## 3 Carbon-Based NPs in Water Treatment
### Carbon Nanospheres (CNs)  
Carbon nanospheres (CNs), or fullerenes, refer to spherical carbon particles without a specific molecular structure. They can be solid or hollow. These vary in composition and may include amorphous carbon or graphite-like structures. They are not limited to a specific number of carbon atoms in a closed cage. They can be classified into three categories: the Cn family and carbon onions (2-20 nm), carbon nanosized spheres (50 nm to 1 mm), and carbon beads (one to several microns) [89]. Carbon onions, according to [PERSON]'s textural classification, are viewed as concentric, nested hyperfullerenes, with C60 being the smallest [90]. The unique structure of carbon onions, with a large surface-to-volume ratio, contributes to their stability by eliminating dangling bonds.  
CNs can be fabricated via the arc discharge process, laser ablation, and autoclave methodology. Carbon sphere-derived materials offer a promising alternative to activated carbon in the water treatment industry. When hydroxylated, these materials exhibit enhanced contaminant trapping capacity attributed to an abundance of surface sites, variable specific surface area, and favorable surface charging [91]. The versatile applications of carbon spheres and their functionalized counterparts extend across various fields, with significant relevance in water treatment [54; 55].  
Fullerenes are a family of carbon allotropes with molecules in the form of a hollow sphere, tube, or ellipsoid that is composed entirely of carbon. Fullerenes have been extensively used for photocatalytic wastewater treatment, including disinfection and pollutant degradation [92]. The most well-known fullerene is C60, a molecule consisting of 60 carbon atoms with low solubility and dispersibility in water and easy agglomeration [93; 94].  
Their synthesis methods include arc discharge, laser ablation, and solvent-based methods. Fullerenes are suggested as a foundation for advancing technologies that harness nanomaterials for enhanced oxidation and disinfection, improved membrane processes, superior adsorbents, and surfaces resistant to biofilm formation [95]. Their nanostructures with high adsorption capacities revealed their importance in the field of membranes [96]. Table 3 shows recent studies on the removal of different pollutants using different types of CNPs.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Type of CNPs** & \begin{tabular}{c} **Synthesis** \\ **Methods** \\ \end{tabular} & **Pollutant Type** & \begin{tabular}{c} **Surface** \\ **Area(m\({}^{2}\)/g)** \\ \end{tabular} & \
\begin{tabular}{c} **Optimum** \\ **Condition** \\ \end{tabular} &
\begin{tabular}{c} **Adsorption** \\ **Capacity (mg/g)** \\ \end{tabular} & **Removal (\%)** & **Ref.** \\ \hline \multirow{3}{*}{MWCNTs-KOH@Ni} & \multirow{3}{*}{NA} & Pb (II) & \multirow{3}{*}{242} & pH: 5.5 & 481 & 91.2 & \multirow{3}{*}{[97]} \\ \cline{3-3} \cline{5-7}  & & As (V) & & T: 303 K & 415.8 & 88.5 & \\ \cline{3-3} \cline{5-7}  & & Cd (II) & & 30 min & 440.9 & 80.6 & \\ \hline \multirow{3}{*}{rGO hydrogel (rGOH)} & Improved & \multirow{3}{*}{Pb (II)} & \multirow{3}{*}{553.69} & pH: 4 & \multirow{3}{*}{80} & \multirow{3}{*}{[98]} \\ \cline{3-3} \cline{5-7}  & Hummer’s & & & T: 298 K & & 1185.27 & \\ \cline{3-3} \cline{5-7}  & method & & & 5 min & & \\ \cline{3-3} \cline{5-7}  & & & & pH: 4 & & \\ \cline{3-3} \cline{5-7}  & Hummer’s & & & T: 298 K & 158.73 & 99 & \\ \cline{3-3} \cline{5-7}  & method & & & 1080 min: & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Recent studies on the removal of different pollutants from water treatment using CNFs.