In situ chemical reduction involves the placement of a reductant or reductant generating material in the subsurface for the purpose of degrading toxic organic compounds to potentially nontoxic or less toxic compounds, immobilizing metals such as Cr (VI) by adsorption or precipitation, and degrading non-metallic oxyanions such as nitrate. The most commonly used reductant is zero valent iron (ZVI), which is used to remediate halogenated ethenes and ethanes, energetics, and some metals/metalloids (Chromium (VI), Arsenic, and Uranium) [ITRC 2011]. Other reductants that are used to address metals include ferrous iron, sodium dithionite, sulfide salts (calcium polysulfide), and hydrogen sulfide (Dresel et al. 2011). The introduction of substrates to microbially produce reducing conditions favorable to microbial reduction of iron and sulfates also has been used to treat dissolved metal contamination (Waybrant et al. 2002).

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Inorganics

Dissolved trace metals generally can be divided into two groups: those that are present as anions and oxyanions (arsenic, chromium, molybdenum, selenium, uranium) and divalent cations (cadmium, copper, lead, mercury, nickel, zinc). The solubility of metals depends on pH, reduction potential, aqueous concentrations of reacting species, availability of sorption sites, and reaction kinetics. The solubility, and thus mobility, of many inorganic compounds, such as chromium, copper, zinc, and nickel, is reduced in a range of neutral to slightly basic pH, while the solubility and mobility can increase in either very acidic or very basic pH solutions. The solubility of divalent metals generally increases under reducing conditions, unless sufficient sulfides are present to remove the metals as metal sulfide minerals (ITRC 2011).

In situ reduction of metals and metalloids is accomplished by a stabilization mechanism where the target subsurface area geochemistry is manipulated to bring about the direct precipitation, co-precipitation, or indirect adsorption and precipitation (e.g., adsorption to iron hydroxides) of the target chemicals (ITRC 2011 and Horst et al. 2010). Table 1 presents a list of elements and their potential precipitates under various geochemical conditions.

While dissolved metal contamination can be addressed by these stabilization mechanisms, it is important to determine if the stabilized metal species will continue in a stabilized state when subsurface conditions return to a natural condition. Horst et al. (2010) discuss the long term stability of metals precipitated in situ and suggest in some cases that "long term stability may be achieved by incorporating the targeted compound in a matrix of other precipitates formed through the treatment process." Deflaun et al. (2009) explored the long term stability of arsenic precipitation using anaerobic biostimulation (sulfate reduction) and concluded that the arsenic should remain sequestered when the aquifer returned to naturally occurring aerobic conditions. Ford et al. (2007) provide a discussion of the conditions necessary for the natural attenuation of selected metals, metalloids, and oxyanions, which can be used as a screening tool for active stabilization techniques.

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Inorganic Contaminants

Anaerobic Biostimulation

Remediation of metals using anaerobic biostimulation does not generally involve microbial action with the metals directly, but rather some of the products of their respiration can be used for metal stabilization through direct or co-precipitation. Under strong anaerobic reducing conditions, which can be created using an electron donor and carbon source (e.g., methanol, lactate, molasses) sulfate reduction conditions can be created in an aquifer (or vadose zone). The microbial reduction of sulfate produces H₂S, releases HCO₃-, and results in an increase in alkalinity and pH (Waybrant et al. 2002). Iron reducing conditions (ferric to ferrous) can also be induced by biostimulation. Ferrous iron is particularly useful in treating hexavalent chromium, while sulfide causes the precipitation of generally insoluble divalent metal sulfides.

Following a long period of pump and treat (1994-2008) of a hexavalent chromium (Cr(VI)) groundwater plume, the Track Four and Merck & Co. site began using alcohol biostimulation to stabilize the chromium in place. Consumption of the alcohol by microbes creates an anaerobic reducing environment that converts Cr(VI) to Cr(III), which has a low solubility. The system began operations in April 2008 and cleanup was expected to be completed in 2011 (California Central Valley Regional Water Quality Control Board 2011).

