FAQ

‌Activated carbon, made from carbon-rich materials through high-temperature carbonization and activation, is a hydrophobic adsorbent. It contains a large number of micropores and has a huge specific surface area, enabling it to effectively remove color, odor, and most organic pollutants in secondary effluent, as well as certain inorganic substances, including some toxic heavy metals. ‌The factors affecting the adsorption of activated carbon include its characteristics, the characteristics and concentration of the adsorbate, the pH of the wastewater, the content of suspended solids, the contact system, and the operation mode. Activated carbon adsorption is the most important and effective treatment technology in advanced urban sewage treatment and is widely used‌. Activated carbon can effectively adsorb chlorinated hydrocarbons, organophosphorus and carbamate insecticides, as well as phenyl ethers, ortho-nitrochlorobenzene, naphthalene, ethylene, xylenol, phenol, DDT, aldrin, alkylbenzenesulfonic acid, and many esters and aromatic compounds. However, secondary effluent also contains organic substances that are not adsorbed by activated carbon, such as intermediate degradation products of proteins, which are more difficult to be adsorbed than the original organic substances. Activated carbon has a relatively low removal capacity for THMS, only reaching 23-60%. The activated carbon adsorption method can also be combined with other treatment methods, such as the ozone-activated carbon method, coagulation-adsorption activated carbon method, Habberer process, and activated carbon-diatomite method, which significantly extend the adsorption cycle of activated carbon, reduce its usage, and greatly improve the treatment effect and scope.

‌Strong Adsorption Capacity‌:

Honeycomb activated carbon is widely used in various organic waste gas purification systems with low concentrations and high air volumes. When the treated waste gas passes through the square holes of the honeycomb activated carbon, it can fully contact with the activated carbon, achieving an adsorption efficiency of up to 80% for various waste gases. It can be extensively used for purifying gases containing toluene, xylene, benzene, and other aromatic hydrocarbons, phenols, esters, alcohols, aldehydes, and other organic gases, as well as malodorous gases and various gases containing trace metals. Environmental protection equipment utilizing honeycomb activated carbon offers high purification efficiency, a small adsorption bed volume, low energy consumption, reduced construction and operating costs, and the purified gas fully meets environmental emission requirements‌.

‌Low Wind Resistance‌:

One of the main reasons why honeycomb activated carbon can replace columnar and granular activated carbon is its low wind resistance coefficient (reaching 0.8 m/s), allowing waste gas to enter and exit stably without affecting the subsequent waste gas inlet volume and minimizing the impact on equipment lifespan. It exhibits excellent adsorption, desorption performance, and gas dynamic performance‌.

‌Common Specifications of Honeycomb Activated Carbon in Waste Gas Treatment‌:

Classified by pore size or number of pores: It is divided into three types: large pores (5mm, 400 pores) with low wind resistance and fast filtration speed; medium pores (3mm, 625 pores) with moderate wind resistance and filtration speed; and small pores (1.5mm, 1600 pores) with the best effect and most common use.

Classified by size: It is divided into three types: 100100100mm (most commonly used), 10010050mm, and 505050mm.

Classified by water resistance: It is divided into water-resistant honeycomb activated carbon (suitable for high air humidity or processes without a drying device after water spraying) and non-water-resistant honeycomb activated carbon (suitable for low air humidity)‌

‌Technical Indicators of Honeycomb Activated Carbon in Waste Gas Treatment‌:

Iodine value: An important indicator of adsorption capacity, representing the amount of iodine (in milligrams) adsorbed by each gram of carbon at 0.1mol/L.

Specific surface area: Refers to the total area per unit mass of the material.

Carbon tetrachloride (CTC): Commonly used as an indicator for adsorbing organic waste gas molecules.

Benzene adsorption rate: Commonly used to indicate the adsorption capacity for small molecular organic compounds‌.

In summary, different organic waste gas components and concentrations require different treatment methods. Currently, considering factors such as technological maturity, economy, and equipment maintenance, the most widely used method is honeycomb activated carbon adsorption. When using honeycomb activated carbon, it is important to avoid high temperatures and high dust content in the gas to ensure optimal adsorption effect and prolong its service life‌.

There are four conventional methods for wastewater treatment: physical treatment, chemical treatment, physicochemical treatment, and biological treatment. Physical treatment primarily addresses insoluble and suspended substances in industrial wastewater through methods such as stirring, centrifugation, and filtration. Chemical treatment involves adding specific chemicals to the wastewater to precipitate and separate specific ions, followed by settlement or filtration to remove insolubles. Common techniques include chemical precipitation, neutralization, coagulation, and oxidation-reduction methods. Physicochemical treatment, which includes methods like adsorption, membrane separation, and ion exchange, is employed to treat harmful substances in industrial wastewater, with activated carbon adsorption being one such method. Biological treatment, on the other hand, utilizes microbial metabolism to consume and convert organic substances in wastewater into stable, harmless compounds, categorized into anaerobic and aerobic processes.

Activated carbon adsorption falls under the physicochemical category of industrial wastewater treatment methods. Notably, it not only physically adsorbs many insoluble harmful substances but also effectively chemically adsorbs harmful ions in ionic state, combining the advantages of both physical and chemical treatment methods. Furthermore, activated carbon exhibits exceptionally high efficiency and quality in treating industrial wastewater. Its porous structure endows it with a vast relative surface area, granting it powerful adsorption capabilities. Additionally, its carbon chain structure provides significant rigidity, allowing the activated carbon to maintain its shape and structure during use, thereby effectively exerting its adsorption function.

‌1. Principle of Treating Industrial Wastewater with Activated Carbon Adsorption‌

Activated carbon is produced by activating carbon-containing materials such as wood, coal, and nut shells under high temperature and oxygen-deficient conditions. It features numerous micropores and a huge specific surface area, often reaching 500-1500 square meters per gram, conferring strong physical adsorption capabilities for organic pollutants in wastewater. Moreover, during the activation process, oxygen-containing functional groups such as carboxyl (-COOH), hydroxyl (-OH), and carbonyl groups form on the non-crystalline portions of the activated carbon surface. These groups enable the activated carbon to perform chemical adsorption and catalytic oxidation-reduction, effectively removing metal ions from wastewater. The strong adsorption capacity of activated carbon allows it to adsorb fine particles in industrial wastewater, facilitating their precipitation and removal, thus achieving wastewater treatment goals. The porous structure of activated carbon creates a multitude of tiny pores on its surface, with diameters typically in the nanometer range, resulting in an enormous relative surface area and a powerful adsorption effect on external fine particles. Besides direct adsorption in wastewater, activated carbon's adsorption capacity can be enhanced by providing continuous oxygen through ventilation and heating, thereby improving wastewater treatment efficacy.

‌2. Advantages of Treating Industrial Wastewater with Activated Carbon Adsorption‌

(1) Effective and stable treatment results.

(2) Enhanced microbial resistance to organic toxins and heavy metals.

(3) Formation of cohesive carbon bodies and microorganisms, producing solid and dense sludge and improving the operational conditions of activated sludge processes.

(4) Adsorption of surface-active substances by activated carbon, resolving the issue of foaming in aeration tanks.

(5) Applicability to wastewater with complex compositions, varying concentrations, and fluctuating water volumes.

(6) Cost-effectiveness of powdered carbon.

‌3. Applications of Activated Carbon Adsorption in Special Industrial Wastewater Treatment‌

(1) ‌Treatment of Oily Industrial Wastewater‌: Oily industrial wastewater, such as seawater pollution from oil tanker spills and oily waste from factories, requires multi-stage treatment to meet standards. Since oil is an organic substance, only materials with good compatibility can effectively adsorb and remove it. However, activated carbon has better hydrophilicity and poorer oleophilicity, limiting its adsorption capacity for oil. Therefore, oily industrial wastewater should undergo organic adsorption and multi-stage treatment before final treatment with activated carbon to completely remove the oil.

(2) ‌Treatment of Industrial Wastewater Containing Heavy Metal Ions‌: Industrial wastewater often contains unreacted toxic heavy metals like mercury and chromium, which pose severe threats to plant growth and animal protein metabolism and respiratory systems if discharged untreated. Activated carbon strongly adsorbs low-concentration mercury, the most toxic component in industrial wastewater, effectively removing mercury ions. However, for high-concentration mercury, chemical precipitation is required first, followed by activated carbon adsorption to thoroughly remove residual mercury ions. Although the mechanism of chromium removal by activated carbon is complex, involving physical adsorption, chemical reduction, and physicochemical adsorption, it is highly effective. Generally, chromium ions in wastewater can be completely removed by activated carbon adsorption alone.

(3) ‌Treatment of Dye-Containing Industrial Wastewater‌: Incomplete treatment of wastewater generated by the textile industry, particularly the clothing and paper industries, has led to increasing discharges of dye-containing industrial wastewater. Due to its complex composition, high concentration, and deep color, this type of wastewater is challenging to treat solely with activated carbon. The common treatment approach involves initial oxidation and adsorption, followed by membrane separation and multi-stage degradation, and finally, depth decolorization with activated carbon. Activated carbon's strong adsorption capacity for colors makes it highly promising for the treatment of dye-containing wastewater.

‌Coal Powder Preparation‌: Lignite or bituminous coal and coking coal are separately crushed to a particle size of ≤2mm. They are then mixed in a mass ratio of lignite or bituminous coal to coking coal of 1:0.2-0.5 and further ground to a particle size below 74μm to obtain uniformly mixed coal powder‌.

