Multinuclear Surface Complexes Research Articles

Surface complexation modeling of neptunium(V) sorption to lepidocrocite (γ-FeOOH)

Abstract Lepidocrocite (γ-FeOOH), an important iron-bearing mineral found to exist with a relatively high abundance in the soils of the Chinese nuclear test sites, has rarely been studied for its sorption of transuranic elements from the nuclear wastes. This work develops a quantitative surface complexation model describing the sorption and speciation of Np(V) on synthetic lepidocrocite (γ-FeOOH). Batch sorption experiments on γ-FeOOH were performed under a range of conditions (25 ℃, 0.1 M NaClO4, pH = 4 – 11, 5 μM Np(V), and atmospheric conditions). The Diffuse Layer Model (DLM) was applied to describe the surface-complexation reaction. The data were best-fitted with a single surface-complexation reaction, (≡XOH + NpO2 + = XONpO2 0 + H+) that occurred at the lepidocrocite/water interface to form an inner-sphere Np(V) complex with lepidocrocite. The results are complemented by the Np L III-edge EXAFS data that show that Np(V) was absorbed on γ-FeOOH as monomeric neptunyl ions, with no observations of multinuclear surface complexes or surface precipitates. A prominent peak at ∼3 Å in the EXAFS Fourier Transform spectra can be attributed to a Np-Fe scattering path, consistent with the formation of an inner sphere Np(V)-lepidocrocite surface complex. Formation of aqueous NpO2(CO3) x 1–2x complexes prevents Np(V) sorption at higher pH values but it is unclear if ternary lepidocrocite-Np-carbonate complexes may also form. These data indicate that there are subtle differences in Np(V) interactions with hematite, goethite, and lepidocrocite which is likely a manifestation of the differences in surface reactivity of the three minerals.

Layer charge and heavy metals structures in hydrated 2 : 1 silicates: state of the art and new advances on cadmium

Abstract This study will discuss how layer charge can affect chemical speciation and topology of heavy metals adsorbed to 2 : 1 layer silicates, by providing: i) an overview of literature data; ii) experimental data on Cd complexes adsorbed by 2 : 1 layer silicates with different layer charge (montmorillonite and vermiculite); iii) a comparison between our results and literature data. This study will also be supported by several different experimental techniques such as chemical and thermal analyses, X-ray powder diffraction and X-ray absorption spectroscopy. Based on our data Cd atoms were found to complex water molecules in both clay minerals and to show four-fold coordination in montmorillonite (Cd–O distances of 2.24 Å) and six-fold coordination in vermiculite (Cd–O distances of 2.16 and 2.28 Å). Furthermore our models clearly suggest that Cd mainly bonds to interlayer water, without neglecting the more limited, but still significant, Cd multinuclear surface complexes at the octahedral broken edges. Both clay minerals show H2O/Cd ratio, as evidenced by thermal analyses, drastically higher than expected from X-ray adsorption spectroscopy data, thus implying that most of the water molecules are only loosely coordinated to interlayer cations.

EXAFS Study of Zn Sorption Mechanisms on Montmorillonite

Extended X-ray absorption fine structure (EXAFS) analysis of zinc sorption on montmorillonite showed that different types of surface complexes or surface precipitates were formed depending on the reaction time. With an initial zinc concentration of 10(-3) M at neutral pH, zinc remained octahedrally coordinated with about six oxygen atoms at Zn-O bond distances of 2.02-2.07 A for up to six months. For samples aged up to 11 days, the Zn-Zn contribution in the second shell suggested formation of multinuclear surface complexes or surface precipitates. For samples aged 20 days and more, Zn-Zn and Zn-Si contributions in the second shell suggested formation of mixed metal coprecipitates such as a Zn phyllosilicate-like phase. Formation of these mixed metal solids probably accounts for the slow continuous sorption reaction at aging times exceeding 20 days. Sequestration of Zn in mixed metal precipitates and the stability of these phases can reduce the concentration, mobility, and toxicity of Zn in soils or sediments.

