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Treatment Technologies
Permeable Reactive Barriers
This page identifies general resources that contain detailed information on the design and implementation of permeable reactive barriers (PRBs). Information on applications of this technology specific to a chemical class can be found in the class subsections listed to the right. More resources on PRBs for a wide range of contamination problems can be found in the Permeable Reactive Barriers pages of Technology Focus .
A subsurface PRB is an emplacement of reactive materials through which a dissolved contaminant plume must move as it flows, typically under natural gradient. Treated water exits the other side of the PRB. This in situ method for remediating dissolved-phase contaminants in groundwater combines a passive chemical or biological treatment zone with subsurface fluid flow management.
PRBs can be installed as permanent or semi-permanent units. The most commonly used PRB configuration is that of a continuous trench in which the treatment material is backfilled. The trench is perpendicular to and intersects the groundwater plume. Another frequently used configuration is the funnel and gate, in which low-permeability walls (the funnel) direct the groundwater plume toward a permeable treatment zone (the gate). Some gates are in situ reactors that are readily accessible to facilitate the removal and replacement of reactive media. These PRBs use collection trenches, funnels, or complete containment to capture the plume and pass the groundwater, by gravity or hydraulic head, through a vessel containing either a single treatment medium or sequential media (ITRC 2005).
The majority of installed PRBs use zero-valent iron (ZVI) as the reactive medium for converting contaminants to non-toxic or immobile species. ZVI, a mild reductant, has the ability to reductively dehalogenate many halogenated hydrocarbons. Dehalogenation rates will vary for the different halogenated contaminants. The primary determinant of degradation rate is the specific surface area, or the surface area of iron per unit volume of pore water. The reaction pathways by which ZVI reduces halogenated hydrocarbons have been determined for a few major classes of chlorinated hydrocarbons. This information is significant to the optimal design of a PRB, as incomplete dechlorination of a highly chlorinated ethene, for example, could produce an intermediate product (e.g., vinyl chloride) that is more hazardous and more persistent than the parent compound. Even very low concentrations of undesirable byproducts in the reactive barrier effluent must be avoided (Powell 1998).
A variety of other media can be used in PRBs to address a wide range of organic and inorganic contaminants (Powell 1998). For example, organic materials—activated charcoal, cottonseed meal, peat moss, lignite, humite, and compost—have been placed in PRBs to treat groundwater affected by solvent-related compounds, primarily by promoting or enhancing biologically mediated destruction of the target analytes. The treatment zone materials concentrate and either degrade or retain the contaminants and may need to be replaced periodically (ITRC 2005).
In situ redox manipulation (ISRM) is a passive barrier technology based upon the in situ manipulation of natural processes to change the mobility or form of dissolved contaminants in the subsurface. ISRM was developed to remediate groundwater that contains chemically reducible metallic and organic contaminants (i.e., chlorinated solvents). ISRM creates a permeable treatment zone by injection of chemical reagents and/or microbial nutrients into the subsurface downgradient of the contaminant source. The type of reagent is selected according to its ability to alter the oxidation/reduction state of the groundwater, thereby destroying or immobilizing specific contaminants. Because unconfined aquifers are usually oxidizing environments and many of the contaminants in these aquifers are mobile under oxidizing conditions, appropriate manipulation of the redox potential can result in the immobilization of redox-sensitive inorganic contaminants and the destruction of organic contaminants (U.S. DOE 2000).
PRBs can be adapted to sequential treatments to address groundwater plumes that contain a mixture of contaminants. Sequenced reactive barriers have been constructed to treat a variety of mixed contaminant plumes.
- ZVI to treat chlorinated hydrocarbons followed by aerobic bioremediation to treat aromatic hydrocarbons.
- ZVI to treat carbon tetrachloride and chloroform followed by MNA for dichloromethane.
- ZVI to treat chlorinated hydrocarbons followed by nutrient addition or solid carbon sources to promote anaerobic biodegradation of VOCs that cannot be degraded by the iron.
- Solid carbon sources to treat nitrate followed by granular iron to treat volatile organics (ITRC 2005).
