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1.
The surface structure of α-Fe2O3(0 0 0 1) was studied using crystal truncation rod (CTR) X-ray diffraction before and after reaction with aqueous Fe(II) at pH 5. The CTR results show the unreacted α-Fe2O3(0 0 0 1) surface consists of two chemically distinct structural domains: an O-layer terminated domain and a hydroxylated Fe-layer terminated domain. After exposing the α-Fe2O3(0 0 0 1) surface to aqueous Fe(II), the surface structure of both co-existing structural domains was modified due to adsorption of Fe at crystallographic lattice sites of the substrate, resulting in six-coordinated adsorbed Fe at the surface. The average Fe-O bond lengths of the adsorbed Fe are consistent with typical Fe(III)-O bond lengths (in octahedral coordination), providing evidence for the oxidation of Fe(II) to Fe(III) upon adsorption. These results highlight the important role of substrate surface structure in controlling Fe(II) adsorption. Furthermore, the molecular scale structural characterization of adsorbed Fe provides insight into the process of Fe(II) induced structural modification of hematite surfaces, which in turn aids in assessing the effective reactivity of hematite surfaces in Fe(II) rich environments.  相似文献   

2.
Many geochemical reactions that control the composition of natural waters, contaminant fate and transport, and biogeochemical element cycling take place at the interface between minerals and aqueous solutions. A fundamental understanding of these important processes requires knowledge of the structure of mineral-water interfaces. High-resolution specular X-ray reflectivity was used to determine the structure of the hematite (0 1 2)-water interface. Relaxation of the surface was observed to be minor, and water was found to order near the hematite surface. Two sites of adsorbed water are inferred to be ordered laterally, one bridging between triply coordinated functional groups and the other bridging between the singly coordinated functional groups on the surface, as steric constraints limit the possible arrangements of water molecules occurring at the observed heights above the hematite surface. Relaxations of the hematite and corundum (0 1 2) surfaces, which are isostructural, are similar and limited primarily to the top most layer of the structures. No significant changes to the interfacial stoichiometry (i.e., partial occupancy of surface species) are observed in either case. The structure of interfacial water is similar on the hematite and corundum (0 1 2) surfaces as well, although water appeared to be less well ordered on the hematite surface. This may be due to expected differences in the oxygen exchange rates from surface functional groups or the apparent better matching of the corundum oxygen lattice to the natural structural ordering in water, and suggests that the dielectric constant gradients of interfacial water may differ on the two surfaces. Similar charging behavior is expected for these surfaces as similar types of surface functional groups are exposed. Although generally similar, subtle differences in the reactivity of hematite and corundum (0 1 2) surfaces to arsenate adsorption, and possibly the adsorption of other species, may be related to the difference in ordering of interfacial water observed in this study.  相似文献   

3.
Ordering of interfacial water at the hematite and corundum (0 0 1)-water interfaces has been characterized using in situ high resolution specular X-ray reflectivity measurements. The hematite (0 0 1) surface was prepared through an annealing process to produce a surface isostructural with corundum (0 0 1), facilitating direct comparison. Interfacial water was found to display a similar structure on this pair of isostructural surfaces. A single layer of adsorbed water having a large vibrational amplitude was present on each surface and additional ordering of water extended at least 1 nm into the bulk fluid, with the degree of ordering decreasing with increasing distance from the surfaces. Consistent with prior studies of the (0 1 2) and (1 1 0) surfaces of hematite and corundum, the configuration of water above the (0 0 1) surfaces is primarily controlled by the surface structure, specifically the arrangement of surface functional groups. However, interfacial water at the (0 0 1) surfaces displayed significantly larger vibrational amplitudes throughout the interfacial region than at other isostructural sets of hematite and corundum surfaces, indicating weaker ordering. Comparison of the vibrational amplitudes of adsorbed water on a series of oxide, silicate, and phosphate mineral surfaces suggests that the presence or absence of a substantial interfacial electrostatic field is the primary control on water ordering and not the surface structure itself. On surfaces for which charge originates dominantly through protonation-deprotonation reactions the controlling factor appears to be whether conditions exist where most functional groups are uncharged as opposed to the net surface charge. The doubly coordinated functional groups on hematite and corundum (0 0 1) surfaces are largely uncharged under slightly acidic to circumneutral pH conditions, leading to weak ordering, whereas singly coordinated groups on (0 1 2) and (1 1 0) surfaces of these phases are always charged, even when the net surface charge is zero, and induce strong water ordering. Surfaces lacking structural charge can thus be divided into two distinct classes that induce either strong or weak ordering of interfacial water. Surface functional group coordination is the ultimate control on this division as it determines the charge state of such groups under different protonation configurations. Ion adsorption and electron transfer processes may differ between these classes of surfaces because of the effect of water ordering strength on interfacial capacitances and hydrogen bonding.  相似文献   

