Anthony Beauvois
email: anthony.beauvois@univ-rennes1.fr

Delphine Vantelon

Jacques Jestin

Martine Bouhnik-Le Coz

Charlotte Catrouillet

Valérie Briois

Thomas Bizien

Mélanie Davranche

Martine Bouhnik- Le Coz

How crucial is the impact of calcium on the reactivity of iron-organic matter aggregates? Insights from arsenic

Keywords: Fe-OM colloids, adsorption isotherms, ferrihydrite, Fe-oligomers, EXAFS, SAXS, Ca interactions

Environmental iron-organic matter (Fe-OM) aggregates play a major role in the dynamic of pollutants. Nowadays, there is a lack of information about the control exerted by their structural organization on their reactivity towards metal(loid)s and in particular, the impact of major ions, such as calcium. The sorption capacity of mimetic environmental Fe-OM-Ca aggregates was investigated relative to the Fe/organic carbon (OC) and Ca/Fe ratios using As as a probe. It was shown that Fe speciation is the key factor controlling the reactivity of Fe-OM-Ca aggregates regarding the high affinity of Fe(III)-oligomers towards As and the high sorption capacity of ferrihydrite-like nanoparticles. Moreover, when it occurs at high concentration, Ca competes with Fe for OM binding leading to an increase in the amount of ferrihydrite-like nanoparticles and binding site availability. As a consequence, Ca not only J o u r n a l P r e -p r o o f impacts the ionic strength but it also has a dramatic impact on the structural organization of Fe-OM aggregates at several scales of organization, resulting in an increase of their sorption capacity. In the presence of high amounts of Ca, Fe-OM-Ca aggregates could immobilize pollutants in the soil porous media as they form a micrometric network exhibiting a strong sorption capacity.

Introduction

Environmental colloids are of major importance given their high reactivity towards metal and metalloid pollutants [START_REF] Kretzschmar | Metal Retention and Transport on Colloidal Particles in the Environment[END_REF]. They are nanometric particles, either aggregated or not, with a high specific surface area (SSA), mainly composed of minerals and/or organic matter (OM). Among them, those particles composed of iron (Fe) and OM, usually called "ironorganic matter (Fe-OM) aggregates", are widely encountered in natural systems such as wetlands where they are mainly produced by drying/wetting cycles in the uppermost soils [START_REF] Wigginton | Aquatic environmental nanoparticles[END_REF][START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF][START_REF] Al-Sid-Cheikh | Interactions between natural organic matter, sulfur, arsenic and iron oxides in re-oxidation compounds within riparian wetlands: NanoSIMS and X-ray adsorption spectroscopy evidences[END_REF][START_REF] Pédrot | Insights into colloid-mediated trace element release at the soil/water interface[END_REF][START_REF] Ratié | Iron speciation at the riverbank surface in wetland and potential impact on the mobility of trace metals[END_REF].

For many years, the structure and organization of Fe-OM aggregates was were only simply described as poorly crystallized iron oxyhydroxides embedded in an OM matrix [START_REF] Poggenburg | Microbial reduction of ferrihydrite-organic matter coprecipitates by Shewanella putrefaciens and Geobacter metallireducens in comparison to mediated electrochemical reduction[END_REF].

The overall multiscale complexity of the organization between Fe and OM was poorly investigated. Only recently, several studies have demonstrated that within Fe-OM aggregates, OM in the form of molecules and colloidal aggregates [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF][START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF][START_REF] Gentile | Ferrihydrite Nanoparticle Aggregation Induced by Dissolved Organic Matter[END_REF] constrains the growth and crystallinity of the Fe phases [START_REF] Pédrot | How does organic matter constrain the nature, size and availability of Fe nanoparticles for biological reduction?[END_REF][START_REF] Vantelon | Iron speciation in iron-organic matter nanoaggregates: A kinetic approach coupling Quick-EXAFS and MCR-ALS J o u r n a l P r e -p r o o f chemometry[END_REF]. The result is that Fe forms Fe(III) monomers, small oligomers and ferrihydrite-like nanoparticles (Fh-like Nps), bound to the OM involving its carboxylic (COOH) and phenolic (PhOH) sites [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF][START_REF] Vantelon | Iron speciation in iron-organic matter nanoaggregates: A kinetic approach coupling Quick-EXAFS and MCR-ALS J o u r n a l P r e -p r o o f chemometry[END_REF][START_REF] Karlsson | Complexes with aquatic organic matter suppress hydrolysis and precipitation of Fe(III)[END_REF][START_REF] Karlsson | Characterization of Iron(III) in Organic Soils Using Extended X-ray Absorption Fine Structure Spectroscopy[END_REF][START_REF] Kleber | Mineral-Organic Associations: Formation, Properties, and Relevance in Soil Environments[END_REF][START_REF] Chen | Properties of Fe-Organic Matter Associations via Coprecipitation versus Adsorption[END_REF][START_REF] Karlsson | Coordination chemistry and hydrolysis of Fe(III) in a peat humic acid studied by X-ray absorption spectroscopy[END_REF]. Fh-like Nps exhibit a fractal organization, composed of primary beads (radius ~0.8 nm) forming primary aggregates (radius ~4 nm) that can aggregate to form secondary aggregates (radius > 100 nm) bound to large OM aggregates [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF][START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. As a consequence, despite the fact that they are known to react strongly with pollutants, many questions still remain with regards to the binding capacities of such heterogeneous and polymorph aggregates. For many years, it was accepted that the sorption capacities of such entities follow an additive rule, i.e. for a given chemical element, the total sorption capacity was the sum of the ability of the components of each aggregate to sorb this element. However, although some experimental data was were successfully modelled and reproduced following this additive model [START_REF] Zachara | Influence of humic substances on Co2+ sorption by a subsurface mineral separate and its mineralogic components[END_REF][START_REF] Bruggeman | Influence of Boom Clay organic matter on the adsorption of Eu 3+ by illitegeochemical modelling using the component additivity approach[END_REF], many other data were not J o u r n a l P r e -p r o o f [START_REF] Vermeer | Metal Ion Adsorption to Complexes of Humic Acid and Metal Oxides: Deviations from the Additivity Rule[END_REF][START_REF] Lippold | Effect of humic matter on metal adsorption onto clay materials: Testing the linear additive model[END_REF][START_REF] Janot | Modelling Eu(III) speciation in a Eu(III)/PAHA/α-Al2O3 ternary system[END_REF][START_REF] Saito | Analysis of Copper Binding in the Ternary System Cu 2+ /Humic Acid/Goethite at Neutral to Acidic pH[END_REF]. As an example, Weng et al. [START_REF] Weng | Effects of Fulvic and Humic Acids on Arsenate Adsorption to Goethite: Experiments and Modeling[END_REF] observed an additivity behavior for fulvic acid and goethite while Christl and Kretzschmar [START_REF] Christl | Interaction of copper and fulvic acid at the hematite-water interface[END_REF] observed a non-additive behavior for fulvic acid and hematite. The most probable explanation is that the structural organization between components strongly influences the global surface reactivity of the aggregate (e.g. in response to the surface covering). More recently, Guénet et al. [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF] demonstrated that the sorption capacity of the Fe-OM aggregates increased with the increasing Fe/organic carbon (OC) ratio while the SSA of the Fe-NPs remained constant, providing evidence that SSA was not the only parameter that controls the reactivity of Fe-OM aggregates towards chemical elements.

These authors considered that with the increasing Fe/OC ratio, the distance between the Fe primary aggregates within the Fe secondary aggregates increases increased, and that this involved a decrease in the Fe coating rate by the OM and an increase in the availability of the binding sites. In light of the recent evidence in support of the Fe-OM structural and organizational complexity of Fe-OM aggregates, these results bring the simplistic additive model into question and highlight the importance of the structural organization of Fe-OM aggregates in their sorption reactivity towards chemical elements.

Furthermore, in natural systems, major ions such as Ca coexist with Fe and OM [START_REF] Iglesias | Analysis of copper and calciumfulvic acid complexation and competition effects[END_REF].

In a recent study, we demonstrated that Ca can strongly influence the structural organization of the Fe-OM aggregates, depending on the Ca concentration [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. When low amounts of Ca are present, the overall structural organization of the Fe-OM aggregates is not modified. By contrast, for high Ca concentrations (i.e. Ca/OC ≥ 0.026), Ca drives the structural transition of the Fe-OM aggregates from a colloidal state to a non-colloidal micrometric Ca-branched OM network in which Fh-like Nps remained embedded. This dramatic change is mainly involved occurs through the binding of Ca to the OM COOH sites. For high amounts of Ca, the divalent cation could thus compete against metals and metalloids for OM binding. In these conditions, the interactions between Fe and OM are partially screened and two antagonist J o u r n a l P r e -p r o o f results are expected: an increase in the availability of Fe binding sites, free from OM, or an increase in the size of the Fh-like Nps [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF][START_REF] Davis | Role of Calcium in the Coagulation of NOM with Ferric Chloride[END_REF] involving a decrease in their SSA and subsequently, their decreased sorption capacity. Iron reactive sites are free from OM leading to an increase their binding site availability. Moreover the size of the Fh-like Nps increases [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF][START_REF] Davis | Role of Calcium in the Coagulation of NOM with Ferric Chloride[END_REF] involving a decrease in their SSA and a subsequent decrease of their sorption capacity.

To understand the overall adsorption capacity of the Fe-OM aggregates, it is necessary to investigate the reactivity of the Fe and OM phases independently. In the present study, we investigate the reactivity of the Fe phases within Fe-OM aggregates at several Fe/organic carbon (OC)OC and Ca/Fe ratios. Thus, our goal is to provide a clear description of the parameters that control their sorption capacity. For this purpose, arsenic (As) was used as a probe of the Fe surface reactivity of Fe as it arsenic is known to have a stronger affinity for Fe than for OM. Iron-OM-Ca aggregates were synthesized at various Fe/OC and Ca/Fe ratios.

The speciation of Fe was studied by X-ray absorption spectroscopy (XAS) at the Fe K-edge and the size and specific surface of the particulate Fe in the Fe-OM-Ca aggregates was were investigated using small-angle X-ray scattering (SAXS). To evaluate the binding capacity of the Fe-OM aggregates and the parameters controlling this process, As sorption experiments were carried out and coupled with (ultra)filtrations to investigate the distribution of As relative to the size evolution of the Fe-OM aggregates.

