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Poster De Conférence Année : 2024

Knowledge gaps in thermodynamic data needed to predict the aqueous speciation of platinum-group elements

Résumé

The current tenet of metal bioavailability stipulates that the uptake and toxicity of a metal are not related to its total concentration in solution but rather to its free-ion activity (the other main toxicity modifying factors being hardness and pH). Ideally, the free-metal ion activity (or concentration) would be measured analytically. However, currently available analytical techniques do not allow for routine measurements as most are metal-specific, labour-intensive, subject to matrix interferences or lack sensitivity. On the other hand, aqueous metal speciation can be effectively predicted using the appropriate thermodynamic data (formation constants). Typically, metals will form inorganic complexes with ligands such as hydroxide, carbonate, chloride and fluoride ions. For many transition elements (e.g. Cu, Pb, Zn), thermodynamic data for these complexes are readily available and speciation can be calculated with a high degree of confidence. Even in the presence of polyfunctional heterogeneous natural organic matter (humic and fulvic acids), complexation can normally be estimated with reasonable confidence. The growing demand for less traditional elements (e.g. technology critical elements) is driving environmental regulators to develop new criteria for environmental protection. However, these cannot be derived without the appropriate ecotoxicological data, and the production of such data requires basic knowledge of metal aqueous chemistry. We reviewed the available formation constants for platinum group elements (Ru, Rh, Pd, Os, Ir and Pt) and identified key missing data. Among these metals, palladium is the one for which the most comprehensive data set is available. At the other end of the spectrum, there are currently no thermodynamic data available for iridium. We also explored the use of Linear Free-Energy Relationships (LFER) to fill data gaps. These involve the establishment of empirical relationships between complexes. For example, the binding constants of the first hydroxo-complex of metals (MOH) are usually available and can be correlated with the formation constant with a more highly substituted complex (e.g. M(OH)2) or with another ligand (e.g. MCO3). LFER can thus be used to estimate missing data or to select a value when large discrepancies are observed among available values. Examples will be provided with a focus on Pt(II). Future research priorities will be discussed.
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Dates et versions

hal-04652424 , version 1 (18-07-2024)

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  • HAL Id : hal-04652424 , version 1

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Claude Fortin, Peter Campbell, Séverine Le Faucheur. Knowledge gaps in thermodynamic data needed to predict the aqueous speciation of platinum-group elements. SETAC Europe 34th Annual Meeting, May 2024, Seville (Spain), Spain. ⟨hal-04652424⟩
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