The Adsorption Principle of Activated Carbon

Introduction of activated carbon

Activated carbon is a black powder or granular carbon material. Because of the irregular arrangement of microcrystalline carbon in the structure of activated carbon, there are pores between cross-connections, and carbon structure defects will be generated during activation, so it is a kind of porous carbon with low bulk density and large specific surface area. The main material of the filter.

Production of activated carbon

The main raw material of activated carbon can be almost all carbon-rich organic materials, such as coal, wood, fruit shell, coconut shell, walnut shell, apricot shell, jujube shell, etc. These carbonaceous materials are converted into activated carbon by pyrolysis at high temperature and certain pressure in an activation furnace. During this activation process, a huge surface area and a complex pore structure are gradually formed, and the so-called adsorption process is carried out in and on these pores. The size of the pores in the activated carbon has a selective adsorption effect on the adsorbate, which is Because macromolecules cannot enter into the pores of activated carbon smaller than its pores. Activated carbon is a hydrophobic adsorbent made of carbon-based materials as raw materials, which are carbonized and activated at high temperature. Activated carbon contains a large number of micropores and has a huge surface area, which can effectively remove color and odor, and can remove most organic pollutants and some inorganic substances in secondary effluent, including some toxic heavy metals.

The principle of activated carbon

1) Filtering principle

The activated carbon filter is a process of intercepting the pollutants in the suspended state in the water, and the intercepted suspended matter fills the gaps between the activated carbons. The pore size and porosity of the filter layer increase with the increase of 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 solids. It is manifested as enhanced filtration capacity, increased dirt holding capacity, and increased dirt interception. 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, making the middle and lower filter layers more efficient. The interception function is well exerted, and the amount of pollutant interception of the unit increases.

Strictly speaking, the retention capacity of activated carbon for suspended solids comes from the surface area provided by activated carbon. When the flow rate is low, the filtration capacity of the unit mainly comes from the screening effect of activated carbon, and when the flow rate is fast, the filtration capacity comes from the adsorption effect on the surface of activated carbon particles. The stronger the adhesion.

2) The principle of adsorption

According to the different forces between activated carbon molecules and pollutant molecules during the adsorption process, 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 bonds, it is called chemisorption. . The adsorption strength of physical adsorption is mainly related to the physical properties of activated carbon, and has little to do with the chemical properties of activated carbon. Because the van der Waals force is weak, it has little effect on the structure of pollutant molecules. This force is the same as the intermolecular cohesion force, so physical adsorption can be compared to agglomeration phenomenon. The chemical properties of the pollutants remain unchanged upon physical adsorption.

Due to the strong chemical bond, it has a great influence on the structure of pollutant molecules, so chemisorption can be regarded as a chemical reaction, which is the result of the chemical interaction between pollutants and activated carbon. Chemisorption generally involves electron pair sharing or electron transfer, rather than simple perturbation or weak polarization, and is an irreversible chemical reaction process. The fundamental difference between physisorption and chemisorption is the force that creates the adsorption bond.

The adsorption process is a process in which pollutant molecules are adsorbed to the solid surface, and the free energy of the molecules will decrease. Therefore, the adsorption process is an exothermic process, and the heat released is called the adsorption heat of the pollutant on the solid surface. Due to the different forces of physical adsorption and chemical adsorption, they show certain differences in adsorption heat, adsorption rate, adsorption activation energy, adsorption temperature, selectivity, adsorption layer number and adsorption spectrum.

Activated carbon adsorption technology has been used in the refining and decolorization of pharmaceutical, chemical and food industries for many years in China. It has been used for industrial wastewater treatment since the 1970s. Production practice shows that activated carbon has excellent adsorption to trace organic pollutants in water, and it has good adsorption effect on industrial wastewater such as textile printing and dyeing, dye chemical industry, food processing and organic chemical industry. Under normal circumstances, it has a unique ability to remove organic compounds represented by comprehensive indicators such as BOD and COD in wastewater, such as synthetic dyes, surfactants, phenols, benzenes, organochlorines, pesticides and petrochemical products. Therefore, activated carbon adsorption has gradually become one of the main methods for secondary or tertiary treatment of industrial wastewater.

Adsorption is the slow-acting process of one substance attaching to the surface of another. Adsorption is an interfacial phenomenon, which is related to changes in surface tension and surface energy. There are two driving abilities that cause adsorption, one is the repulsion of solvent water to hydrophobic substances, and the other is the affinity attraction of solids to 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 affect its adsorption capacity. When selecting activated carbon, it should be determined through experiments according to the quality of wastewater. For printing and dyeing wastewater, carbon species with developed transition pores should be selected. In addition, the ash content also has an influence. The smaller the ash content, the better the adsorption performance; the closer the size of the adsorbate molecule is to the carbon pore diameter, the easier it is to be adsorbed; the adsorbate concentration also affects the adsorption capacity of activated carbon. Within a certain concentration range, the adsorption capacity increases with the increase of the adsorbate concentration. In addition, water temperature and pH also play a role. The adsorption capacity decreased with the increase of water temperature.