Lydie Le Forestier
email: lydie.leforestier@univ-orleans.fr

Fabrice Muller

Frédéric Villieras

Manuel Pelletier

TEXTURAL AND HYDRATION PROPERTIES OF A SYNTHETIC MONTMORILLONITE COMPARED WITH A NATURAL NA-EXCHANGED CLAY ANALOGUE Authors

Keywords: Hydrothermal synthesis, Wyoming montmorillonite, hydraulic conductivity, swelling, N 2 /Ar/water adsorption

An aluminous montmorillonite has been synthesized at hydrothermal conditions (623 K, 120 MPa) and compared to the natural Na-exchanged Wyoming montmorillonite (Na-SWy2). The synthetic product is characterized by a high cation exchange capacity (83 meq/100g) and ethylene glycol specific surface area (764 m 2 /g) similar to Na-SWy2 but with a higher purity with one crystalline phase. Compacted smectite samples were percolated with water by using an oedometer cell equipped with a separated injection system, in order to simulate the behaviour of an engineered clay barrier in the context of subsurface waste landfill. Percolation experiments showed very low hydraulic conductivities (10 -12 m/s) for both synthetic and natural smectites, in addition to the non macroscopic swelling of the synthetic montmorillonite, in contrast to Na-SWy2. At the nanometric scale, this synthetic clay mineral is characterized by higher hydration states and a better organization in the distribution of water molecules in particles, as proven by X-ray diffraction technique and continuous water vapor gravimetry method. Moreover, adsorption-desorption isotherms of nitrogen revealed the classical feature of swelling clay minerals, and the Brunauer-Emmet-Teller calculation gives a nitrogen specific area of 87 m 2 /g for the synthetic smectite, twice higher than for the Wyoming clay, due to its higher mesoporosity. Finally, low-pressure argon adsorption coupled with the Derivative Isotherm Simulation procedure highlighted classical adsorption energy distributions of natural phyllosilicates, and thus the synthetic montmorillonite was characterized by an average of 11 layers per particle in the dry state, with basal and edge surface areas different from Na-SWy2.

Introduction

Bentonites are used in numerous industrial, agricultural and environmental applications [START_REF] Murray | Applied clay mineralogy today and tomorrow[END_REF], in particular, as engineered barrier for high-level nuclear waste repositories or subsurface waste disposals. These clays are of great interest due to their rather low permeability when compacted, their ability to expand and their capacity to retain pollutants. Bentonites take advantage of the hydration, swelling, cation exchange, and surface complexation capacity of montmorillonite, its main mineral component [START_REF] Caglar | Characterization of the cation-exchanged bentonites by XRPD, ATR, DTA/TG analyses and BET measurement[END_REF].

Bentonites, being natural materials, contain impurities and different clay minerals, and their intrinsic features may induce chemical and mineralogical variations [START_REF] Galan | Genesis of clay minerals[END_REF]. On the contrary, synthetic clay minerals are reproducible and homogeneous in terms of the chemical composition, mineralogy, texture, and thus, they become useful for systematic studies aimed at a better understanding of the interaction mechanisms between clay minerals and metal ions or organic pollutants. Before considering synthetic smectites as a suitable model of natural bentonites, it is necessary to elucidate their basic characteristics such as cation exchange capacity, surface properties, morphology, and in particular their behaviour at different scales when contacted with water. Indeed, smectites are characterized at the nanometric scale by a crystalline swelling in that water molecules can be intercalated between individual 2:1 layers (0 to 4 water layers), and also by a macroscopic swelling. At the macroscopic scale, the measured swelling of smectites results from a combination of several simultaneous processes: crystalline swelling, double-layer swelling, breakup of crystals, cation demixing, co-volume swelling, and Brownian swelling [START_REF] Laird | Influence of layer charge on swelling of smectites[END_REF].

Although dioctahedral smectites, such as montmorillonites, are predominant in soils and sediments and they are widely used in many industrial applications, most of the clay studies have been devoted to the synthesis of trioctahedral smectites, such as hectorite [START_REF] Carrado | A study of organo-hectorite clay crystallization[END_REF][START_REF] Spagnuolo | Coprecipitation of trace metal ions during the synthesis of hectorite[END_REF], and saponite [START_REF] Delevoye | 23 Na 2D 3QMAS NMR and 29 Si, 27 Al MAS NMR investigation of Laponite and synthetic saponites of variable interlayer charge[END_REF].

Beidellites (Kloprogge et al., 1993;Lantenois et al., 2008a) and nontronites [START_REF] Decarreau | Hydrothermal synthesis, between 75 and 150°C, of high-charge, ferric nontronites[END_REF] have been synthesized as dioctahedral smectites, but the synthesis of montmorillonite in high purity is rarely achieved due to the difficulty to achieve it [START_REF] Reinholdt | Synthesis and characterization of montmorillonite-type phyllosilicates in a fluoride medium[END_REF]Lantenois et al., 2008b).

Therefore, in the present study we focus on the comparison of textural, cation exchange, hydration and swelling properties of a synthetic montmorillonite with those of the sodium-exchanged Wyoming smectite which is considered as its natural clay analogue.

Materials and methods

Starting materials

Natural clay

The natural clay used for this study is the SWy-2 Wyoming montmorillonite supplied by the Source Clay Minerals Repository of the Clay Minerals Society. The impurities identified as quartz, feldspar, gypsum, mica, illite in the SWy-2 sample represent 5% after purification by < 2 μm size separation [START_REF] Chipera | Baselines studies of the clay minerals society source clays: Powder X-ray diffraction analyses[END_REF]. The SWy-2 montmorillonite was sodium-saturated by exchange in 1M NaCl. 2 g of SWy-2 and 50 mL of 1M NaCl solution were mixed on a SRT1 Stuart Scientific roller mixer for 16 h, and then the mixture was centrifuged. The supernatant chloride solution was filtered off and the tube was refilled with the 1M NaCl solution. This exchange procedure was repeated five times. The sample was then transferred into a dialysis membrane tubing and placed in deionised water to remove chlorine. The water was daily refreshed until the disappearance of chlorine (AgNO 3 test).

After dialysis and decantation, the < 2 μm fraction of Na-SWy2 samples was recovered, dried and finely hand ground in an agate mortar. Electron microprobe analyses using a CAMECA SX50 electron microprobe (EMPA) operating at 15 kV accelerating voltage and 6 nA intensity, as well as ICP-OES analysis were carried out to quantify the structural formula of the < 2 μm Na-exchanged SWy-2, called afterwards Na-SWy2 (Table 1). The structural formula of this clay mineral is close to that previously determined by [START_REF] Michot | Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy[END_REF].

Synthetic smectite

The starting material for the hydrothermal syntheses was a gel of anhydrous composition 0.33Na 2 O-1.67Al 2 O 3 -0.66MgO-8SiO 2 and prepared using a procedure adapted from [START_REF] Hamilton | The preparation of silicate compositions by a gelling method[END_REF][START_REF] Hamilton | The preparation of silicate compositions by a gelling method[END_REF]Lantenois et al. (2008b). The chemicals for the gel preparation, with a 99% minimum grade purity, were Na 2 CO 3 (Prolabo, 0.5 g/10g dry gel), tetraethylorthosilicate (TEOS, Fluka, 23.87 g/10g dry gel) as the silicon source, solutions of Al(NO 3 ) 3 .9H 2 O (27.53 g/10 g dry gel) and Mg(NO 3 ) 2 .6H 2 O (3.72 g/10g dry gel), whose titration is 88.6 mg Al 2 O 3 /g and 102.5 mg MgO/g respectively, NH 4 OH (Aldrich), and ethanol (Aldrich, 30 mL/10g dry gel), were used as solvent. After the dissolution of sodium carbonate in Mg-and Al-nitrate solution (pH=2), TEOS and ethanol are added. NH 4 OH is added up to the formation of a precipitate. After the complete hydrolysis of the TEOS (16 h), the precipitate is progressively heated up to 873 K to remove water, organic matter, NO 2 , and CO 2 , and to obtain a Na-Mg-Al-Si oxide gel. This gel is finally ground in an agate mortar.

Experimental charges were prepared as following: deionised water and gel (weight ratio 1:5) were placed into a gold capsule welded on one end. The capsule was closed by arcwelding, while its main body was constantly cooled in liquid nitrogen. Hydrothermal syntheses were carried out in Tuttle-type, externally heated, cold-seal pressure vessels, using argon as the pressure medium at 623 K and 120 MPa, for the duration of 28 days. All capsules were leak-checked before the start of the experiment (test with oil), and after by reweighing.

Percolation experiments

Percolation experiments have been performed using oedometer cells equipped with a separated injection system characterized by a controlled mechanical pressure. This specific equipment has been developed, in order to mimic the subsurface landfill conditions at laboratory scale [START_REF] Jullien | Dispositif perméamétrique à injecteur à pression contrôlée séparé[END_REF][START_REF] Jullien | Hydro-chemio-mechanical coupling effects on permeability and swelling behaviour of a Ca smectite soaked by Cu solutions[END_REF]. The oedometer cell, used in this study, is characterized by a 10 mm diameter, which requires low sample masses (ca. 400 mg for each experiment), and so adapted for synthetic clay minerals. Besides, this cell is equipped with a piston for mechanical stress control. The axial displacements of the piston are measured with a dial indicator of a 0.5 µm precision. Porous sintered bronze plates surrounded by teflon waterproof rings are used at the top and bottom of the cell. The porous plates are covered with filter paper to prevent pore obstruction by the clay sample.

Concerning solute injection, the syringe, made of inert materials, allows to apply a constant mechanical pressure to the solution with a piston. The syringe piston displacements, which can be related to the injected volume, are picked up from a dial indicator. Negligible strains of the syringe are obtained in the range of applied injection pressures (below 1 MPa). Therefore, the injection pressure is separately controlled from the mechanical pressure applied to the clay sample based upon the experimental parameters optimized to reproduce the hydro-mechanical conditions of subsurface landfill well [START_REF] Pothier | Etude des phénomènes de transfert de métaux lourds dans une smectite[END_REF]. So, for each experiment, the airdried smectite sample is firstly compacted up to 0.5 MPa and totally unloaded, in order to simulate the over-consolidation of engineered clay barriers. Then, deionised water is injected into the compacted clay sample under a constant pressure of 0.3 MPa, applied to the upper boundary while the lower boundary is exposed to a mechanical stress of 0.3 MPa. Such test conditions lead to an over-consolidation ratio of 1.66. The sample thickness and the injected volume were monitored during testing, as well as the weight of the possible output solutions.

