C Xing

B Bernicot

G Arrachart
email: guilhem.arrachart@umontpellier.fr

S Pellet-Rostaing
email: stephane.pellet-rostaing@cea.fr

Application of ultra/nano filtration membrane in uranium rejection from fresh and salt waters

Application of ultra/nano filtration membrane in uranium rejection from fresh and salt waters

C Xing, B Bernicot, Guilhem Arrachart, S Pellet-Rostaing

Introduction

Uranium is mainly used as fuel in nuclear power plants to produce electricity, given the increase in energy needs, the demand for uranium will therefore also increase. Whether or not it is possible to obtain sufficient fuel will become a key factor in the sustainable development of nuclear industry. To ensure the long-term development of nuclear power production, it is crucial to exploit nonconventional uranium resources, such as uranium in seawater. 71% of the surface of earth is covered by oceans and seas, there are some 50 quadrillion tons of minerals and metals dissolved in all the world's seas and oceans [START_REF] Loganathan | Mining valuable minerals from seawater: A critical review[END_REF]. Thus uranium, calcium, magnesium, potassium, sodium represents a much larger amount of storage compared to the land and underground resources [START_REF] Bardi | Extracting minerals from seawater: An energy analysis[END_REF]. Uranium can be found in seawater at a concentration of 3.3 μg.L-1, forming highly stable Ca-UO2-CO3 and Mg-UO2-CO3 species [START_REF] Lee | Formation, stability and structural characterization of ternary MgUO2(CO3)3 2and Mg2UO2(CO3)3(aq) complexes[END_REF][START_REF] Lee | Formation of ternary CaUO2(CO3)3 2and Ca2UO2(CO3)3(aq) complexes under neutral to weakly alkaline conditions[END_REF]. Although dilute, this equates to an about 4.5 billion tons of uranium, about 1000 times more than is available from conventional sources such as earth ores. These reserves greater than the terrestrial uranium reserves, offer a substantial potential source of nuclear fuel. Therefore, uranium in seawater will be a near-limitless resource for nuclear fuel in the future, and its recovery will avoid the deleterious environment impact of terrestrial mining. However, efficient and selective extraction of uranium from seawater is particularly challenging because of the complex solution chemistry in seawater including high salt concentration (average salinity around 35 g.L-1 mainly due to NaCl), high carbonate concentration, basic pH (7.5-8.5), temperature •C), low uranium concentration (~3.3 μg. L-1), and other metal ions at similar or higher concentrations. Although there are many difficulties to overcome, the recovery of U from seawater is one of the most promising solutions given the importance of this resource [START_REF] Loganathan | Mining valuable minerals from seawater: A critical review[END_REF][START_REF] Bardi | Extracting minerals from seawater: An energy analysis[END_REF][START_REF] Kim | Recovery of Uranium from Seawater: A Review of Current Status and Future Research Needs[END_REF][START_REF] Diallo | Mining Critical Metals and Elements from Seawater: Opportunities and Challenges[END_REF].

To achieve the goal to concentrate U from seawaters, various methods have already been evaluated. Many materials with specific functions have been investigated, particularly including recent developments in inorganic materials, polymer adsorbents and related research belong to amidoxime, and nanostructured materials such as metal 􀀀 organic frameworks, porous-organic polymers, and mesoporous carbons [START_REF] Abney | Materials for the Recovery of Uranium from Seawater[END_REF]. Among them, amidoxime-based materials serve as one of the most promising candidates and are the main tool used for uranium extraction from aqueous systems owing to their special affinity for uranium [START_REF] Zeng | New Amidoxime-Based Material TMP-g-AO for Uranium Adsorption under Seawater Conditions[END_REF][START_REF] Xu | Preparation of amidoxime-based PE/PP fibers for extraction of uranium from aqueous solution[END_REF][START_REF] Tang | Amidoxime-based materials for uranium recovery and removal[END_REF]. More recently, new methods based on the use of DNA-based uranium extraction hydrogel adsorbent [START_REF] Yuan | DNA nano-pocket for ultraselective uranyl extraction from seawater[END_REF], hydrogel-like spidroin-based protein fiber [START_REF] Yuan | Ultrafast and Highly Selective Uranium Extraction from Seawater by Hydrogel-like Spidroin-based Protein Fiber[END_REF] and biofouling-resistant polymeric peptide materials [START_REF] Yuan | Selective extraction of uranium from seawater with biofouling-resistant polymeric peptide[END_REF] shows interesting perspectives in the context of uranium recovery from seawaters. Compared to these methods, membrane technologies are known to be flexible, automatable and can work continuously. Moreover, extraction, separation or purification takes place through purely physical without phase change [START_REF] Vatanpour | Fouling reduction and retention increment of polyethersulfone nanofiltration membranes embedded by amine-functionalized multi-walled carbon nanotubes[END_REF]. Membrane separation processes have been used for several years to concentrate or fractionate suspended particles and dissolved substances. Reverse osmosis (RO), now in widespread use to prepare irrigation water from briny waters, and ultrafiltration (UF) both constitute a valuable aid for the fractionation and concentration of colloidal substances. Nanofiltration (NF) membranes have intermediate molecular weight cut-offs (MWCOs) between UF and RO membranes. NF using organic or inorganic membranes has been found to be useful recently for recycling and recovery metals from urban mine [START_REF] Song | A critical review on membrane extraction with improved stability: Potential application for recycling metals from city mine[END_REF], from seawater desalination brine [START_REF] Zhang | A review of resource recovery from seawater desalination brine[END_REF], from industrial wastewater [START_REF] Nakhjiri | Recovery of precious metals from industrial wastewater towards resource recovery and environmental sustainability: A critical review[END_REF], selectively remove of minor actinides from lanthanides [START_REF] Favre-Réguillon | Nanofiltration assisted by complexation: A promising process for the separation of trivalent long-lived minor actinides from lanthanides[END_REF][START_REF] Chitry | Separation of lanthanides(III) by nanofiltrationcomplexation in aqueous medium[END_REF][START_REF] Chitry | Separation of americium (III) from lanthanides(III) by nanofiltration-complexation in aqueous medium[END_REF][START_REF] Sorin | Rejection of Gd(III) by nanofiltration assisted by complexation on charged organic membrane: Influences of pH, pressure, flux, ionic strength and temperature[END_REF][START_REF] Borrini | Water Soluble PDCA Derivatives for Selective Ln(III)/An(III) and Am(III)/Cm(III) Separation, Solvent Extraction and Ion[END_REF][START_REF] Borrini | Separation of lanthanides(III) by inorganic nanofiltration membranes using a water soluble complexing agent[END_REF], removing uranium from drinking water [START_REF] Favre-Réguillon | Selective removal of dissolved uranium in drinking water by nanofiltration[END_REF] and from seawater [START_REF] Favre-Reguillon | Selective Concentration of Uranium from Seawater by Nanofiltration[END_REF]. Some renewable energy powered membrane technologies have been investigated for water treatment and ions separation [START_REF] Ramkumar | Selective permeation of Cu2+ and UO22+ through a Nafion ionomer membrane[END_REF][START_REF] Mccleskey | Asymmetric membranes with modified gold films as selective gates for metal ion separations[END_REF][START_REF] Richards | Renewable energy powered membrane technology: Salt and inorganic contaminant removal by nanofiltration/reverse osmosis[END_REF][START_REF] Boussouga | Renewable energy powered membrane technology: System resilience under solar irradiance fluctuations during the treatment of fluoride-rich natural waters by different nanofiltration/reverse osmosis membranes[END_REF]. Ultra/nano filtration process using inorganic membranes can concentrate multivalent metal ions by rejection attributed to a combination of various mechanism including steric, electric, Donnan, dielectric and transport effects [START_REF] Oumar Anne | Nanofiltration of seawater: Fractionation of mono-and multi-valent cations[END_REF]. Steric effect and electric effect are two major factors that affect the rejection. A chemical species will be retained by the membrane if its size is larger than that of the pores [START_REF] Favre-Réguillon | Nanofiltration assisted by complexation: A promising process for the separation of trivalent long-lived minor actinides from lanthanides[END_REF]. In addition to size exclusion, charged species in solution can be rejected by electrical interactions with charges in the membrane material. Membranes in contact with an aqueous solution acquire an electric charge by dissociation of surface functional groups, adsorption of ions and ionised solutes from the solution. The origin of this charge comes from the ionization of sites on the surface of the membrane generally resulting from the acid-base properties of the surface groups and the ionic strength of the solution. The electrical interactions can take place on the surface of the membrane or through the pore [START_REF] Teixeira | The role of membrane charge on nanofiltration performance[END_REF]. Therefore, the UF/NF membrane can reject charged solutes of much smaller size than the dimensions of the pores. This Donnan exclusion mechanism can be particularly used to remove ions from waters and to separate them according to their ionic valences [START_REF] Schaep | Influence of ion size and charge in nanofiltration[END_REF]. Indeed, surface groups are strongly influenced by the pH of the contacting solution since the membrane exhibits an IsoElectric Point (IEP) [START_REF] Childress | Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes[END_REF]. The IEP is the pH value of the solution for which there are as many positive and negative ions around the surface of the membrane. The surface will be positively charged if the pH is lower than the IEP and conversely, it will be negatively charged if the pH is higher than the IEP. For the ceramic membrane used in this study, the manufacturer provides a value for IEP ≈ 5.5. The purpose of this study is to evaluate the opportunities of inorganic TiO2/ZrO2 membranes with a molecular weight cut-off (MWCO) of 1 or 5 kDa for the selective concentration of valuable metal ions, such as U from different water solutions, for example, from seawater. The choice of inorganic membranes is based on its advantages of amphoteric behaviour, excellent tolerance to wide pH and temperature ranges, outstanding mechanical strength, longer operational lifetime, and lower clogging tendency [START_REF] Das | Preparation and characterization of novel ceramic membranes for micro-filtration applications[END_REF]. First, the influence of various parameters (MWCO, pH, temperature, applied pressure, composition of solutions, ionic strength) to rejections of U and Na have been studied with simple synthetic solutions of uranyl ion or uranyl carbonate complex and sodium. Then, the comparison of the influence of pH to rejections of U, Mg, Ca, Na, and K was studied with the corresponding solutions. Finally, the filtration experiments are performed with synthetic and real seawaters or river waters to see the possibility of separating U from the other competitive species. At the same time, the species distributions of all the solution systems have been simulated using PHREEQC software for a better understanding of the mechanism of interactions of metal cations with the membrane.

Experiments

Materials and methods

Ultra/nano filtration experiments are investigated in a 1 L volumetric reactor filled with synthetic or real solutions. Experiments are performed with a SIVA laboratory-scale membrane system which is illustrated in Fig. 1 with detailed caption. The filtration membrane system consists of three main parts: i) the feed tank, ii) the membrane cell and iii) the pump which pumped the feed solution from the feed tank to reach the membrane cell body. The feed of volume 1 L is maintained at constant composition and volume during each filtration by totally recycling the permeate and the retentate back to feed. Feed solutions are filtered through the membranes in a crossflow mode for minimizing the clogging at the membrane surface. During filtration, the metal ions in the feed are convectively driven to the membrane surface by transmembrane pressure (TMP), thus forming a concentrated polarization boundary layer near the membrane. A feed flowrate of 0.1 m 3 /h is applied, the pressure is set from 1 to 3 bars. A cooling system is implemented to maintain a constant experiment temperature during the experiment. The pH of the solution is constant during the filtration. Flowmeters and pressure meters monitor the flow rates and pressures of each part of system.

