According to statistics, China ’s annual industrial wastewater discharge is about 8 ¡Á 108 m3, which not only contains highly toxic components such as cyanide, but also contains metal ions such as chromium, zinc, and nickel.
There are many methods for wastewater treatment, including chemical precipitation method, electrolytic method and membrane treatment method. This article introduces activated carbon adsorption method. Activated carbon has a large surface area and has high physical and chemical adsorption functions. Therefore, activated carbon adsorption method is widely used in wastewater treatment. And it has the characteristics of high efficiency and good effect.
With the advancement of science and technology and the special requirements of wastewater treatment, the research of activated carbon has gradually developed from its pore structure and specific surface area to the study of the effect of surface functional groups on the adsorption performance of activated carbon.
For example, activated carbon fiber (referred to as ACF) has received the attention of researchers in the treatment of wastewater in recent years. Its diameter is generally 5-20 μm. Its preparation principle is the same as that of traditional activated carbon. Steam or carbon dioxide activation treatment. The pore structure of fibrous activated carbon is mainly micropores, few mesopores, almost no macropores, and the specific surface area can reach 2500m2 / g. It has the characteristics of adsorption and desorption rate, large adsorption capacity and high conductivity.
Experiments show that the adsorption capacity of ACF for phenol is 248 mg / g. After adsorption saturation, the adsorption capacity is almost unchanged after repeated regeneration, and the adsorption performance is better than that of activated carbon. At room temperature, under acidic or neutral conditions, 0.5 g of activated carbon fiber was added to 100 mL of phenol-containing simulated wastewater with a concentration of 282 mg / L, and shaking at constant temperature for 30 min, the phenol removal rate could reach 91%.
Recently, it has been discovered that activated carbon not only has adsorption characteristics, but also exhibits catalytic characteristics. The catalytic oxidation method developed from this has received increasing attention, and its research is continuously deepening. In order to improve the treatment efficiency, starting from studying the mechanism of catalytic oxidation, changing the surface structure of activated carbon, improving the capacity of activated carbon, and looking for an ideal adsorbent.
Activated carbon is a specially treated carbon with numerous fine pores and a large surface area. The surface area per gram of activated carbon is 500-1500 square meters.
Activated carbon has strong physical and chemical adsorption functions, and also has a detoxifying effect. The detoxification effect uses its huge area to adsorb poisons into the micropores of activated carbon, thereby preventing the absorption of poisons. At the same time, activated carbon can be combined with a variety of chemicals to prevent the absorption of these substances.
1 classification of activated carbon
There are many types of activated carbon used in production. Generally made into powder or granules.
Powdered activated carbon has strong adsorption capacity, easy preparation, and low price, but it is difficult to regenerate and generally cannot be reused.
Granular activated carbon is more expensive, but it can be reused after being regenerated, and the labor conditions during use are better, and the operation and management are convenient. Therefore, granular activated carbon is mostly used in water treatment .
2 activated carbon adsorption
Activated carbon adsorption refers to the use of the solid surface of activated carbon to adsorb one or more substances in water to achieve the purpose of purifying water.
3 Factors affecting adsorption of activated carbon
Adsorption capacity and adsorption speed are the main indicators to measure the adsorption process . The size of adsorption capacity is measured by the amount of adsorption. The adsorption speed refers to the amount of substance adsorbed by a unit weight of adsorbent in a unit time. In water treatment, the adsorption speed determines the contact time between the sewage and the adsorbent.
The adsorption capacity of activated carbon is related to the pore size and structure of activated carbon. In general, the smaller the particles, the faster the pore diffusion rate, and the stronger the adsorption capacity of activated carbon.
The pH and temperature of sewage also have an effect on the adsorption of activated carbon. Activated carbon generally has a higher adsorption capacity under acidic conditions than under alkaline conditions . The adsorption reaction is usually an exothermic reaction, so a low temperature is beneficial for the adsorption reaction.
Of course, the adsorption capacity of activated carbon is related to the concentration of sewage. At a certain temperature, the adsorption amount of activated carbon increases as the equilibrium concentration of the adsorbed substance increases.
Application of activated carbon in sewage treatment
Because activated carbon requires high water pretreatment and the price of activated carbon is high, in wastewater treatment, activated carbon is mainly used to remove trace pollutants in wastewater to achieve the purpose of deep purification.
