Camille Francois
email: camille.francois@u-bourgogne.fr

Vincent Placet
email: vincent.placet@univ-fcomte.fr

Yves Gaillard
email: yves.gaillard@univ-fcomte.fr@www.femto-st.fr

Sylvie Pourchet
email: sylvie.pourchet@u-bourgogne.fr

Gilles Boni
email: gilles.boni@u-bourgogne.fr

Laurent Plasseraud
email: laurent.plasseraud@u-bourgogne.frwww.icmub.fr

Pierre Ouagne
email: pierre.ouagne@enit.fr@www.enit.fr

Joseph Samec
email: joseph.samec@kemi.uu.se

Sari Rautiainen
email: sari.rautiainen@su.se

Lucie Fournier
email: lucie.fournier@chimie-paristech.fr

C François

C Robert

C M Thomas

S Fontaine

Design and development of 100 % bio-based high-grade hemp/epoxy composites

Keywords: Bio-composite, Hemp fibres, Bio-based epoxy thermosets, Mechanical performances

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

INTRODUCTION

Today, the development of bio-based products, substituting petroleum-based materials has become a major focus of academic or industrial scientists. To face the depletion of fossil resources and to preserve the environment, this change is essential. This growing awareness is especially strong in composites sectors, in particular in the field of transport, sport and leisure activities. Traditional composites made of synthetic fibres impregnated with a petrochemical matrix are left out in favour of materials extracted from biomass. A totally bio-based composite can be defined as a material made of a natural reinforcement and a polymer matrix fully synthesized from biomass.

This study focuses on developing composites with unidirectional hemp fibres and bio-based epoxy matrix for secondary structural applications. To meet the requirements of such applications, the constituents have to reach a certain level of performance. For these reasons, in this work, we proposed to work with unidirectional non-twisted hemp yarns and epoxy polymers relating to their good mechanical properties.

One of the main issue is to substitute petrochemical resins. In fact, the most commonly used epoxy resin is a diglycidylether of bisphenol A (DGEBA). The replacement of DGEBA is of great concern to the polymers field because it is a petroleum-based epoxy resin and bisphenol A-based material have been suspected to be toxic for health [START_REF] Fenichel | Bisphenol A: an endocrine and metabolic disruptor[END_REF][START_REF] Michalowicz | Bisphenol A-sources, toxicity and biotransformation[END_REF]. Over the past decades more and more researchers have been engaged in developing bio-based resins to replace DGEBA [START_REF] Fache | Epoxy thermosets frommodel mixtures of the lignin-tovanillin process[END_REF][START_REF] Baroncini | Recent advances in bio-based epoxy resins and bio-based epoxy curing agents[END_REF]. This study focuses on processing composites with bio-based epoxy matrix and unidirectional hemp reinforcement.

The precursor used to the polymerization reaction is the Eugenol. It is extracted from cloves and then converted into its diglycidylether derivative (DGE-Eu). The synthesis and characterization of this biobased epoxy resin was described by Qin & al. [START_REF] Qin | Use of eugenol and rosin as feedstocks for biobased epoxy resins and study of curing and performance properties[END_REF]. Recently we resolved the crystal structure of DGE-Eu by single-crystal X-ray diffraction [START_REF] Vigier | Crystal Structure of the Diglycidyl Ether of Eugenol[END_REF]. In this work, the prepolymer was cured with several acids anhydrides to obtain polyepoxides with good properties in order to process hemp fibres composites. The reinforcement was processed using different technologies to produce individualized and aligned hemp fibres in the form of combed sliver laps. This reinforcement structure ensure a maximisation of the mechanical properties at the scale of the composite material. In our ongoing studies, we herein report the polymerization and the characterization of DGE-Eu-based polyepoxides cured with different acids anhydrides and the processing of hemp/epoxy bio-based composites. We also evaluate the performance of the bio-based epoxy matrix, the sliver laps of hemp fibres and the final composite for structural applications. The structural performance of DGE-Eu are compared with those obtained with the DGEBA petro-sourced resin.

MATERIALS AND METHODS

Materials

The polymer matrix used in this project is a polyepoxide, which was obtained from the reaction of the prepolymer DGE-Eu with acids anhydrides hardeners. A commercial anhydride was purchased from Sigma-Aldrich, 1,2-cyclohexanedicarboxylic anhydride (HHPA, 95% purity) and the second, maleopimaric anhydride (MPA) was prepared according to published procedures [START_REF] Qin | Use of eugenol and rosin as feedstocks for biobased epoxy resins and study of curing and performance properties[END_REF]. As the prepolymer, MPA comes from biomass. The catalyst, 1-Ethyl-4-methylimidazole (EMID; 95%), was purchased from Sigma-Aldrich.

Long hemp fibres in the form of combed sliver laps were provided by an Italian company. Their linear mass was about 15000 tex.

Processing of polyepoxides

The bio-based epoxy thermosets studied in this work are composed of DGE-Eu and anhydrides hardeners. Two different anhydrides are used. MPA is synthesized from biomass (rosin derivatives) and HHPA is a commercial petroleum-based anhydride. The protocol for curing the epoxy monomers was the same as the one already described by Qin et al. [START_REF] Qin | Use of eugenol and rosin as feedstocks for biobased epoxy resins and study of curing and performance properties[END_REF]. The molar ratio corresponding for the epoxy/anhydride/catalyst combination was 1-0.8-0.012 with MPA and 1-0.8-0.08 with HHPA. These ratios are not in stoichiometry proportion because of simultaneous reactions [START_REF] Dusek | Network formation in curing of epoxy resins[END_REF].

Processing of hemp fibres

Combed sliver laps were drawn to obtain slivers with a linear mass of about 2200 tex, and then obtained a size of reinforcement compatible with expected applications [START_REF] Placet | Industrial hemp transformation for composite applications: influence of processing parameters on the fibre properties[END_REF].

Processing of bio-composites

The manufacturing of bio-composites was performed by thermocompression method. They were heated at 120°C under a pressure of 0.25 bar. These parameters have been selected based on the features of the polymerization reaction of the epoxy matrix. The dimensions of the manufactured specimens are approximately 50 × 10 × 1.5 mm. The fibres mass fraction was around 23%. DSC analyses were carried out using TA MDSC 2920 under nitrogen flow (60 mL.min -1 ) with a sample mass of 10 ± 3 mg. To study curing reaction, samples were heated from -20 °C to 180 °C at a heating rate of 5 °C min -1 . To determine glass transition temperature (Tg), the cured resin was heated from -20 °C to 180 °C at a heating rate of 20 °C min -1 . The two combinations of polyepoxide were characterized using this technique.

Thermogravimetric analysis (TGA)

Thermogravimetric analyses were performed on a TA Instruments TGA Q600 thermoanalyser using aluminium pans. Samples (5-10 mg) were heated from room temperature to 800 °C at a rate of 20 °C min -1 under air flow (100 mL min -1 ). Weight loss percentages were determined using the TA Universal Analysis 2000 software accompanying the instrument. The two combinations of polyepoxide were characterized using this technique.

Tensile tests on elementary fibres

Tensile tests were performed with a dynamic mechanical analyser (DMA Bose Electroforce 3230). Thirty to fifty single fibres, manually extracted from combed sliver laps, with a diameter of about 20 microns were tested at a constant crosshead displacement rate of 5 µm.s -1 . A gauge length of 10 mm was chosen. The tensile tests were carried out at a temperature of 21°C ± 2°C and a relative humidity of 50 % ± 5%. The paper frame supporting each fibre was clamped onto the testing machine and was cut before initiating each test [START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF]. The applied force was measured using a 20 N load sensor, with a resolution of approximately 1 mN, and the displacement was measured using a LVDT with a resolution ranging from 0.1 µm. The DMA chamber was pair up with a HumiSys humidity generator from InstruQuest INC to control the relative humidity. Longitudinal mechanical properties (Young's modulus, ultimate strength and failure strain) of hemp fibres were determined. The fibre strain was determined using the displacement measurements and the initial length of the fibre, the tensile stress was determined using the cross-section of each fibre and the Young's modulus was computed from the initial linear section of the stress-strain curve. The cross-section area of the fibre was approximated using the average diameter and considering the fibre as perfectly cylindrical and the lumen neglected.

