What Are the Different Uses of Activated Charcoal?

Activated carbon is a kind of specially treated carbon. It heats organic raw materials (husk, coal, wood, etc.) in the air to reduce non-carbon components (this process is called carbonization), and then reacts with the gas, and the surface is Erosion produces microporous structures (this process is called activation). Because the activation process is a microscopic process, that is, a large amount of molecular carbide surface erosion is point-like erosion, so there are numerous fine pores on the surface of activated carbon. The diameter of micropores on the surface of activated carbon is mostly between 2 and 50 nm. Even a small amount of activated carbon has a huge surface area. The surface area per gram of activated carbon is 500 to 1500 m2. All applications of activated carbon are based on this characteristic of activated carbon. ^[1]

Chinese name: Activated carbon
Foreign name: active carbon
Character: Powdery or granular porous amorphous carbon

Features: Developed microporous structure, large specific surface area and adsorption activity
Application: Wastewater treatment, electrodes, flue gas treatment, etc.
Preparation: Chemical activation, physical activation, etc.
Regeneration method: Thermal regeneration method, electrochemical regeneration method, etc.

Introduction of activated carbon

Activated carbon is prepared by pyrolysis and activation processing of carbon-containing raw materials such as wood, coal, and petroleum coke. It has a developed pore structure, a large specific surface area, and rich surface chemical groups, and has a strong specific adsorption capacity. Collective name for carbon materials. ^[2]

It is usually powdery or granular porous amorphous carbon with strong adsorption capacity. Carbonized solids (such as coal, wood, hard husks, nuts, resins, etc.) are carbonized at a temperature of 600 to 900 ° C under the condition of isolated air, and then air, carbon dioxide, water vapor or It is obtained after the mixed gas of the three is oxidized and activated. ^[3]

Carbonization volatilizes substances other than carbon, and oxidative activation can further remove the remaining volatile substances, generate new and expand the original pores, improve the microporous structure, and increase activity. Activated carbon at low temperature (400 ° C) is called L-carbon, and activated carbon at high temperature (900 ° C) is called H-carbon. H-char must be cooled in an inert atmosphere, otherwise it will be converted to L-char. The adsorption performance of activated carbon is related to the chemical properties of the gas and its concentration, activation temperature, degree of activation, composition and content of inorganic substances in activated carbon, etc., which mainly depends on the nature of the activated gas and the activation temperature.

The carbon content, specific surface area, ash content of the activated carbon and the pH value of its aqueous suspension all increase with the increase of the activation temperature. The higher the activation temperature, the more complete the remaining volatile substances are volatilized, the more developed the microporous structure, and the larger the specific surface area and adsorption activity. ^[3]

The composition and content of ash in activated carbon have a great influence on the adsorption activity of carbon. The ash content is mainly composed of K ₂ O, Na ₂ O, CaO, MgO, Fe ₂ O ₃ , Al ₂ O ₃ , P2O ₅ , SO ₃ , Cl ^- and so on. The ash content is related to the raw materials for making activated carbon. The removal of volatiles in the carbon increases the ash content in the charcoal. ^[3]

As of 2007, the annual output of activated carbon in the world has reached 900kt, of which coal-based (quality) activated carbon accounts for more than 2/3 of the total output; while China's annual output has exceeded 400kt, ranking first in the world, and the United States, Japan and other major active carbon producing countries . ^[4]

Physical and Chemical Properties of Activated Carbon

According to the shape of activated carbon, it is usually divided into two categories: powdery and granular. Granular activated carbon includes cylindrical, spherical, hollow cylindrical and hollow spherical, and irregular shaped broken carbon. With the development of modern industry and science and technology, many new varieties of activated carbon have appeared, such as carbon molecular sieves, microsphere carbon, activated carbon nanotubes, activated carbon fibers, and so on. ^[5]

Activated carbon pore structure

Activated carbon is composed of graphite microcrystals, single planar network carbon and amorphous carbon. The graphite microcrystals are the main part of the activated carbon. The microcrystalline structure of activated carbon is different from that of graphite. The interlayer spacing of the microcrystalline structure is between 0.34 and 0.35 nm, and the gap is large. It is difficult to convert into graphite even at a temperature as high as 2000 . This microcrystalline structure is called non-graphite microcrystal. Most activated carbon belongs to non-graphite structure. The crystal structure of the graphite structure is more regular and can be converted into graphite after processing. The non-graphite microcrystalline structure makes the activated carbon have a developed pore structure, and the pore structure can be characterized by the pore size distribution. The pore size distribution of activated carbon is wide, from less than 1nm to thousands of nm. Some scholars have proposed to divide the pore size of activated carbon into three types: micropores with a pore diameter of less than 2nm, mesopores with a pore diameter of 2-50nm, and macropores with a pore diameter of more than 50nm. ^[5]

The specific surface area of micropores in activated carbon accounts for more than 95% of the specific surface area of activated carbon, which largely determines the adsorption capacity of activated carbon. The mesoporous specific surface area accounts for about 5% of the specific surface area of activated carbon. It is the adsorption site of larger molecules that cannot enter the micropores, and capillary condensation occurs under relatively high pressure. The large pore specific surface area generally does not exceed 0.5 m ² / g, and it is only the channel for the adsorbate molecules to reach the micro and mesopores, which has little effect on the adsorption process. ^[5]

