Activated carbon

Bright field microscopy of granulated activated carbon. The brittle structure of the coal particles indicates the enormous size of the surface. Each particle in the image, about 0.1 mm in diameter, has a surface area of ​​several square meters.

Scanning electron microscopy of an activated carbon pellet

Activated carbon , briefly activated carbon , also medicinal charcoal (lat. Carbo medicinalis ), is porous, fine-grained carbon with a large internal surface area , which as adsorbent among other things, in chemistry , medicine , drinking water treatment , waste water treatment and ventilation - and air conditioning systems is used. Activated charcoal is granulated or pressed in tablet form (charcoal complete). It also serves as a carrier material for catalysts for heterogeneous catalysis .

properties

Activated charcoal is flammable and consists mainly of carbon (mostly> 90%) with a highly porous structure. The pores are open-pored and connected to one another like a sponge . The inner surface is between 300 and 2000 m ² / g carbon, so the inner surface of four grams of activated carbon corresponds approximately to the area of ​​a football field . The density of activated carbon is in the range from 0.2 to 0.6 g / cm ³ .

The pore size and the pore size distribution are divided into four orders of magnitude: submicropores (<0.4 nm), micropores (0.1 to 2 nm), mesopores (also called transition pores, 2 to 50 nm) and macropores (> 50 nm).

Macro and mesopores are the access routes for gases or liquids into the interior of the coals and are essentially involved in diffusion and material transport processes into deeper areas of the grain. Most of the adsorption takes place on the surface of the micropores. The size of this area determines the effective surface and thus the adsorption properties of a coal. The following data shows the size of the inner surface in relation to the volume of an activated carbon. In the case of a cube with an edge length of 1 cm, the inner surface exceeds the outer surface by more than a factor of 100,000.

Basically, the adsorbability of a compound increases with it

• increasing molecular weight
• increasing number of functional groups such as double bonds or halogen ligands
• increasing polarizability of the molecule.

Iodine number

Activated carbon mostly adsorbs smaller molecules. The iodine number is the decisive parameter for the adsorption capacity of an activated carbon. The activation of the activated carbon is measured. The adsorption capacity is often given in mg / g. This is the standard procedure for liquid applications.

The iodine number is defined as the number of milligrams of iodine that is adsorbed by one gram of activated carbon. Typically, water treatments have an iodine number of 600–1100. This parameter is also used to measure the utilization of activated carbon. However, this can only be recommended if the adsorbate is free of chemical interactions and the correlation between the iodine number and the degree of utilization has been proven for the desired field of application.

Molasses number

Other types of activated carbon are more efficient at adsorbing larger molecules. The molasses number indicates the efficiency of the adsorption of larger molecules. A high molasses number corresponds to a high adsorption of larger molecules.

Tanning agent

Tannins are a mixture of large and medium-sized molecules. Activated carbon in combination with macro- and mesopores adsorb tannins. The adsorption efficiency of tannins is measured in parts per million. The recorded values ​​range between 200 ppm-362 ppm.

Dechlorination / dechlorination

Activated carbon can also contribute to dechlorination. The half-life for dechlorination is the decisive indicator. This measures the efficiency of activated carbon in removing chlorine. 

Abrasion number

The abrasion number measures the resistance of the activated carbon to wear and tear. There are big differences between different types of activated carbon. The abrasion rate is significantly influenced by the original raw material and the activation. 

Grain size distribution

The finer the particles, the better the access to the surface and the faster it is adsorbed. A good evaluation of the grain size can lead to significantly better results of the adsorption performance. For the adsorption of minerals such as gold, the particle size should be between 1.4 and 3.35 mm. Particles below 1 mm would no longer be sufficient for the washout process. During this process, the minerals are removed from the activated carbon. 

Extraction

Activated carbon

Activated charcoal is made from vegetable, animal, mineral or petrochemical substances such as brown , hard charcoal or various plastics . Activated carbon is a by-product of hydrogen production using the Kværner process . Activated charcoal from raw materials such as wood , peat , coconut fiber and nutshells is also known as biochar . As animal charcoal , lat . carbo animalis is the name given to activated charcoal made from animal blood (blood charcoal) or from bones (bone charcoal). With sugar carbon, an activated carbon is referred to, consisting of glucose or another sugar as a raw product.

Two processes can be used for production and activation:

• the gas activation and
• chemical activation

In the production of chemical activation a mixture of unverkohltem starting material is treated with chemicals. This is generally done through use with dehydrating agents (e.g. zinc chloride or phosphoric acid ) at 500-900 ° C. Another method is dry distillation ( coking ), in which the material is heated in an oxygen-free atmosphere and volatile components are expelled at temperatures around 800 ° C. The raw activated carbon obtained in this way is then oxidatively activated at 700–1000 ° C. with water vapor or carbon dioxide, and sometimes with air. In this activation , a part of the carbon is after the water-gas process in carbon monoxide converted, so that additional pores are formed and the surface of the coal is increased.