Flexural tests on thermosets and bio-composites

Flexural strength and stiffness were determined for the DGE-Eu-based thermosets and the DGE-Eubased hemp fibres reinforced composites, using three-point bending test method. Tests were performed on an electodynamical testing machine (DMA Bose Electroforce 3230). The span between supports was 40.0 mm and the crosshead speed was 10 µm.s -1 . Flexural strength is calculated by the following equation.

(

) 1
Where F is the maximum load (N); L is the distance between the supports or span (mm); b is the width (mm) and h is the thickness (mm) of the specimen.

Flexural modulus is determined from the initial part of the stress-deflection curve. One specimen of DGE-Eu polyepoxide cured with HHPA and three of hemp fibres reinforced composites were tested.

RESULTS AND DISCUSSIONS

Characterization and performance evaluation of bio-based epoxy thermosets

The values of glass transition temperatures for each polyepoxide combination are reported in the table 1. They are indicators of the level of stiffness of the material and they depend on both the chemical structure of the polymer chain and the architecture of the polymer. We can note that the glass transition temperatures for bio-based epoxy resin (DGE-Eu) are in the same order of magnitude as values for DGEBA-based polymers (Tab. 1) [START_REF] Qin | Use of eugenol and rosin as feedstocks for biobased epoxy resins and study of curing and performance properties[END_REF][START_REF] Vigier | Crystal Structure of the Diglycidyl Ether of Eugenol[END_REF][START_REF] Dusek | Network formation in curing of epoxy resins[END_REF][START_REF] Placet | Industrial hemp transformation for composite applications: influence of processing parameters on the fibre properties[END_REF][START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF][START_REF] Hussain | Toughening if anhydride cured epoxy resins[END_REF]. We can see that MPA gives a better stiffness to the thermosets than HHPA. Thermogravimetric analyses point out a degradation temperature corresponding to 5% weight loss of around 317°C and 321°C for DGE-Eu/MPA and DGE-Eu/HHPA polyepoxides respectively. The origin of the thermosets, bio-based or petroleum-based do not significantly influence the thermal degradation temperature. Indeed, there is no significant variation between combination with MPA or HHPA. Moreover, this result also applies to fully petroleum-based epoxy polymers [START_REF] Su | Thermal properties of phtalic anhydride-and phenolic resincured rigid rod epoxy resins[END_REF]. The values for the degradation temperature corresponding to 5% weight loss are about 320°C.

Flexural tests were performed on DGE-Eu/HHPA polyepoxide in order to evaluate the properties of the matrix used to develop composite materials. As well as its good properties, this combination exhibits a liquid physical aspect, which facilitates the manufacturing of bio-composites. The DGE-Eu/HHPA epoxy thermosets show a flexural strength of about 90 MPa and a stiffness of approximately 3.6 GPa. They are in good accordance with typical values encountered for classical polymer materials [START_REF] Lee | Handbook of Epoxy Resins[END_REF].

Tensile characterization of hemp fibres

Plant fibres exhibit generally a high scattering in their mechanical properties that need to be contained by controlling the processing stages. The tensile properties of single hemp fibres with an average diameter of 20.2 ± 4.5 µm, taken from the slivers (2200 tex) are reported in the table 3. Results show that their stiffness is about 18 GPa, the mean strength is 325 MPa and the strain at failure is approximately equal to 2.05 %. Their mechanical properties are lower than those already reported in literature [START_REF] Placet | The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres[END_REF][START_REF] Marrot | Analysis of the hemp fiber mechanical properties and their scattering (Fedora 17)[END_REF]. This might be explained by the process used for their transformation into slivers.

Processing and characterization of bio-composites

DGE-EU/HHPA hemp reinforced composites were tested using flexural characterization. Stressdeflection curves are reported in Figure 3. Results show a mean strength of 171 ± 19 MPa and a mean stiffness of 8.8 ± 1.5 GPa for an average fibre volume fraction of 22.4 ± 1.6 % (Tab. 4). This volume fraction is low due to the small quantities of the synthetized polymers, which make possible only very small composite specimens to be manufactured. In such conditions, the control and tuning of constituents volume fractions are particularly difficult during the manufacturing process. In any case and as expected, we observe that incorporating hemp slivers in the bio-based matrix greatly enhances its mechanical properties. Indeed, the strength is improved by 90% and the apparent modulus by 144%. In comparison to the previous work of Islam & al. [START_REF] Islam | The Effect of Accelerated Weathering on the Mechanical Properties of Alkali Treated Hemp Fibre/Epoxy Composites[END_REF], we note that bending properties are in the same order of magnitude but with a lower fibre ratio. It is foreseeable that the mechanical properties of the manufactured 100% bio-based composite will be significantly improved when increasing the fibre ratio. 4 synthetizes the mechanical properties experimentally determined and the estimation made using the ROM (Rule Of Mixture) and the properties of the fibres and matrix. This calculation does not take into account the composite's porosity. We can observe that the experimental values are higher than the ones estimated using the ROM. This difference can certainly be attributed to the error made when determining the tensile properties of individual fibres, and in particular due to the over-estimation of the fibre cross-section area (due to lumen neglecting). This is promising results. High and interesting properties should be reached by increasing the fibres volume fraction.

Strength

CONCLUSIONS

The development of fully bio-based epoxy resins from Eugenol is an interesting alternative to the DGEBA-based resins. This work shows that, in addition to its bio-based origin, it exhibits physicochemical and mechanical properties that meet the needs for secondary structural applications. It makes these bio-based polymers an excellent alternative to petrochemical resin. Results also show, at the scale of composite material reinforced with unidirectional hemp fibres, mechanical properties in the same order of magnitude as some of the petroleum-based matrices reinforced with different natural fibres exposed in the literature. Therefore, these results are very promising for the development of fully bio-based epoxy matrix for composites applications. In our ongoing studies on fully bio-based composites, we work to the development of bio-composites with a prepolymer extracted from lignin and anhydrides derived from plant-based acids [START_REF] Francois | Diglycidylether of iso-eugenol: a suitable lignin-derived synthon for epoxy thermoset applications[END_REF].

Figure 1 :

 1 Figure 1: Constituents of bio-based epoxy thermosets

Figure 2 :

 2 Figure 2: Manufacturing of bio-composites studied in this work

Figure 3 :

 3 Figure 3: Stress-Deflection curve of bio-based thermoset and composite Table4synthetizes the mechanical properties experimentally determined and the estimation made using the ROM (Rule Of Mixture) and the properties of the fibres and matrix. This calculation does not take into account the composite's porosity. We can observe that the experimental values are higher than the ones estimated using the ROM. This difference can certainly be attributed to the error made when determining the tensile properties of individual fibres, and in particular due to the over-estimation of the fibre cross-section area (due to lumen neglecting). This is promising results. High and interesting properties should be reached by increasing the fibres volume fraction.

Table 1 :

 1 Values of glass transition temperatures for all polyepoxides combinations

		Glass transition temperature (Tg °C)
	Epoxy resin	DGE-Eu DGEBA [5] DGEBA [10]	MPA HHPA HHPA MTHPA	155 114 106 129

Table 3 :

 3 Tensile properties of single hemp fibres from combed sliver laps compared to data from literature

Table 4 :

 4 Average tensile properties determined experimentally and using the Rule Of Mixture

			Stiffness	Fibres
		(MPa)	(GPa)	content
	Experiments (mean value ± Std Deviation)	171 ± 19	8.8 ± 1.5	22.4 ± 1.6
	Estimation using ROM	143	6.9	22.5

ACKNOWLEDGEMENTS

The authors are grateful for financial supports from the Regional Council of Burgundy (PhD scholarship for CF) and the University of Bourgogne Franche-Comté (BQR PRES 2014-19 project). The French Biobased Laminates Research Consortium (BioLAM) is also thanked for the fruitful and critical discussions.