At the end of each experiment, the clay sample was removed, and the final mass and thickness were measured.

Hydration experiments

X-ray diffraction

Structural investigation of the clay minerals, as well as the study of the hydration properties, was carried out using X-ray diffraction (XRD) technique. XRD patterns were recorded using a Thermo Electron ARL'XTRA diffractometer equipped with a Cu anode and a Si(Li) solid detector. The vertical θ:θ goniometer supports two Sollers slits. A VTI RH 200 relative humidity generator device coupled with an Anton Paar THC chamber allows to select variable environmental conditions. Experimental measurement parameters were 10 s counting time per 0.05° 2θ step. The divergence, the incident beam scatter, the diffracted beam scatter and the receiving slits are 1.0, 1.5, 0.45 and 0.30 mm, respectively. 30-60 mg of powder sample is necessary for XRD analysis. Data collection was performed at 303 K and 50% relative humidity (RH) after establishing an equilibrium period of 1 h prior to the measurement. Concerning hydration, the first step, at 5% RH, was obtained after an equilibrium period of 24 h before the pattern was recorded. Then the sample was gradually hydrated from 5 to 99% RH for 1 h of equilibrium period before measurement.

Quasi-equilibrium water vapor adsorption-desorption

Water vapor adsorption-desorption measurements were carried out in a continuous gravimetric apparatus built around a Setaram MTB 10-8 symmetric microbalance with a precision of 1 μg [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite: 1. The sodium-exchanged form[END_REF]. 43 mg of Na-MMT sample was outgassed overnight under vacuum (0.01 Pa) at 383 K before the measurement. The sample was exposed to water vapor was supplied to the sample through an Edwards microleak valve, at a constant slow flow rate to maintain quasi-equilibrium conditions during the experiment. Water pressure is measured by a DRUCK 0-13333 Pa absolute pressure sensor. The adsorption-desorption isotherm, that is the plot of the mass of H 2 O adsorbed or desorbed at 303 K versus pressure, was directly recorded on a computer. Experimental data were further analyzed using the BET equation. Specific surface areas were calculated referring to the two cross-sectional area values of the adsorbed water molecules, 0.106 and 0.148 nm 2 , corresponding to various packings of liquid water on mineral surfaces [START_REF] Harkins | Surface of solids. XIII. A vapor adsorption method for the determination of the area of a solid without the assumption of a molecular area, and the areas occupied by nitrogen and other molecules on the surface of a solid[END_REF][START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite: 1. The sodium-exchanged form[END_REF].

Complementary characterization 2.4.1. CEC measurements

The Cation Exchange Capacity (CEC) values of Na-SWy2 and Na-MMT were measured using the copper complex method [START_REF] Ammann | Determination of the cation exchange capacity of clays with copper complexes revisited[END_REF], for which 100 mg of dry clay sample was suspended in 8 mL of 0.01 M copper triethylenetetramine ([Cu(trien)] 2+ ) solution.

The solution was shaken for 15 h and then centrifuged at 6000 rpm for 15 min. The Cu concentration of each supernatant was measured (i) by colorimetry using a Hitachi U-1100 spectrophotometer, at 577 nm, and also (ii) by atomic absorption spectrometry (AAS) using a Hitachi Z-8100 spectrophotometer.

Specific surface area

The specific surface areas of Na-SWy2 and Na-MMT, representing the complete unimolecular surface coverage of montmorillonite by ethylene glycol, were experimentally determined [START_REF] Eltantawy | Ethylene glycol sorption by homoionic montmorillonites[END_REF].

N 2 adsorption

The nitrogen adsorption-desorption volumetry at 77K was performed using a lab-built automatic apparatus described by Marrochi et al. (2005). 1 g of sample was outgassed at 383 K under a residual pressure of 0.01 Pa. Specific surface area was determined using the BET equation [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF] based upon the cross-sectional area of nitrogen (0.163 nm 2 ) at 77K [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity[END_REF]. The presence of micropores in the sample was assessed using the tplot method [START_REF] De Boer | The t-curve of multimolecular N 2 adsorption[END_REF]. Pore size distributions were calculated following the BJH method [START_REF] Barrett | The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms[END_REF].

Ar adsorption

High-resolution low-pressure argon adsorption was carried out on an automatic quasiequilibrium volumetric set-up [START_REF] Michot | Surface heterogeneity studied by a quasiequilibrium adsorption procedure[END_REF], Villiéras et al., 1992, 1997a). 1 g of sample was outgassed overnight at 383 K under a residual pressure of 10 -4 Pa. After outgassing, a slow, constant and continuous flow of argon was introduced into the adsorption cell through a microleak. The adsorption isotherms were then derived from the recording of quasi-equilibrium pressures versus time. The data were then treated using the Derivative Isotherm Simulation (DIS) procedure designed by Villiéras et al. (1992Villiéras et al. ( , 1997aVilliéras et al. ( , 1997b) ) to examine the surface energetic heterogeneity of the Na-MMT sample. Having the large number of experimental data points acquired by the quasi-equilibrium technique, the experimental derivative of the adsorbed amount as a function of the logarithm of relative pressure could be calculated accurately. The total derivative adsorption isotherm on a heterogeneous surface is simulated by the sum of local theoretical derivative adsorption isotherms on heterogeneous surfaces [START_REF] Michot | Assessment of surface energetic heterogeneity of synthetic Na-saponites. The role of layer charge[END_REF]Tournassat et al., 2003;[START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF].

Results and discussion

Similar characteristics

X-ray diffraction patterns revealed that smectite mineral is the only crystalline phase existing in Na-MMT sample (Fig. 1). This phase was clearly identified by (00l) reflections at 5.7° (1.55 nm), 17.4° (0.508 nm), 28.75° (0.31 nm), and (02.11), (20.13), (06.33) reflections with the maxima at 19.8° (0.448 nm), 35.3° (0.254 nm), 62.05° (0.149 nm), respectively. The position of the (06.33) reflection at 0.149 nm confirms the predominance of dioctahedral domains in the octahedral sheets of the synthetic Na-MMT smectite similar to the natural Na-SWy2 smectite. The (02.11), (20.13) reflections and the ( 002) and ( 004) reflections are also located at the same positions with those of Na-SWy2. Quartz and pyroxene were also identified in the Na-SWy2 sample. This natural bentonite still contains accessory phases despite the purification, while the synthetic Na-MMT smectite reserves the main advantage of being pure. Besides, the synthesis of montmorillonite is reproducible since ten syntheses of Na-MMT samples have been successfully performed.

As expected, the Na-SWy2 sample develops a high exchange capacity of 85 meq/100g (Table 1) in agreement with the previous value obtained by [START_REF] Duc | Coupled chemical processes at clay/electrolyte interface: A batch titration study of Na-montmorillonite[END_REF]. The CEC value of the synthetic Na-MMT sample which is 83 meq/100g also confirms the high potential of exchange capacity.

The synthesized product, called Na-MMT, has the theoretical structural formulae: Na 0.66 (Al 3.34 Mg 0.66 )(Si 8 )O 20 (OH) 4 .nH 2 O. The chemical analysis using ICP-OES coupled with electron microprobe analyses reveals that the charge deficit is mainly located in the octahedral sheets (Table 1), as previously shown by Lantenois et al. (2008b) for a similar synthetic smectite represented by SM 0.33 -1200. However, the structural formula of the SM 0.33 -1200 sample which was derived by Lantenois et al. (2008b) as Na 0.68 (Al 3.32 Mg 0.72 )(Si 7.94 Al 0.06 )O 20 (OH) 4 highlights the non-equilibrium between the interlayer charge (0.68 per unit cell) and the total charge deficit (0.72 + 0.06 = 0.78). Thus, it may be deduced from these data that Na + is not the only cation involved in the interlayer charge. To elaborate this, all the constituent elements (Na, Al, Mg, Si) of the synthetic Na-MMT sample were measured in the supernatant by AAS after the exchange of Na-MMT with the copper complex for the CEC determination. Magnesium may have been located in the interlayer spacing of the mineral since a minor fraction of this element is exchangeable. This allowed us to accurately calculate the structural formulae of Na-MMT as 1) and are in agreement with expected values of swollen clay minerals changing in the range of 700-800 m 2 /g [START_REF] Helmy | Surface area evaluation of montmorillonite[END_REF]. The specific surface area (S) can be estimated by assuming a perfect crystal structure and ignoring the layer thickness via the equation:

S = (N a . a . b . 2.10 -20 ) / M
where N a is the Avogadro number, a and b are the unit cell parameters and M is the molar mass of the clay mineral per unit cell. The calculation results in the specific surface areas of 756.7 and 750.2 m 2 /g for Na-MMT that contains smectite particles only and Na-SWy2, respectively, which are in parallel with the experimental ones (Table 1).

Hydraulic conductivity

The permanent hydro-mechanical regime is defined by the leachate production and strain equilibrium [START_REF] Jullien | Hydro-chemio-mechanical coupling effects on permeability and swelling behaviour of a Ca smectite soaked by Cu solutions[END_REF]. The solute intake shown as a function of time in Fig. 2 displays a constant flow for both experiments after about 50 h, which landmarks the end of the transient hydraulic state. The hydraulic conductivity, k, was then calculated from the Darcy's law, using the expression elaborated for a saturated medium [START_REF] Domenico | Ground water movement[END_REF]): k = Q / i S, where Q is the measured volumetric flow rate (m 3 /s) at the steady state, i is the hydraulic gradient and S is the cross-sectional area of the oedometer (m 2 ).

The hydraulic conductivity k of the compacted synthetic Na-MMT smectite percolated with water (3.8 10 -12 m/s) is slightly higher than the value obtained for the natural Na-SWy2 smectite (0.9 10 -12 m/s). These results are in the same magnitude with the permeability (k = 1.1 10 -12 m/s) of Fo-Ca, a bentonite sample compacted and soaked with water under the same conditions [START_REF] Jullien | Hydro-chemio-mechanical coupling effects on permeability and swelling behaviour of a Ca smectite soaked by Cu solutions[END_REF].

