In situ chemical reduction
Encyclopedia
In situ chemical reduction (ISCR) is a new type of environmental remediation technique used for soil and/or groundwater remediation
to reduce the concentrations of targeted environmental contaminants to acceptable levels. It is the mirror process of In Situ Chemical Oxidation (ISCO). ISCR is usually applied in the environment by injecting them in liquid form into the contaminated area or placing a solid medium of chemical reductants in the path of a contaminant plume. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation.
The in situ in ISCR is just Latin for "in place", signifying that ISCR is a chemical reduction reaction that occurs at the site of the contamination. Like ISCO, it is able to decontaminate many compounds, and, in theory, ISCR could be more effective in ground water remediation than ISCO.
Chemical reduction is one half of a redox
reaction, which results in the gain of electrons. One of the reactants in the reaction becomes oxidized, or loses electrons, while the other reactant becomes reduced, or gains electrons. In ISCR, reducing compounds, compounds that accept electrons given by other compounds in a reaction, are used to change to change the contaminants into harmless compounds.
, endrin
, chloroform
, and hexachlorocyclopentadiene
to name a few. His work has been the basis of ISCR today.
In the 1990s, Gillham, Tratnyek, Kriegman, Zhang, and Batchelor all made significant contributions in testing different metals and oxides for the use of ISCR. Gillham and Tratnyek in particular applied the reductive chemistry to groundwater treatment with the emplacement of ZVI barriers. Although it has been shown that other metals like aluminum and magnesium can produce the same effect in the laboratory, ground water treatment most generally focuses on the use of iron. Other major contributions in this field includes Zhang, who researched nanoscale iron, and Batchelor, who researched zero-valent iron clay (ZVI Clay). This past decade, more aspects of ISCR have been researched and new methods of implementation, such as ZVI clay and emulsified ZVI (EZVI), have been created. Scientists have also found that certain iron minerals, like green rust, magnetite, and pyrite, also have reductive capabilities although they contain ferrous iron rather than ZVI.
(TCE) and Hexavalent chromium
(Cr(VI)). ZVMs are usually implemented by a permeable reactive barrier
. For example, iron that has been embedded in a swellable, organically-modified silica (commercially available as Osorb
) creates a permanent soft barrier underground to capture and reduce small, organic compounds as groundwater passes through it.
, pyrite
, and glauconite
. The most reactive of the iron minerals are the iron sulfide
s and oxides
. Pyrite, an iron sulfide, is able to dechlorinate carbon tetrachloride
in suspension. These substances are very interesting because they are naturally present, and learning about how they produce reductive zones could lead to the development of better reductants for ISCR.
can also be used as a reductant. It is usually used in addition to iron reduce contaminants. A number of reactions take place and eventually the contaminant is removed. In the process, ditionite is consumed and the final product of all the reactions is 2 sulfur dioxide anions. The dithionite is not stable for a long period of time.
are materials that are made out of two different metals or alloys that are tightly bonded together. A good example of a bimetallic material would be a bimetallic strip which is used in some kinds of thermometers. In ISCR, bimetallic materials are small pieces of metals that are coated lightly with a catalyst such as palladium, silver, or platinum. The catalyst drives a faster reaction and the small size of the particles allows them to effectively move into and remain in the target zone.
Another material worth mentioning is EZVI (emulsified ZVI) which is a NASA technology. EZVI is used mainly to treat halogenated hydrocarbons and DAPNLs. EZVI is nanoscale iron that is placed into a biodegradable oil emulsion
. The emulsion is then injected into the substratum.
, β-elimination, hydrogenation
, α-elimination, and electron transfer
. The specific combination of reductive processes that actually take place in the subsurface depends on the species of contaminant that is present and also the type of reduction being used. The natural and biological processes that take place in the substratum also effect the kinds of reductive processes that are found.
Combining bacterial action and biological processes with iron is also known to be effective. The most evident uses of biological processes are with the EZVI technology created by NASA and with the EHC product created by Adventus. Both of these materials have iron within some biological matrix (iron is suspended in vegetable oil in EZVI and in organic carbon in EHC) and use microbial organisms to enhance the reduction zone and to create a more anaerobic environment for the reactions to take place in.
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
to reduce the concentrations of targeted environmental contaminants to acceptable levels. It is the mirror process of In Situ Chemical Oxidation (ISCO). ISCR is usually applied in the environment by injecting them in liquid form into the contaminated area or placing a solid medium of chemical reductants in the path of a contaminant plume. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation.
The in situ in ISCR is just Latin for "in place", signifying that ISCR is a chemical reduction reaction that occurs at the site of the contamination. Like ISCO, it is able to decontaminate many compounds, and, in theory, ISCR could be more effective in ground water remediation than ISCO.
Chemical reduction is one half of a redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reaction, which results in the gain of electrons. One of the reactants in the reaction becomes oxidized, or loses electrons, while the other reactant becomes reduced, or gains electrons. In ISCR, reducing compounds, compounds that accept electrons given by other compounds in a reaction, are used to change to change the contaminants into harmless compounds.