4.
Experiments were performed herein to investigate the rates and products of heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to hematite and goethite, and by Fe(II) associated with a dithionite-citrate-bicarbonate (DCB) reduced natural phyllosilicate mixture [structural, ion-exchangeable, and edge-complexed Fe(II)] containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by DCB-reduced phyllosilicates were not sensitive to pH or to added that adsorbed to the clay. The reduction kinetics were orders of magnitude slower than observed for the Fe(III) oxides, and appeared to be controlled by structural Fe(II). The following affinity series for heterogeneous Tc(VII) reduction by Fe(II) was suggested by the experimental results: aqueous Fe(II) ∼ adsorbed Fe(II) in phyllosilicates [ion-exchangeable and some edge-complexed Fe(II)] ? structural Fe(II) in phyllosilicates ? Fe(II) adsorbed on Fe(III) oxides. Tc-EXAFS spectroscopy revealed that the reduction products were virtually identical on hematite and goethite that were comprised primarily of sorbed octahedral TcO2 monomers and dimers with significant Fe(III) in the second coordination shell. The nature of heterogeneous Fe(III) resulting from the redox reaction was ambiguous as probed by Tc-EXAFS spectroscopy, although Mössbauer spectroscopy applied to an experiment with 56Fe-goethite with adsorbed 57Fe(II) implied that redox product Fe(III) was goethite-like. The Tc(IV) reduction product formed on the DCB-reduced phyllosilicates was different from the Fe(III) oxides, and was more similar to Tc(IV) oxyhydroxide in its second coordination shell. The heterogeneous reduction of Tc(VII) to less soluble forms by Fe(III) oxide-adsorbed Fe(II) and structural Fe(II) in phyllosilicates may be an important geochemical process that will proceed at very different rates and that will yield different surface species depending on subsurface pH and mineralogy.  相似文献   