Surface chemical properties of activated carbon

The activated carbon has a crystal structure and a pore structure inside, and the surface of the activated carbon also has a certain chemical structure. The adsorption performance of activated carbon depends not only on the physical (pore) structure of the activated carbon, but also on the chemical structure of the activated carbon surface. During the preparation of activated carbon, the edge chemical bonds of the aromatic sheet formed during the carbonization stage are broken to form edge carbon atoms with unpaired electrons. These marginal carbon atoms have unsaturated chemical bonds and can react with heterocyclic atoms such as oxygen, hydrogen, nitrogen and sulfur to form different surface groups. The presence of these surface groups will undoubtedly affect the adsorption performance of activated carbon. X-ray studies have shown that these heterocyclic atoms and carbon atoms bind to the edges of the aromatic sheet, producing oxygen-, hydrogen-, and nitrogen-containing surface compounds. When these edges become the main adsorption surface, these surface compounds change the surface characteristics and surface properties of the activated carbon. The surface groups of activated carbon are divided into three types: acidic, basic and neutral. The acidic surface functional groups are carbonyl, carboxyl, lactone, hydroxyl, ether, phenol, etc., which can promote the adsorption of alkaline substances by activated carbon; the basic surface functional groups are pyranone (cycloketone) and its derivatives, which can promote activated carbon Adsorption of acidic substances. ^[5]

The surface of activated carbon prepared by acidic activators such as phosphoric acid is mainly acidic groups, and it has good adsorption on alkaline substances; the surface of activated carbon prepared by alkaline activators such as KOH, K ₂ CO ₃ is mainly basic groups, which is suitable for adsorption Acidic materials; and the activated carbon surface functional groups prepared by physical activation methods such as CO ₂ and H ₂ O are generally neutral. ^[5]

Activated carbon adsorption mechanism

Activated carbon adsorption refers to the use of the solid surface of activated carbon to adsorb one or more substances in water to achieve the purpose of purifying water. The adsorption capacity of activated carbon is related to the pore size and structure of activated carbon. In general, the smaller the particles, the faster the pore diffusion rate, and the stronger the adsorption capacity of activated carbon.
Adsorption capacity and velocity are the main indicators to measure the adsorption process. The amount of adsorption capacity is measured by the amount of adsorption, and the rate of adsorption refers to the amount of adsorbed by the adsorbent per unit weight per unit time. In water treatment, the adsorption speed determines the contact time between the adsorbent and the sewage. ^[6]

The main occurrence of activated carbon is physical adsorption, most of which are single-layer molecular adsorption. The adsorption amount and the concentration of the adsorbed substance obey the Langermuir monolayer adsorption isotherm equation:

In the formula:

(Coverage)-the fraction of the total surface area of the adsorbed molecules on the solid surface at a certain temperature;

-The partial pressure of the adsorbate in the gas phase;

-The ratio of adsorption and desorption speeds;

Adsorption of gas on solid surface. ^[6]

Classification and naming of activated carbon

Chinese national standards categorize activated carbon according to two parts: one is based on the main raw materials used in manufacturing, and the other is based on the raw materials used in manufacturing and the corresponding product shape combinations.
Activated carbon is divided into four categories according to the main raw materials used in manufacture: coal-based activated carbon, wood activated carbon, synthetic material activated carbon, and other types of activated carbon. It is divided into 16 types according to the combination of the main raw materials used for manufacturing and the corresponding product shape. Among them, coal-based activated carbon is divided into: columnar coal-based granular activated carbon, crushed coal-based granular activated carbon, powdered coal-based granular activated carbon, and spherical coal-based granular activated carbon. Wooden particle activated carbon is divided into: columnar wooden particle activated carbon, broken wooden particle activated carbon, powdery wooden particle activated carbon, spherical wooden particle activated carbon. Synthetic material activated carbon is divided into: columnar synthetic material granular activated carbon, crushed synthetic material granular activated carbon, powdery synthetic material granular activated carbon, shaped activated carbon, spherical synthetic material granular activated carbon, cloth synthetic material activated carbon (carbon fiber cloth), felt synthetic material Activated carbon (carbon fiber felt). Other types of activated carbon refer to activated carbon prepared from other raw materials (such as coal pitch, petroleum coke, etc.) in addition to the above three types of activated carbon. This type of activated carbon is temporarily listed in the product shape classification of pitch-based microsphere activated carbon. The detailed classification is shown in the following table ^[1] :

Classification of manufacturing raw materials	Product shape classification
Coal activated carbon	Columnar Coal Granular Activated Carbon
	Crushed coal granular activated carbon
	Powdered coal granular activated carbon
	Spherical coal granular activated carbon
Wooden activated carbon	Cylindrical wood granular activated carbon
	Broken wood granulated activated carbon
	Powdery wood granular activated carbon
	Activated carbon with spherical wood particles
Synthetic material activated carbon	Granular activated carbon
	Broken synthetic material granular activated carbon
	Powdered synthetic material granular activated carbon
	Shaped activated carbon
	Spherical synthetic material granular activated carbon
	Cloth synthetic material activated carbon (carbon fiber cloth)
	Felt Synthetic Material Activated Carbon (Carbon Fiber Felt)
Other types of activated carbon	Pitch-based microsphere activated carbon

