Dense Nonaqueous Phase Liquids (DNAPLs)

For more information on the DNAPL Website, please contact:

Linda Fiedler
Technology Assessment Branch
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fiedler.linda@epa.gov

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Treatment Technologies

In Situ Oxidation

This page identifies general resources that contain detailed information on ISCO design and implementation. Information on applications of the technology specific to a chemical class can be found in the class subsections listed to the right. More resources on this technology for a wide range of contaminants can be found in the In Situ Oxidation pages of Technology Focus.

ISCO is an aggressive remediation technology that has been applied both to DNAPL source zones and to the dissolved-phase chemicals emanating from the source zones. Chemical oxidation typically involves reduction/oxidation (redox) reactions that chemically convert hazardous contaminants to nonhazardous or less toxic compounds that are more stable, less mobile, or inert. Redox reactions involve the transfer of electrons from one chemical to another. Specifically, one reactant is oxidized (loses electrons) and one is reduced (gains electrons). There are several oxidants capable of degrading contaminants. Commonly used oxidants include potassium or sodium permanganate, Fenton’s catalyzed hydrogen peroxide, hydrogen peroxide, ozone, and sodium persulfate. Each oxidant has advantages and limitations, and while applicable to soil contamination and some source zone contamination, they have been applied primarily toward remediating groundwater.

The selection of an oxidant for site cleanup involves the following key concepts:

• Is the oxidant capable of degrading the contaminant of concern? Is a catalyst or other additive required to increase effectiveness?
• What is the soil oxidant demand (SOD)? SOD is a measure of how the naturally occurring materials in soil will affect the performance of some of the oxidants. For non-selective oxidants, high SOD will increase the cost of cleanup, as more oxidant will be required.
• What is the naturally occurring pH of the soil/groundwater system? Some oxidants require an acidic environment to work. If the soil is basic, an acid needs to be applied in addition to the oxidant.
• How will the decomposition rate of the oxidant affect application strategies? Some unreacted oxidants may remain in the subsurface for weeks to months, while others naturally decompose within hours of injection.

The type of delivery system selected depends upon the depth of the contaminants, the physical state of the oxidant (gas, liquid, solid), and its decomposition rate. Backhoes, trenchers, and augers have been used to work liquid and solid oxidants into contaminated soil and sludge. Liquids can be delivered either by gravity through wells and trenches or by injection. For vadose zones, gravity has the drawback of a relatively small area of influence. Pressurized injection of liquids or gases, either through the screen of a well or the probe of a direct-push (DP) rig, will force the oxidant into the formation. The DP rig offers a cost-effective way of delivering the oxidant, and if needed, the hole can be completed as a small-diameter well for later injections. Potassium permanganate and other solid-phase chemical oxidants have also been added by hydraulic or pneumatic fracturing.

The site stratigraphy plays an important role in the distribution of oxidants. Fine-grained units redirect oxidants to more permeable areas and are difficult to penetrate; hence, they can be the source of rebound later on as contaminants diffuse out. Long-lived oxidants (e.g., permanganate) have the potential to remain active as this diffusion occurs, and they can mitigate some of the potential rebound.

Chemical oxidation usually requires multiple applications. The table provides a qualitative list of oxidant reactivities with common site contaminants.

Source: ITRC 2005 and Brown 2003
¹ Peroxide without a catalyst must be applied at higher concentrations, which are inherently hazardous, and the reactions are more difficult to predict and control.

In the special case of NAPLs, oxidants that are in a water-based solution will be able to react only with the dissolved phase of the contaminant, since the two will not mix. This property limits their activity to the oxidant solution/NAPL interface. Cost estimates depend on the heterogeneity of the site subsurface, soil oxidation demand, stability of the oxidant, and type and concentration of the contaminant. Care should be taken when comparing different technologies on a cubic yard basis without considering these site attributes. Cost data can be found in ITRC (2005) and Brown (2003). In situ chemical oxidation has been used at dozens of sites, and oxidizing compounds are available from a variety of vendors.