Eric Boonaert

Alain Valtz

Christophe Coquelet
email: christophe.coquelet@mines-paristech.fr

Vapour-liquid equilibria of n-butane and ethyl mercaptan: experiments and modelling

Keywords: Equation of state, Static analytic method, data treatment, gas processing

Design of debutaniser in fractionation train in gas processing requires accurate knowledge of phase equilibrium properties of n-butane with sulphur compounds like mercaptan. In this paper, we report high-quality isothermal vapor-liquid equilibrium data for n-butane + ethyl mercaptan (or ethanethiol) between 298.14 K to 388.18 K and pressures up to 2.0029 MPa. An equipment whose experimental technique is based on static-analytic method was considered. The equipment is composed by an agitated equilibrium cell with two online micro samplers connected to a Gas Chromatograph. The data was correlated with the Peng Robinson equation of state with classical alpha function. A comparison was also performed with two predictive models, PPR78 and PSRK UNIFAC.

Introduction

Worldwide total energy requirements are increasing and natural gas appears to be the best temporary solution in the context of the energy transition. In order to have a safe and environmentally-friendly production, challenges related to processing and environmental protection arise. The presence of significant quantities of sulphur compounds in the exploited gas resources is one of these challenges. Mercaptans are sulphur compounds are present in natural gas. One major operation in Gas processing is to remove the acid gases (in an acid gas removal unit) [START_REF] Kohl | Gas purification[END_REF]. During this operation some mercaptans are also removed. Hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) are removed by amine treating but the majority of amine solvents remove few to no mercaptans [START_REF] Stewart | Gas sweetening and processing field manual[END_REF]. Mercaptans may then be removed in dehydration units. Any remaining mercaptan in the natural gas tends to accumulate in hydrocarbon condensates After dehydration, fractionation is performed. A fractionation train is composed of a series of distillation columns; a deethaniser, a depropaniser and debutaniser. Several products can be obtained: GNL, gaseous methane, Ethane/Propane mixtures (EP), Commercial Propane, Propane/Butane mixture (LPG), Butane and isobutene, Natural Gasoline and Mixtures with a vapour pressure specification. Mercaptans are not the only contaminants: Hydrogen sulphide, Carbon dioxide, Carbonyl sulphide, Carbon disulphide, Organic sulphides, Nitrogen, Water, methanol (prevention of hydrate formation) can also be present. Determining thermodynamic properties of systems composed of mercaptans and hydrocarbons is required to better understand how organic sulfur compounds in raw feed gas are distributed in the products of NGL fractionation systems. The main properties of interest are vapour-liquid equilibria in the range of pressure and temperature of the operating conditions in the train of fractionation. In the last 6 years, our research group has published VLE data of binary and ternary systems involving mercaptans (without water) in several articles: Awan et al. [START_REF] Awan | Phase Equilibria of Three Binary Mixtures: Methanethiol + Methane, Methanethiol + Nitrogen, and Methanethiol + Carbon Dioxide[END_REF] (Phase Equilibria of Three Binary Mixtures: Methanethiol + Methane, Methanethiol + Nitrogen, and Methanethiol + Carbon Dioxide), Awan et al. [START_REF] Awan | Phase Equilibrium Measurements and Modeling of 1-Propanethiol+ 1-Butanethiol+ CH4 in Methane Ternary System at 303, 336, and 368 K and Pressure Up to 9 MPa[END_REF] (1-Propanethiol+ 1-Butanethiol+ CH 4 ), Afzal et al. [START_REF] Afzal | Vapour-Liquid Equilibria of Ethane and Ethanethiol: Experiments and Modelling[END_REF] (Ethane + Ethyl mercaptan). Mercaptans have similar molecular structure to alcohols with the oxygen (O) atom replaced by a sulfur (S) atom. This change gives mercaptans a more acidic behaviour compared to their alcohol counterparts. Mercaptans are only slightly acidic, and this acidity decreases with their molecular weight. Moreover, the knowledge of thermodynamic properties of mixtures of sulfur compounds with hydrocarbons is not only important in gas processing but also in the petroleum and chemical industries. The thermophysical properties and equilibrium data are important for the rational design of processes for the removal of sulfur compounds from petroleum streams but also for the purification of sulfur compounds for use in chemical processes. Because all of these sulphur compounds are highly toxic and volatile, obtaining reliable thermodynamic data like vapour-liquid equilibrium (VLE) data is not straight-forward. The Centre of Thermodynamics of Processes (CTP) at MINES ParisTech is one of the few places where toxic fluids can be studied to produce reliable data. About the binary system ethylmercaptan n-butane, the authors have found one set of data in the open literature from Giles and Wilson [START_REF] Giles | Phase equilibria on seven binary mixtures[END_REF]. It concerns PTx data obtained using a synthetic method.

The classical Peng Robinson Equation of state [START_REF] Peng | A new two-constant equation of state[END_REF] with its classical mixing rule was used to correlate the data. A comparison was performed with two predictive models: PPR78 [START_REF] Privat | Addition of the sulfhydryl group (-SH) to the PPR78 model (predictive 1978, Peng-Robinson EOS with temperature dependent kij calculated through a group contribution method)[END_REF] and PSRK UNIFAC [START_REF] Horstmann | PSRK group contribution equation of state: revision and extension III[END_REF].

Materials and methods

Materials

In Table 1 are listed the chemicals used for the VLE measurements. Research grade n-butane with minimum purity of 99.95 % (obtained from Air Products) and ethyl mercaptan with minimum purity 99 % (obtained from Acros) were used. No further purification of the chemical products was needed, only degassing when loading the chemicals into the equilibrium cell.

2.2.Methods

In order to measurements in the whole range of pressure, we have used two different equipments with an experimental technique based on the analytic-static method.

At low pressure, we have used an equipment previously presented and described by Zhang et al. [START_REF] Zhang | An improved static-analytic apparatus for vapor-liquid equilibrium (PTxy) measurement using modified in-situ samplers[END_REF] and Théveneau et al. [START_REF] Théveneau | Vapor Liquid Equilibrium Data for the Furan-Toluene Binary System between 313[END_REF]. The equilibrium cell is immersed in a thermoregulated liquid bath (LAUDA Proline RP 3530 C). Two (Pt-100) platinum probes are used to measure the temperature at the bottom and top of the cell. The two probes were calibrated by comparison with a 25 Ω reference platinum probe (Tinsley, France). The accuracy of the two probes was estimated to ± 0.02 K. Also, the pressure was measured by one pressure transducer (General Electric, model UNIK 5000) with a maximum absolute pressure of 10 bar. The pressure transducer is maintained at a constant temperature (353 K) by means of a PID regulator (FUJI, model PXE-4). The accuracy of the pressure transducer was estimated to ± 3 mbar after calibration. Both temperature and pressure signals were transmitted to a data acquisition unit (Agilent 34972A). Two samplers are connected to the equilibrium cell in order to take samples of vapor and liquid phases. Liquid and vapor samples were analyzed by means of a gas chromatograph (Perichrom, model PR-2100). A thermal conductivity detector (TCD) was calibrated and used to determine the molar composition of the two phases.

At high pressure (P>0.5 MPa), we have used an equipment similar to the equipment used by Afzal et al. [START_REF] Afzal | Vapour-Liquid Equilibria of Ethane and Ethanethiol: Experiments and Modelling[END_REF]. Briefly, the equilibrium cell consists of one sapphire tube, held between two flanges in Hastelloy with suitable o-rings. On each flange are located valves for loading, vaccum and cleaning of the cell. An agitation assembly is installed in the sapphire tube for the stirring of the two phases with two propellers. The agitation assembly is magnetically coupled to an external agitation motor capable of producing the desired level of agitation inside the cell.

