1. Permeable Reactive Barriers
Shu-Chi Chang, Ph.D., P.E., P.A.
Assistant Professor1 and Division Chief2
1Department of Environmental Engineering
2Division of Occupational Safety and Health,
Center for Environmental Protection and Occupational Safety and Health
National Chung Hsing University
May 9, 2007

2. Permeable Reactive Barriers
Technology Background
Types of Barriers
Barrier Emplacement
Barrier Design
Reactive Media Selection
Zero-valent iron
Aquifer oxidation
Aquifer reduction
Microbial Barriers (Biocurtains)
Biostimulation
Bioaugmentation
Conclusions

4. Permeable Barrier Configuration

5. Permeable Barrier Configuration

6. Funnel-and-Gate with Single and Serial Reactive Medium

7. Installation of Funnel-and-Gate Barrier: funnel walls: sheet-piling; gates: pea gravel and iron zones
GW flow

8. Deep Soil Mixing
Use of augers in series, which simultaneously mixes up soil (permeable barrier) and injects bentonite (impermeable wall)
Depth to 40 m.

9. Zerovalent Iron Reaction stoichiometry 3Feo  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!

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

11. 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•h
Where V volume discharge, L the length of the reactive medium, A the cross-sectional area, and h the hydraulic head difference

12. Current Status – Chlorinated Solvents Sites

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

14. Principal Observations
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
PCA
TCA
TCE
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 Dam
Emplacement/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.

15. Principal Observations
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 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.

16. 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

17. 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

18. Impact of Physical-Chemical Properties on the Potential for Bioventing

19. 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

20. 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 mg/kg/day obtained depending on contaminant and soil characteristics.

21. Experimental Configuration for In Situ Respiration Tests

22. Sample Data Sets for Respiration Tests

23. In Situ Respiration Tests

24. Soil Analysis Before and After Venting

25. Bioventing Efficacy Assessment Based on Respirometric Assays

26. Isotopic Fractionation During Bioventing Operations

27. 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

28. ORC Implementation Results

29. 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

30. 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

31. Cometabolic degradation of TCE and toluene during cometabolic bioventing

32. Biomineralization during Cometabolic Bioventing

33. 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)

34. 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

35. Example: HRC amendment

36. 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.

37. Example: Moffett Naval Air Station

38. Moffett NAS: Methane Oxidation

39. Moffett NAS: Chloroethene Cometabolism

40. 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)

41. 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+ + l