Intercomparison of Ta and Ti solid-electrodes for pH measurements under oxic and anoxic conditions in reconstituted waters of a future nuclear waste disposal

International audienceIn France, the deep geological disposal has been chosen to manage long-lived high and middle activity nuclear waste. This project is supervised by " the Agence National pour la Gestion des Déchets Radioactif" (ANDRA-French national radioactive waste management agency). An underground research laboratory (URL), dedicated to host-rock properties characterization was built at approximately-490m depth in the Callovo-Oxfordian geological formation (COx), which is a potential candidate for nuclear waste disposal. Based on this overall strategy, Andra has analysed the technical requirements that must be met by adapted monitoring equipment. First, these must be able to provide information on key THMCR (Thermal-Hydraulic-Mechanical-Chemical and Radiological) processes, to provide a three-dimensional image of its behaviour and thus to understand the underground installation functioning, in particular the cell interactions with the near-field. This study aims to develop innovative all-solid-state electrodes made of iridium (Ir), ruthenium (Ru), tantalum (Ta), titanium (Ti), tungsten (W), niobium (Nb) and a tin-lead alloy (Sn/Pb), for pH measurement. Moreover, the potentiometric response of these electrodes is mainly based on Metal Oxide (M x O y)/Metal (M) equilibriums, which make them react to changes of oxygen partial pressures. For now, the antimony electrode (Sb) showed excellent results regarding the measure of pH under oxic and anoxic conditions. Nevertheless, it is necessary to multiply the electrode materials considered for pH measurements, for preventing a dysfunction of electrodes which could occur over time. By multiplying the electrode materials for pH measurement, we indirectly increase, firstly, the measurement reliability by giving the possibility to inter-compare the potentiometric responses of the set of the electrodes dedicated to pH, and secondly, the monitoring duration, in the case of electrode surface alterations, which could be caused by variations of the physical and chemical parameters within the nuclear waste repository. In other words, this would provide alternatives electrode materials for pH measurements, in prevention to possible electrode surface alterations. Thus, the influence of pH from 5 to 13, on the potentiometric responses of the several electrodes was investigated by means of pH buffers solutions or by continuous and regulated addition of H 2 SO 4 or NaOH, under both oxic (exposed at air) and anoxic conditions (in glove box: 99% N 2 , 1% CO 2 and [O 2 ] < 2 ppm, 25°C). Titanium (Ti) and tantalum (Ta) electrodes showed a potentiometric linear response to pH variations, as shown on figures 1 and 2. In absence of oxygen, the Ti electrode showed a linear response to pH (from 5.8 to 11), close to the theory. After pH 11, a drop of potential is observed. In presence of oxygen, the response to pH is linear but further to the theory. In the same way, the potentiometric behaviour of the Ta electrode to pH variations is linear, but much more sensitive under anoxic conditions

Multi-Parametric Devices with Innovative Solid Electrodes for Long-Term Monitoring of pH and Redox-Potential of the actual pore water of COx formation in a future Nuclear Waste Repository

International audienceWe present innovative electrochemical probes for the monitoring of pH and redox potential in pore water in near-field rocks of a future deep geological radioactive waste repository at 500 m depth within the clayey Callovian-Oxfordian (COx) formation. The conceived experimental setup assembles two multi-parameter probes (MPPs), used together throughout two series of several months duration measurements in situ into the underground research laboratory of Andra at Bure, France. The two MPPs, connected in series, were up-flow fed with actual pore water of COx formation during several with a very low flowrate. Each MPP is composed of different individual probes containing the following: two monocrystalline antimony electrodes for pH sensing; eight AgCl/Ag-based reference or Cl − selective electrodes; four Ag 2 S/Ag-based reference or S 2− selective electrodes; eight platinum electrodes; two gold electrodes; two glassy-carbon electrodes; two ruthenium and two inox 316 electrodes, for redox potential measurements. The Open Circuit Potential (OCP) measurements of the developed sensors under different conditions and in quasi-actual conditions were compared to conventional reference electrode and pH electrodes in terms of performance, reliability and robustness and allowed to create calibration curves. Conductivity measurements, carried out along MPPs, will not be presented here. Overall, the conceived bundle of electrodes as designed works reliably during a timescale that is promising for monitoring the COx formation during its envisaged use for hosting a nuclear waste repository

Influence of temperature and surfactants on the solubilization of hexachlorobutadiene and hexachloroethane

The solubilization of hexachlorobutadiene (HCBD) and hexachloroethane (HCA) in water as a function of temperature and in the presence of surfactants was investigated in order to predict their fate in groundwater and to increase their recovery. HCBD and HCA solubility data were experimentally determined at five temperatures in the range from (285.15 to 318.15) K. Thermodynamic parameters for dissolution (ΔsolG°, ΔsolH°, and ΔsolS°) have been calculated in order to propose a physical explanation of the minimum solubility observed between 293.15 and 298.15 K for both compounds. The solubilization process appeared to be influenced by the network of water molecules rather than by physical and chemical properties of HCBD or HCA, due to an opposite effect of temperature onto Brownian motion, which increases with temperature, and hydrogen-bond network, which collapses with temperature. Concerning the influence of surfactants, determination of the micelle–water partition coefficients (Kmw) and the molar solubilization ratio (MSR) has shown that the solubilization per micelle was more important for nonionic surfactants Triton X-100 and Tween 80 than for anionic SDBS. Also, the increase of solubility was 1 order of magnitude higher for liquid HCBD than for crystalline HCA irrespective of surfactant

