Activated carbon is a kind of commonly used adsorbent, catalyst or catalyst carrier. When it is widely used in water treatment, how to choose active carbon? Let's analyze it together!

What is activated carbon

Activated carbon is a kind of microcrystalline carbon material with black appearance, developed pore structure, large specific surface area and strong adsorption capacity. It is a commonly used adsorbent, catalyst or catalyst carrier. Activated carbon can be divided into: wood activated carbon, shell activated carbon, animal bone / blood activated carbon, mineral raw material activated carbon, synthetic resin activated carbon, rubber / plastic active carbon, recycled activated carbon, etc; Activated carbon can be divided into powder, granular, irregular, cylindrical, spherical and fibrous according to appearance. Activated carbon is widely used, and its application involves almost all national economic departments and people's daily life, such as water purification, gold extraction, sugar liquor decolorization, drug injection extraction, blood purification, air purification, human safety protection, etc.

Composition and manufacture of activated carbon

Activated carbon is amorphous carbon which is processed and has a large specific surface area, and has good adsorption capacity for gas, inorganic or organic matter in solution and colloidal particles. The active carbon materials mainly include activated carbon (a C) and activated carbon fibers (ACF). As a kind of adsorbent with excellent performance, activated carbon material is mainly determined by its unique adsorption surface structure and chemical properties. The chemical properties of activated carbon materials are stable, high mechanical strength, acid resistance, alkali resistance, heat resistance, insoluble in water and organic solvents, and can be recycled. It has been widely used in chemical industry, environmental protection, food processing, metallurgy, drug refining, military chemical protection and other fields. At present, the modified activated carbon materials are widely used in the fields of sewage treatment and air pollution prevention, and it has been more and more attractive in the treatment of environmental pollution.

More than 80% to 90% of activated carbon is composed of carbon elements, which is also the reason why activated carbon is hydrophobic adsorbent. Besides carbon elements, there are two kinds of admixtures: one is chemical binding elements, mainly oxygen and hydrogen. These elements are left in carbon due to incomplete carbonization, or in the activation process, foreign non-carbon elements are chemically combined with the surface of activated carbon, such as when activated by steam gas, the surface of activated carbon is oxidized or water vapor oxidized; The other kind of admixture is ash, which is the inorganic part of activated carbon.

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

The principle of activated carbon

1. filtering principle

Activated carbon filter is the process of intercepting pollutants in suspended state in water, and the suspended substance is filled with the void between activated carbon. The pore size and porosity of the filter layer increase with the increase of the particle size of the activated carbon. That is, the coarser the particle size of activated carbon, the larger the space for the suspension. The results show that the filtration capacity, the capacity of pollution absorption and the amount of wastewater interception are increased. At the same time, the larger the pore of the activated carbon filter, the more the suspended matter in water can be further transported to the next layer of activated carbon filter. Under the condition of sufficient protection thickness, the suspended substance can be more intercepted, so that the middle and lower layer filter layer can play a better role in interception and the unit's pollution intercepting capacity will increase.

In strict theory, the ability of activated carbon to intercept suspended matter 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 action of the activated carbon, while the filtration capacity is from the adsorption of the activated carbon particles surface when the flow rate is fast. 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. adsorption principle

According to the different forces between activated carbon and pollutant molecules in the adsorption process, adsorption can be divided into two categories: physical adsorption and chemical adsorption (also known as active adsorption). In the process of adsorption, when the interaction between activated carbon and pollutant molecules is van der Waals force (or electrostatic attraction), it is called physical adsorption; When the interaction between activated carbon 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 is not related to the chemical properties of activated carbon. Because van der Waals force is weak, it has little influence on the structure of pollutant molecules. This force is the same as the intermolecular cohesion, so physical adsorption can be compared to condensation. The chemical properties of pollutants remain unchanged during physical adsorption.

Because of the strong chemical bond, it has a great influence on the structure of pollutant molecules, so the chemical adsorption can be regarded as chemical reaction, which is the result of the chemical interaction between pollutants and activated carbon. Chemical adsorption generally includes electron to share 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 is the force of the adsorption bond.

Adsorption process is the process of adsorption of pollutant molecules to the solid surface, and the free energy of molecules will be reduced. Therefore, the adsorption process is exothermic process. The heat released is called the adsorption heat of the pollutant on the solid surface. Due to the different physical and chemical adsorption forces, they have some differences in adsorption heat, adsorption rate, adsorption activation energy, adsorption temperature, selectivity, number of adsorption layers and adsorption spectrum. Activated carbon adsorption technology has been used in the refining and decolorization of medicine, chemical industry and food industry for many years. It was used in industrial wastewater treatment in 1970s. The production practice shows that activated carbon has excellent adsorption on trace organic pollutants in water, and it has good adsorption effect on textile printing and dyeing, dye chemical industry, food processing and organic chemical industry wastewater. Generally, organic substances in wastewater represented by BOD and COD, such as synthetic dyes, surfactants, phenols, benzene, organochlorine, pesticides and petrochemical products, have unique removal capacity. Therefore, activated carbon adsorption has gradually become one of the main methods of secondary or tertiary treatment of industrial wastewater.

