Activated carbon is a non-toxic and odorless, excellent adsorbent with a well-developed pore structure and a large specific surface area. In the early 1960s, European and American countries began to use a large amount of activated carbon adsorption to treat urban drinking water and industrial wastewater. At present, activated carbon adsorption has become urban sewage,
An effective means for advanced treatment of industrial wastewater and purification of contaminated water sources. In the 1960s, China has used activated carbon for the treatment of carbon disulfide wastewater. Since the early 1970s, the use of granular activated carbon to treat industrial wastewater has progressed rapidly in terms of application and processing scale. For example, in the areas of refinery wastewater, gunpowder wastewater, printing and dyeing wastewater, chemical wastewater and electroplating wastewater treatment, it has been applied on a large scale and achieved satisfactory results.
With the increasing application range of activated carbon, the recovery of activated carbon has begun to face people's attention. If the used activated carbon can not be recycled, in addition to the treatment cost per ton of wastewater will increase by 0.83 ~ 0.90 yuan [1], it will cause secondary pollution to the environment. Therefore, the regeneration of activated carbon is of particular importance.
1 traditional activated carbon regeneration method
1.1 Thermal regeneration method
The thermal regeneration method is currently the most widely used and the most mature activated carbon regeneration method in the industry [2, 3]. The activated carbon after treatment of organic wastewater is generally divided into three stages of drying, high temperature carbonization and activation according to the change of organic matter when heated to different temperatures. In the drying stage, the volatile components on the activated carbon are mainly removed. In the high-temperature carbonization stage, a part of the organic matter adsorbed on the activated carbon is boiled, vaporized and desorbed, and a part of the organic matter is decomposed, and the small-sized hydrocarbons are desorbed, and the residual components remain in the pores of the activated carbon to become "fixed carbon". At this stage, the temperature will reach 800-900 ° C, in order to avoid oxidation of activated carbon, generally under vacuum or inert atmosphere. In the next activation stage, a gas such as CO2, CO, H2 or water vapor is introduced into the reaction vessel to clean the activated carbon micropores to restore the adsorption function, and the activation phase is the hub of the entire regeneration process. Although the thermal regeneration method has the characteristics of high regeneration efficiency and wide application range, in the regeneration process, additional energy heating is required, and the investment and operation cost are high.
1.2 Biological regeneration method
The biological regeneration method utilizes domesticated bacteria to analyze the organic matter adsorbed on the activated carbon and further digest and decompose into H2O and CO2 [1, 2]. The biological regeneration method is similar to the biological method in sewage treatment, and there are also aerobic methods and anaerobic methods. Because the pore size of activated carbon itself is very small, some only a few nanometers, microorganisms can not enter such pores, usually it is thought that during the regeneration process, cell autolysis will occur, that is, the cell enzyme flows to the extracellular, and activated carbon has an adsorption effect on the enzyme. Therefore, the enzymatic center is formed on the surface of the carbon, thereby promoting the decomposition of the pollutants and achieving the purpose of regeneration.
The biological method is simple and easy to operate, and the investment and operation cost are low, but it takes a long time and is greatly affected by water quality and temperature. Microbial treatment of pollutants is highly targeted and needs to be specifically domesticated for specific substances. In the degradation process, it is generally impossible to completely decompose all the organic matter into CO2 and H2O, and the intermediate product remains on the activated carbon and accumulates in the micropores, and the regeneration efficiency is significantly reduced after repeated rounds.
Therefore, the industrial application of the biological regeneration method is limited.
1.3 wet oxidation regeneration method
Under the premise of high temperature and high pressure, using oxygen or air as an oxidant, a treatment method for oxidative decomposition of organic matter adsorbed on activated carbon in a liquid phase into small molecules is called a wet oxidation regeneration method [4]. The premise of regeneration is generally 200 to 250 ° C, 3 to 7 MPa, and the regeneration time is mostly within 60 min. The wet oxidation regeneration method has a wide range of treatment targets, a short reaction time, and no regenerative efficiency, and no additional heating is required after the start of regeneration. However, for some refractory organics, more toxic intermediates may be produced.
