Adsorption Of Rare Earth Elements Research Articles

Interactions of microorganisms with rare earth ions and their utilization for separation and environmental technology

In recent years, rare earth elements (REEs) have been widely used in various modern technological devices and the global demand for REE has been increasing. The increased demand for REEs has led to environmental exposure or water pollution from rare earth metal mines and various commercial products. Therefore, the development of a safe technology for the separation and adsorption of REEs is very important from the perspective of green chemistry and environmental pollution. In this review, the application and mechanisms of microorganisms for the removal and extraction of REEs from aqueous solutions are described. In addition, the advantages in using microorganisms for REE adsorption and future studies on this topic are discussed.

Recovery and Separation of Rare Earth Elements Using Columns Loaded with DNA-filter Hybrid

Given that the supply of several rare earth elements (REEs) is sometimes limited, recycling REEs used in various advanced materials, such as Nd magnets, is important for realizing efficient use of REE resources. In the present work, the feasibility of using DNA for REE recovery and separation was examined, along with the identification of the binding site of REEs in DNA. In particular, a DNA-cellulose filter paper hybrid was prepared so that DNA-based materials can be used for the separation of REEs using columns loaded with DNA. N,N'-Disuccinimidyl was used as a cross-linker reagent for the fixation of DNA onto a fibrous cellulose filter. The results showed that (i) the DNA-filter hybrid has a sufficiently high affinity to adsorb REEs; (ii) the adsorption capacity was 0.182 mg/g for Nd; and (iii) the affinity of REEs for DNA was stronger for REEs with larger atomic numbers. The difference of the affinity among REEs in the third result was compared with the adsorption patterns of REEs discussed in the literature. The comparison suggests that phosphate in the DNA-filter paper hybrid was responsible for REE adsorption onto the hybrid. The results were supported by the Nd, Dy, and Lu L(III)-edge EXAFS; the REE-P shell was identified for the second neighboring atom, showing the importance of the phosphate site as REE binding sites. The difference in the affinity among REEs suggest that group separation of REEs (such as La, Ce, (Pr and Nd), (Ho, Dy, and Er), (Tb and Gd), (Sm, Eu), Tm, Yb, and Lu) is possible, although complete isolation of each REE from a solution containing all REEs may be difficult. For practical applications, Nd and Fe(III) were successfully separated from a synthetic solution of Nd magnet waste using columns loaded with the DNA-filter hybrid.

A study of differentiation pattern and rare earth elements migration in geochemical and hydrogeochemical environments of Airekan and Cheshmeh Shotori areas (Central Iran)

The Airekan and Cheshmeh Shotori areas are located about 60 km northeast of Khour, in Isfahan province from Central Iran. Research on characteristics and rare earth elements (REE) pattern in hydrogeochemical environments of these areas suggests the same origin for the elements dissolved in groundwater in these areas. Investigation of migration pattern of REE in hydrogeochemical environments shows that the migration and transportation of REE has occurred through chloride complexes. REEs, leached by water/rock interaction from the Airekan granite, are transported by groundwater and then precipitated in the Cheshmeh Shotori area. Study of the Cheshmeh Shotori sediments shows the presence of a sequence of red oxidized and dark layers. Geochemical characteristics of these sediments reveal that their REE characteristics are mainly inherited from the Airekan granite. Changes in the REE pattern of these sediments with depth show that changes in oxidation and reduction process have not played a significant role in controlling their behavior. It is crucial to note that adsorption of REEs dissolved in water by hydrosilicate increases these elements in depth. The REE behavior shows water/rock interaction between the granitic rocks and groundwater as the main factor of solution, migration and precipitation of REEs in the Cheshmeh Shotori area.

