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Soil Vapor Extraction Limitations and Enhancements

LeeAnn Racz

AgE 558

Semester Project

April 2001

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Outline

• Theory– SVE Mass Removal Performance– Gas Extraction Methods

• Application– Limitations to SVE– Enhancements to SVE

• Remaining Uncertainties/Challenges

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Theory

• Removes soil gas under vacuum from soil matrix

• Mass transfers from aqueous and sorbed phases to gas phase in order to re-establish equilibrium

• Model assumes mass transfer between gas and solid phases occurs via continuous film of water (wetting fluid)

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Water/Solid Partitioning (Desorption)

Advecting Air

Air/Water Partitioning (Volatilization)

Dissolved Contaminant Sorbed Contaminant

Soil Moisture

Mass Transfer Processes in the Vadose Zone (Armstrong et al. 1994)

Soil Grain

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SVE Mass Removal Performance

• First Stage– Removes pure product– System in equilibrium– High off-gas concentrations– Relatively short duration– Henry’s law dominates– High organic content can have partitioning

between liquid and solid phases in equilibrium

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SVE Mass Removal Performance

• Henry’s lawPi = HiCi

or

Hi = CwCa

where Pi = partial pressure in gas phase

Ci and Cw = concentration in aqueous phase

Hi = Henry’s law constant for phase partitioning of i

Ca = concentration in gas phase

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SVE Mass Removal Performance• Partitioning between liquid and solid phases

– Expressed as linear Freundlich isotherm– Valid for soils with >0.1% organic carbon

Kd = Cs/Cw

and

Kd = foc/Koc

where Kd = distribution coefficient

Cs = concentration in sorbed phase

foc = mass fraction of organic carbon

Koc = organic carbon partitioning coefficient

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SVE Mass Removal Performance

• Second Stage– Transition from first to third stages– System is in non-equilibrium– Quickly declining mass removal rates

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SVE Mass Removal Performance

• Third Stage– Also in non-equilibrium– Partitioning between soil gas, soil moisture and

soil solids limit the mass transfer rate to mobile gas pathways

– Non-zero asymptote

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SVE Mass Removal Performance• Non-equilibrium mass transfer (second and third

stages)– Rate limiting factors in mass transfer process

– Modeled as first-order kinetic mass transfer relationships

• Diffusive mass transfer between air and water driven by concentration gradient between average concentration in water phase and equilibrium concentration at water/air interface

• Kinetically limited desorption from soil grains to water phase

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Gas Extraction Methods• Active

– Involves introducing fresh air into soil– Apply vacuum by mechanical means to draw soil gas

from soil matrix

• Passive– Screened well installed– Open to atmosphere– Gas flows from soil matrix out through open well when

subsurface gas pressure greater than barometric pressure

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Limitations to SVE

• Well suited for:– Vapor removal from moist sand and granular

soils– Soils with increased gas permeability– Removing VOCs and LNAPLs

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Limitations to SVE

• Not so well suited for:– Removing contaminants from capillary fringe

• Low relative permeability to soil gas flow makes diffusion the rate-limiting process

– Sites with high water tables– Mixed contamination

• Includes nonvolatile compounds and DNAPLs

– Sites without sufficient moisture• If too dry, increases sorption capacity of soil

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Enhancements to SVE

• Synergistic Effects– Pump and treat ground water

• Leaves dewatered area treatable by SVE

– SVE vacuum produces air flow• Enhances effects of aerobic microbial activity

– Air sparging• Injects air into ground water and extracts volatile

portion to unsaturated zone

• Extracts gas and controls vapor migration

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Enhancements to SVE• Soil Heating

– Increases volatility of contaminant to gas phase– Reduces mass transfer limitations in non-equilibrium

conditions– Useful for removing chlorinated compounds and

compounds with higher boiling points– Methods

• Hot air • Electrical heating• Microwave energy

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Enhancements to SVE

• Pulse Pumping– Theory: turn off vacuum at tailed portion of

effluent curve and allow air phase concentrations to recover, then reapply vacuum

– Intended to give lower energy costs and effluent treatment costs

– However, slow but continuous pumping gives best performance

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Enhancements to SVE• Passive SVE

– Cap well with check valve to prevent air from flowing into subsurface through well

• Lightweight ball in conical seat

• Solenoid valve

– Install surface cover around well • Prevents short-circuiting adjacent to well to increase horizontal

flow to well

• Prevents clean air from entering subsurface diluting contaminant concentrations

• Increases differential between surface and subsurface gas pressures

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Remaining Uncertainties/Challenges

• Difficult to use in removing DNAPLs and other recalcitrant compounds

• Difficult to use in certain soil types– Low porosity– High moisture content– Capillary fringe

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Remaining Uncertainties/Challenges

• Combine with other methods for synergistic effects

• Nonzero asymptotic characteristic of nonequilibrium

• Better measuring techniques to obtain data for better designs– Uncertainties in heterogeneous media– Otherwise, rely on further refined curve-fitted

models

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E-mail:

leeann.racz@osan.af.mil