5.
The fate of the oxoanion arsenate in diverse systems is strongly affected by its adsorption on the surfaces of iron (oxyhydr)oxide minerals. Predicting this behavior in the environment requires an understanding of the mechanisms of arsenate adsorption. In this study, the binding site and adsorption geometry of arsenate on the hematite (0 1 2) surface is investigated. The structure and termination of the hematite (0 1 2)-water interface were determined by high resolution X-ray reflectivity, revealing that two distinct terminations exist in a roughly 3:1 proportion. The occurrence of multiple terminations appears to be a result of sample preparation, and is not intrinsic to the hematite (0 1 2) surface. X-ray standing wave (XSW) measurements were used to determine the registry of adsorbed arsenate to the hematite structure, and thus the binding site and geometry of the resulting surface complex. Arsenate forms a bridging bidentate complex on two adjacent singly coordinated oxygen groups on each of the two distinct terminations present at the hematite surface. Although this geometry is consistent with that seen in past studies, the derived As-Fe distances are longer, the result of the topology of the FeO6 octahedra on the (0 1 2) surface. As EXAFS-derived As-Fe distances are often used to determine the adsorption mechanism in environmental samples (e.g., mine tailings, contaminated sediments), this demonstrates the importance of considering the possible sorbent surface structures and arrangements of adsorbates when interpreting such data.As multiple functional groups are present and multiple binding geometries are possible on the hematite (0 1 2) surface, the XSW data suggest that formation of bridging bidentate surface complexes on singly coordinated oxygen sites is the preferred adsorption mechanism on this and most other hematite surfaces (provided those surfaces contain adjacent singly coordinated oxygen groups). These measurements also constrain the likely reaction stoichiometry, with only the protonation state of the surface complex undetermined. Although bridging bidentate inner-sphere surface complexes comprised the majority of the adsorbed arsenate present on the hematite (0 1 2) surface, there is an additional population of sorbed arsenate species that could not be characterized by the XSW measurements. These species are likely more disordered, and thus more weakly bound, than the bridging bidentate complexes, and may play a role in determining the fate, transport, and bioavailability of arsenate in the environment. Finally, the possibility of obtaining species-specific XSW measurements by tuning the incident beam energy to specific features in a XANES spectrum is described.  相似文献   

6.
The chemical reduction of U(VI) by Fe(II) is a potentially important pathway for immobilization of uranium in subsurface environments. Although the presence of surfaces has been shown to catalyze the reaction between Fe(II) and U(VI) aqueous species, the mechanism(s) responsible for the enhanced reactivity remain ambiguous. To gain further insight into the U-Fe redox process at a complexing, non-conducting surface that is relevant to common organic phases in the environment, we studied suspensions containing combinations of 0.1 mM U(VI), 1.0 mM Fe(II), and 4.2 g/L carboxyl-functionalized polystyrene microspheres. Acid-base titrations were used to monitor protolytic reactions, and Fe K-edge and U L-edge X-ray absorption fine structure spectroscopy was used to determine the valence and atomic environment of the adsorbed Fe and U species. In the Fe + surface carboxyl system, a transition from monomeric to oligomeric Fe(II) surface species was observed between pH 7.5 and pH 8.4. In the U + surface carboxyl system, the U(VI) cation was adsorbed as a mononuclear uranyl-carboxyl complex at both pH 7.5 and 8.4. In the ternary U + Fe + surface carboxyl system, U(VI) was not reduced by the solvated or adsorbed Fe(II) at pH 7.5 over a 4-month period, whereas complete and rapid reduction to U(IV) nanoparticles occurred at pH 8.4. The U(IV) product reoxidized rapidly upon exposure to air, but it was stable over a 4-month period under anoxic conditions. Fe atoms were found in the local environment of the reduced U(IV) atoms at a distance of 3.56 Å. The U(IV)-Fe coordination is consistent with an inner-sphere electron transfer mechanism between the redox centers and involvement of Fe(II) atoms in both steps of the reduction from U(VI) to U(IV). The inability of Fe(II) to reduce U(VI) in solution and at pH 7.5 in the U + Fe + carboxyl system is explained by the formation of a transient, “dead-end” U(V)-Fe(III) complex that blocks the U(V) disproportionation pathway after the first electron transfer. The increased reactivity at pH 8.4 relative to pH 7.5 is explained by the reaction of U(VI) with an Fe(II) oligomer, whereby the bonds between Fe atoms facilitate the transfer of a second electron to the hypothetical U(V)-Fe(III) intermediate. We discuss how this mechanism may explain the commonly observed higher efficiency of uranyl reduction by adsorbed or structural Fe(II) relative to aqueous Fe(II).  相似文献   

