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Activated carbon

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Activated carbon

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.[1]

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.

Production

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:

  1. 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 air (usually in inert atmosphere with gases like argon or nitrogen)
    • Activation/Oxidation: Raw material or carbonised 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.
  2. 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.

Classifications

Activated carbons are complex products which are difficult to classify on the basis of their behaviour, surface characteristics and preparation methods. However, some broad classification is made for general purpose based on their physical characteristics.

Powdered activated carbon (PAC)

A micrograph of activated charcoal under bright field illumination on a light microscope. Notice the fractal-like shape of the particles hinting at their enormous surface area. Each particle in this image, despite being only around 0.1 mm wide, has a surface area of several square metres. This image of activated charcoal in water is at a scale of 6.236 pixels/μm, the entire image covers a region of approximately 1.1 by 0.7 mm.

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 (GAC)

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 vapours as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodourisation 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 vapour 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 (EAC)

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.

Impregnated carbon

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.

Polymer coated carbon

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.

Other

Activated carbon is also available in special forms such as cloths and fibres. The "carbon cloth" for instance is used in personnel protection for the military.

Properties

A gram of activated carbon can have a surface area in excess of 500 m2, with 1500 m2 being readily achievable.[3] Carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications.

File:Activated-carbon.jpg
Activated carbon, as viewed by an electron microscope

Under an electron microscope, the high surface-area structures of activated carbon are revealed. Individual particles are intensely convoluted and display various kinds of porosity; there may be many areas where flat surfaces of graphite-like material run parallel to each other, separated by only a few nanometers or so. These micropores provide superb conditions for adsorption to occur, since adsorbing material can interact with many surfaces simultaneously. Tests of adsorption behaviour are usually done with nitrogen gas at 77 K under high vacuum, but in everyday terms activated carbon is perfectly capable of producing the equivalent, by adsorption from its environment, liquid water from steam at 100 °C and a pressure of 1/10,000 of an atmosphere.

Physically, activated carbon binds materials by van der Waals force or London dispersion force.

Activated carbon does not bind well to certain chemicals, including alcohols, glycols, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.

Activated carbon does adsorb iodine very well and in fact the iodine number, mg/g, (ASTM D28 Standard Method test) is used as an indication of total surface area.

Contrary to a claim repeated [citation needed] throughout the web, activated carbon can capture gaseous ammonia. In fact, it is utilized for that very function in many diazo copier machines.

Activated carbon can be used as a substrate for the application of various chemicals to improve the adsorptive capacity for some inorganic (and problematic organic) compounds such as hydrogen sulfide (H2S), ammonia (NH3), formaldehyde (HCOH), radioisotopes iodine-131(131I) and mercury (Hg). This property is known as chemisorption.

Iodine number

Many carbons preferentially adsorb small molecules. Iodine number is the most fundamental parameter used to characterize activated carbon performance. It is a measure of activity level (higher number indicates higher degree of activation), often reported in mg/g (typical range 500–1200 mg/g). It is a measure of the micropore content of the activated carbon (0 to 20 Å, or up to 2 nm) by adsorption of iodine from solution. It is equivalent to surface area of carbon between 900 m²/g and 1100 m²/g. It is the standard measure for liquid phase applications.

Iodine number is defined as the milligrams of iodine adsorbed by one gram of carbon when the iodine concentration in the residual filtrate is 0.02 normal. Basically, iodine number is a measure of the iodine adsorbed in the pores and, as such, is an indication of the pore volume available in the activated carbon of interest. Typically, water treatment carbons have iodine numbers ranging from 600 to 1100. Frequently, this parameter is used to determine the degree of exhaustion of a carbon in use. However, this practice should be viewed with caution as chemical interactions with the adsorbate may affect the iodine uptake giving false results. Thus, the use of iodine number as a measure of the degree of exhaustion of a carbon bed can only be recommended if it has been shown to be free of chemical interactions with adsorbates and if an experimental correlation between iodine number and the degree of exhaustion has been determined for the particular application.

Molasses

Some carbons are more adept at adsorbing large molecules. Molasses number or molasses efficiency is a measure of the mesopore content of the activated carbon (greater than 20 Å, or larger than 2 nm) by adsorption of molasses from solution. A high molasses number indicates a high adsorption of big molecules (range 95–600). Caramel dp (decolorizing performance) is similar to molasses number. Molasses efficiency is reported as a percentage (range 40%–185%) and parallels molasses number (600 = 185%, 425 = 85%). The European molasses number (range 525–110) is inversely related to the North American molasses number.

