What is activated carbon?
Activated carbon or activated charcoal is a porous carbon that traps compounds, mainly organic, present in a gas or liquid. It does so effectively, that it is the most widely used purifier.
Organic compounds are derived from the metabolism of living beings, and their basic structure consists of chains of carbon and hydrogen atoms. Among them are all the derivatives of the vegetable and animal world, including oil and the compounds obtained from it.
The property of a solid to adhere a flowing molecule to its walls is called "adsorption". The solid is called "adsorbent" and the molecule, "adsorbate."
After filtration – which aims to retain solids present in a fluid – there is no single purification process with more applications than activated carbon. Among them are:
- Potabilization of water (the carbon retains pesticides, fats, oils, detergents, disinfection by-products, toxins, color-producing compounds, compounds caused by the decomposition of algae and vegetables or by animal metabolism)
- Air deodorization and purification (e.g. in cartridge respirators, air recirculation systems in public spaces, drainage vents and water treatment plants, paint application booths, spaces hat store or apply organic solvents)
- Treatment of people with acute intoxication (activated carbon is considered the “most universal antidote”, and is applied in emergency rooms and hospitals)
- Sugar refining (coal retains the proteins that give color to the cane juice; the main objective of this process is to prevent sugar from fermenting and spoiling)
- Discoloration of vegetable oils (such as coconut oil), corn glucose and other liquids for food.
- Discoloration and deodorization of alcoholic beverages (such as grape wines and distillates of any origin)
- Gold recovery (gold that cannot be separated from minerals by the flotation process, is dissolved in sodium cyanide and adsorbed onto activated carbon)
What gives activated carbon the property of adsorbing, mainly organic molecules?
Any carbon particle has the ability to adsorb. That is why some people put carbon in the refrigerator to eliminate odors. The same is true if you put carbon in a container of water: it removes color, taste and odor. 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 carbon has a surface area of about 50 m2. With the activation process, it reaches 600 or 800 m2. That is to say, it increases between 12 and 16 times.
The carbon atoms that form a solid that we call “carbon” are bound together by covalent bonds. Each atom shares an electron with four other carbon atoms (remember that in ionic bounds, the most electronegative atom steals one or more electrons from the other).
The atoms that are not on the surface distribute their four bonds in all directions. But the surface atoms, although they are bound to four other atoms, 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 de Waals Forces”. It is considered a physicochemical bond, strong enough to retain the adsorbate, but not so strong as to be considered an irreversible chemical bond that forms a new molecular structure. Therefore, the adsorption is reversible and the activated carbon can be reactivated for use again.
As we said, the molecules that carbon adsorbs tend to be covalent; not ionic, as the latter would try to steal or donate electrons to the carbon atoms. The bonds between carbon and hydrogen atoms are covalent, and that is why carbon is a good adsorbent of organic molecules.
Not all organic molecules tend to be covalent. They usually contain oxygen, Sulphur 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 those with a covalent tendency. Such is the case of gold dicyanide, which makes activated carbon an essential part of the extraction process of this precious metal.
From which raw materials can an activated carbon be obtained?
In theory, any carbon particle could be activated. However, if the carbon is very ordered (as is the case with diamond or graphite), it is difficult to remove some carbon atoms to generate pores.
One way of classifying carbons, is based on the degree of “coking” or ordering of their carbon atoms. The less ordered, the less hard the carbon is and the more easily it can be activated.
The most commonly used raw materials to make activated carbon are: soft woods (such as pine), mineral carbons "coal" (lignite, bituminous and anthracite) and vegetable shells or bones (coconut shells, olive or peach pits, walnut shells).
Activated carbons made from soft woods, form large diameter pores and are particularly suitable for discoloring liquids.
Those made from coal, tend to form a wide range of pores; they are usually more suitable for applications where the compounds to be retained are of different molecular sizes.
Those that come from hard shells or bones, form small pores, and are applied in the treatment of gases or in the purification of water coming from wells.
What is the physical form of an activated carbon?
Carbon can be produced in the form of powder, granules or cylindrical pellets.
Dust is only applied in the purification of liquids; the coal is dosed in a tank with agitation and then separated from the liquid by means of a suitable filter to retain small particles (such as the filter press).
In the case of granular coal, it is produced in different particle ranges, which are specified based on the particle size or mesh number. A 4 mesh, for example, is one that has four holes in every linear inch. They are applied both in the purification of liquids and gases.
Pellets are normally applied in gas treatment, as they cylindrical shape produces a lower pressure drop.
If a granular carbon or pellet is desired, if the raw material is not hard enough, it can be re-agglomerated with a binding agent that imparts hardness to it to prevent it from breaking when the fluid passes through.
How to activate the carbon?
Carbon can be activated by thermal or chemical processes. Thermal processes consist of provoking a partial oxidation of the carbon, to achieve that the pores are formed, but avoiding that it gets gasified and loses more carbon than necessary. This occurs at temperatures between 600 and 1100 °C (1112 °F and 2012 °F), and in a controlled atmosphere (achieved by injecting an appropriate amount of water vapor or nitrogen).