Calcium Polysulfide

Calcium polysulfide (CaS_x) is a strong reductant with a pH around 11. It is routinely used to precipitate metals in wastewater treatment systems. When injected into the ground it causes precipitation of cations as sulfides (FeS, ZnS, PbS, CdS, and CuS) and reduces oxidized metals such as hexavalent chromium, which typically precipitate as a hydroxide (Petersen and Hedquist 2006 and Zawislanski et al. 2010).

Arsenic is an exception with CaS_x use in that unlike other metals and metalloids that occur predominantly as oxyanions in groundwater, such as selenium and uranium, arsenic solubility is lowest in its most oxidized state [As(V)], and increases in its slightly reduced state [As(III)] (ITRC 2011). The predominant forms of arsenic under normal conditions are the oxyanions of trivalent arsenite and pentavalent arsenate with solubility increasing with increased pH (Zawislanki 2010). CaS_x with its strong reducing action and high pH actually makes arsenic more mobile.

Calcium polysulfide was employed at the Coast Wood Preserving Superfund site to remediate hexavalent chromium in the groundwater. Arsenic was also present at the site. The CaS_x was successful in lowering dissolved chromium concentrations but the reduced conditions caused by its application resulted in mobilization of arsenic and manganese. This mobilization is expected to be temporary as the dissolved arsenic and manganese leave the reduced zone (USEPA 2011a).

Zero Valent Iron

Zero valent iron (ZVI) can be used to treat both metal oxyanions and divalent metals but the mechanisms of action are somewhat different between species. For hexavalent chromium, the mechanism appears to be reduction to trivalent chromium followed by precipitation of sparingly soluble chromium hydroxides and oxides (Cr(OH)₃ Cr₂O₃) (ITRC 2011). Trivalent chromium can also be removed from solution by co-precipitation with iron phases such as FeOOH or Fe2O3. Yang et al. (2007) as cited in ITRC (2011).

Adsorption of both arsenic (III) and arsenic (V) appears to be the key mechanism when ZVI is used (Manning et al. 2002). Arsenic forms inner sphere complexes with ZVI corrosion products, including ferrous hydroxide, mixed valence iron oxides and hydroxides and ferric oxyhydroxides (Manning et al. 2002). Arsenic (III) and arsenic (V) that are adsorbed onto ZVI surfaces are consequently occluded by successive layers of corrosion products (Kanel et al. 2006) (in ITRC 2011).

The presence of ZVI particles in the subsurface provides an environment for the removal of divalent metallic cations through reductive precipitation, surface adsorption or complexation, or co-precipitation with iron oxyhydroxides (Cundy et al. 2008). The corrosion of the ZVI by water and dissolved oxygen also produce a reduced environment and increased pH (alkalinity). In packed column experiments, Kishimoto et al. (2011) hypothesized that the removal of Zn by ZVI occurred with the oxidation of the ZVI to produce ferrous iron ions (Fe⁺²). The Fe⁺² is further oxidized to Fe⁺³ and precipitated as iron hydroxide onto the surface of the ZVI and the Zn is adsorbed by or co-precipitated with the iron hydroxide. The iron hydroxide is finally transformed into iron oxide.

Fiore and Zanetti (2009) researched the ability of ZVI to treat acid mine drainage water that contained among other ions Al, Ba, Cu, Cr, Fe, Mn, Pb, and Zn. All of these ions were removed in the ZVI column to levels below the appropriate Italian regulatory levels.

With the exception of use in permeable reactive barriers, ZVI, as a stand alone injection technique, has not been widely used for the remediation of metal contamination.

Combined Reductants

Combining reductant techniques can often improve the efficiency of the cleanup. Two more common combinations are using iron with sodium dithionite and iron with biostimulation.

Ferrous Iron and Sodium Dithionite

Sodium dithionite is a strong reductant and is used primarily in the textile and paper industries. It also is used as a metal reductant in wastewater treatment. Ferrous iron is also an effective reductant. When a ferrous iron solution is injected into the subsurface alone it has a tendency to rapidly precipitate out of solution as a ferric iron product (USEPA 2011b). This precipitation can be prevented by acidifying the solution (USEPA 2000) or it can be accomplished by using buffered sodium dithionite as a co-injectant to the ferrous iron solution.

