Karima Sini

Damien Bourgeois

Madjid Idouhar

Madjid IDOUHAR b Michaël Carboni
email: michael.carboni@cea.fr

Daniel Meyer

Karima Sini Ab

Metal-organic frameworks cavity size effect on the extraction of organic pollutants

Keywords: Perfluorinated Pollutants, Hydrophobic interaction, Waste water treatment, Metal-Organic Framework, Porous materials

Introduction

Fluorosurfactants are com osed o ver strong F ond (116 kcal.mol -1 ) and lead to very low surface tension in aqueous solutions. [START_REF] Richardson | Water Analysis: Emerging Contaminants and Current Issues[END_REF] Owing to these properties and despite their high costs, fluorocarbons have been used during several decades in many applications, eg cooling agents, fire-fighting foams or in the photolithographic industry. [START_REF] Mcculloch | Fluorocarbons in the global environment: a review of the important interactions with atmospheric chemistry and physics[END_REF] Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) (denoted as PFOX along the manuscript) are very stable and considered as almost nonbiodegradable, generating a large pollution in natural environment. Several studies have reported that PFOX are widely present in surface water, in ground, marine or drinking water at mg.L -1 to low g.L -1 levels. [START_REF] Rumsby | Perfluorooctane sulphonate and perfluorooctanoic acid in drinking and environmental waters[END_REF] They have received great attention because of their bioaccumulation, and potential health concerns due to their toxicity and carcinogenicity have been largely discussed in the literature. [START_REF] Viberg | Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA)[END_REF] Recently, PFOS has been added to the list of Persistent Organic Pollutants (POPs) under the Stockholm Convention, [START_REF] Mclachlan | Can the Stockholm convention address the spectrum of chemicals currently under regulatory scrutiny? Advocating a more prominent role for modeling in POP screening assessment[END_REF] and the European Food Safety Authority (EFSA) has established a tolerable daily intake (TDI) for both PFOS and PFOA present in food. [START_REF] Noorlander | Levels of Perfluorinated Compounds in Food and Dietary Intake of PFOS and PFOA in The Netherlands[END_REF] Numerous methods have been developed for the degradation of PFOX in aqueous environments, such as sonochemical, [START_REF] Moriwaki | Sonochemical Decomposition of Perfluorooctane Sulfonate and Perfluorooctanoic Acid[END_REF] irradiation [START_REF] Yamamoto | Photodegradation of Perfluorooctane Sulfonate by UV Irradiation in Water and Alkaline 2-Propanol[END_REF] or oxidation methods. [START_REF] Zhuo | Efficient Electrochemical Oxidation of Perfluorooctanoate Using a Ti/SnO 2 -Sb-Bi Anode[END_REF] However, these techniques have high operating costs, and extraction has also been evaluated with adsorbents such as granular activated carbon (GAC) or resins. But their adsorption capacities remain rather low (112 -160 and 46 -41 mg of PFOA and PFOS respectively per gram of GAC and resin). [START_REF] Yu | Sorption of perfluorooctane sulfonate and perfluorooctanoate on activated carbons and resin: Kinetic and isotherm study[END_REF] Hybrid organic-inorganic materials, metal-organic frameworks (MOFs), have attracted considerable interest because of their large specific surface areas and pore volumes. [START_REF] Zhou | Metal-Organic Frameworks (MOFs)[END_REF] Zirconium-MOFs, UiO-66 and 67 built with bdc (benzene-dicarboxylic acid) and bpdc (biphenyl-dicarboxylic acid) ligands respectively, have shown to possess very high thermal, chemical and water stability, [START_REF] Katz | A facile synthesis of UiO-66, UiO-67 and their derivatives[END_REF] making them suitable as sorbents for environmental purposes. [START_REF] Amador | BTEX removal from aqueous solution with hydrophobic Zr metal organic frameworks[END_REF][START_REF] Amador | Sorption and photodegradation under visible light irradiation of an organic pollutant by a heterogeneous UiO-67-Ru-Ti MOF obtained by postsynthetic exchange[END_REF] Recently, we have shown that Zirconium-MOF UiO-66, with small cavity, can efficiently adsorb PFOX. [START_REF] Sini | Metal-organic framework sorbents for the removal of perfluorinated compounds in an aqueous environment[END_REF] In this study, we evaluated the efficiency of UiO-67, with larger cavity, for the removal of PFOX in aqueous solution. Comparison with UiO-66 revealed that the size of the cavity is essential to efficient sorption of pollutants inside the material. Results show that the sorption mechanism is based on a physical process, with very rapid sorption of pollutants in aqueous solution (around 1 hour), but different mechanism according to the cavity size.

Experimental

Synthesis and characterizations

All reagents were purchased from Aldrich and were used without further purification. UiO-66 and UiO-67 were synthesized as reported in the literature [START_REF] Katz | A facile synthesis of UiO-66, UiO-67 and their derivatives[END_REF] and characterized by PXRD and SEM (Supplementary information).

Sorption experiments

Sorption experiments were performed at 298 K in an aqueous solution at pH 4 with a 1 mg/mL MOF concentration. After centrifugation, the solution was analyzed with 19 F NMR (Brucker) with the use of an internal standard (trifluoroethanol), and the concentration of PFOX determined after integration of characteristic peaks as previously reported. [START_REF] Sini | Metal-organic framework sorbents for the removal of perfluorinated compounds in an aqueous environment[END_REF] All experiments were at least duplicated. Calibration curves were performed for [PFOX] range of 100 to 1000 mg.L -1 (Figure S1).

Results and discussion

Both materials have been obtained after solvothermal syntheses with the use of HCl as modulator in order to increase the crystallinity of the materials. [START_REF] Katz | A facile synthesis of UiO-66, UiO-67 and their derivatives[END_REF] The UiO phase was confirmed for both materials by SEM and optical microscopy through the formation of discrete octahedral crystals (Figure S2), and by diffractograms recorded at high angle peaks ( e ond 2θ o 30º) in agreement with the theoretical pattern (Figure S3). Sorption kinetics of fluorinated pollutants with both MOFs was first studied (Figure 1, top-left). A large amount of pollutant is adsorbed in less than 20 minutes and equilibrium is reached within 60 minutes. Pseudo-first order (1) and pseudo-second order (2) equations were used to model the experimental data (q e and q t are the sorption capacities at equilibrium and any time t, respectively, and k 1 and k 2 are the pseudo-first-and pseudo-second-order rate constants, respectively). The pseudosecond order model (Figure 1, top-right) leads to much better fits in comparison with the pseudo-first order (Table S1) with correlation coefficients of 0.999 to 0.996.

(

) 1
(2)

Temperature effect on sorption capacities of PFOA and PFOS was studied over a range of 298 K to 318 K. A sorption increase with an increase of the temperature was observed, and is attributed to a higher diffusion of PFOX molecules into the MOFs cavities. The thermodynamic parameters of the sorption including were calculated from the Van't Ho equation ( 3) and are summarized in Table S2.

Ln Kd = ΔS°/ R -ΔH°/ RT (3)
In all cases, ΔG and ΔH values indicate respectively that the sorption process is spontaneous and endothermic, whereas ΔS values suggest that the randomness at the solid-liquid is largely compensated by the release of the freedom in the water phase. UiO-66 best fit with Langmuir model:

UiO-67 best fit with Freundlich model:

( Fits are performed using the Langmuir model for UiO-66 and the Freundlich model for UiO-67.

m ax m ax 1 1 q K q C q C L e e e   
Sorption isotherms of both PFOX by both MOFs show an increase with the PFOX concentration until saturation (Figure 2). Experimental data were then fitted using the linear form of the Langmuir model (Equation 4, Figure S4) and the Freundlich model (Equation 5, Figure S4). Fitting results are summarized in Table 1, where q e is the sorption capacity at equilibrium, q m is the saturation sorption capacity, C e is the PFOX concentration in solution at equilibrium, K L is the Langmuir constant related to binding site affinity, K f is the Freundlich constant and n (dimensionless; 0 < n < 10) is a Freundlich intensity parameter. Complete saturation was observed only for UiO-66. In this case, the Langmuir model seems adequate and supposes that sorption occurs on a homogeneous surface by a monolayer deposition. In the case of UiO-67, the increase of cavity size leads to a change in sorption mechanism, possibly related to a multi-layer process, partially limited by diffusion inside the cavity. Also, higher sorption (up to 743 mg.g -1 in the case of PFOA) is observed.

Figure 1 :

 1 Figure 1: Top: (left) Sorption kinetics of PFOX with UiO-66 and UiO-67; (right) Application of the pseudo-second-order kinetic model. Bottom: (left) Evolution of sorption according to temperature; (right) Van't Ho lot (pH = 4, [PFOX] = 500 mg.L -1 , [MOF] = 1 mg/mL, 60 min).

