Iron Adsorption on Clays Inferred from Atomistic Simulations and X-ray Absorption Spectroscopy.

The atomistic level understanding of iron speciation and the probable oxidative behavior of iron (Feaq2+ → Fesurf3+) in clay minerals are fundamental for environmental geochemistry of redox reactions. Thermodynamic analyses of wet chemistry data suggest that iron adsorbs on the edge surfaces of clay minerals at distinct structural sites commonly referred as strong and weak sites (with high and low affinity, respectively). In this study, we applied ab initio molecular dynamics simulation to investigate the structure and the stability of the edge surfaces of trans- and cis-vacant montmorillonites. These structures were further used to evaluate the surface complexation energy and to calculate reference ab initio X-ray absorption spectra (XAS) for distinct inner-sphere complexes of iron. The combination of ab initio simulations and XAS allowed us to reveal the Fe-complexation mechanism and to quantify the Fe partitioning between the high and low affinity sites as a function of the oxidation state and loadings. Although iron is mostly present in the Fe3+ form, Fe2+ increasingly co-adsorbs at increasing loadings. Ab initio structure relaxations of several different clay structures with substituted Fe2+/Fe3+ in the bulk or at the surface site showed that the oxidative sorption of ferrous iron is an energetically favored process at several edge surfaces of the Fe-bearing montmorillonite.

Characterization of Structural Iron in Smectites - An Ab Initio Based X-ray Absorption Spectroscopy Study.

Fe-bearing clay minerals are abundant in argillaceous rocks as their redox-active structural iron may control the sorption mechanism of redox sensitive elements on the surface of clay minerals. The extent and efficiency of the redox reactions depend on the oxidation state (Fe2+/Fe3+ ratio) and structural distribution of the substituting cations in the TOT-layer of clay minerals. Even smectites with similar structure originating from different locations might have a distinct arrangement of isomorphic substitutions (e.g., individual iron or Fe-Fe pairs). In this study, the proportion of different iron distribution in Milos-, Wyoming-, and Texas-montmorillonite was determined by combining X-ray absorption spectroscopy (XAS) with ab initio calculations. The relaxed atomic structures of the smectite models with different arrangement of individual Fe atoms and Fe-Fe/Fe-Mg clusters served as the basis for the calculations of the XAS spectra. The combination of simulation results and measured Fe K-edge XAS spectra of Wyoming-, Milos- and Texas-montmorillonites suggested that iron is present as Fe3+ in the octahedral sheet. Fe3+ in Texas-montmorillonite has a tendency to form clusters, while no definitive statement about clustering or avoidance of Fe-Fe and Fe-Mg pairs can be made for Milos- and Wyoming-montmorillonite.

Combined XAFS Spectroscopy and Ab Initio Study on the Characterization of Iron Incorporation by Montmorillonite.

Iron occurs in clay minerals in both ferric and ferrous forms. Depending on its oxidation state and the environmental conditions, it can participate in redox reactions and influence the sorption processes at surfaces of clay minerals. Knowing the oxidation state and the preferential structural position of Fe2+ and Fe3+ is essential for the detailed understanding of the mechanism and kinetics of such processes. In this study, molecular dynamics (MD) calculations based on density functional theory (DFT+U) were applied to simulate the incorporated Fe in bulk montmorillonite and to explain the measured Fe K-edge X-ray absorption fine structure (XAFS) spectra. The analysis of the experimental data and simulation results suggested that iron in montmorillonite is preferentially incorporated as Fe3+ into the octahedral layer. The simulations showed that there is no preferential occupation of cis- or trans-sites by Fe2+ and Fe3+ in bulk montmorillonite. A very good agreement between the ab initio simulated and the measured XAFS spectra demonstrate the robustness of the employed simulation approach.

Competitive Fe(II)-Zn(II) uptake on a synthetic montmorillonite.

