Rare Earth Elements Speciation Research Articles

Rare earth elements sorption to iron oxyhydroxide: Model development and application to groundwater

Iron oxyhydroxides are among the most important colloids that control rare earth elements (REE) concentrations and transport in natural hydrosystems. In this study, REE surface complexation to iron oxyhydroxides (Fe(OH)3(a)) was described by using the Donnan diffuse layer model and a two-site (i.e. ≡FesOH and ≡FewOH) model. The specific surface area and pH of zero charge were fixed as 100000 m2/mol and 8.0, respectively. The surface site density for weak and strong binding sites were fixed at 0.1 moL/mol Fe (≡FewOH) and 0.001 moL/mol Fe (≡FesOH) respectively. The two site types were used with pKa1int = 7.29 and pKa2int = 8.93. Using linear free energy relationship, the estimated equilibrium surface complexation constants (log K) increased from light REE (LREE) to heavy REE (HREE). Results of REE modeling calculation using the determined log K revealed a good fit of experimental data, showing an order of sorption on iron oxyhydroxides: HREE > MREE > LREE and preferential sorption of HREE at a lower pH. However, sorption edges only showed a slight change with ionic strength (0.1–0.7 moL/L) for the whole REE series. The generalized model was subsequently used to evaluate the impact of iron oxyhydroxides on REE speciation in groundwater. Application of the model to “model groundwater” showed that iron oxyhydroxide complexes of REE were significant in near neutral and weakly alkaline pH. This study contributes to putting forward a comprehensive database which would be useful for the application of surface complexation model to describe REE sorption by amorphous ferric hydroxides in nature.

Petrographic and geochemical features of the dolostones at the Gölboğazı Formation (Upper Devonian), Southwest Konya, Turkey

This paper describes the occurrence of dolostone and the mechanism of dolomitization of the Upper Devonian Golbogazi Formation in the allochthonous Taurus Mountains Aladag unit in Turkey. The Upper Devonian Golbogazi Formation carbonates, with dominant ostracod-bearing mudstone and wackestone, formed tidal and subtidal environments, and some of these rocks were dolomitized from shallow to deep burial. On the basis of the field, the petrographic and geochemical features, four different replaceable and cement dolostone phases have been recognized. The replacive dolostones contain (1) very fine to fine crystalline planar-s dolostone (df1), (2) medium to coarse crystalline planar-s to planar-e dolostone (df2), (3) coarse to very coarse crystalline non-planar-a dolostone (df3), and (4) coarse to very coarse crystalline planar dolostone cement (df4). The replacive dolostones are disordered to moderate the ordered and calcium-rich. They are non-stoichiometric and have 46–59 mol% CaCO3 and 41–54 mol% MgCO3 total contents. The df1 dolostones have MgCO3 contents of 41–54 mol%, the df2 dolostones have 41–53 mol%, the df3 dolostones have 49 mol%, and the df4 dolostones have 49–50 mol%, respectively. The Golbogazi dolostones have δ18O values of −9.44 to −2.20‰ Vienna Pee Dee Belemnite (VPDB) and δ13C values of −1.58 to +2.52 VPDB. Sr, Na, Mn, and Fe concentrations of replacive dolostones are 74–184, 148–593, below detection level (bdl)–619, and 1049–9233 ppm, respectively. The petrographic and geochemical data demonstrate that the replacive dolostones occurred prior to the chemical compaction at shallow to intermediate burial depths from Late Devonian seawater and/or seawater lightly modified by water–rock interaction process and later recrystallized by basinal brines at increasing burial depths and temperature. The North American Shale Composite-normalized rare earth element values of both limestone and dolostone show very similar rare earth element patterns characterized by slightly or considerably negative cerium (Ce) anomalies and a clear depletion in all rare earth element species. The dedolomitization observed in the Golbogazi Formation is thought to occur by the oxidizing effect of the meteoric water in the shallow burial environment during the telodiagenesis.

