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1.
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2.
The complexation of Cd(II) and Cd(II)-phthalate at the goethite/water interface were investigated by EXAFS and IR spectroscopy, by batch adsorption experiments and by potentiometric titrations at 298.15 K. The EXAFS spectra showed Cd(II) to form only inner-sphere corner-sharing complexes with the goethite surface sites in the presence and absence of phthalate. EXAFS spectra also showed the presence of Cd(II)-chloride complexes in 0.1 mol/L NaCl. IR spectra also showed phthalate to form (1) an inner-sphere complex with adsorbed corner-sharing Cd(II) surface complexes in the pH 3.5 to 9.5 and (2) an outer-sphere complex with the same type of corner-sharing Cd(II) complex however at pH > 6, in addition to the inner- and outer-sphere complexes of phthalate reported in a previous study. The potentiometric titration and the batch adsorption data were used to constrain the formation constants of the different Cd(II)-phthalate surface complexes on the dominant {110} and the {001} planes of the goethite. The models were carried out with the Charge Distribution Multisite Complexation model coupled to the Three Plane Model and can predict the molecular-scale speciation of cadmium and phthalate in the presence of goethite. Cd(II) adsorption models calibrated on a 90 m2/g goethite also could accurately predict experimental data for a 37 m2/g goethite of slightly different basic charging properties.  相似文献   

3.
《Geochimica et cosmochimica acta》1999,63(19-20):2929-2938
The competitive sorption of Cu(II) and Pb(II) to colloidal hematite was investigated as a function of pH and total metal concentration. Acid–base titrations of the hematite and single-metal sorption experiments for Cu and Pb at low to medium surface coverages were used to calibrate two surface complexation models, the triple layer model, and a 2-pK basic Stern model with ion-pair formation. The surface site density was systematically varied from 2 to 20 sites/nm2. Three different metal surface complexes were considered: (1) an inner-sphere metal complex; (2) an outer-sphere metal complex; and (3) an outer-sphere complex of singly hydrolyzed metal cations. Both models provided excellent fits to acid–base titration and single-metal sorption data, regardless of the surface site density used. With increasing site density, ΔpK of the stability constants for protonation reactions increased and metal surface complexes decreased steadily. The calibrated models based on different site densities were used to predict competitive sorption effects between Cu and Pb and single-metal sorption at higher total metal concentrations. Precipitation of oversaturated solid phases was included in the calculations. Best predictions of competitive sorption effects were obtained with surface site densities between 5 and 10 sites/nm2. The results demonstrate that surface site density is a key parameter if surface complexation models are exposed to more complex, multicomponent environments. We conclude that competitive metal sorption experiments can be used to obtain additional information about the relevant surface site density of oxide mineral surfaces.  相似文献   

4.
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5.
The types and structures of adsorption complexes formed by oxalate at boehmite (γ-AlOOH)/water and corundum (α-Al2O3)/water interfaces were determined using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy and quantum chemical simulation methods. At pH 5.1, at least four different oxalate species were found at or near the boehmite/water interface for oxalate surface coverages (Γox) ranging from 0.25 to 16.44 μmol/m2. At relatively low coverages (Γox < 2.47), strongly adsorbed inner-sphere oxalate species (IR peaks at 1286, 1418, 1700, and 1720 cm−1) replace weakly adsorbed carbonate species, and a small proportion of oxalate anions are adsorbed in an outer-sphere mode (IR peaks at 1314 and 1591 cm−1). IR peaks indicative of inner-sphere adsorbed oxalate are also observed for oxalate at the corundum/water interface at Γox = 1.4 μmol/m2. With increasing oxalate concentration (Γox > 2.47 μmol/m2), the boehmite surface binding sites for inner-sphere adsorbed oxalate become saturated, and excess oxalate ions are present dominantly as aqueous species (IR peaks at 1309 and 1571 cm−1). In addition to these adsorption processes, oxalate-promoted dissolution of boehmite following inner-sphere oxalate adsorption becomes increasingly pronounced with increasing Γox and results in an aqueous Al(III)-oxalate species, as indicated by shifted IR peaks (1286 → 1297 cm−1 and 1418 → 1408 cm−1). At pH 2.5, no outer-sphere adsorbed oxalate or aqueous oxalate species were observed. The similarity of adsorbed oxalate spectral features at pH 2.5 and 5.1 implies that the adsorption mechanism of aqueous HOx− species involves loss of protons from this species during the ligand-exchange reaction. As a consequence, adsorbed inner-sphere oxalate and aqueous Al(III)-oxalate complexes formed at pH 2.5 have coordination geometries very similar to those formed at pH 5.1.The coordination geometry of inner-sphere adsorbed oxalate species was also predicted using quantum chemical geometry optimization and IR vibrational frequency calculations. Geometry-optimized Al8O12 and Al14O22 clusters with the reactive surface Al site coordinated by three oxygens were used as model substrates for corundum and boehmite surfaces. Among the models considered, calculated IR frequencies based on a bidentate side-on structure with a 5-membered ring agree best with the observed frequencies for boehmite/oxalate/water samples at Γox = 0.25 to 16.44 μmol/m2 and pH 2.5 and 5.1, and for a corundum/oxalate/water sample at Γox = 1.4 μmol/m2 and pH 5.1. Based on these results, we suggest that oxalate bonding on boehmite and corundum surfaces results in 5-coordinated rather than 4- or 6-coordinated Al surface sites.  相似文献   

