AGK Applied Geosciences University of Karlsruhe Karlsruhe University of Applied Sciences...

AGK Applied Geosciences University of KarlsruheKarlsruhe

Universityof Applied Sciences

FachhochschuleNordostniedersachsen

LüneburgBuxtehudeSuderburg

An Introduction ToPermeable Reactive Barriers (PRB)

Volker Birke Ernst Karl Roehl



University of Karlsruhe Applied GeosciencesKarlsruhe





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."





Source: http://www.eti.ca/eti.html





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





"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:





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





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





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:





Types of reactive walls:

a) Continuous Barrier (CRB) b) Funnel-and-gate (F&G) system





Source: Gavaskar et al. 1998





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





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





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





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





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





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:





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






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






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





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.






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-






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






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-






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






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-






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






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






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






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).






Silicon dioxide

Distribution of dissolved silicon dioxide in a Fe0 wall (Moffett Naval Station, Mountain View, CA).

Source: Gavaskar et al. (2000)






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






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






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)






Activated carbon:

• Adsorption of organic contaminants

• Specific surface: approx. 1000 m2/g

• Granular

Reaction kinetics: Diffusion controlled

Critical parameter: contact time!

Activated Carbon





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





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





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





PRB Construction





Karlsruhe, Germany





Monitoring

Targets:

Validation of Performance

Longevity





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





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.





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





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.





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.





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.

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