Molasses Number is a measure of the degree of decolorization of a standard molasses solution that has been diluted and standardized against standardized activated carbon. Due to the size of color bodies, the molasses number represents the potential pore volume available for larger adsorbing species. As all of the pore volume may not be available for adsorption in a particular waste water application, and as some of the adsorbate may enter smaller pores, it is not a good measure of the worth of a particular activated carbon for a specific application. Frequently, this parameter is useful in evaluating a series of active carbons for their rates of adsorption. Given two active carbons with similar pore volumes for adsorption, the one having the higher molasses number will usually have larger feeder pores resulting in more efficient transfer of adsorbate into the adsorption space.

Tannin

Tannins are a mixture of large and medium size molecules. Carbons with a combination of macropores and mesopores adsorb tannins. The ability of a carbon to adsorb tannins is reported in parts per million concentration (range 200 ppm–362 ppm).

Methylene blue

Some carbons have a mesopore (20 Å to 50 Å, or 2 to 5 nm) structure which adsorbs medium size molecules, such as the dye methylene blue. Methylene blue adsorption is reported in g/100g (range 11–28 g/100g).

Dechlorination

Some carbons are evaluated based on the dechlorination half-value length, which measures the chlorine-removal efficiency of activated carbon. The dechlorination half-value length is the depth of carbon required to reduce the chlorine level of a flowing stream from 5 ppm to 3.5 ppm. A lower half-value length indicates superior performance.

Apparent density

Higher density provides greater volume activity and normally indicates better quality activated carbon.

Hardness/abrasion number

It is a measure of the activated carbon’s resistance to attrition. It is important indicator of activated carbon to maintain its physical integrity and withstand frictional forces imposed by backwashing, etc. There are large differences in the hardness of activated carbons, depending on the raw material and activity level.

Ash content

It reduces the overall activity of activated carbon. It reduces the efficiency of reactivation. The metal oxides (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water soluble ash content is more significant than total ash content. Soluble ash content can be very important for aquarists, as ferric oxide can promote algal growths, a carbon with a low soluble ash content should be used for marine, freshwater fish and reef tanks to avoid heavy metal poisoning and excess plant/algal growth.

Carbon tetrachloride activity

Measurement of the porosity of an activated carbon by the adsorption of saturated carbon tetrachloride vapour.

Particle size distribution

The finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption kinetics. In vapour phase systems this needs to be considered against pressure drop, which will affect energy cost. Careful consideration of particle size distribution can provide significant operating benefits.

Examples of adsorption

Heterogeneous catalysis

The most commonly encountered form of chemisorption in industry, occurs when a solid catalyst interacts with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the catalyst surface creates a chemical bond, altering the electron density around the reactant molecule and allowing it to undergo reactions that would not normally be available to it.

Adsorption refrigeration

Adsorption refrigeration and heat pump cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating. The adsorbent acts as a "chemical compressor" driven by heat and is, from this point of view, the "pump" of the system. It consists of a solar collector, a condenser or heat-exchanger and an evaporator that is placed in a refrigerator box. The inside of the collector is lined with an adsorption bed packed with activated carbon adsorbed with methanol. The refrigerator box is insulated filled with water. The activated carbon can adsorb a large amount of methanol vapours in ambient temperature and desorb it at a higher temperature (around 100 degrees Celsius). During the daytime, the sunshine irradiates the collector, so the collector is heated up and the methanol is desorbed from the activated carbon. In desorption, the liquid methanol adsorbed in the charcoal heats up and vaporizes. The methanol vapour condenses and is stored in the evaporator.

At night, the collector temperature decreases to the ambient temperature, and the charcoal adsorbs the methanol from the evaporator. The liquid methanol in the evaporator vaporizes and absorbs the heat from the water contained in the trays. Since adsorption is a process of releasing heat, the collector must be cooled efficiently at night. As mentioned above, the adsorption refrigeration system operates in an intermittent way to produce the refrigerating effect.

Helium gas can also be 'pumped' by thermally cycling activated carbon 'sorption pumps' between 4 kelvins and higher temperatures. An example of this is to provide the cooling power for the Oxford Instruments AST series dilution refrigerators. 3He vapour is pumped from the surface of the dilute phase of a mixture of liquid 4He and its isotope 3He. The 3He is adsorbed onto the surfaces of the carbon at low temperature (typically <4K), the regeneration of the pump between 20 and 40 K returns the 3He to the concentrated phase of the liquid mixture. Cooling occurs at the interface between the two liquid phases as 3He 'evaporates' across the phase boundary. If more than one pump is present in the system a continuous flow of gas and hence constant cooling power can be obtained, by having one sorption pump regenerating while the other is pumping. Systems such as this allow temperatures as low as 10 mK (0.01 kelvin) to be obtained with very few moving parts.

