Permeable reactive barrier
Encyclopedia
A permeable reactive barrier (PRB), also referred to as a permeable reactive treatment zone (PRTZ), is a developing technology that has been recognized as being a cost-effective technology for in situ (at the site) groundwater remediation
. PRBs are barriers which allow some—but not all—materials to pass through. One definition for PRBs is an in situ treatment zone that passively captures a plume of contaminants and removes or breaks down the contaminants, releasing uncontaminated water. The primary removal methods include: (1) sorption
and precipitation
, (2) chemical reaction, and (3) reactions involving biological mechanisms.
was done in Base Borden, Ontario by Robert Gillham and Stephanie O'Hannesin of the University of Waterloo
. The design, typical of many PRBs, consisted of a treatment zone formed by excavating an area isolated by sheet piles, refilling the hole with a mixture of granular iron and sand, and removing the sheet pile to leave an in situ, permeable, iron-bearing treatment zone. The contaminants, chlorinated ethylenes (PCE and TCE), were removed, leaving, for the most part, fully dechlorinated groundwater (little vinyl chloride
was observed).
The success of the field test at Base Borden eventually led to commercialization of this technology. Since then, a great deal of interest has developed in the groundwater remediation
community over the prospects of new treatment strategies (especially PRBs based on contaminant reduction by granular iron and other zero valent metals (ZVMs)). There have now been many feasibility studies, pilot tests, demonstration projects, and full-scale applications performed by numerous groups.
groundwater. The two main processes are immobilization (AKA sequestration) and transformation.
to the barrier materials or precipitation
from the dissolved state. Organic compounds tend to be undergo sorption due to hydrophobic expulsion from the surrounding water. Metals, however, tend to sorb through electrostatic attraction or surface complexation reactions. Sorption and precipitiation are potentially reversible and may thus require removal of the reactive medium and gathered products in order to continue with remediation.
reaction. The medium may directly supply electrons for reduction or stimulate microorganisms to facilitate electron transfer.
. It continues to be the main material used in the construction of these barriers. In additional to conventional scale iron, nanoscale-iron may also be used.
. Many environmental pollutants are highly reduced
, thus, the oxidation
of these pollutants to harmless compounds is thermodynamically
viable. Other pollutants, such as chlorinated solvents, are highly oxidized
and as such are easily reduced. Microorganisms commonly facilitate such redox
reactions, exploiting contaminant degradation as a means to obtain energy and materials for cell synthesis.
Oxidative biodegradation necessitates electron
acceptors that microbes use to "respire" the electrons removed from target contaminants. This transfer of electrons releases energy to drive microbial life functions. Under aerobic
conditions, molecular oxygen is used for this purpose. When oxygen is not present, a variety of other molecules can serve as electron acceptors. Oxygen is preferentially utilized over the anaerobic
electron acceptors because using oxygen gives more energy and, as an added benefit, results in faster contaminants oxidation rates. Unfortunately, the available oxygen is often not sufficient for the contaminants in highly contaminated areas, and as a result the anaerobic
electron acceptors must be utilized. Reactive barriers containing oxygen-releasing compounds have been used successfully to stimulate aerobic
biodegradation of monoaromatic
hydrocarbons.
s, and other natural material have a high capacity for cation exchange. They do this by creating a net negative charge by substituting lower-valent cations (e.g. Al3+) with a higher-valent cation (e.g. Si4+) within the mineral structure. Adding sorbed
surfactant
s can change the affinity for anions and nonpolar organic compounds. Surfactants that have accumulated at the surface will create a hydrophobic organic coating that promotes sorption
of non-polar organic compounds. Surfactant Modified Zeolites (SMZs) are promising for treating non-polar organic contaminants. However, clay's low permeability means it cannot be used in flow-through PRBs, but have been proposed for use in slurry walls, landfill liners, and containment barriers. Zeolites; however, have cavities to maintain hydraulic conductivity
, allowing their use in PRBs.
