Dense Nonaqueous Phase Liquids (DNAPLs)
Treatment Technologies
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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. More resources on this technology for a wide range of contaminants can be found in the In Situ Chemical Reduction pages of Technology Focus.
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."
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.
Fracture-Emplacement and 3-D Mapping of a Microiron/Carbon Amendment in TCE-Impacted Sedimentary Bedrock
Bures, G.H., J.A. Skog, D. Swift, J. Rothermel, R. Starr, and J. Moreno.
Proceedings of the Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2010). Battelle Press, ISBN: 978-0-9819730-2-9, Paper & presentation D-067, 9 pp + 23 slides, 2010
An in situ pilot remediation project was carried out on behalf of the U.S. Army Corps of Engineers (Omaha District) at the F.E. Warren AFB in Colorado. The pilot featured an innovative application of drilling, fracture emplacement, treatment, and geophysical technologies to mitigate impacts from chlorinated solvents. The former missile site complex is underlain by silty sandstone bedrock sediments affected by TCE >2,000 µg/L and associated VOCs. Pilot tests of biotic and abiotic in situ chemical reduction (ISCR) were conducted in the source area and dissolved plume to evaluate technology performance prior to developing the proposed remedy. The pilot involved the emplacement of over 100 tons of EHC™, a micro-iron/complex-carbon treatment amendment, into deep bedrock sediments to attain optimal distribution throughout the contaminant plume, including beneath the former Launch and Service Building. The radius of fracture emplacement in the bedrock was up to 60 ft, with a typical fracture overlap of 30 to 50%. Following placement of the amendment, physical, chemical, and microbiological processes combined to create very strong reducing conditions that stimulated chemical and microbiological dehalogenation of the contaminants. View longer abstract. Additional information: Presentation Slides
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Field Profile
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.
Treatment of Chlorinated Hydrocarbon Contaminated Groundwater with Injectable Nanoscale Bimetallic Particles: Lessons Learned
D.S. Liles.
ESTCP Project ER-0017, 8 pp, 2009
The reductive dechlorination of TCE by multiple types of nanoscale zero-valent iron (NZVI) was evaluated using particles obtained from multiple manufacturers. The manufacturing methods used to produce the NZVI particles tested result in particles that fall into two structural categories defined here as particles with amorphous atomic structures and those with crystalline atomic structures. These structural differences can lead to very different properties during the use of the NZVI for reductive dechlorination of TCE. The investigators learned that initial rates of reaction of different batches of iron provided by the same manufacture under the same brand name can differ dramatically, which likely reflects the continuing evolution of the manufacturing technology; each individual batch of iron should undergo kinetics testing as a quality control step before application in the field. Contrary to expectation, in almost all cases the TCE removal performance of NZVI particles was better in the non-palladized form compared to palladized particles. This brief report documents other lessons learned in the study and from the literature concerning NZVI reactivity, longevity, injectability, and potential for treatment of DNAPL.
Workshop on In Situ Biogeochemical Transformation of Chlorinated Solvents
Environmental Security Technology Certification Program (ESTCP), 66 pp, 2008
This workshop provided an active discussion of ongoing research focused on the role of in situ biogeochemical transformation of chlorinated solvents, defined here as processes where contaminants are degraded by abiotic reactions with naturally occurring and biogenically formed minerals in the subsurface.
Zero Valent Iron Injection Tool
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools Website, 61 pp, 2011
This Web tool focuses on ZVI injection for treating chlorinated solvents and is designed to assist RPMs in the development and implementation of effective ZVI injection applications. Users can learn about 1) the scientific concepts related to the use of ZVI and the types of ZVI media available for injection, 2) different injection methods used to deliver ZVI to the subsurface, 3) which factors influence the applicability of the ZVI technologies, and 4) lessons learned during ZVI implementation.
Performance Monitoring for In Situ Reduction
In situ chemical reduction involves the introduction of a reducing agent into the subsurface to destroy target compounds. The most widely used reducing agent is zero-valent iron (ZVI), which is iron metal, often in fine granular or powder form. Other zero-valent metals, such as zinc (Dolfing et al. 2008), have been shown to work in laboratory settings. When introduced in sufficient quantities, ZVI is effective in treating chlorinated ethenes and some halogenated ethanes, ethenes, and methanes, among other chemicals (ITRC 2005).
The usual methods for introducing ZVI to the subsurface are direct mixing and pressure injection (ITRC 2005). Direct mixing may involve using a large-diameter auger. Pressure injection forces a slurry of ZVI and water or ZVI and emulsified oil into the formation target zone where the iron can create geochemical conditions favorable to the destruction of contaminants of concern (COCs).
Performance monitoring for in situ chemical reduction has several goals. The primary goal, which is to ascertain the areal extent of the iron introduced to the subsurface and whether the targeted area has been fully covered, is accomplished by determining the presence or absence of indicator parameters, such as dissolved oxygen and nitrate. Both of these parameters are consumed by ZVI, and their presence indicates incomplete ZVI coverage or insufficient ZVI mass (Gavaskar et al. 2005).