6.
《Geochimica et cosmochimica acta》1999,63(19-20):3039-3048
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7.
Interactions between aqueous amino acids and mineral surfaces influence many geochemical processes from biomineralization to the origin of life. However, the specific reactions involved and the attachment mechanisms are mostly unknown. We have studied the adsorption of l-aspartate on the surface of rutile (α-TiO2, pHPPZC = 5.4) in NaCl(aq) over a wide range of pH, ligand-to-solid ratio and ionic strength, using potentiometric titrations and batch adsorption experiments. The adsorption is favored below pH 6 with a maximum of 1.2 μmol of adsorbed aspartate per m2 of rutile at pH 4 in our experiments. The adsorption decreases at higher pH because the negatively charged aspartate molecule is repelled by the negatively charged rutile surface above pHPPZC. At pH values of 3-5, aspartate adsorption increases with decreasing ionic strength. The adsorption of aspartate on rutile is very similar to that previously published for glutamate (Jonsson et al., 2009). An extended triple-layer model was used to provide a quantitative thermodynamic characterization of the aspartate adsorption data. Two reaction stoichiometries identical in reaction stoichiometry to those for glutamate were needed. At low surface coverages, aspartate, like glutamate, may form a bridging-bidentate surface species binding through both carboxyl groups, i.e. “lying down” on the rutile surface. At high surface coverages, the reaction stoichiometry for aspartate was interpreted differently compared to glutamate: it likely involves an outer-sphere or hydrogen bonded aspartate surface species, as opposed to a partly inner-sphere complex for glutamate. Both the proposed aspartate species are qualitatively consistent with previously published ATR-FTIR spectroscopic results for aspartate on amorphous titanium dioxide. The surface complexation model for aspartate was tested against experimental data for the potentiometric titration of aspartate in the presence of rutile. In addition, the model correctly predicted a decrease of the isoelectric point with increased aspartate concentration consistent with previously published studies of the aspartate-anatase system. Prediction of the surface speciation of aspartate on rutile indicates that the relative proportions of the two complexes are a strong function of environmental conditions, which should be taken into account in considerations of geochemical systems involving the interactions of biomolecules and minerals in electrolyte solutions.  相似文献   

8.
The distribution and dynamics of water molecules and monovalent cations (Li+, Na+, K+, Cs+, and H3O+) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs+ and Cl− ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li+ ions near the muscovite surface. For Na+, K+, Cs+, and H3O+ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li+, Na+, K+, and H3O+ ions adsorbed on the muscovite surfaces. The water molecules around Cs+ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs+ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li+ ions and decreased with an increase in the ionic radius.  相似文献   

9.
Despite the fact that the bulk compositions of most low temperature natural surface waters, groundwaters, and porewaters are heavily influenced by alkaline earths, an understanding of the development of proton surface charge in the presence of alkaline earth adsorption on the surfaces of minerals is lacking. In particular, models of speciation at the mineral-water interface in systems involving alkaline earths need to be established for a range of different minerals. In the present study, X-ray standing wave results for Sr2+ adsorption on rutile as a tetranuclear complex [Fenter, P., Cheng, L., Rihs, S., Machesky, M., Bedyzk, M.D., Sturchio, N.C., 2000. Electrical double-layer structure at the rutile-water interface as observed in situ with small-period X-ray standing waves. J. Colloid Interface Sci.225, 154-165] are used as constraints for all the alkaline earths in surface complexation simulations of proton surface charge, metal adsorption, and electrokinetic experiments referring to wide ranges of pH, ionic strength, surface coverage, and type of oxide. The tetranuclear reaction
4>SOH+M2++H2O=(>SOH)2(>SO-)2_M(OH)++3H+  相似文献   

