In water purification, activated carbon is primarily used as an adsorbent to remove undesired colouring, flavouring or odorous substances. Activated carbon is mostly delivered in the form of granulate, powder and pellets.
Unipolar and especially organic contaminants but also inorganic compounds or elements such as chlorine or ozone from the water can be removed by means of activated carbon (no salts or other ions).
Especially if such substances are present at very low concentrations so that other – technical – procedures would not be used effectively, activated carbon is used.
All active carbons are characterised by a spongy construction. The single pores of the “sponge” are connected with each other and differ in size. Here, a distinction is made between three pore size regions:
- Macropores at a diameter of > 50 nm (> 0.0000050 mm)
- Mesopores at a diameter of 2 to 50 nm
- Micropores at a diameter of < 2 nm (< 0.0000002 mm)
Macropores serve as the main access pathway for water to infiltrate carbon. Such pores are of no considerable practical importance with regard to the adsorption process. In carbon material, adsorption mainly takes place at the surface area of the micropores and here especially at microscopic graphite crystals or “tiny crystals”. In some parts they are so tiny that they reach molecular sizes. The surface of micropores forms the effective surface and determines the adsorption properties of carbon. The larger the molecule that is to be removed, the better AC functions in water purification. The count of so-called functional groups (especially reactive parts within the molecules) or the electric polarisability of the molecule also plays a significant role.
This can also be imagined this way: once molecules have accessed activated carbon they are locked inside. This is due to the fact that once they have left the site they were previously attached to, they immediately “stick” to another site due to the small pores.
A huge surface area is formed by the structures of the internally connected channels. As a rule of thumb, only 4 g of activated carbon correspond approximately to the surface of one football pitch.
Apart from carbon itself, the temperature, the accessibility as well as the reaction time also play a role in the purification process. The adsorption capacity decreases with an increase in temperature. The other parameters can be influenced, for example, by the grain size of the particles (pellets/granulate) or the technical parameters such as flow velocity or the amount of activated carbon/water volume.
Adsorption is a physical process in which substances (generally molecules) get attached to the surface of another substance where they get enhanced. Due to adhesive forces (attractive force between two different types of molecules), the natural desire to get attached is common to all gaseous or liquid substances. Those forces that cause molecules to attach themselves to a surface are not considered chemical bonds but only van der Waals forces (relative loose attachment of materials on surfaces, such as, for example, a hair on a wall).
In general, activated carbon can be manufactured from all materials with a sufficient carbon content in their matrix. These are especially current or previous herbal raw materials now occurring in the form of peat, wood, lignin, bituminous coal, petroleum coke, brown coal, coconut shell, fruit stones or sugar.
Activated carbon is manufactured by means of two common procedures, called water vapour activation or chemical activation:
- In chemical activation, the raw material is dried and mixed with chemicals such as phosphoric acid, potassium hydroxide, sodium carbonate, zinc chloride or sodium sulphate. At a temperature between 450° and 600° C and in a closed atmosphere, the chemicals extract hydrogen and oxygen atoms from the raw material. The residual carbon has a more or less large pore volume which can be controlled by means of the reactive conditions and the source chemicals.
- In water vapour activation, the carbon of a raw material is partially gasified at temperatures between 800° and 1000° C under a nitrogen atmosphere by bringing it into contact with oxidizing gases. In this process, the reaction temperature and time as well as the oxidants’ concentration influence the quality and thus the range of application of activated carbon.
Activated carbons from Euroquarz are always designated with a letter in the beginning. At the moment, the letters K and S are used indicating the raw material employed in the manufacturing process. K indicates that the activated carbon has been manufactured from coconut shell and S stands for the raw material hard coal.
The grain size of coal is generally measured in the American unit mesh. The number indicates how many theoretical meshes are allowed to occur at a diameter of 1 inch (25.4 mm) so that all grains fall through. Mesh thus indicates the largest grain. The smaller the particles, the more meshes can occur in one “sieve”. In practice, the thickness of the sieve wire also plays a role with regard to all measurements, but discussing that would go into too much detail here.
Table 1 Conversion of mesh to millimetres
| U.S. mesh|
Activated carbon K-8-35 has a grain size of 0.500 – 2.35 mm and K-8-14 has a grain size of 1.410 – 2.35 mm.
There are a number of measurable values each describing specific properties of activated carbon. Here, a particular emphasis is laid on the active surface. The following properties are generally specified:
Here, the iodine count is defined as the mass of iodine in milligrams that has been adsorbed in a liquid solution by 1 g of activated carbon. In practice, a liquid iodine solution and a defined amount of activated carbon are dispensed into a volumetric flask that is then intensively shaken. Then, the coal is removed by filtration and the filtrate (from which iodine is now missing as it has been adsorbed by the coal) is examined. On the basis of the missing amount of iodine, the iodine count is determined for each calculation. The higher the count, the more iodine was adsorbed and the more “active” the coal.
As laboratory devices as well as the laboratory assistant have a high influence on the results at different points during the determination process (e.g. how long and how intensely and at what temperature it was shaken), the iodine count shall only be considered a reference value rather than an absolute value for the comparison of quality.
The BET surface area indicates the active surface of activated carbon, which is determined by a fairly complicated model in which the uptake and release behaviour of liquid nitrogen plays a significant role. The higher the active surface area, the “better” the activated carbon.
The ash content measures the mineral content of activated carbon in wt%. The ash content is measured by transferring the contained minerals into the corresponding oxides at a temperature of 800° C. The ash mainly consists of silicone dioxide and aluminium oxide. The amount is dependent on the used basic raw material in the manufacturing of activated carbon.