‌Emulsification of Coal Tar‌:

‌Step 1‌: High-temperature coal tar with an asphalt content of ≥55% is preheated to 120-150℃‌

‌Step 2‌: An emulsifier aqueous solution is prepared by adding 2-6% of the mass of high-temperature coal tar as a surfactant and 0.5-2% as a stabilizer into water, and preheating the mixture to 80-90℃ to form the emulsifier aqueous solution‌

‌Step 3‌: High-temperature coal tar is added to the emulsifier aqueous solution under stirring conditions in a mass ratio of high-temperature coal tar to emulsifier aqueous solution of 1:0.3-0.7 to produce emulsified coal tar‌

‌Processing of Plastic Coal Mud‌: The ingredients are mixed in a mass ratio of coal powder to emulsified coal tar to water to water-soluble binder to lubricant of 34-57:30-36:6-29:3-5:2-3. This mixture is added to a kneader and stirred uniformly to produce plastic coal mud‌

‌Mud Kneading‌: The plastic coal mud is processed through a vacuum mud kneader or an extrusion molding machine to ensure uniform mixing. It is then filtered through a 60-100 mesh screen in a molding machine to remove large particles‌

‌Extrusion Molding‌: The kneaded mud is added to an extrusion molding machine equipped with a honeycomb steel mold for shaping, resulting in a wet honeycomb-shaped body‌

‌Drying‌: The wet honeycomb-shaped body is dried until its moisture content is less than 2%‌

‌Carbonization and Activation‌: Using inert gas as a protective gas, the material is heated at a rate of 3-5℃/min to 600-800℃ and carbonized for 2-3 hours. Subsequently, it is heated at a rate of 5-15℃/min to 750-850℃ and activated with steam for 3-15 hours. After multiple processes, coal-based honeycomb-shaped activated carbon is finally produced‌

‌The mechanism of action of activated carbon for alcoholic beverages involves addressing various issues in the production of alcoholic beverages such as Baijiu, beer, and other related products. Here's a detailed explanation‌:

‌Issue in Alcoholic Beverage Production: Turbidity, Precipitation, and Low-Temperature Discoloration‌

‌Problem‌: When the alcohol content of Baijiu is reduced or the temperature drops, the solubility of higher fatty acid ethyl esters changes, leading to turbidity, flocculent precipitation, suspended solids, and low-temperature discoloration.

‌Mechanism of Activated Carbon‌: Activated carbon for alcoholic beverages, produced through a special process with a unique pore structure, selectively adsorbs higher fatty acid ethyl esters such as ethyl palmitate, ethyl oleate, and ethyl linoleate, as well as large molecules that cause turbidity. It has minimal adsorption of the main flavor components of Baijiu, ensuring that the original flavor remains unchanged while removing turbidity. This results in a clear, bright, and stable beverage that does not become turbid at low temperatures.‌

‌Issue in Alcoholic Beverage Production: Harsh, Spicy, and Incoordinated Taste‌

‌Problem‌: Newly produced alcoholic beverages often have a harsh, spicy, and incoordinated taste. To produce high-quality beverages, a certain storage period is required, during which both physical and chemical changes occur, a process that is very slow.

‌Mechanism of Activated Carbon‌: The surface of activated carbon for alcoholic beverages contains numerous oxygen-containing functional groups and trace metal elements and ions. These promote the association of water and ethanol molecules, accelerating the formation of molecular clusters, thereby restraining alcohol molecules and reducing their activity. Simultaneously, it accelerates a series of chemical reactions such as esterification, oxidation, and condensation, resulting in a balance of alcohol, acid, and ester substances. This leads to a reduction in harshness and spiciness, an increase in aroma, and a pure, mellow, and soft taste.‌

‌Issue in Alcoholic Beverage Production: Unpleasant Odors and Flavors in New Alcohol‌

‌Problem‌: Newly produced alcohol (ethanol) often contains unpleasant odors and flavors such as bitterness, furan-like flavors, earthy and musty tastes.

‌Mechanism of Activated Carbon‌: Activated carbon for alcoholic beverages can adsorb organic compounds and aldehydes that produce unpleasant odors, such as fusel oil, furfural, dimethyl sulfide, etc., effectively removing these odors and flavors. This results in a pure and aromatic beverage.

‌1. The Impact of Temperature on the Adsorption Capacity of Coal-based Columnar Activated Carbon‌

Coal-based columnar activated carbon, made from high-quality anthracite through advanced processing techniques, appears as black cylindrical particles with a reasonable pore structure, good adsorption performance, high mechanical strength, and easy regeneration. Studies have shown that adsorption is a dynamic equilibrium reaction. Changes in temperature increase the K value, indicating an increase in adsorption rate and the achievement of a new equilibrium, thereby altering the adsorption capacity of the coal-based columnar activated carbon. The saturated adsorption capacity (Xm) represents the adsorption amount when the adsorbent surface is fully covered with a monomolecular layer, and it is a fixed value not influenced by other factors. Generally, adsorption is an exothermic process, so an increase in temperature reduces the adsorption capacity and weakens the adsorption ability. In practical applications, the influence of temperature should be comprehensively considered based on specific conditions.‌

‌2. The Impact of Pressure on the Adsorption Capacity‌

An increase in pressure leads to an increase in gas adsorption capacity, especially for gases with low adsorptivity under normal pressure conditions. The increase in pressure positively promotes adsorptivity, serving as the theoretical basis for pressure swing adsorption.‌

‌3. The Influence of Adsorbate Concentration‌

The properties of the adsorbate, including its solubility, molecular polarity, and relative molecular weight, all affect the adsorption performance. For the same substance, the adsorption capacity initially increases linearly with an increase in adsorbate concentration, then increases slowly until it reaches a certain level and remains constant.‌

‌4. The Impact of pH Value‌

The influence of pH value on different adsorbates varies. For non-ionic adsorbates, the adsorption capacity is not significantly related to pH value. For cationic adsorbates, the adsorption capacity increases with an increase in pH value. Conversely, for anionic adsorbates, the adsorption capacity decreases with an increase in pH value. When using coal-based columnar activated carbon, comprehensive consideration should be given to the specific application, process, equipment, and other factors. The influence of these factors should be weighed, and experimental research should be conducted to find the optimal application conditions.‌

In summary, the adsorption performance of columnar activated carbon is influenced by multiple factors, including temperature, pressure, adsorbate concentration, and pH value. Understanding and controlling these factors are crucial for optimizing the adsorption process and achieving the desired adsorption effect.

Activated carbon is one of the commonly used substances in organic synthesis experiments, primarily used for decolorization and removal of impurities from compounds. According to standard chemical laboratory practices, the typical method of using activated carbon is to perform hot filtration for decolorization before recrystallization. However, activated carbon has other uses as well. By skillfully utilizing the properties of activated carbon and distinguishing its advantages and disadvantages, organic synthesis experiments can be greatly facilitated.

The main functions of activated carbon in organic synthesis are decolorization, impurity removal, and adsorption. In a single operation involving activated carbon, one of these functions is usually predominant, while the others play a secondary role.

‌Decolorization‌: The most common function of activated carbon is decolorization. Based on polarity analysis, activated carbon can be considered as a non-polar material, capable of adsorbing non-polar and slightly polar pigments, making it suitable for use in highly polar solvents. Most of the pigments present in substances are non-polar or slightly polar, so activated carbon is often the preferred decolorizing agent, with water and alcohols being the most commonly used solvents. When decolorization is needed, the polarity of the pigment is usually not a concern. Activated carbon is directly used for decolorization, and the effect is judged by observing the change in the solution before and after decolorization. The general procedure is as follows: the substance to be decolorized is added to a certain amount of solvent, heated until completely dissolved, then a certain amount of activated carbon is added, and the mixture is stirred for a period of time before being hot-filtered and the filtrate is concentrated. The substances to be decolorized are usually solids or liquids containing visible pigments, with solids being more common. The amount of solvent is generally 3-10 times the amount of the substance, as too little makes the operation difficult and results in significant loss during hot filtration, while too much increases costs unnecessarily. The solvent is typically a highly polar one, such as methanol, ethanol, or water. If crystallization is needed after decolorization, the most suitable solvent is usually selected. The amount of activated carbon added is generally 5-10% of the solute (i.e., the substance to be decolorized), with adjustments made as needed. The stirring time is usually 30 minutes to 2 hours, with adjustments made as needed. The filtrate is processed accordingly. If there is little change in appearance before and after decolorization, more activated carbon can be added for repeated decolorization. If recrystallization is needed, it can be achieved by direct cooling or cooling after appropriate concentration. If the substance to be decolorized is a liquid, it is usually concentrated to dryness.‌

‌Impurity Removal‌: The impurities referred to here mainly include insolubles such as inorganic salts, dust, and physical impurities. Decolorization by activated carbon also falls under the category of impurity removal, but it specifically targets organic pigments that absorb visible light. The process of impurity removal is very simple and similar to the decolorization process. After complete dissolution, activated carbon is added, stirred, and then directly filtered and concentrated. For simple impurity removal, activated carbon may not be necessary, and direct filtration after complete dissolution is sufficient. The main reason for adding activated carbon is to utilize its filtering aid properties, which facilitate filtration.‌

‌Adsorption‌: Adsorption is mainly used for tar and sticky impurities. If these substances are not treated with activated carbon, direct filtration can clog the filtration medium. After adsorption by activated carbon, the effect is usually significant. When adsorption is the primary function, activated carbon can be replaced by silica gel or kieselguhr, with little difference in effect. During the use of activated carbon, it often simultaneously exhibits the three functions of decolorization, impurity removal, and adsorption. Colored impurities enter the internal structure of the activated carbon particles, while tar and sticky impurities are trapped between the particles. During filtration, activated carbon aids in the filtration process of insoluble impurities. These three functions cannot be viewed separately.

Activated carbon is highly effective in treating various types of wastewater due to its excellent adsorption properties for organic compounds. Its developed pore structure and large specific surface area enable it to strongly adsorb dissolved organic pollutants in water, such as benzene compounds, phenolic compounds, petroleum and petroleum products. It also effectively removes organic pollutants that are difficult to remove by biological or other chemical methods, including color, odor, methylene blue surfactants, herbicides, insecticides, pesticides, synthetic detergents, synthetic dyes, amines, and many artificially synthesized organic compounds.

