Control and treatment of indoor formaldehyde pollution
Brief introduction: The control measures and treatment technologies of indoor formaldehyde pollution are summarized. The main purification technologies of indoor formaldehyde pollution are introduced in detail, including physical adsorption technology, catalysis technology, chemical neutralization technology, air negative ion technology, ozone oxidation technology, room temperature catalytic oxidation technology, biotechnology, material sealing technology and their research progress, and their advantages and disadvantages are analyzed.
Keywords: formaldehyde pollution indoor air air purification technology
Formaldehyde is one of the main pollutants in indoor air. It is a colorless irritating gas with a boiling point of 19.5℃. It is volatile at room temperature and easily soluble in water. It mainly comes from all kinds of artificial boards and furniture, adhesives, wall covering, paints, curtains, sofas, Simmons mattresses and other decorative materials, as well as the smoke generated by smoking. Formaldehyde is very harmful to human health. Indoor formaldehyde content greater than 0. 1 mg/m3 will do harm to respiratory system. High concentration of formaldehyde is harmful to nervous system, immune system and liver, especially to children, pregnant women and the elderly. Other studies have found that air pollution such as formaldehyde is also the chief culprit of general "office syndrome". Formaldehyde ranks second in the list of toxic chemicals in China and has been identified as a suspected teratogenic and carcinogenic substance by the World Health Organization (WHO) [1]. The Hygienic Standard for Formaldehyde in Indoor Air (GB/T16127-1995) stipulates that the indoor formaldehyde content should be less than 0.08 mg/m3, but the average formaldehyde concentration after decoration in general houses in China is 0.2 mg/m3, and the highest formaldehyde concentration can reach 0.8 1 mg/m3. At present, many technical methods have been adopted to reduce the free formaldehyde in building materials. Although some achievements have been made, the formaldehyde pollution in indoor air is still very serious due to technical barriers and economic conditions. Therefore, it is very important to control and control indoor formaldehyde pollution.
1 Internal environment of control room
Because the release of formaldehyde is a long-term process, the research of Yokohama National University in Japan shows that the release period of indoor formaldehyde is generally 3 ~ 15 years, and it is related to indoor temperature, relative humidity, indoor air exchange times and indoor building materials. Reasonable environment in the control room can reduce the concentration of formaldehyde.
1. 1 ventilation
Ventilation is an effective method to remove formaldehyde, and introducing fresh air can effectively dilute polluted air. Choosing air ventilation device or natural ventilation is beneficial to the emission of formaldehyde in indoor materials. Zhang et al. [3] found that mixed ventilation can better maintain indoor air quality than displacement ventilation. Indoor ventilation should depend on the season, weather and the number of people in the room. Usually, proper ventilation openings should be left in spring, summer and autumn, and windows should be opened for ventilation for at least 30 min every day in winter.
Disadvantages: Generally, places with serious formaldehyde pollution are not well ventilated, and the problem of pollution sources has not been solved.
1.2 control indoor temperature and humidity
It was found that the formaldehyde emission increased with the increase of humidity and temperature [4]. When the temperature drops from 30℃ to 25℃, the formaldehyde content can be reduced by 50%, and when the relative humidity drops from 70% to 30%, the indoor formaldehyde content can be reduced by 40%. The influence of temperature and humidity mainly depends on reducing the diffusion of pollution sources [5]. In order to release formaldehyde from indoor materials as soon as possible, the temperature and humidity should be increased. Therefore, baking in a newly decorated room or putting a pot of water indoors can accelerate the release of formaldehyde. To reduce indoor formaldehyde concentration, it is necessary to reduce its temperature and humidity. This is also the main reason for the decrease of formaldehyde emission from indoor pollution sources in winter.
Disadvantages: it only solves the problem of formaldehyde release time from pollution sources, and the actual application effect is not ideal.
1.3 Plant Purification
The related experiments of the National Space Technology Laboratory of the United States [6] proved that indoor foliage plants such as chlorophytum, aloe, cactus, tiger tail flower and hibiscus flower have good absorption effect on formaldehyde. Therefore, placing the above plants indoors not only beautifies the environment but also purifies the air. However, China's testing foundation is not sufficient. The preliminary test shows that it is effective, but the effect is quite limited, which is far less obvious than that obtained by ventilation.