Hydration properties

1. Macroscopic swelling

The axial swelling strain, ε 1 , deduced from the measured axial displacements of the piston, is illustrated as a function of time for both percolation experiments with water (Fig. 3).

For Na-SWy2, the mechanical permanent regime, corresponding to the asymptotic ε 1 value, is obtained after 10 days of percolation. The natural Na-SWy2 sample shows a high swelling strain at equilibrium with a value of 37%, whereas the Na-MMT synthetic smectite sample has a very low macroscopic swelling strain, around 1%. Moreover, we have tested the swelling ability of both Na-MMT and Na-SWy2, without any mechanical constraint. For this purpose, 200 mg of powdered clay sample was gradually added to 10 mL of deionised water in a cylinder and the swelling was recorded after 24 h of contact time. In the presence of water, the natural Na-SWy2 smectite was transformed into a gelling phase and swollen with a volume gain of 8.5 mL, whereas the synthetic Na-MMT sample remained unchanged.

Crystalline swelling

X-ray diffraction technique has been used to determine the mineralogy of both Na-MMT and Na-SWy2 samples, as well as to quantify the crystalline swelling by measuring the d(001) basal spacing. Under the same conditions of adsorption (T=303 K and P/P 0 =0.5), the (001) reflection came at 1.21 nm for Na-SWy2, but shifted at 1.55 nm for Na-MMT (Fig. 1), revealing that the synthetic Na-MMT smectite has a higher hydration state (double water layer) than the natural Na-SWy2 smectite (single water layer). The variation of the d( 001) basal spacing as a function of the relative water partial pressure is presented in Fig. 4. At the lowest pressure (P/P 0 =0.05), the basal distance of Na-MMT reaches 1.1 nm which is higher than a dry situation of 1.0 nm implying that the crystalline swelling has already begun with a partial intercalation of the first water layer. Similar trends were followed by the natural Na-SWy2 and the synthetic Na-MMT samples up to a pressure of 0.44 in the presence of single water layer. At 0.53, Na-SWy2 still retains single water layer, whereas the XRD pattern of Na-MMT reveals two distinct reflections at 1.3 and 1.5 nm, corresponding to the coexistence of particles associated with single and double water layers. The same feature is obtained at the highest water pressure (P/P 0 =0.98), where the appearance of a new reflection at 1.9 nm points out that the crystalline swelling of Na-MMT has reached three water layers whereas the Na-SWy2 particles only contain two water layers in the interlayer spacing. Na-MMT hydrates more than Na-SWy2 at lower water partial pressures. Whatever the relative humidity is, the (001) reflection of Na-MMT is always finer than Na-SWy2, as clearly shown on Fig. 1 for P/P 0 equal to 0.5, or on Fig. 4 at 0.65 where the (001) distance at 1.35 nm for Na-SWy2 in fact corresponds to an average value between single and double water layer species. These results outline that the synthetic particles have a better organization than the natural ones.

Water sorption isotherms for the synthetic Na-MMT montmorillonite (Fig. 5) display irregular shapes characteristic of swelling material which suggest complex water adsorption mechanisms [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite: 1. The sodium-exchanged form[END_REF]. More precisely, the sorption isotherms of Na-MMT exemplify those typical for Na-exchanged montmorillonites [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite: 1. The sodium-exchanged form[END_REF][START_REF] Bérend | Mechanism of adsorption and desorption of water vapour by homoionic montmorillonites: 2. The Li + , Na + , K + , Rb + , and Cs + -exchanged forms[END_REF]. At relatively low pressures (P/P 0 <0.2), adsorption of water occurs only on external surfaces which is followed by a sharp increase at P/P 0 >0.25, the behaviour ascribed to the adsorption of water molecules in the interlamellar spacing. The application of the BET equation using a water cross-sectional area of 0.148 nm 2 yields an energetic constant of the BET theory equal to 14 and a total specific surface area of 149 m 2 /g which is significantly higher than the value (106 m 2 /g) obtained for Na-SWy2. Therefore the synthetic Na-MMT particles, owing to a greater external surface area, become accessible by more water molecules during the adsorption.

Geometric and energetic surface properties

Nitrogen adsorption-desorption at 77 K

Nitrogen adsorption-desorption experiments are used to obtain further information on the specific surface area of dried particles, resulting of the stacking of several individual sheets, and also on pore size distribution of the synthetic Na-MMT sample. Adsorptiondesorption curves of Na-MMT and Na-SWy2 (Fig. 6) reveal the classical features observed for swelling clay minerals having both microporosity and mesoporosity [START_REF] Michot | Assessment of surface energetic heterogeneity of synthetic Na-saponites. The role of layer charge[END_REF][START_REF] Neaman | The effects of exchanged cation, compression, heating and hydration on textural properties of bulk bentonite and its corresponding purified montmorillonite[END_REF]Marrochi et al., 2005;[START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF]. The adsorption isotherms of Na-MMT and Na-SWy2 are of type II with a hysteresis loop, originating from the capillary condensation of liquid nitrogen in mesopores [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity[END_REF]. The low pressure region (P/P 0 < 0.42) represents to the filling of micropores, the completion of the first monolayer on external particle faces which is followed by the multilayer adsorption. In the relatively high pressure region (P/P 0 > 0.42), the shape of the hysteresis loop corresponding to the filling of the mesoporosity is typical of type H3 with any limiting adsorption taking place at high P/P 0 , which has been observed with aggregates of plate-like particles giving rise to slit-shaped pores [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity[END_REF]. Moreover, the vertical closure of the hysteresis loop around 0.44 in P/P 0 is generally observed for such materials as a typical feature of closed mesoporosity inside clay aggregates [START_REF] Neaman | The effects of exchanged cation, compression, heating and hydration on textural properties of bulk bentonite and its corresponding purified montmorillonite[END_REF]. The adsorptiondesorption isotherms of Na-MMT demonstrate a very important hysteresis loop (P/P 0 > 0.7), which points out a well-calibrated mesoporosity. The BET and t-plot calculations applied to the nitrogen adsorption isotherm of Na-MMT give a total specific area of 87.1 and 92.8 m 2 /g, respectively; which are twice higher than those of Na-SWy2 (Table 2). BET specific surface areas are slightly smaller than those derived from t-plot calculation, which is in agreement with the fact that both samples are slightly microporous. The specific surface areas of micropores are equivalent in both cases, with the values of 17 and 21 m 2 /g, respectively, which show that the microporosity is not quantitatively important in comparison to activated carbon or zeolites. However, Na-MMT is characterized by a higher mesoporosity, as this synthetic smectite develops an external specific surface area three times higher than the natural Na-SWy2 smectite. To confirm this proposal, pore size distributions (PSD) were calculated according to the BJH method by applying the parallel pore model [START_REF] Barrett | The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms[END_REF] for Na-MMT and Na-SWy2 (Fig. 7). Calculated PSD from the adsorption and desorption parts, which correspond to true and access PSD values, respectively [START_REF] Neaman | The effects of exchanged cation, compression, heating and hydration on textural properties of bulk bentonite and its corresponding purified montmorillonite[END_REF] signify that pore access is linked to mesopores smaller than 2.6 nm in both cases, but the synthetic Na-MMT sample includes an additional access size around 5.1 nm. This difference may be explained by referring to the mesoporosity. Indeed, particle arrangement and PSD of swelling clay minerals depend on sample preparation, such as dehydration conditions [START_REF] Michot | Assessment of surface energetic heterogeneity of synthetic Na-saponites. The role of layer charge[END_REF]. The pore size distribution of the synthetic Na-MMT montmorillonite is found to be similar to the previously obtained result for the natural Prassa smectite [START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF].

When the specific surface area is known, the mean number of layers (n) contained by a clay particle may be determined by the following equation [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite: 1. The sodium-exchanged form[END_REF]: S (m 2 /g) = 801.3/n + 5.13

In dry condition, the particles of the synthetic Na-MMT smectite are made up of 9 to 10 layers, which is less than the average number of 20 layers found for the natural Na-SWy2 smectite. Our results are in agreement with those of [START_REF] Lantenois | Thermodynamic assessment of the variation of the surface areas of two synthetic swelling clays during adsorption of water[END_REF] who have also obtained 9 layers constituting a particle of a synthetic montmorillonite by using X-ray data and the Scherrer equation [START_REF] Moore | X-Ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF].

Low-pressure argon adsorption at 77 K

The experimental argon derivative isotherm of Na-MMT is displayed in Fig. 8, together with the four local isotherms, using the DIS procedure. Fitting parameters are given in Table3. The curves of the synthetic Na-MMT smectite resemble classical adsorption energy distributions of natural phyllosilicates (Villiéras et al., 1992(Villiéras et al., , 1997a(Villiéras et al., , 1997b;;[START_REF] Bardot | High resolution gas adsorption study on illites permuted with various cations: assessment of surface energetic properties[END_REF][START_REF] Michot | Assessment of surface energetic heterogeneity of synthetic Na-saponites. The role of layer charge[END_REF]Tournassat et al., 2003;[START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF], with a main peak located at -4.11 (domain 2), assigned to the adsorption on basal surfaces, and a shoulder centred at -7.05 (domain 3), corresponding to the adsorption on lateral (or edge) surfaces. As classically obtained with argon measurements, the broad high-energy peak centred at -10.66 (domain 4), is also assigned to edge surfaces of the particles, whereas the low-energy contribution located at -2.27 (domain 1) is due to the adsorption on basal surfaces.

