Oleksandra Ganzenko

Clément Trellu

Nihal Oturan

David Huguenot

Yoan Pechaud

Eric D Van Hullebusch

Mehmet A Oturan
email: mehmet.oturan@univ-paris-est.fr

Yoan Péchaud

Electro-Fenton treatment of a complex pharmaceutical mixture: Mineralization efficiency and biodegradability enhancement

Keywords: Electro-Fenton, Pharmaceutical pollutants, Degradation, Mineralization, Biodegradation, Toxicity

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

Introduction

Electro-Fenton process is an electrochemical advanced oxidation process (EAOP) with an elevated interest for the removal of recalcitrant organic compounds form wastewaters [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF]Oturan and Aaron, 2014;[START_REF] Martínez-Huitle | Single and coupled electrochemical processes and reactors for the abatement of organic water pollutants: A critical review[END_REF]. This process is based on the well-known Fenton's reaction (Eq. ( 1)) for in-situ generation of hydroxyl radicals ( OH).

Contrarily to the conventional Fenton process, Fe 2+ is continuously regenerated at the cathode (Eq. ( 2)). It means that a catalytic amount of iron is initially added and steadily regenerated at the cathode throughout the treatment. Besides, aeration of the treated solution ensures continuous in situ production of H2O2 from reduction of dissolved oxygen at a suitable cathode material (Eq. ( 3)). Depending on the nature of both the anode material and inorganic matrix, further oxidant species such as OH, SO4 -, active chlorines can be also generated at the anode [START_REF] Panizza | Direct and mediated anodic oxidation of organic pollutants[END_REF][START_REF] Rodrigo | Use of conductive-diamond electrochemical oxidation for wastewater treatment[END_REF][START_REF] Martínez-Huitle | Single and coupled electrochemical processes and reactors for the abatement of organic water pollutants: A critical review[END_REF][START_REF] Salazar | Abatement of the fluorinated antidepressant fluoxetine (Prozac) and its reaction by-products by electrochemical advanced methods[END_REF][START_REF] Garcia-Segura | Electrochemical oxidation remediation of real wastewater effluents -A review[END_REF][START_REF] Nidheesh | Environmental applications of boron-doped diamond electrodes: 1. Applications in water and wastewater treatment[END_REF]. Therefore, electro-Fenton process has demonstrated a great potential for degradation and mineralization of different types of pollutants, including dyes, pesticides, pharmaceuticals etc. [START_REF] Sirés | Electro-Fenton degradation of antimicrobials triclosan and triclocarban[END_REF][START_REF] Özcan | Degradation of picloram by the electro-Fenton process[END_REF][START_REF] Oturan | Electrocatalytic destruction of the antibiotic tetracycline in aqueous medium by electrochemical advanced oxidation processes: Effect of electrode materials[END_REF][START_REF] Nidheesh | Magnetite as a heterogeneous electro Fenton catalyst for the removal of Rhodamine B from aqueous solution[END_REF][START_REF] Brillas | Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review[END_REF]Mousset et al., 2018).

Fe 2+ + H2O2 → Fe 3+ + OH + OH¯

(1)

Fe 3+ + e -→ Fe 2+ (2)

O2 + 2H + + 2e -→ H2O2 (3)
Research on the degradation of single compounds by the electro-Fenton process is an important step to understand the underlying mechanisms [START_REF] Oturan | Complete destruction of p-nitrophenol in aqueous medium by electro-Fenton method[END_REF][START_REF] Yang | Electrocatalytic generation of homogeneous and heterogeneous hydroxyl radicals for cold mineralization of anti-cancer drug Imatinib[END_REF]. Besides, there is also a need to assess the efficiency of the process on solutions containing a mixture of organic compounds as well as on real effluents. Thus, the goal of this study was to complement existing knowledge on electro-Fenton process by optimizing it for the treatment of a synthetic pharmaceutical mixture composed of 13 pharmaceuticals belonging to different therapeutic and pharmacological classes (Table SM-1). Each pharmaceutical selected was regarded as a representative of drugs with similar pharmaceutical action. The selection was made based on several publications that investigated the composition of hospital wastewater [START_REF] Thomas | Source to sink tracking of selected human pharmaceuticals from two Oslo city hospitals and a wastewater treatment works[END_REF][START_REF] Kosma | Occurrence and removal of PPCPs in municipal and hospital wastewaters in Greece[END_REF][START_REF] Verlicchi | Hospital effluents as a source of emerging pollutants: An overview of micropollutants and sustainable treatment options[END_REF][START_REF] Verlicchi | Hospital effluent: Investigation of the concentrations and distribution of pharmaceuticals and environmental risk assessment[END_REF][START_REF] Escher | Environmental toxicology and risk assessment of pharmaceuticals from hospital wastewater[END_REF][START_REF] Sim | Occurrence and distribution of pharmaceuticals in wastewater from households, livestock farms, hospitals and pharmaceutical manufactures[END_REF]. Among the multitude of identified compounds from these studies, a selection was done based on (i) the most frequently detected drugs with highest concentrations (Table SM-2) and, (ii) the risk or hazard they represent for humans and ecosystems (genotoxicity, antibiotic resistance in bacteria and various adverse effects on living organisms) [START_REF] Martínez | Antibiotics and antibiotic resistance genes in natural environments[END_REF][START_REF] Allen | Call of the wild: antibiotic resistance genes in natural environments[END_REF][START_REF] Załęska ; Radziwiłł | Ecotoxicity of chosen pharmaceuticals in relation to micro-organisms-risk assessment[END_REF][START_REF] Dias | Assessing the antibiotic susceptibility of freshwater Cyanobacteria spp[END_REF][START_REF] Diniz | Ecotoxicity of ketoprofen, diclofenac, atenolol and their photolysis byproducts in zebrafish (Danio rerio)[END_REF][START_REF] Kovács | Assessment of toxicity and genotoxicity of low doses of 5-fluorouracil in zebrafish (Danio rerio) two-generation study[END_REF][START_REF] Zivna | Effect of salicylic acid on early life stages of common carp (Cyprinus carpio)[END_REF].

The objective of the electro-Fenton process detailed in this study is to provide an efficient solution for the removal of pharmaceutical in the range of mg L -1 concentrations. Such concentrations can be observed in effluents from the pharmaceutical industry effluents where contamination mainly arises from washing and rinsing procedures of tanks. For example, sulfonamide drugs were detected in an untreated drug waste at 400 -700 mg L -1 (Kumar [START_REF] Gupta | Treatment of pharmaceutical wastes[END_REF]. High concentration of pharmaceutical compounds can be also analyzed in nanofiltration and/or reverse osmosis concentrates [START_REF] Urtiaga | Removal of pharmaceuticals from a WWTP secondary effluent by ultrafiltration/reverse osmosis followed by electrochemical oxidation of the RO concentrate[END_REF]. Hospitals are also another source of uncontrolled release of drugs into the environment with potential negative consequences [START_REF] Mendoza | Pharmaceuticals and iodinated contrast media in a hospital wastewater: A case study to analyse their presence and characterise their environmental risk and hazard[END_REF]. Hospital effluents are normally discharged to the common sewage system and undergo conventional treatment at municipal wastewater treatment facilities which do not constitutes an effective barrier for the removal of hardly biodegradable compounds [START_REF] Heberer | Occurrence, fate, and removal of pharmaceutical residues in the aquatic environment: a review of recent research data[END_REF][START_REF] Radjenović | Fate and distribution of pharmaceuticals in wastewater and sewage sludge of the conventional activated sludge (CAS) and advanced membrane bioreactor (MBR) treatment[END_REF].

Particular emphasis has been given in this study to the potential combination of the electrochemical process with a biological post-treatment in order to improve the costefficiency of the process [START_REF] Ganzenko | Electrochemical advanced oxidation and biological processes for wastewater treatment: A review of the combined approaches[END_REF]Olvera-Vargas et al., 2016a, 2016b;Monteil et al., 2018;[START_REF] El Kateb | Electrochemical advanced oxidation processes using novel electrode materials for mineralization and biodegradability enhancement of nanofiltration concentrate of landfill leachates[END_REF]. Therefore the following objectives were pursued:

(i) assessment of the influence of the mixture on degradation kinetics of two components of the pharmaceutical mixture (caffeine and 5-fluorouracil), (ii) determination of the effect of operating parameters (current, catalyst concentration, treatment time) on mineralization rate of the pharmaceutical mixture and determination of energy consumption, (iii) investigation of the evolution of inorganic species of nitrogen during the treatment, (iv) evolution of the acute toxicity of treated solutions and, (v) determination of the biodegradability enhancement of the solution after the electro-Fenton treatment.