Nomenclature of activated carbon

Activated carbon is named after its material and shape. The naming method is based on the content stipulated by the naming principle. There are three levels of content: the first layer represents the main raw materials for the manufacture of activated carbon, which is capitalized by the first letter of the English word of the main raw material; the second layer represents the shape of the activated carbon, and the first word of the shape English word The letters are capitalized; the third layer is the name of the activated carbon, which consists of Chinese characters. ^[1]

Active carbon raw material classification symbol

The classification symbols for the names of the raw materials for the manufacture of activated carbon are capitalized by the first letter of the English word of the material name. If the first letter of the name is repeated, a lowercase English letter is suffixed to the first letter of the English word, which is derived from the English word of the material name (consonant priority). In the classification of manufacturing raw materials, because there are many types of processing raw materials that belong to wood activated carbon, and the properties of activated carbon after the manufacture of various wood raw materials have certain differences, the raw materials for manufacturing wood activated carbon are divided into four categories: wood chips Activated carbon, nutshell activated carbon, coconut shell activated carbon, biomass activated carbon. The classification symbols of these four types of wood activated carbon are collectively represented by the capitalization of the first letter of the English word of the raw material classification symbol (W) and its specific raw materials (sawdust, nut shell, coconut shell, biomass). For the classification symbols, please refer to the Chinese National Standard GB / T 32560-2016 "Classification and Naming of Activated Carbon" issued in 2016. ^[1]

Active carbon shape classification symbol

The classification symbols of various shapes of activated carbon are expressed by capitalizing the first letter of the English word of the shape name. If the first letter of the shape name is repeated, a lowercase English letter is suffixed to the first letter of the English word, the letter is derived from the English word of the shape (consonant priority) ). For broken activated carbon, in addition to woody broken activated carbon, there are three types of coal-based broken activated carbon. These three types of broken coal-shaped activated carbon have different production processes, and there are also large differences in quality indicators and application fields. In order to facilitate manufacturers and applications The customer distinguishes the broken coal-based activated carbon. The standard specifies the following classification rules for the shape of the broken activated carbon: The shape classification symbol of the broken activated carbon is represented by the initial letter of the English word of G and the specific types of broken activated carbon. At the lower foot mark of G, they are collectively expressed, for example, briquette crushed activated carbon (coal quality) is expressed as GB ^[1] . For the specific classification of shape names, please refer to the Chinese National Standard GB / T 32560-2016 "Classification and Naming of Activated Carbon" published in 2016 ^[2] .

Activated carbon preparation technology

Activated carbon chemical activation

The chemical activation method is to prepare activated carbon by uniformly mixing various carbon-containing raw materials with chemicals and undergoing carbonization, activation, recycling of chemicals, rinsing, and drying at a certain temperature. Phosphoric acid, zinc chloride, potassium hydroxide, sodium hydroxide, sulfuric acid, potassium carbonate, polyphosphoric acid, and phosphate esters can all be used as activating agents. Although different chemical reactions occur, some have erosion, hydrolysis or dehydration effects on the raw materials. Some play an oxidation role, but these chemicals can promote the activation of raw materials to a certain extent. Among them, the most commonly used activators are phosphoric acid, zinc chloride and potassium hydroxide. The activation principle of the chemical activation method is not very clear. It is generally believed that chemical activators have the effect of eroding and dissolving cellulose, and can decompose and separate hydrogen and oxygen contained in the hydrocarbons in the raw materials, such as H ₂ O, CH ₄ and the like. Small molecule forms escape, creating a lot of pores. In addition, the chemical activator can inhibit the formation of tar by-products and prevent tar from blocking the pores generated during the pyrolysis process, thereby improving the yield of activated carbon. ^[2]

China's lignophosphoric acid powder activated carbon has achieved large-scale, automated and clean production, and the overall technology has reached the international leading level. ^[2]

(1) Phosphoric acid activation method

In the process of preparing activated carbon by the phosphoric acid method, the action mechanism of phosphoric acid and lignocellulosic raw materials can be divided into the following aspects: swelling, accelerated activation, dehydration, oxidation, and aromatic condensation. ^[2]

The basic process of the phosphoric acid activation method includes the steps of screening wood chips, drying, preparing phosphoric acid solution, mixing (or impregnating), carbonizing, activating, recycling, rinsing (including acid treatment and water washing), centrifugal dehydration, drying and milling, etc. Activated carbon also needs to increase the kneading process. In addition, a special exhaust gas purification system is attached to recover phosphoric acid and carbon powder in the flue gas to reduce environmental pollution. In the production process of the phosphoric acid activation method, it is necessary to pay attention to the degree of control in the carbonization section, allowing the phosphoric acid to fully penetrate into the wood chips, and then coordinated control with the activation section, which can significantly improve the activated carbon adsorption capacity and stable product quality. advantageous. The multi-stage liquid phase recovery of carbon activated tail gas can increase the recovery of phosphoric acid and fine carbon powder, and the high-voltage electrostatic method is also beneficial to the removal of tar in the tail gas. ^[2]

