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An Introduction ToPermeable Reactive Barriers (PRB)
Volker Birke Ernst Karl Roehl
Universityof Applied Sciences
FachhochschuleNordostniedersachsen
University of Karlsruhe Applied GeosciencesKarlsruhe
AGK Applied Geosciences University of KarlsruheKarlsruhe
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EPA (1999), Remedial Technology Fact Sheet, 542-R-99-002
Definition:
Permeable Reactive Barriers are
"passive in situ treatment zones of reactive material that degrades or immobilizes contaminants as ground water flows through it. PRBs are installed as permanent, semi-permanent, or replaceable units across the flow path of a contaminant plume. Natural gradients transport cont-aminants through strategically placed treatment media. The media degrade, sorb, precipitate, or remove chlo-rinated solvents, metals, radionuclides, and other pollutants."
AGK Applied Geosciences University of KarlsruheKarlsruhe
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Source: http://www.eti.ca/eti.html
AGK Applied Geosciences University of KarlsruheKarlsruhe
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GW
DNAPL
plume
Aquitard
contamination source
heavy metals
Aquifer
LNAPL
reactive barrier
clean groundwater
LNAPL = light non-aqueous phase liquidsDNAPL = dense non-aqueous phase liquids
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"Emission oriented remediation approach"
Decontamination of the plume(vs. removal of the contaminant source)
Passive system
No active pumping of groundwater
Low maintenance following installation
PRB Concept:
AGK Applied Geosciences University of KarlsruheKarlsruhe
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Basic Concept:
"Emission oriented remediation approach" Clean-up of the plume, not the source
Passive system: No pumping required
Application:
Unclear location of source(s) Slow contaminant release from source Low solubility of contaminants Large volumes of contaminated soil Built-up areas
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Treatability Study:
Choice of attenuation mechanism and reactive material Column tests Determination of required residence time Calculation of barrier thickness
Site Characteristics:
Flow field (hydraulics) Contaminant concentrations Total contaminant mass expected Groundwater characteristics
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Degradation: Chemical and/or biological reac-tions converting the contaminants to harmless by-products.
Sorption: Contaminant removal from ground-water through adsorption or complexation.
Precipitation: Fixation of contaminants in insoluble compounds and minerals.
Types of reactive walls:
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Types of reactive walls:
a) Continuous Barrier (CRB) b) Funnel-and-gate (F&G) system
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Source: Gavaskar et al. 1998
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High contaminant attenuation
Good selectivity for target contaminants
Fast reaction rates
High hydraulic permeability
Long-term stability
Environmental compatibility
Sufficient availability in homogenous quality
Cost-effectiveness
Reactive Material Requirements
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Barrier Materials Contaminants Processes Development status
Fe0, Fe0/Al0, Fe0/Pd-mixtures,Fe0/pyrite-mixture, Fe/Ni
CHC, FCHC,chlorinated aromatics
abiotic reductive dehalogenation lab tests, pilot plants,commercial application
Fe0 and methanothrophicbacteria
CHC abiotic reductive dehalogenationand microbial degradation
lab tests
Zeolites and methanothrophicbacteria
TCE sorption coupled withmicrobial degradation
lab tests
Zeolites MTBE, CHCl3, TCE sorption lab tests
Surface-modified zeolites PCE, PAH sorption lab tests, pilot scale
Fe0/surface-modified zeolites PCE sorption, reduction lab tests
Organobentonites TCE, benzene, phenols sorption lab tests
ORC (oxygen releasingcompounds)
BTEX oxidative degradation, microbial lab tests, field tests
Activated carbon PAH sorption possibly coupled withmicrobial degradation
lab tests
Main source: Dahmke et al. (1996)+ own additions
Reactive Materials targeting Organic Contaminants
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Barrier Materials Contaminants Processes Development status
Fe0 CrO42- reduction and precipitation lab tests, pilot plants
Surface-modified zeolites CrO42-, SO4
2- sorption, reduction, surfaceprecipitation (?)
