Transcript Lecture notes 445 - Simon Fraser University

Objectives:
to formalize the relationship between the properties
of the chemical and its environmental behaviour.
to apply these relationship to develop tools for the
assessment of the fate of the chemical in the
environment.
Environmental Partitioning
What is Partitioning?
Partitioning of DDT
Oil
Concentration in oil:
Co = 10,000 mol/m3
Concentration in water
Water
Cw = 2 mol/m3
Kow = Co/Cw = 5,000
Partitioning of NaCl
Oil
Concentration in oil:
Co = 0.001 mol/m3
Water
Concentration in water:
Cw = 10,000 mol/m3
Kow = Co/Cw = 0.0000001
Partitioning
Partitioning is the phenomenon where a chemical
substance distributes itself based on its ability to
dissolve in the media involved.
K12 = C1/C2 = S1/S2
K12 : Chemical Partition Coefficient between media 1 and 2 (unitless)
C1 : Concentration in medium 1 (mol/m3)
C2 : Concentration in medium 2 (mol/m3)
S1 : Solubility of chemical in medium 1 (mol/m3)
S2 : Solubility of chemical in medium 2 (mol/m3)
Equilibrium
•End result of a partitioning process.
•Concentrations in media reflect the chemical’s solubilities
of the chemical substance in the media involved
•A situation where the concentrations in the two media do
no longer change with time.
•i,A = i,B
i,A : Chemical potential of chemical i in medium A
•fi,A = fi,B
fi,A : Fugacity of chemical in medium A (Pa)
•K12 = C1/C2 = S1/S2
i,B : Chemical potential of chemical i in medium B
fi,B : Fugacity of chemical in medium B (Pa)
What is an “evaluative” environment?
Mass Balance
Total Mass = S Mi = S (Ci.Vi)
Total Mass = CW.VW + CA.VA + CAE.VAE + CBS.VBS +
CSS.VSS + CS.VS + CAB.VAB + CTB.VTB
M : Mass (moles)
Subscripts:
C : Concentration (moles/m3) W : Water
V : Volume
(m3)
K : Partition Coefficient
AB : Aquatic Biota
AE : Aerosol
BS : Bottom Sediments
S : Soil
SS : Suspended Sediments
A : Air
TB : Terrestrial Biota
KAW = CA/CW
KAEW = CAE/CW
KBSW = CBS/CW
KSSW = CSS/CW
KSW = CS/CW
KABW = CAB/CW
KTBW = CTB/CW
Substitute the partition coefficients in the Mass Balance Equation
Total Mass = CW.VW + CA.VA + CAE.VAE + CBS.VBS +
CSS.VSS + CS.VS + CAB.VAB + CTB.VTB
Total Mass = CW. VW + KAW. CW. VA + KAEW. CW. VAE +
KBSW. CW. VBS + KSSW. CW. VSS + KSW. CW. VS + KABW. CW.
VAB + KTBW. CW. VTB
UNKNOWN
Total Mass = CW.(VW + KAW.VA + KAEW.VAE + KBSW.VBS
+ KSSW.VSS + KSW.VS + KABW.VAB + KTBW.VTB)
Application
Chemical Name:
Amount (moles):
Temperature (C):
Molecular Weight (g/mol):
Water Solubility (g/m3):
Vapor Pressure (Pa):
log Kow (no units):
Concentration C (mol/m3) = f.Z
Mass (moles) = C.V
% Mass
dioxin
1
25
322
1.93E-05
2.00E-07
6.8
Soil
Sediment
Biota
S.Sedim.
Water
Air
1.15E-05
0.515514
0.515514
2.29E-05
0.481146
0.481146
4.47E-05
0.000313
0.000313
2.29E-05
0.000802
0.000802
1.48E-10
0.001033
0.001033
1.99E-13
0.001192
0.001192
Total
1
What are the merits & limitations of the Environmental
Partitioning Approach for Evaluative Environments?
Environmental Partitioning in Evaluative Environments
Merits:
•Provides assessments of the environmental distribution
of chemicals based on chemical properties
•Can be used for comparing/ranking chemicals
Environmental Partitioning in Evaluative
Environments
Limitations:
•Closed System
•Describes an end-situation, achieved after a long
time when equilibrium is reached.
