Nanostructured calcite precipitated under hydrothermal conditions in the presence of organic and inorganic selenium
Résumé
Selenium is an important trace metalloid, whose global cycle is controlled by fluid-rock interactions in the Earth's upper crust, interactions with bio-molecules in soils and living systems, and atmospheric transport in ashes. The cycling of selenium is often intimately associated with carbonate phases, with Se being generally incorporated as an impurity in calcite crystals or adsorbed on carbonate nanoparticles. In order to better understand the interaction of aqueous selenium species with carbonates, we studied the precipitation of calcite under hydrothermal conditions (30-90 °C, 25-90 bar) in a CO2-H2O-Ca(OH)2 medium in the presence of aqueous inorganic and organic selenium compounds. Aqueous carbonation reactions in the presence of selenium at elevated temperatures and pressures, relevant for long-term CO2 sequestration in reservoirs and other natural geological systems, have until now not been investigated to the best of our knowledge. Electron microscopy (FESEM and TEM) and synchrotron X-ray absorption spectroscopy (XAS) were used in a complementary manner to investigate crystal size, structural order (crystallinity), morphology of crystal faces, crystal organization, and selenium speciation in the calcite samples. XAS data analysis showed clear evidence for the incorporation of selenite oxyanion (SeO32−) into the calcite crystal structure. At low Se content (1.3 mg/g calcite), a single site was observed with Se surrounded by six Ca atoms, whereas additional sites, probably corresponding to surface sorption sites, were found with increasing Se content. XAS also showed that seleno-L-cystine (Secys) was chemically fragmented during carbonation, and the solid phase contained elemental and oxidized Se, in hexagonal or amorphous form depending on the experimental conditions, with a minor proportion of Se(IV). Moreover, FESEM and TEM measurements revealed a very complex effect of Secys on the particle size and aggregation/agglomeration process, leading to the following calcite morphologies: rhombohedra, elongated rhombohedra (c-axis elongation), scalenohedra, star-like and shell-like crystal aggregates, and irregular calcite polycrystals. The aggregates and irregular polycrystals, which we designate as nanostructured calcite material, were constituted of nanometer-sized calcite crystallites (< 100 nm). The star and shell-like crystal aggregates, which were observed only in the presence of Secys, may be due to crystal growth in the presence of associated secondary organic compounds due to a simultaneous chemical fragmentation of Secys. Overall, the results from this study show that selenium (of biotic or abiotic origin) can be integrated into the crystallographic structure of calcite under hydrothermal conditions. This has relevance for geological processes in diverse environments, such as hydrothermal systems along mid-ocean ridges, or underground reservoirs associated with massive injection of CO2 for long-term geological sequestration.