Activated carbon, also known as Activated charcoal or Activated char. It is a black powdery or granular carbon substance. Activated carbon is a porous carbon with low packing density and large specific surface area due to the irregular arrangement of microcrystalline carbon and the presence of pores between cross connections, which can cause carbon structure defects during activation. It is also the main material for making filters.
Production of activated carbon
The main raw materials for activated carbon can be almost all organic materials have a large content in carbon, such as coal, wood, fruit shells, coconut shells, walnut shells, apricot shells, jujube shells, etc. These carbon containing materials are converted into activated carbon through pyrolysis at high temperature and certain pressure in an activation furnace. During this activation process, a huge surface area and complex pore structure gradually form, and the so-called adsorption process occurs in these pores and on the surface. The size of the pores in activated carbon has a selective adsorption effect on the adsorbate, because large molecules can not enter the pores of activated carbon that are smaller than its pores. Activated carbon is a hydrophobic adsorbent produced by high-temperature carbonization and activation of substances mainly containing carbon as raw materials. Activated carbon contains a large number of micropores and has an incredibly large surface area, which can effectively remove color and odor. It can also remove most organic pollutants and certain inorganic substances, including toxic heavy metals, from secondary effluent.
The principle of activated carbon
1. Filtering principle
Activated carbon filter is the process of intercepting suspended pollutants in water, and the intercepted suspended solids fill the gaps between activated carbons. The pore size and porosity of the filter layer increase with the particle size of the activated carbon material. The coarser the particle size of activated carbon, the larger the space that can accommodate suspended solids. It performs as enhanced filtering ability, increased pollutant holding capacity, and larger interception capacity. At the same time, the larger the pores in the activated carbon filter layer, the deeper suspended solids in the water can be transported to the next layer of activated carbon filter layer. With sufficient protective thickness, more suspended solids can be intercepted, allowing the middle and lower layers of filter layer to better play the interception role and increase the interception capacity of the unit.
Generally speaking, the ability of activated carbon to intercept 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, while when the flow rate is fast, the filtration capacity comes from the adsorption effect on the surface of activated carbon particles. During the filtration process, the larger the particle surface area provided by activated carbon, the stronger the adhesion to suspended solids in water.
2. Adsorption principle
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). During the adsorption process, when the interaction force between activated carbon molecules and pollutant molecules is van der Waals force (or electrostatic attraction), it is called physical adsorption; When the interaction force between activated carbon molecules and pollutant molecules is a chemical bond, it is called chemical adsorption. 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. Due to the weak van der Waals force, it has little effect on the structure of pollutant molecules. This force is similar to intermolecular cohesion, so physical adsorption can be analogized as a condensation phenomenon. The chemical properties of pollutants remain unchanged during physical adsorption.
Due to the strong chemical bonds, chemical adsorption has a significant impact on the structure of pollutant molecules. Therefore, chemical adsorption can be regarded as a chemical reaction, which is the result of the chemical interaction between pollutants and activated carbon. Chemical adsorption 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 physical adsorption and chemical adsorption lies in the force that generates adsorption bonds.
The adsorption process is the process in which pollutant molecules are adsorbed onto a solid surface, and the free energy of the molecules decreases. Therefore, the adsorption process is an exothermic process, and the heat released is called the adsorption heat of the pollutant on this solid surface. Due to the different forces of physical adsorption and chemical adsorption, they exhibit certain differences in adsorption heat, adsorption rate, adsorption activation energy, adsorption temperature, selectivity, adsorption layers, and adsorption spectra.
Activated carbon adsorption technology has been used for refining and decolorization in industries such as pharmaceuticals, chemicals, and food in China for many years. It began to be used for industrial wastewater treatment in the 1970s. Production practice has shown that activated carbon has excellent adsorption properties for trace organic pollutants in water, and it has good adsorption effects on industrial wastewater such as textile printing and dyeing, dye chemical industry, food processing, and organic chemical industry. In general, organic compounds represented by comprehensive indicators such as BOD and COD in wastewater, such as synthetic dyes, surfactants, phenols, benzenes, organochlorine, pesticides, and petrochemical products, have unique removal capabilities. Therefore, activated carbon adsorption has gradually become one of the main methods for secondary or tertiary treatment of industrial wastewater.
Adsorption is a slow process in which a 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. Adsorption in wastewater treatment is mostly the result of the combined action 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 based on the water quality of the wastewater. It is advisable to choose charcoal with well-developed transition pores for printing and dyeing wastewater. In addition, ash content also has an impact, the smaller the ash content, the better the adsorption performance; The closer the size of adsorbate molecules is to the pore diameter of carbon, the easier it is to be adsorbed; The concentration of adsorbate also has an impact on the adsorption capacity of activated carbon. Within a certain concentration range, the adsorption capacity increases with the increase of adsorbate concentration. In addition, water temperature and pH value are also related. The adsorption capacity decreases with the increase of water temperature.