The method of using ferrous iron with sodium dithionite is well suited for groundwater contaminated with Cr(VI) where the source of the Cr(VI) has already been addressed and the pH is >6.0. Once injected, the ferrous iron effectively interacts with solid-phase and dissolved-phase Cr(VI) in the aquifer formation, converting it to Cr(III). Excess ferrous iron attaches itself to aquifer particles (through sorption or precipitation reactions) allowing for long-term treatment of contaminated water subsequently entering the treatment zone from up-gradient locations. Reduced sulfur species resulting from the decomposition of dithionite may also attach themselves to aquifer solids and thereby assist in providing long-term treatment in the injected formation (USEPA 2011b). This method is currently being employed to remediate Cr(VI) at the Macalloy Corporation Superfund Site in Charleston, South Carolina (USEPA 2010).

Many aquifers contain iron rich soil. In oxidizing aquifers, the minerals are often coated with ferric iron containing products of weathering such as oxides and oxyhydroxides. It is this surface ferric iron, along with the ferric iron in certain clay minerals, that is chemically reactive and available for use in forming a reductive treatment zone (Fruchter et al. 2000). The addition of a buffered sodium dithionite to an iron rich aquifer results in the conversion of the ferric iron to a reduced ferrous iron environment. Chromate reacts with the ferrous iron and is precipitated as a solid hydroxide. Similar precipitation reactions will occur for other oxidized redox sensitive metal species (Fruchter et al. 2000). The amount of iron in the soil can be a limiting factor (DOE 2004).

Biostimulation and Iron

As discussed above, both biostimulation and iron (Fe⁰ and Fe⁺²) can be effective treatment technologies for divalent metals and metal oxyanions. Depending upon the target metals and the subsurface conditions, combining these technologies can produce better results—especially with multiple contaminants of concern. Both ZVI and biostimulation contribute to a reducing environment that is conducive to metals precipitation and provide a wide range of precipitation, co-precipitation, or indirect adsorption and precipitation opportunities. EHC-M™ and ABC^® Plus ZVI are commercial examples of combining ZVI with biostimulants. The use of ZVI plus emulsified oil has also found use in appropriate subsurfaces.

Co-injection of ferrous sulfate with a biostimulant can also be employed to ensure a proper level of sulfate and reduced iron. This technique was approved for use of deep soil vadose zone cleanup at the Track Four and Merck & Co. hexavalent chromium site in Merced County, CA (Houston et al. 2010 and California Central Valley Regional Water Quality Control Board 2011). The injection solution was made up of potable water mixed with corn syrup, methyl alcohol and ferrous sulfate.

Other

Another commercial product, MRC™ uses an organosulfur compound as the active metals immobilization agent. This compound is part of a viscous polylactate matrix. Upon injection into an aquifer, the organosulfur compound is slowly released along with lactic acid. The lactic acid provides a carbon source for naturally-occurring bacteria to create a reduced environment that enhances the precipitation of metal sulfides (Gilmore et al. 2010).

Performance Monitoring

Table 2 lists available approaches for monitoring changes in subsurface conditions as a result of applying ISCR. The listed parameters do not address process issues such as pumping rates and pressures. Note if organic substrate is being used to address organic contamination through reductive bioremediation in addition to inorganic contaminants refer to the performance monitoring table in the DNAPL anaerobic bioremediation section for additional parameters.

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Additional Information

A Pilot Study for the Reduction of Dissolved Copper in Shallow Groundwater
Gilmore, C., J. Hess, M. Sorensen, and F. Fadullon
Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2010).

This paper reports on a pilot scale study comparing calcium polysulfide and MRC® to precipitate copper from groundwater. While both were successful, the study found that MRC® also aided in the reduction of the groundwater Eh to attain geochemical conditions within the stability field for copper sulfide (Cu₂S) precipitation and hence obtained better results.

Challenges Associated with Arsenic Remediation in Media Affected by Multiple Metals
Zawislanski, P., E. Kalve, J. Horst, J. Gillow, and D. Liles
Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2010).