The behaviour of zirconium, hafnium and rare earth elements during the crystallisation of halite and other salt minerals

Halite crystals from Messinian and Tortonian evaporites from Sicily and Spain and current precipitated halite crystals and the relative parent brines (active evaporation systems) were investigated in order to evaluate the Zr, Hf and Rare Earth Element (REE) behaviour.Halite crystallisation from evaporating brines fractionates Zr, Hf and REE through a two-step process. During the first step, dissolved complexes of studied elements are scavenged onto the surfaces of crystallising halite. During the second step, elements are co-precipitated into the crystal lattice as it grows. The first step mechanism is determined by the dissolved REE speciation. In saltworks where carbonate-REE complexes occur, surface complexation of REE onto halite crystals does not occur. On the contrary, surface REE-complexes onto halite crystals are forming in the Dead Sea water where aqueous REE speciation is dominated by chloride-complexes. Under the latter conditions, halite crystallises with cubic and cubic-octahedral composite habitus. Octahedral planes involve the formation of strong coulombic interactions, mainly with [Hf(H2O)3(OH)5]− rather than with [Zr(H2O)4(OH)4]0 complexes. As a consequence, newly formed halite in the Dead Sea shows strong subchondritic Zr/Hf ratios.Based on these indications, analyses carried out on salt minerals from Messinian and Tortonian evaporites in Sicily and Spain show that their overall REE content can be considered a discriminating parameter between authigenic minerals and diagenetic modified materials. However, features of shale-normalised REE patterns are driven by the mineralogical composition of evaporites rather than their authigenic or secondary nature. On the contrary, the Zr/Hf signature of salt minerals is influenced by their origin. Indeed, subchondritic Zr/Hf values are found in primary salt minerals, whereas larger Zr/Hf values are recognised in those diagenetically modified.Calculated distribution coefficients of Zr, Hf and REE are employed for modelling the REE distribution in halite equilibrated with the deep-sea brines from Typo, Medee and Thetis basins (Eastern Mediterranean). The obtained indications allow us to discriminate brines formed by dissolution of evaporites relative to those representing relics of fossil evaporated seawater.

Milli-channel mixer and phase separator for solvent extraction of rare earth elements

A process intensification study was performed on the standard mixer–settler unit used in recycling of rare earth elements (REE) in lamp phosphors. A phase separation design was constructed to operate coupled to a milli-channel mixer. For this particular design, the phase separation is not driven by differences in density but by differences in wetting and pressure drop across a hydrophobic membrane. The phase separation device was selected since it is easier to scale than other devices seen in literature. The device was shown to decrease the passage time required to separate the phases. The whole mixing and phase separation process was completed in 10–20s in contrast to ∼25min observed in mixer–settlers used in industry. A full factorial parametric study was also performed to observe how the distribution coefficients and separation coefficient (S) of the REE species as well as the time needed for phase separation (τ) is affected by the REE concentration, organic to aqueous phase ratio and initial pH of the REE solution. Results of parametric studies show that the S values vary between 50 and 3 while τ values vary between 17s and 5s. The results also show that increasing phase ratio has a desired decreasing effect on τ but also decreases S, which is undesired. An optimization calculation was also performed to illustrate the tradeoff between S and τ.

Donnan membrane speciation of Al, Fe, trace metals and REEs in coastal lowland acid sulfate soil-impacted drainage waters