(2) Zinc chloride activation method

During the activation process of ZnCl _2, the lignocellulosic raw material undergoes a dehydrogenation reaction and is further aromatized, thereby forming a preliminary pore structure. After the zinc chloride is eluted by water, a pore structure is formed. In addition, some scholars believe that zinc chloride forms a skeleton of nascent carbon deposits during carbonization. After it is washed away, the surface of the carbon is exposed, forming the inner surface of activated carbon with adsorption force. ^[2]

The zinc chloride activation process is basically similar to the phosphoric acid activation process. Zinc chloride activated carbon has been favored by domestic and foreign markets due to its relatively concentrated pore size distribution and strong adsorption force, and its demand has increased year by year. ^[2]

(3) potassium hydroxide activation method

KOH activation method is a kind of activation process for the preparation of activated carbon with high specific surface area. The activation process is to mix raw material carbon with KOH or NaOH with several times the mass of carbon, and then dehydrate it at 500 at 800 . Calcined for several times, and after cooling, the product is washed to be neutral to obtain activated carbon. The reaction mechanism is that the carbon consumed in the activation process mainly generates potassium carbonate, and at about 800 ° C, the potassium metal (boiling point 762 ° C) reduced by the carbon is precipitated, and the vapor of the metal potassium continuously enters the layer composed of carbon atoms. Activated between these two reactions, the product has a large specific surface area. ^[2]

KOH activated carbon is mainly used in the field of supercapacitors. The specific surface area of the activated carbon prepared with coconut shell as the main raw material can approach 3000m ² / g, and the specific capacitance can exceed 200F / g. At the same time, it can also show very good hydrogen and methane storage capacity. Under the conditions of 77K and 100kPa The hydrogen storage capacity can reach 2.94%, the pressure can be increased to 1MPa, and the hydrogen storage capacity can reach 4.82%. ^[2]

Activated carbon physical activation method

The physical method is also commonly called the gas activation method. It is the process of carbonizing raw materials with water vapor, flue gas (mixed gas of water vapor, CO ₂ , N ₂ etc.), CO or air at a high temperature of 800 to 1000 ° C. The process of activating reaction by contacting the activating gas. The basic process of the physical activation method mainly includes carbonization, activation, impurity removal, crushing (ball milling), refining and other processes. The preparation process is clean and the liquid phase is less polluted. ^[2]

During the preparation process, the disordered carbon atoms and heteroatoms of the oxidizing high-temperature activated gas first react to open the originally closed pores, and then the surface of the basic crystallite is exposed, and then the activation gas continues with the carbon atoms on the surface of the basic crystallite. An oxidation reaction occurs, which continuously expands the pores. Some unstable carbons generate CO, CO ₂ , H ₂ and other carbon compound gases due to gasification, thereby generating new pores, at the same time, tar and uncarbonized materials are also removed, and finally activated carbon products are obtained. The developed specific surface area of activated carbon is derived from the increase in the pore volume of mesopores and macropores, and the formed macropores, mesopores, and micropores are interconnected. Because the physical method process is relatively simple, the waste gas produced is mainly CO ₂ and water vapor, which has less environmental pollution, and the final activated carbon product has a high specific surface area, a developed pore structure, and a wide range of applications. More than 70% of the manufacturers use physical methods to produce activated carbon. A large amount of waste heat is generated during the carbon activation process, which can meet the required heat energy for raw material drying, high-temperature steam from waste heat boilers, and washing and drying of products. ^[2]

- Activated carbon physical-chemical activation method

(1) Physical-chemical integrated preparation technology

The physical-chemical activation method, as its name implies, is a combination of physical and chemical activation methods, that is, carbon is first treated by chemical method, and then further activated by physical method (water vapor or CO ₂ ). Foreign researchers have prepared a super activated carbon with a specific surface area of up to 3700m ² / g by a combined activation method of H ₃ PO ₄ and CO _{2. The} specific steps are to soak the wood raw materials with H ₃ PO ₄ at 85 ° C, and then carbonize for 4h at 450 ° C. Activated with CO ₂ . The physical method and the chemical method are combined, and the carbonized tail gas of the physical method is used to supply heat for chemical production, so that no coal combustion is consumed in the production process, and physical activated carbon and chemical activated carbon are obtained at the same time. ^[2]

(2) Microwave-assisted chemical activation

Due to the disadvantages of labor-consuming, time-consuming and uneven heating of materials in the traditional furnace heating during the preparation of activated carbon, the introduction of microwaves can achieve uniform heating inside the materials, and at the same time can be started and stopped quickly and conveniently. Much shorter. Therefore, microwave-assisted chemical activation can significantly shorten production time, thereby greatly improving production efficiency and reducing environmental pollution. The ordinary phosphoric acid method, zinc chloride method, and potassium hydroxide activation method can all adopt microwave heating, and research shows that microwave heating method can also obtain high-performance activated carbon, especially suitable for KOH activation method to prepare supercapacitor activated carbon. However, the preparation of activated carbon by microwave heating is still in the experimental stage, mainly due to the large equipment investment and high energy consumption. ^[2]

(3) Catalytic activation

Metal-based catalysts can form active sites on the surface of carbon-containing raw materials, reducing the activation energy of the reaction between carbon and water or CO ₂ , thereby reducing the activation temperature, increasing the reaction rate, and forming well-developed pores. At the same time, pores will also be generated when metal particles move. The catalyst can reduce the activation temperature and greatly increase the reaction rate when preparing super activated carbon, and can also make the pore size distribution of the prepared activated carbon uniform. Although there are many advantages mentioned above for the preparation of activated carbon by catalytic activation method, too fast reaction speed may burn through the microporous wall surface, thereby destroying the microporous structure. ^[2]

