Search

International audience; Although the use of iron oxide nanoparticles (IONPs) has high potential in remediation and agriculture, a major hindrance to their use includes the risk of contamination of soil and water resources with underexplored effects of IONPs on biota. The fate, phytotoxicity and remediation potential of IONPs are investigated with soil column experiments using 7 nm-sized magnetite (Fe 3 O 4) nanoparticles (magnNPs) and sunflower (Helianthus annuus). Control soil, magnNP-containing soil (10 g magnNPs per kg soil), copper-polluted soil (500 mg Cu per kg soil) and copper-polluted soil containing magnNPs (10 g magnNPs per kg soil and 500 mg Cu per kg soil) support sunflower growth for 57 and 95 days. In magnNP-exposed plants, the occurrence of magnNPs does not affect the growth of the vegetative aerial parts and photosynthetic efficiency. Decreased lipid peroxidation indicates an enhanced antioxidant enzymatic response of magnNP-exposed plants. In plants grown in Cu-and magnNP-Cu-soils, the physiological and biochemical impacts of excess copper are clearly identified, resulting in growth retardation, decreased pigment contents and photosynthetic efficiency, and increased lipid peroxidation and peroxidase (POD) activities. Based on magnetic susceptibility, a higher amount of magnNPs is detected after 57 days in the roots of magnNP-exposed plants (1400 mg kg À1) than in the roots of magnNP-Cu-exposed plants (920 mg kg À1). In the latter, magnNP internalization is likely hampered because of the plants' physiological responses to Cu toxicity. At the working Cu and magnNP concentrations, magnNPs neither decrease Cu accumulation in the plant tissues nor alleviate the overall growth retardation of sunflowers and certain phytotoxic effects induced by excess Cu. However, this study highlights several positive environmental aspects relative to magnNP use, including the harmless effects of magnNPs on sunflowers (1% magnNPs in soil) and the ability of magnNPs to influence Cu mobility in the soil (which could be even more pronounced at lower Cu concentration).

International audience; Magnetite (Fe(III)2Fe(II)O4) nanoparticles are fascinating nanoparticulate minerals for their electronic, magnetic and chemical properties. Ubiquitous, in the environment, they are also among the most used ferromagnetic nanomaterials in environmental, industrial and biomedical applications. Their intriguing structural and reactivity features do not only arise from the “nano-effect” but also, from the occurrence of Fe2+ ions in their structure. Previous studies showed that partial oxidation of (nano)magnetite may occur. However, such transformations were only monitored under either oxidizing or very acidic conditions. Here, we report that 10 nm-sized stoichiometric magnetite particles (Fe(II)/Fe(III) = 0.5) are in fact not stable in aqueous solutions over a biologically- and environmentally- relevant pH range (4-7). In the absence of O2, an H+-promoted dissolution process is responsible for the preferential release of Fe(II) into solution, which leads to partial oxidation of magnetite to a magnetite-maghemite solid-solution. Long-term kinetic investigations combined with XMCD measurements reveal that the dynamic exchange of Fe(II) between the surface and the solution is key in determining the magnetite stoichiometry even at circumneutral pH. Based on this finding, we developed a thermodynamic model for the magnetite-maghemite solid-solution able to predict the chemical stability of the 10 nm-sized magnetite. This model enables to rationalize and predict the behavior and transformation of magnetite nanoparticles in aqueous solutions, which is crucial for a broad range of applications (medicine, biology, chemistry, environment, etc.).

International audience; The nanoplastics presence in the ocean and soil demonstrates their global distribution in the environment. Due to their colloidal properties, nanoplastics could influence trace metal speciation in natural matrices, which has been poorly addressed due to the lack of methodology to assess the nanoplastics -metal association in the natural environment. To give a global picture of this property for nanoplastics, this work aims at using rare earth elements (REE) as a chemical probe to understand metal adsorption mechanisms that could occur onto nanoplastics surfaces, which is expected to be highly oxidized in the environment. Comparison between environmentally relevant nanoplastics’ models and REE-COOH complexes patterns demonstrated that REE are adsorbed through the COOH sites onto the nanoplastics. These adsorptions pathways follow a mono-ligand process for light REE and bi-ligand for heavy REE complexes, chelate being excluded. We demonstrated that the denticity of the complexes is influenced by the pH, REE loading, and competitive ions. Bi-ligand complexes with heavy REE dominate at pH>4.5 and low REE loading. This study is a novel approach in understanding NPs-metals interaction and highlights REE as a powerful tracer of sorption mechanisms.

International audience; Potentiometric acid–base titration curves collected on humic (nano)particles as a function of pH and salt concentration reflect the electrostatics of the particles and the amount of chemical charges (Q) they carry. In turn, the interpretation of titration data helps quantify their reactivity toward metals provided that both intrinsic chemical and nonspecific electrostatic contributions to proton binding are correctly unraveled. Establishing a titration curve requires several steps, i.e., blank subtraction, relative curve positioning with respect to the electrolyte concentration, and absolute curve positioning achieved by the estimation of particle charge Q0 at low pH. Failure to properly establish each step may lead to the misevaluation of nanoparticle charging behavior. Here, we report (i) a simple procedure to measure and position titration curves for humic substances (HS) versus salt concentration and (ii) an original approach for absolute curve positioning upon the exploitation of proton affinity spectra. The latter do not depend on Q0 and they thus constrain the titration data analysis using the soft Poisson–Boltzmann-based titration (SPBT) formalism for nanoparticles in the thick electric double-layer regime. We illustrate the benefits of our approach by analyzing titration measurements for a large range of humic nanoparticles and by comparing the outcome with results from the literature.

International audience; The nanoscale size of plastic debris makes them potential efficient vectors of many pollutants and more especially of metals. In order to evaluate this ability, nanoplastics were produced from microplastics collected on a beach exposed to the North Atlantic Gyre. The nanoplastics were characterized using multi-dimensional methods: asymmetrical flow field flow fractionation and dynamic light scattering coupled to several detectors. Lead (II) adsorption kinetics, isotherm and pH-edge were then carried out. The sorption reached a steady state after around 200 min. The maximum sorption capacity varied between 97% and 78.5% for both tested Pb concentrations. Lead (II) adsorption kinetics is controlled by chemical reactions with the nanoplastics surface and to a lesser extent by intraparticle diffusion. Adsorption isotherm modeling using Freundlich model demonstrated that NPG are strong adsorbents equivalent to hydrous ferric oxides such as ferrihydrite (log against 11.76 for NPG and ferrihydrite, respectively). The adsorption is dependent upon pH, in response to the Pb(II) adsorption by the oxygenated binding sites developed on account of the surface UV oxidation under environmental conditions. They could be able to compete with Fe or humic colloids for Pb binding regards to their amount and specific areas. Nanoplastics could therefore be efficient vectors of Pb and probably of many other metals as well in the environment.