Materials and methods

Sample syntheses and chemical analyses

Sample synthesis

Samples were synthesized at three Fe/OC ratios (0.02, 0.05 and 0.08 mol./mol.) and four Ca/Fe ratios (0, 0.1, 0.5 and 1 mol./mol.) following the procedure described by Guénet et al. [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF]. Samples are labelled Fexx-Cayy, where xx and yy represent the Fe/OC and Ca/Fe J o u r n a l P r e -p r o o f ratios, respectively. The OM used was Leonardite Humic Acid (LHA) purchased from the international humic substance society (IHSS) with the following elemental composition C = 63.81%, O = 31.27%, H = 3.70% and N = 1.23% (as mass fraction). An iron(II) stock solution of 1.79×10 -2 mol L -1 was prepared with FeCl 2 .4H 2 O (Sigma Aldrich). From this solution, three Fe(II)-Ca(II) solutions were prepared at [Ca] = 2.50×10 -3 mol L -1 , 1.25×10 -2 mol L -1 and 2.50×10 -2 mol L -1 with CaCl 2 .2H 2 O (Sigma Aldrich). The Fe-OM-Ca aggregates were synthesized through a titration of the LHA suspension at [OC] = 1.00×10 -1 mol L -1 with the Fe(II)-Ca(II) solution at 0.05 mL min -1 in 5×10 -3 mol L -1 of NaCl using an automated titrator (Titrino 794, Metrohm). The pH was monitored at 6.5 ± 0.04 with a 0.1 M NaOH solution using a second titrator (Titrino 794, Metrohm) at a set pH mode. A solution of pure LHA was prepared by dissolving 25 mg of the LHA in a 1×10 -3 mol L -1 NaCl solution at pH = 6.0 as a reference for the chemical analyses.

The Fe-OM-Ca aggregates were synthesized following the procedure described by Guénet et al. [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF] and Beauvois et al. [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. The Leonardite Humic Acid (LHA), purchased from the international humic substance society (IHSS), was used as the OM and has the following elemental composition C = 63.81%, O = 31.27%, H = 3.70% and N = 1.23% (as mass fraction). A LHA solution at [OC] = 1.00×10 -1 mol L -1 was prepared by dissolving the powder at pH = 9. After being stirred for 24 h, the pH of the LHA stock solution was adjusted to 6.5 using a 1 mol L -1 HCl solution. An iron(II) stock solution of 1.79×10 -2 mol L -1 was prepared with FeCl 2 .4H 2 O (Sigma Aldrich) at pH = 2 to avoid Fe(II) oxidation. From this solution, three Fe(II)-Ca solutions were prepared adding CaCl 2 .2H 2 O powder (Sigma Aldrich) to reach

[Ca] = 2.50×10 -3 mol L -1 , 1.25×10 -2 mol L -1 and 2.50×10 -2 mol L -1 , that corresponds to Ca/Fe molar ratios of 0.1, 0.5 and 1, respectively. The ionic strength of all of the solutions was fixed with 5×10 -3 mol L -1 of NaCl. The synthesis of the samples consisted of titrating the LHA solution with the Fe(II) or the Fe(II)-Ca solutions at 0.05 mL min -1 using an automated titrator J o u r n a l P r e -p r o o f (Titrino 794, Metrohm). The added volume of the Fe(II) or the Fe(II)-Ca solutions was determined in order to reach three Fe/OC molar ratios (0.02, 0.05 and 0.08). During the synthesis, the pH was monitored at 6.5 ± 0.04 with a 0.1 mol L -1 NaOH solution using a second titrator (Titrino 794, Metrohm) in a set pH mode. The samples were labelled Fexx-Cayy, where xx and yy represent the Fe/OC and Ca/Fe ratios, respectively.

Arsenic sorption experiments

Arsenic sorption isotherms were performed in triplicate for all of the samples at for five As/Fe ratios (i.e. 0.0007, 0.004, 0.04, 0.15 and 0.4 mol/mol). For this purpose, various As(III) solutions were prepared from NaAsO 2 (Sigma Aldrich) at 5.00×10 -2 mol L -1 , 5.00×10 - 3 mol L -1 and 5.00×10 -4 mol L -1 . An aliquot of each As(III) solution was added to the suspension of Fe-OM-Ca aggregates to reach the fixed set As/Fe ratio. The pH of the suspensions was adjusted to 6.5 ± 0.04 using 1 mol L -1 HCl or NaOH. The equilibrium time for As adsorption onto goethite in the presence of with HA and onto hematite nanoparticles was demonstrated reported to be 4 h and 24 h, respectively [START_REF] Grafe | Adsorption of Arsenate (V) and Arsenite (III) on Goethite in the Presence and Absence of Dissolved Organic Carbon[END_REF][START_REF] Dickson | Adsorption kinetics and isotherms of arsenite and arsenate on hematite nanoparticles and aggregates[END_REF]. Similarly, Raven et al. [START_REF] Raven | Arsenite and Arsenate Adsorption on Ferrihydrite: Kinetics, Equilibrium, and Adsorption Envelopes[END_REF] observed an equilibrium time for the As adsorption of As by Fh of 24 h, irrespective of the pH or the initial As concentration. The suspensions were therefore stirred for 24 h to ensure that the equilibrium is reached. A fraction of each solution was ultrafiltrated at 30 kDa (Vivaspin VS2022, Sartorius). The ultrafiltrate corresponds to the dissolved species. The arsenic concentration was measured in the bulk fraction and in the <30 kDa fraction in order to determine the quantity of adsorbed As on Fe phases.

For an As/Fe ratio = 0.004 and 0.15, additional (ultra)filtrations were performed at 0.2 µm (cellulose acetate membrane filter, Sartorius), 1000 kDa (Vivaspin VS2062, Sartorius) and 100 kDa (Vivaspin VS2042, Sartorius). Each 0.2 µm filter was rinsed with 200 mL of Milli-Q water (Millipore) before use. Each ultrafiltration cell was washed several times with 15 mL of a 0.1 mol L -1 NaOH solution and Milli-Q water until the DOC concentration in the J o u r n a l P r e -p r o o f ultrafiltrate was below 0.2 mg L -1 . An As/Fe ratio = 0.004 was chosen since it is close to the ratio observed in riparian wetland soil solutions [START_REF] Dia | The distribution of rare earth elements in groundwaters: assessing the role of source-rock composition, redox changes and colloidal particles[END_REF][START_REF] Olivie-Lauquet | Release of Trace Elements in Wetlands: Role of Seasonal Variability[END_REF] and an As/Fe ratio = 0.15 was chosen because it ensured a high As adsorption rate [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF][START_REF] Dixit | Comparison of Arsenic(V) and Arsenic(III) Sorption onto Iron Oxide Minerals: Implications for Arsenic Mobility[END_REF].

Chemical analyses

The chemical analyses were performed on the bulk fraction and all of the (ultra)filtrates.

The OC concentrations were determined using an OC analyzer (Shimadzu TOC-V CSH). The uncertainty of the C measurement was determined to be ± 5% using a standard solution of potassium hydrogen phthalate (Sigma Aldrich). The Fe, Ca and As concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent Technologies 7700x instrument at the University of Rennes 1. In order to solubilize the Fe, Ca and As and to eliminate the OM, the samples were predigested with 14.6 mol L -1 distilled HNO 3 at 90°C and evaporated. A second oxidative predigestion with a mixture of 14.6 mol L - 1 distilled HNO 3 and Suprapur 30% H 2 O 2 (a HNO 3 /H 2 O 2 volume ratio of ~75/25) at 90°C was necessary to completely dissolve the samples. Finally Lastly, the samples were evaporated to complete dryness and solubilized with HNO 3 at 0.37 mol L -1 prior to the ICP-MS measurements. The detection limits of the ICP-MS measurements were 1×10 -2 , 2×10 -3 and 5×10 -5 mol L -1 for Ca, Fe and As, respectively.

All of the aqueous solutions were prepared with Milli-Q water (Millipore).

Adsorption isotherm modelling

As adsorption isotherms were fitted with the Langmuir equation (eq. 1): The adsorption isotherms were fitted with the Freundlich and Langmuir isotherm. The Langmuir isotherm results were significantly better than for Freundlich isotherm, as previously observed for several Fe (hydr)oxides sorption studies [START_REF] Hao | A critical review on arsenic removal from water using iron-based adsorbents[END_REF]. The Langmuir model was therefore used in the present sutdy. It can be described by the following equation (eq. 1):

(eq. 1)

J o u r n a l P r e -p r o o f where q is the quantity of adsorbed As per unit mass of adsorbent (µmol g -1 ), q max is the maximal quantity of adsorbed As per unit mass of adsorbent (µmol g -1 ), Langmuir K ads is a constant related to the binding strength of adsorbent (L µmol -1 ) and C is the final concentration of As in solution (µmol L -1 ), i.e. in the <30 kDa fraction as described above [START_REF] Sparks | Environmental soil chemistry[END_REF]. The isotherms were fitted using the KaleidaGraph software with the Levenberg-Marquardt algorithm in order to calculate the best curve fit using an iterative procedure.

Fe K-edge XAS data acquisition and analysis

Before the adsorption experiments were performed, a fraction of Prior to the adsorption experiments, an aliquot of each sample was freeze dried (Freeze dryer Alpha 1-2 LD plus, Christ). The powder was then mixed with cellulose (Merck) and pressed into a 6 mm pellet.

The Fe K-edge spectra were recorded on the ROCK [START_REF] Briois | ROCK: the new Quick-EXAFS beamline at SOLEIL[END_REF] beamline at the SOLEIL synchrotron (Saint-Aubin, France). The monochromator used was a Si(111) channel-cut. The spectra were recorded in transmission mode using three ionization chambers (Ohyo Koken) filled with N 2 .

The energy was calibrated using a Fe foil located between the 2 nd and 3 rd ionization chambers and measured simultaneously with the samples. The calibration was done by setting the maximum of the first derivate of the Fe foil to 7112 eV. The reference used for further the subsequent data analysis was a Fh synthesized following as per the procedure described in Schwertmann and Cornell [START_REF]Iron Oxides in the Laboratary[END_REF].

All of the XAS data were processed using the Athena software [START_REF] Ravel | HEPHAESTUS : data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF] including the Autbk algorithm (Rbkb = 1, k-weight = 3). The Fe spectra were normalized by fitting the pre-edge region with a linear function and the post-edge region with a quadratic polynomial function.

The Fourier transforms of the k 3 -weighted EXAFS spectra were calculated over a range of 2-12.5 Å -1 using a Hanning apodization window (window parameter = 1). Back Fourier filters were extracted over the R-range of 1.15-4.1 Å, using the same apodization window shape.

