# A Brief Review of Formaldehyde Removal through Activated Carbon Adsorption
## 3 Physical (Operational) Influence Factors  
The adsorption behavior of AC is classified according to chemisorption and physisorption. Chemisorption occurs through chemical bonding between the chemical and adsorbent, while physisorption occurs through bonding of the adsorbate on the surface of the adsorbent due to weak van der Waals forces and capillary interactions. Typically, AC is a porous material with abundant micropores, and thus AC has a high surface area. AC may be produced from myriad sources, such as fossil fuel residues and various types of biomass. Furthermore, coconut shells and charcoal are often considered as common examples of activated carbon materials [32; 33]. In studies on the adsorption of formaldehyde (FA) on AC under humid conditions, three different pathways of adsorption of FA on AC have been suggested in the presence of water vapor [34]. First, the competitive adsorption of FA with H\({}_{2}\)O due to their similar polarities, which takes place on active sites located on the pore surface of the AC, can have a negative effect on FA removal. Recently, the FA removal efficiency of nitrogen- and sulfur-modified AC was evaluated under humid conditions [35]. Even though the modified AC showed an overall improvement in efficiency of FA removal, the presence of water vapor resulted in a decrease in adsorption capacity over all adsorbents (ca. 3-40% decrease, based on FA (mg)/g adsorbent). Interestingly, it was found that the introduction of organosilane onto the AC surface improved the adsorption performance for the adsorption of FA over a wide range of humidities [36; 37]. In fact, the FA removal efficiency of the organosilane-modified AC increased to 48% at a relative humidity (RH) of 80%, while the FA removal efficiency of pristine AC decreased by 86% at a relative humidity of 80% when compared to that at RH 30%. Other studies have also presented good examples of competitive adsorption of FA with H\({}_{2}\)O [38; 39; 40; 41; 42; 43]. [PERSON] et al. [12] reported that an increase in the water affinity of adsorbents could be obtained by introducing polar functional groups. However, this may lead to a considerable decrease in the adsorption capacity of the AC for FA due to the adsorbents with preferential adsorption of water. Therefore, an optimized amount of polar functional groups should be considered for FA adsorption by reducing water affinity. A second pathway for adsorption of FA on AC suggests that the exposed surface area may be reduced by the capillary condensation of water molecules within the micropores of the AC, which may further reduce the adsorption capacity of AC. [PERSON] et al. reported a higher capillary condensation of water vapor within the micropores of AC under higher RH conditions, resulting in a decrease in the FA adsorption capacity [11]. It was indicated that nano-sized pores with less than 2.4 nm diameter were unfavorably blocked due to capillary condensation at RH 80%. Similar results also demonstrated that increase in relative humidity decreased the FA breakthrough capacity in the case of nitrogen-modified AC [44]. A third pathway for the adsorption of FA on AC suggests that the FA molecules can be co-absorbed by the condensed and/or adsorbed water on the surface. If this is true, the FA removal efficiency may increase because the boiling point of FA is much lower than that of water vapor, which is more difficult to adsorb. The co-adsorption of FA and water has a positive effect on FA removal, whereas the opposite is true for the first and second pathways mentioned above. Therefore, the phenomenon of FA adsorption remains contentious. The authors of the present manuscript have attempted to improve the adsorption performance of bead-type AC (BAC) and two acid-modified BACs (sulfuric acid-modified BAC (SA-BAC) and hydrochloric acid-modified BAC (HA-BAC)) at different RH values. The results are shown in Figure 1. The adsorption data (mg-FA/g-AC) showed a similar trend in all three cases: a higher capacity at a higher RH (70%) and a lower capacity at a lower RH (30%).