In situ chemical reduction

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In situ chemical reduction (ISCR) is a 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 chemically reductive additives in liquid form into the contaminated area or placing a solid medium of chemical reductants in the path of a contaminant plume. [1] It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation.

Contents

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 the contaminants into harmless compounds.

History

Early work examined the dechlorinations with copper. Substrates included DDT, endrin, chloroform, and hexachlorocyclopentadiene. Aluminum and magnesium behave similarly in the laboratory. Ground water treatment most generally focuses on the use of iron. [2]

Reductants

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 be up to ten times more effective at eradicating the contaminants than ZVI. [3] Some applications of ZVMs are to clean up Trichloroethylene (TCE) and Hexavalent chromium (Cr(VI)). [4] ZVMs are usually implemented by a permeable reactive barrier. For example, iron that has been embedded in a swellable, organically modified silica creates a permanent soft barrier underground to capture and reduce small, organic compounds as groundwater passes through it. [5]

Iron minerals

Iron minerals can active for dechlorination. These minerals use Fe2+
. Particular minerals that can be used include green rust, magnetite, pyrite, and glauconite. [6] The most reactive of the iron minerals are the iron sulfides and oxides. Pyrite, an iron sulfide, is able to dechlorinate carbon tetrachloride in suspension. [2]

Polysulfides

Polysulfides are compounds that have chains of sulfur atoms. This reactant 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. [7]

Dithionite

Dithionite (S
2O2−
4) 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 materials 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. [8]

Proprietary materials

One proprietary material 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. [9]

Another material worth mentioning is EZVI (emulsified ZVI) which is a NASA technology. EZVI is used mainly to treat halogenated hydrocarbons and DNAPLs. EZVI is nanoscale iron that is placed into a biodegradable oil emulsion. The emulsion is then injected into the substratum. [10]

Reactions in ISCR

Reductive processes

In ISCR, many reductive processes can take place. There are hydrogenolysis, 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 affect the kinds of reductive processes that are found. [6]

Surface catalyzed reactions

Iron Remediation Reaction Processes.jpg
Iron Remediation Reaction Processes II.jpg
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. [2]

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. [11]

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. [12]

Related Research Articles

<span class="mw-page-title-main">Inorganic chemistry</span> Field of chemistry

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

<span class="mw-page-title-main">Redox</span> Chemical reaction in which oxidation states of atoms are changed

Redox 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 decrease in the oxidation state.

In chemistry, a reducing agent is a chemical species that "donates" an electron to an electron recipient. Examples of substances that are common reducing agents include the alkali metals, formic acid, oxalic acid, and sulfite compounds.

<span class="mw-page-title-main">Environmental remediation</span> Removal of pollution from soil, groundwater etc.

Environmental remediation deals with the removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water. Remedial action is generally subject to an array of regulatory requirements, and may also be based on assessments of human health and ecological risks where no legislative standards exist, or where standards are advisory.

<span class="mw-page-title-main">Bioremediation</span> Process used to treat contaminated media such as water and soil

Bioremediation broadly refers to any process wherein a biological system, living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluents etc., in natural or artificial settings. The natural ability of organisms to adsorb, accumulate, and degrade common and emerging pollutants has attracted the use of biological resources in treatment of contaminated environment. In comparison to conventional physicochemical treatment methods bioremediation may offer considerable advantages as it aims to be sustainable, eco-friendly, cheap, and scalable. Most bioremediation is inadvertent, involving native organisms. Research on bioremediation is heavily focused on stimulating the process by inoculation of a polluted site with organisms or supplying nutrients to promote the growth. In principle, bioremediation could be used to reduce the impact of byproducts created from anthropogenic activities, such as industrialization and agricultural processes. Bioremediation could prove less expensive and more sustainable than other remediation alternatives.

Fenton's reagent is a solution of hydrogen peroxide (H2O2) and an iron catalyst (typically iron(II) sulfate, FeSO4). It is used to oxidize contaminants or waste water as part of an advanced oxidation process. Fenton's reagent can be used to destroy organic compounds such as trichloroethylene (TCE) and tetrachloroethylene (perchloroethylene, PCE). It was developed in the 1890s by Henry John Horstman Fenton as an analytical reagent.

Organohalide respiration (OHR) (previously named halorespiration or dehalorespiration) is the use of halogenated compounds as terminal electron acceptors in anaerobic respiration. Organohalide respiration can play a part in microbial biodegradation. The most common substrates are chlorinated aliphatics (PCE, TCE), chlorinated phenols and chloroform. Organohalide-respiring bacteria are highly diverse. This trait is found in some Campylobacterota, Thermodesulfobacteriota, Chloroflexota (green nonsulfur bacteria), low G+C gram positive Clostridia, and ultramicrobacteria.