The EXAFS data were analyzed by linear combination fitting (LCF) procedure available in J o u r n a l P r e -p r o o f the Athena software over the range 3-12.5 Å -1 ; all the component weights were forced to be positive. Beauvois et al. [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF] demonstrated that the EXAFS data at the Fe K-edge could be reproduced by performing LCF using Fe(III)-oligomers and Fe(III)-Np isolated by Vantelon et al. [START_REF] Vantelon | Iron speciation in iron-organic matter nanoaggregates: A kinetic approach coupling Quick-EXAFS and MCR-ALS J o u r n a l P r e -p r o o f chemometry[END_REF] as well as Fh. The total LCF weight for each sample varied between 0.95 and 1.05 without any constraint. In order to facilitate the comparison between each sample, the total LCF weight was arbitrary fixed to 1. The EXAFS data were also fitted in the range 1.15-4.1 Å with the Artemis [START_REF] Ravel | HEPHAESTUS : data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF] interface to IFEFFIT using a least-squares refinement procedure. The theoretical backscattering paths used were calculated from a goethite structure [START_REF] Hazemann | Rietveld Studies of the Aluminium-Iron Substitution in Synthetic Goethite[END_REF] using the FEFF6 algorithm [START_REF] Newville | EXAFS analysis using FEFF and FEFFIT[END_REF] included in the Artemis interface.

SAXS measurements

The small-angle X-ray scattering measurements were performed on the SWING beamline at the SOLEIL synchrotron. Using two sample-to-detector distances (1 and 6 m) at a wavelength of 1.03 Å, a momentum transfer q range of 2.0×10 -3 -0.7 Å -1 was reached using a EIGERX 4M detector. Measurements were also carried out on the XEUSS 2.0 spectrometer from Xenocs (CEA-LIONS/LLB, Saclay, France). With two sample-to-detector distances (0.33 and 2.49 m) and a wavelength of 1.54 Å (Cu X-ray source), a momentum transfer range of 4.5×10 -3 -0.5 Å -1 was reached.

In the high q domain of the SAXS curves, the so-called Porod region, the scattered intensity is proportional to the surface scattering per unit volume S/V and can be modelled according to the following equation (eq. 2):

( ( ))
where φ 0 is the volume fraction of the primary beads, Δρ² is the contrast term (cm -4 ), both estimated determined by Beauvois et al. [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. By dividing S/V by the mean density of the Fe aggregates (i.e. 3.96 g cm -3 [START_REF] Hofmann | Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing[END_REF]), we were able to calculate the SSA of the Fh-like Nps.

(eq. 2)

J o u r n a l P r e -p r o o f

Results

OC, Fe, Ca and As distribution in the size fractions

The OC, Ca, Fe and As concentrations in each fraction are reported in Tables SI 1-7, respectively. For the samples at the Ca/Fe ratio = 0, 0.11 mmol L -1 of Ca was detected which corresponded to the concentration of Ca naturally present in the LHA (Table SI 8). The ICP-MS measurements of the LHA alone also displayed [Fe] = 0.04 mmol L -1 , which is low as compared to the lowest Fe/OC with in which [Fe] = 1.23 mmol L -1 and [As] was below the detection limit.

The % distribution of OC, Ca, Fe and As in each size fraction is reported in Figure 1 (for As/Fe = 0.004) and Figure SI-1 (for As/Fe = 0.15). In the <30 kDa fraction, ~5% of the OC and less than 1% of the Fe were present. The dissolved As% decreased with the increasing Fe/OC ratio for all of the Ca/OC ratios. By contrast, irrespective of the Fe/OC ratio, the Ca% in the <30 kDa fraction remained < 10% for the Ca/Fe ratios ≤ 0.1. Then, it increased with the increasing Ca/Fe ratio. This increase was less pronounced for Fe/OC = 0.02 compared to Fe/OC ≥ 0.05. The results provided evidence of a perfect co-distribution of OC, Ca, Fe and As in the >30 kDa fractions. For Fe/OC = 0.02, ~80% of the OC, Ca, Fe and As in the >30 kDa fraction were distributed in the 1000-100 kDa fraction, ~15% were in the >0.2 µm fraction and the remaining 5% were distributed in the other fractions for both Ca/Fe = 0 and 0.1. With the increasing Ca/Fe ratio, the amount of OC, Ca, Fe and As in the 1000-100 kDa fraction decreased until ~60% for Ca/Fe = 1, and simultaneously increased in the >0.2 µm fraction (~30%) and to a lower extent in the 0.2 µm-1000 kDa fraction (~10%). The same behavior was observed for the higher Fe/OC values. For Fe/OC = 0.05, with the increasing Ca/OC ratio from 0 to 0.1, the proportion of OC, Ca, Fe and As decreased from ~75% to ~65% from between the 1000-100 kDa fraction and to the >0.2 µm fraction for which the % increased from ~20% to ~30%. For Ca/Fe = 0.5, the concentration was below the J o u r n a l P r e -p r o o f ICP-MS detection limit in the 1000-100 kDa fraction while ~30% and ~65% were observed in the 0.2 µm-1000 kDa and >0.2 µm fractions, respectively. This distribution was higher for Ca/Fe = 1, as almost all of the OC, Fe, Ca and As were was distributed in the >0.2 µm fraction. For the highest Fe/OC = 0.08, OC, Fe, Ca and As in the 1000-100 kDa fraction decreased from ~60% for Ca/Fe = 0 to ~0% for Ca/Fe ≥ 0.5 and increased in the >0.

Main text

↗ [Fe] ↗ [Ca]
J o u r n a l P r e -p r o o f

Arsenic adsorption isotherms

The As adsorption isotherms exhibited an L-type curve shape for which the slope decreased with the increasing As concentration. The isotherms were fitted with the Langmuir equation (eq. 1) (Figure 2, Table 1). The good quality of the fit (R² > 0.975) demonstrated that the Langmuir isotherm is suitable for As sorption by the Fe-OM-Ca aggregates. The Fe-OM-Ca aggregates exhibited a sorption capacity, q max , varying from ~665 µmol g -1 for Fe0.02-Ca0.0 to 3240 µmol g -1 for Fe0.08-Ca1.0. The q max value increased with the increasing Fe/OC ratio. These q max were in the range of those calculated for Fh under similar pH conditions (Table 2). However, they are were around one order of magnitude J o u r n a l P r e -p r o o f higher than crystallized Fe-oxides such as magnetite or goethite (Table 2). By contrast, the Langmuir K ads value for the Fe-OM-Ca aggregates Langmuir K ads decreased with the increasing Fe/OC ratio. For the Fe/OC ratio = 0.02, the Langmuir K ads value decreased with the increasing Ca/Fe ratio while remaining constant irrespective of the Ca/Fe ratio for Fe/OC ≥ 0.05. The calculated Langmuir K ads values are 10 times lower than those reported for Fh.

For the lowest Fe/OC and Ca/Fe ratios, the Langmuir K ads values are were similar to the Langmuir K ads values for Fh bound with OM, but decreased by one order of magnitude with the decreasing increasing Fe/OC and Ca/Fe ratios. Xue et al. [START_REF] Xue | Arsenite and arsenate binding to ferrihydrite organo-mineral coprecipitate: Implications for arsenic mobility and fate in natural environments[END_REF] demonstrated that the increase in the amount of OM leads to a decrease in the Fh Langmuir K ads value for Fh. Thus, the presence of OM and Ca may be responsible for the decrease in the Langmuir K ads of value for As caused by the Fe Ca-OM aggregates.

Table 1 -Parameters used to fit the adsorption isotherms with the Langmuir model.

Sample q max (µmol g -1 )

Langmuir K ads (L µmol -1 ) R² Fe0.02-Ca0.0 6.7×10 2 10×10 -3 0.995 Fe0.02-Ca0.1 6.7×10 2 8×10 -3 0.993 Fe0.02-Ca0. [START_REF] Pédrot | Insights into colloid-mediated trace element release at the soil/water interface[END_REF] 7.6×10 2 7×10 -3 0.991 Fe0.02-Ca1.0 1.5×10 *IS: ionic strength in mol L -1

Speciation of Fe within the Fe-OM-Ca aggregates

The maximum amplitude of the EXAFS spectra was reached at 6.3 Å -1 (Figure 3). The Fe(III)-oligomer reference displayed a close monotonous damping shape. The Fh-like Np and Fh spectra showed a shoulder at 5.1 Å -1 and an oscillation at 7.5 Å -1 that was more pronounced for Fh compared to Fh-like Np. The spectra of Fh-like Np and Fh showed a shoulder at 5.1 Å -1 and an oscillation at 7.5 Å -1 . Both shoulder and oscillation were far more pronounced for Fh as compared to Fh-like Np. For Fe/OC = 0.02, the shape of the spectra were monotonous but a shoulder occurred at 7.5 Å -1 . For Fe/OC ≥ 0.05, the EXAFS spectra were more structured with an additional shoulder and a more pronounced shoulder occurring at 5.1 Å -1 and 7.4 Å -1 , respectively.

J o u r n a l P r e -p r o o f

For Fe/OC ≥ 0.05, the LCF reproduced well the EXAFS signal well. The percentage of nanoparticulate Fe, i.e. the percentage of Fe organized as Fh-like Np and Fh, was calculated from the LCF results (Table 3). It This value was ≥ 80% and increased with the increasing Ca/Fe ratio, as already reported [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. For Fe/OC = 0.02, the LCF results reported returned ~70% of nanoparticulate Fe whatever the Ca/Fe ratio. However, the LCF exhibited a shoulder at 7.5 Å -1 that was much more pronounced than in the experimental data (highlighted by the red arrow in Figure 3). This shoulder was mostly due to the neighbors siting at a distance larger than 3 Å (Figure SI-2). Therefore, when the size of the Fe particle was smaller, it was expected that the shoulder at 7.5 Å -1 would be less pronounced. This effect was observed for the Fh-like Np; the EXAFS spectrum exhibited a shoulder at 7.5 Å -1 lower than Fh (Figure 3). The strongly pronounced shoulder at 7.5 Å -1 provided evidence that the size of the Fe nanoparticle used for LCF is too large. This assumption was also supported by the size of the particulate Fe reported from the SAXS experiments showing that they were smaller for Fe/OC = 0.02 than for Fe/OC ≥ 0.05. Therefore, it was expected that the shoulder at 7.5 Å -1 would be less pronounced for a smaller Fe particle. This effect could be observed for the Fhlike Np which EXAFS spectrum exhibited a less intense shoulder at 7.5 Å -1 than Fh (Figure 3). The strongly pronounced shoulder at 7.5 Å -1 provided by the LCF evidenced that the size of the Fe nanoparticle used for the LCF was too large. This assumption was also supported by our previous study on the structural organization of Fe-OM aggregates in the presence of Ca showing that the size of the particulate Fe reported from the SAXS experiments showing that they were smaller for Fe/OC = 0.02 than for Fe/OC ≥ 0.05 [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF].

