Enhancement of formaldehyde removal by activated carbon fiber via in situ growth of carbon nanotubes

Highlights

• Carbon nanotubes (CNTs) were tested for formaldehyde removal for the first time.

• A CNT/activated carbon fiber (ACF) filter media was fabricated.

• The CNT/ACF can remove 3 times more formaldehyde than the ACF.

• Compared with other fiber materials, the CNT/ACF presented higher removal capacity.

Abstract

Formaldehyde is a known gaseous pollutant that has a carcinogenic effect on human health. Carbon nanotubes (CNTs) are herein proposed as a potentially new technology for the removal of formaldehyde from indoor air. A CNT/activated carbon fiber (ACF) filter medium was fabricated via in situ growth of CNTs on a pristine ACF by using the chemical vapor deposition method. The formaldehyde removal efficiencies of the CNT/ACF and the ACF filter media were tested with a low inlet formaldehyde concentration. The amounts of formaldehyde adsorbed on the materials were calculated and compared, and the pressure resistances of the two filter media were also determined. The results showed that the CNT/ACF material had a higher initial formaldehyde removal efficiency and removed three times more formaldehyde per filter weight than the ACF, while the pressure resistance of the former was only 50% higher than that of the latter. Compared with the recently reported fiber filter media in the literature, the CNT/ACF material exhibited a higher formaldehyde adsorption capacity. In comparison with the previously reported carbon nanotube/quartz fiber film-based gas filter medium, the CNT/ACF material demonstrated superior formaldehyde adsorption capacity and a significantly lower pressure resistance.

Introduction

Formaldehyde is one of the most ubiquitous and dangerous indoor gaseous pollutants [1]. It can originate from building materials [2], environmental tobacco smoke [3], certain consumer products [4], and indoor chemical reactions [5]. The World Health Organization recommends that the 30-min average concentration of formaldehyde in the air should be less than 0.10 mg m⁻³ [6]. The Chinese national standards agency has also adopted this value as a 1-h exposure limit in the indoor environment [7]. However, previous field surveys have reported that the indoor formaldehyde concentration in 70% of the newly built or redecorated houses exceeded this limit, with maximum levels higher than 1.50 mg m⁻³ [8]. Exposure to formaldehyde is closely related to human health issues, leading to eye, nose, and throat irritation as well as severe allergic reactions [9], [10], [11]. Even worse, formaldehyde is considered to have a carcinogenic effect on humans [12]. Therefore, it is necessary to develop effective technologies for removal of formaldehyde.

Several methods have been proposed for the removal of formaldehyde apart from ventilation [13]. Generally, these methods are based on three mechanisms: catalytic (including photocatalytic) oxidation, biological or botanical filtration, and adsorption [14]. In the case of the catalytic oxidation of formaldehyde, several noble metals such as Pt [15], [16] and Ag [17], and metal oxides such as MnO₂ [18], [19] and TiO₂ [20], [21], have been chosen as catalysts for formaldehyde decomposition with or without thermal treatment. Theoretically, formaldehyde can be oxidized to CO₂ and water in the presence of a catalyst. However, the effectiveness of this method is reduced at low temperatures as a result of the deactivation of the catalyst [22]. Some studies have introduced ultraviolet techniques such as photocatalytic oxidation to assist the oxidation process, which has demonstrated high formaldehyde removal efficiency [23], [24], [25]. Nevertheless, harmful byproducts such as CO were generated in the photocatalytic decomposition of formaldehyde, which is a potential health concern [26]. Previous studies have investigated the performance of indoor plants and botanical/biological filters for formaldehyde removal [27], [28]. However, the decomposition kinetics of formaldehyde by plants is still unclear and the effectiveness of the filters used for removing formaldehyde under typical room conditions has not been addressed [14]. The adsorption of formaldehyde by different adsorbents is a more commonly used method for its removal. Zeolite, granular activated carbon, and activated carbon fiber (ACF) are typical adsorbents that are expected to adsorb formaldehyde by physisorption and chemisorption owing to their large surface area and abundant surface functional groups [29], [30], [31], [32]. Although previous studies have reported high formaldehyde adsorption by the adsorbents, the formaldehyde concentrations in the experiments were quite excessive, at 20 ppm or higher, for representing typical conditions of indoor environments [33], [34], [35], [36]. Therefore, the evaluation of new approaches for potentially improved removal of formaldehyde, particularly at low concentrations, is a matter of great interest.

Owing to their superior properties, carbon nanotubes (CNTs) have been applied in many fields since their discovery [37], [38]. Recently, they have been introduced in the field of air purification. Previous studies have demonstrated that CNT-based films can effectively filter particulate matter [39], [40]. In the category of the removal of gaseous pollutants, CNTs have been shown to be more effective in the removal of ozone compared to granular activated carbon and potassium iodide solution with the same weight [41]. Several researchers have found that CNTs are capable of effectively adsorbing volatile organic compounds (VOCs), particularly acetone and benzene [42], [43], [44], [45]. However, to the best of our knowledge, interactions between CNTs and formaldehyde have not been investigated thus far.

This study aims to address the question of the removal of formaldehyde using CNTs. Thus, an experimental investigation on the active formaldehyde removal performance of CNTs has been performed. A CNT/ACF filter medium was fabricated via in situ growth of CNTs on a pristine ACF by the chemical vapor deposition method. The formaldehyde removal efficiency of the CNT/ACF material was tested until the breakthrough time and compared with that of the pristine ACF. The inlet formaldehyde concentration in the experiment was lower than that reported previously in the literature and consistent with the concentrations in actual formaldehyde-polluted indoor environments. The pressure resistances of the two filter media were also measured and the formaldehyde adsorption amounts were calculated and compared.