International audience; Colloids have been recognized as key vectors of pollutants in aqueous environment. Amongst them, those formed by iron (Fe) and organic matter (OM) are of major importance due to their ubiquity in the surface environment and strong affinity for metals. In the recent years, Fe stable isotopes have been increasingly used to elucidate the sources and biogeochemical cycling of Fe in Earth's surface environments. In this study, we aim to elucidate (i) the possible Fe isotopic signature resulting from the Fe/OM colloid formation and (ii) the mechanisms involved in the development of such isotopic signature. For this purpose, Fe-OM associations were synthesized through binding and titration experiments. Various pH levels were used in order to study the isotope behavior of Fe occurring as free species at pH 1, as Fe-OM complexes at pH 2 and as mixed Fe-oxyhydroxide/OM nanoaggregates or particles at pH 6.5. Organic matter-free, Fe-free and OM membrane-deposition experiments were also performed. These suspensions were (ultra)filtered at 0.2 µm, 30 kDa and 5 kDa to evidence the possible Fe isotope fractionation between fractions. This protocol allowed also testing the potential of (ultra)filtration techniques to generate isotope fractionation. The results provided evidence that abiotic Fe precipitation, (ultra)filtration techniques and OM deposition were not able to produce significant Fe isotope fractionation under the experimental conditions. However, at circum-neutral pH, the Fe-OM binding and titration experiments displayed a significant enrichment of heavy Fe isotopes in the < 30 kDa fractions relative to the total Fe pool δ56Fe = 0.35 ± 0.05‰ and 0.26 ± 0.05‰ (95% confidence interval, 2σ and relative to international standard IRMM-14), respectively. Mass balance and error propagation calculation showed Fe isotope fractionation in binding and titration experiments between the > 30 kDa and < 30 kDa fractions for -0.35 ± 0.05‰ and -0.27 ± 0.05‰, respectively. This Fe isotope fractionation could be due to the complexation of Fe by OM in the < 30 kDa fractions. At pH 2, the OM-free experiment, the < 30 kDa fraction showed Fe isotope ratio δ56Fe = 0.75 ± 0.03‰ with an enrichment in heavy Fe isotopes of δ56Fe’ = 0.14 ± 0.04‰ relative the total Fe pool (δ56Fe’ is δ56Fe value which was corrected by δ56Fe of total fraction). This enrichment in heavy Fe isotopes induced an isotopic fractionation factor of -0.87 ± 0.26‰ between the > 30 kDa and < 30 kDa fractions produced by the complexation between the heavy Fe isotopes and OH- ligands in the < 30 kDa fraction. Natural Fe-OM associations were further investigated through oxidation experiments of a reduced wetland soil solution. The oxidized soil solution was (ultra)filtered at 5 µm, 3 µm, 0.2 µm, 30 kDa and 5 kDa. The highest δ56Fe was obtained in the smallest size fraction, i.e. < 5 kDa fraction, yielding a negative isotopic fractionation Δ56Fe >5kDa

International audience; UV-vis spectrophotometric acid-base titration can characterize dissolved organic matter (DOM) acid-base properties. However, it requires incremental pH adjustment, which make the procedure time consuming and results are subjected to dilution effect. This study brings forth a new approach, referred as the “buffer method” for pH adjustments by using carefully selective pH-buffers to adjust the pH. This, statistically validated method minimizes the pH adjustment time and lightens the laboratory work load. Chemical product cost associated with this novel method is slightly increased as compared to the previous approach, due to the necessity to use pH-buffers.• Buffer method: Acid-base titration by using buffer for pH adjustment• Buffer method validated by statistical means• Rapid, reliable and economical method

International audience; Rare Earth Elements (REE) are emergent anthropogenic contaminants. Currently, there are knowledge gaps in assessing the environmental risks posed by these contaminants. High demand for modern technology has led to increased REE use, resulting in elevated REE concentrations in the environment. Therefore, demonstrating a need to understand the behavior, fate, and cycling of REEs as emergent pollutants.The physical and chemical speciation of REE is (i) reflective of their origin and the encountered processes within the environment and (ii) potentially influences their availability and toxicity. To date, very few publications have focused on how the physical and chemical speciation of REE controls phyto-availability. Thus, an evaluation of the spontaneous vegetation at three abandoned mining sites in Portugal was carried out in November 2020 (Figure 1). Spontaneous vegetation and related media (soils and surface waters) were collected from inside and outside of mining areas with the intention of analyzing REE dynamics in rhizosphere soils and subsequent REE dissemination in plants. Trace element concentrations were analyzed in soil and plant samples using inductively coupled plasma mass spectroscopy (ICP-MS). Measurements of key parameters (pH, Eh, and major anions) and determination of dissolved organic carbon and trace element concentrations were performed in water samples by multi-meter, ion chromatography, total carbon-analyzer, and ICP-MS, respectively. Measurements were then used to decipher how the chemical compositions of waters and soils can be linked to that of plants and, more precisely, to the fate of REE in planted environments. Contrasting REE concentrations and patterns were observed in water samples regarding the prevailing environmental conditions.In addition, the complex biogeochemical interactions in rhizospheres potentially alter the speciation and phyto-availability of REE. Therefore, rhizosphere soil samples were paired with bulk soil samples to elucidate the impact of plant and microorganism bioweathering processes on REE speciation.Ultimately, this field study aims to evaluate the environmental risk posed by anthropogenic REE contamination and contribute to a body of knowledge on the circular economy of REE. Further study of plant REE uptake mechanisms and toxicological responses to REE contamination is planned for future laboratory experiments.