Like the low-pressure equipment, two calibrated 100 Ω platinum resistance thermometer sensors (Pt-100) are used to measure equilibrium temperatures at the lower and upper parts of the cell. The accuracy of the two probes was estimated to ± 0.03 K. The pressures are measured using one calibrated Druck pressure transducer (Druck, model PTX 611) maintained at constant temperature (higher than the maximum temperature of study). The temperature is controlled by a PID regulator (WEST instrument, model 6100). The accuracy of the pressure transducer was estimated to ± 9 mbar after calibration. Two capillary samplers (ROLSI®, Armines's patent) were vertically mounted on the equilibrium cell, one for liquid phase and the other for vapour phase. The samplers are connected to the Gas chromatograph. A thermal conductivity detector (TCD) is used for analysis.

The two GC of the two equipments were equipped with a 4-meter long packed Porapack Q column. The WINILAB III (Perichrom, France) data acquisition software was used for peak integration. The relative accuracies of mole numbers are ±1.1% for n-butane and ±2.4 for ethylmercaptan. Consequently the maximum calibration uncertainty of mole fractions, calculated at x butane = 0.5 is u max (x,y) = 0.007. At equilibrium conditions, each composition was determined 6-8 times for each phase and average values are reported along with relative standard deviation (σ). Equation 1reminds the expression of the composition of component i (ncomp is the number of component in the mixture).

  ncomp i i i i n n x ( 1
)
Equation 2 is used for the calculation of the uncertainty of mixture composition of component i.

                 ncomp j j i j j i i cal n u n x x u 2 2 (2)
Consequently, for a binary system, one can calculate uncertainty on mole fraction x 1 by using equation 3.

        2 2 2 2 1 1 1 1 1 1                    n n u n n u x x x u cal (3)
Equation 4 reminds the combined standard temperature uncertainty on molar composition.

      2 2 i rep i calib i x u x u x u    (4)
u rep is obtained by considering the standard deviation (σ) accounting for the repeatability of the measurements. The experimental results are presented in Table 2 with the calculated uncertainties.

Results and discussion

Data treatment

The critical temperatures (Tc), pressures (Pc) and acentric factors ( ) for pure n-butane and Ethylmercaptan are provided in Table 3. The experimental VLE data of (ethane + Ethylmercaptan) system were correlated using the Peng Robinson equation of state (Peng and Robinson, 1978) with the classical van der Waals mixing rules (model 1). PPR78 and PSRK Original alpha function is considered. Simulis Thermodynamics™ software by PROSIM (France) was used. k ji is adjusted directly to the VLE data using the objective function given in Eq. ( 5). Calculations are based on a bubble point algorithm. The parameters are presented in Table 4 and experimental and modelling results are presented on Fig. 1. Relative volatility is presented on Fig. 2. We can see on figure 2 that the evolution of relative volatility follows the good trend.

(5) The variance of the binary interaction parameter was calculated using experimental data.

Considering the objective function f given in Eq. ( 6), the variance is calculated using Eq. ( 7).

The variance of k ij is given by Eq. 8.

(

With the residual variance. We have done the calculation at each temperature. The standard deviation for the binary interaction is the square root of the variance. Its value is almost constant and equal to 0.004 (k=2). The binary interaction parameters are plotted on Fig. 3. In order to evaluate the quality of the data treatment, the Mean Relative Deviation (MRDU), and the BiasU were calculated based on pressures and vapour phase mole fractions, as defined in Eqs. ( 9) and ( 10).

where N is the number of data points, and U = P or y 1 . The MRDU and BiasU indicators, which give information on the agreement between model and experimental results, are presented in Table 5. The Gibbs-Duhem equation (Eq. ( 11)) is considered to examine the consistency of the data. Temperature /K

Where v E and h E are the excess volume and excess enthalpy respectively and  i the activity coeffcient of component i. At constant temperature and after integration, Eq. ( 12) is obtained. [START_REF] Van Ness | Vapor-liquid equilibrium: Part I. An appraisal of data reduction methods[END_REF] By neglecting the excess volume v E , we obtained Eq. ( 13) wich can be used to test the consistency of our data: consistent data means A=0.

(

) 13
The activity coefficient is estimated from Eq. ( 14). This equation can only be considered for low pressure data. Consequently, it is difficult to use the test with our data.

(

) 14
Where is the pure component vapor pressure of component i. Therefore, the van Ness test (van Ness et al. [START_REF] Van Ness | Vapor-liquid equilibrium: Part I. An appraisal of data reduction methods[END_REF]) was preferred, using our model and our data treatment. This test, also called modelling capability test consists of applying a thermodynamic model and calculating pressure and vapour deviations as defined by Eqs. ( 15) and ( 16).

(15) (16) where N is the number of data points. If and are less than 1, the test is validated. The results of the van Ness test are presented in Table 5. As we can see, our data passes the test and so consistency of our data is validated.

Comparison with literature data

We have compared our results with literature data (Giles and Wilson [START_REF] Giles | Phase equilibria on seven binary mixtures[END_REF]) and two predictive models, PPR78 and PSRK UNIFAC. Using literature data, obtained by a synthetic method (PTx data) as explained in the paper, we have applied the Eq. ( 17) to estimate the value of the binary interaction parameter as a function of temperature. The parameters of the Eq. ( 17) are determined by using parameters presented in Table 3.

(17) Figure 4 presents the comparison and Table 6 the Mean Relative Deviation (MRDU), and the BiasU, applied on equilibrium pressure. The deviations are less than 1% and so the two sets of data, from literature and this work are in very good agreement.

Conclusion

Isothermal (P-x-y) VLE data for n-butane + Ethylmercaptan binary system were measured at four temperatures ranging from 298.14 K to 388.18 K using a static-analytic method. Two equipments for low and high pressure measurements were used to generate the data. The measured data are in good agreement with literature data from Gils and Wilson [START_REF] Giles | Phase equilibria on seven binary mixtures[END_REF] and are very well predicted using PPR78 and PSRK models. This work is in the continuity of experimental and modelling work realised in the Centre of Thermodynamics from Mines ParisTech on the study of binary system phase diagrams involving sulphur components for gas processing industries.

List of tables

Fig. 1 :

 1 Fig. 1: Pressure as a function of n-butane mole fraction in the n-butane (1) + Ethylmercaptan (2) mixture at different temperatures. : 298.14 K,  : 328.04 K, ■: 358.11 K, +: 388.18 K. Dashed lines: calculated with PR EoS with binary intercation parameters from Table 4.

Fig. 3 :

 3 Fig. 3: Evolution of the Binary interaction parameter k ij as a function of temperature. Error bar: 0.004.

Fig. 4 :

 4 Fig. 4: Pressure as a function of n-butane mole fraction in the n-butane (1) + Ethylmercaptan (2) mixture at 323.15 (•) and 373.15 K (Δ) and comparison with the data from Giles and Wilson [6].

Fig. 3 :Fig. 4 :

 34 Fig. 3: Evolution of the Binary interaction parameter k ij as a function of the temperature. Error bar: 0.004.Fig. 4: Pressure as a function of n-butane mole fraction in the n-butane (1) + Ethylmercaptan (2) mixture at 323.15 and 373.15 K and comparison with the data from Giles and Wilson [6].

Table 1 . Purities and suppliers of the chemicals used in this work Chemicals Purity (GC) Supplier

 1

	n-butane	99.95 vol%	Air Products
	Ethylmercaptan	≥ 99 vol%	Acros

Table 2 : Experimental isothermal VLE data for the n-Butane (1) + Ethylmercaptan (2) mixture system and their standard uncertainties. Maximum calibration uncertainty on composition u(x,y, k=2)=0.008.

