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Abstract
Mineral carbonation, a silicates/oxides reaction with CO2-bearing fluids to form
stable carbonates, is a promising strategy to mitigate rising CO2 concentrations in
the atmosphere and the relative climate effects. Serpentinite that is considered as a highly suitable feedstock for mineral carbonation due to its high Mg-content minerals and worldwide abundance naturally transforms into Mg-carbonates,
providing insight into the carbonation process. Understanding of natural analogues of mineral carbonation is an important step in the implementation and optimization of processes for industrial carbon storage in minerals.
Tuscany (Italy) provides outstanding examples of natural analogues for carbon dioxide mineral sequestration. Here, atmospheric-CO2 is spontaneously
sequestered through the weathering and subsequent carbonation of serpentinite outcrops at Montecastelli (Tuscany, Italy). Three sub-areas of Montecastelli have been selected: (i) a large steep escarpment placed on the western side of the Pavone River; (ii) the underground works of the Montecastelli copper mine; (iii) a small mine dump placed near the exit of the upper adit of Montecastelli mine. This Ph.D. study focusses on multidisciplinary approach involving petrographic, geochemical and isotopic characterization of selected solid, liquid and vapor matrices of the Montecastelli sub-areas.
The occurrence of brucite-rich serpentinized dunites, hosted by brucite-free serpentinized harzburgites, is pivotal to enhance the different carbonation effects observed at Montecastelli. Carbonation of serpentinites is strongly catalysed by these brucite-rich lithotypes, showing a pervasive effect with the precipitation of Mg-Fe LDHs and hydrous Mg-carbonates. By contrast, the brucite-free serpentinized harzburgites are not significantly affected by carbonation. Coatings and crusts of hydrous Mg-carbonates ± aragonite can form on free surfaces of this rock, but they never reach the pervasive effect observed in brucite-rich serpentinized dunites.
Here, the infiltrated rainwaters underwent multiple interactions with the soil, acquiring negative carbon isotope signature, and with the hosting serpentinized ultramafic rocks, changing their chemical composition (Mg content > 55 mg/L) and pH (~8.5).
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The resultant waters show a similar constant chemical characteristic (both in the escarpment and in the mine) suggesting an underground reservoir and a broad circulation. The outflow and the circulation of the waters in the serpentinites are highlighted by a diffuse aragonite precipitation, as confirmed by our modelling. In correspondence with the brucite-rich serpentinized dunite bodies, the circulating waters dissolve the additional amounts of the brucite, increasing their Mg content. Different combinations of the brucite dissolution, together with local evaporation, and/or temperature increase, result in hydromagnesite precipitation.
Air moisture could contribute, in a small extent, to the hydromagnesite formation with its condensation and evaporation on the external surface of the serpentinized dunite rocks. This condensed thin water film reacts with brucite or mixes with infiltrated waters, resulting in the hydromagnesite formation.
This process, that has a marginal role in the outcropping carbonated serpentinized dunite, acquires a fundamental role in the mine. Here, the percolating modified waters are saturated in aragonite (as confirmed by the aragonite flowstone formation), but cannot react locally with additional serpentinized dunites to reach hydromagnesite saturation. Indeed, the condensation of air moisture and its reaction with the external surface of the serpentinized harzburgite produce the observed thin hydromagnesite crusts.
Our genetic models have been confirmed by the PHREEQC geochemical code and the Rayleigh-type isotope distillation model.
Finally, evidence of microbial activity has been observed in the carbonate crust, at the interface with serpentine and inside the carbonate texture. The role of the observed photosynthetic and non-photosynthetic microorganisms is still unclear. Only in some places, they seem to affect the carbonation process, whereas in others they play a passive role in the process.
