# A Brief Review of Formaldehyde Removal through Activated Carbon Adsorption
## 1 Introduction  
Exposure to high levels of pollutants and hazardous gases adversely affects human health, and a significant challenge is to mitigate the adverse effects by reducing the levels of the pollutants and hazardous gases in areas of human habitation. In particular, toxic volatile organic compounds (VOCs) pose a grave threat to human health, and prolonged exposure to VOCs can result in a number of adverse effects on human health [1; 2; 3]. Formaldehyde is a major indoor VOC and is classified as a group 1 carcinogen according to the WHO International Agency for Research on Cancer (IARC) [4]. Furthermore, it has been reported that formaldehyde may cause irritation of the eye, nose, skin, throat, or combinations thereof. Moreover, long-term exposure to formaldehyde can be linked to asthma, sinonasaldisorders, and even some cancers [5]. Formaldehyde is present in many objects, such as building materials, home interior decorative materials, and burning instruments [1,4], and may be emitted from the objects into their surroundings. Due to increased construction of structures such as bridges, roadways, buildings, and other infrastructure, the use of building materials has correspondingly increased, leading to an overall increase in levels of formaldehyde present in the environment. Many countries have regulated lower levels of formaldehyde concentrations in indoor air, including as low as two-digit ppb(ug/m\({}^{3}\)) levels [6]. To conform to such regulations, numerous approaches have been used to improve the effectiveness of the removal of formaldehyde from the air, including its recovery and destruction. Methods for the recovery of formaldehyde include physisorption and chemisorption, and methods for the destruction of formaldehyde include thermal and photocatalytic oxidation. The merits/demerits of each of the methods for formaldehyde removal are presented in Table 1. Among the methods, thermal oxidation requires high energy to substantially oxidize formaldehyde (HCHO) to carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O). Photocatalytic oxidation of formaldehyde may require noble metals as catalysts, thereby making the process expensive. Further, photocatalysis of formaldehyde may generate undesirable byproducts. As alternates, non-thermal plasma and biomaterial-filtration methods have been proposed to reduce the concentration of formaldehyde in the air [7,8]. Non-thermal plasma can efficiently oxidize low concentrations of formaldehyde. However, the process may result in the generation of undesirable byproducts, such as ozone and NOx. Furthermore, while biomaterial filtration has been proposed as an eco-friendly method to reduce the concentration of formaldehyde, the methodology of biomaterial filtration is still required development of a design for the effective microbial consumption of formaldehyde [9]. Some unconventional methods, such as membrane and condensation methods, have been explored and have been demonstrated to remove formaldehyde when it occurs at low concentrations. However, such methods are very expensive. In contrast, the adsorption methods offer attractive alternatives for reducing formaldehyde owing to its simplicity, ease of operation, and low operational cost [3,5,10,11,12,13,14,15,16,17,18,19]. Various adsorbents have been studied to regulate the indoor VOC content. Recently, some adsorbents with homogeneous nano-characteristics have been presented that have shown superior adsorption performance compared to that of conventional adsorbents [20]. However, this special nano-adsorbent requires expensive handling. Therefore, the modification of conventional adsorbents, especially activated carbon (AC), is considered to achieve efficient formaldehyde removal in practical applications. In general, AC shows effective adsorption of VOCs with boiling points above 0 \({}^{\circ}\)C but is inefficient when the boiling points of VOCs is lower than 0 \({}^{\circ}\)C; in these cases, the physical adsorption is dominant.