J o u r n a l P r e -p r o o f SI -8). This low number of neighbors suggests that Fe(III)oligomers remain in the signal as this amount of neighbors is too low to be provided by particulate Fe. The same is observed for Fe0.02-Ca0.0; the signal for these nanoparticles, after subtracting 30% of the Fe(III)-oligomers, exhibited less than 1 Fe (0.8) at 3.03 Å (Figure SI-5 and Table SI-8). These results demonstrate that the amount of Fe(III)-oligomers was underestimated by LCF in both samples. In order to isolate the EXAFS signal of the particulate Fe (i.e. a signal containing a comparable number of Fe first neighbors as in Fh and Fh-like Np), it is necessary to subtract 60% of the Fe(III)-oligomers (Figure 4) from Fe0.02-Ca0.5 and 70% of the Fe(III)-oligomers (Figure 4) from Fe0.02-Ca0.0. In both cases, the number of Fe at 3.47 Å was lower than for the Fh-like Nps, demonstrating that the Nps from Fe0.02-Ca0.0 and Fe0.02-Ca0.5 were smaller than the Fh-like Nps initially used for the LCF calculations.

In order to analyze the data for Fe/OC = 0.02, a procedure based on the linear combination principle was implemented. For Fe0.02-Ca1.0, the LCF reported that the Fe is constituted of 30% of Fe(III)-oligomers and 70% of Np. The experimental EXAFS signal (exp. EXAFS) of Fe0.02-Ca1.0 could be expressed as followed:

Exp. EXAFS = 0.3×(Fe(III)-oligomers EXAFS signal) + 0.7×(Np EXAFS signal) However, the LCF results are not as convincing as for the samples with a Fe/OC ratio ≥ 0.05, as discussed above. The EXAFS signal of the Fe organized as Np within Fe0.02-Ca1.0 was extracted with the following procedure:

J o u r n a l P r e -p r o o f

( ) ( ( ) )
The obtained signal, denoted "Np from Fe0.02-Ca1.0", was fitted and reported in Figure 4 along with the shell-by-shell fit results. The corresponding Fourier transform was reported in As a consequence, the proportion of the nanoparticulate Fe increased with both the Fe/OC and Ca/Fe ratios the Fe/OC and with the Ca/Fe ratio for Fe/OC = 0.02 (Table 3). This increased ratio is much more pronounced with the increasing Ca/Fe ratio for Fe/OC = 0.02 than for Fe/OC ≥ 0.05. J o u r n a l P r e -p r o o f The amplitude reduction factor S 0 2 and the energy shift ΔE were set to 0.75 and -3.3 eV, respectively. N is the coordination number, R is the interatomic distance (Å) and σ² is the Debye-Weller factor. The error on N, R and σ² are usually estimated to be ± 10%, ± 1% and ± 20%, respectively. '*' denotes the fixed parameters. ⁑ The σ² were forced to be equal to each other. ⁂ The coordination number for the Fe-O2 path was constrained as followeds: N Fe-O2 = N Fe-Fe1 + N Fe-Fe2 .

J o u r n a l P r e -p r o o f

Specific surface area

The SAXS curves displayed a shoulder between 10 -2 and 10 -1 Å -1 providing evidence of the existence of Fe nanoparticles (Figure 5). The size of these Fe nanoparticles corresponded to Fe primary aggregates (radius between 2 and 5 nm) comprised assemblages of Fe primary beads (radius = 0.8 nm) [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF][START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. However, the increase in the scattered intensity at low q indicates that the Fe nanoparticles are were aggregated as into larger objects [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF][START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. For Fe/OC ≥ 0.05 and Ca/Fe ≥ 0.5, the shoulder is less pronounced indicating a dispersity in the Fe nanoparticle size and a high contribution of the larger objects to the total scattered intensity. Without Ca, the SSA remains remained constant ~1300 m² g -1 irrespective of the Fe/OC ratio, as demonstrated by Guénet et al. [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF] For Fe/OC = 0.02, the SSA remains J o u r n a l P r e -p r o o f remained constant while it increased with the increasing Ca/Fe ratio for Fe/OC ≥ 0.05 (Table 5).

Discussion

The huge structural transition from aggregates to a micrometric network observed for the Fe-OM aggregates in the presence of Ca is not accompanied by a dramatic evolution of their sorption capacity as we expected [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. However, the impact of Ca is far more complex.

Calcium interacts with the OM COOH sites that partially screen the interactions between Fe and OM [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF]. As a consequence, the size of the Fh-like Nps increases, which should involve a decrease in their SSA and q max [START_REF] Hofmann | Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing[END_REF][START_REF] Wang | Effect of particle size on the adsorption and desorption properties of oxide nanoparticles[END_REF][START_REF] Wang | Effect of Ferrihydrite Crystallite Size on Phosphate Adsorption Reactivity[END_REF][START_REF] Hiemstra | Evolution of the reactive surface area of ferrihydrite: time, pH, and temperature dependency of growth by Ostwald ripening[END_REF]. Surprisingly, our results did not confirm these expectations: q max and the SSA increased concomitantly with the increasing size of Fh-like Nps (Figure 6). Guénet et al. [START_REF] Guénet | Characterization of iron-organic matter nanoaggregate networks through a combination of SAXS/SANS and XAS analyses: impact on As binding[END_REF] assumed that these unexpected data result from the decrease in the OM coating of the Fe aggregates.

J o u r n a l P r e -p r o o f Since LHA is composed of 7.5×10 -3 mol of COOH per gram of C [START_REF] Ritchie | Proton-binding study of standard and reference fulvic acids, humic acids, and natural organic matter[END_REF], an apparent COOH concentration can be calculated for the >30 kDa fraction for each sample. Knowing the OM COOH density, the Fe and Ca concentrations in the >30 kDa fraction and given that Ca is bound to OM with a molar ratios (COOH/Ca) >30 kDa = 3 (Beauvois et al. [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF] and Figure SI 6), the quantity of available OM COOH that are able to bind the Fe phases can be calculated (Figure 7). For Fe/OC = 0.02, the SSA remained constant whereas for Fe/OC ≥ 0.05, the SSA tended to decrease with the increasing availability of the OM COOH sites.

In summary, at a high [Ca], the coating of the Fe phases by OM decreased and the site availability of the Fe phases increased.

J o u r n a l P r e -p r o o f Another major parameter that controls the adsorption capacity of the Fe-OM aggregates is the Fe speciation (Figure 8). An exponential fit of the dataset demonstrated that when Fhlike Nps were not formed, As adsorption could occur through the Fe(III)-oligomers. This assumption was in agreement with previous results demonstrating that As binding by Fe(III) monomers or tetramers may have occured occur, leading to the formation of As-Fe-OM ternary complexes [START_REF] Hoffmann | Arsenite Binding to Natural Organic Matter: Spectroscopic Evidence for Ligand Exchange and Ternary Complex Formation[END_REF][START_REF] Mikutta | Spectroscopic Evidence for Ternary Complex Formation between Arsenate and Ferric Iron Complexes of Humic Substances[END_REF][START_REF] Catrouillet | Does As(III) interact with Fe(II), Fe(III) and organic matter through ternary complexes?[END_REF]. Thus, the adsorption capacity of Fe(III)-oligomers can thus be estimated as q max ~250 µmol g -1 with a Langmuir K ads value of ~20×10 -3 L µmol -1 . The q max value increases with the increasing Fh-like Np% which suggests that Fh-like Nps exhibit higher adsorption capacities than Fe(III)-oligomers. For the As-Fe-OM ternary complexes, in which the Fe speciation was dominated by the Fe(III)-oligomers, As is only bound to Fe as a bidentate mononuclear edge-sharing complex ( 2 E) due to geometric constraints [START_REF] Guénet | Evidence of organic matter control on As oxidation by iron oxides in riparian wetlands[END_REF]. For Fhlike Nps, As is bound as 2 E complexes and as bidentate binuclear corner-sharing ( 2 C) complexes [START_REF] Ona-Nguema | EXAFS Analysis of Arsenite Adsorption onto Two-Line Ferrihydrite, Hematite, Goethite, and Lepidocrocite[END_REF]. Therefore, As can bind to Fh-like Nps in two ways as compared to Fe(III) oligomers. Moreover, the Fe vacancies in the Fh structure provide a higher density in terms of adsorption sites for Fh-like Nps than for Fe(III)-oligomers [START_REF] Ona-Nguema | EXAFS Analysis of Arsenite Adsorption onto Two-Line Ferrihydrite, Hematite, Goethite, and Lepidocrocite[END_REF][START_REF] Jambor | Occurrence and Constitution of Natural and Synthetic Ferrihydrite, a Widespread Iron Oxyhydroxide[END_REF]. However, the Langmuir J o u r n a l P r e -p r o o f K ads value decreased decreases with the increasing Fh-like Nps % suggesting that As has higher affinity for Fe(III)-oligomers than for Fh-like Nps. As a consequence, the speciation of Fe impacts the oxidation-reduction behavior of the adsorbed As given that the formation of 2 E stabilizes As(III) while whereas the 2 C complexes lead to the oxidation of As(III) as As(V) [START_REF] Guénet | Evidence of organic matter control on As oxidation by iron oxides in riparian wetlands[END_REF]. Moreover, these results demonstrate that the speciation of Fe within the Fe-OM aggregates should thus be taken into account in order to evaluate and predict their ability to control metal(loid) mobility under environmental conditions. Since this distribution is dependent on the amounts of Ca and OM, their concentrations must be systematically determined. To reach this objective, the distribution of Fe between Fe(III)-oligomers and Fh-like Nps could, for example, be used to improve the existing geochemical models via the implementation of new parameters.

Conclusions

The overall reactivity of the Fe-OM aggregates strongly depends on their complex structural organization that which varies in the presence of Ca. In this study, we developed a complete description of the parameters that control the surface reactivity of Fe within synthesized Fe-OM-Ca aggregates using arsenic as a probe. We demonstrated that the Fe(III)- Combined with our previous work [START_REF] Beauvois | How does calcium drive the structural organization of iron-organic matter aggregates? A multiscale investigation[END_REF], this study has emerged as an efficient approach to understand and predict the impact of Ca on the reactivity and mobility of Fe-OM aggregates in light of their structural organization. This global approach, applied to other element-Fe-OM interactions, will provide valuable information for understanding the pollution fate of natural systems.

  2 µm fraction, which reached 100% for Ca/Fe ≥ 0.5. In summary, for Ca/Fe = 0, OC, Fe, Ca and As primarily occurred mainly in the 1000-100 kDa fraction. When the [Fe] and [Ca] increased, they OC, Fe, Ca and As were transferred to the >0.2 µm fraction. The Ca/Fe ratio had a greater influence than that of Fe/OC. The [Ca] had a greater influence than that of [Fe]. This result is illustrated in Figure SI-2 by plotting the evolution of the As % distributed in the 1000-100kDa fraction relative to [Fe] and [Ca].Therefore, the size of the Fe-OM-Ca association increased with the increasing Ca concentration.

Figure 1 -

 1 Figure 1 -Percentage of the distribution of OC (black), Fe (blue), Ca (green) and As (red) distributions relative to the size fraction for each Fe/OC and Ca/OC ratio at As/Fe = 0.004.

