Adsorption principle of activated carbon

Activated carbon (English: Active charcoal), also known as activated carbon, activated charcoal, activated charcoal or activated carbon, is a black powdered or granular carbon substance. Activated carbon is structurally due to the microcrystalline carbon is irregularly arranged, there are fine pores between the cross-connections, in the activation will produce carbon organisational defects, so it is a kind of porous carbon, low packing density, large specific surface area, but also to do a filter of the main material.

Activated carbon is prepared from wood, coal, petroleum coke and other carbon-containing raw materials by pyrolysis, activation processing, with a developed pore structure, a large specific surface area and rich surface chemical groups, specific adsorption capacity of the carbon material of the strong general term.

Production of Activated Carbon

The main raw materials for activated carbon can be almost all organic materials rich in carbon, such as coal, wood, fruit shells, coconut shells, walnut shells, apricot shells, date shells and so on. These carbon-containing materials are converted into activated carbon by pyrolysis under high temperature and certain pressure in an activation furnace. In this activation process, a huge surface area and complex pore structure is gradually formed, and the so-called adsorption process is carried out in these pores and on the surface, the size of the pores in the activated carbon has a selective adsorption effect on the adsorbent, which is due to the fact that macromolecules can't get into the activated carbon aperture that is smaller than the one in which it is porous. Activated carbon is a hydrophobic adsorbent produced by high temperature carbonisation and activation of carbonaceous materials as raw materials. Activated carbon contains a large number of micropores, has a huge surface area, can effectively remove colour, odour, can remove most of the organic pollutants in the secondary effluent water and some inorganic substances, including some toxic heavy metals.

Principle of activated carbon

1、The principle of filtration

Activated carbon filter is the process of retaining pollutants in suspension in water, and the retained suspended matter is filled up in the space between the activated carbon. The size of the pore scale and porosity of the filter layer increases with the increase in the particle size of the activated carbon material. That is, the coarser the particle size of the activated carbon, the larger the space that can accommodate the suspended matter. This is manifested in the enhancement of filtration capacity, increased dirt-holding capacity, and increased dirt-holding capacity. At the same time, the larger the pores of the activated carbon filter layer, the deeper the suspended solids in the water can be transported to the next layer of activated carbon filter layer, under the condition of sufficient protection thickness, the suspended solids can be retained more, so that the middle and lower layers of the filter layer can better play the role of retention, and the unit intercepted dirt increased.

From a strictly theoretical point of view, the retention capacity of suspended solids possessed by activated carbon comes from the surface area provided by the activated carbon. When the flow rate is low, the filtration capacity of the unit mainly comes from the sieving effect of the activated carbon, while when the flow rate is fast, the filtration capacity comes from the adsorption effect on the surface of the activated carbon particles, and the larger the surface area of the particles provided by the activated carbon during the filtration process, the stronger the adhesion to the suspended matter in the water.

2. Principle of adsorption

According to the different forces between activated carbon molecules and pollutant molecules in the adsorption process, the adsorption can be divided into two categories: physical adsorption and chemical adsorption (also known as active adsorption). In the adsorption process, when the force between activated carbon molecules and pollutant molecules is van der Waals force (or electrostatic attraction), it is called physical adsorption; when the force between activated carbon molecules and pollutant molecules is chemical bonding, it is called chemical adsorption. The adsorption strength of physical adsorption is mainly related to the physical properties of the activated carbon, but not to the chemical properties of the activated carbon. Due to the weak van der Waals force, the structure of the pollutant molecules do not have much influence, this force is the same as the intermolecular cohesion, so the physical adsorption can be analogous to the cohesion phenomenon. The chemical properties of the pollutants remain unchanged during physical adsorption.

Due to the strong chemical bond, the structure of the pollutant molecules have a greater impact, so the chemical adsorption can be regarded as a chemical reaction, which is the result of the chemical interaction between the pollutant and the activated carbon. Chemisorption generally involves electron pair sharing or electron transfer, rather than simple perturbation or weak polarisation, and is an irreversible chemical reaction process. The fundamental difference between physical and chemical adsorption is the force that produces the adsorption bond.

Adsorption is a process in which pollutant molecules are adsorbed onto a solid surface, and the free energy of the molecules decreases; therefore, the adsorption process is exothermic, and the heat released is called the heat of adsorption of the pollutant on this solid surface. Due to the different forces involved in physical and chemical adsorption, they show certain differences in heat of adsorption, rate of adsorption, activation energy of adsorption, temperature of adsorption, selectivity, number of adsorption layers and adsorption spectra.

Activated carbon adsorption technology has been used for many years for refining and decolourisation in the pharmaceutical, chemical and food industries in China, and has been used for industrial wastewater treatment since the 1970s. Production practice shows that activated carbon has excellent adsorption of trace organic pollutants in water, and it has good adsorption effect on industrial wastewater of textile printing and dyeing, dyestuff chemical industry, food processing and organic chemical industry. In general, it has unique removal ability for organic substances expressed in wastewater by BOD, COD and other comprehensive indicators, such as synthetic dyes, surface agents, phenols, benzene, organochlorine, pesticides and petrochemicals. Therefore, activated carbon adsorption has gradually become one of the main methods for secondary or tertiary treatment of industrial wastewater.

Adsorption is a slow-acting process in which one substance adheres to the surface of another substance. Adsorption is an interfacial phenomenon that is related to changes in surface tension and surface energy. There are two driving forces that cause adsorption, one is the repulsive force of solvent water on hydrophobic substances, and the other is the affinity attraction of solids on solutes. Most of the adsorption in wastewater treatment is the result of the combined effect of these two forces. The specific surface area and pore structure of activated carbon directly affects its adsorption capacity, and when selecting activated carbon, it should be determined by test according to the water quality of wastewater. For printing and dyeing wastewater, it is preferable to choose the charcoal with developed transition pores. In addition, ash also has an effect, the smaller the ash, the better the adsorption performance; adsorbent molecule size and carbon pore diameter is closer to the more easily adsorbed; adsorbent concentration of activated carbon adsorption also has an effect. In a certain concentration range, the adsorption amount is increased with the increase of adsorbent concentration. In addition, the water temperature and pH value also have an effect. The adsorption amount decreases with the increase of water temperature.