7.
Using a simple ionic model with polarizable oxygen ions and dissociating water molecules, we have calculated the energetics governing the distribution of Fe(II)/Fe(III) ions at the reduced (2 × 1) surface of α-Fe2O3 (hematite) (0 1 2) under dry and hydrated conditions. The results show that systems with Fe(II) ions located in the near-surface region have lower potential energy for both dry and hydrated surfaces. The distribution is governed by coupling of the ferrous iron centers to positive charge associated with missing oxygen atoms on the dry reduced (2 × 1) (0 2 1) surface. As the surface is hydroxylated, the missing oxygen rows are filled and protons from dissociated water molecules become the positive charge centers, which couple more weakly to the ferrous iron centers. At the same time, the first-layer iron centers change from fourfold or fivefold coordination to sixfold coordination lowering the potential energy of ferric iron in the first layer and favoring migration of ferrous iron from the immediate surface sites. This effect can also be understood as reflecting stronger solvation of Fe(III) by the adsorbed water molecules and by hydrolysis reactions favoring Fe(III) ions at the immediate surface. The balance between these two driving forces, which changes as a function of hydration, provides a compelling explanation for the anomalous coverage dependence of water desorption in ultra-high vacuum experiments.  相似文献   

8.
Computer modelling techniques were used to elucidate the hydration behaviour of three iron (hydr)oxide minerals at the atomic level: white rust, goethite and hematite. A potential model was first adapted and tested against the bulk structures and properties of eight different iron oxides, oxyhydroxides and hydroxides, followed by surface simulations of Fe(OH)2, α-FeO(OH) and α-Fe2O3. The major interaction between the adsorbing water molecules and the surface is through interaction of their oxygen ions with surface iron ions, followed by hydrogen-bonding to surface oxygen ions. The energies released upon the associative adsorption of water range from 1 to 17 kJ mol−1 for Fe(OH)2, 26 to 80 kJ mol−1 for goethite and 40 to 85 kJ mol−1 for hematite, reflecting the increasing oxidation of the iron mineral. Dissociative adsorption at goethite and hematite surfaces releases larger hydration energies, ranging from 120 to 208 kJ mol−1 for goethite and 76 to 190 kJ mol−1 for hematite.The thermodynamic morphologies of the minerals, based on the calculated surface energies, agree well with experimental morphologies, where these are available. When the partial pressures required for adsorption of water from the gas phase are plotted against temperature for the goethite and hematite surfaces, taking into account experimental entropies for water, it appears that these minerals may well be instrumental in the retention of water during the cyclic variations in the atmosphere of Mars.  相似文献   

9.
Structural characterization of iron oxide-water interfaces provides insight into the mechanisms through which these minerals control contaminant fate and element cycling in soil, sedimentary, and groundwater systems. Ordering of interfacial water and structural relaxations at the hematite (1 1 0) surface have been investigated in situ using high-resolution specular X-ray reflectivity. These measurements demonstrate that relaxations are constrained to primarily the top ∼5 Å of the surface. Near-surface iron atoms do not relax substantially, although the uppermost layer displays an increased distribution width, while the undercoordinated oxygens on the surface uniformly relaxed outward. Two sites of adsorbed water and additional layering of water farther from the surface were observed. Water fully covers the (1 1 0) surface and appears to form a continuous network extending into bulk solution, with positional order decreasing to that of a disordered bulk fluid within 1 nm. The arrangement of water is similar to that on the hematite (0 1 2) surface, which has a similar surface topography, although these surfaces display different vibrational amplitudes or positional disorder of adsorbed water molecules and average spacings of near-surface layered water. Comparison between these surfaces suggests that interfacial water ordering on hematite is controlled primarily by surface structure and steric constraints and that highly ordered water is likely common to most hematite-water interfaces.  相似文献   