 2

			T: 298.14 K	u(T, k=2)= 0.02 K u(P, k=2) = 0.0004 MPa
	P/ MPa	n	x 1	x 1	u(x 1 )	n	y 1	y 1	u(y 1 )
	0.0942	7	0.0644	0.0007	0.0009	5	0.2932	0.0007	0.003
	0.1261	7	0.161	0.003	0.002	5	0.5174	0.0002	0.004
	0.1439	7	0.236	0.003	0.003	5	0.6041	0.0005	0.004
	0.1691	7	0.387	0.002	0.004	5	0.7158	0.0008	0.003
	0.183	8	0.484	0.004	0.004	5	0.7558	0.0007	0.003
	0.2044	11	0.666	0.007	0.004	6	0.8308	0.0002	0.002
	0.2126	13	0.733	0.006	0.003	8	0.8598	0.0002	0.002
	0.2226	3	0.8358	0.0002	0.002	5	0.904	0.002	0.001
	0.2394	5	0.9606	0.0002	0.0006	4	0.9794	0.0001	0.0003
			T: 328.04 K	u(T, k=2)= 0.02 K u(P, k=2) = 0.0004 MPa
	0.2300	6	0.0411	0.0002	0.0006	5	0.1845	0.0002	0.002
	0.2395	6	0.0513	0.0006	0.0008	6	0.2122	0.0006	0.003
	0.2752	6	0.0965	0.002	0.001	5	0.3525	0.0004	0.004
	0.3030	6	0.149	0.001	0.002	6	0.437	0.001	0.004
	0.3181	5	0.176	0.003	0.002	5	0.4835	0.0003	0.004
	0.3409	6	0.226	0.001	0.003	6	0.539	0.001	0.004
	0.3632	8	0.282	0.005	0.003	6	0.5892	0.0004	0.004
	0.3947	5	0.360	0.002	0.004	5	0.6466	0.0002	0.004
	0.4180	5	0.428	0.004	0.004	4	0.692	0.002	0.003
	0.4212	4	0.4353	0.0003	0.003	4	0.6964	0.0007	0.002
	0.4423	7	0.505	0.003	0.004	7	0.7374	0.0006	0.003
	0.4596	4	0.5569	0.0004	0.003	6	0.760	0.002	0.002
	0.5188	5	0.775	0.002	0.002	6	0.862	0.005	0.001
	0.5333	4	0.8298	0.0003	0.002	5	0.892	0.004	0.001
	0.5475	5	0.8893	0.0008	0.001	5	0.9284	0.0009	0.0008
	0.5573	5	0.9477	0.0003	0.0006	6	0.9632	0.0003	0.0004
			T: 358.11 K	u(T, k=2)= 0.02 K u(P, k=2) = 0.0009 MPa

Table 3 . Thermal properties for ethane and Ethylmercaptan pure components (Source Simulis Thermodynamics™)

 3

	Component	T c /K	P c /MPa	Acentric factor ω
	n-butane	425.12	3.796	0.200164
	EthylMercaptan	499.15	5.490	0.187751

Table 4 .

 4

		2.5
		2
	MPa	1.5
	Pressure/	1
		0.5
		0
		0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1
							x1, y1

Fig. 2: Relative volatility of the n-butane (1) + Ethylmercaptan (2) binary system at different temperatures. : 298.14 K,  : 328.04 K, ■: 358.11 K, +: 388.18 K.

Dashed: calculated with PR EoS with binary interaction parameters from Table 4.

Table 4 : Values of the binary interaction parameters and objective function at each temperature.

 4

	T/ K	k 12	F
	298.14	0.035	0.001
	328.04	0.044	0.002
	358.11	0.037	0.001
	388.18	0.033	0.001

Table 5 : Mean Relative deviation MRDU and BiasU obtained in fitting experimental VLE data with PR EoS and Van Ness test.

 5

						Van Ness test
	T/K	BiasP % MRDP % BiasY % MRDY %
	298.14	-0.19	0.62	-0.18	1.50	0.8	0.6
	328.04	-0.42	1.10	0.77	1.74	0.9	1.1
	358.11	0.10	0.99	0.02	0.32	0.2	1.0
	388.18	-0.03	0.53	-0.17	0.20	0.1	0.5

Table 6 : Mean Relative deviation MRDU and BiasU obtained in application of our model to the data from Giles and Wilson [6].

 6

	T/K	Bias P % MRD P %
	323.15	-0.16	0.56
	373.15	-0.01	0.21

Table 7

 7 presents the Mean Relative Deviation (MRDU), and the BiasU, applied on pressures and vapour phase mole fractions for the two predictive models. The performance of the two models is very similar with a slightly lower deviation for PSRK UNIFAC model.

		1.4
		1.2
	MPa	1
	Pressure/	0.6 0.8
		0.4
		0.2
		0
		0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1
							x 1 , y 1

Table 7 : Mean Relative deviation MRDU and BiasU obtained in fitting experimental VLE data with PPR78 and PSRK UNIFAC.

 7

	T/K	BiasP % MRDP % BiasY % MRDY %
		PPR78
	298.14	-5.24	5.24	-1.98	3.38
	328.04	-1.40	1.91	0.22	1.94
	358.11	-1.29	1.72	-0.90	1.47
	388.18	-1.35	1.36	-1.15	1.50
		PSRK UNIFAC
	298.14	2.08	2.28	1.35	2.49
	328.04	3.39	3.39	4.20	4.52
	358.11	0.82	0.82	0.52	1.14
	388.18	-0.44	0.44	-0.25	0.53

Table 2 .

 2 Purities and suppliers of the chemicals used in this work

Table 2 :

 2 Experimental isothermal VLE data for the n-Butane (1) + Ethylmercaptan (2) mixture system and their standard uncertainties. Maximum calibration uncertainty on composition u(x,y, k=2)=0.008. Table3. Thermal properties for ethane and Ethylmercaptan pure components (Source Simulis Thermodynamics™)

Table 4 :

 4 Values of the binary interaction parameters and objective function at each temperature.

Table 5 :

 5 Mean Relative deviation MRDU and BiasU obtained in fitting experimental VLE data with PR EoS.

Table 6 :

 6 Mean Relative deviation MRDU and BiasU obtained in application of our model to the data from Giles and Wilson[START_REF] Giles | Phase equilibria on seven binary mixtures[END_REF].

Table 7 :

 7 Mean Relative deviation MRDU and BiasU obtained in fitting experimental VLE data with PPR78 and PSRK UNIFAC.Fig. 1: Pressure as a function of n-butane mole fraction in the n-butane (1) + Ethylmercaptan (2) mixture at different temperatures. : 298.14 K,  : 328.04 K, ■: 358.11 K, +: 388.18 K. Solid lines: calculated with PR EoS with parameters from Table 3.Fig. 2: Relative volatility for the binary n-butane (1) + Ethylmercaptan (2) mixture at different temperatures. : 298.14 K,  : 328.04 K, ■: 358.11 K, +: 388.18 K. Solid lines: calculated with PR EoS with parameters from Table4.

	List of figures

Acknowledgments

This research did not receive any specific grant from funding agencies in the public, commercial or non-profit sectors.


Marie Thoby

Kurt O Konhauser

Philip W Fralick

Wladyslaw Altermann

Pieter T Visscher

Stefan V Lalonde

Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates

Sedimentary molybdenum (Mo) isotope compositions are a promising paleoredox indicator as the Mo isotope composition of seawater reflects the balance between anoxic and oxic sinks. Most available data are from shales, however the Mo isotope composition of carbonates also reflects the composition of ancient seawater. Here we provide an expanded dataset of carbonate Mo isotope compositions, including the first data for carbonates older than 2.64 Ga, which we evaluate against a compilation of published data for carbonates, shales, and iron formations spanning geological time. Archean carbonate samples reveal maximum d 98 Mo values that are generally above 1‰. These heavy values indicate that Mn(IV)-or Fe(III)-oxide sinks were sufficiently important to influence the Mo isotope composition of seawater as far back as 2.93 Ga. Comparison of Mo isotope and rare earth element data, as well as residence time considerations, indicates that this metal oxide influence was likely global. Available Mo isotope data for shales over the same time period generally show crustal values, which we attribute to negligible authigenic enrichment of Mo from seawater due to low ambient concentrations and a paucity of euxinic conditions. Our work demonstrates that the carbonate record provides important new insights into marine paleoredox, especially when shale records are absent or unsuitable, and re-enforces the emerging paradigm that oxic Mo sinks were important in the marine realm prior to 2.7 Ga.