Typically, PRBs are designed to provide adequate residence time in the treatment zone for the degradation of the parent compound and all toxic intermediate products that are generated. At sites where the groundwater contamination includes a mixture of chlorinated hydrocarbons, the design of the PRB usually is determined by the least reactive constituent (Powell 1998).
The use of a passive PRB requires an unusually comprehensive hydrologic characterization so that the design can be based on a thorough understanding of subsurface heterogeneity rather than on average values for hydraulic parameters. Given the level of investigation required, design costs likely will increase, and the pre-design field work may demonstrate that a passive PRB is not suitable for a particular site (Korte 2001).
General Resources
Final Design Guidance for Application of Permeable Reactive Barriers for Groundwater Remediation
A. Gavaskar, N. Gupta, B. Sass, R. Janosy, and J. Hicks.
Environmental Security Technology Certification Program (ESTCP), Arlington, VA. 247 pp, 2000
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.
Permeable Mulch Biowalls
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, May 2007
A biobarrier is a biologically active flow-through zone in an aquifer that is established downgradient of a source zone or on the leading edge of a contaminant plume. As contaminated groundwater passes through the biobarrier, the contaminants are converted by microorganisms into innocuous byproducts, such as carbon dioxide and water. The microbial population is established with biostimulation and/or bioaugmentation. Biobarriers can be used to create either aerobic or anaerobic conditions. Configurations include biowalls, bioborings, or injection wells to add substrates to groundwater as it passively flows through the biologically active zone. Permeable mulch biowalls are used to remediate chlorinated organic compounds and other contaminants in ground water via the reaction of natural organic substrates, such as mulch, compost, and/or vegetable oil. The organic media create an anaerobic reaction zone that enhances bioremediation within the aquifer. This Web tutorial reviews design and installation considerations for permeable mulch biowalls and highlights case study results at Navy and Air Force sites.
Permeable Reactive Barrier Technologies for Contaminant Remediation
R.M. Powell, et al.
EPA 600-R-98-125, 102 pp, 1998
This document provides sufficient background in the science of PRB technology to allow a basic understanding of the chemical reactions that transform contaminants. It contains sections on PRB-treatable contaminants and the treatment reaction mechanisms; information on feasibility study, site characterization, design, emplacement, and monitoring issues specific to PRBs; and summaries of several field installations.
Permeable Reactive Barriers: Lessons Learned/New Directions
Interstate Technology & Regulatory Council (ITRC) Permeable Reactive Barriers Team.
PRB-4, 202 pp, 2005
While ZVI is the most common medium placed in PRBs to treat a variety of chlorinated organics, metals, and radionuclides, other reactive media—carbon sources (compost), limestone, granular activated carbon, and zeolites—also have been used to address metals and some organic compounds. This document contains compiled information and data on PRBs generated over the last 10 years of technology development and research, as well as information on non-iron-based reactive media that can be used in PRBs. This report also provides an update on a developing technology related to PRBs in which source zone contamination is addressed with iron-based reactive media via backfilling, soil mixing, or injection.
PRB Tool
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, 80 pp.
This tutorial is designed to allow the user to learn about the history and key scientific concepts related to the use of PRBs, to understand the steps needed to ensure the efficient design and effective performance of PRBs, to anticipate longevity issues and their relationship to the economic benefits of PRB use, and to benefit from lessons learned at other sites related to the installation and use of PRBs. The tutorial is available through the Navy's Multimedia Web Tools Portal along with a data sheet that briefly describes PRB configurations, construction methods, monitoring considerations, and advantages and disadvantages of the technology.
Zero-Valent Iron Permeable Reactive Barriers: A Review of Performance
N.E. Korte.
ORNL/TM-2000/345, 36 pp, 2001
This review reports on performance issues with PRBs installed at DOE sites. The principal conclusion of the review is that the most significant problems with installed PRBs have been the result of insufficient characterization, which resulted in poor engineering implementation.
U.S. DOE, 2000. In Situ Redox Manipulation: Innovative Technology Summary Report
DOE/EM-0499.
Return to "In Situ Reduction Introduction"
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Last updated on Monday, November 10, 2008