Filtration experiments

For the filtration experiment, the valves VFA, VFR and VFP are open. The permeate and retentate can be collected at the end of the pipes. For cleaning the membrane, the valves VCA, VCR are open. The circulation pump ensures the circulation of the solution in the system. Rejection of metal ions is evaluated from the retentate and permeate concentration of samples collected during the experiments. The rejection Ri (%) of a substance i is calculated with the following equation (1):

R i (%) = 100 × (1 - C 𝑝 𝑖 C 𝑅𝑒𝑡 𝑖 ) (1)
where CRet i is the concentration of i in the retentate (flux that has not passed through the pores of the membrane) and Cp i is the concentration of i in the permeate (corresponding to the flux that has passed through the pores of the membrane). The concentration of retentate can be considered equal to the feed concentration because the flux rate of retentate (0.1 m 3 /h) is far higher than that of permeate (0.05-0.5 L/h).

Fig. 1 Schematic representation of the SIVA laboratory-scale membrane filtration system used in this study.

Membrane

The inorganic filtration membrane used was a multi-layer structure with a TiO2/ZrO2 active layer and a tubular channel titanium support (CéRAMTM -TAMIS INDUSTRIES, Nyons, 26 -France). They consist of several elements. The filter layer is based on a titanium tubular support of a length of 25 cm with an outer diameter of 10 mm. Mono canal membranes are used with a hydrodynamic diameter of 6 mm, allowing the passage of fluid within the hollow portion. The range of MWCO employed varies between 1 and 8 kDa.

Sample preparation

Synthetic solutions of UO2 2+ or uranyl carbonate complex are prepared from 10 g.L -1 uranyl nitrate UO2(NO3)2⋅6H2O, in HNO3 2%, and NaHCO3 powder in ultra-pure water (milli-Q water), synthetical grade salts of NaCl, MgCl2, CaCl2, KCl, Na2SO4, NaHCO3 are added to corresponding solutions depending on the desired composition. Then, pH of the filtration feed was measured using a pH meter (Metrohm 781 pH meter) and adjusted to the desired value by adding 2 mol.L -1 NaOH or 1 mol.L -1 HCl.

Simple synthetic solutions of UO2 mol.L -1 , the concentration of NaHCO3 is of 1.0 × 10 -2 mol.L -1 , and the concentration of CaCl2 is controlled from 2.16 × 10 -5 to 5.82 × 10 -4 mol.L -1 .

Species distribution by PHREEQC

Speciation diagrams of U in all the solution systems studied are built with the software PHREEQC with database "NEA_TDB_phreeqc_December2020" to predict species distribution in each solution with the aim to highlight a relationship between the filtration electric effect and the solution composition, then to allow to consider the different rejections behaviour in the different solutions.

Uranium (VI) carbonate systems are often very complicated because they consist of several different complex ions that are in rapid equilibrium with each other and with aquo ions or hydrolysed species [START_REF] Favre-Reguillon | Selective Concentration of Uranium from Seawater by Nanofiltration[END_REF]. Examples of speciation diagrams are illustrated in Fig. 2.

.L -1 . [U] = 8.40×10 -2 mmol.L -1 , [Na] = 1.0×10 - 2 mol.L -1 , [C] = 1.0×10 -2 mol.L -1 . (b) Seawater A: Mediterranean seawater. I = 0.534 mol.L -1 . [U] = 1.426×10 -5 mmol.L -1 , [Na] = 0.319 mol.L -1 , [Mg] = 5.225×10 -2 mol.L -1 , [K] = 1.218×10 -2 mol.L -1 , [Ca] = 1.19×10 -2 mol.L -1 , [Cl] = 0.437 mol.L -1 , [C] = 2.388×10 -3 mol.L -1 , [S] = 9.897×10 -3 mol.L -1 . (c) Seawater S- D (2): Synthetic Baltic seawater doped with U. I = 0.135 mol.L -1 , [U] = 4.201×10 -3 mmol.L -1 , [Na] = 9.112×10 -2 mol.L -1 , [Mg] = 1.034×10 -2 mol.L -1 , [K] = 2.57×10 -3 mol.L -1 , [Ca] = 2.72×10 -3 mol.L -1 , [Cl] = 0.115 mol.L -1 , [C] = 8.524×10 -4 mol.L -1 , [S] = 1.999×10 -3 mol.L -1 . (d) Fresh water F-B: Rhone River doped with U. I = 6.944×10 -3 mol.L -1 , [U] = 8.405×10 -3 mmol.L -1 , [Na] = 5.458×10 -4 mol.L -1 , [Mg] = 2.385×10 -4 mol.L -1 , [K] = 5.92×10 -5 mol.L -1 , [Ca] = 1.693×10 -3 mol.L -1 , [Cl] = 5.841×10 -4 mol.L -1 , [C] = 2.753×10 -3 mol.L -
pH UO 2 2+ (UO 2 ) 2 CO 3 (OH) 3 - Ca 2 UO 2 (CO 3 ) 3 UO 2 OH + UO 2 Cl + UO 2 (CO 3 ) (aq) pH=8.25 CaUO 2 (CO 3 ) 3 2- MgUO 2 (CO 3 ) 3 2- Seawater D (2). Synthetic Baltic seawater doped U 1 mg.L -1 UO 2 (CO 3 ) 3 4- UO 2 (CO 3 ) 2 2-
The species distribution in the seawater A and B are relatively similar despite the difference in U concentration. In seawater A, NaCl, MgCl2, Na2SO4, CaCl2 and KCl are the principal salts. At pH 8.25, Ca2UO2(CO3)3, CaUO2(CO3)3 2-, MgUO2(CO3)3 2-and UO2(CO3)3 4-, representing respectively 60%, 20%, 15% and 5% of proportion, make the composition of U. As for seawater B, Ca2UO2(CO3)3, CaUO2(CO3)3 2- , MgUO2(CO3)3 2-and UO2(CO3)3 4-, representing respectively 50%, 30%, 15% and 5% of proportion.

However, in freshwater F, U is 85% in the form of Ca2UO2(CO3)3 and 15% in the form of CaUO2(CO3)3 2 .

The more general feature with NF membranes is the separation of salts according to their size and valence. Thus, a high rejection of these highly charged U species can be expected.

Analytical procedures and error analysis

For doped-U solutions, quantification of U, Na, Ca, Mg, K in simulated seawater was performed with ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy, iCAP 7000 Series) with a standard deviation of ± 2%. Each filtration experiment was repeated at least 2 times. Analyte concentrations were determined with respect to calibration standards in dilute nitric acid. The absence of interferences was checked during the analysis. Analysis of uranium in real seawater and in river waters is performed using ICP-MS (Inductively Coupled Plasma Mass Spectrometer, iCAP RQ). Analyte concentrations were determined using matrix-matched calibration. Errors were estimated with ICP measurement calibration, the verification of the mass-balance at each step of the filtration also allows to establish the uncertainty on the process. The detail for the error estimation is provided in the supporting information.

Evaluation of membrane performance

The filtration experiment was proceeded in a closed filtration system. Closed filtration system is defined by the constant feed volume during the whole filtration process. The filtration is conducted with 1 L of feed solution. During the whole process, the retentate and the permeate go back to the feed, keeping the almost constant volume. The recuperation of permeate and retentate, both about 10 mL (relatively negligible to 1 L), is carried out about each 30 min to evaluate the rejections towards ICP measurement. The experiments were repeated several times to improve the reliability of the results. Closed filtrations are conducted to evaluate the influence of various physical-chemical parameters (pH, MWCO, pressure, temperature, sodium salt types, the solution salinity, solution composition) to filtration performance. Concentration of each element in solution was measured by ICP-OES and ICP-MS. U and Na are both rejected driven by electrical repulsion. Na rejection decreases from 50% at pH 3 to 0% at pH 5.5 and increases to 35% at pH 8.25. The separation of U to Na can be realized with the optimal efficiency at pH 8.25 or 3 in solution containing U and NaHCO3. The minimum rejection was found at pH 5.5 that corresponds to the IEP of ceramic membrane. However, the rejection of U decreases from 95% at pH 3 to its minimum value 30% at pH 5, then it increases from 30% to 100% while pH increasing from 3 to 8.25. According to Donnan effect, the more pH value is close to IEP, the weaker is the electric repulsion. This outline illustrates that the electric effect contributes to the species-membrane interaction and is explained by Donnan effect (Equation ( 2)) [START_REF] Afonso | Transport of MgSO4, MgCl2, and Na2SO4 across an amphoteric nanofiltration membrane[END_REF][START_REF] Violleau | D´emin´eralisation par ´electrodialyse en pr´esence d'un complexant: Application au Lactoserum[END_REF].

Results and discussion

( [𝐴] 𝑚 [𝐴] 𝑠 ) 1 𝑍𝑎 ⁄ = ( [𝐶] 𝑠 [𝐶] 𝑚 ) 1 𝑍𝑐 ⁄ (2)
where [A]m and [C]m represents the concentrations of the anion and the cation at the surface of the membrane, [A]s and [C]s their concentrations within the solution. Za and Zc represent the valences of anion and cation respectively. The higher the charge of the co-ion to the membrane, the higher is the rejection. On the other hand, the higher the charge of the counterion, the lower is the rejection of the co-ion [START_REF] Condom | Use of ultra-and nanofiltration ceramic membranes for desalination[END_REF][START_REF] Van Gestel | Salt retention in nanofiltration with multilayer ceramic TiO2 membranes[END_REF][START_REF] Peeters | Retention measurements of nanofiltration membranes with electrolyte solutions[END_REF][START_REF] Tsuru | Ion Separation by Porous Silica-Zirconia Nanofiltration Membranes[END_REF]. When the pH is high in solution, (

[𝐻 + ] 𝑠 [𝐻 + ] 𝑚 ) 1 1 ⁄ tends to 0, ( [𝑈(𝑉𝐼) 𝑛-] 𝑚 [𝑈(𝑉𝐼) 𝑛-] 𝑠 ) 1 𝑍 𝑈 ⁄
also tends to 0, in the membrane the concentration of negatively charged U species tend to 0, the co-ions cannot practically penetrate the membrane. It is the mathematical translation of Donnan exclusion principle.

As observed, U is more rejected than Na whatever the pH. From pH 3 to 5, U is in the form of UO2 2+ , having larger charge than Na + , as co-ions of the positively charged membrane, both are rejected.

