguity via multiple AO images using a non-convex shape model with a spin pole at (λecl, βecl) = (208 ± 1°,-3 ± 3°) and Psid = 6.081435 h. They estimated a diameter of 296 ± 25 km, which is within 1σ of the earlier published radiometric diameter of 273 ± 11 km(Müller & Blommaert 2004b) and also that published by Alí-Lagoa et al. (2020). Ali-Lagoa et al. (2020) analysed IRAS, AKARI and Herschel-PACS measurements using the latest spin-shape solution (ADAM model by Viikinkoski et al. (

Figure 1c (

 1c Figure 1c (right image) and 1d (right plot) for the resultant TPM output. The inclusion of the SPIRE data contributed to confirm that Cybele shows a normal sub-mm (spectral) emissivity behaviour similar to many other large regolith-covered main-belt asteroids like Ceres, Pallas and Juno (Müller & Lagerros 1998).

Figure 1 :

 1 Figure 1: Averaged HIFI Spectra and Thermophysical modelling output. HRS averaged spectra for (a) asteroid Themis and (b) asteroid Cybele with TPM outputs (c) for Themis (left) and Cybele (right). These outputs represent the Herschel view with orientation "ecliptic-sky" at the specific HIFI observing start epochs. Plots (d) show for Themis (left) and Cybele (right) the observations divided by the TPM predictions using the best solution described in the text as a function of wavelengths. For Themis, the data shown are the good photometric data points coming from Spitzer-IRS (□), Subaru (X), Akari below 5 μm and above 18 μm (Δ) and WISE-W4 at 22 μm (♦) while for Cybele: IRAS (□), AKARI (Δ), PACS (♦), SPIRE (♦), ISOCAM (X), Spitzer-IRS (♦), with identification linked to wavelength.

  7 and xne = 0 (see text). All other estimates are performed with the optimal values vexp = 0.5 and xne = 0.2. Previously published data references are as follows : (i) Jewitt & Guilbert-Lepoutre (2012) (ii) Lovell et al. (2010) (iii) Mckay et al. (2017).

Table 4 :

 4 Summary of main results of this paper where Q(H2O) is the water production rate upper limits, Ncol is the Column density upper limits, f ice is the fraction of ice coverage depending if it is an intimate or areal mixture, Γ is the thermal inertia, D is the effective Diameter and pV is the V-band Geometric and bond Albedo.

Figure 2 :

 2 Figure 2 : Derived allowed values for the ice coverage for (a) Themis and (b) Cybele as a function of the Bond albedo of surface ice. The solid curve represents the Q(H2O) constraint derived for each asteroid. Acceptable solutions must lie on or below this curve as otherwise gas production through sublimation would have been large enough for detection by HIFI. The dotted curves reflect the albedo constraint (see text) for geometric albedos of non-icy (dust) terrains pv(dust) in the range 0.02 to 0.04 for Themis and 0.02 to 0.03 for Cybele. Acceptable solutions for the areal mixing case must lie along or

Table 1 : Observations & Observing conditions for the previously unpublished measurements used in this paper.

 1 Columns : Target (the 2 asteroids), Observatory & Instruments used, OBSID (Observation ID reference number), Date and time in UT, rh (heliocentric distance), Δ (Earth to asteroid distance), α (Phase angle).

	Target	Observatory &	OBSID	Date/Time	rh	Δ	α
		Instrument		(UT)	(au)	(au)	(°)
		Herschel HIFI
		(556.936 GHz)	1342262585	2013.01.30 17:30:38	2.92	2.37	18.1
	Themis	Subaru COMICS
		(N-band 8.8 μm)	n/a	2003.06.20 06:46:30	2.87	2.64	20.7
		Subaru COMICS
		(N-band 11.7 μm)	n/a	2003.06.20 06:43:30	2.87	2.64	20.7
		Herschel HIFI	134225765	2012.12.21 16:53:19	3.52	3.11	15.5
		(556.936 GHz)
		Herschel SPIRE	134187509	2009.12.01 13:14:28	3.45	3.27	16.6
		(Large Map) (250/350/500 μm)	134187511	2009.12.01 13:44:47	3.45	3.27	16.6
	Cybele		134228344	2011.09.08 16:48:43	3.80	4.09	14.1
			134230779	2011.10.10 22:51:21	3.79	3.61	15.2
		Herschel SPIRE	134244177	2012.04.12 19:47:23	3.72	3.86	15.1
		(Small Map)	134244840	2012.04.22 18:30:51	3.72	3.99	14.4
		(250/350/500 μm)	134254082	2012.10.28 19:50:08	3.57	3.93	14.2
			134255059	2012.11.15 00:37:22	3.56	3.68	15.6
			134257368	2012.12.17 23:39:25	3.53	3.16	15.7

Table 2 :

 2 Output

		Herschel/HIFI	Subaru/COMICS			Herschel/SPIRE
	Target	Line area	Line area	Air	Flux	Err	PSW	PSW	PMW	PMW	PLW	PLW
		1σ mK km s -1	1σ mK km s -1	mass	(Jy)	(Jy)	Flux	Err	Flux	Err	Flux	Err
		(HRS)	(WBS)				(Jy)	(Jy)	(Jy)	(Jy)	(Jy)	(Jy)
				1.27	3.81	0.22
	Themis	2.98	2.82	1.26	11.50 0.66			n/a
							1.680	0.092	0.896	0.049	0.450	0.025
							1.181	0.065	0.630	0.035	0.307	0.017
							0.993	0.055	0.528	0.029	0.260	0.015
							0.857	0.047	0.459	0.025	0.229	0.013
	Cybele	2.96	2.68		n/a		0.937	0.052	0.493	0.027	0.246	0.014
							1.040	0.057	0.546	0.030	0.271	0.015
							0.795	0.044	0.411	0.023	0.201	0.012
							1.673	0.092	0.883	0.049	0.431	0.024
							1.669	0.092	0.874	0.048	0.431	0.024

results following processing of the data in Table

1

. Columns : Target (the two asteroids), Herschel/HIFI Instrument (the calculated line areas for the HRS and WBS spectra for both asteroids), Subaru/COMICS instrument (the airmass, flux & error obtained for Themis), Herschel/SPIRE Instrument (the fluxes and errors derived from the SPIRE 250/350/500 μm observations of Cybele).

https://ird.konkoly.hu/

The Herschel Science Archive web page : https://herschel.esac.esa.int/Science_Archive.shtml

https://astro.troja.mff.cuni.cz/projects/damit/asteroid_models/view/1006

https://astro.troja.mff.cuni.cz/projects/damit/asteroid_models/view/783

https://pdssbn.astro.umd.edu/tools/ma-evap/

Acknowledgments

This study was based in part on data collected using the Subaru Telescope, which is operated by NAOJ. S.H. & H.F. would like to acknowledge H. Kataza, T. Ootsubo, T. Sekiguchi, and I. Yamamura for their helpful comments and discussions. The work of S.H was supported by the Hypervelocity Impact Facility, ISAS, JAXA. TM has received funding from the European Union's Horizon 2020 Research and Innovation Programme, under Grant Agreement no 687378, as part of the project "Small Bodies Near and Far" (SBNAF). We acknowledge the use of the GILDAS CLASS software (http://www.iram.fr/IRAMFR/GILDAS) in the processing of the HIFI data. We would like to thank the anonymous referee for the insightful and helpful comments provided.


Patricia Garcia-Muñoz
email: garciamunoz@unistra.fr

Fernando Fresno

Javier Ivanez

Didier Robert

Nicolas Keller
email: nkeller@unistra.fr

Activity enhancement pathways in TiO 2 @LaFeO 3 heterojunction photocatalysts for visible and solar light driven degradation of myclobutanil pesticide in water

Keywords: LaFeO3/TiO2 heterojunction, visible light, solar light, myclobutanil pesticide, photocatalysis, water treatment

TiO2@LaFeO3 heterojunction composites with a core-shell porous structure and LaFeO3 contents in the 2.5-25 wt.% range have been synthesized via consecutive sol-gel syntheses and tested for the photocatalytic oxidation of the myclobutanil pesticide in water under solar light and pure visible light.

