M.D Hamadi Ghezal

M.D Sophie Bouvet

M.D Sarah Kabani

M.D Jacques Ripart

M.D Philippe Cuvillon
email: philippe.cuvillon@chu-nimes.fr

Philippe Pr

Cuvillon

Ropivacaine versus placebo on postoperative analgesia and chronic pain following third molar extraction: a prospective randomized controlled study

Keywords: ropivacaine, local anesthetic, molar extraction, outcome

Purpose

The present study aimed at assessing the efficiency of ropivacaine on post-operative pain for extraction of third molars.

Methods

In a single centre, prospective, parallel, double blind randomised trial, patients scheduled for removal of all four third molars, ASA I-III patients < 65 year-old patients were included.

After intubation under general anesthesia (using intravenous remifentanil and propofol), for each of the third molars, 2mL of ropivacaine (7.5mg/ml) or placebo (0.9% saline solution) was injected into the vestibular capsule (total: 8 mL) before extraction. At the end of surgery, similar analgesia was injected for both groups (intravenous paracetamol 1g and ketoprofene 100 mg).

The primary outcome was postoperative pain assessed by Visual Analog Scale (VAS).

Postoperative consumption of analgesics (morphine titration in post-operative care unit when VAS > 3/10, followed by oral tramadol 50 mg after discharge), patient satisfaction, chronic pain (1-3 month), time in PACU and total hospitalization time were also recorded.

Results

50 patients were analysed in each group with similar characteristics (ropivacaine vs control, for age (years) 18 [17 -21] vs 18 [17 -21], for sex (female) 33 (66%) vs 25 (50%), and BMI (Kg/m2): 20 [19-23] vs 21 [19 -23]). Area Under the Curve for VAS pain (0 to 4h) was lower for Ropivacaine group: 0.43 [0.19-0.66] vs 0.63 [0.43-0.87], p=0.005. Use of morphine in PACU (8 vs 18, p =0.02) and median length of stay in ambulatory setting (5 vs 6 h, p=0.03) were reduced in Ropivacaine vs Placebo group. At days 1 and 4, VAS of pain was higher in Ropivacaine group (respectively 4 vs 2, p=0.006 and 3 vs 2, p=0.05). At month 1 and 3, pain and 4 DN4 score were similar between groups, with a median VAS pain score at 0 for both groups (p=0.42). No difference was observed for patient satisfaction and adverse events.

Conclusions: Ropivacaine provides an immediate efficient pain relief after extraction of third molars without benefit after discharge.

5

Introduction

Removal of the third molars is a frequent performed surgery with significant short-term pain; yet as it is performed as day surgery, patients have limited recourse to analgesics post-operatively.

There are also reports of chronic pain affecting 2.2% patients following surgery. [START_REF] Berge | Incidence of chronic neuropathic pain subsequent to surgical removal of impacted third molars[END_REF] Anaesthesia can take the form of local, regional (truncal infiltration of the dental nerve, with or without sedation) or combined with general anaesthetic. The prolonged action of local anaesthetic into the postoperative period confers an additional advantage, with the various treatments possessing differing durations and quality of pain relief. There is currently no "gold standard" of anaesthetic for such procedures, although lidocaine appears to be most commonly used in the USA and UK. [START_REF] Moore | Dental therapeutic practice patterns in the U.S. I. Anesthesia and sedation[END_REF][START_REF] Devine | Current practice in mandibular third molar surgery. A national survey of British Association of Oral Surgeons membership[END_REF] However, this is a short-acting product, thus alternatives with better properties would be welcome.

Ropivacaine, a long-acting local anaesthetic has been recently considered as a potential replacement (vs lidocaine). Previous studies on ropivacaine have compared it against other nerve blocks (lidocaine, mepivacaine, and articaine), although no studies have yet compared ropivacaïne against a placebo control. [START_REF] Budharapu | Ropivacaine: a new local anaesthetic agent in maxillofacial surgery[END_REF][START_REF] Crincoli | The Effectiveness of Ropivacaine and Mepivacaine in the Postoperative Pain after Third Lower Molar Surgery[END_REF][START_REF] Krzemiński | Comparison of ropivacaine and articaine with epinephrine for infiltration anaesthesia in dentistry -a randomized study[END_REF] Previous studies have identified a dose of 0.75% as the best adapted. [START_REF] Ernberg | Ropivacaine for dental anesthesia: a dose-finding study[END_REF]8 However, no study evaluated benefit after hospital discharge for ambulatory day case surgery and incidence on chronic pain.

The purpose of this study was to compare ropivacaine against placebo as local analgesia on postoperative short-and long-term pain following extraction of all four third molars. Our hypothesis was that ropivacaine will be superior to placebo for reducing post-operative pain.

6

Methods

Study design

We designed a one single centre, 1:1 two-parallel arms, prospective, placebo-controlled doubleblind trial comparing ropivacaine (7.5mg/mL) versus placebo (0.9% saline solution) regional anaesthesia. Patients were followed for 3 months.

According to the French law, the official Institutional Review Board approved the study (Comité Protection Personne, Sud Méditerranée III, 2012, Hôpital Universitaire de Nîmes, France, Chairman: Pr Thierry Lavabre Bertrand and registered at EudraCT: 2011-004972-13). Prior to first patient enrollment, the study was registered at ClinicalTrials.gov (NCT01541059; 29 feb. 2012, principal investigator: Pr Jacques Ripart, see author). [START_REF] Toulouse | French legal approach to clinical research[END_REF] A written informed consent was obtained for every participant before inclusion, this study followed the guidelines of Helsinki declaration for human investigation.

.

Participants

Subjects requiring bilateral surgery of inferior and superior third molars only under general anaesthesia were recruited from the University Hospital of Nîmes. Were included 16 to <65 yo patients with an ASA physical status grade I-III and after having given an informed consent (guardians also signed for patient < 18 year old). Were not included: refusal, pregnant and parturient patients, patients using ineffective contraception, or suffered from addiction or chronic pain treated with opioid, patients with a contra-indication for regional anaesthesia or the local or general anaesthetics and analgesics used in the protocol, or if a hypersensitivity to ropivacaine or 7 other local anaesthetics with amide links was reported. Patients with predicted difficulties in cooperation or psychiatric disorders that could interfere with assessments were also excluded.

Management of general anaesthesia

All patients underwent oral premedication with hydroxyzine (1mg/kg) one hour before the operation. Intravenous antibiotic prophylaxis was used prior to induction. Standard general anaesthesia was used: briefly, anaesthesia was induced using propofol (2.5-3 mg/kg) and remifentanil via target-controlled infusion (TCI). Depth of anaesthesia was monitored, with objective between 40-60 according to bispectral index (BIS). The use of intubation was left at the discretion of the anaesthetist. Arterial pressure and heart rate were maintained at ±20% of baseline just before induction using ephedrine. For postoperative analgesia, paracetamol (1g four times daily/five days) and ketoprofene (100mg/12 hours for three days) were systematically given to patients prior to awakening. In post-anesthesia care unit (PACU), after approximately 30 minutes post-awakening, patients declaring pain to be >3/10 were offered iv morphine administered in line with standard procedures (3mg plus 2-3mg/5 minutes until VAS pain score was < 3/10). Patients were discharged from the PACU according to normal criteria; modified Aldrete score >12. Additional 'rescue' analgesia was available for patients during hospitalization and upon return home: tramadol (50mg / 4 hours orally as needed). Then, patients were given mouthwash to be used three times daily for 10 days.

Intervention

The same operator (experience: >15 yrs) performed extraction in all patients. For each of the third molars, 2 mL of ropivacaïne (7.5mg/mL) or placebo (0.9% saline solution) was injected into the vestibular capsule (total of 8 mL). The surgeon and operating room nurse who prepared the 8 product were aware of the nature of the injection. The anaesthetist, anaesthetic nurse and the outcomes assessors were blinded to patient group. The syringe was prepared in a separate room to ensure blindness of other staff. Surgical notes and anaesthetic notes were completed independently for respecting the double blinded design.

Primary outcome

The primary outcome of this study was patient-reported pain by visual analogue scale (VAS) (0-10) at 0 (time of awakening), 1.5, 3, 4, 6, 8 and 24 hours after awakening, evaluated by the anaesthetic team (blinded to patient allocation). For the evaluation 24 hours after awakening, a call was made to the patient's home by a clinical study technician.

Secondary outcomes

Secondary outcomes measures were: the consumption of propofol and remifentanil peroperatively; post-operative pain at day 1 (D1), D4, Month 1 (M1) and M3; consumption of morphine and tramadol post-operatively; time until first request for tramadol; consumption of analgesics in hospital; time spent in PACU; hospitalization duration; consumption of analgesics at home until D3; total analgesic consumption; patient satisfaction at D4; chronic post-operative pain as assessed by the DN4 questionnaire (10 pain measures: Yes/No; cut-off for positive pain = 4) (10) and the secondary effects of medications and regional anaesthetic (pruritus, nausea, vomiting, bleeding of the mouth, loss of feeling, respiratory difficulties, drowsiness, headache, sweating, palpitations, cutaneous rash, swallowing difficulties and dysarthria).