Preparation, characterization and adsorptive study of rare earth ions using magnetic GMZ bentonite

The magnetic GMZ bentonite adsorbent was prepared by the co-precipitation method to study adsorption of rare earth elements (REE)(La3+,Eu3+ and Yb3+), especially Eu3+. Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, magnetization measurement, N2 adsorption–desorption isotherms and zeta potential were used to characterize its structure, morphology, surface chemistry and magnetic properties. The removal of REE (%) was determined as a function of contact time, pH, and adsorbent and metal ion concentration in batch experiments. The optimal adsorption condition was pH 6.5 and adsorbent content of 1.0g/L. Use of the Langmuir isotherm showed that maximum adsorption capacity was 18.4mg/g for La (III), 26.1mg/g for Eu (III) and 71.4mg/g for Yb (III). Pseudo-second-order rare equation fit the data well and adsorption was found to occur via chemisorption. The magnetic composite could be regenerated with a 0.5mol/L NaCl solution (pH 2.0) and reused for three times with limited magnetic loss.

Syntheses, Structures, Photoluminescence, and Gas Adsorption of Rare Earth–Organic Frameworks Based on a Flexible Tricarboxylate

Three new organic-lanthanide frameworks based on a flexible H3L acid, namely, [Er3(L)3(H2O)4]·5.64H2O (1), [Dy3(L)3(H2O)4]·2H2O (2), and [Tb3(L)3(H2O)4]·2H2O (3), where H3L = 5-(benzonic-4-ylmethoxy)isophthalic acid, have been synthesized under mild solvothermal conditions. Compounds 1–3 are isostructural, in which L ligands linked the lanthanide(III) ions (M) to generate two kinds of cages, [M2L]6 and [M2L2]12. These cages are further connected through sharing the M(III) atoms to yield a 3D framework with 1D channels. Topologically, the 3D framework belongs to a binodal (3,6)-connected net with the point symbol of (42.6)(44.62.86.103). Furthermore, compounds 1 and 2 display interesting emissions in the near-IR regions. Compound 3 exhibited strong green luminescence upon 249 nm excitation. The adsorption property of 1 has also been investigated.

Geochemical and mineralogical characteristics of ion-adsorption type REE mineralization in Phuket, Thailand

Geochemical and mineralogical studies were conducted on the 12-m-thick weathering profile of the Kata Beach granite in Phuket, Thailand, in order to reveal the transport and adsorption of rare earth elements (REE) related to the ion-adsorption type mineralization. The parent rock is ilmenite-series biotite granite with transitional characteristics from I type to S type, abundant in REE (592 ppm). REE are contained dominantly in fluorocarbonate as well as in allanite, titanite, apatite, and zircon. The chondrite-normalized REE pattern of the parent granite indicates enrichment of LREE relative to HREE and no significant Ce anomaly. The upper part of the weathering profile from the surface to 4.5 m depth is mostly characterized by positive Ce anomaly, showing lower REE contents ranging from 174 to 548 ppm and lower percentages of adsorbed REE from 34% to 68% compared with the parent granite. In contrast, the lower part of the profile from 4.5 to 12 m depth is characterized by negative Ce anomaly, showing higher REE contents ranging from 578 to 1,084 ppm and higher percentages from 53% to 85%. The negative Ce anomaly and enrichment of REE in the lower part of the profile suggest that acidic soil water in an oxidizing condition in the upper part mostly immobilized Ce4+ as CeO2 and transported REE3+ downward to the lower part of the profile. The transported REE3+ were adsorbed onto weathering products or distributed to secondary minerals such as rhabdophane. The immobilization of REE results from the increase of pH due to the contact with higher pH groundwater. Since the majority of REE in the weathered granite are present in the ion-adsorption fraction with negative Ce anomaly, the percentages of adsorbed REE are positively correlated with the whole-rock negative Ce anomaly. The result of this study suggests that the ion-adsorption type REE mineralization is identified by the occurrence of easily soluble REE fluorocarbonate and whole-rock negative Ce anomaly of weathered granite. Although fractionation of REE in weathered granite is controlled by the occurrence of REE-bearing minerals and adsorption by weathering products, the ion-adsorption fraction tends to be enriched in LREE relative to weathered granite.

