Permeable Reactive BarriersShu-Chi Chang, Ph.D., P.E., P.A.Assistant Professor1 and Division Chief21Department of Environmental Engineering2Division of Occupational Safety and Health, Center for Environmental Protection and Occupational Safety and HealthNational Chung Hsing University

May 9, 2007

Permeable Reactive BarriersTechnology BackgroundTypes of BarriersBarrier EmplacementBarrier DesignReactive Media SelectionZero-valent ironAquifer oxidationAquifer reductionMicrobial Barriers (Biocurtains)BiostimulationBioaugmentationConclusions

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

Zerovalent IronReaction 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 dechlorinationRequired 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 bof reactive cell:b = Vx tw,corr.Note: Vx,reactive cell > Vx,groundwaterHydraulic conductivity of reactive medium:K = VL/AthWhere 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

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

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

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

Microbial Barriers: Biostimulation

1. Growth ProcessesVadose zone: BioventingSaturated zone: BiospargingElectron acceptor amendments2. Cometabolic processes/Reductive DechlorinationVadose zone: Cometabolic bioventingSaturated zone:Electron donor amendmentsInduction 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 compoundsnote: 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 hexanenote: 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 ratesKb = -Ko A Do C/100where 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 AmendmentMonitoring 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 ratesDegradation kinetics:generally based on zero- or first-order Monod-type rate expressions, with respect to either contaminant or electron acceptor concentrationdepends on which parameter is being monitoredno '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 AssessmentMonitoring 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 (

Accelerated DechlorinationPrinciple: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 InterpretationMonitoring Strategies:oxygen and primary substrate utilizationformation 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 affinitiessubstrate disappearance kineticsproduct formation observed on the order of hours (alkyl halides) to days (aryl halides)

Microbial Barriers: BioaugmentationBioaugmentation 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