Figure 2 -

 2 Figure 2 -Arsenic adsorption isotherms for (a) Fe/OC = 0.02, (b) Fe/OC = 0.05 and (c) Fe/OC = 0.08. The circles represent the experimental points and the dotted lines represent the Langmuir isotherm.

Figure 3 -Fe

 3 Figure 3 -Fe K-edge EXAFS spectra of the references used to perform the LCF and for each sample. The solid lines are experimental data and the dotted lines are the LCF results. The red arrow highlights the main discrepancy between the LCF result and the experimental data.For Fe0.02-Ca1.0, the EXAFS signal was subtracted from the experimental data

Figure

  Figure SI-3a. The results of the fit (Table4) reported 6.5 O at 1.98 Å and 1.6 Fe at 3.04 Å, in

Figure 4 -

 4 Figure 4 -Fe K-edge EXAFS spectra of the nanoparticulate Fe obtained by subtracting 30% of the EXAFS signal for the Fe(III)-oligomers from the raw signal of Fe0.02-Ca1.0, 60% from the raw signal of Fe0.02-Ca0.5 and 70% from the raw signal of Fe0.02-Ca0.0. The gray solid lines are the EXAFS signals after this subtraction and the black dotted lines are the fit results.

Figure 5 -

 5 Figure 5 -SAXS curves obtained for all samples.

Figure 6 -

 6 Figure 6 -Variations in (a) the As q max calculated from the Langmuir isotherm and (b) the SSA relative to the radius of Fh-like Nps for Fe/OC = 0.02 (blue circles), 0.05 (red squares) and 0.08 (green triangles).

Figure 7 -

 7 Figure 7 -Evolution of the SSA (m 2 g -1 ) relative to the quantity of available COOH (mmol L -1 ).

Figure 8 -

 8 Figure 8 -Variations of (a) q max and (b) Langmuir K ads relative to the percentage of Fe organized as Fh-like Np for Fe/OC = 0.02 (blue circles), 0.05 (red squares) and 0.08 (green triangles).

J o u r

  n a l P r e -p r o o f oligomers and Fh-like Nps exhibit a high affinity for As and a high sorption capacity, respectively. The percentage of Fe distributed as Fe(III)-oligomers and Fh-like Nps is therefore a major parameter controlling As binding by Fe-OM aggregates. Moreover, our study provided evidence that Ca not only controls the ionic strength but also plays a significant role in the reactivity of the Fe-OM aggregates by acting as a competitor of Fe for binding OM. In that respect, the partial screening of the interaction between Fe and OM by Ca leads to an increase in the percentage of Fh-like Nps for low Fe/OC values and to the decrease in the Fh-like Nps coating rate by OM for high Fe/OC values. As a consequence, the availability of the Fe binding sites increases and the Fe-OM sorption capacity for As are enhanced by Ca. These results are of fundamental importance to model and predict As mobility in natural systems and will be efficient inputs to study As speciation and more specifically the oxidation state and thus the toxicity with oxidation-reduction cycles.

Table 2 -

 2 q max and Langmuir K ads of various Fe-oxyhydroxides.

	Fe oxyhydroxides	q max (µmol g -1 )	Langmuir K ads	Experimental conditions	Reference
	Fh	1.1×10 3	7.0×10 -2	pH = 6.0 IS* = 0.02	Zhu et al. [42]
	Fh	5.0×10 3	1.5×10 -2	pH = 7.0 IS* = 0.01	Xue et al. [41]
	Fh-HA (5 wt%)	3.6×10 3	1.1×10 -2	pH = 7.0 IS* = 0.01	Xue et al. [41]
	Fh-HA (15 wt%)	2.7×10 3	8.8×10 -3	pH = 7.0 IS* = 0.01	Xue et al. [41]
	Magnetite	1.4×10 1	1.0×10 -1	pH = 6.1 IS* = 0.01	Yean et al. [43]
	Nano-magnetite	1.4×10 2	5.4×10 -1	pH = 6.1 IS* = 0.01	Yean et al. [43]
	Goethite	1.7×10 2	4.0×10 -2	pH = 4 IS* = 0.01	Dixit et al. [32]
	HFO	2.7×10 2	9.0×10 -2	pH = 4 IS* = 0.01	Dixit et al. [32]

Table 3 -

 3 Percentage of Fh-like Nps. For Fe/OC ≥ 0.05, the % was calculated by LCF. For Fe/OC = 0.02, the % was deduced from the isolation of the Fh-like Nps EXAFS signal. The error is estimated to be ± 10%. n.d.: not determined

	Fh-like Nps %	Ca/Fe ratio 0.0	0.1	0.5	1.0
	Fe/OC	0.02	30	n.d.	40	70
		0.05	82	80	80	83
		0.08	85	86	87	88

Table 4 -

 4 Parameters used to fit the EXAFS signals at the Fe K-edge.

	Sample	Fh-like Np	Fh	Np from Fe0.02-Ca0.0	Np from Fe0.02-Ca0.5	Np from Fe0.02-Ca1.0
	Fe-	N	6.3	4.4	6.8	6.4	6.5
	O1
		R	1.99	1.97	1.94	1.96	1.98
		(Å)
		σ²	0.009	0.010	0.007	0.008	0.010
	Fe-	N	1.3	1.2	1.4	1.7	1.6
	Fe1
		R	3.04	3.02	3.04	3.05	3.04
		(Å) σ²	0.009 ⁑	0.009 ⁑	0.007 ⁑	0.009 ⁑	0.010 ⁑
	Fe-	N	2.4	3.5	2.1	2.0	1.5
	Fe2
		R	3.47	3.47	3.50	3.48	3.47
		(Å) σ²	0.009 ⁑	0.009 ⁑	0.007 ⁑	0.009 ⁑	0.010 ⁑
	Fe-	N	3.7 ⁂	4.7 ⁂	3.5 ⁂	3.7 ⁂	3.1 ⁂
	O2
		R	3.63	3.61	3.70	3.66	3.69
		(Å)
		σ²	0.005	0.007	0.006	0.012	0.015
	Fe-	N	4.0*	4.0*	4.0*	4.0*	4.0*
	O3
		R	4.41	4.45	4.37	4.39	4.39
		(Å)
		σ²	0.015	0.015	0.002	0.006	0.010
	R-factor	3.5	5.6	16.8	11.6	9.7
	(×10 -3 )

Table 5 -

 5 Specific surface area (SSA, in m 2 g -1 ) calculated for each sample. The errors on the parameters were 15%.

	SSA (m 2 g -1 )	Ca/Fe 0.0	0.1	0.5	1.0
	Fe/OC	0.02	1300	1100	1300	1300
		0.05	1250	1250	1650	1600
		0.08	1400	1800	2000	1700

Acknowledgments

This study is part of a thesis project funded by the French administrative region of Brittany and by the SOLEIL-LLB through the "ORPHREA" project. This study was funded by the French "Institut national des sciences de l'Univers" (INSU) through the "Initiative Structurante EC2CO -BIOHEFECT" allocated to Dr. Delphine Vantelon via the 'ISAAP' project. The authors acknowledge SOLEIL for beamtime allocation at the ROCK beamline (proposal 20170795) and SWING beamline (proposal 20181932) and the ROCK, SWING and LUCIA staff for their support. The work at ROCK was supported by a public grant overseen J o u r n a l P r e -p r o o f by the French National Research Agency (ANR) as a part of the "Investissements d'Avenir" program ref: ANR-10-EQPX-45. Dr. Sara Mullin is acknowledged for post-editing the English style (https://www.proz.com/profile/677614).

Statement of contributions

 As binding affinity is higher for Fe(III)-oligomers than for Fh-like Nps.

J o u r n a l P r e -p r o o f


Brina Madingou Tchibinda
email: tchibinda@cerege.fr

Gwenaël Hervé

Mireille M Perrin

Freda M'mbogori

Djimet Guemona

Pierre-Etienne Mathé

Pierre Rochette

David Williamson

Vincent Mourre

Caroline Robion-Brunner

Brina Tchibinda Madingou

Caroline Robion- Brunner

Brina Tchibinda Madingou

First archeomagnetic data from Kenya and Chad: Analysis of iron furnaces from Mount Kenya and Guéra Massif

Keywords: archeomagnetism, secular variation, archeointensity, Africa, iron metallurgy

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

Introduction

Precise knowledge of the Earth's magnetic field through time is necessary to understand the geodynamo evolution. Direct observations of the geomagnetic field are provided by satellites and ground-based measurements for the last 500 years for declination, 400 years for inclination and since the 1830s for intensity. Using these direct data together with historic data from navigators, a global model gufm1 was calculated for the period 1590-1990 by spherical harmonic analysis of the geomagnetic field [START_REF] Jackson | Four centuries of geomagnetic secular variation from historical records[END_REF]. This model is nevertheless not well constrained in intensity before 1840. It is therefore needed to J o u r n a l P r e -p r o o f extend knowledge of the Earth's magnetic field, and especially of its secular variation, further back in time and this can be done using either volcanic rocks or baked archeological artefacts that record the local direction and intensity of the geomagnetic field present at the time and place of their last cooling. Sedimentary rocks that also record the direction of the field, can only provide relative paleointensities.

Archeomagnetic data are compiled in databases such as GEOMAGIA50.v3.3 [START_REF] Brown | GEOMAGIA50.v3: 2. A new paleomagnetic database for lake and marine sediments[END_REF] and several authors (e.g [START_REF] Constable | Persistent high paleosecular variation activity in southern hemisphere for at least 10 000 years[END_REF][START_REF] Hellio | Time-correlation-based regression of the geomagnetic field from archeological and sediment records[END_REF][START_REF] Arneitz | A Bayesian iterative geomagnetic model with universal data input: Self-consistent spherical harmonic evolution for the geomagnetic field over the last 4000 years[END_REF][START_REF] Campuzano | Emergence and evolution of the South Atlantic Anomaly revealed by the new paleomagnetic reconstruction SHAWQ2k[END_REF] have developed global spherical harmonic models covering the last 3 to 14 millennia. However, all models are likely to be biased by a strongly uneven geographic distribution, with about 90% of GEOMAGIA50.v3.3 data coming from the Northern hemisphere, and a large majority (∼90%) from the mid-latitudes. Africa and South America are the greatest landmasses where we can retrieve data from the equatorial and southern latitudes to address this problem.

