Modern agriculture replaces the use of organic fertilizers with a large amount of chemical fertilizers, replaces the use of agricultural waste with artificial feed, and the development of modern agricultural intensification and scale, breaking the recycling of waste in traditional agriculture, resulting in The accumulation of agricultural waste has led to serious environmental problems and waste of resources. Therefore, the rational use of agricultural waste resources has increasingly become a common problem faced by most countries in the world. Practices at home and abroad have shown that the resource utilization and harmless treatment of agricultural waste is an effective way to control agricultural environmental pollution, improve rural environment, develop circular economy, and achieve sustainable development of agriculture.
Activated carbon is a carbonaceous material with a special microcrystalline structure, a developed pore structure, a large specific surface area and a strong adsorption capacity. It has good chemical stability and is resistant to acid, alkali and high temperature. As an excellent adsorbent, people have more and more research on the application and development of activated carbon. Before the 1970s, the application of activated carbon in China was mainly concentrated in the sugar, pharmaceutical and MSG industries: later expanded to the water treatment and environmental protection industries; in the 1990s, in addition to the above areas, expanded to solvent recycling, food and beverage Purification, air purification, desulfurization, carrier, pharmaceutical, gold extraction, semiconductor and many other applications [1-5].
2 Status of agricultural waste utilization
Agricultural waste (agriculturalresidue) refers to the production of by-products in the production and processing of agriculture and forestry, a large number of renewable, short regeneration cycle, biodegradable, environmentally friendly and many other advantages, is an important biomass resource. There are mainly bark, nut shell, sawdust, straw, bagasse and so on. According to relevant information, the agricultural waste generated in China can be converted into biogas 311.55 billion m3 according to the current biogas technology level, and the average household income is 1275.2 m3, which can solve the rural energy shortage. Taking crop straw as an example, the current 650 million tons of straw is converted into electrical energy, and 1 kilowatt hour of electricity is generated by 1 kg of straw, which has the potential to generate 650 million kilowatt-hours of electricity; as a fertilizer, it can provide about 22.644 million tons of nitrogen and phosphorus 459.1. 10,000 tons and potassium 27.157 million tons; as feed, only corn stalks can provide 1.9 to 220 million tons. However, the utilization rate of agricultural waste in China is currently low or even not utilized. Therefore, agricultural waste has become one of the largest shelving resources on the one hand, and has become a huge source of pollution on the other hand [6].
From the perspective of resource economics, agricultural waste itself is a carrier of certain substances and energy, and it is a special form of agricultural resources, which contains abundant energy and nutrients. At present, with the increasing shortage of non-renewable resources such as oil and coal, more and more countries, especially developed countries, have included the conversion and utilization of renewable resources such as agricultural waste into an important strategy for sustainable socio-economic development. The research on the preparation of new industrial products by using renewable resources such as waste as raw materials has attracted the attention of countries all over the world. In China, with the rapid development of the economy, the development and utilization of agricultural waste resources and the gradual supplement or replacement of fossil resources are major issues related to the sustainable development of China's social economy.
3 Agricultural waste preparation activated carbon and its modification
At present, the use of activated carbon preparation raw materials is also fully utilized from wood chips and wood chips to coal and various agricultural and forestry products. The product is developed from a single variety to a variety of varieties: from low-grade activated carbon to high-grade activated carbon. The process of preparing activated carbon for agricultural waste generally involves several steps such as raw material pulverization, pressure bar, carbonization, activation, rinsing, drying and activated carbon pulverization. At the same time, the modification of the surface physical structure or the modification of the surface chemical properties can be carried out in different steps according to different requirements.
3.1 Surface physical structure modification
The modification of the physical structure of the adsorption surface of the activated carbon material means that the specific surface area of ​​the activated carbon material, the pore size and the distribution thereof are increased by physical or chemical methods in the preparation process of the activated carbon material, so that the adsorption surface structure of the activated carbon material is changed, thereby increasing Physical adsorption properties of activated carbon materials. Commonly used activators include alkali metal, alkaline earth metal hydroxides, inorganic salts and some acids. Currently, more mature chemical activators include KOH, NaOH, ZnCl2, CaCl2 and H3PO4 [7-10]. .