This presentation uses case histories to discuss the difficulties of remediating a site that has arsenic oxyanions—best approached under oxidizing conditions and divalent metals which are generally remediated under reducing conditions. Solidification/stabilization approaches are evaluated as well as groundwater treatment technologies.

Creation of a Subsurface Permeable Treatment Zone for Aqueous Chromate Contamination Using In Situ Redox Manipulation
Fruchter, J. et al.
Ground Water Monitoring and Remediation, Vol. 20, No. 2, p 66-77, 2000

This paper discusses a demonstration project that used a buffered sodium dithionite solution to reduce naturally occurring iron in the subsurface. The reduced iron reacts with hexavalent chromium converting it to trivalent chromium which drops out of solution.

Decreasing Toxic Metal Bioavailability with Novel Soil Amendment Strategies
Jardine, Philip M., et al.
SERDP Project ER 1350, 22 pp, 2007

The report discusses a number of ways, including in situ chemical reduction, to reduce mobility and bioavailability of metals in soils. The approaches are tested in bench scale experiments.

Evaluating Long-Term Stability of Metals Precipitated In Situ
Horst, John, Suthan Suthersan, Jeff Gillow, and Rick Wilkin
Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2010).

This presentation reviews the key concepts underpinning the long-term stability of metals immobilized through a variety of in situ remedial techniques (e.g., precipitation by hydroxides and sulfides, and matrix effects), and consider methods of evaluating that stability.

Evaluation of Amendments for Mending the ISRM Barrier Technical Assistance Project #33
DOE
DOE, Office of Environmental Management, 68 pp, 2004

The ISRM technology creates a treatment zone within the aquifer by injection of sodium dithionite, a strong reducing agent that scavenges dissolved oxygen (DO) from the aquifer and reduces ferric iron [Fe(III)], related metals, and oxy-ions. The reduced iron/metal environment can convert Cr⁺⁶ to less soluble trivalent chromium. Several years after initial and secondary treatment, the groundwater in approximately 17 wells was found to contain elevated Cr⁺⁶ concentrations. Potential causes of Cr⁺⁶ breakthrough are related to physical and chemical heterogeneity within the aquifer (including loss of reductive capacity within preferential flow paths) and the presence of other oxidants (DO and nitrate) that significantly affect the reductive capacity of the treated aquifer. This report presents analyses and recommendations of potential amendments and delivery options to improve the performance of the ISRM barrier.

Final Report: Anaerobic Biostimulation for the In Situ Precipitation and Long-Term Sequestration of Metal Sulfides
DeFlaun, Mary et al.
SERDP Project ER-1373, 176 pp, 2009

This report discusses using anaerobic biostimulation to produce sulfide that in turn reacts with metal/metalloid compounds such as arsenate to reduce them to a metal sulfide. The study showed that the resulting sulfide compound would remain immobilized even as the aquifer returned to aerobic conditions.

In Situ Treatment of Soil and Groundwater Contaminated with Chromium. Technical Resource Guide
Cook, K.R., EPA 625-R-00-005, 2000.

This report provides detailed discussions of in situ technologies that can used to treat chromium. Each discussion consists of a technology description with its advantages and disadvantages, status (as of 2000), and performance and cost data. In situ technologies included are geochemical fixation, permeable reactive barriers (PRBs), reactive zones. enhanced extraction, electrokinetics, and biological processes.

In-Situ Chromium Treatability Study Results Report, Nevada Environmental Response Trust Site, Henderson, Nevada: Revision 1
Nevada Environmental Response Trust, Chicago, IL. 1139 pp, 2018

For the biological reduction treatability study (Nov. 2016-Oct. 2017) in the Central Retention Basin, three separate injection events of carbon substrates—EOS_PRO^®, industrial sugar wastewater, granular sugar and/or molasses—were conducted to promote in situ biological reduction of Cr(VI). For the chemical reduction study conducted August 7-8, 2017, the injection and monitoring wells installed as part of the Ammonium Perchlorate Area Up and Down Flushing Treatability Study were used for a single chemical injection event of a total of 600 gal of a calcium polysulfide solution.