Disadvantages: the adsorption capacity of plants to formaldehyde is quite limited, and the effect is better when the air is slightly polluted. In places with serious pollution, there are also reports of plants dying from excessive inhalation pollution.
In a word, only by adjusting the indoor environment to reduce the indoor formaldehyde concentration can not achieve the ideal effect, especially in the early stage of formaldehyde release or in places with serious air formaldehyde pollution. At best, these are only auxiliary means to purify the air. To really solve the problem of air pollution, air purification technology is also needed. At the same time, adjusting the indoor environment to treat the symptoms but not the root cause has not fundamentally solved the pollution source problem.
2 Indoor formaldehyde pollution control technology
At present, many methods have been adopted to control indoor formaldehyde pollution at home and abroad, and now some products have come out. The air purification technology of indoor formaldehyde pollution can be summarized as: physical adsorption technology, catalysis technology, chemical neutralization technology, air negative ion technology, ozone oxidation technology, room temperature catalytic oxidation technology, biotechnology, material sealing technology and so on.
2. 1 physical adsorption technology
Physical adsorption mainly uses some substances with adsorption capacity to adsorb harmful substances, so as to achieve the purpose of removing harmful pollution, including various air purifiers. Commonly used adsorbents include granular activated carbon, activated carbon fiber, zeolite, molecular sieve, porous clay mineral, silica gel and bamboo charcoal popular in recent years. Sonia aguado and others [7] found that zeolite membrane has a good removal effect on indoor pollutants such as formaldehyde and benzene. Activated carbon fiber is the most striking carbonaceous adsorbent among adsorbents. Cai Jian et al. [8] found that under appropriate conditions, ACF modified with H2O2 can improve the adsorption performance of formaldehyde. Rong et al. [9] made a preliminary study on the formaldehyde adsorption performance of modified polyacrylonitrile (PAN)-based activated carbon fiber (ACF), and found that the formaldehyde adsorption capacity of samples impregnated with PAN-ACF and subsequent heat treatment was significantly higher than that of untreated samples. The improvement of physical adsorption technology is mainly to find adsorbents with large specific surface area and faster adsorption and desorption rates, and to combine them with other technologies. Sawada et al [10] cultivated plants with formaldehyde purification performance in flowerpots filled with activated carbon, and its in addition to formaldehyde effect was better than that of pure activated carbon adsorption. Physical adsorption can also be used on building materials. Kazunori et al. [1 1] developed a biodegradable carbon plate, which can completely absorb 20× 10-6 formaldehyde within 2 hours, and the waste carbon plate is biodegradable. Strong physical adsorption and enrichment ability, no secondary pollutants, simple and easy to popularize, and effective for low-concentration harmful gases.
Disadvantages: the adsorption speed of physical adsorption is slow, and the formaldehyde removal in heavily polluted rooms newly renovated for several months is not obvious. Once the adsorbent is saturated, it may re-release the adsorbed odor substances with the change of environmental conditions, so it is necessary to replace the adsorbent regularly. As for the statement that it can be desorbed after exposure, there is a lack of credible scientific data. Desorption requires special equipment. Especially when consumers use activated carbon or bamboo charcoal products, they can't know when such products are saturated, which leaves great security risks for consumers' healthy consumption. To put it mildly, activated carbon or bamboo charcoal products on the market are a bit like "time bombs" hidden around you, which is one of the reasons why many of these products have poor deodorization effect on rooms after decoration.
2.2 Catalytic technology
Catalysis technology is mainly based on catalysis, combined with ultra-micro filtration, which ensures that many harmful odorous gases are decomposed into harmless and odorless substances at normal temperature and pressure, and changes simple physical adsorption into physical and chemical adsorption without secondary pollution. At present, some of these products are on the market.
Disadvantages: Electrostatic adsorption is effective in a small space. If used in air purifiers, it should be the direction of future product development.
