Electrokinetics is a method that uses direct electric current to remove organic, inorganic, and heavy metal particles from soil. This technique helps clean the soil without disturbing the ground's surface while treating pollution below the surface.
System Components
A basic electrokinetics remediation site includes an external direct current power source, a positively charged electrode (called an anode), and a negatively charged electrode (called a cathode) placed in the ground. The placement of these electrodes depends on the size and shape of known contaminant plumes. Removing contaminants and stopping plumes from spreading influence how the electrodes are arranged. Each electrode is placed inside a reservoir well where an electrolytic solution can be added. This solution is used both to conduct electricity (as a pore fluid) and to remove contaminants or add chemicals and biological materials. The solution also helps control and manage reactions at the electrodes. When submerged in the solution, the anode causes oxidation, and the cathode causes reduction. These reactions create an acidic area, which can affect the system in different ways. By pumping, processing, and testing the electrolytic solution at each electrode location, the system's lifespan and effectiveness can be improved.
Method
When electricity is applied from a power source to electrodes, movements occur beneath the soil surface. While many types of movements happen at the same time, two main processes drive electrokinetics: ionic migration and electrophoresis. When an electrolytic solution is injected into the soil, part of the solution forms a thin layer of charged particles around the soil and contaminants. This layer helps move ions through the soil and surrounding liquid as electricity flows, a process called electroosmosis. The thickness of this thin layer depends on the types of ions in the solution and the average charge of particles in the soil. As the concentration of the electrolyte increases, the thickness of this layer decreases. Electrophoresis is the movement of charged particles in response to an electric field. Both electroosmosis and electrophoresis occur simultaneously but in opposite directions. Charged particles are pulled by electrophoresis from the cathode to the anode, while the electrolyte solution moves from the anode to the cathode. Of the two main processes, electrophoresis (or electromigration) is more significant than electroosmosis. Electrophoresis causes electroosmosis to flow in the opposite direction. Electromigration is the primary method for removing ionic contaminants. For electromigration to happen, materials in the soil must first dissolve into ionic forms, whether they are metals, organic, or inorganic particles. The flow of liquid between electrodes during electroosmosis creates a low-pH environment in the soil. This acidic environment prevents metallic contaminants from attaching to soil particles, which helps form compounds that make electrokinetics effective. This process allows the soil to become more acidic, which can release heavy metals that are absorbed into the soil.
Applications
Electrokinetic remediation is used to remove many types of contaminants that dissolve in groundwater. Heavy metals are a major group of contaminants that this process helps remove. For example, cadmium (II) can be removed, but this process uses a lot of energy. Chromium (III) can also be removed, but it is not very efficient because it easily breaks down in water and attaches to other substances. Chromium (IV) can be treated with electrokinetics, but its movement is slowed when sulfur is present because it changes into chromium (III). Other heavy metals that can be treated include mercury, zinc, iron, lead, copper, and magnesium.
Electrokinetics can also be used for alkali and alkaline earth metals, which move faster through most materials than heavy metals. In tests using 20 to 30 volts, alkali metals moved about 50 to 60 centimeters per day for every volt, while heavy metals moved about 10 to 20 centimeters per day for every volt. This difference may be due to the slower release and dissolving of heavy metals. Electrokinetics can also treat other substances, such as polar organic compounds (like phenol and acetic acid), radionuclides (like radium), toxic anions (such as nitrates and sulfates), dense, non-aqueous-phase liquids (DNAPLs), cyanide, petroleum hydrocarbons (including diesel fuel, gasoline, kerosene, and lubricating oils), halogenated pollutants, explosives, halogenated and polynuclear aromatic hydrocarbons.
Advantages
Electrokinetics is a method used to clean polluted soil. It can be done in situ, which means the cleaning happens directly at the site where the pollution is located. This is especially helpful for treating contaminants in areas of the soil that are hard to reach or where the soil does not allow easy movement of water or treatment solutions. Alternatively, the cleaning can be done ex situ, where the soil is moved to a laboratory for testing and treatment. This flexibility in where the cleaning occurs can save money. Electrokinetics works well in both wet and dry soils because it uses water within the soil’s tiny spaces to help move contaminants. It can also be used in soils with different layers or uneven structures. In soils that are difficult to clean, such as kaolinite and clayey sands, this method can remove up to 90% of heavy metal pollutants. Before starting the cleaning process, it is often necessary to test the soil to understand its conditions and prepare for the work.
The way electricity moves through soil depends on how ions (tiny charged particles) are spread in the water between soil particles. Since the arrangement of ions affects how well electrokinetics works, engineers like John Dzenitis have studied the chemical reactions that occur near the electrodes used in the process. These studies help create models that predict how ions will move during cleaning. These models can then be used to decide if electrokinetics is the best method for cleaning a specific area.
Limitations
A major problem with using electrokinetics to clean soil is that it requires adding an outside liquid to the soil. If the pollution in the soil cannot dissolve in this liquid, the method cannot remove the harmful substance. When electricity is used near the electrodes for a long time, it can change the soil's pH, making it more acidic. This acidity can sometimes react with the pollution. If this increased acidity is not safe for the environment, the electrokinetics process should be reconsidered. Large metal objects buried underground can also cause problems by creating a shortcut for the electric current, which disrupts the system. These metal objects can also change how the electric current flows through the soil. Removing certain types of pollution, such as volatile organic compounds, can increase the amount of vapor in the soil. Surprisingly, soils that allow water and chemicals to pass through easily, like sandy soils, make the electrokinetics process less effective. In contrast, soils that are harder for water to pass through, like clay, can remove up to 90% of pollution at first, while soils like peat can only remove about 65% of pollution initially.
Another important issue with electrokinetics is that the electric current in the system can weaken over time. Different types of electrical problems can affect how well the system works. For example, activation polarization happens when gas bubbles form on the surface of the electrodes during the process. Resistance polarization occurs after the process starts, when a white layer forms on the electrodes. This layer is made of salt and other materials that block the flow of electricity. Concentration polarization happens when hydrogen ions near the positive electrode move toward the negative electrode, and hydroxide ions near the negative electrode move toward the positive electrode. If these ions neutralize each other, the electric current between the electrodes weakens. Uneven areas in the electric current can also cause differences in how pollution moves through the soil.
Case Studies
The study of how electrokinetics remediation works at specific sites can help improve the technology. Often, this method is used together with other cleanup techniques to solve problems unique to each site. In a case in Denmark, copper pollution in the soil occurred in two forms: as ions mixed with other substances in the soil or as part of soil mineral structures. At this site, soil pH was very important because a large amount of copper was present as ions. Scientists used barriers or membranes to stop ions from moving from the electrode areas into the soil. Separating the soil from the electrodes helped prevent the soil from becoming too acidic near the cathode and reduced the loss of electrical current caused by mobile ions moving through the soil.
In 1995, at the Paducah site in Kentucky, USA, a new method called the Lasagna Process was developed to remove heavy metals from soil. This method creates several horizontal layers that allow treatment chemicals to move through the contaminated soil. Materials like sorbents, catalysts, buffering solutions, and oxidizing agents are added to the electrolytic solution through a vertical system, with the anode placed near the bottom and the cathode near the top. This setup makes it easier to reuse water and treat the soil. The layers in the Lasagna Process form because horizontal electrodes cause cracking in very dense clay. Combining horizontal electrodes with a vertical pressure system helps this method remove pollutants from deep soil layers. The first test of this process successfully removed 98% of trichloroethylene from the Paducah site. Electrokinetic remediation can also be used at sites with radioactive contamination.