International audience

International audience; The Okavango Delta in northern Botswana is an endorheic ecosystem formed by 150,000 islands surrounded by a flood plain. The vegetation of these islands displays an external tree ring enclosing a barren interior domain with evaporitic deposits. We sampled the groundwater into piezometers across an island at the southwest of the Delta, and drilled sediments. The geochemical analysis of the water exhibits two different compositions: a fresh water in the floodplain and a saline water in the interior part. These inner samples display enrichments in heavy metals, metalloids and critical metals such as, Rare Earth Elements (REE). The latest show a continuous enrichment from LREE to HREE and positive Ce and Eu anomalies, associated to alkaline pH. Overall, the two types of displayed REE patterns fingerprint two different water sources. The geochemistry of sediments indicates the same composition whatever the sample location and no traces of evaporitic deposits was found. The variations in the water table depth in the piezometers shows different hydrological behaviors in between the floodplain and the barren zone which does not follow the flood cycle. Thus, we document two different aquifers with independent hydrological behaviors: a surface fresh‐water aquifer in the floodplain connected to the channels, and a salty confined aquifer below the islands. The contrasted geochemical composition of the waters explains the distribution of the vegetation which survives through access to fresh water. The hydrological architecture of this island in the southwestern Delta differs from other islands, highlighting the hydrological complexity of the Okavango Delta.

International audience; Engineered oxide nanoparticles have appeared as a highly promising tool for soil remediation. However, their behaviour under complex environmental exposures (i.e. in natural soils) remains poorly understood and a relevant issue to complete their risk assessments. Prior using iron oxide nanoparticles in soil remediation scenarios, it is crucial to evaluate their possible role on the release of natural organic matter (NOM) and trace elements (TEs) as a vector of trace elements (TEs) as it could contribute to the contamination of aquifers. This work presents leaching experiments demonstrating the effects of magnetite (i.e. nFe3O4) on TEs and natural organic matter (NOM) transport within an organo-mineral horizon from a natural wetland. nFe3O4 (~10nm) with two different surface compositions (i.e. uncoated and dimercapto-succinic acid (DMSA) coating) were used in the experiments and both forms impacted the mobility of the NOM within the organo-mineral horizon. The mobility of the coated nFe3O4 within the horizon was higher than the uncoated nFe3O4 and impacted differently the mobility of TEs and NOM. TEs mobility within the horizon increased in presence of uncoated nFe3O4 with a transfer of elements into the colloidal fraction. This distinctive transfer might be explained by interactions between the surface of nanoparticles with NOM. We also suggest that some of the different behaviours observed might be due to the destabilization of the DMSA coating, highlighting the critical importance of understanding the aging processes of nanoparticles under environmental exposures to assess the risks associated with their use in environmental situations.

International audience; The 20.000 km2 swamp of the Delta is organized into islands, flood plains and permanent and seasonal channels. Most of these islands display a surprising vegetation distribution composed of tree rings surrounding the islands and limiting an inner domain with scarce vegetation. Whereas the hydrology of the Okanvango wetlands is governed by a series of drivers such as, sedimentation, climate, tectonic and biological processes, the potential of the use of multi-chemical tracing has not been so far much investigated. The conducted study as part of a multidisciplinary project dedicated to the understanding of the functioning of the Delta, involved water samples collected both upstream and downsream the river, close to one of these islands and also recovered within the island as well. The main objective of this geochemical investigation was to better constrain the interactions prevailing in between these islands and the water chemical record. pH, conductivity (C), dissolved organic and inorganic carbon (DOC & DIC) concentrations were measured as well as those of major anion and cation and trace cation concentrations as well. Whichever the tracers are considered, two contrasted groups of samples were evidenced depending on their sampling positioning regarding the island. The samples recovered only within the island displayed pH around and over 9 and higher conductivities, whereas the other showed lower circumneutral pH values and conductivities as well. The high conductivities of the water samples fom the island also correspond to the highest DOC and DIC concentrations. The strong relationship linking the high DIC values and the high pH in the island samples records probably alkaline CO32- et HCO3- -rich waters resulting from water-rock interactions with carbonates. The marked DOC enrichment has mostly to be related to microbial or photo-degradation of plant-derived organic matter and/or hydrological condition variations promoting DOC release. Significant, Cl-, SO42-, NO2-enrichments as well as major cation ones were also evidenced in the same group of samples within the island. However, the most surprising results are sourced in the trace element fingerprinting. This latter includes huge enrichment in heavy, critical metals and metalloids as well (e.g. Cr, Pb, V, REE, U, Th or As). Beyond the only marked REE-spike, Upper Continental Crust-normalized REE patterns displayed markedly contrasted shapes exhibiting two types of waters with circumneutral pH ones with MREE-enrichment, whereas the alkaline waters evidenced a classical continuous enrichment throughout the whole series from LREE to HREE and a positive Ce anomaly. The use of such multi-tracing allowed an efficient fingerprinting of two distint types of waters to get clues to further constrain both the dynamics of such islands and the functioning of the water system. Still in progress, the study will be completed by (i) the stable isotope analysis, (ii) the modeling of the minerals possibly at equilibrium with the waters and of the organic matter-trace element interactions, (iii) the speciation analysis of some enriched elements, (iv) the comparison between water and solid samples analyses and (v) the understanding of the relations in between the concentrations and locations in the hydrological system.

The 20.000 km2 swamp of the Delta is organized into islands, flood plains and permanent and seasonal channels. Most of these islands display a surprising vegetation distribution composed of tree rings surrounding the islands and limiting an inner domain with scarce vegetation. Whereas the hydrology of the Okanvango wetlands is governed by a series of drivers such as, sedimentation, climate, tectonic and biological processes, the potential of the use of multi-chemical tracing has not been so far much investigated. The conducted study as part of a multidisciplinary project dedicated to the understanding of the functioning of the Delta, involved water samples collected both upstream and downsream the river, close to one of these islands and also recovered within the island as well. The main objective of this geochemical investigation was to better constrain the interactions prevailing in between these islands and the water chemical record. pH, conductivity (C), dissolved organic and inorganic carbon (DOC & DIC) concentrations were measured as well as those of major anion and cation and trace cation concentrations as well. Whichever the tracers are considered, two contrasted groups of samples were evidenced depending on their sampling positioning regarding the island. The samples recovered only within the island displayed pH around and over 9 and higher conductivities, whereas the other showed lower circumneutral pH values and conductivities as well. The high conductivities of the water samples fom the island also correspond to the highest DOC and DIC concentrations. The strong relationship linking the high DIC values and the high pH in the island samples records probably alkaline CO32- et HCO3- -rich waters resulting from water-rock interactions with carbonates. The marked DOC enrichment has mostly to be related to microbial or photo-degradation of plant-derived organic matter and/or hydrological condition variations promoting DOC release. Significant, Cl-, SO42-, NO2-enrichments as well as major cation ones were also evidenced in the same group of samples within the island. However, the most surprising results are sourced in the trace element fingerprinting. This latter includes huge enrichment in heavy, critical metals and metalloids as well (e.g. Cr, Pb, V, REE, U, Th or As). Beyond the only marked REE-spike, Upper Continental Crust-normalized REE patterns displayed markedly contrasted shapes exhibiting two types of waters with circumneutral pH ones with MREE-enrichment, whereas the alkaline waters evidenced a classical continuous enrichment throughout the whole series from LREE to HREE and a positive Ce anomaly. The use of such multi-tracing allowed an efficient fingerprinting of two distint types of waters to get clues to further constrain both the dynamics of such islands and the functioning of the water system. Still in progress, the study will be completed by (i) the stable isotope analysis, (ii) the modeling of the minerals possibly at equilibrium with the waters and of the organic matter-trace element interactions, (iii) the speciation analysis of some enriched elements, (iv) the comparison between water and solid samples analyses and (v) the understanding of the relations in between the concentrations and locations in the hydrological system.