Specifically, activated carbon is suitable for treating the following types of wastewater:

‌Wastewater Containing Cr(VI)‌

With the rapid development of the electroplating industry, a large amount of electroplating wastewater poses an increasingly serious threat to the environment. Activated carbon, with its highly developed microporous structure and high specific surface area, has strong physical adsorption capabilities and can effectively adsorb Cr(VI) in wastewater. Additionally, the oxygen-containing groups on the surface of activated carbon, such as hydroxyl (-OH) and carboxyl (-COOH) groups, have electrostatic adsorption functions and can chemically adsorb Cr(VI), making it suitable for treating electroplating wastewater containing Cr(VI).‌

‌Cyanide-Containing Wastewater‌

Industries such as electroplating, coking, blast furnace gas scrubbing, and gold and silver ore processing discharge cyanide-containing wastewater. Cyanide is highly toxic and poses great harm to humans and fish. Activated carbon, with its large specific surface area, effectively adsorbs cyanide-containing wastewater, making it a suitable treatment option.‌

‌Mercury-Containing Wastewater‌

In the chlor-alkali industry, mercury is used as the cathode to produce chlorine and caustic soda. Mercury is also used as a catalyst in the synthesis of polyvinyl chloride, acetaldehyde, and vinyl acetate. The electronic instrumentation industry also frequently uses mercury, resulting in the discharge of mercury-containing wastewater. Activated carbon can effectively adsorb mercury in wastewater, making it a viable treatment option, especially for low-concentration mercury-containing wastewater.‌

‌Phenol-Containing Wastewater‌

Phenol-containing wastewater is widely generated in petrochemical plants, plastic factories, synthetic fiber factories, coking plants, textile factories, nitrogen plants, and refinery-chemical plants. Activated carbon has good adsorption properties for phenol and can successfully treat phenol-containing wastewater. It is particularly suitable for treating medium- and low-concentration phenol-containing wastewater, with removal rates reaching over 99% and outlet phenol concentrations below 0.1mg/L.‌

‌Dye Wastewater‌

The development of the textile industry has driven the production of dyes. Each year, a significant amount of dye is produced worldwide, with a portion entering water bodies directly as wastewater and another portion lost during the subsequent textile dyeing process. Dye wastewater is complex in composition, with varying water quality, deep color, and high concentration, making it difficult to treat. Activated carbon, with its large specific surface area, can effectively remove the color from dye wastewater, helping to restore the ecological balance of the water.‌

‌I. Classification of Common VOCs‌

VOCs (Volatile Organic Compounds) are a type of common atmospheric pollutant generated in industries such as paint production, chemical fiber manufacturing, metal coating, chemical coatings, shoemaking and leather making, plywood manufacturing, tire manufacturing, and others. Harmful VOCs mainly include acetone, toluene, phenol, dimethylaniline, formaldehyde, n-hexane, ethyl acetate, ethanol, etc. In industrial enterprises, VOCs can be classified based on their sources as follows:

‌Paint Exhaust‌: Mainly composed of volatile organic compounds such as acetone, butanol, xylene, toluene, ethyl acetate, and butyl acetate. It is mainly generated in surface treatment enterprises such as paint spraying. Common treatment methods include oil curtain absorption, water curtain absorption, combined with secondary and tertiary activated carbon adsorption.

‌Plastic and Rubber Exhaust‌: Mainly composed of polymer monomers volatilized during the heat processing of plastics, rubbers, and other particles. Due to the complex composition of plastics and rubbers, the exhaust mainly contains olefinic plastic polymer monomers such as ethylene, propylene, styrene, acrylonitrile, and butadiene, but the concentration is generally low and the air volume is large. The involved enterprises mainly include plastic pelletizing enterprises, chemical fiber production enterprises, injection molding enterprises, rubber production enterprises, etc. The treatment methods mainly include activated carbon absorption and plasma purification.

‌Shaping Exhaust‌: Mainly composed of aldehydes, ketones, hydrocarbons, fatty acids, alcohols, esters, lactones, heterocyclic compounds, and aromatic compounds. The involved enterprises are mainly dyeing and finishing enterprises and chemical fiber production enterprises. Water spraying treatment process and electrostatic adsorption treatment process are commonly used.

‌Chemical Organic Exhaust‌: Mainly emitted by chemical enterprises. The composition of the exhaust is closely related to the types of chemical products designed and produced by the chemical enterprises. Condensation recovery and catalytic combustion technologies are commonly used for purification and collection.

‌Printing Exhaust‌: Mainly composed of toluene, non-methane total hydrocarbons, ethyl acetate, ethanol, etc., volatilized from inks. The involved enterprises are mainly those with ink printing processes, such as packaging and printing companies. Activated carbon adsorption is commonly used.

‌II. Summary of Common VOCs Purification and Treatment Methods‌

Priority should be given to exhaust purification and treatment methods that are low in cost, energy consumption, and secondary pollution, and that fully utilize the waste heat of the exhaust to achieve resource recycling. Generally, due to the specificity of their production activities, petrochemical enterprises have high exhaust concentrations and often adopt methods such as condensation, absorption, and combustion for exhaust purification and treatment. For industries such as printing with low exhaust concentrations, methods such as adsorption and catalytic combustion are often used for exhaust purification and treatment. The following is a brief overview of these methods:

‌Condensation Recovery Method‌: The condensation method involves directly introducing industrial exhaust into a condenser, where it undergoes adsorption, absorption, desorption, separation, and other processes to recover valuable organic substances, recover waste heat, and purify the exhaust to meet emission standards. This method is suitable for exhaust with high VOC concentrations, low temperatures, and small air volumes, and is commonly used in pharmaceutical and petrochemical enterprises. Usually, one or more additional stages of organic exhaust purification equipment are installed after the condensation recovery unit to ensure compliance with emission standards.

‌Absorption Method‌: Physical absorption is commonly used in industrial production, where the exhaust is introduced into an absorption liquid for absorption and purification. After the absorption liquid becomes saturated, it is heated, desorbed, condensed, and other processes are carried out to recover waste heat. The absorption method is suitable for situations with low concentrations, low temperatures, and large air volumes, but it requires a heating and desorption recovery device, resulting in a large investment. The commonly used methods of oil curtain and water curtain absorption for paint mist in paint coating operations are examples of organic exhaust absorption methods.

‌Direct Combustion Method‌: This method involves igniting the exhaust with auxiliary materials such as gas to convert harmful substances into harmless substances through high-temperature combustion. This method is low in investment, simple in operation, and suitable for exhaust with high concentrations and small air volumes, but it has high safety requirements.

‌Catalytic Combustion Method‌: This method involves heating the exhaust and converting it into harmless carbon dioxide and water through catalytic combustion. It is suitable for the purification and treatment of organic exhaust with high temperatures and high concentrations. It has advantages such as low combustion temperature, energy saving, high purification efficiency, and small footprint, but the investment is relatively large.

‌Adsorption Method‌: The adsorption method can be divided into three types:

(1) ‌Direct Adsorption Method‌: Activated carbon is used to adsorb and purify the organic exhaust, with a purification rate of over 95%. This method is simple in equipment and low in investment, but the activated carbon needs to be replaced frequently, increasing operating costs due to frequent loading, unloading, and replacement procedures.

(2) ‌Adsorption-Recovery Method‌: Fiber-activated carbon is used to adsorb the organic exhaust, and it is desorbed and regenerated by overheated steam counter-blowing when it approaches saturation.

(3) ‌New Adsorption-Catalytic Combustion Method‌: This method combines the advantages of the adsorption method and the catalytic combustion method, with stable operation, low investment, low operating costs, and simple maintenance. It uses new adsorption materials to adsorb the organic exhaust when it approaches saturation, and then introduces the exhaust into a catalytic combustion bed for flameless combustion treatment to achieve complete purification. This method is suitable for the purification and treatment of exhaust with low concentrations and large air volumes and is currently the most widely used exhaust purification and treatment method in China.

‌Low-Temperature Plasma Purification Method‌: Low-temperature plasma is the fourth state of matter after solid, liquid, and gas. When the applied voltage reaches the discharge voltage of the gas, the gas is broken down, generating a mixture including electrons, various ions, atoms, and free radicals. Although the electron temperature is high during the discharge process, the temperature of the heavy particles is low, and the entire system is in a low-temperature state, hence the name low-temperature plasma. Low-temperature plasma degrades pollutants by using these high-energy electrons, free radicals, and other active particles to decompose pollutant molecules in a very short time and trigger subsequent reactions to achieve degradation.

The process in which the adsorbed toxic and harmful gas molecules are released from the micropores of activated carbon is called "desorption" or "regeneration" of activated carbon. The "desorption" of activated carbon requires complex technological methods such as thermal regeneration, chemical elution, solvent extraction regeneration, and biological regeneration to be carried out in specific equipment. Therefore, in the natural environment, the toxic and harmful gas molecules adsorbed in the micropores of activated carbon ‌will not leak out‌.

‌Impurity Removal and Crushing‌:

After cooling, the activated carbon is conveyed to a crusher, typically a ball mill or a universal crusher, using a belt conveyor.

The suction force of an exhaust fan is utilized to draw the activated carbon on the conveyor belt into the crusher, while heavier impurities such as sand, stones, and metal fragments remain on the belt and are removed‌.

The crushed carbon is required to have no more than 5%-8% of particles larger than 120 mesh. This crushed carbon can then proceed to the next step or be sold directly as a finished product based on customer requirements‌.

‌Acid Washing, Water Washing, and Dehydration‌:

The carbon contains impurities such as ash and iron salts, which can be removed by washing with hydrochloric acid.

Acid washing and water washing are conducted in an acid washing tank, which is a rectangular tank made of acid-resistant cement and coated with epoxy resin, measuring 1.65 meters in length, 1.15 meters in width, and approximately 3 meters in depth.

During acid washing, a small amount of hot water is first added to the tank, followed by the crushed carbon. More hot water is then added to fully wet the carbon, and about 20% hydrochloric acid is added. The mixture is heated and stirred with direct steam to ensure uniform mixing.

The carbon material is then suctioned into a high-level tank for storage using a vacuum system. After cooling to below 60 degrees Celsius, the carbon is transferred to another acid washing tank and filtered using a suction filter with microporous plastic tubes. The waste acid is suctioned out through the microporous plastic tubes, while the carbon is adsorbed onto the filter tube walls.

The suction filter, along with the carbon, is then transferred to a clean water tank for continued filtration. Clean water passes through the carbon layer to wash away the acid and impurities until the pH value reaches 5-6. The resulting wet carbon, containing about 50% moisture, can be sold as a finished product‌.

‌Drying, Mixing, and Packaging‌:

For producing dry carbon with a moisture content below 10%, a tunnel-type drying room can be used.