•Absolute values of concentrations are irrelevant
•Well mixed environment
•Assumes chemical losses (through transformation
and transport) do not occur
Environmental Partitioning in Evaluative
Environments
Limitations:
•Closed System
•Describes an end-situation, achieved after a long
time when equilibrium is reached.
•Absolute values of concentrations are irrelevant
•Well mixed environment
•Assumes chemical losses (through transformation
and transport) do not occur
What is fugacity?
What is fugacity?
then
Glass of Water
What is fugacity?
Glass of Water
What is fugacity?
Glass of Water
Glass of Water
What is fugacity?
Equilibrium:
Pair
Pair = Pwater
Pwater
fair = fwater
Pair : Pressure of water in air
Pwater : Pressure of water in liquid water
fair : Fugacity of water in air
fwater : Fugacity of water in liquid water
What is fugacity?
Equilibrium:
Pair
Pair = Pwater
Pwater
fair = fwater
Pair : Pressure of benzene in air
Pwater : Pressure of benzene in liquid water
fair : Fugacity ofbenzene in air
fwater : Fugacity of benzene in liquid water
Measuring fugacity
fa
fm
fa
fm
POG cylinder (68 mm o.d., 64 mm i.d., 70 mm tall)
coated with EVA solution
fe
fm
gap between upper and lower
stainless steel bowls to promote air
circulation
5/16 “ steel support rod
eight, 3/16 “ air circulation holes
Fugacity
•Escaping Tendency of the chemical
•The partial pressure that the chemical substance exerts
•Referred to as f
•Measured in units of pressure (Pa)
•Applies to all media
•Expresses chemical potential or activity in a
measurable quantity
What is the Relationship
between?
Fugacity
&
Concentration
Relationship between
Fugacity & Concentration:
C = f.Z
C : Concentration (mol/m3)
f : fugacity (Pa)
Z : fugacity Capacity (mol/Pa.m3)
What is Z?
•Z is the number of moles of a substance that you can
add to 1m3 of a phase or medium in order to raise the
fugacity of the chemical in that phase by 1 Pa.
•Expresses the ability of a medium to “dissolve” a
chemical substance
•The ratio of Z values for a chemical substance is
equivalent to the chemical’s partition coefficient K.
KAW = CA/CW
fA.ZA/fW.ZW =
ZA/ZW
Since fA = fZ
ANALOGY :
Fugacity Capacity(mol/m3.Pa)
amount of substance (in moles) that you can add to 1m3 of a
phase or medium in order to raise the fugacity of the
chemical in that phase by 1 Pa.
Heat Capacity (J/m3.K)
amount of heat (in Joules) that you can add to 1m3 of a
phase or medium in order to raise the temperature of the
medium by 1 degree Kelvin.
Mass Balance
Total Mass = Sum (Ci.Vi)
Total Mass = Sum (fi.Zi.Vi)
At Equilibrium : fi are equal
Total Mass = M = f.Sum(Zi.Vi)
f = M/Sum (Zi.Vi)
C : Concentration (mol/m3)
f : fugacity (Pa)
Z : fugacity Capacity (mol/Pa.m3)
Recipes for Z
Air:
Ideal Gas Law : p.V = n.R.T
p = (n/V).R.T
p = C.R.T
f = C.R.T.
C/f = 1/RT
Z = (C/f) = 1/RT
Recipes for Z
Water:
fW= fA
CW/ZW = CA/ZA
ZW = CW.ZA /CA
ZW = CW/R.T.CA
ZW = 1/KAWR.T
H = KAW R.T
ZW = 1/H
Recipes for Z
Particulate:
fW= fS
Phases, i.e.
CW/ZW = CS/ZS
Soil,
ZS = CS.ZW /CW
Sediment
ZS = KSW.ZW
Susp. Sed.
ZS = KSW/H
ZS = K*SW.dS/H
ZS = fOC.KOC.dS/H
ZS = fOC.0.41.KOW.dS/H
Recipes for Z
Biological
fW= fB
Phases
CW/ZW = CB/ZB
ZB = CB.ZW /CW
ZB = KBW.ZW
ZB = KBW/H
ZB = K*BW.dB/H
ZB = LB.KOW.dB/H