Laurent Charlet

Surface catalysis of uranium(VI) reduction by iron(II)

Abstract Colloidal hematite (α-Fe2O3) is used as model solid to investigate the kinetic effect of specific adsorption interactions on the chemical reduction of uranyl (UVIO22+) by ferrous iron. Acid–base titrations and Fe(II) and uranyl adsorption experiments are performed on hematite suspensions, under O2- and CO2-free conditions. The results are explained in terms of a constant capacitance surface complexation model of the hematite–aqueous solution interface. Two distinct Fe(II) surface complexes are required to reproduce the data: (≡FeIIIOFeII)+ (or ≡FeIIIOFeII(OH2)n+) and ≡FeIIIOFeIIOH0 (or ≡FeIIIOFeII(OH2)n−1OH0). The latter complex represents a significant fraction of total adsorbed Fe(II) at pH > 6.5. Uranyl binding to the hematite particles is characterized by a sharp adsorption edge between pH 4 and pH 5.5. Because of the absence of competing aqueous carbonate complexes, uranyl remains completely adsorbed at pH > 7. A single mononuclear surface complex accounts for the adsorption of uranyl over the entire range of experimental conditions. Although thermodynamically feasible, no reaction between uranyl and Fe(II) is observed in homogeneous solution at pH 7.5, for periods of up to three days. In hematite suspensions, however, surface-bound uranyl reacts on a time scale of hours. Based on Fourier Transformed Infrared spectra, chemical reduction of U(VI) is inferred to be the mechanism responsible for the disappearance of uranyl. The kinetics of uranyl reduction are quantified by measuring the decrease with time of the concentration of U(VI) extractable from the hematite particles by NaHCO3. In the presence of excess Fe(II), the initial rate of U(VI) reduction exhibits a first-order dependence on the concentration of adsorbed uranyl. The pseudo-first-order rate constant varies with pH (range, 6–7.5) and the total (dissolved + adsorbed) concentration of Fe(II) (range, 2–160 μM). When analyzing the rate data in terms of the calculated surface speciation, the variability of the rate constant can be accounted for entirely by changes in the concentration of the Fe(II) monohydroxo surface complex ≡FeIIIOFeIIOH0. Therefore, the following rate law is derived for the hematite-catalyzed reduction of uranyl by Fe(II), d[U(VI)] dt =−k[≡ Fe III OFe II OH 0 ][U(VI)] ads where the bimolecular rate constant k has a value of 399 ± 25 M−1 min−1 at 25°C. The hydroxo surface complex is the rate-controlling reductant species, because it provides the most favorable coordination environment in which electrons are removed from Fe(II). Natural particulate matter collected in the hypolimnion of a seasonally stratified lake also causes the rapid reduction of uranyl by Fe(II). Ferrihydrite, identified in the particulate matter by X-ray diffraction, is one possible mineral phase accelerating the reaction between U(VI) and Fe(II). At near-neutral pH and total Fe(II) levels less than 1 mM, the pseudo-first-order rate constants of chemical U(VI) reduction, measured in the presence of the hematite and lake particles, are of the same order of magnitude as the highest corresponding rate coefficients for enzymatic U(VI) reduction in bacterial cultures. Hence, based on the results of this study, surface-catalyzed U(VI) reduction by Fe(II) is expected to be a major pathway of uranium immobilization in a wide range of redox-stratified environments.

A surface complexation model of the carbonate mineral-aqueous solution interface

A surface complexation model for the chemical structure and reactivity of the carbonatewater interface is presented. The model postulates the formation of the hydration species >CO3H0 and >MeOH0 at the surface of a divalent metal carbonate MeCO3 (Me = Ca, Mn, Fe, etc.). The existence of these primary hydration species is supported by spectroscopic data. The following reactions are proposed to govern surface speciation in the MeCO3(s)-H2O-CO2 system: CO3H0a2>CO3− + H+(1) CO3H0 + Me2+a2>CO3Me+ + H+(2) MeOH2+a2 >MeOH0 + H+(3) MeOH0a2 >MeO− + H+(4) MeOH0 + CO2a2 >MeHCO3o (5) MeOH0 + CO2a2 >MeCO3−+ H+(6) It is shown in this paper that the surface complexation model provides a systematic explanation of surface charge development and dissolution kinetics of carbonate minerals. The intrinsic stability constants of the surface complexation reactions agree well with the equilibrium constants of the corresponding complexation reactions in homogeneous solution.

