Activated Carbon H2O2
CAS No. 7722-84-1, activated charcoal, activated coal
Activated carbon, also called activated charcoal or activated coal is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.
The word activated in the name is sometimes replaced with active. Due to its high degree of microporosity, just 1 gram of activated carbon has a surface area in excess of 500 m2 (about one tenth the size of a football field), as determined typically by nitrogen gas adsorption. Sufficient activation for useful applications may come solely from the high surface area, though further chemical treatment often enhances the absorbing properties of the material. Activated carbon is usually derived from charcoal.
Activated carbon is carbon produced from carbonaceous source materials like nutshells, peat, wood, coir, lignite, coal and petroleum pitch. It can be produced by one of the following processes:
- Physical reactivation: The precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:
- Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600-900°C, in absence of oxygen (usually in inert atmosphere with gases like argon or nitrogen)
- Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (carbon monoxide, oxygen, or steam) at temperatures above 250°C, usually in the temperature range of 600-1200°C.
- Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Then, the raw material is carbonized at lower temperatures (450-900°C). It is believed that the carbonization / activation step proceeds simultaneously with the chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.
Traditionally, active carbons are made in particular form as powders or fine granules less than 1.0 mm in size with an average diameter between .15 and .25 mm.[2] Thus they present a large surface to volume ratio with a small diffusion distance. PAC is made up of crushed or ground carbon particles, 95-100% of which will pass through a designated mesh sieve or sieve. Granular activated carbon is defined as the activated carbon being retained on a 50-mesh sieve (0.297 mm) and PAC material as finer material, while ASTM classifies particle sizes corresponding to an 80-mesh sieve (0.177 mm) and smaller as PAC. PAC is not commonly used in a dedicated vessel, owing to the high head loss that would occur. PAC is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters.
Granular activated carbon has a relatively larger particle size compared to powdered activated carbon and consequently, presents a smaller external surface. Diffusion of the adsorbate is thus an important factor. These carbons are therefore preferred for all adsorption of gases and vapors as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodorization and separation of components of flow system. GAC can be either in the granular form or extruded. GAC is designated by sizes such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10 for vapor phase applications. A 20×40 carbon is made of particles that will pass through a U.S. Standard Mesh Size No. 20 sieve (0.84 mm) (generally specified as 85% passing) but be retained on a U.S. Standard Mesh Size No. 40 sieve (0.42 mm) (generally specified as 95% retained). AWWA (1992) B604 uses the 50-mesh sieve (0.297 mm) as the minimum GAC size. The most popular aqueous phase carbons are the 12×40 and 8×30 sizes because they have a good balance of size, surface area, and head loss characteristics.
Extruded activated carbon combines powdered activated carbon with a binder, which are fused together and extruded into a cylindrical shaped activated carbon block with diameters from 0.8 to 130 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content.
Porous carbons containing several types of inorganic impregnant such as iodine, silver, cations such as Al, Mn, Zn, Fe, Li, Ca have also been prepared for specific application in air pollution control especially in museums and galleries. Due to antimicrobial/antiseptic properties, silver loaded activated carbon is used as an adsorbent for purification of domestic water. Drinking water can be obtained from natural water by treating the natural water with a mixture of activated carbon and Al(OH)3, a flocculating agent. Impregnated carbons are also used for the adsorption of H2S and thiols. Adsorption rates for H2S as high as 50% by weight have been reported.
This is a process by which a porous carbon can be coated with a biocompatible polymer to give a smooth and permeable coat without blocking the pores. The resulting carbon is useful for hemoperfusion. Hemoperfusion is a treatment technique in which large volumes of the patient's blood are passed over an adsorbent substance in order to remove toxic substances from the blood.
Activated carbon is also available in special forms such as cloths and fibres. Activated carbon is used in gas purification, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and respirators, filters in compressed air and many other applications.
One major industrial application involves use of activated carbon in the metal finishing field. It is very widely employed for purification of electroplating solutions. For example, it is a main purification technique for removing organic impurities from bright nickel plating solutions. A variety of organic chemicals are added to plating solutions for improving their deposit qualities and for enhancing properties like brightness, smoothness, ductility, etc. Due to passage of direct current and electrolytic reactions of anodic oxidation and cathodic reduction, organic additives generate unwanted break down products in solution. Their excessive build up can adversely affect the plating quality and physical properties of deposited metal. Activated carbon treatment removes such impurities and restores plating performance to the desired level.
Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as:
- Spill cleanup
- Groundwater remediation
- Drinking water filtration
- Air purification
- Volatile organic compounds capture from painting, dry cleaning, gasoline dispensing operations, and other processes.
In 2007, West-Flanders University (in Belgium) began research in water treatment after festivals.[6] A full scale activated carbon installation was built at the Dranouter music festival in 2008, with plans to utilize the technology to treat water at this festival for the next 20 years. Activated charcoal is also used for the measurement of radon concentration in air.
Activated carbon is used to treat poisonings and overdoses following oral ingestion. It is thought to bind to poison and prevent its absorption by the gastrointestinal tract. In cases of suspected poisoning, medical personnel administer activated charcoal on the scene or at a hospital's emergency department. Dosing is usually empirical at 1 gram/kg of body mass (for adolescents or adults, give 50-100 g), usually given only once, but depending on the drug taken, it may be given more than once. In rare situations activated charcoal is used in Intensive Care to filter out harmful drugs from the blood stream of poisoned patients. Activated charcoal has become the treatment of choice for many poisonings, and other decontamination methods such as ipecac-induced emesis or stomach pumping are now used rarely.
While activated carbon is useful in acute poisoning, it has been shown to not be effective in long term accumulation of toxins, such as with the use of toxic herbicides.