The same DIS procedure applied on experimental Ar low-pressure adsorption was used for the Na-SWy2 sample (Fig. 8b), whose adjusted parameters are presented in Table 3, as well as those previously obtained for other natural Na-exchanged smectites, MX-80 (Tournassat et al., 2003) and Prassa [START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF], respectively. Five local isotherms are necessary to model the experimental curves of Prassa with the presence of a higher energy adsorption site on edge faces (domain 5) at -12.80. The peak positions corresponding to domains 1 and 2 (basal faces) are quite lower for the synthetic Na-MMT montmorillonite in comparison with the natural samples. However, the situation is different for edge faces as adsorption energy for domain 4 increases in the order: Na-SWy2 < Prassa < MX-80 < Na-MMT. These high-energy adsorption sites of the less important domain 4 (and also of 5 for Prassa) could be attributed to chemical variability or non-uniform morphology such as local defects, corners or ridges of particle surfaces [START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF]. As the two low-energy peaks (domains 1 and 2) are assigned to the adsorption on basal faces, and the three high-energy peaks (domains 3, 4 and 5) to the adsorption on edge faces, the basal and lateral surface areas can be calculated separately (Table 4). The proportion of lateral surface is similar for Na-MMT, MX-80, and Prassa, with the total surface area values of 26.5%, 24.2%, and 27%, respectively, whereas Na-SWy2 is completely different with a majority of edge faces (54%). Moreover, it may be pointed out that the synthetic Na-MMT smectite is characterized by basal and edge surface areas very close to those obtained for Prassa previously. In order to have more information on the geometric properties, mean characteristic length (l) and thickness (t) of the particles have where ρ is the density (2.7 g/cm 3 ) and S l and S b , are the lateral and basal surface areas, respectively. Calculations give a length of 65 and 58 nm for Na-MMT and Na-SWy2, respectively, but the thickness increases from 12 to 34 nm for these samples (Table 4). The smaller thickness of Na-MMT particles is likely to be responsible partly for the greater mesoporosity of this sample as derived from nitrogen adsorption/desorption experiments.

Taken into account the theoretical thickness of one dry smectite unit layer (0.95 nm), the average number of 2:1 layers per stack under dry condition is around 12-13 for Na-MMT, which is almost in the same order with Prassa (11 layers), but much lower than those of MX-80 and Na-SWy2 (29 and 36 layers per particle, respectively) (Table 4). The present results highlight textural similarities between Na-MMT and the Prassa smectite, resulting from the low-temperature alteration of a rhyolitic rock in Greece [START_REF] Christidis | Formation and growth of smectites in bentonites: a case study from Kimolos Island, Aegean, Greece[END_REF]. The latter clay mineral appears to be a better natural clay analogue for the synthetic Na-MMT montmorillonite than the Wyoming Na-SWy2 smectite which is a classical model used in the literature.

Conclusions

The hydrothermal synthesis of a pure dioctahedral smectite was performed successfully. The synthetic Na-MMT montmorillonite exhibits similar high cation exchange capacity and total ethylene glycol specific surface area which are comparable to the natural Na-exchanged Wyoming smectite. The synthetic montmorillonite is characterized by a better crystalline swelling but no macroscopic swelling is observed when exposed to water. The hydro-mechanical properties of smectites, such as hydraulic conductivity and macroscopic swelling, measured at a macroscopic scale, are obviously linked to the multi-scale organization of porosity and also to the geometric and energetic properties of surface particles. The interlayer position of some magnesium cations could contribute to the nonmacroscopic swelling of the synthetic Na-MMT montmorillonite, by preventing from the delamination of layers in the presence of water. However, the interlayer space of Na-MMT is mainly occupied by sodium cations, which may prevail for the swelling behaviour against magnesium. The particles of the synthetic Na-MMT smectite do not have sufficient length in contrast to the natural Na-SWy2 sample, in order to obtain a gel phase, and textural anisotropy has to be taken into account to reach the sol/gel transition. Our future work will focus on the investigation of the dependence of macroscopic swelling and sol/gel transition on particle size and exfoliation which will mainly be searched through laser granulometry and transmission electron microscopy. Parameters adjusted to the experimental Ar derivative isotherm of Na-MMT and Na-SWy2 presented in Fig. 8, then compared to those of other natural Na-exchanged samples: the MX-80 bentonite (Tournassat et al., 2003), and the Prassa smectite [START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF] 3.

(

  Na 0.68 Mg 0.03 )(Al 3.35 Mg 0.65 )(Si 7.91 Al 0.09 )O 20 (OH) 4 . The specific surface area measured by the adsorption of ethylene glycol represents the total surface area of individual sheets of the clay sample. The ethylene glycol specific surface areas of the synthetic Na-MMT and the natural Na-SWy2 montmorillonites are close to each other (Table

  been calculated from basal and lateral surface areas by assuming a simple square base, as proposed by Tournassat et al. (2003): l = 4 / ρ . S l and t = 2 / ρ . S b ,

c

  Total specific area (±4 m 2 /g) d Equivalent specific surface area of micropores (±20 %) e External specific surface area (= S tot -S μpores , ±20 %) f Argon-specific surface area derived from the modelling of the low-pressure argon adsorption isotherm Table 3

Figure captions Fig. 1 .

 captions1 Figure captions

Fig. 2 .Fig. 3 .

 23 Fig. 2. Solute intake during percolation experiments of water in Na-MMT and Na-SWy2,

Fig. 4 .

 4 Fig. 4. Evolution of the basal distance d(001) as a function of the relative water partial

Fig. 5 .

 5 Fig. 5. Water adsorption-desorption isotherms obtained at 303 K for (a) the synthetic Na-

Fig. 6 .

 6 Fig. 6. Adsorption-desorption isotherms of nitrogen obtained at 77K for (a) the synthetic Na-

Fig. 7 .

 7 Fig. 7. Pore size distribution (PSD), calculated from nitrogen adsorption-desorption isotherms

Fig. 8 .

 8 Fig. 8. Adsorption energy distributions (thin solid lines) derived from low-pressure argon

Fig

  Fig. 1.

Fig

  Fig. 2.

Table 1

 1 Relevant features of the synthetic montmorillonite (Na-MMT) and the Na-exchanged Wyoming smectite (Na-SWy2). From ICP-OES and electron microprobe analyses of the solid clay, and AAS analyses of the supernatant after exchange with copper complex b From ICP-OES and electron microprobe analyses c Measured by colorimetry and AAS after exchange with copper complex d Specific surface area measured by adsorption of ethylene glycol.

	Structural formula		CEC (meq/100g	SSA d
			of calcined clay)	(m 2 /g)
	Na-MMT (Na 0.68 Mg 0.03 ) (Al 3.35 Mg 0.65 )(Si 7.91 Al 0.09 )O 20 (OH) 4	a	83.0 ± 0.3 c	763.8
	Na-SWy2 Na 0.68 (Al 3.06 Fe III 0.42 Mg 0.58 )(Si 7.90 Al 0.10 )O 20 (OH) 4 b		85.0 ± 2.7 c	736.8

a

Table 2

 2 Numerical results derived from the BET and t-plot methods of the N 2 adsorption-desorption isotherms, and from the DIS method used for Ar adsorption. Previous data obtained for natural Na-exchanged smectites are given for comparison with the synthetic Na-MMT

	sample.
	BET		t-plot				DIS
	V m a	C b S tot c	S μpores	d	S ext e	S tot	S Ar f
	(cm 3 /g)	(m 2 /g)	(m 2 /g)	(m 2 /g)	(m 2 /g)	(m 2 /g)
	Na-MMT 19.93	451 87.1	16.7		74.6	92.8	85.6
	Na-SWy2 10.39	685 45.4	20.6		28.5	49.1	47.2
	a Monolayer volume

b BET energy constant

  .

	Sample			Domain 1 Domain	Domain	Domain	Domain	Total
					2	3	4	5
		Peak	ln P/P 0 -2.27	-4.11	-7.05	-10.66
		position
	Na-	V monolayer (cm 3 /g) 4.18	12.76	4.51	1.62
	MMT	ω	(kT)	0.0	1.4	0.7	-0.2
		SSA a	(m 2 /g) 15.5	47.3	16.7	6.0	85.5
	Na-SWy2	Peak position	ln P/P 0 -2.73	-4.38	-7.05	-9.63
		SSA	(m 2 /g) 2.6	19.2	20.6	4.8	47.2
	MX-80 b	Peak position	ln P/P 0 -2.52	-4.14	-7.05	-10.29
		SSA	(m 2 /g) 2.0	24.6	5.6	2.9	35.1
	Prassa c	Peak position	ln P/P 0 -2.50	-4.60	-6.60	-10.20	-12.80
		SSA	(m 2 /g) 18.9	48.6	20.3	2.9	1.8	92.5
	a Specific surface area
	b Data from Tournassat et al., 2003

c Data from

[START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF]

Table 4

 4 Geometrical characteristics of the dry particles of the synthetic Na-MMT sample compared with three natural Na-exchanged smectites, Na-SWy2, MX-80 and Prassa, respectively.

	Na-MMT Na-SWy2 MX-80 a Prassa b

a Data from

Tournassat et al., 2003

b Data from

[START_REF] Perronnet | Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion[END_REF]

Acknowledgments

The authors are grateful to Patrick Lecomte for the conception of the oedometer cell and Fabien Veillon for the technical assistance. The Editor and two anonymous reviewers are gratefully acknowledged for their useful comments and suggestions on the paper. The authors express their gratitudes to the French National Agency for Research (ANR) for the financial support. This publication is a contribution to the ANR-07-JCJC-0013-01 (Metalclay) project allocated to L. Le Forestier.

montmorillonites particles: Estimation of the clay edge sorption site density by lowpressure gas adsorption and AFM observations. American Mineralogist 88, 1989Mineralogist 88, -1995. . Villiéras, F., Cases, J.M., François, M., Michot, L.J., Thomas, F., 1992. Texture and surface energetic heterogeneity of solids from modeling of low pressure gas adsorption isotherms. Langmuir 8, 1789-1795.

Villiéras, F., Michot, L.J., Cases, J.M., Bérend, I., Bardot, F., François, M., Gérard, G., Yvon, J., 1997a. Static and dynamic studies of the energetic surface heterogeneity of clay minerals, in: Rudzinski, W., Steele, W.A., Zgrablich, G. (Eds.), Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces, Studies in Surface Science and Catalysis. Elsevier Science Publishers B. V., Amsterdam,104, Villiéras, F., Michot, L.J., Bardot, F., Cases, J.M., François, M., Rudzinski, W., 1997b. An improved derivative isotherm summation method to study surface heterogeneity of clay minerals. Langmuir 13, 1104Langmuir 13, -1117. .


