## 3 Carbon-Based NPs in Water Treatment
### Carbon Nanotubes (CNTs)  
In recent years, CNTs have attracted great attention in wastewater treatment due to their extensive surface area and the presence of active sites. CNTs are proficient in absorbing diverse contaminants such as heavy metals, organic pollutants, and dyes. Their versatility extends to membrane technology, where CNTs can enhance filtration processes by creating membranes with nanoscale pores, improving both selectivity and permeability [56; 57]. This integration is called CNT-based membranes, which have been classified into three categories, including (1) Vertically Aligned (VA-CNT), which has well-ordered cylindrical pores and the outer layer is made of polymeric using the VCD Method, (2) Mixed Matrix (MM-CNT), which consists of CNTs incorporated within polymeric or inorganic materials, and (3) Bucky Paper (BP), which refers to self-standing membranes based on the use of the highest percentage of CNTs in the membrane structure [58]. BP membranes have shown promise for diverse applications, including membrane distillation, oil-water separation, and heavy metal ion adsorption, though the issues regarding their uniform pore sizes within these membranes must be considered [59]. In addition, CNTs have shown their ability in water disinfection due to their antibacterial properties, inhibiting the growth of microorganisms. Their catalytic activity makes them suitable for advanced oxidation processes, aiding in the degradation of organic pollutants. CNTs also hold promise for addressing emerging contaminants like pharmaceuticals [60]. The significant features of CNTs make them a focal point for addressing environmental concerns and improving processes related to the purification of water [61]. The cylindrical structures of CNTs are composed of carbon atoms arranged in hexagonal patterns, forming a tubular nanostructure. These tubes have single or multiple layers \begin{table}
\begin{tabular}{c c l c} \hline \hline
**CNP Type** & & **Synthesis Methods** & **Ref.** \\ \hline \multirow{4}{*}{CNTs} & \(\bullet\) & Electric arc discharge & \multirow{4}{*}{[48]} \\  & \(\bullet\) & CVD \({}^{*}\) & \\  & \(\bullet\) & Laser ablation method & \\ \hline \multirow{4}{*}{GNPs} & \(\bullet\) & Exfoliation and cleavage & \multirow{4}{*}{[49; 50]} \\  & \(\bullet\) & Liquid-phase mechanical exfoliation & \\  & \(\bullet\) & Intercalation of small molecules by mechanical exfoliation & \\  & \(\bullet\) & CVD & \\ \hline \multirow{4}{*}{CQD} & \(\bullet\) & Hydrothermal & \multirow{4}{*}{[51]} \\  & \(\bullet\) & Laser irradiation & \\ \cline{1-1}  & \(\bullet\) & Heating processes & \\ \cline{1-1}  & \(\bullet\) & Electrochemical processes & \\ \cline{1-1}  & \(\bullet\) & Microwave irradiation & \\ \cline{1-1}  & \(\bullet\) & Ultrasonic & \\ \hline \multirow{4}{*}{CNFs} & \(\bullet\) & CVD & \multirow{4}{*}{[52; 53]} \\  & \(\bullet\) & Electrospinning/carbonization & \\ \hline \multirow{4}{*}{CNSs} & \(\bullet\) & Arc discharge process & \multirow{4}{*}{[54; 55]} \\  & \(\bullet\) & Laser ablation & \\ \cline{1-1}  & \(\bullet\) & Autoclave methodology & \\ \hline \hline \end{tabular} \({}^{*}\)Chemical vapor deposition.  
\end{table} 
Table 2: Different types of CNPs and their synthesis methods. of carbon atoms, leading to the distinction between single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) [62]. The SWCNTs, featuring a diameter ranging from 0.4 to 2 nm, are formed by rolling a single graphene sheet into a cylindrical shape. In contrast, MWCNTs exhibit an outer diameter ranging from 2 to 100 nm, an inner diameter of 1 to 3 nm, and a length extending to several microns. MWCNTs are composed of two or more graphene sheets encapsulating a hollow core, a structural arrangement analogous to that of SWCNTs [63; 64]. The structure of SWCNTs and MWCNTs is shown in Figure 4.  
CNTs can be synthesized using different methods, including CVD, arc discharge, and laser ablation. Usually, in all synthesis methods, generated CNTs need energy and carbon sources.