Whatever the light spectrum, the kinetic rate constants for both myclobutanil degradation and TOC conversion exhibited a volcano-like profile with increasing the narrow band-gap (2.1 eV) LaFeO3 content, the optimum composite strongly overperforming both single phases, with full myclobutanil mineralization achieved in 240 min in the best case. The light spectrum influenced the optimum LaFeO3 content in the composite, being observed at 5 wt.% and 12.5 wt.% under solar and visible light, respectively. This has been attributed to the existence of different light-mediated reaction mechanisms. The optimum LaFeO3/TiO2 composite photocatalyst was active and stable after several runs under solar light with leached iron concentration below 0.1 mg/L in solution.

INTRODUCTION

Advanced Oxidation Processes (AOPs) emerged as feasible alternatives or complements to conventional water treatment processes for treating refractory compounds such as notably the pesticides, that can provoke ecological risks or adverse effects on living organisms even at low concentrations [START_REF] Westlund | Investigation of the presence and endocrine activities of pesticides found in wastewater effluent using yeast-based bioassays[END_REF][START_REF] Vu | Toxicological effects of fungicide mixtures on the amphipod Austrochiltonia subtenuis[END_REF][START_REF] Malato | Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends[END_REF]. Among them, photocatalysis has gained important attention due to a high efficiency and relatively low costs, when working with low pollutant charges, as well as to its ability to achieve complete mineralization of biorecalcitrant and emerging contaminants in an extended pH range, thanks to its ability to oxidize the short-chain acids [START_REF] Malato | Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends[END_REF][START_REF] Esplugas | Comparison of different advanced oxidation processes for phenol degradation[END_REF][START_REF] Malato | Treatment of 2,4-Dichlorophenol by Solar Photocatalysis: Comparison of Coupled Photocatalytic-Active Carbon vs. Active Carbon[END_REF].

TiO2 remains nowadays the photocatalyst par excellence due to its good characteristics.

However, it requires the use of ultraviolet-A (UV-A) as activation light source, as a result of its wide band-gap value (~3.2 eV). One approach for overcoming this issue consists in the elaboration of TiO2based heterojunction photocatalysts through the coupling of TiO2 with another semiconductor of narrower band-gap used as visible light photosensitizer [START_REF] Robert | Photosensitization of TiO2 by MxOy and MxSy nanoparticles for heterogeneous photocatalysis applications[END_REF][START_REF] Low | Heterojunction Photocatalysts[END_REF][START_REF] Jang | Heterojunction semiconductors: A strategy to develop efficient photocatalytic materials for visible light water splitting[END_REF]. Further, building heterojunctions is a strategy for enhancing the activity under solar light by favoring inter-semiconductor charge transfer that allows a lowering of charge recombination [START_REF] Chong | Recent developments in photocatalytic water treatment technology: A review[END_REF][START_REF] Ong | Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?[END_REF][START_REF] Kumar | Recent developments and challenges in practical application of visible-light-driven TiO2-based heterojunctions for PPCP degradation: A critical review[END_REF]. Many works investigated on the coupling of TiO2 with semiconductors such as e.g. oxide, sulfide or, more recently, carbon nitride for degrading organic pollutants in water under visible or solar light [START_REF] Perovic | Recent Achievements in Development of TiO2-Based Composite Photocatalytic Materials for Solar Driven Water Purification and Water Splitting[END_REF][START_REF] Garcia-Muñoz | Ferrite Materials for Photoassisted Environmental and Solar Fuels Applications[END_REF]. WO3 [START_REF] Rey | WO3-TiO2 based catalysts for the simulated solar radiation assisted photocatalytic ozonation of emerging contaminants in a municipal wastewater treatment plant effluent[END_REF], Fe2O3 [START_REF] García-Muñoz | Evaluation of photoassisted treatments for norfloxacin removal in water using mesoporous Fe2O3-TiO2 materials[END_REF][START_REF] Moctezuma | Photocatalytic degradation of phenol with Fe-titania catalysts[END_REF][START_REF] Zarazua-Morin | Synthesis, characterization, and catalytic activity of FeTiO3/TiO2 for photodegradation of organic pollutants with visible light[END_REF], Bi2O3 and ferrites are among the most popular metal oxides associated within binary heterojunctions [START_REF] Zarazua-Morin | Synthesis, characterization, and catalytic activity of FeTiO3/TiO2 for photodegradation of organic pollutants with visible light[END_REF][START_REF] Li | Direct Formation of Reusable TiO2/CoFe2O4 Heterogeneous Photocatalytic Fibers via Two-Spinneret Electrospinning[END_REF].

In this frame, ferrite semiconductor materials have recently received a special attention for use in photocatalysis because of the ability of most of them to absorb and use a part of visible light for performing surface redox reactions, due to band gaps of ca. 2 eV (wavelengths lower than 620 nm) [START_REF] Garcia-Muñoz | Ferrite Materials for Photoassisted Environmental and Solar Fuels Applications[END_REF]. Further, most of them display band energy positions suitable for oxidizing water and reducing dioxygen. Beside a high stability, they also benefit from a large composition versatility that allows their physico-chemical properties to be tuned via adequate synthetic strategies, such as e.g. the band-gap energy, the n/p-type character or the properties of lattice oxygen vacancies, whose controlled formation was reported to inhibit the charge recombination in the perovskite structures [START_REF] Masunga | Recent advances in copper ferrite nanoparticles and nanocomposites synthesis, magnetic properties and application in water treatment: Review[END_REF][START_REF] Kefeni | Application of spinel ferrite nanoparticles in water and wastewater treatment: A review[END_REF][START_REF] Magalhães | LaMn1-xFexO3 and LaMn0.1-xFe0.90Mox O3 perovskites: synthesis, characterization and catalytic activity in H2O2 reactions[END_REF][START_REF] Peña | Chemical Structures and Performance of Perovskite Oxides[END_REF][START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF].

Among the ABO3 perovskite type oxides, lanthanum ferrite (LaFeO3) is an interesting p-type semiconductor with a band-gap of ca. 2.1 eV, and the ability to absorb a part of the visible light from the solar spectra [START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF][START_REF] Li | Photoinduced charge property of nanosized perovskitetype LaFeO3 and its relationships with photocatalytic activity under visible irradiation[END_REF][START_REF] Thirumalairajan | Controlled synthesis of perovskite LaFeO3 microsphere composed of nanoparticles via self-assembly process and their associated photocatalytic activity[END_REF][START_REF] Kumar | Facile hydrothermal synthesis and characterization of LaFeO3 nanospheres for visible light photocatalytic applications[END_REF]. However, when used as heterogeneous catalyst, LaFeO3 displays a relatively low activity [START_REF] Li | Photoinduced charge property of nanosized perovskitetype LaFeO3 and its relationships with photocatalytic activity under visible irradiation[END_REF][START_REF] Thirumalairajan | Controlled synthesis of perovskite LaFeO3 microsphere composed of nanoparticles via self-assembly process and their associated photocatalytic activity[END_REF][START_REF] Su | Synthesis of large surface area LaFeO3 nanoparticles by SBA-16 template method as high active visible photocatalysts[END_REF][START_REF] Ding | Microwave-assisted synthesis of perovskite ReFeO3 (Re: La, Sm, Eu, Gd) photocatalyst[END_REF][START_REF] Tang | Microwave-assisted synthesis of nanoparticulate perovskite LaFeO3 as a high active visible-light photocatalyst[END_REF], mainly due to poor intrinsic charge transport in the bulk and high charge recombination rates, so that following a semiconductor heterojunction strategy is a valuable approach.