In Africa, archeomagnetic and volcanic data, as well as relative data from lake and marine sediments (e.g. Di Chiara, 2019), are rare. Moreover, the large majority of data is concentrated in Northern Africa (e.g. [START_REF] Odah | Improvement of the secular variation curve of the geomagnetic field in Egypt during the last 6000 years[END_REF][START_REF] Leonhardt | Archaeomagnetic field intensity during the Roman period at Siwa and Bahryn oasis, Egypt: Implications for the fidelity of Egyptian archaeomagnetic data[END_REF][START_REF] Gómez-Paccard | Archaeomagnetic and rock magnetic study of six kilns from North Africa (Tunisia and Morocco)[END_REF][START_REF] Casas | New Archaeomagnetic Data from Tunisia: Dating of Two Kilns and New J o u r n a l P r e -p r o o f Archaeointensities from Three Ceramic Artifacts[END_REF] and in the Canary Islands (e.g. [START_REF] De Groot | High paleointensities for the Canary Islands constrain the Levant geomagnetic high[END_REF]Kissel et al., 2015aKissel et al., , 2015b)). Sub-Saharan Africa accounts for only 0.1% of GEOMAGIA50.v3.3 database despite recent additions (e.g. [START_REF] Tanguy | Archeomagnetism of Piton de la Fournaise: Bearing on volcanic activity at La Réunion Island and geomagnetic secular variation in Southern Indian Ocean[END_REF][START_REF] Mitra | Two thousand years of archeointensity from West Africa[END_REF]Donadini et al., 2015;[START_REF] Tarduno | Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo[END_REF][START_REF] Kapper | Reconstructing the Geomagnetic Field in West Africa: First Absolute Intensity Results from Burkina Faso[END_REF][START_REF] Kapper | Novel insights on the geomagnetic field in West Africa from a new intensity reference curve (0-2000 AD)[END_REF].

Sub-Saharan Africa is also crucial to understand one of the most remarkable features of the modern geomagnetic field: the South Atlantic Anomaly (SAA). The SAA, currently located in Southern Brazil, is characterized by intensity about 30% weaker at the surface of the globe than if the field was purely dipolar [START_REF] Thébault | International Geomagnetic Reference Field: the twelfth generation[END_REF]. It is generally assumed that the SAA is related to reversed flux patches at the surface of the outer core (e.g. Terra-J o u r n a l P r e -p r o o f [START_REF] Terra-Nova | Relating the South Atlantic Anomaly and geomagnetic flux patches[END_REF]. Direct measurements of the geomagnetic field show that since 1840 the SAA intensity minimum drifted to the southwest, about 4500 km at the Earth's surface, associated to an increase of its extent area (e.g. [START_REF] Hartmann | Time evolution of the South Atlantic Magnetic Anomaly[END_REF][START_REF] Pavon-Carrasco | The South Atlantic Anomaly: The Key for a Possible Geomagnetic Reversal[END_REF]. Data from Sub-Saharan Africa are crucial to better constrain the path and amplitude of the SAA beyond this date, and especially to confirm if it appeared in the Indian Ocean [START_REF] Hellio | Time-correlation-based regression of the geomagnetic field from archeological and sediment records[END_REF][START_REF] Campuzano | Emergence and evolution of the South Atlantic Anomaly revealed by the new paleomagnetic reconstruction SHAWQ2k[END_REF] or in Southern Africa [START_REF] Tarduno | Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo[END_REF].

Contrary to what the scarcity of archeomagnetic data suggests, the archeological potential is high in Central and Eastern Africa. For archeomagnetic studies, the most frequent well-preserved baked structures are iron furnaces (e.g. [START_REF] Killick | A global perspective on the pyrotechnologies of Sub-Saharan Africa[END_REF][START_REF] Robion-Brunner | L'Afrique des m taux[END_REF].

In the current state of knowledge, the oldest sites clearly associated to iron metallurgy in Central and Eastern Africa are dated to the first half of the first millennium BCE. This pyrotechnology might even have emerged earlier in the modern Central African Republic at the end of the third millennium BCE [START_REF] Alpern | Did They or Didn't They Invent It? Iron in Sub-Saharan Africa[END_REF]Zangato and Holl, 2010). In this paper, we present archeomagnetic results of iron furnaces dated to the last six centuries in the Mbeere region in Kenya and in the Guéra Massif in Chad (Figure 1a).

Archeological setting and sampling

All furnaces were sampled with the plaster cap method (Figures 1b and1c). Blocks were levelled horizontally with a bubble level and oriented using a magnetic and eventually a sun compass. Between three and six large blocks were collected per furnace. In the laboratory, the blocks were cut into cubic specimens, 2 cm in side, after consolidation in sodium silicate.

Two blocks from Kenya and six from Chad could not be used for directional analysis because J o u r n a l P r e -p r o o f their orientation was not precise enough. This is was due either to a damaged plaster cap during transport or to the fragile nature of the samples.

Mbeere region, Central Kenya

The Kamuturi 1 archeological site (0°25'53" S, 37°47'3" E) that we sampled is located in the Ishiara district of the Mbeere region, close to Mount Kenya (Figure 1a). The site, located at the bottom of a small hill, is assumed to cover an area of 25,000 m 2 , based on the extent of visible surface materials. Excavations by F. M'Mbogori between 2015 and 2016 revealed in situ furnaces on the eastern side of the site, with several fragments of slag and tuyere pieces in between. To the west of the furnaces, iron working materials are found isolated, more fragmented and in association with daily use objects such as pottery, beads, cowry shells, and metallic artefacts.

All furnaces excavated at Kamuturi 1 site were described as bowl furnaces, made using irregular bricks of clay with maximum length of 15 cm and thickness of 1-4 cm (M'Mbogori et al., in preparation). As some of the best-preserved furnaces were removed for display in a museum, we could only sampled three of the remaining furnaces. Furnace 1 (KAM1-1 in our nomenclature) is the largest with a maximum diameter of 80 cm and a minimum diameter (at bottom) of 42 cm (Figure 1b). The depth from the bottom to the uppermost edge measured 85 cm but bricks scattered on the surface suggest that the furnace stood taller above the ground than the excavated remains. Furnaces 2 and 3 (KAM1-2 and KAM1-3 in our nomenclature), partially excavated, have similar morphology, brick sizes and shapes but they are smaller in size. These differences highlight the complexity of the smelting technologies in use [START_REF] Iles | Iron production in second millennium AD pastoralist contexts on the Laikipia plateau, Kenya[END_REF][START_REF] Iles | Pastoralist iron production on the Laikipia Plateau, Kenya: wider implications for archaeometallurgical studies[END_REF] and it can be noted that the morphology of the excavated furnaces differs from the ethnographic furnaces as described by J o u r n a l P r e -p r o o f [START_REF] Brown | Traditional metalworking in Kenya[END_REF]. M'Mbogori (2013) points out that the smelters of Kamuturi 1 may have affinities with past communities who lived in this region before the arrival of Bantu groups.

In order to spot additional furnaces, magnetic mapping was done in the area. It is a convenient technique for archeological prospection since buried magnetic objects generate, by contrast with the substratum, a magnetic anomaly. Several EW transects were carried out every 50 cm covering a survey area of 37 m by 20 m around the Kamuturi 1 site. The maps of anomaly gradients (Figures S1a andS1b in the supplementary material) show heterogeneous zones with localized and intense anomalies (maybe related to the slags and the tuyeres), superimposed on a background signal with weaker anomalies and greater wavelength (maybe due to vegetation and soil thickness). The interpretation of these maps is also complicated by previous excavations and by a substratum made of red clay that has an induced magnetization on the same order of magnitude as the furnaces. However, one furnace (KAM1-4) is clearly indicated by an association of positive and negative anomalies, characterizing a magnetic dipole that is remanent since it is not aligned with the local geomagnetic field (Figure S1b). This furnace, comparable in size to KAM1-2, was sampled. After filtering of the lower probe map, the presence of other potential furnaces is suggested (shaded areas in Figure S1c). Three to six blocks were collected per structure for a total of 17 blocks, a fairly low number compared to other archeomagnetic studies but that could not be enhanced due to preservation constraints. Sampling was also limited by suspicions of post-heating movements due to the surrounding vegetation in some part of the furnaces. The colour of the baked clay used for the walls of the furnaces varies from grey to dark-brown and more reddish parts but without clear relationships with the temperature gradient.

KAM1-1 and KAM1-2 furnaces were dated by radiocarbon on charcoals found in their filling (M'Mbogori et al., in preparation). As the site is located at the equator, the calibration process can be performed using either IntCal13 [START_REF] Reimer | IntCal13 AND Marine13 radiocarbon age calibration curves 0-50,000 years Cal BP[END_REF] or SHCal13 (Hogg et al., J o u r n a l P r e -p r o o f 2013) curves, which were designed for the Northern and the Southern hemispheres respectively. The two probability density functions that can be obtained have different shapes and therefore provide slightly different time intervals at 95% of confidence (see Figure S2 in the supplementary material). For example, in the case of KAM1-1, IntCal13 provides two time-intervals of [1452-1528] and [1543-1634] cal CE, while SHCal provides only one cal CE. In order to cover the two possibilities, the dating of KAM1-1 and KAM1-2 furnaces was cautiously fixed between 1450 and 1650 CE and, considering the usually short use of this type of metallurgical sites in this area (low quantity of scoriae and low number of furnaces per site), we assumed that KAM1-3 and KAM1-4 furnaces are dated from the same period.

Guéra Massif, central Chad

The Guéra Massif, mainly composed of granites [START_REF] Shellnutt | Timing of collisional and post-collisional Pan-African Orogeny silicic magmatism in southcentral Chad[END_REF]

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Bogrom site (12°09'29.4'N; 18°25'50.2''E) is in the Bitkine district, about 30 km west of Mongo. The four furnaces that were uncovered are conical in shape, with a diameter of about 40 cm and walls about 6 cm thick (Figure 1c). Four furnaces (BOG1, BOG2, BOG3 and BOG4) were sampled.

Djogolo site (12°12'22.4''N; 18°47'55.2''E) is close to Bankakotch site about 12 km east of Mongo. The furnaces here are more semi-circular than those of Bankakotch and Bogrom, with a long axis around 40 cm and a thin wall thickness around 4 cm. Three furnaces (DJO1, DJO2 and DJO3) were sampled.

Between 2 and 4 dark-coloured blocks were collected per furnace, for a total of 34 blocks at the three sites.

Only two furnaces from Bankakotch (BANK1 and BANK2) and three from Bogrom (BOG1, BOG2 and BOG4) could be dated per radiocarbon on charcoals found at the base of the structures (filled with charcoal trapped in scoria). All charcoals were first studied by anthracology by Barbara Eichhorn (Goethe-Universität, Frankfurt/Main). Almost all of them belong to Prosopis Africana, a common shrub of savannah in Sub-Saharan Africa. AMS dating at Poznan radiocarbon laboratory were preferentially performed on charcoals from twigs, because they are likely not affected by a significant old-wood effect. Calibration was processed with IntCal13 curve (Figure S3 in the supplementary material). Due to a plateau effect on the calibration curve, 14 C intervals are very large (between 1650 and 1950 CE, see supplementary material for details). They agree with the proposed archeological age but do not specify it much. We attributed the maximum 14 C range (1650( -1950 CE) CE) to all others undated furnaces.

