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1.
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   

2.
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   

3.
The adsorption of five toxic metallic cations, Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), onto montmorillonite was investigated as a function of pH and ionic strength and a two-site surface complexation model was used to predict the adsorption data. The results showed that in the lower pH range, 3∼6 for Cd, Cu, Ni and Zn, and 3∼4.5 for Pb, the adsorption was greatly affected by ionic strength, while in the higher pH range, the adsorption was not. In the lower pH range, the metallic cations were mainly bound through the formation of outer-sphere surface on the permanently charged basal surface sites (≡X−), while in the higher pH range the adsorption occurred mainly on the variably charged edge sites (≡SOH) through the formation of inner-sphere surface complexes. Acid-base surface constants and metal binding constants for the two sites were optimized using FITEQL. The adsorption affinity of the five metallic cations to the permanently charged sites of montmorillonite was Pb > Cu > Ni ≈ Zn ≈ Cd, while that to the variable charged sites was Pb ? Cu > Zn > Cd > Ni.  相似文献   

4.
《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   

5.
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   

6.
The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO22+) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10−3 M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)6] octahedral sites over [Al(O,OH)6] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO2. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO2.  相似文献   

7.
The adsorption of oxalate and malonate at the water-goethite interface was studied as a function of pH and total ligand concentrations by means of quantitative adsorption measurements and attenuated total reflectance Fourier transform infrared spectroscopy. The obtained results conclusively showed that oxalate and malonate both form outer-sphere and inner-sphere surface complexes on goethite, and that these complexes coexist over a broad pH interval. The inner-sphere complexes were favored by low pH, while the relative concentrations of the outer-sphere species increase with increasing pH. Based on comparisons with model complexes characterized by Extended X-Ray Adsorption Fine Structure (EXAFS) and results from theoretical frequency calculations, the structures of the inner-sphere complexes of oxalate and malonate were best described as mononuclear five- and six-membered ring chelate structures, respectively. The stability of the inner-sphere complexes followed the trend expected from solutions studies, with the oxalate five-membered ring yielding the more stable complexes compared to the six-membered ring of malonate. The increased stability of the inner-sphere complex of oxalate was manifested in a greater extent of adsorption at acidic pH values. Despite the fact that significant amounts of oxalate and malonate inner-sphere surface complexes were formed, no ligand-promoted dissolution was observed at the experimental conditions in the study.  相似文献   

8.
We have characterized the adsorption of Suwannee River humic acid (SRHA) and Cu(II) on calcite from preequilibrated solutions at pH 8.25. Sorption isotherms of SRHA on calcite follow Langmuir-type behavior at SRHA concentrations less than 15 mg C L−1, whereas non-Langmuirian uptake becomes evident at concentrations greater than 15 mg C L−1. The adsorption of SRHA on calcite is rapid and mostly irreversible, with corresponding changes in electrostatic properties. At pH 8.25, Cu(II) uptake by calcite in the presence of dissolved SRHA decreases with increasing dissolved SRHA concentration, suggesting that formation of Cu-SRHA aqueous complexes is the primary factor controlling Cu(II) sorption at the calcite surface under the conditions of our experiments. We also observed that surface-bound SRHA has little influence on Cu(II) uptake by calcite, suggesting that Cu(II) coordinates to calcite surface sites rather than to surface-bound SRHA.Cu K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopic results show that the local coordination of Cu adsorbed at the calcite surface is very similar in the presence and absence of SRHA. Ca backscatterers at ∼3.90 Å indicate that Cu(II) forms tetragonally distorted inner-sphere adsorption complexes in both binary and ternary systems. Subtle differences in the XANES and EXAFS between binary sorption samples and ternary sorption samples, however, prevent us from ruling out the formation of ternary Cu-SRHA surface complexes. Our findings demonstrate that SRHA plays an important role in controlling the fate and transport of Cu(II) in calcite-bearing systems.  相似文献   

9.
The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite–water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7–9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, binuclear bidentate, complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8–9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0 mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27 Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)–Fe bond distances of ∼2.92–2.94 and 3.41–3.44 Å, respectively. The As–Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As–Fe bonding mechanisms.  相似文献   

10.
Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10−8–10−6 M) in an Ar atmosphere using batch sorption with radioactive 63Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (Kd = ∼10−3 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces.  相似文献   

