Uranium speciation and mobility in soils and lake sediments downstream from former uranium mines

L’objectif de ce travail de thèse était d’améliorer les connaissances sur le comportement géochimique et la mobilité de l’uranium (U) dans des sédiments lacustres et des sols enrichis en U, dont les conditions réductrices sont à priori propices à son piégeage. Des analyses de spectroscopie d’absorption des rayons X et de microscopie électronique à balayage combinées à des analyses géochimiques ont été mises en œuvre. Pour les sédiments lacustres, la réduction progressive de U(VI) en U(IV) sous l’interface eau-sédiment est mise en évidence. Les données de spéciation et la modélisation géochimique suggèrent un contrôle important de la réduction par le Fe(II) issu de la diagenèse précoce, au travers de la réduction microbienne du Fe(III) structural des argiles. Pour les sols, une réduction brutale de U(VI) est observée, contrôlée par le niveau de saturation en eau. Pour les deux sites étudiés, des complexes mononucléaires adsorbés de U(IV) et des minéraux phosphatés de U(IV) ont été mis en évidence. Pour l’étude des sols, une redistribution de U au niveau du front redox, via la dissolution oxydative des minéraux phosphatés de U(IV) suivie de la complexation de U(VI) à la matière organique du sol est suggérée. Des incubations de sol ont permis de confirmer ces mécanismes de redistribution. Ces travaux appellent à considérer les phases de U(IV) non-cristallines et les minéraux phosphatés de U(IV) comme espèces contrôlant la solubilité de l’uranium dans les environnements contaminés. Il apparaît donc nécessaire de prendre en compte ces formes chimiques pour modéliser la mobilité de l’uranium dans ces systèmes et permettre une gestion adéquate de ces environnements contaminés.Uranium scavenging in soils and sediments located downstream from former U mines is expected to naturally limit uranium dispersion in downstream waterways. However, uranium mobility in such contaminated sites may depend on the identity of U traps as well as the geochemical conditions. The aim of this thesis was to improve our knowledge on the geochemical behavior and the mobility of uranium in U contaminated lacustrine sediments and wetland soils, whose reducing conditions is expected to mitigate uranium mobility because U(IV) species are less soluble than U(VI) ones. X-ray absorption spectroscopy and scanning electron microscopy analyzes combined with geochemical analyzes were carried out. In U contaminated lake sediments, we show that indirect reduction of U(VI) by Fe(II) associated to clay minerals may be a major diagenetic process responsible for the scavenging of uranium. For organic-rich weltand soils, we show a sharp uranium redox boundary mainly controlled by the water-table. For both sites, U(IV) mononuclear species and U(IV)-phosphate minerals were identified as the major species controlling uranium solubility, while uraninite is virtually absent. For the highly U-contaminated wetland soil, we suggest a major uranium redistribution via the oxidative dissolution of U(IV)-minerals followed by U(VI) organic matter complexation. Soil incubation experiments have confirmed these redistribution mechanisms and suggest different geochemical behaviors for lermontovite (U(PO4)(OH)•H2O) and ningyoite (CaU(PO4)2•2H2O). These experiments also highlight the role of organic matter in the control of uranium mobility, favoring the remobilization of U(IV) organic complexes under reducing conditions. Altogether, our results call for the need to consider both non-uraninite U(IV) minerals and mononuclear U(IV) complexes in such anoxic environments as major species controlling uranium solubility

Spéciation et mobilité de l'uranium dans des sols et des sédiments lacustres en aval d'anciens sites miniers