Materials and methods

Chemicals

Analytical grades of 5-fluorouracil, acetylsalicylic acid (aspirin), atenolol, caffeine, diclofenac sodium salt, diatrizoate meglumine, erythromycin, naproxen, norfloxacin, acetaminophen (paracetamol), ranitidine, sulfamethoxazole and tetracycline hydrochloride were obtained from Sigma-Aldrich. Iron (II) sulfate heptahydrate and sodium sulfate were reagent grade obtained from Acros Organics and Sigma-Aldrich, respectively. All reagents listed in literature for COD and BOD analyses were reagent grade. For toxicity assessment with Microtox ® , Vibrio fischeri bacteria were purchased from Hach Lange.

Preparation of the synthetic pharmaceutical mixture

The synthetic pharmaceutical solution was prepared by mixing 0.1 mM of each compound listed in sub-section 2.1 and Table SM-1 in water. All the solutions were prepared with ultrapure water produced by a Millipore Milli-Q (simplicity 185) system with resistivity >18.2 MΩ cm. Solution was stirred until complete dissolution of all compounds and stored at 4 °C for maximum one week before use.

Experimental setup

Open undivided electro-Fenton glass reactor was operated in a batch mode at ambient temperature (T = 20±1 °C) and was filled with 200 mL of the synthetic pharmaceutical solution containing 0.05 M sodium sulfate and iron sulfate at concentration in the range 0.1 -0.5 mM. The pH of the solution was adjusted to 2.9 ±0.1 with 1 M sulfuric acid. The carbon felt cathode (18.5 cm × 4.5 cm) circled the internal wall of the reactor. The boron-doped diamond (BDD) anode (6 cm × 4 cm) was positioned in the center on equal distances from the encircling cathode. The solution was continuously stirred at a speed of 450 rpm and aerated with compressed air (0.2 L min -1 ) throughout the experiment. Aeration started 5 min prior to electrolysis in order to saturate the solution with oxygen. The current and voltage were monitored in real time using a power supply (HM8040-3, Hameg Instruments).

Instruments and analytical procedures

Total organic carbon (TOC)

TOC analysis was performed in order to determine the mineralization rate of the solution during the treatment. It was measured on a Shimadzu VCSH TOC analyzer by combustion with catalytic oxidation at 680 °C. A non-dispersive infrared detector was used. Standard potassium hydrogen phthalate solution was used for calibration. The injection volume was 50 μL.

Caffeine and 5-fluorouracil analysis by high performance liquid chromatography (HPLC)

Experiments on degradation kinetics of caffeine and 5-fluorouracil were followed by a reverse-phase HPLC. The equipment (Merck Hitachi) consisted of a column Purospher STAR RP-18 endcapped (5 μm), a pump (Elite LaChrome, L-2130), UV detector (Elite LaChrome, L-2400) and a thermostat set at 40 °C (Jetstream Plus, series 140310). The mobile phase was a mixture of methanol and ultrapure water both buffered with 1% acetic acid. Injection volume was equal to 20 μL. UV detector was set to 275 nm. 5-Fluorouracil and caffeine exhibited well-defined peaks at retention time of 10 and 25.4 min, respectively, under gradient elution conditions (details of the procedure was given in Text SM-1)

Ion chromatography

The evolution of inorganic ions was followed with a Dionex ICS-1000 ion chromatography system equipped with an ASRS-ULTRA II (for anions) or CSRS-ULTRA II (for cations)

self-regenerating suppressor to improve the sensitivity of the detector. The system was equipped with a DS6 conductivity detector containing a cell heated at 35 °C. An anionexchange column (IonPac AS4ASC, 25 cm × 4 mm) was used for NO3 -and NO2 -analyses.

NH4 + analysis was performed on a cation-exchange column (IonPac CS12A, 25 cm × 4 mm).

A solution composed of 1.8 mM Na2CO3 and 1.7 mM NaHCO3 at 2.0 mL min -1 was used as mobile phases for anion analysis. Mobil phase for cation analysis was a 9.0 mM H2SO4 solution at 1.0 mL min -1 . Identification and quantification of ions was done by comparison with elution time and peak areas of standard solutions.

Acute toxicity

Toxicity of solutions was measured by means of Microtox ® test on bioluminescent bacteria Vibrio fischeri. Microtox ® Model 500 Analyzer was used for this analysis. Each sample was adjusted to the pH in the range of 6.5-7.5 and filtered (0.2 μm regenerated cellulose filter) in order to remove precipitated iron. Sodium chloride 22% was added to the tubes with reactivated Vibrio fischeri for osmotic protection of bacteria. Finally, prepared samples were added to the tubes in order to measure luminescence inhibition (as % of the initial luminescence of bacteria) after 5 min of incubation.

Chemical oxygen demand (COD)

Chemical oxygen demand was measured using a reflux method in a closed system (tubes)

with potassium dichromate followed by a colorimetric dosage using a Spectroquant ® TR 420 (Merck).

Biological oxygen demand (BOD)

The biological oxygen demand (BOD) was measured in order to follow the evolution of the biodegradability of the solution. The analysis was performed following the method detailed by [START_REF] Rodier | Analyse de l'eau[END_REF]. The inoculum was activated sludge from a French municipal wastewater treatment plant. Before the beginning of the experiment, sample, dilution water and bacterial inoculum were aerated to reach oxygen saturation. The BOD values were measured by respirometric method using OxiTop® Control System (WTW, Germany). Each sample bottle of OxiTop® of 432 mL volume contained 100 mL of sample, 3 mL of buffer solution (amount calculated to keep the ratio COD:N close to 100:5), 1 mL of allylthiourea solution (to prevent nitrification), 273 mL of dilution water and 5 mL of biomass from activated sludge. Before adding the inoculum, the solution was adjusted to circumneutral pH.

All the bottles contained a rubber sleeve containing sodium hydroxide pellets to absorb the carbon dioxide produced during the bacterial respiration. Continuously stirred bottles were incubated in dark place with temperature in the range 20(±1) °C during 5 days in order to determine the BOD at 5 days (BOD5) as well as during 17 days in order to observe the effect of bacteria acclimation on the biodegradability of the solution. For each batch of analysis, a blank (ultrapure water) was analyzed for evaluation of the endogenous respiration of bacteria.

Endogenous respiration was subtracted from each value obtained for the different samples.

Results and discussion

Influence of the mixture on degradation kinetics of caffeine and 5-fluorouracil.

The degradation kinetic of two components of the mixture was followed in order to determine the apparent rate constant (kapp) of the reaction with hydroxyl radicals. The degradation kinetics of pharmaceutical compounds fitted well with the pseudo-first order model (R 2 > 0.99) (Eqs. ( 4) and ( 5)):

d[Pharma]/dt = -kapp [Pharma], ( 4
)
kapp = kabs [ • OH] (5)
Apparent rate constants for caffeine and 5-fluorouracil are given in Table 1 as a function of current applied. Apparent rate constants in the pharmaceutical mixture were compared with values obtained from the previous studies on individual molecules of caffeine and 5fluorouracil [START_REF] Ganzenko | Removal of psychoactive pharmaceutical caffeine from water by electro-Fenton process using BDD anode: Effects of operating parameters on mineralization efficiency[END_REF][START_REF] Ganzenko | Fast and complete removal of the 5-fluorouracil drug from water by electro-Fenton oxidation[END_REF]. As the experimental setup was identical, all these data allowed visualizing a change in the degradation rate of given compounds depending on the organic matrix of the solutions (compounds alone vs mixture of compounds).