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Magnetic Mineralogy

Rock magnetic experiments were performed, in order to investigate the ferromagnetic mineralogy and its stability to temperature. All experiments were conducted at CEREGE laboratory.

K/T curves

The susceptibility of the samples was measured through two successive heatingcooling cycles up to medium (350°C) and high (650°C) temperatures using an AGICO Multifrequency Kappabridge (MFK1-CS3) apparatus. The experiments were done on 30 and 32 powdered specimens of ~200 mg from Kenyan and Chadian furnaces, respectively. The Curie temperature was estimated at the point at which the curvature of the concave part of the heating curve is maximal (e.g. [START_REF] Dunlop | Rock Magnetism[END_REF].

In both cases, we observe mainly two characteristic behaviours in the K/T curves. The first behaviour, that represents ~35% of the Kenyan specimen and ~30% of the Chadian specimens, shows a single ferromagnetic phase that corresponds to an almost pure magnetite with Curie temperatures close to 580°C (Figures 2a and2b). The mostly reversible curves highlight the absence of mineralogical transformation during heating. It is worth noting that for the Kenyan samples, this behaviour is mainly found in the samples that present a reddish colour. The other specimens, grey or dark-brown in colour, present at least another phase besides this almost pure magnetite. This second behaviour, also seen in the Chadian samples, is illustrated in Figures 2c and2d. The K/T curves present a first phase that seems to be rather stable at least up to 350-400°C, with Curie temperature around 400-500°C. This is probably a titanomagnetite that will transform at higher temperature into magnetite, explaining the increase of susceptibility in the cooling curves.

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Hysteresis parameters

Hysteresis curves up to 0.5 Tesla and backfield curves were performed on eight Kamuturi 1 specimens and eight Chadian specimens of ~ 5 mm diameter using the Micromag Vibrating Sampling magnetometer 2900 (Figure S4 in supplementary material). Hysteresis parameters were calculated after removal of the paramagnetic component: M s is the saturation magnetization, M rs the saturation remanent magnetization, B c the coercive force and B cr the remanent coercive force (Table S1 in supplementary material). In Figure 3, the Day diagram [START_REF] Day | Hysteresis properties of titanomagnetites: Grain size and composition dependence[END_REF] that presents the remanence ratio M rs /M s versus the coercivity ratio B cr /B c can be used to estimate roughly the domain state of magnetite grains (e.g. [START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF]. All the samples that were measured plot in the pseudo single domain (PSD) range, indicating a common occurrence of large grains within our samples.

Directional analysis

All magnetic moments were measured with a 2G cryogenic magnetometer. To retrieve the direction of the remanent magnetizations, 100 cubic specimens (38 from Kenya and 62 from Chad) were demagnetized. Stepwise alternating field (AF) demagnetization was made from 3 to 90 mT (every 3 mT up to 50 mT and every 10 mT between 50 and 90 mT), with the AF demagnetizer built in line with the 2G cryogenic magnetometer. Stepwise thermal demagnetization was done in an MMTD80A oven with 14 steps up to 585°C (every 50°C from 100 to 400°C, every 30°C from 400°C to 520°C, every 20°C from 520°C to 560°C and finally 585°C). For Kenya, 20 specimens were AF and 18 thermally demagnetized. AF demagnetization was in average less noisy than thermal treatment (e.g. Figures 4a and4c and Table S2). This difference was due to temporary experimental problems with our MMTD80A oven, and we therefore favoured AF demagnetization (56 specimens) for the Chadian J o u r n a l P r e -p r o o f samples. However, a few thermal demagnetizations (6 specimens) were done to determine their unblocking temperature spectrum (e.g. Figures 4b). Results were interpreted with the Puffin Plot software [START_REF] Lurcock | PuffinPlot: A versatile, user-friendly program for paleomagnetic analysis: technical brief[END_REF]. Most of the plots and the calculation of the mean directions by Fisher statistic were done with the PmagPy3.0 software [START_REF] Tauxe | PmagPy: Software package for paleomagnetic data analysis and a bridge to the Magnetics Information Consortium (MagIC) Database[END_REF].

Most of the specimens (80% and 75% from Kenya and Chad respectively) present a single component of magnetization (Figures 4a, 4c and4d), while a secondary component is also observed for the rest of the samples. As this secondary component is usually removed at fairly low fields or temperatures (Figure 4b, 4e and 4f), it is probably of viscous origin. The directions of the characteristic remanent magnetization (ChRM) per demagnetized specimen are listed in Table S2 of the supplementary material, and the equal area plots of Figure S5 (supplementary material) present the distribution of the ChRM directions per furnace (Table 1). Eight specimens with an aberrant orientation were rejected (with more than 30° to their sister specimens). This is probably related to an error in the transfer of orientation to the specimen.

Most furnaces show a good grouping of their ChRM directions, as evidenced by large kappa. For some furnaces, the dispersion is larger, especially in declination, likely because of slight post-heating wall movements due to tree roots (e.g. for KAM1-3) or during the excavations (e.g. KAM1-1). Despite these dispersions at the cooling unit level, the directions are coherent at the site level (Figure 5, Table 1) and therefore were all considered as representative of the ancient geomagnetic field. They are the first archeomagnetic directions ever obtained from Central and Eastern Africa.

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Archeointensity determination

For archeointensity determination, we used the classical Thellier-Thellier method (Thellier and Thellier, 1959) with 7-12 temperature steps up to 560°C, distributed according to the unblocking temperature range of the specimens. The protocol consists of heating and cooling the specimens twice at each temperature step with a laboratory field (40μT) applied along the +Z and -Z axis of the specimen successively. To assure a better reproducibility of the experiments, we invert the field and not the specimens. Partial thermoremanent magnetization (pTRM) checks are performed every two steps to monitor the mineralogical transformations. All heatings were performed in an ASC TD48-SC oven with a temperature reproducibility within 1°C. The archeointensity results were interpreted using ThellierTool4.0 software [START_REF] Leonhardt | Analyzing absolute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0[END_REF].

A total of 19 oriented cubic specimens (2 cm in side) from Kenya and 41 from Chad were analysed. In the case of Kenyan furnaces, in order to test if the differences in ferromagnetic mineralogy and colour of the blocks play a role on the results, we also prepared 21 non-oriented grey, brown or reddish small chips embedded in non-magnetic plaster.

A paleointensity estimate is considered as reliable only when the Arai plot is linear over a large proportion of the unblocking temperatures and when the orthogonal diagram shows a single component of magnetization in this interval of temperature. Our acceptance criteria are a quality factor q higher than 5 and a paleointensity error lower than 10%. We also consider the consistency within a given furnace and fix an upper limit of ±10% for the standard deviation on the average value.

Only three specimens from one Kenyan furnace (KAM1-1) meet our acceptance criteria (Table 2). KAM1-1.1A11 and KAM1-1.1A21 especially provide convincing results (Figure 6a). The paleointensity estimation of KAM1-1.2A11 is of lower quality with a q factor of 7 but is consistent with the two other accepted specimens (Figure 6b).

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Five specimens from Chad have rather linear Arai plots but show two components of magnetization in orthogonal plots (Figure 6c). As the sister-specimens that were AF and thermally demagnetized carry only one component of magnetization, this behaviour is likely explained by mineralogical alteration and specimens must be rejected. The two apparent components of magnetization indicate the acquisition of a chemical remanent magnetization during the laboratory experiment that results in the deviation of the direction along the laboratory field direction (+Z). Finally, most other specimens from Kenya and Chad show two slopes or a concave-up shape on their Arai plot associated to a non-linear orthogonal diagram (Figure 6d). This behaviour can be related to the presence of coarse grains (e.g. [START_REF] Levi | The effect of magnetite particle size on paleointensity determinations of the geomagnetic field[END_REF], in line with the Day plot (Figure 3). For Kenya, we checked if there was any relationship between the paleointensity results and the ferromagnetic properties or the colour of the samples without clear results.

The archeointensities of the three accepted specimens from KAM1-1 furnace were corrected for the effects of TRM anisotropy and cooling rate with the protocol of [START_REF] Chauvin | Paleointensity of the geomagnetic field recovered on archaeomagnetic sites from France[END_REF]. The TRM anisotropy tensor was determined at 550°C with six different positions (+X, -X, +Y, -Y, +Z and -Z) of the specimens. The anisotropy correction could not be applied to KAM1-1.2A11, likely altered by mineralogical transformations, but this is not really crucial with an anisotropy effect of 4.5% and 0.8% for the two others specimens.

The cooling rate correction was performed at the same temperature as the anisotropy correction. It consists in the comparison of two pTRMs acquired during a rapid cooling, over one hour (i.e. the duration used in routine during the Thellier-Thellier procedure) and a slow cooling, to better approximate the original cooling of the furnace. Here the duration of the slow cooling was 5 hours due to experimental constraints of our laboratory oven. It may underestimate the initial archeological cooling but this approximation will have a smaller impact on the accuracy of the average archeointensity than the absence of correction (Hervé et J o u r n a l P r e -p r o o f al., 2019). The cooling rate correction factors of the three specimens range between 8 and 11%. The corrected mean archeointensity of KAM1-1 furnace is 43.5 ± 1.8 μT. Even though the success rate of this study is low, we propose the first full vector determination ever published in Eastern Africa.

Interpretation

Over the Holocene, the current update of GEOMAGIA50.v3.3 database [START_REF] Brown | GEOMAGIA50.v3: 2. A new paleomagnetic database for lake and marine sediments[END_REF] reports 40 archeointensities and 48 archeodirections in Sub-Saharan Africa, to which have been recently added 18 new archeointensities by [START_REF] Kapper | Novel insights on the geomagnetic field in West Africa from a new intensity reference curve (0-2000 AD)[END_REF]. All these data are dated in the last 3 millennia and most were acquired in Western Africa [START_REF] Mitra | Two thousand years of archeointensity from West Africa[END_REF][START_REF] Donadini | Directional changes of the geomagnetic field in West Africa: Insights from the metallurgical site of Korsimoro[END_REF][START_REF] Kapper | Reconstructing the Geomagnetic Field in West Africa: First Absolute Intensity Results from Burkina Faso[END_REF][START_REF] Kapper | Novel insights on the geomagnetic field in West Africa from a new intensity reference curve (0-2000 AD)[END_REF], La Réunion Island [START_REF] Tanguy | Archeomagnetism of Piton de la Fournaise: Bearing on volcanic activity at La Réunion Island and geomagnetic secular variation in Southern Indian Ocean[END_REF] and South Africa [START_REF] Neukirch | An archeomagnetic analysis of burnt grain bin floors from ca. 1200 to 1250 AD Iron-Age South Africa[END_REF][START_REF] Tarduno | Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo[END_REF]. Data are very rare in Central and Eastern Africa near our sites, emphasizing again the impact of our study. In Cameroon, three directions have been published on volcanic lava flows erupted in the 20 th century CE [START_REF] Herrero-Bervera | Paleomagnetic and paleosecular variation study of the Mt. Cameroon volcanics (0.0-0.25 Ma), Cameroon, West Africa[END_REF]. In Eastern Africa, [START_REF] Skinner | The recent secular variation of declination and inclination in Kenya[END_REF] acquired 13 directions on lava flows but only two of them (data named S and T5) are independently dated with precision to the 19 th century CE. An intensity data point has also been published for Ethiopia on bricks produced at the beginning of the 17 th century CE [START_REF] Osete | First archaeomagnetic field intensity data from Ethiopia, Africa (1615±12AD)[END_REF].