10.
Macro- and molecular-scale knowledge of uranyl (U(VI)) partitioning reactions with soil/sediment mineral components is important in predicting U(VI) transport processes in the vadose zone and aquifers. In this study, U(VI) reactivity and surface speciation on a poorly crystalline aluminosilicate mineral, synthetic imogolite, were investigated using batch adsorption experiments, X-ray absorption spectroscopy (XAS), and surface complexation modeling. U(VI) uptake on imogolite surfaces was greatest at pH ∼7-8 (I = 0.1 M NaNO3 solution, suspension density = 0.4 g/L [U(VI)]i = 0.01-30 μM, equilibration with air). Uranyl uptake decreased with increasing sodium nitrate concentration in the range from 0.02 to 0.5 M. XAS analyses show that two U(VI) inner-sphere (bidentate mononuclear coordination on outer-wall aluminol groups) and one outer-sphere surface species are present on the imogolite surface, and the distribution of the surface species is pH dependent. At pH 8.8, bis-carbonato inner-sphere and tris-carbonato outer-sphere surface species are present. At pH 7, bis- and non-carbonato inner-sphere surface species co-exist, and the fraction of bis-carbonato species increases slightly with increasing I (0.1-0.5 M). At pH 5.3, U(VI) non-carbonato bidentate mononuclear surface species predominate (69%). A triple layer surface complexation model was developed with surface species that are consistent with the XAS analyses and macroscopic adsorption data. The proton stoichiometry of surface reactions was determined from both the pH dependence of U(VI) adsorption data in pH regions of surface species predominance and from bond-valence calculations. The bis-carbonato species required a distribution of surface charge between the surface and β charge planes in order to be consistent with both the spectroscopic and macroscopic adsorption data. This research indicates that U(VI)-carbonato ternary species on poorly crystalline aluminosilicate mineral surfaces may be important in controlling U(VI) mobility in low-temperature geochemical environments over a wide pH range (∼5-9), even at the partial pressure of carbon dioxide of ambient air (pCO2 = 10−3.45 atm).  相似文献   

11.
The ability to predict the fate and transport of arsenic in aquatic environments, its impact on water quality and human health, and the performance and cost-effectiveness of water treatment systems relies on understanding how it interacts with solid surfaces. In situ resonant surface X-ray scattering measurements of arsenate adsorption at pH 5 in 0.01 M NaCl on corundum and hematite (012) surfaces demonstrate that arsenate surface complexation is unexpectedly bimodal, adsorbing simultaneously as inner- and outer-sphere species. In addition, this bimodal behavior is found to be independent of the total arsenate solution concentration, and thus surface coverage, over the range of 10−6 to 10−3 M. Alternative mechanisms to produce the observed As distributions, such as arsenate dimerization or surface precipitation of an aluminum or ferric arsenate, are inconsistent with the experimentally-determined total and As-specific density profiles. Based on the location of the outer-sphere arsenate in relation to the surfaces studied, possible binding mechanisms include electrostatic attraction, hydrogen bonding to surface oxygen functional group, and configurational stabilization by interfacial water. Although the observation of outer-sphere arsenate surface complexes on a metal oxide surface is unprecedented, it is unclear if such species were absent in previous molecular-scale studies, as it is difficult for methods commonly used to investigate the mechanisms of arsenate adsorption to conclusively identify or rule out the presence of outer-sphere species when inner-sphere species are also present.  相似文献   

12.
Adsorption of Rb+ and Sr2+ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb+ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K+ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å2). These results are consistent with an ion-exchange reaction in which Rb+ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO3)2 solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr2+ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr2+ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb+, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr2+ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr2+ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr2+ on orthoclase. Differences in the partitioning of Sr2+ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.  相似文献   

