Permeable Reactive Barriers

Shu-Chi Chang, Ph.D., P.E., P.A.Assistant Professor1 and Division Chief2

1Department of Environmental Engineering2Division of Occupational Safety and Health,

Center for Environmental Protection and Occupational Safety and Health

National Chung Hsing University

May 9, 2007

Permeable Reactive Barriers

1. Technology Backgrounda. Types of Barriersb. Barrier Emplacementc. Barrier Design

2. Reactive Media Selectiona. Zero-valent ironb. Aquifer oxidationc. Aquifer reduction

3. Microbial Barriers (Biocurtains)a. Biostimulationb. Bioaugmentation

4. Conclusions

Permeable Barrier Configuration

Permeable Barrier Configuration

Funnel-and-Gate with Single and Serial Reactive Medium

Installation of Funnel-and-Gate Barrier:funnel walls: sheet-piling; gates: pea gravel and iron

zones

GW flow

Deep Soil Mixing

Use of augers in series, which simultaneously mixes up soil (permeable barrier) and injects bentonite (impermeable wall)

Depth to 40 m.

Zerovalent Iron

Reaction stoichiometry3Feo 3Fe2+ + 6e-

C2HCl3 + 3H+ + 6e- C2H4 + 3Cl-______________________________

3Feo + C2HCl3 + 3H+ 3Fe2+ + C2H4 + 3Cl-

Due to OH- production (from iron-mediated reduction of water), the pH in a reactive cell increases (up to 9)

OH- production increases carbonate dissolution and ferric carbonate (FeCO3) precipitation

Decreases reactivity!

Degradation Rates (Half lives per 1 m2 iron surface per mL)

Barrier Design (based on column tests)

Assume column VOC profile during iron-mediated dechlorination

Required residence time for permeable barrier (based on t3):

tw = (1/k)ln(Co/C) Include correction factor for temp. (Q10

= 2-2.5): tw,corr.

Determine required thickness ‘b’of reactive cell:

b = Vx • tw,corr.

Note: Vx,reactive cell > Vx,groundwater

Hydraulic conductivity of reactive medium:

K = V•L/A•t•hWhere V volume discharge, L the length

of the reactive medium, A the cross-sectional area, and h the hydraulic head difference

Current Status – Chlorinated Solvents Sites

Type of Application

Contaminant Mixtures

Oxidant Principal Observation Reference Number

Fluid Injection

Petroleum Hydrocarbons Petroleum Hydrocarbons Chlorinated Alkenes (PCE, TCE) Vinyl Chloride 1,2 DCE

H2O2 (10-50% v/v) H2O2/Fe2+ (Fenton’s Reagent) H2O2/Fe2+/H+

Poor oxidant persistence and limited zone of oxidative influence Limited short-term injection zone influence, very pH dependent oxidation efficiency Documented local contaminant removal; in some cases, anoxic conditions returned within a month

1, 22, 23, 39 8 25, 38

Permeable Reactive Barrier

Aromatic Hydrocarbons (Benzene Toluene) Aromatic Hydrocarbons (Benzene, Toluene, Ethyl Benzene, Xylene)

MgO2 (ORC) MgO2

Benzene, Toluene degradation occurred, elevated dissolved O2 near source though rapidly utilized Elevated O2 levels near application Dissolved contaminants degraded; oxygen levels dropped after several months

7 14, 24, 26

Microbial Niche Adjustment: Maintaining Oxidizing Subsurface Conditions in Shallow Glacial Aquifer Systems

Microbial Niche Adjustment: Maintaining Reducing Conditions in Shallow Glacial Aquifer Systems

Type of Application Contaminant Mixture

Reductant Principal Observations

In-Situ Grout Injection/Permeable Barrier

PCATCATCE

Proprietary polylactate polymer releases lactate and microorganisms produce hydrogen at 2 to 10 nM level (Hydrogen Releasing Compound - HRC)

At two field sites, hydrogen levels were maintained for at least five months and reductive dechlorination of 50% of dissolved contaminant mass. Anaerobic bacteria counts increased in treated zone.