Rare earth elements in suspended and bottom sediments of the Mandovi estuary, central west coast of India: Influence of mining

Rare earth elements (REEs) in the suspended particulate matter (SPM) of the Mandovi estuary indicated that the mean total-REEs (∑REE) and light REE to heavy REE ratios are lower than that of the average suspended sediment in World Rivers and Post-Archean average Australian shale. High ∑REE were associated with high SPM/low salinity and also with high SPM/high salinity. Although the ∑REE broadly agree with SPM levels at each station, their seasonal distributions along transect are different. SPM increased seaward in the estuary both during the monsoon and pre-monsoon, but consistently low at all stations during the post-monsoon. The mean ∑REE decreased marginally seaward and was <25% at sea-end station than at river-end station. Spatial variations in ∑REE are maximum (64%) during the pre-monsoon. Strong to moderate correlation of ∑REE with Al, Fe and Mn in all seasons indicates adsorption and co-precipitation of REEs with aluminosilicate phases and Fe, Mn-oxyhydroxides. The ratio of mean ∑REE in sediment/SPM is low during the monsoon (1.27), followed by pre-monsoon (1.5) and post-monsoon (1.62). The middle REE- and heavy REE-enriched patterns with positive Ce and Eu anomalies are characteristic at every station and season, both in SPM and sediment. They also exhibit tetrad effect with distinct third and fourth tetrads. Fe-Mn ore dust is the most dominant source for REEs. However, the seasonal changes in the supply of detrital silicates, Fe-Mn ore dust and particulates resuspended from bottom sediments diluted the overall effect of salinity on fractionation and distribution of REEs in the estuary.

Study on the diffusion kinetics of adsorption of heavy rare earth with Cyanex272-P507 impregnated resin

Kinetics of adsorption of heavy rare earth with Cyanex272-P507 impregnated resin was studied. The influences of temperature, concentration, granularity of resin on the exchange degree were investigated by the limited bath method. The result showed that the controlling factor of RE 3+/H + diffusion on Cyanex272-P507 extraction resin was particles diffusion. The process of diffuse kinetics fitted well with the expression developed by Body model. The apparent activation energy was determined as 23.20 kJ/mol. The reaction order was 0.452. In addition, the constant of particles diffusion ( D 0) was 1.352×10 −8 m 2/S and the entropy of diffusion was −47.055 J/(mol·K).

Combination of on-line preconcentration by di(2-ethylhexyl) phosphoric acid resin in the presence of complexing agent with microwave plasma-atomic emission spectrometry for the determination of rare earths

The adsorption of rare earths, lanthanum, cerium, neodymium, and yttrium by di(2-ethylhexyl) phosphoric acid (D2EHPA) resin in the presence of a complexing agent, EDTA, was investigated. Flow injection was selected as the injecting method. Various parameters, sample pH, sample loading time, sample flow rate, EDTA concentration, EDTA flow rate, eluent concentration, and eluent flow rate, were studied and optimized. Under the optimum conditions, the method allowed the determination of rare earths with detection limits of 1.09 μg/L (lanthanum), 3.31 μg/L (cerium), 2.05 μg/L (neodymium), and 1.25 μg/L (yttrium). High repeatabilities were obtained for all the four rare earths (RSD<5%). The proposed method was applied to the determination of the rare earths in water samples with a recovery range of 95.8%-103.3%.

Rare earth elements adsorption onto Carrizo sand: Influence of strong solution complexation