The interaction of Fe(II) with clay minerals is of particular relevance in global geochemical processes controlling metal and nutrient cycles and the fate of contaminants. In this context, the influence of competitive sorption effects between Fe(II) and other relevant transition metals on their uptake characteristics and mobility remains an important issue. Macroscopic sorption experiments combined with surface complexation modeling and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to elucidate competitive sorption processes between divalent Fe and Zn at the clay mineral-water interface. Sorption isotherms were measured on a synthetic iron-free montmorillonite (IFM) under anoxic conditions (O2 <0.1 ppm) for the combinations of Zn(II)/Fe(II) and Fe(II)/Zn(II), where the former metal in each pair represents the trace metal (<10(-7) M) and the latter the competing metal at higher concentrations (10(-7) to 10(-3) M). Results of the batch sorption and EXAFS measurements indicated that Fe(II) is competing with trace Zn(II) for the same type of strong sites if Fe(II) is present in excess, whereas no competition between trace Fe(II) and Zn(II) was observed if Zn(II) is present at high concentrations. The noncompetitive behavior suggests the existence of sorption sites which have a higher affinity for Fe(III), where surface-induced oxidation of the sorbed Fe(II) to Fe(III) occurred, and which are not accessible for Zn(II). The understanding of this competitive uptake mechanism between Fe(II) and Zn(II) is of great importance to assess the bioavailability and mobility of transition metals in the natural environment.

Fe(II) uptake on natural montmorillonites. I. Macroscopic and spectroscopic characterization.

Iron is an important redox-active element that is ubiquitous in both engineered and natural environments. In this study, the retention mechanism of Fe(II) on clay minerals was investigated using macroscopic sorption experiments combined with Mössbauer and extended X-ray absorption fine structure (EXAFS) spectroscopy. Sorption edges and isotherms were measured under anoxic conditions on natural Fe-bearing montmorillonites (STx, SWy, and SWa) having different structural Fe contents ranging from 0.5 to 15.4 wt % and different initial Fe redox states. Batch experiments indicated that, in the case of low Fe-bearing (STx) and dithionite-reduced clays, the Fe(II) uptake follows the sorption behavior of other divalent transition metals, whereas Fe(II) sorption increased by up to 2 orders of magnitude on the unreduced, Fe(III)-rich montmorillonites (SWy and SWa). Mössbauer spectroscopy analysis revealed that nearly all the sorbed Fe(II) was oxidized to surface-bound Fe(III) and secondary Fe(III) precipitates were formed on the Fe(III)-rich montmorillonite, while sorbed Fe is predominantly present as Fe(II) on Fe-low and dithionite-reduced clays. The results provide compelling evidence that Fe(II) uptake characteristics on clay minerals are strongly correlated to the redox properties of the structural Fe(III). The improved understanding of the interfacial redox interactions between sorbed Fe(II) and clay minerals gained in this study is essential for future studies developing Fe(II) sorption models on natural montmorillonites.

Fe(II) sorption on a synthetic montmorillonite. A combined macroscopic and spectroscopic study.

Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy combined with macroscopic sorption experiments were employed to investigate the sorption mechanism of Fe(II) on an iron-free synthetic montmorillonite (Na-IFM). Batch sorption experiments were performed to measure the Fe(II) uptake on Na-IFM at trace concentrations as a function of pH and as a function of sorbate concentration at pH 6.2 and 6.7 under anoxic conditions (O2 < 0.1 ppm). A two-site protolysis nonelectrostatic surface complexation and cation exchange sorption model was used to quantitatively describe the uptake of Fe(II) on Na-IFM. Two types of clay surface binding sites were required to model the Fe(II) sorption, the so-called strong (≡S(S)OH) and weak (≡S(W)OH) sites. EXAFS data show spectroscopic differences between Fe sorbed at low and medium absorber concentrations that were chosen to be characteristic for sorption on strong and weak sites, respectively. Data analysis indicates that Fe is located in the continuity of the octahedral sheet at trans-symmetric sites. Mössbauer spectroscopy measurements confirmed that iron sorbed on the weak edge sites is predominantly present as Fe(II), whereas a significant part of surface-bound Fe(III) was produced on the strong sites (∼12% vs ∼37% Fe(III) species to total sorbed Fe).