The LaFeO3-based heterojunctions are however not the most studied ones so far. In most of the studies, LaFeO3 was associated with narrow band gap semiconductors such as g-C3N4 [START_REF] Jin | Boosting visible light photocatalytic performance of g-C3N4 nanosheets by combining with LaFeO3 nanoparticles[END_REF][START_REF] Xu | Superior photocatalytic performance of LaFeO3/g-C3N4 heterojunction nanocomposites under visible light irradiation[END_REF][START_REF] Wu | Quasi-polymeric construction of stable perovskite-type LaFeO3/g-C3N4 heterostructured photocatalyst for improved Z-scheme photocatalytic activity via solid p-n heterojunction interfacial effect[END_REF], and to a lesser extent e.g. SnS2 [START_REF] Luo | Rational design of Zscheme LaFeO3/SnS2 hybrid with boosted visible light photocatalytic activity towards tetracycline degradation[END_REF] , AgBr [START_REF] Song | Enhanced photocatalytic decomposition of an organic dye under visible light with a stable LaFeO3/AgBr heterostructured photocatalyst[END_REF], CdS [START_REF] Hoseini | A novel n-type CdS nanorods/p-type LaFeO3 heterojunction nanocomposite with enhanced visible-light photocatalytic performance[END_REF], Ag2CO3 [START_REF] Pirzada | Synthesis of LaFeO3/Ag2CO3 Nanocomposites for Photocatalytic Degradation of Rhodamine B and p-Chlorophenol under Natural Sunlight[END_REF] or ZnIn2S4 [START_REF] Gao | In situ Synthesis of p-n LaFeO3/ZnIn2S4 Heterojunctions for Enhanced Photocatalytic Activity[END_REF], or the ternary oxide Bi3NbO7 [START_REF] Xu | Synthesis of LaFeO3/Bi3NbO7 p-n heterojunction photocatalysts with enhanced visible-light-responsive activity for photocatalytic reduction of Cr(Ⅵ)[END_REF], for water treatment or H2 production under visible light. In most of the cases, a Z-scheme mechanism was proposed, implying the interfacial recombination of the conduction band electrons or of the valence band holes of the ferrite with their counterparts in the second semiconductor, depending on the relative band positions. Scarce works focused on the building of p-n heterojunctions between LaFeO3 and wide band gap semiconductors, e.g. ZnO [START_REF] Ren | Highly efficient and stable p-LaFeO3/n-ZnO heterojunction photocatalyst for phenol degradation under visible light irradiation[END_REF] and TiO2 [START_REF] Humayun | Exceptional Visible-Light Activities of TiO2-Coupled N-Doped Porous Perovskite LaFeO3 for 2,4-Dichlorophenol Decomposition and CO2 Conversion[END_REF][START_REF] Jing | Synthesis of High-Activity TiO2-Based Photocatalysts by Compounding a Small Amount of Porous Nanosized LaFeO3 and the Activity-Enhanced Mechanisms[END_REF][START_REF] Kumar | Synthesis and characterization of LaFeO3/TiO2 nanocomposites for visible light photocatalytic activity[END_REF][START_REF] Lv | In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production[END_REF]. Regardless of the coupled semiconductor, a two-step synthesis approach was mostly followed for synthesizing LaFeO3based composites, the associated semiconductor material being synthesized in presence of sol-gel LaFeO3 grains. The reverse synthetic strategy was rarely reported, since obtaining good crystallization rates for LaFeO3 requires a high-temperature oxidative treatment that most of the associated phases cannot withstand [START_REF] Xu | Superior photocatalytic performance of LaFeO3/g-C3N4 heterojunction nanocomposites under visible light irradiation[END_REF][START_REF] Xu | Synthesis of LaFeO3/Bi3NbO7 p-n heterojunction photocatalysts with enhanced visible-light-responsive activity for photocatalytic reduction of Cr(Ⅵ)[END_REF]. Heterojunction composites were also scarcely prepared by mechanical or ultrasonic treatment of crystallized powders in suspension with final temperature treatment [START_REF] Wu | Quasi-polymeric construction of stable perovskite-type LaFeO3/g-C3N4 heterostructured photocatalyst for improved Z-scheme photocatalytic activity via solid p-n heterojunction interfacial effect[END_REF].

In the frame of the search for efficient narrow/wide band-gap heterojunction materials, we report on the two-step sol-gel synthesis and the photocatalytic activity of LaFeO3/TiO2 composite catalysts under solar and pure visible light, taking the myclobutanil pesticide degradation in water as test reaction of interest [START_REF] Garcia-Muñoz | Reaction pathways, kinetics and toxicity assessment during the photocatalytic degradation of glyphosate and myclobutanil pesticides: Influence of the aqueous matrix[END_REF]. Indeed, the Cl-containing triazole fungicide myclobutanil (C15H17ClN4)with both C-C and N-C aromatic rings and an aliphatic branch -, used to avoid fungi infection, is one of the most frequently detected pesticides in both surface water and groundwater in the Upper Rhine river area, in Central Europe [START_REF] Peschka | Trends in pesticide transport into the River Rhine[END_REF], [START_REF] Chen | Occurrence, fate and ecological risk of five typical azole fungicides as therapeutic and personal care products in the environment: A review[END_REF]. Here the influence of both the LaFeO3 content and the light spectrum on the myclobutanil degradation performances, has been studied to gain insight into the role of both semiconductor phases in the degradation mechanism.

MATERIALS AND METHODS

Materials

Analytical grade chemicals were used without further purification. Myclobutanil (>99%) was purchased from HPC Standards GmbH (Cunnersdorf, Germany). La(NO3)3 hexahydrate (99.99%), Fe(NO3)3 nonahydrate (>98%), citric acid hydrate (C6H8O7xH2O, 99.5%) and titanium tetraisopropoxide (TTIP, 97%) were purchased from Sigma Aldrich.

Synthesis of LaFeO3

The reference LaFeO3 (LFO) material was obtained following a classical sol-gel Pechini synthesis.

In this method, lanthanum nitrate hexahydrate and iron nitrate nonahydrate were dissolved in 30 mL distilled water at 108.3 g/L and 133 g/L, respectively, and the metallic ions underwent complexation by subsequent addition of citric acid at 70 g/L, corresponding to a stoichiometric molar ratio in respect of the metallic ions. The resulting suspension was maintained at 80°C for 2 days (2°C/min heating rate) in order to induce the complete gel formation. The calcination of the dried gel was performed using a heating cycle as reported by Gosavi et al. [START_REF] Gosavi | Pure phase LaFeO3 perovskite with improved surface area synthesized using different routes and its characterization[END_REF] with a heating rate of 5°C/min, consisting in holding the temperature at 600°C for 2 h, before further maintaining a temperature plateau at 800°C for 2 h.

The stoichiometric reaction forming the LaFeO3 perovskite structure is as follows (Eq.1) [START_REF] Qi | A simple way to prepare nanosized LaFeO3 powders at room temperature[END_REF]:

𝐿𝑎(𝑁𝑂 & ) & • 6𝐻 + 𝑂 + 𝐹𝑒(𝑁𝑂 & ) & • 9𝐻 + 𝑂 + 𝐶 1 𝐻 2 𝑂 3 • 𝑥𝐻 + 𝑂 → 𝐿𝑎𝐹𝑒𝑂 & + 6𝐶𝑂 + + 3𝑁 + + 𝑛𝐻 + 𝑂 [Eq.1]

Synthesis of LaFeO3/TiO2 composites

The LaFeO3/TiO2 composites were prepared by synthesizing TiO2 in the presence of calcined LFO, according to a sol-gel method under basic conditions using TTIP as precursor. In a typical synthesis, 17.8 g of TTIP was first added to a 40 mL ethanolic solution at room temperature, before 40 mL of water was added dropwise to the solution and the calcined LaFeO3 powder was further introduced under stirring into the aqueous solution. After rectification of the pH to 9 by adding ammonia (Carlo Erba, 30% aqueous solution), the solution was left under agitation at room temperature until a dried paste was obtained. The resulting powder was further dried at 100°C for 12 h, and finally calcined at 450°C for 2 h with a rate of 4.5°C/min. Depending on the amount of LFO material introduced into the aqueous solution, the so-obtained composites were labelled as x-LFO/TiO2, x being the theoretical LFO wt.% content of the final composites in respect of the theoretical amount of the composite, with x values of 2.5, 5.0, 7.5, 12.5 and 25.0.

Sol-gel TiO2 was synthesized as LFO-free reference material by following the same protocol without any addition of LFO powder.

Characterization techniques

X-ray diffraction (XRD) patterns have been recorded at room temperature in a θ/θ mode on a Bruker D8 Advance diffractometer equipped with a monochromatic copper radiation source (Kα = 1.54056 Å) with a scan step of 0.02°. The mean crystallite sizes, that are the average sizes of the coherently diffracting domains, were calculated by using the Scherrer equation with the usual assumption of spherical crystallites.