History
ISCR is a relatively new type of ground water remediation technology. The most work on this method of remediation has been done in the past 10–15 years, so there are still many gaps in the understanding of the chemistry behind this process. The development of ISCR started out when K.H. Sweeney conducted research with zero-valent copper and iron in the late 1970s. He was able to treat a number of different chlorinated substances such as DDTDDT
DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
, endrin
Endrin
Endrin is an organochloride that was primarily used as an insecticide. It is a colourless odorless solid, although commercial samples are often off-white. It is also a rodenticide. This compound became infamous as persistent organic pollutant and for this reason is banned in many...
, chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
, and hexachlorocyclopentadiene
Hexachlorocyclopentadiene
Hexachlorocyclopentadiene is an organochlorine compound that is a precursor to several pesticides. This colourless liquid is an inexpensive reactive diene. Many of its derivatives proved to be highly controversial, as studies showed them to be persistent organic pollutants. Collectively, the...
to name a few. His work has been the basis of ISCR today.
In the 1990s, Gillham, Tratnyek, Kriegman, Zhang, and Batchelor all made significant contributions in testing different metals and oxides for the use of ISCR. Gillham and Tratnyek in particular applied the reductive chemistry to groundwater treatment with the emplacement of ZVI barriers. Although it has been shown that other metals like aluminum and magnesium can produce the same effect in the laboratory, ground water treatment most generally focuses on the use of iron. Other major contributions in this field includes Zhang, who researched nanoscale iron, and Batchelor, who researched zero-valent iron clay (ZVI Clay). This past decade, more aspects of ISCR have been researched and new methods of implementation, such as ZVI clay and emulsified ZVI (EZVI), have been created. Scientists have also found that certain iron minerals, like green rust, magnetite, and pyrite, also have reductive capabilities although they contain ferrous iron rather than ZVI.
Zero Valent Metals(ZVMs)
Zero Valent Metals are the main reductants used in ISCR. The most common metal used is iron, in the form of ZVI (zero valent iron), and it is also the metal longest in use. However, some studies show that zero valent zinc (ZVZ) could actually be up to ten times more effective at eradicating the contaminants than ZVI. Some applications of ZVMs are to clean up TrichloroethyleneTrichloroethylene
The chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. It is a clear non-flammable liquid with a sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, which is commonly known as chlorothene.The IUPAC name is...
(TCE) and Hexavalent chromium
Hexavalent chromium
Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately of hexavalent chromium were produced in 1985...
(Cr(VI)). ZVMs are usually implemented by a permeable reactive barrier
Permeable reactive barrier
A permeable reactive barrier , also referred to as a permeable reactive treatment zone , is a developing technology that has been recognized as being a cost-effective technology for in situ groundwater remediation. PRBs are barriers which allow some—but not all—materials to pass through...
. For example, iron that has been embedded in a swellable, organically-modified silica (commercially available as Osorb
Osorb
Osorb is a swellable, organically-modified silica or glass capable of absorbing volatile organic compounds and other contaminants from water. The glass was discovered by Dr. Paul L. Edmiston and is trademarked by ABSMaterials, Inc.-History of discovery:...
) creates a permanent soft barrier underground to capture and reduce small, organic compounds as groundwater passes through it.
Iron Minerals
There are also many iron minerals that can actively be used in dechlorination. These minerals use . Particular minerals that can be used include green rust, magnetiteMagnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, and glauconite
Glauconite
Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry...
. The most reactive of the iron minerals are the iron sulfide
Iron sulfide
Iron sulfide or Iron sulphide refers to a chemical compound of iron and sulfur with a wide range of stoechiometric formulae and different crystalline structures.-Natural minerals:By increasing order of stability:...
s and oxides
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
. Pyrite, an iron sulfide, is able to dechlorinate carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
in suspension. These substances are very interesting because they are naturally present, and learning about how they produce reductive zones could lead to the development of better reductants for ISCR.
Polysulfides
Polysulfides are compounds that have chains of sulfur atoms. This is a relatively new reactant, but it has been tested on the field in treating TCE and in comparison to EHC. The use of Polysulfides is a type of abiotic reduction and works best in anaerobic conditions where iron (III) is available. The benefit of using polysulfides is that they do not produce any biological waste products; however, the reaction rates are slow and they require more time to create the DVI (dual valent iron) minerals that are needed for the reduction to occur.Dithionite
DithioniteDithionite
The dithionite anion , is an oxoanion of sulfur formally derived from dithionous acid, H2S2O4.-Chemistry:Dithionous acid has not been detected either as a pure compound or in solution....
can also be used as a reductant. It is usually used in addition to iron reduce contaminants. A number of reactions take place and eventually the contaminant is removed. In the process, ditionite is consumed and the final product of all the reactions is 2 sulfur dioxide anions. The dithionite is not stable for a long period of time.