13.
A combination of macroscopic experiments and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to study Cd(II)-sulfate interactions on the goethite-water interface. The presence of SO4 dramatically promoted Cd adsorption at lower pH (pH 5.5-6.5) and had a smaller effect at higher pH. ATR-FTIR studies indicated sulfate adsorption on goethite occurred via both outer- and inner-sphere complexation. The relative importance of both complexes was a function of pH and sulfate concentration. ATR-FTIR spectra provided direct evidence of the formation of Cd-SO4 ternary surface complexes on goethite. In addition to ternary complexes, Cd specifically sorbed on goethite promoted SO4 adsorption via changing the surface charge, and caused additional SO4 adsorption as both inner- and outer-sphere complexes. The relative importance of ternary complexes versus electrostatic effects depended upon pH values and Cd concentration. Ternary complex formation was promoted by low pH and high Cd levels, whereas electrostatic effects were more pronounced at high pH and low Cd levels. A portion of SO4 initially sorbed in inner-sphere complexes in the absence of Cd was transformed into Cd-SO4 ternary complexes with increased Cd concentration.  相似文献   

14.
The structure and dynamics of water on muscovite mica (0 0 1) surfaces have been investigated by molecular dynamics simulations. X-ray reflectivity profiles highly reflecting the interfacial structure are directly calculated and compared with those of experiments. The direct comparison has validated the usefulness of MD simulations to understand the real interfacial structure of the mica−water system. We observed five distinguished peaks in the density profile of oxygen present in water, and these peaks are attributable to the water molecules directly adsorbed on mica, hydrated to the K+ ions on the mica surface, and ordered due to hydrogen bonds between hydrated K+ ions. The hydrated K+ ions make an inner-sphere complex and have an explicit first hydration shell with a radius of 3.6 Å and a hydration number of 2.9. The change of the viscosity of water located above 1 nm apart from the mica surface was not observed. This feature is in good agreement with a recent experimental study in which the shear measurement was conducted using a surface forces apparatus. The increase of the viscosity by a factor of ca. 2-3 relative to that of the bulk water was observed at water located within 1 nm from the isolated mica surface.  相似文献   

15.
《Geochimica et cosmochimica acta》1999,63(19-20):3121-3131
Because of a recent increase in interest in the properties of the calcite surface, there has also been an increase in activity toward development of mathematical models to describe calcite’s surface behaviour, particularly with respect to adsorption and precipitation. For a mathematical model to be realistic, it must be based on a sound conceptual model of atomic structure at the interface. New observations from high resolution techniques have been combined with previously published data to resolve the apparent conflict with results from electrokinetic studies and to present a picture of what the calcite surface probably looks like at the atomic scale.In ultra-high vacuum (10−10 mbar), a cleaved surface remains unreacted for at least an hour, but the unreacted surface does not remain as a termination of the bulk structure. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) show that the outer-most atomic layer relaxes and the surface slightly restructures. In air, dangling bonds are satisfied by hydrolysed water. XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveal the presence of adsorbed OH and H. In AFM images, the features so typical of calcite, namely, alternate-row offset, pairing and height difference, as well as the consistent dependence of these features on the force and direction of tip scanning, are best explained by OH filling of the vacant O sites created during cleavage on the Ca octahedra. Thus there is solid evidence to indicate the presence of OH and H chemi-sorbed at the termination of the bulk calcite structure.Wet chemical studies, however, show that calcite’s pHpzc (zero point of charge) varies with sample history and solution composition. Electrophoretic mobility measurements indicate that the potential-determining ions are not H+ and OH−, but rather Ca2+ and CO32− (or HCO3− or H2CO30). This apparent conflict is resolved by a slight modification of the electrical double layer (EDL) model. At the bulk termination, hydrolysis species are chemi-bonded. At the Stern layer, adsorption attaches Ca2+ and CO32− (or other carbonate species), but the hydrolysis layer keeps them in outer-sphere coordination to the surface. With dehydration, loss of the hydrolysis species results in direct contact between adsorbed ions and the bulk termination, therefore, inner-sphere sorption is equivalent to extension of the three dimensional bulk network, which is precipitation. Attachment of ions with size and charge compatible with Ca and CO3 likewise results in coprecipitation and solid–solution formation.  相似文献   