Figure 2 :

 2 Figure 2 : Sorption isotherms of PFOA and PFOS by UiO-66 and UiO-67 in water (pH 4) at 298 K.

ConclusionA

  Zirconium-MOF based on bpdc (biphenyl-dicarboxylic acid) ligand, UiO-67, has been used for the extraction of fluorinated pollutants from aqueous solutions. It has shown higher capacities than the equivalent MOF based on bdc (benzene-dicarboxylic acid) ligand, UiO-66, which possesses smaller cavities. These results are in full agreement with a sorption mechanism based on hydrophobic interactions between the pollutant and the MOF cavity, instead of coordination of the pollutant on the metal node. The increase of the cavity size also leads to a change in sorption mechanism: this highlights the importance of detailed studies on such materials. MOFs are indeed simple and scalable materials, and this works demonstrates their potential use for the decontamination of organic pollutants from aqueous solutions.

Table 1 :

 1 Calculated parameters for sorption of PFOA and PFOS by UiO-66 and UiO-67, using Langmuir and Freundlich models.

	Adsorbent		Langmuir			Freundlich
			q m	K L	R 2	Kf	n	R 2
			(mg.g -1 )	(L.mg -1 )		(mg/g)(
						L/mg) 1/n
	UiO-66	PFOA	388	0.0182	0.9718	1.833	2.150	0.858
		PFOS	160	0.0466	0.9803	1.834	2.597	0.940
	UiO-67	PFOA	700	0.1143	0.9740	2.239	5.650	0.912
		PFOS	580	0.0884	0.9805	2.200	5.235	0.883


Malek Saidi

Florence Fourcade

Nizar Bellakhal

Abdeltif Amrane

Florence Geneste

Nickel foam as a new material for chlortetracycline electrochemical oxidation: biodegradability improvement and biological treatment

Keywords: Chlortetracycline, electrochemical oxidation, dechlorination, biodegradability, coupling process

Chlortetracycline CTC is an organochlorine compound released from industrial pharmaceutical waste. This pollutant had dangerous effects in aquatic organisms and human health, since it is responsible for inhibition of microbial activity and growth. In this study, the elimination of CTC from water is A c c e p t e d m a n u s c r i p t targeted. Biodegradability, based on the BOD 5 / COD ratio was studied, showing the non-feasibility of a direct biological treatment. Cyclic voltammetry analysis on a nickel electrode revealed a significant electrochemical activity of CTC. An electrochemical pre-treatment was therefore carried out in a percolation cell using a nickel foam as a working electrode at the potential of 0.5 V /SCE. The operating parameters were optimized. A degradation yield of 99% was achieved for an initial concentration of pollutant of 50 mg L -1 and a flow rate of 5 mL min -1 in alkaline conditions. After electrolysis, a total dechlorination of CTC was observed. The biodegradability ratio was improved, from 0.13 to 0.33 after electrolysis; while the dissolved organic carbon (DOC) analysis revealed that the level of mineralization remained low 27%. A 21 days biological treatment with activated sludge was then carried out, leading to an overall mineralization yield of 78%.

Introduction

The low cost and good oral absorption of tetracycline (TC) derivatives make it the most used antimicrobial in veterinary medicine and agriculture with 14 % of the total antibiotic usage in medicine and animal husbandry [START_REF] Albero | Persistence and availability of veterinary antibiotics in soil and soil-manure systems[END_REF][START_REF] Wang | Occurrence and risk assessment of acidic pharmaceuticals in the Yellow River, Hai River and Liao River of north China[END_REF]. Chlortetracycline CTC (Fig. 1) is the most heavily consumed owing to its low price, broad antibiotic effect, and potential for promoting the growth of livestock [START_REF] Research | Novel Drug Approvals for 2016[END_REF]. Therefore, CTC has the most negative impact on the environment of all tetracycline derivatives [START_REF] Daghrir | Tetracycline antibiotics in the environment: a review[END_REF] . Furthermore, this chlorinated tetracycline is a persistent compound that can be found at a relatively A c c e p t e d m a n u s c r i p t high concentration in nature [START_REF] Liu | Bio-removal of tetracycline antibiotics under the consortium with probiotics Bacillus clausii T and Bacillus amyloliquefaciens producing biosurfactants[END_REF]. Its toxicity is hundreds of times higher than that of other veterinary antibiotics such as sulfamethazine [START_REF] Kümmerer | Pharmaceuticals in the Environment[END_REF]. Although pharmaceutical residues are only the subject of a watch list, the very low biodegradability of CTC makes it viewed as a significant environmental pollutant [START_REF] Han | Effects of tetracycline on growth, oxidative stress response, and metabolite pattern of ryegrass[END_REF]8]. Chlortetracycline has been detected in different aquatic environments, such as surface water (122.3 ng L -1 ) and ground water (86.6 ng L -1 ) [START_REF] Tong | Occurrence of antibiotics in the aquatic environment of Jianghan Plain, central China[END_REF]. These high levels of CTC have bad effects on ecological system and human health since it is responsible for inhibition of microbial activity and growth [START_REF] Liu | Effects of chlortetracycline on soil microbial communities: Comparisons of enzyme activities to the functional diversity via Biolog EcoPlates TM[END_REF], phytoplankton toxicity [START_REF] Guo | Phytoplankton toxicity of the anti biotic chlortetracycline and its UV light degradation products[END_REF], and inhibition of protein synthesis [START_REF] Anderson | Qualitative and quantitative drug residue analyses: Chlortetracycline in white -tailed deer (Odocoileus virginianus) and supermarket meat by liquid chromatography tandem-mass spectrometry[END_REF]. CTC removal from water becomes then an urgent issue.

Biodegradation cannot be an effective remediation process to remove CTC from water, since the presence of chlorine atoms on the phenyl ring is a factor that favours the toxicity of aryl compounds [START_REF] Liao | Biodegradation of chlortetracycline by acclimated microbiota[END_REF][START_REF] Fontmorin | Direct electrochemical oxidation of a pesticide, 2,4-dichlorophenoxyacetic acid, at the surface of a graphite felt electrode: Biodegradability improvement[END_REF]. In that case, a pre-treatment method is necessary to increase CTC biodegradability prior to a biological treatment.

Chemical and physicochemical techniques have been widely studied in order to remove tetracycline from water. Adsorption is one of the most effective and low cost methods. In Ahmad et al. study [START_REF] Alahabadi | Comparing adsorption properties of NH4Cl -modified activated carbon towards chlortetracycline antibiotic with those of commercial activated carbon[END_REF], commercial and modified activated carbons have been implemented showing good removal efficiency. Biochar, product of biomass pyrolysis, is also an efficient material for CTC adsorption; especially when it is coupled with flocculation process [START_REF] Yue | Biochar accelerates the removal of tetracyclines and their intermediate s by altering soil properties[END_REF][START_REF] Xu | Interaction between chlortetracycline and calcium-rich biochar: Enhanced removal by adsorption coupled with flocculation[END_REF]. However, supplementary degradation processes are needed for the elimination of biorecalcitrant pollutants. Advanced oxidation processes (AOPs) such as electro-Fenton, electrooxidation, ozonation and photo-catalysis have been applied in order to enhance CTC removal efficiency [START_REF] Khan | Ozonation of chlortetracycline in the aqueous phase: Degradation intermediates and pathway confirmed by NMR[END_REF][START_REF] Daghrir | Removal of chlortetracycline from spiked municipal wastewater using a photoelectrocatalytic process operated under sunlight irradiations[END_REF][START_REF] Kanakaraju | Advanced oxidation process-mediated removal of pharmaceuticals from water: A review[END_REF][START_REF] Zaviska | De La Rochebrochard d'Auzay, Electrochemi cal Oxidation of Chlortetracycline Using Ti/IrO2 and Ti/PbO2 Anode Electrodes: Application of Experimental Design Methodology[END_REF][START_REF] Qian | Electrochemical Degradation of Tetracycline Antibiotics Using a Ti/SnO2-Sb2O3/PbO2 Anode: Kinetics, Pathways, and Biotoxicity Change[END_REF]. These processes are based on the generation of hydroxyl radicals (°OH), known for their high oxidation capacity but also non-selectivity for organic pollutants [START_REF] Mansour | Relevance of a combined process coupling electro-Fenton and biological treatment for the remediation of sulfamethazine solutions -Application to an industrial pharmaceutical effluent[END_REF]. Moreover, combined processes involving photocatalysis coupled to a biological treatment have been considered for TC removal, showing good degradation yields and biodegradable by-products [START_REF] Yahiat | Removal of antibiotics by an integrated process coupling photocatalysis and biological treatment -Case of tetracycline and tylosin[END_REF].