has a large specific surface area
(>200 m2/g) and a high porosity
. Metals are taken up by peat through an ion exchange reaction where the metal displaces a proton if the pH
is low or an existing metal if the pH
is high from the anionic function group. Anions, such as and are removed more effectively at pH
< 3 because of the positively charged surface created by the addition of protons onto the surface functional groups, whereas cations, such as , , are more effectively removed at higher pH
values. Peat moss seems to be an effective ion-exchange material for removing heavy metals and some anions. Removal efficiency of cations approaches 100% at low pH, but the strong dependency on pH and the initial metal ion concentration have to be considered.
: Fig. A, a conventional PRB made with mm-sized granular iron and Fig. B, a "reactive treatment zone" formed by sequential injection of nano-sized iron to form overlapping zones of particles absorbed by the grains of native aquifer material. In A, groundwater flows through the barrier and is remediated. In B, nanoparticles of iron are represented by black dots; the nanoparticles have little mobility in the porous medium. Note that reaction will only occur when contaminants, either dissolved in the groundwater
or as DNAPL, come into contact with the iron surfaces.
. This method may be slow (and therefore expensive) and is only via for plumes less than 35 feet deep.
and iron. The guar gum holds the iron in place before degrading, leaving a permeable zone of iron (the PRB).
s. This process creates a series of columnar treatment zones that form a PRB when lined up. This method can treat plumes to a depth of 100 feet, but the treatment zone is relatively low in the proportion of iron.
, influent [pH], and influent concentrations of [alkalinity], [nitrate ], and [chloride Cl-] are the strongest predictors of possible diminished performance of PRBs. The reactivity of the media, rather than a reduction in permeability is more likely the factor that limits field PRB longevity. Because this technology is relatively new, it is still hard to predict the longevity of sites. Depending on assumptions of controlling factors, longevity estimates can differ by a factor of magnitude (e.g. 10–100 years).
, at the site of a previously operating semi-conductor plant. At the time, the best available remediation
technology was pump and treat technology. PRBs presented a more cost-effective solution to the problem at hand, being able to passively remediate the groundwater. Granular metal was chosen as the reactive media after laboratory testing using contaminated water from the site. After installation contaminants were reduced to target levels. As a result, the pump and treat machinery was able to be removed and the above ground was free to be used for commercial purposes. The savings from using the PRB as opposed to pump and treat were able to pay for the installation in about three years.
a contaminant plume of trichloroethylene
(TCE) and hexavalent chromium
(Cr (VI)). The PRB took only 6 hours to install using a continuous trenching technique, which simultaneously removed the pre-existing sediment while installing the reactive medium (granular iron). The PRB was configured as a continuous wall as opposed to a funnel-and-gate setup because 3D computer simulations suggested that the two would have the same effectiveness, but cost analyses showed that the continuous setup would be cheaper to install. The total cost of installation was approximately $1 million, while the U.S. Coast Guard predicts that over 20 years $4 million will be saved compared to a pump-and-treat system.
There are also a variety of companies available for implementing this technology. Here are just a few of them:
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
. PRBs are barriers which allow some—but not all—materials to pass through. One definition for PRBs is an in situ treatment zone that passively captures a plume of contaminants and removes or breaks down the contaminants, releasing uncontaminated water. The primary removal methods include: (1) sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
and precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
, (2) chemical reaction, and (3) reactions involving biological mechanisms.
The first application of PRBs
The first field-scale application of PRBs in groundwater remediationGroundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
was done in Base Borden, Ontario by Robert Gillham and Stephanie O'Hannesin of the University of Waterloo
University of Waterloo
The University of Waterloo is a comprehensive public university in the city of Waterloo, Ontario, Canada. The school was founded in 1957 by Drs. Gerry Hagey and Ira G. Needles, and has since grown to an institution of more than 30,000 students, faculty, and staff...
