Most people spend most time of life indoors. Therefore, quality of the indoor air plays a very important role in people's life. With the continuously increase of population, the deteriorative shortage of natural resources, and the worsening environmental pollution, the concentration of volatile organic compounds (VOCs) in the air which is one of the main causes to formation of PM2.5 particles that affect the air quality and human health increases significantly. The volatile organic compounds in the air include formaldehyde, acetaldehyde, various ketones, aromatic hydrocarbon, and the like. In these volatile organic compounds, formaldehyde is the most common one, and comes from synthetic fibers such as artificial carpet and artificial cloth), resin such as adhesive for furniture), paint (used for wall decoration and furniture paint), microorganism degradation (indoor space is particularly suitable for microorganisms' growth and reproduction because of the suitable humidity and rich nutrition), meat, fruit and vegetables, and various wood.
Irritation caused by formaldehyde may lead to acute lesion of headache, nausea, asthma and skin rash, and formaldehyde is the organic matter that most easily changes protein and the genetic material such as DNA and RNA, leads to tumor and cancer, and causes great harm to the human body. Removing the formaldehyde indoor (including in the car) and reducing the formaldehyde to a safety concentration range are very important for protecting the human life and the genetic substances.
The concentration of formaldehyde indoor that is acceptable by the World Health Organization (WHO) is 0.08 ppm, and formaldehyde concentration in most Chinese indoor space is much higher than this value. The concentration of formaldehyde in new furniture and decoration housing even reaches hundreds of ppm and goes beyond the standard by tens of thousands of times, which becomes the main cause to the increase of incidence of tumor and cancer in China in recent years.
In order to eliminate the hazards caused by the formaldehyde, one way readily occurred to people is to avoid the substances which product formaldehyde. Urea-formaldehyde resin polymer (UF) is considered a major villain. However, the Canadian research shows that the concentration of formaldehyde is the same in a room where the UF is used and as in a room where the UF material is not used (Ref. Indoor Air Quality Control Techniques, by Jeffrey M. Lemn, in 1987, William Andrew Press). Subsequently, the ban on use of this substance was canceled one year later (in 1983). Afterwards, the materials containing formaldehyde were in use all the time for the low cost and high performance (very high performance cost ratio) thereof. Therefore, people have attempted a variety of coating technologies, and these technologies have not been widely used due to high cost and poor effect. Some other methods for actively removing formaldehyde, including dehumidification, air washing, ammonia fumigation, and the like, have not been promoted due to poor effect, large energy consumption, or secondary pollution. For example, the ammonia water elution may directly cause injury to human body and damages to furniture and housing).
Therefore, there are two main research directions at present, adsorption, and oxidation.
The absorbent materials include activated carbon, carbon fiber, activated aluminum oxide, treated calcium carbonate, sepiolite, ceramic materials, wool, lignin, mesoporous silica materials, and the like. Meanwhile, the adsorption materials have been modified using a variety of chemicals to further strengthen the formaldehyde adsorption capability, for example, immersion in urea or ammonia sulfate, atmospheric oxidation, carry onto zinc chloride, treatment with hydrochloric acid and ferrous powder after treatment with nitric acid, modification by amino silane or nanometer silver powder, phosphoric acid activation, treatment with potassium hydrogen phosphate, and the like.
All of these methods have failed to be widely employed because adsorption effects of formaldehyde are poor, or the raw materials are toxic to the environment and human, or the cost is too high, except that the active carbon is accepted as a type of formaldehyde adsorption product by most people. According to the research of the United States in 1983 (Ref. Indoor Air Quality Control Techniques, by Jeffrey M. Lemn, in 1987, William Andrew Press), each family needs to remove hundreds of grams of formaldehyde a year. The adsorption capability of the activated carbon is limited due to the absorption using the activated carbon is reversible and is subject to impacts from the ambient temperature, humidity, and other molecules. An annual cost of each family reaches $600 according to the price of activated carbon at that time. In consideration of factors such as inflation, the annual cost of using active carbon amounts up to $2,000.
Because formaldehyde can be oxidized to formic acid, the oxidation method is an important way to remove formaldehyde. The method includes using oxidant directly, producing ozone or free radical by using a high voltage electric field, and the chemical catalytic oxidation method invented by the inventor of the present invention. With respect to the oxidation method, since the ionization potential of formaldehyde is up to 10.88 eV which is very close to that of the oxygen (12.07 eV), strong oxidants need to be used to oxidize formaldehyde. These oxidants include potassium permanganate, peroxy-acid, peroxy-alcohol, hydrogen peroxide, dichromate, copper peroxide, chromium oxide, and the like. However, these oxidants have significant drawbacks. In one aspect, these oxidants have such strong oxidation capabilities that other components in the air may be oxidized during oxidation of formaldehyde, causing quick deactivation of the oxidants, and therefore the cost is very high (there is application of carrying the oxidants to the solid carrier. In another aspect, all of the oxidants may cause secondary pollution, and the metal oxidants may cause heavy metal pollution to the environment. Furthermore, the peroxy-acid and peroxy-alcohol are volatile and the volatile substance may corrode the other articles and cause damages the human body. In still another aspect, the by-products of formaldehyde oxidization are more volatile, and thus more harmful to environment and human body. Meanwhile, the strong oxidants tend to explode. In view of the above, the oxidation method can not be promoted. The bi-enzymatic method belongs to the oxidation methods, but the cost is too high for household use. The high voltage electric field method includes negative ion, photo-catalytic, ozone, and the like. These methods all have drawbacks. The formaldehyde removal effect of the negative ion method is only 18-30% due to a weak oxidization capability thereof, and thus the negative ion method needs to be run for a very long time at a low indoor temperature (The process of negative on generating produces heat, leading to a high overall energy consumption). Like the bi-enzymatic method, the cost of using photo-catalytic is the highest because the carrier is nanotitania.
In addition, the hydroxyl free radical that produced in the photo-catalytic method is harmful to human body and environment. Although the cost of ozone method is relatively low, ozone will cause damages the human health and pollute the environment. The capability of removing formaldehyde in the chemical catalytic oxidation method is several to ten times higher than that of the activated carbon.
Therefore, to invent a method of removing formaldehyde which is cost-efficiency, causes no secondary pollution, and achieves a superior effect over the activated carbon is social and economically and socially significant in improving quality of human life, safeguarding human health, and protecting environment.