Carbon Adsorption In Water Treatment How Adsorption Works

Activated carbon is a crude form of graphite, the substance used for pencil leads. It differs from graphite by having a random imperfect structure which is highly porous over a broad range of pore sizes from visible cracks and crevices to molecular dimensions. The graphite structure gives the carbon it's very large surface area which allows the carbon to adsorb a wide range of compounds. Activated carbon can have a surface of greater than 1000 m2/g. This means 5g of activated carbon can have the surface area of a football field. Adsorption is the process by which liquid or gaseous molecules are concentrated on a solid surface, in this case activated carbon. This is different from absorption, where molecules are taken up by a liquid or gas. Activated carbon can made from many substances containing a high carbon content such as coal, wood and coconut shells. The raw material has a very large influence on the characteristics and performance activated carbon.

The term activation refers to the development of the adsorption properties of carbon. Raw materials such as coal and charcoal do have some adsorption capacity, but this is greatly enhanced by the activation process. There are three main forms of activated carbon.

• Granular Activated Carbon (GAC) - irregular shaped particles with sizes ranging from 0.2 to 5 mm. This type is used in both liquid and gas phase applications.

• Powder Activated Carbon (PAC) - pulverized carbon with a size predominantly less than 0.18mm (US Mesh 80). These are mainly used in liquid phase applications and for flue gas treatment.

• Pelleted Activated Carbon - extruded and cylindrical shaped with diameters from 0.8 to 5 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength

Activated carbon is also available in special forms such as a cloth and fibres. Activated Charcoal Cloth (ACC) represents a family of activated carbons in cloth form. These products are fundamentally unique in several important ways compared with the traditional forms of activated carbon and with other filtration media that incorporate small particles of activated carbon. Developed in the early 1970's ACC products are

similar to the traditional activated carbon products in that they are 100 % activated carbon. This gives the products the same high capacity for adsorption of organic compounds and other odorous gases as the more traditional, pelletised, granular and powder forms of activated carbon. As with the traditional forms of activated carbons, ACC products can be impregnated with a range of chemicals to enhance the chemisorption capacity for selected gases. By being constructed of bundles of activated carbon filaments and fibres in a textile form, several important advantages are imparted to ACC. The diameter of these fibres is approximately 20 mm, so the kinetics for ACC products are similar to that of a very tine carbon particle. Gases and liquids can flow through the fabric and the accelerated adsorption kinetics mean that the ACC can retain the advantages of mass transfer zones associated with deeper filter beds. Faster adsorption rates mean smaller adsorption equipment and up to twenty times less carbon on line.

Adsorption is the process where molecules are concentrated on the surface of the activated carbon. Adsorption is caused by London Dispersion Forces, a type of Van der Waals Force which exists between molecules. The force acts in a similar way to gravitational forces between planets. London Dispersion Forces are extremely short ranged and therefore sensitive to the distance between the carbon surface and the adsórbate molecule They are also additive, meaning the adsorption force is the sum of all interactions between all the atoms. The short range and additive nature of these forces results in activated carbon having the strongest physical adsorption forces of any material known to mankind. All compounds are adsorbable to some extent. In practice, activated carbon is used for the adsorption of mainly organic compounds along with some larger molecular weight inorganic compounds such as iodine and mercury. In general, the adsorbability of a compound increases with : (1) increasing molecular weight, (2) a higher number of functional groups such as double bonds or halogen compounds, and (3) increasing polarisability of the molecule. This is related to electron clouds of the molecule. Refer to Figure 6. The most common manufacturing process is high temperature steam activation though activated carbon can also be manufactured with chemicals. Along with the raw material, the activation process has a very large influence on the characteristics and performance of activated carbon. Figure 7 illustrates the production of granular

Gas Phase Adsorption - This is a condensation process where the adsorption forces condense the molecules from the bulk phase within the pores of the activated carbon. The driving force for adsorption is the ratio of the partial pressure and the vapour pressure of the compound.

Liquid Phase Adsorption - The molecules go from the bulk phase to being adsorbed in the pores in a semi-liquid state. The driving force for adsorption is the ratio of the concentration to the solubility of the compound.

Reactivated, normally off-site

# Functional Groups - The molecular structure of the resin is such that it must contain a macroreticular tissue with acid or basic radicals. These radicals are the basis of classifying ion exchangers into two general groups: (1) Cation exchangers, in which the molecule contains acid radicals of the HS03 or HCO type able to fix mineral or organic cations and exchange with the hydrogen ion H + ; (2) Anion exchangers, containing basic radicals (for example, amine functions of the type NH2) able to fix mineral or organic anions and exchange them with the hydroxyl ion OH" coordinate to their dative bonds. The presence of these radicals enable a cation exchanger to be assimilated to an acid of form H-R and an anion exchanger to be a base of form OH-R when regenerated. In-situ regenerated. This is possible for most gas phase and some liquid phase applications.

It may also be replaced with new carbon and disposal of the exhausted carbon Most adsorbers are pressure vessels constructed in carbon steel, stainless steel or plastic. Large systems for drinking water are often constructed in concrete. In some cases, a moving or pulsed bed adsorber is employed to optimize the use of the granular activated carbon.

The main factors in the design of an adsorption system are the: (1) Carbon consumption - The amount of carbon required to treat the liquid or gas, normally expressed per unit of the fluid treated; and (2) Contact time - For a fixed flow rate, the contact time is directly proportional to the volume of carbon and is the main factor influencing the size of the adsorption system and capital cost. With powder activated carbon, in most cases, the carbon is dosed into the liquid, mixed and then removed by a filtration process. In some cases, two or more mixing steps are used to optimise the use of powder carbon. Powder activated carbon is used in a wide range of liquid phase applications and some specific gas phase applications such as Incinerator flue gas treatment and where it is bonded into filters such as fabrics for personnel protection.

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