Data collection 9

Before surgery, patient information was collected (age, weight, height, sex), physical status was scored according to the ASA score, and ease of intubation was scored in consultation by Mallampati score. Patient's VAS was recorded at anaesthesia consultation and the morning of intervention. In addition, each patient had to complete the DNA survey composed of four questions and 10 items which generates a score between 0 and 10. The surgeon was asked to anticipate the difficulty of the operation according to the Pederson Difficulty Index (0-10 scale) pre-operatively and then to rate the difficulty encountered on the same scale post-operatively.

During surgery, modified Cormack-Lehane score, times of induction, infiltration, incision-end of surgery, end of anaesthesia, consumption of propofol and remifentanil and use of anti-emetics were collected. Bleeding in per-and post-operation was ranked as minimal, average or high (subjective evaluation performed by surgeon), and complications encountered during surgery were recorded. After surgery, patient's VAS was collected at awakening (0), 1.5, 4 and 24 hours after awakening and then at day 4 (D4), month 1 (M1) and month 3 (M3) post-surgery. The consumption of morphine in PACU and tramadol after PACU, the first time of tramadol request, the use of anti-emetics and the secondary effects of medications and regional anaesthetic were collected during hospitalization. At day 1 and day 4, patients were contacted to request pain evaluation by VAS, use of antalgic and secondary effects and satisfaction. Finally, at M1 and M3 patients were called to collect information on pain by VAS and DNA score.

Sample size and randomization

Sample size was determined based on the results of a prospective pilot study from our team. Pain was evaluated by VAS until 5.5 hours post-surgery, we found that patients treated with ropivacaine (n=18) had an average area under the curve (AUC) of 0.28±0.31 compared to 0.52 ± 35 in placebo patients (n=11). In order to detect a superiority of ropivacaine with an alpha risk of 10 0.05 and a power of 0.9, we required 33 patients in each group. To account for potential 20% missing data for the primary outcome and approximation of estimations, a total of 50 patients per group were required. The randomization list was computer generated using SAS Software version 9.4 (SAS Institute Inc. Cary, USA) by the methodologist of the Biostatistics Department of Nimes University Hospital appointed to the study and not involved in-group allocation. The allocation was made using an online randomization application designed for clinical research projects. Randomization was performed in random blocks of 4 or 6 without stratification with a 1:1 allocation ratio. Randomization was performed following recruitment of eligible participants by the surgeon who was the only person aware of which group to assign the patient.

Data analysis

All analyses were conducted following the intent-to-treat (ITT) principle. Studied parameters were expressed as means ± standard deviations (SD), or medians with interquartile, or in frequencies and percentages when appropriate. Analysis of unbalanced percentages when appropriate. The group differences were not subjected to statistical tests due to the randomization principle (CONSORT guidelines). [START_REF] Bouhassira | Development and validation of the Neuropathic Pain Symptom Inventory[END_REF] Comparisons for pain (for primary or secondary endpoints) were one-sided and a p-value of <0.05 was required to exclude the null hypothesis. Other comparisons were two-sided with the same p-value threshold. For the primary endpoint, area under curve (AUC) was estimated using the trapezoidal method. AUC were estimated using VAS evaluation at 0, 1.5, 3 and 4 hours after awakening. Evaluation of pain at 24 hours was compared by a Wilcoxon test. For both these elements of the primary endpoint, potential covariates (identified when comparing groups) were tested in univariate analysis and a multivariate general linear regression model with appropriate distribution was realized. For secondary endpoints, 11 comparisons between groups were performed using unpaired Student's t-tests, Wilcoxon tests and chi-square tests ± Fischer exact tests when appropriate. To estimate the time until tramadol request, a survival analysis was performed and the comparison was made with a log-rank test.

Statistical analysis was conducted using SAS (release 9.4; SAS institute, Cary, USA).

12

Results

From June 2012 to May 2015, 103 patients were recruited to the study, with a follow-up of three months (Fig1). 100 patients were finally included in the study and allocated to the two groups (Figure 1). Baseline patient characteristics and surgical duration are shown in table 1. There were more women in ropivacaïne group.

Primary end point

The median AUC values of the VAS from H0 to H+4 were 0.63 [0.43 -0.87] and 0.43 [0.19 -66] (p<0.01) in the Placebo and Ropivacaine groups, respectively. As there were more women in Ropivacaine group, sex was studied as a variable in univariate analysis with a lower AUC in men (median 0.45 [0.22 -0.57]) than in women (median 0.660 [0.37 -0.96]) (p=0.0078). After having performed a general linear regression model with gamma distribution using sex and randomization arm as covariables, sex (men)(p<0.01) and randomisation arm (Ropivacaine)(p=0.03) were associated with a lower AUC of VAS. At H24 and D4, VAS pain score was higher in the Ropivacaine group than Placebo group (Table2).

By month 1 and month 3, few patients in either group reported pain (median 0), and the intergroup differences were not significant (p=0.42 and p=0.29 respectively).

Secondary end points

More patients in the Placebo group used morphine between H0-H1.5 than the Ropivacaine group (36.7% vs. 16.3%, p=0.002) or tramadol between H0-H4 (6.7 % vs 15.2 %, p< 0.01) and tramadol until H24 (Table 2). Multivariate analysis was performed using tramadol usage and 13 treatment arm as variables. Tramadol (p=0.03) and ropivacaine (p<0.01) were associated with a lower AUC of VAS. Satisfaction, length of time in PACU and in hospital, the number of patients with complication did not differ between groups (Table 2).

14

Discussion

The results of this study demonstrate that infiltration of ropivacaine for the four third molar removal improved early postoperative pain and time discharge from hospital, but we did not find a significant difference after day 1.

Third molar removal is the most common surgical procedure and many patients need time off work and their quality of life is significantly affected for several days due to severe pain, swelling and trismus. [START_REF] Bailey | Ibuprofen and/or paracetamol (acetaminophen) for pain relief after surgical removal of lower wisdom teeth, a Cochrane systematic review[END_REF][START_REF] Korn | Comparison of rofecoxib and oxycodone plus acetaminophen in the treatment of acute pain: a randomized, double-blind, placebo-controlled study in patients with moderate to severe postoperative pain in the third molar extraction model[END_REF][START_REF] Gammaitoni | Randomized, double-blind, placebo-controlled comparison of the analgesic efficacy of 20 oxycodone 10 mg/acetaminophen 325 mg versus controlled-release oxycodone 20 mg in postsurgical pain[END_REF][START_REF] Brković | Efficacy and safety of 1% ropivacaine for postoperative analgesia after lower third molar surgery: a prospective, randomized, double-blinded clinical study[END_REF] Various types of medication and local anaesthetics have been used to control postoperative pain following surgical third molar removal. Previous studies have suggested that maximal pain is felt during the first 3 to 5 hours postoperatively, which is in line with our results (Table 2) (11). This time (3-5h) exceeded duration of short acting LA (lidocaine, mepivacaine), reducing the interest for these agents. In contrast, we demonstrated an efficient potent of long acting local anaesthetic on pain relief compared to placebo. Because early discharge and pain relief are considered as quality makers in ambulatory unit, the use of ropivacaine (or similar long acting LA) in this surgery should be considered.

Few studies have shown the effectiveness of ropivacaine in dental anaesthesia. El-Sharrawy et al. found that 0.5% and 0.75% concentrations were effective for intra oral nerve blockade, with a rapid onset and prolonged duration of pain control. 8 Similar findings were reported by Brkovic et al with high dose of ropivacaïne (1%), by Budharapu et al, by Krzeminski et al, and by Ernberg et al with various concentrations (0.375 to 0.75%). [START_REF] Budharapu | Ropivacaine: a new local anaesthetic agent in maxillofacial surgery[END_REF][START_REF] Krzemiński | Comparison of ropivacaine and articaine with epinephrine for infiltration anaesthesia in dentistry -a randomized study[END_REF][START_REF] Ernberg | Ropivacaine for dental anesthesia: a dose-finding study[END_REF][START_REF] Brković | Efficacy and safety of 1% ropivacaine for postoperative analgesia after lower third molar surgery: a prospective, randomized, double-blinded clinical study[END_REF][START_REF] Krzemiński | Comparison of ropivacaine and lidocaine with epinephrinee for infiltration anesthesia in dentistry. A randomized study[END_REF] However, all of these studies were limited to onset and duration of LA without any data for early (D1, D4) or delayed (1-3 months) evaluations. The present study is the first one to demonstrate that ropivacaine is, as expected, effective on pain relief in the first 4 postoperative hours (on VAS pain score, on opioid rescue, on time discharge); but also highlighted that secondary outcomes recorded in the study after day 15 case surgery, were similar vs placebo. Also, no benefit of 0.75% ropivacaine on pain relief at D1 and D4 or on chronic pain at D30 and D90 was reported.