It can be seen that the apparent rate constants for both molecules are close when comparing for same current intensities and same matrix. These results might be ascribed to a similar initial reaction mechanism and show that hydroxyl radicals really act as non-selective oxidants in this case. Besides, apparent rate constants decreased in the pharmaceutical mixture by a factor in the range 18 -30 compared to experiments performed on single molecules. In fact, initial TOC of individual caffeine and 5-fluorouracil solutions was around 9.6 and 4.8 mg L -1 respectively, while the TOC of the synthetic mixture was 220 mg L -1 (including the same concentration of caffeine and 5-fluorouracil). These results show that hydroxyl radicals really act as non-selective oxidants. A slightly higher apparent reaction rate was observed for caffeine, as expected from absolute rate constants previously determined (2.48 × 10 -9 L mol -1 s -1 for caffeine and 1.52 × 10 -9 L mol -1 s -1 for 5-florouracil) [START_REF] Ganzenko | Removal of psychoactive pharmaceutical caffeine from water by electro-Fenton process using BDD anode: Effects of operating parameters on mineralization efficiency[END_REF][START_REF] Ganzenko | Fast and complete removal of the 5-fluorouracil drug from water by electro-Fenton oxidation[END_REF]. As hydroxyl radicals are non-selective oxidants, they can react with caffeine and 5-florouracil as well as with the other pharmaceuticals present in the mixture. Since 5florouracil and caffeine accounted for only 2.2 and 4.4% of the TOC of the mixture, the apparent rate constants were strongly decreased because of the competition of other organics for hydroxyl radicals. It can be noted that the apparent rate constants for both molecules were close in the mixture, highlighting the non-selective potential of the electro-Fenton process. An interesting observation was also linked to the fact that the optimal current for degradation of both compounds in the pharmaceutical mixture was 400 mA. By comparison, the optimal current for degradation of individual caffeine and 5-fluorouracil was 300 mA, with a strong decrease of the apparent rate constant at 400 mA compared to 300 mA. This difference in the optimal current can be mainly attributed to the higher TOC of the mixture that leads to a different ratio between the amount of organic matter available for oxidation and the amount of hydroxyl radicals produced. The existence of an optimal value of current can be associated to the production of high amount of hydroxyl radicals leading to enhancement of the rate of parasitic reactions (Eqs. ( 4)-( 7)) to the detriment of reaction with the organic matter [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF]. Thus, the higher TOC in the mixture shifted the optimal current to a higher value as a greater fraction of hydroxyl radicals was consumed for the degradation of organic matter instead of propagation of parasitic reactions.

Fe 2+ + • OH → Fe 3+ + OH¯ (4) 2BDD( • OH) → 2 BDD + H2O2 (5)
• OH + HO2 • → H2O + O2 (6)

• OH + H2O2 → H2O + HO2 • (7)

Effect of operating parameters on mineralization of the pharmaceutical mixture.

Degradation of pharmaceutical compounds is not the only objective to achieve during the treatment of such effluent. It is also necessary to remove degradation by-products that can be potentially even more toxic than parent compounds. Total mineralization of organic compounds ensures a total decontamination of the solution due to the complete transformation of organic compounds into harmless inorganics such as CO2, H2O and inorganic ions. Therefore mineralization kinetics was followed through TOC analyses and the influence of two main operating parameters (catalyst concentration and current) on process efficiency was assessed (Fig. 1).

First, the influence of Fe 2+ (catalyst) concentration, which influences greatly the formation of hydroxyl radicals, was analyzed. Results obtained for three different Fe 2+ concentrations (0.1, 0.2 and 0.5 mM) were depicted in Fig. 1A. It can be seen that a double increase in Fe 2+ concentration from 0.1 to 0.2 mM led to a faster TOC removal. However, a further increase to 0.5 mM showed that the mineralization was slower than for 0.2 mM Fe 2+ . The existence of an optimal value for catalyst concentration during electro-Fenton process for the removal of organic pollutants is mainly attributed to enhancement of the parasitic reaction between hydroxyl radicals and Fe 2+ in the presence of excess Fe 2+ (Eq. ( 4)) [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF][START_REF] Yang | Electrocatalytic destruction of pharmaceutical imatinib by electro-Fenton process with graphene-based cathode[END_REF]. Therefore, the concentration of 0.2 mM Fe 2+ was chosen for all further experiments.

Then, the influence of current was studied in the range 100 to 1500 mA and the results were presented in Fig. 1B. It was observed that the higher the current the faster the mineralization of pharmaceuticals, without existence of an optimal threshold value. For example, after 0.5 h of electrolysis the mineralization rate was at 20, 33, and 50% for 100, 500 and 1500 mA, respectively. This behavior is different from the mineralization of single pharmaceuticals (caffeine and 5-florouracil) for which an optimal value of 400 mA was observed. These results can be explained by the fact that degradation is mainly ascribed to oxidation of organic compounds in the bulk through the electro-Fenton process, while mineralization is also strongly enhanced through anodic oxidation at the BDD anode. While there is an optimal current for the electro-Fenton process (because of parasitic reactions, Eqs. ( 4)-( 7)), it is usually observed a continuous increase of mineralization rates when increasing current during anodic oxidation process [START_REF] Panizza | Direct and mediated anodic oxidation of organic pollutants[END_REF].

However, it is worth noticing that the mineralization rate was much faster at the beginning of the treatment compared, particularly at high current. After 1 h of electro-Fenton treatment with an intensity of 1000 mA almost 62% of organic matter was removed, while during next 1 h only additional 21% mineralization was achieved. Such behavior can be attributed to a stronger mass transfer limitation after removal of the main portion of the organic matter. In fact, the level of production of hydroxyl radicals was constant throughout the treatment, while the organic matter was being gradually mineralized. The ratio between the amount of hydroxyl radicals and the amount of organic matter becomes higher, resulting in excess of hydroxyl radicals and thus promoting parasitic/ wasting reactions instead of reaction with organic compounds [START_REF] Mousset | Charge transfer/mass transport competition in advanced hybrid electrocatalytic wastewater treatment: Development of a new current efficiency relation[END_REF].

This phenomenon led to a decrease of the energy efficiency (EC) of the process during the treatment. Thus, the EC was calculated according to Eq. ( 7) in terms of kWh consumed per kg of TOC removed during the process:

= . . ( ) . ( 7
)
where Ecell is the cell voltage (V), I is the applied current (A), t is the treatment time (h),

(ΔTOC)t is the decay of TOC (g L -1 ) at time t and Vs is the volume of the solution treated (L).

At 500 mA, energy consumption was 97.8, 130, 181, 289 and 407 kWh per kg of TOC removed after 0.5, 1, 2, 4 and 6 hours of treatment, respectively. This strong increase in energy consumption with treatment time is directly related to the slower mineralization kinetics.

Formation and monitoring of inorganic nitrogen species

Mineralization of organic compounds results in the formation of inorganic species depending on the nature of heteroatoms in the structure of parent compounds. In the pharmaceutical mixture, 0.2 mM of fluorine, 0.2 mM of chlorine, 0.3 mM of iodine, 0.2 mM of sulfur and 2.6 mM of nitrogen were present as heteroatoms. N being the main heteroatom present in organic molecules, the formation of different forms of inorganic N was followed during the treatment. The evolution of nitrite (NO2 -), nitrate (NO3 -) and (NH4 + ) is presented in Fig. 2 as a percentage of the initial N concentration (2.6 mM). The concentration of these ions gradually increased till the end of the treatment, when total inorganic nitrogen reached 95% of total initial N content while the TOC removal rate was 91% at the end of the treatment.

These results show that almost all the organic nitrogen was converted into inorganic nitrogen.

A little fraction of N might be also converted into gaseous products such as N2, NO2 or N2O5 [START_REF] Garcia-Segura | Could NOx be released during mineralization of pollutants containing nitrogen by hydroxyl radical? Ascertaining the release of N-volatile species[END_REF]Mousset et al., 2018). It can also be noticed that organic nitrogen was transformed mainly into nitrate (73%) and ammonium (19% N), while the percentage of nitrite was low throughout the whole treatment (<3%). These results are consistent with previous studies on mineralization of single compounds by the electro-Fenton process [START_REF] Ganiyu | Efficiency of plasma elaborated sub-stoichiometric titanium oxide (Ti4O7) ceramic electrode for advanced electrochemical degradation of paracetamol in different electrolyte media[END_REF]. The amount and nature (oxidation state) of inorganic nitrogen species accumulated in the solution is related to (i) the release of N from degradation of organic compounds (ii) oxidation and reduction of inorganic N species at the electrodes (particularly the oxidation of NH4 + and NO2 -into NO3 -at the anode) and, (iii) reduction of NH4 + at the cathode or its reaction with HClO that can be generated from oxidation of Cl -at the anode (however, initial Cl concentration from heteroatoms in organic compounds was much lower than N concentration (0.2 and 2.6 mM, respectively) (Mousset et al., 2018). The evolution of inorganic N has to be carefully monitored since it could constitute a secondary contamination after mineralization of organic compounds. For example, it could be necessary to implement a post-biological nitrification and denitrification process for the removal of inorganic N species.