International audience; The occurrence of nanoplastics in the environment is now known and presents new threats linked to plastic debris issues. New questions about the formation pathways and life cycle assessment of nanoplastics in continental and marine environments have been raised. In the present study, we focused on the Guadeloupe island coastal system. For the first time, we demonstrated the presence of nanoplastics in sand water extracts (SWEs). We also investigated the potential of rare earth elements (REE) in tracing nanoplastics. Based on the REE patterns of SWEs, beached plastics and modern corals, we demonstrated that nanoplastics are important components in the REE signature of coastal sand (29 to 73%). These original results demonstrate the relevance of developing geochemical tracers for determining the fate of missing plastic litter.

Environmental contextThe origin of organic matter at Earth’s continental surface can be either terrestrial or microbial, and its precise composition can influence its reactivity towards metals. We investigated the potential of rare earth elements to fingerprint the origin of various organic matters through their reactivity and composition. The rare earth element patterns can be useful tools to determine the reactivity and also pristine source of natural organic matter. AbstractRare earth elements (REEs) have been shown to be efficient tracers of the functional sites and/or complexes formed on humic molecules. In the present study, we test the potential of REEs to be used as tracers of the sources of humic substances (HSs). Three types of organic matter (OM) of terrestrial and microbiological origin were tested. The experiments of REEs binding to the HSs were combined with size-fractionation experiments. The REE patterns were the most fractionated in the

International audience; The evolution of rare earth element (REE) speciation between reducing and oxidizing conditions in a riparian wetland soil was studied relative to the size fractionation of the solution. In all size fractions obtained from the reduced and oxidized soil solutions, the following analyses were carried out: organic matter (OM) characterization, transmission electron microscopy (TEM) observations as well as major and trace element analyses. Significant REE redistribution and speciation evolution between the various size fractions were observed. Under reducing conditions, the REEs were bound to colloidal and dissolved OM (2 μm size fraction), colloidal (

International audience; Understanding the processes involved in the control of arsenic (As) dynamics within soils has become a challenging issue for soil and water quality preservation. Interactions between mineralogical phases, organic ligands and bacterial communities - closely linked to the chemical conditions of the medium - were thus investigated through a geochemical and microbiological experimental study involving the reduction of As(V)-doped lepidocrocite within the soil. Reducing conditions were established as soon as the experiment started, followed by a release of dissolved organic carbon corresponding to a release of acetate. Scanning Electron Microscopy observations pointed out a large bacterial colonization occurring on the lepidocrocite leading to a 3-dimensionally shaped biodissolution of lepidocrocite. The taxonomic diversity evolved throughout the experiment, and thus it demonstrated the evolution of the metabolic activities of the bacteria. At the beginning, lepidocrocite was mainly colonized by bacteria belonging to the Geobacter genus (Deltaproteobacteria) (26%) and Bacillus and Oxalophagus fermentative related genera (Firmicutes) (72%). After two weeks, Geobacter spp. and Firmicutes represented 54% and 30% of the bacterial community, respectively. Although still dominated by Geobacter spp. (34%) at the end of the experiment, the bacterial diversity had increased. After 3 and 8 weeks of incubation, the presence of the arsB and ACR3(1) genes, encoding transporters involved in As detoxification processes, indicated that this community harbored As-resistant or As-transforming genera able to contribute to the transformation of As(V) into As(III).

International audience; Nanoparticle (NP) stability in aqueous environments is dependent upon many parameters including environmental conditions, NP concentrations as well as NP intrinsic characteristics. In this study, the effects of pH and surface modifications on the colloidal and chemical stabilities of nanosized magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3) are investigated. Because changes in surface charge affect the size distribution of NP, pH plays a key role in driving the colloidal stability. More NP aggregation is observed at pH values close to the pH of zero point of charge (pHzpc). Coating of magnetite with humic acid (HA) and phosphatidylcholine (PC) affects the electrostatic interactions and then the colloidal behavior of NP. The rapid transformation of magnetite into maghemite through air oxidation results in significant modification of both surface charge and specific surface area of NP. Because the maghemite almost exclusively formed µm-scale aggregates, the colloidal stability of magnetite is expected to be hindered in oxic environments. For hematite, the particle size distribution data emphasize the influence of both pH and intrinsic surface properties in the colloidal stability. These findings may have strong implications for an accurate prediction of the transformation and mobility of Fe-nanoparticles under environmentally relevant conditions and thus their fate in nature.

International audience; Vanadium (chemical symbol, V) is a d-block transition metal,silver in color, appearing in the first long period of the peri-odic table between titanium and chromium. Vanadium hastwo stable isotopes: 50V and 51V, with atomic abundance of0.25 % and 99.75 %, respectively. Vanadium has several oxidation forms (between 2+ and 5+). In the lithosphere, Voccurs as reducing V(III) form, whereas in oxidizing con- ditions V prevails under V(IV) form. Vanadium(II) is partic- ularly unstable in the environment. Vanadium(III) is more stable than V(II), but it is also gradually oxidized by the air or dissolved oxygen. Vanadium(V) is expected to be the prevailing form in waters exposed to atmospheric oxygen, whereas V(IV) may be present in reducing environments. Depending upon geometry and environment, V ionic radii vary between 36 pm and 79 pm. Vanadium has a high melting point of 1910 42 C and is a mildly incompatible, refractory,lithophile (siderophile in the iron core and chondrites) ele- ment. Vanadium has an electronegativity of 1.63 on the Pau- ling scale and displays a first ionization potential of 6.74 eV. More details can be found in Richards (2006) and Haynes (2015).

