Partitioning of VOCs:Why do we care?

Partitioning of VOCs:Why do we care?

Determines how best to treat a site vapor extraction pump and treat remove contaminated soil

Determines feasibility of phase changes gas stripping tower sorption to activated carbon from liquid or gas

phase

Determines how best to treat a site vapor extraction pump and treat remove contaminated soil

Determines feasibility of phase changes gas stripping tower sorption to activated carbon from liquid or gas

phase

FRTRFRTR

Gas/Liquid partitioningGas/Liquid partitioning

PartitioningPartitioning

gas-liquid Henry’s law constant

gas-solid distribution coefficient

liquid-solid distribution coefficient

gas-liquid Henry’s law constant

gas-solid distribution coefficient

liquid-solid distribution coefficient KL

S CL

KLS

CL

KGS

CG

KGS

CG

HLG

CGCL

HLG

CGCL

= mass of solute sorbed per mass of solid

Liquid volumeLiquid volumeGas volumeGas volume

Gas volumeGas volumeSolid massSolid mass

Gas-Solid PartitioningGas-Solid Partitioning

soilsoilorganiccarbonorganiccarbon

VOCVOC

molecules in gas phasemolecules in gas phase

moleculessorbed to

soil

moleculessorbed to

soil

linearlinear inhibitioninhibition condensationcondensation

Linearity of PartitioningLinearity of Partitioning

Linear at low VOC concentrations The fraction of VOC that sorbs to soil is

independent of the concentration of VOC in the gas or liquid phase

Interference from previously sorbed VOC molecules is small

(a reasonable assumption at the concentrations we will be using in the laboratory)

Linear at low VOC concentrations The fraction of VOC that sorbs to soil is

independent of the concentration of VOC in the gas or liquid phase

Interference from previously sorbed VOC molecules is small

(a reasonable assumption at the concentrations we will be using in the laboratory)

Non-linearity of PartitioningNon-linearity of Partitioning

Non-linear at high VOC concentrations partition coef. = f(VOC concentration) VOCs prefer sorption to soil over sorbing to

previously sorbed VOC molecules (_________)

Condensation occurs at very high VOC concentrations

Non-linear at high VOC concentrations partition coef. = f(VOC concentration) VOCs prefer sorption to soil over sorbing to

previously sorbed VOC molecules (_________)

Condensation occurs at very high VOC concentrations

inhibitioninhibition

gas phase

gas phase

Partitioning during RemediationPartitioning during Remediation

Equilibrium conditions? assume the kinetics of partitioning are fast

relative to the rate that concentrations change in any of the phases

Function of temperature increase temperature - compounds become

more volatile -increased preference for ____ _______

Equilibrium conditions? assume the kinetics of partitioning are fast

relative to the rate that concentrations change in any of the phases

Function of temperature increase temperature - compounds become

more volatile -increased preference for ____ _______

organic carbonorganic carbon

Partitioning during RemediationPartitioning during Remediation

Reversible reactions if the concentration in one phase decreases the

concentration in the other phase will decrease to reach a new equilibrium

Soil type is an important variable! VOCs prefer sorption to sites containing

_______ ________ over sorption to inorganic soil surfaces

Reversible reactions if the concentration in one phase decreases the

concentration in the other phase will decrease to reach a new equilibrium

Soil type is an important variable! VOCs prefer sorption to sites containing

_______ ________ over sorption to inorganic soil surfaces

Remediation of VOC Contaminated Site

Removal techniques pump and treat water

contaminant sticks to soil or is in gas phase pump and treat air (vapor extraction)

contaminant sticks to soil or is in aqueous phase

Effect of partitioning on transport retardation factor R = velocity of water/velocity of pollutant, but

pollutant when in fluid moves at ______ ________ so R = 1/fraction of pollutant in fluid.

2

2

x

CE

x

Cu

t

C

2

2

xCE

xCu

tC

R R

fluid velocity

Cause slow removal

Techniques to Measure Partitioning

Techniques to Measure Partitioning

Batch Air Stripping (BAS) purge dissolved VOC from water using an inert

gas The concentration of VOC in the gas stream

should be related to the partitioning between gas and liquid

Batch Air Stripping (BAS) purge dissolved VOC from water using an inert

gas The concentration of VOC in the gas stream

should be related to the partitioning between gas and liquid

Completely mixedCompletely mixedGas-Liquid equilibriumGas-Liquid equilibrium

Batch Air Stripping: Experimental SetupBatch Air Stripping: Experimental Setup

Water jacketWater jacket

ExhaustExhaust

Aqueous Sample with VOCAqueous Sample with VOC

SaturatorSaturator

Air Supply

Air Supply

To Temperature Control Bath

To Temperature Control Bath

To FID detectorTo FID detector

6-Way Sampling Valve6-Way Sampling Valve

Assumptions...•________________•____________________

Assumptions...•________________•____________________

Equilibrium Partitioning in Closed Systems (EPICS)

Equilibrium Partitioning in Closed Systems (EPICS)

Load two bottles containing different liquid volumes with the same mass of VOC

Shake the bottles to achieve equilibrium between gas and liquid phase

Sample the headspace to measure the concentration of VOC in the gas phase

Use mass balance to determine the amount of VOC in the liquid phase

Load two bottles containing different liquid volumes with the same mass of VOC

Shake the bottles to achieve equilibrium between gas and liquid phase

Sample the headspace to measure the concentration of VOC in the gas phase

Use mass balance to determine the amount of VOC in the liquid phase

Equilibrium Partitioning in Closed Systems (EPICS)

Equilibrium Partitioning in Closed Systems (EPICS)

water

•Add same mass of VOC to both bottles.•Equilibrate by shaking.•Measure concentration of VOC in gas phase.