Evolution of acute toxicity of the pharmaceutical mixture

The evolution of acute toxicity of the pharmaceutical mixture during the electro-Fenton process was monitored through a series of Microtox ® tests and results are presented on Fig. 3.

The initial solution had already a rather high acute toxicity with a Vibrio Fisheri luminescence inhibition of 70%. After 30 min of treatment, Vibrio Fisheri luminescence inhibition increased to 100%, indicating the formation of degradation by-products with higher toxicity [START_REF] Le | Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media[END_REF]. Then, luminescence inhibition started to decrease from 4 h of treatment at 500 and 1000 mA reaching 42% inhibition (40% lower than initial inhibition) at the end of 6 h treatment. This result might be ascribed to the degradation of toxic by-products into harmless by-products such as carboxylic acids [START_REF] Le | Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media[END_REF]. Contrarily to 500 and 1000 mA, a lower decrease was observed at 100 mA because of the slower oxidation/mineralization kinetics.

Biodegradability enhancement

The possibility to combine the electro-Fenton process with a post-biological process for removing organic compounds in a more cost-effective way has been investigated. In fact, operation of a conventional biological process requires lower energy consumption than electrochemical processes. However, conventional heterotrophic biological processes are only effective for the removal of biodegradable organic compounds. First, the ratio BOD5/COD was used as indicator of the biodegradability of the effluent under examination.

It is usually assumed that a value above 0.33 is required for treatment of an effluent by a biological process. The ratio BOD5/COD of the initial pharmaceutical mixture was only 0.017, indicating that it would not be efficient to implement directly a biological treatment.

Then, it was investigated the evolution of the biodegradability of the effluent during the electro-Fenton treatment. The factor of biodegradability enhancement was calculated from Eq. ( 8).

Biodegradability enhancement = Rt / Ri (8

)
where Rt is BOD5/COD ratio at time t and Ri is initial BOD5/COD ratio.

Evolution of biodegradability of pharmaceutical mixture during electro-Fenton treatment is presented on Fig. 4A. It was observed that the biodegradability after 5 days of incubation for 100 and 500 mA was enhanced in the range of 3-9 times from the initial value and tends to increase with longer treatment duration. When a current of 1000 mA was applied, the biodegradability enhancement reached 5, 11 and 16 after 1, 2 and 3 h of treatment, respectively. These results highlight that the electro-Fenton process is able to degrade initial compounds into more biodegradable by-products. Particularly, short-chain carboxylic acids are well known to be easily biodegradable. Finally, their formation requires multiple steps of degradation of pharmaceutical compounds as described in studies on single compounds.

Therefore, longer treatment times and higher currents allow for greater enhancement of biodegradability. However, even after 3 h of treatment at 1000 mA, the BOD5/COD value was only 0.27. Values of BOD5 can strongly depend on the nature of the inoculum chosen for the analyses since different bacterial population could result in different biodegradation rate of organic compounds. In this study, it was chosen to use an activated sludge from a municipal wastewater treatment plant in order to obtain data that could be useful in view of the combination of the electrochemical process with a conventional biological process. However, acclimation of the biomass to this specific effluent might result in different biodegradation rate. Therefore, it has been investigated the evolution of BOD/COD ratio over incubation time longer than the conventional 5 days. A set of BOD experiments was conducted for 17 days and results are graphically presented on Fig. 4B. Prolonged incubation time did not significantly change the biodegradability of the initial effluent. However, it was observed a different behavior in the effluents pre-treated by electro-Fenton process. First, BODt/COD ratio increased rapidly during the first 3 days of incubation, most probably owing to the biodegradation of short-chain carboxylic acids. For instance, a maximal BODt/COD value of 0.18 was reached after 1 d of incubation of electro-Fenton pre-treated mixture at 1000 mA for 3 h. In fact, the higher amount of carboxylic acids was expected for this effluent because of the higher current and treatment time used for the pre-treatment. Interestingly, a second phase of fast increase of BODt/COD ratio was observed from days 5-6, particularly for the effluents pre-treated at 500 and 1000 mA currents. That means that an acclimation time of the biomass was required for degradation of some degradation by-products generated during the electro-Fenton pre-treatment. Results from Microtox ® tests indicating that some degradation byproducts have higher acute toxicity than the parent compound are consistent with these results showing the necessity of an acclimation time of the biomass to the pre-treated effluent. [START_REF] Trellu | Combination of anodic oxidation and biological treatment for the removal of phenanthrene and Tween 80 from soil washing solution[END_REF] observed similar results for the treatment of soil washing solutions by combining anodic oxidation and biological treatment. Thus, after 17 days of incubation, BODt/COD ratio reached 0.59 and 0.91 for effluents pre-treated at 500 and 1000 mA respectively. These values are much higher than the threshold value of 0.33, indicating that a conventional biological treatment might be applied to these pre-treated effluents. Such combined treatment would allow the application of the electro-Fenton process only for short treatment times in order to keep the electrical energy consumption at a minimal level, while the biological process would allow the removal of the biodegradable residual organic matter in a more cost-effective way than the electro-Fenton process. Acclimation of the biomass to the pre-treated effluent would be a crucial parameter. However, using continuous biological reactors will easily solve this issue. The results obtained also confirm the limited relevance of using the BOD5/COD ratio alone to assess the biodegradability of an effluent. Finally, it can be noticed that NO3 -released from mineralization of organic compounds could also act as electron acceptors in place of O2 for heterotrophic bacteria degrading organic compounds.

Thus, presence of NO3 -would be also an advantage as it can decrease the aeration cost of the biological treatment.

Conclusions

This study has investigated the potential of the electro-Fenton process for degradation and mineralization of a complex mixture of pharmaceuticals. Degradation/mineralization of 2 of the 13 pharmaceuticals was also comparatively studied in single compound solutions and in the mixture. Optimal Fe 2+ concentration and current for degradation of the pharmaceutical mixture were found as 0.2 mM and 400 mA, respectively. However, the mineralization rate was continuously increased when increasing the current without any threshold value owing of the strong contribution of anodic oxidation process with BDD anode. Mineralization of pharmaceuticals was confirmed through monitoring of TOC removal and nitrogen ions released into the solution mainly under the form of NO3 -and NH4 + . These results highlight that optimal operating conditions strongly depends on hydroxyl radical scavenging. Such competition phenomena depend on the nature of each specific effluent and should be further study for the different kinds of effluent for which the EF process could be applied.

The energy consumption related to electro-Fenton process is a crucial parameter for cost efficiency of the process. Thus, the possibility to combine this process with a post-biological treatment was investigated through monitoring the evolution of the biodegradability of electro-Fenton pre-treated effluent. Acclimation of the biomass to the pre-treated effluent was observed to be an important parameter since some degradation by-products were observed to have higher acute toxicity than the parent compounds. Short treatment time (3 h maximum) at 500 and 1000 mA were effective operating conditions for increasing sufficiently the biodegradability of the effluent. Therefore, the electro-Fenton process allows the degradation of non-biodegradable pharmaceuticals into more biodegradable compounds that could be removed in a more cost-effective way by a post-biological treatment.

Mixture of 13 pharmaceutical compounds

Biological+ treatment+

High mineralization rate and treatment of inorganic nitrogen

Fig. 1 -

 1 Fig. 1 -Effect of operating parameters on TOC removal during the treatment of the pharmaceutical mixture by electro-Fenton process. A: Effect of Fe 2+ concentration; [Fe 2+ ] = 0.1 -0.2 -0.5 mM; operating conditions: I = 300 mA, [Na2SO4] = 50 mM, V = 200 mL. B: Effect of current; I = 100 -300 -500 -1000 -1500 mA; operating conditions: [Fe 2+ ] = 0.2 mM, [Na2SO4] = 0.05 M, V = 200 mL.