Nano-photocatalytic technology is an air purification technology developed in recent years. It mainly uses the photocatalytic performance of titanium dioxide to oxidize formaldehyde to produce carbon dioxide and water. The application of this technology in air pollution control under ultraviolet irradiation has been paid more and more attention, and it has become a research hotspot of air pollution control technology. In order to improve the degradation rate of formaldehyde, a series of studies were carried out on the influencing factors of its reaction. The kinetic study of photocatalytic degradation of formaldehyde by titanium dioxide shows that the photocatalytic degradation of formaldehyde follows the first-order reaction kinetics law, the reaction rate is controlled by reactant concentration, and the photocatalytic reaction is controlled by surface chemical reaction [12]. When the concentration of formaldehyde is lower than 10 mg/m3, it can be completely degraded by TiO2 _ 2 under ultraviolet irradiation and oxidized to formic acid at higher concentration [13]. The experiment of Stevens et al [4] shows that the removal rate of low-concentration formaldehyde by nano-TiO2 _ 2 photocatalytic reactor is 100% under ultraviolet irradiation, while the purification efficiency is only 35% under sunlight irradiation. Qian Yu et al. [14] studied the photocatalytic degradation effect of nano-TiO _ 2 on formaldehyde in the air, and found that the effect of TiO _ 2 loading on non-woven fabrics and nickel nets was better than that on glass fiber cloth; Adding proper amount of activated carbon can obviously improve the photocatalytic degradation rate of formaldehyde; When sodium silicate is used as binder, the photocatalytic degradation rate of formaldehyde can be effectively improved. In addition, many scholars continue to develop new methods, and coating a layer of sol-gel TiO2 film on the surface of borosilicate glass has a good effect on the removal of indoor formaldehyde. Under UVA irradiation of 1.56 mW/cm2, the maximum reflectivity constant is 0.148 min-1[15]. Liu et al. [16] prepared uniform and transparent cerium-doped nano-TiO _ 2 thin films on the surfaces of glass and porous ceramics by sol-gel method. It was found that the absorbance of the nano-TiO _ 2 thin films under near ultraviolet light was obviously improved and the photocatalytic degradation rate of formaldehyde was very high. Yang Yang et al. [17] prepared a water-based coating with nano-TiO2 _ 2, which is completely free of organic matter, and can effectively decompose harmful gases for a long time when coated on the inner wall. In practical application, visible light is easier to obtain than ultraviolet light. By compounding Fe-TiO2 _ 2 photocatalyst with photocatalytic oxidation-resistant potassium silicate binder, a composite architectural coating which can effectively and permanently degrade formaldehyde in ordinary fluorescent lamp environment can be obtained [18].
Catalytic technology can be combined with physical adsorption technology or other technologies, and the effect is better. The combination of catalytic technology and physical adsorption technology can provide a high-concentration reaction environment for catalytic technology, which can degrade formaldehyde and regenerate adsorbent. Nano-titanium dioxide photocatalyst is combined with some gas adsorbents (zeolite, activated carbon, silica, etc.). ) can effectively degrade low-concentration harmful gases under the excitation of weak ultraviolet light. Hou Yining et al. [19] studied the purification of indoor formaldehyde pollution by TiO _ 2- activated carbon fiber mixture, and found that the effect of TiO _ 2-ACF mixture was better than that of TiO _ 2 TiO _ 2-ACF alone. When the mass ratio of TiO _ 2-ACF was 1:0.5, the removal effect of formaldehyde was the best. Fumihide et al. [20] combined photocatalytic technology with continuous adsorption and desorption technology using activated carbon to invent an improved photocatalytic reactor, which can degrade formaldehyde with low concentration below 1 mg/m3 in a closed room of 10 m3 to below the WHO standard (0. 1 mg/m3) within 90 minutes. Rare-earth activated air purification materials combine chemical adsorption, physical adsorption, photocatalysis and other multiple catalytic technologies to achieve lasting purification of formaldehyde [2 1]. Zhang Zengfeng et al. [22] studied the removal of indoor formaldehyde by low-temperature plasma-catalysis, and found that the removal rate of formaldehyde by plasma technology increased with the increase of voltage under the conditions of room temperature, atmospheric pressure and dielectric barrier discharge. Filling dielectric spheres with large specific surface area was beneficial to the removal of formaldehyde, and titanium dioxide could produce catalytic activity in plasma atmosphere. The combination of catalytic technology and other technologies can complement each other and achieve better purification effect.
