H-terminated polycrystalline boron doped diamond electrode for geochemical sensing into underground components of nuclear repositories

Nuclear waste repositories are being installed in deep excavated rock formations in some places in Europe to isolate and store radioactive waste. In France, Callovo-Oxfordian formation (COx) is potential candidate for nuclear waste repository. It is thus necessary to measure in situ the state of a structure's health during its entire life. The monitoring of the near-field rock and the knowledge of the geochemical transformations can be carried out by a set of sensors for a sustainable management of long-term safety, reversibility and retrievability. Among the chemical parameters, the most significant are pH, conductivity and redox potential. Wide band gap semiconductors are favored materials for chemical sensing because of their high stability to many chemical agents. Among the wide band gap materials, Chemical Vapor Deposition (CVD) boron doped diamond (BDD) benefits from a large band gap (5.45 eV), which gives rise to a wide electrochemical potential window (~3 V/Saturated Calomel Electrode(SCE)) (Angus et al. 1999). It is moreover described as a radiation, corrosion and bio-corrosion resistant. These remarkable properties, in addition to a low double layer capacity and a low residual current, make BDD a promising material for geochemical sensor elaboration. This work aimed to investigate BDD- based electrodes coated with p-type polycrystalline BDD- hydrogen-terminated surfaces (1 cm2) for pH and/or redox measurements into the underground components of nuclear repositories. The boron-doped p-type channel was grown in a microwave plasma reactor (BJS 150) (Silva et al. 2009). The boron-doped channel was hydrogen terminated by a hydrogen plasma treatment in the CVD reactor, resulting in full saturation of the surface carbon bonds with hydrogen atoms. Figure 1 shows the Scanning Electron Microscopy (SEM) of the polycrystalline BDD coating with a Bore/Carbon ratio of 500 ppm and its Raman spectrum. SEM micrograph illustrates the typical columnar growth of the polycrystalline CVD diamond. A homogeneous surface was observed concerning the crystallite size which average was 1.5 microns. On the Raman spectrum of a single crystal diamond intrinsic film (undoped), the diamond peak is usually observed at 1332 cm-1. In Figure 1, the intense peak at 1327 cm-1 corresponding to diamond is shifted due to the "Fano" effect according to doping, which is observed through a broad peak at 910 cm-1. Its intensity shows that the investigated sample was highly doped. Gheeraert et al. (1993) suggested that the peaks at 500 and 1230 cm-1 appears when the boron concentration reaches the critical value of 3×1020 at.cm-3 corresponding to a metallic conductivity. The lack of peak around 1350 cm-1 and 1570 cm-1, which corresponds respectively to D and G graphite peak of impurity phases of non-diamond carbon (sp2), attests to the crystalline quality of the deposit. The slight width at half maximum of the characteristic peak of diamond compared to that of natural diamond reflects the degree of organization and structural perfection of this phase indicating that the coating was of high quality. Electrodes made in this way have been used for 8 month without any surface treatments or conditioning. The electrochemical behavior of Hydrogen-terminated BDD was studied by cyclic voltammetry. Electrodes showed a wide potential range of about 2 V/SCE. They also showed and a rapid reversible charge transfer in the presence of redox probes such FeCN63-/4- and Ru(NH)63+/2+. Performances, reliability and robustness for pH or redox monitoring were examined by potentiometric measurements at 25°C under anaerobic conditions (oxygen-free atmosphere, 100 % nitrogen) in a glove box. Investigation has been limited in pH, ranging from 5.5 to 13.5, close to those encountered in the environment of the nuclear repositories. The feasibility of measuring pH with BDD electrodes was first tested in NH4Cl/NH3-NaCl (0.1mol L-1) buffer solutions, leading to electrode calibration over the widest range of pH, from around neutral to basic pH. Experiments were also conducted in NaHCO3/Na2CO3 buffer samples, similar to conditions prevailing in the COx formation. For redox measurements, [Fe3+]/[Fe2+] ratios were analysed at different pH and/or ionic strengths (supporting electrolytes concentration ranged from 0.05 to 1 mol.L-1). The same measurements were also done using a 10-mm disk platinum electrode with a surface of 78.54 mm². No pH sensitivity was observed, thus the energy level of the state was not moved. However, for redox measurements the potential acquired by Hydrogen-terminated BDD and Platinum electrode converged to a value of the same order of magnitude, independently of the sample. This fact demonstrates that, under the same experimental conditions, the redox couples fixe identically the potential of the electrodes. Investigations with reference to ionic strength in thermodynamically equilibrated Fe(III)/Fe(II) samples were highly interesting. Independently of the electrode, the voltage measurement was not or little affected, whereas both the solution conductivity as well as the speciation were affected, due to the increase in salinity. This means that the term [Fe3+]/[Fe2+] is practically unaffected. This implies that assuming the ratio of the activity coefficients, γFe3+/ γFe2+ as equal to 1 has a minor effect on the measured redox potential. H-terminated BDD electrode appears well suited for redox monitoring. Work is in progress to demonstrate the robustness of the H-terminated BDD electrode for redox monitoring into COx over a long period