3.2 Surface chemical properties modification
The chemical composition of the surface of the activated carbon material has an effect on the acidity and alkalinity, wettability, adsorption selectivity and catalytic properties of the activated carbon material. The modification of the chemical properties of the adsorption surface of the activated carbon material means that the functional group of the adsorption surface of the activated carbon material and the surrounding atmosphere are improved by a certain method, so that it becomes an active point in a specific adsorption process, thereby controlling the hydrophilic/hydrophobic property. And the ability to combine with metals or metal oxides. The modification of the surface chemical properties of the activated carbon material can be modified by surface oxidation modification, surface reduction modification, and loading metal modification.
3.2.1 Oxidation modification
Oxidation modification mainly uses a strong oxidizing agent to oxidize the functional groups on the surface of activated carbon at a suitable temperature, thereby increasing the content of oxygen-containing acidic groups (such as carboxyl groups, phenolic hydroxyl groups, ester groups, etc.) on the surface, and enhancing the surface of the material. Sexual and hydrophilic. Commonly used oxidants are mainly HNO3, HClO3 and H2O2. Tsutsumi [11] believes that HNO3 is the strongest oxidant and produces a large number of acidic groups. The oxidizing property of HClO3 is mild, and the surface acidity of activated carbon can be adjusted to an appropriate value. The geometry of the activated carbon surface becomes more uniform after oxidation. Liu Wenhong et al [12] used concentrated HNO3 to modify activated carbon at room temperature and boiling state respectively. The results show that the specific surface area and pore volume of activated carbon are obviously improved after HNO3 modification, and the boiling HNO3 is changed. After the properties, the specific surface area and pore volume are significantly reduced, but both modification methods produce more oxygen-containing groups on the surface of the activated carbon. Han Bin [13] and other selected diammonium phosphate as activator to prepare activated carbon at different activation temperatures and pre-oxidation conditions. The results showed that the sample prepared by pre-oxidation and pre-oxidation at 700 ° C had a specific surface area of ​​1078.21 m 2 /g, and the yield and iodine adsorption values ​​were 39.75% and 636 mg / g, respectively.
3.2.2 reduction modification
The surface reduction modification means that the surface functional group of the activated carbon material is reduced and modified by a reducing agent at a suitable temperature, thereby increasing the specific content of the oxygen-containing basic group and enhancing the non-polarity of the surface. Sexual substances have stronger adsorption properties. Commonly used reducing agents are H2, N2, NaOH, KOH, and the like. Menendez et al [14] believe that the basicity of activated carbon is mainly due to its oxygen-free Lewis base, which can be obtained by high temperature treatment under inert gas such as reducing gas H2 or N2 to obtain activated carbon with a large content of basic groups. Krisztinalaszlo et al. [15] studied the adsorption of phenol and 2,3,4-trichlorophenol in solution by activated carbon treated with N2. The results showed that when the pH of the solution was 3, the adsorption amount was the highest. When the pH of the solution was 11, The amount of adsorption decreases. Haghserssht et al [16] found that the adsorption of p-cresol, nitrobenzene and p-nitrophenol in aqueous solution by H2 and N2 reduction of alkaline activated carbon is larger than that of untreated activated carbon.
3.2.3 Modified metal and metal oxide modification
The metal modification of the load mostly utilizes the reducibility and adsorption of the metal ions by the activated carbon, so that the metal ions are first adsorbed on the surface thereof, and then reduced to the elemental or low-valent ions, and the metal ions or the metal are adsorbed by the metal ions or the metal. Strong binding force increases the adsorption performance of activated carbon on the adsorbate. The Central South University of Forestry and Technology studied the process of using agricultural waste cotton straw as raw material [17], using activated zinc chloride to prepare activated carbon, and the influence of various factors on the adsorption performance of activated carbon, and obtained suitable process conditions. The concentration of zinc chloride solution is 40°Be', the ratio of solid to liquid is 1:2, carbonization at 400 °C for 180 min, and activation at 650 °C for 60 min. Garg et al [18] used concentrated sulfuric acid to treat Indian rosewood sawdust at 150 °C for 24 h, and removed residual acid to prepare activated carbon adsorbent. Compared with formaldehyde-treated sawdust, this adsorbent has better Cr(VI) removal ability. .
4 Application of activated carbon for agricultural waste preparation
The application of activated carbon has a long history. Activated carbon was originally used for the decolorization of sugar, and then gradually expanded to various industries of production and life, and constantly developed new products according to market demand. Activated carbon prepared from agricultural waste has been used in sewage treatment, water purification, smoke control, etc.