Electrochemical oxidation of biorecalcitrant compounds has also been reported as a pre-treatment to improve their biodegradability prior to a biological treatment [START_REF] Geneste | Catalytic electrochemical pre-treatment for the degradation of persistent organic pollutants[END_REF]. Thus, previous studies dealing with direct electrooxidation of tetracycline have been carried out in a homemade percolation flowcell involving a graphite felt electrode [START_REF] Belkheiri | Feasibility of an electrochemical pre-treatment prior to a biological treatment for tetracycline removal[END_REF]. Results showed an almost total TC removal, and biodegradability has been improved for a final BOD 5 /COD of 0.39. Graphite felt electrodes modified by nickel electro-deposition have been also used for TC pre-treatment leading to the same conversion yield 99%. Mineralization yields remained low, but biodegradability has been well improved for a BOD 5 /COD ratio increasing from 0 to 0.45. After a biological treatment, the overall mineralization yield reached 69% [START_REF] Belkheiri | Combined process for removal of tetracycline antibiotic -Coupling pre-treatment with a nickel-modified graphite felt electrode and a biological treatment[END_REF].

A c c e p t e d m a n u s c r i p t

Three-dimensional porous electrodes are very commonly used in electrochemical treatment of persistent organic pollutants, since they show high specific surface area [START_REF] Farinos | Development of Three-Dimensional Electrodes of PbO2 Electrodeposited on Reticulated Vitreous Carbon for Organic Eletrooxidation[END_REF]. The metallic foams are undoubtedly very interesting as porous electrode for their catalytic properties [START_REF] Langlois | Flow-through and flow-by porous electrodes of nickel foam. II. Diffusionconvective mass transfer between the electrolyte and the foam[END_REF]. Thus, Nickel based materials have been intensively investigated and considered as good potential electrode materials due to their high chemical and thermal stability, ready availability, environmentally benign nature and relatively low cost [START_REF] Feng | Recent progress in nickel based materials for high performance pseudocapacitor electrodes[END_REF]. In addition to the properties of the bulk material, nickel foam has a desirable three-dimensional network structure with large specific surface area and good electrical conductivity [START_REF] Zhang | Preparation and characterization of palladium/polypyrrole-reduced graphene oxide/foamed nickel composite electrode and its electrochemical dechlorination of triclosan[END_REF].

The encouraging results previously obtained with TC [START_REF] Belkheiri | Feasibility of an electrochemical pre-treatment prior to a biological treatment for tetracycline removal[END_REF], led us to study ele ctrooxidation on nickel to degrade the most toxic tetracycline derivative, CTC. This coupled process combining an electrochemical oxidation pretreatment with a biological process for CTC treatment is reported for the first time in this work. It was interesting to look into the chlorine atom conversion during the treatment to check that toxic chlorinated by-products were not formed. CTC electroactivity was first investigated by cyclic voltammetry analysis on nickel electrode. Its electrochemical oxidation was carried out in a percolation cell involving nickel foam as a working electrode, in view of biodegradability improvement. The biodegradability of the electrolyzed solution was then evaluated by a biological treatment.

Materials and methods

Chemicals

7-chlortetracycline hydrochloride (≥ 75% HPLC-grade) was purchased from Sigma Aldrich (Saint-Quentin Fallavier, France). Acetonitrile was HPLC grade from Fisher Scientific (Illkirch, France), and formic acid (LC-MS grade, 98%) was purchased from J.T. Baker (Deventer, Holland). All standards were prepared with ultra-pure water (Purelab Options-Q7/15, Elga, 18.2 MU cm -1 ). All other reagents were of analytical grade. The nickel foam was purchased from Lizhiyuan Battery Sales Department, China (Ni purity: 99.9%, density: 320 ± 20 g/m 3 , thickness: 1 mm, diameter: 4 cm)

Analysis

Cyclic voltammetry analysis

The electrochemical behaviour of chlortetracycline was studied by cyclic voltammetry analysis. Nickel electrode (20 mm 2 ) was used as a working electrode in a conventional three ele ctrode-cell with a platinum wire as counter electrode. The reference electrode was a saturated calomel electrode located near the working electrode. The electrochemical behaviour of a 20 mL CTC solution was examined in alkaline (NaOH 0.1 mol L -1 ), acidic (H 2 SO 4 0.5 mol L -1 ) and neutral (Na 2 SO 4 0.1 mol L 1 ) media at ambient temperature under nitrogen atmosphere. Voltammograms were obtained by cyclic voltammetry (100 mV s -1 ) using an e-daq potenstiostat linked to an e-corder 401 converter.

A c c e p t e d m a n u s c r i p t

UPLC

The residual chlortetracycline concentration was determined by a UPLC (ultra-high-pressure liquid chromatography) system involving a Waters Acquity UPLC® H-Class gradient pump and a PDA (photo diode array) UV detector. The λ max was 273 nm for CTC. A C18 BEH (bridged ethylene hybrid) column, 1.7 mm (2.1 × 50 mm) was used. A gradient elution was carried out with 0.1% formic acid in ultrapure water (solvent A) and acetonitrile (solvent B) as follows: 90/10% from 0 to 1 min; from 1 to 8 min elution was linearly modified from 90/10 to 0/100%, maintained at 0/100% from 8 to 8.5 min; from 8.5 to 9 min elution was linearly modified from 0/100 to 90/10%, where it was maintained from 9 to 10 min for solvents A and B, respectively. CTC signal was detected at 3.5 min retention time.

Ion-exchange chromatography (IC)

A DIONEX DX120 ion chromatography, equipped with a conductivity detector, was used to identify the generated inorganic ions (Cl -and NO 3 -). The stationary phase was a DIONEX AS19 (4 × 250 mm) ion-exclusion column. The sample was eluted with potassium hydroxide at a flow rate of 1 mL min -1 . The detection was carried out by conductivity with a self-regenerating suppressor (SRS).

Dissolved Organic Carbon (DOC) and Total nitrogen measurements

Dissolved organic carbon and total nitrogen were measured by means of a Schimadzu total organic analyzer (TOC-V CPH TOC-V CPN / TOC-V WP ). HCl (2 N) and H 3 PO 4 (25%) were considered for CO 2 and NO production. Organic carbon compounds were combusted and converted to CO 2 , which was detected by a non-dispersive infrared detector (NDIR). Dissolved nitrogen compounds were combusted and converted to NO, which was then mixed with ozone chemiluminescence for detection by a photomultiplier. Analyses were run in duplicate.

Chemical oxygen demand (COD) measurements

Chemical oxygen demand (COD) was measured by means of a Test Nanocolor® CSB 160 from Macherey-Nagel (Düren, Germany). The amount of oxygen required for the oxidation of the organic and mineral matter at 148°C for 2 h was quantified after oxidation with K 2 Cr 2 O 7 at acidic pH and with heating. Analyses were duplicated.

Biochemical oxygen demand (BOD 5 ) measurements

BOD 5 measurements was carried out in an Oxitop IS6 (WTW, Ales, France), in order to evaluate biodegradability. Activated sludge was provided by a local wastewater treatment plant (Rennes Beaurade, Bretagne, France). The flasks were inoculated by activated sludge initially concentrated at 0.05 g L -1 . The total volume of sample was deduced from the expected BOD 5 range, which was calculated from the equation BOD 5 =COD/1.46 [START_REF] Fontmorin | Electrochemical oxidation of 2,4-Dichlorophenoxyacetic acid: Analysis of by-products and improvement of the biodegradability[END_REF]. The composition of the mineral solution was (in g L -1 ): MgSO 4 .7H 2 O, 22.5; CaCl 2 , 27.5; FeCl 3 , 0.15; NH 4 Cl, 2.0; and that of the phosphate solution was (g L -1 ): Na 2 HPO 4 , 6.80; KH 2 PO 4 , 2.80. Regarding the blank solution, a similar protocol was considered except for the sample, it was replaced by water to deduce the biological oxygen demand corresponding to the endogenous respiration. The control flask was prepared in the same way, but A c c e p t e d m a n u s c r i p t this time the sample was replaced by a solution of easily biodegradable compounds, namely glutamic acid (150 mg L -1 ) and glucose (150 mg L -1 ). Before use, KOH was added to achieve neutral pH (7.0 ± 0.2). All BOD 5 measurements were duplicated.