Sodium or Potassium Permanganate. Permanganate is a non-specific oxidizer of contaminants with low standard oxidation potential and high SOD. It can be used over a wide range of pH values and does not require a catalyst. Permanganate tends to remain in the subsurface for a long time, allowing for more contaminant contact and the potential of reducing rebound. As permanganate oxidizes organic materials, manganese oxide forms as a dark brown to black precipitate. During the treatment of large bodies of NAPL with high concentrations of permanganate, this precipitate may form a coating that reduces contact between oxidant and NAPL. The extent to which this reduction negatively affects contaminant oxidation has not been quantified. Potassium permanganate has a much lower solubility than sodium permanganate and generally is applied at lower concentrations. Commercial-grade permanganates may contain elevated concentrations of heavy metals, and they may lower the pH of the treated zone (U.S. EPA 2004). If bioremediation is planned as a polishing step, permanganate will have an adverse effect on microbial activity and may cause a change in microbe distribution. This effect is generally transitory. Also, there is some evidence that permanganates may be inhibitory to Dehalococcoides ethenogenes, the microbial species that completely dechlorinates PCE and TCE (Hrapovic et al. 2005).

Fenton's Catalyzed Hydrogen Peroxide. Fenton's reagent uses hydrogen peroxide in the presence of ferrous sulfate to generate hydroxyl radicals that are powerful oxidants. The reaction is fast, releases oxygen and heat, and can be difficult to control. Because of the fast reaction, the area of influence around the injection point is small. In conventional application, the reaction needs to take place in an acidified environment, which generally requires the injection of an acid to lower the treatment zone pH to between three and five. The reaction oxidizes the ferrous iron to ferric iron and causes it to precipitate, which can result in a loss of permeability in the soil near the injection point. Over time, the depletion of the ferrous ion can be rate limiting for the process. Chelated iron can be used to preserve the iron in its ferrous state at neutral pH, thus eliminating the acid requirement. The byproducts of the reaction are relatively benign, and the heat of the reaction may cause favorable desorption or dissolution of contaminants and their subsequent destruction. It also may cause the movement of contaminants away from the treatment zone or allow them to escape to the atmosphere. There are safety concerns with handling Fenton’s reagent on the surface, and the potential exists for violent reactions in the subsurface. In many cases, there may be sufficient iron or other transition metals in the subsurface to eliminate the need to add ferrous sulfate.

Hydrogen Peroxide. While catalysts can be added to increase oxidation potential, hydrogen peroxide can be used alone to oxidize contaminants. Peroxide oxidation is an exothermic reaction that can generate sufficient heat to boil water. The generation of heat can assist in making contaminants more available for degradation, as well as allowing them to escape to the surface. With its high reaction and decomposition rates, hydrogen peroxide is not likely to address contaminants found in low permeability soil. Solid peroxides (e.g., calcium peroxide) in slurry form moderate the rate of dissolution and peroxide generation, thereby allowing a more uniform distribution.

Ozone. One of the stronger oxidants, ozone can be applied as a gas or dissolved in water. As a gas, ozone can degrade a number of chemicals directly in both the dissolved and pure forms, and it provides an oxygen-rich environment for contaminants that degrade under aerobic conditions. It also degrades in water to form radical species that are highly reactive and non-specific. Ozone may require longer injection times than other oxidants, and vapor control equipment may be needed at the surface. Because of its reactivity, ozone may not be appropriate for slow diffusion into low-permeability soil.

Sodium Persulfate. Persulfate is a strong oxidant with a higher oxidation potential than hydrogen peroxide and a potentially lower SOD than permanganate or peroxide. Persulfate reaction is slow unless placed in the presence of a catalyst, such as ferrous iron, or heated to produce sulfate free radicals that are highly reactive and capable of degrading many organic compounds. At temperatures above 40°C, persulfate becomes especially reactive and can degrade most organics (Block et al. 2004). Like Fenton's reagent, the ferrous iron catalyst (when used) will degrade with time and precipitate (U.S. EPA 2006).

Technology Advantages

• Contaminant mass can be destroyed in situ.
• Rapid destruction/degradation of contaminants (measurable reductions in weeks or months).
• Produces no significant wastes (VOC offgas is minimal), except Fenton's.
• Reduced mobilization, operation, and monitoring costs due to rapid results.
• Compatible with post-treatment monitored natural attenuation.
• Has potential to enhance aerobic and anaerobic biodegradation of residual hydrocarbons.
• Likely to cause minimal disturbance to site operations.

Technology Limitations

• Potentially higher initial and overall costs relative to other source area solutions.
• Contamination in low permeability soils may not be readily contacted and destroyed by chemical oxidants.
• Fenton's reagent can produce a significant quantity of explosive offgas. Special precautions (i.e., an SVE system) are required for appropriate implementation of remedial action involving Fenton's chemistry.
• Dissolved contaminant concentrations may rebound weeks or months following chemical oxidation treatment.
• Dissolved contaminant plume configuration may be altered by chemical oxidation application.
• Significant health and safety concerns are associated with applying oxidants.
• May not be able to reduce contaminants to background or very low concentrations due to limitations of technology or cost.
• Significant losses of chemical oxidants may occur as they react with soil/bedrock material rather than contaminants.
• May significantly alter aquifer geochemistry; can cause clogging of aquifer through precipitation of minerals in pore spaces.