The surface area measurements were carried out on a Micrometrics Tristar 3000 using N2 as adsorbent at -196 °C with a prior outgassing at 250°C overnight to desorb the impurities or moisture.

UV-vis diffuse reflectance spectra were acquired in a UV/Vis/NIR Perkin Elmer Lambda 1050 spectrometer, recording the reflectance between 250 and 800 nm. Band gaps were estimated from the Tauc plots by representing (αhν) n vs. photon energy, where α= -Ln(Reflectance), with n=2 for LaFeO3 (direct band gap semiconductor) [START_REF] Luo | Rational design of Zscheme LaFeO3/SnS2 hybrid with boosted visible light photocatalytic activity towards tetracycline degradation[END_REF] and n=1/2 for TiO2 (indirect band gap semiconductor).

Fluorescence lifetime measurements by time-correlated single photon counting (TCSPC) were carried out with a Mini-t equipment from Edinburgh Instruments. For measurements under UV excitation, a laser diode of 372.2 nm wavelength and a pulse width of 61.2 ps was employed, using an emission band pass filter at 400 ± 25 nm. As visible excitation source, a laser diode with a wavelength of 445.2 nm and a pulse width of 90.7 ps was used, and a band pass filter at 600 ± 25 nm was placed before the detector. Both lasers were used at a repetition rate of 1 MHz.

Scanning electron microscopy (SEM) was performed in both secondary and back-scattered electron modes on a JEOL JSM-6700 F FEG microscope.

X-ray photoelectron spectroscopy (XPS) characterization was performed on a ThermoVGMultilabESCA3000 spectrometer (Al Kα anode at hλ = 1486.6 eV). The energy shift due to electrostatic charging was subtracted using the contamination sp 2 carbon C 1s band at 284.8 eV. Contributions with Doniach-Sunjic shape [START_REF] Doniach | Many-electron singularity in X-ray photoemission and X-ray line spectra from metals[END_REF] and a S-shaped Shirley type background [START_REF] Shirley | High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold[END_REF] were used.

Typical reaction procedure

The myclobutanil degradation experiments were carried out in a batch beaker-type glass reactor at atmospheric pressure, natural pH, at a constant temperature of 20°C ± 2°C (using a cooling bath). In a typical reaction procedure, 100 mg of catalyst was dispersed under stirring in a 100 mL aqueous solution of myclobutanil at 20 mg/L concentration, corresponding to an initial organic carbon concentration of 18 mg/L and to a catalyst load of 1.0 g/L. Prior to irradiation, the catalyst suspension had been stirred for 1 h in the dark to ensure the establishment of the adsorption/desorption equilibrium. The experiments were carried out within a Suntest XLS+ reaction chamber (Atlas Material Testing Technology BV, Gelnhausen, Germany) equipped with a Xenon arc lamp of 1700 W adjustable power and a Solar ID65 filter to limit the UV radiation at 320 nm for simulating solar exposition according to ICH Q1B guidelines. The suspension was exposed to simulated solar light at 250 Wm -2 which corresponds to an average solar radiation in a summer day in southern Europe, or to pure visible light through the use of 420-nm cut-off filter (GG420 Yellow Schott Optical Filters). Solar light and visible light spectral distributions, measured consequently with or without the use of the cut-off filter respectively, are reported in Fig. S1. At each time interval, 8 mL of solution were sampled and then filtered through 0.20 μm porosity filter to remove the photocatalyst powder if any, before the concentration of myclobutanil was determined by UV-Visible spectrophotometry by monitoring the disappearance of the main absorption peak at λ=220 nm. Total Organic Carbon measurements were performed using a Shimadzu TOC-L analyzer to determine the organic carbon load. The Fe leached during time-course of the reaction was quantified by the ortho-phenanthroline method [START_REF] Sandell | Colorimetric Determination of Traces of Metals[END_REF].

The efficiency of the UV cut-off filter was evidenced by performing a photocatalytic test of myclobutanil degradation (at 20 mg/L) under pure visible light (i.e. under an irradiation > 420 nm) using the commercially available Aeroxide© TiO2 P25 (Evonik) and sol-gel TiO2 as reference photocatalysts.

As it can be seen in Figure S2, no activity was observed under pure visible light for both reference TiO2 photocatalysts, neither in the myclobutanil degradation nor in the TOC conversion.

All degradation experiments have been triplicated, with standard deviations remaining within the ± 5% relative range. The associated error bars are reported in the activity graphs for the myclobutanil and TOC concentration evolution and for the corresponding kinetic rate constants.

RESULTS AND DISCUSSION

LaFeO3/TiO2 composite catalyst characterization

The XRD patterns of the reference catalysts and of the LaFeO3/TiO2 composite catalysts are shown in Figure 1. First, the XRD pattern of the reference titania showed only anatase TiO2 as crystalline phase with the main diffraction peaks at 2θ = 25.4°, 37.8°, 48.1°, 53.9° and 55.2° corresponding to the diffraction of the (101), (004), ( 200), ( 105) and (211) planes, respectively, according to the ICDD card no. 01-089-4921. Also, the XRD pattern of the pristine LFO catalyst was characterized by the most intense diffraction peaks at 2θ = 22.6°, 32.2°, 39.7°, 46.1°, 57.4° and 67.3° corresponding to the diffraction of the (100), ( 110), ( 111), ( 200), ( 210) and (220) planes of the LaFeO3 phase (ICDD card 070-7777), respectively. In turn, the XRD patterns of the as-synthesized LaFeO3/TiO2 composites could be fully indexed to pure anatase TiO2 and LaFeO3 phases. The higher the LFO weight content, the higher the (110)LFO/(101)A intensity ratio for the composites, as reported in Table 1, whereas no LFO phase was detected for the catalysts with a low ferrite content, probably due to low bulk iron contents.

Further, no diffraction peak shift was observed for the composites, suggesting that neither insertion of Ti atoms in the LFO unit cell nor that of La or Fe atoms in the anatase crystal occurred, as expected from the soft synthesis method and the low temperature of the final calcination step compared to the that used for stabilizing the LFO. In a first approximation, the mean size of the anatase crystallites was globally increasing from 13 nm to 21-25 nm with the increase in the LFO content, the general trend being that the formation of smaller anatase crystallites was favoured at low ferrite contents.

Figure 2 shows SEM images of the heterojunction materials and of both titania and LFO reference materials. The images recorded in both secondary and back-scattered electron modes evidenced that synthesising TiO2 in the presence of the LFO phase did not influence the overall granular morphology of the titania phase. Mapping analysis showed the distribution of the LFO phase in the composite materials while, considering the accuracy of the measurement, EDX analysis corroborated the equimolar La:Fe ratio within the composites, as well as a homogeneous and parallel distribution of titania and ferrite phases, with a minor formation of ferrite-rich regions. respectively. For the composites, in addition to the clear edge corresponding to the titania phase, a second absorption edge with low intensity assigned to the ferrite phase was observed in the spectra recorded on the samples with a low ferrite content, that was more pronounced as the LFO content increased and could be clearly evidenced for both 12.5-and 25.0-LFO/TiO2 samples. The band gap values estimated from the Tauc plots (Fig. S3) for an indirect transition in TiO2 and a direct one in LaFeO3 [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF] are shown in Table 1. The band gap of the ferrite phase was not influenced by the presence of the anatase phase, and was calculated at ca. 2.10 eV for spectra with a clear absorption edge.

