Direct carbon dioxide uptake from the atmosphere: examples from Montecastelli serpentinites (Tuscany, Italy)

Atmospheric CO2 is spontaneously sequestered through the carbonation of serpentinites at Montecastelli (Tuscany, Italy). In the studied area, hydrated Mg-carbonates frequently form crusts, coating and spherules on exposed surfaces, in rock fractures, and on clast surfaces in mining dumps. XRD analyses indicated that precipitated minerals are hydromagnesite, nesquehonite, manasseite, pyroaurite, brugnatellite, often associated with aragonite. No silica has been detected. Brucite-rich serpentinites do not evidence any pervasive alteration, and water/rock interaction seems to be restricted to the fracture surfaces along which carbonation occurred. The sampled spring waters show high pH, relatively high TDS (Total Dissolved Solids) and high Mg/Ca molar ratio with respect to the local rainwater, indicating a chemical equilibration with serpentinites. Isotopic analyses (?18O, ?2H) of rainwaters, emerging waters and carbonates confirmed that the carbonates have been precipitated from superficial waters circulating into the serpentinite bodies. Overall, our data point to an on-going rapid uptake of atmospheric CO2, triggered by water flow in fractures and humid airflow on surfaces of the brucite-rich serpentinites. This natural process introduces new applications on the CO2 mineralogical sequestration and its study could open new solutions for the environmental sustainability and GHG reduction.