According to Donnan relation, (

[𝑈𝑂 2 2+ ] 𝑚 [𝑈𝑂 2 2+ ] 𝑠 ) 1 𝑍 𝑈 ⁄ = ( [𝑂𝐻 -] 𝑠 [𝑂𝐻 -] 𝑚 ) 1 𝑍 𝑂𝐻 ⁄ ( [𝑁𝑎 + ] 𝑚 [𝑁𝑎 + ] 𝑠 ) 1 𝑍 𝑁𝑎 ⁄ = ( [𝑂𝐻 -] 𝑠 [𝑂𝐻 -] 𝑚 ) 1 𝑍 𝑂𝐻 ⁄ .
When pH is low,

[𝑂𝐻 -] 𝑠 [𝑂𝐻 -] 𝑚 tends to negligeable. As a result, ( [𝑈𝑂 2 2+ ] 𝑚 [𝑈𝑂 2 2+ ] 𝑠 ) 1 𝑍 𝑈 ⁄
and

( [𝑁𝑎 + ] 𝑚 [𝑁𝑎 + ] 𝑠 ) 1 𝑍 𝑁𝑎 ⁄ also tends to 0. Zc(UO2 2+ ) = 2, Zc(Na + ) = 1, Zc(OH -) = 1. If [𝑂𝐻 -] 𝑠 [𝑂𝐻 -] 𝑚 is 0.01, then [𝑈𝑂 2 2+ ] 𝑚 [𝑈𝑂 2 2+ ] 𝑠 is 0.0001. [𝑁𝑎 + ] 𝑚 [𝑁𝑎 + ] 𝑠 is 0.01. U is
thus 100 times more rejected than Na. While pH is from 5 to 8.25, the proportion of negatively charged U species UO2(CO3)2 4-, UO2(CO3)2 2-and (UO2)2CO3(OH)3 -increases with the increase of pH value. At the same time, the negative charge on the membrane surface increases. Clearly, according to Donnan effect, U in the form of carbonate complex is largely rejected and reaches the maximum rejection around pH 8. However, Na + , as the counterion of membrane, should pass through the membrane freely. As a result, it has smaller rejection than U. The overall salt rejection is heavily dependent on the rejection of co-ions. Therefore, a higher Donnan potential with the presence of negatively charged uranyl carbonate complex leads to an increase overall salt rejection of the membrane [START_REF] Peeters | Retention measurements of nanofiltration membranes with electrolyte solutions[END_REF][START_REF] Ong | Evaluation of feed concentration effects on salt/ion transport through RO/NF membranes with the Nernst-Planck-Donnan model[END_REF]. That explains that the Na+ rejection is not 0% even that the global membrane charge is negative. The observed trends are in good agreement with the membrane IEP. The given IEP of industrial membrane supplier is 5.5, corresponding to the pH of minimum Na+ rejection. The minimum rejection of U is found at pH 5, not at 5.5. At pH 5.5, green-yellow precipitation was observed in the retentate solutions.

This could be the precipitation of UO2CO3. According to the speciation diagram, the dominant species in solution of U and NaHCO3 at pH 5.5 is UO2CO3(aq), this specie could be precipitated when its maximum solubility is reached. According to PHREEQC calculation, when pH is from 4.75 to 6.0, the saturation index is superior to 0 (> +0.31 when pH is from 5 to 5.5). Concerning a water solution containing CaCO3, when the saturation index is high enough (+0.31 or higher), water has too much CaCO3 in it. Water must precipitate CaCO3 to get itself back under + 0.30 and into the balanced range.

As a result, the abnormal high rejection of U observed at pH 5.5 is due to the UO2CO3 precipitation since U is rejected by the steric effect.

Depending on the pH, different filtration time is required for the filtration to reach the steady state (See Table S1 and Fig. S1.1 in the supporting information). When solution pH value is 5, 5.5, 6 or 6.5, the steady state is more difficult to be reached. When pH is close to IEP, the fluctuation can be very important with a slight change of solution environment. The interaction of the membrane surface and the solution species is not predictable. At pH 5.5, 6 or 6.5, the filtration does not reach the steady state even after 2 h. Close to IEP, the rejection is not stable, it is possible that during filtration there is sometimes a slight excess of positive or negative charge on the membrane surface. However, for pH 3, 4, 7 and 8, the steady state was reached after 1 h. Therefore, at least 2 h for a filtration experiment allows considering a credible rejection. Furthermore, the rejection of U is not depending on the molecular weight cut-off of the ceramic membranes (MWCO). Indeed, the same observation was observed for inorganic membranes of MWCO 1 and 5 kDa with minimum permeate flow rate always found at pH 5.5 (Table S2 and from filtration experiments conducted at different applied pressure. The variation of the applied pressure has no significant influence on the U rejection. The electrical interaction between the membrane and U (co-ion) is very strong and is not affected by the pressure. However, rejection of Na (counter-ion) increases largely with the pressure (Fig. S1.4 in the supporting information). This phenomenon can be explained by the following reason. Transport of ions through an ultra/nano filtration membrane is composed by diffusion and convection [START_REF] Hilal | A comprehensive review of nanofiltration membranes: Treatment, pretreatment, modelling, and atomic force microscopy[END_REF]. At a higher pressure, the permeate flux is higher and the contribution of diffusion becomes less important in comparison to convection, therefore an increase in rejection is observed for monovalent metal ions (counter-ions) [START_REF] Al-Zoubi | Rejection of salt mixtures from high saline by nanofiltration membranes[END_REF]. However, researchers believe that by increasing the pressure and permeate flux, more species are transferred to the surface of the membrane [START_REF] Abdelkader | Nanofiltration as a Pretreatment Step in Seawater Desalination: A Review[END_REF]. As the permeate flux increases, concentration polarization also intensifies, and this will reduce co-ion rejection. In our case, U species in form of UO2(CO3)3 4-, UO2(CO3)2 2-are co-ions of the membrane but are nearly not affected by the pressure. In another study, similar phenomenon is reported, i.e., the rejections of divalent ions (Ca 2+ , Mg 2+ and SO4 2-) are less influenced by the increase of pressure than the monovalent ion (Na + and Cl -) [START_REF] Abdelkader | Nanofiltration as a Pretreatment Step in Seawater Desalination: A Review[END_REF].

Effect of the ionic strength

The nature of the sodium salts by varying the counter-ions as NaCl, Na2SO4 or NaHCO3 has no influence on the rejection of U in the solution of pH 8.25 (Fig. S1.5, S1.6 and Table S1 in the supporting information). The effect of the ionic strength to the rejection of U is evaluated with the solution of U, Despite the different concentration of NaCl, U is always in the form of negatively charged species when the solution pH is 8.25. The only reason for which U is less rejected is the increase of ionic strength. The reason for the decrease of rejections is that the increase of the ionic strength produces a shielding effect responsible for the decrease of the membrane negative charge along the surface.

Accumulation of solute ions at the membrane/solution interface causes screening of the membrane surface charges and compression of the electric double layer. This phenomenon may reduce the ability of the membranes to reject contaminants through electrostatic effects. In this case, with the increase of NaCl concentration, the Na+ couples with the negative charge on the membrane surface, shielding the negative charges and reduces the electrical repulsion (Donnan exclusion) between the membrane and negatively charged U species, as a result leading to lower rejection. M. Oo et al [START_REF] Oo | Implication of zeta potential at different salinities on boron removal by RO membranes[END_REF] have studied on the influence of NaCl salt concentration on boron (B(OH)4 -) rejection with RO membranes at pH 9

and have measured the zeta potential at each salinity. The rejection of boron decreased with the increase of NaCl salt concentration. The zeta potential increased from 􀀀 20 to 50 mV when increasing the NaCl salt concentration from 0.5 g.L -1 to 10 g.L -1 . At higher NaCl concentrations, zeta potentials generally shifted towards the positive region. They assumed that this phenomenon could be attributed to a stern layer which was densely shielded by positive ions. A major limitation is therefore the salinity of the solution and therefore can be problematic in the context of the recovery of U from seawater.

However, with these considerations, it seems interesting to see the applicability on seawaters of weaker ionic strength for concentrating U by ultra/nano filtration. Baltic Sea has a relative low average salinity around 7 g.L -1 [START_REF] Müller | Ocean acidification in the Baltic Sea: involved processes, metrology of pH in brackish waters, and calcification under fluctuating conditions[END_REF]. As presented in Fig. 4, in the solutions of U and NaCl at 0.17 mol.L -1 (10 g.L - 1

), the rejection of U is still 40%, while the rejection of Na is only 2%. The efficient separation of U from Na is still guaranteed in the solution of this salinity.

Study on competitive metal ions in seawater

The rejections of predominant metal elements in seawater, i.e. Ca, Mg, Na, K are compared with the rejection of U obtained from the filtration experiments conducted with the solution of U and NaHCO3 (U (VI) at 8.40 × 10 -5 mol.L -1 , NaHCO3 at 1.0 × 10 -2 mol.L -1 ). Fig. 5 shows the rejections of different metal elements influenced by the solution pH value. The rejection of Mg decreased with the increase of the pH value. The rejection was maximal of 80% when the pH value was close to 3, it decreased successively with the increase of pH value and reached to 0% when pH value is 8. The rejection of Ca decreases from over 60% to 0% when the pH value evolved from 3 to 5.5, then the rejection was found at 20% when pH value is 6, the rejection was of 0% for pH 7 and 8.25. For monovalent alkali metal specie Na, the rejection decreases from 10% to 5% when pH value increases from 3 to 8.25. The rejection of K decreases from 5% at pH 3 to 0% at pH 5.5, then increases to 5% at pH 8.25. At acidic environment, the membrane cannot separate Ca 2+ from Mg 2+ , since the membrane cannot distinguish the solutes of same valence, but it can separate divalent metal cations from monovalent metal cations. Na+ can neither be separated from K + . According to the species distribution of Ca, Mg, Na, K (Fig. S2.1 in the supporting information), all these divalent or monovalent metal species are in the form of cations and none of them precipitates for the whole pH range. As a result, the electric effect contributes to influence the rejection. At low pH (pH < IEP), the membrane is positively charged, X"Cl2 (X"=Ca, Mg) rejection is high because the co-ion is divalent (X 2+ ) and the counterion is monovalent (Cl -). The latter lightly screens the membrane charge. At high pH (pH > IEP), the membrane is negatively charged. XCl2 rejection is low because the co-ion is monovalent (Cl -

) and the counterion is divalent (X 2+ ). The latter screens the membrane charge more strongly. In the case of X'Cl (X'=Na, K), X + and Cl -screen both lightly the membrane at weak concentration. As a result, compared to Mg and Ca, the rejections of Na, K are less influenced by the pH value. Finally, it should be noticed that being studied separately, only U as negatively charged uranyl carbonate complex at pH 8.25 is rejected while all the other metal cations can pass through the membrane. The rejection of U from the other solutions containing only U and Na are also shown in Fig. 6 for a comparison. The rejections of U and the other metals of all these solutions are summarised in Table 1.