Donnan dialysis has been applied to forty filtered drainage waters collected from five coastal lowland acid sulfate soil (CLASS) catchments across north-eastern NSW, Australia. Despite having average pH values<3.9, 78 and 58% of Al and total Fe, respectively, were present as neutral or negatively-charged species. Complementary isotope dilution experiments with 55Fe and 26Al demonstrated that only soluble (i.e. no colloidal) species were present. Trivalent rare earth elements (REEs) were also mainly present (>70%) as negatively-charged complexes. In contrast, the speciation of the divalent trace metals Co, Mn, Ni and Zn was dominated by positively-charged complexes and was strongly correlated with the alkaline earth metals Ca and Mg. Thermodynamic equilibrium speciation calculations indicated that natural organic matter (NOM) complexes dominated Fe(III) speciation in agreement with that obtained by Donnan dialysis. In the case of Fe(II), however, the free cation was predicted to dominate under thermodynamic equilibrium, whilst our results indicated that Fe(II) was mainly present as neutral or negatively-charged complexes (most likely with sulfate). For all other divalent metals thermodynamic equilibrium speciation calculations agreed well with the Donnan dialysis results. The proportion of Al and REEs predicted to be negatively-charged was also grossly underestimated, relative to the experimental results, highlighting possible inaccuracies in the stability constants developed for these trivalent Me(SO4)2− and/or Me–NOM complexes and difficulties in modeling complex environmental samples. These results will help improve metal mobility and toxicity models developed for CLASS-affected environments, and also demonstrate that Australian CLASS environments can discharge REEs at concentrations an order of magnitude greater than previously reported.

Rare earth element fractionation in heterogenite (CoOOH): implication for cobalt oxidized ore in the Katanga Copperbelt (Democratic Republic of Congo)

Heterogenite (CoOOH) is the most abundant cobalt oxide mineral in the Katanga Copperbelt, which hosts around half of the world's known reserves of mineable cobalt. Heterogenite formed by the oxidation of Co-sulfides and accumulated as residual deposits during a Pliocene weathering event. Bulk analysis samples of oxidized cobalt ore samples containing with variable heterogenite concentration display two rare earth element (REE) patterns: (i) Type 1 is enriched in middle REE, with a negative cerium anomaly and a relatively low REE content; (ii) Type 2 is variably enriched in light REE (LREE), without a cerium anomaly and with higher REE content. However, in situ LA-ICP-MS reveals that the Type 2 patterns are due to the mixing of heterogenite with a LREE-rich mineral.Weathering processes leading to heterogenite formation mainly consist of water–rock interactions at high Co activity, in the near-surface environment. These result in the formation of a lateritic deposit. Heterogenite precipitates at near-neutral pH conditions together with manganese oxides. REE are mainly fractionated between these Co- and Mn-oxide minerals. In the deeper part of the oxidized profile, cobalt activity decreases and the heterogenite stability field shifts to alkaline pH conditions due to the dissolution of dolostone in the bedrock. In such an alkaline environment, REE speciation is mainly driven by carbonate complexation/precipitation. This environment would be favorable for the formation of REE-rich carbonate which is intimately associated with heterogenite (LREE-rich Type 2 patterns).The combined whole-rock and in situ geochemical analyses presented here clearly help (1) to distinguish the REE signature of the Co oxidized ore (mineral paragenesis comprising heterogenite) and of heterogenite itself, and subsequently (2) to highlight two different chemical environments for the formation of heterogenite in supergene ores. This study therefore improves the understanding of REE behavior and metal mobility in near-surface environments, and more particularly in environments where the supergene ores form. In the future, the approach developed here can be applied to other Co–Ni–Mn lateritic deposits such as those in New Caledonia and Cameroon.

Rare earth element mineralogy and geochemistry in a laterite profile from Madagascar

In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE+Y concentrations are typical of granitoids (299–363ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9wt% SO3 and 7wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite–jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE+Y reach 2194ppm with high Ce concentrations (1638ppm; 9*Cebedrock) compared to other REE (3–4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing.

Geochemical behavior and speciation modeling of rare earth elements in acid drainages at Sarcheshmeh porphyry copper deposit, Kerman Province, Iran

