Continental shelves as potential resource of rare earth elements

International audienceThe results of this study allow the reassessment of the rare earth elements (REE) external cycle. Indeed, the river input to the oceans has relatively flat REE patterns without cerium (Ce) anomalies, whereas oceanic REE patterns exhibit strong negative Ce anomalies and heavy REE enrichment. Indeed, the processes at the origin of seawater REE patterns are commonly thought to occur within the ocean masses themselves. However, the results from the present study illustrate that seawater-like REE patterns already occur in the truly dissolved pool of river input. This leads us to favor a partial or complete removal of the colloidal REE pool during estuarine mixing by coagulation, as previously shown for dissolved humic acids and iron. In this latter case, REE fractionation occurs because colloidal and truly dissolved pools have different REE patterns. Thus, the REE patterns of seawater could be the combination of both intra-oceanic and riverine processes. In this study, we show that the Atlantic continental shelves could be considered potential REE traps, suggesting further that shelf sediments could potentially become a resource for REE, similar to metalliferous deep sea sediments.

Rare earth element sorption onto hydrous manganese oxide: A modeling study

International audienceManganese oxides are important scavengers of rare earth elements (REE) in hydrosystems. However, it has been difficult to include Mn oxides in speciation models due to the lack of a comprehensive set of sorption reactions consistent with a given surface complexation model (SCM), as well as discrepancies between published sorption data and predictions using the available models. Surface complexation reactions for hydrous Mn oxide were described using a two surface site model and the diffuse double layer SCM. The specific surface area, surface side density, and pHzpc were fixed to 746 m2/g, 2.1 mmol/g, and 2.2, respectively. Two site types (triple bond; length of mdashXOH and triple bond; length of mdashYOH) were also used with pKa2 values of 2.35 (triple bond; length of mdashXOH) and 6.06 (triple bond; length of mdashYOH). The fraction of the high affinity sites was fixed at 0.36. Published REE sorption data were subsequently used to determine the equilibrium surface complexation constants, while considering the influence of pH, ionic strength, and metal loading. Log K increases from light REE to heavy REE and, more specifically, displays a convex tetrad effect. At low metal loading, the triple bond; length of mdashYOH site type strongly expresses its affinity toward REE, whereas at higher metal loading, the same is true for the triple bond; length of mdashXOH site type. This study thus provides evidence for heterogeneity in the distribution of the Mn oxide binding sites among REE

Characterization of metal binding sites onto biochar using rare earth elements as a fingerprint

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It’s time to replace the term “heavy metals” with “potentially toxic elements” when reporting environmental research

International audienceEven if the Periodic Table of Chemical Elements is relatively well defined, some controversial terms are still in use. Indeed, the term "heavy metal" is a common term used for decades in the natural sciences, and even more in environmental sciences, particularly in studies of pollution impacts. As the use of the term appears to have increased, we highlight the relevance of the use of the term "Potentially Toxic Element(s)", which needs more explicit endorsement, and we illustrate the chemical elements that need to be considered

Rare earth elements complexation with humic acid

The binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE­HA complexation experiments were performed at various pH conditions (ranging from 2 to 10.5) using a standard batch equilibration method. Results show that the amount of REE bound to HA strongly increases with increasing pH. Moreover, a Middle-REE (MREE) downward concavity is evidenced by REE distribution patterns at acidic pH. Modelling of the experimental data using Humic Ion Binding Model VI provided a set of log KMA values (i.e., the REE­HA complexation constants specific to Model VI) for the entire REE series. The log KMA pattern obtained displays a MREE downward concavity. Log KMA values range from 2.42 to 2.79. These binding constants are in good agreement with the few existing datasets quantifying the binding of REE with humic substances but quite different from a recently published study which evidence a lanthanide contraction effect (i.e., continuous increase of the constant from La to Lu). The MREE downward concavity displayed by REE­HA complexation pattern determined in this study compares well with results from REE­fulvic acid (FA) and REE­acetic acid complexation studies. This similarity in the REE complexation pattern suggests that carboxylic groups are the main binding sites of REE in HA. This conclusion is further illustrated by a detailed review of published studies for natural, organic-rich, river- and ground-waters which show no evidence of a lanthanide contraction effect in REE pattern. Finally, application of Model VI using the new, experimentally determined log KMA values to World Average River Water confirms earlier suggestions that REE occur predominantly as organic complexes (= 60%) in the pH range between 5­5.5 and 7­8.5 (i.e., in circumneutral pH waters). The only significant difference as compared to earlier model predictions made using estimated log KMA values is that the experimentally determined log KMA values predict a significantly higher amount of Light-REE bound to organic matter under alkaline pH conditions