Fig. 2 -

 2 Fig. 2 -Evolution of inorganic species of nitrogen (nitrite, nitrate, ammonium) during electro-Fenton treatment of the pharmaceutical mixture. Data are presented as a mass balance (%) of the initial amount of organic nitrogen (2.6 mM) in the pharmaceutical mixture. Operating conditions: I = 100 mA, [Fe 2+ ] = 0.2 mM, [Na2SO4] = 50 mM, V = 200 mL.

Fig. 3 -

 3 Fig. 3 -Evolution of luminescence inhibition of Vibrio fisheri as a function treatment time during electro-Fenton process as a function of f current (100, 500 and 1000 mA). Operating conditions: [Fe 2+ ] = 0.2 mM, [Na2SO4] = 0.05 M, V = 200 mL.

Fig. 4 -

 4 Fig. 4 -A -Biodegradability enhancement (after 5 days of incubation) after electro-Fenton treatment of the pharmaceutical mixture under 3 different as a function of current applied. B -evolution of BODt / COD ratio as a function of incubation time during BOD tests, depending on the current applied for the electro-Fenton treatment of the pharmaceutical mixture during 3 h (B). The red dotted line indicates the value 0.33 of the ratio BODt / COD. Operating parameters: [Fe 2+ ] = 0.2 mM, [Na2SO4] = 50 mM, V = 200 mL.

Table 1 -

 1 Apparent (pseudo-first order) rate constants obtained for degradation of 5-flourouracil and caffeine at different current intensities in the synthetic pharmaceutical mixture (Mix) and in solutions of individual compounds (Alone). Optimal values for different types of solutions are indicated in bold characters. Operating conditions: [Fe 2+ ] = 0.2 mM, [Na2SO4] = 0.05 M, V = 200 mL.

		5-fluorouracil	Caffeine
	Current (mA)	Mix	Alone	Mix	Alone
		kapp (min -1 ) kapp (min -1 ) kapp (min -1 ) kapp (min -1 )
	200	0.038	0.61	0.034	0.73
	300	0.040	0.74	0.036	1.14
	400	0.042	0.60	0.038	0.64
	700	0.03	-	0.023	-

Acknowledgment

The authors thank the EU for providing financial support through the Erasmus Mundus Joint Doctorate Program ETeCoS3 (Environmental Technologies for Contaminated Solids, Soils and Sediments, grant agreement FPA no. 2010-0009).


V Szczepan

C Petitjean

P J Panteix

M Vilasi

Corrosion and depassivation of a chromia-forming Ni-based alloy in the 0.75Na2O-B2O3-2.75SiO2 melt

Keywords: High temperature corrosion, depassivation, Ni-based alloy, chromia-forming alloy, corrosion potential

Corrosion resistance of nickel based alloys in molten silicates is linked to the temperature. Above a certain temperature, the protection of the alloy by a chromia layer is no more ensured: this depassivation phenomenon leads to active corrosion. Electrochemical methods were used for in situ characterization, and Scanning Electron Microscopy (SEM) and Scanning Electron Transmission Microscopy (STEM) were used in order to measure the oxide thickness after immersion. Corrosion tests with increasing temperature showed that above a certain temperature, the oxide thickness decreased homogeneously leading to the depassivation of the alloy. The different steps of this phenomenon are discussed here.

1

Introduction

Contact between alloys and molten silicates based glasses are encountered in many industrial applications, for instance, in the production of glass fiber [START_REF] Martino | Corrosion of metals and alloys in molten glasses. Part 1: glass electrochemical properties and pure metal (Fe, Co, Ni, Cr) behaviours[END_REF][START_REF] Martino | Corrosion of metals and alloys in molten glasses. Part 2: nickel and cobalt high chromium superalloys behaviour and protection[END_REF] or for the vitrification of nuclear wastes [START_REF] Sengupta | Interaction study between nuclear waste-glass melt and ceramic melter bellow liner materials[END_REF]. Nickel-based or cobalt-based chromia forming alloys are widely used in these applications due to theirs good mechanical properties at high temperature and their ability to develop a protective chromia layer [START_REF] Schmucker | Oxidation of Ni-Cr alloy at intermediate oxygen pressures. I. Diffusion mechanisms through the oxide layer[END_REF].

In order to enhance the protective behavior of chromia-forming nickel-based alloys, a pre-oxidation step is performed to form a passivating chromia layer before immersion.

Lizarazu noticed that a nickel based alloy with 30% mass of chromium (noted here as Ni-30Cr) exhibits a better corrosion resistance after two hours of immersion at 1100 °C when pre-oxidized 35 min in air at 1100 °C instead of being directly immersed [START_REF] Lizarazu | Comportement en oxydation à l'air et comportement électrochimique dans le verre fondu d'alliages nickel-chrome[END_REF]. Pre-oxidation was then commonly used for corrosion resistance studies [START_REF] Abdullah | Corrosion of Pure Cr and Ni-30Cr Alloy by Soda-Lime-Silicate Melts: Study of Simplified Systems[END_REF][START_REF] Carton | Corrosion of Chromium in Glass Melts[END_REF]. The efficiency of the preoxidation is explained by the compactness of the chromia layer formed by oxidation in air as well as the low solubility of chromia in molten silicates [START_REF] Manfredo | Solubility of Refractory Oxides in Soda-Lime Glass[END_REF], which induces a limited recession of the passive layer and thus confers its protective properties against high temperature corrosion by molten silicates [START_REF] Kim | Enthalpies of Chromium Oxide Solution in Soda Lime Borosilicate Glass Systems[END_REF][START_REF] Khedim | Redox-Control Solubility of Chromium Oxide in Soda-Silicate Melts[END_REF][START_REF] Khedim | Solubility of chromium oxide in binary soda-silicate melts[END_REF]. The knowledge of the conditions of stability of this protective oxide layer is the key to predict the corrosion rate of the material and thus its durability.

Electrochemical methods can help to study this subject. First, these methods can be used in silica-based liquids due to the high ionic conductivity of these media. Second, as these methods are in situ measurements, they give access to corrosion properties of materials in conditions. As an example, corrosion potential measurements give information about the corrosion state of the material i.e. passive or active [START_REF] Martino | Corrosion of metals and alloys in molten glasses. Part 1: glass electrochemical properties and pure metal (Fe, Co, Ni, Cr) behaviours[END_REF][START_REF] Martino | Corrosion of metals and alloys in molten glasses. Part 2: nickel and cobalt high chromium superalloys behaviour and protection[END_REF]. Moreover, the results are not subject to deterioration by quenching or polishing as for electronic microscopy characterization.

Among the previous works dealing with the corrosion study of nickel based alloys by silica-based liquids, some of them studied and highlighted the importance of temperature on the stability of the chromia layer. Carton et al. [START_REF] Carton | Corrosion of Chromium in Glass Melts[END_REF] proposed an experimental procedure in which the temperature is slowly increased at 5°C/h and 10°C/h while the pre-oxidized alloy is immersed in the liquid. Corrosion potential measurements showed that above a certain temperature, the chromia layer stops to ensure its protective properties and is dissolved by the melt. This phenomenon was called depassivation and its origin is not understood but it seems to appear in various kinds of silicates, e.g. soda-lime-silicates [START_REF] Abdullah | Corrosion of Pure Cr and Ni-30Cr Alloy by Soda-Lime-Silicate Melts: Study of Simplified Systems[END_REF] and boro-silicates [START_REF] Abdullah | Study of the redox and acid-base properties of soda-lime silicate glass : appliacation to the high temperature corrosion of the Ni-based alloys and ceramic materials[END_REF].

In the experimental procedure proposed by Carton et al. [START_REF] Carton | Corrosion of Chromium in Glass Melts[END_REF], the influence of parameters such as pre-oxide thickness, immersion temperature or temperature increase rate was not verified. In their works, Schmucker et al. had shown that the formation of the oxide occurs continuously during the immersion [START_REF] Abdullah | Study of the redox and acid-base properties of soda-lime silicate glass : appliacation to the high temperature corrosion of the Ni-based alloys and ceramic materials[END_REF]. As a consequence, a slow temperature increase rate involves a longer time immersion, and thus a significant increase of the oxide thickness during immersion.