Experimental procedures 2.3.1. Electrochemical pre-treatment

A home-made flow cell was used for the electrochemical pre-treatment. As shown in Fig. 2, the continuous percolation system was composed from two cationic exchange membranes (Ionac 3470 -Lanxess SAS, Courbevoie, France) separating the working electrode from two interconnected stainless steel counter electrode compartments. The working electrode was composed of three juxtaposed nickel foams (0.3 cm thickness and 4 cm diameter), corresponding to a volume of 3.77 cm 3 . SEM image obtained by SEM JEOL 6301F (9 kV) microscope, showed a smooth surface of the nickel foam (Fig. 3). The working electrode was located between two counter-electrodes in order to provide a good homogeneity of the potential distribution [START_REF] Moinet | Electrosynthèse organique en continu. Méthodes directes et indirectes[END_REF]. The reference electrode (SCE) was positioned in the middle of the foam. The potential control was performed using an e -daq potentiostat linked to an e-corder 401 converter (AD Instruments Pty Ltd., Castle Hill, Australia). 0.1 mol L -1 NaOH was used as supporting electrolyte. The chlortetracycline soluti on (50 mg L -1 ) percolated the three-dimensional electrode at a constant flow rate monitored by a Gilson minipuls 2 peristaltic pump (Middleton, WI, USA). The electrochemical behaviour of CTC on nickel electrode was studied in basic (0.1 mol L -1 NaOH) medium by cyclic voltammetry. According to Fig. 4, voltammograms displayed a quasi-reversible system at 0.4 V/SCE (0.64 V/SHE at pH 13) for 0.1 mol L -1 NaOH solution, which is in agreement with the study of Belkheiri et al. [START_REF] Belkheiri | Combined process for removal of tetracycline antibiotic -Coupling pre-treatment with a nickel-modified graphite felt electrode and a biological treatment[END_REF]. Since the pH was above 12, this signal can be related to the reversible couple nickel hydroxide/nickel oxyhydroxide (Ni(OH) 2 /NiOOH).

Results and discussion

Electroactivity of CTC on Nickel Foam electrode

Ni(OH) 2 NiOOH + H + + e - (1)
In presence of CTC, an increase of the anodic peak was observed for the quasi-reversible system in alkaline medium. This increase showed the electrocatalytic effect of the nickel electrode on the oxidation of the target molecule. In alkaline medium, NiOOH groups were probably responsible for the CTC oxidation. It can be therefore concluded that in alkaline medium, an electrochemical oxidation on nickel foam electrode can be an efficient method for chlortetracycline pre -treatment.

Electrochemical pre-treatment of CTC

Effect of the oxidation potential

50 mg L -1 of CTC solution was treated in a homemade flow cell using a nickel foam electrode in alkaline medium (pH 12). The electrolysis was carried out for 5 h with a constant flow rate of 5 mL min -1 at different potential values, chosen in the potential range corresponding to the formation of the NiOOH groups (Fig. 4). The studied values were chosen based on the voltammogram of Fig. 4, showing a significant electrochemical activity of chlortetracycline in oxidation at 0.5 V/SCE. As expected, a negligible CTC degradation was observed (Fig. 5) when the electrolysis was performed at 0.3 V/SCE. More than 99% of CTC degradation was achieved at a potential of 0.5 V/SCE. However, when the potential reached 0.7 V/SCE, the oxidation of the target molecule decreased to 5%, showing a high competition with the oxidation of water. Indeed, pH measurement after electrolysis showed a pH decrease from 12 (before electrolysis) to 8. Therefore, NiOOH can no more be formed (Eq. 1), inhibiting the catalytic effect of nickel electrode on CTC oxidation. It is relevant to investigate the CTC degradation during the oxidation pre-treatment under different initial concentrations, in order to gain a better characterization of the oxidative power of the process. Anodic oxidation was performed in NaOH alkaline medium (pH 12) at 0.5 V/SCE for 5h. The flow rate was set to 5 mL min -1 . After 180 min of electrolysis, the oxidation yield exceeded 99% for 50 mg L -1 initial CTC concentration. A higher concentration of CTC (100 mg L -1 ) required a longer electrolysis time, since the degradation of chlortetracycline was only 42% after 180 min of electrolysis. After that, CTC concentration remained constant till the end of electrolysis. Similarly to the impact of the potential (cf. 2.1), pH value decreased significantly after electrolysis; from 12 (before electrolysis) to 8 that prevents NiOOH formation and then the CTC catalytic oxidation. As displayed in Fig. 6b, CTC mineralized slowly. Indeed, a significant DOC decrease was only detecte d after 3h and the mineralization levels remained low even at the end of the runs; only 9% of the organic carbon was removed for a 100 mg L -1 initial CTC solution, while 27% of DOC removal was reached for a CTC A c c e p t e d m a n u s c r i p t concentration of 50 mg L -1 . To achieve a high mineralization rate, which reduces the environmental risks of effluent discharges [START_REF] Asgari | O 3 , O 3 /UV and O 3 /UV/ZnO for abatement of parabens in aqueous solutions: Effect of operational parameters and mineralization/biodegradability improvement[END_REF], subsequent biological treatment seemed therefore relevant, owing to the significant residual organic carbon content. Indeed, organic CTC transformation products can be used as a carbon source by biomass [START_REF] Aydın | Chlortetracycline removal by using hydrogen based membrane biofilm reactor[END_REF]. The electrochemical pre-treatment was performed in order to improve the CTC biodegradability. Aerobic biodegradability study was carried out to assess the relevance of a subsequent biological treatment with activated sludge. Therefore, BOD 5 was measured before and after electrolysis.

Effect of the initial CTC concentration 3.2.2.1. Degradation and mineralization efficiency

CTC biodegradability study

As expected, the untreated chlortetracycline was non-biodegradable for both 50 and 100 mg L -1 initial concentrations with BOD 5 /COD ratios equal to 0.13 and 0.07, respectively. It is important to note that the considered concentrations had an effect on the target molecule biodegradability. It was in agreement with previous findings showing that tetracycline is biodegradable up to 18 mg L -1 initial concentration [START_REF] Yahiat | Removal of antibiotics by an integrated process coupling photocatalysis and biological treatment -Case of tetracycline and tylosin[END_REF]; while, beyond this value TC inhibited the bacterial growth. After electrolysis, the BOD 5 /COD ratio increased and reached 0.33 for the oxidized 50 mg L -1 CTC solution. This value was close to the limit of biodegradability (0.4) usually admitted [START_REF] Moreira | Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters[END_REF][START_REF] Sarria | Recent developments in the coupling of photoassisted and aerobic biological processes for the treatment of biorecalcitrant compounds[END_REF], showing that a biological treatment with activated sludge can be envisaged. According to Fig. 7a, by doubling the initial CTC concentration, the electrolyzed solution remained very difficult to biodegrade, according to the low BOD 5 /COD ratio, 0.16. Consequently, a biological treatment seemed not relevant because the microorganisms were unable to assimilate the by-products generated during the electrolysis of CTC and CTC in a still high concentration. Furthermore, COD removal yields decreased from 19.7% to A c c e p t e d m a n u s c r i p t 11.2% when increasing the initial CTC amount from 50 to 100 mg L -1 (Fig. 7b). Consequently, chemical oxidation appeared more effective for lower initial CTC concentration. Thus, the oxidative pretreatment seemed more suitable for a 50 mg L -1 CTC concentration in the experimental conditions used in this work. The flow rate is an influential parameter in the flow electrochemical cell. CTC oxidation was first studied for 5 mL min -1 . The target molecule was totally removed after 100 min of electrolysis. Electrolysis was also examined at a lower flow rate in order to see if the kinetics of the oxidation reaction can be improved. According to Fig. 8, the oxidation rate decreased and only 76% of elimination was achieved after 100 min of electrolysis at 3 mL min -1 . When the flow rate was higher, the mass transport of compounds to the electrode surface probably increased, leading to a more effective CTC oxidation. After 100 min of electrolysis, a pH decrease (7.6) appeared to be responsible for the plateau displayed in Fig. 8 for 3 and 5 mL min -1 flow rates. Thus, the formation of NiOOH groups at the electrode surface was inhibited and the oxidation of water became favored. The oxidation reaction was consequently stopped for 3 and 5 mL min -1 flow rates. These results showed that the reaction was not limited by the kinetics of the reaction occurring at the surface of the nickel foam. Interestingly, even if 24% of CTC remained in solution for the lower flow rate, the BOD 5 on COD ratio was significantly improved if compared to the non-treated CTC (Table 1). Indeed, the final CTC concentration was 12 mg L -1 , namely below the inhibitory level shown for TC, 18 mg L -1 [START_REF] Yahiat | Removal of antibiotics by an integrated process coupling photocatalysis and biological treatment -Case of tetracycline and tylosin[END_REF].