Similarly, taking into account the moderate accuracy of the graphical method due to the difficulty of estimating the low-energy region baseline because of the simultaneous ferrite absorption, the band gap of the TiO2 phase remained unchanged, being centered around that of the reference TiO2 within the 3.14-3.26 eV range. Figure S4 shows the N2 adsorption-desorption isotherms of the composite materials and of both single phase references, which are characterized by a IV-type, with a H3-type hysteresis, characteristic of aggregates or agglomerates of particles forming slit shaped pores with non-uniform size and/or shape. The reference LFO has a low specific surface area of 8 m 2 /g, that increased notably up to the 14-77 m 2 /g values for the composite materials, so that the composite photocatalysts globally possess a higher surface area than both single phase counterparts (Table 1). This suggests that the LFO/TiO2 materials differ strongly from a mechanical mixing of segregated phases and can consequently be considered as (nano)composite materials providing good inter-phase contacts. In particular, the highest specific surface areas (at 66 m 2 /g and 77 m 2 /g) with the highest pore volumes, were obtained for the 2.5-and 5.0-LFO/TiO2 samples, respectively, in agreement with the smallest mean crystallite size of 13 nm of the main anatase phase. This might directly result from the interaction of the titanium precursor with the LFO crystallites during the sol-gel synthesis. Further, the presence of interconnected phases can consequently contribute to the enhanced surface area and pore volume properties. A similar increase in surface area was already observed for LFO-based composites compared to both single phase materials, for instance in the case of heterojunctions obtained by synthesizing CdS nanorods [START_REF] Hoseini | A novel n-type CdS nanorods/p-type LaFeO3 heterojunction nanocomposite with enhanced visible-light photocatalytic performance[END_REF] and g-C3N4 [START_REF] Jin | Boosting visible light photocatalytic performance of g-C3N4 nanosheets by combining with LaFeO3 nanoparticles[END_REF] in the presence of crystallized LFO. Also, Mrotek et al. prepared a coreshell structured sol-gel TiO2-based composite with an enhanced surface area in comparison to that of ZnFe2O4/SiO2 used as core material [START_REF] Mrotek | Improved degradation of etodolac in the presence of core-shell ZnFe2O4/SiO2/TiO2 magnetic photocatalyst[END_REF]. Fluorescence lifetime measurements under UV excitation (Fig. 3A) show, for TiO2, a decay curve that can be fitted to a monoexponential function with a lifetime of ca. 1.05 ns, while LFO data (not shown) are better described as a biexponential decay with considerably longer lifetime (1.96 ns).

Composite catalysts give rise, in all cases, to decay data fitting monoexponential curves with lifetimes between ca. 1.1 and 1.3 ns, suggesting that the fluorescence of titania is mainly observed above that of the ferrite. The comparison of the composites with TiO2 shows, as a general trend, an enlargement in this fluorescence lifetime when both phases coexist, indicating an increased population in the conduction band of titania and therefore suggesting an electron transfer from the excited ferrite phase to the TiO2 one, in accordance to the previously described relative band positions between both semiconductors [START_REF] Lv | In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production[END_REF]. By contrast, the fluorescence lifetimes under visible light excitation (Fig. 3B), corresponding only to the excitation of the ferrite phase [START_REF] Garcia-Muñoz | Highly robust La1-xTixFeO3 dual catalyst with combined photocatalytic and photo-CWPO activity under visible light for 4chlorophenol removal in water[END_REF], are shorter in all composites compared to that observed for the pure ferrite, indicating a decreased population of electrons in the conduction band in this phase. These observations therefore suggest that a type-II heterojunction [START_REF] Robert | Photosensitization of TiO2 by MxOy and MxSy nanoparticles for heterogeneous photocatalysis applications[END_REF][START_REF] Lv | In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production[END_REF][START_REF] Low | Heterojunction Photocatalysts[END_REF][START_REF] Jang | Heterojunction semiconductors: A strategy to develop efficient photocatalytic materials for visible light water splitting[END_REF] is established between both components, as described in Figure S5 in Supplementary Material. XPS surface investigation has been performed for selected LFO/TiO2 composites as well as for both sol-gel TiO2 and LFO single phase references, and Figure 4 and Figure S6 show the XPS spectra profiles for the different core orbitals. They evidence that the pure LFO phase was characterized by the presence of La 3+ and Fe 3+ ions at the surface [START_REF] Garcia-Muñoz | Highly robust La1-xTixFeO3 dual catalyst with combined photocatalytic and photo-CWPO activity under visible light for 4chlorophenol removal in water[END_REF][START_REF] García-Muñoz | Synergy Effect Between Photocatalysis and Heterogeneous PhotoFenton Catalysis on Ti-Doped LaFeO3 Perovskite for High Efficiency Light-Assisted Water Treatment[END_REF]. Indeed, the La 3d5/2 -La 3d3/2 orbital doublet contribution at 833.6 eV and 850.3 eV (spin orbit splitting of 16.7 eV [START_REF] Humayun | Synthesis of ZnO/Bi-doped porous LaFeO3 nanocomposites as highly efficient nano-photocatalysts dependent on the enhanced utilization of visible-light-excited electrons[END_REF][START_REF] Phokha | Polymerized complex synthesis and effect of Ti dopant on magnetic properties of LaFeO3 nanoparticles[END_REF][START_REF] Thirumalairajan | Novel Synthesis of LaFeO3 Nanostructure Dendrites: A Systematic Investigation of Growth Mechanism, Properties, and Biosensing for Highly Selective Determination of Neurotransmitter Compounds[END_REF] and multiplet splitting of 3.8 eV [START_REF] Petrović | LaMO3 (M=Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation[END_REF]) is characteristic of La 3+ species (Fig. 4C), while the Fe 2p3/2 and Fe 2p1/2 orbital doublet contribution at 710.5 eV and 723.9 eV, with a 13.4 eV spin orbit coupling [START_REF] Humayun | Synthesis of ZnO/Bi-doped porous LaFeO3 nanocomposites as highly efficient nano-photocatalysts dependent on the enhanced utilization of visible-light-excited electrons[END_REF][START_REF] Petrović | LaMO3 (M=Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation[END_REF] a the low intensity and broad shake-up satellite peak at about 718 eV, is characteristic of Fe 3+ ions [START_REF] Yamashita | Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials[END_REF][START_REF] Hawn | Deconvolution as a correction for photoelectron inelastic energy losses in the core level XPS spectra of iron oxides[END_REF] (Fig. 4D). The LFO surface is also characterized by the broad multi-contribution envelope of the O 1s core orbital spectra, in agreement with the literature [START_REF] Parrino | Cu-substituted lanthanum ferrite perovskites: Preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation[END_REF], and with a main contribution at 528.8 eV assigned to lattice oxygen species [START_REF] Gong | Highly active and humidity resistive perovskite LaFeO3 based catalysts for efficient ozone decomposition[END_REF][START_REF] Phan | Heterogeneous photo-Fenton degradation of organics using highly efficient Cu-doped LaFeO3 under visible light[END_REF]. Regarding the TiO2 reference, the Ti 2p spectra shows the usual doublet related to the Ti 2p3/2-Ti 2p1/2 spin-orbit components of octahedral symmetry Ti 4+ (Ti-O) in a typical TiO2 crystalline structure, at 458.6 eV and 464.3 eV respectively, with no contribution attributed to Ti 3+ species, indicating the presence of few surface defects [START_REF] Alonso-Tellez | A parametric study of the UV-A photocatalytic oxidation of H2S over TiO2[END_REF][START_REF] Kubo | Local Structure of TiO2-Derived Nanotubes Prepared by the Hydrothermal Process[END_REF][START_REF] Yan | On the role of BmimPF6 and P/F-containing additives in the sol-gel synthesis of TiO2 photocatalysts with enhanced activity in the gas phase degradation of methyl ethyl ketone[END_REF]. Also the O 1s spectra envelope exhibits a main contribution at 529.8 eV, usually attributed to O 2-species in O-Ti bonds of the TiO2 lattice.

As far as the LFO/TiO2 catalysts are concerned, it is worth noting that the presence of both La 3+ and Fe 3+ cations is not evidenced at the surface of the different composites, whatever the wt.% content of the LFO phase in the materials. By contrast, both Ti 2p and O 1s core orbital spectra are characteristic of the TiO2 phase, with binding energies being similar to those recorded for the pure sol-gel TiO2 material, ie. at 458.6 ± 0.2 eV and 529.8 ± 0.2 eV for Ti 2p3/2-Ti 2p1/2 and O 1s orbital peaks, respectively.

