The biggest feature of activated carbon is its rich pore structure. There are countless tiny pores on the surface of activated carbon. The diameter distribution range of these micropores is very wide, ranging from less than one nanometer to thousands of nanometers. They can be divided into three categories: macropores (d>50nm), mesopores (2nm<d<50nm) and micropores (d<2nm). Therefore, activated carbon has a huge specific surface area, and the surface area of each gram of activated carbon can reach 1000m². The pore characteristics are most related to the physical adsorption properties of activated carbon. The size, shape and distribution of the pores contained in activated carbon will affect the physical adsorption of activated carbon. Activated carbon also has a wealth of surface chemical groups, which are divided into three types: acidic, alkaline and neutral, which are mainly related to the chemical adsorption of activated carbon.
Due to the rich pore structure, huge specific surface area and rich surface chemical groups, activated carbon has a high adsorption capacity. In order to improve the adsorption efficiency of activated carbon, additional modification processes can be carried out, such as sulfidation, halogenation, loading of precious metals or metal oxides, additional oxygen-containing functional groups or loading of catalysts.
Activated carbon is easily saturated during the adsorption process. After reaching the saturated state, the pore structure inside the activated carbon is blocked by the adsorbed substances and no longer has adsorption activity. At present, the treatment methods of saturated activated carbon include landfill, incineration and regeneration, but simple landfill will pollute the surrounding soil and water bodies, and incineration will produce highly toxic substances such as dioxins, causing air pollution. Therefore, the desorption, regeneration and harmless treatment of waste activated carbon are of great significance to protecting the environment, improving economic benefits and reducing resource waste.
Activated carbon regeneration (or activation) refers to the use of physical or chemical methods to remove the adsorbate adsorbed in the micropores of activated carbon without destroying the original structure of the activated carbon, restore its adsorption performance, and achieve the purpose of reuse.
At present, the main activated carbon regeneration technologies at home and abroad are thermal regeneration, chemical regeneration and biological regeneration. Chemical regeneration is divided into solvent regeneration and electrochemical regeneration.
Thermal regeneration has the advantages of high regeneration efficiency and stability, and is usually not limited by the type of substances adsorbed on the waste activated carbon, and has a wide range of applications. However, the thermal regeneration method requires the activated carbon to be heated to a high temperature, which consumes a lot of energy. If it is used in industry, it will waste resources and increase costs; if the heating temperature is too high, there is a risk of spontaneous combustion; high temperature will also cause the microcrystalline structure inside the activated carbon to expand and the mesopores to collapse, resulting in a decrease in the specific surface area of the activated carbon, shrinkage of the pores, and a decrease in the pore volume; high temperature will also cause the decomposition of some functional groups on the activated carbon, and functional groups are one of the key factors affecting the adsorption of inorganic substances by activated carbon, which will inhibit the adsorption of activated carbon. In addition, the mechanical strength of the activated carbon after direct thermal regeneration will be damaged, and the adsorption efficiency will also decrease to a certain extent. After repeated regeneration, it will no longer have adsorption capacity.
According to the thermal regeneration medium, it can be divided into: inert gas thermal regeneration, water vapor thermal regeneration, electric current thermal regeneration, and physical wave (microwave, ultrasonic) thermal regeneration.
The research on thermal regeneration with air, inert gas and water vapor as the medium is very mature, and it is the main application technology for industrial activated carbon desorption regeneration at this stage. It has the advantages of high and stable regeneration efficiency, but high energy consumption, large heat loss of activated carbon itself, and the risk of spontaneous combustion when the activated carbon is heated too high.
Electric current thermal regeneration is due to the resistivity of carbonaceous materials. The Joule effect directly converts the current into thermal energy, that is, heating from the inside rather than the external energy.
Electric current thermal regeneration is most suitable for removing non-strong adsorbents and highly volatile adsorbents. This method is rarely used in traditional GAC (granular activated carbon) activated carbon beds because the activated carbon particles are not in uniform contact, and it is more suitable for other forms of activated carbon.
Microwaves induce molecular motion to generate heat through dipole rotation and ion migration. Compared with conventional thermal regeneration technology, microwave thermal regeneration technology has the advantages of fast heating speed, basically no heating gradient, less time consumption, low energy loss, and high efficiency. The research of this method is still in the laboratory stage, and the microwave regeneration mechanism needs to be deepened. The practicality and safety of microwave generators and large-scale microwave regeneration devices need to be confirmed and improved.
Ultrasonic waves are absorbed by the medium during propagation, and then weaken to produce “cavitation bubbles” that continue to expand until they “explode” and release energy. This energy causes three effects: ① The local temperature rises rapidly to form a “hot spot”; ② High-pressure shock waves appear; ③ High-speed micro-jets appear, and the liquid is sprayed onto the adsorbent surface at a speed of hundreds of kilometers per hour.
Compared with traditional thermal regeneration, ultrasonic technology is immature and the regeneration efficiency is unstable. It is more suitable for physical adsorption and is not suitable for widespread industrial production at this stage.
There are two main mechanisms in the electrochemical regeneration process: ① The local pH change, local salinity concentration change, and electrostatic repulsion enhance the desorption effect and cause the electrolysis of organic matter; ② The adsorption saturated activated carbon is used as the electrode, and the ion exchange membrane and electrolyte solution are arranged as the medium for ion exchange. When the power is turned on, the electrode undergoes oxidation or reduction reaction.
Compared with other traditional methods, the electrochemical regeneration method has unique advantages and has great development prospects in terms of energy efficiency, selectivity, cost-effectiveness and environmental compatibility. However, most of the advanced results at this stage remain at the laboratory scale, and industrial applications are limited by integration, economy and effectiveness.
The oxidative regeneration method is very feasible in practical applications. The temperature and pressure of the oxidative regeneration conditions are easy to achieve under oxidation, especially the wet oxidation method is often used in the treatment of wastewater saturated activated carbon.
The solvent regeneration method depends on the interaction between activated carbon, adsorbent and solvent. The solvent must diffuse and penetrate into the adsorbent structure to reach the active sites on the surface or in the micropores, and break the original adsorption balance through chemical reactions or dissolution extraction, thereby separating the adsorbent from the activated carbon. The solvent regeneration method does not need to disassemble the activated carbon first and then reinstall it after regeneration. It can be carried out directly in situ, which not only saves downtime and maintenance time, but also reduces the loss of activated carbon caused by wear. Compared with the thermal regeneration method, due to the lack of high-temperature carbonization and reoxidation process, the solvent regeneration method has no heat loss, and the mechanical strength and pore structure of the activated carbon are maintained.
Supercritical regeneration does not have secondary pollution problems, but the supercritical conditions required for the fluid are difficult to achieve, the operating conditions are harsh, and the types of pollutants that can be removed are limited, so there is a lack of industrial-scale application research.
The principle of the vacuum regeneration method is that the adsorption equilibrium of the adsorbed activated carbon is broken in a vacuum environment, and it is mechanically transferred to desorption to restore the regeneration capacity.
The advantages of the VSA process include a long service life of the adsorbent, simple technology, and low energy consumption, but the process requires a multi-stage vacuum system to continuously operate to maintain the regeneration environment, so the regeneration cost is expensive, the requirements for the equipment valves are high, and the post-operation maintenance is difficult.
The biological regeneration method mainly uses the degradation of microorganisms to desorb various organic pollutants.
The regeneration time of the biological regeneration method is often very long, the cultivation and domestication of organisms are difficult to control, and the activated carbon is difficult to regenerate in situ. The regeneration efficiency is greatly affected by changes in the external environment, so the scope of practical application is limited.
