What is activated carbon (active carbon)?
Activated (active) charcoal is a carbon sorbent, produced industrially. Active charcoal has standardized quality indicators. The sorption capacity, specific area of pores of activated carbon, particle sizes and a number of other indicators are specified by the Standards or Production Specifications.
Raw materials for the production of active coal
Activated carbon can be produced from any carbonaceous material. Mainly active carbon is produced from coconut shell – activated coconut charcoal, coal - mineral activated carbon and wood - activated charcoal.
The production of activated carbon from low-porous raw material consists from activation, grinding and sieving on fractions. During activation, a structure, contained a large number of pores, is formed. There can be others operations in the production of special grades of activated carbon.
Methods of charcoal activation
There are used two activation methods in the production of activated carbon:
Activation with water vapor at 700 – 900 °C. Pores are formed in the internal structure of activated carbon. In the result a finely porous activated carbon is formed. During steam activation, partial oxidation of coal takes place.
The raw materials are mixed with a dehydrating substance (acid or zinc chloride) and is heated to 400 – 600 °C. In the result, there is formed coarse-porous activated carbon, which is used, for example, for bleaching.
Adsorption and desorption
The accumulation of substances in the pores of the adsorbent is called adsorption. Adsorption occurs when the gas or liquid passes through activated charcoal. Desorption – the release of substances accumulated in the adsorption process from the sorbent.
There are physical adsorption and chemisorption:
Physical adsorption occurs primarily under Van der Waals forces, and chemical properties of adsorbed substances are not changed. Physical adsorption is reversible, the adsorbed substances can be separated from the sorbent.
In chemisorption, the substance enters into chemical reaction with the sorbent. Its chemical properties are changed as well as the properties of the sorbent. Chemisorption is irreversible.
Substances adsorbed by activated carbon
Activated carbon can adsorb organic and non-polar substances such as solvents chlorinated hydrocarbons, coloring materials, oil and petroleum products. Macromolecular substances and substances with non-polar structure are adsorbed better.
The ability of sorption of the activated carbon increases with decreasing of substance solubility in water (for substances with non-polar structure) and increasing of molecular mass.
A graphical representation of the adsorption of activated carbon in the form of adsorption isotherms
Adsorption, as a function of the substance concentration that must be adsorbed, is presented in the form of isotherms. Isotherm describes the equilibrium between the substance that must be adsorbed in the liquid or in the air (residual concentration) and the substance adsorbed in the activated carbon (the maximum amount in this residual concentration). Usually, the maximum capacity increases with increasing of initial concentration.
Raw materials for the production of active coal
Activated carbon can be produced from any carbonaceous material. Mainly active carbon is produced from coconut shell – activated coconut charcoal, coal – mineral activated carbon and wood – activated charcoal.
The essential characteristic in activated carbon is pores!
Active charcoal has a porous structure and large internal surface area. Due to these properties, activated charcoal is used as a sorbent. Activated carbon is able to hold molecules of pollutants during water purification, air cleaning, purification of liquids and gases.
The pore volume of activated carbon by definition exceeds 0.2 ml/g; the inner surface is more than 400 sq.m/g. The pores can have a size from 0.3 nanometers to several thousand nanometers (1 nanometer = 10 -9 cm).
The structure of activated carbon
The molecular structure of activated charcoal contains carbon in the form of platforms or rings of several atoms. They form the walls of the molecular pores of the active carbon. Usually rings have gaps. Because of this structural defect, there can be reaction in the gap of the ring.
The pores of active carbons classified by diameter:
The micropores of the activated carbon are less than 1 nanometer.
The mesopores of activated carbon are from 1 to 25 nanometers.
The macropores of the activated carbon are more than 25 nanometers.
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