To better understand the mobility and transport of rare earth elements (REE) in groundwater flow systems, the influence of carbonate ions as well as humic substances on REE adsorption onto Carrizo Aquifer sand was investigated as a function of CO 2 partial pressures ( P CO 2 ), pH, ionic strength, and initial REE concentrations. Batch adsorption experiments were carried out at room temperature (23° ± 1 °C) in a glove box under different P CO 2 (i.e., 0 < P CO 2 ≤ 10 − 1.3 atm). Our experiments show that carbonate ions can either increase or decrease REE adsorption depending on the carbonate ion concentrations or the initial REE concentrations. More specifically, the degree that REEs are adsorbed onto the aquifer sand is inversely related to the amount of carbonate species of REEs. For example, an increase of P CO 2 from 0 to 10 − 3.5 atm, or the presence of carbonate ions in low initial REE concentrations (e.g.,10 μg/kg), generally result in enhanced REE adsorption, owing to the low amount of carbonate species of REEs in solution. With further increase of P CO 2 at constant pH (7) and dissolved REE concentrations, solution complexation of REEs with carbonate ions increases and readily outcompetes mineral surface sites for REEs, leading to less REE adsorption. In addition, our experiments show that in the presence of carbonate ions, increasing pH from neutral pH to more alkaline pH leads to decreases in REE adsorption. The presence of humic substances can also either increase or decrease REE adsorption, depending on the pH range of the solution. Humic substances enhance REE adsorption at acidic pH range (i.e., pH < 4) and decrease REE adsorption when pH is above 4. Ionic strength has a negligible effect on REE adsorption in dilute solutions, and only when ionic strength was increased to 0.1 M was a decrease in REE adsorption noticeable. REE adsorption onto Carrizo sand can be successfully described by a model that considers both surface complexation and surface exchange reactions. Specifically, the modeling studies point out the important role that surface exchange reactions play in REE adsorption onto Carrizo sand at high REE loading conditions (i.e., high initial REE concentration at low solid/solution ratio). Model calculations show that strong solution complexation of dissolved REEs with carbonate ions competes for uncomplexed REE ions, [Ln 3+] F, where Ln is any of the lanthanides, and thus decreases REE adsorption at high P CO 2 levels (> 10 − 2.3 atm) or at high pH (> 8.0). Model calculations also indicate that competition between Na + and uncomplexed REE ions, [Ln 3+] F, for exchange sites results in less REE adsorption at high ionic strength (e.g., I = 0.1 M). Both experimental and modeling studies show that REE adsorption onto Carrizo sand is strongly influenced by REE solution complexation reactions, which are sensitive to P CO 2 levels, pH, and the amount and type of solution complexing ligands.

Observations on the Influence of Secondary Me Oxide Additives (Me=Si, Al, Mg) on the Microstructural Evolution and Mechanical Behavior of Silicon Nitride Ceramics Containing RE2O3(RE=La, Gd, Lu)

The evolution of β‐Si3N4microstructures is influenced by the adsorption of rare earth (RE) elements at grain surfaces and by the viscosity of the intergranular phases. Theoretical and scanning transmission electron microscopy studies show that the RE atoms exhibit different tendencies to segregate from the liquid phase to grain surfaces and different binding strengths at these surfaces. When combined with MgO (or Al2O3) secondary additions, the rare earth additives are combined in low‐viscosity intergranular phases during densification and the α‐ to β‐phase transformation and microstructural evolution are dominated by the RE adsorption behavior. On the other hand, a much higher viscosity intergranular phase forms when the RE2O3are combined with SiO2. While the RE adsorption behavior remains the same, phase transformation and microstructure are now dominated by Si3N4solubility and transport in the high liquid phase. By understanding these additive effects, one can develop reinforced microstructures leading silicon nitride ceramics with greatly improved mechanical behavior.

Determination of preferential rare earth adatom adsorption geometries on Si(001)

The adsorption patterns of rare earth atoms on Si(001) were investigated using scanning tunneling microscopy measurements and density functional calculations. Stable configurations were systematically determined via calculation of binding energies of various adatom coverage and adsorption geometry. Competition between inter-adatom hybridization and Coulomb repulsion is the mechanism contributing to binding energy minima associated with commonly observed rare earth adsorption geometries. Comparison of stable configurations with experimental scanning tunneling microscopy images demonstrated accuracy of the theoretical models. This paves a way for the understanding of self-assembly of rare earth disilicide nanowires on vicinal Si(001) substrates.