The extent to which the oxidation-reduction potential (ORP) shifts to the negative indicates whether sufficient active ZVI is in place to cause the desired abiotic destruction of the target compounds. Gavaskar et al. (2005) theorize that an ORP of -400 mv or less is needed for the abiotic reaction (β-elimination). At sites where anaerobic conditions were achieved with an ORP greater than -250 mv, the destruction of target compounds occurred through biodegradation as evidenced by the presence of daughter products (e.g., dichloroethene and vinyl chloride in the case of trichloroethene), which do not occur in abiotic degradation (Gavaskar et al. 2005).
To measure treatment success, baseline contaminant concentrations are compared with those present at the completion of the injection and periodically afterward. A rise in contaminant concentrations over time indicates rebound and incomplete treatment. Indicator parameters that show trends may suggest whether or not the ZVI is still active (e.g., a rise in ORP values might indicate a falling-off of activity (Gavaskar et al. 2005). To measure performance success directly, statistically derived soil core samples are taken from the source area and tested for the presence of target compounds.
The performance parameters in the table below are drawn from several studies and documents to convey the approaches and methods that investigators have used to evaluate the performance of in situ reduction activities. Determination of which parameters to use for monitoring and monitoring frequency requires site-specific information. For example, ORP is a good field indicator of whether a ZVI water emulsion is achieving an appropriate geochemical state, and it can be measured on a near-continuous basis. ORP also can indicate whether more or a more concentrated emulsion is needed during injection�a useful feedback measure. Measurement of COCs and their degradation products generally is used less frequently than the other methods (e.g., consider the sites described in Gavaskar et al. 2005).
Several other indirect methods can be used to measure performance:
- Contaminant flux measurements in the downgradient groundwater;
- Geophysical techniques, such as electrical resistivity tomography, which can indicate where the injected ZVI has gone (i.e., identify preferential pathways); and
- Tracer tests, such as the partitioning interwell tracer test, for estimating subsurface contaminant mass before and after treatment.
For other potentially useful nonspecific performance monitoring techniques, see Remediation Measurement Tools.
Performance Parameter |
Method |
Data Use |
Performance Expectation |
Recommended Frequency of Analysis |
|---|---|---|---|---|
COCs and Degradation Products |
VOCs, EPA SW-846: 8021B (GC), Field GC headspace technique; 8260B (GC/MS) SVOCs, SW-846: 8270D. Depending upon the detection limits desired, chemical class-specific testing methods might be used (e.g., SW-846: 8330 for nitroaromatics). |
In water, determine the areal extent of the injection and amount of degradation. In both saturated and unsaturated soil, (by coring) determine if remedial goals were met. |
The reaction can proceed abiotically, biotically, or both. The presence of degradation products (e.g., vinyl chloride, dichloroethene,) indicates biotic degradation is occurring and possibly that an insufficient amount of iron was applied (Gavaskar et al. 2005, Zhang 2003). In either case, the target compound concentrations should fall. |
Site specific. The Navy at several sites performed baseline measures followed by resampling at 2, 6, and 12 weeks (Gavaskar et al. 2005). |
Dissolved Gases (e.g., Hydrogen, Ethane, Ethene, Methane) |
Dissolved hydrogen can be measured using a Hach hand-held meter. SW-846: 5021A (gasoline range hydrocarbons) Robert S. Kerr Laboratory RSK-175 SOP and GC or GC/MS . |
Indicator parameters for abiotic and biotic degradation of halogenated ethenes, ethanes, and methanes. |
Hydrogen is produced during the reduction of contaminants by iron (Dolfing 2008). Methane can be produced by microbes in the active zone (ITRC 2005). Ethenes and ethanes are end-products of both abiotic and anaerobic degradation of halogenated ethenes and ethanes (ITRC 2005). |
Site specific, but generally would be done at the same time as the COCs. |
Oxidation Reduction Potential (ORP) |
Hand-held meter, flow-through cell, down-hole electrode |
Indicator parameter for presence of ZVI. |
An abundance of ZVI causes the ORP to fall dramatically. Gavaskar et al. (2005) theorize that a fall to -200mV indicates poor distribution of ZVI, while a fall to -400mV or less indicates good distribution. |
Site specific. This is a field parameter that can be used to monitor water ORP changes on a near-continuous basis if desired. |
Dissolved Oxygen (DO) |
Flow-through cell, down-hole electrode |
Indicator parameter for presence of ZVI. |
DO is highly reactive with ZVI. Its presence downgradient or within the reaction zone indicates poor distribution of the ZVI (ITRC 2005). |
Site specific. This is a field parameter that can be used to monitor water DO changes on a near-continuous basis if desired. |