lab tests
Fe0/surface-modified zeolites CrO42- sorption, reduction lab tests
Hydroxylapatite Pb2+ precipitation lab tests, field tests
Hydroxylapatite Zn2+ sorption/ co-precipitation lab tests
Hydroxylapatite Cd2+ co-precipitation lab tests
Lime, fly ash UO2+ co-precipitation lab tests
Fe0 UO2+ reduction and precipitation lab tests
Fe0 TcO4- reduction and precipitation lab tests
Cellulose UO2+ reduction and precipitation lab tests
Peat, Fe(III) oxides MoO42- sorption, co-precipitation lab tests
Zeolites 90Sr2+ sorption lab tests
Fe0 NO3- reduction lab tests
Sawdust NO3- reduction field tests
Fe/Ca oxides PO4 sorption, co-precipitation lab tests
Main source: Dahmke et al. (1996)+ own additions
Reactive Materials targeting Inorganic Contaminants
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Hydraulic conductivity:
A minimum permeability must be guaranteed during barrier operation to avoid that contaminated groundwater by-passes the system.
Homogeneity:
In areas of favoured flow-paths there is the danger of a fast consumption of the reactive material's contaminant attenuation capability.
PRB Operating Requirements
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Period during which the reactive material keeps its ability to remove the target contaminants from the groundwater.
Barrier life-time:
Period during which the PRB keeps its hydraulic performance.
PRB Operating Requirements
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Type and concentration of contaminants
Type and kinetics of sorption and/or degradation processes.
Type and mass of reactive material
Hydraulic characteristics of the site (flow velocity)
Geochemical characteristics of the ground-water (Eh, pH, composition)
Long-term Performance Aspects
The barrier life-time is governed by:
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Considerations on mass flux
Hydraulic model of the former gas works site in Portadown, Northern Ireland.
Source: Kalin, R., presentation at PRB-net Workshop, April 2001, Belfast, Northern Ireland
Long-term Performance Aspects
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Coatings on the particle surface of the reactive material by precipitation of secondary minerals corrosion ("rust")
Processes that might impair the long-term performance of PRBs:
Clogging of the pore space between the particles by precipitation of secondary minerals gas formation (H2)
Biomass production
Consumption of the reactivity by arriving at the material's sorption capacity dissolution of the reactive material
Long-term Performance Aspects
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granular Fe0 foamed Fe0 aggregates
Organic contaminants: abiotic reductive degradation of chlorinated hydrocarbons (e.g., PCE, TCE, VC)
Inorganic contaminants: abiotic reductive immobilisation of heavy metals and others (e.g., Cr, U, Mo, Tc, As, NO3).
Costs: 200 - 400 €/t
Zero-valent Iron (Fe0) Walls
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Source: Gillham & O'Hannesin, 1994
Results of column tests conducted using commercial iron and groundwater from a contaminant plume at an industrial site. PCE dechlorination, formation of cDCE, and subsequent cDCE degradation.
Zero-valent Iron (Fe0) Walls
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Degradation of chlorinated hydrocarbons
Electron transfer from Fe0 surface (oxidation) to the chlorinated hydrocarbon (reduction, dehalogenation):
2Fe0 2Fe2+ + 4e-
3H2O 3H+ + 3OH-
2H+ + 2e- H2
X-Cl + H+ + 2e- X-H + Cl-
2Fe0 + 3H2O + X-Cl 2Fe2+ + 3OH- + H2 + X-H + Cl-
Zero-valent Iron (Fe0) Walls
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Source: http://www.doegjpo.com/perm-barr/index.htm
Removal of uranium and molybdenum from contaminated groundwater in porous Fe0 aggregates of a PRB system (Durango uranium mill tailings, Colorado, USA).
Uranium Molybdenum
Zero-valent Iron (Fe0) Walls
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Reductive immobilisation of heavy metals
Reduction of mobile and oxidised metal compounds followed by mineral precipitation
Chromium: Fe0 Fe2+ + 2e-
2H2O 2H+ + 2OH-
2H+ + 2e- H2
Fe0 Fe3+ + 3e-
Cr(VI)O42- + 4H2O + 3e- Cr(III)(OH)3 + 5OH-
Fe0 + Cr(VI)O42- + 4H2O Fe(III)Cr(III)(OH)6 + 2OH-
Zero-valent Iron (Fe0) Walls
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Source: Powell & Associates Science Services http://www.powellassociates.com/
Coatings might block access to the reactive surfaces. Further precipitation blocks the pore spaces between some iron particles increa-sing flow velocity and decrea-sing the residence time.