Activated carbon application

A brief history of activated carbon applications

(1) Brief History of Foreign Applications

In about 3750 BC, charcoal was recorded in ancient Egypt. ^[4]

In 1900, the British first invented a method of carbonizing plants with metal chlorides to make activated carbon. ^[4]

In 1917 both sides of the First World War used activated carbon in gas masks. ^[4]

A foul odor accident occurred in the Chicago Waterworks in 1927. Since then, activated carbon has been widely used in tap water deodorization. ^[4]

In 1930, the first water plant using granular activated carbon adsorption tank to deodorize was built in Philadelphia, USA. ^[4]

In the late 1960s and early 1970s, due to the advent of large-scale production and regeneration equipment for coal-like carbon particles, developed countries carried out research on the use of activated carbon to remove trace organic matter in water to further treat drinking water. Granular activated carbon purification devices have been completed and put into operation in the United States, Europe, Japan and other countries. More than 90% of water plants in the United States that use surface water as the source have adopted activated carbon adsorption processes. ^[7]

(2) Brief history of domestic applications

In the early 1950s, China began to produce activated carbon. ^[4]

In the late 1960s, activated carbon was used to remove odor and odor from polluted source water. ^[7]

Activated carbon is mainly used as a solid adsorbent, and is used in chemical, pharmaceutical, and environmental aspects to adsorb substances with higher boiling points and critical temperatures and organic substances with larger molecular weights. Applications in the fields of air purification and water treatment have also shown an increase in the amount of application. Special high-end carbons such as high specific surface area carbon, high benzene carbon, and fiber carbon have penetrated into aerospace, electronics, communications, energy, biological engineering and life sciences. field. ^[7]

Application fields of activated carbon

(1) Treatment of oily sewage

Adsorption method for oil-water separation uses lipophilic materials to adsorb dissolved oil and other dissolved organic matter in wastewater. The most commonly used oil-absorbing material is activated carbon, which can absorb dispersed oil, emulsified oil and dissolved oil in wastewater. Due to the limited adsorption capacity of activated carbon for oil (generally 30 to 80 mg / g)), the cost is high and regeneration is difficult. Usually it is only used as the last stage of multi-stage treatment of oily wastewater. The effluent oil concentration can be reduced to 0.1 to 0.2 mg / L. ^[6]

Because activated carbon has high requirements for water pretreatment and the price of activated carbon is high, in wastewater treatment, activated carbon is mainly used to remove trace pollutants in wastewater to achieve the purpose of deep purification. The oily wastewater of the refinery is first treated with oil barrier, air flotation and biological treatment, and then further treated by sand filtration and activated carbon filtration. The phenol content in the wastewater decreased from 0.1 mg / L (after biological treatment) to 0.005 mg / L, the cyanide content decreased from 0.19 mg / L to 0.048 mg / L, and the COD decreased from 85 mg / L to 18 mg / L. ^[6]

(2) Treatment of dye wastewater

Dye wastewater has complex components, large changes in water quality, deep chroma, and high concentration, making it difficult to handle. Treatment methods include oxidation, adsorption, membrane separation, flocculation, and biodegradation. These methods have their own advantages and disadvantages, among which activated carbon can effectively remove the chroma and COD of wastewater. Active carbon treatment of dye wastewater has been studied at home and abroad, but most of them are coupled with other processes. Active carbon adsorption is mostly used for advanced treatment or activated carbon as a carrier and catalyst, and there are few studies on the use of activated carbon alone to treat higher concentration of dye wastewater. ^[6]

Activated carbon has a good decoloring effect on dye wastewater. The decolorization rate of dye wastewater increased with the increase of temperature, but the pH value did not have much influence on the decolorization effect of dye wastewater. Under the optimal adsorption process conditions, both the decolorization rate of acid fuchsin and alkaline fuchsin wastewater are> 97%, the chromaticity dilution factor of the effluent is 50 times, and the COD <50 mg / L, which meets the national first-level emission standard. ^[6]

(3) Treatment of mercury-containing wastewater

Among the heavy metal pollutants, mercury is the most toxic. When mercury enters the body, it will destroy the functions of enzymes and other proteins and affect its resynthesis. Activated carbon has the ability to adsorb mercury and mercury-containing compounds, but its adsorption capacity is limited, which is only suitable for treating wastewater with low mercury content. If the concentration of mercury is high, it can be treated by chemical precipitation method. After treatment, the mercury content is about 1mg / L, and it can reach 2 ~ 3mg / L when it is high, and then activated carbon is used for further processing. ^[6]

(4) Treatment of chromium-containing wastewater

There are a large number of oxygen-containing groups on the surface of activated carbon, such as hydroxyl (-OH), carboxyl (-COOH), etc. They all have electrostatic adsorption function, produce chemical adsorption on hexavalent chromium, and can effectively adsorb hexavalent chromium in wastewater. The adsorbed wastewater can meet national discharge standards. ^[6]