Acid mine drainage is a major source of water pollution in the Sarcheshmeh porphyry copper mine area. The concentrations of heavy metals and rare earth elements (REEs) in the host rocks, natural waters and acid mine drainage (AMD) associated with mining and tailing impoundments are determined. Contrary to the solid samples, AMDs and impacted stream waters are enriched in middle rare earth elements (MREEs) and heavy rare earth elements (HREEs) relative to light rare earth elements (LREEs). This behavior suggests that REE probably fractionate during sulfide oxidation and acid generation and subsequent transport, so that MREE and HREE are preferentially enriched. Speciation modeling predict that the dominant dissolved REE inorganic species are Ln3+, Ln(SO4)2−, LnSO4+, LnHCO32+, Ln(CO3)2− and LnCO3+. Compared to natural waters, Sarcheshmeh AMD is enriched in REEs and SO42−. High concentrations of SO42− lead to the formation of stable LnSO4+, thereby resulting in higher concentrations of REEs in AMD samples. The model indicates that LnSO4+ is the dissolved form of REE in acid waters, while carbonate and dicarbonate complexes are the most abundant dissolved REE species in alkaline waters. The speciation calculations indicate that other factors besides complexation of the REE's, such as release of MREE from dissolution and/or desorption processes in soluble salts and poorly crystalline iron oxyhydroxy sulfates as well as dissolution of host rock MREE-bearing minerals control the dissolved REE concentrations and, hence, the MREE-enriched patterns of acid mine waters.

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

Competitive mechanisms between rare earth elements (REE) and iron (Fe) for humic acid (HA) binding were 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 (L- and 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 pH3, 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 pH6, i.e. under pH conditions at which hydrolyzed Fe species bind to HA, Fe appears to compete equally for every REE, thereby indicating that Fe has the same relative 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.

Separation of Lanthanides by Continuous Chromatography

Rare earth elements (REE) are widely used in industrial products, such as magnets, superconductors, and batteries. As the worldwide demand of REEs was increasing drastically during the past decade and the applications require very high purity materials, more efficient purification processes are required. Among the purification methods proposed for REE, chromatography has been considered initially, but was then replaced by extraction in the production scale because of its higher cost. In this work, we investigate the possibility of using continuous countercurrent chromatography to decrease the purification cost. In particular, a model mixture of three REE species (praseodymium, cerium, and lanthanum) has been considered with the aim of purifying all three components in a single step to >95% with high yield. It is found that this performance can be achieved with a yield of 75.4% and a productivity increase of 5 to 15-fold with respect to batch chromatography. However, the productivity value remains too low (approximately 1 g per liter stationary phase and day) for large scale applications. This is due to the low loading caused by the limited solubility of the REE in the adopted buffer used. Possibilities to overcome this limitation are discussed.

Speciation of rare earth elements in surface sediments of Lagoon of Aveiro (N Portugal)

Martins, V.A., Dias, J.A., Laut, L.M., Sobrinho, F., Santos, J.F., Rodrigues, M. A., Frontalini, F., Miranda, P., Terroso, D., Fernandez-Fernandez, S., Bernardes, C., Figueira, R., Sousa, S., Amaral, P., Mahiques, M., Bernabeu, A., Rey, D., Rubio, B., and Rocha, F., 2013. Speciation of rare earth elements in surface sediments of Lagoon of Aveiro (N Portugal). In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 64-69, ISSN 0749-0208. The goal of this work is to determine the distribution of the rare earth elements in the surface sediments in the Ria de Aveiro (N Portugal) coastal lagoon. Concentrations of REE (La, Ce, Nd, Sm) and related elements (Th, Y and Sc) were evaluated by total digestion of sediments in the fine fraction of 33 grab-samples of surface sediments. The same method was followed for Al, Ca, S and Zr, related to terrigenous sources, but also, in the cases of Ca and S, to biogenic and quimiogenic processes. Concentrations of REE and related elements were additionally assessed by a three-step sequential chemical extraction (E1-3) procedure coupled with ICP–MS determination. The geochemical results were processed by statistical analysis combining textural and mineralogical data (XRD techniques). The results show that mobile La, Ce, Nd, Sm and Y are mostly adsorbed by amorphous Mn hydroxide (E3), secondly associate with organic matter (E2) and only in a very small extent are adsorbed on clay mineral structural sheets or associated with carbonates (E1). Thorium and Sc are almost exclusively linked with E2. Ytrium shows a very high percentage of availability, whilst Ce, La, Sc and Th are by far less mobile. Results of cluster analysis reveals that sediments are enriched in: i) quartz, feldspars and anhydrite in the most hydrodynamic areas with good connection with the ocean, where tidal currents are stronger and water is salty and welloxygenated; ii) mud, TOC, phyllosilicates, siderite, pyrite and S in low hydrodynamic areas. In these areas, pyrite and siderite can result from reducing bacterial activity in subsurface sediments. Early diagenetic changes in the sediments would also influence the elements adsorption to clay minerals and their co-precipitation with carbonates (E1). The highest values of REE, in E1, occur in distal locations in relation to the mouths of rivers and streams. The retention of the elements in this phase may occur mainly in the lagoon, depending on the availability of carbonates and clay minerals and their assemblage’s composition. Concentrations of REE and related elements adsorbed by organic matter (E2) and by amorphous Mn hydroxide (E3) tend to rise near the rivers mouths and may be related to geochemical processes that take place upstream in the course of the rivers before the entry of sediments into the lagoon. Elemental concentrations retained in the structure of mineral phases (R), also tend to reach high values near the rivers mouths but in low hydrodynamic areas. There is some fractionation between Th and the light REE which is more evident in the Murtosa channel that is fed mainly by the Antua river.

