Activated carbon is produced by high-temperature carbonization and activation of plant-derived carbon-containing raw materials such as wood chips, anthracite, lignite, and husk. Carbonization is to heat the raw material in the case of isolating air, so as to liberate gases such as water vapor, carbon monoxide, carbon dioxide and hydrogen to form a pore structure composed of carbon atom microcrystals. Activation is the action of an oxidant (usually using water vapor or carbon dioxide as an oxidant) to heat the carbonized material, perfect its pore structure, and become a stable new structure with a well-porous structure and a large specific surface area. There are numerous fine pores inside the activated carbon particles. It has a pore size of 1-1000 nm and a specific surface area of ​​more than 1000 m2/g. The physical characteristic of activated carbon is that the adsorption amount of organic matter is larger than that of other adsorbents.
In addition to carbon, activated carbon also contains hydrogen and oxygen. Hydrogen and oxygen and carbon are chemically bonded to the surface of activated carbon to form various surface oxide complexes, which have a great influence on the adsorption properties of activated carbon, so that activated carbon and adsorbate The chemistry of the molecule shows the selective adsorption characteristics of the activated carbon during the adsorption process.
A variety of surface oxide composites on the surface of activated carbon generally divide the surface oxide into acidic and alkaline types. The type and quantity of surface oxides on activated carbon are closely related to the raw materials, activation conditions, pretreatment methods and redox reactions during use. Among them, the activation process is greatly affected. It is generally considered to be activated by wet air below 300-500 degrees. The activated carbon is mainly composed of acidic oxides, and the acidic oxides have acidic oxide groups, such as carboxyl groups and hydrocarbon groups, which make the activated carbon have polar properties and easily adsorb polar compounds such as phenol and halogenated hydrocarbons. It hinders the adsorption process of non-polar substances. At 800-900 degrees, activated carbon activated by air, steam or carbon dioxide is mainly alkaline oxide, while activated between 500-800 degrees has amphoteric properties.
[Relationship between specific surface area of ​​activated carbon and adsorption capacity]
Activated carbon is mainly made of substances with high carbon content, such as wood, coal, nut shell, bone, petroleum residue and so on. With coconut shell as the most commonly used raw material, under the same conditions, the active quality and other characteristics of coconut shell activity are the best, because it has the largest specific surface.
In general, the larger the specific surface area (BET) of activated carbon, the greater the adsorption force, but sometimes it is not.
BET is an application parameter for measuring the total surface area of ​​activated carbon by a nitrogen or butane adsorption method. The larger the BET, the greater the adsorption force. However, this concept has limitations in practical applications because the pores of activated carbon have the distinction of large pores, mesopores and micropores, and sometimes only some of the pores are suitable for the entry of certain types of adsorbates.
In liquid phase applications, the adsorption value of an organic substance generally increases as the molecular weight (molecular size) increases. Until the molecule is too big to enter the hole. The most preferred activated carbon is one having a large number of pores that are just slightly larger than the adsorbate molecules. The pores are too small for the adsorbate to enter; the pores are too large to reduce the surface area per unit volume.
In gas phase applications, small molecules are adsorbed into the micropores. The concept of total surface area is then used. As for the adsorption of activated carbon on metal complexes, the formation of chemical bonds is not the bigger the BET, the better.
[activated carbon solvent regeneration method]
The solvent regeneration method utilizes a phase equilibrium relationship between activated carbon, a solvent and an adsorbed substance, and breaks the adsorption equilibrium by changing the temperature and the pH of the solvent to desorb the adsorbate from the activated carbon. This regeneration process is generally accomplished in three ways: by changing the chemical nature of the contaminant; by using a solvent that has a stronger affinity for the contaminant than the activated carbon; and using a substance that has a stronger affinity for the activated carbon than the contaminant (generally only Used for the purpose of adsorption of adsorbate). Depending on the solvent used, it can be classified into an inorganic solvent regeneration method and an organic solvent regeneration method.
The inorganic solvent regeneration method mainly uses a mineral acid (H2SO4, HCl, etc.) or a base (NaOH or the like) as a regenerating solvent. Ye Liyi of Xiamen University studied the adsorption equilibrium relationship between phenol and p-chlorophenol aqueous solution on activated carbon, the effect of solution pH on the adsorption performance of activated carbon, and the adsorption and desorption kinetics of phenol on a fixed bed. At the same time, the process of activated carbon alkali regeneration after adsorption of phenol and the effect of multiple regeneration on the regeneration efficiency of activated carbon were studied by batch method and fixed bed continuous method. The preliminary rule of activated carbon regeneration by alkaline solvent was discussed. School of Materials Science and Engineering, Nanjing University of Chemical Technology Zhang Guojin and Zhou Yongzhen used a new type of organic regeneration solvent (ZL) to regenerate activated carbon in printing and dyeing wastewater treatment. The regenerant is a colorless transparent compound organic solvent, which can be repeatedly used after being distilled, and has high promotion value for some recyclable waste heat manufacturers.