The wet carbon is loaded into aluminum trays, placed on trolleys, and pushed into the tunnel-type drying room, where it is dried using 70-degree Celsius hot air as the drying medium.

After drying, the carbon is pulled out from the other end of the drying room, with trolleys loaded with carbon being pulled out and pushed in every two hours.

A certain batch of dried carbon is mixed uniformly before being packaged‌.

‌In summary‌, the post-treatment procedures for activated carbon include impurity removal and crushing, acid washing, water washing, and dehydration, as well as drying, mixing, and packaging‌.

‌Gas activation, also known as physical activation, is a method of manufacturing activated carbon by using oxidizing gases such as water vapor, carbon dioxide, flue gas, air, etc., as activating agents at high temperatures‌.

In the process of carbonization, wood materials form a surface area and capillaries, which are the reasons for the adsorption capacity of activated carbon. However, due to the adsorption of some tar and hydrocarbons generated during carbonization, the specific surface area becomes smaller, and the activity is lost. The use of activating agents such as water vapor, carbon dioxide, flue gas, and air for activation can oxidize and decompose the residual tar and other carbon-containing compounds in the carbon, remove surface impurities, and reopen the originally blocked pores. Additionally, activating agents such as water vapor can also erode the carbon surface, forming new pores. At the same time, the thin walls between the original pores may be destroyed, enlarging the pores and forming a more developed pore structure, which significantly increases the specific surface area and enhances the adsorption capacity of the carbon.

The temperature required for activation varies depending on the type of activating agent. The activation temperature is approximately 800-950°C when using water vapor, and about 900-950°C when using flue gas. When air is used as the activating agent, activation is generally carried out at around 600°C due to the intense reaction between carbon and oxygen at high temperatures.

The formation of pores is closely related to the degree of carbon oxidation, and carbon oxidation necessarily consumes carbon. Therefore, the burn-off rate, which is the percentage reduction in carbon weight during activation, is often used to measure the degree of carbon activation. Some scientists believe that when the burn-off rate is less than 50%, microporous activated carbon is obtained; when the burn-off rate is greater than 75%, macroporous activated carbon is obtained; and when the burn-off rate is between 50% and 75%, the activated carbon has a mixed structure of macropores and micropores‌.

‌Advantages‌:

‌High Product Yield‌: Only 2.1 to 2.5 tons of absolutely dry wood chips are required to produce one ton of activated carbon‌.

‌Low Activation Temperature‌: The activation temperature is generally around 500-600°C, which reduces the difficulties associated with high-temperature operations‌.

‌Adjustable Product Specifications‌: By adjusting the ratio of zinc to wood chips, the pore volume and pore size distribution of the activated carbon can be regulated‌.

‌Unique Properties‌: Activated carbon produced using the zinc chloride method has certain unique properties that are difficult to replicate with other methods. Particularly, it differs from activated carbon produced by the water vapor method in physicochemical properties and is more suitable for liquid-phase applications, especially for the decolorization of sugar solutions‌.

‌Disadvantages‌:

The main disadvantage of the zinc chloride method is that it causes more environmental pollution and equipment corrosion compared to physical methods. Therefore, when using the zinc chloride method, it is necessary to consider the treatment of waste gas and wastewater, as well as equipment corrosion prevention during the production process.

‌Chemical activation is a method of producing activated carbon using zinc chloride and other chemicals as activating agents, also known as the chemical reagent activation method or simply the chemical method‌. Commonly used chemical reagents include zinc chloride, phosphoric acid, potassium sulfate, and potassium sulfide, with zinc chloride and phosphoric acid being the primary activating agents used in China.

The activation effects of zinc chloride are mainly as follows:

‌Swelling and Dissolution‌: Zinc chloride causes the cellulose in plant materials to swell, gelatinize, and eventually dissolve, allowing the solution to penetrate into the material and form pores by dissolving the cellulose‌.

‌Catalytic Dehydration‌: At high temperatures, zinc chloride catalyzes the dehydration of hydrogen and oxygen atoms from the raw material, releasing them as water and retaining more carbon in the solid product, thereby increasing the yield of activated carbon. The use of zinc chloride as an activating agent also lowers the activation temperature and results in a lighter color of the produced tar, indicating a different activation process compared to conventional pyrolysis‌.

‌Skeleton Formation‌: During carbonization, zinc chloride acts as a skeleton, providing a structure for the newly formed carbon to deposit onto. The fresh carbon has initial bonds and adsorption capacity, enabling it to bind with zinc compounds such as zinc chloride. After washing off the inorganic components like zinc chloride with acid and water, the carbon surface is exposed, creating an internal surface area with adsorption capabilities. This effect is most evident in the increase of the total pore volume of activated carbon with an increasing ratio of zinc to wood chips. A higher zinc-to-wood chip ratio produces activated carbon with more developed transitional pores, while a lower ratio results in activated carbon with more developed micropores‌.

The production process of powdered activated carbon using the zinc chloride continuous method is as follows:

‌Screening and Drying of Wood Chips‌: Wood chips are delivered to a vibrating screen by a bucket elevator for screening. Chips of 6-40 mesh are selected and transported to a cyclone separator by a blower. The separated wood chips fall into a storage silo. The chips are then dried using a pneumatic dryer, where they are continuously fed from the silo into a spiral feeder by a disk feeder. Hot air from a hot air furnace is used to dry the chips, reducing their moisture content from about 40% to 15%-20%. The dried chips are separated in a cyclone separator and fall into a dry wood chip storage silo‌.

‌Preparation of Zinc Chloride Solution‌: The zinc chloride solution is prepared according to production requirements. The recovered zinc solution with a concentration of about 40°Baume is pumped into a zinc preparation tank, where fixed zinc chloride and hydrochloric acid are added to achieve the desired concentration and pH. Alternatively, it can be directly prepared with water. The prepared solution is then pumped into a concentrated zinc tank for later use‌.

‌Mixing‌: The zinc chloride solution from the concentrated zinc tank is pumped into a high-level tank. Wood chips from the silo are lifted to a metering tank by a bucket elevator. A certain amount of wood chips is placed into a mixer and blended with a measured amount of concentrated zinc solution from the high-level tank. The mixture is then poured into the hopper of a rotary kiln‌.

‌Carbonization and Activation‌: The wood chips are fed into the rotary kiln by a disk feeder and spiral feeder at the bottom of the hopper. Hot flue gas is introduced from the other end of the kiln to carbonize and activate the wood chips. The activated material falls into the discharge chamber, where it is periodically removed and transported to the bucket elevator loading point of the recovery process by a trolley‌.

‌Recovery and Washing‌: The activated material is added to a recovery tank to recover zinc chloride by operating the bucket elevator. It is first washed with a 25-30°Baume zinc chloride solution, and the resulting concentrated zinc solution is sent for zinc chloride solution preparation. It is then washed with a diluter zinc solution, with hydrochloric acid added during washing. The solution is heated to above 70°C to convert zinc oxide into zinc chloride. The concentration of the washing solution is finally reduced to below 1°Baume. The recovered carbon is rinsed into a washing tank with water and washed with hot water above 90°C. Hydrochloric acid is added during the second wash, and the solution is heated to boiling to remove iron from the carbon until the washing solution is iron-free‌.

‌Centrifugal Dewatering, Drying, and Grinding‌: The activated carbon is dewatered in a centrifuge, then dried to a moisture content of 4-6% in an externally heated rotary dryer, and finally sent to a ball mill for grinding to produce the finished product. Additionally, a dedicated waste gas and wastewater treatment system is set up to recover zinc chloride and hydrochloric acid from the flue gas and eliminate environmental pollution.

The carbonization of wooden materials, such as wood, wood chips, tree roots, nut shells, and fruit shells, involves placing them in a carbonization device and heating them for thermal decomposition. During this pyrolysis process, a series of complex chemical reactions occur, producing many new products and causing changes in the wooden materials. Based on the temperature changes and the characteristics of the products generated during the thermal decomposition process, the carbonization process can be roughly divided into the following four stages:

‌Drying Stage‌: This stage occurs at a temperature range of 120-150°C. The pyrolysis rate is very slow, mainly involving the evaporation of moisture in the wood through externally supplied heat. The chemical composition of the wooden material remains almost unchanged‌.

‌Pre-carbonization Stage‌: This stage occurs at a temperature range of 150-275°C. The thermal decomposition reaction of the wooden material becomes more pronounced, and its chemical composition begins to change. Unstable components, such as hemicellulose, decompose to produce substances like carbon dioxide, carbon monoxide, and a small amount of acetic acid. Both of these stages require external heat supply to ensure the rise in pyrolysis temperature, and are therefore also known as the endothermic decomposition stages‌.

‌Carbonization Stage‌: This stage occurs at a temperature range of 275-400°C. During this stage, the wooden material undergoes rapid thermal decomposition, generating a large amount of decomposition products. The liquid products contain a significant amount of acetic acid, methanol, and wood tar, while the gaseous products gradually decrease in carbon dioxide content and increase in combustible gases like methane and ethylene. This stage releases a large amount of reaction heat and is therefore also known as the exothermic reaction stage‌.

‌Calcination Stage‌: The temperature rises to 450-500°C. In this stage, external heat is supplied to calcine the charcoal, expelling volatile substances remaining in the charcoal and increasing its fixed carbon content. At this point, the production of liquid products is minimal‌.

It should be noted that, in practice, the boundaries between these four stages are difficult to clearly define. Due to the different heat exposure in various parts of the carbonization equipment and the low thermal conductivity of wooden materials, the position of the wooden material within the equipment, and even the interior and exterior of a large piece of wood, may be at different stages of pyrolysis‌.

The raw materials for carbonization are diverse, including firewood, forest residues, miscellaneous trees removed during forest tending, and wood processing residues such as wood chips. Apart from wood chips, which are granular and require special carbonization furnaces, most other raw materials are in the form of wood segments and are suitable for carbonization in most carbonization furnaces or kilns.

The tree species used as carbonization raw materials can be classified into three categories:

‌Hard broad-leaved species‌, such as beech, oak, chinkapin, and elm;

‌Soft broad-leaved species‌, such as poplar, willow, and basswood;

‌Coniferous species‌, such as masson pine, southern pine, and slash pine.