Uranium scavenging in soils and sediments located downstream from former U mines is expected to naturally limit uranium dispersion in downstream waterways. However, uranium mobility in such contaminated sites may depend on the identity of U traps as well as the geochemical conditions. The aim of this thesis was to improve our knowledge on the geochemical behavior and the mobility of uranium in U contaminated lacustrine sediments and wetland soils, whose reducing conditions is expected to mitigate uranium mobility because U(IV) species are less soluble than U(VI) ones. X-ray absorption spectroscopy and scanning electron microscopy analyzes combined with geochemical analyzes were carried out. In U contaminated lake sediments, we show that indirect reduction of U(VI) by Fe(II) associated to clay minerals may be a major diagenetic process responsible for the scavenging of uranium. For organic-rich weltand soils, we show a sharp uranium redox boundary mainly controlled by the water-table. For both sites, U(IV) mononuclear species and U(IV)-phosphate minerals were identified as the major species controlling uranium solubility, while uraninite is virtually absent. For the highly U-contaminated wetland soil, we suggest a major uranium redistribution via the oxidative dissolution of U(IV)-minerals followed by U(VI) organic matter complexation. Soil incubation experiments have confirmed these redistribution mechanisms and suggest different geochemical behaviors for lermontovite (U(PO4)(OH)•H2O) and ningyoite (CaU(PO4)2•2H2O). These experiments also highlight the role of organic matter in the control of uranium mobility, favoring the remobilization of U(IV) organic complexes under reducing conditions. Altogether, our results call for the need to consider both non-uraninite U(IV) minerals and mononuclear U(IV) complexes in such anoxic environments as major species controlling uranium solubility.L’objectif de ce travail de thèse était d’améliorer les connaissances sur le comportement géochimique et la mobilité de l’uranium (U) dans des sédiments lacustres et des sols enrichis en U, dont les conditions réductrices sont à priori propices à son piégeage. Des analyses de spectroscopie d’absorption des rayons X et de microscopie électronique à balayage combinées à des analyses géochimiques ont été mises en œuvre. Pour les sédiments lacustres, la réduction progressive de U(VI) en U(IV) sous l’interface eau-sédiment est mise en évidence. Les données de spéciation et la modélisation géochimique suggèrent un contrôle important de la réduction par le Fe(II) issu de la diagenèse précoce, au travers de la réduction microbienne du Fe(III) structural des argiles. Pour les sols, une réduction brutale de U(VI) est observée, contrôlée par le niveau de saturation en eau. Pour les deux sites étudiés, des complexes mononucléaires adsorbés de U(IV) et des minéraux phosphatés de U(IV) ont été mis en évidence. Pour l’étude des sols, une redistribution de U au niveau du front redox, via la dissolution oxydative des minéraux phosphatés de U(IV) suivie de la complexation de U(VI) à la matière organique du sol est suggérée. Des incubations de sol ont permis de confirmer ces mécanismes de redistribution. Ces travaux appellent à considérer les phases de U(IV) non-cristallines et les minéraux phosphatés de U(IV) comme espèces contrôlant la solubilité de l’uranium dans les environnements contaminés. Il apparaît donc nécessaire de prendre en compte ces formes chimiques pour modéliser la mobilité de l’uranium dans ces systèmes et permettre une gestion adéquate de ces environnements contaminés

Spéciation et mobilité de l'uranium dans des sols et des sédiments lacustres en aval d'anciens sites miniers

Uranium scavenging in soils and sediments located downstream from former U mines is expected to naturally limit uranium dispersion in downstream waterways. However, uranium mobility in such contaminated sites may depend on the identity of U traps as well as the geochemical conditions. The aim of this thesis was to improve our knowledge on the geochemical behavior and the mobility of uranium in U contaminated lacustrine sediments and wetland soils, whose reducing conditions is expected to mitigate uranium mobility because U(IV) species are less soluble than U(VI) ones. X-ray absorption spectroscopy and scanning electron microscopy analyzes combined with geochemical analyzes were carried out. In U contaminated lake sediments, we show that indirect reduction of U(VI) by Fe(II) associated to clay minerals may be a major diagenetic process responsible for the scavenging of uranium. For organic-rich weltand soils, we show a sharp uranium redox boundary mainly controlled by the water-table. For both sites, U(IV) mononuclear species and U(IV)-phosphate minerals were identified as the major species controlling uranium solubility, while uraninite is virtually absent. For the highly U-contaminated wetland soil, we suggest a major uranium redistribution via the oxidative dissolution of U(IV)-minerals followed by U(VI) organic matter complexation. Soil incubation experiments have confirmed these redistribution mechanisms and suggest different geochemical behaviors for lermontovite (U(PO4)(OH)•H2O) and ningyoite (CaU(PO4)2•2H2O). These experiments also highlight the role of organic matter in the control of uranium mobility, favoring the remobilization of U(IV) organic complexes under reducing conditions. Altogether, our results call for the need to consider both non-uraninite U(IV) minerals and mononuclear U(IV) complexes in such anoxic environments as major species controlling uranium solubility.L’objectif de ce travail de thèse était d’améliorer les connaissances sur le comportement géochimique et la mobilité de l’uranium (U) dans des sédiments lacustres et des sols enrichis en U, dont les conditions réductrices sont à priori propices à son piégeage. Des analyses de spectroscopie d’absorption des rayons X et de microscopie électronique à balayage combinées à des analyses géochimiques ont été mises en œuvre. Pour les sédiments lacustres, la réduction progressive de U(VI) en U(IV) sous l’interface eau-sédiment est mise en évidence. Les données de spéciation et la modélisation géochimique suggèrent un contrôle important de la réduction par le Fe(II) issu de la diagenèse précoce, au travers de la réduction microbienne du Fe(III) structural des argiles. Pour les sols, une réduction brutale de U(VI) est observée, contrôlée par le niveau de saturation en eau. Pour les deux sites étudiés, des complexes mononucléaires adsorbés de U(IV) et des minéraux phosphatés de U(IV) ont été mis en évidence. Pour l’étude des sols, une redistribution de U au niveau du front redox, via la dissolution oxydative des minéraux phosphatés de U(IV) suivie de la complexation de U(VI) à la matière organique du sol est suggérée. Des incubations de sol ont permis de confirmer ces mécanismes de redistribution. Ces travaux appellent à considérer les phases de U(IV) non-cristallines et les minéraux phosphatés de U(IV) comme espèces contrôlant la solubilité de l’uranium dans les environnements contaminés. Il apparaît donc nécessaire de prendre en compte ces formes chimiques pour modéliser la mobilité de l’uranium dans ces systèmes et permettre une gestion adéquate de ces environnements contaminés