Study on natural/synthetic water systems

Comparison of rejections in different solutions

According to the results in Table 1 and Fig. 6, the conclusion can be drawn that the rejection of U is high in solutions containing only U and sodium salts. However, in natural or synthetic solutions containing U, Ca, Mg, Na, and K, the rejection of U is close to 0%, other metal cations Ca, Mg, Na, K are neither rejected at all. U is not rejected at all in the solutions seawater A, seawater B, is 5% rejected in freshwater F.

The separation of U from the other metals in seawaters or in freshwater cannot be realised. Negative rejection is a normal phenomenon and has been reported in other research on organic membranes [START_REF] Jitsuhara | Rejection of inorganic salts by charged ultrafiltration membranes made of sulfonated polysulfone[END_REF][START_REF] Gilron | Experimental analysis of negative salt rejection in nanofiltration membranes[END_REF][START_REF] Yaroshchuk | Negative rejection of ions in pressure-driven membrane processes[END_REF][START_REF] Garcia-Aleman | Permeation of mixed-salt solutions with commercial and porefilled nanofiltration membranes: membrane charge inversion phenomena[END_REF], especially for multi-component electrolyte solutions in the presence of multivalent ions. The membrane properties (e.g., charge and material), the ions in the solution and the operating conditions are some of the parameters that can lead to the membrane surface charge reversal thus to the negative rejections [START_REF] Yaroshchuk | Negative rejection of ions in pressure-driven membrane processes[END_REF]. However, the negative rejections observed here should be the errors, or might be contributed by some complicate electrical mechanisms. The evolution of the rejection of U with time is different depending on the solutions as shown in Table 2. For the solutions containing only U and Na, the rejection of U remains at the same value despite the time. In the solutions containing Ca or Mg, U is partly rejected at the first 30 min of filtration experiment, but after 1.5 h, U is no more rejected at all. The example of freshwater F (Rhone River doped with U of 2 mg.L -1 ).

Table 1 Composition of solution and respective rejection of U, Na, Mg, Ca and K. Table 2 The uranium rejection of the first and the final samples. At the first 30 min, U is 60% rejected, then the rejection decreases to 5% after 2 h. The 0% rejection of U in the Mediterranean seawater is due to the extreme high solution ionic strength (>35 g.L-1) despite the species distribution. However, the uranium species distributions of the synthetic Baltic seawater shows that at least 50% of uranium species are in the form of Ca2UO2(CO3)3, CaUO2(CO3)3 2-, UO2(CO3)3 4- , MgUO2(CO3)3 2-, and Mg2UO2(CO3)3 in alkaline environment (pH 7-9). As discussed previously, the electric effect of the filtration is no longer sufficient for the separation of U from sodium salts when the salinity is higher than 10 g/ L. In the Baltic seawater, the salinity is around 7-8 g/L, if Mg or Ca salts have no other effect to the electric effect, the rejection of U should be at least 50%. In the freshwater F, with even a lower ionic strength, only 5% of the U is rejected. According to its speciation, U is 85% in the form of Ca2UO2(CO3)3 and 15% in the form of CaUO2(CO3)3 2-. The incoherence between the experimental results and the theoretical species distribution highlight that the electric repulsion is more strongly shielded in seawaters and freshwaters. The electric effect is shielded more easily when there is Mg or Ca in the solution.

Influence of Ca or Mg in solution to the rejection of U

To estimate the influence of divalent cations Mg 2+ and Ca 2+ on their ability to shield the electric effect, the filtration experiments with the solution of U (8.4 × 10 -5 mol.L-1), Na (1.0 × 10 -2 mol.L -1 ) with CaCl2 of different concentration (2.16 × 10 -5 -5.82 × 10 -4 mol.L -1 ) are implemented. MgUO2(CO3)3 2-decreases sharply, the proportion of Ca2UO2(CO3)3 is over 80% when [X] is higher than 3 mmol.L -1 . The rejection of U in solution of low ionic strength containing U, Mg, Ca and Na should be at least higher than 15%, which is not observed with fresh water. To conclude, U can be separated and concentrated by filtration process by electrical repulsion in solutions at alkaline environment of salinity weaker than 10 g.L -1 with no presence of Mg or Ca.

Interpretations and discussions of the observed phenomenon

Three possible reasons which caused the 0% rejection of U when Ca or Mg is in the solution are proposed as following. The first possible reason is that, despite the existence of negatively charged complexes CaUO2(CO3)3 2-and MgUO2(CO3)3 2-, they are bonded to cations present in solutions. As a result, together with the cations around these complexes, the global charge is not negative to be rejected by the membrane (Fig. 8 (a)). Satoru Tsushima et al found that the uranyl tricarbonate complex, which has high negative charge (-4), is expected to have strong interactions with the counter ions. It suggests that counter ion(s) play a very important roles in the electronic and geometric structures of the uranyl tricarbonate complex [START_REF] Tsushima | A theoretical study on the structures of UO2(CO3)43-, Ca2UO2 (CO3)3, and Ba2UO2(CO3)3[END_REF]. Shelly D. Kelly et al demonstrated with X-ray absorption spectroscopy measurement that the number of calcium and/or sodium atoms coordinated to a uranyl carbonate complex will determine the net charge of the complex. On the average, approximately 2 calcium atoms are bound to the uranyl carbonate species at the highest concentrations and 2-4 sodium atoms are bound to the uranyl carbonate species when calcium is absent. The effect on the bond strength with more single charged sodium atoms (4Na + atoms), as compared to fewer doubly charged calcium atoms (2 Ca 2+ atoms) bound to the UO 2 (CO3)3 4-moiety, would suggest that the UO2(CO3)3 4-Na4 4+ bond is weaker than the UO2(CO3)3 4-Ca2 4+ bond [START_REF] Kelly | X-ray absorption spectroscopy identifies calcium-uranylcarbonate complexes at environmental concentrations[END_REF]. Weihong Wu et al suggested that the structure of the Ca2UO2(CO3)3 complex is very stable in the seawater model. A Na + ion was found to be closely associated with the Ca2UO2(CO3)3 complex at a U -Na distance of 5.25 Å, while the Cl ion is at least 11 Å away from U. The Na + ion interacts indirectly with one axial oxygen atom of the UO2 group bridged by a water molecule.

In addition, the Na + ion interacts more closely with one Ca 2+ ion than the other. The present simulations revealed the key role of common ions such as Na+ in impacting the solvation, structure, and apparent charge of the Ca2UO2(CO3)3 complex in seawater [START_REF] Wu | Solvation of the Ca2UO2 (CO3)3 Complex in Seawater from Classical Molecular Dynamics[END_REF]. Another possible reason is that the Ca 2+ or Mg 2+ have been adsorbed on the membrane surface. As a result, the membrane surface is less negatively charged. At the same time, Donnan potential is known to be seriously influenced by the valance of ions present in the feed. Specifically, the Donnan potential is weakest in solutions with a higher concentration of divalent cations [START_REF] Higa | A novel measurement method of Donnan potential at an interface between a charged membrane and mixed salt solution[END_REF]. The influence of the electrolyte types and concentration (1 mM KCl, 0.1 mM CaCl2, 1 mM CaCl2 and 1 mM MgSO4) to the membrane zeta potential along the surface in the pH range 4.0-8.3, with the electrolytes have been implanted by M. Teixeira et al [START_REF] Teixeira | The role of membrane charge on nanofiltration performance[END_REF]. In the presence of Ca 2+ , the tested membrane was more positively charged over the entire pH range and the IEP shifts from 4.2 to 5-6. When both divalent cations (Mg 2+ ) and anions (SO4 2-) were present in solution, the effect of the divalent anion was opposite to the effect of the divalent cation, Mg 2+ adsorbs less than Ca 2+ , therefore the zeta potential curve was intermediate between the curves obtained for KCl and CaCl2. S. Deshmukh et al [START_REF] Deshmukh | Zeta potential of commercial RO membranes: Influence of source water type and chemistry[END_REF] has investigated the influence of feed water on the zeta potential of the membrane surface. They also observed the zeta potential has increased with river water. They assumed that the divalent cations (Ca 2+ , Mg 2+ ) have been absorbed on the membrane surface. M.

Giagnorio et al [START_REF] Giagnorio | Achieving low concentrations of chromium in drinking water by nanofiltration: membrane performance and selection[END_REF] have worked on the rejection of CrO4 2-with polyamide membranes which are negatively charged under natural pH conditions. The effect of the ionic composition and of the salt concentration has been evaluated. They observed that Cr(VI) rejection decreased with the increase of salt concentration, but NaCl had less effect on Cr(VI) rejection than CaCl2 at the same ionic strength and the same permeate flux. As a result, this phenomenon was not attributed to concentration polarization. They assumed that the presence of divalent cations in solution might induce the formation of ion pairs with the negative functional groups at membrane surface, thus lowering its overall charge and allowing the passage of Cr(VI) [START_REF] Giagnorio | Achieving low concentrations of chromium in drinking water by nanofiltration: membrane performance and selection[END_REF]. Their measurements on the zeta potential corroborated this discussion. At neutral pH, the potential was much lower in magnitude in CaCl2 solutions compared with NaCl solutions with similar ionic strength. This result strengthens the hypothesis of specific chemical interactions of calcium with membrane moieties, resulting neutralization of charges and causing the easier passage of Cr(VI) through membranes governed by electrostatic effects as separation mechanism. The larger effect of calcium compared to sodium at similar ionic strengths is attributed to chemical pairing of this divalent cation with the functionalities of the membrane, which lowers the overall surface potential. Illustrated in Fig. 8 (b), in our case, the inorganic membranes used in our research have amphoteric behaviour of mineral oxides, hydroxyl groups on the material surface can also react with cations in the solution. Ca 2+ , Mg 2+ could react with the membrane surface then shield the negative charges at the membrane interface significantly decreased the rejection of co-ions (U species) at pH 8.25. To confirm this hypothesis, zeta potential measurements should be conducted. The final and the most probable reason is the 0% rejection of U with the presence of Ca 2+ and Mg 2+ could be the continuous formation of Ca2UO2(CO3)3 and Mg2UO2(CO3)3 on the membrane surface for establishing the reaction equilibrium during the filtration (Fig. 8 (c)). This hypothesis is based on the hypothesis that the membrane surface charge is not screened by Ca 2+ or Mg 2+ : The kinetics of mixed U-Ca-CO3 complex formation due to the very high association constant between UO2 2+ , Ca 2+ and CO3 2-is clearly higher than the transport rate of the permeate at the outlet of the membrane towards the reservoir of the feed caused by the big difference between the flow rates of permeate and retentate, and by the long distance for permeate to return to the feed tank. The permeate flux is around 0.1-0.3 L/h, negligible to the retentate flux of 100 L/h. The membrane carter is far to the feed tank, there is a large difference between the time for the permeate and the retentate to return into the feed. Meanwhile, the space between the membrane and the membrane carter can be considered as an intermediate recipient of a capacity to contain 50-100 mL permeate solution. As a result, when the species composition of permeate is totally different to the retentate, the chemical equilibrium shift at this intermediate recipient could happen. Fig. 8 (c) shows a schematic explanation of this hypothesis. At the beginning of the filtration experiment, more than 50% of U under Ca2UO2(CO3)3 form passes through the membrane, while CaUO2(CO3)3 2-is rejected. The "lack" of Ca2UO2(CO3)3 on the membrane surface becomes a stress of the equilibrium. According to Le Chatelier principle, and based on the continuous equilibrium displacement, the system continues to proceed toward a new equilibrium, leading to encourage the reaction to produce more Ca2UO2(CO3)3 from CaUO2(CO3)3 2-. As a result, the chemical equilibrium is never reached until all CaUO2(CO3)3 2- transfers to Ca2UO2(CO3)3. Once Ca2UO2(CO3)3 is produced, the reverse reaction to form CaUO2(CO3)3 2- is unfavourable to proceed since the chemical equilibrium constant for producing Ca2UO2(CO3)3 is higher than that of CaUO2(CO3)3 2-[58]. The same trend is applicable with Mg-UO2-CO3 species, as represented in equations ( 3)-( 6) [3,4,]. Therefore, all U passes through the membrane since there is no more enough negatively charged U species to induce the electrical repulsion interaction.

