Uranium(6+) sorption on montmorillonite: Experimental and surface complexation modeling study

Sorption interactions with montmorillonite and other clay minerals in soils, sediments, and rocks are potentially important mechanisms for attenuating the mobility of U(6+) and other radionuclides through the subsurface environment. Batch experiments were conducted (in equilibrium with atmospheric % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% acbiGaiWiG-bfadaWgaaWcbaacbaGaa43qaiaa+9eadaWgaaqaaiaa% +jdaaWqabaaaleqaaaaa!400D!\[P_{CO_2 } \])to determine the effects of varying pH (2 to 9), solid-mass to solution-volume ratio (M/V = 0.028 to 3.2 g/L), and solution concentration (2 × 10^?7 and 2 × 10^?6 M ²³³U) on U(6+) sorption on SAz-1 montmorillonite. The study focused on U(6+) surface complexation on hydroxylated edge sites as the sorption mechanism of interest because it is expected to be the predominant sorption mechanism at pHs typical of natural waters (pH ≈6 to ≈9). Thus, the experiments were conducted with a 0.1 M NaNO₃ matrix to suppress ion-exchange between U(6+) in solution and interlayer cations. The results show that U(6+) sorption on montmorillonite is a strong function of pH, reaching a maximum at near-neutral pH (≈6 to ≈6.5) and decreasing sharply towards more acidic or more alkaline conditions. A comparison of the pH-dependence of U(6+) sorption with that of U(6+) aqueous speciation indicates a close correspondence between U(6+) sorption and the predominance field of U(6+)-hydroxy complexes. At high pH, sorption is inhibited due to formation of aqueous U(6+)-carbonate complexes. At low pH, the low sorption values indicate that the 0.1 M NaNO₃ matrix was effective in suppressing ion-exchange between the uranyl (UO₂ ²⁺) species and interlayer cations in montmorillonite. At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH. The experimental results were used to develop a thermodynamic model based on a surface complexation approach to permit predictions of U(6+) sorption at differing physicochemical conditions. A Diffuse-Layer model (DLM) assuming aluminol (>AlOH^?) and silanol (>SiOH^?) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)-H₂O-CO₂-montmorillonite system at an ionic strength of 0.1 M and pH > 3.5. A comparison of model predictions with data from this study and from published literature shows good agreement and suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling by providing a better alternative to the use of constant K _d s in transport calculations.     

Iron release in aqueous environment by fresh volcanic ash from Mount Etna (Italy) and Popocatépetl (Mexico) volcanoes

In this study, we performed leaching experiments for timescales of hours-to-months in deionized water on fresh volcanic ash from Mt. Etna (Italy) and Popocatépetl (Mexico) volcanos to monitor Fe release as a function of ash mineral chemistry and size, with the aim of clarifying Fe release mechanisms and eventually evaluating the impact of volcanic ash on marine and lacustrine environments. To define sample mineralogy and Fe speciation, inclusive characterization was obtained by means of XRF, SEM, XRPD, EELS and Mössbauer spectroscopies. For Etna and Popocatépetl samples, glass proportions were quantified at 73 and 40%, Fe₂O₃ total contents at 11.6–13.2 and 5.8 wt%, and Fe³⁺/Fe_Tot ratios at 0.33 and 0.23, respectively. Leaching experiments showed that significant amounts of iron, ~?30 to 150 and ~?750 nmol g^?1 l^?1 for pristine Etna and Popocatépetl samples, respectively, are released within the first 30 min as a function of decreasing particle size (from 1 to 0.125 mm). The Popocatépetl sample showed a very sustained Fe release (up to 10 times Etna samples) all along the first week, with lowest values never below 400 nmol g^?1 l^?1 and a maximum of 1672 nmol g^?1 l^?1 recorded after 5 days. This sample, being composed of very small particles (average particle size 0.125 mm) with large surface area, likely accumulated large quantities of Fe-bearing sublimates that quickly dissolved during leaching tests, determining high Fe release and local pH decrease (that contributed to release more Fe from the glass) at short timescale (hours-to-days). The fractional Fe solubility (Fe_S) was 0.004–0.011 and 0.23% for Etna and Popocatépetl samples, respectively, but no correlation was found between Fe released in solution and either ash Fe content, glass/mineral ratio or mineral assemblage. Results obtained suggest that volcanic ash chemistry, mineralogy and particle size assume a relevant role on Fe release mostly in the medium-to-long timescale, while Fe release in the short timescale is dominated by dissolution of surface sublimates (formed by physicochemical processes occurring within the eruption plume and volcanic cloud) and the effects of such a dissolution on the local pH conditions. For all samples, a moderate to sustained Fe release occurred for leaching times comparable with their residence time within the euphotic zone of marine and lacustrine environments (variable from few minutes to few hours), revealing their possible contribution to increase Fe bioavailability.     

Continental evaporites and the search for evidence of life on Mars

Continental evaporites are deposits that originate from the evaporation of saline waters in the low areas of saline lakes from all continents, except Europe, and mainly consist of chloride, sulphate and potash minerals. In recent years, the discovery on the Martian surface of hydrated salt minerals, including sulphates and chlorides, interpreted as deriving from the desiccation of preexisting large bodies of water, such as lakes, has provided further convincing evidence of liquid water activity on the surface of Mars and, consequently, it has reinforced the plausibility of finding life. Because evaporites require short‐term aqueous processes for their formation, they can trap and preserve over geologic times a biological record made up of halophilic extremophiles—such as microalgae, bacteria, and their remains—that recent research on Earth has shown to be characterized by unexpectedly high biodiversity. This record may consist of varying types of fossils, including morphological fossils, chemofossils and biominerals. As a consequence, continental evaporite environments and their saline deposits are now a primary target for the near future astrobiology missions devoted to the search for fossil Martian life. Lacustrine evaporite deposits and minerals have, therefore, been identified as primary targets for the NASA–ESA joint programme of the Mars sample return, planned for the end of the current decade. Copyright © 2011 John Wiley & Sons, Ltd.