10.
The distribution of Mo between seawater and marine ferromanganese oxides has great impacts on concentration and isotopic composition of Mo in modern oxic seawater. To reveal the adsorption chemistry of Mo to ferromanganese oxides, we performed (i) detailed structural analyses of Mo surface complexes on δ-MnO2, ferrihydrite, and hydrogenetic ferromanganese oxides by L3- and K-edge XAFS, and (ii) adsorption experiments of Mo to δ-MnO2 and ferrihydrite over a wide range of pHs, ionic strengths, and Mo concentrations. XAFS analyses revealed that Mo forms distorted octahedral (Oh) inner-sphere complexes on δ-MnO2 whereas it forms a tetrahedral (Td) outer-sphere complex on ferrihydrite. In the hydrogenetic ferromanganese oxides, the dominant host phase of Mo was revealed to be δ-MnO2. These structural information are consistent with the macroscopic behaviors of Mo in adsorption experiments, and Mo concentration in modern oxic seawater can be explained by the equilibrium adsorption reaction on δ-MnO2. In addition, the large isotopic fractionation of Mo between seawater and ferromanganese oxides detected in previous studies can be explained by the structural difference between and adsorbed species on the δ-MnO2 phase in ferromanganese oxides. In contrast, smaller fractionation of Mo isotopes on ferrihydrite is due to little change in the Mo local structures during its adsorption to ferrihydrite.The structures of Mo species adsorbed on crystalline Fe (oxyhydr)oxides, goethite, and hematite were also investigated at pH 8 and I = 0.70 M (NaNO3). Our XAFS analyses revealed that Mo forms inner-sphere complexes on both minerals: Td edge-sharing (46%) and Oh double corner-sharing (54%) for goethite, and Td double corner-sharing (14%) and Oh edge-sharing (86%) for hematite. These structural information, combined with those for amorphous ferrihydrite and δ-MnO2, show the excellent correlation with the magnitude of adsorptive isotopic fractionation of Mo reported in previous studies: the proportion of Oh species or their magnitude of distortion in Mo surface complexes become larger in the order of ferrihydrite < goethite < hematite < δ-MnO2, a trend identical to the magnitude of isotopic fractionation.Based on the comparison with previous reports for Mo surface species on various oxides, the chemical factors that affect Mo surface complex structures were also discussed. The hydrolysis constant of cation in oxides, log KOH (or the acidity of the oxide surfaces, PZC) is well correlated with the mode of attachment (inner- or outer-sphere) of Mo surface complexes. Furthermore, the symmetric change in Mo species from Td to Oh is suggested to be driven by the formation of inner-sphere complexes on specific sites of the oxide surfaces.  相似文献   

11.
Sorption and catalytic oxidation of Fe(II) at the surface of calcite   总被引：1，自引：0，他引：1  
The effect of sorption and coprecipitation of Fe(II) with calcite on the kinetics of Fe(II) oxidation was investigated. The interaction of Fe(II) with calcite was studied experimentally in the absence and presence of oxygen. The sorption of Fe(II) on calcite occurred in two distinguishable steps: (a) a rapid adsorption step (seconds-minutes) was followed by (b) a slower incorporation (hours-weeks). The incorporated Fe(II) could not be remobilized by a strong complexing agent (phenanthroline or ferrozine) but the dissolution of the outmost calcite layers with carbonic acid allowed its recovery. Based on results of the latter dissolution experiments, a stoichiometry of 0.4 mol% Fe:Ca and a mixed carbonate layer thickness of 25 nm (after 168 h equilibration) were estimated. Fe(II) sorption on calcite could be successfully described by a surface adsorption and precipitation model (Comans & Middelburg, GCA51 (1987), 2587) and surface complexation modeling (Van Cappellen et al., GCA57 (1993), 3505; Pokrovsky et al., Langmuir16 (2000), 2677). The surface complex model required the consideration of two adsorbed Fe(II) surface species, >CO3Fe+ and >CO3FeCO3H0. For the formation of the latter species, a stability constant is being suggested. The oxidation kinetics of Fe(II) in the presence of calcite depended on the equilibration time of aqueous Fe(II) with the mineral prior to the introduction of oxygen. If pre-equilibrated for >15 h, the oxidation kinetics was comparable to a calcite-free system (t1/2 = 145 ± 15 min). Conversely, if Fe(II) was added to an aerated calcite suspension, the rate of oxidation was higher than in the absence of calcite (t1/2 = 41 ± 1 min and t1/2 = 100 ± 15 min, respectively). This catalysis was due to the greater reactivity of the adsorbed Fe(II) species, >CO3FeCO3H0, for which the species specific rate constant was estimated.  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.
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   

