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Treatment Technologies
In Situ Reduction
This page identifies general resources that contain information on the design and implementation of in situ reduction technology. Information on applications of this technology specific to a chemical class can be found in the class subsections listed to the right; however, in situ reduction as described in this section is a young technology and thus far has been applied primarily to chlorinated solvents, i.e., to compounds within the classes of halogenated alkenes and alkanes.
The in situ reduction of halogenated organic compounds dissolved in groundwater utilizing zero-valent iron (ZVI) has typically relied on the flow of groundwater through a subsurface permeable reactive barrier (PRB). In its simplest form, a PRB consists of a zone of reactive material, such as granular iron, installed in the path of a plume of dissolved-phase contaminants, such as chlorinated solvents. As the groundwater flows through this PRB, the contaminants contact the reactive media and are degraded to potentially nontoxic hydrocarbons and inorganic chloride. The main advantage of this system is that no pumping or aboveground treatment is required; the barrier acts passively after its installation. PRB technology, however, does not focus on the source of contamination (soils or sediments containing residual or free-phase toxic compounds) and depends entirely on the desorption or dissolution of the contaminants into the groundwater and the subsequent migration to the PRB for treatment. In addition, the emplacement of the PRB requires trenching and intensive construction activities, which can result in significant disturbances to the ecosystem.
Innovators have expanded upon the above PRB approach through the development of in situ remediation processes that involve the injection of specific quantities of highly reactive iron powder directly into contaminant zones. Pneumatic or hydraulic injection have been successful in introducing reactants to contaminants in zones of low permeability. Injection by direct push rigs has been used successfully to introduce treatment media rapidly to the groundwater or a soil source area. These efforts have advanced the knowledge base of the iron powder dehalogenation technology through the identification of critical parameters affecting the reaction performance. By emplacing the iron powder by means of injection, rather than in the form of a reactive wall, soluble, absorbed-phase, and free-phase halogenated hydrocarbons all can be reduced to targeted levels (U.S. EPA 2002). Note that iron injected as part of a water emulsion can treat only contaminants that are accessible by water and will not treat free-phase contaminants directly.
One approach to source treatment consists of mixing ZVI and clay into a source zone for the reductive dehalogenation of chlorinated solvents. The purpose of mixing clay into the source zones is to create a stagnant hydrologic environment to inhibit transfer of contaminants from the source zone to groundwater while the reaction with ZVI occurs inside the source zone (NRC 2004).
Emulsified Zero-Valent Iron (EZVI) can be used to enhance the destruction of chlorinated solvent DNAPL in source zones by creating intimate contact between the DNAPL and the nanoscale ZVI. The EZVI is composed of food-grade surfactant, biodegradable vegetable oil, water, and ZVI particles (either nano- or micro-scale iron). EZVI forms emulsion particles that contain the ZVI in water surrounded by an oil/liquid membrane. The exterior oil membrane has hydrophobic properties similar to that of DNAPL; therefore, the emulsion is miscible with the DNAPL. Encapsulating the ZVI in a hydrophobic membrane protects the nanoscale iron from other groundwater constituents that otherwise would exhaust much of the iron's reducing capacity. This approach reduces the mass of EZVI required for treatment relative to unprotected ZVI. EZVI will combine directly with the target contaminants until the oil membrane is consumed by biological activity. In addition to the abiotic degradation associated with the ZVI, EZVI injection will result in enhanced biodegradation of dissolved chlorinated ethenes because the vegetable oil and surfactant act as electron donors to promote anaerobic biodegradation processes (ESTCP Project CU-0431).
Bimetallic nanoscale particle (BNP) technology consists of submicron particles of ZVI with a trace coating of palladium (approximately 0.1% by weight) that acts as a catalyst. Rapid destruction of a wide range of recalcitrant contaminants by BNPs can be accomplished either in situ or ex situ and is based on a redox process whereby the ZVI serves as the electron donor. A BNP/water mixture can be injected under pressure into the area where treatment is needed. Due to the extremely small size of the particles (on the order of 10 to 100 nanometers), they can be transported by groundwater to establish in situ treatment zones, thus addressing not only dissolved contaminant plumes but also highly concentrated dissolved contaminants within source areas. Given the mobility of the particles, BNP can be used to treat contaminant areas that generally are inaccessible to conventional technologies—e.g., beneath buildings and in deep aquifers. Unlike PRB technology, BNP treatments are not limited by contaminant depth below ground surface. A substantial body of research on elemental iron transformation processes strongly suggests that BNP reactivity is surface-mediated (U.S. Navy 2003).