. The design, typical of many PRBs, consisted of a treatment zone formed by excavating an area isolated by sheet piles, refilling the hole with a mixture of granular iron and sand, and removing the sheet pile to leave an in situ, permeable, iron-bearing treatment zone. The contaminants, chlorinated ethylenes (PCE and TCE), were removed, leaving, for the most part, fully dechlorinated groundwater (little vinyl chloride
Vinyl chloride
Vinyl chloride is the organochloride with the formula H2C:CHCl. It is also called vinyl chloride monomer, VCM or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride . At ambient pressure and temperature, vinyl chloride...
was observed).
The success of the field test at Base Borden eventually led to commercialization of this technology. Since then, a great deal of interest has developed in the groundwater remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
community over the prospects of new treatment strategies (especially PRBs based on contaminant reduction by granular iron and other zero valent metals (ZVMs)). There have now been many feasibility studies, pilot tests, demonstration projects, and full-scale applications performed by numerous groups.
Reactive processes
There are a variety of ways that permeable reactive membranes can be used in order to remediateGroundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
groundwater. The two main processes are immobilization (AKA sequestration) and transformation.
Immobilization
Immobilization of the contaminant may occur through sorptionSorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
to the barrier materials or precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
from the dissolved state. Organic compounds tend to be undergo sorption due to hydrophobic expulsion from the surrounding water. Metals, however, tend to sorb through electrostatic attraction or surface complexation reactions. Sorption and precipitiation are potentially reversible and may thus require removal of the reactive medium and gathered products in order to continue with remediation.
Transformation
Transformation involves taking the contaminant and transforming it to a less harmful or non-toxic form. One of the chief benefits of transformation is that it does not necessarily require removal of the reactive medium (unless the reactive medium must be replaced due to decreased effectiveness or clogging occurs). Transformation most commonly takes the form of an irreversible redoxRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reaction. The medium may directly supply electrons for reduction or stimulate microorganisms to facilitate electron transfer.
Reactive Materials
In addition, there are many several different material which may be used. Here are the more prominent:Zerovalent iron
Zerovalent Iron was the first material to be used in PRBs for groundwater remediationGroundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
. It continues to be the main material used in the construction of these barriers. In additional to conventional scale iron, nanoscale-iron may also be used.
Biological barriers
Sometimes material will be put into the ground to stimulate the growth of microbes that facilitate the groundwater remediationGroundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
. Many environmental pollutants are highly reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, thus, the oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of these pollutants to harmless compounds is thermodynamically
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
viable. Other pollutants, such as chlorinated solvents, are highly oxidized
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
and as such are easily reduced. Microorganisms commonly facilitate such redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
reactions, exploiting contaminant degradation as a means to obtain energy and materials for cell synthesis.
Oxidative biodegradation necessitates electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
acceptors that microbes use to "respire" the electrons removed from target contaminants. This transfer of electrons releases energy to drive microbial life functions. Under aerobic
Aerobic
Aerobic is a word that means "requiring air", where "air" usually means oxygen.Aerobic may also refer to:* Aerobic exercise, prolonged exercise of moderate intensity* Aerobics, a form of aerobic exercise...
conditions, molecular oxygen is used for this purpose. When oxygen is not present, a variety of other molecules can serve as electron acceptors. Oxygen is preferentially utilized over the anaerobic
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
electron acceptors because using oxygen gives more energy and, as an added benefit, results in faster contaminants oxidation rates. Unfortunately, the available oxygen is often not sufficient for the contaminants in highly contaminated areas, and as a result the anaerobic
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
electron acceptors must be utilized. Reactive barriers containing oxygen-releasing compounds have been used successfully to stimulate aerobic
Aerobic
Aerobic is a word that means "requiring air", where "air" usually means oxygen.Aerobic may also refer to:* Aerobic exercise, prolonged exercise of moderate intensity* Aerobics, a form of aerobic exercise...
biodegradation of monoaromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
hydrocarbons.