The chemical processes start from the raw material before it is carbonized. The reagents are dehydrating agents (such as phosphoric acid) that break the bonds that bind the cellulose chains together. After this stage, the material is carbonized at a relatively low temperature (about 550 °C or 1022 °F) and then washed to remove reagent residues and other by-products.
Furnaces in which a carbon is thermally activated or in which a carbon previously treated with a chemical is carbonized, can be either rotary or vertical (staged).
What is the adsorption capacity of activated carbon?
The capacity of an activated carbon to retain a given substance is not only given by its surface area, but also by the proportion of pores that are the right size, i.e. a suitable carbon has a diameter of between one and five times the molecule to be adsorbed.
If this condition is met, the capacity of an activated carbon can be between 20% and 50% of its own weight.
How does activated carbon remove free chlorine from water?
Dechlorination is a complicated mechanism that can follow different reaction paths in which activated carbon can intervene as a reactive or a catalyst.
Free chlorine can be added to water in the form of chlorine gas, sodium hypochlorite solution, or calcium hypochlorite tables (granules).
In any of these cases the chlorine is dissolved in the form of hypochlorous acid (HOCl), a weak acid that tends to partially dissociate.
The distribution between hypochlorous acid and hypochlorite ion depends on the pH and concentration of these species. Both molecular forms are defined as free chlorine.
Both are strong oxidants that when added to water react almost immediately with organic and inorganic impurities, and exert a biocidal effect on microorganisms.
The chlorine that reacts and the one that intervenes in this stage of disinfection, stops being free and remains combined and stops being free. Once this stage is finished, it is necessary to eliminate the residual free chlorine, by means of granular activated carbon.
When the carbon is exposed to free chlorine, reactions take place in which the HOCl or OCl is reduced to chloride ion. This reduction is the result of different possible reactions paths.
In two of the most common, the GAC acts according to the following reactions:
Where C* represents activated carbon. C*O and C*O2 are surface oxides, which gradually occupy spaces that, when blocked, no longer participate in the reaction. Some of these oxides are released into solution as CO and CO2. This leaves spaces available again which therefore increase the capacity of the granular activated carbon for this reaction.
As for Cl, it also accumulated on the surface of the coal during the first moments of operation. As HOCl or OCl continues to reach the surface of the carbon, the reaction slows down a little, and then Cl begins to be released. This slowdown is due to the poisoning of the carbon by surface oxides. This poisoning continues gradually, while the capacity for both adsorption and dechlorination of activated carbon decreases.
In the above reactions you can intervene instead of HOCl, with the difference that no H+ is produced. It can be seen that the activated carbon reacts and therefore disappears. If there were no accumulation of surface oxides, the reaction would continue until the complete disappearance of the carbon.
What type of carbon is best suited for bleaching?
The colors that appear in liquids are usually relatively large molecules. Therefore, they are adsorbed in large pores, which makes the carbon more suitable for retaining them the ones with the highest macroporosity.
Wood charcoals, particularly those from not very hard woods (such as pine) that are chemically activated, are the most macro-porous and, are therefore, the most suitable for discoloring.
The problem with these carbons is that they are not very hard and not very resistant to abrasion, which means that they have to be applied in powder form. When bleaching carbon is required to be granular, the best alternative is usually lignite carbon. This is the carbon with the highest macroporosity.
What type of activated carbon is the most suitable for water potabilization?
The contaminants typically present in well water are usually of low molecular weight and, for these cases, the most suitable carbon is one with high microporosity.
The carbons that best meet this condition are, firstly, coconut shell carbons and, subsequently, bituminous minerals.
Why does the pH of the water vary when a virgin carbon is installed?
When a carbon is chemically activated, it is impractical and unnecessary for the manufacturer to remove all the chemical used from the final product. Therefore, if the chemical was an acid, it will lower the pH of the first few liters of water that come in contact with the carbon. The opposite will occur if the chemical used was an alkali.
In the case of thermally activated carbon (without the presence of chemicals other than water vapors and combustion gases), the pH of the first liters of water treated with it increases.
This is because all vegetables have significant amounts of sodium, potassium, calcium and other cation that, in the carbonization process, remain in the carbon in the form of oxides. These oxides are converted into hydroxides when they come into contact with water, dissolve in the water and increase its pH.
When the pH of the first liters of water that come into contact with a carbon does not change, it can be a pH-adjusted carbon or an ultra-pure carbon (free of soluble).
What type of activated carbon is best suited to purify air and gases?
All contaminants in the gaseous state have molecular diameters less than 2 nm. This means that they are preferably adsorbed in micropores. Coconut shell carbons have the highest microporosity and are therefore the most commonly used in air and gas purification.
There is modified structure activated carbons, special activated carbons, which are used when a standard activated carbon cannot retain other non-organic compounds.