Investigation of dental amalgam electrode behaviour for the long term monitoring of nuclear waste disposals

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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

Modeling the dioctahedral smectites layer charge variation versus structural Iron reduction level

Iron is one of the most common redox species in soils and sedimentary rocks. Amongst iron-bearing phases, phyllosilicates might play key roles in various bio-geochemical processes involving redox reactions, where structural Fe (Festr) can act as a renewable source/trap of electron. A large set of data from kinetics, spectroscopic or electrochemical studies on dioctahedral smectites demonstrates that reduction of Festr impacts many clay properties such as colour, layer charge, swelling pressure, colloidal properties that are linked to layer structural changes. Experiments also suggest that this mechanism is partly reversible, depending on type and properties of the primary oxidized clay, on how the reduction is induced (chemically and/or biologically) and on extent of iron reduction level. The complexity of the involved mechanisms makes the prediction of Festr redox properties challenging. For instance, only empirical models are currently available to quantify structural changes as a function of reduction level. However, a predictive and mechanistic model of these changes is a prerequisite to develop a thermodynamic model for Festr redox properties. In this contribution, we propose a mechanistic statistical model to explain 2:1 layer excess negative charge changes induced by structural Fe(III) to Fe(II) chemical reduction (by dithionite). This model completes this published by Drits and Manceau (2000) and was calibrated on data from our own and from the literature. Actually, a large number of studies on Festr redox properties (Eh and kinetics) neglects the major structural changes that occur during redox reactions of this material and that are partially reversible, and are focused in measuring a single Eh value. Actually, the complex relationship that exist between the different structural iron sites should lead to consider that not only one but several Fe(II)/Fe(III) poles (classes) must exist in the structure, thus exhibiting gradually decreasing Eh values. Hence, further developments of our model will include Crystal Field Theory (CFT) calculation to identify the variety of Festr redox potential, which arises from the varying Festr neighbouring inside the same structure and along the redox processes

Remediation of Chlorinated Organic Compounds: Single- and Multi-Component Approaches

International audienceChlorinated organic compounds (COCs) represent a major concern and are widespread distributed in soil and groundwater. Due to their strong hydrophobicity and their density higher than water, these COCs infiltrate through aquifers and form DNAPL pools. A large part of DNAPL can be removed by physical technologies (mainly pumping), but an important part will remain trapped and adsorbed in the aquifer matrix. In situ remediation technologies have been then developed in order to destroy in situ the remaining COCs.This study aims at characterizing the chemical reductive dechlorination of a mixture of COCs, mainly composed of hexachlorobutadiene (HCBD) and hexachloroethane (HCA). Many studies have shown the great efficiency of bimetallic Pd/Fe particles for the remediation of COCs (Colombo et al., 2015; Kim and Carraway, 2003; Lien and Zhang, 2007). Preliminary studies have been conducted in order to select the most appropriate reactants, which are Pd/Fe microparticles dispersed in a polyacidic hydrophobic matrix (Rodrigues et al., 2015).First, HCBD and HCA were individually investigated in a monophasic single-component system, i.e. in presence of one dissolved pollutant in deionized water:methanol (99.9:0.1% v/v) solutions. Several analytical parameters were studied: pollutant/reactant ratios, temperatures and presence or absence of surfactants. Similar experiments were performed in a monophasic multi-component system, containing dissolved HCBD and HCA, to characterize the impact of a hydrophobic mixture.Then, as most part of the pollutant is present as a DNAPL in groundwater, HCBD and HCA were investigated in a polyphasic single-component system, i.e. in presence of one pure pollutant in deionized water. This second approach combines both reduction reactions and transport processes, especially solubilization. The aim is to understand the influence of temperature and the presence of a surfactant in a polyphasic system to define the rate-determining step of the global remediation process. Finally, polyphasic multi-component systems were performed in presence of a mixture of HCBD and HCA, and in presence of a DNAPL taken from a polluted site.These two approaches allowed the determination of degradation pathways and kinetic laws for the two compounds individually and in mixture.ReferencesColombo, A., Dragonetti, C., Magni, M., Roberto, D., 2015. Degradation of Toxic Halogenated Organic Compounds by Iron-Containing Mono-, Bi- and Tri-Metallic Particles in Water. Inorganica Chim. Acta 431, 48–60.Kim, Y.H., Carraway, E.R., 2003. Reductive Dechlorination of TCE by Zero Valent Bimetals. Environ. Technol. 24, 69–75.Lien, H.-L., Zhang, W.-X., 2007. Nanoscale Pd/Fe Bimetallic Particles: Catalytic Effects of Palladium on Hydrodechlorination. Appl. Catal. B Environ. 77, 110–116.Rodrigues, R., Betelu, S., Garnier, F., Colombano, S., Joubert, A., Cazaux, D., Masselot, G., Tzedakis, T., Ignatiadis, I., 2015. SILPHES – Investigation of Chemical Treatments for the Remediation of Recalcitrant Chlorinated Solvents. In: 13th International UFZ-Deltares Conference on Groundwater-Soil-Systems and Water Resource Management