Fig. 2

 2 Fig. 2 The species distribution of aqueous U complexes in solution 1, seawater A, seawater S-D (2), fresh water F-B. (a) Solution 1: U 20 mg.L -1 , NaHCO3 0.84 g.L -1 . [U] = 8.40×10 -2 mmol.L -1 , [Na] = 1.0×10 - 2 mol.L -1 , [C] = 1.0×10 -2 mol.L -1 . (b) Seawater A: Mediterranean seawater. I = 0.534 mol.L -1 . [U] = 1.426×10 -5 mmol.L -1 , [Na] = 0.319 mol.L -1 , [Mg] = 5.225×10 -2 mol.L -1 , [K] = 1.218×10 -2 mol.L -1 , [Ca] = 1.19×10 -2 mol.L -1 , [Cl] = 0.437 mol.L -1 , [C] = 2.388×10 -3 mol.L -1 , [S] = 9.897×10 -3 mol.L -1 . (c) Seawater S-D (2): Synthetic Baltic seawater doped with U. I = 0.135 mol.L -1 , [U] = 4.201×10 -3 mmol.L -1 , [Na] = 9.112×10 -2 mol.L -1 , [Mg] = 1.034×10 -2 mol.L -1 , [K] = 2.57×10 -3 mol.L -1 , [Ca] = 2.72×10 -3 mol.L -1 , [Cl] = 0.115 mol.L -1 , [C] = 8.524×10 -4 mol.L -1 , [S] = 1.999×10 -3 mol.L -1 . (d)Fresh water F-B: Rhone River doped with U. I = 6.944×10 -3 mol.L -1 , [U] = 8.405×10 -3 mmol.L -1 , [Na] = 5.458×10 -4 mol.L -1 , [Mg] = 2.385×10 -4 mol.L -1 , [K] = 5.92×10 -5 mol.L -1 , [Ca] = 1.693×10 -3 mol.L -1 , [Cl] = 5.841×10 -4 mol.L -1 , [C] = 2.753×10 -3 mol.L -

1 ,

 1 [S] = 5.613×10 -4 mol.L -1 . Drawn by PHREEQC, based on the formation constants of Ca-UO2-CO3, Mg-UO2-CO3, NEA TDB database updated in December 2020. Minor U species are not shown.

Fig. 2 (

 2 Fig. 2 (a), (b), (c) and (d) show respectively the species distribution of solution 1, seawater A, seawater B, freshwater F. In solution 1, U is present in the form of UO2(CO3)3 4-, UO2(CO3)2 2-and (UO2)2CO3(OH) 3- at pH 8.25, which are all negative charge species.

3. 1 .

 1 Uranium and sodium rejections 3.1.1. Effect of the pH, MWCO, temperature and pressure Various parameters (pH, temperature, Pressure, MWCO) have been investigated to study the possibility of U/Na separation by membrane filtration, for this purpose a solution consisting of U(VI) at 8.40 × 10 -5 mol/L and NaHCO3 at 1.0 × 10 -2 mol/L was studied. The effect of pH, ranging from 2.3 to 8.25, on U and Na rejection was studied using 5 kDa membrane, with an applied pressure of 3 bars, temperature around 22 ± 2 •C, a retentate flux of 0.1 m 3 /h. The rejections of U and Na according to pH have similar profiles as depicted in Fig. 3.

Fig. 3 .

 3 Fig. 3. Rejections of U and Na as a function of pressure with solutions of pH 8.25; Feed solution: solution 1, pH varies from 2.3 to 8.25.

  Fig. S1.2 in the supporting information). The influence of temperature to the rejection of U is evaluated with experiments at various temperature (10, 22 and 35 •C), the temperature has no influence on the uranium rejection when pH value is very different from IEP (Fig. S1.3 in the supporting information). Fig. S1.4 shows the rejections of U and Na in solution of pH 8.25

NaCl of different concentrations ( 1 . 7 ×

 17 10 -2 to 0.60 mol.L -1 (1-35 g.L -1 , i.e. I = 0.017 to 0.609 mol.Kg-1 ) and NaHCO3 (0 for [NaCl] from 0 to 8.5 × 10 -2 mol.L -1 or 1.0 × 10 -2 mol.L -1 for [NaCl] > 8.5 × 10 -2 mol.L-1). The addition of NaHCO3 guarantees that there is no uranium precipitation in the solution. The effect of salt concentration was studied using 5 kDa membrane, with an applied pressure of 3 bars, temperature around 22 ± 2 •C, a retentate flux of 0.1 m 3 /h. According to Fig.4, the ionic strength controlled by the NaCl salt concentration has a remarkable influence on the rejection of U. The rejection of U decreases from 100% to 0% with the increase of NaCl concentration from 1.7 × 10 -2 to 0.60 mol.L -1 (1-35 g.L -1 , i.e. I = 0.017 to 0.609 mol.Kg -1 ). At the same time, the rejection of Na decreases from 10% to 0%. At 8.5 × 10 -2 mol.L -1 of NaCl, U rejection is about 50% while Na is only 2.5% rejected. Species distribution of these solutions of U, NaCl and NaHCO3 have been simulated and are shown in the supporting information.

Fig. 4 .

 4 Fig. 4. Rejections of U and Na as a function of NaCl salt concentration.

Fig. 3

 3 Fig. 3 Influence of pH on the rejections for U, Ca, Mg, Na, K from different solutions, with pH varying from 3 to 8.25: solution 1. [U] = 8.4×10 -5 mol.L -1 , [NaHCO3] = 1.0×10 -2 mol.L -1 ; solution 2. [Ca] = 5.00×10 - 4 mol.L -1 (CaCl2); solution 3. [Mg] = 8.23×10 -4 mol.L -1 (MgCl2); solution 4. [Na] = 8.69×10 -3 mg.L -1 , [K] = 5.13×10 -4 mol.L -1 (NaCl, KCl).

Filtration experiments of naturalFig. 6 .

 6 Fig. 6. The rejection of U in the solutions containing only U and Na, and in the solutions containing U, Na, Ca and Mg.

Fig. 7 ( 1 of

 71 a) displays the comparison of experimental rejections and theoretical rejections of U and Ca. The experimental rejections are calculated with formula (1) with the concentrations of U and Ca in retentate and permeate samples measured by ICP-OES. Both the rejection of U and Ca decreases from 90% to about 10% when Ca concentration increases from 2.16 × 10 -5 to 5.82 × 10 -4 mol.L -1 (CaCl2 from 2.4 mg.L -1 to 64 mg.L -1 ). The theoretical rejection of U equals to the proportion of negatively charged U species in the form of UO2(CO3)3 4-and CaUO2(CO3)3 2-in solution, calculated by ([UO2(CO3)34-] + [CaUO2(CO3)3 2- ])/[U] at each solution composition (Fig. 7 (b)), the theoretical rejection of Ca equals to the proportion of the negatively charged Ca species, i.e., CaUO2(CO3)3 2-in solution, calculated with [CaUO2(CO3)3 2- ]/[Ca] at each solution composition (Fig. 7 (c)). Fig. 7 (b) and (c) shows respectively the species distribution of U and Ca as a function of CaCl2 concentration. According to Fig. 7 (a), the theoretical rejection of Ca can fit the experimental rejection of Ca. However, the theoretical rejection of U doesnot fit to its experimental rejection, demonstrating that the species distribution of U cannot predict the rejections when Ca coexists with U in the solution. These experiments demonstrates that the rejection of U decreases sharply with the increase of Ca concentration, and U is only 15% rejected when Ca concentration is of 5.82 × 10 -4 mol.L -1 , corresponding to 23 mg.L -1 . Generally speaking, Ca concentration is over 50 mg.L -1 in fresh waters (e.g. river waters) and over 500 mg.L-1 in salty waters (e.g. seawater), the Ca concentration in these natural waters are higher. Accordingly, it is logic that the rejection of U is only 5% in Rhone water and 0% in seawaters of low salinity. The same calculations of species distribution of U and Mg in solutions containing U and MgCl2 are shown in Fig.S2.12 and Fig.S2.13 in the supporting information. Like Ca, the species distribution of U cannot explain the 0% rejection of U in fresh waters. According to the speciation diagrams, the proportion of negatively charged uranium species, i.e., UO2(CO3)3 4-and MgUO2(CO3)3 2-is always 100% despite different concentration of Mg, predicting that U is supposed to be 100% rejected in any solutions of low ionic strength only containing U, Na and Mg. In a filtration experiment conducted with the solution containing 20 mg.L -1 of U, 4.2 g.L -1 of MgCl2 and 0.84 g.L -1 of NaHCO3, the rejection of U was 0%. In another experiment conducted with the solution containing 20 mg.L -1 of U, 1.0 g.L -1 of MgCl2, 5.0 g.L -NaCl and 0.84 g.L -1 of NaHCO3, the rejection of U was 4.6%. Fig. S2.14 and Fig. S2.15 show the species distribution of U, Ca and Mg as function of the concentration of divalent salt XCl2 (X = Ca, Mg) in solution. With the increase of concentration [X], the total proportion of UO2(CO3)3 4-, CaUO2(CO3)3 2-and

Fig. 4

 4 Fig. 4 (a) Theoretical rejection and experimental rejection of U and Ca of filtration with solution 13. (b) Species distribution of U in solution 13. (c) Species distribution of Ca in solution 13. Solution composition: U 8.4×10 -5 mol.L -1 , NaHCO3 1.0×10 -2 mol.L -1 , CaCl2 2.16×10 -5 ~5.82×10 -4 mol.L -1 . Drawn by PHREEQC, based on the formation constants of Ca-UO2-CO3, Mg-UO2-CO3, NEA TDB database updated in December 2020. Minor U species are not shown.

Fig. 5

 5 Fig. 5 Illustration of three hypothesis for the 0% rejection of U. (a) Negatively charged uranium species bonded to sodium cation. (b) The membrane surface screened by Mg 2+ or Ca 2+ . (c) The equilibrium shift.

