# An Overview of Nanofiltration and Nanoadsorption Technologies to Emerging Pollutants Treatment
## 4 Synthesis and Fabrication of Nanomaterials for the Remediation of Emerging Pollutants
### Carbon-Based Nanoadsorents  
Carbon-based nanomaterials have been commonly applied in the remediation of pollutants by adsorption processes. The most used nanoadsorbents for this function are nanotubes, fullerenes, nanodiamonds, and graphene nanomaterials [107]. Carbon nanotubes consist of sheets of graphene structured in a cylindrical shape, formed by carbon atoms with sp2 configuration [108]. These nanomaterials can be single-walled or multi-walled (association of single-walled nanotubes) [109].  
Carbon nanotubes are commonly produced by the arc discharge method with graphite electrodes. The process consists of passing an electric current between the electrodes (anode and cathode) inside a chamber containing inert gas (helium gas is generally used) [110]. In this way, carbon atoms move from the anode to the cathode, forming carbon nanotubes at the negative electrode [111]. In the chemical vapor deposition method, the production of high purity nanotubes consists of the decomposition of hydrocarbons in the vapor phase or carbon monoxide due to the application of high temperatures (700 and 1000 \({}^{\circ}\)C) in the presence of catalysts [112]. The process is carried out in a quartz chamber giving rise to free carbon atoms. These diffuse and deposit on a metallic surface, reorganizing and forming carbon nanotubes [108]. Furthermore, the adsorption capacity of carbon nanotubes can be improved through surface modifications. For this, oxidizing treatments and chemical surface functionalization techniques are used, adding functional groups (polymers, surfactants) [113].  
Graphene is composed of a single layer of carbon atoms arranged in a two-dimensional \"honeycomb\" structure [114, 115]. This compound is widely used in the extraction of organic compounds due to the benzene ring in its structure [116]. The functionalization of graphene is performed to improve the pollutant removal capacity of this compound. Generally, it is applied in the form of graphene oxide (functionalized or reduced). Graphene oxide is characterized by the greater predominance of functional groups (such as phenolic hydroxyl (-OH), carboxyl (-COOH), epoxy groups (-COC-), which improve the absorption of pollutants [113].  
Graphene oxide can be synthesized by oxidation of graphite or by exfoliation. Staudemmaier, Brodie, and Hummers' methods are commonly used to synthesize this compound. The oxidation of graphite is carried out with potassium chlorate and nitric oxide ([PERSON] and [PERSON]) and potassium permanganate and sulfuric acid (Hummers) [117]. In the oxidation of graphite, the stacked sheets of graphene are pulled apart, due to the breakage of the \(\pi\) bond. The GO resulting from this process can also be modified into reduced graphene oxide through chemical, photocatalytic, hydrothermal, or microwave reduction. Furthermore, the functionalization of GO can be performed by doping or by covalent or non-covalent bonds [108].  
Carbon nanofibers are mainly developed by electrospinning, chemical vapor deposition, and modeling techniques. Regarding fullerenes, they can be synthesized similarly to carbon nanotubes. Nano-diamonds can be produced by microwave plasma-assisted chemical vapor deposition, laser and ion deposition, blasting, hydrothermal processes and ultrasound. However, the most used technique is chemical vapor deposition [107].