Coatings
Zero-valent Iron (Fe0) Walls
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Iron corrosion
Anoxic: Fe0 Fe2+ + 2e-
2H2O 2H+ + 2OH-
2H+ + 2e- H2
Fe0 + 2H2O Fe2+ + H2 + 2OH-
Oxic: Fe0 Fe2+ + 2e-
H2O H+ + OH-
½O2 + 2e- O2-
Fe0 + H2O + ½O2 Fe2+ + 2OH-
Zero-valent Iron (Fe0) Walls
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Precipitation of secondary minerals
Carbonates
HCO3- + OH- CO3
2- + H2O
Fe2+ + CO32- FeCO3 (s)
Ca2+ + CO32- CaCO3 (s)
Iron minerals
Fe2+ + 2OH- Fe(OH)2 (s)
3Fe(OH)2 (s) Fe3O4 (s) + 2H2O + H2
Magnetite
Calcite
Siderite
Zero-valent Iron (Fe0) Walls
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Stability fields for the system Fe-CO2-H2O with the following solid phases:
• Am. iron hydroxide Fe(OH)3
• Siderite FeCO3
• Iron hydroxide Fe(OH)2
• Zero-valent iron Fe(25°C, Fetotal = 10-5 M, Ctotal = 10-3 M, from: Stumm & Morgan 1996).
Iron geochemistry
Zero-valent Iron (Fe0) Walls
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Source: McMahon, P.B., Dennehy, K.F. & Sandstrom, M.W. (1999), Ground Water, 37, 396-404.
Carbonate, Ca and Fe concentration in ground-water passing through a Fe0 wall.Obvious precipitation of calcite and siderite, especially in the upstream pea gravel (Denver Federal Center, Denver, USA).
Clogging
Zero-valent Iron (Fe0) Walls
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Carbonate precipitation
Source: Vogan, J.L. et al. (2000), J. Haz. Mat., 68, 97-108.
Carbonate concentrations in the zero-valent iron filling of a Fe0 wall (industrial site contaminated by chlorinated hydrocarbons, New York, USA).
Zero-valent Iron (Fe0) Walls
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Silicon dioxide
Distribution of dissolved silicon dioxide in a Fe0 wall (Moffett Naval Station, Mountain View, CA).
Source: Gavaskar et al. (2000)
Zero-valent Iron (Fe0) Walls
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Dissolved iron with pH in Fe0 column experiments (ZVI): Clear dissolution of iron, but only relevant at pH values < 7.
Source: U.S. Department of Energy Grand Junction Office (GJO)http://www.doegjpo.com/perm-barr/
Consumption
Zero-valent Iron (Fe0) Walls
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Decrease of concentration in the wall:Ca, Mg, Si, bicarbonate, sulphate, H+
Showing some influence on the reaction kinetics (corrosion, dehalogenation):Bicarbonate, sulphate, nitrate, phosphate, chloride, dissolved oxygen
Groundwater constituents
Zero-valent Iron (Fe0) Walls
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Mass balancing
Precipitation in a Fe0 wall, Copenhagen, Denmark (Kiilerich et al., 2000):
13,3 kg iron hydroxides, 2,7 kg CaCO3, 2,7 kg FeCO3 and 0,8 kg FeS per 1000 kg iron filling per year
Loss of porosity in a Fe0 wall, Denver Federal Center, Denver, USA (McMahon et al., 1999):
0,35 % of total porosity per year (calculated only for the assumed precipitation of calcite and siderite)
Zero-valent Iron (Fe0) Walls
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Activated carbon:
• Adsorption of organic contaminants
• Specific surface: approx. 1000 m2/g
• Granular
Reaction kinetics: Diffusion controlled
Critical parameter: contact time!