Petrographic and Geochemicial Characteristics of the Kızıloren Formation (Upper Triassic‐Lower Jurassic) in the Akpınar (Konya, Turkey) Area

Abstract:This paper describes the occurrence of dolomite and the mechanism of dolomitization of the Upper Triassic‐Lower Jurassic Kızıloren Formation in the autochthonous Bolkardagı unit of the middle Taurus Mountains in south western Turkey. Dolomites were analyzed for geochemical, isotopic and crystallographic variation. Dolomites occur as a replacement of precursor carbonate and cement. The dolomite crystals range from &lt;10 to ∼1000 μm existing as both replacements and cements. Sr concentrations range between 84 and 156 ppm, and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 0.0066 to 0.013 ratios. Dolomites are Ca‐rich (with average CaCO3 and MgCO3 equal to 56.43 and 43.57 mol%, respectively) and they are non‐stoichiometric, with an average Sr=116 ppm, Na=286 ppm, Mn=81 ppm, Fe=1329 ppm, and δ18O and δ13C ranges from −0.6‰ to −6.1‰ Pee Dee Belemnite [PDB], and +1.2 to +3.9‰ PDB. The North American Shale Composition [NASC]‐normalized rare earth element (REE) values of the both limestone and dolomite sample groups show very similar REE patterns characterized by small positive Eu (mean=1.32 and mean=1.42, respectively) and slightly or considerably negative Ce (mean=0.61 and mean=0.72, respectively) anomalies and a clear depletion in all REE species. The Kızıloren Formation dolomites have been formed as early diagenetic from mixing zone fluids at the tidal‐subtidal environment and at the late diagenetic from basinal brines at the shallow‐deep burial depths.

Geochemistry of rare earth elements in groundwater from the Taiyuan Formation limestone aquifer in the Wolonghu Coal Mine, Anhui province, China

To investigate the geochemistry of rare earth elements in groundwater from the Taiyuan Formation limestone aquifer, the concentrations of rare earth elements (REEs) were determined by ICP-MS for groundwater and limestone samples collected from the Taiyuan Formation limestone aquifer in the Wolonghu Coal Mine, Anhui province, China. The results shown that the groundwater is characterized by warm temperature (ranging from 35.8 to 39.1°C), circum-neutral pH (7.34–8.27) and high total dissolved solids (TDS, 1380–2850mg/L). The REEs in groundwater are characterized by light rare earth elements (LREEs) depletion relative to heavy rare earth elements (HREEs), positive Eu anomalies and negative Ce anomalies. The fractionation of REEs in groundwater can be effected by the inorganic species of REEs. The negative Ce anomalies probably reflect the signature of the aquifer rocks (limestone rocks), whereas positive Eu anomalies are probably attributed to the redox conditions. The similarities of REE patterns between groundwater and aquifer rocks suggest that aquifer rocks play important roles in controlling the REE characteristics of groundwater. The characteristics of REEs and major ions in groundwater from the Taiyuan Formation aquifer are different from other aquifers in the Wolonghu Coal Mine.