Surfactant-modified zeolites
Clays, zeoliteZeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s, and other natural material have a high capacity for cation exchange. They do this by creating a net negative charge by substituting lower-valent cations (e.g. Al3+) with a higher-valent cation (e.g. Si4+) within the mineral structure. Adding sorbed
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
s can change the affinity for anions and nonpolar organic compounds. Surfactants that have accumulated at the surface will create a hydrophobic organic coating that promotes sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
of non-polar organic compounds. Surfactant Modified Zeolites (SMZs) are promising for treating non-polar organic contaminants. However, clay's low permeability means it cannot be used in flow-through PRBs, but have been proposed for use in slurry walls, landfill liners, and containment barriers. Zeolites; however, have cavities to maintain hydraulic conductivity
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
, allowing their use in PRBs.
Peat moss
Peat MossSphagnum
Sphagnum is a genus of between 151 and 350 species of mosses commonly called peat moss, due to its prevalence in peat bogs and mires. A distinction is made between sphagnum moss, the live moss growing on top of a peat bog on one hand, and sphagnum peat moss or sphagnum peat on the other, the...
has a large specific surface area
Specific surface area
Specific surface area is a material property of solids which measures the total surface area per unit of mass, solid or bulk volume, or cross-sectional area...
(>200 m2/g) and a high porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
. Metals are taken up by peat through an ion exchange reaction where the metal displaces a proton if the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
is low or an existing metal if the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
is high from the anionic function group. Anions, such as and are removed more effectively at pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
< 3 because of the positively charged surface created by the addition of protons onto the surface functional groups, whereas cations, such as , , are more effectively removed at higher pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
values. Peat moss seems to be an effective ion-exchange material for removing heavy metals and some anions. Removal efficiency of cations approaches 100% at low pH, but the strong dependency on pH and the initial metal ion concentration have to be considered.
Groundwater modeling
Modeling groundwater flow is important for optimizing the design of a PRB. Most importantly, by modeling the flow, the hydraulic capture zone width (HCZW) and the residence time can be determined. The HCZW is the width of the zone of groundwater that will pass through the reactive cell or gate (for funnel-and-gate configurations). The residence time is the time that the contaminated groundwater will spend in the treatment zone for decontamination. Contamination outside the capture zone or that does not have a long enough residence time will not be properly decontaminated. Groundwater modeling can also be used for the following:- Determining the location of the PRB
- Determining the a suitable configuration
- Determining the width of the reactive cell (and funnel for funnel-and gate)
- Evaluating potential for underflow, overflow, or flow across aquifers
- Providing knowledge of groundwater flow fluctuations (velocity and direction) for use in the design
- Determining reactive media selection (based on hydraulic conductivity) to match the conductivity of the aquiferAquiferAn aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
- Evaluating possibilities for flow bypass due to reduced porosity
- Helping determine monitoring well locations and monitoring frequencies
Iron barriers
The accompanying figure shows two approaches to application of iron particles for groundwater remediationGroundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
: Fig. A, a conventional PRB made with mm-sized granular iron and Fig. B, a "reactive treatment zone" formed by sequential injection of nano-sized iron to form overlapping zones of particles absorbed by the grains of native aquifer material. In A, groundwater flows through the barrier and is remediated. In B, nanoparticles of iron are represented by black dots; the nanoparticles have little mobility in the porous medium. Note that reaction will only occur when contaminants, either dissolved in the groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
or as DNAPL, come into contact with the iron surfaces.