To produce high-quality wooden charcoal suitable for use in industries such as the metallurgical industry and the carbon disulfide industry, hard broad-leaved species should be selected as the carbonization raw material. Coniferous species are often used to produce pine charcoal, which is used in the production of activated carbon.

The carbonization material should be of uniform size, with a diameter generally not exceeding 10 centimeters. If the diameter is too large, it should be split, with the split line length required to be less than 12 centimeters. The length of the carbonization material is determined by the height of the carbonization furnace or kiln. If large pieces of wood are not split, due to the poor thermal conductivity of wood, the gaseous mixture produced during carbonization will have a long path to travel from the inside to the outside of the wood, resulting in a longer carbonization time and reduced mechanical strength of the wood.

The firewood used for carbonization mostly comes from sprouting forests, so it is best to harvest it in autumn or winter. During this period, the trees are in a dormant stage, the sap stops flowing, and the stored substances in the roots are not damaged, which is conducive to sprouting and regeneration in the following year. Moreover, the weather in autumn is sunny, with low relative humidity and low moisture content in the wood. The harvested firewood is easy to dry, which can shorten the carbonization time, reduce fuel consumption, and produce high-quality wooden charcoal with fewer cracks.

Additionally, decayed wood, diseased, or dead wood should not be used as carbonization raw materials. This is because during the carbonization of decayed wood, the resulting charcoal is loose, fragile, and prone to self-ignition, significantly reducing the quality of the wooden charcoal.

The reason why activated carbon has such strong adsorption capacity is that its adsorption ability is far superior to other adsorbents such as silica gel, zeolite, and activated clay. The reasons are as follows:

‌Developed Pore Structure and Large Specific Surface Area‌: Activated carbon has a developed pore structure with a wide range of pore size distribution, which enables it to adsorb various substances of different molecular sizes. It also has a large number of micropores, resulting in a large specific surface area and strong adsorption capacity. The activation method has a significant impact on the pore size of the produced activated carbon.‌

‌Surface Characteristics of Activated Carbon‌: The surface properties of activated carbon vary depending on the activation conditions. Activated carbon activated by high-temperature steam contains more alkaline oxides on its surface, while activated carbon activated by zinc chloride contains more acidic oxides, which have a particularly strong adsorption capacity for alkaline compounds. The different surface chemical properties, pore size distributions, and pore shapes of activated carbon are the main reasons for its selective adsorption.‌

‌Catalytic Properties‌: Activated carbon is used as a contact catalyst in various isomerization, polymerization, oxidation, and halogenation reactions. Its catalytic activity is due to the role of the carbon surface, surface compounds, and ash content. In the chemical industry, activated carbon is often used as a catalyst support, where catalytically active substances are deposited on the activated carbon and used together as a catalyst. At this time, the role of activated carbon is not limited to supporting the activator; it also has a significant impact on the activity, selectivity, and service life of the catalyst, exhibiting a co-catalytic effect.‌

‌Chemically Stable and Easy to Regenerate‌: Activated carbon is chemically stable, resistant to acids and bases, and can be applied within a wide range of pH values. It is insoluble in water and other solvents and can be used in aqueous solutions and many solvents. Activated carbon can withstand high temperatures and pressures and is often used as a catalyst or support in organic synthesis due to its catalytic activity. When activated carbon becomes ineffective, it can be regenerated repeatedly using various methods to restore its adsorption capacity and reuse in production. If regenerated properly, it can reach its original adsorption level.‌

‌The types and uses of activated carbon, and the characteristics of activated carbon with developed pore structure, large specific surface area, and strong adsorption capacity‌.

Activated carbon is a type of carbon with developed pore structure, large specific surface area, and strong adsorption capacity. The total surface area of each gram of activated carbon can reach over 1500 square meters.

‌Types of Activated Carbon‌:

‌Based on Raw Materials‌: Plant-based carbon, coal-based carbon, petroleum-based carbon, bone carbon, blood carbon, etc.‌

‌Based on Manufacturing Methods‌: Physical gas activation, chemical activation, chemo-physical activation.‌

‌Based on Appearance‌: Powdered activated carbon, irregular granular activated carbon, shaped granular activated carbon, spherical carbon, fibrous carbon, fabric-like carbon, etc.‌

‌Based on Uses‌: Gas-phase adsorption carbon, liquid-phase adsorption carbon, sugar carbon, industrial carbon, catalyst and catalyst support carbon, etc.‌

‌Uses of Activated Carbon‌:

Activated carbon is widely used in various fields due to its excellent properties as an adsorbent. It is used in environmental protection, water treatment, air purification, industrial production, pharmaceutical manufacturing, food processing, and more. Specifically, it can effectively adsorb and remove pollutants such as organics, heavy metals, pigments, odors, and harmful gases from water and air, improving water quality and air quality. It is also used in industrial processes such as decolorization, deacidification, catalysis, and the preparation of pharmaceutical intermediates. In addition, activated carbon is used in medical fields such as drug poisoning treatment, kidney dialysis, and oral cleaning, as well as in applications such as gas masks, industrial waste gas treatment, metal extraction, food decolorization, and hydrogen storage.‌

‌Characteristics of Activated Carbon‌:

Activated carbon is a carbon material with a highly porous structure obtained through special activation processes. It has a large specific surface area and strong adsorption capacity. It is usually made from carbon-containing raw materials such as wood, coal, and coconut shell through carbonization and activation treatments. The rich micropores and mesopores in activated carbon give it excellent adsorption capabilities. Activated carbon is insoluble in water and other solvents and has physical and chemical stability. Except for reactions with strong oxidants such as oxygen at high temperatures, ozone, chlorine, and dichromate, it is extremely stable under practical usage conditions.‌

It's worth noting that the selective adsorption of activated carbon varies depending on the manufacturing method, each with its own range of applicability. Improper use may result in poor performance. In recent years, foreign countries have made significant progress in developing new products with activated carbon, including using various raw materials and processes to develop activated carbon with special properties and secondary processing methods to create high-efficiency, multi-functional activated carbon products.

The question of which is better among coconut shell carbon, bamboo charcoal, and activated carbon often puzzles consumers when purchasing activated carbon. So, how to make a choice?

Firstly, activated carbon can be classified into wooden carbon, coal-based carbon, petroleum carbon, and recycled carbon based on different raw materials, with coconut shell carbon belonging to the category of wooden carbon. Coconut shell carbon is produced through a series of production processes such as activation and carbonization using high-quality coconut shells as raw materials. Due to its developed pores, excellent adsorption performance, and high strength compared to other types of carbon, coconut shell carbon is also the most effective type of activated carbon.

On the other hand, the bamboo charcoal available on the market is simply made by burning bamboo at high temperatures without undergoing processing by specialized equipment and instruments. Moreover, because bamboo charcoal has a loose texture and low hardness, it is prone to micro-pore blockage and deformation under external forces, making it difficult to maintain its original shape. Effective air purification requires numerous micro-pores to absorb harmful substances. Reputable activated carbon manufacturers do not choose bamboo as a raw material.

Therefore, ‌coconut shell carbon is the best type of activated carbon in terms of adsorption effectiveness‌!‌

Taking their adsorption effect on tannic acid as an example.‌

Experimental results demonstrate that, compared to coconut shell activated carbon, nut shell activated carbon exhibits better adsorption performance for tannic acid. It is therefore suitable for removing humic acids from natural water bodies, thereby reducing the generation of disinfection byproducts in drinking water.

A study was conducted to investigate the equilibrium adsorption behavior of nut shell activated carbon and coconut shell activated carbon towards tannic acid. Under the experimental conditions, the relationship between the equilibrium adsorption capacity of nut shell activated carbon for tannic acid and its equilibrium concentration was found to be more consistent with the Langmuir isothermal adsorption equation. The adsorption isotherm of tannic acid on coconut shell activated carbon, however, conformed to both the Langmuir equation and the Freundlich equation.

Furthermore, changes in temperature had a certain impact on the equilibrium adsorption behavior of both types of activated carbon. Specifically, an increase in temperature favored the adsorption of tannic acid, with a particularly significant effect on nut shell activated carbon. This indicates that nut shell activated carbon has a stronger adsorption capacity for large molecules such as tannic acid.

The adsorption kinetics of both types of activated carbon for tannic acid were well fitted by the Lagergren pseudo-second-order rate equation, suggesting that the entire adsorption process consisted of a dual-speed process comprising rapid adsorption and slow adsorption. Additionally, nut shell activated carbon had a larger adsorption rate constant, indicating a faster adsorption rate for tannic acid.

The activation energy of nut shell activated carbon was higher than that of coconut shell activated carbon, indicating that the reaction rate of nut shell activated carbon was more sensitive to temperature and increased rapidly with rising temperature.

‌In summary, experiments have shown that nut shell activated carbon has higher adsorption capacity and activation energy than coconut shell activated carbon.‌‌

In recent years, with the continuous development of China's economy and the constant improvement of people's living standards, everyone has become increasingly attentive to the quality of their indoor environment. Everyone aspires to have a fresh, comfortable, and healthy home. However, new home renovations and newly purchased furniture have become major sources of indoor air pollution. Why is it so difficult to prevent decoration pollution? How can we eliminate toxic and harmful substances such as formaldehyde and benzene compounds without causing secondary pollution? Nowadays, people have started to use activated carbon to purify indoor air. Through the adsorption capability of activated carbon, harmful substances in newly renovated rooms can be absorbed, thereby reducing indoor pollution.

The use of activated carbon adsorption is currently the most widely used, mature, safest, most reliable, and versatile method for removing indoor pollutants. Although there are many types of activated carbon in terms of appearance and application, they all share a common characteristic, which is "adsorption". How can we briefly identify the adsorption capacity of activated carbon? Experts provide us with the following tips:

‌Check the density‌: Hold it in your hand and feel its weight. The lighter the activated carbon, the stronger its adsorption performance‌;

‌Observe bubbles‌: Put a small handful of activated carbon into water, and you will see a series of extremely fine bubbles, forming a delicate bubble line, accompanied by a slight bubbling sound. The more intense and longer-lasting this phenomenon, the better the adsorption performance of the activated carbon‌;

‌Assess decolorization ability‌: Activated carbon has the unique ability to turn colored liquids into light or colorless ones, as it adsorbs the pigment molecules in the colored liquid. Take two transparent cups, fill one with pure water, and then drop in a drop of red ink. After stirring well, pour half of the colored water into the other cup for comparison. Put activated carbon into the colored water, with the amount reaching half or more of the water. This will make the effect more noticeable. After letting it stand for 10-20 minutes, compare it with the control water sample. Under the same conditions, the stronger the decolorization effect, the better the adsorption performance of the activated carbon‌.