The Environmental College of Tongji University takes the change of phenol adsorption isotherm as the evaluation scale, systematically studies the main influencing factors in the process of activated carbon wet oxidation regeneration, and discusses its regularity in theory; discusses the synergy between the main factors. The possibility of multiple cycles of saturated carbon regeneration was investigated. The changes of the structure of activated carbon in the wet oxidation process were studied. The optimal regeneration conditions of the activated carbon obtained by the experiment are: regeneration temperature 230 ° C, regeneration time 1 h, oxygenation pO20.6 MPa, carbon addition 15 g, water addition 300 mL. The regeneration efficiency reached (45 ± 5)%, and the regeneration efficiency decreased by only 3% after 5 cycles of regeneration. The oxidation of the micropores in the surface of activated carbon is the main reason for the decline in regeneration efficiency.
In addition to their respective drawbacks, the traditional activated carbon regeneration technology usually has three common defects: (1) the loss of activated carbon tends to be large during the regeneration process; (2) the adsorption capacity of activated carbon after regeneration is significantly reduced; (3) the regeneration occurs. The exhaust will cause secondary pollution of the air. Therefore, people can either improve on traditional regeneration techniques or explore new regeneration technologies.
2 The current emerging activated carbon regeneration technology
2.1 Solvent regeneration method
The solvent regeneration method utilizes the phase equilibrium relationship between the activated carbon, the solvent and the adsorbed substance, and breaks the adsorption equilibrium by changing the temperature and the pH value of the solvent, and desorbs 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 [5], 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 alkaline solvent refilling charcoal 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) [6] 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 methods are 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.
2.2 electrochemical regeneration method
Electrochemical regeneration is a new type of activated carbon regeneration technology being studied [7]. 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 the large portion of the pollutant adsorbed on the activated carbon is decomposed, and the small portion is desorbed by the electrophoresis force.
The method has the advantages of convenient operation, high efficiency, low energy consumption, and limited limitation on the object to be treated. If the treatment process is perfect, secondary pollution can be avoided.
Zhang Huiping and Fu Zhihong of the Department of Chemical Engineering of Xiamen University studied the effect of pH on the adsorption equilibrium of phenol on activated carbon, the electrochemical regeneration efficiency of activated carbon on different electrodes and the effect of regenerative regeneration on the regeneration efficiency of activated carbon. They combined with the research results to conclude that the electrochemical regeneration process of activated carbon includes electrical desorption, NaOH alkali regeneration, and NaClO chemical oxidation. The experimental results show that the electrochemical refilling charcoal has a high regeneration efficiency and can reach 90%. In addition, the research on process parameters shows that the regeneration position is the most important factor in the regeneration process of activated carbon. The concentration of electrolyte NaCl is an important factor. The regeneration current and regeneration time also have a certain influence on the electrochemical regeneration of activated carbon.
2.3 Supercritical fluid regeneration method
When the temperature and pressure of a substance are above its critical temperature and critical pressure, it is called a supercritical fluid. Many substances have little ability to dissolve certain solute at normal pressure and normal temperature, but have an unusually large solvency in a subcritical state (near critical state) or supercritical state. In the supercritical state, slightly changing the pressure, the solubility will produce a number of changes [8]. Using this property, the supercritical fluid can be used as an extractant to achieve separation of the solute by adjusting the operating pressure, that is, supercritical fluid extraction technology. The critical temperature of carbon dioxide is 31 ° C, close to normal temperature, critical pressure (7.2 MPa) is not very high, non-toxic, non-flammable, non-polluting environment and easy to obtain supercritical conditions, is the first choice for supercritical fluid extraction technology applications. Extractant. According to recent research data, the regeneration efficiency changes greatly around the critical point of CO2; for activated carbon that is not dried, it is necessary to extend the regeneration time. For p-aminobenzenesulfonic acid, the optimum temperature for CO2 supercritical fluid regeneration is 308K. When the temperature exceeds 308K, regeneration is not affected; when the flow rate is greater than 1.47×10-4m/s, the flow rate does not affect regeneration; After the treatment with HCl solution, the regeneration effect of activated carbon is significantly improved. For benzene, the regeneration efficiency decreases with decreasing temperature at low pressure; the optimum regeneration temperature at 16.0 MPa is 318 K; at the experimental flow rate, the regeneration efficiency increases with the increase of flow rate [9].