Mechanistic Consideration of Zinc Ion Removal by Zero-Valent Iron
Kishimoto, Naoyuki, Shiori Iwano, and Youhei Narazaki
Water, Air, & Soil Pollution Volume 221, Issue 1-4, pp 183-189, October 2011

This packed column experiment explored zinc removal responses with ZVI powder. The removal was enhanced by dissolved oxygen and ferric ion addition. The mechanism of action was inferred to be ZVI was firstly corroded and oxidized into ferric ion by dissolved oxygen. The ferric ion was precipitated as iron hydroxide onto the surface of the zero-valent iron powder. Zinc ion was adsorbed on and/or coprecipitated with the iron hydroxide. The iron hydroxide was finally oxidized and transformed into iron oxides.

Mechanistic Evidence and Efficiency of the Cr(VI) Reduction in Water by Different Sources of Zero-Valent Irons (Abstract)
Yang, J. E., J. S. Kim, Y. S. Ok, and K. R. Yoo
Water Science and Technology 55(1–2): 197–202, 2007

This bench scale experiment assessed the mechanism of action between ZVI and Cr(VI) in water. The Cr and Fe in precipitates were exclusively in the Cr(III) or Fe(III) states with the respective forms of Cr(OH)₃ or Cr₂O₃ and FeOOH or Fe₂O₃. Electrons produced from ZVI oxidation reduced Cr(VI) to Cr(III), with Cr(III) precipitated or co-precipitated with Fe(III) to form Fe(III)-Cr(III) hydroxide or Fe(III)-Cr(III) oxyhydroxide.

Monitored Natural Attenuation of Inorganic Contaminants in Ground Water: Volume 2 Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium
Ford, Robert G., Richard T. Wilkin, and Robert W. Puls eds.
USEPA, EPA/600/R-07/140, 124 pp, 2007

This document provides information on geochemical conditions necessary for attenuation of various metals and for evaluating whether metal or metalloids that are removed from solution by chemical addition will remain removed when the subsurface geochemistry returns to normal conditions.

Nanoscale Zero Valent Iron and Bimetallic Particles for Contaminated Site Remediation
O'Carroll, D., B. Sleep, M. Krol, H. Boparai, and C. Kocur.
Advances in Water Resources 51:104-122(2013)

This review provides background on the NZVI/bimetallic technology, summarizes NZVI reactions with chlorinated solvents and metals, examines the factors affecting NZVI reactivity, discusses studies on subsurface transport of bare and coated NZVI particles, and identifies field implementation challenges.

Order No. R5-2011-0006 Track Four Inc. and Merck & Co. Merced County
California Central Valley Regional Water Quality Control Board. 2011

This order reports on the use of alcohol biostimulation to stabilize hexavalent chromium in place. Consumption of the alcohol by microbes creates an anaerobic reducing environment that converts Cr(VI) to Cr(III), which has a low solubility. The treatment has been successful and is expected to end in 2011.

Preliminary Tests Concerning Zero-Valent Iron Efficiency in Inorganic Pollutants Remediation
Fiore, Silvia and Maria Zanetti
American Journal of Environmental Sciences 5 (4), p 556-561, 2009

The degradation efficiency of Brown size 8/50 ZVI (Peerless Powders and Abrasive Inc., Detroit) was evaluated in a leaching column test with analysis of the oxidation/reduction potential, metals, nitrates, chlorides, and sulfates profiles along the column. The study allowed simulation of the solid/liquid contact characteristic of PRB behavior and indicated a possible solution for the remediation of inorganic pollutants in groundwater. The results showed ZVI efficiency above 99% for metals removal, although further tests involving biotic processes and more reducing conditions are necessary to improve the degradation of sulfates and nitrates.

Review: Technical and Policy Challenges in Deep Vadose Zone Remediation of Metals and Radionuclides (Abstract)
Dresel, P.E., D. Wellman, K. Cantrell, and M. Truex
Environmental Science & Technology, 45, 4207-4216, 2011

This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments; hence, efforts to remove contaminants generally have been unsuccessful, although partial removal can reduce downward flux. Abiotic and biotic reactions or physical encapsulation have the potential to reduce contaminant mobility, and hydraulic controls can limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.

