Investigation of mechanically induced CO2 storage and conversion driven by Olivine weathering process

The aim of this PhD thesis is to study the conversion of CO2 into hydrocarbons as a consequence of the interaction with Olivine in presence of water, simulating the Serpentinization process. Olivine is a natural mineral constituted by a solid solution of fayalite (Fe2SiO4) and forsterite (Mg2SiO4). In nature around 100 million tons of carbon per year, according to a slow weathering process, is bound by these minerals. The CO2 sequestration in natural silicates sometimes is accompanied by the occurrence of the serpentinization process. This is a widespread phenomenon on the Earth’s mantle that occurs generally at temperatures less than 300°C and during which mineral-based silicates of Fe and Mg react with water to give H2 and minerals of the serpentine group [(Mg, Fe)3Si2O5(OH)4] (Eq. 1.1). This involves the formation of extremely reducing fluids, rich in hydrogen, so any species present, such as inorganic C, can be reduced. 6 (Mg,Fe)2SiO4 + 12 H2O + 6 CO2 → 2 (Mg,Fe)3[(Si)]2O5(OH)4 + 2 Fe3O4 + 8 H2 + 6 [(MgCO)]3 + 2 SiO2 Eq. 1.1 Therefore, CO2 can react with H2, through a Fischer-Tropsch type (FTT) or Sabatier mechanism, to form CH4 and light hydrocarbons (Eq. 1.2). CO2 + 4 H2 → CH4 + 2 H2O Eq. 1.2 Although the whole process is thermodynamically favoured, the rate of the reaction is very slow, and, accordingly, the natural process does not allow the control of CO2 emission levels into the atmosphere. Then, the possibility to increase the kinetics of such processes deserves interest, and, to this regard, preliminary treatment of the olivine mineral, such as mechanical activation, has been demonstrated to be successful. If, on the one hand, the CO2 absorption process during olivine serpentinization has been experimentally investigated, on the other hand, literature data are not homogeneous and refer to the effects of mechanical activation on olivine, focusing mainly on its structural and surface transformation, in order to facilitate CO2 storage. In particular, it has been shown that the mechanical grinding of olivine significantly increases its ability to form iron and magnesium carbonates making the CO2 capture a potential stable storage method for long periods. The mechanical activation was found to be more effective if the treatment occurs in the presence of liquids such as water or ethanol, as a consequence of the greater surface area generated in wet conditions. Conversely, less attention has been paid, in such studies, to the chemical reduction of CO2, to yield light hydrocarbons and corresponding oxygenated compounds. The present work just deals with such issues, and, for the first time to our knowledge, the attention has been focused on the mechanically induced production of methane and light hydrocarbons during the interaction between olivine and water under CO2 atmosphere. So, the purposes of this project are: • a detailed study of the Olivine serpentinization process induced by mechanical grinding, and of the production of methane and hydrocarbons; • investigate the mechanism of the different stages of the serpentinization process and their dependence on experimental conditions; • quantify and optimize the production of methane and light hydrocarbons; • evaluate potential applications.