X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface: II. Adsorption, coprecipitation, and surface precipitation on hydrous ferric oxide

Abstract The sorption of Cr(III) by hydrous Fe oxides involves adsorption, surface precipitation, and coprecipitation phenomena. These phenomena lead to different phases which are here compared with regard to both their Cr solubility and their local structure. The former is simulated by the “surface precipitation model” and the latter is derived from X-ray absorption fine-structure (EXAFS) spectroscopic data interpreted by the “polyhedral approach method.” The adsorption of a Cr(III) atom onto goethite or hydrous ferric oxide (HFO) occurs via the formation of inner-sphere surface complexes. In such complexes, Cr atoms are never isolated, but present as small surface hydroxy polymers. This polymerization has been catalyzed by the surface, and it occurs when Cr(III) bonds only 10% of the surface “active” sites (i.e., when it covers 1% of the BET surface area). In these surface polymers, Cr(III) atoms are surrounded by three metal (Fe or Cr) shells at 3.00–3.05, 3.40–3.46, and 3.94–4.03 A, as in a mixed α- and γ-MeOOH local structure (Me = Fe or Cr). These polymers act as nuclei for the precipitation of a surface hydrous Cr oxide, a precipitation that starts under conditions undersaturated with respect to the homogeneous precipitation of the same oxide, a phenomenon well described by the surface precipitation model. This surface precipitate and the pure hydrous chromium oxide have identical solubility products and the same local structure (γ-CrOOH). On the contrary, when Cr(III) is coprecipitated with Fe(III), only two cation shells are detected around Cr(III) at 2.99 and 3.40 A, indicating that Cr substitutes for Fe in an α-(Fe, Cr)OOH framework. A difference in local structure (α-CrOOH versus γ-CrOOH) therefore accounts for the difference in solubility products of the surface-precipitated and coprecipitated Cr hydroxides.

X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface: I. Molecular mechanism of Cr(III) oxidation on Mn oxides

Abstract The oxidation of Cr(III) to Cr(VI) at Mn(IV) and Mn(III) oxide/water interfaces was investigated by X-ray absorption spectroscopy. The sample [Cr(VI)]/[Cr(VI) + Cr(III)] ratios within the solid-phase samples were accurately determined by two independent methods: the intensities of preabsorption edge spectra and the extended X-ray absorption fine-structure (EXAFS) contribution of the Cr-O atomic pair. Modifications of the local structure around chromium atoms along the redox process were followed by EXAFS. The oxidation process was elucidated at the birnessite (Na4Mn14O27 · 9H2O) surface by means of a kinetic study of the Cr sorption process at pH 4. After a 30-s reaction time, while chromate ions are still few (≈5%), sorbed chromium octahedra are bonded to birnessite by sharing five or six edges with MnO6 octahedra. Ninety seconds later, the chromate concentration is high, and the Mn(IV)-O-Cr(III) bridges are no longer detected. According to these results the mechanism of the oxidation can be depicted as follows: (1) Cr(III) aqua ions diffuse toward Mn(IV) vacancies present in the sheet of MnO6 octahedra; (2) the coupled Cr(III) oxidation/Mn(IV) reduction occurs; (3) Cr(VI) ions are released into the solution. Experimental data suggest that the oxidation process on Mn(III) oxides is similar to that depicted for birnessite. This study provides the first evidence of a double solid-state diffusion, first toward and then backward from the sorbent. These results illustrate how the structure of the sorbent, through the nature of the active site, may control the speciation of elements in aqueous media.

Nanomorphology of montmorillonite particles: Estimation of the clay edge sorption site density by low-pressure gas adsorption and AFM observations

