Electrokinetic remediation
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Electrokinetics remediation, also termed electrokinetics, is a technique of using direct
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
to remove organic, inorganic and heavy metal particles from the soil by
electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
.R. Iyer, "Electrokinetic Remediation," Particulate Science and Technology: An International Journal, vol. 19, 2001, p. 219. The use of this technique provides an approach with minimum disturbance to the surface while treating subsurface
contaminant Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc. Types of contamination Wi ...
s.


System Components

A basic electrokinetics remediation site contains an external
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
source, a
positively charged Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
(or
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
) and a
negatively charged Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectiv ...
electrode (or a
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
) placed into the ground. Placement of electrodes are based on the size and shape of known contaminant plumes. The removal of contaminants and prevention of plume migration are big influences in determining the arrangement of electrodes. Each electrode is encased in a reservoir well in which an
electrolytic An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
solution can be injected.H.D. Sharma, Geoenvironmental Engineering: Site Remediation, Waste Containment, and Emerging Waste Management Technologies, Hoboken, N.J: Wiley, 2004. The electrolytic solutions serve both as a conducting media (or pore fluid) and as a means to extract contaminants and introduce
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
s or biological entities. Another use of the electrolytic solution is for control and/or
depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
of electrode reactions. Immersed in a solution the electrodes can result in
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
at the anode site and the reduction at the cathodic site. The oxidation and formation of an acidic front are byproducts of the process and cause varying degree of influence to the system. By pumping, processing and testing the electrolytic solution at each electrode site you can extend the life and efficiency of the system.


Method

When current is applied, by the direct power source, to the electrodes, migrations occur beneath the
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
surface. Although many types of migrations occur in tandem with the current there are two driving migrations within electrokinetics; ionic migration and
electrophoresis Electrophoresis, from Ancient Greek ἤλεκτρον (ḗlektron, "amber") and φόρησις (phórēsis, "the act of bearing"), is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric fie ...
. When the electrolytic solution is injected within the soil part of the
ionic solution An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...
forms a diffuse double layer with soils and contaminants. This diffused double layer will aid in the ionic drift that will occur as the current passes through the soil and surrounding liquid, this process is called
electroosmosis Electroosmotic flow (or electro-osmotic flow, often abbreviated EOF; synonymous with electroosmosis or electroendosmosis) is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or an ...
. The thickness of the diffused double layer is a function of ionic composition of bulk solution and the average
charge density In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in co ...
of
hydration Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...
products. As the electrolyte concentration increases the diffuse double layer thickness decreases. Electrophoresis is the mass flux of
charged particle In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary particle, ...
s under an
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
. Both processes work at the same time but in a counter current manner. The charged particles are driven by electrophoresis flow from the cathode to the anode while electrolytic solution flows from the anode to the cathode. Of the two main processes electrophoresis (or electromigration) is more dominant than electroosmosis. Electrophoresis serves as the driving factor that induces the flow of electroosmosis in the opposite direction. Electromigration also serves as the major component for ionic contaminant removal. For electromigration to occur absorbed material must be dissolved to an ionic form whether they are metals, organic or inorganic particles. Electroosmotic flow between the electrodes promotes the development of a low-pH environment in the soil. This low pH environment inhibits metallic contaminants from being sorbed onto soil particle surfaces which aids in the formation of compounds making electrokinetics possible. By this thought it is possible to acidify the soil and induce the release of absorbed heavy metals.


Applications

Electrokinetic remediation is applied to many contaminants that can be dissolved within
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
. Heavy metals are one of the main contaminants that are removed by the electrokinetics process. Some metals like
cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
(II) can be removed with high consequences on energy expenditure. Chromium (III) can be removed but with low efficiency because of the ease of
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
allowing it to sorb to other substances. Chromium (IV) is also a candidate for electrokinetics removal although chromium (IV) migration is retarded in the presence of
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
because it will break down into
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...
(III). Other heavy metal species that are tested and reported to be treatable include;
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
,
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
,
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, and
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
. Electrokinetics is also possible with
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
and alkaline earth metals which travel faster in most mediums than heavy metals. At tests between 20 and 30 volts, alkali metals were known to move between 50 and 60 cm per day per volts whereas heavy metal moved at velocities between 10 and 20 cm per day per volts. It is possible that this difference could be because of the slow
desorption Desorption is the physical process where a previously adsorbed substance is released from a surface. This happens when a molecule gains enough energy to overcome the activation barrier of the bounding energy that keeps it in the surface. There ...
and dissolution of heavy metals. Electrokinetics can also be used to treat polar organic compounds (
phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...
and
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
) and
radionucleotide Radionucleotide may refer to: * In experimental biochemistry, a nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid ...
s (
radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rather t ...
), toxic anions (nitrates and sulfates), dense, non-aqueous-phase liquids (DNAPLs),
cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
, petroleum hydrocarbons (diesel fuel,
gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
,
kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
and lubricating oils), halogenated pollutants, explosives, halogenated and polynuclear aromatic hydrocarbons.