UO2 2 -

 2 +Ca 2+ +3CO3 2-→ CaUO2(CO3)3 2-log β • 113 = 25.4±0.25 (3) UO2 2-+2Ca 2+ +3CO3 2-→ Ca2UO2(CO3)3 log β • 213 = 30.45±0.35 (4) UO2 2-+Mg 2+ +3CO3 2-→ MgUO2(CO3)3 2-log β • 113 = 25.8±0.30(5)UO2 2-+2Mg 2+ +3CO3 2-→ Mg2UO2(CO3)3 log β • 213 = 27.1±0.60[START_REF] Diallo | Mining Critical Metals and Elements from Seawater: Opportunities and Challenges[END_REF]

Acknowledgment

The authors are grateful to the China Scholarship Council (CSC) for PhD funding. The authors acknowledge Béatrice Baus-Lagarde, for her help regarding the ICP/OES and ICP/MS experiments.

Conclusions

The objective of this study was to examine the possibility to separate and/or concentrate uranium from other salts in water systems by UF/NF filtration process with inorganic membranes, and evaluating the rejection mechanism, the interaction of membrane with solution species. Influence of various physio-chemical parameters to solute rejections during filtration were studied, for example, MWCO, pH value of solutions, temperature, applied pressure, ionic strength/salinity, composition of the solutions, etc. Several conclusions were drawn based on the above experiments. Electrical repulsion and attraction is the key mechanism for the solute rejection, therefore pH value is an important determining factor of solute rejection since the species distribution could be quite different when pH changes. The separation of U to Na can be performed at pH 3 and 8.25 in solutions of U and NaHCO3. MWCO, temperature does not influence the rejection of U, higher applied-to-membrane pressure dose not permeate higher U rejection at pH 8.25. Nevertheless, fluctuations can be enormous when pH value is close to IEP. With filtration experiments of solutions with U, NaHCO3 and NaCl (1-35 g.L -1 ), a huge impact of solution ionic strength to rejections was observed. U rejection decreases from 100% to 0% when increasing the NaCl concentration from 1 g.L -1 to 35 g.L -1 . The separation of U from Na can still be guaranteed when the salinity is lower than 10 g.L -1 . The filtration experiments of natural and synthetic solutions including seawaters, Rhone River show that none of the solutes (U, Na, Mg, Ca, K) can be rejected by Donnan electrical exclusion. Finally, filtration experiments on solutions of U with CaCl2 at different concentration demonstrated that U rejection decreases with the increase of Ca/Mg concentration. Three hypotheses have been proposed to explain the invalidity of electric effect to reject U in solutions with Ca or Mg. Further studies on applying complexation with ultra/nano filtration to concentrate U from seawaters are under investigation.


Childbirth as an Anticipated Trauma during Pregnancy: Pretraumatic Stress Symptoms in Primiparous Women

Keywords: Pretraumatic stress, Pregnancy, Primiparous, Traumatic reactions, Anticipation

Introduction: While childbirth is predictable, some aspects of delivery remain unknown, which can lead women to perceive childbirth threatening. Thus, women can develop anticipated traumatic reactions related to the forthcoming delivery through "pretraumatic stress" symptoms. The present cross-sectional study aims at extending prior findings by: a) exploring the frequency of probable pretraumatic stress disorder in primiparous pregnant women, b) evidencing associated features of pretraumatic stress symptoms and c) exploring which specific components of antenatal anxiety are associated with pretraumatic stress symptoms. Methods: A sample of 100 primiparous pregnant women completed an online questionnaire assessing pretraumatic stress, fear of childbirth, depressive and anxiety symptoms. Socio-demographic and pregnancy-related data were also gathered. Results: 8 % of women met all criteria for probable pretraumatic stress disorder with the forthcoming delivery as the traumatic event. Increased depressive symptoms (ß = 0.48, p< .05), childbirth concerns (ß = 0.47, p< .05) and worry about self (ß = 0.74; p< .05) were associated with the intensity of pretraumatic stress disorder symptoms. Conclusion: This study contributes to the very limited literature on pretraumatic stress symptoms. To the best of our knowledge, this is the first study conducted exclusively on primiparous pregnant women. Thus, it is noteworthy that pretraumatic stress is not a reactivation of a former postpartum PTSD or associated with a prior negative experience of childbirth. Future studies conducted on primiparous women with no history of traumatic exposure could allow to provide additional evidences of the existence of anticipated traumatic reactions of childbirth.

Introduction

Pregnancy is an important event for women that might lead to psychological impairments [START_REF] Asselmann | Changes in psychopathological symptoms during pregnancy and after delivery: A prospectivelongitudinal study in women with and without anxiety and depressive disorders prior to pregnancy[END_REF][START_REF] Johnson | Screening for mental health disorders among pregnant women availing antenatal care at a government maternity hospital in Bengaluru city[END_REF]. Many studies have been conducted on pregnancy-related psychopathological disorders with a special attention paid to depression that affects 15% of women (see [START_REF] Okagbue | Systematic Review of Prevalence of Antepartum Depression during the Trimesters of Pregnancy[END_REF], for a systematic review).

Anxiety is also quite common during pregnancy [START_REF] Grigoriadis | A systematic review and meta-analysis of the effects of antenatal anxiety on postpartum outcomes[END_REF] with a prevalence of self-reported anxiety symptoms ranging from 18.2 to 24.6 % depending on the trimesters of pregnancy (see Dennis, et al., 2018, for a meta-analytic review).

To date, while the majority of research focuses on anxiety and depressive disorders during pregnancy, traumatic reactions are receiving an increasing attention. Given that mothers can perceive a threat to their own life and/or the infant's life (American Psychiatric Association, APA, 2013), giving birth can lead to Posttraumatic Stress Disorder (PTSD) symptoms [START_REF] Dekel | Childbirth induced posttraumatic stress syndrome: a systematic review of prevalence and risk factors[END_REF]. As for any other traumatic events, symptoms of PTSD following childbirth include avoidance, intrusions, negative alterations in cognitions and mood as well as alterations in arousal and reactivity (APA, 2013). According to meta-analytic reviews, the mean prevalence of postpartum PTSD ranges from 3.1%, to 4.7 depending on the reviews [START_REF] Grekin | Prevalence and risk factors of postpartum posttraumatic stress disorder: a meta-analysis[END_REF][START_REF] Heyne | Prevalence and risk factors of birth-related posttraumatic stress among parents: A comparative systematic review and metaanalysis[END_REF].

Experiencing PTSD symptoms is not only distressing for the mother but also for the entire family. Indeed, it is now acknowledged that maternal PTSD symptoms can lead to marital difficulties (e.g. [START_REF] Garthus-Niegel | The impact of postpartum posttraumatic stress and depression symptoms on couples' relationship satisfaction: a population-based prospective study[END_REF][START_REF] Parfitt | The effect of post-natal symptoms of post-traumatic stress and depression on the couple's relationship and parent-baby bond[END_REF]. Postpartum PTSD can also have a negative impact on mother-infant bonding (Muziket al., 2013;[START_REF] Parfitt | The effect of post-natal symptoms of post-traumatic stress and depression on the couple's relationship and parent-baby bond[END_REF] or on the infant's socio-emotional development (e.g. [START_REF] Enlow | Maternal posttraumatic stress symptoms and infant emotional reactivity and emotion regulation[END_REF][START_REF] Garthus-Niegel | The impact of postpartum post-traumatic stress disorder symptoms on child development: a population-based, 2-year follow-up study[END_REF]. Thus, early detection of postpartum PTSD appears of prime importance.

Consistent with findings from the general trauma literature, (see. [START_REF] Brewin | Meta-analysis of risk factors for posttraumatic stress disorder in trauma-exposed adults[END_REF] for a meta-analytic review on risk factors for PTSD), studies focusing on postpartum PTSD have suggested that traumatic history is an important risk factor to take into account (e.g. [START_REF] Lev-Wiesel | The role of peripartum dissociation as a predictor of posttraumatic stress symptoms following childbirth in Israeli Jewish women[END_REF][START_REF] Lev-Wiesel | Childhood sexual abuse as a predictor of birth-related posttraumatic stress and postpartum posttraumatic stress[END_REF][START_REF] Oh | Comorbid trajectories of postpartum depression and PTSD among mothers with childhood trauma history: Course, predictors, processes and child adjustment[END_REF]. Unlike the majority of potentially traumatic events that cannot be predictable, childbirth is an expected event that paves the way for pregnant women to develop anticipated traumatic reactions regarding their upcoming delivery. While childbirth is predictable, some aspects of labour remain unknown. Thus, this aspect of delivery may lead pregnant women to perceive childbirth as a threat [START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF]. Even though research in this area is still at an early stage, such reactions named "pre traumatic stress symptoms", have been found in pregnant women.

To date, pretraumatic stress disorder is not considered as a disorder per se and is not mentioned in any of the classifications of mental disorders (e.g. Diagnostic and Statistical Manual of Mental Disorders 5th edition-DSM-5, APA, 2013). Nevertheless, an adaptation of PTSD diagnostic criteria was proposed to explore clinical features of probable pretraumatic stress disorder. Thus, based on DSM-5 criteria for PTSD adapted for a forthcoming traumatic event, this proposed disorder consists of 4 categories of symptoms: intrusion, avoidance of thoughts and behaviours, changes in arousal and reactivity as well as negative changes in thoughts and mood. As an example, regarding intrusive symptoms, women can experience pre-experiencing repeated, unwanted and disturbing images of the forthcoming delivery such as flash-forwards. Avoidance symptoms consist of avoiding thoughts related to the upcoming delivery, or conversations related to childbirth. Thus, pretraumatic stress reactions are similar to PTSD symptoms but, instead of being related to a prior traumatic event, they develop as a result of the anticipated traumatic appraisal of the forthcoming childbirth. As for any other trauma-and stressor-related disorder, the severity of symptoms can negatively affect daily functioning.

A study conducted on a sample of 1,224 Swedish women has found that of all participants, 2.3% met all DSM-IV criteria for PTSD related to their forthcoming delivery and 5.8% fulfilled criteria B, C, and D [START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF]. A study conducted on a sample of 102 French women has also reported similar results with 8.8% of women meeting all criteria, 34.3% meeting criterion B, 14.7% meeting criterion C and 64.7% meeting criterion D [START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF]. Pretraumatic stress symptoms have also been found in a sample of 610 male soldiers before deployment to Afghanistan [START_REF] Berntsen | Pretraumatic stress reactions in soldiers deployed to Afghanistan[END_REF].

Noteworthy, there was a positive correlation between pretraumatic stress symptoms and several well-known risk factors for PTSD (e.g. Neuroticism, depressive symptoms).

Interestingly, findings from this longitudinal study have also evidenced PTSD symptoms during and after a 6-month deployment to Afghanistan and the intensity of pretraumatic stress symptoms before deployment predicted subsequent PTSD symptoms [START_REF] Berntsen | Pretraumatic stress reactions in soldiers deployed to Afghanistan[END_REF]. Taking together, these findings suggest that pretraumatic reactions might exist in the context of a predictable trauma, regardless of the event or the gender [START_REF] Berntsen | Pretraumatic stress reactions in soldiers deployed to Afghanistan[END_REF][START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF][START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF].