Analgesics including paracetamol or acetaminophen and/or non-steroidal anti-inflammatory drugs (NSAIDs) have been widely used to control pain after this surgery. [START_REF] Bailey | Ibuprofen and/or paracetamol (acetaminophen) for pain relief after surgical removal of lower wisdom teeth, a Cochrane systematic review[END_REF] Therefore, the present study used paracetamol and NSAIDs to reduce postoperative pain. But, our results demonstrated that many patients needed additional tramadol after PACU and that ropivacaine group had higher pain score at 24h. Similar results were observed when single nerve blocks were performed in orthopaedic surgery: rebound effect. If ropivacaïne becomes adopted as the local anaesthestic of choice, the anesthetist would need to ensure that the patient was adequately informed prior to discharge (block duration) that they should keep to the pain medication schedule regardless of whether they had yet experienced pain or not. High doses of oral opiod are not commonly used for analgesia in dentistry probably as they are not suitable for ambulatory surgery because of high incidence of drug-related adverse effects, especially nausea and vomiting. [START_REF] Korn | Comparison of rofecoxib and oxycodone plus acetaminophen in the treatment of acute pain: a randomized, double-blind, placebo-controlled study in patients with moderate to severe postoperative pain in the third molar extraction model[END_REF][START_REF] Gammaitoni | Randomized, double-blind, placebo-controlled comparison of the analgesic efficacy of 20 oxycodone 10 mg/acetaminophen 325 mg versus controlled-release oxycodone 20 mg in postsurgical pain[END_REF] In the present study, ropivacaïne led to lower dose of opioid in PACU allowing a more rapid hospital discharge (one hour less). We did not assess the impact of this shorter hospital duration on cost.

In this study, we performed infiltration before surgery, but we did not reported intra operative opioid or hypnotic sparing. Remifentanil and propofol consumption were similar in both groups (Table 2).

The rate of adverse events did not differ between ropivacaine and placebo, although we believe that this LA is an optimal choice for analgesia because it has extended duration of analgesia and negligible toxicity. Effect of sex, surgical experience and level of anxiety were reported for this surgery in previous study. [START_REF] Renton | Factors predictive of difficulty of mandibular third molar surgery[END_REF][START_REF] Conrad | Patients'perception of recovery after third molar surgery[END_REF][START_REF] Reebye | Comparing the efficiencies of third molar surgeries with and without a dentist anesthesiologist[END_REF][START_REF] Sisk | Complications following removal of impacted third molars: the role of the experience of the surgeon[END_REF][20] Similarly, our results found sex effect on postoperative pain and tramadol rescue.

16

Limitations of this study were the lack of data collection beyond four hours after surgery, preventing us from more precisely evaluating the time line of benefit from ropivacaine. Moreover, we choose high dose of ropivacaine, lower concentrations may produce similar results.

Bias arising from surgeon experience was avoided by using the same surgeon for all extractions. [START_REF] Reebye | Comparing the efficiencies of third molar surgeries with and without a dentist anesthesiologist[END_REF] In conclusion, our results support the theory that ropivacaine represents an effective LA for third molar extraction without major adverse events, with the quicker discharge affording a potential economical benefit to the institute.

  22

  Figure legend

20: Fillingim RB, King CD, Ribeiro-Dasilva MC, Rahim-Williams B, Riley JL. Sex, gender, and pain: a review of recent clinical and experimental findings. J Pain 2009; 10: 447-485.

Acknowlegements:

Financial Disclosures: no grants was received for this study that was supported solely by our institution.

This work was supported by the Department of Anesthesiology at the Caremeau University Hospital (Nîmes, France).

Allocation

Analysis*

Follow-Up

Included (n=100)


Régine Mosser-Ruck

Jérôme Sterpenich

N Michau

Marie-Camille Jodin-Caumon

A Randi

M Abdelmoula

Odile Barres

Michel Cathelineau

M.-C Jodin-Caumon

Serpentinization and H2 production during an iron-clay interaction experiment at 90C under low CO2 pressure

Keywords: serpentinization, iron-clay interaction, hydrothermal experiment, corrosion, H2, geochemical modelling

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

Introduction

In the concept of high-level radioactive waste repositories, the claystone of the geological barrier could come into contact with alloyed or non-alloyed steel elements (liners of the disposal cells, waste containers and overpacks). Steel will be corroded in the presence of interstitial water coming from the geological formation and this reaction will produce ironrich fluids and molecular hydrogen. The enhanced availability of Fe(II) resulting from this corrosion under reducing conditions will favor the transformation of some minerals in the claystone. Even if CO2 (produced during oxidation of the claystone in the course of drilling of galleries and disposal cells) and H2 (produced during corrosion of the steel in contact with the claystone) have already been identified in previous studies, their relationship with the mineralogical transformations of the claystone has been rarely investigated in recent research.

A detailed "state of the art" concerning i) experimental iron-clay/claystone interactions, ii) effect of carbonates on iron corrosion and iii) serpentinization of silicates is thus detailed hereafter.

Experimental iron-smectite interactions

and the corrosion rate of iron. Newly formed minerals of the serpentine group, associated with the formation of magnetite, siderite and the dissolution of quartz, illite and interstratified illite/smectite are reported in experimental studies of iron corrosion (powder or foil) ± glass in contact with the COx claystone under an oxygen-free atmosphere and in the presence of pure water at 90°C during 6 to 18 months [START_REF] De Combarieu | France perturbation on mineralogy and pore water composition in a clay-rock: an experimental and modelling study[END_REF], 2011). In the case of rods of low-allow carbon steel corroded at 90°C under anoxic condition in contact with the COx formation saturated with a synthetic clay porewater, the nature of the corrosion products can both depend on the duration of the experiment and the damage area at the C-steel/clay interface. Schlegel et al. 2019, reported the formation of Fe(hydr)oxide and (Fe,Si) (hydr)oxide phases, ankérite, akaganeite, later replaced by (Fe,Si,O) corrosion products containing Fe sulfide and chukanovite. Another original experimental procedure by step cooling from 90°C to 40°C [START_REF] Pignatelli | A multi-technique characterisation of cronstedtite synthetised by iron-clay interaction in a stepby-step cooling procedure[END_REF](Pignatelli et al. , 2014b) ) confirms without doubt, the formation of cronstedtite at 90°C, with an increase of crystallinity occurring from 90°C to 60°C, and the crystallization of chukanovite at the surface of iron plates (Pignatelli et al. 2014a). Specific surface area of metallic iron on the reaction pathway and its impact on corrosion rate and on Fe-enrichment of the clay particles, as well as the effect of a thermal gradient (150°C-300°C or 80°C-150°C) were also studied [START_REF] Bourdelle | Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway[END_REF][START_REF] Jodin-Caumon | Iron-Clay interactions under various H2 pressures. Clays in Natural and Engineered Barriers for Radioactive Waste Confinement[END_REF]. These studies demonstrate that the initial clay minerals, mostly illite and I/S, are replaced by chlorite, Fe-serpentine or mixed-layer smectite-(chlorite or serpentine) depending on the temperature.