16.
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17.
Boron is an important micronutrient for plants but is toxic at high pore solution concentrations. Its mobility and migration in many geochemical environments is often controlled by reactions with mineral surfaces, and thus its speciation on mineral surfaces has been extensively investigated. Most previous studies have used IR spectroscopy to characterize the surface B-environments. We present here the first 11B MAS NMR study of surface sorbed boron on minerals. The results demonstrate the capability of this method to effectively probe the local structure of the sorption sites at total B-concentrations in the samples as small as 0.03 wt% and to provide insight into the mechanisms of sorption. Signal is readily resolved for both trigonal (B(3)) and tetrahedral (B(4)) boron exchanged onto boehmite, silica gel and illite, and the resonances are readily assigned on the basis of chemical shift and quadrupole coupling constant. Boron surface densities on illite are approximately order of magnitude greater than on silica gel or boehmite. For boehmite, both B(3) and B(4) occur dominantly as inner-sphere complexes formed by ligand exchange reaction with surface aluminol sites. The B(3)/[B(3) + B(4)] ratio of approximately 0.87 does not vary significantly with pH from 3 to 11, with solution B-concentration, or with washing. The occurrence of B(3) and B(4) as inner-sphere complexes is in agreement with previous suggestions from IR studies of B-sorption on iron hydroxide, allophone, kaolinite, and hydrous ferric oxide. For silica gel, B(3) and B(4) occur principally as outer-sphere complexes or as residual precipitate from un-removed solution. The B(3)/B(4) ratio decreases with increasing pH paralleling the speciation in solution, but the relative abundance of B(4) is greater than in solution. A small fraction of the B(4) occurs as inner-sphere complexes with B(4)-O-Si linkages formed by ligand exchange reaction with silanol sites. For illite, surface boron occurs as outer-sphere B(3) and B(4), as for silica gel, and as inner-sphere B(3) and B(4), as for boehmite. Outer-sphere B(3) and B(4) are dominant at pH 3 and 5, whereas inner-sphere B(3) and B(4) are dominant at pH 9 and 11. The inner-sphere complexes probably form dominantly by ligand exchange reactions involving sites on the broken edges of illite layers.  相似文献   

18.
《Geochimica et cosmochimica acta》1999,63(19-20):2957-2969
Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAPS) spectroscopic measurements were performed on Pb(II)ethylenediaminetetraacetic (EDTA) adsorbed on goethite as a function of pH (4–6), Pb(II)EDTA concentration (0.11–72 μM), and ionic strength (16 μM–0.5 M). FTIR measurements show no evidence for carboxylate-Fe(III) bonding or protonation of EDTA at Pb:EDTA = 1:1. Both FTIR and EXAFS spectroscopic measurements suggest that EDTA acts as a hexadentate ligand, with all four of its carboxylate and both of its amine groups bonded to Pb(II). No evidence was observed for inner-sphere Pb(II)-goethite bonding at Pb:EDTA = 1:1. Hence, the adsorbed complexes should have composition Pb(II)EDTA2−. Because substantial uptake of PbEDTA(II)2− occurred in the samples, we interpret that Pb(II)EDTA2− adsorbed as outer-sphere complexes and/or as complexes that lose part of their solvation shells and hydrogen bond directly to goethite surface sites. We propose the term “hydration-sphere” for the latter type of complexes because they should occupy space in the primary hydration spheres of goethite surface functional groups and to distinguish this mode of sorption from common structural definitions of inner- and outer-sphere complexes. The lack of evidence for inner-sphere EDTA-Fe(III) bonding suggests that previously proposed metal/ligand-promoted dissolution mechanisms should be modified, specifically to account for the presence of outer-sphere precursor species.  相似文献   

19.
The distribution of yttrium and the rare earth elements (YREE) between natural waters and oxide mineral surfaces depends on adsorption reactions, which in turn depend on the specific way in which YREE are coordinated to mineral surfaces. Recent X-ray studies have established that Y3+ is adsorbed to the rutile (1 1 0) surface as a distinctive tetranuclear species. However, the hydrolysis state of the adsorbed cation is not known from experiment. Previous surface complexation models of YREE adsorption have suggested two to four cation hydrolysis states coexisting on oxide surfaces. In the present study, we investigate the applicability of the X-ray results to rare earth elements and to several oxides in addition to rutile using the extended triple-layer surface complexation model. The reaction producing a hydrolyzed tetranuclear surface species
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20.
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 LnOH2+, LnSO4+, and LnCO3+, but not that of Ln(CO3)2- or LnPO4o 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 (Ln3+) 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 (Ln3+) but also as aqueous complexes (e.g., as Ln(CO3)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.  相似文献   

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