Dynamics of rare earth elements in water–soil systems: The case study of the Pineta San Vitale (Ravenna, Italy)

The European Union Site of Community Importance Pineta San Vitale (Ravenna, Italy) is located in a complex lagoon system nearby the Adriatic Sea. This area is affected by relevant environmental pressures related to subsidence, groundwater salinization and industrial contamination. The occurrence of rare earth elements (REE), and the characterization of processes that control their migration in the water–soil system were performed for implementing the knowledge of the present environmental status in the area.Nine soil profiles were opened in the Pineta San Vitale and each horizon was characterized. In the parent material, dominant quartz, followed by plagioclase, mica, chlorite and calcite were recognized by XRD. Topsoil samples were characterized by neutral or sub-acid pH, relatively high organic carbon content (LOI up to 43wt.%; TOC: 38÷61g/kg), and low carbonate (11÷20g/kg CaCO3). Subsoil horizons were sub-alkaline or moderately alkaline, rich in carbonate (up to 114g/kg CaCO3), and depleted in organic matter. The electrical conductivity (EC: 0.2÷4.2 mS/cm) values along the soil profiles did depend on the water table fluctuations and the salinity of groundwater. Sequential extraction tests on two soil profiles showed the highest Y and ΣREE occurrence in the residual fraction of soils, with values generally increasing at depth, and total amounts similar to values estimated in the upper continental crust (UCC). Poor fractionation of REE was observed when the bulk soil horizon was considered, but different REE patterns occurred in the organic, oxide, and carbonate soil fractions, suggesting REE fractionation during weathering and pedogenic processes.Physical–chemical parameters and concentrations of total and dissolved elements were determined in 44 water samples. Waters showed near-neutral or slightly alkaline pH (6.8÷8.0) and total dissolved solids mostly in the range of 2 to 24g/L, with values increasing with increasing seawater influence. Concentrations of ΣREE determined in waters filtered through 0.4μm pore size filters varied from 0.1 to 189μg/L (0.7·10−9÷1.3·10−6mol/L ΣREE), with the highest values observed in soil waters. REE values were not related to salinity, nor to major ions, while were positively correlated to Al concentrations. Further filtration through 0.015μm pore size filters removed about 90% of REE compared to the 0.4μm filtered samples, i.e. the studied waters showed a small quantity of truly dissolved REE species. Speciation modeling results by Visual MINTEQ indicated that truly dissolved REE species are dominated by bi-dentate organic complexes in the soil waters rich in dissolved organic carbon (up to 74mg/L DOC); aqueous complex with the carbonate and bicarbonate ligands increased in waters having low DOC. UCC-normalized patterns of REE in the colloidal fraction of soil waters showed slight enrichments in the middle REE (MREE), likely due to preferential adsorption of the MREE onto <0.4μm particles. UCC-normalized patterns of REE truly dissolved in soil waters showed slight enrichments in the heavy REE (HREE), probably due to dominant low molecular weight DOC in the truly dissolved fraction.For evaluating the potential REE contribution from the atmosphere, 12 vegetation samples (ivy leaves) were submitted to leaching tests using acidulated water as a proxy of rain water. The REE leached from leaves were in the range of 0.5 to 10μg/kg ΣREE, suggesting a significant atmospheric contribution to the water–soil system. UCC-normalized patterns of REE leached from leaves were nearly flat, possibly indicating a dominant geogenic source of atmospheric REE.

