# A Brief Review of Formaldehyde Removal through Activated Carbon Adsorption
## 4 Approaches to Chemical Modifications of the AC Surface  
Generally, carbonaceous materials exhibit a good adsorptive performance and mechanical stability. Apart from physical properties such as surface area and pore characteristics, surface functional groups that are used for surface modification of AC play an important role in the adsorption capacity of AC [50; 51]. When we consider the direct FA adsorption on the surface of AC, the hydrophobic surface of AC is not favorable. However, surface modification of AC can enable the AC to efficiently adsorb hydrophilic FA. Therefore, the adsorptive performance of AC may be enhanced by surface modification, which may be achieved through three means: acid or base treatment, impregnation with hydrophilic chemicals, and grafting of hydrophilic functional groups connected to the AC surface [52; 53]. Considering the first option, modification to brace acidic or basic functional groups on the AC surface has been extensively evaluated [16; 31; 54; 55; 56; 57; 58]. Generally, the surface chemistry of AC depends on the presence of O, N, S, and H within the functional groups on its surface. Returning to Figure 1, the results support the evidence that sulfur- or halogen-modified BAC (bead-type AC) has better adsorption capacity than pristine BAC. When  
Figure 1: FA removal efficiency on the (modified) bead-type activated carbon at the various RH contents: (**a**) non-modified bead-type activated carbon, (**b**) halogen modified bead-type activated carbon, and (**c**) sulfur modified bead-type activated carbon. Operation conditions: 100 ppmv; space velocity: 40,000/h; temperature: ambient. we consider effective FA removal, the oxygen functional groups of the Bronsted acidic or basic sites, as well as nitrogen-containing functional groups, can be the most important sites for FA adsorption. This point is also supported by the adsorption isotherms of the Langmuir model, which was found suitable for the adsorption of FA on surface-modified ACs. Hence, modifying the surface functional groups of AC is a key strategy for designing the surface chemistry. Considering second option, nitrogen-containing functional groups can be introduced as dopants in the AC framework with heat treatment in the presence of nitrogen sources (such as NH\({}_{3}\), HNO\({}_{3}\), etc.). The term dopant used here refers to the active chemical that is impregnated into the AC surface structure and forms physical bonds: Van der Waals and hydrogen bonds. Figure 2 shows the conceptual impregnation of aniline into the AC surface structure. A similar conceptual drawing can be seen elsewhere [59]. These physical bonds between aniline and the AC surface may be much weaker than the chemical bonds, but these bonds are still sufficient to hold dopants within the AC surface structure without detaching during the adsorption process [60]. The dopant was safely located within the AC surface structure with basic characteristics and surface polarity (\(\pi\)-electrons). Other nitrogen-containing functional groups include -NH\({}_{2}\), -NH, -C=N, and -C-N. Notably, -NH\({}_{2}\) (amine functional group) is attributed to the amine-aldehyde conjugation through covalent bonding and shows an enhanced adsorption capacity for FA [60]. Such nitrogen-containing functional groups have a strong affinity for water molecules on the adsorbent [20]. In addition, oxidation and sulfurization can successfully modify the AC surface by increasing the functional groups, such as -OH, -COOH, -C=O, -C-O, -C-S, -C=S, and -S=O, on the AC surface. Considering the third option, the grafting of hydrophilic functional groups can be achieved by a direct reaction between a hydrophilic reactant and intrinsic hydrophilic functional groups on the AC surface. Two-stage reactions can also be applied to build grafting structures. For example, carboxylic functional groups can be used to explain the generation of a grafting structure.