Effect of bicarbonate/carbonate ions on iron corrosion processes

The knowledge of the p(CO2) in claystones is a key parameter to understand the thermodynamic equilibria governing the carbonate system. [START_REF] Gaucher | CO2 partial pressure in clayrocks: a general model[END_REF], measuring the p(CO2) in four geological sites, demonstrated that it ranges between 3 mbar and 13 mbar and that it is governed by an assemblage of calcite, dolomite and Mg-clay minerals. These values are higher than the atmospheric p(CO2) (around 0.35 mbar) and favor the formation of iron carbonates as corrosion products when the claystone is in contact with steel elements in radioactive waste repositories. The wide variation in the nature of alteration products identified in numerous experiments is due to various experimental set-up defined to simulate the geochemical conditions in the storage (steel + groundwater ± clays or claystone ± CO2 overpressure). For investigations carried out on iron corrosion and ferrous archaeological artefacts in the presence of carbonate-containing aqueous solutions and under anoxic conditions [START_REF] Blengino | Physico-chemical characterization of corrosion layers formed on iron in a sodium carbonate-bicarbonate containing environment[END_REF][START_REF] Savoye | Experimental investigations of iron corrosion products formed in bicarbonate/carbonatecontaining solutions at 90°C[END_REF][START_REF] Musić | The effect of bicarbonate/carbonate ions on the formation of iron rust[END_REF][START_REF] Kohn | Longevity of granular iron in groundwater treatment processes: corrosion product development[END_REF][START_REF] Refass | Localised corrosion of carbon steel in NaHCO3/NaCl electrolytes: role of Fe(II)-containing compounds[END_REF][START_REF] Saheb | Long-term behavior of lowcarbon steel in anoxic environment: Characterization of archaeological artefacts[END_REF][START_REF] Saheb | Multisecular corrosion behavior of low carbon steel in anoxic soils: Characterization of corrosion system on archaeological artefacts[END_REF][START_REF] Saheb | Iron corrosion in an anoxic soil: Comparison between thermodynamic modelling and ferrous archaeological artefacts characterized along with the local in situ geochemical conditions[END_REF][START_REF] Saheb | Raman study of a deuterated iron hydroxycarbonate to assess long-term corrosion mechanisms in anoxic soils[END_REF][START_REF] Schlegel | Metal corrosion and argillite transformation at the water-saturated, high-temperature iron-clay interface: A microscopic-scale study[END_REF][START_REF] Schlegel | Anodic activation of iron in clay media under water-saturated conditions at 90°C: Characterisation of the corrosion interface[END_REF][START_REF] Nishimura | Corrosion behaviour of carbon steel for overpack in groundwater containing bicarbonate ions[END_REF][START_REF] Lee | Iron hydroxyl carbonate formation in zerovalent iron permeable reactive barriers: Characterization and evaluation of phase stability[END_REF][START_REF] Ruhl | Identification, quantification and localization of secondary minerals in mixed Fe 0 fixed bed reactors[END_REF][START_REF] Rémazeilles | Fe(II) hydroxycarbonate Fe2(OH)2CO3 (chukanovite) as iron corrosion product: synthesis and study by Fourier transform infrared spectroscopy[END_REF], the formation of siderite and/or Fe II (OH -)x(HCO3 -/CO3 -)y species, sometimes identified as chukanovite is reported. However, the conditions of formation of these minerals are mostly described at 25°C, pH of 6.3-7 [START_REF] Saheb | Long-term behavior of lowcarbon steel in anoxic environment: Characterization of archaeological artefacts[END_REF][START_REF] Ruhl | Identification, quantification and localization of secondary minerals in mixed Fe 0 fixed bed reactors[END_REF]) or 8.3-9.5 (Blengino et al. 1995;[START_REF] Refass | Localised corrosion of carbon steel in NaHCO3/NaCl electrolytes: role of Fe(II)-containing compounds[END_REF][START_REF] Nishimura | Corrosion behaviour of carbon steel for overpack in groundwater containing bicarbonate ions[END_REF] and up to 11 [START_REF] Azoulay | Determination of standard Gibbs free energy of formation of chukanovite and Pourbaix diagrams of iron in carbonated media[END_REF]. The proportion of the newly-formed iron hydroxide-carbonate is controlled either by the [CO3 2- ]/[OH -] or [Fe 2+ ]/[OH -] ratios and its stability field would be reduced by a low carbonate activity [START_REF] Azoulay | Determination of standard Gibbs free energy of formation of chukanovite and Pourbaix diagrams of iron in carbonated media[END_REF]). In the context of a nuclear waste repository, the corrosion of iron and/or low alloyed steel at the contact of water-saturated bricks of clay was also investigated under anaerobic conditions and in a solution representative of the pore COx water with a pH fixed to 7.3 and a HCO3 -concentration of 1.6 mmol/kgw [START_REF] Martin | Corrosion of iron and low alloyed steel within a water saturated brick of clay under anaerobic deep geological disposal conditions: An integrated experiment[END_REF][START_REF] Schlegel | Metal corrosion and argillite transformation at the water-saturated, high-temperature iron-clay interface: A microscopic-scale study[END_REF][START_REF] Schlegel | Anodic activation of iron in clay media under water-saturated conditions at 90°C: Characterisation of the corrosion interface[END_REF]. From iron to claystone, various corrosion layers are observed: an internal thin layer of magnetite, an intermediate Fe2CO3(OH)2 (chukanovite) layer, an outer layer of Fe-rich 7Å-phyllosilicates, and a transformed matrix layer containing siderite (FeCO3). Microcrystalline chukanovite often mixed with other iron-rich minerals, is identified in other studies reproducing the corrosion of steel containers [START_REF] Saheb | Long-term behavior of lowcarbon steel in anoxic environment: Characterization of archaeological artefacts[END_REF][START_REF] Saheb | Iron corrosion in an anoxic soil: Comparison between thermodynamic modelling and ferrous archaeological artefacts characterized along with the local in situ geochemical conditions[END_REF][START_REF] Nishimura | Corrosion behaviour of carbon steel for overpack in groundwater containing bicarbonate ions[END_REF]Pignatelli et al. 2014a). In the experiments of Pignatelli et al. (2014b), the p(CO2) is not measured but considered at equilibrium with carbonates of the claystone, i.e. around 0.038 ± 0.02 bar according to an experimental test [START_REF] Bourdelle | Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway[END_REF] which is a value quite close to that of experiments at 80°C described by Gailhanou et al. (2017).

Serpentinization of silicates

Two thermal fields (Lost City and Rainbow) were especially well studied [START_REF] Charlou | Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36_14'N, MAR)[END_REF][START_REF] Douville | The Rainbow vent fluids (36_14'N, MAR): the influence of ultramafic rocks and phase separation on trace metal contents on Mid-Atlantic Ridge hydrothermal fluids[END_REF], Früh-Green at al. 2003[START_REF] Kelley | A serpentinite-hosted ecosystem: the Lost City hydrothermal field[END_REF][START_REF] Seyfried | Redox evolution and mass transfer during serpentinization: an experimental and theoretical study at 200°C, 500 bar with implications for ultramafic-hosted hydrothermal systems at mid-ocean ridges[END_REF]Foustoukos et al. 2008) and demonstrated that hydrothermal alteration of ultramafic rocks at mid-ocean ridges involves H2 production because of the reduction of water together with the oxidation of ferrous iron in olivine and/or pyroxene. Magnetite and magnesium hydroxides are the by-products of this reaction. The hydrothermal source of H2 attracted much attention in recent years and both experimental studies and thermodynamic modelling were developed to improve our understanding of the influence of different parameters (such as temperature, pressure, water/rock ratio, availability of silica, partitioning and oxidation state of iron) on this mineralogical transformation [START_REF] Allen | Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: an experimental study at 400°C, 500 bars[END_REF]Seyfried 2003, Seyfried et al. 2007;[START_REF] Maccollom | Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks[END_REF][START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15_N on the Mid-Atlantic Ridge[END_REF][START_REF] Marcaillou | Mineralogical evidence for H2 degassing during serpentinization at 300°C/300 bar[END_REF][START_REF] Andreani | µXANES study of iron state in serpentine during oceanic serpentinization[END_REF].

Serpentinization mechanisms are also evoked in the case of CM chondrites to explain the formation of cronstedtite [START_REF] Pignatelli | Multiple precursors of secondary mineralogical assemblages in CM chondrites[END_REF]2017). However, unlike the reactions taking place in hydrothermal fields, the serpentinization of olivines or pyroxenes in chondrites requires a large contribution of ferrous iron and water. This extraterrestrial environment is after all closer to that of the experiment described below.

The study presented here measures the H2 production and the effect of a low p(CO2) on the formation of newly-formed iron-rich minerals (silicates, carbonates), during the corrosion of iron under experimental conditions close to that of a nuclear waste repository and using a low solution/COx ratio. The experiment was carried out at 90°C in the presence of pure water and NaHCO3 to produce a p(CO2) about 20 mbar. This value of p(CO2) is chosen from the geochemical calculations with the PHREEQC code by [START_REF] Bourdelle | Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway[END_REF] in their study of iron-clay interactions under hydrothermal conditions. This value is also close to the maximal p(CO2) modelled by [START_REF] Gaucher | A robust model for pore-water chemistry of clayrock[END_REF] for the Bure pore water (15.8 mbar). The total pressure was monitored throughout the experiment. The minerals and the composition of the gas phase were analysed to identify the reaction processes involved in the conditions of steel corrosion of nuclear waste repositories. Geochemical modelling was performed to decipher the reaction path followed by the system during the experiment.

Materials and experimental conditions

Geological reactant

The claystone comes from the Callovo-Oxfordian formation (COx) in the underground research laboratory of Andra (Bure, France, borehole EST 26457 FOR1118) located in the east of the Paris basin. The sample was taken in a borehole in the maximum claystone zone.

According to different authors [START_REF] Rousset | Etude de la fraction argileuse de séquences sédimentaires de la Meuse et du Gard[END_REF][START_REF] Gaucher | ANDRA underground research laboratory: interpretation of the mineralogical and geochemical data acquired in the Callovo-Oxfordian formation by investigative drilling[END_REF][START_REF] Rivard | Contribution à l'étude de la stabilité des minéraux constitutifs de l'argilite du Callovo-Oxfordien en présence de fer à 90[END_REF]), the claystone is composed of 41 wt% to 44 wt% of clay minerals (illite and mixed-layered illite/smectite as predominant clay minerals, and minor amounts of kaolinite and chlorite), 21 wt% to 25 wt% of quartz, 27 wt% to 31 wt% of carbonates (calcite and minor dolomite), with the remaining proportions made up of pyrite, phosphates, K-feldspar and organic matter.

Claystone was ground to 40 µm and stored in a glove box under Ar. Iron was used as a powder (< 50 µm, Riedel-de-Haën, purity> 99.5%) and as plates (Sigma-Aldrich, thickness 0.25 mm, > 99.99%). The reactants were mixed in pure Milli-Q water (18.2 MΩ.cm).

NaHCO3 powder (Sigma-Aldrich, > 99.7%) was added to buffer the initial p(CO2) (cf. below).

Experiment

The starting mixture was made up of 55.5 g of ground claystone and 55.5 g of iron powder.

Three iron plates (∼ 200 mg) were also placed at the bottom of the stainless steel reactor. The liquid/rock and the iron(powder)/claystone ratios were equal to unity. Claystone and iron reacted at 90°C during 107 days in a polytetrafluoroethylene (PFTE) foil placed to protect the reactor from corrosion. The total volume of the reactor was 170 cm 3 . The pressure was monitored by a calibrated pressure transducer (± 0.25 bar). The device was heated using a MICA heater band. The temperature was controlled by a thermocouple (± 1°C) set in a stainless steel thimble dipping into the autoclave. The temperature and pressure data were recorded every three minutes throughout the experiment. Ten milligrams of NaHCO3 were added to the mixture to obtain a p(CO2). Indeed, at T > 70°C, the decomposition of NaHCO3 yields CO2 in agreement with the following reaction (1): 2 NaHCO3 (powder) Na2CO3(solid) + H2O(g) + CO2(g)

(1)

The mass of NaHCO3 (powder) added in the experimental medium allowed to reach a pressure of ∼ 20 mbar at the beginning of the experiment. This value was estimated from (i) the free gas volume in the reactor after filling it with claystone, iron and water (around 86 cm 3 ), (ii) the fraction of CO2 dissolved in water at 90°C (estimated at around 10% of CO2(g) produced).

