Adsorption by Activated Carbon

The applications of importance are the removal of color, taste and odor in drinking water or other organic or inorganic substances that may be contained in it; but it is also used in industry, in the purification of air and gases, etc.

The activated carbons can be used in various forms: powder, granulate or currently also in fibrous form. Powdered activated carbons (CAP) generally have a particle diameter of the order of 44 micrometers, which gives a rapid adsorption, but are difficult to handle in fixed beds, generate higher load losses and are difficult to regenerate. For its part, in its granulated form (CAG), the particle diameters are from 0.6 to 4 mm, they are hard, resistant to abrasion, relatively denser, generate lower pressure loss, and can be regenerated by using steam. Activated carbon fibers (CAF) are expensive materials for water treatment, but have the advantage of being easily molded to the shape of the adsorption matrix and produce very low head loss.

Activated carbons include a wide variety of processed amorphous materials. Actually, they are not amorphous, but have a micro crystalline structure. These compounds have a very high porosity and specific surface. Its production involves two stages: the first, the carbonization of raw carbonaceous material at temperatures below 800 ° C in an inert atmosphere and then the activation of the charred product. Thus, all carbonaceous materials can be converted into activated carbon, although the final properties thereof will be different depending on the nature of the raw material used, the nature of the activating agent, the carbonization conditions and the activation process thereof.

During the carbonization process, most non-carbonaceous elements such as oxygen, hydrogen and nitrogen are eliminated as gaseous volatile species by the pyrolytic decomposition of the raw material. The elemental carbon atoms are grouped in piles of flat aromatic leaves crisscrossed in a random manner. These aromatic leaves are arranged irregularly, leaving free interstices. These are ultimately pores, which convert activated carbon into a large adsorbent. The result of coal activation (high temperatures in oxidic atmospheres and with CO2) is a material with a very high internal surface, which in some cases can reach 2500 m2/g.

The typical elemental composition of an activated carbon is  88% C, 0,5% H, 0,5% N, 1,0% S y 6-7% O.

Adsorption

This phenomenon arises as a result of unsaturated and unbalanced molecular forces that are present on every surface of a solid. In this way, when a solid surface is brought into contact with a liquid or gas, there is an interaction between the field forces of the surface and those of the liquid or gas. The solid surface tends to satisfy these residual forces by attracting and retaining on its surface the molecules, atoms or ions of the gas or liquid.

Depending on the nature of the forces involved, the adsorption can be of two types: physical or chemical. In the case of physical adsorption, the adsorbate is bound to the surface by relatively weak forces such as Van der Waals, which are similar to the molecular forces of cohesion that are involved in the condensation of vapors on liquids. The chemical adsorption, on the other hand, involves the exchange of electrons between the adsorbate molecules and the surface of the adsorbent, resulting in a chemical reaction. The bond formed between adsorbate and adsorbent is much stronger than in physical adsorption.

The adsorption information can be represented by several isothermal equations, the most important ones: Langmuir, Freundlich, Brunauer-Emmett-Teller (BET) and Dubinin. The first two isotherms apply for both physical and chemical adsorption.

The adsorption on activated carbon has wide applications in industries such as pharmaceutical or food.

Adsorption on activated carbon

The surface of the coal has a unique characteristic. It has a porous structure which determines its adsorption capacity, has a chemical structure that influences its interaction with polar or non-polar adsorbates, has active sites in its structure, etc. In this way, activated carbon is an excellent and versatile adsorbent.

The applications of importance are the removal of color, taste and odor in drinking water or other organic or inorganic substances that may be contained in it; but it is also used in industry, in the purification of air and gases, etc.

The activated carbons can be used in various forms: powder, granulate or currently also in fibrous form. Powdered activated carbons (CAP) generally have a particle diameter of the order of 44 micrometers, which gives a rapid adsorption, but are difficult to handle in fixed beds, generate higher load losses and are difficult to regenerate. For its part, in its granulated form (CAG), the particle diameters are from 0.6 to 4 mm, they are hard, resistant to abrasion, relatively denser, generate lower pressure loss, and can be regenerated by using steam. Activated carbon fibers (CAF) are expensive materials for water treatment, but have the advantage of being easily molded to the shape of the adsorption matrix and produce very low head loss.

The factors that affect the adsorption equilibrium can be listed below. The main factors that affect the shape of the adsorption isotherm include:

  • Nature of the adsorbate.
  • Total surface of the adsorbent.
  • Chemical characteristics of the adsorbent surface.
  • pH
  • Temperature
  • Adsorbates competition.
  • Reversibility of adsorption.

Use of activated carbon in the field of drinking water:

The use of powdered activated carbon (CAP) is usually carried out in water treatment plants permanently or to face short-term emergencies in the plant or temporary seasonal problems, with the following objectives:

  • Improve the efficiency of the removal of organic matter in conventional processes.
  • Act as a coagulation aid.
  • Eliminate flavors and odors

Although the application of the CAP is based on the same adsorption principles as the granular activated carbon (CAG), it is not used as a filtering medium, but is added directly to the water in different points during the treatment process. In many establishments its use is common to mitigate effects of algae in the water. However, the main application of CAP in water treatment plants has been the removal of odors and flavors. The doses used can range from a few milligrams per liter to values ​​higher than 100 mg / L, however, the usual typical range is below 25 mg / l.

In contrast, in granular activated carbon (CAG) columns, adsorption occurs in a filling strip generally referred to as the mass transfer zone. The activated carbon located behind the mass transfer zone is completely saturated, while the amount of adsorbate retained per unit mass of activated carbon corresponds to that expressed by the adsorption isotherm for the input concentration to the column. On the other hand, the coal located beyond the mass transfer zone has not yet been exposed to the inlet adsorbate concentration, resulting in zero both the concentration of the treated water and the amount adsorbed on the activated carbon. Within the saturation zone, the degree of saturation of the bed varies from 0% to 100%

Granular activated carbon can be used in open downflow filters operated by gravity, or in closed systems operating at pressures above atmospheric. The granular activated carbon systems may be of the fixed or expanded bed type. On the other hand, the flow of water to be treated may have an ascending or descending direction, although in water treatment the most usual mode of operation is downflow.

When the treatment system belongs to small plants in general, a single stage is used; in this case, the activated carbon must be renewed when the mass transfer zone begins to “leave” the column, which is reflected in the quality of the effluent. When this moment arrives, normally only a fraction of the activated carbon contained in the unit has reached the condition of equilibrium (saturation) with the concentration of the feed. The greater the fraction of saturated carbon at the time of renewal, the greater the use of the adsorbent will have been and therefore the operation of the unit will be more economical. But also in order to increase the treatment capacity, columns can be arranged in series so that the mass transfer zone leaving the first unit is completely included between the units located downstream.

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