2.4 ultrasonic regeneration method
Because the thermal regeneration of activated carbon requires heating all activated carbon, adsorbed substances and a large amount of water to a relatively high temperature, and sometimes even a vaporization temperature, energy consumption is large, and the process equipment is complicated. In fact, if the energy is applied to the adsorption surface of the activated carbon to obtain enough energy for the adsorbed material to be separated from the adsorption surface and returned to the solution, the purpose of refilling the carbon can be achieved. Ultrasonic regeneration is proposed for this purpose. The most important feature of ultrasonic regeneration is that it only applies energy locally, without the need to heat a large amount of aqueous solution and activated carbon, so the applied energy is very small [10].
Studies have shown that the temperature of the regeneration effluent is only increased by 2 to 3 ° C after ultrasonic regeneration. Each treatment of 1L of activated carbon uses a 50W ultrasonic generator for 120min, which is equivalent to 100kWh per m3 of activated carbon regeneration. The regeneration loss per regeneration is only 0.6%~0.8% of the dry mass, and the water consumption is 10 times of the activated carbon volume. . Wang Sanfang of Lanzhou Railway Institute conducted an experiment on ultrasonic regeneration. The results show that ultrasonic regeneration has the advantages of low energy consumption, simple process and equipment, low loss of activated carbon, and recoverable useful materials. However, it is only effective for physical adsorption. At present, the regeneration efficiency is only about 45%, and the pore size of activated carbon has a great influence on the regeneration efficiency.
2.5 microwave irradiation regeneration method
The microwave irradiation regeneration method is an activated carbon regeneration technology developed on the basis of the thermal regeneration method. The principle is to use electricity as energy source and use microwave irradiation to achieve regeneration [11]. Fu Dafang, from Southeast University, used the change of new carbon iodine value as the evaluation scale to study the premise of microwave regeneration of activated carbon adsorbed by sodium dodecylbenzene sulfonate.
The relationship between the regeneration efficiency of activated carbon and microwave power, microwave irradiation time and adsorption capacity of activated carbon was discussed by orthogonal experiment. The optimum regeneration efficiency in the test was at a power of HI (W) and an irradiation time of about 80 s. Comparing the extremely poor S, it is known that the maximum impact on the recovery of activated carbon iodine value after regeneration is microwave power, followed by irradiation time, and finally the adsorption amount of activated carbon. The microwave irradiation method has a short time for substituting carbon. Low energy consumption, simple equipment construction, and a good application prospect. However, in the process of microwave heating to desorb organic matter, whether or not other intermediate products are produced remains to be further studied.
2.6 Catalytic Wet Oxidation
The traditional wet oxidation method has low regeneration efficiency and high energy consumption. The regeneration temperature is the main factor affecting the regeneration efficiency, but the advanced regeneration temperature increases the surface oxidation of the activated carbon, thereby reducing the regeneration efficiency. Therefore, it is considered to use a high-efficiency catalyst to catalyze wet oxidation to refill the charcoal. Research staff in the State Key Laboratory of Water Environment Control and Resource Research at Tongji University are conducting research in this area. With the deepening of the concept of sustainable development, activated carbon regeneration technology and technology are increasingly being recognized. Some traditional activated carbon regeneration technologies and processes have made new improvements and breakthroughs in recent years. At the same time, new regeneration technologies are also emerging. Although these emerging technologies are still immature in the process route, they are not yet ready for industrial use. However, their emergence has brought new ideas and new ideas for the regeneration of activated carbon.
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