In situ reduction is believed to have a high potential for meeting a variety of remediation goals when it is used on appropriate sites. The chemistry of the contaminant degradation reactions that this technology depends upon is well-documented and established. This technology has shown high potential for achieving mass removal, concentration reduction, mass flux reduction, reduction of source migration potential, and a substantial reduction in toxicity; however, due to its status as one of the newer remediation innovations, cost and performance data derived from field applications of in situ reduction are still few in number.
An overview of different nanoscale iron particle technologies with interviews of the principals appeared in the April 23, 2005, issue of Science News, "Special Treatment: Tiny Technology Tackles Mega Messes."
General Resources
Get to the Source with Emulsified Zero-Valent Iron (EZVI)
NASA Success Story, 2 pp, 2006
Emulsified zero-valent iron (EZVI) technology (U.S. Patent No.6,664,298) was developed for the in situ treatment of DNAPL source zones by scientists at NASA and the University of Central Florida. This technology provides an effective method of combining two mechanisms (abiotic reductive dechlorination due to ZVI and biological reductive dechlorination) for degrading chlorinated solvents present in pools or as residual organic liquid. Based on the success of laboratory and field tests at Kennedy Space Center's Launch Complex 34, NASA licensed EZVI to five companies that are producing their own versions of the technology.
In Situ Chemical Treatment: Technology Evaluation Report
Yujun Yin and Herbert E. Allen.
Ground-Water Remediation Technologies Center (GWRTAC). TE-99-01, 82 pp, 1999
In situ chemical treatment techniques are useful for treatment of source areas to reduce the mass of contaminants and intercept plumes to remove mobile organics and metals. Chemical injection treatment mechanisms can be oxidative, reductive/precipitative, or desorptive/dissolvable, depending upon the chemical/contaminant interaction. Chemicals can be delivered to the subsurface via well injection techniques, deep soil mixing and hydraulic fracturing, or installation of permeable chemical treatment walls. The main chemical injection in situ treatments discussed are oxidation, flushing, and reduction and immobilization. Treatment wall reactions include immobilization of inorganics and organics via sorption, immobilization of inorganics via precipitation, and degradation of inorganic anions and organics. This report discusses the chemistry and the engineering aspects of available in situ chemical treatment technologies and provides information on costs, lessons learned, and regulatory issues.
In Situ Reactive Zones (IRZ) Data Sheet
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, 16 pp, 2005
IRZ treatment involves the active manipulation of subsurface conditions to promote the transformation and/or degradation of a target contaminant into a less mobile or less toxic form. This can be accomplished through geochemical and/or biological mechanisms. This brief Web tutorial provides an introduction to IRZ treatment, including a discussion of the role of redox and site conditions, the selection of IRZ amendments and delivery mechanisms, and the overall advantages and limitations of the technology.
Nanoscale Zero Valent Iron Training Tool
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, 32 pp, 2005
Zero-valent iron (ZVI) is a strong reducing agent. Nanoscale iron particles typically have surface areas up to 30 times greater than larger-sized granular iron and are up to 1,000 times more reactive for the degradation of chlorinated organic compounds. NZVI is ideally suited for treating chlorinated organic compounds and dense nonaqueous-phase liquid (DNAPL) "hot spots" through injection directly into the source area of contamination. A slurry of NZVI can be distributed into the subsurface using a variety of carrying fluids that help the iron powders disperse into the subsurface and create contact between the contaminants and the iron particles. This training tool discusses injection methods, specific aspects of implementation, NZVI economics, advantages and limitations of the technology, and lessons learned.
Remediation of Chlorinated Organic Contaminants in Fractured Aquifers Using Zero-Valent Metal: Report on Laboratory Trials
C. Merly and D.N. Lerner.
Environment Agency, Bristol, UK. P2-182/TR, ISBN: 1-85705-795-3, 113 pp, 2002
This document describes laboratory trials using fine-grained ZVI in sandstone fracture systems to assess the potential for in situ reduction of dissolved-phase TCE.
Return to "In Situ Oxidation Introduction"
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Last updated on Monday, November 10, 2008