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

Sources, production et transfert du carbone organique dissous dans les bassins versants élémentaires sur socle (apports des isotopes stables du carbone)

En dépit de son importance pour les écosystèmes aquatiques, l origine et les mécanismes de production du carbone organique dissous (COD) sont toujours sujets à discussion. Cette thèse vise à mieux comprendre le rôle joué par l hydrologie tant sur les mécanismes de production et la location des sources du COD que sur les flux et les processus de transfert du sol vers les cours d eau. Un suivi haute fréquence des eaux du sol et de rivière a été réalisé sur la totalité d un cycle hydrologique dans le bassin versant expérimental de Kervidy-Naizin (Morbihan, Observatoire de Recherche en Environnement (ORE) AgrHys). Le COD contenu dans ces eaux a été caractérisé par sa composition isotopique en carbone ( 13C), et ce suivi a été complété par l analyse de la dynamique de la nappe. L analyse des variations saisonnières de concentration et de composition du COD révèle l existence d un relais dans les sources et les mécanismes de production du COD dans les sols des zones de fond de vallée en phase avec les changements de régime hydrologique du bassin. Ainsi, lors de la période automnale de remontée de la nappe, le compartiment COD présente un caractère faiblement aromatique et une composition isotopique particulière, suggérant une origine microbienne. Ce réservoir de très faible taille (5% du flux annuel de COD exporté par le cours d eau) est entièrement épuisé par les premières crues d automne. La mise en charge de la nappe en versant conduit à la mobilisation d un second réservoir de COD caractérisé par une aromaticité élevée et une signature isotopique proche de celle de la matière organique des sols. Ce compartiment correspond au fond humique ancien des sols et présente une taille nettement plus important que le premier (90% du flux annuel). Cependant ce compartiment ne se limite pas au COD produit dans sols des zones de bas-fond mais comprend aussi des apports de matières organiques issues des sols de bas de versant. Enfin, le rabattement de la nappe au milieu du printemps marque le début de la période d assèchement du bassin versant et le retour dans les sols de bas-fond d un COD faiblement aromatique et isotopiquement similaire à celui observé en automne. En conséquence, la teneur et la composition du COD véhiculé par le cours d eau varient fortement à l échelle saisonnière (variabilité temporelle des processus de production du COD). Les horizons superficiels des sols des zones de bas-fond sont la principale source de COD, par lesquels transitent entre 60 et 80% du flux de COD exporté lors des événements de crues. Concernant le réservoir humique, un résultat important est qu il semble limité dans les sols de bas de versant, alors qu il semble au contraire illimité dans les sols de bas fond. Au final, les résultats obtenus dans le cadre de cette thèse démontrent le contrôle majeur de la dynamique de la nappe sur les mécanismes de production de COD par le sol ainsi que sur la localisation spatiale et les flux de COD transférés de ces sols vers les cours d eau à l échelle de la saison. Ils valident également l hypothèse émise selon laquelle l exportation du COD par les cours d eau ne résulterait pas d un simple flushing du COD produit dans les sols de bas-fond mais mobiliserait plutôt un ensemble de sources localisées le long du continuum rivière-zone de bas-fond-zone de versant, et dont le caractère plus moins limité du réservoir expliquerait les dynamiques des concentrations en COD annuelles observées à l exutoire de ces bassins. Un autre résultat majeur de cette thèse est la qualification des isotopes comme outil de traçage des sources et de la dynamique du COD dans les bassins versants. Un point clé de ce travail est la différenciation isotopiques des réservoirs COD des sols de bas-fond et des sols de bas de versant, qui a permis pour la première fois de révéler la participation de l'un et de l autre au flux exporté à l exutoire et d estimer quantitativement leur contribution respective à ce même flux.Despite its importance to aquatic ecosystems, origin and production mechanisms of dissolved organic carbon (DOC) are still subject to discussion. This thesis aims to better understand the role of hydrology in controlling both DOC production in soils and export by stream waters. High frequency monitoring of soils and streams waters was carried out during an entire hydrological cycle in the Kervidy-Naizin catchment (Morbihan). The DOC was characterized by its stable carbon isotopic composition, and groundwater table dynamic was also analyzed during the same period. The analysis of seasonal variations in both DOC concentration and composition reveals a succession in the sources and mechanisms of production of DOC in soils of valley bottom in line with changes in the hydrological regime of the basin. Thus, during the autumn period of rising water-table, the COD compartment has a low aromatic character and a particular isotopic composition, suggesting a microbial origin. This reservoir of very small size (5% of the annual flux of DOC exported by the stream) is fully exhausted by the first autumn storm events. The increase of water-table in the upland domain leads to the mobilization of a second DOC pool characterized by a high aromaticity and a isotopic signature similar to those of soil organic matter. This compartment corresponds to "old" humic fraction of the soil and has a size much larger than the first (90% of annual flux). However, this compartment includes contributions of DOC from riparian soils and also upland soils. Finally, the water-table drawdown in the middle of spring marks the beginning of the drying period of the catchment and the return to the soil of the fresh DOC pool. As a result, concentration and composition of DOC transported by the stream vary greatly across a seasonal scale (temporal variability of production processes DOC). The surface horizons of soils riparian soils are the main source of DOC, which handle between 60 and 80% of stream DOC flux during flood events. An important result is that the DOC pool seems limited in upland soils, while its seems no-limited in riparian soils. Finally, the results obtained in this thesis demonstrate the major control of the dynamics of the water-table on the mechanisms of production of DOC from the soil as well as the spatial location of DOC sources and DOC flux transferred from the soil to stream through the hydrological cycle. They also validate the hypothesis that DOC export by rivers mobilizes rather a set of sources located along the upland-riparian-stream continuum. Another major result of this thesis is the validation of stable carbon isotopes as a tool for tracing sources and dynamics of DOC in the catchment.RENNES1-Bibl. électronique (352382106) / SudocSudocFranceF