Generation of Ce anomalies in SW Sardinian Mn ores

Sedimentary (pedogenic) and hydrothermal Mn ores are hosted by a volcanic succession in south-western Sardinia. The sedimentary Mn ores are characterized by chondrite-normalized Ce anomalies (Ce/Ce*) of 1.08–3.40, whereas the hydrothermal Mn ores have Ce anomalies of 0.35–3.15.The presence of both negative and positive Ce anomalies in the hydrothermal Mn ores is related to the remobilization of rare earth elements (REEs) within the hydrothermal system, which occurred via the dissolution and redistribution of REE-bearing accessory minerals by mineralizing fluids. The REEs were removed from wall-rocks and redeposited within the Mn ores as apatite, monazite, and cerianite, which have textures characteristic of growth into a Mn-oxide matrix during mineralization. In addition, the redistribution of cerium led to a progressive increase in the magnitude of the Ce anomaly from E to W, suggesting that mineralizing fluids moved along an E–W-striking fault close to the Mn ore.In the sedimentary Mn ores, the spatial variability of Ce/Ce* values probably relates to changes in the deposition environment, from organic-rich to organic-poor. Positive Ce anomalies with Ce/Ce* values of 1.08–1.75 are associated with samples from thick sedimentary layers within a paleosoil or samples from veins related to paleosoils, and potentially formed in a environment where organic speciation of the REEs acted to limit cerianite precipitation. Conversely, spheroidal concretions of Mn-oxides within pumice and ash layers have larger positive Ce anomalies (Ce/Ce*>2.5) and likely formed in an organic-free environment matter, with REE speciation in circulating fluids dominated by inorganic species.Finally, given the large range in Ce/Ce* ratios of Mn ores, we stress the need to carefully consider sampling methodologies, to ensure the accurate interpretation and determination of ore deposit models, the identification of mechanisms involved in Ce distribution, and the relationships between Ce and Mn phases.

Behavior of rare earth elements in acid coal mine drainage in Shanxi Province, China

Rare earth element (REE) concentrations were determined in acid mine drainage (AMD), bedrock, pyrite, and coal samples from the Sitai coal mine and the Malan coal mine in Shanxi province, China. The AMD displayed high REE concentrations with typical convex shale-normalized patterns. The REE concentrations in the bedrock samples are one order of magnitude higher than those found in pyrite and coal samples. The high REE concentrations in AMD most likely come from the acidic solution leached out REE in bedrock. Results from laboratory and field experiments show that pH is the most important factor controlling the fractionation of REE; but Fe, Al, and Mn colloids and secondary minerals also affects their fractionation. As the pH increased from 4 to 6, the concentrations of total dissolved REE decreased from 520 to 0.875 µg L−1. Fe and Al in AMD has less influence on the fractionation of dissolved REE than low concentrations of Mn. HREE were preferentially removed by secondary minerals and colloids, followed by MREE. Rare earth element’s speciation modeling indicates that sulfate complexes (LnSO4 + and Ln(SO4) 2 − , 79–91%) and free-metal species (Ln3+, 8.8–21%) are the dominant REE species in the AMD, but the REE-sulfate complexation could not explain the MREE-enriched patterns.

Geochemical characteristics of coal mine discharge vis-à-vis behavior of rare earth elements at Jaintia Hills coalfield, northeastern India

Geochemical characteristics of dissolved metals including the rare earth elements (REEs) in acid mine drainage (AMD) from Jaintia Hills coalfield, Meghalaya were studied. The mine drainage is characterized by low pH with high concentrations of Fe, Al, Mn, Ni, Pb and SO 4 2− and REEs. The average total REE concentration in AMD is 714.7 μg/L, which is much higher in comparison to Sitai coalmine drainage. Mine drainage and most of the stream waters from Jaintia coalfield showed a middle REE-enriched pattern when normalizing against North American Shale Composite (NASC). Speciation calculation revealed that Fe 2+ is the predominant free cation whereas Fe 3+ and Al 3+ prevailed as their sulfate complexes. Further, in REE species, REE–sulfate complex (LnSO 4 +) accounts for > 62%, while free metal (Ln 3+) accounts for 24% to 40%. REE pattern indicates that streams are highly impacted by the AMD. Geochemical modeling indicated the supersaturation of various Fe or Al bearing secondary minerals, which possibly control the mobility of metals and REEs.