Analytical techniques and geochemical modelling

3.1. Solid analysis 9/48 FTIR and Mossbauer spectroscopies, XRD, TEM and SEM analytical techniques are described in [START_REF] Mosser-Ruck | Effects of temperature, pH, and iron/clay and liquid/clay ratios on experimental conversion of dioctahedral smectite to berthierine, chlorite, vermiculite, or saponite[END_REF], 2016).

Gas analysis

The gases were collected in a cell equipped with a sapphire window and analysed by Raman spectrometry. Raman spectra of the gaseous phase were recorded using a RXN1 Raman spectrometer (Kaiser Optical Systems, Inc.) equipped with a 532 nm Nd:YAG laser at 80 mW, a multiplex holographic grating, and a CCD detector cooled to -40°C by the Peltier effect. The spectral resolution was of ~ 3 cm -1 at 1400 cm -1 . The Raman spectra were acquired using 20x objective (Olympus) to focalize inside the cell and connected to the spectrometer through an optical head and an optical fiber.

Geochemical modelling

Thermodynamic equilibria and reaction paths were calculated with the PHREEQC geochemical code (Parkhurst and Appelo 1999) using the ThermoChimie_PHREEQC_SIT_v9b0 database of Andra [START_REF] Giffaut | Andra thermodynamic database for performance assessment: ThermoChimie[END_REF]. The data base using the SIT theory (Specific ion Interaction Theory) for the calculation of aqueous species activities was chosen because of the high ionic strength expected in our system (I≈1.6 mol/kgw). Carbonates under reducing conditions in the presence of H2 can transform to CH4 but this reaction is kinetically unfavorable and furthermore CH4 remained undetected by Raman spectroscopy in our experiment. In consequence, thermodynamic data concerning CH4

were removed of the database before calculations of the modelling in order to avoid the formation of CH4. The first step of the modelling was to mix COx with water at 90°C to determine the mineral phases expected to crystallize (saturation index SI ≥ 0). Among these phases, those observed in the experiment and others not observed but relevant in the context were allowed to crystallize and thus added to the input file. Table 1 shows the input file for modelling, which includes the following data: i) minerals initially present in the COx sample (according to the mineralogical determination of Rivard 2011) and considered as reactive; ii) secondary minerals allowed to precipitate, including those likely to crystallize at 90°C (Fe(s), cronstedtite-Th, greenalite, siderite, gibbsite, magnetite, nontronite-Ca, saponite-FeCa, pyrrhotite). No zeolites were added to the list of secondary minerals because when tested in the modelling, only phillipsite-K crystallized in the first step of the modelling and led to the dissolution of all the initial clays of the COx. This result was not consistent with the observations on the experimental products; iii) the water chemistry. The aqueous solution contained 2.14 mmol/kgw of Na + and HCO3 -to simulate the addition of NaHCO3 in the experiment. The number of moles for each mineral was recalculated for 1 kg of COx, and iron (as Fe 0 ) was added step by step to 18 moles (∼ 1 kg of iron), to mimic a reaction pathway. The amount of iron added at each step was small at the beginning of the modelling (0.01 mole to 0.1 mole) when mineral transformations were more sensitive to iron content, and then increased progressively until the total quantity of iron in the experiment was attained (18 moles). Because of its high reactivity, iron added at each step is completely dissolved before proceeding to the next iteration (additional iron step) in the modelling. pH and pe (negative log of the conventional activity of the electron) were free to vary. The temperature was fixed at 90°C. Solid-solutions and ion exchange were not considered by the simulation program.

Mineralogical composition of the

Results

Pressure monitoring

Before heating, a vacuum was created in the reactor to remove air and especially oxygen. The pumping step was almost instantaneous (several tens of seconds) to obtain a vacuum of 0.01 bar avoiding massive water evaporation. Notice that at room temperature the evaporation rate of water is low and the estimated amount of removed water is negligible (about 10 -4 mole for a saturated vapour pressure of 0.032 bar at 25°C). After ten minutes, the temperature reached 88°C and the pressure was equal to 0.73 bar, which is roughly the saturated vapour pressure of pure water. In half a day, the pressure reached 1.93 bar. This result implies that the experimental mixture quickly reacted. A pressure of 6 bar was reached within three days, rising to about 50 bar at the end of experiment. This increase in pressure was due to the production of gas by chemical reactions between the different components in the autoclave.

The Raman spectrum showed only H2 bands while CO2 remained below the limit of detection (~0.5 bar).

Mineralogical evolution

Figures 1a and1b present the diffractograms of the initial starting mixture and the reaction product. The comparison of the two diffractograms in Fig. 1a shows that carbonate minerals (calcite and dolomite) were apparently not very reactive. During the experiment, neither magnetite nor iron-rich TOT or TOTO clays (e.g. minnesotaite, griffithite, bowlingite, nontronite and chamosite) crystallized in sufficient quantity to be detected by X-ray diffraction. On the other hand, dissolution of iron, quartz and clays (mainly I/S and illite) were observed (Fig. 1a and Fig. 1b). 7Å-clay phases and chlorite were still present but always 12/48 in small proportions. However, the modifications of the mineralogical proportions have to be considered with caution due to possible bias during sampling operations (heterogeneous material). The diffractogram of the fine fraction of the reaction product (clay fraction < 2 µm)

shows peaks with a weak intensity (multiplication factor x5000), indicating a low crystallinity of the clays. Iron is considered as Fe(II). A well-marked evolution was observed: i) silicon depletion coupled with a decrease of the interlayer charge (Fig. 2a), ii) a decrease of the interlayer charge correlated with the release of potassium (Fig. 2a and Fig. 2c), iii) a substantial enrichment in iron explained by a replacement of aluminum and magnesium in the octahedral sites (Fig. 2b). Iron enrichment and silicon depletion argue in favor of a transformation of I/S (± illite) minerals of the starting COx into chlorite or 7Å iron-bearing serpentines (Fig. 2d).

The clay particles sampled in the bulk and from the iron plates in the reaction product cover a wide range of chemical compositions. Figure 3 shows that their mineral composition changes from interstratified clays (I/S) and/or illite to greenalite. These new argillaceous phases were interpreted as a mixture of dioctahedral and trioctahedral clays and/or complex mixed-layer clays, more or less enriched in iron and generally depleted in aluminum and potassium. The iron enrichment in the newly-formed clay particles probably depends on the distance from the iron source: it was higher in the clay particles collected close to the iron plates or close to iron powder.

4.

In fact, two main morphologies were observed both in the bulk and near the iron plate. The first morphology "cornflakes"-like, is typical of little transformed clays and weakly enriched in iron (points 1 and 2 of Fig. 3). It was visible on TEM images a) and b) of Fig. 4. The second morphology appeared as a "hairy" assemblage of small tabular particles (TEM images b) and c) of the Fig. 4), already observed by other authors [START_REF] Bourdelle | Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway[END_REF]). These particles are then very enriched in iron (points 3 and 4 of Fig. 3). phases. This was explained by a wide variety of contacts between the COx and the iron plate, the claystone being itself very heterogeneous. Fig. 5a shows a superposition of at least two different mineral layers on the iron plate. A first relatively homogeneous layer of smooth and cracked appearance (Fig. 5b) is composed of silicates (Fig. 5c) characterized by an iron enrichment (Fig. 5d). Some of the clay particles on the iron plate are well crystallized, with a polygonal platy morphology (see particles in the white squares of Fig. 5c). minerals are observed side by side (Fig. 6a). These last minerals are mainly composed of Fe and S in proportions close to those of pyrrhotite (Fig. 6b). Meanwhile, pyrites most often occur in the form of highly corroded framboids. The transformation of pyrite into pyrrhotite was already demonstrated at hydrogen pressures exceeding 30 bar [START_REF] Jodin-Caumon | Iron-Clay interactions under various H2 pressures. Clays in Natural and Engineered Barriers for Radioactive Waste Confinement[END_REF][START_REF] Truche | Sulphide mineral reactions in clay-rich rock induced by high hydrogen pressure Application to disturbed or natural settings up to 250°C and 30 bar[END_REF]. After ultrasonic cleaning to eliminate clay particles, the iron powder also shows corrosion traces but no characteristic octahedral crystals of magnetite are observed either in the bulk or on the iron plates of the reaction product. Other minerals resulting from the corrosion of iron (iron oxy-hydroxides, ferrihydrite or other iron silicates) were not detected by the techniques used in this study maybe because of very low abundances.