Mechanisms of action:
- Binding of the toxin to prevent stomach and intestinal absorption. Binding is reversible so a cathartic such as sorbitol may be added as well.
- It interrupts the enterohepatic and enteroenteric circulation of some drugs/toxins and their metabolites
Incorrect application (e.g. into the lungs) results in pulmonary aspiration which can sometimes be fatal if immediate medical treatment is not initiated.[8] The use of activated charcoal is contraindicated when the ingested substance is an acid, an alkali, or a petroleum product.
For pre-hospital (paramedic) use, it comes in plastic tubes or bottles, commonly 12.5 or 25 grams, pre-mixed with water. The trade names include InstaChar, SuperChar, Actidose, Charcodote, and Liqui-Char, but it is commonly called activated charcoal.
Ingestion of activated charcoal prior to consumption of alcoholic beverages appeared to reduce absorption of ethanol into the blood. 5 to 15 milligrams of charcoal per kilogram of body weight taken at the same time as 170 ml of pure ethanol (which equals to about 10 servings of an alcoholic beverage), over the course of one hour, seemed to reduce potential blood alcohol content. Yet other studies showed that this is not the case, and that ethanol blood concentrations were increased because of activated charcoal use.[10]
Charcoal biscuits were sold in England starting in the early 19th century, originally as an antidote to flatulence and stomach trouble. Tablets or capsules of activated charcoal are used in many countries as an over-the-counter drug to treat diarrhea, indigestion, and flatulence. There is some evidence of its effectiveness as a treatment for irritable bowel syndrome (IBS), and to prevent diarrhea in cancer patients who have received irinotecan. It can interfere with the absorbency of some medications, and lead to unreliable readings in medical tests such as the guaiac card test. Activated charcoal is also used for bowel preparation by reducing intestinal gas content before abdominal radiography to visualize bile and pancreatic and renal stones. A type of charcoal biscuit has also been marketed as a pet care product.
Research is being done testing various activated carbons' ability to store natural gas and hydrogen gas. The porous material acts like a sponge for different types of gasses. The gas is attracted to the carbon material via Van der Waals forces. Some carbons have been able to achieve bonding energies of 5-10 kJ per mol. The gas may then be desorbed when subjected to higher temperatures and either combusted to do work or in the case of hydrogen gas extracted for use in a hydrogen fuel cell. Gas storage in activated carbons is an appealing gas storage method because the gas can be stored in a low pressure, low mass, low volume environment that would be much more feasible than bulky on board compression tanks in vehicles. The United States Department of Energy has specified certain goals to be achieved in the area of research and development of nano-porous carbon materials. As of yet all of the goals are yet to be satisfied but numerous institutions, including the Alliance for Collaborative Research in Alternative Fuel Technology (ALL-CRAFT, http://all-craft.missouri.edu) program, are continuing to conduct work in this promising field.
Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odors, and other hydrocarbons from the air. The most common designs use a 1 stage or 2 stage filtration principle in which activated carbon is embedded inside the filter media. Activated charcoal is also used in spacesuit Primary Life Support Systems. Activated charcoal filters are used to retain radioactive gases from a nuclear boiling water reactor turbine condenser. The air vacuumed from the condenser contains traces of radioactive gases. The large charcoal beds adsorb these gases and retains them while they rapidly decay to non-radioactive solid species. The solids are trapped in the charcoal particles, while the filtered air passes through.
Activated carbon is commonly used to purify homemade non-dangerous chemicals such as sodium acetate.
Activated carbon filters can be used to filter vodka and whiskey of organic impurities which can affect color, taste, and odor. Passing an organically impure vodka through an activated carbon filter at the proper flow rate will result in vodka with an identical alcohol content and significantly increased organic purity, as judged by odor and taste.
Activated carbon, often impregnated with iodine or sulfur, is widely used to trap mercury emissions from coal-fired power stations, medical incinerators, and from natural gas at the wellhead.
The mercury laden activated carbon presents a disposal dilemma. If the activated carbon contains less than 260 ppm mercury, Federal regulations allow it to be stabilized (for example, trapped in concrete) for landfilling. However, waste containing greater than 260 ppm is considered to be in the high mercury subcategory and is banned from landfilling (Land-Ban Rule).[ It is this material which is now accumulating in warehouses and in deep abandoned mines at an estimated rate of 1000 tons per year.
The problem of disposal of mercury laden activated carbon is not unique to the U.S. In the Netherlands this mercury is largely recovery and the activated carbon is disposed by complete burning.
The regeneration of activated carbons involves restoring the adsorptive capacity of saturated activated carbon by desorbing adsorbed contaminants on the activated carbon surface.
The most common regeneration technique employed in industrial processes is thermal regeneration. The thermal regeneration process generally follows three steps.
- Adsorbent drying at approximately 105°C
- High temperature desorption and decomposition (500-900°C) under an inert atmosphere
- Residual organic gasification by an oxidizing gas (steam or carbon dioxide) at elevated temperatures (800°C)
The heat treatment stage utilizes the exothermic nature of adsorption and results in desorption, partial cracking and polymerization of the adsorbed organics. The final step aims to remove charred organic residue formed in the porous structure in the previous stage and re-expose the porous carbon structure regenerating its original surface characteristics. After treatment the adsorption column can be re-used. Per adsorption-thermal regeneration cycle between 5-15 weight % of the carbon bed is burnt off resulting in a loss of adsorptive capacity. Thermal regeneration is a high energy process due to the high required temperatures making it both an energetically and commercially expensive process. Plants that rely on thermal regeneration of activated carbon have to be of a certain size before it is economically viable to have regeneration facilities onsite.