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

Reductive dechlorination of hexachlorobutadiene by a Pd/Fe microparticle suspension in dissolved lactic acid polymers: degradation mechanism and kinetics

Reductive dechlorination of hexachlorobutadiene (HCBD) was performed by a suspension of scattered spots of palladium nanoparticles deposited on iron microparticles (nPd/μZVI) in a mixture of dissolved lactic acid polymers and oligomers (referred to as PLA). The effects of nPd/μZVI loading, temperature, HCBD initial concentration, and PLA content were investigated as to define the best conditions for the dechlorination. HCBD dechlorination by nPd/μZVI occurred in a two-step process: first, HCBD adsorbed onto the nPd surface, which resulted in a rapid initial disappearance of pollutant in solution, and, second, it degraded chemically by atomic hydrogen H*, which resulted from the dissociative adsorption of H2 on nPd. HCBD remained adsorbed on the surface until its complete degradation in nonchlorinated product, in agreement with the formation of an ordered activated complex on the nPd/μZVI surface as suggested by the negative entropy of activation calculated from the Eyring equation. Hence, a minimum amount of nPd/μZVI was required to enable simultaneously HCBD adsorption and H2 production. In these cases, pseudo-first order rate equations were suitable to model HCBD disappearance kinetics. The increase in PLA content resulted in enhancing initial pH decrease such as to maintain acidic conditions and thus high reactivity over a longer period of time. It also resulted in enhancing the contact between HCBD and nPd/μZVI, which was characterized by a more important initial adsorption. As a consequence, deviations from pseudo-first order kinetics were observed and a more representative model with a two-phase decay was proposed

Linear sweep voltammetry coupled to a quartz crystal microbalance for investigating the catalytic activity of the Mg (II) –water electrochemical system and managing the Mg oxy-hydroxide hydration state

This study provides an insight into the catalytic activity of the Mg(II)–water electrochemical system, from aqueous magnesium chloride hexa-hydrate to magnesium oxy-hydroxide electro-nucleation, growth and evolution, using Linear Sweep Voltammetry (LSV) coupled to a Quartz Crystal Microbalance (QCM) (−0.80>EV/SCE>−1.80). Interfacial phenomena occur at the gold resonator during cathodic polarization (−0.90>EV/SCE>−1.80).MgH2O62+→−0.90>EV/SCE>−1.25MgH2O4OH2→−1.25>EV/SCE>−1.50MgH2O2OH2→−1.50>EV/SCE>−1.70MgOH2 From −0.90V, reduction of the intramolecular water of the hexa-coordinated Mg(H2O)62+ cluster enhanced the cathodic current i°MgH2O62+/i°H2O. In the range −1.25>EV/SCE>−1.70, electrosynthesis of Mg(H2O)4(OH)2 and its simultaneous gradual dehydration to Mg(OH)2 take place. At lower potentials, experiments suggest intramolecular dehydroxylation of Mg(OH)2 to MgO. The applied potential thus allows fine control of the hydration state of the Mg oxy-hydroxide

Impact de la production de biosurfactant sur la remobilisation et la biodégradation des HAP par une communauté bactérienne issue d'un site contaminé

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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