INTRODUCTION

Molybdenum (Mo) stable isotopes have emerged as a powerful proxy for marine redox evolution that is based on the redox sensitive nature of Mo sources and sinks.

Under modern oxic conditions, seawater shows a homogenous value of 2.36‰ [START_REF] Siebert | Molybdenum isotope records as a potential new proxy for paleoceanography[END_REF]Figure DR1 in the GSA Data Repository 1 ). Modern Mo seawater sources are principally derived from the oxidative weathering of continental crust sulfide minerals (~90%; mean δ 98 Mo value of 0.20‰; [START_REF] Siebert | Molybdenum isotope records as a potential new proxy for paleoceanography[END_REF][START_REF] Voegelin | The impact of igneous bedrock weathering on the Mo isotopic composition of stream waters: Natural samples and laboratory experiments[END_REF] variably modified during transport to values as high as 2.39‰ (e.g., [START_REF] Voegelin | The impact of igneous bedrock weathering on the Mo isotopic composition of stream waters: Natural samples and laboratory experiments[END_REF].

Modern Mo sinks are dominated by Mn(IV) and Fe(III) oxide minerals that sequester Mo by adsorption and co-precipitation. With an equilibrium Mo fractionation of 3.3‰ onto Mn(IV) oxides (Siebert et al., 2003, Barling andAnbar, 2004) and from 0.83 to 2.19‰ onto Fe(III) oxides [START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr) oxides[END_REF], they constitute the isotopically lightest sink and are responsible for the heavy Mo isotope enrichment of seawater. The second most important sink is sedimentary Mo scavenging under suboxic, anoxic, and euxinic conditions, which also favor the lighter isotope but to a lesser degree (see Figure DR1).

As sulfide increases, sediment isotope compositions approach that of seawater as Mo scavenging becomes near-quantitative (e.g., [START_REF] Neubert | Sulfidity controls molybdenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea[END_REF]. This explains why black shales are used to track marine paleoredox using Mo isotopes.

The Mo isotope proxy has been used to provide critical insights into the relative importance of oxic and anoxic marine waters through geological time. [START_REF] Arnold | Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans[END_REF] Ga. Conversely, prior to 2.60 Ga, shales generally show crustal Mo isotope values [START_REF] Siebert | Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth[END_REF][START_REF] Wille | Evidence for a gradual rise of oxygen between 2.6 and 2.5 Ga from Mo isotopes and Re-PGE signatures in shales[END_REF], suggesting that either oxic Mo sinks were minimal prior to the Great Oxygenation Event (GOE) ca. 2.45 Ga, or that Mo in shales is a poor proxy for Archean oxygenation. Interestingly, [START_REF] Planavsky | Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event[END_REF] demonstrated that iron formation (IF) as old as 2.95 Ga show highly fractionated δ 98 Mo values ranging from -0.71‰ to 2.32‰, which vary as a function of Fe/Mn ratio. The lightest values are best explained by syndepositional adsorption of Mo to Mn(IV) oxides, implying that sufficient O2 for Mn(II) oxidation, and thus the evolution of oxidative photosynthesis, had already occurred by 2.95 Ga.

Here we turn here to another sedimentary proxy capable of recording ancient Mo cycling: the molybdenum isotope composition of Mo hosted in carbonates (δ 98 Mocarb; [START_REF] Voegelin | Molybdenum isotopic composition of modern and carboniferous carbonates[END_REF][START_REF] Voegelin | Molybdenum isotopes in late Archean carbonate rocks: implications for early Earth oxygenation[END_REF][START_REF] Wen | Molybdenum isotopic records across the Precambrian-Cambrian boundary[END_REF][START_REF] Eroglu | Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58[END_REF][START_REF] Romaniello | Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas[END_REF]. Although carbonates are a negligible Mo sink, they may record the δ 98 Mo signature of seawater from which they were formed [START_REF] Voegelin | Molybdenum isotopic composition of modern and carboniferous carbonates[END_REF][START_REF] Romaniello | Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas[END_REF]. Our study provides an expanded dataset for modern and Archean carbonate Mo isotopic compositions, which we compare to published data from carbonate, shale, and iron formation records to shed new light on marine Mo cycling in the Archean.

Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G45706.1 Page 5 of 19

SAMPLES AND METHODS

The newly acquired Mo isotopic data presented here include modern thrombolites and stromatolites from the Bahamas, 2.52 Ga stromatolites from the Gamohaan Formation (Ghaap Group, S. Africa), crystal fans and stromatolites from the 2.80 Ga Mosher Carbonate (Steep Rock Group, Canada) and 2.93 Ga Ball Assemblage (Red Lake Greenstone Belt, Canada), and 2.97 Ga stromatolitic and non-stromatolitic carbonates from the Chobeni Formation (Nsuze Group, S. Africa). See Figure 1 and Data Repository for descriptions and locations of each unit and sample. Major element concentrations were measured on a HORIBA Ultima 2 ICP-AES after HF-HNO3 total digestion, trace elements on carbonate leaches (5% acetic acid) using a ThermoFisher Scientific Element2 HR-ICP-MS, and Mo isotopic compositions on a ThermoFisher Scientific Neptune MC-ICP-MS after 6N HCl digestion, double spike (DS, 97 Mo 100 Mo) addition, and chromatographic purification of Mo. Mo concentrations were calculated by isotope dilution. All analyses were performed at the Pôle Spectrometry Ocean, Brest, France. For all data (new and compiled), δ 98 Mo values are reported relative to the NIST3134 standard set to 0.25‰ [START_REF] Nägler | Proposal for an International Publisher[END_REF]

RESULTS AND DISCUSSION

The Phanerozoic Mo Isotope Record

The modern seawater δ 98 Mo value of 2.36‰ (±0.1‰, [START_REF] Siebert | Molybdenum isotope records as a potential new proxy for paleoceanography[END_REF] is reflected in the maximal δ 98 Mo values of both modern black shales and carbonates (See Figure 2A, Tables DR3 andDR4). With a maximum value of 1.76‰, our new data from modern Bahamian microbialites fall within the range observed for contemporaneous carbonates, but due to detrital impurities, or local production of sulfide, they do not reach the modern seawater value (see sample description in the DR; Figure 2A and Table DR2). For all available carbonate data, δ 98 Mo values are never heavier than modern seawater and are thus useful for constraining the minimum value of seawater at any given time (Figure 2A).

During the Phanerozoic, carbonates record a range of δ 98 Mo values from 2.42‰ to -0.91‰ (see Figure 2A), with maximum values universally attaining the modern seawater value of 2.36±0.1‰ (Figure 2A). Shales record a range of δ 98 Mo values similar to carbonates (from 2.44‰ to -1‰), but also include the few outliers lower than -1‰.

Only about one-fifth of compiled

Mo Cycling in the Precambrian

For the Proterozoic, no Mo isotope data are available for carbonate rocks.