Advantages

One of the advantages of electrokinetics is that the remediation can be conducted in situ (within the remediation site) to treat contaminants in low permeability zones to overcome accessibility of contaminants or delivery of treatment. Remediation can also occur ex situ (removed from the natural site) to have contaminants tested and treated within a
laboratory A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physicia ...
. This versatility of treatment location can be very cost effective. Electrokinetics has the advantage of use in saturated or unsaturated soils because of the insertion of pore fluid. Remediation can also occur despite soil stratifications or
homogeneity Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
. For soils that are low in permeability like kaolite and clayey sands it is possible to remove up to 90% of heavy metal contaminants. In many cases pretreatment of soil should be made to determine appropriate working conditions of the soil. One thing to note is that the potential profile in soils can be determined by the ionic distribution of pore fluid. Because ion distribution effects the efficiency of the electrokinetics system, engineers like John Dzenitis have done comprehensive study to find key reactions around the electrodes that can be used to create models for ionic flowrate prediction. These models can then be interpreted to determine if electrokinetics remediation is the best choice for a given site.


Limitations

A major limitation of electrokinetics is the introduction of an external fluid into the soil. If the contaminant cannot be dissolved it is not possible to extract the contaminant of interest. Electrolysis near the electrodes can alter the pH of the soil especially if the current is induced for an extended period of time. Extended use of electrokinetics system can also cause acidic conditions around the electrodes sometimes reacting with contaminants. If increased acidification of the soil is not environmentally acceptable the use of electrokinetics should be reevaluated. Large metal objects that are buried underground also pose as a limitation to electrodes because they offer a path to short circuit the system. Buried metal objects can also change the voltage gradient and decrease or stop the flow. The removal of
volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s from the soils can increase the soil vapor concentration. Counterintuitively, highly permeable soils lower the efficiency of electrokinetics. Where a low permeable soil like clay can receive up to 90% initial contaminant removal a low permeable soil like peat achieves about 65% removal of initial contaminants. Another major limitation of the electrokinetics process is the decrease in the electric potential of the system. Different polarization effects can decrease how the system runs. For instance: Activation polarization can occur during the electrokinetic remediation process removing gas bubbles that form on the surface of the electrodes during conductivity. Resistance polarization can occur after the electrokinetic remediation process has started a white layer can be observed. Just like in hard water stains this layer may be the insoluble salt and other impurities that inhibit conductivity.
Concentration polarization Concentration polarization is a term used in the scientific fields of electrochemistry and membrane science. In electrochemistry In electrochemistry, concentration polarization denotes the part of the polarization of an electrolytic cell resultin ...
happens when hydrogen ions generated at the anode are attracted to the cathode and the hydroxide ions generated at the cathode are attracted to the anode. If neutralization occurs the potential between the system drops. Local flattening of the electrical potential profile can also cause the difference in migration.


Case Studies

In many cases the study of specific site occurrences with electrokinetics remediation can lead to new advancements in the technology. Many times electrokinetics remediation will be coupled with other remediation forms to handle site specific issues. At a Danish Wood Perseveration copper was a heavy metal that polluted the soil in two forms; ionic solution with different complexes within the soil or a crystal lattice of soil minerals. For this site
soil pH Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...
was a parameter of great importance because of an increased amount of copper present as ionic solution. By creating active barriers or exchange membranes that prevent ions from traveling from the electrode compartment into the soil. The separation of the soil from the electrode is designed to deter the acidification of the cathode and the current loss as highly mobile ions pass from electrode site through the soil. In 1995 at the Paducah site, in Kentucky, USA a new technique was developed for removing heavy metals from soils. Called the Lasagna Process, it simply is the creation of several horizontal permeable zones used to provide treatment through the contaminated soil matrix by adding different admixtures to the electrolytic solution. Admixtures like sorbents, catalytic reagents, buffering solutions, oxidizing agents in this system are applied through a vertical system with the anode near the bottom and the cathode near the top. The orientation of vertical anode and cathode system make recycling of fluid and system treatment easier. The formation of the lasagna layers is due to fracturing in over-consolidated clays because of horizontal electrodes. Coupling of the horizontal electrodes with a vertical pressuring system make this method especially effective in removing contaminants from deeper layers of the soil. The first test of this process proved 98% effective of removing
trichloroethylene The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, ...
from the Paducah site. Electrokinetic remediation is also suitable for use at sites with radioactive contamintion.


References

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