Speciation and reactivity of uranium products formed during in situ bioremediation in a shallow alluvial aquifer

In this study, we report the results of in situ U(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U(VI), and acetate to stimulate the growth of indigenous microorganisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy and chemical extractions. Circa 90% of the total uranium was reduced to U(IV) in each reactor. Noncrystalline U(IV) comprised about two-thirds of the U(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U(IV) species are more suceptible to remobilization and reoxiation than crystalline U(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U(IV) formation across a wide range of aquifer parameters when designing in situ remediation plans

Development of a flux DMT with integrated adsorptive stripping voltammetry for free metal ions detection in solution

Development of a flux DMT with integrated adsorptive stripping voltammetry for free metal ions detection in solution. Interfaces Against Pollutio

Rare earth elements binding to organic matter: an experimental and modelling study

International audienceThe biogeochemical cycle of Rare Earth Elements (REEs) is being affected by their increased use in modern technologies an increased concentrations of certain REEs have been measured in natural waters. Presently there are no regulatory thresholds for REE concentrations and emissions into the environment and knowledge on their environmental behaviour and effects is limited.The fate and behaviour of REEs in the environment is determined by their binding to reactive components such as organic matter, metal oxides, and clays. All of them are considered of key important. However, humic substances (HS, humic and fulvic acids) are the most reactive fractions of natural organic matter, having a high affinity for metal ions. Thus, the use of thermodynamic models which could predict the partitioning and speciation of trace metals in terrestrial and aquatic systems is of key importance. Here, we use the NICA-Donnan (ND) model which is one of the advanced models for ion binding to HS. Nevertheless, ND parameters for REEs binding to HS (except Eu) are still lacking.Here we studied Nd, Gd and Yb binding to a groundwater FA in batch titration experiment in the pH range 3-7. Free metal ion concentrations were determined using the Donnan Membrane Technique, being the first time this technique was used for REEs. Using the experimental data for FA together with published data on REE binding to HA we derived generic ND parameters for all the 14 stable lanthanides. The new model parameters were evaluated with published data

Studying rare earth elements binding to natural organic matter

International audienceStudying rare earth elements binding to natural organic matter. 29. Annual Goldschmidt Conferenc

Free Eu3+ determination in natural waters

International audienceThe aim of this study was to develop an analytical method to determine free concentration of the rare earth element Europium in natural waters. Detection of Eu in solution using electroanalytical methods was performed by adsorptive cation stripping voltammetry after complexation with N-nitroso-N-phenylhydroxylamine (cupferron). Optimization of analytical parameters allowed us to detect nanomolar level of Eu(III) in solution.To detect the amount of free Eu3+ in solution, we use the Donnan Membrane Technique (DMT) in which a natural solution (the “donor” side) is separated from a ligand-free (“acceptor”) solution by a cation-exchange membrane. This membrane allows only non-complexed cations to pass through it, and after a given equilibration time free metal concentration are equal in both sides. However, due to its high charge Eu3+ tends to get adsorbed onto the cation-exchange membrane. We analyzed solutions from different Eu3+ and Ca2+ (as background-ion) concentrations to determine physico-chemical conditions for which the method can be used. Results showed that 100 mM of Ca2+ prevented any adsorption of Eu from the solution. The optimized set-up was then used to determine Eu complexation in a Eu-fulvic acid solution

Solid-solution partitioning of Rare Earth Elements in mine-tailings and soils in China: experimental results and multi-surface modelling

International audienceSolid-solution partitioning of Rare Earth Elements in mine-tailings and soils in China: experimental results and multi-surface modelling. Interfaces Against Pollutio

The influence of organic complexation on Ni isotopic fractionation and Ni recycling in the upper soil layers