An improved description of the interactions between rare earth elements and humic acids by modeling: PHREEQC-Model VI coupling

The Humic Ion Binding Model VI (Model VI) – previously used to model the equilibrium binding of rare earth elements (REE) by humic acid (HA) – was modified to account for differences in the REE constant patterns of the HA carboxylic and phenolic groups, and introduced into PHREEQC to calculate the REE speciation on the HA binding sites. The modifications were shown to greatly improve the modeling. They allow for the first time to both satisfactorily and simultaneously model a large set of multi-REE experimental data with the same set of equations and parameters. The use of PHREEQC shows that the light rare earth elements (LREE) and heavy rare earth elements (HREE) do not bind to HA by the same functional groups. The LREE are preferentially bound to carboxylic groups, whereas the HREE are preferentially bound to carboxy–phenolic and phenolic groups. This binding differentiation might lead to a fractionation of REE–HA patterns when competition between REE and other metals occur during complexation. A survey of the available data shows that competition with Al 3+ could lead to the development of HREE-depleted HA patterns. This new model should improve the hydrochemical modeling of the REE since PHREEQC takes into account chemical reactions such as mineral dissolution/precipitation equilibrium and redox reactions, but also models kinetically controlled reactions and one-dimensional transport.

Organic species of lanthanum in natural environments: Implications to mobility of rare earth elements in low temperature environments

Abstract Naturally occurring organic ligands, such as acetate, citrate, malonate, oxalate, and succinate, play important roles in mobility and accumulation of La and other rare earth elements in low temperature systems under Earth surface conditions. However, a comprehensive and consistent thermodynamic database covering the complexes of rare earth elements with those naturally occurring organic ligands is lacking. In this study, thermodynamic data of organic species of rare earth elements (REE) represented by La, with an emphasis on their aqueous complexes with organic ligands, are critically reviewed. The organic ligands covered by this study include acetate, citrate, malonate, oxalate and succinate. In this critical review, the Specific Interaction Theory (SIT) model is adopted for extrapolation to infinite dilution. This model is a reliable activity coefficient model valid for a wide range of ionic strengths. These critically reviewed data, including complex formation constants, SIT interaction coefficients and solubility product constants, would enable accurate modeling of the speciation and solubility of REE in various environments including high ionic strength environments, providing insight into mobility and enrichment of REE in various environments.

The geochemical characteristics of aqueous rare-earth elements in shallow karst groundwater in Guiyang City, China

Fifty-seven shallow groundwater samples were collected from Guiyang karst basin, China, to analyze the aqueous rare-earth elements in low-water seasons and it is shown that the total amount of rare-earth elements (ΣREE) in karst groundwater is exceedingly low compared with that in carbonate rocks or weathering crusts of carbonate rocks, and ranges from 0.01 to 0.43, from 0.03 to 0.27, from 0.03 to 0.19 and from 0.05 to 1.38 μg·L−1 for dolomite, dolomitic & limestone, limestone and clastic rock aquifer, respectively. Both distributions and contents of rare-earth elements (REE) in karst groundwater reflect the lithology of host rocks or weathering crusts of carbonate rocks through which groundwater flows. The chondrite-normalized patterns show a non-flat profile with higher enrichment of slightly light rare-earth elements (LREE) than heavy rare-earth elements (HREE), prominent fractionation between LREE and HREE, negative Ce anomalies and negative or positive Eu anomalies. There is more obvious fractionation between LREE and HREE in groundwater than that in carbonate rocks and their weathering crusts due to high contents of HCO3− and PH in groundwater. In shallow karst groundwater, REE(CO3)n2n−3 (n=1 and 2) is the main inorganic species of REE. But for a clastic rock aquifer, both REESO4+ and REECO3+ are the main inorganic species of REE. Species of REE in groundwater is closely associated with the hydrochemical type of groundwater which is predominated by the lithology of host rocks, groundwater-rock interaction and weathering-pedogenesis of carbonate rocks.