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

Organo-colloidal control on major- and trace element partitioning in shallow groundwaters : confronting ultrafiltration and modelling

International audienceUltrafiltration experiments using new small ultracentifugal filter devices were performed at different pore size cut-offs to allow the study of organo-colloidal control on metal partitioning in water samples. Two shallow, circumneutral pH waters from the Mercy site wetland (western France) were sampled: one dissolved organic carbon (DOC)- and Fe-rich and a second DOC-rich and Fe-poor. Major- and trace-element cations and DOC concentrations were analysed and data treated using an ascendant hierarchical classification method. This reveals the presence of three groups: (i) a "truly" dissolved group (Na, K, Rb, Ca, Mg, Ba, Sr, Si and Ni); (ii) an inorganic colloidal group carrying Fe, Al and Th; and (iii) an organic colloidal group enriched in Cr, Mn, Co, Cu and U. However, REE and V have an ambivalent behaviour, being alternatively in the organic pool and in the inorganic pool depending on sample. Moreover, organic speciation calculation using Model VI were performed on both samples for elements for which binding constants were available (Ca, Mg, Ni, Fe, Al, Th, Cr, Cu, Dy, Eu). Calculation shows relatively the same partitioning of these elements as ultrafiltration does. However, some limitations appear such as (i) a direct use of ultrafiltration results which tends to overestimate the fraction of elements bound to humic material in the inorganic pool as regards to model calculations as well as, (ii) a direct use of speciation calculation results which tends to overestimate the fraction of elements bound to humic material in the organic pool with regard to ultrafiltration results. Beside these limitations, one can consider that both techniques, ultrafiltration and speciation calculation, give complementary information, especially for more complex samples where inorganic and organic colloids compete

Uncertainties in assessing annual nitrate loads and concentration indicators. Part 1: Impact of sampling frequency and load estimation alogorithms

International audienceThe objectives of this study are to evaluate the uncertainty in annual nitrate loads and concentrations (such as annual average and median concentrations) as induced by infrequent sampling and by the algorithms used to compute fluxes. A total of 50 watershed-years of hourly to daily flow and concentration data gathered from nine watersheds (5 to 252 km2) in Brittany, France, were analyzed. Original (high frequency) nitrate concentration and flow data were numerically sampled to simulate common sampling frequencies. Annual fluxes and concentration indicators calculated from the simulated samples were compared to the reference values calculated from the high-frequency data. The uncertainties contributed by several algorithms used to calculate annual fluxes were also quantified. In all cases, uncertainty increased as sampling intervals increased. Results showed that all the tested algorithms that do not use continuous flow data to compute nitrate fluxes introduced considerable uncertainty. The flowƒ]weighted average concentration ratio method was found to perform best across the 50 annual datasets. Analysis of the bias values suggests that the 90th and 95th percentiles and the maximum concentration values tend to be systematically underestimated in the long term, but the load estimates (using the chosen algorithm) and the average and median concentrations were relatively unbiased. Great variability in the precision of the load estimation algorithms was observed, both between watersheds of different sizes and between years for a particular watershed. This has prevented definitive uncertainty predictions for nitrate loads and concentrations in this preliminary work, but suggests that hydrologic factors, such as the watershed hydrological reactivity, could be a key factor in predicting uncertainty levels

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

Sources of dissolved organic matter during storm and inter-storm conditions in a lowland headwater catchment: constraints from high-frequency molecular data

International audienceThe transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of mi-cropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochem-istry. DOM delivery to streams during storms is assumed to come from the flushing of preexisting soil DOM reservoirs mobilized by the modification of water flow paths. We tested this hypothesis by investigating the evolution of the composition of stream DOM during inter-storm conditions and five storm events monitored with high-frequency sampling. The composition of DOM was analyzed using thermally assisted hydrolysis and methylation (THM) with tetramethylammo-nium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer. In inter-storm conditions, stream DOM is derived from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms due to the increase in water velocity, resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks, leading to a partition of organic matter between particulate and dissolved phases. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partition between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of the water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. These results are the most important factors in understanding the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events

Does As(III) interact with Fe(II), Fe(III) and organic matter through ternary complexes?

International audienceUp until now, only a small number of studies have been dedicated to the binding processes of As(III) with organic matter (OM) via ionic Fe(III) bridges; none was interested in Fe (II). Complexation isotherms were carried out with As(III), Fe(II) or Fe(III) and Leonardite humic acid (HA). Although PHREEQC/Model VI, implemented with OM thiol groups, reproduced the experimental datasets with Fe(III), the poor fit between the experimental and modeled Fe(II) data suggested another binding mechanism for As(III) to OM. PHREEQC/Model VI was modified to take various possible As(III)-Fe(II)-OM ternary complex conformations into account. The complexation of As(III) as a mononuclear bidentate complex to a bidentate Fe(II)-HA complex was evidenced. However, the model needed to be improved since the distribution of the bidentate sites appeared to be unrealistic with regards to the published XAS data. In the presence of Fe(III), As(III) was bound to thiol groups which are more competitive with regards to the low density of formed Fe(III)-HA complexes. Based on the new data and previously published results, we propose a general scheme describing the various As(III)-Fe-MO complexes that are able to form in Fe and OM-rich waters

Thiol groups controls on arsenite binding by organic matter: New experimental and modeling evidence

International audienceAlthough 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

Effects of Fe competition on REE binding to humic acid: Origin of REE pattern variability in organic waters

International audienceCompetitivemechanisms 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