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The characterization of the mineralogical assemblages of the reaction product by Mössbauer spectroscopy was recorded in the ±11 mm/s velocity range because this was ideally suited to distinguish iron in the clay minerals (paramagnetic doublets) from metallic iron or iron oxides [START_REF] Rivard | Reactivity of Callovo-Oxfordian claystone and its clay fraction with metallic iron: Role of non-clay minerals on the mechanism of interaction[END_REF]. The fitting of the spectra was reasonably described in terms of a magnetic contribution corresponding to metallic α-Fe in addition to a superimposition of elemental subspectra (paramagnetic components): two doublets assigned to Fe 3+ and Fe 2+ clays (53% and 7.5%, respectively) and a doublet characterized by a centre shift and quadrupole splitting (1.08 mm/s and 1.98 mm/s) which correspond to Fe 2+ in siderite (2.5%) (Figure 7a). The high percentage of Fe 3+ in the clays is surprising, and was explained by the presence of original inherited clays (illite or chlorite) of the claystone which would have not reacted and did not transform into Fe 2+-rich clays. The influence of atmospheric water and carbon dioxide was always subtracted. Few differences between the spectra obtained from the starting COx and the reaction product are observed (Fig. 7b). In the reaction product, FTIR spectroscopy showed that quartz and carbonates (calcite) are still present as well as clays. But clays are less hydrated than those of the starting claystone. A magnification between 1000 cm - 1 and 600 cm -1 (Fig. 7c) highlights that most of the infrared bands in the 600-950 cm -1 region, characteristic of OH bending modes of clays are hidden in the background noise. The new band at 630 cm -1 could be attributed to the bending vibration of OH of the hydroxide sheet in the serpentine-like minerals and the small shoulder at 737 cm -1 , only visible on the spectrum of the reaction product (Fig. 7c), could correspond to the stretching band of siderite (Farmer 1974). material and of the reaction product (collected on the iron plates). Analyses were made using 1 mg of powder sample with 150 mg of pure potassium bromide (KBr) and were scanned for 22/48 about 2 minutes with a 2 cm -1 spectral resolution. c) Magnification between 1000 cm -1 and 600 cm -1 .

Geochemical modelling

Variations in pH and pe (Fig. 8a) indicate that the reaction medium becomes alkaline (up to pH ∼11.6) and reduced (pe around -12.9). The modelling predicts that 54% (by mass) of water is consumed by both iron corrosion and mineralogical transformations (Fig. 8b). The modelling also highlights several points (Fig. 9a and Fig. 9b): i) siderite, cronstedtite and nontronite-Ca do not crystallize, ii) at the first steps of iron addition (<0.4 mole), calcite, quartz, kaolinite, saponite-FeCa, illite and pyrrhotite crystallize whereas ripidolite_Cca-2, dolomite, illite/smectiteISCz-1 and pyrite rapidly dissolve, iii) next, quartz dissolves and greenalite crystallizes, iv) up to 5.5 moles of added iron, quartz continues to dissolve and greenalite to crystallize. From this step, quartz is nearly totally dissolved and kaolinite and saponite begin to dissolve supplying enough Si and Al to form a little ripidolite, gibbsite and always greenalite. Illite is still stable, v) illite starts to dissolve after 9 moles of iron were added. When illite is totally dissolved, formation of greenalite and gibbsite stops and magnetite crystallizes by adding more iron, vi) the final mineralogical assemblage modelled consists of greenalite, calcite, gibbsite, magnetite, pyrrhotite, and ripidolite. The pressure calculated at the end of the modelling, related to H2 produced during the dissolution of the metal iron added step by step (up to 17.9 moles) is around 390 bar. It is much higher than that measured at the end of the experiment stopped at 107 days (around 50 bar). This difference can be explained by the fact that after 107 days, thermodynamic equilibrium is not reached in the experiment and also because PHREEQC works with a constant volume of gas, which could not be the case in the experiment because of mineral transformations leading to variations of the total volume of solid phases. However calculations of the volume occupied by minerals and water at the various stages of modelling show that the volume occupied by these phases varies little (around 8% maximum).

Discussion

Iron corrosion

It is known that the generation of H2 is commonly due to the anoxic corrosion of metallic iron in presence of water, assuming two possible reactions ( 2) and (3):

2H2O + Fe 0 Fe 2+ + 2OH -+ H2(g) (2) 3H2O + Fe 0 Fe 3+ + 3OH -+ 3/2 H2(g) (3)
From these equations, and using the ideal gas law, it can be predicted that only 9 to 14% of iron and 4.5 to 9% of water would be consumed during the experiment to produce a partial pressure of around 50 bar of H2 measured after 107 days and assuming that the free volume for gases in the reactor is that at the beginning of the experiment. However, at the end of the experiment, after opening the reactor in the glove box, the reaction product appeared as a solid granular mass of greenish colour and apparently dry (no liquid water was observed).

This result displays that the mineral transformations previously described had taken place together with the iron oxidation (equations 2 and 3) and that they involve a partial dissolution of some silicates and an additional water consumption. Assuming no change of the free volume for gases, a relative uncertainty of 0.25 bar for gas pressure and a specific surface area of the iron powder of 0.07 m 2 /g (± 0.03), the initial rate of corrosion during the first ten days of heating at 90°C is about 0.97 ± 0.66 mmol/m 2 /day. This calculated initial rate of iron alteration in the presence of COx is lower than the values established by [START_REF] Bourdelle | Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway[END_REF]2017) i.e. 3.66 to 9.16 mmol/m 2 /day and 4.5 to 11.25 mmol/m 2 /day, respectively in experiments with liquid/rock ratios of 10 or 100 and iron/rock ratios of 0.1 or 1. As the same iron powder was used by these authors (same specific surface area), it appears that the initial corrosion rate is very dependent on many factors as the liquid/rock mass ratio (with a strong impact on ionic strength and pH in the experimental aqueous solution) and the iron/rock mass ratio. The corrosion rate calculated by [START_REF] De Combarieu | France perturbation on mineralogy and pore water composition in a clay-rock: an experimental and modelling study[END_REF] in their experiment using an iron/rock ratio equal to 1 but iron with a higher specific surface area (0.26 m 2 /g) and a higher water/rock ratio (10) is 0.52 mmol/m 2 /day. This result compared to our data and those of [START_REF] Bourdelle | Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway[END_REF]2017) confirms that specific surface area of iron also has to be considered in the evaluation of the corrosion rate of iron. Determining a kinetic law of corrosion of iron over long times of experiment is therefore complex because a lot of parameters have to be taken into account but are not always known or measured and consequently force the authors to do some hypotheses. As a kinetic law cannot be determined from the stoichiometry of a single reaction from our experiment and because the evolution of the chemistry of the experimental solution is not known, the kinetics of our reaction can rather be modeled from experimentally measured parameters. By continuously measuring the pressure corresponding to the evolution of hydrogen content linked to the dissolution of iron, it was possible to propose an equation for the rate of iron corrosion over the 107 days of our experiment. Assuming that the volume of the gas phase remains constant over time, the evolution of the concentration of H2 produced during the dissolution of iron and calculated from the ideal gas law, was plotted as a function of time (Fig. 10). If corrosion products develop on the surface of iron grains, the transport of corroding agents toward fresh iron can be slowed down. Diffusion could be then the rate limiting step of iron corrosion as also demonstrated by [START_REF] Wilson | Thermodynamic and fullycoupled reactive transport of a steel-bentonite interface[END_REF]. These authors modelled the control of steel corrosion rate in bentonite by H2 diffusion and showed that the rate could be decreased by two orders of magnitude. In our case, the kinetic law of H2 production should be a function of the square root of time and written as follows: n = k.t 0.5 with n the mole number of H2, t the time (in days) and k the rate constant (in mol.day -0.5 ). Assuming a 0.5 order kinetic law, the rate of iron dissolution (r) can be deduced from the derivative of the equation n = k.t 0.5 using the same method described by [START_REF] Jerz | Pyrite oxidation in moist air[END_REF] for pyrite oxidation: r = dn/dt = k/2.

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This kinetic law of dissolution of iron shows that the rate decreases when the reaction progresses. The corrosion rate of iron would decrease over time because of i) the variations of the proportions of the reagents and reaction products (dissolution and precipitation of minerals and consequently modification of their reactive specific surface area, water consumption) and/or ii) the passivation of iron due to the covering of secondary products limiting the access of water to fresh iron and/or iii) the change of the pH of the solution.

These assumptions are supported by observed mineral transformations, both the dissolution of quartz, iron, interstratified clays and illite as shown by XRD, as well as the significant decrease in the amount of "free" water (i.e. unrelated to the crystallographic structure of mineral phases produced during the experiment). The consumption of hydrogen during pyrite to pyrrhotite conversion and its adsorption by clays [START_REF] Didier | Adsorption of Hydrogen Gas and Redox Processes in Clays[END_REF] were also considered to be negligible in our experiment.

Precipitation of iron carbonates

Under the experimental conditions presented here, i.e. a low p(CO2) of about 20 mbar (0.02 atm) developed by adding a very small initial amount of NaHCO3 (about 1.2 10 -4 mol/kgw), it was a challenge to detect the presence of iron carbonates. Indeed, in other experimental studies at 80°C and 90°C, thin layers of iron carbonates are always observed but the concentration of NaHCO3 in solutions is much higher (1 to 0.1 mol/kgw NaHCO3 at pH 8.4 to 10 in [START_REF] Savoye | Experimental investigations of iron corrosion products formed in bicarbonate/carbonatecontaining solutions at 90°C[END_REF]; 1 mol/kgw NaHCO3 or 0.5 mol/kgw Na2CO3 at pH 9.5 in [START_REF] Blengino | Physico-chemical characterization of corrosion layers formed on iron in a sodium carbonate-bicarbonate containing environment[END_REF]. In the experimental studies of de Combarieu ( 2007) and [START_REF] Schlegel | Metal corrosion and argillite transformation at the water-saturated, high-temperature iron-clay interface: A microscopic-scale study[END_REF], the concentration of CO2 is < 2.94 10 -4 mol/kgw in solution, which induces a p(CO2) of 0.43 atm. This is the same pressure fixed by [START_REF] Beaucaire | A thermodynamic model for the prediction of pore water composition of clayey rock at 25°C and 80°C -Comparison with results from hydrothermal alteration experiments[END_REF] in their thermodynamic model for the prediction of pore water compositions in COx at 80°C. While the initial p(CO2) in our experiment is much lower, it nevertheless corresponds to the value experimentally measured at 90°C, in equilibrium with the COx carbonates and in the presence of iron [START_REF] Bourdelle | A new view on iron-claystone interactions under hydrothermal conditions (90°C) by monitoring in situ pH evolution and H2 generation[END_REF]). As a result, its impact is weak and we can assume that the stability of calcite and, to a lesser extent of the other minor carbonates of the COx, is due to the high pH of the experimental solution (as predicted by modelling).

