Performance of MoxOy pH sensor prepared by thermal oxidation for the long term monitoring of nuclear waste disposals
Résumé
Cigéo is the future French industrial project for the deep geological disposal of High-Level Long-Lived Radioactive Waste. The Callovo-Oxfordian clay formation (Cox) has been selected at a depth of 500 m as a natural barrier to isolate the waste from living beings and the environ ment. This project is supervised by the French National Agency for Radioactive Waste Man agement (Andra). The Cox formation is a layer of clay-rich rock approximately 130 m thick, formed around 160 million years ago. It is located at depths ranging from 400 to 600 m under ground. This formation is characterized by its high water saturation, very low permeability, po rosity, and hydraulic conductivity. The temperature, pH, and CO2 partial pressure of the Cox pore-water solution remain consistent at 25°C, 7.3 (±0.1), and 8. 10−3 atm, respectively [1]. In order to effectively monitor the near-field evolution around radioactive waste, the use of dependable all-solid-state reference electrodes and sensors is crucial for ensuring safety and reversibility. Parameters such as pH and redox potential play a significant role in physical, chemical, and electrochemical (redox) transformations, which, consequently, need to be mon itored. Due to reversible interfacial redox processes involving only protons, metal oxy-hydroxides (MxOy) potentiometric electrodes are considered promising for pH measurements. This is be cause of their inherent mechanical stability in various conditions such as temperature, pres sure, and harsh environments. The development of all-solid-state pH electrodes is attracting considerable attention because such electrodes are more favorable for process monitoring where robustness and resistance to high temperature, high salinity, and high pressure are required. This work aims to develop pH innovative all-solid-state electrodes based on molybdenum Ox ide (MoxOy)/molybdenum (Mo). Understanding the electrochemical behavior of such metallic coatings is the key point of our research, as provides the tools for constructing and exploiting the Potential-pH abacuses, which are necessary for developing robust and reliable pH sen sors. Among the molybdenum sensors developed in the literature, one study conducted by Shuk et al., [2] focuses on pH sensors based on single crystals of molybdenum oxide bronzes for mon itoring pH titrations. In this previously cited study, the pH electrodes responded reproducibly to pH changes in the range of 4 to 9, with a response time of less than 5 seconds. In our case, the sensors were prepared by thermal oxidation at 370, 400, 500 and 600°C. The sensibility of electrodes was then evaluated by potentiometric measurements during electrode immersion in pH buffers solutions, at 25°C, under both oxic (at atmospheric pressure) and anoxic conditions (in a glove box (GB), pN2 = 1 atm; pO2 = 10-6 atm), in the presence and the absence of sulfides at various concentrations (ranging from 10-7 to 10-3 mol/L). We employed various heat treatment parameters in our study because our aim is also to di versify the electrodes for pH measurements to prevent any dysfunction that may occur over time. By multiplying the electrodes dedicated to pH measurement, we indirectly enhance, firstly, the reliability of the measurements by allowing comparison of potentiometric responses across all electrodes, and secondly, the monitoring duration, in the event of alterations to elec trode surfaces caused by variations in the physical and chemical parameters within the nuclear waste repository. MoxOy films seem to be robust and exhibit a good sensitivity under oxic and anoxic conditions, and, furthermore, the absence of oxygen (in GB) leads to an increase in sensitivity and a de crease in the experimental standard potential E°Exp. The sensors were subsequently observed using a scanning electron microscope (SEM); the films appear to be adhesive. The electrodes made at 500°C seem to be more homogeneous. The immersion of the MoxOy/Mo sensors in milliQ water for 70 days allows the hydration of the MoxOy layer, which is responsible of an increase of sensitivity. XPS analyses were carried out before and after hydration, to determine the main redox couples responsible for electrode re sponse to pH variations. Finally, the performances of MoxOy - pH sensors show their utility and ability for long-term pH monitoring. Work is in progress to demonstrate the robustness of the MoxOy-/Mo electrodes into actual Cox pore water solution over a long period.
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