In the present work the depassivation of Ni-30Cr alloy in a ternary Na2O-B2O3-SiO2 was studied. The selected glass composition was 0.75Na2O-B2O3-2.75SiO2 (0.75NB2.75S) which was first formulated as a model nuclear glass [START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF]. Various values of the temperature increase rate, the pre-oxide thickness and the starting immersion temperature were tested in order to optimize the experimental procedure. Ex situ characterization (Scanning Electron Microscopy SEM and Scanning Transmission Electron Microscopy STEM) coupled with in situ electrochemical measurements were used in order to characterize the oxide layer during the depassivation. These are the first observations of alloys subject to this phenomenon reported in the literature to our knowledge.

Materials and methods

Alloy and glasses synthesis

Ni-30Cr alloy was elaborated by high frequency induction melting in neutral atmosphere (argon O2 < 2 ppm). Ingots of 40 -50 g were melted from pure chromium (Alfa Aesar chromium pieces, 3 -8 mm (0.1 -0.31 in), 99.99%) and pure nickel (Alfa Aesar nickel shot, 3 -25 mm (0.1 -0.98 in), 99.95%). Two melting steps were performed to ensure compositional homogeneity. Then, molten ingots were aspirated into silica pipes (inner diameter: 6 mm, length: 150 mm) using a primary vacuum pump. This device allows a slow quenching of the alloy giving time for an important grain growth. Thus, the rods microstructure presents grains of several hundreds (up to several thousands) micrometers.

Rods were cut and shaped to obtain suitable dimensions (diameter: 5.5 mm, length: 25 mm).

They were then ground with P1200 SiC paper for reproducible surface condition and ultrasonically cleaned in absolute ethanol.

Glass was elaborated by the CERFAV (Centre Européen de Recherche et de Formation aux Arts Verriers). SiO2, Na2CO3 and H3BO3 powders were mixed and heated at high temperature to obtain homogenous liquids. The low sodium concentration limits the volatilization of that element from the melt and ensures the stability of the composition during long time exposure at high temperature [START_REF] Cable | Kinetics of Volatilization of Sodium Borate Melts[END_REF]. Composition was controlled by EPMA, results are available in ref [START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF]. Acid-base properties of the silica-based liquids are expressed in the well-known concept of optical basicity [START_REF] Duffy | A review of optical basicity and its applications to oxidic systems[END_REF]. With a value of 0.52 this glass is more acidic than common nuclear glasses. Viscosity of the melt was calculated using the Factsage software [START_REF] Bale | FactSage thermochemical software and databases -recent developments[END_REF]. A value close to the one corresponding to the R7T7 nuclear glass was found (117 dPa.s for 0.75NB2.75S and 157 dPa.s for R7T7 at 1150°C [START_REF] Abdullah | Study of the redox and acid-base properties of soda-lime silicate glass : appliacation to the high temperature corrosion of the Ni-based alloys and ceramic materials[END_REF]).

Corrosion measurements

Corrosion potential (i.e open circuit potential) of alloys was measured during immersion. A classical three electrodes device was adapted to high temperature molten silicates. This device was described elsewhere [START_REF] Martino | Corrosion of metals and alloys in molten glasses. Part 1: glass electrochemical properties and pure metal (Fe, Co, Ni, Cr) behaviours[END_REF][START_REF] Martino | Corrosion of metals and alloys in molten glasses. Part 2: nickel and cobalt high chromium superalloys behaviour and protection[END_REF]. Working electrode was made of Ni-30Cr, counter electrode was a platinum plate and reference electrode was an yttria stabilized zircona (YSZ) stick. A platinum wire (diameter: 0,5 mm) was welded to rod to ensure electrical conductivity across mullite tube; in the case of reference electrode, platinum lacquer was used as junction between ceramic and platinum wire. Electrodes were sealed in mullite tubes with alumina cement (Resbond 989FS).

Immersion procedure

Immersion tests of Ni-30Cr alloys were performed at high temperature. The following procedure was used in order to study depassivation phenomenon. 600 g of pieces of the studied glass were placed in a Pt-10%Rh crucible (inner diameter: 115 mm, depth: 80 mm) and molten in a furnace in air; temperature was increased to 1150 °C in 2 hours, maintained at 1150 °C for 10 min and decreased to room temperature in 2 hours. Then Ni-30Cr electrode was placed in the furnace above the molten silicate bath and heated at 942 °C for 64 hours. It was thus pre-oxidized in air in order to obtain an initial chromia layer with an expected thickness of 2 µm. The working, counter and reference electrodes were immersed at the end of the pre-oxidation time. Corrosion potential of the working electrode was measured during immersion; temperature was increased at a constant rate of 50°C/h until the measured potential fell down to active state (< -1000 mV/Ref above 1000 °C) [START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF]. Temperature was measured in the liquid with an S-type thermocouple (Pt/Pt-10Rh) during the experiment to be sure of the thermal equilibrium in the liquid. Ni-30Cr working electrode was then quenched and embedded in resin. All the immersion procedure is summarized in figure 1.

Three parameters can be adjusted in this experiment: pre-oxidation duration (and thus the initial oxide thickness), temperature increase rate and initial immersion temperature. Two pre-oxidation durations of 64 h and 16 h were tested at 942 °C. Due to the well-known parabolic growth of chromia layer on alloys at high temperature [START_REF] Schmucker | Oxidation of Ni-Cr alloy at intermediate oxygen pressures. I. Diffusion mechanisms through the oxide layer[END_REF][START_REF] Wagner | Beitrag zur Theorie des Anlaufvorgangs[END_REF][START_REF] Ecer | Oxidation of high-chromium Ni-Cr alloys[END_REF][START_REF] Ramanarayanan | Transport through chromia films[END_REF], the thickness of the pre-oxidized layer is two times lower after 16 h than after 64 h of oxidation. Temperature increase rates from 10 °C/h up to 150 °C/h and immersion temperature from 890 °C to 1018 °C were also tested.

Electronic microscopy characterization 2.4.1. Metallographic preparation

After quenching, rods were embedded in resin and cut in order to realize cross section characterizations of glass/oxide/alloy interface. Samples were polished using SiC paper (grid P120 to P1200), then with diamond gel (9 µm to 1 µm) and finally OPS (colloidal suspension of amorphous silica 40 nm). Embedded samples were ultrasonically cleaned in absolute ethanol.

SEM and STEM analysis

Scanning electron microscopy was performed with a JEOL 6010LA at an acceleration voltage of 15 kV and a current beam of 6 nA in Back Scattered Electron (BSE) mode to obtain a compositional contrast. The apparatus is equipped with an Energy Dispersion Spectrometer (EDS) to get information about the elemental composition of the sample. Gold metallization was carried out with aim to avoid polarization of the non-conducting surface.

Gold was chosen due to its high electronic conductivity providing good micrograph quality.

The thicknesses of the oxide layers were measured specifically on non-deteriorated areas of the sample (no cracks or unsticking of the glass/oxide interface) to ensure the reliability of the results. The average oxide thickness of a sample was calculated from 10 to 15 measurements, and the uncertainty is evaluated as the standard deviation.

In order to perform scanning transmission electron microscopy analysis, a thin slice (10 × 2 × 0.2 µm) of the glass/alloy interface was extracted from in the analyzed sample using a scanning electron microscope coupled with a focused ion beam (FIB) (Helios Nanolab 600i FEI). STEM was performed with a JEOL ARM 200F TEM in STEM mode using the HAADF (dark field) detector for imaging with an acceleration tension of 200 kV and EDS detector for elemental analysis. The electron beam focus on the sample in STEM mode allows probing an area of 1 nm in diameter and 200 nm in depth (across the thin slice).

Results and discussions

Immersion and characterization

Features of the corrosion potential

Electrochemical measurements were performed in the reference silica-based liquid. The procedure described in the previous section was applied with a pre-oxidation duration of 64 h at 942 °C, an immersion temperature of 942 °C and a temperature increase rate of 50 °C/h.

SEM and STEM characterization of the sample

Since the corrosion potential of the alloy versus the temperature varies in a specific way, other measurements of the corrosion potential of the Ni-30Cr alloy in the 0.75NB2.75S liquid were performed. Samples were quenched at specific moments according to the shape of the potential curve, and characterized by SEM.

