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International audience; Excavation operations during construction produce millions of tons of soil sometimes with high leachable molybdenum (Mo) contents, that can lead to risks for both human health and the environment. It is therefore necessary to immobilize the Mo in excavated soils to reduce pollution and lower the costs of soil disposal. This paper studies the immobilization of Mo by three cementitious binders. To this end, one Ordinary Portland cement (OPC), one binder composed of 90% ground granulated blast furnace slag (GGBS) and 10% OPC, and one supersulfated GGBS binder were spiked with sodium molybdate at six different Mo concentrations from 0.005 wt% to 10 wt% before curing. In addition, to gain mechanistic insights, the capacity of synthetic calcium silicate hydrates (C-S-H) to immobilize Mo was studied. This study was completed by thermodynamic modeling to predict the immobilization of Mo at low Mo concentrations (95%) by the coprecipitation of powellite. Thermodynamic modeling was in good agreement with measured values when the equilibrium constant of powellite was modified to LogK = −7.2. This suggested that powellite is less stable in cementitious environments than would be expected from thermodynamic databases. Moreover, modeling showed that, for a solution at equilibrium with portlandite or C-S-H, the Mo concentration is limited to 1.7 mg/L by powellite precipitation. In contrast, for a solution saturated with respect to ettringite, the threshold concentration for powellite precipitation is 6.5 mg/L.

The spatial distribution of soil constituents at the micrometer scale is of great importance to understand processes controlling the formation of micro-aggregates and the stabilization of organic carbon. Here, the spatial distribution of organic and mineral constituents in Podzol horizons is studied by concerted measurements of (i) the content of various forms of Fe, Al, Si and C determined by selective extraction in the fine earth fraction of soil (f < 2 mm); (ii) the elemental composition of the clay fraction (f < 2 um) with lateral resolution using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and with surface selectivity using X-ray photoelectron spectroscopy (XPS); (iii) the specific surface area (SSA) of fine earth and clay fractions by krypton physisorption. The SSA of the fine earth in illuvial horizons is predominantly due to finely divided Fe oxides, including goethite, characterized by an equivalent particle size of about 10 mu m. Kaolinite platelets of about 2 gm size account for a large volume proportion in the clay fraction but have a minor contribution to SSA. Fe oxides and organic matter (OM) are intimately associated. Heterogeneity at the um scale is created by local variations in the relative amounts of kaolinite and Fe-OM associations. These two kinds of physical entities are in random mixture. Moreover, variation of C/Fe atomic ratios reveals sub-mu m scale heterogeneity. The latter is due to variation in the relative proportion of organic compounds and Fe oxides, indicating that aggregation of nanoparticles, and not only mere adsorption or pore filling, plays a role in these associations. In this regard, our results highlight that OM associated with Fe protects Fe oxides against physical displacement and that part of this associated OM is oxidizable by NaOCl treatment. These findings demonstrate that the concept of OM stabilization through association with Fe must be revisited when considering the sub-mu m scale level because fine Fe oxide particles can be easily dispersed during oxidation of associated carbon. Combination of physical fractionation and microanalysis (e.g. SEM-EDS, vibrational spectroscopy) offer promising perspectives to clarify the relationship between chemical composition and sub-inn scale architecture, and to better understand soil processes.

Este artículo contiene 9 páginas, 1 figura, 1 tabla., The recent surge in research on organic carbon sequestration by seagrass ecosystems has begun to reveal the complexity of the carbon cycle within these ecosystems. In this prospective we discuss two areas of investigation that require further scrutiny: (1) why organic carbon is stabilized in seagrass sediments, and (2) how long organic carbon resides within these sediments. By delving into these topics, pointing out current pitfalls, and highlighting methodological advances, our motive is to focus future efforts and provide a frame work to manage the complexity found within the diverse seagrass bioregions. The high rate of seagrass degradation and loss, coupled with increasing atmospheric CO2 concentrations gives precedence to these lines of research, which require rigorous reevaluation if we are to substantially advance our understanding of OC dynamics in seagrass ecosystems., The work leading to this publication was supported by the German Academic Exchange Service (DAAD) with funds from the German Federal Ministry of Education and Research (BMBF) and the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n◦ 605728 (PRIME—Postdoctoral Researchers International Mobility Experience), and the projects SUMILEN (CTM2013-47728-R) and PALEOPARK (1104/2014), funded by the Spanish National Parks and State Research Schemes.

cited By 15; International audience; Low size dispersity silver nanoparticles (ca. 6 nm) have been synthesized by the hydrogenolysis of silver amidinate in the presence of hexadecylamine. Combining NMR techniques with SERS and DFT modeling, it is possible to observe an original stabilization mechanism. Amidine moiety is strongly coordinated to the Ag0 nanoparticles surface whereas HDA ligand is necessary to prevent agglomeration, although it is only weakly interacting with the surface.