International audience; Competitivemechanisms between rare earth elements (REE) and iron (Fe) for humic acid (HA) bindingwere investigated by coupling laboratory experiments and modeling calculations using PHREEQC/Model VI. This study aims, firstly, at determining the effect of Fe on REE-HA binding, in order to explain the REE pattern variability observed in natural organic-rich waters. Secondly, it has previously been shown that light and heavy REE (Land HREE) speciation with HA molecules differ with pH. Therefore, REE-HA complexation patterns have been used as a probe of Fe-HA binding mechanisms. At pH 3, i.e. pH conditions at which Fe3+ binds to HA, Fe is shown to be a strong competitor for heavy REE (HREE), suggesting that Fe3+ has a marked affinity for the few strong HA multidentate sites. At pH 6, i.e. under pH conditions atwhich hydrolyzed Fe species bind to HA, Fe appears to compete equally for every REE, thereby indicating that Fe has the samerelative affinity for carboxylic and phenolic HA sites as LREE and HREE, respectively. Fractionation of REE in organic-rich natural waters depends mainly on the coupling of two factors: (i) the total dissolved metal concentration (i.e. the HA metal loading) and (ii) the competition between REE and major cations (i.e. Fe and Al). The pH, which regulates the speciation of these competitive metals, is, therefore, indirectly the main controlling factor of REE fractionation in organic-rich waters.

International audience; The Kaluvelly watershed is a coastal area (Tamil Nadu, India) where water abstraction has resulted in a dramatic fall in the level of the water table and a piezometric depression in the most exploited aquifer, the Vanur aquifer. In addition, intensification/mechanization of agriculture may have affected the quality of recharge water. An initial hydrodynamic study showed that the Vanur aquifer is highly vulnerable to salinization due to potential seawater intrusion, and our aim was to determine the source of salinity recorded in the groundwater of this multilayered aquifer. Our approach involved the use of existing boreholes and of a moderate number of samples, with the aim of developing appropriate water resource management techniques. Major elements, 18O/16O, 2H/1H, and 87Sr/86Sr ratios were measured in rainwater, surface water and groundwater collected during five sampling campaigns over a two-year period. Geochemical data indicate that the Vanur aquifer is recharged and that small mixings between aquifers fluctuate according to monsoon intensity. There was no evidence of seawater intrusion. The range of recorded salinity originated mainly from water-rock interaction but a disconnection of some deeper parts of the aquifer was apparent. Strontium isotopic ratios in the recharge area suggest an anthropogenic influence, possibly related to fertilizer use. A high SO4/Cl ratio was observed in the aquifer; in the deeper parts, the influence of a formation containing lignite is hypothesized, whereas near the surface, sulphate may partly originate from fertilizer use and fossil fuel combustion. Water isotopic data suggest that the origin of precipitation in this region has been unchanged for several hundreds or thousands of years

International audience; Whereas humic substances are known to play a key role in controlling metal speciation and trace element mobility within soils and waters, the understanding of their structure is still unclear and remains a matter of debate. Several models of humic substance structure have been proposed, where humic substances were composed of either: (i) macromolecular polyelectrolytes that can form molecular aggregates or (ii) supramolecular assemblies (molecular aggregates) of small molecules without macromolecular character, joined together by weak attraction forces. This experimental study was designed and dedicated: (i) to follow the size of organic molecules versus ionic strength or pH by the combined means of ultrafiltration and aromaticity data and rare earth element (REE) fingerprinting, and (ii) to investigate the pH and ionic strength effect on the distribution of associated rare earth elements in soil solution. This study supports the presence of supramolecular associations of small molecules and probably the presence of macromolecules in the bulk dissolved organic matter. By contrast to ionic strength, pH appeared to be the major parameter playing on the stability of the humic substance structure. Humic substances displayed dynamic structures, which evolved with regard to pH. Low pH led to a destabilization of the humic substance conformation. This destabilization had an impact on the trace element distribution in soil solution, as assessed by REE data, and conversely, the destabilization degree of humic substances seemed to be influenced by the metal ion charge.

International audience; A 7-year monitoring period of rare earth element (REE) concentrations and REE pattern shapes was carried out in well water samples from a 450 m long transect setup in the Kervidy/Coët-Dan experimental catchment, France. The new dataset confirms systematic, topography-related REE signatures and REE concentrations variability but challenges the validity of a groundwater mixing hypothesis. Most likely, this is due to REE preferential adsorption upon mixing. However, the coupled mixing­adsorption mechanism still fails to explain the strong spatial variation in negative Ce anomaly amplitude. A third mechanismƒnamely, the input into the aquifer of REE-rich, Ce anomaly free, organic colloidsƒis required to account for this variation. Ultrafiltration results and speciation calculations made using Model VI agree with this interpretation. Indeed, the data reveal that Ce anomaly amplitude downslope decrease corresponds to REE speciation change, downhill groundwaters REE being mainly bound to organic colloids. Water table depth monitoring shows that the colloid source is located in the uppermost, organic-rich soil horizons, and that the colloid input occurs mainly when water table rises in response to rainfall events. It appears that the colloids amount that reaches groundwater increases downhill as the distance between soil organic-rich horizons and water table decreases. Topography is, therefore, the ultimate key factor that controls Ce anomaly spatial variability in these shallow groundwaters. Finally, the