Redox Cycling of Uranium Phosphate Minerals in a Mining-Contaminated Wetland

International audienceReduction of uranium (VI) to low soluble uranium (IV) species in wetlands is expected to limit the transfers of this toxic element to downstream waterways. However, in such environments, long-term uranium (U) scavenging may beperturbed by hydrological and redox fluctuations. Here, based on field [1] and laboratory investigations, we detailthe mechanisms of uranium redistribution from U-phosphate minerals in a heavily contaminated wetland from Brittany,France.Using U LIII-edge (micro-) X-ray absorption spectroscopy, electron microscopy and geochemical analyses, we show that uranium released by the oxidative dissolution of U(IV)-phosphate minerals, especially ningyoite CaU(PO4)2·2H2O, is rapidly converted to organicbound mononuclear U(VI) species. These latter can be then reduced to organic-bound U(IV) species under watersaturated conditions [1]. Moreover, oxic and anoxic incubations of soil samples reveal that specific U-phosphate minerals, autunite Ca(UO2)2(PO4)2·11H2O and lermontovite U(PO4)(OH)·H2O, are the most resistant uranium species to redox cycling occurring in the studied soils.Altogether, the results of this study bring important informations to assess the long-term stability of uranium inseasonally saturated organic-rich mining-impacted environments.[1] Stetten et al. (2018) Environ. Sci. Technol. 52 (22), 13099–1310

Redox Cycling of Uranium Phosphate Minerals in a Mining-Contaminated Wetland

International audienceReduction of uranium (VI) to low soluble uranium (IV) species in wetlands is expected to limit the transfers of this toxic element to downstream waterways. However, in such environments, long-term uranium (U) scavenging may beperturbed by hydrological and redox fluctuations. Here, based on field [1] and laboratory investigations, we detailthe mechanisms of uranium redistribution from U-phosphate minerals in a heavily contaminated wetland from Brittany,France.Using U LIII-edge (micro-) X-ray absorption spectroscopy, electron microscopy and geochemical analyses, we show that uranium released by the oxidative dissolution of U(IV)-phosphate minerals, especially ningyoite CaU(PO4)2·2H2O, is rapidly converted to organicbound mononuclear U(VI) species. These latter can be then reduced to organic-bound U(IV) species under watersaturated conditions [1]. Moreover, oxic and anoxic incubations of soil samples reveal that specific U-phosphate minerals, autunite Ca(UO2)2(PO4)2·11H2O and lermontovite U(PO4)(OH)·H2O, are the most resistant uranium species to redox cycling occurring in the studied soils.Altogether, the results of this study bring important informations to assess the long-term stability of uranium inseasonally saturated organic-rich mining-impacted environments.[1] Stetten et al. (2018) Environ. Sci. Technol. 52 (22), 13099–1310

Platinum Nanoparticle Extraction, Quantification, and Characterization in Sediments by Single-Particle Inductively Coupled Plasma Time-of-Flight Mass Spectrometry