The use of activated carbon to treat chromium-containing wastewater is the result of the combined effects of physical adsorption, chemical adsorption, and chemical reduction of activated carbon on hexavalent chromium in solution. Activated carbon treats chromium-containing wastewater with stable adsorption performance, high treatment efficiency, low operating cost, and certain social and economic benefits. Therefore, the treatment of chromium-containing wastewater with activated carbon has been widely used. ^[6]

(5) Catalysis and supported catalyst

Graphitized carbon and amorphous carbon are components of the crystalline form of activated carbon, and because they have unsaturated bonds, they exhibit functions similar to crystal defects. Activated carbon is widely used as a catalyst because of the existence of crystal defects. At the same time, because of its large specific surface area and porous structure, activated carbon is also widely used as a catalyst support. ^[8]

The commercial activated carbon is treated with gamma rays. This process can change the surface chemical properties of the activated carbon without affecting the physical properties of the activated carbon. The role of activated carbon surface chemistry in photocatalysis was studied by ultraviolet radiation and simulated solar radiation. The results show that activated carbon can play a photocatalytic role whether it is ultraviolet or simulated solar radiation. The determination of hydroxyl radicals and superoxide anion radicals in UV / activated carbon and simulated sunlight / activated carbon systems shows that the use of activated carbon as a photocatalyst and light-induced reactant can effectively eliminate the effects of impurities on the reaction. The hydroxyl radicals and superoxide in the system Anion radicals are obtained much higher than using light radiation alone. This provides new possibilities for developing free radical chemistry and finding new free radical reactions. ^[8]

The activated sludge has a slow composition due to its complex composition. Some scholars have used granular activated carbon for anaerobic decay of activated sludge, which has increased the methane yield by 17.4% during the activated sludge decay process, while increasing the activated sludge decay rate by 6.1%. In addition, the introduction of -SO ₃ H on the surface of activated carbon has a catalytic effect on the synthesis of methyl tert-amyl ether. The catalyst is easy to prepare, has high catalytic activity and is not easy to decompose, reflecting the great application potential of modified activated carbon catalysts. Studies have shown that the use of granular activated carbon-loaded ozone system enables the catalytic oxidation rate of humic acid to reach 48.1%, which provides a new way for the degradation of humic acid. The activated carbon-supported alumina was used as the modified activated carbon paste electrode for the study of the electrocatalytic oxidation of phenol. It showed good stability and reusability, and had a relatively low detection limit and a wide detection range. ^[8]

(6) Clinical medicine

Activated carbon can be used for acute clinical gastrointestinal detoxification due to its good adsorption performance. It has the advantages of not being absorbed by the gastrointestinal tract, non-irritating, direct oral administration, simple and convenient; meanwhile, activated carbon is also used for blood purification and cancer Treatment, etc. Colorectal cancer is a common malignant tumor. Studies have shown that the use of nano-active carbon as a tracer can effectively increase the number of lymph node detection in colorectal cancer patients. Activated carbon fiber has two characteristics: one is adsorption performance; the other is far-infrared radioactive energy. The silver was adsorbed on activated carbon fibers and used to treat patients with chronic wounds. There were no adverse reactions in the wound within months of treatment. Some scholars used coconut shell activated carbon as a carrier to support gatifloxacin. The results show that it has a good load capacity for gatifloxacin and can be used as a slow-release carrier for gatifloxacin. Studies on the use of paracetamol and ibuprofen as model drugs and activated carbon as a drug carrier have shown that activated carbon particles exhibit very low cytotoxicity. This study provides support for activated carbon as an amorphous drug carrier. Some scholars simply use the local rectal injection of highly active granular activated carbon twice a day to treat simple chronic anal fistulas. The results show that this treatment is effective and safe, and it is easier for patients to accept than other treatments. The cure for chronic anal fistula offers new strategies. ^[8]

(7) for super capacitor electrodes

Supercapacitors are mainly composed of electrode active materials, electrolytes, current collectors, and separators. The electrode materials directly determine the performance of the capacitor. Activated carbon has the advantages of large specific surface area, developed pores, and easy preparation, and has become the earliest carbonaceous electrode material for supercapacitors. New and high-performance activated carbon electrode materials can be prepared by modifying traditional activated carbon. Using polyvinylidene chloride as a precursor, porous carbon with a specific surface area of 1200 m ² · g ^-1 and a pore volume of 0.48 cm ³ · g ^-1 was prepared through carbonization treatment without other post-treatment. The highest specific capacitance is 262F · g ^-1 , electrode density is about 0.8g · cm ^-3 , volume specific capacitance can reach 214F · cm ^-3 , it is a promising supercapacitor electrode material. In addition, carbonization of waste tea leaves was followed by activation with KOH to prepare activated carbon with amorphous characteristics, which has a porous structure with a specific surface area between 2245 and 2184 m ² · g ^-1 . It was used as a supercapacitor electrode and KOH aqueous solution was used. As an electrolytic solution, the specific capacitance is as high as 330F · g ^-1 . After 2000 charges and discharges, the capacitance slightly decreases, which is 92% of the initial capacitance, showing good cycle performance. If lotus pollen is used as a carbon source and a self-template, activated carbon particles are prepared using CO ₂ as an activator. The prepared activated carbon has a porous hollow structure composed of a three-dimensional nano-grid skeleton. This special activated carbon is used as a supercapacitor electrode, and its specific capacitance is As high as 244F · g ^-1 , there is no attenuation of the capacitance after 10,000 times of charging and discharging. ^[8]