Mechanisms of the mineralogical transformations

The single corrosion of iron in pure water is not sufficient to explain i) the consumption of water in the experimental system, ii) the granular greenish appearance of the experimental reaction product and iii) the mineralogical transformations highlighted by XRD, TEM and SEM, as well as Mössbauer and FTIR spectroscopies (Figs. 1 to 7). The reaction [START_REF] Blengino | Physico-chemical characterization of corrosion layers formed on iron in a sodium carbonate-bicarbonate containing environment[END_REF] in pure water can be initially invoked because it quickly induces an increase of pH in the experimental solution promoting the dissolution of quartz; this reaction also consumes water, changes the concentration of elements (Si, Fe) and produces H2. But other mineralogical transformations also may have an indirect effect on iron corrosion. The reactions (4), ( 5), ( 6) and ( 7 by-products of these reactions. In reactions (4) and ( 5), Fe 2+ in greenalite requires a prior corrosion of iron metal. But it is noteworthy that the neoformation of this Fe-rich serpentine mineral leads to a consumption of H2O, additional to the moles necessary for the dissolution As no "free" water was found that could be sampled for analysis at the end of the experiment, the hypothesis is that the water was trapped in the structure of the new clay mineral phases described above and/or adsorbed on the minerals. The processes alluded to the evolution of clay minerals towards 7 Å iron-rich clays may involve the dissolution of several layers of the starting clays (mechanism already suggested by Wilson et al. 2006aWilson et al. , 2006b)). No cronstedtite precipitation was observed (confirmed by geochemical modelling), likely because of the excessively low water/rock ratio. Indeed, [START_REF] Pignatelli | Iron-clay interactions: Detailed study of the mineralogical transformation of claystone with emphasis on the formation of iron-rich T-O phyllosilicates in a step-by-step cooling experiment from 90°C to 40°C[END_REF] mentioned that the crystallization of this mineral is due to a dissolution/precipitation mechanism that requires a high solution/rock mass ratio (10 in their COx-iron interaction experiments).

Modelling of the mineralogical evolution

The geochemical modelling shows some convergences and also differences with the results of the experiment. Quartz, illite and interstratified clays totally dissolve during the first steps of the modelling providing the silicon needed for greenalite to crystallize. This results is explained by the rapid increase in pH (> 10) as soon as iron is added step-by-step to the system and by the total dissolution of iron in the geochemical code. In the reaction product, a few illite and quartz still remain in the experiment (Fig. 1). Modelling also shows that magnetite can crystallize at a late stage when initial clays are totally dissolved (illite, kaolinite and I/S) and if iron is still supplied to the solution. Magnetite was not observed in our experiment maybe because there was not enough water in the medium and/or the geochemical equilibrium calculated by the code was not achieved in our experiment. De Combarieu et al.

(2007) showed that magnetite was only observed after 80 days in an experimental study with Fe/COx=1 but with a higher water/rock ratio (10). Under the pe-pH conditions of the modelling (parameters free to evolve), pyrite is totally consumed to yield pyrrhotite. This is not the case in the experiment, pyrite and pyrrhotite co-exist in the reaction product as shown by SEM observations. Siderite does not crystallize in the modelling, likely because of the very pH in aqueous solution and the formation of secondary minerals. The modelling of [START_REF] Bildstein | Gaining insight into corrosion processes from numerical simulations of an integrated ironclaystone experiment[END_REF] was performed using the coupled reactive transport code Crunchflow, which was applied to an experimental iron-claystone (COx) interaction at 90°C for 2 years (Arcorr 2008 experiment described in [START_REF] Schlegel | Corrosion of metal iron in contact with anoxic clay at 90°C: characterization of the corrosion products after two years of interaction[END_REF]. The results of this modelling show some similarities with both our experiment and modelling: i) an initial rapid increase in pH (9.5) at the iron-claystone interface, ii) partial pressure (due to H2 production) up to 40 bar, which is stabilized until the end of the simulation, iii) a preferential precipitation of greenalite at the surface of the iron. However, some differences also exist between the modelling of [START_REF] Bildstein | Gaining insight into corrosion processes from numerical simulations of an integrated ironclaystone experiment[END_REF] and our simulation results. Magnetite and Ca-siderite are predicted as dominant Fe-rich products by Crunchflow. These phases were neither observed in our experiment nor in the first steps of our simulations (moreover, Ca-siderite is not in the database we used). The discrepancies could be interpreted by the fact that, in our experiment and simulation, a sufficient supply of silica (alteration of quartz and primary clays) would favor the crystallization of greenalite at the expense of magnetite. The differences between modelling studies of the literature and our own results could be due to the method we chose to introduce iron into the calculation ( i.e. step-by-step addition of iron). In addition, the stability fields of hematite, magnetite and greenalite in the pe vs. pH phase diagram were calculated at 90°C

and for three values of the activity of SiO2 in water (aSiO2(aq)=10 -5 mol/kgw, 10 -3 mol/kgw and 5.10 -4 mol/kgw). The comparison of the three phases diagrams shows that the stability fields of greenalite and magnetite are present together only when aSiO2(aq) is equal to 5.10 -4 mol/kgw which is a value close to the solubility of quartz at 90°C (Fig. 11). The modelled reaction paths (black points and black dotted line) for iron/claystone interactions under hydrothermal conditions (90°C), simulated by the step-by-step addition of iron, are superimposed on each other and remain within the stability field of greenalite (Fig. 11). The values of pe and pH at the beginning and at the end of the modelling lead to a decimal 34/48 logarithm of oxygen fugacity (logfO2) between -84.7 and -99.1. For these values of logfO2 and assuming the solubility of quartz at 90°C, greenalite is stable as already demonstrated by [START_REF] Pignatelli | Iron-clay interactions: Detailed study of the mineralogical transformation of claystone with emphasis on the formation of iron-rich T-O phyllosilicates in a step-by-step cooling experiment from 90°C to 40°C[END_REF]. The results of the characterization and modelling of the mineralogical evolution indicate that transformation of the clay fraction of the COx claystone plays a significant role in the corrosion of iron, thus supporting the assumptions of [START_REF] Bourdelle | A new view on iron-claystone interactions under hydrothermal conditions (90°C) by monitoring in situ pH evolution and H2 generation[END_REF] regarding the consumption of water placed in the autoclave, and favors the increase of porosity.

Conclusion

In the presence of an initial low p(CO2) (20 mbar), the iron-claystone interaction experiment carried out at 90°C (with 1/1/1 as water/iron/rock ratio) mainly leads to the formation of ironrich 7 Å clays with a low degree of crystallinity at the expense of the starting material made up of illite-smectite mixed-layered clays and quartz. Pyrite is altered and pyrrhotite is formed on the surface of the iron plates. The crystallization of iron hydroxycarbonate (possibly siderite or a chukanovite-like phase) in very small amounts is suspected considering the results of Mössbauer and infrared spectroscopies. Calcite remains unchanged. It is probable that pH rapidly rises above at least 9 at the beginning of the experiment leading to the dissolution of quartz. This increase of pH is due to the corrosion of iron by water. The water consumption observed at the end of the experiment and the production of H2 are explained by both the corrosion of iron and mineralogical reactions such as the transformation of illite and I/S into newly-formed 7Å iron-rich clays (greenalite-like) and the likely formation of hydrated amorphous hydroxides. Magnetite is not observed under our experimental conditions. However, the transformations and crystallinity of the reaction minerals are likely limited by the small quantity of water in the experimental system. Since the experiment presented here was carried out under a low p(CO2), the impact of CO2 on mineral transformations is also difficult to estimate, the gaseous phase of the experimental system being largely dominated by H2. A very low oxygen fugacity is indicated by the presence of sparse iron carbonates and pyrrhotite crystallized on the surface of the iron plates, together with iron-rich clays with a composition lying along a trend between I/S and greenalite. This study confirms that water/rock and water/iron ratios remain very sensitive parameters which must be taken into account to understand the reactivity of the COx in contact with steel containers. The most important result is the demonstration that, during the iron-claystone interaction experiments, water consumption is not only due to iron corrosion but also to mineralogical transformations and, in particular, the formation of Fe-serpentine. This consumption of water will have an impact on the hydration state of the claystone and necessarily on the long term behaviour of this geological barrier in the high-level radioactive wastes repositories.

Fig. 1

 1 Fig.1: a) Diffractograms of the starting mixture of claystone and Fe (black) and of the runproduct (grey). b) Diffractograms of the air-dried oriented clay fractions (<2 μm) of the starting claystone (black) and of the run-product (grey). A multiplication factor (×5000) is applied to the diffractograms of the fine fraction of the reaction product for sake of clarity.