14.
Batch and column experiments were conducted to examine the capability of naturally formed hematite and siderite to remove As from drinking water. Results show that both minerals were able to remove As from aqueous solutions, but with different efficiencies. In general, each material removed arsenate much more efficiently than As–DMA (dimethylarsinic acid), with the lowest adsorption efficiency for arsenite. The best removal efficiency for As species was obtained using a hematite, with a grain size range between 0.25 and 0.50 mm. The adsorption capacity for inorganic As(V) reached 202 μg/g. The pH generally had a great impact on the arsenate removal by the Fe minerals studied, while arsenite removal was slightly dependent on the initial pH of between 3 and 10. The presence of phosphate always had a negative effect on arsenate adsorption, due to competitive adsorption between them. A column packed with hematite in the upper half and siderite in the lower half with a grain size range of 0.25–0.5 mm proved to be an efficient reactive filter for the removal of all As species, causing a decrease in As concentration from 500 μg/L (including 200 μg/L As(V) as arsenate, 200 μg/L As(III) as arsenite and 100 μg/L As(V) as DMA) to less than 10 μg/L after 1055 pore volumes of water were filtered at a flow rate of 0.51 mL/min. After 2340 pore volumes passed through the column filter, the total inorganic As in the effluent was less than 5 μg/L. The total As load in the column filter was estimated to be 0.164 mg/g. Results of μ-synchrotron X-ray fluorescence analysis (μ-XRFA) suggest that coatings of fresh Fe(III) oxides, formed on the surface of the siderite grains after two weeks of operation, greatly increased the adsorption capacity of the filling material towards As.  相似文献   

15.
Electric potentials of the (0 0 1) surface of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4-14 range, and develops a positive surface potential below pH 4 due to protonation of μ-OH0 sites (pK1,1,0,int = −1.32). This site is resilient to deprotonation up to at least pH 14 (−pK−1,1,0,int ? 19). The associated Stern layer capacitance of 0.31-0.73 F/m2 is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO3 etches the (0 0 1) surface, yielding a convoluted surface populated by sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (0 0 1) dissolution. Associated electric potentials were strongly negative, with values as large as −0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7-11 range due to surface complexation and/or precipitation of iron species (0.0038 Fe/nm2) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite surfaces.  相似文献   

16.
The interaction of Pu3+ bearing solutions with the muscovite (0 0 1) basal plane is explored using a combination of ex-situ approaches including alpha-counting, to determine the Pu3+ adsorption isotherm, and X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR), to probe the interfacial structure and Pu-specific distribution, respectively. Pu uptake to the muscovite (0 0 1) surface from Pu3+ solutions in a 0.1 M NaClO4 background electrolyte at pH 3 follows an approximate Langmuir isotherm with an apparent adsorption constant, Kapp = 5 × 104 M−1, and with a maximum coverage that is consistent with the amount needed to fully compensate the surface charge by trivalent Pu. The XR results show that the muscovite surface reacted with a 10−3 M Pu3+ solution (at pH 3 with 0.1 M NaClO4) and dried in the ambient environment, maintains a 30-40 Å thick layer, indicating the presence of a residual hydration layer (possibly including adventitious carbon). The RAXR results indicate that Pu sorbs on the muscovite surface with an intrinsically broad distribution with an average height of 18 Å, substantially larger than heights expected for any specifically adsorbed inner- or outer-sphere complexes. These results are discussed in the context of recent studies of cation adsorption trends on muscovite and the possible roles of Pu hydrolysis species in controlling the Pu-muscovite interactions.  相似文献   