Applications

Activated carbon is used in gas purification, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and filter masks, 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.

Analytical chemistry applications

Activated carbon, in 50% w/w combination with celite, is used as stationary phase in low-pressure chromatographic separation of carbohydrates (mono-, di- trisacchardes) using ethanol solutions (5–50%) as mobile phase in analytical or preparative protocols.

Environmental applications

Activated carbon is usually used in water filtration systems. In this illustration, the activated carbon is in the fourth level (counted from bottom).

Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as:

In 2007, West-Flanders University (in Belgium) began research in water treatment after festivals [4]. A full scale activated carbon installation was built at the Dranouter music festival in 2008 [citation needed], with plans to utilize the technology to treat water at this festival for the next 20 years [citation needed].

Activated charcoal is also used for the measurement of radon concentration in air.

Medical applications

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 weight (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.

Activated charcoal for medical use.

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.[5]

Mechanisms of action:

Incorrect application (e.g. into the lungs) results in pulmonary aspiration which can sometimes be fatal if immediate medical treatment is not initiated.[6] 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.[7] Yet other studies showed that this is not the case, and that ethanol blood concentrations were increased because of activated charcoal use.[8]

Charcoal biscuits were sold in England starting in the early 19th century, originally as an antidote to flatulence and stomach trouble.[9]

Tablets or capsules of activated charcoal are used in many countries as an over-the-counter drug to treat diarrhea, indigestion, and flatulence.[10] There is some evidence of its effectiveness as a treatment for irritable bowel syndrome (IBS),[11] and to prevent diarrhea in cancer patients who have received irinotecan.[12] It can interfere with the absorbency of some medications, and lead to unreliable readings in medical tests such as the guaiac card test.[13] 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.

Fuel storage

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.

Gas purification

Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapours, odours, 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.

Chemical purification

Activated carbon is commonly used to purify homemade non-dangerous chemicals such as sodium acetate. It is then filtered out. It is afterwards mostly placed in a jug to spice up meals.

Distilled alcoholic beverage purification

See also: Lincoln County Process

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.[citation needed]

Mercury scrubbing

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. This carbon is a specialty product costing more than $4.00 per kg. However, it is often not recycled.

Disposal in the USA after absorbing mercury

The mercury laden activated carbon presents a disposal dilemma.[citation needed] If the activated carbon contains less than 260 ppm mercury, Federal regulations allow it to be stabilized (for example, trapped in concrete) for landfilling.[citation needed] However, waste containing greater than 260 ppm is considered to be in the high mercury subcategory and is banned from landfilling (Land-Ban Rule).[citation needed] It is this material which is now accumulating in warehouses and in deep abandoned mines at an estimated rate of 1000 tons per year.[citation needed]

The problem of disposal of mercury laden activated carbon is not unique to the U.S. In the Netherlands this mercury is largely recovered[14] and the activated carbon is disposed by complete burning.

Regeneration

The regeneration of activated carbons involves restoring the adsorptive capacity of saturated activated carbon by desorbing adsorbed contaminants on the activated carbon surface.

Thermal regeneration

The most common regeneration technique employed in industrial processes is thermal regeneration [15]. The thermal regeneration process generally follows three steps [16]:

  • Adsorbent drying at approximately 105 °C
  • High temperature desorption and decomposition (500–900°C) under an inert atmosphere
  • Residual organic gasification by an oxidising gas (steam or carbon dioxide) at elevated temperatures (800°C)

The heat treatment stage utilises 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 reused. Per adsorption-thermal regeneration cycle between 5–15 wt% of the carbon bed is burnt off resulting in a loss of adsorptive capacity.[17]. Thermal regeneration is a high energy process due to the high required temperatures making it both an energetically and commercially expensive process [16]. 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. As a result it is common for smaller waste treatment sites to ship their activated carbon cores to a specialised facility for regeneration, increasing the processes already significant carbon footprint [18].