Potential measurements of those samples are presented in figure 3 Oxide needles can be observed on these samples. In the second part corresponding to the micrograph (d), the oxide thickness increases and the oxide layer becomes thicker than the initial pre-oxidized sample. The last part corresponds to micrographs (e) and (f). A clear decrease of the oxide thickness can be noticed, this one leading to a complete loss of the oxide layer. The standard deviation of thickness measurements decreases with the immersion duration in figure 3, which can be a sign of the formation and dissolution of the oxide layer: as both phenomena are diffusion controlled, the heterogeneities of the layer are smoothed during the long time immersions. These variations of the oxide thickness are interpreted as follows:

• During the first moments of immersion, the formation of the oxide is limited by its own thickness [START_REF] Wagner | Beitrag zur Theorie des Anlaufvorgangs[END_REF] while the absence of chromium in the liquid generates a high concentration gradient of chromium. Schmucker et al. assumed that the dissolution is limited by the diffusion of chromium in the liquid [START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF]. In this case the high chromium gradient involves a fast dissolution rate overcoming the formation rate. This leads to a loss of the oxide thickness (Figure 4b and4c). Thus, the first part of the immersion corresponds to a light dissolution of the oxide layer.

• The dissolution rate decreases with immersion time because of the flattening of the concentration profile of chromium in the liquid with time [START_REF] Crank | The mathematics of diffusion[END_REF]. A regime of formation slowed down by the dissolution takes place and yields a light increase of the oxide thickness (Figure 4d). Indeed, in previous works, when the temperature is lower than the depassivation temperature, a significant increase of the thickness of the oxide layer is observed. For example [START_REF] Abdullah | Corrosion of Pure Cr and Ni-30Cr Alloy by Soda-Lime-Silicate Melts: Study of Simplified Systems[END_REF], the thickness of the chromia layer of a preoxidized Ni-30Cr sample immersed in a soda-lime silicate at 1100°C increases from 4 µm to 10 µm in 24 h. Consequently, we can expect an increase of the thickness of the chromia layer in the low temperature range. Furthermore, at higher temperature the growth rate of the protective layer becomes important in comparison to its dissolution due to a parabolic growth constant of the oxide increasing faster [START_REF] Schmucker | Oxidation of Ni-Cr alloy at intermediate oxygen pressures. I. Diffusion mechanisms through the oxide layer[END_REF] than the dissolution rate [START_REF] Khedim | Solubility of chromium oxide in binary soda-silicate melts[END_REF][START_REF] Bale | FactSage thermochemical software and databases -recent developments[END_REF] with the temperature.

• The oxide thickness decreases on the sample (e) which was quenched just before the final potential falls down to the active state. This means that the formation/dissolution competition is overcome by dissolution. The protective layer is no longer present on sample (f), which concords with the very low potential of this sample.

Two hypotheses can be proposed to explain the last point: either the dissolution of the chromia layer, or the formation rate become respectively greater and lower than expected from the extrapolation of lower temperature values. For the first hypothesis, data on solubility and diffusion of chromium in molten silicates, so as silica-based liquids viscosity are pretty well known [START_REF] Khedim | Redox-Control Solubility of Chromium Oxide in Soda-Silicate Melts[END_REF][START_REF] Khedim | Solubility of chromium oxide in binary soda-silicate melts[END_REF][START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF][START_REF] Abdullah | Dissolution equilibrium of chromium oxide in a soda lime silicate melt exposed to oxidizing and reducing atmospheres[END_REF][START_REF] Fluegel | Glass viscosity calculation based on a global statistical modelling approach[END_REF]. In each case, an Arrhenius type law describes the evolution of these parameters with temperature. No deviation from these behaviors was reported.

Regarding the second hypothesis, a lack of the oxygen (O ) supply towards the growing oxide layer could decrease the formation rate of the oxide by hampering its oxidation, and thus lead to a complete loss of the passivating layer. This explanation has already been proposed to explain the low corrosion resistance of chromia-forming alloys in molten silicates in argon atmosphere [START_REF] Allemand | Etude de la corrosion de materiaux metalliques et ceramiques par le verre de confinement des dechets nucleaires fondu[END_REF]. Activity measurements of the oxygen (O ) in silica-based liquids have been reported in the literature [START_REF] Rüssel | Interaction between oxygen activity of Fe2O3 doped soda-lime-silica glass melts and physically dissolved oxygen[END_REF], but the link between such measurements and the concentration of dissolved oxygen has not been established yet. The origin of depassivation cannot be clearly defined due to the lack of data on the dissolved oxygen.

As expected, the selective oxidation of chromium leads to a pronounced concentration gradient over several tens of microns. The depassivated sample (f) was also analyzed by STEM. Composition of the alloy was checked close to the glass/alloy interface using an EDS detector coupled with STEM. The analyzed area and the concentration profiles are presented in figure 5 and6 respectively. Each concentration profile appears flat due to the high magnification. Silicon was detected at about 2 at.% in the alloys with an homogeneous distribution over the 600 nm of analysis. The presence of this element near the interface has already been observed by Abdullah et al. [START_REF] Abdullah | Corrosion of Pure Cr and Ni-30Cr Alloy by Soda-Lime-Silicate Melts: Study of Simplified Systems[END_REF] and can be attributed to the diffusion of metallic silicon produced by reduction of silica from the liquid. The formal potentials of different elements were studied by Petitjean et al. [START_REF] Petitjean | Electrochemical Behavior of Glass Melts: Application to Corrosion Processes[END_REF]. The potential of Si VI /Si 0 couple is around -1150 mV/(YSZ reference electrode) in silica-based liquids above 1000 °C.

The corrosion potential of a depassivated Ni-30Cr alloy is lower than this value (< -1200 mV/ref) involving that the reduction of silica is thermodynamically allowed. Moreover, the formal potential of Cr II /Cr 0 in these media and in this range of temperature is around -1300 mV/ref. So, silica is a strong enough oxidant to be reduced by chromium on the surface of the alloy according to the following reaction.

2Cr ( ) + Si ( ) ⇒ 2Cr ( ) + Si ( ) (1)
Thus, this reaction leads to Cr II in the liquid and Si 0 in the Ni-Cr CFC solid solution.

Usually, the main oxidant compound in the melt is the oxygen (O ) which is a strong enough oxidant to oxidize Cr 0 in Cr2O3. The reaction of chromium and silica involves a low oxygen activity at the liquid/alloy interface which is in agreement with the lack of oxygen supply stated in the previous section.

The diffusion profiles of chromium and nickel (figure 6) show a concentration gradient of nickel with a negative value from the bulk to the interface. This involves a diffusion of the nickel towards the liquid/oxide interface and so a consumption of this element by the corrosion process of the depassivated alloy. Nickel was detected in the glass by EDS analysis so as chromium which confirms the consumption of this two element during the active corrosion process. Nickel in the glass and silicon in the alloy were not detected in the passivated samples.

Parameters influencing the depassivation

Temperature increase rate

Four experiments were performed with the following temperature increases rates: 10, 50, 75, 150 °C/h and unchanged pre-oxidation conditions (942 °C during 64 h). Potential curves are shown in figure 7. The curves exhibit the same trend of the potential evolution in these conditions. At a temperature above 1150 °C, two behaviors appear. The corrosion potential of the sample heated at 10 °C/h falls down to -800 mV/ref, which corresponds to the passive state range of Ni-30Cr [START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF]. SEM micrograph confirms this result as shown in figure 8: a homogeneous oxide thickness (11.8 ± 1.7 µm) was formed probably due to the long immersion duration before the critical temperature of depassivation. The potential peaks at the end of this curve were identified as being due to the penetration of the molten silicate into the YSZ reference electrode which alters the measured potential. For 50, 75 and 150 °C/h the corrosion potential decreases faster to a value below -1200 mV/ref which is typical of the active state [START_REF] Schmucker | Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy[END_REF]. A same depassivation temperature of 1240 ± 10 °C was measured in these three cases. SEM analysis confirms the absence of the oxide layer on this sample. These faster temperature increase rates correspond to relatively shorter immersion time (3-7 hours) compared to 10 °C/h (40 hours of immersion), so the oxide growth is limited, leading to the same depassivation temperature. Moreover, the shape of the potential curve is similar for 50, 75 and 150 °C/h indicating the depassivation is more related to temperature and oxide thickness than to immersion duration. The reason of this better behavior during the slow temperature increase would probably be related to the better relaxation of the expansion constrain at the alloy/oxide interface.