4.1 sewage treatment
The main advantages of activated carbon in wastewater treatment are high degree of treatment and stable effluent quality. It can be used in combination with other methods to obtain high quality effluent water quality. Zheng Xuyu et al. [19] studied the photocatalytic degradation properties of activated carbon-loaded nano-TiO2 and affected methyl groups. The main factors of orange wastewater treatment show that the TiO2 activated carbon catalyst prepared by sol-gel method has the advantages of large specific surface area, high dispersibility, good photocatalytic degradation performance and reusability. Jun et al. [20] reported that the effect of enhancing the adsorption of organic acids can be achieved by using various activated carbons loaded with platinum in the redox process. Inorganic industrial wastewater treatment [21-22] Some activated carbons have certain selective adsorption capacity for inorganic heavy metal ions in wastewater. It is used to treat drinking water and micro-polluted water purification. The ozone-bio-activated carbon process [23] has attracted much attention and attention for its efficient removal of dissolved organic matter and carcinogenic mutants in water, and safe and high quality water.
4.2 Water purification
Activated carbon not only has good color and odor removal effect in purifying water supply, but also has high adsorption capacity for synthetic detergent ABS, trihalomethane, halogenated hydrocarbon and free chlorine, and can effectively remove almost indecomposable carbamate. Insecticides, etc. Activated carbon can effectively remove free chlorine and some heavy metals (such as Hg, Sb, Sn) in water and is not easy to cause secondary pollution. It is often used in the purification process of domestic water and drinking water [24].
4.3 exhaust gas treatment
At present, SO2 and NOx emitted from coal combustion in China are the main atmospheric pollutants, and the activated carbon materials have good desulfurization and denitrification treatment effects, low investment operation cost, and easy recycling. Desulfurization and denitrification of modified activated carbon materials Firstly, the adsorption properties of activated carbon materials are used to physically adsorb the polluted gases SO2 and NOx in the flue gas on the surface of activated carbon materials. Under the catalytic action of surface functional groups or supported metals of activated carbon materials, SO2 and NOx conversion For SO3 and non-polluting N2 or O2. In the presence of water vapor, SO3 will combine with water to form sulfuric acid recovery. Wey et al. [25] studied the desulfurization performance of carbon-supported metal copper and antimony desulfurizer. Qiu Lin et al [26] studied that the activated carbon modified with sodium carbonate solution increased the sulfur capacity of the pure activated carbon desulfurizer by nearly 30%. Wang et al [27] studied the effect of metal modified activated carbon fiber on the removal performance of sulfur dioxide.
As a porous carbonaceous material, activated carbon has a well-developed void structure and a large specific surface area. The surface has a special functional surface functional group containing elements such as oxygen, and the application field is wider and wider. Since it was put into industrial production in the early 20th century, it has been widely used as an adsorbent and catalyst carrier in the fields of electronics, chemical industry, food processing, medical and health, transportation and energy, agriculture, national defense, etc., especially recently, in order to prevent and control air pollution and water pollution. And odors and other public hazards to protect the environment, so that the production and research of activated carbon has developed faster. Today, about 50 countries around the world produce activated carbon, and the United States, Japan, the United Kingdom, Germany, France and Russia are at the leading level. By 1990, the annual consumption of the United States was 105,491 tons, with an average annual growth rate of 4% to 5%. Japan also consumes 75,251 tons, while the annual production capacity of activated carbon in Western Europe is 100,000 tons [1]. China's activated carbon industry started in the 1960s, and the output in the 1970s was only 10,000 tons. By the end of the 1980s, the output reached 40,000 tons. In recent years, China's activated carbon industry has developed greatly, with an annual output of 80,000 tons, but the quality of activated carbon is far less than that of developed countries, and a large amount of high-quality activated carbon needs to be imported [2].
2 Preparation materials for activated carbon
All the raw materials for the production of activated carbon are carbonaceous materials. At present, the raw materials for the manufacture of activated carbon selected at home and abroad are divided into five categories.
2.1 Plant raw materials (wood raw materials)
Activated wood has a wide range of wood materials. Commonly used are: charcoal, coconut shell, wood chips, bark, walnut shell, fruit core, cotton shell, rice husk, bamboo, coffee bean stem, oil palm shell, furfural residue and pulp waste. Liquid etc. [3~13]. Wood raw materials occupy a very important position in China's activated carbon industry. Among them, coconut shell and walnut shell are the best, but due to the limited raw materials, their development is restricted.