MODELING LANTHANIDE BINDING SITES ON HUMIC ACID

International audienceLanthanide (Ln) binding to humic acid (HA) has been investigated by combining ultrafiltration and ICP-MS techniques. A Langmuir sorption isotherm metal complexation model was used in conjunction with a linear programming method (LPM) to fit experimental data representing a wide range of experimental conditions both in HA/Ln ratio (varying between 5 and 20) and in pH range (from 2 to 10) with a ionic strength of 10-3 mol L-1. The LPM approach, not requiring prior knowledge of surface complexation parameters, was used to solve the existing discrepancies in LnHA binding constants and site densities. The application of the LPM to experimental data revealed, for the first time, the presence of two discrete metal binding sites at low humic acid concentrations, (5 mg L-1), with log metal complexation constants (log KS,j) of 2.65 ± 0.05 and 7.00 (depending on Ln). The corresponding site densities were 2.71 ± 0.57 x 10-8 and 0.58 ± 0.32 x 10-8 moles of Ln3+/mg of HA (depending on Ln). Total site densities of 3.28 ± 0.28 x 10-8 mol mg-1, 4.99 ± 0.02 x 10-8 mol mg-1 and 5.01 ± 0.01 x 10-8 mol mg-1 were obtained by LPM for humic acid, for HA concentration of 5 mg L-1, 10 mg L-1 and 20 mg L-1, respectively. These results confirm that lanthanide binding occurs mainly at weak sites (i.e., ca. 80%) and secondly at strong sites (i.e., ca. 20%). The first group of discrete metal binding sites may be attributed to carboxylic groups (known to be the main binding sites of Ln in HA), and the second metal binding group to phenolic moieties. However, this study evidences heterogeneity in the distribution of the binding sites among Ln. Eventually, the LPM approach produced feasible results, as it was less sensitive to error and did not require an a priori assumption of the number and concentration of binding sites

Negative cerium anomalies in manganese (hydr)oxide precipitates due to cerium oxidation in the presence of dissolved siderophores

International audienceWe present experimental results on the sorption behavior of rare earth elements and yttrium (REY) on precipitating manganese (hydr)oxide in the presence of the biogenic siderophore desferrioxamine B (DFOB). In marked contrast to inorganic systems, where preferential adsorption of HREY and depletion of LREY is commonly observed in manganese (hydr)oxide precipitates, sorption of REY in presence of the DFOB siderophore leads to HREY-depleted and LREY-enriched patterns in the precipitates. Moreover, our data indicate that surface oxidation of Ce(III) to Ce(IV) during sorption onto manganese (hydr)oxides and the resulting development of a positive Ce anomaly, which are commonly observed in inorganic experiments, are prevented in the presence of DFOB. Instead, Ce(III) is oxidized to Ce(IV) but associated with the dissolved desferrioxamine B which forms complexes with Ce(IV), that are at least twenty orders of magnitude more stable than those with Ce(III) and REY(III). The overall result is the formation of a positive Ce anomaly in the solution and a negative Ce anomaly in the Mn (hydr)oxides. The distribution of the strictly trivalent REY and Eu(III) between the manganese (hydr)oxide phase and the remaining ambient solution mimics the distribution of published stability constants for complexes of REY(III) with DFOB, i.e. the heavy REY form more stable complexes with the ligand and hence are better shielded from sorption than the LREY. Surface complexation modeling corroborates our experimental results. Negative Ce anomalies in Mn precipitates have been described from biogenic Mn oxides. Our results provide experimental evidence for the development of negative Ce anomalies in abiogenic Mn (hydr)oxide precipitates and show that the presence of the widespread siderophore desferrioxamine B during mineral precipitation results in HREY-depleted Mn (hydr)oxides with negative Ce anomalies