Effect of the flow rate

Table1

[CTC], COD, and BOD5/COD determination after 300 min of CTC electrolysis on a nickel foam electrode. Flow rate (mL min -1 )

[CTC] (mg L -1 ) COD (mg O2 L -1 ) BOD5 (mg O2 L - According to Fig. 1, chlortetracycline CTC presents three heteroatoms in its chemical structure, 2 nitrogens and 1 chlorine atoms. The removal of the C-Cl and C-N bonds during the electrolysis was checked. The qualitative and quantitative monitoring of free Cl -, NO 2 -and NO 3 -were carried out by ion-exchange chromatography (IC). The oxidation of CTC on nickel foam electrode released free Cl - and NO 3 -in the solution, while nitrite ions were not detected. Fig. 9 shows that the chloride concentration progressively increased during the 5 hours of electrolysis until reaching 7.36±0.01 mg L -1 , corresponding to 99.4 % of the chloride ions expected for a total conversion. The high abundance of Cl -evidenced that CTC underwent a dechlorination process and that chloride ions were not oxidized into chlorine in the conditions of the electrolysis [START_REF] Shoiful | Degradation of organochlorine pesticides (OCPs) in water by iron (Fe)-based materials[END_REF]. It is important to note that only few CTC treatment methods lead to its total dechlorination, such as ozonation and thermal decomposition [START_REF] Khan | Ozonation of chlortetracycline in the aqueous phase: Degradation intermediates and pathway confirmed by NMR[END_REF][START_REF] Cervini | Thermal decomposition of tetracycline and chlortetracycline[END_REF]. Gaseous compounds of nitrogen (NO x and N 2 ) were not released since the total amount of nitrogen TN remained constant (3±0.1 mg L -1 ) throughout the electrochemical pretreatment. 2.80±0.01 mg L -1 of NO 3 -were accumulated after electrolysis, corresponding to a quasitotal conversion of the nitrogen content. Since the DOC value decreased (Fig. 6b), it can be concluded that the carbon skeleton was also partially degraded and that the main formed by-products contained only carbon and oxygen atoms. The microbial biomass could be an adsorbent of organic compounds [START_REF] Singh | Water purification by using Adsorbents: A Review[END_REF]. Therefore, possible absorption of CTC on activated sludge was studied. 0.5 g L -1 activated sludge was washed in the same conditions as the biological treatment and injected in 100 mL medium. The fl asks were stirred at 250 rpm at 25°C. For 2 hours, samples were taken and filtered (0.45 μm pore size) in order to perform DOC measurements. The experiments were duplicated to ensure the reproducibility of the results. As displayed in Fig. 10, DOC removal reached 25% in the first two hours. CTC adsorption on the bacterial biomass was therefore significant, as previously observed for TC [START_REF] Belkheiri | Combined process for removal of tetracycline antibiotic -Coupling pre-treatment with a nickel-modified graphite felt electrode and a biological treatment[END_REF][START_REF] Pulicharla | Toxicity of chlortetracycline and its metal complexes to model microorganisms in wastewater sludge[END_REF]. These results are also in agreement with previous studies, since CTC biosorption on activated sludge have been concluded as a thermodynamically favorable process [START_REF] Li | Adsorption behavior of chlortetracycline on activated sludge[END_REF].

Evolution of the generated inorganic ions

Biological treatment

Prior to biological treatment, 50 mg L -1 CTC was electrolyzed in basic medium during 300 min at 0.5 V/SCE with a flow rate of 5 mL min -1 . After CTC oxidative pre-treatment, a total removal of the target molecule was reached and biodegradability was improved (BOD 5 /COD=0.33). An activated sludge culture may be then promisingly considered.

Concerning the untreated chlortetracycline, DOC values decreased during the first 2 hours of activated sludge culture, which is in agreement with the previous biosorption study results. As shown in Fig. 11, DOC removal remained constant (around 30%) until the 9 th day and slightly increased from 33 to 42.2 mg L -1 between the 9 th and 13 th day respectively, most likely due to CTC desorption process. After that, dissolved organic carbon removal remained low and constant (21%) until the 21 st day. These results confirmed that activated sludge culture was ineffective for non-treated CTC mineralization. The decrease of DOC was mainly due to the biosorption of the target molecule on the microorganisms during the first hours of culture. The non-biodegradability of CTC was therefore confirmed.

On the other hand, in the presence of the pre-treated CTC solution, experimental results showed that DOC values decreased during the first 2 days, reaching 18% of DOC removal. The decrease can be most likely related to the biosorption of the by-products and to the biodegradation of the more readily biodegradable ones [START_REF] Yahiaoui | The feasibility of combining an electrochemical treatment on a carbon felt electrode and a biological treatment for the degradation of tetracycline and tylosinapplication of the experimental design methodology[END_REF]. These values remained constant during the next 9 days and then continuously decreased until the 21 st day. The period between the 2 nd and 9 th days corresponded to an acclimatization step of the microorganisms to the residual by-products and the assimilation of intracellular material released by the autolyzed cells [45]. These results fitted with recent research in which a similar period of acclimatization was observed [START_REF] Yahiaoui | The feasibility of combining an electrochemical treatment on a carbon felt electrode and a biological treatment for the degradation of tetracycline and tylosinapplication of the experimental design methodology[END_REF]. After that, the DOC removal continuously decreased until the last day of treatment (day 21). It can be concluded that microorganisms activity did not seem to be limited by the presence of inhibitor by -products. Therefore, a longer biological treatment is expected to still improve the mineralization yield. Interestingly, 70% of mineralization was reached at the end of the biological treatment. From this, the combined process led to 78% global mineralization efficiency for 50 mg L -1 CTC solution, with 28% corresponded to the oxidative pretreatment and 70% of the remaining by-products to the biological treatment.

•

Conclusions

A c c e p t e d m a n u s c r i p t

Since the non-biodegradability of CTC was confirmed with a value of 0.13 as an initial BOD 5 /COD ratio, a combined process involving an electrochemical pre-treatment, expected to improve biodegradability, prior to biological treatment was highly recommended. The feasibility of CTC oxidation on nickel foam electrode was confirmed. In alkaline medium, an electrochemical oxidation on nickel foam electrode was concluded to be an efficient method for chlortetracycline pretreatment. The oxidation yield exceeded 99% for a 50 mg L -1 initial CTC concentration, 0.5 V/SCE potential and 5 mL min -1 flow rate. Interestingly, the mild conditions used for electrooxidation did not lead to the formation of chlorine and so to toxic chlorinated by-products. Total dechlorination and denitrogenation were achieved making the solution less toxic for the microorganisms. Indeed, the BOD 5 on COD ratio was significantly improved for only 27% of DOC removal. A subsequent biological treatment became then relevant; it led to 70% of mineralization after 21 days of activated sludge culture. Consequently, the combined process led to 78% global mineralization efficiency for 50 mg L -1 CTC solution.

Fig. 1 .

 1 Fig. 1. Chemical structure of the chlortetracycline molecule

  A c c e p t e d m a n u s c r i p t

Fig. 2 .Fig. 3 .

 23 Fig. 2. Schematic diagram of the percolation cell: 1. cationic membranes; 2. saturated calomel electrode (SCE); 3. Working electrode (disc of nickel foam: 4 cm diameter, 0.3 cm thickness); 4. Auxiliary counter electrodes,

Fig. 4 .

 4 Fig. 4. Cyclic voltammogram of 1 g L -1 CTC in 0.1 mol L -1 NaOH on a nickel electrode (S = 20 mm 2 ), under nitrogen atmosphere and T = 298 K.Current-potential curves in the absence (red) and in the presence of CTC (black line). (100 mV s -1 )

A c c e p t e d m a n u s c r i p tFig. 5 .

 5 Fig. 5. Effect of the electrolysis potential on the degradation (%[CTC] removal) of CTC by the oxidative pre -treatment on a nickel foam electrode for 300 min of electrolysis at 5 mL min -1 and [CTC]0= 50 mg L -1 . Error bars are based on duplicate measurements.

Fig. 6 .

 6 Fig. 6. Effect of the initial CTC concentration on the degradation (a), and mineralization (b) for 300 min of electrolysis at 0.5 V/SCE and 5 mL min -1 . Error bars are based on duplicate measurements.

Fig. 7 .

 7 Fig. 7. Effect of the CTC initial concentration on BOD5/COD ratio (a) after 5 hours of the oxidative pre-treatment (b) %CTC removal, %DOC removal and %COD remova l (b). Error bars are based on duplicate measurements.

Fig. 8 .

 8 Fig. 8. Effect of the flow rate on the CTC removal for 300 min of electrolysis at 0.5 V/SCE and [CTC]0= 50 mg L -1 . Error bars are based on duplicate measurements.

A c c e p t e d m a n u s c r i p tFig. 9 .