Catalytic technology has the advantages of mild reaction conditions, low energy consumption and less secondary pollution, and can oxidize and decompose organic compounds with stable structure at normal temperature and pressure. Generally, the indoor formaldehyde concentration is low, and coating the surface of building materials such as rooms, glass and ceramics or placing TiO2 air purification equipment can effectively degrade formaldehyde. However, it needs nano-TiO2 _ 2 and ultraviolet radiation, which has economic and technical limitations and has not yet entered the stage of large-scale use and popularization. Recent studies have found that the nano-coating composed of nano-titanium dioxide and nano-silver is less dependent on ultraviolet light, and it can deodorize even under visible light irradiation, and has the effect of sterilization and mildew prevention, which is worth looking forward to.
2.3 Chemical neutralization technology
Chemical neutralization technology generally uses complexing technology to destroy the molecular structure of harmful gases such as formaldehyde and benzene, neutralize harmful gases in the air, and then gradually eliminate them. At present, experts have developed various deodorants and formaldehyde scavengers, all of which belong to this technical product. This technology is best combined with decoration engineering, which can effectively reduce the free formaldehyde in wood-based panels. Domestic good products include formaldehyde, paint companion and paint companion, which can completely solve the pollution source problem. After testing, the formaldehyde emission of the treated board can be reduced to below 0.3mg/l, which is far lower than the emission standard of E0 grade board.
Disadvantages: it needs professional construction, and it is best to use it in the process of decoration or furniture production. General consumers lack awareness of prevention, and the cost after renovation is relatively high. It is an essential product for pollution control.
2.4 Air negative ion technology
Rare mineral stones with obvious thermoelectric effect are mainly selected as raw materials and added to wall materials, which ionize air and moisture in the air to generate negative ions when contacting with air; Polarization can occur, discharge outwards, and indoor air can be purified. The "Rino Air Ion Bao" sold in the market belongs to this kind of product. Jin Zongzhe et al. [23] research shows that rare earth activated tourmaline can purify more than 95% of formaldehyde, which combines negative ion technology with physical adsorption and chemical adsorption technology. Negative ion technology can also be applied to building materials, such as negative ion coatings, which can continuously neutralize and degrade negative ions and harmful gases (positive ions) continuously released by indoor pollution sources and remove formaldehyde for a long time. Feng et al. [24] developed a healthy and environment-friendly interior wall coating by using the modification and activation technology of natural minerals and nano-rare earth activation technology, which not only has superior conventional properties, but also integrates the functions of pollution-free, antibacterial, mildew-proof, far-infrared radiation, negative ion release and so on, which are beneficial to human health. The coating can generate a large number of negative ions only under the excitation of visible light, which increases the number of indoor negative ions by 200 ~ 400 /cm3. The negative ion generator can also be used to directly generate negative ions.
Disadvantages: the negative ion dust removal effect is better than the odor removal effect, especially the positively charged smoke and odor substances, but the oxidative decomposition ability of odor is limited, and the low concentration pollution effect is better. Can be used as an auxiliary deodorizing product.
2.5 ozone oxidation method
Ozone reacts with polar organic compounds, such as formaldehyde, which leads to the rupture of unsaturated organic molecules, so that ozone molecules combine with the double bonds of organic molecules to generate smelly oxides, thus achieving the purpose of decomposing formaldehyde molecules. Wang Yaozhu et al. [25] found that the purification rate of formaldehyde gas by low concentration ozone (ultraviolet lamp irradiation) was 0.050 ~ 0.075 mg/m3, and the formaldehyde concentration was 3.03 ~ 8.70 mg/m3. After 5 minutes, the purification rate of formaldehyde by ozone was 4 1.74%. Ozone generator has the ability of sterilization, disinfection, deodorization and decomposition of organic matter, but the efficiency of ozone purification of formaldehyde is low, and ozone is easy to decompose and unstable, which may produce secondary pollutants.
Disadvantages: ozone itself is also an air pollutant, and the country also has corresponding limit standards. Its carcinogenicity and acute toxicity are even stronger than formaldehyde. If the ozone amount is not well controlled, it will be counterproductive.
2.6 room temperature catalytic oxidation method
Also known as cold catalysis, it mainly uses the special catalytic oxidation performance of some precious metals to convert indoor pollutants into CO2 and H2O. Commonly used carriers are zirconia, cerium oxide, silica, activated carbon, molecular sieve and so on. Commonly used precious metals are palladium, platinum, rhodium, ruthenium and iridium. In recent years, Japan has conducted in-depth research on low-temperature catalysts, and a series of patents have come out. The air purifier developed by Yushika et al. [26, 27] contains manganese oxide (MnO2 _ 2 is 77%), which has HCOOH effect on formaldehyde removal in newly decorated houses. In more than seven months, the indoor formaldehyde in newly-built houses decreased from 0.2 1× 10-6, and no harm was found.