(8) For hydrogen storage

Commonly used hydrogen storage methods include high-pressure gaseous hydrogen storage, liquefied hydrogen storage, metal alloy hydrogen storage and organic liquid hydride hydrogen storage, carbon material hydrogen storage, etc. Among them, carbon materials mainly include super activated carbon, nano-carbon fiber, and carbon nanotubes. Activated carbon has attracted widespread attention due to its abundant raw materials, large specific surface area, surface chemical modification, large hydrogen storage capacity, fast desorption speed, long cycle life, and easy industrialization. Some scholars have used CO _{2 to} activate templates to prepare porous carbon, and have obtained super pores between 0.7 to 1.3 nm, mesopores between 2 to 4 nm, specific surface area of 2829 m ² · g ^-1 and pore volume of 2.34 cm ³ · g ^-1 Activated carbon materials can absorb 0.95% of hydrogen at room temperature of 298K and medium pressure of 8MPa. ^[8]

Since the 21st century, porous solid materials similar to metal-organic frameworks have opened up new development directions for the absorption and storage of hydrogen. Some scholars have introduced activated carbon into metal-organic framework materials under mild conditions, and synthesized an activated carbon-metal-organic framework hybrid material with a high specific surface area. Under the conditions of 77K and 10 MPa, the hydrogen adsorption capacity has increased from 8.2%. It was 13.5%. Controlling the preparation process of super activated carbon to obtain a suitable specific surface area, pore size and distribution of hydrogen storage, and then surface modification. At room temperature and medium pressure, increasing the hydrogen storage capacity is the key to the research and application of super activated carbon hydrogen storage. ^[8]

(9) For flue gas treatment

In the process of desulfurization and denitrification of activated carbon materials, it has attracted attention because of its good treatment effect, low investment and operating costs, realization of resources, and easy recycling. However, single activated carbon desulfurization has a slow speed and low efficiency. In the process of improving the desulfurization performance of activated carbon, modified activated carbon has attracted attention. It can overcome some of the shortcomings and limitations of ordinary activated carbon, and is considered to be one of the most promising desulfurizers. Other studies have shown that ferrous salts and The activated carbon treated with copper salt formula has good adsorption performance for ammonia. ^[7]

(10) Other applications

In various applications of activated carbon, Appendix A of the national standard "Classification and Naming of Activated Carbon" provides a comparison table of the main uses of different types of activated carbon. This comparison table is convenient for guiding different users to select different types of activated carbon and their applications. See the following table ^{[9] for} details:

Classification of manufacturing raw materials	product type	use
Coal activated carbon	Columnar Coal Granular Activated Carbon	Gas separation and purification, solvent recovery, flue gas purification, desulfurization and denitrification, water purification, sewage treatment, catalyst carrier, etc.
	Crushed coal granular activated carbon	Gas purification, solvent recovery, water purification, sewage treatment, environmental protection and other powdery coal activated carbon water pollution emergency treatment, waste incineration, chemical decolorization, flue gas purification, etc.
	Spherical coal granular activated carbon	Carbon molecular sieve, catalyst carrier, gas mask, gas separation and purification, military adsorption, etc.
Wooden activated carbon	Cylindrical wood granular activated carbon	Gas separation and purification, gold extraction, water purification, food and beverage decoloration, etc.
	Broken wood granulated activated carbon	Air purification, solvent recovery, water purification, MSG refining, vinyl acetate synthesis catalyst, etc.
	Powdered wood activated carbon	Water purification, injection injection decolorization, sugar solution decolorization, MSG and beverage decolorization, medicinal, etc.
	Activated carbon with spherical wood particles	Carbon molecular sieve, blood purification, beverage refining, gas separation, gold extraction, etc.
Synthetic material activated carbon	Granular activated carbon	Gas separation and purification, water purification, flue gas purification, sewage treatment, environmental protection, etc.
	Broken synthetic material granular activated carbon	Air purification, odor removal, environmental protection, water and sewage treatment, etc.
	Powdery synthetic material activated carbon	Water purification, waste incineration, chemical decolorization, flue gas purification, etc.
	Shaped activated carbon	Water purification filter element, water purification filter rod, empty honeycomb body, environmental protection, filtration and adsorption, etc.

Activated carbon regeneration

Activated carbon regeneration principle

Activated carbon regeneration refers to the process of removing the adsorbate adsorbed on the micropores of activated carbon and restoring its adsorption performance by using physical or chemical methods without destroying its original structure. In the process of activated carbon adsorption, it has an adsorption effect on both the adsorbate and the solvent. Due to the different affinity, the adsorption equilibrium is reached after a certain period of adsorption. Activated carbon regeneration is to take measures to destroy this equilibrium relationship, which is mainly based on the following aspects: change the chemical properties of the adsorbent; extract with a solvent with a strong affinity for the adsorbent; The substance replaces the adsorbed substance, and then the desorbed substance is desorbed, and the activated carbon is regenerated; The equilibrium conditions are changed by external heating and the temperature is increased; Regenerated by reducing the concentration (or pressure) of the solute in the solvent; The Adsorbed matter (organic matter) is decomposed or oxidized to be removed. ^[10]