Maximum values for shales are highly variable, ranging from -0.09‰ to 1.98‰ and averaging 0.81±1.31‰; [START_REF] Arnold | Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans[END_REF] inferred a Proterozoic δ 98 Mosw value of 1.08‰. Available IF data during this period are limited, but universally heavy in δ 98 Mo, with maximum values for each deposit ranging from 1.03‰ to 2.08‰ (n=3). Archean Mo isotope data are more abundant in literature for all three lithologies, and a significant contrast in Mo isotope compositions between the three lithologies appears to occur prior to ~2.6 Ga. Maximum shale values range from 1.72‰ to 1.76‰ and average 1.73±0.05‰ from 2.50 Ga to 2.60 Ga, yet are universally low (ranging from -0.46‰ to 0.89‰, averaging 0.31±0.52‰) before 2.64 Ga. In contrast, and with the exception of the Chobeni carbonates, maximum values for carbonates and IF are consistently positive from 2.50 Ga to 2.97 Ga (0.82‰ to 1.97‰ with an average of 1.31±0.72‰ for carbonates; 0.97‰ to 1.87‰ with an average of 1.52±0.79‰ for IF; see Figure 2B). As discussed below, this contrast likely relates to the chemical conditions that enable the capture of seawater molybdenum in shale records. adjacent Transvaal basin (up to 1.97%; [START_REF] Eroglu | Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58[END_REF]; Figure 2B). The general agreement in maximum δ 98 Mo values for available shale, carbonate, and IF records during this period is remarkable. All three archives appear to record seawater that is isotopically heavy due to sequestration of light Mo isotopes by Mn(IV)-and Fe(III)-oxide exit channels, consistent with evidence for increasing surface redox potential in the runup to the GOE. Enhanced supply of sulfate (and likely Mo) to the oceans at this time and the development of euxinic conditions (e.g., [START_REF] Reinhard | A late Archean sulfidic sea stimulated by early oxidative weathering of the continents[END_REF] would have promoted the authigenic enrichment of seawater Mo into organic-rich shales.

In several carbonate samples measured by others [START_REF] Voegelin | Molybdenum isotopes in late Archean carbonate rocks: implications for early Earth oxygenation[END_REF][START_REF] Eroglu | Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58[END_REF], δ 98 Mo compositions are isotopically light (as low as -0.82‰), indicating the former presence of Mn(IV)-or Fe(III)-oxides in the samples. As chemical sediments that often contain little detrital material, Precambrian carbonates are generally characterized by low Mo concentrations (<200 ppb), with average isotope-dilution whole rock Mo concentrations of 140 ± 470 ppb for the period 2.50 to 2.97 Ga [START_REF] Voegelin | Molybdenum isotopes in late Archean carbonate rocks: implications for early Earth oxygenation[END_REF][START_REF] Eroglu | Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58[END_REF]this study). By contrast, shales generally reflect crustal Mo abundances (1.1 ppm;[START_REF] Rudnick | Composition of the continental crust: Treatise on geochemistry[END_REF], unless authigenic enrichment from seawater occurs. Accordingly, detritus-poor carbonate rocks should be more sensitive to the addition of small amounts of isotopically light Mo associated with Mn(IV)-or Fe(III)-oxide sedimentary inputs.

Our dataset includes the first available Mo isotope data for carbonates older than 2.64 Ga. Microbialitic calcite and aragonitic crystal fans (now replaced by calcite) from the 2.80 Ga Mosher Formation at Steep Rock (Canada) show a maximum δ 98 Mo value of 1.22±0.04‰, comparable to dolomitic stromatolites and dolomitized crystal fans from the Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G45706.1 Page 9 of 19 2.93 Ga Ball Assemblage at Red Lake (Canada) that display a maximum δ 98 Mo value of 1.03±0.03‰ (Figure 2B). Both of these values are remarkably heavy. While the low Mo contents of these samples might make their δ 98 Mo values more susceptible to diagenetic alteration, multiple lines of evidence indicate this was not responsible for heavy isotope enrichments prior to 2.64 Ga. Briefly, there is no relationship between δ 98 Mo and δ 18 Ocarb, and δ 98 Mo data tend toward crustal values as a function of degree of silicification (see discussion in Data Repository and Figures DR4 andDR5).

Furthermore, burial of light Mo with organic carbon at concentrations typical of Archean shale is unlikely to have affected the global marine Mo cycle (see Data Repository).

Instead, these heavy values suggest that Mn(IV)-and Fe(III)-oxide exit channels for Mo were sufficiently important to impart a heavy Mo isotope signature on Mesoarchean seawater. This conclusion is similar to that of [START_REF] Planavsky | Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event[END_REF] and Ossa Ossa et al.

(2018), who also concluded that an Mn(IV)-oxide exit channel was in operation ca. 2.95 Ga. Crucially, the isotopically light data of these studies simply implies some unknown degree of Mo removal by an oxide-based exit channel, while the isotopically heavy data in our new dataset (also present in the IF examined by [START_REF] Planavsky | Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event[END_REF] reflects the composition of residual seawater.

If both carbonates (this study) and iron formations [START_REF] Planavsky | Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event[END_REF] are characterized by Mo isotope compositions reflecting an important role for metal oxidebased Mo sinks prior to 2.7 Ga, then with the exception of recently analyzed shales from the Pongola Supergroup [START_REF] Ossa Ossa | Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea[END_REF], why has this not been detected in the larger shale record? Several possible explanations for this discrepancy exist, notably the possibility of spatial heterogeneity in seawater δ 98 Mo (see below and supplementary Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G45706.1

Page 10 of 19 discussion). However, considering that nearly all available data for shales older than 2.64 Ga show δ 98 Mo values in the crustal range, we suggest that the most parsimonious explanation is that most shales simply did not experience authigenic Mo enrichment prior to 2.64 Ga. Indeed, shales older than ~2.64 Ga rarely show signs of enrichment in Mo above crustal values (Figure 2B) and generally reflect detrital Mo sources. This can be explained by the fact that euxinic conditions that strongly promote Mo enrichment in shales were generally absent, whereas after 2.63 Ga, when the first euxinic conditions were established (e.g., [START_REF] Reinhard | A late Archean sulfidic sea stimulated by early oxidative weathering of the continents[END_REF], carbonates and shales show comparable δ 98 Mo records. Moreover, shales deposited under anoxic but non-euxinic conditions today still become authigenically enriched in Mo; it would appear that lower marine Mo concentrations rendered this process less effective in the Archean. In this context, carbonates that contain a smaller Mo contribution from detrital components, and do not depend on local euxinia or a large marine Mo reservoir for authigenic enrichment, provide unique insight into the upper limit of d 98 Mo evolution of seawater prior to 2.63 Ga that is otherwise cryptic in the shale sedimentary record. The rare cases where authigenic signals are recorded in shales (e.g., Ossa [START_REF] Ossa Ossa | Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea[END_REF] point to the intriguing possibility of localized Mo isotope responses to local redox conditions.

The new carbonate d 98 Mo data presented here show no relation with Mn content or Mn/Fe ratio, and considered in light of other redox indicators such as the presence of negative Ce anomalies (represented here as Pr/Pr* > 1; see Data Repository), it becomes apparent that interpretation of carbonate d 98 Mo data may not always be straightforward.

Simple mixing calculations reveal that the Mo isotope composition of carbonates is more resistant to dilution by detrital material than REE-based signatures (Figure 3). However, Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G45706.1