2.2 Coal raw materials
Coal is the raw material for the manufacture of activated carbon. Activated carbon can be produced from almost all coals. Among them, young anthracite, weak coal, lignite and peat with short coal formation time are excellent raw materials for the production of activated carbon. Due to the abundant coal resources, wide distribution and low price, it has a good prospect to produce activated carbon from coal as raw material [2].
2.3 Petroleum raw materials
Petroleum raw materials mainly refer to carbon-containing products and waste materials in the petroleum refining process. For example, petroleum asphalt, petroleum coke, petroleum oil residue, etc. [4~17]. In the early 1990s, the Shanxi Institute of Coal Chemistry of the Chinese Academy of Sciences used petroleum-based bitumen with low ash content and low impurity content (<0.01%) as raw material to prepare activated carbon with a specific surface area of ​​3 600 m2/g by KOH chemical activation method [18~ 19].
2.4 Plastic PVC, polypropylene, furan resin, phenolic resin, urea formaldehyde
Resin, polycarbonate, polytetrafluoroethylene, etc. [20~23]. These raw materials mainly refer to industrial recycling waste, which is not fully utilized in China.
2.5 other
Old tires, animal bones, animal blood, sucrose, molasses, etc. [24]. Among the above raw materials, China currently mainly uses coconut shell and peach apricot kernel as raw materials for wood activated carbon. Because they have the advantages of low ash content, developed pores, large specific surface area, good strength and good adsorption performance, they are ideal wood-based activated carbon raw materials, but their large amount of development is affected by the limitation of the amount of raw materials. Coal has many advantages such as variety, low price, stable quality and abundant resources. Therefore, activated carbon with coal as raw material has developed rapidly, and the application range and quantity of coal-based activated carbon are gradually expanding.
3 Domestic and foreign activated carbon manufacturing methods
At present, the manufacturing methods of activated carbon at home and abroad can be divided into three categories in principle.
3.1 Gas activation method
The gas activation method is a method of activating carbon dioxide, carbon dioxide, air, and flue gas at 600 to 1,200 ° C after carbonization of the raw material [25]. More than 70% of the world's manufacturers of activated carbon use gas activation. China mainly produces activated carbon by gas activation method.
It is generally believed that the reaction mechanism of water vapor activation is as follows:
C*+H2O C[H2O]
C[H2O] H2+C[O]
C[O] CO
Here, C* represents a carbon atom located at an active site, and [] represents a chemical adsorption state. The reaction is prevented by hydrogen bonding at the active site:
C+H2C[H2]
Also, there are side reactions as shown in the following formula:
The CO+H2O→H2+CO2 gas activation method forms a pore structure by consuming carbon atoms, and thus the yield is low. The process features high activation temperature and large equipment investment, but no pollution to the environment.
3.2 Chemical activation method
The chemical activation method involves adding a chemical to a raw material in a certain ratio and then heating it in an inert gas medium while carbonizing and activating [25]. Finally, the added chemicals are recovered. In the activation process, the carbonaceous material is etched with a chemical agent, and elements such as hydrogen and oxygen are mainly released as small molecules such as H2O and CH4, and the formation of by-product tar is suppressed, and the yield of the activated carbon can be improved. The main chemical agents used are KOH, KCNS, H3PO4, H2SO4, ZnCl2, NaOH, and the like.
At present, the most reported chemical activation method is the preparation of high specific surface area activated carbon by KOH activation [18~19] [26~39]. In the mid-1980s, Amoka Corporation of the United States used KOH as an activator to produce activated carbon with a specific surface area greater than 2 500 m2/g by chemical activation [26-28]. Japan's Osaka Gas Company used mesophase pitch microspheres as raw materials and similar activation methods to produce activated carbon with a specific surface area of ​​up to 4 000 m2/g [29]. Kansai thermochemistry in Japan also has this product called Maxsorb [18]. The Shanxi Institute of Coal Chemistry of the Chinese Academy of Sciences carried out research work in this area in the early 1990s and successfully produced high specific surface area activated carbon (SBET~3 600 m2/g) [19][30~31]. The chemical activation method will be briefly described below by taking KOH activation as an example.
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