XPS study of dissolution of birnessite by humate with constraints on reaction mechanism

The present study is an investigation into the surface reactions of synthetic birnessite [Mn(II) 0.05Mn(III) 0.25Mn(IV) 0.7O 1.7(OH) 0.25] when reacted with dilute (5 × 10 −4 M) Na-humate solution using X-ray Photoelectron Spectroscopy (XPS). Mn2p 3/2, C1s and O1s spectra of reacted surfaces reveal that Mn(IV) of birnessite is reduced to Mn(III) while humate is oxidized to CO 2. Initial rapid production of CO 2 results in accumulation of CO 2 at the reaction interface. After approximately 15 min, the reaction rate decreases to the point where desorption keeps pace with accumulation until after 5 h of reaction, when CO 2 is no longer detected at the reaction surface. Reduction of Mn(IV) of birnessite to Mn(III) indicates that the MnO 2-oxalate redox reaction proceeds as a transfer of one electron per metal centre. There is no XPS evidence for reduction of Mn(III) of the solid to Mn(II) in the presence of humate. The carboxyl group of humate apparently inhibits this step, most likely through formation of strong Mn(III)-carboxyl surface complexes. This postulate is consistent with Mn 3+(aq) stabilization by oxalate in aqueous solutions. Further study using X-ray absorption spectroscopy (XAS) is required for a better understanding of the structure of the surface complexes. The rate of release of soluble Mn(II) into dilute humate solutions (5 × 10 −4 M) is lower by more than two orders of magnitude compared with rate of release to aerated, distilled water at similar pH. The process of proton promoted dissolution of the soluble Mn(II) component of birnessite in distilled water would appear to be impeded by the addition of humate. This could be attributed to the formation of strong, multinuclear surface complexes between Mn(II, III) and the adsorbed carboxyl groups.

Reactivity at the mineral-water interface: dissolution and inhibition

The reactivity of the mineral-water interface is often interpreted with the help of models used in electrochemistry, in solution coordination chemistry and in crystallography. Progress in understanding mechanisms of growth and dissolution of crystals and the inhibition of these processes depends on a better integration of these models. It is shown that dissolution can be explained in terms of a ligand exchange process; simplified rate laws for proton- and ligand-protonated dissolution rates, being related to surface bound protons and ligands, respectively, can be derived. Further refinement in interpreting surface reactivity comes from an appreciation of the molecular structures at the mineral water interface; here significant advances have been made by X-ray absorption spectroscopy, especially EXAFS (Extended X-ray Absorption Fine Structure spectroscopy), which permit distinction to be made between outer-sphere and inner-sphere surface complexes, and in many cases to determine the structure of the surface species at different crystallographic planes (e.g., bi-nuclear or mono-nuclear linkage of ligands on metal ions to surface metal centers). Such information coupled with solution-chemical studies on the extent of adsorption can provide new insight into the mechanisms of dissolution reactions and their inhibition and surface poisoning. A few experimental results are given to exemplify the factors that enhance and inhibit the non-reductive (EDTA) and reductive dissolution (by H 2S) of Fe(III)(hydr)oxides. Binuclear surface complexes by multivalent cations and by oxoanions, such as phosphate, arsenate and borate, are believed to be efficient inhibitors for oxide dissolution because they form bi- or multinuclear innersphere surface complexes that can bridge two or more metal centers in the surface lattice; the simultaneous removal of such bi- or multinuclear surface complexes from the surface is energetically unfavorable. Proton and ligand promoted dissolution reactions and their inhibition by oxoanions and bi- or multinuclear surface complexes are not only relevant in geochemistry (weathering, soil-formation, transfer of elements and pollutants) but also in metallic corrosion, formation and breakdown of passive oxide films.