Abstract Dry and in situ (fluid-cell) Atomic Force Microscopy (AFM) and Low-Pressure Gas Adsorption experiments were used to investigate the surfaces of pure Na-smectite particles. These two techniques permit the identification of different surfaces of the platelets (lateral, basal, and interlayer surfaces) and to quantify their surface area. Calculation of the surface area was done for AFM, by measuring directly the dimensions of the clay particles on AFM images, and for gas adsorption experiments, by applying the Derivative Isotherm Summation (DIS) procedure designed by Villiéras et al. (Villiéras et al. 1992, 1997a, 1997b). In the present study, we find a discrepancy between measurements of the basal and interlayer surface area. This difference is due to the stacking of platelets in dry conditions compared to their dispersion in aqueous suspension. A particle is estimated to be formed of nearly 20 stacked layers in the dehydrated state used in the gas adsorption experiment, whereas it is estimated to be composed of only 1 or 2 layers in aqueous suspension, on the basis of AFM measurements. However, the two techniques give similar results for the lateral surface area of the platelets (i.e., about 8 m2/g) and the perimeter to area ratio value of the particles because the stacking of platelets does not alter these values. This correlation confirms the effectiveness of the interpretation of the gas adsorption experiments lowest pressure domains as the adsorption on lateral surfaces. The lateral surface area has important implications in the calculation of specific sorption site density on clay material. The relevance of the lateral surface area value (8 m2/g) was tested subsequently with sorption data found in the literature. Based on those results, we show that one essential parameter for the calculation of particle edge-site density is the mean perimeter to area ratio value. This parameter can be obtained by microscopic techniques but the measurement is tedious. The good correlation between the AFM results and the DIS-method results confirms that the latter procedure offers a quick and reliable alternative method for the measurement of the lateral surface area. AFM experiments can be further conducted to constrain the dispersion around the DIS value and the anisotropy of suspended particles.

In situ atomic force microscopy study of hectorite and nontronite dissolution: Implications for phyllosilicate edge surface structures and dissolution mechanisms

Abstract The dissolution behavior of two smectite minerals, hectorite (trioctahedral) and nontronite (dioctahedral), was observed in situ, in acid solutions, using atomic force microscopy. As expected, the crystallites dissolved inward from the edges, and the basal surfaces appeared to be unreactive during the timescale of the experiments. The hectorite (010) faces appeared to dissolve about 6× more slowly than the lath ends, usually broken edges. The edges visibly dissolved on all sides, and appeared to roughen somewhat. On the other hand, the (010), (110), and (11̄0) faces on nontronite crystals were exceptionally stable, so that any dissolution fronts originating at broken edges or defects would quickly become pinned along these faces, after which no more dissolution was observable. These observations can be explained by using periodic bond chain theory to predict the topology of the surface functional groups on the edge faces of these minerals. If a certain amount of predicted surface relaxation is allowed on the (110) and (11̄0) faces of nontronite, an important difference between the exceptionally stable faces and the others becomes apparent. That is, the oxygen sites connecting the octahedral and tetrahedral sheets are all fully bonded on the nontronite (010), (110), and (11̄0) edge faces, whereas all hectorite edge faces and nontronite broken edges would have coordinatively unsaturated connecting O atoms. This explanation for the differential reactivity of these crystal faces implies that the rate limiting step of the dissolution process is the breaking of bonds to connecting O atoms.

X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by FeII-bearing minerals.

The long-lived radionuclide 79Se is one of the elements of concern for the safe storage of high-level nuclear waste, since clay minerals in engineered barriers and natural aquifer sediments strongly adsorb cationic species, but to lesser extent anions like selenate (SeVIO4(2-)) and selenite (SeIVO3(2-)). Previous investigations have demonstrated, however, that SeIV and SeVI are reduced by surface-associated FeII, thereby forming insoluble Se0 and Fe selenides. Here we show that the mixed FeII/III (hydr)oxides green rust and magnetite, and the FeII sulfide mackinawite reduce selenite rapidly (< 1 day) to FeSe, while the slightly slower reduction by the FeII carbonate siderite produces elemental Se. In the case of mackinawite, both S(-II) and FeII surface atoms are oxidized at a ratio of one to four by producing a defective mackinawite surface. Comparison of these spectroscopic results with thermodynamic equilibrium modeling provides evidence that the nature of reduction end product in these FeII systems is controlled by the concentration of HSe(-); Se0 forms only at lower HSe(-) concentrations related to slower HSeO3(-) reduction kinetics. Even under thermodynamically unstable conditions, the initially formed Se solid phases may remain stable for longer periods since their low solubility prevents the dissolution required for a phase transformation into more stable solids. The reduction by Fe2+-montmorillonite is generally much slower and restricted to a pH range, where selenite is adsorbed (pH < 7), stressing the importance of a heterogeneous, surface-enhanced electron transfer reaction. Although the solids precipitated by the redox reaction are nanocrystalline, their solubility remains below 6.3 x 10(-8) M. No evidence for aqueous metal selenide colloids nor for Se sorption to colloidal phases was found. Since FeII phases like the ones investigated here should be ubiquitous in the near field of nuclear waste disposals as well as in the surrounding aquifers, mobility of the fission product 79Se may be much lower than previously assumed.