These tips can help you choose high-quality activated carbon for effective indoor air purification.

With a wide variety of activated carbon products available on the market, varying significantly in quality, how can ordinary consumers assess the quality of the activated carbon they purchase? Below are several simple methods to help you determine the quality of activated carbon:

‌Distinguish by Production Method‌:

Activated carbon is produced through either physical or chemical methods. Most activated carbon for civilian use on the market is produced through physical methods. You can break a piece of activated carbon and bring the fractured surface close to your tongue. If you feel a strong adsorption force, it indicates good quality activated carbon. The stronger the adsorption, the better the quality.

‌Conduct a Bubble Test‌:

At home, you can perform a bubble test. Place a small amount of activated carbon into a transparent glass. As water penetrates the pores of the activated carbon, it forces the air out, generating a series of fine bubbles and creating bubble lines in the water. The more intense and longer-lasting this phenomenon, the better the quality of the activated carbon.‌

‌Measure Water Absorption Capacity‌:

Weigh a certain amount of activated carbon and pour it into water. After it becomes fully saturated (no more bubbles appear on the surface), filter out the excess water and weigh the saturated activated carbon. The more water the activated carbon absorbs, the better its quality.

‌Check Volume and Weight‌:

The strong adsorption capacity of activated carbon is due to its numerous pores. More pores make the activated carbon more porous and lighter in weight. Therefore, high-quality activated carbon feels lighter, and for the same weight, good-quality activated carbon will have a much larger volume than inferior products.‌

‌Examine Particle Size‌:

Smaller activated carbon particles have a larger surface area in contact with the air, resulting in better adsorption. It is recommended not to purchase activated carbon with particles larger than 2mm.‌

‌Check Packaging‌:

The outer packaging of activated carbon must be sealed to prevent it from absorbing moisture from the air during storage, which would reduce its adsorption effectiveness.

By following these simple methods, you can more accurately assess the quality of activated carbon and make informed purchasing decisions.

Activated carbon, characterized by strong adsorption capacity, resistance to acids and alkalis, heat resistance, insolubility in water and organic solvents, high safety, and easy regeneration, is an environmentally friendly adsorbent. It is widely used in the treatment of industrial waste (waste water, waste gas, and solid waste), solvent recovery, purification of food and beverages, as a carrier in various applications, in medicine, gold extraction, semiconductor applications, batteries, and energy storage. Adjusting the pore structure of activated carbon and modifying its surface groups play a crucial role in enhancing its specific properties and targeted adsorption and catalytic effects.

‌I. Production Processes of Activated Carbon‌

The majority of carbon-containing materials can be used to produce activated carbon, such as wood, sawdust, coal, peat, nut shells, fruit cores, sugarcane bagasse, rice husks, petroleum waste, waste plastics, waste leather, waste tires, papermaking waste, and urban waste. Currently, nut shells are generally considered the best raw material for activated carbon production. However, due to their limited availability, high cost, and difficulties in collection and storage in China, alternative sources are being explored.

In recent years, domestic and international research has focused on using various low-cost and widely available waste materials to produce activated carbon. Although the performance of activated carbon produced from waste materials is not yet high, and its practical applications are limited, it is increasingly favored due to its low cost, high carbon content, easy availability, abundance, and environmental friendliness. Effective utilization of waste materials for activated carbon production not only conserves resources but also benefits ecological environmental protection.

Research in China has explored the use of bamboo, tobacco stems, sugarcane bagasse, wheat straw, rice husks, cotton stalks, walnut shells, and other waste materials for activated carbon production. To ensure stable performance and low cost of the produced activated carbon, the raw materials must be of stable quality and easily accessible, while the production process should be simple, economical, and environmentally friendly. Therefore, key research topics include developing low-cost granulation methods, finding new binders, exploring uses for low-grade activated carbon, and utilizing special waste materials to discover unique functionalities.

Various methods have been developed domestically and internationally for activated carbon production, including chemical activation, physical activation, combined chemical-physical activation, catalytic activation, supercritical technology, and microwave heating. Microwave heating, a rapidly developing method in recent years, involves high-frequency reciprocating motion of dipolar molecules within the heated material, generating "internal friction heat" to raise the material's temperature. This method eliminates the need for any heat conduction process, resulting in fast and uniform heating, no additional heat consumption, high energy efficiency, and a processing time that is only a fraction of that required by traditional heating methods. Activated carbon produced by microwave heating has high yield and mesoporosity (characterized by methylene blue decolorization rate). Generally, smaller activated carbon particles have a larger specific surface area and stronger adsorption capacity. Therefore, activated carbon products are often highly crushed and screened to obtain fine powders. However, excessively small particles and an overly large specific surface area can lead to particle aggregation, increasing particle size and affecting the adsorption performance of the activated carbon. During chemical activation, if the raw material particles are too large and fully exposed to the activating agent, they may be over-activated, resulting in the burnout of pore walls, increased average pore size, and decreased specific surface area and yield. Therefore, a particle size of 74-149 micrometers is generally chosen for activated carbon.

‌II. Modification and Structural Characterization of Activated Carbon‌

The main factors affecting the performance of activated carbon are its specific surface area, pore volume, and pore size distribution. Generally, a larger specific surface area and pore volume result in stronger adsorption capacity. The International Union of Pure and Applied Chemistry (IUPAC) recommends classifying pores as micropores (radius < 2 nm), mesopores (radius 2-50 nm), and macropores (radius > 50 nm). Macropores serve as channels for adsorbate molecules; mesopores not only serve as channels for adsorbate molecules, governing the adsorption rate, but also undergo capillary condensation under certain relative pressures, adsorbing molecules that cannot enter micropores; micropores dominate the adsorption capacity. A more concentrated pore size distribution in activated carbon leads to better performance. As production methods have certain limitations in adjusting microstructure, modifications to the pore structure and surface groups are often made to impart specific properties and uses to the activated carbon.

(I) Surface Chemical Modification

Surface modification of activated carbon includes surface oxidation modification, surface reduction modification, and loading of heteroatoms and compounds. Oxidizing activated carbon to alter its surface properties and porous structure is one of the most popular methods. This process increases the number of existing oxygen-containing functional groups and forms new ones. After surface modification, the weak equilibrium formed by various oxidation products (such as carboxyl and phenolic groups) and the heteroatoms infiltrating the carbon pores can significantly alter the physicochemical properties of the adsorbent. Researchers believe that the abundant oxygen-containing groups on the surface of activated carbon can prevent the rapid destruction and sintering of chelate structures. The type of oxidant and oxidation conditions can significantly change the number of surface oxygen-containing functional groups, accompanied by slight or significant changes in the pore structure. Therefore, modifying activated carbon requires selecting appropriate oxidants and operating conditions based on the characteristics of the adsorbate. Commonly used gaseous oxidants include oxygen and ozone, while liquid oxidants include nitric acid, phosphoric acid, and hydrogen peroxide. Taking nitric acid oxidation as an example, it can significantly increase the content of surface acidic groups. Activated carbon oxidized with HNO3 and then heat-treated at 300-400°C develops more surface acidic groups, resulting in higher cation exchange capacity and excellent adsorption and exchange capacity for heavy metal ions such as Cr. However, oxidation with concentrated nitric acid increases the hydrophilicity of the activated carbon due to the increased number of surface acidic groups, which is不利 for the adsorption of organic compounds such as phenol, aniline, and humic acid in water. For activated carbon surface modification aimed at removing organic pollutants, it is necessary to reduce the content of acidic oxygen-containing functional groups such as surface lactones and carboxyl groups, thereby increasing the hydrophobicity of the activated carbon surface. The acidic groups such as carboxyl groups added to the surface of activated carbon through HNO3 oxidation can be removed by high-temperature treatment (>700°C) in inert gases such as H2 and N2. It has also been found that treating oxidized activated carbon with ammonia at a low temperature of 200°C can result in a basic surface with strong ion exchange performance. High-temperature treatment (>800°C) can also remove acidic groups, generating more basic groups and providing higher anion exchange capacity, thus exhibiting stronger adsorption and exchange capacity for anions. Adjusting the relative content of acidic and basic groups allows for the selective adsorption of different adsorbates.The adsorption of activated carbon for specific adsorbates can also be enhanced by increasing specific surface heteroatoms (such as P, Cl, S, N, CO, Ni) or compounds (such as sodium dimethyldithiocarbamate).

‌Microstructure Modification‌:

Methods such as microwave heating, neutron flux radiation, and low-temperature plasma technology can ideally achieve microstructure modification of activated carbon. For boron-doped activated carbon dried with neutron flux radiation, the arbitrary bombardment of α particles generated by neutron flux radiation increases the destruction of fine pores and/or enhances the micropore area, thereby increasing the specific surface area and adsorption capacity of the activated carbon. The results indicate that the moisture absorption capacity of the modified activated carbon is increased by 18.8% compared to the original sample. Microwave heating also produces good effects in the preparation and modification of activated carbon. After microwave modification, many blocked pores are opened, and the mesopore volume changes significantly, promoting enhanced adsorption. The microwave power and irradiation time are key factors determining the adsorption performance of the modified activated carbon. After microwave irradiation, the surface of the activated carbon becomes rough with concave-convex features, many blocked pores are opened and extend inward, attachments around the pores are removed, the carbon framework shrinks, pores of different sizes shrink, and the pore size distribution shifts towards micropores, which is highly beneficial for adsorption.

‌Applications of Activated Carbon‌:

Activated carbon, as an excellent adsorbent, has wide applications in water purification, advanced wastewater treatment, gas purification, or storage.