 9 Fig. 9. Time-course of the concentrations of Cl -, NO3 -and TN during 5 hours of CTC pre-treatment. Experimental conditions: [CTC] 0= 50 mg L -1 , T= 298 K, E = 0.5 V/SCE, 5 mL min -1 flow rate. The inset represents the targeted heteroatoms on the CTC chemical structure. Error bars are based on duplicate measurements.

Fig. 10 .Fig. 11 .

 1011 Fig. 10. Biosorption study of the non-pretreated CTC (50 mg L -1 ) on activated sludge (0.5 g L -1 ). Error bars are based on duplicate measurements.

A c c e p t e d m a n u s c r i p t A c c e p t e d m a n u s c r i p t

  5 g L -1 was inoculated and the following minerals were used for all experiments (mg L -1 ): K 2 HPO 4 , 208; KH 2 PO 4 , 85; Na 2 HPO 4 .2H 2 O, 154.4; MgSO 4 .7H2O, 22.6; CaCl 2 , 27.6; FeCl 3 .6H2O, 0.26; NH 4 Cl, 75[START_REF] Fontmorin | Combined process for 2,4-Dichlorophenoxyacetic acid treatment-Coupling of an electrochemical system with a biological treatment[END_REF]. The pH was then fixed to 7.0 with NaOH solution (2 mmol L -1 ). Samples were taken regularly for DOC measurements and filtered on 0.45 μm Chromafil® Xtra PET -

	45/25.


Ph.D, "partner searcher Julien Gueslin
email: juliengueslin@free.fr

  L'archive ouverte pluridisciplinaire

The First World War profoundly changed French society, which understandably was focused on the Western Front and the confrontations taking place there. From 1915 onwards, the epic of the Latvian soldiers was to play an important role in the assertion of Latvian identity and national conscience.

Because of this, from 1917, Latvians in the West believed they could invoke the Latvian people's contribution to the Allied victory, and most of all, that they could therefore obtain the swift recognition of the declaration of independence made in November 1918. However, it is interesting to see that this argument had only limited impact. On the eve of the Peace Conference, the image of Latvia remained very vague and ambiguous: who represented what, in a country pulled between a seemingly very weak civil government dependent on Germany, and some famous soldiers who had gone over to the Red Army ? If, at that time, Lithuania had a more negative image (one could assert it was fast an invention by the Germans to weaken Poland ?), Estonia, on the contrary, had a better reputation, as it had already obtained a certain amount of autonomy from the provisional government, it had proclaimed its independance, and it had created attention at the time of the German invasion of the country and the Treaty of Brest-Litovsk i . This paper thus tries to explain the reasons for this difference in perception, by showing that the chronology of events in Latvia up to 1919 was never in phase with the moments when French attention was on the Eastern Baltic. The military events, and in particular the assessment of the situation in Latvia, were strongly dependent on the international conjuncture and especially the way France perceived the balance of power according to its own interests at a particular moment.

We will not recall here to what extent before 1914, public opinion in France, Germany and Britain was unaware of what was happening in the Eastern Baltic ii . The French Consul in Riga dealt principally with economic questions, and his reports on political affairs were not very detailed: they seem in any case to have attracted little attention, even if this seems to have evolved progressively in 1913. At the time of the German invasion in 1915, the French Consul thus evoked the determining role of French influence in the acceleration of the work on the new rail bridge in Riga in 1913 following his reports on the strategic importance of the region, and the decisive results for the mass transport of Russian troops iii .

It seems that French officers visited had attended the Vilna military school and had developed contacts with Russian, Estonian and Latvian officers studying there. During their time there, it seemed that studies were carried out on the strategic importance of the region and of the Russian defence lines. This question deserves deeper archival research, but in any case, no knowedge of it was mentioned.

After the beginning of the war, the Tsarist regime, just like the French censors, clearly controlled the distribution of information. French military circles had very little first hand information about operations and about what was happening exactly on the different fronts: most of the time, the French military representatives were stationed in Saint Petersburg, or at the large general staff headquarters, the Stavka. According to circumstances, and particularly personal affinities, they could nevertheless benefit from special access to the Grand Duke Nicolas or the Russian command. But it was often a case of fragmented information of a strategic nature and exaggeratedly optimistic opinions on the desire to fight to the end, the popularity of the war, or the satisfying state of the army despite the retreat in Lithuania and Courland, a retreat explained by the simple lack of artillery munitions iv . In January 1916, Joffre therefore had to again ask General Pelé, who had been sent to Russia, to repeat the request of the French command to be informed not only of operations, but also of the projects and intentions of the Russian High Command. In July 1916, he complained of having had no news at all from the Russian front and asked by exemple if the Russian command had given up its great offensive from Dvinsk v .

The French Consul in Riga sent extremely useful reports, notably on the state of public opinion in Riga and Livonia, but he had very little means at his disposal, and no link with the military authorities.

And in August 1915, the German threat led to the evacuation of the consular authorities to Dorpat, and thus the suppression of all French sources of direct information vi . All in all, it was no accident if nothing was known at this time of the heroic action of the 20th Latvian Corps in Eastern Prussia, and especially, very little about the events of summer 1915, notably the role played by the Latvian soldiers in the stabilisation of the military front in front of Riga and the setting-up of Latvian batallions.

Later, after 1918, the Russian communiqués would be referred to, as would the fear of the German invasion, the fact that Kovno/Kaunas and Lithuania were heard of for the first time, the heroic defence of Riga, and the Latvians. But these Russian references to heroic Latvian fighters would soon be drowned or forgotten by the multitude of other events which were just as agonising vii . Also, the communiqués were certainly positive, but typically they mainly referred to bold moves by Latvian troops (for example, surging out of trenches and killing the enemy with bayonets) but which had little strategic effect. Through a certain irony of fate, several communiqués translated into French and taken up by the main French newspapers spoke of « Lithuanian » soldiers. At the end of the war, the ignorance of journalists would be recalled, but in the military archives, texts can be found containing exactly the same mistake. Beyond the confusion, it is especially important to note that no-one then seemed to notice the mistake, a sign undoubtledly of the confusion and undoubtledly of the difficulty at that time of bringing concrete reality to these names.

In addition, both the military and diplomats relayed quite quickly the vision of a Russian general staff which did not see the German offensive in the Baltic provinces as a major threat to its security, as the Germans were unwilling to concentrate substantial numbers there. Thus in the summer of 1915, General Laguiche, following his meeting with General Alexieff and the command of the North West army, recalled the « perfect calm and trust » which existed, or the natural protection (by the sea and the marshes) of Riga: the « German project » in the Riga area seemed « vague », and the destruction of railway lines seemed the first objective viii . Knowing, as we do now after the event, the tensions which existed at this time between the Germany military authorities, this judgment was not necessarily wrong, but of course it also had the advantage of hiding the tactical mistakes and the totally deficient organisation of the Russian army ix .

It was therefore logical that the dramatisation of the situation in Riga in the summer of 1915 was less well perceived by the French military who were in Russia or Paris. The efforts of the Latvian and Russian units was less well highlighted than the apparent weakness of the German troops, which again can be explained logically as part of a global vision within which they tried to persuade themselves of certain German weaknesses and thus of the possibility of obtaining conclusive victories. The French press therefore, in major articles, often referred to the desperate efforts by the Germans to reach the Dvina or to take Riga or Dvinsk, but other than Russian resistance, they never failed to mention the marsh country or the predominance of the Anglo-Russian fleet in the Riga Gulf, etc. The strict control and the rarity of this information, led to less focus on the Russian army than on the German army: the maps and descriptions multiplied, but with hardly ever a reference to the situation or the history of the region, or to the current efforts of the 12th Army.

Furthermore, contrary to Poland, it was of course until at least 1917 that the loss of the Baltic provinces was not seen in the short term as a political problem; the question of Baltic autonomy was not an issue, pan-German ambitions were linked to a hypothetical German victory which had to be avoided in all cases. And in French military thought, reasoning was less in terms of territory (except to regret the loss of railway crossroads which hindered troop movement and thus the management of armed action on the different fronts), than in terms of military potential and number of divisions.

Germany having failed to nip the Russian army in the bud, and the army seeming to have retreated in good order, German advantages derived from marching forward seemed to be an illusion x . Also, for the French military authorities, the question was most of all the coordination of Allied efforts and the possibility of helping French troups on the offensive or defensive by fixing German units on the Russian front: where the liberation of Courland by a strong offensive was dreamt of, local, well-targeted and mortal action could suffice.