International audience; The competitive binding of rare earth elements (REE) to purified humic acid (HA) and MnO2 was studied experimentally using various HA/MnO2 ratios over a range of pH (3 to 8). MnO2, humic acid and REE solutions were simultaneously mixed to investigate the kinetics of the competitive reactions. Aqueous REE­HA complex is the dominant species whatever the experiment time, pH and HA/MnO2 ratio. The value of the distribution coefficients between MnO2 and solution (log KdRee/Mno2) increases with the HA/MnO2 ratio, indicating that part of the REE­HA complexes are adsorbed onto MnO2. The development of a Ce anomaly appears strongly limited in comparison with inorganic experimental conditions. Throughout the experimental run time, for HA/MnO2 ratios of less than 0.4, MnO2 acts as a competitor leading to a partial dissociation of the REE­HA complex. The majority of the dissociated REE is readsorbed onto the MnO2 surface. The readsorption of REE is expressed by an increased Ce anomaly on the log KdRee/Mno2 pattern as well as a change in shape of the coefficient distribution of REE between soluble HA and solution pattern (log KdRee/HA decrease for the heavy rare earth elements ƒ HREE). Thus, REE are not only bound to MnO2 as a REE­HA complex, but also as REE(III). Moreover, the competition between HA and MnO2 for REE binding is shown to be higher at low pH (< 6) and low DOC/Mn ratio. This study partially confirms previous work that demonstrated the control of REE adsorption by organic matter, while shedding more light on the impact of pH as well as complexation reaction competition on long-term REE partitioning between solid surface and organic solutions. The latter point is important as regards to REE speciation under conditions typical of rock and/or mineral alteration.

International audience; Soils in riparian wetlands are periodically flooded, resulting in the establishment of reducing conditions and the solubilization of As, subsequently to the reductive dissolution of Fe(III)-oxyhydroxides. When the water level decreases, the wetlands are reoxidized. However, although the behavior of As under the reducing period is well documented, there is a lack of information regarding its behavior during the oxidizing period.In this study, we investigated As speciation in oxidation products from an initially reduced wetland soil solution recovered from the Naizin-Kervidy riparian wetland (France). The oxidation products were studied using NanoSIMS analysis and synchrotron X-ray techniques. These products were enriched in organic carbon, Fe and As compared with the soil and soil solution. The NanoSIMS analysis showed a colocalization of As and Fe but also revealed the presence of As hotspots where As was either associated with Fe or organic matter (OM). X-ray absorption spectroscopy (XAS) showed that As was sorbed to Fe(III)-oxyhydroxides. The linear combination fitting (LCF) of the As K-edge XANES revealed that As was not totally oxidized (i.e. between 65 and 100% of As(V)). Shell-by-shell fits of the As K-edge EXAFS showed that As formed binuclear edge-sharing 2E (RAs-Fe = 2.74–2.95 Å) and corner-sharing 2C (RAs-Fe = 3.28–3.43 Å) complexes with Fe.In addition to study of natural samples, oxidized reference samples were analyzed and demonstrated the role of OM on As speciation. The persistence of As(III) was explained by OM control on the As carrying phase during the oxidation, via the formation of nano-lepidocrocite and small Fe-clusters bound to OM. The small size of the Fe phase led to an increased capacity for As adsorption and an increase in 2E sites compared to 2C active sites for As(III) oxidation.

International audience; Although it has been suggested that several mechanisms can describe the direct binding ofAs(III) to organic matter (OM), more recently, the thiol functional group of humic acid (HA)was shown to be an important potential binding site for As(III). Isotherm experiments onAs(III) sorption to HAs, that have either been grafted with thiol or not, were thus conducted toinvestigate the preferential As(III) binding sites. There was a low level of binding of As(III) toHA, which was strongly dependent on the abundance of the thiols. Experimental datasetswere used to develop a new model (the modified PHREEQC-Model VI), which defines HA asa group of discrete carboxylic, phenolic and thiol sites. Protonation/deprotonation constantswere determined for each group of sites (pKA = 4.28 ± 0.03; ΔpKA = 2.13 ± 0.10; pKB = 7.11 ±0.26; ΔpKB = 3.52 ± 0.49; pKS = 5.82 ± 0.052; ΔpKS = 6.12 ± 0.12 for the carboxylic, phenolicand thiols sites, respectively) from HAs that were either grafted with thiol or not. The pKSvalue corresponds to that of single thiol-containing organic ligands. Two binding models weretested: the Mono model, which considered that As(III) is bound to the HA thiol site asmonodentate complexes, and the Tri model, which considered that As(III) is bound astridentate complexes. A simulation of the available literature datasets was used to validate2the Mono model, with log KMS = 2.91 ± 0.04, i.e. the monodentate hypothesis. This studyhighlighted the importance of thiol groups in OM reactivity and, notably, determined theAs(III) concentration bound to OM (considering that Fe is lacking or at least negligible) andwas used to develop a model that is able to determine the As(III) concentrations bound toOM.

The foundation of nanoscience is that the properties of materials change as a function of their physical dimensions, and nanotechnology exploits this premise by applying selected property modifications for a specific benefit. However, to investigate the fate and effect of the engineered nanoparticles on toxic metal (TM) mobility, the analytical limitations in a natural environment remain a critical problem to overcome. Recently, a new generation of size exclusion chromatography (SEC) columns developed with spherical silica is available for pore sizes between 5 and 400 nm, allowing the analysis of nanoparticles. In this study, these columns were applied to the analysis of metal-based nanoparticles in environmental and artificial samples. The new method allows quantitative measurements of the interactions among nanoparticles, organic matter, and metals. Moreover, because of the new nanoscale SEC, our method allows the study of these interactions for different size ranges of nanoparticles and weights of organic molecules with a precision of 1.2 × 10(-2) kDa. The method was successfully applied to the study of nanomagnetite spiked in complex matrixes, such as sewage sludge, groundwater, tap water, and different artificial samples containing Leonardite humic acid and different toxic metals (i.e., As, Pb, Th). Finally, our results showed that different types of interactions, such as adsorption, stabilization, and/or destabilization of nanomagnetite could be observed using this new method.

International audience; Wetlands are specific areas able to regulate metals mobility in the environment. Among metals, rare earth elements (REE) appear to be particularly interesting because of the information that could be provided by the REE patterns. Moreover, as REE are becoming a matter of great economic interest, their significant release into the environment may be expected over the next few decades. Wetlands would then play a key role in the regulation of their concentration in the environment. This review demonstrated that REE are released in wetland bound to colloidal organic matter. During the flood season, the released REE concentrations are largely higher than those released during the wet period. This solubilisation is related to the organic matter desorption causedby the pH rise imposed by the reducing reactions. The resulting REE patterns depend on the heterogeneity of the humic acid (HA) binding sites and the presence of potential competitive cations, such as Fe(III) and Al(III). At high REE loading, REE are bound to HA carboxylic groups and the pattern exhibit a MREE downward concavity. At low loading, REE are bound to phenolic and chelate groups and the pattern exhibits a lanthanide contraction. At low loading, REE seem to act as cationic bridges between two organic molecules, whereas at high loading they seem to be engaged in strong multidentate bonding. Moreover, the REE patterns can be modified with the competitive cations amount and speciation. The prime factor governing all these processes is pH, which drives the organic colloid production, REE loading and solubility of competitive cations.