11.
Bacteria are very efficient sorbents of trace metals, and their abundance in a wide variety of natural aqueous systems means biosorption plays an important role in the biogeochemical cycling of many elements. We measured the adsorption of Cu(II) to Bacillus subtilis as a function of pH and surface loading. Adsorption edge and XAS experiments were performed at high bacteria-to-metal ratio, analogous to Cu uptake in natural geologic and aqueous environments. We report significant Cu adsorption to B. subtilis across the entire pH range studied (pH ∼2-7), with adsorption increasing with pH to a maximum at pH ∼6. We determine directly for the first time that Cu adsorbs to B. subtilis as a (CuO5Hn)n−8 monodentate, inner-sphere surface complex involving carboxyl surface functional groups. This Cu-carboxyl complex is able to account for the observed Cu adsorption across the entire pH range studied. Having determined the molecular adsorption mechanism of Cu to B. subtilis, we have developed a new thermodynamic surface complexation model for Cu adsorption that is informed by and consistent with EXAFS results. We model the surface electrostatics using the 1pK basic Stern approximation. We fit our adsorption data to the formation of a monodentate, inner-sphere RCOOCu+ surface complex. In agreement with previous studies, this work indicates that in order to accurately predict the fate and mobility of Cu in complex biogeochemical systems, we must incorporate the formation of Cu-bacteria surface complexes in reactive transport models. To this end, this work recommends log K RCOOCu+ = 7.13 for geologic and aqueous systems with generally high B. subtilis-to-metal ratio.  相似文献   

12.
《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   

13.
The adsorption of two model siderophores, desferrioxamine B (DFOB) and aerobactin, to lepidocrocite (γ-FeOOH) was investigated by attenuated total reflection infrared spectroscopy (ATR-FTIR). The adsorption of DFOB was investigated between pH 4.0 and 10.6. The spectra of adsorbed DFOB indicated that two to three hydroxamic acid groups of adsorbed DFOB were deprotonated in the pH range 4.0-8.2. Deprotonation of hydroxamic acid groups of adsorbed DFOB at pH values well below the first acid dissociation constant of solution DFOB species (pKa = 8.3) and well below the point of zero charge of lepidocrocite (pHPZC = 7.4) suggested that the surface speciation at the lower end of this pH range (pH 4) is dominated by a surface DFOB species with inner-sphere coordination of two to three hydroxamic acids groups to the surface. Maximum adsorption of DFOB occurred at approximately pH 8.6, close to the first pKa value of the hydroxamic acid groups, and decreased at lower and higher pH values.The spectra of adsorbed aerobactin in the pH range 3-9 indicated at least three different surface species. Due to the small spectral contributions of the hydroxamic acid groups of aerobactin, the interactions of these functional groups with the surface could not be resolved. At high pH, the spectral similarity of adsorbed aerobactin with free aerobactin deprotonated at the carboxylic acid groups indicated outer-sphere complexation of the carboxylate groups. With decreasing pH, a significant peak shift of the asymmetric carboxylate stretch vibration was observed. This finding suggested that the (lateral) carboxylic acid groups are coordinated to the surface either as inner-sphere complexes or as outer-sphere complexes that are strongly stabilized at the surface by hydrogen bonding at low pH.  相似文献   

14.
A mixing of metal-loaded acid mine drainage with shallow groundwater or surface waters usually initiates oxidation and/or hydrolysis of dissolved metals such as iron (Fe) and aluminum (Al). Colloidal particles may appear and agglomerate with increasing pH. Likewise chemical conditions may occur while flooding abandoned uranium mines. Here, the risk assessment of hazards requires reliable knowledge on the mobility of uranium (U). A flooding process was simulated at mesocosm scale by mixing U-contaminated acid mine water with near-neutral groundwater under oxic conditions. The mechanism of U-uptake by fresh precipitates and the molecular structure of U bonding were determined to estimate the mobility of U(VI). Analytical and spectroscopic methods such as Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy at the Fe K-edge and the U LIII-edge, and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy were employed. The freshly formed precipitate was identified as colloidal two-line ferrihydrite. It removed U(VI) from solution by sorption processes, while surface precipitation or structural incorporation of U was not observed. EXAFS data suggest a mononuclear inner-sphere, edge-sharing complex of U(VI) with ferrihydrite in the absence of dissolved carbonate. By employing a novel EXAFS analysis method, Monte Carlo Target Transformation Factor Analysis, we could for the first time ascertain a 3-D configuration of this sorption complex without the necessity to invoke formation of a ternary complex. The configuration suggests a slightly tilted position of the adsorbed unit relative to the edge-sharing Fe(O, OH)6 octahedra. In the presence of dissolved carbonate and at pH ∼8.0, a distal carbonate O-atom at ∼4.3 Å supports formation of ternary U(VI)-carbonato surface complexes. The occurrence of these complexes was also confirmed by ATR-FTIR. However, in slightly acidic conditions (pH 5-6) in equilibrium with atmospheric CO2, the U(VI) sorption on ferrihydrite was dominated by the binary complex species Fe(O)2UO2, whereas ternary U(VI)-carbonato surface complexes were of minor relevance. While sulfate and silicate were also present in the mine water, they had no detectable influence on U(VI) surface complexation. Our experiments demonstrate that U(VI) forms stable inner-sphere sorption complexes even in the presence of carbonate and at slightly alkaline pH, conditions which previously have been assumed to greatly accelerate the mobility of U(VI) in aqueous environments. Depending on the concentrations of U(VI) and carbonate, the type of surface complexes may change from binary uranyl-ferrihydrite to ternary carbonato-uranyl-ferrihydrite complexes. These different binding mechanisms are likely to influence the binding stability and retention of U(VI) at the macroscopic level.  相似文献   