17.
The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 × 10−9 atm O2). The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Fe/Tc(IV) form was observed when 11 μmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. Heterogeneous reduction predominated after approximately 0.01 mmol/L of Fe(II) was oxidized. The heterogeneous reaction was more rapid, and was catalyzed by Fe(II) that adsorbed to the Fe/Tc(IV) redox product. Wet chemical and Fe-X-ray absorption near edge spectroscopy measurements (XANES) showed that Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox products after reaction termination. 57Fe-Mössbauer, extended X-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements revealed that the Fe/Tc(IV) solid phase was poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with an octahedral Tc(IV) dimer bound in bidentate edge-sharing mode to octahedral Fe(III) associated with surface or vacancy sites in ferrihydrite. The precipitate maintained Tc(IV)aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O2·nH2O(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O2·nH2O(s) as a result of its intraparticle/intragrain residence. Precipitates of this nature may form in anoxic sediments or groundwaters, and the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.  相似文献   

18.
The sorption of 57Fe(II) onto an Fe-free, mineralogically pure and Ca-saturated synthetic montmorillonite sample (structural formula: Ca0.15(Al1.4Mg0.6)(Si4)O10(OH,F)2), was studied as a function of pH under strictly anoxic conditions (N2 glove box atmosphere, O2 content <1 ppm), using wet chemistry and cryogenic (T = 77 K) 57Fe Mössbauer spectrometry. No Fe(III) was detected in solution at any pH. However, in pH conditions where Fe(II) is removed from solution, a significant amount of surface-bound Fe(III) was produced, which increased with pH from 0% to 3% of total Fe in a pre-sorption edge region (i.e. at pH < 7.5 where about 15% of total Fe is sorbed) to 7% of total Fe when all Fe is sorbed. At low pH, where the pre-sorption edge plateau occurs (2 < pH < 7.5), the total sorbed-Fe amount remained constant but, within this sorbed-Fe pool, the Fe(III)/Fe(II) ratio increased with pH, from 0.14 at pH 2 up to 0.74 at pH 7. The pre-sorption edge plateau is interpreted as cation exchange on interlayer surfaces together with a sorption phenomenon occurring on highly reactive (i.e. high affinity) surface sites. As pH increases and protons are removed from the clay edge surface, we propose that more and more of these highly reactive sites acquire a steric configuration that stabilizes Fe(III) relative to Fe(II), thereby inducing a Fe to clay particle electron transfer. A sorption model based on cation exchange combined with surface complexation and electron transfers reproduces both wet chemical as well as the Mössbauer spectrometric results. The mechanism is fully reversible: sorbed-Fe is reduced as pH decreases (Mössbauer solid-state analyses) and all Fe returned to solution is returned as Fe(II) (solution analyses). This would not be the case if the observed oxidations were due to contaminant oxidizing agents in solution. The present work shows that alternating pH may induce surface redox phenomena in the absence of an electron acceptor in solution other than H2O.  相似文献   

19.
Macroscopic sorption edges for Cu2+ were measured on hematite nanoparticles with average diameters of 7 nm, 25 nm, and 88 nm in 0.1 M NaNO3. The pH edges for the 7 nm hematite were shifted approximately 0.6 pH units lower than that for the 25 nm and 88 nm samples, demonstrating an affinity sequence of 7 nm > 25 nm = 88 nm. Although, zeta potential data suggest increased proton accumulation at the 7 nm hematite surfaces, changes in surface structure are most likely responsible for the preference of Cu2+ for the smallest particles. As Cu2+ preferentially binds to sites which accommodate the Jahn-Teller distortion of its coordination to oxygen, this indicates the relative importance of distorted binding environments on the 7 nm hematite relative to the 25 nm and 88 nm particles. This work highlights the uniqueness of surface reactivity for crystalline iron oxide particles with decreasing nanoparticle diameter.  相似文献   

20.
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   

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