The development of activated carbon regeneration technology

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Abstract: Activated carbon is an effective adsorbent commonly used in wastewater treatment, and its regeneration is of great significance. Traditional methods for activated carbon regeneration, such as thermal regeneration and biological regeneration, are reviewed. Meanwhile, emerging activated carbon regeneration technologies, including electrochemical methods, supercritical fluid methods, catalytic wet oxidation, and ultrasonic methods, are also introduced and discussed. Keywords: activated carbon, regeneration, water treatment. Activated carbon is an excellent adsorbent that is non-toxic and odorless, featuring a well-developed porous structure and a large specific surface area. In the early 1960s, European and American countries began to use activated carbon adsorption on a large scale to treat urban drinking water and industrial wastewater. Currently, activated carbon adsorption has become an effective method for the advanced treatment of urban sewage and industrial wastewater, as well as for the purification of polluted water sources. China began using activated carbon for the treatment of carbon disulfide wastewater in the 1960s. Since the early 1970s, granular activated carbon has been employed for treating industrial wastewater, and significant progress has been made in terms of both technology and the scope as well as scale of application. It is now widely used in the treatment of refinery wastewater, **wastewater, printing and dyeing wastewater, chemical industry wastewater, and electroplating wastewater, with satisfactory results achieved. 　　As the application scope of activated carbon becomes increasingly wide, the recovery of activated carbon has begun to attract attention. If the used activated carbon cannot be recycled, in addition to an increase in the treatment cost per ton of wastewater of 0.83–0.90 yuan, it will also cause secondary pollution to the environment. Therefore, the regeneration of activated carbon is of particular importance. 　　1 Traditional methods for activated carbon regeneration 1.1 Thermal regeneration Thermal regeneration is the most widely used and most mature method for activating carbon regeneration in industry. During the regeneration of activated carbon used to treat organic wastewater, depending on the changes in organic substances at different heating temperatures, it is generally divided into three stages: drying, high-temperature carbonization, and activation. During the drying stage, the volatile components on the activated carbon are primarily removed. The high-temperature carbonization stage causes some of the organic substances adsorbed on the activated carbon to boil and vaporize and be desorbed; some of these organic substances undergo decomposition reactions, producing small-molecule hydrocarbons that are also desorbed, while the remaining components remain in the pores of the activated carbon as \"fixed carbon\". At this stage, the temperature will reach 800–900 °C; to prevent the oxidation of activated carbon, the process is generally carried out under vacuum or in an inert atmosphere. During the subsequent activation phase, gases such as CO2, CO, H2, or water vapor are introduced into the reaction vessel in order to clean the micropores of the activated carbon and restore its adsorption capacity; the activation phase is crucial to the entire regeneration process. Although the thermal regeneration method features high regeneration efficiency and a wide range of applications, it requires external energy for heating during the regeneration process, resulting in high investment and operating costs. 　　1.2 Biological regeneration method The biological regeneration method is a process that uses domesticated bacteria to break down the organic substances adsorbed on activated carbon, and further digest and decompose them into H2O and CO2. The biological regeneration method is similar to the biological methods used in wastewater treatment, and it also includes aerobic and anaerobic approaches. Due to the very small pore sizes of activated carbon, some of which are only a few nanometers in diameter, microorganisms cannot enter such pores. It is generally believed that during the regeneration process, cell autolysis occurs – that is, cellular enzymes flow outside the cells. Since activated carbon has an adsorptive effect on enzymes, enzyme-mediated centers are formed on the surface of the carbon, thereby facilitating the decomposition of pollutants and achieving regeneration. 　　The biological method is simple and easy to implement, with low investment and operating costs, but it requires more time and is highly affected by water quality and temperature. Microbial treatment of pollutants is highly targeted, requiring the specialized domestication of microorganisms for specific substances. Moreover, during the degradation process, it is generally not possible to completely break down all organic substances into CO2 and H2O; the intermediate products remain on the activated carbon and accumulate in its micropores, resulting in a significant decrease in regeneration efficiency after multiple cycles. Thus, it limits the industrial application of biological regeneration methods. 　　1.3 Wet oxidation regeneration method: A treatment method in which, under high temperature and pressure conditions, oxygen or air is used as an oxidant to oxidize and decompose the organic substances adsorbed on activated carbon in its liquid state into smaller molecules; this method is known as wet oxidation regeneration. The regeneration conditions are generally 200–250 °C and 3–7 M Pa, with the regeneration time usually being within 60 minutes. The wet oxidation regeneration method can be applied to a wide range of substances, features a short reaction time, maintains stable regeneration efficiency, and requires no additional heating once the regeneration process begins. However, for certain hard-to-degrade organic compounds, more toxic intermediate products may be generated. 　　