Particulate emissions from vehicle exhaust catalysts are the primary contributors to platinum group elements (PGEs) being released into roadside environments, especially platinum (Pt) particles. With increasing traffic density, it is essential to quantify the emission, accumulation, and potential health effects of traffic-emitted Pt particles. In this study, three procedures were investigated to extract Pt nanoparticles (NPs) from sediments and characterize them by single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOF-MS). For this purpose, a reference sediment sample was spiked with manufactured Pt NPs. Pt NPs’ extraction recoveries reached from 50% up to 102%, depending on the extraction procedure and whether the particle mass or number was used as the metric. Between 17% and 35% of the Pt NPs were found as unassociated Pt NPs and between 31% and 78% as Pt NPs hetero-aggregated with other sediment particles. Multi-elemental analysis of Pt-containing NPs in the pristine sediment revealed frequently co-occurring elements such as Au, Bi, and Ir, which can be used to determine a natural background baseline. Our results demonstrated that spICP-TOF-MS elemental characterization allows for distinguishing anthropogenic Pt NPs from the natural background. In the future, this could enable the sensitive monitoring of PGE release from anthropogenic sources such as vehicle exhausts

Towards Standardization for Determining Dissolution Kinetics of Nanomaterials in Natural Aquatic Environments: Continuous Flow Dissolution of Ag Nanoparticles

The dissolution of metal-based engineered nanomaterials (ENMs) in aquatic environments is an important mechanism governing the release of toxic dissolved metals. For the registration of ENMs at regulatory bodies such as REACH, their dissolution behavior must therefore be assessed using standardized experimental approaches. To date, there are no standardized procedures for dissolution testing of ENMs in environmentally relevant aquatic media, and the Organisation for Economic Co-operation and Development (OECD) strongly encourages their development into test guidelines. According to a survey of surface water hydrochemistry, we propose to use media with low concentrations of Ca2+ and Mg2+ for a better simulation of the ionic background of surface waters, at pH values representing acidic (5 < pH < 6) and near-neutral/alkaline (7 < pH < 8) waters. We evaluated a continuous flow setup adapted to expose small amounts of ENMs to aqueous media, to mimic ENMs in surface waters. For this purpose, silver nanoparticles (Ag NPs) were used as model for soluble metal-bearing ENMs. Ag NPs were deposited onto a 10 kg.mol−1 membrane through the injection of 500 µL of a 5 mg.L−1 or 20 mg.L−1 Ag NP dispersion, in order to expose only a few micrograms of Ag NPs to the aqueous media. The dissolution rate of Ag NPs in 10 mM NaNO3 was more than two times higher for ~2 µg compared with ~8 µg of Ag NPs deposited onto the membrane, emphasizing the importance of evaluating the dissolution of ENMs at low concentrations in order to keep a realistic scenario. Dissolution rates of Ag NPs in artificial waters (2 mM Ca(NO3)2, 0.5 mM MgSO4, 0–5 mM NaHCO3) were also determined, proving the feasibility of the test using environmentally relevant media. In view of the current lack of harmonized methods, this work encourages the standardization of continuous flow dissolution methods toward OECD guidelines focused on natural aquatic environments, for systematic comparisons of nanomaterials and adapted risk assessments

HERFD-XANES spectroscopy at the U M4-edge applied to the analysis of U oxidationstate in a heavily contaminated wetland soil

International audienceDetermining U oxidation state in contaminated (sub)surface soils and sediments is essential to depict the geochemical processes affecting U in natural media. This information is also mandatory to infer the mechanisms governing the mobilization and transfer of this toxic radionuclide to the environment. Here, in attempt to detect U(IV), U(V) and U(VI) in wetland soil samples contaminated by past mining activities, we have performed high-resolution fluorescence detected X-Ray absorption near edge structure (HERFD-XANES) measurements at the U M4-edge. Linear combination fitting (LCF) analysis of the spectra have been conducted using reference samples representative of the wetland geochemistry, in which U occurs as U-phosphate minerals and mononuclear U complexes. Our experimental constraints for HERFD measurements at low energy (3.7 keV) implied to limit the thickness of the Kapton® foil used to protect the samples, which lead to slow oxidation by air during the measurements. In this context, U(IV) appeared to partly oxidize into U(VI) and/or U(V) within a few tens of hours. Nano-crystalline reference samples showed contrasted oxidation pathways for U(IV), transforming into U(V)/U(VI)-uranate in biogenic nano-uraninite, and into U(VI)-uranyl in nano-U(IV)-rhabdophane. In the wetland soils samples, uranium was mainly present as U(IV) and U(VI) with detection32 of minor U(V) (< 13 % of total U), possibly pristine and/or resulting from oxidation during the measurements. Our results thus show that U(V) may result from oxidation of mononuclear or nano-crystalline U(IV) after moderate air exposure, which challenges unambiguous detection of U(V) in environmental samples and calls for further U M4-edge HERFD-XANES measurements under strict anoxia

Early diagenesis in the Congo deep-sea fan sediments dominated by massive terrigenous deposits: Part I – Oxygen consumption and organic carbon mineralization using a micro-electrode approach

International audienc