Biodegradation Processes for Chlorinated Solvents

Dehalogenation Stripping halogens (generally Chlorine) from an organic molecule Generally an anaerobic process, and is often referred to as reductive dechlorination R–Cl + 2e– + H+ ––> R–H + Cl– Can occur via Dehalorespiration (anaerobic) Cometabolism (aerobic)

Dehalorespiration Certain chlorinated organics can serve as a terminal electron acceptor, rather than as a donor Confirmed only for chlorinated ethenes Rapid, compared to cometabolism High percentage of electron donor goes toward dechlorination Dehalorespiring bacteria depend on hydrogen-producing bacteria to produce H2, which is the preferred primary substrate

Reductive Dechlorination of Chlorinated Ethenes CCl =CCl PCE CHCl=CCl TCE CHCl=CHCl 1,2 DCE CH =CHCl VC H 2 2 H 2 H H 2 Ethylene CH = CH CO Carbon dioxide 2 2 2

Added Danger Dechlorination of PCE and TCE should be encouraged, but monitored closely The dechlorination products of PCE are more hazardous than the parent compound DCE is 50 times more hazardous than TCE Vinyl Chloride is a known carcinogen

Cometabolism Fortuitous transformation of a compound by a microbe relying on some other primary substrate Generally a slow process - Chlorinated solvents don’t provide much energy to the microbe Most oxidation is of primary substrate, with only a few percent of the electron donor consumption going toward dechlorination of the contaminant Not all chlorinated solvents susceptible to cometabolism (e.g., PCE and carbon tetrachloride)

Cometabolic Transformations of Chlorinated Aliphatic Hydrocarbons (CAHs) 2 - MMO NADH, O Secondary Reaction O CH 4 NADH, O CO , H O Primary Reaction CCl =CHCl Cl C CHCl CO , Cl ,H O

Classification System for Chlorinated Solvent Plumes Type 1 : Anaerobic due to anthropogenic carbon Type 2 : Anaerobic due to naturally occurring carbon Type 3 : Aerobic due to no fermentation substrates

Dechlorination Zones

Will it work for Chlorinated Solvents? Natural Attenuation Will it work for Chlorinated Solvents?

Natural Reductive Dechlorination Natural dechlorination of solvents in aquifers with rich organic load and low redox potential Not frequently found Many chlorinated solvent plumes located in low organic load, aerobic aquifers

to be broadly used for some compounds, especially chlorinated solvents Natural Attenuation Not fast enough Not complete enough Not frequent enough to be broadly used for some compounds, especially chlorinated solvents

Enhanced Bioattenuation Engineered system to increase the intrinsic biodegradation rate to reduce contaminant mass Usually addition of electron acceptors (oxygen, nitrate, sulfate) or electron donors (organic carbon, hydrogen) Could involve bioaugmentation - adding the catalyst for bioattenuation

Enhanced Bioattenuation of Chlorinated Solvents Inadequate electron donor concentrations Determine methods of adding electron donors

In Situ Biodegradation of Chlorinated Solvents Electron Donor Addition Nutrient (if necessary) Injection Well To: • Treatment • Treatment / Recycle • Recycle DNAPL Recovery In Situ Biodegradation Zone

Enhanced Bioattenuation Petroleum Chlorinated Technology Hydrocarbons Solvents (e– acceptor) (e– donor) Liquid Delivery Oxygen Benzoate Nitrate Lactate Sulfate Molasses Carbohydrates Biosparge Air (oxygen) Ammonia Hydrogen Propane Slow-release Oxygen Hydrogen (ORC) (HRC)

Selective Enhancement of Reductive Dechlorination Competition for available H2 in subsurface Dechlorinators can utilize H2 at lower concentrations than methanogens or sulfate-reducers Addition of more complex substrates that can only be fermented at low H2 partial pressures may provide competitive advantage to dechlorinators

Electron Donors Alcohols and acids Almost any common fermentable compound Hydrogen apparently universal electron donor, but no universal substrate Laboratory or small-scale field studies required to determine if particular substrate will support dechlorination at particular site

Electron Donors Acetate Hydrogen - Pickle liquor Acetic acid biochemical Polylactate esters Benzoate electrochemical Propionate Butyrate gas sparge Propionic acid Cheese whey Humic acids - Sucrose Chicken manure naturally occurring Surfactants - Corn steep liquor Isopropanol Terigitol5-S-12 Ethanol Lactate Witconol 2722 Glucose Lactic acid Tetraalkoxsilanes Hydrocarbon Methanol Wastewater contaminants Molasses Yeast extract Mulch