The School of Environment at Tongji University used changes in phenol adsorption isotherms as evaluation criteria to systematically study the main influencing factors in the wet oxidation regeneration process of activated carbon, and theoretically explored their patterns. It examined the synergistic effects among these various factors, investigated the possibility of multiple cycles of regeneration for saturated carbon, and studied the changes in the structure of the activated carbon itself during the wet oxidation process. The optimal regeneration conditions for the activated carbon obtained through experiments are: regeneration temperature of 230 °C, regeneration time of 1 hour, oxygen partial pressure of pO2 at 0.6 MPa, carbon addition amount of 15 g, and water addition amount of 300 mL. The regeneration efficiency reached (45±5)%, and after 5 cycles of regeneration, it decreased by only 3%. Partial oxidation of the micropores on the surface of activated carbon is the main reason for the decrease in regeneration efficiency. 　　In addition to their respective drawbacks, traditional activated carbon regeneration techniques generally have three common defects: (1) significant loss of activated carbon occurs during the regeneration process; (2) the adsorption capacity of the activated carbon decreases markedly after regeneration; (3) the exhaust gases generated during regeneration cause secondary air pollution. Therefore, people either improve traditional regeneration techniques or explore entirely new ones. 　　2 Current emerging technologies for activated carbon regeneration 2.1 Solvent regeneration method The solvent regeneration method makes use of the phase equilibrium relationship among activated carbon, solvent, and the adsorbate; by changing conditions such as temperature and the pH value of the solvent, this method disrupts the adsorption equilibrium and enables the adsorbate to be desorbed from the activated carbon. This regeneration process is generally achieved through three methods: altering the chemical properties of the pollutants; using solvents with a greater affinity for the pollutants than activated carbon to carry out extraction; or using substances with a greater affinity for activated carbon than for the pollutants to effect displacement (this method is usually applied only for the purpose of recovering the adsorbate). Depending on the solvent used, it can be divided into inorganic solvent regeneration methods and organic solvent regeneration methods. 　　The inorganic solvent regeneration method primarily uses inorganic acids (such as H2SO4, HCl, etc.) or bases (such as NaOH, etc.) as regeneration solvents. Ye Liyi and others from Xiamen University studied the adsorption equilibrium of phenol and p-chlorophenol in aqueous solutions on activated carbon, the effect of solution pH on the adsorption capacity of activated carbon, and the adsorption and desorption kinetics of phenol on a fixed-bed. Meanwhile, the alkaline regeneration process of activated carbon after phenol adsorption was studied using both batch and fixed-bed continuous methods, as well as the effect of multiple regenerations on the regeneration efficiency of activated carbon; the preliminary principles for regenerating activated carbon with alkaline solvents were explored. Zhang Guojin, Zhou Yongzhang, and others from the School of Materials Science and Engineering at Nanjing University of Chemical Technology used a new type of organic regeneration solvent (ZL) to regenerate activated carbon in the treatment of printing and dyeing wastewater. This regenerant is a colorless and transparent composite organic solvent that can be reused after distillation; it holds significant promotional value for manufacturers with recoverable waste heat. 　　The solvent regeneration method is more suitable for reversible adsorption, such as the adsorption of high-concentration, low-boiling-point organic wastewater. It is quite specific in its application; often, a single solvent can only remove certain types of pollutants. Since there are many different types of pollutants present in water treatment processes, and these types can vary, the scope of application for any given solvent is relatively limited. 　　2.2 Electrochemical regeneration method The electrochemical regeneration method is a new type of activated carbon regeneration technology that is under investigation. In this method, activated carbon is placed between two main electrodes. A direct current electric field is applied in the electrolyte, causing the activated carbon to become polarized – one end acting as the anode and the other as the cathode – thereby forming a microelectrolytic cell. Reduction reactions occur at the cathodic side of the activated carbon, while oxidation reactions take place at the anodic side. As a result, most of the pollutants adsorbed on the activated carbon are decomposed, with a small portion being desorbed due to electrostatic forces. This method is easy to operate, efficient, and has low energy consumption. It imposes few restrictions on the materials to be processed, and with a proper treatment process, secondary pollution can be avoided. 　　Zhang Huiping, Fu Zhihong, and others from the Department of Chemical Engineering at Xiamen University studied the effect of pH value on the adsorption equilibrium of phenol on activated carbon, as well as the electrochemical regeneration efficiency of activated carbon on different electrodes and the impact of cyclic regeneration on its regeneration efficiency. Based on relevant research findings, they concluded that the mechanism of the electrochemical regeneration process of activated carbon includes processes such as electrodesorption, alkaline regeneration with NaOH, and chemical oxidation with NaClO. Experimental results show that electrochemical regeneration of activated carbon achieves a high regeneration efficiency of up to 90%. Furthermore, studies on process parameters have shown that the regeneration position is the most important influencing factor in the activated carbon regeneration process; the concentration of the electrolyte NaCl is another significant influencing factor. Regeneration current and regeneration time also have a certain impact on the electrochemical regeneration of activated carbon. 　　