Adsorption is a slow process of the adsorption of one substance on the surface of another. Adsorption is an interface phenomenon, which is related to the change of surface tension and surface energy. There are two kinds of driving force that cause adsorption, one is the repellent force of solvent water to hydrophobic substances, the other is the affinity attraction of solid to solute. The adsorption in wastewater treatment is mostly the result of the comprehensive 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 by the test according to the water quality of wastewater. For printing and dyeing wastewater, carbon species with developed transition holes should be selected. In addition, ash content also has an effect, the smaller ash content, the better adsorption performance; The closer the size of adsorbate molecules to the pore diameter of carbon, the easier it is to be adsorbed; The concentration of adsorbate also has an effect on the adsorption capacity of activated carbon. In a certain concentration range, the adsorption amount increases with the increase of the concentration of adsorbate. In addition, the water temperature and pH value also have an effect. The adsorption capacity decreases with the increase of water temperature.

The characteristics of activated carbon

1. adsorption characteristics

Activated carbon adsorption is a method that uses porous activated carbon to make one or more substances in water be absorbed on the surface of activated carbon and removed. The removal objects include soluble organic substances, synthetic detergents, microorganisms, viruses and certain amount of heavy metals, and can decolorize, deodorize and air purification.

Activated carbon, sulfonated coal, zeolite, coke are all commonly used adsorbents for water treatment. After activated, activated carbon lattice forms developed pores with different shapes and sizes, which greatly increases the specific surface area and improves the adsorption capacity. The effective radius of the pore is 1-10000nm, the pore radius is below 2nm, the transition hole radius is 2-100nm and the large pore radius is 100-10000nm. The volume of the small hole is generally 0.15-0.90ml/g, and the area of the transition hole is generally 0.02-0.10ml/g; The volume of large pore is generally 0.2-0.5ml/g.

Activated carbon is a kind of very small carbon particle, which has a large surface area, and there are also smaller pores in the carbon particles capillary. This capillary has a strong adsorption capacity, because of the large surface area of carbon particles, it can fully contact with gas (impurities). When these gases (impurities) are adsorbed when they touch the capillary, they play a role in purification. The surface area of activated carbon is very important. Only the BET method is used to detect the specific surface area data of activated carbon. There are many domestic instruments that can only be used for direct comparison. At present, the multi-point BET method is adopted for specific surface area test at home and abroad. The specific surface area measurement standards developed at home and abroad are based on bet test methods. Please refer to the method of measuring specific surface area of solid matter by gas adsorption bet principle (gb/t 19587-2004).

Specific surface area detection is actually a time consuming work. Due to the different adsorption capacity of samples, some samples may take a whole day to test. If the test process is not fully automated, the tester will not leave at all times, and it is necessary to focus on the instrument panel and control the knob, which will lead to failure of the test process, This will waste a lot of valuable time on the tester. The F-Sorb 2400 specific surface area tester is a real instrument (with direct comparison method) which can realize the test function of BET method. The more important F-Sorb 2400 specific surface area tester is the only fully automatic and intelligent specific surface area testing equipment in China so far. The test results are consistent with the international standards, and the stability is also good, and the human error is reduced, Improve the accuracy of test results.

The factors influencing the adsorption of activated carbon are as follows: the characteristics of activated carbon; The characteristics and concentration of adsorbed substance; PH value of wastewater; The characteristics of suspended solid content; Contact system and operation mode, etc. Active carbon can effectively absorb chlorohydrocarbon, organophosphorus and carbamate insecticides, and also adsorb benzene ether, n-nitrochlorobenzene, naphthalene, ethylene, xylenol, phenol, DDT, aldrin, alkylbenzene sulfonic acid and many ester and aromatic compounds. The secondary effluent also contains organic substances that are not adsorbed by activated carbon, such as intermediate degradation substances of protein, which is more difficult to be adsorbed by activated carbon than the original organic matter. The removal capacity of activated carbon to THMs is lower, only 23-60%. The adsorption cycle of activated carbon is obviously prolonged, the amount of which is reduced and the treatment effect and range are greatly improved.

2. chemical properties

Besides physical adsorption, adsorption of activated carbon also has chemical adsorption. The adsorption of activated carbon depends on both pore structure and chemical composition.

Activated carbon contains not only carbon, but also a small amount of chemical combination, oxygen and hydrogen starting from functional groups, such as carbonyl, carboxyl, phenol, lactone, quinone and ether. The oxides and complexes on these surfaces, some of which are derived from raw materials, are formed by the action of air or water vapor during and after activation. Sometimes, surface sulphides and chlorides are also produced. The minerals contained in the raw materials are concentrated in the activated carbon to become ash. The main components of ash are alkali metals and alkaline earth metals, such as carbonate and phosphate.