Sophie Cornu
email: sophie.cornu@aix.inra.fr

J A Cattle

A Samouëlian

C Laveuf

L R G Guilherme

P Albéric

Impact of redox cycles on Mn, Fe, Co and Pb in nodules

Impact of redox cycles on Mn, Fe, Co and Pb in nodules

Introduction

Nodules of Mn and Fe oxides are commonly found in different soil types where they have been extensively studied [START_REF] Childs | Interelement relationships in iron-manganese concretions from a catenary sequence of yellow-grey earth soils in loess[END_REF][START_REF] Dawson | Distribution of elements in some Fe-Mn nodules and an iron-pan in some gley soils of New Zealand[END_REF][START_REF] Latrille | Physical speciation of trace metals in Fe-Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)[END_REF][START_REF] Palumbo | Trace metal partitioning in Fe-Mn nodules from Sicilian soils, Italy[END_REF][START_REF] Liu | Trace elements in manganese-iron nodules from a Chinese Alfisol[END_REF]. They are generally made of both Mn and Fe oxide cements, both elements having a different spatial distribution within nodules [START_REF] Dawson | Distribution of elements in some Fe-Mn nodules and an iron-pan in some gley soils of New Zealand[END_REF][START_REF] Palumbo | Trace metal partitioning in Fe-Mn nodules from Sicilian soils, Italy[END_REF][START_REF] Liu | Trace elements in manganese-iron nodules from a Chinese Alfisol[END_REF], surrounding detrital grains of varying nature according to the soil context [START_REF] Latrille | Physical speciation of trace metals in Fe-Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)[END_REF][START_REF] Palumbo | Trace metal partitioning in Fe-Mn nodules from Sicilian soils, Italy[END_REF][START_REF] Liu | Trace elements in manganese-iron nodules from a Chinese Alfisol[END_REF][START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF].

Iron oxides of nodules consist in goethite [START_REF] Fitzpatrick | Iron compounds as indicators of pedogenic processes: example from southern hemisphere[END_REF] and either hematite [START_REF] Herbillon | Laterites and lateritization processes[END_REF] or ferrihydrite [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF] according to the soil type.

Hematite is mostly found in nodules formed in tropical soils while ferrihydrite is found in those formed under temperate conditions. Manganese oxides in nodules consist mainly of birnessite and lithiophorite [START_REF] Taylor | The association of manganese and cobalt in soils-further observations[END_REF][START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scale resolution[END_REF][START_REF] Manceau | Molecular-scale speciation of Zn and Ni in soil ferromanganese nodules from loess soils of the Mississippi Basin[END_REF][START_REF] Neaman | Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations[END_REF].

Nodules are well-known for their high content in trace elements. Cobalt, Ni, and Zn are frequently associated with the Mn-rich parts of nodules [START_REF] Childs | Interelement relationships in iron-manganese concretions from a catenary sequence of yellow-grey earth soils in loess[END_REF][START_REF] Dawson | Distribution of elements in some Fe-Mn nodules and an iron-pan in some gley soils of New Zealand[END_REF][START_REF] Latrille | Physical speciation of trace metals in Fe-Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)[END_REF][START_REF] Liu | Trace elements in manganese-iron nodules from a Chinese Alfisol[END_REF][START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scale resolution[END_REF][START_REF] Neaman | Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations[END_REF][START_REF] Neaman | Trace element associations with Fe-and Mn-oxides in soil nodules: Comparison of selective dissolution with electron probe microanalysis[END_REF], while Cr [START_REF] Liu | Trace elements in manganese-iron nodules from a Chinese Alfisol[END_REF] and Pb [START_REF] Latrille | Physical speciation of trace metals in Fe-Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)[END_REF][START_REF] Palumbo | Trace metal partitioning in Fe-Mn nodules from Sicilian soils, Italy[END_REF][START_REF] Neaman | Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations[END_REF][START_REF] Neaman | Trace element associations with Fe-and Mn-oxides in soil nodules: Comparison of selective dissolution with electron probe microanalysis[END_REF] are associated with Fe-rich parts. [START_REF] Mckenzie | An electron microprobe study of the relationships between heavy metals and manganese and iron in soils and ocean floor nodules[END_REF], [START_REF] Childs | Interelement relationships in iron-manganese concretions from a catenary sequence of yellow-grey earth soils in loess[END_REF], [START_REF] Dawson | Distribution of elements in some Fe-Mn nodules and an iron-pan in some gley soils of New Zealand[END_REF], [START_REF] Latrille | Physical speciation of trace metals in Fe-Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)[END_REF], [START_REF] Palumbo | Trace metal partitioning in Fe-Mn nodules from Sicilian soils, Italy[END_REF] and [START_REF] Liu | Trace elements in manganese-iron nodules from a Chinese Alfisol[END_REF] showed that Cu and Mn partitioning between Fe and Mn oxides in nodules is variable. In some soils, nodules can even be the main trace element scavenger [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF].

These nodules are formed in horizons undergoing past or present seasonal waterlogging (McKenzie, 1989 and references herein;[START_REF] Khan | Saturated zones and soil morphology in a Mollisol catena of Central Iowa[END_REF][START_REF] Zhang | Characterization of iron-manganese concretions in Kentucky Alfisols with perched water tables[END_REF]. In environments still undergoing seasonal waterlogging, nodules are sensitive to variation in pH and Eh, and can be both source and sink of trace elements according to the intensity, and possibly also the duration, of the reduction phase [START_REF] Jenne | Controls on Mn, Fe, Co, Ni, Cu and Zn concentrations in soils and waters: the significant role of hydrous Mn and Fe oxides[END_REF][START_REF] Zaidel'man | Manganese-iron concretions in soils and their change under the effect of gleyification on parent materials of different genesis[END_REF][START_REF] Cambier | Influence of reducing conditions on the mobility of divalent trace metals in soils[END_REF]. This process is of prime importance in wetlands undergoing seasonal waterlogging periods that can last for several months [START_REF] D'amore | Saturation, reduction and formation of iron-manganese concretions in the Jackson-Frazier Wetland, Oregon[END_REF]. [START_REF] Grybos | Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxydes reduction?[END_REF] demonstrated that the fate of Fe-Mn oxides in wetlands was entirely responsible for the fate of Co and partially for that of Pb and Ni, while the impact of redox conditions on Cu and Cr was less clear.

Climate change has affected the duration of the annual waterlogging period, such that it is either reduced or increased according to the latitude (IPCC, 2001). An understanding of the dynamics and the fate of Fe and Mn in nodules, as well as their associated trace elements is of importance in predicting their environmental impact in ecosystems such as wetlands.

Several studies report the impact of redox cycles on the fate of Fe oxides [START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF], and Fe and Mn oxides and their associated trace elements [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF][START_REF] Salvador-Blanes | Déterminisme de la distribution spatiale des éléments majeurs et traces dans les sols en contexte métamorphique[END_REF][START_REF] Feder | In situ Mossbauer spectroscopy: Evidence for green rust (fougerite) in a gleysol and its mineralogical transformations with time and depth[END_REF][START_REF] Contin | Enhanced soil toxic metal fixation in iron (hydr)oxides by redox cycles[END_REF][START_REF] Grybos | Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxydes reduction?[END_REF]. However, these oxides were dispersed into the soil matrix and, to our knowledge, no attempt was made to understand the sorption and release of trace elements from soil nodules that are the main trace element scavengers in soils [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF] and have different accessibility to soil solution than the dispersed oxides.

We aim here to investigate the effect of redox cycles on the mobility of Fe and Mn in soil nodules and of the associated Co and Pb coming from temperate climate. For that an original experimental design using nodule column experiments was designed in order to mimic natural conditions of redox cycles that occur in subsurface soil horizons.

Material and Methods

Site and soil

The study site, located on the Aigurande plateau in the Massif Central, is underlain by amphibolite and gneiss. On the footslope, Planosols [START_REF] Fao | World reference base for soil resources. A framework for international classification[END_REF] -or Albaqualfs (Soil Taxonomy, 1999) -developed on gneiss [START_REF] Salvador-Blanes | Déterminisme de la distribution spatiale des éléments majeurs et traces dans les sols en contexte métamorphique[END_REF]. The upper horizons of the Planosols are allochthonous and derived from silty, colluvial materials of amphibolitic and gneissic origin, while the deeper clayey B horizon (below 50 cm depth) and the alterite are developed in gneiss. Colluvial materials differentiate into three horizons: a sandy-loam to clay-loam organic-rich A horizon (25 to 30 cm thick) and two E horizons, one of them being rich in Fe/Mn-nodules and gravels [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF]. The nodule-rich horizon is a sink for Fe, Mn, Co and Pb (Table 1; [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF]. [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF] showed that Mn oxides preferentially scavenge Co and Pb. The E horizons are waterlogged during winter and spring due to the texture contrast with the underlying B horizon. The duration of the waterlogging period increases down the slope. This horizon is draining most of the water of a 6-ha plot.

Soil water quality was monitored on this site by Penfrêne (2008). From the data they provided (Penfrêne, 2008 and references herein), we estimated that the average effective rainfall for the region ranges from 200 to 300 mm.

The nodule rich E horizon and the associated top horizon were sampled from a midslope position equivalent to that of pit 2 in [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF].

Column preparation and characterisation

Nodules were sorted from the rest of the soil and carefully cleaned. They were then separated from the bulk soil matrix by wet sieving. For practical reasons, only the 2-5 mm fraction was collected and oven dried at 40°C. Nodules were manually sorted with tweezers until ~500 mL of nodules were obtained. To remove nodule clay coatings, nodules were then placed into 250-mL screw cap bottles containing ultrapure water (nodules:water 1:1 v/v). The bottles were placed on an end-over-end shaker and rotated at 100 rpm for eight hours. Water was changed after ~ four hours by tipping nodules onto a 1-mm sieve, followed by discarding and rinsing excess clay suspension through the sieve with ultrapure water. Nodules were then washed (as above) on the sieve and left covered in a tray overnight. This process was repeated for seven days. Nodules were then sieved into 3-4 mm and 4-5 mm fractions (nodules <3mm comprised only 1.5% of the total and were not used). Each column contained equivalent amounts of each fraction. To keep the natural soil nodule particle-size distribution, 34.2 g of 3-4 mm nodules and 43.5 g of 4-5 mm nodules were weighed into each column (Table 2). We thus ensured that all the columns had the same characteristics in terms of chemistry, pore volume and surface area. A subset of nodules not used in the column experiment was kept aside for determination of BET (Brunauer, Emmett and Teller) surface area measurement, total elemental analysis by ICP-AES after triacid HF dissolution, and solid density with a pycnometer (Table 1).

Quartz sand (purchased commercially) was used as a control. It was sieved to be in the size fraction 0.4-1 mm. This size fraction was used to favour the formation of nodules around sand particles (this sand also contained small amounts of micas and feldspars as shown by Xray diffraction; results not shown). Quartz sand was packed into the column on a volume basis matching that of the nodule columns, i.e. ~98 g (Table 2). A sub-sample was kept for characterisations as described for the nodules (Table 1).