Activated Carbon
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Retardation factor:
f(c) = adsorption isotherm (linear, Freundlich, Langmuir)
va = groundwater flow velocity
vS = contaminant transport velocity
Retardation:
S
a
vv
c)c(f
n1R
PAH: R > 3000 (Schad & Grathwohl, 1998)
Trichloroethene: R 5000 - 20000
Chlorobenzene: R 10000 - 20000(Köber et al., 2001)
Activated Carbon
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d = reactive wall thickness
va = groundwater flow velocity
R = retardation factor
Maximum barrier life-time estimation:
Horizontal flow through an activated carbon reactor of 1,8 m diameter with a flow velocity of 0,5 m/d and a retardation factor of R = 3000: maximum life-time = 30 years
Rvd
ta
S
Activated Carbon
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Groundwater composition
Competition effects: Natural groundwater constituents and contaminants compete for the adsorption sites
Precipitation of secondary minerals: Coatings block the access to the particle surfaces and alter the reaction kinetics
Formation of biomass
Negative effect: clogging of the free pore space
Positive effect: biological degradation of sorbed contaminants possible
Factors influencing barrier life-time:
Activated Carbon
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PRB Construction
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Karlsruhe, Germany
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Monitoring
Targets:
Validation of Performance
Longevity
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Checking of hydraulics
Checking groundwater chemistry
Hydrochemical parameters: pH, electr. conductivity
cations: Ca2+, Mg2+, Fet,
anions: HCO3-, SO4
2-, Cl-, PO42-, NO3
-
Investigation of the reactive material
Coring: carbonate, XRD, REM
Longevity:
Monitoring
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Focus of current R&D:
Selection of appropriate materials and processes for selective and efficient removal of groundwater pollutants.
Current Research
Evaluation of longevity and long-term performance; development of models.
Upscaling – applicability and transfer of lab-scale results into the field
Hydraulics of PRBs.
AGK Applied Geosciences University of KarlsruheKarlsruhe
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Current Research: Tri-Agency-Initiative
Tri-Agency Initiative, USA:
US EPA US DOE US DoD
USCG Base,Elizabeth City, NC
Y-12 Plant, OakRidge, TN
Dover AFB, Dover,DE
Denver Fed.Center, Denver, CO
Kansas City Plant,Kansas City, MO
Lowery AFB,Denver, CO
SomersworthLandfill,Somersworth, NH
DOE Uranium Mill,Monticello, UT
Moffett NavalStation, MountainView, CA
Alameda NavalSta., Alameda, CA
Watervliet Arsenal,Watervliet, NY
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Current R&D
„Reaktionswände und -barrieren im Netz-werkverbund“ („RUBIN“), BMBF, Germany
PRB projects co-operating in a network (RUBIN) Launched May 2000, 3 years Financial means: ca. 4 Mill. Euro. Coordination: University of Applied Sciences (Prof. H.
Burmeier, Dr. V. Birke, Dipl.-Ing. D. Rosenau) 11 projects 8 projects dealing with design, erection and operation
of pilot- or full-scale PRBs in Germany and/or important general preparatory R&D work
3 projects addressing general issues and missions.
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Conclusions
PRB long-term behaviour is a function of the deployed reactive material.
PRB longevity is influenced by the pollutants to be treated and the groundwater ingredients, i.e., groundwater chemistry.
The main groundwater components reveal a specific, important influence predominantly due to their higher concentrations compared to the pollutant´s concentrations.
Surface reactions at the reactive material cause significant changes in geochemical conditions (pH, Eh) regarding pore space that is passed by groundwater and therefore hydrochemical changes in the composition of the groundwater.
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Conclusions
Mineral formation (coatings), alteration of surfaces, gas evolution and biomass can influence reactivity and permeability of a PRB.
Alteration of surfaces and mineral formation can be mostly observed directly upgradient of a PRB.
However, only pertaining to a few cases, detrimental effects regarding efficiency of the PRB have been observed so far.
Geochemical processes are predominantly well-known and well understood. However, quantitative approaches for long-term behaviour/performance are still lacking. Current R&D projects address these issues.