Rare earth elements in seawater during an iron-induced phytoplankton bloom of the western subarctic Pacific (SEEDS-II)

During an iron-enrichment experiment in the western subarctic Pacific (SEEDS-II), concentrations of dissolved and acid-soluble rare earth elements (REEs) were determined in shallow waters. Detailed vertical profiles of dissolved REEs were compared with those of nutrients in a preliminary survey. The results showed good correlation, except for Ce and Y. Along with phytoplankton growth within the iron-enriched patch, dissolved REEs were decreased in surface waters. The changes in REE concentrations were small but systematic: light REEs were adsorbed preferentially over heavy REEs. This systematic trend is consistent with results of earlier studies of REE adsorption onto biogenic particle surfaces. However, we were unable to detect a decrease of acid-soluble REEs in surface waters. During the phytoplankton bloom decline phase, dissolved REEs were elevated to concentration levels at the preliminary survey, although nutrients were decreased. The liberation of REEs from the suspended particles might be related to disaggregation from large particles to colloid particles, desorption from the particulate phase, and remineralization from organic matter. Outside the Fe-patch, time variations of REEs were also observed during SEEDS-II, indicating that both artificially induced phytoplankton blooms and natural variation of biogenic particles can affect the biogeochemical cycles of REEs in shallow waters within a short time.

First-principles study of rare earth adsorption atβ-Si3N4interfaces

Structural characterization of rare earth adsorption at surfaces or interfaces of $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}$ grains within silicon nitride ceramics has recently been reported by three different groups using $Z$-contrast scanning transmission electron microscopy (STEM) imaging. Here we report the electronic structure basis for these observations and discuss the origin of similarities and differences among the lanthanides characterized in that work. Along with the features that are well described by a first-principles cluster and surface slab models, we identify those differences in the experiment and theory that warrant further investigation. Stereochemical bonding factors are found to determine adsorption site preferences as opposed to ionic size effects. The set of possible bond sites is a characteristic of the $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}$ interface; however the strength of the rare earth--interface bonding is determined by the electronic structure of the nitride surface and the specific adsorbate. This is the principal factor controlling the effects of dopants on the $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\beta}$ phase transformation and on the $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}$ grain growth at high temperature as well as the subsequent microstructure of the ceramic.

Competitive binding of REE to humic acid and manganese oxide: Impact of reaction kinetics on development of cerium anomaly and REE adsorption

The competitive binding of rare earth elements (REE) to purified humic acid (HA) and MnO 2 was studied experimentally using various HA/MnO 2 ratios over a range of pH (3 to 8). MnO 2, humic acid and REE solutions were simultaneously mixed to investigate the kinetics of the competitive reactions. Aqueous REE–HA complex is the dominant species whatever the experiment time, pH and HA/MnO 2 ratio. The value of the distribution coefficients between MnO 2 and solution (log K d Ree/Mno 2 ) increases with the HA/MnO 2 ratio, indicating that part of the REE–HA complexes are adsorbed onto MnO 2. The development of a Ce anomaly appears strongly limited in comparison with inorganic experimental conditions. Throughout the experimental run time, for HA/MnO 2 ratios of less than 0.4, MnO 2 acts as a competitor leading to a partial dissociation of the REE–HA complex. The majority of the dissociated REE is readsorbed onto the MnO 2 surface. The readsorption of REE is expressed by an increased Ce anomaly on the log K d Ree/Mno 2 pattern as well as a change in shape of the coefficient distribution of REE between soluble HA and solution pattern (log K d Ree/HA decrease for the heavy rare earth elements — HREE). Thus, REE are not only bound to MnO 2 as a REE–HA complex, but also as REE(III). Moreover, the competition between HA and MnO 2 for REE binding is shown to be higher at low pH (< 6) and low DOC/Mn ratio. This study partially confirms previous work that demonstrated the control of REE adsorption by organic matter, while shedding more light on the impact of pH as well as complexation reaction competition on long-term REE partitioning between solid surface and organic solutions. The latter point is important as regards to REE speciation under conditions typical of rock and/or mineral alteration.

A rare earth element signature of bacteria in natural waters?