•Add same mass of VOC to both bottles.•Equilibrate by shaking.•Measure concentration of VOC in gas phase.

Measure M by taking sample from control bottle to help account for losses to bottle surfaces.

Measure M by taking sample from control bottle to help account for losses to bottle surfaces.

mass VOC addedmass VOC added volume gasvolume gas

volume liquidvolume liquid

Control

M VGCG

VLCL

M V C + V CL L G G

HCCL

G G

L

CM V C

VLG G

L

EPICS extended to gas-solid partitioning

EPICS extended to gas-solid partitioning

soil

KGS

CG

KGS

CG

M VGCG

MS

M VGCG

MS

M VGCG +MS M VGCG +MS

VGCG

MS

M

Identical analysis to gas-liquid partitioning

EPICS extended to liquid-solid partitioning

EPICS extended to liquid-solid partitioning

water

M VGCG VLCL MS M VGCG VLCL MS

VGCG VGCG

VLCL VLCL

VGCG VGCG

VLCL VLCL

M VGCG +VLCL M VGCG +VLCL

HL

G CG

CL HL

G CG

CL

M VGCG +VL

CG

HLG

M VGCG +VL

CG

HLG

M VGCG VLCL

MS

M VGCG VLCL

MS

M VGCG VL

CG

HLG

MS

M VGCG VL

CG

HLG

MS

KL

S CL

KLS

CL

SM SM

HCCL

G G

L

EPICS Error AnalysisEPICS Error Analysis

water water

low solubilit

y

low solubilit

y

high solubilit

y

high solubilit

y

Assume 10% error in measuring gas concentrations

What are the maximum and minimum values of mass in liquid phase?

Assume 10% error in measuring gas concentrations

What are the maximum and minimum values of mass in liquid phase?

(10 ± 1) – (9 ± 0.9)

11 – 8.1 = 2.9

9 – 9.9 = -0.9

(10 ± 1) –( 1 ± 0.1)

11 – 0.9 = 10.1

9 – 1.1 = 7.9

EPICS Low solubility Limitations

EPICS Low solubility Limitations

Henry’s Law constant determination is inaccurate for compounds of low solubility

Method Detection Limit... The fraction of VOC in the liquid phase must

be greater than the coefficient of variation of the mass of VOC added

The fraction in gas phase must be less than 1 minus the coefficient of variation

Henry’s Law constant determination is inaccurate for compounds of low solubility

Method Detection Limit... The fraction of VOC in the liquid phase must

be greater than the coefficient of variation of the mass of VOC added

The fraction in gas phase must be less than 1 minus the coefficient of variation

EPICS LimitationsEPICS Limitations

High solubility limitations The fraction of VOC in the liquid phase can be

accurately determined as long as sufficient VOC is added so that the gas phase concentrations are greater than the GC detection limits

Similar conclusions for gas-solid partitioning

High solubility limitations The fraction of VOC in the liquid phase can be

accurately determined as long as sufficient VOC is added so that the gas phase concentrations are greater than the GC detection limits

Similar conclusions for gas-solid partitioning

Liquid/Solid partitioning: Error Analysis

Liquid/Solid partitioning: Error Analysis

fraction in gas phase close to 1(low solubility)High Henry’s law constant

fraction in gas phase close to 1(low solubility)High Henry’s law constant

fraction in liquid phase close to 1(very soluble)Low Henry’s law constant

fraction in liquid phase close to 1(very soluble)Low Henry’s law constantfs= coefficient of variationLousy data!

gasgas

liquidliquid

liquidliquid

Expectations based on EPICS limitations

Expectations based on EPICS limitations

OctaneOctane

AcetoneAcetone

TolueneToluene

123123

0.00650.0065

0.2750.275

fs too small?fs too small?

goodgood

goodgood

fs too smallfs too small

fs too smallfs too small

goodgood

HLG

HLG

KLS

KLS

KGS

KGS

soil

Where’sVOC?

Where’sVOC?

EPICS ProcedureEPICS Procedure

airair

air + waterair + water

air + water + soilair + water + soil

air + soilair + soil

OctaneOctane

AcetoneAcetone

TolueneToluene

Test vials each loaded with 3 VOCs

Test vials each loaded with 3 VOCsVOC Source vialsVOC Source vials

Batch Air Stripping ExpectationsBatch Air Stripping Expectations

dCdt

C QV

L G G

L

dC

dtC QV

L G G

L

CCHL

G

LG

dCdt

H C QV

G LG

G G

L

dCC

H QV

dtG

GC

C

LG

G

L

t

G

G

0 0

XZY

XZY

lnCC

H Q tV

G

G

LG

G

L0

FHG

IKJ

C CG G

H Q t

VeLG

G

L

0

CG=f(CL)CG=f(CL)

Identify the important parameters…Identify the important parameters…