Activated Carbon Regeneration (Reactivation) Methods
In the process of use, activated carbon can be saturated and loses its adsorption capacity. Other than replacing activated carbon frequently to achieve the use effect but cause a waste of resource, activated carbon regeneration has more economic and environmental benefits.
What is Activated Carbon Regeneration
Activated carbon regeneration (i.e. reactivation) refers to the removal of adsorbed material from activated carbon by physical or chemical methods without destroying the original structure of activated carbon, and restoring its adsorption performance for the purpose of reuse. Before choosing the activated carbon regeneration method, it is important to have a clear understanding of the treated object and the scale of treatment.
The process of adsorption is a process in which the activated carbon, adsorbate and solvent form an adsorption equilibrium. To let the activated carbon desorb and restore its activity, the original equilibrium conditions must be broken, whose basis are as follows:
- External heating to raise the temperature to change the equilibrium conditions, as the amount of adsorbed mass decreases with increasing temperature, thus desorbing the adsorbed mass.
- Changing the chemical properties of the adsorbate.
- Extracting the adsorbate with a solvent that has a strong affinity for the adsorbate.
- Replacing the adsorbate with a substance that has a high affinity for the activated carbon, and then desorbing the replacement substance and regenerating the activated carbon.
- Lowering the concentration (or pressure) of the solute in the solvent to desorb the adsorbate.
- Removing the adsorbed material (organic matter) by decomposition or oxidation.
Regeneration Methods of Activated Carbon
There are various regeneration methods for activated carbon, like thermal regeneration method, biological regeneration method, wet oxidation method, solvent regeneration method, electrochemical regeneration method, catalytic wet oxidation method, etc.
Thermal Regeneration Method
The thermal regeneration method is one of the most mature activated carbon regeneration methods in the industry. During the regeneration process of activated carbon after treating organic wastewater, it is generally divided into three stages: 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, part of the organic matter adsorbed on the activated carbon boils, vaporizes and desorbs, and part of the organic matter decomposes to produce small molecule hydrocarbons and desorbs, while the residual components remain in the pores of the activated carbon to be fixed carbon. In this stage, the temperature will reach 800-900 °C. To avoid oxidation of the activated carbon, the process is usually carried out under vacuum or inert atmosphere. In the next stage of activation, CO2, CO, H2 or water vapor are introduced into the reactor to clean up the micro-pores of the activated carbon and restore its adsorption performance, which is the key to the whole regeneration process. Although the thermal regeneration method has the characteristics of high regeneration efficiency and wide application, it requires additional energy for heating during the regeneration process, which results in high investment and operation costs.
Biological Regeneration Method
The biological regeneration method is to use domesticated bacteria to resolve the organic matter adsorbed on the activated carbon and further digest and decompose it into H2O and CO2. The biological regeneration method is similar to the biological method in wastewater treatment, and there are also aerobic and anaerobic methods. Since the pore size of activated carbon itself is very small, some only a few nanometers, microorganisms cannot enter such pores, and it is usually believed that cell autolysis occurs during the regeneration process, i.e. cell enzymes flow to the extracellular, and activated carbon has adsorption effect on enzymes, thus forming an enzymatic center on the surface of carbon, and promoting the decomposition of pollutants and achieving the purpose of regeneration. The biological method is simple and easy to implement, with low investment and operating costs, but it takes a long time and is greatly influenced by water quality and temperature.
Wet Oxidation Regeneration Method
The wet oxidation regeneration method is that under the condition of high temperature and pressure, oxygen or air is used to oxidize and decompose the organic matter adsorbed on the activated carbon in the liquid phase into small molecules. The best regeneration conditions of activated carbon are as follows: regeneration temperature 230 °C, regeneration time 1 h, oxygenation PO 20.6 MPa, carbon addition 15 g, water addition 300 mL. The regeneration efficiency reached 45±5%, and after 5 cycles of regeneration, the regeneration efficiency only decreased by 3%. The partial oxidation of activated carbon surface micropores is the main reason for the decrease of regeneration efficiency.
Solvent Regeneration Method
The solvent regeneration method uses the phase equilibrium between activated carbon, solvent and adsorbed material to break the adsorption equilibrium by changing the temperature, pH value of solvent and other conditions to desorb the adsorbed material from the activated carbon. Solvent regeneration method is more suitable for those reversible adsorption, such as the adsorption of organic wastewater with high concentration and low boiling point. It is more targeted, often a solvent can only desorb certain pollutants, while the water treatment process of a wide range of pollutants, variable, so the application of a specific solvent is narrow.
Electrochemical Regeneration Method
Electrochemical regeneration method is a new type of activated carbon regeneration technology. In this method, activated carbon is filled between two main electrodes, and a DC electric field is added to the electrolyte. The activated carbon is polarized under the action of electric field, with one end being anode and the other end cathode, forming a microelectrolytic cell. This method is easy to operate and has high efficiency and low energy consumption, and its treatment object is subject to less limitations, and if the treatment process is perfect, secondary pollution can be avoided.
Catalytic Wet Oxidation Method
Traditional wet oxidation method regeneration has relatively low efficiency but large energy consumption. The regeneration temperature is the main reason for the regeneration efficiency, but increasing the regeneration temperature will also increase the surface oxidation of activated carbon, thus reducing the regeneration efficiency. Therefore, it is considered to regenerate activated carbon with the help of high-efficiency catalyst.