The sensitive mobility of Cr in ashes studied by SiO2-Al2O3-Fe2O3-CaO system.

Since incineration is a feasible method for stabilization/solidification of chromium (Cr)-enriched wastes, the species, distribution, and mobility of Cr in ashes deserve more studies, especially as the function of ash composition. Synthetic Cr-bearing ashes (SAs) were synthesized by SiO2-Al2O3-Fe2O3-CaO systems to investigate Cr mobility under 1100 °C. A study from simplicity to complexity. The Cr in SiO2-CaO is of high mobility with CrO42- formation, in contrary to the moderate mobility in SiO2-Al2O3 and poor mobility in SiO2-Fe2O3. However, species and mobility of Cr are affected by the values of CaO/SiO2, Al2O3/SiO2, and Fe2O3/SiO2 ratios. When other oxides are added to the two-phase systems above, the fate of Cr is affected more considerably. With the SiO2 content of 70%, adding a slight amount of CaO (<10%) strengthens the stabilization/solidification of Cr, due to the favorable solid integration under Ca2+ fusion. However, the Cr mobility is higher with increasing the CaO content further. The minimum content of CaO is ∼20% to sufficiently decrease the proportion of residual Cr (QCr-S5) in SiO2-Al2O3-CaO, much lower than in SiO2-Fe2O3-CaO, which confirms the easier release of Cr immobilized in Si-Al matrixes. Considering the opposite effects of Fe2O3 and CaO on Cr mobility, increasing Fe2O3/CaO ratios >3/2 can limit the effect of CaO, leading to the efficient stabilization/solidification of Cr waste. Additionally, the QCr-S5 is 83% with the Fe2O3 content of 15% in SiO2-Al2O3-Fe2O3, higher than in SiO2-Al2O3 and SiO2-Fe2O3. This suggests the intense stabilization/solidification of Cr, probably due to the formation of amorphous Fe-rich glass. Based on these above, an equation is developed to describe the relationship between ash compositions and QCr-S5 (QCr-S5 = -39.37X1 + 24.96X2 + 5.34X3 - 2.51X4 + 54.29).

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy.

Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono- and bidentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling.

Structural insight into iodide uptake by AFm phases.

The ability of cement phases carrying positively charged surfaces to retard the mobility of (129)I, present as iodide (I(-)) in groundwater, was investigated in the context of safe disposal of radioactive waste. (125)I sorption experiments on ettringite, hydrotalcite, chloride-, carbonate- and sulfate-containing AFm phases indicated that calcium-monosulfate (AFm-SO(4)) is the only phase that takes up trace levels of iodide. The structures of AFm phases prepared by coprecipitating iodide with other anions were investigated in order to understand this preferential uptake mechanism. X-ray diffraction (XRD) investigations showed a segregation of monoiodide (AFm-I(2)) and Friedel's salt (AFm-Cl(2)) for I-Cl mixtures, whereas interstratifications of AFm-I(2) and hemicarboaluminate (AFm-OH-(CO(3))(0.5)) were observed for the I-CO(3) systems. In contrast, XRD measurements indicated the formation of a solid solution between AFm-I(2) and AFm-SO(4) for the I-SO(4) mixtures. Extended X-ray absorption fine structure spectroscopy showed a modification of the coordination environment of iodine in I-CO(3) and in I-SO(4) samples compared to pure AFm-I(2). This is assumed to be due to the introduction of stacking faults in I-CO(3) samples on one hand and due to the presence of sulfate and associated space-filling water molecules as close neighbors in I-SO(4) samples on the other hand. The formation of a solid solution between AFm-I(2) and AFm-SO(4), with a short-range mixing of iodide and sulfate, implies that AFm-SO(4) bears the potential to retard (129)I.

Soft X-ray spectromicroscopy of cobalt uptake by cement.