Activated carbon regeneration method

(1) Thermal regeneration method

Thermal regeneration is the most mature method of activated carbon regeneration. During the regeneration process of the activated carbon treated organic wastewater, according to the changes of organic matter when heated to different temperatures, it is generally divided into three stages: drying, high-temperature carbonization and activation. In the drying stage, volatile components such as moisture on the activated carbon are removed. The high-temperature carbonization stage is to vaporize and desorb some of the organic matter adsorbed on the activated carbon, decompose part of the organic matter, and desorb it as small molecular substances, and the remaining components remain in the pores of the activated carbon to become fixed carbon. In the activation stage, CO ₂ , CO, or water vapor is passed in to clear the micropores of the internal structure of the activated carbon to restore its adsorption activity. The core of the regeneration process is the activation phase. ^[10]

The thermal regeneration method has relatively high regeneration efficiency, short time, and wide application range, but the carbon loss during the regeneration process is large, which can reach 5% to 10%. At the same time, the mechanical strength of the carbon after regeneration is reduced, and the adsorption efficiency is also reduced. The adsorption performance is lost after repeated regeneration. ^[10]

(2) Biological regeneration method

The method of utilizing the metabolism of microorganisms to oxidatively degrade pollutants adsorbed on activated carbon is called a biological regeneration method. The pore diameter of activated carbon is generally only a few nanometers. It is difficult for microorganisms to enter the interior of its pores. Generally, microbial cell enzymes can flow outside the cell. Through the adsorption of enzymes by activated carbon, an enzymatic center is formed on the surface of the carbon to decompose pollutants and achieve regeneration . The investment and operating cost of the biological method is relatively low, but the regeneration time is longer, and the water quality and temperature have a great impact on the regeneration effect. At the same time, the selectivity of microorganisms to treat pollutants is very strong, and generally all organic matter cannot be completely decomposed into CO ₂ and H ₂ O. The intermediate products still remain in the micropores, and the regeneration efficiency will be significantly reduced after multiple cycles. ^[10]

(3) Wet oxidation regeneration method

The wet oxidation regeneration method refers to a treatment method that uses oxygen or air as an oxidant to oxidize and decompose organic substances adsorbed in a liquid phase into small molecular substances under the condition of high temperature and high pressure. The effect of capacity is small, and the rate of carbon loss is low. It is usually suitable for treating highly toxic and biologically difficult to degrade organic matter. ^[10]

The above are the traditional regeneration methods. Generally, the traditional activated carbon regeneration methods have the following common disadvantages: the activated carbon has a large loss; the adsorption capacity will be significantly reduced after regeneration; the tail gas generated during regeneration will cause secondary pollution. ^[10] With the development of science and technology, some emerging regeneration methods have emerged:

(4) Microwave radiation regeneration method

Microwave radiation regeneration method is an activated carbon regeneration method that is gradually developed using the principle of thermal regeneration method. Most of the adsorbents adsorbed by activated carbon are strongly polar substances. They have a higher ability to absorb microwaves than activated carbon, so they can be regenerated by thermal desorption. The adsorbed polar molecules are polarized due to the induction of microwave radiation, and collide and friction with each other generate high heat, thereby converting microwave energy into thermal energy. The adsorbed water and organic molecules are volatilized and carbonized by heat, and the pores are reopened to restore the adsorption activity. At the same time, the activated carbon itself absorbs microwaves and heats up. It burns due to the high temperature, which causes part of the carbon to be lost during combustion, and the pore diameter of the carbon is enlarged. ^[10]

The microwave regeneration method is characterized by short heating time and high regeneration efficiency. At the same time, because selective heating is performed during heating, energy consumption is very low. However, the microwave regeneration method is not mature enough, and many important issues need to be solved urgently: The mechanism of microwave heating is not sufficiently studied, and models need to be established to obtain a more uniform microwave field; Most microwave generators are modified by domestic microwave ovens, and professional microwave regeneration The heating device is in urgent need of design and development. ^[10]

(5) Supercritical fluid regeneration method

The advantages of supercritical fluid (SCF) are high density, high solubility, high mass transfer rate, good diffusion performance, and low surface tension. The adsorbed organics are very soluble in SCF solvents. By changing the temperature and pressure, organic matter can be effectively separated from SCF to achieve the purpose of activated carbon regeneration.
In the supercritical fluid (SFE) method, the most commonly used supercritical fluid is supercritical CO ₂ . This method does not have high regeneration efficiency for organic substances whose adsorption type is chemisorption. At the same time, it requires high technology and equipment materials, and has high investment costs. Most of the research on this method is on the laboratory scale, and there is still a certain gap to realize industrialization. ^[10]

(6) Electrochemical regeneration method

The electrochemical regeneration method is a new type of activated carbon regeneration method, and research has been very active in recent years. Between the two electrodes, filled with activated carbon saturated with adsorption, at the same time adding a certain electrolyte, a DC electric field is passed, the activated carbon is polarized under the action of the electric field, one end is an anode, and the other end is a cathode, forming a micro-electrolytic cell, respectively reduction In the reaction and oxidation reaction, most of the pollutants adsorbed on the activated carbon are decomposed, and a small part is desorbed. The method has the advantages of simple operation, high efficiency, low energy consumption, and relatively wide processing objects. ^[10]