15.
Calcium oxalate monohydrate (CaC2O4·H2O—abbreviated as CaOx) is produced by two-thirds of all plant families, comprising up to 80 wt.% of the plant tissue and found in many surface environments. It is unclear, however, how CaOx in plants and soils interacts with metal ions and possibly sequesters them. This study examines the speciation of Sr(II)aq following its reaction with CaOx. Batch uptake experiments were conducted over the pH range 4-10, with initial Sr solution concentrations, [Sr]aq, ranging from 1 × 10−4 to 1 × 10−3 M and ionic strengths ranging of 0.001-0.1 M, using NaCl as the background electrolyte. Experimental results indicate that Sr uptake is independent of pH and ionic strength over these ranges. After exposure of CaOx to Sraq for two days, the solution Ca concentration, [Ca]aq, increased for all samples relative to the control CaOx suspension (with no Sr added). The amount of Sraq removed from solution was nearly equal to the total [Ca]aq after exposure of CaOx to Sr. These results suggest that nearly 90% of the Sr is removed from solution to a solid phase as Ca is released into solution. We suggest that the other 10% is sequestered through surface adsorption on a solid phase, although we have no direct evidence for this. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the molecular-level speciation of Sr in the reaction products. Deconvolutions of the Sr K-edge EXAFS spectra were performed to identify multi-electron excitation (MEE) features. MEE effects were found to give rise to low-frequency peaks in the Fourier transform before the first shell of oxygen atoms and do not affect EXAFS fitting results. Because of potential problems caused by asymmetric distributions of Sr-O distances when fitting Sr K-edge EXAFS data using the standard harmonic model, we also employed a cumulant expansion model and an asymmetric analytical model to account for anharmonic effects in the EXAFS data. For Sr-bearing phases with low to moderate first-shell (Sr-O pair correlation) anharmonicity, the cumulant expansion model is sufficient for EXAFS fitting; however, for higher degrees of anharmonicity, an analytical model is required. Based on batch uptake results and EXAFS analyses of reaction products, we conclude that Sr is dominantly sequestered by a solid phase at the CaOx surface, likely the result of a dissolution-reprecipitation mechanism, to form SrC2O4 of mixed hydration state (i.e. SrOx·nH2O, where n = 0, 1, or 2). Surprisingly, no spectroscopic or XRD evidence was found for a (Sr,Ca)Ox solid solution or for a separate SrCO3 phase. In addition, we found no evidence for Sr(II) inner-sphere sorption complexes on CaOx surfaces based on lack of Sr-Ca second-neighbor pair correlations in the EXAFS spectra, although some type of Sr(II) surface complex (perhaps a type B Sr-oxalate ternary complex or an outer-sphere Sr(II) complex) or some as yet undetected Sr-bearing solid phases are needed to account for approximately 10% of Sr uptake by CaOx. The formation of a hydrated SrOx phase in environments under conditions similar to those of our experiments should retard Sr mobility and could be a significant factor in the biogeochemical cycling of Sr in soils and sediments or in plants and plant litter where CaOx is present.  相似文献   

16.
Arsenic(III) adsorption reactions are thought to play a critical role in the mobility of arsenic in the environment. It is the nature of the As(III) surface species that must be known on a wide variety of minerals and over a range of pH, ionic strength and surface coverage in order to be able to predict adsorption behavior. EXAFS and XANES spectroscopic studies have identified bidentate, binuclear inner-sphere surface species and/or an outer-sphere species, but only a few oxides have been examined. These results need to be integrated with a predictive surface complexation model in order to ascertain the environmental conditions under which the different surface species may be important on a wide range of solids. In the present study, the surface species information from XAFS and XANES studies has been built into a recent extension of the triple-layer model (ETLM) for the formation of inner-sphere complexes of anions that takes into account the electrostatics of water dipole desorption during ligand exchange reactions. The ETLM has been applied to regress surface titration, proton coadsorption, and As(III) adsorption data over extensive ranges of pH, ionic strength, electrolyte type and surface coverage for magnetite, goethite, gibbsite, amorphous hydrous alumina, hydrous ferric oxide (HFO), ferrihydrite, and amorphous iron oxide. Two principal reactions forming inner- and outer-sphere As(III) surface species,
  相似文献   