‌Water Purification‌: Activated carbon not only effectively removes color and odor from water but also has high adsorption capacity for synthetic detergents (ABS), trihalomethanes (THMs), halogenated hydrocarbons, and free chlorine. It can also effectively remove almost non-degradable carbamate insecticides. Activated carbon can effectively remove free chlorine and certain heavy metals (such as Hg, Sb, Sn, Cr) from water without easily causing secondary pollution, making it commonly used in household water and drinking water purification processes.

‌Wastewater Treatment‌: The main advantages of activated carbon in wastewater treatment are high treatment efficiency and stable effluent quality. When used in combination with other methods, it can achieve very high effluent quality, even meeting drinking water standards. The use of biological activated carbon for treating low-concentration methanol wastewater, leveraging the adsorption of activated carbon and the degradation of the biofilm, results in significantly better treatment effects compared to resins and activated carbon adsorption alone. Studies have found that modifying activated carbon with nitric acid and then loading it with copper nitrate for secondary activation to prepare high-performance activated carbon can further enhance the catalytic performance of copper nitrate. Activated carbon also has special effects when used alone. Studies have shown that activated carbon is a promising alternative for catalytic wet oxidation of phenol-containing wastewater that cannot be treated by traditional wastewater treatment plants. Activated carbon without any metal loading exhibits the highest phenol conversion capacity. Research indicates that low-ash activated carbon has a catalytic effect in the wet oxidation of phenol. Currently, scientists are focusing on finding suitable conditions for phenol oxidation to avoid carbon consumption‌.

Various harmful gaseous pollutants can be mainly divided into: (1) Acidic gaseous pollutants, such as SO2, HCl, NO; (2) Volatile organic compounds (VOCs); (3) Greenhouse gases, such as CO2, CH4; (4) Ozone-depleting substances, such as CFCs, etc. ‌

Non-equilibrium plasma technology is one of the effective control technologies for eliminating gaseous pollutants.‌ Many research results have shown that this technology has great potential for the removal of gaseous pollutants due to its high chemical activity, which is suitable for the destruction and decomposition of gaseous pollutants. In recent years, many studies have focused on the removal of SO2, NO, VOCs, etc. from gases using non-equilibrium plasma. These studies have concentrated on two aspects: one is the energy efficiency of the process, and the other is the distribution of reaction products or by-products. 

Generally, studies have shown that the input power of the plasma has the greatest impact on the conversion efficiency. As the input power increases, the degradation rate of pollutants increases. The concentration of polluted gas also has a certain impact on the conversion efficiency. Non-equilibrium plasma is usually used to remove harmful gases with high flow rates and low concentrations (<0.1%). In these processes, highly chemically active particles generated by the carrier gas collide and react with pollutant molecules. As the pollutant concentration increases, the carrier gas concentration correspondingly decreases, and the conversion rate also drops sharply. In addition, since the diluent gas occupies a significant volume in the discharge system and generates a large number of high-energy electrons and active particles during the discharge process, such as N', OH, O(p), and other active radicals, these particles directly or indirectly participate in chemical reactions. The type of diluent gas not only directly affects the decomposition efficiency of pollutants but also influences the generation and distribution of reaction by-products. 

In the process of using plasma to degrade pollutants, besides focusing on the degradation efficiency of pollutants by plasma, it is also necessary to investigate the degradation products after the plasma degradation process of pollutants. Due to the uncontrollable selectivity of certain by-products in the plasma degradation process, some harmful products may be generated. For example, during the degradation of CFCIBr by radio frequency power discharge plasma, it was found that CF2O, Br2, Cl2, and other substances with greater toxicity than the original substance were generated in the products. One way to control the generation of by-products is to change the composition of the carrier gas. For example, controlling the oxygen content in the gas within 3% can control the generation of NO2, and increasing the reaction power can reduce the organic by-products generated during the reaction. The degradation of many pollutants not only depends on the input power but also on the residence time of the pollutants in the plasma. Therefore, the pollutants can be degraded by lowering the voltage and increasing the residence time, thereby optimizing the process. 

Due to the problems of treatment efficiency and by-product generation in plasma pollution control technologies, many attempts have been made to overcome these deficiencies. One of them is to combine plasma technology with catalytic technology to more effectively improve energy efficiency and control the generation of by-products, eliminating or reducing the post-treatment process of the adsorption method. 

Reactors that combine catalytic technology with plasma technology can be divided into two categories: one-step process reactors and two-step process reactors. In the one-step process reactor, the catalyst is placed in the reactor and activated by the plasma to generate photons. Usually, the temperature of this process is lower than the normal thermal catalytic reaction temperature. Therefore, the one-step process is also called the plasma-driven catalytic process. In the two-step process reactor, while the plasma degrades pollutants, it also generates ozone to promote the catalytic reaction, which is also known as the plasma-enhanced or plasma-assisted catalytic process. Another attempt is to improve the discharge mode, including the reactor structure and discharge parameters, such as discharge frequency, voltage waveform, etc.

Although low-temperature plasma is increasingly being used in the field of pollutant treatment to purify air, it is a complex system composed of various particles. There are multiple physical and chemical processes within it and between the plasma and the solid surface, and it is easily affected by various external and self-generated fields (electric field, magnetic field, electromagnetic field, optical field). There are many factors influencing low-temperature plasma in air purification, and the parameter range is large. Therefore, further research is needed on the mechanism and application of low-temperature plasma‌.

‌Wastewater treatment methods can be categorized into four types based on their functions: physical treatment, chemical treatment, physicochemical treatment, and biological treatment.‌

‌Physical Treatment‌: This method utilizes physical processes to separate and recover insoluble suspended pollutants (including oil films and droplets) from wastewater. Commonly used techniques include gravity separation, centrifugal separation, filtration, etc. Physical treatment methods are simple and effective for removing floating matter, suspended solids, and oil from wastewater, while also recovering useful substances‌.

‌Chemical Treatment‌: This involves adding certain chemicals to the wastewater to utilize chemical reactions for separating and recovering pollutants. Common methods include chemical precipitation, coagulation, neutralization, oxidation-reduction (including electrolysis), etc. Chemical treatment can convert toxic and harmful wastewater into non-toxic or low-toxicity water‌.

‌Physicochemical Treatment‌: This method employs physicochemical processes to remove pollutants from wastewater. Main techniques include adsorption, ion exchange, membrane separation, extraction, etc. Physicochemical treatment is effective for removing pollutants through a combination of physical and chemical processes‌.

‌Biological Treatment‌: This method utilizes the metabolic activities of microorganisms to convert organic pollutants in wastewater, which are in solution, colloidal, or finely suspended states, into stable and harmless substances. It can be divided into aerobic biological treatment and anaerobic biological treatment. Biological treatment is one of the most widely used and effective methods for wastewater treatment, especially for organic wastewater.

Water purification refers to the process of removing contaminants from raw water through specific procedures to achieve the effect of water purification, and the water can be used for different purposes. Most of the water is supplied for human consumption, while a small portion is used for other purposes such as medicine, pharmacology, chemistry, and industry. In most regions, disinfectants (traditionally chlorine or chloramines) are used for water disinfection. During the disinfection process, low levels of disinfection byproducts are generated through chemical reactions when disinfectants encounter organic matter in the water. These disinfection byproducts may potentially be carcinogenic. Water purification can remove disinfection byproducts as well as sand, suspended organic particles, parasites, Giardia lamblia, Cryptosporidium, bacteria, algae, viruses, fungi, minerals such as calcium, silica, and magnesium, and toxic metals such as lead, copper, and chromium, etc.‌

‌Seawater purification methods‌: The process of removing salt from seawater to obtain fresh water is called seawater desalination, also known as seawater desalting. Seawater desalination methods are mainly divided into two categories: (1) Extracting fresh water from seawater, including distillation, reverse osmosis, hydrate method, solvent extraction, and freezing method. (2) Removing salt from seawater, including electrodialysis, ion exchange, and pressure filtration. Currently, the first category of methods is predominantly used. As early as the 15th century, some ships used simple distillation devices to solve the problem of fresh water supply during long voyages. Since the 1950s, with the development of industry and agriculture and the increase in urban population, the supply of fresh water has gradually become tight, causing severe water shortages in some coastal cities. Therefore, improving seawater desalination technology has become one of the important ways to develop new water sources‌

‌Drinking water purification methods‌: Drinking water purification refers to the process of treating raw water (supply water) using relevant water treatment technologies or equipment based on the water quality indicators proposed for drinking water, effectively reducing or removing some or all contaminants in the water to obtain drinking water that meets the requirements (drinking water includes water for drinking, cooking, making soup, making ice, etc.). The effects of drinking water purification can be roughly divided into the following four categories:

If the goal is only to remove sediment and improve taste, water purifiers such as those with activated carbon, fiber, or other filter elements can be used. These water purifiers can only block sediment and adsorb some contaminants. These products require regular replacement of filter materials, otherwise, the deposition caused by blocked or adsorbed contaminants will contaminate the subsequent filtered water.

If the goal is only to remove water alkalinity, water purifiers with cation resin filter elements can solve the problem. Since resin can only adsorb calcium and magnesium ions in the water and has no purification effect on other contaminants, it is safer to boil such water before drinking.

If the goal is to remove sediment, rust, and organic contaminants while retaining substances like water alkalinity, water purifiers with hollow fiber filter elements can be used. Due to the small pore size of the hollow fiber filter material, it can block sediment and organic contaminants but does not remove hardness ions, so it can be considered by consumers who have requirements for minerals in the water.

If the goal is not only to remove water alkalinity but also to remove all contaminants, pure water machines with reverse osmosis membranes can solve the problem. Since the pore size of the reverse osmosis membrane is so small that only water molecules and minerals with diameters smaller than that of water molecules can pass through, other substances such as bacteria, viruses, scale, heavy metal ions, and organic contaminants are blocked and discharged during filtration. Because there are still a small amount of minerals, the water tastes sweet and can be consumed directly without boiling.

Activated carbon, a porous carbon-containing material with a highly developed pore structure, is an excellent adsorbent. Each gram of activated carbon has an adsorption area equivalent to that of eight tennis courts. The adsorption of activated carbon is achieved through both physical and chemical adsorption forces. Besides carbon, it also contains small amounts of hydrogen, nitrogen, oxygen, and ash. Its structure is formed by the accumulation of six-ring carbon compounds. The irregular arrangement of these six-ring carbons results in the characteristics of activated carbon with numerous micropores and a high surface area‌.