The Effectiveness of Ferrous Iron and Sodium Dithionite for Decreasing Resin-Extractable Cr(VI) in Cr(VI)-Spiked Alkaline Soils
Cheng, Chia-Jung, Tzu-Huei Lin, Chiou-Pin Chen, Kai-Wei Juang, and Dar-Yuan Lee
Journal of Hazardous Materials 164, p 510–516, 2009

This paper discusses soil experiments to determine the effectiveness of ferrous iron, sodium dithionite, and a mixture of ferrous iron and sodium dithionite in reducing the availability of hexavalent chromium. The ferrous iron and sodium dithionite mixture proved to be the most effective.

Treatability Test Report for Calcium Polysulfide in the 100-K Area
Petersen, S. and K. Hedquist
DOE, DOE/RL-2006-17, 138 p, 2006

This report describes a demonstration of using calcium polysulfide to reduce hexavalent chromium in an aquifer with the precipitate being a chromium hydroxide. A carbon substrate also was used to increase the reducing conditions of the aquifer. The hexavalent chromium reduction was successful and the report discusses lessons learned.

Treatment of Chromium Contamination and Chromium Ore Processing Residue
CL:aire Technical Bulletin TB14, 4 pp, 2007

This bulletin provides information on the treatment of Cr contamination in the environment and summarizes a number of remedial methods that have been used to treat it. In addition, it looks at the wider issue of chromium ore processing residue (COPR) contamination in the south-east of Glasgow, potential methods for its remediation and the next steps for the redevelopment of this area.

Use of Iron-Based Technologies in Contaminated Land and Groundwater Remediation: A Review
Cundy, A., L. Hopkinson, and R. Whitby. 2008.
Science of the Total Environment, 400, 42-51 (2008).

This paper reviews some of the latest iron-based technologies for remedial activities, their current state of development, and their potential applications and limitations.

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Organic Contaminants

Zero-valent iron (ZVI) has been shown to be effective in treating chlorinated alkenes and alkanes and energetic compounds, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 2,4,6-trinitrotoluene (TNT) [ITRC 2011]. Successful treatment of chloroform with ZVI was demonstrated at the Hunter's Point Superfund site during 2008-09 (Forman 2010).

The in situ reduction of organic compounds dissolved in groundwater utilizing 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. As the groundwater flows through this PRB, the contaminants contact the reactive media and are degraded to potentially nontoxic or less toxic compounds. 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 can require trenching and intensive construction activities, which can result in significant disturbances to the ecosystem (ITRC 2005). ZVI based PRB walls have been placed using injection techniques (ITRC 2011).

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 also have been successful in introducing reactants to contaminants in zones of low permeability (Forman 2010 and Bures et al. 2010) . 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 hydrophobic 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). ZVI also can be mixed directly into source zones where the soil has low permeability (Faircloth 2010).

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 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 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 (ITRC 2011, ESTCP 2010, and Krug et al. 2010).

Emulsified oil is not the only substrate that can be mixed or injected with ZVI to achieve both a biological and chemical reactive zone. Several commercial products (e.g., EHC^® and ABC^® Plus ZVI) provide a carbon substrate along with ZVI. While the formulas differ, these products are designed to take advantage of the synergistic abiotic and biotic reduction reactions created by the combination. In addition to combined products, ZVI can be mixed or injected separately with common substrates (lactate, molasses, alcohol) appropriate to the contaminant and hydrogeological setting (ITRC 2011).

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.

Performance Monitoring

Performance monitoring information for in situ reduction of organics can be found in the DNAPL in situ chemical reduction section.

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Additional Information

Information on the effectiveness of zero valent iron on specific halogenated aliphatic and aromatic compounds can be found at the following:

• Halogenated Alkanes
• Halogenated Alkenes
• Halogenated Monoaromatics

Emulsified Zero-Valent Iron (EZVI) Treatment of Chlorinated Solvents (PPT)
Geosyntec Consultants
NAVFAC Remediation Innovation Technology Seminar Spring 2009, 80 slides

This presentation provides an overview of the theory behind ZVI and biostimulation and how EZVI was employed at three sites (Launch Complex 34, Parris Island, and Patrick Air Force Base).

