Fe(II) uptake on natural montmorillonites. II. Surface complexation modeling

Reduction of U(VI) on Chemically Reduced Montmorillonite and Surface Complexation Modeling of Adsorbed U(IV)

Adsorption and subsequent reduction of U(VI) on Fe(II)-bearing clay minerals can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the reactive surface area in many subsurface environments, and montmorillonite-containing materials are used in the storage of spent nuclear fuel. We investigated the extent of reduction of U(VI) by Fe(II)-bearing montmorillonite at different pH values and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly complete reduction of U(VI) to U(IV) occurred at a low sodium concentration at both pH 3 and 6. At pH 6 and a high sodium concentration, which inhibits U(VI) binding at cation-exchange sites, the extent of U(VI) reduction was only 70%. Surface-bound U(VI) on unreduced montmorillonite was more easily extracted into solution with bicarbonate than surface-bound U(IV) generated by reduction of U(VI) on Fe(II)-bearing montmorillonite. We developed a nonelectrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) on Fe(II)-bearing montmorillonite as a function of pH and sodium concentration. These findings establish the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

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 (Fe_aq²⁺ → Fe_surf³⁺) 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.

Understanding the Heterogeneous Nucleation of Heavy Metal Phyllosilicates on Clay Edges with First-Principles Molecular Dynamics

The nucleation and precipitation of heavy metal phyllosilicates can occur in the course of sorption onto clay edges, which will provide a long-term stabilization of heavy metal pollutants. However, a quantitative understanding of their reaction mechanisms is still lacking. Taking Ni²⁺ as the model cation, we characterized the atomic scale structures and thermodynamics of the early stage of nucleation by carrying out systematic first-principles molecular dynamics (FPMD) simulations, and the microscopic nucleation mechanisms were revealed. Two possible nucleation pathways were examined: a stepwise pathway (denoted as Path1) and a synchronous pathway (denoted as Path2). In Path1, Ni(OH)₂ forms first and then transforms to Ni phyllosilicate via silicification; in Path2, Ni phyllosilicate forms on clay edges directly. The computed free energies of complexation and condensation reactions indicate that Path2 is much more thermodynamically favorable than Path1, meaning that, given that the solution contains dissolved Si initially, heavy metal phyllosilicates will nucleate on clay edges through Path2. By comparing these free energies with their counterpart values of the reaction in bulk solution, the effect of the surface has been uncovered. These findings provide valuable insights for an improved understanding of the stabilization and transformation of heavy metal elements in nature. The derived results form a quantitative basis for future studies on the heterogenous nucleation and precipitation of heavy metal cations.

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.

Retention of selenium by calcium aluminate hydrate (AFm) phases under strongly-reducing radioactive waste repository conditions

Strong Se(-ii) sorption mainly in the interlayer of hemicarbonate (AFm-HC). Weak Se(-ii) sorption restricted to sorption sites on the surface of monocarbonate (AFm-MC).

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.

Kinetics and Products of Chromium(VI) Reduction by Iron(II/III)-Bearing Clay Minerals

Hexavalent chromium is a water-soluble pollutant, the mobility of which can be controlled by reduction of Cr(VI) to less soluble, environmentally benign Cr(III). Iron(II/III)-bearing clay minerals are widespread potential reductants of Cr(VI), but the kinetics and pathways of Cr(VI) reduction by such clay minerals are poorly understood. We reacted aqueous Cr(VI) with two abiotically reduced clay minerals: an Fe-poor montmorillonite and an Fe-rich nontronite. The effects of ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) [Fe(II)/Fe(total)] were examined. The last variable had the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than 3 orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This study helps to quantify our understanding of the kinetics of Cr(VI) reduction by Fe(II/III)-bearing clay minerals and may improve predictions of Cr(VI) behavior in subsurface environments.

MINFIT: A Spreadsheet-Based Tool for Parameter Estimation in an Equilibrium Speciation Software Program

Determination of equilibrium constants describing chemical reactions in the aqueous phase and at solid-water interface relies on inverse modeling and parameter estimation. Although there are existing tools available, the steep learning curve prevents the wider community of environmental engineers and chemists to adopt those tools. Stemming from classical chemical equilibrium codes, MINEQL+ has been one of the most widely used chemical equilibrium software programs. We developed a spreadsheet-based tool, which we are calling MINFIT, that interacts with MINEQL+ to perform parameter estimations that optimize model fits to experimental data sets. MINFIT enables automatic and convenient screening of a large number of parameter sets toward the optimal solutions by calling MINEQL+ to perform iterative forward calculations following either exhaustive equidistant grid search or randomized search algorithms. The combined use of the two algorithms can securely guide the searches for the global optima. We developed interactive interfaces so that the optimization processes are transparent. Benchmark examples including both aqueous and surface complexation problems illustrate the parameter estimation and associated sensitivity analysis. MINFIT is accessible at http://minfit.strikingly.com .

Sorption of trivalent lanthanides and actinides onto montmorillonite: Macroscopic, thermodynamic and structural evidence for ternary hydroxo and carbonato surface complexes on multiple sorption sites

The credibility of long-term safety assessments of radioactive waste repositories may be greatly enhanced by a molecular level understanding of the sorption processes onto individual minerals present in the near- and far-fields. In this study we couple macroscopic sorption experiments to surface complexation modelling and spectroscopic investigations, including extended X-ray absorption fine structure (EXAFS) and time-resolved laser fluorescence spectroscopies (TRLFS), to elucidate the uptake mechanism of trivalent lanthanides and actinides (Ln/An(III)) by montmorillonite in the absence and presence of dissolved carbonate. Based on the experimental sorption isotherms for the carbonate-free system, the previously developed 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model needed to be complemented with an additional surface complexation reaction onto weak sites. The fitting of sorption isotherms in the presence of carbonate required refinement of the previously published model by reducing the strong site capacity and by adding the formation of Ln/An(III)-carbonato complexes both on strong and weak sites. EXAFS spectra of selected Am samples and TRLFS spectra of selected Cm samples corroborate the model assumptions by showing the existence of different surface complexation sites and evidencing the formation of Ln/An(III) carbonate surface complexes. In the absence of carbonate and at low loadings, Ln/An(III) form strong inner-sphere complexes through binding to three Al(O,OH)6 octahedra, most likely by occupying vacant sites in the octahedral layers of montmorillonite, which are exposed on {010} and {110} edge faces. At higher loadings, Ln/An(III) binds to only one Al octahedron, forming a weaker, edge-sharing surface complex. In the presence of carbonate, we identified a ternary mono- or dicarbonato Ln/An(III) complex binding directly to one Al(O,OH)6 octahedron, revealing that type-A ternary complexes form with the one or two carbonate groups pointing away from the surface into the solution phase. Within the spectroscopically observable concentration range these complexes could only be identified on the weak sites, in line with the small strong site capacity suggested by the refined sorption model. When the solubility of carbonates was exceeded, formation of an Am carbonate hydroxide could be identified. The excellent agreement between the thermodynamic model parameters obtained by fitting the macroscopic data, and the spectroscopically identified mechanisms, demonstrates the mature state of the 2SPNE SC/CE model for predicting and quantifying the retention of Ln/An(III) elements by montmorillonite-rich clay rocks.

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.