Biomining is the technique of extracting metals from ores and other solid materials typically using prokaryotes, fungi or plants. These organisms secrete different organic compounds that chelate metals from the environment and bring it back to the cell where they are typically used to coordinate electrons. It was discovered in the mid 1900s that microorganisms use metals in the cell. Some microbes can use stable metals such as iron, copper, zinc, and gold as well as unstable atoms such as uranium and thorium. Large chemostats of microbes can be grown to leach metals from their media. These vats of culture can then be transformed into many marketable metal compounds. Biomining is an environmentally friendly technique compared to typical mining. Mining releases many pollutants while the only chemicals released from biomining is any metabolites or gasses that the bacteria secrete. The same concept can be used for bioremediation models. Bacteria can be inoculated into environments contaminated with metals, oils, or other toxic compounds. The bacteria can clean the environment by absorbing these toxic compounds to create energy in the cell. Bacteria can mine for metals, clean oil spills, purify gold, and use radioactive elements for energy.

<span class="mw-page-title-main">Iron nanoparticle</span>

Nanoscale iron particles are sub-micrometer particles of iron metal. They are highly reactive because of their large surface area. In the presence of oxygen and water, they rapidly oxidize to form free iron ions. They are widely used in medical and laboratory applications and have also been studied for remediation of industrial sites contaminated with chlorinated organic compounds.

Advanced oxidation processes (AOPs), in a broad sense, are a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (·OH). In real-world applications of wastewater treatment, however, this term usually refers more specifically to a subset of such chemical processes that employ ozone (O3), hydrogen peroxide (H2O2) and/or UV light.

Green nanotechnology refers to the use of nanotechnology to enhance the environmental sustainability of processes producing negative externalities. It also refers to the use of the products of nanotechnology to enhance sustainability. It includes making green nano-products and using nano-products in support of sustainability.

Groundwater remediation is the process that is used to treat polluted groundwater by removing the pollutants or converting them into harmless products. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. Globally, between 25 per cent and 40 per cent of the world's drinking water is drawn from boreholes and dug wells. Groundwater is also used by farmers to irrigate crops and by industries to produce everyday goods. Most groundwater is clean, but groundwater can become polluted, or contaminated as a result of human activities or as a result of natural conditions.

<span class="mw-page-title-main">Electrical resistance heating</span> Environmental cleanup method

Electrical resistance heating (ERH) is an intensive in situ environmental remediation method that uses the flow of alternating current electricity to heat soil and groundwater and evaporate contaminants. Electric current is passed through a targeted soil volume between subsurface electrode elements. The resistance to electrical flow that exists in the soil causes the formation of heat; resulting in an increase in temperature until the boiling point of water at depth is reached. After reaching this temperature, further energy input causes a phase change, forming steam and removing volatile contaminants. ERH is typically more cost effective when used for treating contaminant source areas.

Electrokinetics remediation, also termed electrokinetics, is a technique of using direct electric current to remove organic, inorganic and heavy metal particles from the soil by electric potential. The use of this technique provides an approach with minimum disturbance to the surface while treating subsurface contaminants.

In situ chemical oxidation (ISCO), a form of advanced oxidation process, is an environmental remediation technique used for soil and/or groundwater remediation to lower the concentrations of targeted environmental contaminants to acceptable levels. ISCO is accomplished by introducing strong chemical oxidizers into the contaminated medium to destroy chemical contaminants in place. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation. The in situ in ISCO is just Latin for "in place", signifying that ISCO is a chemical oxidation reaction that occurs at the site of the contamination.

A permeable reactive barrier (PRB), also referred to as a permeable reactive treatment zone (PRTZ), 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. One definition for PRBs is an in situ treatment zone that passively captures a plume of contaminants and removes or breaks down the contaminants, releasing uncontaminated water. The primary removal methods include: (1) sorption and precipitation, (2) chemical reaction, and (3) reactions involving biological mechanisms.

<span class="mw-page-title-main">1,2,3-Trichloropropane</span> Chemical compound

1,2,3-Trichloropropane (TCP) is an organic compound with the formula CHCl(CH2Cl)2. It is a colorless liquid that is used as a solvent and in other specialty applications.

<span class="mw-page-title-main">Zerovalent iron</span> Term denoting metallic iron, Fe(0), used for permeable reactive barrier in site remediation

Zerovalent iron (ZVI) is jargon that describes forms of iron metal that are proposed for used in Groundwater remediation.