Experimental setting

The experimental design is shown schematically in Figure 1. This consists in two main parts: part A designed to produce a water whose quality mimicked that of a natural one and part B that consists in the column experiment itself.

The percolating water was produced by passing ultrapure water through a topsoil monolith. A topsoil monolith of 20x13x16 cm in size was sampled from the surface A horizon directly above the sampled nodule-rich E horizon. This monolith was removed from the ground in a plastic box. The plastic box had four inlets in the surface, one outlet at the top of one side (used as a drain when necessary) and two outlets at the base on the opposite side.

One of these outlets was used as the source of water to pass through the columns and the other worked as a port for sampling the inflowing column water for separate measurements. The soproduced inflow water was sampled and analysed for each sampling date (see below).

At the start of the experiment the surface horizon monolith had a moisture content of 37% (field moisture content). Prior to the experiment it was saturated from the base over a period of four hours. A total of 1700 mL of ultrapure water was required to saturate the block.

Saturation continued until free water pounded on the soil surface, when the box was then connected to the columns.

Water percolated through the topsoil monolith was then pumped (via a peristaltic pump) through each of five columns. Columns were fashioned from standard syringes (60 mL) with Teflon ® tubing as inlet and outlet and were wrapped in Al-foil to avoid photooxidation. A layer of nylon mesh was placed at the base of each column to prevent fine material blocking the column outlet. A silicon stopper was used in each column to form a seal. A number of multi-feeder taps (5 taps from a single inlet; see Figure 1) linked all columns to the pump to ensure an equivalent inflow rate for each treatment. All equipment was acid-washed and blanks were performed by pumping ultrapure water through the empty system (blank readings were all below the detection limit of the analytical procedures described thereafter).

The experiment was conducted in a glove box under an oxygen-depleted atmosphere (using N 2 ), with treatments as follows:

(i) nodules (in duplicate) and quartz continuously saturated for 6 months;

(ii) nodules and quartz subjected to 14-day wetting and drying cycles. Columns undergoing drying cycles were removed from the glove box to enable exposure to oxygen.

Water sampling and analysis

Column leachates were collected daily over the first month of the experiment and three times a week for the rest of the experiment, which lasted over six months. Electrical conductivity (EC), pH, and redox potential (Eh) were measured immediately on sample collection in the glove box. Redox potential was measured using an Ag/AgCl electrode and Eh measurements were then corrected to the standard hydrogen electrode, by using a correction factor depending on the temperature [START_REF] Kölling | Comparison of different methods for redox potential determination in natural waters[END_REF].

Eh corrected = Eh measured + (207 + 0.7 (25-T))
where T is the temperature in °C, and Eh corrected is the corrected redox potential in mV.

Experiments were performed at room temperature (~25°C).

As only Eh corrected values are presented in the rest of the paper they are referred as Eh for simplicity.

After collection, samples were removed from the glove box for immediate filtering (< 0.2 µm on cellulose acetate membrane). A part of the sample was then kept aside while the rest was acidified with suprapur HNO 3 for subsequent analysis by atomic absorption spectroscopy using flame for Fe and Mn, and graphite furnace for Co and Pb.

Eh-pH diagrams

Data were reported in Eh -pH stability diagrams [START_REF] Brookins | Eh-pH diagrams for geochemistry[END_REF][START_REF] Mcbride | Environmental chemistry of soils[END_REF] for Mn and Fe species. For the manganese diagram, as no clear identification of the Mn oxide present into the nodule was available [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF], the most common Mn oxides found in nodules were taken into account: birnessite (Mn 5 4+ Mn 2 3+ O 13 ) and lithiophorite (Al 2 Mn 3 4+ O 9 ) [START_REF] Taylor | The association of manganese and cobalt in soils-further observations[END_REF][START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scale resolution[END_REF][START_REF] Manceau | Molecular-scale speciation of Zn and Ni in soil ferromanganese nodules from loess soils of the Mississippi Basin[END_REF][START_REF] Neaman | Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations[END_REF]. For iron diagrams, either ferrihydrite (Fe 2 O 3 0.5(H 2 O)) or goethite (FeOOH) with two possibilities γ or α forms were considered as these two last minerals are observed in the nodules by [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF].

Green rusts were also considered in the form (Fe(OH) 3 ) and (Fe 3 (OH) 8 ) even if not identified as these minerals are transitory [START_REF] Trolard | L'influence des oxydes de fer de type « rouilles vertes » sur les séquences d'oxydo-réduction dans les sols[END_REF][START_REF] Feder | In situ Mossbauer spectroscopy: Evidence for green rust (fougerite) in a gleysol and its mineralogical transformations with time and depth[END_REF].

Thermodynamic constants related to the different solids phases are reported in Table 3. Data from column outflows were plotted on Eh-pH diagrams in order to estimate the potential equilibrium or disequilibrium with minerals present in the nodules when redox conditions reached 100 mV.

Statistic treatment

In order to determine if the discrepancies observed between the different columns were statistically significant, ANOVA tests were performed. This test was applied to the data obtained once the reduced conditions were reached. For each column, measurements were then considered as different repetitions. One ANOVA analysis was performed for each of the studied variable: pH, Eh, Fe, Mn, Co, and Pb. The results were combined in Tables 4 and5.

Results and discussion

Characteristics of the water inflow along the experiment: quality and quantity

The solution percolating through the soil monolith (column inflow) had an initial pH fluctuating from 6.6 to 6.95, increasing slightly over time to a more stable pH range of 6.85 to 6.95 (Fig. 2a; average pH value of 6.75). The pH values were of the same order of magnitude as those recorded in the field by [START_REF] Pelfrêne | Spéciation des métaux traces (Cd, Cu, Pb, Zn) dans les eaux d'un Planosol non pollué[END_REF], for the same site.

Redox potential decreased from values higher than 360 mV at the beginning of the experiment to values fluctuating between 90 and 120 mV after 30 days (Fig. 2b). Eh values stayed in this range (moderately reducing conditions) until the end of the experiment. The lag time needed for the re-establishment of reducing conditions in different laboratory experiments is reported to range from 100 hours [START_REF] Grybos | Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxydes reduction?[END_REF] to 20 days [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF]. In the field, D'Amore et al. ( 2004) reported that this time ranged from a few days to three months according to the depth and the amount of rain. Our results (30 days) are in agreement with these data. In addition, these Eh values are of the same order of magnitude as those recorded in the field by D'Amore et al. ( 2004) for wetlands, although these authors did record more reducing conditions during the wettest years. This range of Eh was also reached by [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF] and [START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF] in their experiments, while the experiments of [START_REF] Grybos | Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxydes reduction?[END_REF] reached lower redox potentials. Their Eh values were lower than the minimum value recorded in the field by D'Amore et al. ( 2004). Unfortunately, no Eh values were recorded by [START_REF] Pelfrêne | Spéciation des métaux traces (Cd, Cu, Pb, Zn) dans les eaux d'un Planosol non pollué[END_REF] for this site. [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF] showed that an Eh decrease occurred only in biologically active samples and interpreted it as the result of biological activity.

Iron concentrations were about ~0.018 mmol L -1 over the 10 first days of the experiment, rising to ~0.18 mmol L -1 after 20 days and to 0.36 mmol L -1 after 35 days (Fig. 2c). Concentrations oscillated around this value for the duration of the experiment. [START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF] and [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF] observed Fe oxide dissolution with comparable Eh values. Eh-pH diagrams for Fe (Fig. 3a) show that solutions coming out of the soil monolith are oversaturated with respect to all Fe-oxides present in the nodules.

Manganese concentrations increased rapidly at the beginning of the experiment, reaching a value of 0.27 mmol L -1 after 20 days (Fig. 2d). Manganese concentrations in the soil leachates (column inflow) were of the same order of magnitude as those recorded for Fe.

The inflow data (Table 4) revealed dissolution of Fe and Mn oxides at a molar Mn/Fe ratio of 0.61, whereas the surface horizon had a solid phase molar Mn/Fe ratio of 0.03 (ratio calculated from data in Table 1). This means that Mn oxides were proportionately more dissolved -about 20 folds -than Fe oxides, which is consistent with the higher sensitivity of Mn oxides to redox conditions compared with Fe oxides [START_REF] Jenne | Controls on Mn, Fe, Co, Ni, Cu and Zn concentrations in soils and waters: the significant role of hydrous Mn and Fe oxides[END_REF][START_REF] Stumm | Aquatic Chemistry[END_REF] and also with data reported by [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF]. After 20 days, Mn concentrations gradually decreased to reach a value of 0.09 mmol L -1 at the end of the experiment. The Eh-pH diagram for Mn (Fig. 4) shows that solutions coming out of the soil monolith are undersaturated with respect to both birnessite and lithiophorite. The dissolution of these two minerals is thus probably responsible for the Mn concentrations acquired by the percolating waters.

Trends observed for cobalt paralleled those described for Mn (Fig. 2e). With the exception of two high concentrations, lead concentrations decreased sharply from 0.019 µmol L -1 to about 0.005 µmol L -1 at 40 days and stabilised thereafter (Fig. 2f).

The obtained reducing conditions were sufficient to dissolve both iron and manganese oxides from the topsoil monolith, also observed in the experiments of [START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF] and [START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF]. [START_REF] Patrick | Sequential reduction and oxidation of inorganic nitrogen, manganese and iron in flooded soil[END_REF] give lower Eh values for the reduction of Fe oxides (50 mV), while their data for Mn oxides are in agreement with results reported here.

After 80 days of the experiment the data became very noisy. The monolith structure started to collapse and had to be replaced. This change produced sample oxidation, and reducing conditions were not observed again before the end of the experiment (data not shown). For this reason only data obtained for the 80 first days of the experiment are presented here.

Impact of the redox condition on the release of Fe, Mn and associated trace elements by nodules

As shown in Table 6, the amount of water percolated through the different columns was reasonably similar despite the difficulties in regulating the water flow. The difference between the continuously saturated treatment and the 14-day wetting and drying cycle treatment was due to no solution leaching during the drying cycle. Given that the quartz and nodule columns had such different pore volumes (Table 2), percolated volumes have been expressed in terms of pore volumes hereafter to allow comparison.