ISCO performance at any given site is dependent upon the contact achieved between the oxidant and the contaminants, which in turn is controlled by the DNAPL architecture and other site-specific conditions. Oxidation technologies have the potential for achieving significant mass destruction of organics in the subsurface; however, the results of field and laboratory work indicate that complete removal of contaminants may not be achieved with these technologies even under optimal conditions (NRC 2004). Innovators are combining the benefits of ISCO implementation with compatible technologies, such as solvent flushing and bioremediation, to maximize remediation potential.

Block, P., R. Brown, and D. Robinson. 2004. Novel activation technologies for sodium persulfate in situ chemical oxidation. Proceedings of the Fourth International Conference on the Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA.

Brown, R. 2003. In situ chemical oxidation: performance, practice, and pitfalls. 2003 AFCEE Technology Transfer Workshop, February 25, 2003, San Antonio, TX.

Hrapovic, L. et al. 2005. Laboratory study of treatment of trichloroethene by chemical oxidation followed by bioremediation. Environmental Science & Technology, Vol 39 No 8, p 2888-2897.

ITRC. 2005. Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated Soil and Groundwater, 2nd Edition.

National Research Council (NRC), 2004. Contaminants in the Subsurface: Source Zone Assessment and Remediation. National Academies Press, Washington, DC.

U.S. EPA. 2004. How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers, EPA 510/R-04/002. Office of Underground Storage Tanks.

U.S. EPA. 2006. In Situ Treatment Technologies for Contaminated Soil: Engineering Forum Issue Paper. EPA 542-F-06-013.

General Resources

Chemical Oxidation
Chapter XIII in How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers
U.S. EPA, Office of Underground Storage Tanks
EPA 510-R-04-002, 52 pp, 2004

Concurrent treatment of source area saturated and unsaturated zones with chemical oxidation usually requires its integration of with other remedial technologies that target unsaturated zone contamination (e.g., SVE). SVE is likely to be included as a component of ISCO solutions even if there is no specific need to treat unsaturated soils in the source area because it can help alleviate safety issues associated with controlling and recovering offgas.

Chemical Oxidation Site Profiles

U.S. EPA has developed a searchable database of case studies in which information about completed and ongoing full-scale applications of in situ chemical oxidation to address a variety of contaminants is summarized.

Engineering Issue Paper: In Situ Chemical Oxidation
EPA 600-R-06-072, 60 pp, 2006

This issue paper provides an overview of ISCO remediation technology and fundamentals based on peer-reviewed literature, EPA reports, web sources, current research, conference proceedings, and other pertinent information.

In Situ Chemical Oxidation Multimedia Training Tool
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, 23 pp.

A variety of toxic organics, including DNAPLs, are amenable to destruction or at least partial degradation through chemical oxidation processes initiated by the application of compounds such as potassium permanganate or Fenton's reagent. The most recent advances in the understanding of the application of ISCO for groundwater remediation are presented through this multimedia training tool.

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.

Principles and Practices of In Situ Chemical Oxidation Using Permanganate
R.L. Siegrist, M.A. Urynowicz, O.R. West, M.L. Crimi, and K.S. Lowe.
Battelle Press, Columbus, OH, ISBN:1-57477-102-7, 336 pp, 2001

Provides guidance on the evaluation and design of in situ chemical oxidation systems with a focus on the use of potassium and sodium permanganate for remediation of organically contaminated sites.

Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated Soil and Groundwater, 2nd Edition
Interstate Technology & Regulatory Council (ITRC). ISCO-2, 172 pp, 2005

In addition to technical and regulatory guidance, includes 14 case studies of ISCO implementation, 4 of them at sites (dry cleaners and a wood treatment facility) affected by DNAPL contaminants.

Technology Status Review: In Situ Oxidation
Environmental Security Technology Certification Program (ESTCP), Arlington, VA. 50 pp, 1999

Following a survey of several government sites where ISCO was used, this report was prepared to help establish the basis for selecting and designing the technology, to assess the costs and performance of the technology at specific sites, to assess the reasons for success or failure of ISCO, and to provide guidance on the use of the technology.

Other DNAPLs Treatment Technologies Topics:

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Last updated on Monday, November 10, 2008