N-COMPOUND REDUCTION AND ACTINIDE IMMOBILISATION IN SURFICIAL FLUIDS BY FE(II) : THE SURFACE FEIIIOFEIIOH SPECIES, AS MAJOR REDUCTANT

Abstract Soluble Fe(II) is an important reductant in anoxic surficial fluids, due to fast redox kinetics of the Fe(II)/Fe(III) couple. However, its availability is limited in neutral and alkaline pH range by the solubility of FeS(s), Fe3(PO4)2, FeCO3(s) and other Fe(II) rich minerals. The adsorption of Fe(II) on a variety of mineral phases has been studied. It is shown that, provided enough surface area is available, adsorption is completed before the onset of precipitation, leading to Fe(II) surface species which are able to reduce compounds present in solution in a very efficient way. The abiotic reduction of a variety of N rich compounds (nitrites and nitrobenzenes) by sorbed Fe(II) has been reported in the literature. The observed initial rate of such reduction reactions is shown to be proportional to the FeIIIOFeIIOH° species concentration, in the same manner that the homogeneous oxygenation rate of Fe(II) is proportional to Fe(OH)2°(aq) concentration. The electron transfer in these reactions, appears to occur dominantly via an outer sphere mechanism. In contrast, the abiotic reduction of inorganics, such as U(VI) and Tc(VII), by sorbed Fe(II) involves inner sphere electron transfer mechanism. In the case of uranium reduction, three kinetic steps can be distinguished: the adsorption of the uranyl ion (formation of an inner sphere surface complex), followed by two reductive steps which lead to the formation of a UO2/Fe(OH)3 mixed solid phase. These surface-catalysed reduction reactions may have led to the formation of uranium mineral ores and to the removal of uranium from reducing surface waters.

The surface chemistry of divalent metal carbonate minerals; a critical assessment of surface charge and potential data using the charge distribution multi-site ion complexation model

The Charge Distribution MUltiSite Ion Complexation or CD–MUSIC modeling approach is used to describe the chemical structure of carbonate mineral-aqueous solution interfaces. The new model extends existing surface complexation models of carbonate minerals, by including atomic scale information on the surface lattice and the adsorbed water layer. In principle, the model can account for variable proportions of face, edge and kink sites exposed at the mineral surface, and for the formation of inner- and outer-sphere surface complexes. The model is used to simulate the development of surface charges and surface potentials on divalent carbonate minerals as a function of the aqueous solution composition. A comparison of experimental data and model output indicates that the large variability in the observed pH trends of the surface potential for calcite may in part reflect variable degrees of thermodynamic disequilibrium between mineral, solution and, when present, gas phase during the experiments. Sample preparation and non-stoichiometric surfaces may introduce further artifacts that complicate the interpretation of electrokinetic and surface titration measurements carried out with carbonate mineral suspensions. The experimental artifacts, together with the high sensitivity of the model toward parameters describing hydrogen bridging and bond lengths at the mineral-water interface, currently limit the predictive application of the proposed CD–MUSIC model. The results of this study emphasize the need for internally consistent experimental data sets obtained with well-characterized mineral surfaces and in situ aqueous solution compositions (that is, determined during the charge or potential measurements), as well as for further molecular dynamic simulations of the carbonate mineral-water interface to better constrain the bond lengths and the number plus valence contribution of hydrogen bridges associated with different structural surface sites.

The dissolution of hectorite: In-situ, real-time observations using atomic force microscopy

Abstract The dissolution of individual hectorite (a trioctahedral smectite) particles has been observed at molecular scales in acidic aqueous solution with atomic force microscopy (AFM). A new sample preparation technique was used to attach nanometer-sized hectorite particles to a mica substrate. The reactive surface area of individual hectorite particles was identified and its change during a dissolution experiment was quantified. The dissolution of hectorite under pH 2 conditions takes place exclusively at the edge surfaces. In contrast, the basal surface is completely unreactive within the investigated time scale of several hours. The short edges of the hectorite laths were found to react somewhat more quickly than the long edges. The edge surface area represents 1.5-3.3% of the total surface area. The total surface area has been determined from the actual particle dimensions derived from AFM data to be 730 m2/g. The dissolution rate normalized to the reactive edge surface area (ESA) has been determined to be 7.3 × 10-9 mol hectorite/(m2-s), which represents a total surface area (TSA) normalized dissolution rate of 1.9 × 10-10 mol hectorite/(m2-s). The ESA/TSA ratio increases by about 15% within 1 h exposure to a pH 2 aqueous solution at 22 °C.