Despite the significant contribution of fossil fuels to the global energy network, their uses have been the subject of great challenges due to the large amount of carbon dioxide CO2 released because of many industrial activities (Olajire 2013).
Paris (COP21) and Marrakech (COP22) agreements call for achieving carbon neutrality at the world level by 2050. It confirms the objective of keeping the temperature increase below 2°C, and this cannot be achieved without reducing the use of fossil fuels that produce greenhouse gases, especially CO2 from transport and energy production, CH4 and N2O from the petrochemical industries, etc. It is therefore important to focus on the efficient use of energies with a low CO2 footprint (Narine et al. 2021).
The environmental impact of CO2 and other gases has made it urgent to find effective methods of removing these gases from the atmosphere or mitigating their emissions without compromising ongoing industrial and societal progress. Carbon capture appears to be the main solution for reducing CO2 emissions.
Since 1990, Seifritz et al. and many other researchers have investigated CO2 sequestration as a way of reducing CO2 emissions. This implies the use of the mineral carbonation method, where CO2 is stabilized to generate carbonates. (Lackner; O’Connor et al. 2002; Huijgen et al. 2007; Teir et al. 2007; Sanna et al. 2016).
Different natural and synthetic calcium or magnesium silicate composites have been studied. Wollastonite which is a calcium inosilicate material (CaSiO3), is one of the natural calcium silicates and was used as an important raw material in different applications, especially in CO2 sequestration, because calcium silicates tend to be more reactive to mineral carbonation than those of magnesium.
The overall reaction of mineral carbonation can be written as [8]: Initially, the CO2 in contact with water passes from the gaseous to the aqueous form (1), followed by the production of carbonic acid (H2CO3) (2). Then, H2CO3 releases a proton to produce the bicarbonate ion (CO32−) (3), and finally the carbonate ion (4). Using wollastonite, the reaction mechanism takes place in a gas-solid-aqueous system according to the reactions steps (5) and (6), With M2+ = Ca2+ or Mg2+ [9].
Dissolving CO2 leads to a decrease in pH, but a high pH is necessary to reach equilibrium. Depending on the pH, the CO2 dissolved in water can be found in different forms, H2CO3, HCO3− and CO32−.
CO2(g) ⇨ CO2(aq) (1)
CO2(aq) + H2O ⇨ H2CO3(aq) (2)
H2CO3(aq) ⇨ HCO3-(aq) + H+ (3)
HCO3-(aq) ⇨ CO32-(aq) + H+ (4)
MSiO3 + 2H+ ⇨ M2+ + SiO2 + H2 (5)
M2+ + HCO3- ⇨ MCO3(S) + H+ (6)
The principal disadvantage of mineral carbonation techniques is their high operating cost due to the consumption of large amounts of energy and chemicals, which makes their industrial-scale application impossible [7]. To overcome this disadvantage, the synthesis of materials with the most eco-friendly techniques will be the key to rewarding extra costs.
Until now, various methods have been used to synthesize wollastonite or calcium inosilicate mineral (CaSiO3). However, in the majority of these methods, the precursors used fail to achieve a uniform mixture at the molecular level. This deficiency inevitably leads to suboptimal insertion of specific ions into the mineral structure, thus influencing CO2 fixation performance.
This study aims to identify the factors that favor the mineral carbonation reaction for sequestering carbon dioxide through the utilization of Ca2+ cations derived from wollastonite. For this purpose, we have tried to synthesize wollastonite by three different methods, namely sol-salt synthesis, hydrothermal synthesis and synthesis by combustion in solution, in order to compare the effectiveness of each synthesis route. In addition, we plan to study their effects on the phase purity of wollastonite (CaSiO3) and its behavior in CO2 sequestration, in order to know how to find its application in the natural wollastonite that exists in the earth’s crusts.