Funnel and gate
Funnel and gate systems are used to channel the contaminant plume into a gate which contains the reactive material. The funnels are non-permeable, and the simplest design consists of a single gate with walls extending from both sides. The main advantage of the funnel and gate system is that a smaller reactive region can be used for treating the plume, resulting in a lower cost. In addition, if the reactive media needs to be replaced, it is much easier to do so because of the small gate.Implementation
PRBs are typically installed by digging a long trench in the path of the flow of the contaminated groundwater. The trench is then filled with the reactive materials (typically iron, carbon, or limestone). Sand can be mixed with the reactive material to aid in allowing the water to flow through the materials. Sometimes, there will be a wall that directs the groundwater to the reactive parts of the barrier. After the trench has been filled with reactive material, soil will typically be used to cover the PRB, thus eliminating visibility from the surface.Sheet pile and excavation
Sheet pile and excavation were used for the installation of earlier PRBs. This method involves containing the area of excavation using sheet piles before excavating using a trackhoeExcavator
Excavators are heavy construction equipment consisting of a boom, stick, bucket and cab on a rotating platform . The house sits atop an undercarriage with tracks or wheels. A cable-operated excavator uses winches and steel ropes to accomplish the movements. They are a natural progression from the...
. This method may be slow (and therefore expensive) and is only via for plumes less than 35 feet deep.
Continuous trencher
Continuous trenching involves using a large cutting chain excavator system then using the trench box and hopper to continuously back-fill the trench with reactive media. Continuous trenching can be fast and thus, inexpensive, but can only be used for trenches less than 50 feet deep. In addition, the machinery used for this technique cannot be used effectively for soil with large cobbles.Mendrel emplacement
Mendrel technology involves vertically driving a long hollow beam deep into the ground. The beam is covered as it is driven in, and the cover is removed once the beam has been placed. Next, the hollow is filled with iron filings. The Mendrel is then vibrated as it is removed, allowing the iron to flow to the bottom, forming the PRB. The Mendrel is then moved one width over, the process is repeated, and a continuous PRB is made.Hydraulic fracture
This methods utilizes injected fine-grained iron into fractures below the surface that were created using controlled applications of high pressure. Jets of water scour out a zone that is then filled with guar gumGuar gum
Guar gum, also called guaran, is a galactomannan. It is primarily the ground endosperm of guar beans. The guar seeds are dehusked, milled and screened to obtain the guar gum. It is typically produced as a free-flowing, pale, off-white-colored, coarse to fine ground powder.-Production:Guar gum is an...
and iron. The guar gum holds the iron in place before degrading, leaving a permeable zone of iron (the PRB).
Deep soil mixing
Deep soil mixing adds iron to the native soil and mixing it with large augerAuger
An auger is a drilling device, or drill bit, that usually includes a rotating helical screw blade called a "flighting" to act as a screw conveyor to remove the drilled out material...
s. This process creates a series of columnar treatment zones that form a PRB when lined up. This method can treat plumes to a depth of 100 feet, but the treatment zone is relatively low in the proportion of iron.
Performance assessment
The key component for assessing the success of a PRB is whether it satisfactorily removes the contaminants. This can be done by monitoring the levels in the water immediately downstream of the PRB. If the levels are below maximum contaminant levels, then the PRB has performed its function.PRB failure
In analyzing PRBs, emphasis has been placed on losses of reactivity and permeability in the reactive well; however, flawed hydraulic characterization of the few PRB failures that have been reported. Oxidation-reduction potentialRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, influent [pH], and influent concentrations of [alkalinity], [nitrate ], and [chloride Cl-] are the strongest predictors of possible diminished performance of PRBs. The reactivity of the media, rather than a reduction in permeability is more likely the factor that limits field PRB longevity. Because this technology is relatively new, it is still hard to predict the longevity of sites. Depending on assumptions of controlling factors, longevity estimates can differ by a factor of magnitude (e.g. 10–100 years).
Sunnyvale, CA
The first field-scale implementation of PRB was in Sunnyvale, CaliforniaSunnyvale, California
Sunnyvale is a city in Santa Clara County, California, United States. It is one of the major cities that make up the Silicon Valley located in the San Francisco Bay Area...