Nanoremediation is the use of nanoparticles for environmental remediation. It is being explored to treat ground water, wastewater, soil, sediment, or other contaminated environmental materials. Nanoremediation is an emerging industry; by 2009, nanoremediation technologies had been documented in at least 44 cleanup sites around the world, predominantly in the United States. In Europe, nanoremediation is being investigated by the EC funded NanoRem Project. A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale. During nanoremediation, a nanoparticle agent must be brought into contact with the target contaminant under conditions that allow a detoxifying or immobilizing reaction. This process typically involves a pump-and-treat process or in situ application.

<i>In situ</i> bioremediation

Bioremediation is the process of decontaminating polluted sites through the usage of either endogenous or external microorganism. In situ is a term utilized within a variety of fields meaning "on site" and refers to the location of an event. Within the context of bioremediation, in situ indicates that the location of the bioremediation has occurred at the site of contamination without the translocation of the polluted materials. Bioremediation is used to neutralize pollutants including Hydrocarbons, chlorinated compounds, nitrates, toxic metals and other pollutants through a variety of chemical mechanisms. Microorganism used in the process of bioremediation can either be implanted or cultivated within the site through the application of fertilizers and other nutrients. Common polluted sites targeted by bioremediation are groundwater/aquifers and polluted soils. Aquatic ecosystems affected by oil spills have also shown improvement through the application of bioremediation. The most notable cases being the Deepwater Horizon oil spill in 2010 and the Exxon Valdez oil spill in 1989. Two variations of bioremediation exist defined by the location where the process occurs. Ex situ bioremediation occurs at a location separate from the contaminated site and involves the translocation of the contaminated material. In situ occurs within the site of contamination In situ bioremediation can further be categorized by the metabolism occurring, aerobic and anaerobic, and by the level of human involvement.

References

  1. ↑ "In Situ Chemical Reduction | Risk Management Ground Water Restoration and Protection | US EPA". www.epa.gov. Archived from the original on 2010-08-17.
  2. 1 2 3 Brown, R.A.; Lewis, R.L; Fiacco, J.; Leahy, M.C (May 2006). "The technical basis for in situ chemical reduction". Remediation of Chlorinated and Recalcitrant Compounds. Columbus, OH: Battelle Press. ISBN   1-57477-157-4.
  3. ↑ Cheng, S.F; Wu, S.C (2000). "The enhancement methods for the degradation of TCE byzero-valent metals". Chemosphere. 41 (8): 1263–1270. Bibcode:2000Chmsp..41.1263C. doi:10.1016/S0045-6535(99)00530-5. ISSN   0045-6535. PMID   10901257.[ dead link ]
  4. ↑ Tratnyek, P.G.; Scherer, M.M.; Johnson, T.L.; Matheson, L.J. (8 August 2003). Chemical Degradation Methods for Wastes and Pollutants: Environmental and Industrial Applications. Basel, New York: MARCEL DEKKER, INC. p. 374. ISBN   978-0-8247-4307-9.
  5. ↑ "CLU-IN | Technologies > Remediation > About Remediation Technologies > Nanotechnology: Applications for Environmental Remediation > Application". Archived from the original on 2011-07-27. Retrieved 2011-06-06.
  6. 1 2 R.A., Brown. State of the Practice in In Situ Biogeochemical Reduction Transformations.
  7. ↑ Svendsen, B. G., D. Brown, and E. Dmitrovic. "Chemical Reduction of TCE with EHC and Calcium Polysulfide." http://www.adventusgroup.com/pdfs/presentations/Chemical%20Reduction%20of%20TCE%20with%20EHC%20and%20Calcium%20Polysulfide.pdfERM. Keynote.
  8. ↑ Gavaskar, A.; Tatar, L.; Condit, W. "Cost and Performance Report: Nanoscale Zero-Valent Iron Technologies for Source Remediation" (PDF). NACFAC (Naval Facilities Engineering Command): 3.{{cite journal}}: Cite journal requires |journal= (help)
  9. ↑ "Adventus : Accelerated Bioremediation". www.adventusgroup.com. Archived from the original on 2006-06-17.
  10. ↑ Parrish, Lew. "Emulsified Zero-Valent Iron (EZZVI." Technology. NASA, n.d. Web. 18 Mar 2011. <http://technology.ksc.nasa.gov/technology/TOP12246-EZVI.htm Archived 2011-07-05 at the Wayback Machine >.
  11. ↑ "Chemical oxidation and reduction for chlorinated solvent remediation." In In Situ Remediation of Schlorinated Solvent Plumes; Stroo, H.F.; Ward C.H. (Eds); New York, NY: Springer. pp. 293-294. ISBN   978-1-4419-1400-2.
  12. ↑ "Chlorinated Solvent Source Zone Initiative". SERDP and ESTCP Summary Report. November 2010.
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