Regarding this almost absence of neighbouring data, we first focus the discussion on the comparison with the predictions of four recent global models, CALS10k.2 (Constable et al., 2016), COVARCH [START_REF] Hellio | Time-correlation-based regression of the geomagnetic field from archeological and sediment records[END_REF], BIGMUDI4k.1 [START_REF] Arneitz | A Bayesian iterative geomagnetic model with universal data input: Self-consistent spherical harmonic evolution for the geomagnetic field over the last 4000 years[END_REF] and gufm1 [START_REF] Jackson | Four centuries of geomagnetic secular variation from historical records[END_REF]. The latter is computed only from direct measurements by navigators or in observatories.

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Comparison of our results with global models

Our new Kenyan data are in good agreement with the global models for the declination (Figure 7a). KAM1-3 has a declination that is ~6° lower than the others furnaces, which may indicate that this furnace did not operate at the same period. The average inclinations of the four furnaces are mutually consistent but they are lower than the average prediction of the models (Figure 7b). The better fit in declination than in inclination could be explained by the fact that the direct measurements were recorded by the navigators along the East African coastline since the 16 th century for the declination but only after the 18 th century for the inclination [START_REF] Jonkers | Four centuries of geomagnetic data from historical records[END_REF][START_REF] Arneitz | The HISTMAG database: Combining historical, archaeomagnetic and volcanic data[END_REF].

The new archeointensity obtained on KAM1-1 furnace is around 10 µT higher than the Ethiopian data point of [START_REF] Osete | First archaeomagnetic field intensity data from Ethiopia, Africa (1615±12AD)[END_REF], after relocation to Kamuturi 1 (Figure 7c). Despite the fact that there are only two archeointensity data for East Africa, this difference could indicate a fast secular variation of the geomagnetic field strength in this area at the middle of the second millennium CE, not seen in models due to the lack of intensity data in Africa. New data are needed to obtain more robust models.

The average directions of the ten furnaces from Chad are also compared to global models. The declination values cover the whole range of values predicted by the models since 1600 CE (Figure 8a). However, the average inclinations are consistent with the models only in the recent part of their 14 C dating (Figure 8b). This large uncertainty of the 14 C dating makes our Chadian results not very useful to improve the archeomagnetic database.

Archeomagnetic dating of Chadian furnaces

Considering the young age of the Chadian furnaces, our results can however be used to better constrain the age of the furnaces. Archeomagnetic dating is performed with the J o u r n a l P r e -p r o o f prediction at the sites of the gufm1 global model using RenDateModel software [START_REF] Ph | Bayesian inference of calibration curves: application to archaeomagnetism[END_REF]. The intervals of date at 95% of confidence are calculated on the combination of the two probability density functions (pdfs) obtained with declination and inclination (Figure 8c for BOG1 furnace and Figure S6 in Supplementary material for other furnaces). No results can be obtained for three furnaces (BANK2, BOG3 and DJO3) that have inconsistent declination and inclination pdfs, maybe because of post-heating movements of ovens walls.

For other furnaces, dating provides a single interval of date between the mid-19 th and the 20 th century CE, that is at the end of the "abasia" period (Figure 8d). These recent dates are in very good agreement with oral sources that suggest an abandonment of the local iron production with the establishment of the French colonial administration in 1908. However, the results of BOG2, BOG4 and DJO2 furnaces indicate that the local production could have slightly continued afterwards.

Conclusions

Our archeomagnetic study of 4 furnaces from Kenya and 10 from Chad is the first published in these countries. We obtained 14 new directions (4 for Kenya and 10 for Chad) and one intensity from Kenya. This work illustrates the two complementary applications of archeomagnetism: 1) data acquisition for geomagnetic modelling, 2) archeomagnetic dating.

Sub-Saharan Africa is one of the poorest landmasses in archeomagnetic data. Our new Kenyan data, dated between 1450 and 1650 CE, are a new step to address this problem.

The current global models are consistent with our data in declination but not in inclination and intensity. Our new data will clearly contribute to improve them in the future, in order to better retrieve the secular variation of the geomagnetic field in Sub-Saharan Africa and to better understand the origin of the present South Atlantic Anomaly.

J o u r n a l P r e -p r o o f

The Chadian furnaces are more recent than the Kenyan ones and were used to test the potential of archeomagnetic dating in Africa over the last centuries. The use of gufm1 global model appears a powerful dating tool with a precision much better than the one of radiocarbon for this period.

  , is located in south-central Chad. An archeological survey was carried out by Vincent Mourre and Djimet Guémona in the area near Mongo, the main town of the region. One of their main objectives was to locate iron smelting sites, to better constrain the history of this technology in the region. Information collected among the local population place the development of steel industry in the Guéra Massif during the "abasia" period that started between 1611 and 1635 CE and ended around 1909 CE. They also indicate that the local production of iron has been abandoned at the time of the establishment of the French colonial administration in 1908. Three main smelting sites with furnaces have been identified. Bankakotch site (12°11'01.1''N; 18°49'17.6''E) is located to the south-east of the Abu Telfane chain. It is one of the largest iron sites in the Migami district with a high quantity of scoriae and tuyeres. The three excavated furnaces (BANK1, BANK2 and BANK3) are circular in shape with a diameter between 35 and 58 cm and wall thicknesses between 6 and 12 cm.

Figure 1 :

 1 Figure 1: Location of the study areas (a) and examples of furnaces sampled at Kamuturi 1

Figure 2 :

 2 Figure 2: Representative K/T curves from Kenya (a-c) and Chad (b-d). Heating/cooling

Figure 3 :

 3 Figure 3: Day plot (Day, 1977) of samples from Kenya and Chad.

Figure 4 :

 4 Figure 4: Orthogonal diagrams, AF demagnetization and thermal demagnetization curves for

Figure 5 :

 5 Figure 5: Mean directions of characteristic remanent magnetizations by furnace with their α 95

Figure 6 :

 6 Figure 6: Arai plots of representative archeointensity results with corresponding orthogonal

Figure 7 :

 7 Figure 7: Comparison of the Kamuturi 1 data (red triangles) with existing Eastern Africa data

Figure 8 :

 8 Figure 8: (a, b) Comparison of the new Chadian data with the predictions of gufm1,

  Figure1

Figure 8

 8 Figure8

Table captions Table 1 :

 captions1 Mean directions of the characteristic remanent magnetizations by furnace. Columns from left to right: country name, site name, latitude and longitude of the site, furnace name, number of blocks used/sampled, number of specimens used to calculate the average value over the number of measured specimens, mean declination and mean inclination, Fisher precision parameter, and Fisher 95% confidence circle, archeomagnetic dating of Chad furnace in years CE.

					2			1				e
					BANK 3	2/3	4/4	349. 0	7.6	104 9.1	1827-1950
					BOG 1 3/4	5/5	349. 2	8.8	268 4.7	1848-1911
		Bogrom	12.15 8	18.431	BOG 2 4/4 BOG 3 3/4	7/8 7/7	359. 2 5.0 7.1 1.5	79 6.8 115 5.7	1938-2000 unreliabl e
					BOG 4 2/2	8/8	355. 7	5.7	84 6.1	1907-1996
					DJO 1 4/4	8/9	353. 2	3.8	37 9.2	1871-1996
		Djogolo	12.20 6	18.799	DJO 2 3/3	7/8	353. 3	2.9	125 5.4	1902-1984
		J o u r n a l P r e -p r o o f DJO 3 2/2 4/4 2.9 5.2	395 4.6	unreliabl e
	Count ry	Site	Lat°N	Long°E	Furnac e	N/N 0	n/n 0	Dec °	Inc °	kapp a	α 95 °	AM dating
					KAM1 -1	6/6	12/1 4	1.8	-3.5	61 5.6
	Kenya	Kamuturi 1	-0.431	37.784	KAM1 -2 KAM1 -3	3/3 7/10 4/5 7/7	1.5 0.3 355. 2 0.5	193 4.4 120 5.5
					KAM1 -4	2/3	7/7	0.6	-2.1	144 5.0
	Chad	Bankakot ch	12.18 4	18.822	BANK 1 BANK 2/4 3/4	6/6 3/3	343. 2 339. 2.8 1.8	1829-1929 282 7.4 unreliabl 51 9.5

Table 2 :

 2 Summary of accepted archeointensity results. Columns from left to right: furnace name, specimen name, temperature interval used for the intensity calculation, uncorrected intensity, standard deviation on the intensity, NRM fraction, gap factor, quality factor, intensity corrected for cooling rate effect. Highlights  Africa's Sub-Saharan data represents less than 1% of the global archaeomagnetic database  First archeomagnetic study in Kenya (16 th century CE) and Chad (18-19 th c. CE)  Fourteen new directions and one new intensity data  Inconsistency with geomagnetic models in inclination and intensity

	Furnace Specimen	Interval °C	F (µT) sd (µT)	f	g	q	F CR (µT)
	KAM1-1	1A11 1A21 2A11	150-500 150-500 150-450	47.1 46.7 49.0	0.7 1.1 2.8	0.6 0.6 0.6	0.8 0.8 0.7	31 21 7	44.0 41.5 45.1
								Mean	43.5 ± 1.8

 Archaeomagnetic dating more precise than radiocarbon in the 18-19 th c. CE

Acknowledgements

This work was supported by the CEREGE laboratory through its 2017 APIC, and by the Programme National de Plan tologie (PNP) of CN S INSU, co-funded by CNES, through its 2018 program. The fieldwork in Kenya was made possible through the support of National Museums of Kenya/British Institute in East Africa (BIEA), and the IRD-East Africa representation in Nairobi. The fieldwork in Chad was supported by the ArRéLat program funded by the French Ministry of Foreign Affairs, and by CEREGE/IRD (Pierre Rochette).

We have a very special thought for Barbara Eichhorn, recently passed away, whose help was extremely precious to select the samples from Chad that could be safely dated by 14 C and whom we regret. The manuscript was improved thanks to reviewers' comments.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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