1 activated carbon treatment of chromium-containing wastewater
Chromium is a large amount of metal raw material used in electroplating. Hexavalent chromium in wastewater exists in different forms with different pH values.
Activated carbon has a very developed microporous structure and a high specific surface area. It has a strong physical adsorption capacity and can effectively adsorb Cr (Ⅵ) in wastewater. A large number of oxygen-containing groups such as hydroxyl (-OH) are present on the surface of activated carbon. ), Carboxyl (-COOH), etc., they all have electrostatic adsorption function, and have chemisorption on Cr (Ⅵ). It can be used to treat Cr (Ⅵ) in electroplating wastewater, and the wastewater after adsorption can meet national discharge standards.
The test showed that the mass concentration of Cr (Ⅵ) in the solution was 50 mg / L, pH = 3, and the adsorption time of 1.5 h, the adsorption performance of activated carbon and the removal rate of Cr (Ⅵ) reached the best effect.
Therefore, the process of using activated carbon to treat chromium-containing wastewater is the result of the combined effects of physical adsorption, chemical adsorption, and chemical reduction of activated carbon on Cr (Ⅵ) in solution. Activated carbon treats chromium-containing wastewater with stable adsorption performance, high treatment efficiency, low operating cost, and certain social and economic benefits.
2 activated carbon treatment of cyanide-containing wastewater
In industrial production, cyanide or by-product cyanide is used in industries such as wet extraction of gold and silver, production of chemical fiber, coking, ammonia synthesis, electroplating, and gas production , so a certain amount must be emitted during the production process Cyanide-containing wastewater.
Activated carbon has a long history of purifying wastewater, and there are more and more literature reports on the treatment of cyanide-containing wastewater . However, due to the small adsorption capacity of CN_ and HCN on activated carbon, it is generally 3 mgCN / gAC ～ 8 mgCN / gAC (varies by species), which is not cost-effective in terms of processing costs.
3 activated carbon treatment of mercury-containing wastewater
Activated carbon has the ability to adsorb mercury and mercury-containing compounds, but its adsorption capacity is limited and it is only suitable for treating wastewater with low mercury content. If the concentration of mercury is high, it can be treated by chemical precipitation method. After treatment, the mercury content is about 1 mg / L, and it can reach 2-3 mg / L when it is high, and then activated carbon is used for further processing.
4 Activated carbon treatment of phenol-containing wastewater
Phenol-containing wastewater is widely sourced from petrochemical plants, resin plants, coking plants and refining and chemical plants. It has been proved by experiments that the adsorption performance of activated carbon on phenol is good, and the increase in temperature is not conducive to adsorption, which reduces the adsorption capacity; but the time to increase the adsorption equilibrium is shortened. There are optimal values for the amount of activated carbon and adsorption time. Under acidic and neutral conditions, the removal rate does not change much; under strong alkaline conditions, the removal rate of phenol drops sharply, and the stronger the alkaline, the worse the adsorption effect.
5 Activated carbon treatment of methanol-containing wastewater
Activated carbon can adsorb methanol, but its adsorption capacity is not strong, and it is only suitable for treating wastewater with low methanol content. The results of engineering operation show that the COD of the mixed liquid can be reduced from 40 mg / L to less than 12 mg / L, and the removal rate of methanol reaches 93.16% ~ 100%. Water quality requirements.
6 Advanced treatment of oil refineries
The oily wastewater of the refinery is treated in depth by sand filtration and activated carbon filtration after oil separation, air flotation and biological treatment. The phenol content of the wastewater decreased from 0.1 mg / L (after biological treatment) to 0.005 mg / L, cyanide decreased from 0.19 mg / L to 0.048 mg / L, and COD decreased from 85 mg / L to 18 mg / L.
The current method of treating wastewater using activated carbon adsorption in China is in the initial development stage. Some related theories and technologies are not mature enough. Moreover, in China, the current supply of activated carbon is relatively tight, with few regeneration equipment and high regeneration costs, which limits the widespread use of activated carbon. Different applications require activated carbon with different functions.
The original activated carbon products can not meet the new requirements, so it is very important to continuously develop new activated carbon products. Therefore, it requires the active participation of professional workers and the strong support of the government to adopt a multidisciplinary and integrated research method to make the technology of activated carbon wastewater treatment develop in a more scientific and beautiful direction.