In order to evaluate the spatial variability of the 234U–238U fractionation and its origin in surface waters, (234U–238U) activity ratios (a.r.) and trace element concentrations were measured in 34 streams and springs draining Mount Cameroon volcano. In addition, tangential ultrafiltration techniques were applied to 10 of the samples to assess the role of colloids on the transport of the uranium isotopes. U concentration variations in the different ultrafiltered fractions indicate that inorganic colloids can significantly influence the transport of this chemical element, even in diluted and organic-poor waters. Such influence, which may be related to the chemical and physical nature of colloids, does induce fractionations between U and Sr along with filter size. (234U–238U) a.r. variations among the different ultrafiltered fractions of a sample remain small or even negligible. Only two samples exhibit a significant isotopic difference between colloid-associated and solute U. Colloids of these two samples seem to correspond to a phase which is distinct from the dissolved and particulate loads of the waters. At the scale of the volcano, the (234U–238U) a.r. in surface waters is large, ranging from 1.038 to 1.397. Small 234U–238U fractionations, between 1.038 and about 1.10, are attributed to surface and meteoric weathering of the bedrock. By contrast, the highest 234U–238U fractionations (up to 1.397) are associated with groundwater contributions. Thus, variations in the (234U–238U) a.r. could correspond to the water source and not to the surface weathering processes. Consequently, 234U–238U disequilibrium may represent a powerful tool for detecting and quantifying the influence of groundwater inputs on the chemical fluxes carried by surface waters.

International audience; Several studies dedicated to the aquatic geochemistry of rare earth elements (REEs) have displayed a wide topography-related spatial variability in the REE signatures of shallow groundwater. The aim of this study was to understand the processes leading to this specific REE signature, notably with regard to the size of the Ce anomaly. Soils were sampled in order to encompass the expected topographic variability in the organic carbon (OC)/Fe(Mn) ratio. Leaching experiments were performed with the uppermost horizons of the soil. The REE patterns that developed in the soil leaching solution were similar to the REE patterns for the shallow groundwater collected in this catchment. The negative Ce anomaly evolves in a similar manner according to the topography. This spatial variation is strongly correlated with the soil OC/Fe ratio. For a low OC/Fe ratio, the negative Ce anomaly amplitude in the soil solution is large, whereas a high OC/Fe ratio generates a small or insignificant Ce anomaly. Reductive dissolution experiments using soil with low OC/Fe ratios demonstrated that the REE pattern for soil Fe oxyhydroxides exhibited a positive Ce anomaly and HREE enrichment, indicating a preferential association of these elements with Fe-oxyhydroxides. The rare earth element signature observed in the shallow groundwater is affected by Fe oxyhydroxides present in the upper soil horizons. In contrast, in soil with a high OC/Fe ratio, the REE pattern obtained under reducing conditions did not exhibit any Ce anomaly, suggesting that in the bottomland, the REE signature is affected by the OC content in the uppermost soil. This study highlights the impact of organic matter on the Fe pedofeatures, which control the development of a negative Ce anomaly in shallow groundwater.

International audience; Humic substances are major natural complexing agents that strongly influence the cycling of metal cations in the environment. The metal to humic acid (HA) concentration ratio (metal loading) can significantly impact the overall HA complexation properties in natural soils and waters. At low metal loading, cations bind to strong multi-carboxylate or -phenolate sites of HA. In addition, these strong sites might contain soft Lewis bases such as N or S, the contribution of which is still a matter of debate. Based on the possible ability of Rare Earth Elements (REE) to trace HA binding properties, the present study aimed at evaluating the influence of REE-loading on the HA binding mechanisms of two hard Lewis acids, Sm3+ and Yb3+. Extended X-ray Absorption Fine Structure (EXAFS) at the Sm and Yb LIII edge was used to analyze the local coordination of these intermediate and heavy REE ions, respectively, in a low and a high REE-loading HA sample. Data were interpreted by comparison with EXAFS data for a series of aqueous REE complexes with iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) and a carboxylic-grafted resin. In all these samples, both a Yb–O (~ 2.31 Å) and a Yb–N (~ 2.50 Å) backscattering path could be identified in the first coordination shell of Yb. At low REE loading, the first coordination shell around the Yb bond to HA is described by a single Yb–O backscattering path, i.e. the presence of N could not be evidenced. By contrast, the carboxylic-grafted resin exhibits both a short (2.31 Å) and a long Yb–O average distance (2.44 Å), suggesting the coordination of carboxylate is bidentate. The Fourier transform of the high loading HA samples exhibits a broad first shell peak possibly described by two Yb–O distances (2.29 and 2.43 Å) suggesting two coordination modes of the carboxylic groups. Different Yb–HA complex structures can therefore be observed at different metal loadings. Unfortunately, the Sm EXAFS spectra could only be recorded up to k = 10 Å−1. There was no evidence of either a clear difference between the metal loadings or with Yb–HA complex structures. A comparison between the EXAFS results, PHREEQC-Model VI simulations, and the pattern of the REE complexation constants with simple organic ligands (REE:ligand 1:1 and 1:2 complexes) provides evidence that, at high loading, REE are bound to HA through bi-ligand complexes without any chelation effect in which REE may potentially act as a cation bridge between two organic molecules. At low loading, the fact that N could not be detected in the Yb first shell suggests that, at low loading, Yb binds to O-containing groups of HA rather than N-containing groups. This may call into question the definition of HA strong sites for REE binding such as soft Lewis base-containing sites.