Scanning transmission X-ray microscopy was used to investigate the speciation and spatial distribution of Co in a Co(II)-doped cement matrix. The aim of this study was to improve the understanding of the heavy metals immobilization process in cement on the molecular level. The Co-doped cement samples hydrated for 30 days with a Co loading of 5000 mg/kg were prepared under normal atmosphere to simulate conditions used for cement-stabilized waste packages. Co 2p(3/2) absorption edge signals were used to determine the spatial distributions of the metal species in the Co(II)-doped cement. The speciation of Co was determined by collecting near-edge X-ray absorption fine structure spectra. On the basis of the shape of the absorption spectra, it was found that Co(II) is partly oxidized to Co(III). The correlation, respectively the anticorrelation with elements such as Al, Si, and Mn, show that Co(II) is predominantly present as Co-hydroxide-like phase as well as Co-phyllosilicate, whereas Co(III) tends to be incorporated only into a CoOOH-like phase. Thus, this study suggests that thermodynamic calculations of Co(II)-immobilization by cementitious systems should take into consideration not only the solubility of Co(II)-hydroxides but also Co(III) phases.

Uptake of Np(IV) by C-S-H phases and cement paste: an EXAFS study.

Nuclear waste disposal concepts developed worldwide foresee the use of cementitious materials for the immobilization of long-lived intermediate level waste (ILW). This waste form may contain significant amounts of neptunium-237, which is expected to be present as Np(IV) under the reducing conditions encountered after the closure of the repository. Predicting the release of Np(IV) from the cementitious near field of an ILW repository requires a sufficiently detailed understanding of its interaction with the main sorbing components of hardened cement paste (HCP). In this study, the uptake of Np(IV) by calcium silicate hydrates (C-S-H) and HCP has been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS studies on Np(IV)-doped C-S-H and HCP samples reveal that Np(IV) is predominantly incorporated in the structure of C-S-H phases having different Ca:Si ratios. The two main species identified correspond to Np(IV) in C-S-H with a Ca:Si mol ratio of 1.65 as in fresh cement and with a Ca:Si mol ratio of 0.75 as in highly degraded cement. The local structure of Np(IV) changes with the Ca:Si mol ratio and does not depend on pH. Furthermore, Np(IV) shows the same coordination environment in C-S-H and HCP samples. This study shows that C-S-H phases are responsible for the Np(IV) uptake by cementitious materials and further that incorporation in the interlayer of the C-S-H structure is the dominant uptake mechanism.

Mechanical behavior and phase change of alkali-silica reaction products under hydrostatic compression.

Alkali-silica reaction (ASR) causes severe degradation of concrete. The mechanical property of the ASR product is fundamental to the multiscale modeling of concrete behavior over the long term. Despite years of study, there is a lack of consensus regarding the structure and elastic modulus of the ASR product. Here, ASR products from both degraded field infrastructures and laboratory synthesis were investigated using high-pressure X-ray diffraction. The results unveiled the multiphase and metastable nature of ASR products from the field. The dominant phase undergoes permanent phase change via collapsing of the interlayer region and in-planar glide of the main layer, under pressure >2 GPa. The bulk moduli of the low- and high-pressure polymorphs are 27±3 and 46±3 GPa, respectively. The laboratory-synthesized sample and the minor phase in the field samples undergo no changes of phase during compression. Their bulk moduli are 35±2 and 76±4 GPa, respectively. The results provide the first atomistic-scale measurement of the mechanical property of crystalline ASR products.

Th uptake on montmorillonite: a powder and polarized extended X-ray absorption fine structure (EXAFS) study.