Solvent regeneration is more suitable for reversible adsorption, such as adsorption of high concentration, low boiling organic wastewater. It is highly targeted, and often a solvent can only desorb certain pollutants, and the variety of pollutants in the water treatment process varies, so the application range of a specific solvent is narrow.
2 electrochemical regeneration method
Electrochemical regeneration is a new type of activated carbon regeneration technology under study. The method encloses activated carbon between two main electrodes. In the electrolyte, a direct current electric field is applied, and the activated carbon is polarized under the action of an electric field, one end is an anode, and the other end is a cathode, forming a micro-electrolytic cell at a cathode portion of the activated carbon and The anode portion can undergo a reduction reaction and an oxidation reaction, respectively, and most of the pollutants adsorbed on the activated carbon are decomposed, and a small portion is desorbed by the electrophoresis force.
[On how to identify coconut shell activated carbon]
Because coconut shell activated carbon is much more expensive than coal-based activated carbon, and the finished activated carbon material is generally not easily recognized by the general public. In the market, unscrupulous sellers often use the weakness of consumers' unrecognizable materials and sell them with coal-based activated carbon fake coconut shell activated carbon, which is more serious in civil or industrial fields.
Here are a few ways to easily distinguish between them
1. Coconut shell activated carbon belongs to the category of shell-shell activated carbon. Its main features are low density and light hand feeling. The weight in the hand is obviously lighter than that of coal-based activated carbon. The same weight of activated carbon, coconut shell activated carbon volume is generally larger than coal-based activated carbon.
2, coconut shell activated carbon shape is generally broken granular, flake, and molded activated carbon, such as columnar, spherical activated carbon, mostly coal charcoal.
3. Due to the low density of coconut shell activated carbon and light hand feeling, the activated carbon can be put into the water. The coal charcoal generally sinks faster, while the coconut shell activated carbon floats in the water for a longer period of time. If you add more weight, you will gradually sink into the bottom of the water. When the activated carbon is completely bottomed, you will see that each activated carbon is wrapped with a small bubble on the outside, which is crystal clear and very interesting.
4, coconut shell activated carbon is a small molecular pore structure, the activated carbon is placed in the water, the air discharged when the water molecules are adsorbed will produce many very small blisters (the naked eye can just see), densely floating to the surface. The coal-based activated carbon is generally a macromolecular pore structure, and the bubble phase produced is also large.
[Removal of algal toxin by powdered activated carbon]
The microcystins in the eutrophic lakes (the secondary metabolites of the cyanobacteria) have become one of the major environmental concerns of the world. Microcystin strongly inhibits the activity of protein phosphatase (PP1, PP2A) and is a strong cancer promoter. Recent studies by the Wuhan Institute of Aquatic Biology of the Chinese Academy of Sciences have shown that microcystin is the only target organ in the liver, and the animal gonad is the second target organ for its attack [11, 12]. However, the conventional coagulation process of the water plant has a poor removal effect on the dissolved microcystins, and the removal rate is generally below 20%, which is difficult to meet the requirements. Studies have shown that PAC has a good adsorption of soluble microcystins [13, 14]. Considering the seasonal characteristics of microcystins, PAC adsorption can be used as an emergency treatment for microcystins contamination. It takes a certain time for PAC to adsorb pollutants, and the process can be divided into three stages: rapid adsorption, basic equilibrium and complete equilibrium. Liu Cheng and other studies have shown that [15] PAC requires about 40 min for the rapid adsorption phase of two typical microcystins (MC-RR and MC-LR), and can reach an adsorption capacity of about 80%.
Therefore, for a water plant that has a certain distance from the water intake to the water purification plant, PAC can be added at the water intake to use the pipeline transportation time to complete the adsorption process; and for the water intake to be close to the water plant, it can only be invested in the water plant. In the case of adding powdered activated carbon, the adsorption time is short, and the contact with the coagulant will affect the contact with microcystins in the water, so that the adsorption capacity of the powdered carbon is difficult to play, so it is necessary to increase the dosage of PAC. . With the increase of powdered activated carbon dosage, the removal effect of microcystins was significantly improved. When the PAC dosage is 20 mg/L, the removal rates of MC-RR and MC-LR are 90% and 76%, respectively, that is, the maximum concentration that can occur for two microcystins in normal raw water (10 μg/L). ), adding 20mL of powdered activated carbon can reduce the concentration of the two toxins to 1μg / L and 2.4 μg / L, respectively, plus the removal of microcystins by other water treatment units (coagulation, disinfection, etc.), effluent The water quality can reach the newly issued national standard (the limit of MC-LR is 1μg/L). In addition, the removal rate of microcystins by PAC is independent of the initial concentration of algal toxin, which can be demonstrated by the theory of ideal adsorption solution and the theory of equivalent background compounds [15]. Thus, the desired powdered activated carbon dosage can be determined based on the concentration of the target compound in the raw water and the required value of the standard.
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