For some purposes, the activated carbon is treated (impregnated) with other chemicals in order to improve the separation effect. Filter carbon for breathing filters in gas masks is coated with metal salts , which improves the separation effect for many chemical toxins. Activated charcoal with a silver coating is particularly suitable for filtering drinking water. Contamination of these filters during operation is largely suppressed by the silver.

Areas of application

A water filter: activated carbon forms the 4th layer from the top

Purification of liquids and gases

Activated carbon is primarily used as an adsorbent to remove unwanted color, flavor and odor from gases, vapors and liquids. A great advantage of activated carbons is that they can be thermally reactivated. A biological reactivation is also possible with activated carbon used for wastewater treatment.

Activated carbon is mainly used in the form of granules, as a powder or in pelletized form. Active carbon fabrics are also available on the market.

Toothpaste with activated charcoal

For example, activated charcoal removes:

• Chlorine , ozone and other substances that impair taste and smell as well as bacteria from water, sweeteners, glycerine and chemical liquids
• Coloring substances and contaminants from liquid streams in the chemical or food industry
• Toxins from the air (filters in respiratory masks , ventilation systems for tanks and shelters, exhaust air in production systems)
• Denitrification of flue gases from glass melting furnaces, waste incineration plants and other exhaust gases
• Uranium hexafluoride from the air in nuclear technology (fuel production)
• Petrol vapors from the exhaust air of tank systems
• Chlorinated hydrocarbons from exhaust air and exhaust gases z. B. in dry cleaning
• Tar from cigarette smoke in cigarette filters
• unwanted flavors from vodka and other spirits
• unwanted coloring from white rum and other spirits
• Odor of sweat in shoes (due to insoles containing activated carbon)
• Fermentation by-products such as fusel oils and esters from beer
• Pollutants in aquarium and pond filters
• annoying odors from the exhaust air of Ventilation Systems
• Fine dust , pollen and odors in the supply air of vehicles (see cabin air filter )

• Discoloration of teeth

In the sewage treatment activated carbons are used adsorbable, dissolved wastewater constituents (z. B. trace substances ) out of the water to adsorb. This process step is usually only used when cheaper methods such as biological processes, precipitation and flocculation do not achieve the goal. This method can also be used for cleaning partial flows in industry, with the aim of recovering residues. Wastewater from textile dye works contains dyes that can often only be removed by adsorption with activated carbon. This can be done economically with biologically regenerated activated carbon.

Another important application for activated carbon is in cabin air filters for the automotive industry. This filter class has been used in air conditioning systems since the mid-1990s . The so-called combination filters (this is a specific cabin air filter class) contain a layer of activated carbon, which filters harmful gases out of the air and thus protects the passengers from these pollutants. For this application, more than 5000 tons of activated carbon are processed worldwide every year.

In the exhaust air lines of nuclear power plants , activated carbon filters are used as delay lines for short-lived radioactive noble gases. The noble gases undergo - due to temporary adsorption - the filter sections considerably slower than the rest of the exhaust air. The radioactive decay significantly reduces the amount of radioactive noble gases in the exhaust air.

The water treatment also uses adsorption on activated carbon to purify raw water.

Due to its high adsorption capacity , activated carbon can also be used in sorption pumps to generate vacuums .

Activated carbon has only a limited load capacity. Regeneration usually takes place by heating to several hundred degrees Celsius. On the one hand, part of the load (e.g. organic solvents) evaporates, while another part can also coke, in which case the activated carbon has to be reactivated with steam, as in the production process.

Thermal reactivation

World's largest reactivation plant in Feluy , Belgium

Saturated activated carbon can be reactivated by means of a thermal process at high temperatures (up to 900 ° C), for example in rotary kilns or multi-deck ovens. Due to modern and intensive flue gas cleaning , saturated activated carbon can be recycled from a wide variety of applications.

The complete reactivation process consists of the following steps:

1. Drying of the material up to +105 ° C.
2. Evaporation of the adsorbed volatile components up to +300 ° C.
3. Decomposition of the adsorbed non-volatile components into smaller molecules at up to +600 ° C, which are broken down in the furnace atmosphere by pyrolysis to amorphous carbon on the inner surface.
4. Gasification of amorphous carbon using steam above 800 ° C.

The reaction of the amorphous carbon with water vapor at high temperatures to carbon monoxide (CO) and finally to carbon dioxide (CO ₂ ) creates micropores, which form the large specific surface.