Electron Donor Demand Theoretical demand for 1 g PCE = 0.4 g COD Must use many times more substrate due to competition for electron donors Minimum of 60 mg/L TOC to support dechlorination beyond DCE in microcosm studies in Victoria, TX soils (Lee et al., 1997)

Electron Donor Technology in Field-Scale Pilots Electron Electron Site Reference Donor Acceptor Benzoate CO Victoria, TX Beeman et al 1994 Beeman 1994 Acetate NO Moffett Air Field, CA Semprini et al 1992 Schoolcraft, MI Dybas et al 1997 Yeast Extract SO /CO Niagara Falls, NY Buchanan et al 1995 Methanol / ? FAA facility, OK Christopher et al 1997 Sucrose Tergito15-S-12 SO Corpus Christie, TX Lee et al 1995 Witconol 2722 Methanol ? Breda, Netherlands Spuij et al 1997 2 3 4 2 4

Electron Donor Technology in Field-Scale Pilots Electron Electron Site Reference Donor Acceptor Lactic acid ? Watertown, MA ABB Environmental Lactate Fe Dover AFB, DE Grindstaff 1998 Benzoate / Lactate / ? Pinellas, Fl US DOE 1998 Methanol Molasses ? Eastern PA Nyer et al 1998 Molasses ? Williamsport, PA Nyer & Suthersan 1996 3+

Engineered Delivery Systems Air injection into vadose zone - venting / bioventing Air injection into ground water - air sparging / biosparging Gas, other than air, injection into ground water - ammonia, hydrogen, propane Slow release into ground water - ORC, HRC Liquid addition - infiltration or injection wells, surfactant / cosolvent flush Recirculation - extraction / reinjection systems, UVB wells, pump and treat

Promotes in situ biodegradation - Minimize hydrogen gas entering Hydrogen Sparging Hydrogen gas Blower Vapor Treatment Tiny Bubbles DNAPL SVE Well Promotes in situ biodegradation - Minimize hydrogen gas entering unsaturated zone

Hydrogen Releasing Compound (HRC ) ® A food grade polylactate ester slowly degraded to lactic acid Lactic acid metabolized to acetic acid with production of hydrogen Hydrogen drives reductive dechlorination

Hydrogen Releasing Compound (HRC ) ® A moderately flowable, injectable material Facilitates passive barrier designs Slow hydrolysis rate of lactic acid from ester keeps hydrogen concentration low, may favor reductive dechlorination over methanogenesis

Hydrogen Releasing Compound (HRC ) ®

HRC Application Delivery Systems - bore-hole backfill or injection via direct-push technologies Designs for Barriers and Source Treatment 1. Upgradient 1 2 3 4 barrier 2. Series of barriers 3. Downgradient 4. “Grid” of HRC injection points

Substrates for Bioattenuation of CAHs (Lee et al, 1997) “any substrate that will yield hydrogen under fermentative and/or methanogenic conditions will ... support dechlorination of PCE to DCE if the microbial population is capable of ... the dechlorination reaction” “biotransformation of DCE to VC and ethene ... not ... universal and may require specific substrates or enrichment strategies”

Substrates for Bioattenuation of CAHs (Lee et al, 1998) “No substrate that reliably supports complete dechlorination at all sites has been identified to date.”

Limitations for Application of Bioattenuation Technologies Delivery of materials to the subsurface (contact) Bioavailability of the contaminants Toxicity of contaminants Threshold substrate concentration

Contact in the Subsurface 10 1 0.1 Methane Oxygen Dissolution Sorption / Desorption

Toxicity of Trichloroethylene Air or water in contact with oily phase may exceed toxic limit for microorganisms TCE: > 6 mg/L in water (30% reduction = 1.8 mg/L; Moffett field) > 2 mg/L in air

Maximum Solvent Concentrations for Reductive Dechlorination Solvent Concentration Reference (mg/L) PCE 50 Smatlak et al 1996 cis-DCE 8.0 Haston et al 1994 VC 1.9 - 3.8 DiStefano et al 1991 DCM 66 Freedman & Gossett 1991 TCA 100 Galli & McCarty 1989

What We Don’t Know Should you use a slow, controlled release or large/small periodic dosing of electron donor? Is it redox reduction or electron donor addition that triggers reductive dechlorination? Under field conditions, does competion for hydrogen exist between dechlorinators, methanogens, and sulfate reducers? Does it matter?

Prognosis? Electron Donor Technology for engineered bioattenuation of CAHs will equal the impact of Electron Acceptor Technology on bioremediation of HCs