pH |
Hand-held meter, flow-through cell, down-hole electrode |
Indicator parameter for presence of ZVI/COC interaction. |
The interaction of COC with ZVI tends to raise the pH of the groundwater (Gavaskar et al. 2005, Zhang 2003). |
Site specific. This is a field parameter that can be used to monitor water pH changes on a near-continuous basis if desired. |
Temperature |
Hand-held meter, flow-through cell, down-hole electrode |
Affects the rate of degradation with ZVI. |
Degradation is slower at lower temperatures (ITRC 2005). |
Site specific. This is a field parameter that can be used to monitor water temperature changes on a near-continuous basis if desired. |
Conductivity |
Hand-held meter, flow-through cell, down-hole electrode |
Indicator parameter for presence of ZVI. |
Specific conductance should fall in the vicinity of ZVI (U.S. Navy 2003). |
Site specific. This is a field parameter that can be used to monitor water conductivity changes on a near-continuous basis if desired. |
Chloride |
Flow-through cell, down-hole instrument |
Indicator parameter for presence of chlorinated COC remediation. |
COCs dechlorination should increase the concentration of chloride ions in the groundwater, but the increase may not be distinguishable from background chloride levels (Gavaskar et al. 2005). Bimetallic nanoiron may contain elevated levels of chlorides (U.S. Navy 2003). |
Site specific. This is a field parameter that can be used to monitor water chloride changes on a near-continuous basis if desired. |
Alkalinity |
EPA Method 310.2 (Alkalinity, Colorimetric, Automated Methyl Orange) Ion-selective electrode2 (ISE) for field measurement |
Indicator parameter for presence of ZVI and the buffering capacity of the aquifer. |
Unless the aquifer is highly buffered, alkalinity should decline (ITRC 2005). |
Site specific. |
Nitrate |
ISE or portable colorimeter (Hach Method 8039) for field measurements. Laboratory methods: EPA Methods 353.2 or 300.1, SW-846: 9056A, APHA et al.: 4500-NO3 D |
Indicator parameter for chemical reductive activity. |
Nitrate is reduced to ammonia/ammonium by ZVI. A sharp drop in nitrate in the target area indicates the iron is active (ITRC 2005). |
Site specific. The Hach method can be done in the field. |
Sulfate |
Portable Colorimeter Hach Method 8051 for field measurement; Laboratory methods: EPA Method 300.1 or SW-846: 9056A |
Indicator parameter for biological reductive activity. |
Sulfate is reduced to sulfide by sulfate-reducing bacteria. A sharp drop in sulfate indicates this activity is occurring (ITRC 2005). |
Site specific. The Hach method can be done in the field. |
Iron |
Laboratory: SW-846: 6010C for the measurement of total iron; Field measurement of ferrous iron, Hach Colorimetric Method 8146 |
Indicator parameter for ZVI activity. |
ZVI is reduced to ferrous ions by water, DO, and many halogenated organic compounds (Gavaskar et al. 2000 and 2005). An increase in dissolved iron indicates this reaction is occurring. |
Site specific, and barring site-specific interferences, the colorimetric meter allows for field measurements. |
Total Organic Carbon/ Dissolved Organic Carbon |
EPA SW-846: 9060A APHA et al.: 5310-B, C, or D |
Indicator of natural organic carbon present at site during baseline characterization. |
High background concentrations might affect degradation rate of VOCs with iron (ITRC 2005). |
Site specific. |
1 For injection methods only. Does not apply to physical mixing.
2 ISE manufacturers should be consulted to determine whether a site-specific condition could limit use.
References
Cost and Performance Report: Nanoscale Zero-Valent Iron Technologies for Source Remediation
A. Gavaskar, L. Tatar, and W. Condit.
Naval Facilities Engineering Service Center, Port Hueneme, CA. CR-05-007-ENV, 54 pp, 2005
Final Design Guidance for Application of Permeable Reactive Barriers for Groundwater Remediation
A. Gavaskar, N. Gupta, B. Sass, R. Janosy, and J. Hicks.
Strategic Environmental Research and Development Program (SERDP), Arlington, VA. 247 pp, 2000
In Situ Chemical Reduction (ISCR) Technologies: Significance of Low eH Reactions
Dolfing, J., M. Van Eekert, A. Seech, J. Vogan, and J. Muellers.
Soil & Sediment Contamination 17(1):63-74(2008)
Nanoscale Iron Particles for Environmental Remediation: An Overview
W.-X. Zhang.
Journal of Nanoparticle Research, Vol 5, p 323-332, 2003
Permeable Reactive Barriers: Lessons Learned/New Directions
Interstate Technology & Regulatory Council (ITRC) Permeable Reactive Barriers Team.
PRB-4, 202 pp, 2005
Standard Methods for the Examination of Water and Wastewater, 18th Edition
American Public Health Association (APHA), American Water Works Association, and Water Environment Federation, 1992 Standard Methods commercial Web site
SW-846 On-Line: Test Methods for Evaluating Solid Waste, Physical/Chemical Methods
U.S. EPA, Office of Resource Conservation and Recovery.
Technical Guidance for the Natural Attenuation Indicators: Methane, Ethane, and Ethene
U.S. EPA Region 1, 18 pp, 2002
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