International audienceThe quantification of Ni isotopic fractionation induced by Ni binding to organic acids is a preliminary step to better constrain the mechanisms determining Ni isotopic fingerprint observed in surface soils, waters and plants, as well as the contribution of metal recycling during plant litter degradation. In this study, Ni isotopic fraction induced by reaction with small organic acids, e.g. citric and oxalic acids, and with soil purified humic acids (PHA) was investigated at different Ni-L ratio and pH conditions. The Donnan Membrane Technique was used to separate Ni bound to organic ligands from the free metal. Obtained results highlighted that Ni binding with carboxylic groups produces, in the adopted experimental conditions, a Δ60Nibond-free < 0.2‰. This value is not high enough to justify neither metal fractionation previously observed between soil and hyperaccumulators, nor the fractionation between different plant parts, e.g. roots and leaves. In parallel, leaf degradation experiments of two hyperaccumulating plants, where Ni is mainly present as Ni-citrate, were performed to simulate litter decomposition and to highlight the contribution of plants on Ni isotopic composition in surface soils and waters. In the case of the hyperaccumulator Alyssum murale, the degradation process did not induce any observable fractionation. On the contrary, during Rinorea bengalensis degradation experiment, a fractionation between Ni leached out in the first 10 days and between 10 and 30 days was observed (Δ60Ni10–30day = 0.20 ± 0.05‰). The observed fractionation evidenced a heterogeneous distribution of Ni within the leaves, and/or distinct chemical bonding to the leaf cells, and finally suggested the influence of the chemical bonding on Ni isotopic signature. Although a precise quantification of plant contribution on Ni isotopic signature in surface soils and waters is still not reached, our results produced important progress to elucidate the role of organic matter in regulating Ni isotopic fingerprint in surface layers

ESPECIAÇÃO TERMODINÂMICA DE METAIS TRAÇO COM SUBSTÂNCIAS HÚMICAS: O MODELO NICA-DONNAN

This work presents a critical overview of the NICA-Donnan model for trace metal thermodynamic speciation with humic substances obtained from natural organic matter in soils, natural waters and anthropogenic sources. A first part covers the theoretical aspects of the model, how to obtain good experimental data and perform the modelling. A second part deals with the developments in the last decade namely the use of the generic parameters, criticism and alternatives to the electrostatic modeling, the understanding of the NICA equation and an analysis of the published data, ending with the needs and opportunities for model development

Molecular controls over uranium biogeochemistry in the upper Colorado River Basin: A regional perspective

International audienceOrganic-rich anoxic sediments at DOE’s Rifle, CO site contain relatively high concentrations of uranium. Based on sediment characteristics, we speculated that these ‘naturally reduced zones’ (NRZs) are common and accumulate uranium at similar contaminated sites across the upper Colorado River Basin (CRB). To test this hypothesis, we sampled NRZs at 4 additional sites along a 700 km north-south transect of the upper CRB: Grand Junction and Naturita, CO; Shiprock, NM; and Riverton, WY. This work confirmed our hypothesis and showed that NRZs are also important reservoirs for nutrients and biogeochemical critical elements (BCE), including C, N, S, and Fe. Sulfate-reducing conditions are required for uranium accumulation, suggesting at least a strong indirect control of sulfide on U(VI) reduction. Indeed, the nominal oxidation state of water-soluble soil organic carbon was found to be correlated to sediment sulfide concentration, suggesting that sulfide plays a major role in poising the redox conditions of NRZs regionally.To better understand molecular controls over uranium behavior in NRZs, we performed controlled microcosm experiments designed to mimic sulfate-reducing conditions in NRZs and to more clearly define the potential roles of organic functional groups as uranium binding sites. U(IV) was found to be dominantly associated with surfaces of particulate organic carbon and to exhibit local molecular structure consistent with sorbed complexes. An important implication of this finding is that U(IV) will be readily mobilized in the presence of complexing agents and oxidants. Such conditions are common in NRZ sediments, which experience large variations in saturation state and redox conditions throughout the annual winter (base flow) / summer (meltwater discharge) cycle within the upper CRB.In ensemble, these studies suggest new conceptual and process models for uranium and BCE biogeochemical behavior that are unprecedented in their regional scale, detail, and awareness of variable hydrologic conditions. By studying anoxic sediment systems from a regional perspective, we have gained important insights into the range and intensity of processes that are likely to be important at individual sites