While studies in the area are still lacking, several factors associated with pretraumatic stress symptoms in pregnant have been reported. [START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF] have found that history of psychological problems, prior psychiatric counseling related to childbirth, elevated anxiety, and depressive symptoms were significant risk factors for pretraumatic stress symptoms. [START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF] have evidenced fear of childbirth, fear of labour pain, antenatal anxiety and depressive symptomatology as significant independent predictors of the intensity of pretraumatic stress symptoms. While adding important findings into the field of pretraumatic stress, it is important to note that these studies have been conducted on both primiparous and multiparous women. As a consequence, it cannot be excluded that anticipated trauma of childbirth could be due, for multiparous women, to a prior traumatic experience of childbirth. Thus, replicating these studies on primiparous pregnant women appears of prime importance to fully assess the existence of pretraumatic stress. Prior studies have used the State-Trait Anxiety Inventory (STAI) to measure anxiety traits [START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF], or the anxiety subscale of the Hospital Anxiety and Depression Scale (HADS) to assess anxiety symptoms [START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF]. Even though anxiety appears to be a potential risk factor for pretraumatic stress symptoms, no data is available regarding whether or not such anxiety was specifically related to pregnancy. Pregnancy-related anxiety refers to the fear, and worry related to pregnancy, childbirth, infant's health and other pregnancyrelated financial or social issues [START_REF] Bayrampour | Pregnancy-related anxiety: A concept analysis[END_REF]. Women suffering from pregnancy-related anxiety symptoms can also develop concerns about their physical appearance or their capability as a mother [START_REF] Rini | Psychological adaptation and birth outcomes: the role of personal resources, stress, and sociocultural context in pregnancy[END_REF]. To date, several scales exist for measuring pregnancy-related anxiety (see [START_REF] Brunton | Pregnancy anxiety: A systematic review of current scales[END_REF], for a systematic review). A recent review aiming at identifying available measurement worldwide has evidenced that the Pregnancy-Related Anxiety Scale (PRAS) (Van den [START_REF] Van Den Bergh | The influence of maternal emotions during pregnancy on fetal and neonatal behavior[END_REF]) and the Pregnancy-Related Anxiety Scale (PRAS) (Brunton et al., 2019) were the most commonly used. As a consequence, using measures covering several aspects of pregnancy-related anxiety is needed to provide additional findings in the field of pretraumatic stress.

Recent studies have emphasized the need to focus on pretraumatic stress as it might be involved in the development of postpartum PTSD [START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF][START_REF] Goutaudier | Traumatic anticipation of childbirth and disordered eating during pregnancy[END_REF]. Indeed, pretraumatic stress symptoms were highlighted as a risk factor for subsequent PTSD following childbirth [START_REF] Söderquist | Risk factors in pregnancy for post-traumatic stress and depression after childbirth[END_REF][START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF], suggesting the existence of a probable continuum of traumatic reactions from pre to postpartum and the possibility for traumatic reactions related to childbirth to be early screened and prevented. Thus, identifying factors associated with pretraumatic stress symptoms can have clinical implications not only for pregnant women but also for women in the postpartum period.

The present cross-sectional study aims at extending prior findings by: a) exploring the frequency of probable pretraumatic stress disorder symptoms related to the forthcoming delivery in primiparous pregnant women, b) evidencing associated features of pretraumatic stress symptoms and c) exploring which specific components of antenatal anxiety are associated with posttraumatic stress symptoms. Based on prior studies, we made the hypothesis that fear of childbirth, prior traumatic exposure, antenatal anxiety and depressive symptoms would be correlated with the intensity of pretraumatic stress symptoms related to the forthcoming delivery.

Methods

Participants and procedure

Potential participants, recruited through social networks, were informed that the study aimed at exploring their experience of pregnancy. They were informed that participation will be anonymous and not financially compensated. They were also informed that data will only be analyzed for scientific purposes. Inclusion criteria were: being over 18, speaking French fluently, and being primiparous. A total of 100 primiparous pregnant women (mean (SD) age = 26.13 (4.82); min = 18, max = 41 years old) agreed to participate. Mean week's gestation was 28.17 (range 8-41; SD = 8.6). A consent form was signed by ticking a box before the completion of the online survey. The study received the approval of the French data protection authority and followed the ethical guidelines of the Helsinki declaration (see table 1 for participants' characteristics).

- -------------------------------------Insert table 1 here-------------------------------------------

Measures

Background data. A background data questionnaire was created. Regarding sociodemographic variables, items were used to obtain information about whether or not women lived with their life partner and were employed (yes/no answers). Data about educational level (multiple choice answer) and age (open-ended answer) were also collected.

Concerning psychological, traumatic, and pregnancy-related variables, women were asked whether or not they experienced a prior traumatic event, had a gynecological history, were hospitalized during pregnancy, planned an epidural anesthesia or attended preparation to childbirth classes (yes/ no answers). They were also asked to provide information about week's gestation (open-ended answer) and type of delivery planned (multiple choice answer).

Pretraumatic stress symptoms. Pretraumatic stress symptoms (i.e. traumatic symptoms related to the forthcoming delivery) were assessed using the French version of the Traumatic Event Scale [START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF]. The TES, was developed in accordance with the DSM-IV criteria for PTSD. Four items focus on criterion A for PTSD and 17 items on criteria B, C, and D. In this 21-item self-report questionnaire, the forthcoming delivery is used as the traumatic event (example of item: "I have unpleasant dreams about childbirth").

Items are rated on a 4-point scale (never/not at all, rarely, sometimes, or often) with increased scores indicating an increased level of pretraumatic stress symptoms. This scale measures 5 dimensions of pretraumatic stress symptoms (anticipation of trauma, hyperarousal, numbing, avoidance, intrusive symptoms). In our sample, α = .89.

Fear of Childbirth. Fear of childbirth was assessed using the Wijma Delivery

Experience/Expectancy Questionnaire (W-DEQ) version a [START_REF] Wijma | Psychometric aspects of the W-DEQ; a new questionnaire for the measurement of fear of childbirth[END_REF]. This is a 33item self-report questionnaire with responses rated on a 6-point Likert-type scale ranging from 0 (not at all) to 5 (extremely). Total score range from 0 to 165 with higher score indicating an elevated fear of childbirth (example of item: "How do you imagine it will feel the very moment you give birth to your baby? Extremely frightened?"). As no French validation of this scale currently exists, a French version, translated/back-translated by researchers fluent in both languages, has been used. This version has been used in prior studies and has demonstrated good psychometric properties (e.g. [START_REF] Perier | Le rôle de l'optimisme et de l'anxiété dans la peur de l'accouchement: une étude exploratoire[END_REF][START_REF] Poggi | When fear of childbirth is pathological: the fear continuum[END_REF]. In our sample, α = .73.

Depressive symptoms. Depressive symptoms were assessed using the Patient Health Questionnaire (PHQ-9), a 9-item self-report questionnaire [START_REF] Kroenke | The PHQ-9: validity of a brief depression severity measure[END_REF]. Items are rated on a 4-point scale form 0 (not at all) to 3 (nearly every day) (example of item: "Over the last 2 weeks, how often have you been bothered by any of the following problems? Feeling down, depressed, or hopeless"). Total score range from 0 to 27 with higher score indicating elevated depressive symptoms. This instrument has been previously used in sample of pregnant women [START_REF] Sidebottom | Validation of the Patient Health Questionnaire (PHQ)-9 for prenatal depression screening[END_REF][START_REF] Zhong | Using the Patient Health Questionnaire (PHQ-9) and the Edinburgh Postnatal Depression Scale (EPDS) to assess suicidal ideation among pregnant women in Lima, Peru[END_REF]. A study focusing on psychometric properties of instruments assessing prenatal depressive symptoms has also shown that the PHQ-9 was a reliable instrument for measuring depressive symptoms in both pregnant and postpartum women with few significant differences in performances while comparing with the Edinburgh Postnatal Depression Scale (EPDS) [START_REF] Flynn | Comparative performance of the Edinburgh Postnatal Depression Scale and the Patient Health Questionnaire-9 in pregnant and postpartum women seeking psychiatric services[END_REF]. In our sample α = .79.

Pregnancy-related anxiety. Pregnancy-related anxiety was assessed using the Pregnancy-related Anxiety Scale (PrAS; [START_REF] Brunton | The pregnancy-related anxiety scale: A validity examination using Rasch analysis[END_REF]. This 33-item self-report questionnaire contains 8 subscales: Childbirth concerns, Body image concerns, Attitudes towards childbirth, Worry about self, Baby concerns, Acceptance of pregnancy, Avoidance and Attitudes towards medical staff. Items are scored on a 4-point scale ranging from 1 to 4 (example of item: "I feel afraid of the invasiveness of childbirth"). Higher scores are indicative of increased pregnancy-related anxiety. Scores can be summed to provide a subscore score for each sub-scale. In our sample, α = .89.

Statistical analyses

Pearson correlations were conducted in order to examine the relationship between the PHQ-9, the PrAS, the W-DEQ and the TES. A correlation analysis between age and the TES was also conducted. Groups were established for each participant according to psychological, traumatic, and pregnancy-related variables (e.g. planning a cesarean section vs. vaginal delivery) and sociodemographic variables (e.g. professional activity vs. unemployment).

Groups were then compared by their mean scores of pretraumatic stress through a series of Student's t-test and an ANOVAs for stage of pregnancy (i.e. first, second or third trimester).

Finally, multiple regression analyses were used to identify independent significant predictor for elevated pretraumatic stress symptoms. The choice of statistical predictors to test was made after examination of mean comparisons analyses and correlations. We only included variables that had a significant influence on TES scores. The significance level was set to 0.05 (two-tailed). All the analyses were conducted using STATISTICA version 13.

Results

Prevalence of pretraumatic stress symptoms

Of all participants, 8 % met all criteria for probable pretraumatic stress disorder with the forthcoming delivery used as the traumatic event. Twenty-two percent of women met criterion B (i.e. intrusion), 29% met criterion C (i.e. avoidance and numbing) and 32% met criterion D (i.e. hyperarousal).

Correlations between fear of childbirth, depressive symptoms, anxiety-related symptoms and pretraumatic stress symptoms

Pearson correlations highlighted that the intensity of pretraumatic stress symptoms was positively correlated with depressive symptoms (r=0.62; p< 0.05) and fear of childbirth symptoms (r=0.41; p< 0.05). Regarding pregnancy-related anxiety, the intensity of pretraumatic stress symptoms was also positively correlated with childbirth concerns (r=0.69; p< 0.05), body image concerns (r=0.49; p< 0.05), worry about self (r=0.76; p< 0.05), baby concerns (r=0.64; p< 0.05), avoidance (r=0.42; p< 0.05), attitudes towards medical staff (r=0.52; p> 0.05). No significant correlations were found between pretraumatic stress symptoms and attitudes towards childbirth (r=0.11; p> 0.05) or acceptance of pregnancy (r=0.09; p> 0.05).

