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| Titel |
Influence of Co2+ in CaCO3 polymorphism. |
| VerfasserIn |
Jorge González-López, Ángeles Fernández-González, Amalia Jimenez |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097370
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| Publikation (Nr.) |
 EGU/EGU2014-12944.pdf |
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| Zusammenfassung |
| Cobalt is a trace element in the Earth crust but also a toxic metal. Furthermore, Co2+can
accumulate in some specific areas due to both natural and anthropogenic factors
and hence soils and groundwater can be contaminated. Numerous studies have
demonstrated that toxic elements can be removed from aqueous solution by its
incorporation in mineral structures. In particular, the uptake of divalent metallic cations
in the crystal structure of calcium carbonates has been postulated as a valuable
solution to resolve and / or reduce some environmental problems. Here, we have
investigated the cobalt uptake during calcium carbonate precipitation at ambient
conditions. It is well known that certain anions such as sulphate, chromate and selenite
favour the precipitation of vaterite while other cations as Sr2+ or Mg2+, avoid the
calcite crystallization favouring the aragonite precipitation. Similar scenery can be
depicted for Co2+ behaviour since the precipitation of both aragonite and a low
crystallinity phase from aqueous solution have been described. However, the evolution
of the polymorphic transformations after the precipitation of the phases remains
unknown.
In the present work we have precipitated calcium carbonate at room temperature in the
presence of a certain amount of Co2+. Then, the precipitated solid has been aged in the
remaining aqueous solution for two months. The experiment was carried out by mixing two
different aqueous solutions: a) 50 mL of CoCl2 (0.02M) and CaCl2(0.05M) and b) 50 mL of
Na2CO3 (0.05M). The aging process was monitored after 5 minutes, 1, 5, 24, and 48
hours and 4, 7, 30 and 60 days by analysing both the aqueous solution and the aged
solids. The evolution of the different crystalline phases in the solid was followed by
X-ray Powder Diffraction, their morphology was observed by Scanning Electron
Microscopy and their chemical composition was analysed by Energy-dispersive X-ray
Spectroscopy. Furthermore, the aqueous solution has also been analyzed for the entire aging
time.
The initial solution is supersaturated for a number of crystalline phases that could
precipitate: calcite, aragonite, vaterite, monohydrocalcite sphaerocobaltite... However, we
have observed that the first precipitate is a mixture of calcite and hydrated amorphous
cobalt carbonate. The evolution of this first solid with the aging time is complex.
One of the most relevant changes takes place one hour after precipitation, when
calcite dissolves and monohydrocalcite occurs together with the amorphous phase.
This newly formed monohydrocalcite is also dissolved and aragonite precipitates
4 days after precipitation. Aragonite and the amorphous phase coexisted in the
aqueous solution at least after 30 days of aging. Finally, after 60 days, the crystalline
phase Co2CO3(OH)2 was identified in the solid together with aragonite. This cobalt
hydroxy-carbonate has never been reported to precipitate at room temperature. |
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