This study shows that rare earth element (REE) distribution patterns between bacteria and water for five different bacterial strains ( Bacillus subtilis, Escherichia coli, Alcaligenes faecalis, Shewanella putrefaciens, and Pseudomonas fluorescens) exhibit a steep increase in the heavy REE (HREE) part of the distribution pattern and a weak peak around the middle REE (MREE) part. Though the REE patterns for B. subtilis and E. coli were already reported in our previous paper [Takahashi, Y., Châtellier, X., Hattori, K.H., Kato, K., Fortin, D., 2005. Adsorption of rare earth elements onto bacterial cell walls and its implication for REE sorption onto natural microbial mats. Chem. Geol., 219, 53–67.], the present results clearly show that the characteristic REE pattern is commonly observed for various bacterial species. In addition, similar REE patterns were observed for bioflims (containing neutrophilic iron-oxidizers and other bacteria) formed in a groundwater discharge area in the Budo pond in Hiroshima (Japan), suggesting that the REE distribution pattern found in the laboratory experiments was reproduced in naturally occurring bacteria in fresh waters. In addition, the REE pattern, characterized by a maximum at MREE and an increase at HREE, was also observed in Fe oxyhydroxide precipitates formed below the same biofilm, which contained bacterial remnants. These results show that the presence of bacteria in natural Fe oxyhydroxides, and possibly in other natural minerals can be reflected in the shape of REE patterns. Such results could potentially be extended to geological materials, such as banded-iron formation (BIF), and used to discuss the effect of microbial activity on the formation of BIF. Although there are still many problems related to the extrapolation of the results obtained for Fe oxyhydroxides in modern water systems to BIF, such as abiotic processes and post-depositional alteration of the REE patterns, our study suggests that the REE pattern can be used as another tool to assess the role of bacteria in mineral formation in natural waters.

Impregnation synthesis of a novel macroporous silica-based TODGA polymeric composite and its application in the adsorption of rare earths in nitric acid solution containing diethylenetriaminepentaacetic acid

A macroporous silica-based N, N, N′, N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) polymeric composite (TODGA/SiO 2-P) was synthesized. It was done through impregnation and immobilization of TODGA molecule into the pores of the SiO 2-P particles utilizing a vacuum sucking technique. The macroporous SiO 2-P particles were the silica-based organic/inorganic composite synthesized by immobilizing styrene–divinylbenzene copolymer inside SiO 2 through the complicated polymerization reaction. The adsorption of rare earth (RE(III)) elements onto TODGA/SiO 2-P was investigated in HNO 3 solution containing diethylenetriaminepentaacetic acid (DTPA), an acidic multi-dentate chelating agent. It was found that in the presence of 0.05 M DTPA, NO 3 - and H + had significant effect on the TODGA/SiO 2-P adsorption due to the competition reactions of RE(III) with different species, H 4DTPA − and H 2DTPA 3−. With an increase in the concentration of NO 3 - from 0.115 M to 3.015 M, the adsorption of RE(III) onto TODGA/SiO 2-P increased noticeably. On the other hand, RE(III) showed strong adsorption at 0.1 M H +, weak adsorption at around pH 2, and no adsorption in excess of pH 2.3. In a 0.1 M H +–0.115 M NO 3 - –0.05 M DTPA solution, a change of the distribution coefficient of RE(III) onto TODGA/SiO 2-P with an increase in atomic number of RE(III) from La(III) to Lu(III) was investigated. The silica-based TODGA/SiO 2-P polymeric composite showed strong adsorption for heavy RE(III) over the light one. In a 0.01 M H +–1.0 M NO 3 - –0.05 M DTPA solution, the effect of the ratio of solid phase to liquid one on the relationship of the distribution coefficient of RE(III) with the change in atomic number of RE(III) was also studied. Based on the complicated disassociation equilibrium of DTPA, the influence of the concentrations of NO 3 - and H + on the adsorption of TODGA/SiO 2-P for RE(III) was demonstrated. This makes the partitioning of RE(III) and MA(III) together from high level liquid waste (HLLW) by the polymeric composite TODGA/SiO 2-P promising.