Disadvantages: it is expensive and cannot solve the problem of pollution sources. Can only be used as an auxiliary deodorization means.
2.7 Biotechnology
Biological purification of organic waste gas is a method by which microorganisms use organic matter as carbon source and energy source for their growth and oxidize it into non-toxic and harmless inorganic matter. Li Xiaomei et al. [28] experiments show that the biofilm packed tower made by screening, culturing and inoculating appropriate microbial strains has a good purification effect on formaldehyde waste gas with inlet concentration less than 20 mg/m3, and the purification efficiency is over 90%. During the purification operation, it is beneficial to keep the liquid spraying amount at 20 L/h. The research of Masaki et al [29] shows that biological enzymes have the potential to degrade formaldehyde. This method has the advantages of simple operation, low operating cost, no secondary pollution, wide application in Europe and industrialization.
Disadvantages: the biological activity temperature is generally 10 ~ 40℃, so the indoor temperature must be kept within the activity temperature range of specific microorganisms, which limits its application. At present, there is still a long way to go before industrialization is put on the market, and the selected strains need to undergo strict biosafety identification.
2.8 material sealing technology
Aiming at formaldehyde in various wood-based panels, experts have developed a sealing material called formaldehyde sealant, which is used to seal formaldehyde gas in furniture and wood-based panels. The researchers of Shuangquan Technology found that the paint film of paint has a good sealing effect on formaldehyde in wood-based panels. The formaldehyde emission can be reduced by more than 90% after being painted with three-primer and three-topcoat, which has the function of blocking formaldehyde. China stipulates that E2-level wood-based panels can not be directly used for interior decoration or furniture making, but can be used to produce office furniture such as test benches, computer desks, desks or cabinets after finishing treatment (such as melamine). After facing treatment, they can also play a good role in sealing formaldehyde and beautifying the appearance of wood-based panels. But in the process of making furniture or cabinets, there will always be new sections or nail holes. And the closed formaldehyde will be released into the environment from these gaps with lax sealing at the edges, causing formaldehyde pollution in the ambient air.
Disadvantages: only closed, not absorbed, not decomposed, palliative, not permanent. This is also one of the reasons why the formaldehyde released by furniture after paint treatment will last for more than ten years, and the long-term low-concentration formaldehyde pollution is more serious.
3 Conclusion
With the increasingly strict national environmental protection laws and regulations and the deep-rooted awareness of environmental protection, more and more attention has been paid to the control and treatment of indoor formaldehyde pollution. Many air purification technologies aimed at formaldehyde pollution have been applied in practice at home and abroad, and various new methods and technologies are constantly being studied. Among them, nano-photocatalytic technology is the development trend of air purification technology research. At the same time, because each method has its own advantages and disadvantages, choosing the appropriate technology according to the actual situation, especially the combination of various technologies, can effectively control and control indoor formaldehyde pollution. Recently, Duxiu Technology recently launched a multifunctional flower deodorant, which combines nano-photocatalytic decomposition technology with physical adsorption technology of activated carbon and bamboo charcoal with simulated flowers as the carrier. The three-dimensional structure of artificial flowers not only beautifies the environment and purifies the soul, but also increases the contact area with airflow. Duxiu Technology puts forward the odor decomposition removal that can be decomposed and removed (through nano-silver titanium flowers). What can't be decomposed and removed will be removed by adsorption (compound odor absorber in flowerpot). What is commendable is that the color change detector is directly built into the compound odor absorber. Once the odor absorbent is saturated, it will change from red to gray to prompt its replacement. In addition, Duxiu Technology will recycle and desorb the compound odor absorbent through special equipment and technology to ensure its effective odor removal. This kind of high-tech and product is worth looking forward to solving the problem of formaldehyde pollution in indoor air in China.
In short, to solve the problem of formaldehyde pollution in indoor air, we must first solve the pollution source, reduce the formaldehyde release from the pollution source to the living environment as much as possible through physical or chemical means, and then assist in adopting some comprehensive treatment technologies and products of photocatalysis and physical adsorption to achieve the ideal treatment effect.