While the absence of any signal assigned to the LFO phase for composites with a low LFO content could directly result from this low content, this explanation has to be ruled out for composites containing higher LFO loadings, and we suggest that the LFO phase is fully covered by a TiO2 layer in the different composite photocatalysts whatever the LFO wt.% content. titanium isopropoxide or n-butoxide as precursor in the presence of a host semiconductor phase.

Through the hydrolysis of butoxide, Li et al. synthesized nanocrystalline and nanoporous sol-gel TiO2 shells surrounding microcrystalline (Ba,Sr)TiO3 cores, while Mrotek et al. prepared ZnFe2O4/SiO2/TiO2 photocatalyst [START_REF] Mrotek | Improved degradation of etodolac in the presence of core-shell ZnFe2O4/SiO2/TiO2 magnetic photocatalyst[END_REF][START_REF] Li | Heterostructured Ceramic Powders for Photocatalytic Hydrogen Production: Nanostructured TiO2 Shells Surrounding Microcrystalline (Ba,Sr)TiO3 Cores[END_REF]. Using titanium isopropoxide, Fe2O3@TiO2 core-shell nanostructures were obtained [START_REF] Liu | 3D Flowerlike alpha-Fe2O3@TiO2 Core-Shell Nanostructures: General Synthesis and Enhanced Photocatalytic Performance[END_REF], and Abdel-Wahed et al. recently elaborated a MnFe2O4@rGO@TiO2 photocatalyst with core double-shell structure by performing the sol-gel synthesis in the presence of a pre-structured core-shell MnFe2O4@rGO host [START_REF] Abdel-Wahed | Core double-shell MnFe2O4@rGO@TiO2 superparamagnetic photocatalyst for wastewater treatment under solar light[END_REF]. TEM images shown in Figure 5 are in agreement with the existence of a core-shell structure for the LFO/TiO2 composite photocatalysts. Indeed, Figure 5A shows a LFO crystallite with an interplanar spacing of 0.39 nm consistent with the (110) planes characteristic of the LFO structural crystallographic features, surrounded by a dense and (meso)porous network of anatase TiO2 crystallites with the characteristic interplanar spacing of 0.35 nm corresponding to the (101) planes. This shell of entangled TiO2 crystallites creates tight interfaces with enhanced contact between both semiconductor phases.

It is worth nothing that even in sample zones for which only crystallites assigned to anatase TiO2 were observed, EDX analysis confirmed the presence of La and Fe atoms, with atomic contents in agreement with the theoretical values (Figures 5B,C). Considering the larger analysis depth in EDX when compared to XPS, this is in agreement with the proposed core-shell character of the composites, even in the case of the highly loaded 25-LFO/TiO2 material.

We cannot rule out that the formation of self-standing TiO2 particles is occurring in some extent in parallel to the coating of LFO grains with a TiO2 poly-crystallite shell, and all the more pronounced as the LFO content is lower. Indeed, because of the highly electrophilic nature of titanium centers in titanium alkoxides, they can easily react with nucleophiles like water and are very sensitive to moisture. This consequently leads to fast and homogeneous nucleation resulting in the aggregation of cores, due to the energy change in the system [START_REF] Li | Core-shell structured titanium dioxide nanomaterials for solar energy utilization[END_REF]. 2 reports the corresponding rate constants for both myclobutanil degradation and TOC conversion using a pseudo first and zero order kinetic model, respectively, as typical in a photocatalytic process. First, no detectable degradation of myclobutanil has been observed at room temperature after 240 min under solar light, indicating that the substrate photolysis can be neglected in the experimental conditions (not shown).

Activity of the LaFeO3/TiO2 photocatalysts under solar light

The results evidence that the single phase photocatalysts and the LFO/TiO2 composites are active in the degradation and mineralization of myclobutanil regardless the wt.% content of LFO. The most active composite photocatalyst allowed full removal of myclobutanil to be achieved within 180 min of test, while only 85% of myclobutanil abatement was obtained with pure LFO and TiO2 photocatalysts after 240 min of test. As far as TOC conversion is concerned, complete myclobutanil mineralization was obtained on the most active composite for 240 min of test, while only 77% and 55% of mineralization yield was achieved with the LFO and TiO2 photocatalysts, respectively. Thus, the coupling of both semiconductor oxides enhanced strongly the activity of the photocatalysts under solar light when compared to that of both single phase materials, with the existence of an optimum in performance and in conversion rates for a composite containing 5 wt.% of ferrite. Fig. 6C depicts the volcano-like profile of the influence of the LFO wt.% content in the composite on the kinetic rate constant for the myclobutanil removal and the TOC conversion. In all cases, the constant values of the composites are higher than a linear combination of the values obtained with the pure phases considering the mass percentage of each component, revealing a synergistic effect between phases. The highest apparent rate constants of 0.018 min -1 and 0.071 mg/Lmin, respectively, are obtained for the 5.0-LFO/TiO2 photocatalyst, in both cases higher than those achieved on the LFO and TiO2 counterparts (Table 2). It is worth noting that optimum LFO contents within the 1-5 wt.% range have been mostly reported [START_REF] Jin | Boosting visible light photocatalytic performance of g-C3N4 nanosheets by combining with LaFeO3 nanoparticles[END_REF][START_REF] Xu | Superior photocatalytic performance of LaFeO3/g-C3N4 heterojunction nanocomposites under visible light irradiation[END_REF][START_REF] Pirzada | Synthesis of LaFeO3/Ag2CO3 Nanocomposites for Photocatalytic Degradation of Rhodamine B and p-Chlorophenol under Natural Sunlight[END_REF][START_REF] Gao | In situ Synthesis of p-n LaFeO3/ZnIn2S4 Heterojunctions for Enhanced Photocatalytic Activity[END_REF][START_REF] Jing | Synthesis of High-Activity TiO2-Based Photocatalysts by Compounding a Small Amount of Porous Nanosized LaFeO3 and the Activity-Enhanced Mechanisms[END_REF], although it obviously strongly depends on both the nature and the synthesis strategy of the LFO-based heterojunctions as well as on the light source.

We suggest that the crystallite size is not to be the predominant factor in the enhancement of the activity for the LFO/TiO2 composites compared to both single phases. Indeed, the most active 5-LFO/TiO2 composite has indeed the smallest mean anatase TiO2 crystallite size at 13 nm, but a far lower kinetic rate constant for myclobutanil degradation was obtained on the 2.5-LFO/TiO2 composite with a similar mean anatase size. Also, composites differing in terms of mean crystallite sizes led to similar kinetic rate constants.

The composite photocatalysts were also evaluated in terms of lanthanum and iron leached to the solution for 240 min. While no presence of La was detected in the treated aqueous effluents, a low final released iron concentration of 0.1 mg/L was observed for the most active photocatalyst, ie. 5-LFO/TiO2 composite, while the pristine LFO photocatalyst released iron with a final concentration of 0.7 mg/L (Table 2). The formation of surface complexes between surface iron and carboxylic acids has been proposed to be one of the main mechanisms involved in the release of Fe 2+ /Fe 3+ species from iron-based catalysts in AOP processes [START_REF] Zazo | Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst[END_REF][START_REF] Zazo | UV-LED assisted catalytic wet peroxide oxidation with a Fe(II)-Fe(III)/activated carbon catalyst[END_REF]. They photodegrade with the simultaneous release of free iron cations (Eq. 2). Consequently, if released in substantial amount, iron might contribute to the overall activity after its subsequent complexation by short-chain organic acids (mainly oxalic, acetic and formic acids), through a photo-induced ligand-to-metal charge transfer mechanism occurring in Fe polycarboxylates in homogeneous phase, with the simultaneous production of active oxygenated radicals [START_REF] Liu | Fe(III)-oxalate complexes mediated photolysis of aqueous alkylphenol ethoxylates under simulated sunlight conditions[END_REF][START_REF] Faust | Photochemistry of aqueous iron(III)-polycarboxylate complexes: roles in the chemistry of atmospheric and surface waters[END_REF][START_REF] Abrahamson | Photochemical and spectroscopic studies of complexes, of iron(III) with citric acid and other carboxylic acids[END_REF]. According to this direct solar light activated mechanism, oxalate was reported to be one of the most active ligands.

[𝐹𝑒 999 (𝑅𝐶𝑂𝑂 ; )] += → >?

