## 3 Carbon-Based NPs in Water Treatment
### Graphene-Based Nanoparticles (GNPs)  
GNPs derived from atomically thin sp2-bonded carbon feature a hexagonal lattice structure with gas-impermeable nanopores. Controlled manipulation of graphene nanopores enables selective filtration of specific molecules, making graphene and its composites valuable in pollutant removal from wastewater [69]. Different types of GNPs, such as graphene oxide (GO), reduced graphene oxide (rGO), and graphene quantum dots (GQDs), have been used for water treatment. They are unique due to their high surface area, catalytic effectiveness, adsorption capacity, and reactivity [70]. The two-dimensional structure of graphene-based materials allows for their integration into membranes with superior mechanical strength, high permeability, and selective transport properties, enhancing water filtration processes [71]. The inherent antibacterial properties of graphene-based materials further contribute to water disinfection by inhibiting the growth of microorganisms. These materials also demonstrate catalytic activity, participating in advanced  
Figure 4: Structural representation of (**a**) SWCNTs and (**b**) MWCNTs. Source: Adapted from [57].  
oxidation processes for the degradation of organic pollutants [72]. Figure 5 shows different types of graphene-based materials that can be used in wastewater treatment.  
The different preparation methods used for the industrial preparation of graphene are mechanical exfoliation, electrochemical exfoliation, liquid-phase exfoliation, CVD, chemical reduction in GO, and bottom-up synthesis [73].  
GO is an engineered nanomaterial with high popularity due to its high specific surface area, chemical and mechanical stability, selectivity, and tunable controlled properties. GO contains different oxygen groups (carboxyl, epoxy, and hydroxyl) that can form complexes with different organic contaminants and metal ions [74]. It can be applied in wastewater treatment via hydrogen bonding and electrostatic interaction. Several adsorption mechanisms, including precipitation and electrostatic interactions, are involved in the adsorption process of functionalized CNTs and graphene-based materials [75]. Multiple environmentally friendly approaches are used for the preparation of graphene oxide (GO). The graphene synthesis methods are the Brodie, Staudenmaier, and Hummers-Offeman methods. Hence, the improved Hummer's method shows the most suitability for preparation due to the low toxicity level and high organization in the structure of GO [76]. The chemical reactions of these methods are shown below:  
(i) Brodie method  
\[\text{C}+2\text{H}_{2}\text{SO}_{4}+\text{KMnO}_{4}\rightarrow\text{GO}+ \text{MnSO}_{4}+\text{K}_{2}\text{SO}_{4}+\text{H}_{2}\text{O}\]  
(ii) Staudenmaier method  
\[\text{C}+3\text{H}_{2}\text{SO}_{4}+\text{HNO}_{3}+\text{KClO}_{3}\to \text{GO}+\text{CO}_{2}+\text{K}_{2}\text{SO}_{4}+\text{H}_{2}\text{O}+\text {NO}_{2}\]  
(iii) Hummers-Offeman method  
\[\text{C}+\text{NaNO}_{2}+\text{KMnO}_{4}+\text{H}_{2}\text{SO}_{4}+\text{H}_ {2}\text{O}_{2}\rightarrow\text{GO}+\text{MnSO}_{4}+\text{K}_{2}\text{SO}_{ 4}+\text{NaNO}_{3}+\text{H}_{2}\text{O}\]  
The comparison of the [PERSON] and Hummers methods for the preparation of multilayer GO shows [PERSON] had a less defective nature, a more homogeneous distribution of functional groups over its surface, and a smaller amount of carbonyl and carboxyl groups. Meanwhile, Hummers showed an inhomogeneous solvation and hydration with effects of inter- and intra-stratification, and the GO absorbed 3-4 times more water than the [PERSON] method [77].