, at the site of a previously operating semi-conductor plant. At the time, the best available remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
technology was pump and treat technology. PRBs presented a more cost-effective solution to the problem at hand, being able to passively remediate the groundwater. Granular metal was chosen as the reactive media after laboratory testing using contaminated water from the site. After installation contaminants were reduced to target levels. As a result, the pump and treat machinery was able to be removed and the above ground was free to be used for commercial purposes. The savings from using the PRB as opposed to pump and treat were able to pay for the installation in about three years.
Elizabeth City, NC
In 1996 a 46 m long, 7.3 m deep, .6 m thick PRB was installed at a Coast Guard Facility near Elizabeth City, NC. The goal of this PRB was to remediateGroundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
a contaminant plume of trichloroethylene
Trichloroethylene
The chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. It is a clear non-flammable liquid with a sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, which is commonly known as chlorothene.The IUPAC name is...
(TCE) and hexavalent chromium
Hexavalent chromium
Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately of hexavalent chromium were produced in 1985...
(Cr (VI)). The PRB took only 6 hours to install using a continuous trenching technique, which simultaneously removed the pre-existing sediment while installing the reactive medium (granular iron). The PRB was configured as a continuous wall as opposed to a funnel-and-gate setup because 3D computer simulations suggested that the two would have the same effectiveness, but cost analyses showed that the continuous setup would be cheaper to install. The total cost of installation was approximately $1 million, while the U.S. Coast Guard predicts that over 20 years $4 million will be saved compared to a pump-and-treat system.
Moffett Field, CA
Moffett Field, CA was home to a pilot scale PRB initiated by the U.S. Navy in 1995. The Moffett Field PRB used a funnel and gate design, with the funnel being composed of interlocking steel sheet piles, while the gate consisted of granular zero-valent iron. The primary contaminants were trichloroethene (TCE), cis-1,2 dichloroethene (cDCE), and perchloroethene (PCE). Data from quarterly monitoring, tracer testing, and iron cell coring have been used to determine the effectiveness of the site. Since the first sampling event in June 1996, concentrations of all chlorinated compounds have been reduced to either non-detect levels or below the maximum contaminant levels.Fry Canyon, UT
The Fry Canyon site was selected in 1996 as a field demonstration site to assess the removal capabilities of PRBs for [uranium]. Laboratory experiments were conducted on three potential PRB materials (phosphate, zero-valent iron, and ferric iron) to determine uranium removal efficiencies and hydrologic properties. A PRB material from each class was selected for demonstration. The selected materials had satisfactory hydraulic conductivity, high U removal efficiency, and high compaction strengths. A funnel and gate design was used. The funnels channeled the [groundwater] into the PRB gates. During the first year, zero-valent iron had lowered U concentration by more than 99.9%, while the amount removed in both the phosphate and the ferric iron exceeded 70% for most of the measurements made. Mechanisms for removing uranium are similar to those for removing other inorganic contaminants, meaning that this study has wide applicability.The status of the technology
In 1994, analysts estimated that in the U.S. total clean up costs of groundwater totaled between $500 billion and $1 trillion. Until about 2000, the majority of groundwater remediation was done using "conventional technologies" (e.g., pump-and-treat systems), which have proven costly to meet applicable clean up standards. In the last few years, research on PRBs has increased because of the reduced water and energy demands and the potential to be more economical than conventional methods. While the reactivity of common PRB materials with chlorinated compounds has long been recognized, in situ applications were not considered until recently.External links
Additional information on this topic may be found at the following sites:- Powell and Associates PRB Notebook
- Remediation Technologies Development Forum PRB Action Team
- Federal Remediation Technologies Roundtable
- US EPA Contaminated Site Clean-Up Information
- "Rubin" (German PRB Network)
- OHSU Center for Grounwater Research, Zero-Valent Iron
There are also a variety of companies available for implementing this technology. Here are just a few of them: