Development of Formaldehyde Adsorption Using Modified Activated Carbon – a Review

Gas storage is a technology developed with an adsorptive storage method, in which gases are stored as adsorbed components on the certain adsorbent. Formaldehyde is one of the major indoor gaseous pollutants. Depending on its concentration, formaldehyde may cause minor disorder symptoms to a serious injury. Some of the successful applications of technology for the removal of formaldehyde have been reported. However, this paper presents an overview of several studies on the elimination of formaldehyde that has been done by adsorption method because of its simplicity. The adsorption method does not require high energy and the adsorbent used can be obtained from inexpensive materials. Most researchers used activated carbon as an adsorbent for removal of formaldehyde because of its high adsorption capacity. Activated carbons can be produced from many materials such as coals, woods, or agricultural waste. Some of them were prepared by specific activation methods to improve the surface area. Some researchers also used modified activated carbon by adding specific additive to improve its performance in attracting formaldehyde molecules. Proposed modification methods on activation and additive impregnated carbon are thus discussed in this paper for future development and improvement of formaldehyde adsorption on activated carbon. Specifically, a waste agricultural product is chosen for activated carbon raw material because it is renewable and gives an added value to the materials. The study indicates that the performance of the adsorption of formaldehyde might be improved by using modified activated carbon. Bamboo seems to be the most appropriate raw materials to produce activated carbon combined with applying chemical activation method and addition of metal oxidative catalysts such as Cu or Ag in nano size particles. Bamboo activated carbon can be developed in addition to the capture of formaldehyde as well as the storage of adsorptive hydrogen gas that supports renewable energy.


Introduction
Gas storage technology is the method of "adsorptive storage", where the gases are stored in the adsorbed state on the certain "adsorbent". Gas molecules have a density in the adsorbed state which is close to the density of the melting state. Hydrogen storage describes a method for storing hydrogen for later use. Hydrogen storage is used to provide renewable energy sources as a provider of fuel for transportation. Storage by melting method is superior, but this requires a very high cost in terms of equipment and energy required. The method of storing hydrogen is performed in a gas cylinder tube which was filled with an activated carbon. Gas pressure can be flexibly adapted to the gas source. Subsequent adsorption technology is used for the adsorption of formaldehyde which is a toxic gas.
Gaseous formaldehyde is one of the major indoor gaseous pollutants. This volatile organic compound is derived from the paint, decoration materials, polymerizing plate, binder of furniture, fiber carpet [1,2]. Formaldehyde is a reagent for adhesives such as urea formaldehyde and phenol-formaldehyde. Formaldehyde at low concentration can cause indisposition of throats at a level of about 0.1 mg/m 3 (0.08 ppm), irritation of eyes and noses at about 1 mg/m 3 , difficult breathing at about 10 mg/m 3 , injury of respiratory organs at about 40 mg/m 3 , and asphyxia above 80 mg/m 3 [3]. Even, formaldehyde can also result in lung cancer and eventually death.
Concentration limit allowable that formaldehyde exposed in the air by OSHA Permissible Exposure Limit (PEL) is up to 2 ppm [4]. The above limitation is also supported by the Indonesian government rules. In addition, World Health Organization recommended that the guideline for indoor air quality is 0,1mg/m3 for 30 minutes to formaldehyde [5].
Efforts to reduce the content of formaldehyde indoor must be done, given the dangers posed and minimum allowable concentration limits. Several methods have been made to reduce formaldehyde gas phase at room and will be reviewed in this paper.

Methods of Removing Indoor Formaldehyde Gas
Efforts to reduce pollutants indoor, especially formaldehyde have been studied including the method of photo-catalyst, plasma, enzymes, and adsorption. There are also combined methods, such as adsorption and catalytic oxidation and adsorption and photocatalyst. Photo-catalyst oxidation methods for removing formaldehyde gas were done by using nano-structured TiO 2 , Ag-TiO 2 and Ce-TiO 2 [6] while plasma method were using various discharge plasmas. Liang et al used NaNO 2 ferro-electric packed bed dielectric barrier discharge plasma reactor applying high-frequency AC power supply [7]. There is also DC Corona discharge plasma that can remove low-concentration of formaldehyde in the air by introduction of MnOx/Al 2 O 3 catalyst placed at downstream of the plasma reactor [8,9]. Cycle storage-discharge plasma catalytic process was applied to low-concentration formaldehyde removal from air using a combination of HZSM-5, HZ with Ag, and Cu [10].
Formaldehyde can also be oxidized by alcohol in the present of oxidase enzyme. Alcohol oxidase is isolated from Hansenula polymorpha C-105 mutant overproducing this enzyme in glucose medium. Sigawi et al used a such method for removal and control of airborne formaldehyde [11].
A combination of adsorption and catalytic oxidation method was developed to hybrid catalyst for long lifetime clean removal formaldehyde indoors. This kind of hybrid catalyst was applied by Miyawaki et al using MnOx (functioned as a catalyst) and polyacrylonitrilbased nano fiber Activated Carbon (functioned as an adsorbent) [12].
Wide application of photo-catalysis in indoor air purification is limited due to the low formaldehyde level as contaminant of indoor air that can be handled. Lu et al used the nano size TiO 2 particles immobilized on the surface of activated carbon filter which result in four time better removal of formaldehyde than using TiO 2 on a flat glass [13].

Adsorption of Formaldehyde Gas on Activated Carbon
Adsorptions of formaldehyde gas on activated carbon have been conducted by several researchers. The activated carbons used varied on their origins and the method of physical or chemical activations. Properties of the activated carbon produced and the performance results in adsorbing formaldehyde gas for several researchers are presented in Table 1

Adsorption of Formaldehyde Gas on Modified Activated Carbon
Potential of activated carbon in adsorbing formaldehyde can be increased by adding additives. The additive might be either a material that can generate functional groups to catch formaldehyde molecule on the surface of activated carbon or an oxidative catalyst metal that are also able to increase the attractive force of the activated carbon surface. Results of some studies on characteristics and Formaldehyde Gas adsorption Capacity on modified Activated Carbon are summarized on Table 2.  © IJRED -ISSN : 2252-4940

Future Development
Efforts on finding more efficient adsorption method for removing formaldehyde gas from air have been continuously conducted by many researchers. Focus of the efforts is to improve the surface area of activated carbon by selecting better raw materials and choosing better activation methods.
Recently, agricultural by-products are the most favorable materials used to make activated carbon, the reasons are that these raw materials are renewable and potentially less expensive to manufacture. Table 3 shows research results on developing activated carbon using various agricultural by-product materials and activation methods. It can be concluded based on the table that bamboo is the most favorable material considering that it can produce large number surface area (550-2100 m 2 /g). In addition, chemical activation using KOH also shows better result in term of surface area.
Further improvement to increase the capacity of activated carbon in adsorbing formaldehyde gas can also be done by adding metal oxidative catalyst to increase the attractive force of the activated carbon surface. Among the metal oxidative catalysts, Cu probably is the best choice because it is cheap, no toxic and has highly attractive for formaldehyde molecules or hydrocarbon molecules, in general. Addition of Ag nano particles on the surface of activated carbon, as done Shin and Song, however, is also a good alternative. It will not only increase the attractiveness of the activated carbon surface but also increase the surface area of the carbon, therefore increasing the capacity of the gas formaldehyde adsorption. Even though Ag is less reactive oxidation catalyst than Cu, but the size of a silver metal in nano size will create more adsorption sites that can improve the performance of activated carbon adsorption.

Conclusion
The efforts to improve the performance of the adsorption of formaldehyde can be done by using modified activated carbon. Bamboo seems to be the most appropriate raw materials to produce activated carbon combined with applying KOH chemical activation method. Furthermore, addition of metal oxidative catalysts such as Cu or Ag in nano size particles will enhance the capacity of activated carbon in adsorbing formaldehyde gas. Further development of an active carbon can be used to adsorb and store hydrogen gas for hydrogen renewable energy sources.