## 3 Research Progress on Adsorption and Separation of Hydrocarbon Molecules
### Aromatic Hydrocarbons
#### 3.2.1 Carbon Materials  
Carbon materials include many classifications, such as common biochar, activated carbon (AC), and other configurations of carbon materials such as graphene, carbon nanotubes, and fullerenes [43; 44; 45]. Most carbon materials have porous structures, high specific surface areas, and can accommodate different adsorption molecules by introducing functional groups. Most carbon materials have a porous structure, high specific surface area, and can adapt to different adsorption molecules by introducing functional groups. These characteristics make carbon materials have good performance in the separation of aromatic hydrocarbons. Carbon materials commonly used for adsorptive separation include activated carbon, carbon nanotubes, and graphene materials, which mostly adsorb adsorbates via van der Waals forces, hydrogen bonding, or \(\pi\)-\(\pi\) interactions. [PERSON] [46] studied the adsorption behavior of five different carbon materials (particulate activated carbon material HD4,000, activated carbon ACF10, two single-walled carbon nanotubes SWNT, SWNT-HT, and multi-walled carbon nanotube MWNT) on aromatics. The factors affecting the adsorption behavior of carbon materials toward three aromatics (p-nitrotoluene PNT, p-xylene BP, and phenylphenol 2 PP), the adsorption sites on different carbon materials, and the adsorption mechanism of adsorbates on the surface of carbon materials were quantitatively illustrated. Figure 9 shows the adsorption isotherms of PNT, BP, and 2 PP in different carbon materials. The authors fit the results using the FM adsorption model according to their modeling results, and the fit to the experimental data was better in the linear form (Inqe = Inkf + nInce) than in the nonlinear form. The modified Freundlich model was adopted to normalize the C\({}_{\text{E}}\) as well as the water-soluble S\({}_{\text{W}}\) of adsorbate to obtain the equation:  
\[\text{q}_{\text{e}}=\text{K}_{\text{FS}}(\text{C}_{\text{e}}/\text{S}_{\text{W }})^{\text{n}}\]  
\(\text{q}_{\text{e}}\): the mass of solute adsorbed  
\(\text{C}_{\text{e}}\): the equilibrium concentration of the solute  
\(\text{S}_{\text{W}}\): water solubility  
\(\text{K}_{\text{FS}}\) is a concentration-independent quantity that represents the effective adsorption capacity of the adsorbent, but it can only be used to estimate the effective adsorption amount when the C\({}_{\text{e}}\) of the low-solubility adsorbent reaches saturation. Figure 9: Adsorption isotherms of three organic aromatic compounds on different carbon materials [46]. Symbol: experimental data, line: linear fit of FM. (**(a)** p-nitrotoluene PNT, **(b)** p-xylene BP, **(c)** phenylphenol 2 PP). As can be seen in Figure 9, the adsorption isotherms of PNT on ACF10 (the pore is mostly microporous, mostly \(<\)1 nm) are significantly lower than that on other adsorbates (pore sizes larger than 1 nm diameter of PNT), and the adsorption isotherms of BP and 2 PP are still in the fraction after the specific surface area normalization treatment, especially at low C\({}_{\rm E}\). This indicates that the micropores and effective adsorption area play dominant roles in adsorption. In addition, the relationship between the adsorbed amount of activated carbon and the structural parameters of carbon was investigated, and V\({}_{\rm AOC}\)/V\({}_{\rm N2}\) was used to represent the effective coverage of organic aromatic compounds on the carbon surface, and V\({}_{\rm AOC}\)/V\({}_{\rm t}\) to represent the pore volume occupancy. It was concluded that at low molecular weight chemicals, the higher the microporous component of carbon, the higher the space volume component occupancy. This can be explained by the micropore effect: in the liquid-phase adsorption of low molecular weight chemicals, the adsorption potential overlap of the walls in the opposite position makes the micropores' adsorption energy much larger than the mesopores and macropores [47].