The uptake process of Th(IV) onto montmorillonite was studied using powder and polarized-EXAFS (P-EXAFS) spectroscopy. Sorption samples were prepared in 0.1 M NaClO(4) solutions either undersaturated (pH 2 and 3, [Th](initial): 2.7x10(-6) to 4x10(-4) M) or supersatured (pH 5, [Th](initial): 4.3x10(-5) to 4x10(-4) M) with respect to amorphous ThO(2). Th loading varied between 1-157 micromol/g at pH 3 and 14-166 micromol/g at pH 5 and equaled 41 micromol/g at pH 2. At pH 5 and high surface loading the EXAFS spectrum resembled that of amorphous Th(OH)(4), suggesting the precipitation of a Th hydrous hydroxide. At low and intermediate surface coverage two O coordination shells at approximately 2.24 and approximately 2.48 A, and one Si shell at 3.81-3.88 A, were systematically observed regardless of pH. The formation of Th nucleation products and Th-Si solution complexes and the sorption of Th on a silica precipitate were excluded from the EXAFS spectra analysis and solution chemistry. In these conditions, Th was shown to bond the montmorillonite surface by sharing double corners with Si tetrahedra. This structural interpretation is consistent with surface coverage calculations which showed that the edge sites were saturated in the two highest concentrated samples (34 and 157 micromol/g) at pH 3.

Macro- and microspectroscopic study of Nd (III) uptake mechanisms in hardened cement paste.

Cement is an important component in repositories for low-level and intermediate-level radioactive waste. Nd uptake by hardened cement paste (HCP) has been investigated with the aim of developing a mechanistic understanding of the immobilization processes of trivalent lanthanides and actinides in HCP on the molecular level. Information on the microstructure of HCP, the Nd distribution in the cement matrix, and the coordination environment of Nd in these matrices was gained from the combined use of scanning electron microscopy (SEM), synchrotron-based micro-X-ray fluorescence (micro-XRF), micro-X-ray (micro-XAS), and bulk-X-ray absorption spectroscopy (bulk-XAS) on Nd doped cement samples. The samples were reacted over periods of time between 15 min and 200 days. SEM and micro-XRF investigations suggest preferential Nd accumulation in rims around "inner"-calcium silicate hydrates (C-S-H). The EXAFS data indicate that the coordination environment of Nd taken up by HCP was dependent on reaction time. Changes in the structural parameters derived from EXAFS support the idea of Nd incorporation into the structure of C-S-H phases. The Nd binding mechanisms proposed in this study have implication for an overall assessment of the safe disposal of trivalent actinides in cement-based repositories for radioactive waste.

Strontium uptake by cementitious materials.

Wet chemistry experiments and X-ray absorption fine structure (XAFS) measurements were carried out to investigate the immobilization of nonradioactive Sr and 85Sr in calcite-free and calcite-containing Portland cement. The partitioning of pristine Sr between hardened cement paste (HCP) and pore solution, and the uptake of 85Sr and nonradioactive Sr were investigated in batch-type sorption/desorption experiments. Sr uptake by HCP was found to be fast and nearly linear for both cements, indicating that differences in the compositions of the two cements have no influence on Sr binding. The partitioning of pristine Sr bound in the cement matrix and 85Sr between HCP and pore solution could be modeled in terms of a reversible sorption process using similar Kd values. These findings allow 85Sr uptake to be interpreted in terms of an isotopic exchange process with pristine Sr. Sr K-edge EXAFS measurements on Sr doped HCP and calcium silicate hydrate (C-S-H) samples reveal no significant differences in the local coordination environments of pristine Sr and Sr bound to the cement matrix upon sorption. The first coordination sphere consists of five to six oxygen atoms located at a distance of about 2.6 A, which corresponds to Sr-O distances in the hydration sphere of Sr2+ in alkaline solution. Sr binds to the cement matrix via two bridging oxygen atoms located at a distance of about 3.6 A. No further neighboring atoms could be detected, indicating that Sr is taken up as a partially hydrated species by HCP. Wet chemistry and spectroscopic data further indicate that Sr binding to C-S-H phases is likely to be the controlling uptake mechanism in the cement matrix, which allows Sr uptake by HCP to be predicted based on a Ca-Sr ion exchange model previously developed for Sr binding to C-S-H phases. The latter finding suggests that long-term predictions of Sr immobilization in the cementitious near field of repositories for radioactive waste can be based on a simplified sorption model with C-S-H phases.