*4.2. Carbonation Route (G-L)*

Vaterite particles can be synthesized using carbon dioxide as a reagent. The reaction may be carried out using calcium hydroxide or calcium salt as a source of Ca2<sup>+</sup> ions. The reaction of calcium hydroxide and carbon dioxide can be written as:

$$\text{Ca(OH)}\_{2} + \text{CO}\_{2} \rightarrow \text{CaCO}\_{3} + \text{H}\_{2}\text{O} \tag{2}$$

When gaseous CO2 is introduced into the aqueous solution of calcium salt (e.g., CaCl2), the overall following reactions can be presented as:

$$\rm CaCl\_2 + CO\_2 + 2OH^- \leftrightarrow CaCO\_3 + H\_2O + 2Cl^- \tag{3}$$

The detailed mechanisms of calcium carbonate precipitation are complex and include the transfer of carbon dioxide from the gas phase into the liquid phase, the formation of a carbonic acid, and its hydrolysis to produce carbonate ions that are the reagent for CaCO3 precipitation. The basic pH of the reactive mixture favors both CO2 absorption and carbonate ion formation. In case of calcium hydroxide slurry, the initial pH is about 12.4, and therefore the CO2 absorption is relatively easy. However, when calcium salts (e.g., calcium chloride, CaCl2, or calcium nitrate, Ca(NO3)2) are used, the pH of solution is more acidic and less advantageous to the formation of carbonate ions. Therefore, CO2 absorption promoters, like ammonia [19,20,55,56] or amines [56,57], are added to the initial solution. Moreover, carbamate ions that form during carbon dioxide absorption in ammonia and primary or secondary amine aqueous solutions may stabilize the vaterite phase [56,58]. A modification of this approach is the use of extracts obtained by the leaching of calcium to reach minerals or solid waste with ammonium salt solutions [59]. The formed leachates contain calcium ions and ammonia and can be utilized in the carbonation process.

Recently, an interesting synthesis method using solid CO2 (dry ice) was proposed [60]. The reaction was carried out at a minimum temperature of −50◦C in a water-methanol solution containing calcium oxide. The addition of dry ice pellets into a water-methanol solution kept the temperature low and the use of methanol allowed the liquid state of the reaction system to be maintained. Such conditions promoted the CO2 solubility and enabled the synthesis of vaterite nanoparticles.
