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Activated charcoal is porous carbon that traps compounds, usually organic, present in a gas or a liquid. It does this with such effectiveness that it is the most widely used purifier by humans. Organic compounds are derived from the metabolism of living things, and their basic structure consists of chains of carbon and hydrogen atoms. Among them are all the derivatives of the plant and animal world, including oil and the compounds obtained from it. The property that a solid has of adhering a flowing molecule to its walls is called “adsorption.”

Activated Charcoal and Adsorbing Property

Activated Charcoal powder on white background

Any carbon particle can adsorb. This is why some people put charcoal in the refrigerator to eliminate odours. The same happens if charcoal is placed in a container of water: it eliminates colour, taste, and odour. Or, in the countryside, people burn tortillas and eat them to relieve digestive problems (such as mild infections, indigestion, or flatulence). Activating a carbon consists of making it porous to increase its adsorption capacity. One gram of charcoal has a surface area of ​​around 50 m2. With activation, it reaches 600 or 800 m2. That is, it increases between 12 and 16 times.

The carbon atoms that form a solid that we call “carbon” are linked together by covalent bonds. Each atom shares an electron with four other carbon atoms (remember that in ionic bonds, the most electronegative atom steals one or more electrons from the other). The atoms which are not located on the surface have their four bonds in all directions, but the surface atoms, although they are linked with four others, are forced to do so in less space, and an imbalance of forces remains in them. This imbalance is what leads them to trap a molecule of the fluid that surrounds the carbon.

The force with which the surface carbon atom traps the other is called the “London force,” which is one of the seven types of “Van der Waals forces.” It is considered a physicochemical bond, strong enough to retain the adsorbate, but not so strong that it is considered an irreversible chemical bond that forms a new molecular structure. Therefore, the adsorption is reversible, and the activated charcoal can be reactivated for reuse. As we said, the molecules that carbon adsorbs tend to be covalent, non-ionic since the latter would try to steal or donate electrons to carbon atoms. The bonds between carbon and hydrogen atoms are covalent, and that is why carbon is a good adsorbent for organic molecules.

Not all organic molecules tend to be covalent. They usually contain oxygen, sulfur, and other high electronegativity atoms, which give an ionic tendency to the part of the molecule that contains them. On the other hand, not all inorganic molecules tend to be ionic; there are also covalent tendencies. Such is the case of gold dicyanide, which makes activated charcoal an essential part of the extraction process of this precious metal.

Sources of Activated Charcoal

Activated charcoal in coconut shells on white background

Activated charcoal can be produced from any material of plant or animal origin: wood, coconut shells, cane bagasse, meat, cobs, blood, among thousands. It can also be produced from mineral coals since these come in turn from trees and plants.

There are two activation methods: thermal and chemical. In the first, the formation of the pores is achieved by subjecting the previously carbonized material to a temperature close to 1000 C and in an atmosphere saturated with water vapour. The raw material is flooded with a dehydrating compound solution in the chemical method, such as zinc chloride or phosphoric acid. Dehydration causes molecular chains to separate from each other.

Subsequently, the dehydrated material is calcined at a relatively low temperature, thus obtaining a carbon that is already porous. As the last step in this method, washing the charcoal is required to remove the activating chemical; in this way, on the one hand, the chemical is recovered, and on the other, the coal is purified as far as possible.

Activated Charcoal in the Treatment of Poisonings

Capsules of activated charcoal

When a person becomes intoxicated due to some organic product ingestion, activated charcoal can save their life. However, not just any activated charcoal is the most effective for this application.

Coal must comply with the following:

  1. a) Be microporous
  2. b) Be, preferably, of plant origin
  3. c) Be, preferably, thermally activated

The need for it to be microporous is because the toxic molecules absorbed in the body have a molecular diameter of fewer than 2 nanometers. In contrast, non-harmful molecules such as proteins and most fats tend to be larger. Therefore, if a larger pore carbon is used, it will adsorb these non-harmful molecules. Coconut shell charcoal is the most microporous of those on the market, more than 95% of its pores have a diameter of less than 2 nm, and therefore, it is the one that best meets the microporosity condition. Regarding the plant origin, it is a recommendation that is because the raw material does not contain harmful contaminants. Coals of mineral origin should be avoided, as most of them contain heavy metals, sulfides, and other dangerous elements that can dissolve in gastric juices.

How does Activated Charcoal Act in the Intoxication of the Body?

To guide users of activated charcoal as a detoxification method, the pharmacopeias of different countries, including the Mexican one, have issued technical specifications that the carbon must meet. Activated charcoal can have different presentations, such as granules, pellets, or powder. For its application in patients with severe poisoning, the charcoal must be a powder of less than 200 mesh, that is, less than 74 microns. This is because the smaller the carbon particles, the faster it acts, and therefore the chances of successful treatment are increased. The increase in adsorption speed is due to the reduction in the length of the pores, which are filled by a capillarity effect.

The effect of increasing the adsorption rate by decreasing the carbon’s particle size is inversely proportional to the square of its particle size.

As an example, activated charcoal between the 200 and 325 mesh (average diameter 0.059 mm) adsorbs 3.57 times faster than one between the 100 and 200 mesh (average diameter 0.1115 mm). On the other hand, powdered charcoal is indicated to flow easily in a suspension along with the hoses through which it is administered when the person is intubated.

The recommended dose of activated charcoal for a patient with acute poisoning is one gram per kilogram of body weight. If this dose does not achieve an acceptable reduction in the serum levels of the poison or the signs and symptoms, it must be repeated every four hours and up to a maximum of 36 hours. In order to avoid intestinal constipation, it is important to administer together with the first dose of charcoal, a cathartic of sodium or magnesium sulfate, in a dose of 0.25 g of this salt per kg of body weight. In the case of multi-dose charcoal, the cathartic dose must be repeated every 12 hours.

The treatment is not indicated for poisonings by:

  • Corrosive agents, such as strong acids (hydrochloric, sulfuric, nitric) or alkalis (soda). Carbon does not preferentially adsorb these compounds due to its polarity and low molecular weight. 
  • Petroleum derivatives (diesel, oils, kerosene, gasoline, transparent oil, etc.). Although they are adsorbed by charcoal, these compounds do not cause significant damage. It is better not to take the risk of causing vomiting with charcoal administration (it occurs in 10% of cases). Bronchial aspiration can occur, which would have significant harmful effects.
  • Organic compounds whose molecular weight is less than 55 (methanol, ethanol, formaldehyde, acrolein, etc.), since carbon does not absorb them effectively.
  • Compounds that ionize in aqueous solution, such as most metals, and inorganic compounds in general.