Conception d'un procédé de décontamination de matrices minérales à base de silice utilisées dans l'industrie nucléaire

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Breaking boundaries in Electrochemistry: Unveiling a new dimensionless number to tackle convective transfer effect on the voltamperometric answer

This study introduces the Theokane number (Tk) as a groundbreaking dimensionless number in Electrochemistry. Tk enables the determination of the operating state of an electrochemical (EC) system—indicating whether it is in a transient state (characteristic of cyclic voltammetry) or a steady state (typical of linear sweep voltammetry)—based on a given combination of potential scan rate and residence time. It aims to bridge the gap between various voltamperometric methods. Tk uniquely compares the duration of the potential scan applied to an EC system to the residence time of the reaction mixture at the electrode. This comparison is pertinent in environments ranging from microfluidic setups to macroscale reactors, including stirred vessels.Tk is particularly crucial for understanding the ‘continuous answer’ of an EC system subjected to voltamperometric polarization across a spectrum of potential scan and stirring rates. Voltammograms recorded in a micro-reactor under varying conditions highlight the influence of operating parameters on EC responses. The approach introduced in this study accomplish three key objectives: i) it validates the Tk number through the comparison of experimental and simulation data, ii) it proposes a range for its applicability; and iii) it opens a new mode for analyzing EC responses. It is important to note that Tk is applicable to both quasi-reversible and irreversible systems

Elucidating the dechlorination mechanism of hexachloroethane by Pd-doped zerovalent iron microparticles in dissolved lactic acid polymers using chromatography and indirect monitoring of iron corrosion

International audienceThe degradation mechanism of the pollutant hexachloroethane (HCA) by a suspension of Pd-doped zerovalent iron microparticles (Pd-mZVI) in dissolved lactic acid polymers and oligomers (referred to as PLA) was investigated using gas chromatography and the indirect monitoring of iron corrosion by continuous measurements of pH, oxidation-reduction potential (ORP), and conductivity. The first experiments took place in the absence of HCA, to understand the evolution of the Pd-mZVI/PLA/H2O system. This showed that the evolution of pH, ORP, and conductivity is related to changes in solution chemistry due to iron corrosion and that the system is initially cathodically controlled by H+ mass transport to Pd surfaces because of the presence of an extensive PLA layer. We then investigated the effects of Pd-mZVI particles, temperature, initial HCA concentration, and PLA content on the Pd-mZVI/PLA/HCA/H2O system, to obtain a better understanding of the degradation mechanism. In all cases, HCA dechlorination first requires the production of atomic hydrogen H(*)involving the accumulation of tetrachloroethylene (PCE) as an intermediatebefore its subsequent reduction to non-chlorinated C-2 and C-4 compounds. The ratio between Pd-mZVI dosage, initial HCA concentration, and PLA content affects the rate of H-* generation as well as the rate-determining step of the process. A pseudo-first-order equation can be applied when Pd-mZVI dosage is much higher than the theoretical stoichiometry (600mg for [HCA](0)=5-20mgL(-1)). Our results indicate that the HCA degradation mechanism includes mass transfer, sorption, surface reaction with H-*, and desorption of the product

The Role of Oxygenic Groups and Sp3 Carbon Hybridization in Activated Graphite Electrodes for Vanadium Redox Flow Batteries

Graphite felt is a widely used electrode material for vanadium redox flow batteries. Electrode activation leads to the functionalization of the graphite surface with epoxy, OH, C=O, and COOH oxygenic groups and changes the carbon surface morphology and electronic structure; thus, improving the electrode’s electroactivity relative to the untreated graphite. In this study, we conduct density functional theory (DFT) calculations to evaluate functionalization’s role towards the positive half-cell reaction of the vanadium redox flow battery. The DFT calculations show that oxygenic groups improve the graphite felt’s affinity towards the VO2+/VO2+ redox couple in the following order: C=O > COOH > OH > basal plane. Projected density of states (PDOS) calculations show that these groups increase the electrode’s sp3 hybridization in the same order. We conclude that the increase in the sp3 hybridization is responsible for the improved electroactivity, while the oxygenic groups’ presence is responsible for this sp3 increment. These insights can help in the better selection of activation processes and optimization of their parameters.</p

Non-Aqueous Li-Based Redox Flow Batteries

International audienceRedox flow batteries have gained renewed interest for grid storage applications. This work focuses on the effect of various chemical/physical parameters on the performance of the LiFePO4/LiPF6 EC-DMC/Li redox flow system. A methodical study of the influence of the content of active material and of the flow rate, coupled with electrochemical and hydrodynamic characterizations, have been carried out in order to better understand the various ‘migration’ and ‘diffusion’ limitations, as well as to try to overcome their effects. As a result, power density performances higher than 328 mW.cm−2 at 104 mA.cm−2 were achieved and the feasibility of reaching energy density of 50 Wh.kg−1 demonstrated