Figure 2

 2 Figure2shows the chemical compositions of clay particles analyzed by TEM-EDXS (Energy-Dispersive X-ray Spectroscopy) in the initial claystone and in the reaction product.

Fig. 2 :

 2 Fig. 2: Chemical compositions of clay minerals analyzed by TEM-EDXS in the COx starting material (white circles) and the reaction product (orange squares for clay particles collected in the bulk and red triangles for those collected on the iron plates). Spectra were collected under nanoprobe mode, using a counting time of 40 s over an area of ~10 nm in diameter. Elemental compositions were calculated assuming the thin film criteria (SMTF program: semiquantitative metallurgical thin film program) and using k-factors calibrated with independently analyzed macroscopic micas, with a maximum error of 5% for each element.

Fig. 3 :

 3 Fig. 3: Chemical compositions of clays in the reaction product (orange squares for clay particles collected in the bulk, and red triangles for clay particles collected more or less close to the iron plate), compared to the compositions of the clay minerals of the starting COx (white circles). Formulae are calculated on the basis of 11 oxygens in the diagram Si-FeT(totalFe)-Al. I/S: interstratified illite/smectite. Reference minerals (half formula) are: Chamosite (green star): (Fe 2+ 3)( Fe 2+ 2Al)(Si3Al)O10(OH)8; Greenalite (green square): (Fe 2+ 3) (Si2)O5(OH)4; Cronstedtite (black triangle): (Fe 2+ , Fe 3+ )3 (Si Fe 3+ )O5(OH)4; Berthierine (blue circle): (Fe 2+ , Fe 3+ )3 (Si1.5 Al0.5)O5(OH)4; MX80 (purple star): (Al1.55Fe0.17, Mg0.28) (Si3.98Al0.02)O10(OH)2 Na0.18 Ca0.1, n H2O; Griffithite-ferroan (blue star): (Fe 2+ 0.75, Mg2.25) (Si3Al)O10(OH)2 Na0.1 Ca0.1, 4 H2O;

Fig. 4 :

 4 Fig. 4: TEM images of fine clay particles (<2 µm) recovered in the bulk (image a) and near the iron plate (images b and c). Points 1, 2, 3 and 4 indicated the particles analyzed and referred to the compositions plotted in the diagram of Fig. 3. The iron plates used in the experiment were observed by SEM. The detailed examination of the surface of the plates shows a great heterogeneity of the newly-formed mineralogical

Fig. 5 :

 5 Fig. 5: SEM images in secondary electron mode and backscattered electron mode of the surface of the iron plate (a and b). Both images and spectrum were recorded with an accelerating voltage of 15 kV and a beam current of 10 nA. White rectangles on (c) indicate well crystallized iron-rich silicates with a polygonal platy morphology and (d) correspond to their EDXS analysis.

Fig. 6 :

 6 Fig. 6: (a): SEM image in backscattered electron mode of iron plate covered with iron silicates close to iron sulphides interpreted as pyrrhotite from EDXS analyses (b). (c) and (d) : corroded pyrite framboids. Both images and spectrum were recorded with an accelerating voltage of 15 kV and a beam current of 10 nA.

Fig. 7 :

 7 Fig. 7: a) Room temperature Mössbauer spectrum of the reaction product (bulk) in velocity range of ± 11 mm/s. The uncertainty about the relative abundances of iron in the different phases is ± 2%. b) FTIR spectra in transmission mode recorded on pellets of the COx starting

Fig. 8 :

 8 Fig.8: a) Evolution of pH, pe, b) evolution of mass of water consumed, versus the amount of iron introduced step by step during the PhreeqC modelling.

Fig

  Fig. 9a, b: Dissolution and crystallization of mineralogical phases versus the amount of iron introduced step by step into the PHREEQC modelling. The dashed rectangle in the diagram a) is zoomed in the Fig. b).

Fig. 10 :

 10 Fig. 10: Black curve: Moles of H2 gas generated in the reactor (bar) at 90°C vs. time (days). Dotted curve is the trend curve. Its equation is n=0.0148.t 0.5 .

  ) given below involve potential transformation of the COx clays into greenalite (Si2Fe3O5(OH)4). These transformations need Fe 2+ and may affect the corrosion of iron metal and the production of H2. The mean structural formulae of I/S and illite were calculated from the analyses obtained by TEM-EDXS. Considering iron as ferrous, the mean structural formula of the illite of the COx is [Si3.4Al0.6](Al1.51Fe 2+ 0.375Mg0.23)O10(OH)2K0.83, which rather corresponds to a phengite. The mean structural formula of I/S is [Si3.61Al0.39](Al1.40Fe 2+ 0.54Mg0.23)O10(OH)2K0.54Ca0.05 (by ignoring the water of hydration of the interlayer cations). The following reactions (4) and (5) show a consumption of Fe 2+ and water to form greenalite. Gibbsite, silica (expressed as (SiO2)aq) and cations in solution are

Fig. 11 :

 11 Fig. 11: Stability fields of hematite, magnetite and greenalite on pe vs. pH phase diagrams calculated at 90°C. The activity of Fe in solution is fixed at 10 -5 mol/kgw, p(CO2) = 2.10 -2 bar and [S] = 0 mol/kgw. Activity of SiO2(aq) is 5.10 -4 mol/kgw. The modelling reaction path

Table 1 :

 1 Data of the input file for the thermodynamic simulation of the experiment.* Salts in the natural interstitial water of COx are not taken into account in the modelling

	COx (1 kg)		Secondary minerals	Starting solution (1kg H2O)*
	Quartz	3.49 Mol	Fe(s)
	Calcite	2.3 Mol	Cronstedtite-Th
	Dolomite	0.216 Mol	Greenalite
	Illite_Imt-2	0.45 Mol	Siderite
	Illite/smectiteISCz-1	0.64 Mol	Gibbsite	Na: 2.14 mMol
	Pyrite	0.25 Mol	Magnetite	HCO3 -: 2.14 mMol
	Ripidolite_cCa-2	0.036 Mol	Nontronite-Ca
	Kaolinite	0.11 Mol	Saponite-FeCa
			Pyrrhotite

Acknowledgments

The discussions and comments of the journal editor and of the two anonymous reviewers are gratefully acknowledged. The authors also thank L. Salsi for the acquisition of SEM images and J. Ghanbaja for TEM analyses at the SCMEM and IJL laboratories (Université de Lorraine, France). The research was financially supported by Andra (French national agency for the management of radioactive waste). M.S.N. Carpenter post-edited the English style and grammar.

of iron : another 2.17 moles of H2O for reaction (4) and 1.77 moles of H2O for the reaction (5).

[ Si3.4Al0.6 The most important result highlighted by these transformation reactions of COx clays into greenalite (Eqn. ( 4) to ( 7)) is first of all a significant consumption of water. Thus, iron corrosion is not the only process that consumes water in the experiment. Furthermore, most of these reactions also produce OH -increasing the pH in the experimental medium and consequently the dissolution of quartz. Metallic iron and silica in solution can also promote greenalite precipitation following the reaction (8) that consumes water and produces H2 gas:

2 (SiO2)aq + 3 Fe (s) + 5 H2O Si2Fe3O5(OH)4 + 3 H2(g) ( 8) low activities of carbonate concentration in solution (aHCO3 -and aCO3 2-< 10 -12 mol/kgw at pH>10), the high pH and the stability of calcite. Thus, the modelling confirms the difficulty of observing iron carbonate in the reaction product. Gibbsite is not revealed in the reaction product by the different analytical techniques used in this study. As it is shown by the modelling, it is formed at the end of the modelling, so it is supposed that in the experiment, the progress of the reaction allowing the formation of crystallized gibbsite may not have been achieved. These differences between modelling and experiment can be explained by the fact that i) the reaction kinetics are not considered in the numerical simulations, ii) the system is assumed to reach equilibrium between each step of addition of iron, iii) under the imposed experimental conditions, not all mineral phases are likely to be available to participate in chemical reactions especially at a low water/rock ratio, and iv) the difficulty of identifying newly-formed phases in a poorly crystallized mixture.

A comparison of our modelling results with other studies reveals a lot of similarities but also some divergences. Using the reaction-transport code Crunch to simulate the interactions between iron and clay-rich geological materials, [START_REF] Bildstein | Modelling iron-clay interactions in deep geological disposal conditions[END_REF] show that iron oxides (magnetite) and small amounts of siderite are the main minerals of steel corrosion. Moreover, these authors confirm that the primary clay phases (illite and montmorillonite) are destabilized at the expense of iron-rich serpentine-like minerals (cronstedtite and berthierine).

These authors also highlight that these mineralogical transformations may lead to significant porosity changes. [START_REF] Ngo | Kinetic modelling of interactions between iron and water: Comparison with data from batch experiments[END_REF] used the code KINDIS to simulate batch experiments on iron-claystone interactions (W/R=10 and iron/claystone=0.1), forecasting the dissolution of the more abundant primary minerals of the claystone (quartz, illite and smectite) and the formation of greenalite, chukanovite and saponite as the main secondary minerals, but without magnetite. These authors also stressed the difficulty of accurately calculating the corrosion and dissolution rates for iron, quartz and illite and, consequently, predicting the evolution of