${\displaystyle \mathrm {C\ +\ H_{2}O\ \longrightarrow \ H_{2}\ +\ CO} }$

With the sensible use of natural raw materials through reactivation, a reduction in CO ₂ emissions by a factor of 5 is achieved.

Medical application

In medicine, activated charcoal is mainly used to remove toxins from the gastrointestinal tract. In harmless diarrheal diseases, e.g. B. gastrointestinal flu ( gastroenteritis ), charcoal are usually used. In the event of a poisoning emergency, activated charcoal is used in large quantities to remove orally ingested poisons that are in the digestive tract or are subject to an enterohepatic cycle from the organism. The dosage in such cases is 0.5 to 1 g of charcoal per kilogram of body weight in an adult human.

Non-activated charcoal, such as non-activated birch charcoal, is also used as an intestinal regulator .

Use as a support for catalysts

In chemistry, catalysts based on activated carbon are used. The activated carbon serves as a carrier for transition metals, for example palladium , platinum or rhodium . A typical area of ​​application for these catalysts is catalytic hydrogenation .

Use as an electrode

Activated carbon is used as an electrode material in supercapacitors . Due to the extremely large surface of these electrodes, very high capacities can be achieved.

Food coloring

Activated charcoal can be an ingredient in toothpaste.

Activated charcoal is used in the food industry as the coloring agent E  153 ( Carbo medicinalis vegetabilis ). E 153 is used in fruit juice concentrates, jellies, jams, confectionery and in black wax coatings for cheese. Only activated charcoal of biogenic origin is permitted as a food additive, e.g. from biochar.

Activated carbon heater

Activated charcoal is also found in warming bags .

Cosmetics

Activated charcoal is added to face masks, shower gels and toothpaste to absorb unwanted substances. However, since these are washed away after cleaning anyway, the effect is questionable.

literature

• Hartmut von Kienle, Erich Bäder: Activated carbon and its industrial application. Ferdinand Enke Verlag, Stuttgart 1980, ISBN 3-432-90881-4 .
• Roop Chand Bansal, Jean-Baptiste Donnet, Fritz Stoeckli: Active Carbon. Marcel Dekker, New York 1988, ISBN 0-8247-7842-1 .
• John W. Patrick (Ed.): Porosity in Carbons. Edward Arnold, London 1995, ISBN 0-340-54473-2 .
• Entry to activated carbon. In: Römpp Online . Georg Thieme Verlag, accessed on June 16, 2014.
• Thomas Schneider, Benno Wolcke, Roman Böhmer: Pocket Atlas Emergency & Rescue Medicine - Compendium for the emergency doctor. 3rd edition, Springer Medizin Verlag, Heidelberg 2000/2004/2006, ISBN 3-540-29565-8 .
• Dieter Bathen, Marc Breitenbach: Adsorption technology. Springer-Verlag, Berlin 2001, VDI book, ISBN 3-540-41908-X .

Web links

Wiktionary: activated carbon  - explanations of meanings, word origins, synonyms, translations

Individual evidence

1. ↑ Carbo medicinalis vegetabilis. Retrieved January 19, 2020 . 
2. ↑ Activated Carbon Treatment of Drinking Water , December 3, 1995
3. ↑ Data sheet activated carbon pa, powder (PDF) from Carl Roth , accessed on January 21, 2019.
4. ↑ Product information, brochure from Lurgi ; T 1158 / 2.81 ; P. 3, 4.
5. ↑ What is adsorption? Chemviron Carbon, accessed January 14, 2017 . 
6. ↑ Volker Wiskamp: Inorganic Chemistry . A practical textbook. Thun, Frankfurt am Main 1996, ISBN 978-3-8171-1514-3 , pp. 107 .
7. ↑ Product information, brochure from Lurgi; T 1158 / 2.81 ; P. 7, 8.
8. ↑ Entry on activated carbon. In: Römpp Online . Georg Thieme Verlag, accessed on June 16, 2014.
9. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .
10. ↑ ^{a b} Erwin Thomanetz, Dieter Bardtke and Ellen Köhler; In: Investigation of the discoloration of municipal wastewater by means of biologically regenerating activated carbon. gwt Wasser Abwasser, 128, No. 8, 1987, pp. 432-441.
11. ↑ Exhaust air purification. CarboTech AC GmbH, accessed on January 21, 2016 . 
12. ↑ Detlef Riedel: Coal is not just coal . In: Practice of the natural sciences - chemistry in school . tape 58 , no. 1 , 2009, p. 6–9 ( gvst.de [PDF]).
13. ↑ Entry on E 153: Vegetable carbon in the European database on food additives, accessed on June 16, 2020.
14. ↑ Ulrich Stock: Coal has a future! In: The time . No. 26 , 2019, p. 25 .