In any case, it is clear that from the end of 1915 onwards, the Russian general staff saw the Baltic front as little likely to lead to major gains and preferred concentrating on other zones, in order to hope for decisive action. In a secret memorandum to the French and British, General Alexieff wanted offensives to be stopped in directions where « the enemy » was strategically less vulnerable, and Allied forces to be concentrated in Central Europe in order to carry out a thrust on Budapest, basically to remain on the defensive in the North and to move to the offensive in the South xi . As the French military authorities were themselves increasingly monopolised by the situation in the Balkans and by the need for the Russian army to give subtantial support to the entrance of Romania into the conflict, the Baltic front was by common accord a subsidiary zone xii .

For these political reasons (the different Baltic nations were not perceived as a political factor), for military reasons (no strategic stakes), or simply for diplomatic reasons (the French desire not to ruffle Russian pride by closely following the functioning of the Russian units), the French knew very little of military life on the Baltic front until the end of 1916, and knew even less of the role of the Latvian soldiers.

The situation evolved from the end of 1916 onwards, most of all because of increasing French anxiety regarding the weakening of the Russian empire and particularly its military potential. During 1916, General Janin arrived at the head of a French military mission, and this can be explained in two ways.

Firstly, a wish to favour Russian efforts for the benefit of Romania: this led to the transfer of a number of units from the 12th Army towards the South, and thus to reduced possibilities of breaking through the German military front in front of Riga, or of resisting an offensive xiii . Indeed, tensions became greater and greater between the French and the Russians, accused of reacting too slowly to the Romanian « débâcle ». In 1916, attention was therefore monopolised by the Romanian question and by the Galicia offensive led by General Broussilov. A report by the military mission sums up the beginning of the year 1916 on the Northern front in a few words: because of the entirely defensive temperament of General Kouropatkine and the withdrawals in Galicia, there were only hints of offensives which did not come off, despite huge losses xiv . Only in December 1916, with the famous offensive of the « Mountain of Machine Guns », would the Baltic front and the Latvian soldiers be spoken of more often: while the Broussilov offensive had finally disappointed the hopes it had raised, the first victories of the Latvian soldiers at last enabled a Russian success to be relayed which was as welcome to the general staff as to French public opinion in a tense military context xv . If the military mission emphasised the small territorial gains which were made, it insisted confidentially on the huge losses suffered in the marshes (as much as 70% for some units) xvi .

Secondly, Janin's idea was to give priority to what were the (seen as inadequate) instruction of the Russian army, to the diffusion of modern techniques, and to Western knowledge on the Russian front.

The number of French technical missions (artillery, aviation, etc.) thus multiplied, partly in relation to Russian demands which were often badly coordinated, and the French representatives appointed to each army xvii . The French military thus discovered to their amazement that each Russian army commander was virtually independent of the general staff on all military and organisational questions. Commander Corbel, sent to Dvinsk in September 1916 on the death of a French airman, owed the possibility of visiting the neighbouring front to the « friendliness » of General Dragomirov, commander of the 5th Army. xviii In the same way, by chance, the commander of the 12th army, Radko-Dimitrief, was a great francophile and favoured the actions of a technical mission xix . The Head of Engineering of the 12th Army was sent on a mission to France and decided to improve the organisation of the military front in front of Riga according to the current methods adopted on the french front.

From the end of 1916 onwards, we therefore see the appearance of the first regular reports by the French military on the 12th army, reporting what happened first hand. In March 1917, Captain Aublet referred to the good defensive organisation of the troops of the 12th Army, in spite of difficult conditions, as well superior to that of the 5th army around Dvinsk, for example (good defence of premiers positions, trench networks communicating between each other, the construction of a complete network of roadways in wood and bridges on lakes, the organisation off allback positions, good artillery positions, etc.). But it was written in theses reports that the 12th Army lacked a guiding military plan, and given its fighting ground (lots of marshland), was virtually incapable of leading offensives of great scope. Through lack of being able to proceed with artillery preparations in France or detailled reconnaissances of enemy positions, but taking advantage of Russian superiority in numbers, the dominant idea was to give priority to « surprise attacks » (as in January 1917), which gave random results and were costly in terms of human lives lost xx . However, the French military were stupified to discover the mediocrity and even the negligence of certain officers: Janin, for example, recalled the criminal negligence of a commander of a Siberian regiment of the 12th Army responsible for the death of a thousand soldiers, having ordered them not to take gas masks into the trenches xxi .

The February revolution accelerated the process, France becoming anxious and thus increasingly involved, with the aim of extending the war effort and avoiding an ill-fated peace or débâcle in its interests. In Riga, the French representative to the Northern Army, Lieutenant-Colonel Perchenet, carried out several missions, but it was most of all a polytechnician engineer serving in the Navy, Gueneau, who played an important role: originally having come to electrify the network of barbed wire at the front, he rapidly extended his actions to the whole of the army, by training it and informing it about military questions xxii .

Yet it was precisely when this detailled information began to reach Paris that revolutionary agitation began to reign amongst the Latvian soldiers. For the first time, a French source recalled in detailed fashion the history of the country from 1905 onwards, and the evolution of its troops which were « better instructed and more civilised », having always proved themselves « very courageous », and participating in all attacks. Gueneau particularly evokes the reasons for Latvian resentment, which was linked to bloody losses and to the sentiment of not being supported in their efforts by the Russian command. Germanophobia disappeared progressively in view of Russophobia, to the extent that it was felt that the Russians wanted to exterminate them all to get rid of them. But in the end, the Latvian troops were identified as being the most agitated and as being troublemakers, and those whose military value had the most diminished, even if the excuse of German propaganda, manipulating the Latvian soldiers was often evoked xxiii . From March 1917 onwards, despite the fear of a German offensive and despite the popularity of General Radko, carelessness was apparent, together with a rapid degredation of troop moral, and a failure to maintain the trenches. The French reports signaled the rapid arrival from Petrograd of revolutionary delegates, fraternisation with enemy troops, or German propaganda which was increasingly present (Latvian soldiers taken behind enemy lines to enable them to understand the situation in Courland were evoked, or negotiations concerning Latvian autonomy garanteed by Germany and an agreement on the sharing of territory with the Baltic people). The second Latvian brigade was said to have threatened to take by bayonet the artillery positions which wanted to carry on firing on the Germans xxiv .

From August 1917 onwards, the German attack on the Riga front showed for the French military the negative spirit of the troops who were opposing resistance, seen as derisory despite the efforts of the provisional government officially invoking resistance, which was combative and orderly. If the role of some Latvians units in certain "delaying" battles was evoked, the telegrams sent to Paris mentioned most of all the flight of some of the Latvian troops who showed little readiness to fight xxv . However, following the October Revolution, and in a tense military context, France desperately looked for an alternative solution to the loss of the Russian alliance: interest in the different Russian nationalities and especially their capacity to offer resistance to the German thrust suddenly increased as the emergency became greater. It is very reasonable to think that if the Riga front had been maintained with the essential role that the Latvian forces played, the Latvian cause would undoubtedly have progressed, at least in terms of the notoriety and recognition of the Latvian people. On the other side, the French military could not fail to notice the evolution of the image of the french military action. In April 1916, General de Laguiche could describe for example the great overexcitement of the soldiers at Dvinsk and the spontaneous desire they expressed to be « worthy of the French Army » of Verdun. In 1917, it was more a case of criticising a France which reduced the Russian empire and its nationalities to simple cannon fodder to be mobilised according to the strategic interests of the moment. Certainly, the incompetence of the Russian officers was recalled, and sometimes it was even thought that Latvian blood was sacrificed on purpose, but was this not also the result of a strategy going nowhere? On the French side, care was thus taken to underline the discouragement or irritation of the Baltic soldiers with the mutinies or disputes which had also seized fighters on the Western Front.

At the same time, Latvian propaganda began to achieve its first breakthrough in France, particularly in the political and intellectual circles which favoured the rights of the people and the defence of small nationalities in the context of a new international organisation based on collective law and security. The work carried out firstly in Switzerland then in Latvia by Austra Ozolin and Anna Kenine was to have considerable impact by making the Latvian question known in modest but influential circles, and the historian Seignobos, the linguist Meillet, and the politician Albert Thomas thus committed themselves to this moment in favour of Latvia. German policy in Courland, which aimed for a disguised annexation and favoured colonisation plans, could only increase part of public opinion's sensitivity to these arguments, circulated in brochures and memoirs. In these the first structured accounts of the events since 1905 and of the conflict since 1914 were circulated, which enable the Latvian question to become understandable in both its political and military dimensions xxvi . However, given the disappearance of the Latvian military force and the context of the time, it was much more difficult for the Latvian representatives to use the political opportunities which presented themselves and thus to increase the standing of their constant military commitment since the war.