Mean comparisons according to obstetrical, psychological and sociodemographic variables

Women who reported a prior exposure to a traumatic event had an average score on the TES significantly higher than those who did not report experiencing a traumatic event (M= 44.61; SD= 15.57 vs. M= 38.29; SD= 9.53; t=-2.03; p = .008). No significant correlation was found between pretraumatic stress symptoms and age (r= -0.06; p> 0.05). No differences were observed regarding other obstetrical, psychological and sociodemographic variables (see table 2).

- -------------------------------------Insert table 2 here-------------------------------------------

Predictors of the intensity of pretraumatic stress symptoms.

A multiple regression analysis was conducted for identifying the independent predictors of the intensity of pretraumatic stress symptoms. We only included variables that were associated with TES scores (i.e. Depressive symptoms, fear of childbirth symptomatology, childbirth concerns, body image concerns, worry about self, baby concerns, avoidance, attitude towards medical staff, and prior exposure to traumatic event). The unique contribution of each independent variable was reported with regression coefficient (beta), and p value. The proportion of the variance in frequency of pretraumatic stress symptoms by the independent variables was reported with the multiple R 2 . Prior exposure to traumatic event was entered in a first step and was not significant (ß = 0.05< .05) (see table 3). The final model explained 73% of the variance and was also significant (F(9,90)=31,498 p<0,00001. adjusted R²= 0.73). Increased depressive symptoms (ß = 0.48, p< .05), childbirth concerns (ß = 0.47, p< .05) and worry about self (ß = 0.74; p< .05) were independently associated with pretraumatic stress symptoms (see table 3).

- -------------------------------------Insert table 3 here-------------------------------------------

Discussion

The present study aimed at exploring the frequency of pretraumatic stress disorder in primiparous pregnant women and evidencing associated features of pretraumatic stress symptoms. With 8 % of our study sample meeting all criteria for probable pretraumatic stress disorder, our findings provide additional support for the existence of pretraumatic stress symptoms related to the forthcoming delivery. While prior studies have evidenced pretraumatic stress symptoms in pregnant women [START_REF] Goutaudier | Traumatic anticipation of childbirth and disordered eating during pregnancy[END_REF][START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF][START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF], to the best of our knowledge, this is the first study conducted exclusively on primiparous women. Even though results from Goutaudier et al.'s study have reported no significant difference between primiparous and multiparous women regarding pretraumatic stress reactions, our findings suggest that such reactions might not be related to history of a prior potential traumatic birth.

Pregnant women who reported prior exposure to a traumatic event had an average score on pretraumatic stress symptoms significantly higher than those who did not report such exposure. Nevertheless, contrary to our hypothesis and prior findings (e.g. [START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF] traumatic history was not highlighted in the multiple regression analysis as associated with the intensity of pretraumatic stress symptoms. This lack of consistency in results between both studies might be due to difference in parity (primiparous vs. multiparous). In the present study, we only included women with no prior experience of childbirth. Then, we can make the hypothesis that exposure to traumatic event (and then a potential traumatic experience of a traumatic birth) would mostly increase the likelihood of developing pretraumatic reactions in multiparous women. Even though speculative, this could explain the inconsistency of results and lack of significance of traumatic exposure in the multiple regression analysis.

Based on prior studies, fear of childbirth symptomatology and depressive symptoms were expected to be associated with the intensity of pretraumatic stress symptoms related to the forthcoming delivery. As expected, significant results were found for depressive symptoms. This result is consistent with prior findings evidencing the high comorbidity between depression and PTSD following childbirth or exposure to other type of traumatic event (e.g. [START_REF] Contractor | Relations between PTSD and depression symptom clusters in samples differentiated by PTSD diagnostic status[END_REF][START_REF] Grekin | Prevalence and risk factors of postpartum posttraumatic stress disorder: a meta-analysis[END_REF][START_REF] Nichter | Psychological burden of PTSD, depression, and their comorbidity in the US veteran population: Suicidality, functioning, and service utilization[END_REF]. Noteworthy there is an overlap between depressive and pretraumatic stress symptoms. While avoidance and intrusive symptoms are specific to traumatic reactions, negative alterations in cognition and mood are commonly found not only in women suffering from pretraumatic stress symptoms but also in individuals showing depressive symptoms. For instance, self-blame, negative emotional state or inability to experience happiness can be developed. Marked alterations in arousal and reactivity (e.g. problems with concentration or sleep disturbance) are also shared symptoms. Thus, developing longitudinal studies are of prime importance to evaluate whether or not antenatal depressive symptoms are a risk factor of the development or maintenance of pretraumatic stress. Interestingly, while fear of childbirth was correlated with pretraumatic stress, it was not highlighted in the multiple regression analysis as associated with the intensity of pretraumatic stress symptoms. A prior study aiming at identifying profiles of women based on pretraumatic stress and FOC symptoms has suggested that, while sharing some similarities, these constructs are clearly different [START_REF] Goutaudier | Traumatic anticipation of childbirth and disordered eating during pregnancy[END_REF]. Thus, taking together, these results suggest that, while a comorbidity can exist between both reactions, FOC would not participate into the development of pretraumatic stress but would rather be a comorbid disorder.

It should be noted that, by highlighting specific components of antenatal anxiety associated with pretraumatic stress, our results extent prior findings that only took into account anxiety as a global construct (e.g. [START_REF] Goutaudier | Childbirth as a forthcoming traumatic event: pretraumatic stress disorder during pregnancy and its psychological correlates[END_REF][START_REF] Söderquist | Traumatic stress in late pregnancy[END_REF].

Moreover, anxiety assessed in previous studies was not specially related to In our study, two components of antenatal anxiety (childbirth concerns and worry about self) were associated with elevated pretraumatic stress symptoms. Interestingly, items from the childbirth concerns sub-scale are related to potential injuries (e.g. worrying about being cut or harmed) and meet criterion A for PTSD. This result suggests that the anticipation of childbirth as a life-threatening event plays a part in the development of pretraumatic stress symptoms. Thus, we can assume that, while the exposure to a traumatic event is a mandatory criterion for PTSD, it is a key component of the development of symptoms for anticipated traumatic reactions.

Regarding the "worry about self" subscale, items are related to anxiety regarding possible issues as a mother (e.g. I worry that I won't do a good job as a mother, I worry about caring for the baby once I am home). Prior studies in the field of postpartum traumatic reactions have shown that women with PTSD following childbirth can also have difficulties to invest their mother role (e.g. [START_REF] Goutaudier | Premature birth: subjective and psychological experiences in the first weeks following childbirth, a mixed-methods study[END_REF]. As a consequence, it appears of prime importance to guide women into the investment of their new role as a mother. The avoidance subscale of the antenatal anxiety scale was not associated with pretraumatic symptoms. Given that avoidance is a part of the pretraumatic stress symptomatology, this lack of association appears counterintuitive but might be explained by the nature of the items of this subscale. Indeed, the avoidant subscale contains items related to the type of delivery (e.g. I may consider a caesarian to avoid a vaginal birth) and does not refer to avoidance of delivery per se. Thus, supported by the lack of significant differences between women who plan a c-section and women who a vaginal delivery, pretraumatic stress does not appear related to the type of delivery. As it is acknowledged that having a cesarean delivery is a risk factor for postpartum PTSD (see [START_REF] Olde | Posttraumatic stress following childbirth: a review[END_REF], for a review), this result suggests that, while having an impact on posttraumatic reactions, the type of delivery does not necessarily influence pretraumatic symptoms.

Although this study provides some important insights on pretraumatic stress symptoms, some limitations need to be acknowledged. First, given the small sample (N=100), our results are not representative of all primiparous pregnant women. Then, given the discrepancies between medical cares during pregnancy across countries, our results might not be generalized to non-French women, suggesting the need from cross-cultural replications. Given that it is the most commonly used instrument in the literature on pretraumatic stress in pregnant women and also the only available scale in French language, we used the Traumatic Event Scale (TES) to assess pretraumatic stress symptoms. As this instrument is based on DSM-IV-TR criteria for PTSD, future studies on pretraumatic stress should be conducted using scales or clinical interviews based on DSM-5 criteria. We did not assess a potential indirect exposure to a traumatic birth (e.g learning that a close relative experienced a traumatic birth or working as a health professional in a maternity unit), which could have impacted the results obtained. While it is an efficient strategy for recruiting research participants, data were collected using the Internet which could be considered as an important limitation. Indeed, we cannot exclude the risk of results being biased by the participation of women who felt more involved in the research topic, with potential impact on probable pretraumatic stress disorder. Thus, including women recruited during ultrasound appointments or childbirth preparation classes appears relevant. Finally, as the cross-sectional design precludes drawing conclusion about the direction of the causality longitudinal studies are warranted.

Our findings also have clinical implications. Eye Movement Desensitization and Reprocessing (EMDR) and Cognitive Processing Therapy (CPT) are widely used as psychotherapeutic treatment for PTSD regardless of the traumatic event (see [START_REF] Asmundson | A meta-analytic review of cognitive processing therapy for adults with posttraumatic stress disorder[END_REF]Powers et al., 2010, for meta-analytic reviews). While few studies have focused on specific treatments for postpartum PTSD, CPT and EMDR appear to be promising therapies for PTSD following childbirth [START_REF] Chiorino | The EMDR Recent Birth Trauma Protocol: a pilot randomised clinical trial after traumatic childbirth[END_REF][START_REF] Gobin | Cognitive Processing Therapy for Childbirth-Related Posttraumatic Stress Disorder: A Case Report[END_REF]. Given the similarities between pre and posttraumatic stress symptoms, these psychotherapies could be adapted for traumatic reactions during pregnancy. Clinicians should also consider the overlap between pretraumatic stress and depressive symptoms and targeting both disorders.

To conclude, this study contributes to the very limited literature on pretraumatic stress. As such symptoms were found in primiparous women, it is noteworthy that pretraumatic is not a reactivation of a former postpartum PTSD or associated with a prior negative experience of childbirth. Future studies conducted on primiparous women with no history of traumatic exposures could allow to provide additional evidence of childbirth as a pretraumatic event. Additional longitudinal studies are also needed to evidence whether or not women with pretraumatic stress symptoms during pregnancy develop peritraumatic reactions during labour and PTSD in the aftermath of childbirth.

Disclosure statement: none

Funding: none

Table 1 : Sociodemographic and obstetric characteristics of the study population (N=100)

 1

	Variables

Table 2 : Mean comparison of TES scores according to obstetrical, medical, and psychological, history in a sample of 100 primiparous women (using t tests and ANOVAs)

 2

	Yes	No	t	p
	Mean score	Mean score
	On the TES	On the TES

Table 3 : Results of a linear multiple regression analysis predicting the intensity of pretraumatic stress symptoms in the pre-partum period in a sample of 100 primiparous women

 3

	β	t(90)	p

*Significant results for p< .05 Pretraumatic stress symptoms were assessed using the Traumatic Event Scale