17.
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   

18.
We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO4Hn)n−6 and binuclear (Cu2O6Hn)n−8 inner-sphere complexes on variable-charge ≡AlOH sites and as Cu2+ on ion exchangeable ≡X--H+ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)6 polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)2Cu(OH)20, tridentate (≡Al3O(OH)2)Cu2(OH)30 and ≡X--Cu2+ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions
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19.
Sorption of Cm(III) and Eu(III) at trace concentrations onto Ca-montmorillonite (SWy-1) and Na-illite (Illite du Puy) has been studied under anaerobic conditions by batch sorption experiments and time-resolved laser fluorescence spectroscopy (TRLFS). Comparison of the results from spectroscopic and batch sorption experiments with Cm and Eu indicates the existence of outer-sphere complexes at pH <4 in the experiments with Na-illite (0.25 g/L solid; 2.5 × 10−7 mol/L Cm; 0.1 mol/L NaClO4). In the case of Ca-montmorillonite, (0.25 g/L solid, 2.5 × 10−7 mol/L Cm or 10−6 mol/L Eu, 0.066 mol/L Ca(ClO4)2), Cm/Eu outer-sphere complexes do not form at significant levels due to the Ca2+ competition for the clay mineral cation-exchange sites. TRLFS spectra indicate the formation of inner-sphere surface complexes at pH >5 for both clay minerals. Five H2O/OH− molecules remain in the first metal ion coordination sphere of the sorbed Eu/Cm. Measured fluorescence lifetimes of sorbed Eu/Cm and peak deconvolution of Cm-spectra are consistent with the formation of surface complexes of the form ≡S-O-Eu/Cm(OH)x(2−x)(H2O)5−x. At pH ≥ 12 Cm becomes incorporated into a surface precipitate at the Ca-montmorillonite surface presumably composed of Ca(OH)2 or calcium silicate hydrate. A dramatic shift of the fluorescence emission band by more than 20 nm and a clear increase in the fluorescence lifetime suggests the almost complete displacement of coordinated H2O and OH−. The pH dependent Eu sorption data obtained in batch experiments are consistent with spectroscopic data on Eu and Cm within experimental uncertainties thus demonstrating the validity of Eu as a homologue for trivalent actinides. Parameterization of a two-site protolysis nonelectrostatic surface complexation and cation exchange model using the batch sorption data and spectroscopic results is discussed in Part 2 of this work.  相似文献   

20.
Hexagonal birnessite (δ-MnO2) is a close analogue to the dominant phase in hydrogenetic marine ferromanganese crusts and nodules. These deposits contain ∼0.25 wt.% Cu which is believed to be scavenged from the overlying water column where Cu concentrations are near 0.1 μg/L. Here, we measured the sorption of Cu on δ-MnO2 as a function of pH and surface loading. We characterized the nature of the Cu sorption complex at pH 4 and 8 using EXAFS spectroscopy and find that, at pH 4, Cu sorbs to birnessite by inner-sphere complexation on the {0 0 1} surface at sites above Mn vacancies to give a three to fourfold coordinated complex with 6 Mn neighbors at ∼3.4 Å. At pH 8, however, we find that some Cu has become structurally incorporated into the MnO2 layer by occupying the vacancy sites to give 6 Mn neighbors at ∼2.91 Å. Density functional calculations on and clusters predict a threefold coordinated surface complex and show that the change from surface complexation to structural incorporation is a response to protonation of oxygens surrounding the vacancy site. Consequently, we propose that the transformation between sorption via surface complex and vacancy site occupancy should be reversible. By fitting the Cu sorption as a function of surface loading and pH to the formation of the observed and predicted surface complex, we developed a surface complexation model (in the basic Stern approximation) for the sorption of Cu onto birnessite. Using this model, we demonstrate that the concentration of inorganic Cu in the deep ocean should be several orders of magnitude lower than the observed total dissolved Cu. We propose that the observed total dissolved Cu concentration in the oceans reflects solubilization of Cu by microbially generated ligands.  相似文献   

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