Contrary to the Latvians, it was the Estonians who were able to exploit this temporary evolution of French policy by proclaiming their independence in February 1918 at the time of the German invasion of the country, and by underlining their refusal to resist German hegemony. British and French « de facto » recognition was very prudent in its terms. However, with the perspective of the German defeat and the rise in force of anti-soviet Russian troops, the French authorities were to come rapidly back to the idea of favorising the recognition of a Russian power, according at the very most, a very limited autonomy to the Baltic nations. In their eyes it would thus be capable of facing Germany, and Russia would thus, in its eyes, guarantee peace in Europe. But from now on, Estonia had official representatives in Paris and London, which would not be the case of the Latvians until the Peace Conference.

The Latvians however could take advantage of their past military commitment in only one case: that of the Imanta and Troitska regiments whose formation and most of all separation from Russian units was favoured by General Janin, former head of the French military mission previously in Siberia.

Knowing precisely the operations which the Latvian soldiers had led, the General was able to obtain agreement from Paris on this subject, despite the hostility or considerable reserves of the Russian authorities in Siberia. If the role played by these formations in the civil war was very little emphasized until their return, it was much more the case for the French High Commissioner Damien de Martel, who was to become the first official French representative in Latvia in 1921 xxvii .

But to come back to the autumn of 1918, and while the Latvian representatives demanded substantial armed support to overcome the Bolshevik invasion, the French military and diplomats remained very reserved. The French press and French diplomats referred prolifically to the eminent role played by the Latvian soldiers in the guarding of the « Smolny Institute » and their essential contribution to the defence of the Revolution and the young Soviet state. On the contrary, the Latvian government, soon to take refuge in Libau, lacked everything and seemed incapable of freeing itself from the pressure of the German military authorities and their Germano-Baltic allies xxviii . In this context, it was therefore very difficult for the Latvian government to refer to the contribution of the Latvian people to the Allied war effort and to a community of aims, whilst, as we have seen, very little had been known of all this at the time of the events. On the other hand, it became clear that the Baltic zone and populations were becoming an important part of the Russian equation to be resolved, and also a real geopolitical question to be resolved, especially as the Latvian zone in particular seemed to be torn between different powers and influences.

Once again, just after the global conflict, the Russian civil war and then the Baltic wars of independence, the Baltic front could again seem secondary in relation to other theatres of operations.

There were few military forces capable of existing on their own and not depending totally on Allied aid.

This aid was very difficult to dispatch, and with the end of the conflict, it was reduced as much as possible. Increasingly, it was feared that arming the different Baltic nations would enable them to use the weapons in new clashes with the Russians, leading to a slowing-down of the rebirth of the desired Russian power. All these arguments were of course reinforced by the influence of the tsarist or anti-soviet Russian circles which still had influence with the Quai d'Orsay, hence the very dangerous decision by the Allied governments and commands, for want of a better solution, to allow the German troops after the armistice to ensure the security of the Baltic regions. But if this choice was going to considerably perturbe the evolution of the Latvian state by forcing it to enter into new conflicts, paradoxically it would show the importance of the Eastern Baltic as a geopolitical zone and would create a regional balance which would prevent any neighbouring power from obtaining political hegemony. Once again, it was the role of the military mission and the naval division to definitively perceive the solidity of the Baltic identities and national consciences through the mobilisation and commitment of their troops. This was all the easier as many of the military sent to the Baltic zone had served in Russia and knew themselves the past role of the Latvian soldiers. For example, General Niessel was to play an important role in the evacuation of the "Freikorps", and also Captain Buschenschutz, deputy to the military attaché during the war and who played an important part in Latvia in the training of the Latvian army. These military missions were to constitute an important stage in the adoption of the response which seemed the most relevant: the recognition of Baltic independences.

After 1921, and the progressive integration of the Baltic states into the « new » Europe, this complex evolution and these misunderstandings took rather a back seat. Linked to the commemoration by the Latvian state of the memory of the Latvian soldiers, French publicists speaking of Latvia would certainly refer to their heroic actions. Indeed, they had the merit of perfectly coinciding with the dominant national imagination, both past (the improvised soldiers of the Revolution going to defend the homeland in danger) and present (a miracle of the Marne or an opinionated resistance like at Verdun, but this time in Latvia). But more than this, it was the souvenir of Admiral Brisson and Colonel Duparquet during the struggle against the army of Von der Goltz or Bermondt in 1919 which was evoked, as this of course enabled Franco-Latvian friendship to be shown at a better advantage.

For a historian of today, it is interesting to retrace this slow and confused evolution, these sometimes missed encounters between the evolution of the political and military situation in Latvia, and the variations in the interests of the French elite or public opinion. This shows the gaps or the agreements that can exist between a conflict or a regional theatre, and the global stakes with repercussions on the mentalities of fighters, with their inevitable « boomerang » effects, the despair and radicalisation of the Latvian fighters thus culminating in the modification in return of the vision of the region and the policies linked to this. Far from being linear, or deriding from a certain political or economic determinism, history is made up of detours and of complexe associations of events. i i AMAEF(Archives du Ministère des Affaires étrangères (français)= French Foreign Office Archives), Guerre 1914-1918, vol. 702 ( 27 février 1918), and Jules NOULENS (1933), Mon ambassade en Russie soviétique, Paris , 1933 ,p. 247 sq. ii See by exemple the article of the famous french historian Charles Seignobos : LVVA, F2575/7, 29 (Charles,SEIGNOBOS, « La politique française. La Lettonie », no date ). iii AMAEF, Guerre, 1012(23 septembre 1915). iv SHD (= Service Historique de la Défense= French Military Archives), 16N 1207, Dossier Laguiche, Telegramme n°86 (23 mai 1915). v AMAEF, Guerre, 1011, 15 mai 1915 et SHD, 5N 139 (n°97(17 janvier 1916) , Lettonie, 1 (29 janvier 1919, Lettonie, 1 (29 janvier , 4 février 1919, Lettonie, 1 (29 janvier , 28 avril 1919) )

  and n° 524(16 juillet 1916) vi AMAEF, Guerre, Russie, 641 (22 février 1915) et 1012 (12 aout 1915 and passim) vii AMAEF, Guerre, 1013 (1 er novembre 12 et 24 novembre 1915, 29 décembre, 14 mars 1916, 3 septembre, 6 janvier 1917) viii SHD, 16N 1207 (op. cit, 1 er août 1915) ix Karl-Heinz JANNSEN (1971), « Die baltische Okkupationspolitik des Detuschen Reiches » in HEHN (Jürgen von) et alii, Von den baltischen Provinzen zu den baltischen Staaten, Marburg, p. 217 sq. x See by ex. the article: : « La Situation militaire à la fin de 1915. VIII », Journal des Débats, 28 décembre 1915 xi SHD, 5N118, Dossier Russie (4 aout 1914-30 sept 1916) : Notes relatives au projet du Général Alexeieff (decembre 1915) ; et 7N 757 (6 avril 1916) xii See the letter of General Joffre the 13th april 1916 (SHD, 5N 139) xiii SHD, 16N 3179, Dossier 13, n°33 19 janvier 1917) and Dossier 12 (2 janvier 1917)) xiv SHD, 16N 3179, n°30(Analyse…Au GQG, 4 janvier 1917) xv Ibid, 10 janvier 191 xvi SHD, Ibid, n°45 (21 janvier 1917) : xvii SHD, 16N 3180, Dossier Conference de Petrograd (Fevrier 1917, Rapport…Annexes V et VI (Etude sur le fonctionnement de la Mission militaire française en Russie)) xviii SHD, 16N 3178, n°16bis (4/19-9/22 septembre 1916) xix Cf. Note 17 (« Etude sur le fonctionnement… », Rubrique D, note 4) xx SHD, 16 N 3179, Dossier 15 (N°46bis) xxi Ibid, n°130 (4 janvier 1917) xxii SHD, 16 N3181, Dossier 1 (7 août 1917) xxiii SHD, 16N 2997, Dossier 3 (Colonel Lavergne, 9/22 juin 1917) ; 16 N3181, Dossier 1 (3 août 1917) et Dossier 3 (30/12 août 1917, Gueneau) xxiv SHD, 16 N 1280 (Gueneau (7-20 juin 1917), Perchenet (4/17 juillet 1917) et Janin (26 mai 1917)) xxv 16 N 3181, DOSSIER 2(8 septembre 1917, copies of tel. of the french Foreign Office, 5-6 september 1917) xxvi AMAEF, Guerre, 701 (Lettonie,6 mars 1917 and passim) et AAA, Politsches Archiv, Bern, 474-476 (passim) xxvii AMAEF, Europe 18-40, Lettonie, 1 (14 et 25 novembre 1918, 2 décembre 1918) xxviii See note 1 and AMAEF