2.3 Supercritical fluid regeneration method A substance is considered a supercritical fluid when its temperature and pressure are higher than its critical temperature and critical pressure. Many substances have extremely low solvating capacity for certain solutes at normal pressure and temperature, but exhibit abnormally high solvating capacity in a subcritical state (near the critical state) or a supercritical state. In the supercritical state, a slight change in pressure can cause a change in solubility by an order of magnitude. Taking advantage of this property, supercritical fluids can be used as extractants, and the separation of solutes can be achieved by adjusting the operating pressure – this is the technique known as supercritical fluid extraction. Carbon dioxide has a critical temperature of 31 °C, which is close to room temperature. Its critical pressure (7.2 M Pa) is not very high. It possesses advantages such as being non-toxic, non-flammable, environmentally friendly, and easy to bring into a supercritical state, making it the preferred solvent for applications in supercritical fluid extraction technology. According to recent research, near the critical point of CO2, the regeneration efficiency varies significantly; for activated carbon that has not been dried, it is necessary to extend its regeneration time. For p-aminobenzenesulfonic acid, the optimal temperature for regeneration using CO2 supercritical fluid is 308 K. Beyond this temperature, regeneration is not affected. When the flow rate exceeds 1.47×10-4 m/s, it also has no impact on regeneration. Treatment with HCl solution significantly improves the regeneration efficiency of activated carbon. For benzene, the regeneration efficiency decreases as the temperature drops at low pressures; the optimal regeneration temperature at a pressure of 16.0 M Pa is 318 K. At the experimental flow rate, the regeneration efficiency increases as the flow rate increases. 　　2.4 Ultrasonic regeneration method Since thermal regeneration of activated carbon requires heating all of the activated carbon, the adsorbed substances, and a large amount of water to high temperatures, sometimes even to the vaporization temperature, it results in high energy consumption and complex process equipment. In fact, by applying energy to the adsorption surface of activated carbon, sufficient energy can be provided to the substances that are adsorbed, enabling them to break free from the adsorption surface and return to the solution, thereby achieving the regeneration of the activated carbon. Ultrasonic regeneration was proposed to address this issue. The greatest advantage of ultrasonic regeneration is that energy is applied only locally, without the need to heat large amounts of aqueous solution and activated carbon; as a result, the amount of energy applied is very small. 　　Studies show that after ultrasonic regeneration, the temperature of the regenerated effluent increases by only 2–3°C. To treat 1 L of activated carbon, an ultrasonic generator with a power of 50 W is used for 120 minutes; this corresponds to 100 kWh of electricity being consumed per m³ of activated carbon that is regenerated. The loss of activated carbon after each regeneration is only 0.6%–0.8% of its dry mass, while the water consumption is 10 times the volume of the activated carbon. Wang Sanfan from Lanzhou Railway Institute conducted experiments on the ultrasonic regeneration method. The results show that ultrasonic regeneration has advantages such as low energy consumption, simple process and equipment, minimal loss of activated carbon, and the ability to recover useful substances. However, it is only effective for physical adsorption; currently, the regeneration efficiency is only around 45%, and the pore size of the activated carbon has a significant impact on this efficiency. 　　2.5 Microwave Irradiation Regeneration Method The microwave irradiation regeneration method is an activated carbon regeneration technique developed on the basis of the thermal regeneration method. Its principle is to use electricity as an energy source, and regeneration is achieved through microwave irradiation for heating. Fu Dafang and others from Southeast University studied the microwave regeneration conditions for activated carbon that has adsorbed sodium dodecylbenzenesulfonate, using changes in the iodine value of the new carbon as a evaluation criterion. Through orthogonal experiments, the relationship between the regeneration efficiency of activated carbon and factors such as microwave power, microwave irradiation time, and the adsorption capacity of activated carbon was investigated. The best regeneration efficiency in the experiment was achieved at a power of HI(W) and an irradiation time of approximately 80 s. By comparing the range S, it can be seen that microwave power has the greatest impact on the recovery of the iodine value of the regenerated activated carbon, followed by irradiation time, and then by the adsorption capacity of the activated carbon. The microwave irradiation method for reactivating activated carbon takes a short time. It has low energy consumption and a simple device structure, offering good application prospects. However, issues such as whether other intermediate products are formed during the desorption of organic substances by microwave heating still require further investigation. 　　2.6 Catalytic Wet Oxidation Method The traditional wet oxidation method has a low regeneration efficiency and high energy consumption. Regeneration temperature is the main factor affecting regeneration efficiency, but increasing it leads to increased surface oxidation of the activated carbon, thereby reducing regeneration efficiency. Therefore, people are considering using efficient catalysts to regenerate activated carbon through catalytic wet oxidation. Researchers at the **Key Laboratory of Water Environment Control and Resource Recovery** at Tongji University are conducting studies in this area. As the concept of sustainable development gains greater acceptance, activated carbon regeneration processes and technologies are attracting increasing attention. Some traditional activated carbon regeneration techniques and processes have seen new improvements and breakthroughs in recent years. At the same time, new regeneration technologies are also continuously emerging. Although these emerging technologies are not yet mature in terms of manufacturing processes, they cannot be put into industrial use at present. However, their emergence has brought new ideas and discussions to the regeneration of activated carbon.