𝐹𝑒 += + 𝐶𝑂 + + 𝑅 [Eq.2]
However, the low released Fe concentration observed at 0.1 mg/L in the case of the most active LFO/TiO2 photocatalyst in comparison to those observed for the other photocatalysts with lower activity (composites and bare LFO), suggests that the released Fe does not contribute significantly to the kinetic rate constants measured for myclobutanil and TOC removal.

The possibility for the iron of the LFO/TiO2 core-shell structured photocatalyst to be released from the LFO core to the water, confirmed the porous nature of the external layer surrounding the LFO core.

This feature is in line with the strong increase in the specific surface area of the composites with respect to the single oxides, and with the enhanced contribution of small size mesopores in the most active composites (Fig. S7), that might result from the agglomeration of small size TiO2 crystallites at the surface of the LFO, consequently providing its porous character to the TiO2 crystallite shell. The nanocrystalline and porous character of sol-gel TiO2 shells synthesized on (Ba,Sr)TiO3 cores using titanium n-butoxide as precursor was demonstrated by Li et al [START_REF] Li | Heterostructured Ceramic Powders for Photocatalytic Hydrogen Production: Nanostructured TiO2 Shells Surrounding Microcrystalline (Ba,Sr)TiO3 Cores[END_REF].

Catalytic activity under visible light

A set of reactions for the myclobutanil removal with the composite photocatalysts was performed under pure visible light (l > 420nm). Figure 7 shows the depletion of myclobutanil as well as the TOC reduction obtained with the x-LFO/TiO2 composite photocatalysts in comparison to those observed with the single phase LFO reference. The apparent reaction rate constants for both the myclobutanil removal and the TOC conversion, both myclobutanil and TOC conversions achieved after 4 h of test as well as the concentration of iron released are reported in Table 3.

Similarly to the reactions performed under solar light, the behaviour of the LFO/TiO2 composites under pure visible light is characterized by the existence of an optimum in terms of efficiency and of reaction rates, while (i) the reference anatase TiO2 did not show any activity neither in TOC conversion nor in myclobutanil degradation (Fig. S2) and (ii) low rate constants of 0.00043 min -1 and 0.002 mg/Lmin were observed on the pure LFO photocatalyst for myclobutanil degradation and TOC conversion, respectively. This suggests, on the one hand, that the photoactivation is initiated by the LFO phase and, on the other hand that the TiO2 phase forming the heterojunction might help for a better charge separation with respect to pure LFO, in line with fluorescence lifetime measurements.

The volcano-like profile of the influence of the LFO content in the composite on both kinetic rate constants is shown in Fig. 7C. By contrast to the reactions under solar light, both volcano-like patterns are centered on a composite with a higher LFO content, ie. 12.5 wt.% vs. 5 wt.% under solar light, with the highest rate constant for myclobutanil degradation and TOC degradation of 0.0016 min -1 and 0.0059 mg/Lmin, respectively. Dhinesh et al. also reported an enhancement of the activity of LFO/TiO2 composites under visible light compared to pure LFO, using the degradation of methyl orange dye as test reaction, but in a lesser extent, probably due to a too high content of LFO in the composite photocatalyst (ie. a 1:1 wt. ratio) [START_REF] Kumar | Synthesis and characterization of LaFeO3/TiO2 nanocomposites for visible light photocatalytic activity[END_REF]. However, no kinetic rate constants were calculated for quantifying this improvement, and the dye mineralization was not evidenced by any TOC concentration evolution curve.

Both the lower myclobutanil abatement and the lower myclobutanil mineralization yield achieved after 240 min of test under pure visible evidenced the far smaller activity of the composite photocatalysts when compared to the reactions performed under solar light, as materialized by kinetic rate constants being lower by about one order of magnitude (Table 3). This suggests in a first approximation that the activity resulting directly from the activation of the composites by visible light (ie. through the LFO phase) does not contribute significantly to the overall activity of the composites under solar light.

No leaching of lanthanum to the water occurred during the tests regardless of the photocatalyst composition, while the amounts of iron released from the LFO-based photocatalysts under visible light were substantially lower than those measured under solar light, with a negligible final released iron concentration of 0.02 mg/L for the most active photocatalyst (Table 3). This is attributed to the lower mineralization yield achieved under pure visible light, that unfavours and prevents the surface complexation-induced release of Fe to the solution (Eq. 2), due to the presence of carboxylic acids in a far lower extent. The shift of the optimum content of the visible-light activated LFO within the composite when replacing the incident solar light by pure visible light suggests that the light-mediated reaction mechanism depends on the activation light spectrum. Thus, based on photocatalytic activity and fluorescence lifetime observations, different reaction mechanisms are proposed as follows:

A. Under solar light, the presence of the narrow band gap LFO in the composite allows the composite to be activated not only through the UV-A part of the spectra, but also via a fraction of its visible region, therefore allowing the extension of the light absorption. Further, the mechanism of the photocatalytic degradation is proposed to involve a double photo-activation of the LFO and TiO2 semiconductor phases, leading to the formation of pairs of charges (electron/hole) in both semiconductors (Eq.3-4), with improved charge separation due to the formation of a p-n heterojunction. Thus, the relative band positions of both semiconductors [START_REF] Lv | In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production[END_REF] -with both valence and conduction bands of the ferrite being more cathodic than the corresponding bands of the titania after Fermi level alignment (type-II staggered gap model) -, is proposed to allow the electrons generated in the conduction band of LFO to be transferred to the conduction band of titania, in accordance with fluorescence lifetime measurements, while holes in the valence band of TiO2 would migrate to that of LFO across the junction [START_REF] Lv | In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production[END_REF]. It is worth noting that the increased population of electrons in the conduction band of titania does not support the existence of a Z-scheme mechanism between both semiconductor phases, for which a shorter fluorescence lifetime should have been measured. Then, the larger concentration of holes in the ferrite semiconductor would lead oxidation reactions to occur preferentially on this phase (Eq. 5) [START_REF] Garcia-Muñoz | Ferrite Materials for Photoassisted Environmental and Solar Fuels Applications[END_REF], while the electrons accumulated in the conduction band of titania (Eq.6) would preferentially react with the adsorbed surface oxygen provoking the formation of the superoxide radicals which can lead to the organic matter oxidation too (Eq. 7) [START_REF] Robert | Photosensitization of TiO2 by MxOy and MxSy nanoparticles for heterogeneous photocatalysis applications[END_REF][START_REF] Chong | Recent developments in photocatalytic water treatment technology: A review[END_REF]. Further, the fact that the longer fluorescence lifetime in TiO2 does not match the most active coupled photocatalyst suggests that this optimum occurs as a balance between inter-phase electron transfer and an adequate relative amount of the component phases, taking into account that both are activated under solar light and therefore contribute with reactive electrons and holes. B. Under pure visible light, the mechanism of the reaction can only be initiated by the photoactivation of the narrow band gap ferrite phase leading to the generation of electron and holes (Eq. 8). While the photogenerated holes react with adsorbed water with the production of the active hydroxyl radicals (Eq. 9), the electrons are proposed to undergo inter-phase transfer from the higher-energy conduction band of the ferrite to the lower-energy conduction band of the titania (Eq. 10), whose role is to act as electron sink in the heterojunction material for reducing the charge recombination. The electrons in the conduction band of titania would further allow the reduction of O2 with the formation of the active O2 -° superoxide radicals (Eq. 11) involved also in the mineralisation of the refractory pesticide in water. LFO being here the only phase activated by the visible light, by contrast to reactions performed under solar light, the reactivity and efficiency maximum is shifted towards a higher LFO content in the composite. In these irradiation conditions, electron transfer to the titania conduction band would be the main factor responsible for the improved activity with respect to the pure phases and therefore the highest kinetic rate constants match the most extensive charge transfer according to fluorescence lifetimes. Figure 8A shows the evolution of the myclobutanil concentration with time, taking the time necessary for achieving a complete myclobutanil depletion during the first cycle as standardized test duration (ie. 180 min). After 4 test cycles, the 5-LFO/TiO2 composite photocatalyst was characterized by a slight deactivation, with the myclobutanil conversion being reduced from 100% to 96% for 180 min of test, and the corresponding apparent rate constant only slightly decreasing from 0.018 to 0.016 min -1 . This was corroborated by the lower mineralization degree reached after the 4 th test, ie. 90% of TOC conversion for 4 h of test (Figure 8B). This slight and progressive deactivation could be ascribed to the consecutive loss of iron species to the aqueous solution during the photoreactions, that was associated to the formation of surface iron carboxylate complexes from carboxylic acid-type intermediates as often observed in the case of high mineralization degree materials. However, the leached iron concentration stabilized at the negligible level of 0.07 mg/L after several consecutive test cycles.