Colloidal Control on the Distribution of Rare Earth Elements in Shallow Groundwaters

International audienceA 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 <0.2 µm REE fraction ultimately comes from two solid sources in these groundwaters: one located in the deep basement schist; another located in the upper, organic-rich soil horizon

Rare earth elements as proxies of supergene alteration processes from the giant Imiter silver deposit (Morocco)

International audienceThe giant Imiter silver mine located at the northern edge of the West African craton in Morocco is assumed to be a late Neoproterozoic epithermal deposit mainly characterized by a hypogene paragenesis of Agrich sulphides and sulfosalts, and Ag-Hg alloys occuring preferentially in quartz-rich veins. The secondary enrichment zone at Imiter reaches a thickness of 50 to 150 m below ground surface. The upper levels, famous because of giant native silver crystals, grade up to 300 kg/t. Metallographic observations, SEM-EDS and XRD analyses reveal the presence of a quite complex secondary paragenesis made of acanthite, cinnabar, imiterite, perroudite, cerussite, mimetite, iron oxyhydroxides, synchisite and coronadite. Supergene alteration processes of the giant Imiter silver mine deposit consist of the remobilisation of the primary hypogene paragenesis by (i) deep and old basinal brines and (ii) downward infiltrations of surficial waters becoming progressively more reduced and F-enriched in response to fluid-rock interactions. Development of such a supergene mineralization strongly suggests prevalence of arid to semiarid conditions

Assessment of vanadium distribution in shallow groundwaters

International audienceShallow groundwater samples (filtered at 0.2 μm) collected from a catchment in Western France (Petit Hermitage catchment) were analyzed for their major- and trace-element concentrations (Fe, Mn, V, Th and U) as well as their dissolved organic carbon (DOC) concentrations, with the aim to investigate the controlling factors of vanadium (V) distribution. Two spatially distinct water types were previously recognized in this catchment based on variations of the rare earth element (REE) concentrations. These include: (i) DOC-poor groundwater flowing below the hillslope domains; this type has low V contents; and (ii) DOC-rich groundwater originating from wetlands, close to the river network; the latter water type displays much higher V concentrations. The temporal variation of the V concentration was also assessed in the wetland waters; the results show a marked increase in the V content at the winter-spring transition, along with variations in the redox potential, and DOC, Fe and Mn contents. In order to allow the study of organo-colloidal control on V partitioning in water samples, ultrafiltration experiments were performed at different pore size cut-offs (30 kDa, 10 kDa and 5 kDa). Two shallow, circumneutral waters were sampled: one was both DOC- and Fe-rich and the other was DOC-rich and Fe-poor. In terms of major- and trace-cations and DOC concentrations, the data were processed using an ascendant hierarchical classification method. This revealed the presence of two main groups: (i) a "truly" dissolved group (Na, K, Rb, Ca, Mg, Ba, Sr, Si, Mn, Co, Ni, Cr, Zn and Ni), and (ii) a colloidal group carrying DOC, Fe, Al, Pb, Cu, REE, U, Th and V. Vanadium has an unpredictable behavior; it can be either in the organic pool or in the inorganic pool, depending on the sample. Moreover, V speciation calculations--using Model VI and SCAMP--were performed on both samples. Speciation modeling showed approximately the same partitioning feature of these elements as compared to ultrafiltration data, namely: a slight change of the V speciation in groundwaters along the studied topographic sequence. This implies that vanadium in hillslope groundwater wells occurs as a mixing of organic and inorganic complexes, whereas V in wetland groundwater wells comprises mainly organic species. Using the dataset described above, factors such as aquifer-rock composition or anthropogenic input were demonstrated to probably play a minor role in determining the V distribution in shallow groundwaters. Although an anthropogenic impact can be ruled out at this local scale, we cannot preclude a perturbation in the global V cycle. Most likely, the two dominant factors involved are the organic matter content and the redox state either promoting competition with Fe-, Mn-oxides as V carriers in groundwater or not. In this context, it appears challenging to determine whether organic matter or redox-sensitive phases are the major V carriers involved, and a further study should be dedicated to clarify this partition, notably to address the processes affecting large-scale V transport