In Situ Chemical Reduction (ISCR) Technologies: Significance of Low Eh Reactions (Abstract)
Dolfing, Jan, Miriam Van Eekert, Alan Seech, John Vogan, and Jim Mueller
Soil & Sediment Contamination, 17:63–74, 2008

Zero-valent iron (ZVI) has been employed to transform and detoxify a wide range of environmental contaminants, including chlorinated organics, heavy metals, nitroaromatics, and to some degree, perchlorate. The combined use of ZVI plus controlled release carbon has been shown to generate an in situ chemical reduction (ISCR) phase in the subsurface that facilitates the destruction or removal of contaminants by microbiological, chemical, and/or physical means. An ISCR zone exhibits low redox potential and has a propensity to produce hydrogen. The authors discuss the thermodynamics of these characteristics with special emphasis on ZVI reactions. The paper includes a case study of the cleanup of groundwater impacted with carbon tetrachloride (CT) at concentrations as high as 4,000 µg/L. In March 2005, a total of 22,000 kg of ISCR reagent was injected into an 82-m permeable reactive barrier (PRB). The PRB was located about 150 m downgradient of the suspected source area and extended across the width of the CT plume. The ISCR reagent was applied at a rate of approximately 1% to soil mass into saturated sand units. The ISCR reagent was supplied as a dry powder in 23-kg bags and mixed with water on site into slurry containing about 40% solids. The slurry was injected at 126 injection points advanced via direct push. By August 2006, CT concentrations had decreased from > 1,600 ppb to < 5 ppb, achieving > 99% removal.

Kinetics of RDX Degradation by Zero-Valent Iron (ZVI)
Wanaratna, P., C. Christodoulatos, and M. Sidhoum

This paper describes a bench scale experiment to evaluate the use of zero-valent iron (ZVI) for the remediation of water contaminated with RDX. RDX was found to degrade rapidly in the presence of ZVI. At low ZVI concentrations, reduction of RDX follows pseudo first order kinetics; while at high ZVI concentrations the RDX reduction is zero-order. The main by-products of ZVI reduction of RDX are nitroso compounds (MNX, DNX, and TNX), nitrate, nitrite and nitrous oxide. The nitroso compounds were found to undergo further reduction by ZVI.

Nanoscale Zero Valent Iron and Bimetallic Particles for Contaminated Site Remediation
O'Carroll, D., B. Sleep, M. Krol, H. Boparai, and C. Kocur.
Advances in Water Resources 51:104-122(2013)

This review provides background on the NZVI/bimetallic technology, summarizes NZVI reactions with chlorinated solvents and metals, examines the factors affecting NZVI reactivity, discusses studies on subsurface transport of bare and coated NZVI particles, and identifies field implementation challenges.

Pilot Test Final Report: Bimetallic Nanoscale Particle Treatment of Groundwater at Area I, Volume I of III: Naval Air Engineering Station Site, Lakehurst, New Jersey
U.S. Navy, 2003

This report details the conduction and results of a pilot test of bimetallic (ZVI and palladium) nanoscale particles to reduce aquifer contamination due to PCE, TCE, and 1,1,1-TCA. The injection of the nanoscale particles resulted in a reduction in ORP, a rise in pH, and lowering PCE concentration by up to 90 percent in some monitoring wells.

Remediation of Explosives Contaminated Groundwater with Zero-Valent Iron
Tratnyek, Paul and Richard Johnson
SERDP Project ER-1232, 41 pp, 2011

A series of lab and field studies was performed to evaluate the in situ degradation of TNT and RDX using permeable reactive barriers (PRBs) made with zero-valent iron (ZVI). The disappearance of both TNT and RDX were shown to be rapid in the laboratory and ex situ field columns. Batch experiments performed with C-14-labelled TNT showed that the products of reaction with iron metal are sequestered partly on the metal (oxide) particle surfaces. Most of the bound residue could not be solubilized using a range of extraction procedures.

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