Investigation of Ni Sorption on Pyrophyllite: An XAFS Study

Sorption reactions at the solid-water interface decrease solute mobility and often control the fate, bioavailability, and transport of metal ions in soils and groundwaters. A thorough understanding of the structural environment of metals at the solid-water interface is therefore of fundamental importance. In this study, X-ray absorption fine structure (XAFS) spectroscopy was used to discern the local atomic structure of Ni(II) sorbed onto pyrophyllite. The first coordination shell consists of 6 O atoms at 2.02-2.04 A. For the second shell, XAFS data suggest a single Ni-Al/Si distance (2.96-3.03 A), indicative of edge sharing of Ni and Al octahedra and possibly the presence of mixed nickel-aluminum hydroxides. As Ni surface loading on pyrophyllite increased, the number of Ni second-neighbor atoms at a distance of 2.99-3.00 A increased from N = 1 to N = 5. The presence of multinuclear surface complexes was depicted at low surface loading and at reaction conditions undersaturated with respect to the formation of Ni(OH){sub 2}(s). This observation suggests that the total coverage of surface sites is not necessary for the formation of multinuclear surface complexes and implies that the pyrophyllite surface promotes hydrolysis and multinuclear complex formation. 59 refs., 6 figs., 1 tab.

Surface Complexation Modeling: II. Strategy for Modeling Polymer and Precipitation Reactions at High Surface Coverage

Three triple-layer model (TLM) surface complexation models that allow for the formation of multinuclear surface complexes or precipitates are compared, based on their ability to simulate cobalt sorption on α-Al2O3. These models include: (1) a surface solid solution model, (2) a surface polymer model, and (3) a surface continuum model. The solid solution model accounts for high coverage data by invoking a coprecipitation surface reaction, while the polymer model accomplishes the same task with multinuclear surface complexation reactions. In the continuum model, two polymer reactions and one precipitation reaction are proposed. Modeling results indicate that all of the models work reasonably well at predicting sorption data from moderate to high surface coverage (0.1 to 100%). Because the continuum model is the only one presented which is consistent with spectroscopic data throughout the range of surface coverages examined, this model is suggested as the preferred one for modeling metal ion sorption data. Model predictions of pH-edge and isotherm data are discussed and used in the assessment of the merits of the three TLM models.

X-Ray Absorption Spectroscopy of Cobalt(II) Multinuclear Surface Complexes and Surface Precipitates on Kaolinite

Abstract We have examined the local atomic structure of multinuclear complexes and surface precipitates of Co(II) sorbed to kaolinite (Al2Si2O5(OH)4), a common clay mineral, using extended X-ray absorption fine structure (EXAFS) spectroscopy. Structural information from quantitative analysis of EXAFS spectra is used to examine differences, on a molecular level, among Co species sorbed on kaolinite at different coverages from solutions under- and over-saturated with respect to solid Co(OH)2 phases, and between sorbed species and Co hydroxide solids precipitated from solutions in the absence of kaolinite. The EXAFS results indicate formation of oxide- or hydroxide-bridged multinuclear surface complexes on kaolinite at surface coverages well below a statistical monolayer from undersaturated solutions. As Co surface loading on kaolinite is increased, the number of Co second-neighbor atoms at 3.10-3.12 A increases from ≈2 to 6, the number of second-neighbor Co backscatterers in crystalline Co (OH)2(s), and interatomic distances determined by EXAFS remain constant. These observations suggest that hydroxide-like multinuclear complexes form at low surface coverages in addition to monomeric surface complexes, perhaps as precursors to surface precipitation. At high surface coverages and apparent solubilities (i.e., solution ion activity products) both below and above saturation with respect to stable Co(OH)2(s), the spectra of the sorption products differ from the spectra of Co solids precipitated in the absence of kaolinite, including dry, crystalline Co(OH)2(s) (pink form) and wet, freshly precipitated Co(OH)2(s) (blue and pink forms). Compared to the precipitates, sorption samples have Co-Co distances shorter by 0.050.07 (±0.02) A, lack small beat patterns in normalized spectra, and have reduced backscattering amplitudes from multiple scattering among Co atoms at ≈ 3 and 6 A. Long-term equilibration of oversaturated sorption samples (21 or 45 days) resulted in a decrease in backscattering amplitudes for both single- and multiple-scattering paths among neighboring Co atoms relative to short-term (24-36 h) equilibration. These differences are attributed to a decrease in the average number of Co atoms in a hydroxide-like complex and/or to an increase in disorder in the local atomic structure. This is not expected in typical aging of a freshly precipitated solid phase, where average particle size and degree of long-range order normally increase with time, suggesting that kaolinite influences the sorption product. The contraction in interatomic distances and disruption of long-range structure indicated by EXAFS for sorption samples can be accounted for by small particle sizes of the new surface phase, structural misregistry on an atomic scale between a Co hydroxide phase and kaolinite, or dissolution of the kaolinite substrate and substitution of minor impurities into a Co hydroxide phase. The apparent reduced solubilities of the sorption products relative to bulk solubilities of Co(OH)2(s) phases may be related to Ostwald-step processes, the formation of an impure Co hydroxide phase, or a lower near-surface solvent dielectric constant.