Pre-oxide thickness

Pre-oxidation duration was 16 and 64 h at a temperature of 942 °C. Pre-oxide thicknesses were measured by SEM analysis. The oxide thicknesses were 1.3 ± 0.2 µm and 2.4 ± 0.7 µm (section 3.1.2.) for 16 h and 64 h respectively. Immersions were performed for these two pre-oxidation conditions with the following parameters: temperature increase rate of 50 °C/h and immersion temperature of 942 °C. Potential curves for these two conditions are presented in figure 9. Depassivation temperature is lower for the 16 h pre-oxidized sample than for the 64 h (1200 °C and 1235 °C respectively). This is coherent with the conclusion of the previous section: depassivation phenomenon is strongly linked to oxide thickness and its evolution during the immersion. It is also possible to state that the depassivation is initiated in a temperature range lightly below 1200°C.

Immersion temperature

In the section 3.1.2, evidences that the depassivation occurs above a specific temperature were found. In order to confirm that assumption, different immersion temperatures were tested. As previously, the other conditions were fixed in order to isolate the influence of the immersion temperature. Pre-oxidation temperature and duration were 942 °C and 64 h respectively, temperature increase rate was 50 °C/h. At the end of the pre-oxidation, temperature was adjusted to reach 890 °C or 1018 °C. This step was quickly achieved (less than 10 min) and the oxide thickness was assumed to not be affected during this duration.

Results of the corrosion potential measurements for these three immersion temperatures are shown in figure 10. The depassivation appears in a restricted temperature range (1250 ± 20 °C). Moreover, these values are close to those obtained with different temperature increase rates (i.e. 1240 ± 10 °C for 50, 75, 150 °C/h). No relation was found between immersion temperature and depassivation temperature implying that the depassivation does not occur between 890 °C and 1018 °C in this silica-based liquid.

Conclusion

In this paper, the corrosion of a chromia forming alloy was studied by considering the influence of temperature, pre-oxide thickness. An experimental procedure, based on the tests of the previous studies, was proposed. Corrosion potential measurements of the immersed alloy with a fast increase of the temperature allowed studying the depassivation phenomenon over a wide range of temperature. By coupling electrochemical measurements to postimmersion SEM and STEM characterizations, a mechanism of the depassivation of the chromia forming alloy was proposed. At low temperature the formation is predominant on dissolution leading to an increase of the oxide thickness, whereas at high temperature the dissolution becomes predominant leading to a fast decrease of the oxide thickness until its complete dissolution and an active corrosion of the alloy. Then, silica becomes the main oxidant species which was proved by the presence of silicon in the alloy detected by EDS analysis. Thanks to SEM characterization, it has been shown that depassivation starts before the final potential fall down. Different experimental parameters were varied. From these tests, it had been shown that depassivation is strongly linked to the oxide thickness and its evolution during the immersion. The origins of the depassivation are still debated but this phenomenon is likely due to a decrease of the formation rate of the oxide in the liquid at high temperature.

However, the duration required to complete the depassivation might be affected the viscosity of the liquid and the solubility of chromium. As these parameters are linked to the Na2O and B2O3 contents, the glass composition might also have an influence on the depassivation phenomenon.

Figure 2

 2 Figure 2 shows the evolution of the corrosion potential of the alloy versus the temperature. The curve is monotonously decreasing and can be divided in three parts. A first smooth part (I) extending from the immersion to a temperature around 1150 °C; a second rough part (II) from 1150 °C to 1230 °C and a third part (III) above 1230 °C. The first two parts can be differentiated by a change of aspect but the values of the potential between 0 and -1000 mV/ref indicate that the alloy is in the passive state in these two first stages [13]. The temperature of 1230°C is characterized by a clear potential drop which leads to an active potential implying a loss of the protective oxide layer.

  ; the potential curves exhibit the same decreasing trend. The quenched samples are represented by black dots, the corresponding oxide thicknesses are represented by the red squares. The SEM micrographs are presented in figure 4. The micrograph (a) corresponds to the pre-oxidized sample immersed for one minute. The thickness of the pre-oxide is 2.4 ± 0.7 µm, oxide needles can be noticed on the sample which is characteristic of dissolution/precipitation of the oxide. As for the corrosion potential curve, the evolution of the thickness can be divided in three parts. A first decreasing part characterized by micrographs (a), (b) and (c) with a covering oxide layer inhomogeneous in thickness which corresponds to the part I of the potential curve.

Fig 1 .

 1 Fig 1. Pre-oxidation and immersion procedure of Ni-30Cr (working electrode).

Fig 2 .Fig 3 .

 23 Fig 2. Corrosion potential of Ni-30Cr vs temperature in 0.75NB2.75S. Pre-oxidation: 64h at 942°C; immersion temperature: 942°C; temperature increase rate: 50°C/h. Fig 3. Corrosion potential of pre-oxidized Ni-30Cr (64 h at 942 °C) vs temperature in 0.75NB2.75S (50 °C/h) for various quenching (black dots) and corresponding oxide thickness (red squares).

Fig 4 .

 4 Fig 4. SEM micrographs of Ni-30Cr alloys corresponding to quenched samples in figure 3.

Fig 5 .

 5 Fig 5. STEM micrograph in dark field mode of the glass/alloy interface on a depassivated Ni-30: sample (f).

Fig 6 .

 6 Fig 6. Concentration profiles of nickel, chromium and silicon in the alloy vs the distance from the glass/alloy interface on the depassivated sample (f).

Fig 7 .

 7 Fig 7. Corrosion potential of pre-oxidized Ni-30Cr vs the temperature in 0.75NB2.75S for various temperature increases rates. Pre-oxidation: 64h at 942°C.

Fig 8 .Fig 9 .Fig 10 .

 8910 Fig 8. SEM micrograph of Ni-30Cr pre-oxidized at 942°C for 64h and immersed in 0.75NB2.75S at 10°C/h. Quenching temperature: 1300 °C.Fig 9. Corrosion potential of pre-oxidized Ni-30Cr vs the temperature in 0.75NB2.75S for two pre-oxidation conditions: 16 h and 64 h at 942°C, temperature increase rate: 50°C/h. Fig 10. Corrosion potential of pre-oxidized Ni-30Cr (64 h at 942 °C) vs the temperature in 0.75NB2.75S for various immersion temperatures.

Acknowledgements

The authors wish to thank the « Centre de Compétences en Microscopies, Microsondes et Métallographie » of the Jean Lamour institute for providing the technical support for the FIB/STEM analysis presented in this work.


Mechanisms underlying gravitropic and autotropic recovery in poplar

Introduction

Lodging is a major problem for herbaceous crops and young woody plants, causing significant yield losses. With climate change, the risk of severe storms during the growing season will increase. The risk of lodging will then be even greater. Resilience is one of the strategies evolved by plants to cope with this problem. Resilience is the ability of plants to recover their structures and functions back to their reference states after disturbance. Resilience to lodging involves active movements called tropisms, i.e. a redirection of organs in the direction of or opposite to the physical stimuli. Our study focuses on gravitropism and autotropism. A fine-tuned combination of these two tropisms is required for the postural control of plants. This relies on two perceptive abilities: i) graviperception (more exactly gravi-cline perception) for the perception of orientation with respect to gravity and ii) proprioception for the perception by the plant of its own curvature. Under the control of these perceptions, the plants activate motors that carry out movements. These motors can be differential elongation in primary growing zone and tension wood formation in secondary growing zone. To understand resilience to lodging, we therefore study in this thesis the graviperceptive and proprioceptive sensitivities (with a greater focus on the latter, which is less well known), and the motors of the different movements related to these tropisms. In particular, we test the relevance of a universal model of postural control called the AC model (Moulia et al. 2021).

Conclusion (up to now)

• Graviperceptive and proprioceptive sensitivities can be measured • Tension wood : a movement motor linked to gravitropism and autotropism (control by proprioception)

Materials and Methods

An original Experimental device

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  Isotropic lighting = No phototropism • Clinostat (Rotation) = Suspends graviperception  Separation of graviperception and proprioception • Model-assisted phenotyping : AC model (posture control explained by the perception of the inclination with respect to gravity (graviperception) and the perception of curvatures (proprioception))