## 8 Applications of Calcium Carbonate Nanoparticles
### Heavy Metals Removal  
Heavy metals contamination is one of the serious nuisances in the area of water and wastewater management. The elimination of heavy metals from the aqueous solutions is carried out by numerous methods that we have already seen above in the introduction section. We know that adsorption is a reliable, economical, and efficient technique [94]. Some of the most common adsorbents are zeolites, activated carbon, alumina, zinc oxide, etc. Here the size of adsorbents falls in the size of microns, so the process is less efficient. Figure 13: X-ray powder diffraction (XRD) spectrum of calcium carbonate nanoparticles adapted from [PERSON] et al., 2010 ([PERSON] et al., 2010). When adsorption is done employing biological material like microbes then the process is called bioremediation [95]. When biological material and NPs are used together with either by surface modification or in the form of composite, the process is called nano bioremediation. When the adsorption process is carried out by NPs then the process is called nano remediation [96]. In the above section, we have already seen the advantages of CCPs as an adsorbent. So, here we have highlighted the examples where CCPs have been applied for the removal of heavy metals and other pollutants. [PERSON] et al., 2011, reported the removal of heavy metals (Cu\({}^{2+}\), Cr\({}^{6+}\), Cd\({}^{2+}\), and Pb\({}^{2+}\)) by using calcium carbonate particles extracted from the starfish by using enzymes like alpha-amylase, beta-amylase, and protease. It was found that the obtained adsorbent were comparatively effective in the heavy metal removal than the calcium carbonate obtained from crabs, oyster, and cockle shells. Moreover, the calcium carbonate was highly porous in nature, which again made them suitable and effective candidate for the remediation of heavy metals [91]. The remediation of heavy metals from the aqueous solution by using CCPs synthesized through chemical method metals [97]. In this experiment, CaCO\({}_{3}\) was used to remove the Fe and Pb ions from the aqueous solutions and studied the adsorption mechanism including the kinetic and isotherm models at room temperature. From the above study following findings were obtained: (i) adsorption kinetics follows a pseudo-second-order equation; (ii) Langmuir isotherm revealed that the maximum monolayer adsorption capacity of CaCO\({}_{3}\) for Pb (II) was 1210 \(\pm\) 30 mg/g and for Fe (II) ions, it was 845 \(\pm\) 8 mg/g; (iii) optimal dose of CaCO\({}_{3}\) was 200 mg/L at 25 \({}^{\circ}\)C, and (iv) and the removal efficiency of Fe and Pb ions is enhanced by a precipitation transformation mechanism instead of adsorption. [PERSON] et al. (2018) reported the use of CaCO\({}_{3}\) coated bacterial magnetosomes (biogenic magnetite obtained from the internal structures of magnetotactic bacteria) for remediation of Cr (III) and Ni (II) ions from tannery effluent. The following findings were observed from the experiment. 1. Equilibrium was attained within an hour for Cr (III) ions at pH-6.0 and Ni (II) ions at a pH-8.0. 2. The adsorption process followed a pseudo-second-order reaction kinetics, along with Langmuir and Freundlich adsorption isotherms. 3. Adsorption of metallic ions on calcite microcrystals was spontaneous and endothermic. 4. Almost equal removal efficiency (Cr-94% and Ni-84%) was observed with both magnetic calcite and calcite crystals but it was higher than magnetosomes and activated carbon. 5. Removal of both the metal ions was facilitated by applying an external magnetic field. 6. It is concluded that the magnetic calcite could act as a potential and alternative adsorbent for the elimination of heavy metals from tannery effluents [98]. [PERSON] et al. (2017) reported the elimination of the excess of fluoride ions from different water sources from Sri Lanka using porous CCNPs. The reported approach is quite simple, cost-effective, and innovative for the elimination of fluoride ions from the wastewater.