Excavation Coffer DamEmplacement/Permeable Barrier

Chlorinated Alkenes and Alkanes

Zerovalent iron (Fe0) Elevated H2 levels within permeable barrier by VOC degradation. Iron and calcium carbonates may reduce barrier porosity. HCO3

and SO4=

hasten corrosion and H2 generation. Vinyl chloride may not be degraded.

Microbial Niche Adjustment: Maintaining Reducing Conditions in Shallow Glacial Aquifer Systems

Type of Application

Contaminant Mixture

Reductant Principal Observations

Liquid Injection via wells

TCE, DCE Liquid molasses ~10% in water as electron donor

At several sites very high local elevation in DOC (>1000) and FeII SO4

= and VOC levels were depressed. Degradation indicated to occur

Pressure Grout Addition

MTBE(methyl t-butyl ether)

20% slurry of dried dairy whey in H2O as electron donor (~70% lactose)

Short-term 10-100 fold increase in DOC near barrier immediate O2 depression to <1 mg/L elevated Fe2+. Within 60 days t-butyl alcohol appeared as MTBE breakdown product. O2 depression sustained >400 days. Microbial community underwent major changes.

Microbial Barriers: Biostimulation

1. Growth Processes

Vadose zone: Bioventing

Saturated zone: Biosparging

Electron acceptor amendments

2. Cometabolic processes/Reductive Dechlorination

Vadose zone: Cometabolic bioventing

Saturated zone: Electron donor amendments

Induction of cometabolic reactions

Bioventing-Principle

(i) amendment of unsaturated zone with oxygen or air to stimulate aerobic (heterotrophic) degradative mechanisms---> only pertains to compounds which are aerobically degradable

(e.g. alkanes, aromatic compounds, lesser chlorinated compounds)

(ii) promote volatilization of subsurface contaminants (vapor pressure approx. 1 mm Hg)

---> does not apply to long-chain alkanes (> C10), polycyclic aromatic hydrocarbons (PAH), polysubstituted aromatic compounds

note: oxidation of the high molecular weight compounds using ozonation has been considered as a pretreatment---> not effective for contaminants with vapor pressures less than 1

atm, such as short chain alkanes (> C5), and lesser chlorinated alkyl halides (e.g. vinyl chloride, C2H5Cl). These will volatilize before the onset of biodegradation.

(iii) promote redistribution of the pollutant within soil pores---> restricted applicability in clayey soils

Impact of Physical-Chemical Properties on the Potential for Bioventing

Bioventing – Monitoring Strategies

(i) in situ respiration tests: measurement of oxygen and carbon dioxide consumption

---> estimation of hydrocarbon removal based on chemical oxygen demand (COD) for hexane

note: This results in overestimations as oxygen consumption for respiration is not considered

--> estimations based on CO2 evolution usually less reliable due to multitude of sources (organic, inorganic, mineral)

(ii) total petroleum hydrocarbon (TPH) concentration in soil gas within the sampling well; using partitioning and homogeneity assumptions, this concentration is converted to mg TPH/kg (or m3) soil

(iii) extraction and chromatographic analysis of a statistically representative number of soil samples---> allows for a differentiation in removal efficiency for different

contaminant components within a mixture

Bioventing – Degradation Kinetics

(i) typically calculated using a zero-order rate expression, based on oxygen utilization rates

Kb = -Ko A Do C/100

where Kb: biodegradation rate (mg/kg.d); Ko: oxygen utilization rate (%/d); A: volume of air in soil (L/kg); Do: density of oxygen gas (mg/L); C: ratio of oxygen to hydrocarbon.

(ii) calculated values on the order of 0.02 - 0.1 mg/kg/day obtained depending on contaminant and soil characteristics.

