**5. Effect of foreign ions on CaCO3 precipitation**

The presence of mineral ions can significantly influence the crystal growth and microstructure of CaCO3 [50–59]. Among mineral ions, sulphate and magnesium ions are widely studied [50, 53–59] since they are present at significant concentrations in natural waters. Compared to sulphate and magnesium ions, the impact of iron on the precipitation of calcium carbonate seems to be less studied and it

appears to be a lack of reliable data on its impact on CaCO3 precipitation, especially at elevated temperature and different pH. **Figure 4** shows a tank highly containing iron-rich water. Iron ions can be present in the aqueous phase either from the corrosion of metallic parts in water desalination processes, such as tanks and pipes [60] or they may be present in groundwater at concentration levels depending on the mineral composition of the aquifer [61]. The water treatment processes are selected according to the water salt concentration. Indeed, membrane technological processes are used to desalinate brackish water containing salt concentrations below 10 g/L, however, reverse osmosis and thermal distillation are generally used to desalinate sea water with salt concentration above 30 g/L. Note that the typical iron concentration in the water used for the above mentioned processes is approximately 2.8 mg/L [62]. Groundwaters containing more than 0.2 mg/L of iron should be treated if used for drinking water [63]. Therefore, it is of great importance to understand how iron and calcium ions interact with each other during CaCO3 precipitation. Throughout the followings, I put a special emphasis on the effects of magnesium, sulphate and iron ions on the precipitation kinetics, microstructure, and polymorphism of CaCO3. Also, the mechanisms by which these ions affect the crystal growth of calcium carbonate were pointed out. Knowledge about the effects of these ions is very important in the elucidation of the growth polymorphs during water treatment.