International audience; The presence of arsenic(As)-bearing Fe(III) oxyhydroxides in wetland zones may threat water quality due to the reduction processes that affect these zones. These processes have indeed the potential of releasing As into the soil solutions, and ultimately into the nearby river network, being given the hydrological connectivity that exists between wetlands and rivers. The effective transport of the released As into the river network is however dependent on the behaviour of As during the re-oxidation process that will occur at the wetland-river boundary, which could immobilize the released As into neo-formed Fe(III) oxyhydroxides. One of the key questions is, however, which is the impact of the organic-rich nature of wetland waters on this neoformation, which could instead favour the development of highly mobile As-bearing, organomineral colloids. In this study, we evaluated this possibility by carrying out oxidation experiments on humic acid (HA)- and As(III)/Fe(II)-rich waters. The ultrafiltration results showed that the presence of organic molecules during Fe oxidation events played a major role in the hydrolysis reaction of Fe oxyhydroxides. When Fe microparticles were formed in the absence of HA, the occurrence of HA during Fe oxidation events promoted the formation of amorphous nanosize Fe phase diffusely embedded within the organic matrix. These mixed Fe-HA colloids constrained the fate and distribution of As. These results were confirmed by nanoSIMS images that showed As sorption onto and within Fe microparticles (> 0.2 μm) in the absence of HA. By contrast, with HA, results showed preferential incorporation of As in the bulk of Fe-HA colloids (< 0.2 μm) during their formation rather than surface adsorption onto these colloids.

Arsenic (As) is a toxic and ubiquitous element which can be responsible for severe health problems. Recently, Nano-scale Secondary Ions Mass Spectrometry (nanoSIMS) analysis has been used to map organomineral assemblages. Here, we present a method adapted from Belzile et al. (1989) to collect freshly precipitated compounds of the re-oxidation period in a natural wetland environment using a polytetrafluoroethylene (PTFE) sheet scavenger. This method provides information on the bulk samples and on the specific interactions between metals (i.e. As) and the natural organic matter (NOM). Our method allows producing nanoSIMS imaging on natural colloid precipitates, including (75)As(-), (56)Fe(16)O(-), sulfur ((32)S(-)) and organic matter ((12)C(14)N) and to measure X-ray adsorption of sulfur (S) K-edge. A first statistical treatment on the nanoSIMS images highlights two main colocalizations: (1) (12)C(14)N(-), (32)S(-), (56)Fe(16)O(-) and (75)As(-), and (2) (12)C(14)N(-), (32)S(-) and (75)As(-). Principal component analyses (PCAs) support the importance of sulfur in the two main colocalizations firstly evidenced. The first component explains 70% of the variance in the distribution of the elements and is highly correlated with the presence of (32)S(-). The second component explains 20% of the variance and is highly correlated with the presence of (12)C(14)N(-). The X-ray adsorption near edge spectroscopy (XANES) on sulfur speciation provides a quantification of the organic (55%) and inorganic (45%) sulfur compositions. The co-existence of reduced and oxidized S forms might be attributed to a slow NOM kinetic oxidation process. Thus, a direct interaction between As and NOM through sulfur groups might be possible.

Rare earth element (REE), dissolved organic carbon (DOC) and trace-element (Al, Mn, Fe, Sr, Ba, U and Th) concentrations were measured in fourteen well water samples (

In order to establish the origin of fluids expelled from mud volcanoes in Trinidad, we analyzed their major-element, trace-element, and isotopic (H, O, Sr) compositions. The mineralogical, chemical, and Sr isotope compositions of associated expelled muds were also determined. On the basis of their chemical and Sr isotope compositions, the fluids were divided into two groups—those southwest and northeast of a major right lateral wrench fault (the Los Bajos fault) that both controls the chemical quality of the fluids and acts as a drain. Strontium isotopes were derived via mixing between a radiogenic source (0.71135) and a nonradiogenic source (≤0.70671) for both southwest and northeast groups. However, the nonradiogenic source possibly feeding the northeast group showed a lower Sr concentration than that of the southwest group. H and O isotope data show that the fluids became enriched in δ 18 O through interaction with wall rocks. The fluids were originally oceanic, but their properties were subsequently modified by diagenesis as evidenced by chemical data, and mixing between fluids issued from two deep-seated reservoirs and surface aquifer end-members. The gas phase expelled with the mud and the fluid through the mud volcanoes is composed mainly of methane with minor carbon dioxide. The amount of expelled gas seems the same in samples from both sides of the Los Bajos fault. The almost unique methane content of the gas phase and the large positive δ 18 O shifts of the reservoir end-member, as well as B, Li, and Ba contents, reveal that the fluids from the deep-seated reservoirs inherited their chemical compositions at high-temperature fluid–rock interactions. Estimates of equilibrium temperatures of the two deep-seated reservoirs suggest that, to acquire the estimated higher temperature (150°C), the mud volcanoes must have been fed partly by a reservoir located at a depth of more than 3 km. This depth corresponds to previous geologic information, which located the deep-seated reservoir in Miocene sediments at such a comparable depth. The mixing of the original fluids with a less deep-seated reservoir and meteoric waters on the ascending path suggests the presence of a recharge mechanism and contributes to the dynamics of the expulsion itself.

The complexation of Fe(II) with organic matter (OM) and especially with humic acids (HAs) remains poorly characterized in the literature. In this study, batch experiments were conducted on a pH range varying from 1.95 to 9.90 to study HA-mediated Fe(II) binding. The results showed that high amounts of Fe(II) are complexed with HA depending on the pH. Experimental data were used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log K MA = 2.19 ± 0.16, log K MB = 4.46 ± 0.47 and ΔLK 2 = 3.90 ± 1.30) were validated using the LFER (linear free energy relationship) method and published adsorption data between Fe(II) and Suwannee River fulvic acid (SRFA) (Rose and Waite, 2003). They were then put in PHREEQC-Model VI to determine the distribution of Fe(II) onto HA functional groups. It was shown that Fe(II) forms mainly bidentate complexes, some tridentate complexes and only a few monodentate complexes with HA. Moreover, Fe(II) is mainly adsorbed onto carboxylic groups at acidic and neutral pH, whereas carboxy-phenolic and phenolic groups play a major role at basic pH. The major species adsorbed onto HA functional groups is Fe 2 + ; Fe(OH) + appears at basic pH (from pH 8.13 to 9.9). The occurrence of OM and the resulting HA-mediated binding of Fe(II) can therefore influence Fe(II) speciation and bioavailability in peatlands and wetlands, where seasonal anaerobic conditions prevail. Furthermore, the formation of a cationic bridge and/or the dissolution of Fe(III)-(oxy)hydroxides by the formation of Fe(II)-OM complexes can influence the speciation of other trace metals and contaminants such as As.