Outflow from the columns had a significantly higher pH (around 7.1 overall; Figs. 5a to 8a) than the inflow according to an ANOVA test at a 5% confidence level (Table 4).

The outflow Eh followed the same general trend as the inflow, decreasing sharply over the first month and then stabilising (Figs. 5b to 8b). Once stabilised, the outflow Eh from the quartz column before the drying cycle was not significantly different from that of the inflow, according to an ANOVA test at a 5% confidence level (Table 4). Nodule columns had a significantly higher Eh in the outflow (ANOVA test at a 5% confidence level, Table 4), especially those undergoing drying cycles.

Iron concentrations in both column inflow and outflow increased initially, reaching a relatively stable level at about 35 days (Figs. 5c to 8c). Subsequent Fe concentrations in the outflow were significantly lower than those of the inflow (ANOVA test at a 5% confidence level, Table 4) suggesting Fe fixation within the columns. The outflow from quartz columns contained higher Fe concentrations than that from the nodule columns (ANOVA test at a 5% confidence level, Table 4). The nodule column undergoing wetting and drying cycles had the smallest outflow Fe concentrations, which could be related to the higher Eh values also recorded for this column. As inflow Fe concentrations slightly changed over time, fixation ratios were calculated to compare the fixation of Fe between the different columns as follows:

Fe fixation ratio = ([Fe] in -[Fe] out ) / [Fe] in
An ANOVA test at a 5% confidence level showed that the fixation ratio of the quartz column undergoing wetting and drying cycles was the lowest before drying and the nodule column undergoing wetting and drying cycles fixed more Fe than the other nodule columns (Table 5). Thus nodule columns were able to fix Fe more efficiently than quartz columns. Eh-pH diagrams for Fe (Fig. 3) show that column outflows are oversaturated with respect to goethite, ferrihydrite and one green rust type (Fe(OH) 3 ). The Fe fraction retained in the columns was thus probably precipitate as goethite, ferrihydrite and one green rust type along the experiment. Green rust, which is a transitory phase, was probably then recrystallised into goethite or ferrihydrite as 40% and 25% of the Fe contained in the nodules was into these minerals respectively [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF].

Similarly to Fe, outflow Mn concentrations increased following the increase in inflow concentrations (Figs. 5d to 8d). Once stabilised, Mn concentrations in column outflows were significantly higher than column inflows (ANOVA test at a 5% confidence level, Table 4).

Manganese concentrations in outflows of nodule columns were also higher than those of quartz columns, indicating a release of Mn from nodules into the water. We have expressed the Mn released from the column in terms of a release ratio to better compare the different treatments:

Mn release ratio = ([Mn] out -[Mn] in ) / [Mn] in

An ANOVA test at a 5% confidence level confirmed these results (Table 5). The Eh-pH diagram for Mn shows that nearly all column outflows are undersaturated with respect to birnessite and lithiophorite (Fig. 4). As manganese oxides contained 100% of the Mn present in the nodules [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF], the dissolution of these minerals is responsible for the increase in Mn concentrations in outflows from the nodule columns.

Results show that while Fe was fixed within the columns during the experiment, reduction processes released Mn. Thus, the reducing conditions were strong enough to dissolve Mn oxide but not sufficient to dissolved Fe oxides.

Cobalt concentrations in column outflows increased with those in the inflow and stabilised after 35 days (Figs. 5e to 8e). Once stabilised, inflow and outflow Co concentrations in quartz columns were not significantly different (ANOVA test at a 5% confidence level, Table 4). Outflow Co concentrations from nodule columns were significantly higher than inflow concentrations indicating a net release of Co from nodules.

As for Mn we calculated a Co release ratio. The results for the Co release ratio were equivalent to those from Co concentrations (ANOVA test at a 5% confidence level, Table 5).

As for Mn, 100% of the Co found in the nodules was contained in Mn oxides [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF]. The Co/Mn ratios of the Mn and Co released from the nodule columns were close to those of the nodules, suggesting that dissolution of Mn oxide evidenced previously is responsible for Co losses. It also suggests that readsorption does not occur within the column unless it is in the same proportion for the two elements as that originally present in the solid phases, which disagrees with results of [START_REF] Quantin | Mn-oxide: a major source of easily mobilisable Co and Ni under reducing conditions in New Caledonia Ferralsols[END_REF] who showed that Co concentrations decreased over time. This was interpreted as readsorption. Such behaviour was not exhibited in our experiment.

As observed for the inflow, outflow Pb concentrations decreased sharply over time and stabilised at low values with the exception of some discrepancies (Figs. 5f to 8f). If we exclude these few very high concentrations, outflow Pb concentrations from columns undergoing wetting and drying cycles were significantly higher than those of the inflow, while those of the permanently saturated column were significantly lower (ANOVA test at a 5% confidence level, Table 4). While there was no obvious explanation for this behaviour, the overall concentrations were very low. When the ANOVA was performed with the high concentrations included no significant differences were found (ANOVA test at a 5% confidence level; results not shown). [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF] showed that Pb was present at almost 90% in Mn nodules; however, no Pb was either added to or removed from the water suggesting that either the Mn oxides were not dissolved congruently or that the Pb was readsorbed on the remaining phases in the column. Indeed, McKenzie (1989 and references herein) stated that in general Mn oxides adsorbed more Pb than Co. The same was recorded for Fe-oxides (Schwertmann and Taylor, 1989 and references herein).

Impact of the drying on the following reducing period

The drying cycle caused a slight increase in the outflow pH from the nodule column (Fig. 8a) although this difference was not significant according to an ANOVA test (Table 4).

For the quartz column, the effect of the drying cycles was even smaller (Fig. 7a).

Outflow Eh values increased significantly after the drying period in the quartz column

(ANOVA test at a 5% confidence level, Table 4), becoming equivalent to outflow Eh values recorded for nodule columns always saturated (Table 4). This increase of Eh upon drying was less clear for the nodule column, which already exhibited a higher outflow Eh before drying than the continuously saturated nodule columns.

Iron concentrations in outflows from the quartz column dropped significantly after drying (ANOVA test at a 5% confidence level, Table 4), however the concentrations increased quickly again (Fig. 7c). Only a slight decrease (not significant) was observed in the nodule column, possibly due to its low original Fe concentrations. The fixation ratios, show that both quartz and nodule columns fixed significantly more Fe after drying (ANOVA test at a 5% confidence level, Table 5). For these two columns, the Fe fixation ratio was closely related to Eh values of the column outflow (Fig. 9). The drying cycle clearly increased the efficiency of the nodules to fix Fe. [START_REF] Contin | Enhanced soil toxic metal fixation in iron (hydr)oxides by redox cycles[END_REF] also reported an increase in trace metal retention upon reoxidation of oxides following consecutive redox cycles. Eh-pH diagrams for Fe (Fig. 3b) show that after a drying period, column outflows are also oversaturated with respect to Fe 3 (OH) 8 , which was rarely the case before the drying. Thus, more Fe oxide types are potentially responsible for the Fe uptake from the solution by the column.

The drying cycles caused outflow Mn concentrations to drop significantly (ANOVA test at a 5% confidence level, Table 4). For the quartz column, the concentrations became even lower than those of the inflow, indicating Mn fixation. For the nodule column, outflow Mn concentrations remained higher than those of the inflow, revealing Mn release. When considered in terms of the Mn release ratio, the impact of the drying cycle was smaller in nodule columns than in quartz columns (Table 5). The Eh-pH diagram for Mn (Fig. 4) did not evidence a clear difference in terms of solution equilibrium with respect to birnessite and lithiophorite before and after drying. Both type of solutions were mainly understaturated with respect to these minerals.

The drying period provoked a net decrease in the outflow Co concentrations from the quartz column, which were significantly lower than the inflow concentrations (ANOVA test at a 5% confidence level, Table 4). This fixation of Co within the quartz column (as was seen for Mn) is small and not significant when expressed in terms of ratio (ANOVA test at a 5% confidence level, Table 5). Surprisingly, outflow Co concentrations from the nodule column increased significantly after drying, which could be better seen when expressed in terms of release ratio (ANOVA test at a 5% confidence level, Table 5).

The drying period increased the Eh within nodule columns, inducing a larger Fe fixation. The influence of these cycles on other elements was less clear. A slightly lower Mn release was recorded. Cobalt release was however found to be larger; this result was not consistent with a decrease in Mn release and Co being contained within Mn oxides. All these trends were unable to be clearly interpreted and more wetting and drying cycles should be performed in order to draw a firm conclusion.

Consequences of the redox cycles in terms of soil evolution and environmental impact

The total gains in Fe and respective losses in Mn and Co were calculated by mass balance calculation on the inflow and outflow waters and reported in Table 7. They are several orders of magnitude lower than the stocks of these elements found in the different columns (Table 2) and are thus negligible compared with the amount of these elements initially present in the nodules. This was confirmed by analyses of the solid phase at the end of the experiment, which showed no difference from analyses performed on nodules and quartz sand before the experiment (results not shown). We do not expect much of a change in the forms of Fe and Mn present in nodules, owing to the small magnitude of the gains or losses that occurred throughout the experiment. For this reason we did not perform any sequential extractions after the experiment as was done in some other studies reporting much larger releases of Fe and/or Mn [START_REF] Quantin | Mn-oxide: a major source of easily mobilisable Co and Ni under reducing conditions in New Caledonia Ferralsols[END_REF][START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF]. In these studies the focus was on the fine earth soil fraction. Our results demonstrate that in the case of nodules the rate of release of these elements as a function of redox processes is slow.

Reducing conditions lead to a slow increase in Fe content and dissolution of Mn oxides, which released associated Co. Lead (also contained in Mn oxides of the nodules) was not lost, as it was probably readsorbed on the remaining phases. Despite the over-saturation of the percolating solutions with respect to green rusts, the only Fe oxides identified in these nodules were goethite and ferrihydrite [START_REF] Cornu | Trace element accumulation in Mn-Fe-oxide nodules of a planosolic horizon[END_REF].

We can conclude that the nodules in the studied soils are still active but that their reactivity is very low, as shown by the low amount of Fe and Mn fixed or released with an amount of percolated water that was equivalent to 8 m. However, this study showed that nodules were very efficient Fe scavengers as they fixed 70 to 90% of the Fe initially present in the water (Table 5).