CONCLUSION

LaFeO3/TiO2 composite photocatalysts, prepared by a sol-gel synthesis of TiO2 in the presence of narrow band gap LaFeO3 crystallites previously obtained through a Pechini sol-gel synthesis, have Pore Diameter (nm)

Fig. 1 -

 1 Fig. 1-B shows the UV-vis diffuse reflectance spectra of the composite catalysts together with

Figure 1 .

 1 Figure 1. A) XRD patterns of the LaFeO3/TiO2 composite catalysts and of both LFO and TiO2 reference

Figure 2 .

 2 Figure 2. SEM images of (A) the LFO and (B) the TiO2 materials, and of the x-LFO/TiO2 composites

Figure 3 . 5 - 5 -

 355 Figure 3. Fluorescence decay curves of single-phase and composite catalysts under (A) UV-A light

Figure 4 .

 4 Figure 4. XPS profile recorded for both TiO2 and LFO singe phase catalysts and for the LFO/TiO2

Figure 5 .

 5 Figure 5. TEM images of (A) the 5-LFO and (B and C-zoom) the 25-LFO composite photocatalysts.

Figure 6

 6 Figure 6 depicts the evolution of the relative myclobutanil and TOC concentrations (6A and 6B)

Figure 6 .

 6 Figure 6. Evolution of (A) the relative myclobutanil concentration and fitted lines and (B) the relative TOC concentration and fitted lines, during the time-course of the photocatalysis with the LaFeO3/TiO2 composites under solar light in comparison with the single phase LaFeO3 and TiO2 reference photocatalysts. (C) Kinetic rate constant of myclobutanil disappearance and mineralization under solar light as a function of the wt.% content of ferrite in the composite. Operating conditions: [Myclobutanil]0=20 mg/L; [TOC]0=12.5 mg/L ; T=25°C; [cat]=1.0 g/L; l = 300-800 nm.

Figure 7 .

 7 Figure 7. Evolution of (A) the relative myclobutanil concentration and (B) the relative TOC concentration, during the time-course of the photocatalysis with the LaFeO3/TiO2 composites under pure visible light in comparison with the single phase LaFeO3 reference photocatalyst. (C) Kinetic rate constants of myclobutanil disappearance and of mineralization under visible light as a function of the wt.% content of ferrite in the composite. Operating conditions: [Myclobutanil]0=20 mg/L; [TOC]0=12.5 mg/L; T=25°C; [cat]=1.0 g/L; l > 420 nm.

3 . 4

 34 Stability and reusability of the optimum LaFeO3/TiO2 photocatalyst under solar lightOne of the key-parameters of Fe-containing photocatalysts is their ability to be reused in consecutive test cycles, that can be notably reduced by the release of iron to the solution by leaching during the course of the mineralization process. In order to evaluate the reusability of the composites, 4 sequential cycles of reaction were performed under solar light on the most active photocatalyst, i.e. the 5-LFO/TiO2 composite. After each run, the catalyst was recovered by filtration, dried at 100°C overnight and reused in a new test cycle. No change of the specific surface area of the photocatalyst was observed upon the test cycles.

Figure 8 .

 8 Figure 8. Evolution of (A) the relative concentration of myclobutanil during 4 sequential cycles

20 B

 20 demonstrated to be active for the degradation of the myclobutanil pesticide in water under solar and visible light. They are proposed to be structured as LaFeO3 crystallite cores surrounded by a dense and (meso)porous network of anatase TiO2 crystallites, providing a TiO2@LaFeO3 porous core-shell-type structure with tight interfaces and enhanced contact between both semiconductor phases. Whatever the irradiation light spectra, the photocatalytic activity of LaFeO3/TiO2 composites is characterized by a volcano-like profile in terms of kinetic rate constants for both the myclobutanil degradation and the TOC conversion, but the optimum semiconductor ratio depends on the activating light. In any case, the optimum composite strongly overperforms both single phase LFO and TiO2. Different light-mediated reaction mechanisms are invoked for such performance: (i) a double photoactivation of both semiconductors under solar light with improved charge separation through a type-II heterojunction, and (ii) LFO acting as light harvesting photosensitizer for TiO2 under pure visible light. Finally, the optimum LaFeO3/TiO2 composite photocatalyst is active and stable after several runs under solar light with negligible iron leaching. This study opens up new opportunities for efficient solar water treatment using heterogeneous photocatalysts. Further enhancements from the chemical engineering point of view are expected by applying these materials with adequate macroscopic supports for operating in a continuous treatment mode.

Figure S3 Figure S3 .

 S3S3 Figure S3

Figure S4 Figure S4 .Figure S5 .

 S4S4S5 Figure S4

Figure S6 .

 S6 Figure S6. XPS Survey and C1s core orbital spectra for both TiO2 and LFO single phase catalysts and for

Figure S7 Figure S7 .

 S7S7 Figure S7

Table 1 .

 1 Main physico-chemical properties of the catalysts.

	Catalyst Phases (110)LFO	Crystallite size	SBET	Pore volume	Band-gap	Band-gap
			/(101)A C	(nm) (A/LFO) a	(m 2 /g)	(cm 3 /g)	TiO2 (eV)	LaFeO3 (eV)
	TiO2	A	-	25	24	0.06	3.19	-
	2.50	A	-	13/-b	66	0.15	3.14	-
	5.00	A	-	13/-b	77	0.19	3.16	-
	7.50	A/LFO 0.08	23/-b	14	0.05	3.17	-
	12.50	A/LFO 0.21	25/33	23	0.09	3.26	2.08
	25.00	A/LFO 0.44	21/33	38	0.09	3.25	2.07

Table 2 .

 2 Reaction rates, conversions of myclobutanil and of TOC achieved after 4 h of reaction under solar light (l = 300-800 nm), and released iron concentration after 4 h of test.

	wt. % LFO	k myclo (min -1 )	kTOC (mg/L min)	X4h Myclobutanil (%)	X4h TOC (%) Fe4h (mg/L)
	TiO2	0.007	0.042	85	55	-
	2.5	0.011	0.060	100	86	0.1
	5.0	0.018	0.071	100	100	0.1
	7.5	0.015	0.063	100	92	0.2
	12.5	0.012	0.045	94	76	0.4
	25.0	0.008	0.040	86	64	0.3
	LFO	0.009	0.050	85	77	0.7

Table 3 .

 3 Reaction rates, conversions of myclobutanil and of TOC achieved after 4 h of reaction under pure visible light (l > 420nm), and the released iron concentration after 4h of test.

	wt. % LFO	k myclo (min -1 ) kTOC (mg/Lmin)	X4h Myclobutanil (%)	X4h TOC (%) Fe4h (mg/L)
	2.5	0.00040	0.0020	5	3	0.03
	5.0	0.00080	0.0050	25	11	0.02
	7.5	0.00079	0.0059	17	12	0.03
	12.5	0.00160	0.0109	37	25	0.02
	25.0	0.00081	0.0059	24	12	0.05
	LFO	0.00043	0.0002	10	5	0.25

Acknowledgements.

The authors want to thank the European Fund for regional development (EFRE/FEDER) for the financial support of the PHOTOPUR project which is performed within the framework of Interreg V and the Sciences Offensive. F.F. thanks financial support from the Spanish Ministry of Science, Innovation and Universities (Project SOLPAC: ENE2017-89170-R, MCIU/AEI/FEDER, EU). T. Dintzer and V. Papaefthimiou (ICPEES) are thanked for performing SEM and XPS analysis, respectively. D. Ihiawakrim (IPCMS) is thanked for performing TEM characterization.