Bimetallic Materials
Bimetallic materialsBimetal
Bimetal refers to an object that is composed of two separate metals joined together. Instead of being a mixture of two or more metals, like alloys, bimetallic objects consist of layers of different metals...
are materials that are made out of two different metals or alloys that are tightly bonded together. A good example of a bimetallic material would be a bimetallic strip which is used in some kinds of thermometers. In ISCR, bimetallic materials are small pieces of metals that are coated lightly with a catalyst such as palladium, silver, or platinum. The catalyst drives a faster reaction and the small size of the particles allows them to effectively move into and remain in the target zone.
Proprietary Materials
One proprietary material present today for ISCR is the EHC technology created by Adventus. This particular product is actually a mixture of carbon, nutrients, and zero-valent iron. The theory behind this product is that the carbon in the mixture will promote bacterial growth in the subsurface. The growing bacteria consume oxygen, which easily accepts electrons, present in the subsurface which increases reducing potential. The growing bacteria also ferment and produce fatty acids that act as electron donors to other bacteria and substances. Adventus uses this combination of biotic and abiotic processes to implement ISCR. EHC is injected as a "slurry" (a mixture that is 15 to 40% solids and weight with the rest being liquid) into the substratum.Another material worth mentioning is EZVI (emulsified ZVI) which is a NASA technology. EZVI is used mainly to treat halogenated hydrocarbons and DAPNLs. EZVI is nanoscale iron that is placed into a biodegradable oil emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
. The emulsion is then injected into the substratum.
Reductive Processes
In ISCR, there are many reductive processes that can take place. There are hydrogenolysisHydrogenolysis
Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes "lysis" by hydrogen. The heteroatom may vary, but it usually is oxygen, nitrogen, or sulfur. A related reaction is hydrogenation, where hydrogen is added to the molecule, without...
, β-elimination, hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
, α-elimination, and electron transfer
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
. The specific combination of reductive processes that actually take place in the subsurface depends on the species of contaminant that is present and also the type of reduction being used. The natural and biological processes that take place in the substratum also effect the kinds of reductive processes that are found.
Surface Catalyzed Reactions
The reactions that occur with permeable reactive barriers and ferrous iron are surface based. The surface reactions take three different forms: direct reduction, electron shunting through ferrous iron, and reduction by production and reaction of hydrogen. Pathway A represents direct electron transfer (ET) for Fe0 to the adsorbed halocarbon (RX) at the metal/water point of contact, resulting in dechlorination and production of Fe2+. Pathway B shows that Fe2+ (resulting from corrosion of Fe0) may also dechlorinate RX, producing Fe3+. Pathway C shows that H2 from the anaerobic corrosion of Fe2+ might react with RX if a catalyst is present.Enhancement of Reductive Pathways
The reductive processes discussed above can be enhanced in two ways. One is by increasing the amount of usable iron in the subsurface to increase the rate of the reduction by chemical or biological means. The second method is to enhance the reducing ability of the iron by coupling it with other chemical reductants or using biological reduction with it. Using this processes, scientists combined sodium dithionite with iron to treat Chrominum VI and TCE effectively.Combining bacterial action and biological processes with iron is also known to be effective. The most evident uses of biological processes are with the EZVI technology created by NASA and with the EHC product created by Adventus. Both of these materials have iron within some biological matrix (iron is suspended in vegetable oil in EZVI and in organic carbon in EHC) and use microbial organisms to enhance the reduction zone and to create a more anaerobic environment for the reactions to take place in.
Implementation
The most common type of implementation of ISCR is the installation of permeable reactive barriers (PRBs), but there are instances when the reductant can be directly injected into the subsurface to treat source areas.Semi-Permeable Reactive Barrier
These barriers are usually made out of zero-valent iron (ZVI) but can also be made with any other zero-valent metal. The most common way they are made is by filling a trench with ZVI, nanoscale iron, or palladium. Nanoscale iron particles can also be injected directly into the subsurface to treat plumes, and they have large surface areas and, therefore, high reactivities and can be distributed more evenly in the contamination site. Palladium's reaction rates are rapid. The main advantages of PRBs are that it can reduce many a variety of contaminants and it has no above-ground structure. Problems with PRBs include that even with well constructed barriers, there might be the problem of hydraulic short-circuiting.Direct Injection of Reductants
Nanoscale iron can be directly into the subsurface because they are small enough to be distributed thoroughly. Because the particles are so small, they have a comparatively large reactive surface, providing a more effective reaction. As of now, nanoscale iron is the only material that has been used with this injection strategy, and it is probably the only material that is effective in injection.Future of ISCR
ISCR is a relatively new technology, so there much scope for research and improvement. Right now, although the reactions that make up ISCR have been studied extensively, there is not much background on what factors most contribute to the effectiveness of ISCR. One thing that needs to be done is find out exactly what reactions are taking place in the subsurface. ISCR is fairly more complex than ISCO because there are substances in the subsurface that will naturally reduce contaminants. The pathways that a contaminant can go through are consequently more diverse. Also, questions that need to be kept in mind are:- Which reducing agent will work best with a particular conaminant?
- How can we improve the ISCR technology that is already commercially available?