3. mechanical characteristics

1. Particle size: a set of standard sieve screening method is used to calculate the weight of activated carbon left and passing through each sieve, indicating the particle size distribution.

2. Static density or heap density: it shall be the weight of the unit volume activated carbon of pore volume and interstate void volume.

3. Volume density and particle density: it shall be the pore volume rather than the weight of the unit volume activated carbon of the void volume between particles.

4. Strength: that is, the crushing resistance of activated carbon.

5. Wear resistance: that is, wear resistance or friction resistance.

These mechanical properties directly affect the application of activated carbon, such as: density affects the size of the container; The size of the powder carbon affects the filtration; The particle size distribution of granular carbon affects the fluid resistance and pressure drop; The fragmentation affects the life of activated carbon and the regeneration of waste carbon.

Classification of activated carbon

1. according to the source of raw materials:

1.1 activated carbon 2, animal bone / blood activated carbon 3, mineral raw material activated carbon 4, other raw materials of activated carbon 5. Regenerated activated carbon

2. according to the manufacturing method:

2.1 chemical activated carbon (chemical carbon) 2. Physical activated carbon (physical carbon) 3. Chemical physical method or physical chemical activated carbon

3. according to appearance shape:

Powdered activated carbon

Granular activated carbon

Amorphous granular activated carbon

Cylindrical activated carbon

Spherical activated carbon

Other forms of activated carbon

4. by pore size:

1. Large hole: aperture 1000A ° 2. Transition hole: aperture 20-1000a ° 3. Micropore: pore diameter 20A ° The surface area of activated carbon is mainly provided by micropores,

5. classification by material:

1. Type: raw material 2. Wood activated carbon: activated carbon made from wood chips, charcoal, etc. 3. Active carbon of fruit shell: activated carbon made from coconut shell, walnut shell, apricot core shell, etc. is lignite, peat, bituminous coal The activated carbon petroleum activated carbon made from anthracite, such as asphalt based spherical activated carbon, is recycled from the used waste carbon and reactivated.

Technical indexes of activated carbon

Note: high quality charcoal and sawdust are selected as raw materials, and refined by physical and chemical methods. The product is black powder, non-toxic and tasteless, and the pores are developed, and it does not contain dissolved substances in water which have adverse effects on water quality. It has the advantages of strong adsorption ability, good decolorization effect and fast filtration speed. It can effectively absorb the small and small molecular structure bad substances in liquid phase, and it is widely used in domestic sewage, chemical sewage, medical sewage and urban sewage, etc. especially for decolorization, odor removal and COD reduction of sewage.

Selection of activated carbon for water treatment

Good activated carbon must have the characteristics of large adsorption capacity, long service life, high mechanical strength, low ash, easy to wash and good water quality. It can not only remove odor, peculiar smell and improve color, but also have a high removal rate for various toxic and harmful substances in water, such as chlorine, phenol, mercury, lead, arsenic, chloride, detergent, pesticide, fertilizer and other pollutants.

The specific main technical indicators are as follows:

1. Particle size (10-24 mesh 2.0-0.8mm): ≥ 95%

Note: Generally speaking, the smaller the particle, the larger the specific surface area, that is, the better the adsorption effect, but the smaller the particles, the greater the loss and the more dust.

2. Iodine adsorption value: ≥ 1000mg/g

The results show that the higher the iodine adsorption value, the stronger the adsorption capacity of activated carbon.

3. Specific surface area: 1000-1200m2 / g

Note: if one gram of activated carbon is taken and all the pore walls inside are expanded into a plane, the area will reach 1000 square meters (that is, the specific surface area is 1000g / m2)! The main factors affecting the adsorptivity of activated carbon depend on the developed degree of internal pore structure（ The larger the specific surface area, the better the adsorption effect of activated carbon.

4. Decolorizing power of methylene blue: ≥ 10ml / g

Description: color removal ability.

5. Wear resistance: ≥ 95%

Description: wear resistance or anti friction performance; The higher the strength, the better the performance of activated carbon.

6. Drying loss: ≤ 10%

Note: drying loss and moisture, the lower the value, the better the quality of activated carbon.

7. Burning residue: ≤ 3%

Note: the lower the ignition residue and ash, the better the quality of activated carbon.

8. Filling specific gravity: 0.48-0.55g/ml

Note: filling specific gravity and density, generally the smaller the density, the better the adsorption capacity of activated carbon.

9. PH value: 6-8

Pure water equipment, ultra pure water equipment, purified water equipment, water treatment equipment, reverse osmosis equipment, sewage treatment equipment, water treatment equipment, etc. will be used, the use is very wide.