The effect of humic acid on the adsorption of REEs on kaolin

For a better understanding of adsorption of the rare earth elements (REEs) onto minerals and its controlling factors, adsorption experiments were performed at pH range of from 3 to 10 with kaolin (1500 mg/L) in a matrix of various concentration of NaNO 3 and about 20 μg/L of the total REEs as well as various amounts of humic acid (HA). The adsorption of HA onto the kaolin occurred over a wide pH range and decreases with increasing pH and with increasing HA concentration. The results show that humic acid has ability to either increase or decrease the adsorption of the REEs onto kaolin, depending on pH, which may be related to their speciation distribution, interaction of HA with the mineral surface. Furthermore, the light REEs are more adsorbed onto kaolin in presence of higher concentration of HA, presumably because the increase in HA concentration in the solution enhance stronger complexing of HA with heavy REEs as compare to light REEs. The ionic strength has strong effect on the adsorption of HA and REEs onto the kaolin but little on the REEs fractionation. The results presented here indicate that mineral/water adsorption may generate the enrichment of the dissolved heavy REEs in the presence of a significant amount of humic acid, which is consistent with the fractionation of REEs in the most of natural waters.

Adsorption of rare earth elements onto Carrizo sand: Experimental investigations and modeling with surface complexation

Rare earth element (REE) adsorption onto sand from a well characterized aquifer, the Carrizo Sand aquifer of Texas, has been investigated in the laboratory using a batch method. The aim was to improve our understanding of REE adsorption behavior across the REE series and to develop a surface complexation model for the REEs, which can be applied to real aquifer-groundwater systems. Our batch experiments show that REE adsorption onto Carrizo sand increases with increasing atomic number across the REE series. For each REE, adsorption increases with increasing pH, such that when pH >6.0, >98% of each REE is adsorbed onto Carrizo sand for all experimental solutions, including when actual groundwaters from the Carrizo Sand aquifer are used in the experiments. Rare earth element adsorption was not sensitive to ionic strength and total initial REE concentrations in our batch experiments. It is possible that the differences in experimental ionic strength conditions (i.e., 0.002–0.01 M NaCl) chosen were insufficient to affect REE adsorption behavior. However, cation competition (e.g., Ca, Mg, and Zn) did affect REE adsorption onto Carrizo sand, especially for light rare earth elements (LREEs) at low pH. Rare earth element adsorption onto Carrizo sand can be successfully modeled using a generalized two-layer surface complexation model. Our model calculations suggest that REE complexation with strong surface sites of Carrizo sand exceeds the stability of the aqueous complexes LnOH 2+, LnSO 4 +, and LnCO 3 +, but not that of Ln(CO 3) 2 - or LnPO 4 o in Carrizo groundwaters. Thus, at low pH (<7.3), where major inorganic ligands did not effectively compete with surface sites for dissolved REEs, free metal ion (Ln 3+) adsorption was sufficient to describe REE adsorption behavior. However, at higher pH (>7.3) where solution complexation of the dissolved REEs was strong, REEs were adsorbed not only as free metal ion (Ln 3+) but also as aqueous complexes (e.g., as Ln(CO 3) 2 - in Carrizo groundwaters). Because heavy rare earth elements (HREEs) were preferentially adsorbed onto Carrizo sand compared to LREEs, original HREE-enriched fractionation patterns in Carrizo groundwaters from the recharge area flattened along the groundwater flow path in the Carrizo Sand aquifer due to adsorption of free- and solution-complexed REEs.

Rare-earth adsorption at intergranular interfaces in silicon nitride ceramics: Subnanometer observations and theory

It is shown that the rare earths (REs) have different tendencies to segregate to the grain surfaces in silicon nitride ceramics, and this alters the filling of adsorption sites on the prismatic plane surfaces. Here, the adsorption and binding strengths of RE segregants at prismatic grain surfaces are determined by first-principles calculations. The adsorption of three rare earths (La, Gd, and Lu) is expected to be distinctly different at the predicted adsorption sites, and this is confirmed by atomic-resolution scanning transmission electron microscopy images. This combination of theory and experiment provides a basis for understanding the adsorption behavior of RE elements in silicon nitride ceramics.