Other regeneration techniques

Current concerns with the high energy/cost nature of thermal regeneration of activated carbon has encouraged research into alternative regeneration methods to reduce the environmental impact of such processes. Though several of the regeneration techniques cited have remained areas of purely academic research, some alternatives to thermal regeneration systems have been employed in industry. Current alternative regeneration methods are:

See also

References

  1. ^ "Properties of Activated Carbon", CPL Caron Link, accessed 2008-05-02
  2. ^ http://en.mimi.hu/astronomy/granule.html
  3. ^ Value Added Products from Gasification – Acitivated Carbon, By Shoba jhadhav, The Combustion,Gasification and Propulsion Laboratory (CGPL) at the Indian Institute of Science (IISc)
  4. ^ Sustainable wastewater treatment of temporary events: the Dranouter Music Festival case study</a>. Water science and technology : a journal of the International Association on Water Pollution Research. 2008;58(8):1653–7.
  5. ^ Eddleston M, Juszczak E, Buckley NA; et al. (2008). "Multiple-dose activated charcoal in acute self-poisoning: a randomised controlled trial". Lancet. 371 (9612): 579. doi:10.1016/S0140-6736(08)60270-6. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  6. ^ Elliott C, Colby T, Kelly T, Hicks H (1989). "Charcoal lung. Bronchiolitis obliterans after aspiration of activated charcoal". Chest. 96 (3): 672–4. doi:10.1378/chest.96.3.672. PMID 2766830.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ US 4594249, Procter, Richard & Anondson, Stanton, "Method of altering intoxicating effects of alcohol", issued 1986 
  8. ^ Activated charcoal in oral ethanol absorption: lack of effect in humans., Minocha A, Herold DA, Barth JT, Gideon DA, Spyker DA., J Toxicol Clin Toxicol. 1986;24(3):225–34, PubMED
  9. ^ Rolland, Jacques L. (2006). The Food Encyclopedia: Over 8,000 Ingredients, Tools, Techniques and People. Robert Rose. p. 148. ISBN 0778801500.
  10. ^ Stearn, Margaret (2007). Warts and all: straight talking advice on life's embarrassing problems. London: Murdoch Books. p. 333. ISBN 978-1-92125984-5. Retrieved 2009-05-03.
  11. ^ Hübner WD, Moser EH (2002). "Charcoal tablets in the treatment of patients with irritable bowel syndrome". Adv Ther. 19 (5): 245–52. doi:10.1007/BF02850364. PMID 12539884.
  12. ^ Michael M, Brittain M, Nagai J; et al. (2004). "Phase II study of activated charcoal to prevent irinotecan-induced diarrhea". J Clin Oncol. 22 (21): 4410–7. doi:10.1200/JCO.2004.11.125. PMID 15514383. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  13. ^ Gogel HK, Tandberg D, Strickland RG (1989). "Substances that interfere with guaiac card tests: implications for gastric aspirate testing". Am J Emerg Med. 7 (5): 474–80. doi:10.1016/0735-6757(89)90248-9. PMID 2787993. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  14. ^ BMT-Begemann, Mercury waste treatment facilities
  15. ^ Bagreev, A. (2001). "Thermal regeneration of a spent activated carbon adsorbent previously used as hydrogen sulfide adsorbent". Carbon. 39: 1319–1326. doi:10.1016/S0008-6223(00)00266-9. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  16. ^ a b Sabio, E. (2004). "Thermal regeneration of activated carbon saturated with p-nitrophenol". Carbon. 42: 2285–2293. doi:10.1016/j.carbon.2004.05.007. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Miguel, G. S. (2001). "The regeneration of field spent granular activated carbons". Water Research. 35 (11): 2740–2748. doi:10.1016/S0043-1354(00)00549-2. PMID 11456174. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ Alvarez, P. M. (2004). "Comparison between thermal and ozone regenerations of spent activated carbon exhausted with phenol". Water Research. 38 (8): 2155–2165. doi:10.1016/j.watres.2004.01.030. PMID 15087197. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  19. ^ Martin, R. J. (1997). "The repeated exhaustion and chemical regeneration of activated carbon". Water Research. 21 (8): 961–965. doi:10.1016/S0043-1354(87)80014-3. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  20. ^ Aizpuru, A. (2003). "Biofiltration of a mixture of volatile organic compounds on granular activated carbon". Biotechnology and Bioengineering. 83 (4): 479–488. doi:10.1002/bit.10691. PMID 12800142. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  21. ^ Narbaitz, R. M. (2009). "Electrochemical regeneration of granular activated carbons loaded with phenol and natural organic matter". Environmental Technology. 30 (1): 27–36. doi:10.1080/09593330802422803. PMID 19213463. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  22. ^ Lim, J. (2005). "Regeneration of granular activated carbon using ultrasound". Ultrasonic-Sono-Chemistry. 12 (4): 277–285. doi:10.1016/j.ultsonch.2004.02.003. PMID 15501710. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Shende, R. V. (2002). "Wet oxidative regeneration of activated carbon loaded with reactive dye". Waste Management. 22 (1): 73–83. doi:10.1016/S0956-053X(01)00022-8. PMID 11942707. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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