Experimental Configuration for In Situ Respiration Tests

Sample Data Sets for Respiration Tests

In Situ Respiration Tests

Soil Analysis Before and After Venting

Bioventing Efficacy Assessment Based on Respirometric Assays

Isotopic Fractionation During Bioventing Operations

Saturated Zone: Electron Acceptor Amendment – Oxygen (ORC-Regenesis)

manipulation of terminal electron accepting processes to increase the oxidation capacity (OXC) of aquifers with respect to contaminant degradation

ORC Implementation Results

Saturated Zone Amendment

Monitoring strategies: similar to those suggested for evaluating natural biodegradation

processes of hydrocarbons, e.g. transiently accumulating intermediates (alkylbenzenes), aromatic and aliphatic acids, carbon isotope fractionation...

correlations between electron acceptor utilization rates (inorganic species) and hydrocarbon disappearance rates

Degradation kinetics: generally based on zero- or first-order Monod-type rate expressions,

with respect to either contaminant or electron acceptor concentration

depends on which parameter is being monitored no 'generally-accepted' expression has been adopted

Cometabolic Bioventing - Principle

(i) promote aerobic degradation reactions via injection of air or oxygen at sites co-contaminated with petroleum hydrocarbons and chlorinated solvents

(ii) take advantage of hydrocarbon-induced cometabolic enzymes which are nonspecific with respect to alkyl halides, e.g. phenol hydroxylase, toluene o-monooxygenase (TOM), toluene dioxygenase (TOD), methane monooxygenase (MMO).

(iii) favorable site characteristics: Alkyhalide:BTEX ratio of 1:10 or 1:15, high porosity soils, soil gas O2 conc. < 2 mg/L

Cometabolic degradation of TCE and toluene during cometabolic bioventing

Biomineralization during Cometabolic Bioventing

Cometabolic Bioventing: Efficacy Assessment

Monitoring Strategies: similar to bioventing; however,

since CO2 and O2 measurements do not provide information on which fraction is derived from alkyl halide mineralization, relative to the hydrocarbon, calculations of reaction stoichiometry are difficult

extensive soil and soil gas characterization for chlorinated degradation products such as chloroacetates, ...

Biodegradation/Biotransformation Kinetics:

based on Monod-type cometabolic transformation models, however, very limited information is available on unsaturated zone biodegradation kinetics (<< rates in the saturated zone)

currently being evaluated on a field scale, using ratios of alkyl halide-to-total hydrocarbons of 1:10 - 1:30 (based on laboratory experiments)

Accelerated Dechlorination

Principle:

based on the assumption that dechlorination reactions in the subsurface are either electron donor- or electron acceptor (i.e. TEAP)- limited

---> provide a source of reducing equivalents to (i) increase the overall electron flux in the environment, and (ii) stimulate specific types of respiration for dehalogenation reactions

Example: HRC amendment

Induction of Aerobic Cometabolic Reactions

(i) based on induction of specific cometabolic enzymes (see cometabolic bioventing) present in natural microbial communities using natural (e.g. methane) or regulated (e.g. phenol, toluene) substrates.

(ii) promote cometabolic aerobic co-oxidation of alkyl halides with oxygen injection

(iii) usually applied using pulsed injection mechanism to (i) prevent excessive microbial growth at the wellhead, (ii) minimize competition between the primary and cometabolic substrate for the same enzyme, and (iii) increase the zone of influence, as the pulse travels with the groundwater.

Example: Moffett Naval Air Station

Moffett NAS: Methane Oxidation

Moffett NAS: Chloroethene Cometabolism

Efficacy Interpretation

Monitoring Strategies: oxygen and primary

substrate utilization formation of chlorinated

intermediates (e.g. c-DCE-epoxide, …)

Kinetics: cometabolic Monod-type

kinetics, based on competitive inhibition reactions; highly dependent on primary-to-cometabolic substrate ratios and enzyme affinities

substrate disappearance kinetics product formation observed on

the order of hours (alkyl halides) to days (aryl halides)

Microbial Barriers: Bioaugmentation

Bioaugmentation is based on the ecological principle that natural microorganisms have not established a competitive 'niche' (function) for the contaminant. An inoculum has a high rate of success to establish as long as the contaminant is present, and the niche is unoccupied.

Requirement for some type of 'tracking mechanism' to establish that the degradation is due to biodegradative activity associated with the inoculum.

---> development of specific metabolic (e.g. Biolog), genetic (e.g. DNA and RNA probes) or physiological (e.g. FAME) fingerprints for the inoculum which can be recognized against 'autochthonous' microorganisms

---> development of bioluminescence probes; e.g luciferase genes coupled to biodegradative genes. Induction of the biodegradative enzymes by long chain aldehydes and alcohols will trigger luciferase expression:

ATP + NADH luciferase ADP + NAD+ +