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

Discrimination between different water bodies from a multilayered aquifer (Kaluvelly watershed, India): Trace element signature

International audienceIn the multilayered aquifer of Kaluvelly (India), comprising various sedimentary layers overlying a charnockitic basement, concentrations of trace elements were measured in aquifer formations and in groundwaters to identify geochemical tracers for water bodies. The two main sandstones (Cuddalore and Vanur) originate from the charnockites and the Cuddalore sandstone has experienced lateritization. In the studied area, two charnockite end-members were identified: a dioritic and a granitic one. Mineralogical composition and whole-rock Ti concentrations confirmed the charnockite which displayed the granitic composition as the parent rock of the two sandstones. Titanium distribution indicates that the Cuddalore sandstone originates from a more intense weathering of the parent material than the Vanur sandstone. Despite extensive differences in trace element contents recorded in aquifer formations, only a few trace elements were suitable to distinguish the water bodies. Among soluble elements, Li (in the Vanur aquifer) and Ba (in the charnockite and carbonaceous aquifers) can be used as tracers. As the input of these elements in solution is mainly regulated by the available stock, for a given mineralogical origin there is a direct link between the aquifer formation composition and water signature. With the exception of As, concentrations of redox-sensitive elements were not preserved during pumping because of oxidation, preventing their use as tracers. Low-mobility elements such as La, Ce, Th, Zr, Nb, Hf, or Ta were too insoluble to be detected in waters and/or to record the aquifer formation signature. Their input in solution was not regulated by the available stock but by the dissolution rate of rock-forming minerals. Only Ti can be used to distinguish between two out of the three aquifers (charnockite and Vanur). The specific behavior of Ti recorded in these waters may be linked to rutile inclusions within plagioclases and to the influence of climate on Ti solubility

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

Geochemical modeling of Fe(II) binding to humic and fulvic acids

International audienceThe complexation of Fe(II)with organic matter (OM)and especiallywith 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 HAdepending on the pH. Experimental datawere used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log KMA = 2.19 ± 0.16, log KMB= 4.46± 0.47 and ΔLK2=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 andWaite, 2003). Theywere 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 Fe2+; Fe(OH)+ appears at basic pH (frompH 8.13 to 9.9). The occurrence of OMand 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

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

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

The geochemistry of gem opals as evidence of their origin

International audienceSeventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondritenormalized REE pattern, are the keys to separating sedimentary opals (BaN110 ppm, Eu and Ce anomalies) from volcanic opals (Bab110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si4+ by Al3+ and Fe3+. The induced charge imbalance is compensated chiefly by Ca2+, Mg2+, Mn2+, Ba2+, K+, and Na+. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to "chocolate brown"). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (=1 ppm) induces a green luminescence

Bioconcentration and translocation of rare earth elements in plants collected from three legacy mine sites in Portugal

Rare earth elements (REE), a group of emerging contaminants with commercial and technological applications, share many physical and chemical characteristics and have thus been used as accurate tracers of various environmental samples. They have been shown to increase in receiving waters following the dissolution of host-rock material during mining activities. In this study, spontaneous vegetation and related media were collected from three Portuguese legacy mine sites in November 2020 to evaluate the phytoavailability and fate of REE. Water, soil and plant data were analyzed in the context of the 1) prevailing geochemical context, 2) the mining context, and 3) plant effects. This study presents the REE signatures for different plant species and links the signatures to a potential source of bioavailable REE. The REE accumulated in plant tissue seems to reflect the REE signature of surface waters in the mining areas, showing enrichment in middle REE. Although the soils, sediments, and waters in this study had similar features, certain plants seemed better adapted to translocating Light REE and Eu over others. Given that REE are readily available within the field conditions of a mining site, this study shows how plant physiology and biologic preference towards particular REE contribute to the fractionation of REE and create a unique signature dependent on plant type

From diagenesis to hydrothermal recrystallization : Polygenic Sr-rich fluoropatite from the oolitic ironstone of Saint-Aubin-des-Châteaux (Armorican Massif, France)

International audienceFour generations (I to IV) of Sr-rich fluorapatite were observed in the hydrothermally altered oolitic ironstone interbedded within the lower Ordovician sandstone of Saint-Aubin-des-Châteaux (Armorican Massif, France). Main type I is diagenetic to weak metamorphic; its remobilization along fractures gave type II, which just preceded the hydrothermal pyritization of the ironstone. Types I and II have a quite similar SrO content (~ 4.8 wt.%), with mean formulae (Ca4.73Sr0.24(Mn, Fe)0.02)S=4.99(PO4)3.02[F0.68(OH)0.32] and (Ca4.75Sr0.24(Mn,Fe)0.01)S=5.00(PO4)3.00[F0.63(OH)0.37], respectively. Types III and IV postdate the main hydrothermal process. Type III results from the breakdown of a Sr phosphate, lulzacite. It presents a patchy texture, where each constitutive sub-grain has a relatively homogeneous Sr content (from 1 to 7 wt.%), with an almost constant F ratio (0.83 apfu). The last type IV (geodic) shows a strong oscillatory growth zoning (Sr.Ca substitution) with SrO content ranging from 0 to 18 wt.%, and F close to 0.75 apfu. On the basis of trace analysis of REE and Y by LA-ICPMS, chondrite-normalized spectra of apatite types I, II and IV present dissymmetric convex shapes, with a significant deficit of LREE and a slight one of HREE relatively to MREE (Gd to Dy). Bond valence calculations indicate that this shape is determined mainly by the heterovalent substitution 2 Ca2+ . Na+ + (REE3+, Y), which controls the incorporation of REE and Y in the crystal structure of apatite. The best fit is for REE = Dy or Ho. This crystal chemical constraint favours at low temperature the fractionation of REE and Y between apatite and REE phosphates. This fractionation is stronger with monazite than with xenotime. The normalized spectrum of apatite type III has a symmetric convex shape, with a strong positive Eu anomaly inherited from its precursor lulzacite (close ionic radii of Eu2+ and Sr2+). Owing to its complex geochemistry and geological evolution, the Saint-Aubin ironstone appears as a basic example for the crystal-chemistry of apatite in low-temperature conditions, especially when phosphate-rich sediments and their metamorphic or hydrothermal derivatives are concerned