Page 11 of 19 paired d 98 Mo -Ce anomaly data from the 2.97 Ga Chobeni Formation (Figure 3) reveal the presence of significant negative Ce anomalies, as reported elsewhere [START_REF] Siahi | Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoar-chaean Pongola Supergroup: Implications for seawater composition[END_REF]), yet show no sign of Mo isotope fractionation outside of the crustal range. This apparent contradiction may be reconciled by the significantly shorter residence times of light REE (<300 years; see Data Repository) compared to Mo (likely greater than 17 ky during the Archean; [START_REF] Asael | Dissolved Mo in the Archean oceans -a case study from the 2.63 Ga Jeerinah Formation[END_REF], making REE signals a local proxy, and as a result, the two proxies are necessarily recording redox at different spatial and temporal scales. Thus, one interpretation of the paired Chobeni d 98 Mo-Ce anomaly data is that the basin experienced oxidative Ce cycling at a local scale, while oxic sinks for Mo remained minor in seawater at a global scale. The reverse is true for Red Lake samples, while the Mosher carbonate shows evidence for both local Mn(IV) oxide precipitation (positive Pr/Pr * , consistent with [START_REF] Riding | Identification of an Archean marine oxygen oasis[END_REF]Riding, 2015) as well as global Mn(IV) oxide precipitation (positive d 98 Mo). For the Chobeni, another possibility is that mixing occurred between authigenic Mo from seawater and oxide sources, with the resulting mixture falling within the crustal d 98 Mo range. Some points fall below the crustal d 98 Mo range, supporting this possibility. While caution is warranted as late oxidative alteration of surface samples may be difficult to detect [START_REF] Albut | Modern rather than Mesoarchaean oxidative weathering responsible for the heavy stable Cr isotopic Publisher[END_REF], our Chobeni dataset is consistent with at least some O2 present locally in the basin, similar to recent findings based on REE data [START_REF] Siahi | Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoar-chaean Pongola Supergroup: Implications for seawater composition[END_REF] and Fe, Mo and S isotope data [START_REF] Eickmann | Isotopic evidence for oxygenated Mesoarchaean shallow oceans[END_REF][START_REF] Ossa Ossa | Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea[END_REF]. This study presents the first δ 98 Mo data for carbonates older than 2.64 Ga, with values ranging from -1.54‰ to 1.22‰. By contrast, Mo isotope compositions of shales of similar age generally show crustal values. We suggest that this discrepancy results from low concentrations of Mo in seawater as well as a general absence of euxinic conditions. This, in turn, prevented widespread authigenic Mo enrichment in shales prior to the onset of oxidative continental weathering, an increased Mo reservoir, and the first establishment of localized euxinia ca. 2.7-2.6 Ga (e.g., [START_REF] Reinhard | A late Archean sulfidic sea stimulated by early oxidative weathering of the continents[END_REF]. Carbonate rocks are not subject to these constraints, and along with IF, provide additional perspective on the δ 98 Mo composition of Mesoarchean seawater. Our dataset further indicates minimum global seawater δ 98 Mo values of 1.2‰ and 1.0‰ at 2.80 and 2.93 Ga, respectively, consistent with the idea that Mn(IV)-and Fe(III)-oxide sinks for Mo were globally important as far back as 2.95 Ga [START_REF] Planavsky | Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event[END_REF][START_REF] Ossa Ossa | Aerobic iron and manganese cycling in a redox-stratified Mesoarchean epicontinental sea[END_REF].

CONCLUSION

Despite possible limitations stemming from detrital contamination and mixing of different authigenic components in the same sample, the δ 98 Mo composition of carbonates is a promising proxy for understanding marine molybdenum cycling and paleoredox evolution before conditions permitted widespread authigenic Mo enrichment in shales. 1 GSA Data Repository item 2018xxx, containing supplemental discussion on marine Mo cycling and post-depositional modification, description of samples and their REE spectra, detailed methods and compilation data sources, and data tables, is available online at http://www.geosociety.org/datarepository/2018/, or on request from editing@geosociety.org.

FIGURE CAPTIONS

  reported δ 98 Mo values ~0.69‰ for mid-Proterozoic euxinic sediments of the Velkerri (1.40 Ga) and Wollogorang (1.70 Ga) formations, implying significant areas of seafloor anoxia at the time. Heavy Mo isotope signatures in shales extend back to 2

  Phanerozoic shale values fall within 1‰ of the modern seawater value, in part due to non-quantitative Mo removal. Carbonates are also limited in their ability to record seawater signatures, notably by their low Mo contents that make them susceptible to diagenetic resetting or dilution by detrital materials, by the presence of organic matter or porewater sulfide during diagenesis that may alter whole-rock values, and/or their pervasive re-crystallization at even low metamorphic grades.Nonetheless, carbonates provide complimentary insight into Mo cycling during periods when shale records fall short.

From 2 .

 2 50 to 2.60 Ga, maximum d 98 Mo values in shales and carbonates vary from 1.40‰ to 1.97‰, and their average places a lower limit on global seawater Mo isotopic composition of 1.63±0.48‰ during this period (Figure2B). IF deposits analyzed during this period present a maximum value between 0.97‰ and 1.76‰. New data from 2.52 Ga carbonate samples from the Gamohaan Formation, Griqualand West basin, show a maximum d 98 Mo of 1.11‰, in line with elevated values observed for carbonates of the Publisher

Figure 1 .

 1 Figure 1. Maps showing (A) Canadian and (B) South African Archean carbonate

Figure 2 .

 2 Figure 2. New and compiled d 98 Mo data for shales, iron formations, and carbonates

Figure 3 .

 3 Figure 3. d 98 Mo data for carbonate samples analyzed in this study plotted against PAAS-

Figure 1 Figure 2 N

 12 Figure 1

see Table DR1 for details), uncertainties are reported as two standard deviations (2SD), and datasets are tabulated in Tables DR2 to DR6. See

	Publisher: GSA
	Journal: GEOL: Geology
	DOI:10.1130/G45706.1
	Data Repository for extended methods.
	Page 6 of 19

Acknowledgments

We thank Dawn Sumner for providing samples and Olivier Rouxel for providing Mo standards and double spike. We also thank Emmanuel Ponzevera, Alexis de Prunelé, Yoan Germain, Bleuenn Guegen, Marie-Laure Rouget, and Céline Liorzou for analytical assistance, and Matthieu Plasman for GIS assistance. This work was supported by an NSERC Discovery Grant to K.O.K. (RGPIN-165831), an NSERC Discovery Grant to P.W.F. (RGPIN 16400), and an NSF EAR Grant to P.T.V (1561173). W.A. Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G45706.1 Page 17 of 19 acknowledges financial support by the Exxaro Chair at the University of Pretoria and by NRF Incentive Funding. S.V.L. acknowledges support from the European Union's Horizon 2020 research and innovation programme (grant agreement n o 716515). Ronny Schönberg and two anonymous reviewers are thanked for their constructive and insightful reviews. Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G45706.1


Romain Milotskyi

Christophe C Bliard

Polysaccharide and lignin modification using reactive extrusion (REX)

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

Zero order kinetics

roman.milotskyi@etudiant.univ-reims.fr christophe.bliard@univ-reims.fr

Efficient monitoring

The The reactivity of the hydroxyl group at C 2 toward carboxymethylation is slightly greater than that at C 6 .

For

  Fig 1 : Carboxymethyl groups distribution (%) in different types of starch

Conclusions

  REX is a fast and efficient process for polymer chemical modification  REX provides high reaction efficiency and selectivity  Possibility of continuous on-line monitoring  No apparent viscosity limits compared to other processes  Kinetics of the reaction are optimal (zero order kinetics)  Continuous process production  Possibility to fit production with the demand  Transfer the REX parameters to a bigger industrial scale Perspectives 1 Xiao, B. ; Sun, X.F. ; Sun, RunCang The chemical modification of lignins with succinic anhydride in aqueous systems, Polymer degradation and stability / v.71 no.2, pp.223-231 (2001) 2 A. Ayoub, C. Bliard et al Study of the Thermoplastic Wheat Starch Cationisation Reaction under Molten Condition, Starch/Stärke, 56, 513-519 (2004) 3 S. Senturk-Ozer et al. Biomass pretreatment strategies via control of rheological behavior of biomass suspensions and reactive twin screw extrusion processing, Bioresource Technology 102 9068-9075 (2011) 4 Bhandari P.N., Jones D.D. and Hanna M.A. Carboxymethylation of cellulose using reactive extrusion, Carbohydrate Polymer 87, 2246-2254 (2012) Natural polymers possess physicochemical properties in their native state, which limit their use in many industrial sectors. Even limited chemical modifications radically change the properties of these macromolecular entities. Chemical modifications commonly performed at industriallevel production in solution or in batches [1] are often laborious and difficult to monitor. In addition, these processes suffer from energy cost and environmental issues caused by unreacted components and co-products. Reactive extrusion offers a simple process, using no solvents, shorter reaction time and improved efficacy [2-4].In the present work we describe a method of chemical modification using reactive extrusion process (REX). In this project three families of biopolymers: starch, cellulose and lignin are studied. In the following example carboxymethyl starches (CMS) with various degrees of substitution (DS) were synthesized in one step and with high levels of reaction efficiency (RE). The results on the distribution of the carboxymethyl functions on the three different positions of the glucose monomer as well as the kinetics of the reaction are analyzed and discussed.

  substituted hydroxyls in an anhydroglucose ring