Molecular Structure and Binding Sites of Cobalt(II) Surface Complexes on Kaolinite from X-Ray Absorption Spectroscopy

AbstractX-ray absorption spectroscopy (XAS) was used to determine the local molecular environment of Co(II) surface complexes sorbed on three different kaolinites at ambient temperature and pressure in contact with an aqueous solution. Interatomic distances and types and numbers of backscattering atoms have been derived from analysis of the extended X-ray absorption fine structure (EXAFS). These data show that, at the lowest amounts of Co uptake on kaolinite (0.20–0.32 µmol m−2), Co is surrounded by ≈6 O atoms at 2.04–2.08 Å and a small number or Al or Si atoms (N = 0.6–1.5) at two distinct distances, 2.67–2.72 Å and 3.38–3.43 Å. These results indicate that Co bonds to the kaolinite surface as octahedrally coordinated, bidentate inner-sphere mononuclear complexes at low surface coverages, confirming indirect evidence from solution studies that a fraction of sorbed Co forms strongly bound complexes on kaolinite. In addition to inner-sphere complexes identified by EXAFS spectroscopy, solution studies provide evidence for the presence of weakly bound, outer-sphere Co complexes that cannot be detected directly by EXAFS. One orientation for inner-sphere complexes indicated by XAS is bidentate bonding of Co to oxygen atoms at two Al-O-Si edge sites or an Al-O-Si and Al-OH (inner hydroxyl) edge site, i.e., corner-sharing between Co octahedra and Al and Si polyhedra. At slightly higher surface sorption densities (0.51–0.57/ µmol m−2), the presence of a small number of second-neighbor Co atoms (average NCo < 1) at 3.10–3.13 Å indicates the formation of oxy- or hydroxy-bridged, multinuclear surface complexes in addition to mononuclear complexes. At these surface coverages, Co-Co and Co-Al/Si distances derived from EXAFS are consistent with edge-sharing between Co and Al octahedra on either edges or (001) faces of the aluminol sheet in kaolinite. Multinuclear complexes form on kaolinite at low surface sorption densities equivalent to <5% coverage by a monolayer of oxygen-ligated Co octahedra over the N2-BET surface area. These spectroscopic results have several implications for macroscopic modeling of metal ion uptake on kaolinite: 1) Primary binding sites on the kaolinite surface at low uptake are edge, non-bridging Al-OH inner hydroxyl sites and edge Al-O-Si bridging oxygen sites, not Si-OH sites typically assumed in sorption models; 2) specific adsorption of Co is via bidentate, inner-sphere complexation; and 3) at slightly higher uptake but still a small fraction of monolayer coverage, formation of Co multinuclear complexes, primarily edge-sharing with Al-OH octahedra, begins to dominate sorption.