Activated carbon can be made from various carbon-containing materials, including wood, sawdust, coal, coke, peat, lignin, fruit pits, hard nut shells, sugar cane molasses, bones, lignite, and petroleum residues. Among them, coal and coconut shells are the most commonly used raw materials for the production of activated carbon. The manufacturing process of activated carbon is basically divided into two stages. The first stage includes dehydration and carbonization, where the raw material is heated and dried at a temperature of 170 to 600°C, carbonizing about 80% of the original organic matter. The second stage is the activation of the carbonized material, which is achieved by reacting it with an activating agent such as steam. In the endothermic reaction, a mixture of gases mainly composed of CO and H2 is produced, which is used to burn and heat the carbonized material to an appropriate temperature (800 to 1000°C) to burn off all decomposable substances. This process results in the development of a microporous structure and a huge specific surface area, thus imparting strong adsorption capacity‌.

The pores of activated carbon can be classified into three categories based on their size: macropores (radius of 1000-1000000A), transition pores (radius of 20-1000A), and micropores (radius below 20A). Activated carbon made from different raw materials has pores of different sizes. Activated carbon made from coconut shells has the smallest pore radius, while wooden activated carbon generally has the largest pore radius, which is used to adsorb larger molecules and is almost exclusively used in liquid phases. The first type of granular activated carbon used in urban water treatment is made from wood, known as charcoal. The pore size of coal-based activated carbon is between the two. Among coal-based activated carbons, lignite-based activated carbon has more transition pores and a larger average pore size than anthracite-based activated carbon, making it effective in removing large molecular organic compounds from water. Generally, the surface area of activated carbon used in water treatment does not need to be excessively large, but it should have more transition pores and a larger average pore size. Some activated carbons for liquid phases sold in the Japanese market have the following characteristics: a specific surface area of 850 to 1000m²/g, a pore volume of 0.88 to 1.5ml/g, and an average pore radius of 40 to 50A‌.

Compared with other adsorbents such as silica gel, zeolites, and activated clay, activated carbon has many unique features:

It has a well-developed pore structure and a large specific surface area. Activated carbon has a wide range of pore sizes and can adsorb various substances of different molecular sizes. It also has a large number of micropores, resulting in a large specific surface area and strong adsorption capacity. The activation method has a significant impact on the pore size of the produced activated carbon‌.

The surface properties of activated carbon vary depending on the activation conditions. Activated carbon activated by high-temperature steam has a surface rich in basic oxides, while activated carbon activated by zinc chloride has a surface rich in acidic oxides, which has a particularly strong adsorption capacity for basic compounds. The surface chemical properties, pore size distribution, and pore shape of activated carbon are the main reasons for its selective adsorption‌.

Catalytic properties. Activated carbon is used as a contact catalyst in various isomerization, polymerization, oxidation, and halogenation reactions. Its catalytic activity is due to the role of the carbon surface, surface compounds, and ash. In the chemical industry, activated carbon is often used as a catalyst carrier, where catalytically active substances are deposited on the activated carbon and used together as a catalyst. At this time, the role of activated carbon is not limited to carrying the activator; it also has a significant impact on the catalyst's activity, selectivity, and service life, exhibiting a co-catalytic effect‌.

Chemically stable and easy to regenerate. Activated carbon is chemically stable, resistant to acids and bases, and can be used within a wide range of pH values. It is insoluble in water and other solvents and can be used in aqueous solutions and many solvents. Activated carbon can withstand high temperatures and pressures and is often used as a catalyst or carrier in organic synthesis due to its catalytic activity. When activated carbon is no longer effective, it can be regenerated multiple times using various methods to restore its adsorption capacity and reuse it in production. If regenerated properly, it can reach its original adsorption level‌.

It is well known that formaldehyde has a significant impact on human health. As a highly volatile organic compound with a high level of pollution, formaldehyde is one of the main indoor air pollutants. It has a strong irritating effect on human mucous membranes and skin. When inhaled at high concentrations, it can cause severe respiratory irritation and edema, as well as eye irritation and headaches. Low-concentration exposure can lead to persistent headaches, weakness, insomnia, and other symptoms. Long-term exposure can result in dermatitis, abnormalities in lung and liver function, immune dysfunction, central nervous system effects, and even damage to intracellular genetic material, making it a carcinogen.

According to research, the main sources of human exposure to formaldehyde include:

Industrial emissions, automobile exhaust, and photochemical smog from the production or use of formaldehyde.

Building materials, indoor decoration materials, furniture, smoking, cooking oil fumes, and fuel combustion are the main sources of indoor formaldehyde pollution. For example, urea-formaldehyde insulating foam used as a thermal insulation material can release formaldehyde when it ages; artificial boards such as plywood and particleboard used in decoration and furniture making are often treated with formaldehyde for preservation and use formaldehyde-based adhesives, which can release formaldehyde when exposed to moisture and heat. Other decorative materials such as wallpaper, interior wall coatings, floor coverings, paints, and synthetic carpets can also release formaldehyde indoors.

Formaldehyde is released in hospitals for the disinfection of wards and equipment, as well as in anatomical and pathological laboratories for the fixation and preservation of bodies and tissue samples. Many dental materials used in oral treatments, such as polyformaldehyde-based root canal disinfectants, also release formaldehyde.

Clothing fabrics often contain formaldehyde. To achieve wrinkle resistance, shrink resistance, flame retardancy, or to maintain the durability of prints and dyes and improve texture, formaldehyde is added to the auxiliaries. The formaldehyde content in casual denim clothes, no-iron shirts, wrinkle-resistant cotton clothes, and children's clothing may exceed the standard.

Excessive formaldehyde concentrations in food.

Products soaked in formaldehyde have a beautiful appearance and are less prone to spoilage. For example, soaking aquatic products in formaldehyde can fix the shape of seafood and river fish and maintain their color. However, excessive formaldehyde in food is harmful to the human body.

Harmful gases released from waste incineration contain formaldehyde.

Formaldehyde is a highly toxic substance and ranks second on China's priority list of controlled toxic chemicals. It has been identified by the World Health Organization as a carcinogen and teratogen, is a recognized allergen, and is one of the potential strong mutagens. Studies have shown that formaldehyde has a strong carcinogenic and cancer-promoting effect. Its impact on human health mainly manifests as olfactory abnormalities, irritation, allergies, abnormal lung and liver function, immune dysfunction, and more. When its concentration reaches 0.06-0.07mg/m³ in the air, children may experience mild shortness of breath. At an indoor air concentration of 0.1mg/m³, there is an unpleasant odor and discomfort; at 0.5mg/m³, it can irritate the eyes and cause tearing; at 0.6mg/m³, it can cause throat discomfort or pain; at higher concentrations, it can cause nausea, vomiting, coughing, chest tightness, shortness of breath, and even pulmonary edema; at 30mg/m³, it can be immediately fatal.

Long-term exposure to low doses of formaldehyde can cause chronic respiratory diseases, nasopharyngeal cancer, colon cancer, brain tumors, menstrual disorders, genetic mutations in cell nuclei, DNA single-strand cross-links, DNA-protein cross-links, inhibition of DNA damage repair, pregnancy syndrome, chromosomal abnormalities in newborns, leukemia, and decreased memory and intelligence in adolescents. Among all exposed individuals, children and pregnant women are particularly sensitive to formaldehyde and are therefore at greater risk. According to relevant statistics, 69% of human diseases are related to the indoor environment. In China, 130,000 people die from indoor pollution each year, and more than 90% of pediatric leukemia patients developed the disease within a year of moving into a newly decorated home.

Exposure to formaldehyde and many other similar indoor air pollutants in the air can cause eye, nose, and throat irritation symptoms. Respiratory symptoms, including sore throat or streptococcal pharyngitis, may also occur. Additionally, symptoms similar to asthma may appear. Formaldehyde air pollution can cause difficulty breathing in infants, leading to crying throughout the night. The eyes may also be irritated, potentially causing a runny nose or nosebleeds. Other symptoms may include rashes, lethargy, dizziness, poor memory, depression, nausea, diarrhea, ear pain and ear infections, allergies, and numbness in the limbs or arms. Symptoms caused by formaldehyde may be similar to those caused by the common cold, flu, or other indoor air pollutants. It is important to be vigilant. If symptoms improve when you leave home or the workplace but return when you come back, it may indicate the presence of indoor air pollutants, such as formaldehyde, in these places. Check your surroundings. Have you changed seats, moved houses, or changed workplaces? Have you renovated or installed new cabinets, furniture, curtains, or small rugs? Your symptoms may be caused by the toxic gases emitted by these new items.

The principle of activated carbon in purifying air relies on its developed pore structure and surface area, which allow it to come into contact with the surrounding air to a great extent and passively absorb some pollutants into its pores. Therefore, the larger the specific surface area and the more developed the pore structure of activated carbon, the stronger its adsorption capacity.

In addition, there is a certain relationship between the pore size of activated carbon and the molecular size of the substances it can adsorb. Theoretical research has shown that the larger the molecular weight of harmful substances, the easier they are to be adsorbed by activated carbon. For example, the molecular weight of benzene is 78, while the molecular weight of formaldehyde is 30. Activated carbon has a stronger ability to adsorb benzene than formaldehyde. Therefore, in industrial production, activated carbon is often used as an adsorbent for benzene compounds.

Similar materials include bamboo charcoal, wooden charcoal, and nano-activated minerals, which can also be used to adsorb harmful gases such as formaldehyde, TVOC, and benzene. The method of using activated carbon to remove formaldehyde is known as physical adsorption in the industry, distinguished from chemical neutralization treatments such as gas-phase catalytic oxidation.

As an excellent physical adsorbent, activated carbon is increasingly popular. High-efficiency and environmentally friendly activated carbon packs can adsorb formaldehyde, ammonia, benzene, xylene, radon, and other indoor harmful gas molecules in the air, quickly eliminate decoration odors, and uniformly regulate spatial humidity.