The water running out of the column experiment has concentrations ranging from 0.01 to 0.39 mmol L -1 for Fe, from 0 to 0.43 mmol L -1 for Mn, from 0.01 to 1.43 µmol L -1 for Co and from 10 -4 to 0.07 µmol L -1 for Pb. This water directly runs out in a small river. However, the concentrations in Pb are very low as they never exceed the acceptable limit for drinking water of 0.048 µmol L -1 . This is also the case for Co concentrations, which are lower than the limit concentration for waste water release into the environment. However, Fe and Mn concentrations are generally higher, from one to several orders of magnitude, than the limit given by the European Directive -of June, 16 th 1975 -for surface waters. Thus the only potential threat to the surrounding water will come from Fe and Mn and not from the trace elements. However, as Mn is very sensitive to change in redox conditions, it will precipitate easily and probably shall not reach the river. Indeed [START_REF] Pelfrêne | Spéciation des métaux traces (Cd, Cu, Pb, Zn) dans les eaux d'un Planosol non pollué[END_REF] gives Fe concentrations in water for this horizon that are on the same order of magnitude as the ones obtained in this study, while their Mn concentrations are one order of magnitude lower. At last, only Fe may represent a threat for the environment, as it is the only one exceeding in the soil water the limit concentration given by the European Directive of June, 16 th 1976.

Conclusion

Simulating redox cycles in a laboratory experiment on soil nodules showed that redox conditions similar to those found in soils could be achieved. These conditions allow reduction of Mn oxides, while Fe oxides were still precipitated. Both the experiment and the Eh-pH diagram results were consistent with these findings. The process was relatively slow as only a very small fraction of the Mn and Fe contained in the nodules was either released or fixed.

This suggested that if still active, the process of formation/dissolution of nodules in soils is slow.

These nodules are a sink for trace elements. In this study we showed that while Co was released at the same rate as Mn, Pb also contained in Mn oxides was not released from the column and was probably readsorbed on nodule surfaces. This suggested that the knowledge of the trace element location as determined by sequential extraction is not sufficient to determine the fate of a trace element in the environment. Such extractions must be combined with other types of experiment.

Finally, redox cycles including a drying period would reduce the release of Co, given that Mn oxide formation would occur with concurrent adsorption of Co. Due to technical difficulties in conducting this experiment, only one complete redox cycle could be achieved.

More redox cycles of different duration should be performed in order to be able to predict the impact of waterlogging duration on nodule fate either through agricultural practices (irrigation/drainage) or due to climate change.

In terms of environmental threat this result also suggests that Pb would not be released from soil during nodule dissolution, whereas Co, which is less toxic, would be released.

44

-15 -85 -1.9 Figure 3: Eh-pH stability diagrams for Fe, considering 2 forms of goethite, 2 green rusts and ferrihydrite as Fe oxides, a) for a total Fe concentration in water at 10 -4 mol L -1 and b) for a total Fe concentration in water at 10 -5 mol L -1 .

Figure caption

Figure 4: Eh-pH stability diagram for Mn, considering total Mn concentration in water = 10 -4 mol L -1 and birnessite and lithiophorite as Mn oxides. For lithiophorite, a concentration of Al in solution of 10 -6 mol L -1 was considered. For birnessite a range of stability constant was encountered in the literature (Table 3). The extreme values were used to calculate the stability lines for birnessite and represented as a shaded domain on the figure.

Figure 1 :

 1 Figure 1: Schematic diagram of the experimental design. Qz refers to the quartz columns and C to the nodule ones.

Figure 2 :

 2 Figure 2: Characteristics of the solution percolating through the topsoil monolith as a function of time: A-pH; B-Eh; C-Fe; D-Mn; E-Co; F-Pb. Dark triangles represent the outflow, while open circles are for the inflow.

Figure 5 :

 5 Figure 5: pH, Eh, Fe, Mn, Co and Pb concentrations as a function of the pore volume of the inflow and outflow for the quartz column undergoing permanent saturation. Dark triangles represent the outflow, while open circles are for the inflow.

Figure 6 :

 6 Figure 6: pH, Eh, Fe, Mn, Co and Pb concentrations as a function of the pore volume of the inflow and outflow for nodule columns 1 & 2 undergoing permanent saturation. Dark triangles represent the outflow, while open circles are for the inflow. Black symbols are for the first column, while grey ones state for the second column.

Figure 7 :

 7 Figure 7: pH, Eh, Fe, Mn, Co and Pb concentrations as a function of the pore volume of the inflow and outflow for the quartz column undergoing 14-day cycles. Dark triangles represent the outflow, while open circles are for the inflow.

Figure 8 :

 8 Figure 8: pH, Eh, Fe, Mn, Co and Pb concentrations as a function of the pore volume of the inflow and outflow for the nodule column undergoing 14-day cycles. Dark triangles represent the outflow, while open circles are for the inflow.

Figure 9 :

 9 Figure 9: Iron fixation ratios versus mean Eh values for the different columns.

Table 1 :

 1 Characteristics of the solids used in the experiment †

	Sample name	solid density	BET Surface	Fe	Mn	Co	Pb
		g cm -3	m 2 g -1	g (100g) -1	g (100g) -1	mg kg -1	mg kg -1
	Surface horizon	2.43	nd	3	0.09	13	30
	3-4 mm nodules 2.86 ± 0.00	66 ± 4	22 ± 1	1.45 ± 0.18	144 ± 15	59 ± 2
	4-5 mm nodules 2.80 ± 0.03	60 ± 4	18 ± 1	2.02 ± 0.15	175 ± 34	48 ± 4
	Quartz sand	2.56	0.57 ± 0.09	< DL	<DL	0.54	19.41
	† When available, arithmetic mean values ± standard deviations were provided; nd = no data;
	DL = detection limit

Table 2 :

 2 Main characteristics of the different columns

	Treatment	Columns	3-4mm nodule mass	4-5mm nodule mass	Quartz mass	Solid density	Pore column volume of the	Fe	Mn	Co	Pb
			g	g	g	g cm -3	cm 3	g/column	mg/column
	Saturated	Quartz			96.89	2.56	24	0	0	0.05	1.9
		Nodules 1	34.27	43.53		2.83	34	15	1.4	12	4.1
		Nodules 2	34.2	43.48		2.83	34	15	1.4	12	4.1
	14-day cycles	Quartz			97.145	2.56	24	0	0	0.05	1.9
		Nodule	34.24	43.51		2.83	34	15	1.4	12	4.1

Table 3 :

 3 Equilibrium reactions and associated solubility constants at 25°C and 1 bar, used to construct the Eh-pH diagrams for the different minerals found in the nodules

	Minerals	Reactions		log K References
	γgoethite	γFeOOH + 3H + + e -↔Fe 2+ + 2H 2 0	16.65 Hashimoto et
							Misawa (1973)
	αgoethite	αFeOOH + 3H + + e -↔Fe 2+ + 2H 2 0	14.97 Détournay et al.
							(1975)
	green rust	Fe(OH) 3 + 3H + + e -↔Fe 2+ + 3H 2 0	15.87 Trolard et Bourrié
							(1999)
	green rust	Fe 3 (OH) 8 + 8H + + 2e-↔ 3Fe 2+ + 8H 2 0	46.41 Ponnamperuma et
							al. (1967)
	ferrihydrite	Fe 2 O 3 (0.5H 2 O) + 6H + + 2e -↔ 2Fe 2+ + 3.5H 2 0	34.778 Rabenhorst et
							Castenson (2005)
	birnessite	Mn 5	4+ Mn 2	3+ O 13 + 26H + + 12e -↔7 Mn 2+ +	249.84	after Parc et al.
		13H 2 O			to	(1989)
						258.6
	lithiophorite Al 2 Mn 3	4+ O 9 + 18H + + 6e -↔2Al 3+ + 3Mn 2+ +	140.13 after Parc et al.
		9H 2 O				(1989)

Table 4 :

 4 Average values for the pH and Eh values and Fe, Mn, Co and Pb concentrations according to the water type considered and Fisher's pairwise mean comparison results (ANOVA) , c, d are indicating wether the average are significantly different or not among the different samples with a confident level of 5%

	pH	Eh	Fe	Mn	Co	Pb

Table 5 :

 5 Average fraction fixed/released from the different columns for Fe, Mn and Co and Fisher's pairwise mean comparison results , b, c, d are indicating wether the average are significantly different or not among the different samples with a confident level of 5%

	(ANOVA)
			Fixed Fe	Released Mn	Released
			fraction	fraction	Co fraction
	Continuously
	saturated	Quartz	0.522 c	0.139 bc	0.126 c
		Nodule 1	0.660 b	0.301 bc	1.630 b
		Nodule 2	0.647 b	0.581 a	2.383 a
	14-day cycles	Quartz before drying	0.440 c	0.158 bc	0.182 c
		Quartz after drying	0.800 ab	-0.186 c	-0.012 c
		Nodule before
		drying	0.760 ab	0.394 ab	1.417 b
		Nodule after drying	0.885 a	0.373 abc	2.864 a

a

Table 6 :

 6 Amount of leachate collected from the different columns

	Treatment	Column	Percolated	Percolated	Percolated
			volume (mL)	volume	volume during
				(pore volumes)	the cycle (mL)
	Saturated	Quartz	5704	238	987
		Nodule 1	6183	182	1440
		Nodule 2	6565	187	1411
	14-day cycles Quartz	3970	165	0
		Nodule	4225	124	0

Table 7 :

 7 Total Fe, Mn, Co and Pb gained or lost from the different columns at the end of the experiment, calculated from mass balance calculation on the inflow and outflow waters.

	Treatment type	Column type	Fe	Mn	Co	Pb
			mg	mg	µg	µg
	Continuously saturated	Quartz	31	-8.4	8.3	1
		Nodule 1	69	-11	-102	3.7
		Nodule 2	61	-28	-169	1.4
	14-day cycles	Quartz before drying	15	-6.7	-8.8	0.003
		Quartz at the end of the	25	-5.5	-8.6	0.04
		experiment
		Nodule before drying	27	-11	-57	0.4
		Nodule at the end of the
		experiment

Acknowledgments

The authors would particularly like to thank O. Josière, B. Renaux, P. Courtemanche, A. Besnault and C. Le Lay for technical support. This study was funded by the Conseil Régional of Région Centre through the METALOE project and benefited from a STUDIUM fellowship. J.Cattle is grateful to INRA, Orleans, for providing funding for this project.