Transcript Fugacity-based environmental models

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Fugacity-based environmental models
Level 1--the equilibrium distribution of a fixed quantity of conserved chemical, in a
closed environment at equilibrium, with no degrading reactions, no advective
processes, and no intermedia transport processes (eg. no wet deposition, or
sedimentation).
Level 2-- describes a situation in which a chemical is continuously discharged at a
constant rate and achieves a steady-state and equilibrium condition at which the input
and output rates are equal. Degrading reactions and advective processes are the loss
or output processes treated. Intermedia transport processes (eg. no wet deposition, or
sedimentation) are not quantified.
Level 3--A Level III simulation describes a situation which is one step more complex
and realistic than the Level II model. Like the Level II model, chemical is
continuously discharged at a constant rate and achieves a steady state condition in
which input and output rates are equal. The loss processes are degrading reactions
and advection. Unlike the Level II model, equilibrium between media is not assumed
and, in general, each medium is at a different fugacity. A mass balance applies not
only to the system as a whole, but to each compartment. Rates of intermedia transport
are calculated using D values which contain information on mass transfer
coefficients, areas, deposition and resuspension rates, diffusion rates, and soil runoff
rates. It is now essential to define inputs to each medium separately, whereas in Level
II only the total input rate was requested.
Level 1 program
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Physical-chemical properties are used to quantify a chemical's behavior in an
evaluative environment. Three types of chemicals are treated in this model:
chemicals that partition into all media (Type 1), involatile chemicals (Type 2),
and chemicals with zero, or near-zero, solubility (Type 3). The Level I
Program assumes a simple, evaluative, closed environment with user-defined
volumes and densities for the following homogeneous environmental media
(or compartments): air, water, soil, sediment, suspended sediment, fish and
aerosols.
This model is useful for establishing the general features of a new or existing
chemical's behaviour. A Level I calculation gives the general impression of the
likely media into which a chemical will tend to partition and an indication of
relative concentrations in each medium. The results of changes in chemical
and environmental properties may be explored.
LEVEL I
Chemical:
LEGEND
EQUILIBRIUM
7
Air
Aerosols
1
5
Soil
3
Water
Suspended Sediment
2
Sediment
4
Fish
6
Inputs and outputs
The required input data are:
Chemical Properties:
chemical name
molecular mass
data temperature
Type 1 chemicals
- water solubility
- vapor pressure
- log Kow
- melting point
Type 2 and 3 chemicals
- partition coefficients
Environmental Properties:
volumes for all 7 media
densities for all 7 media
organic carbon content (soil,
sediment & suspended sediment
only)
fish lipid content (Type I chemicals
only)
Emissions:
chemical amount
Model Output:
partition coefficients (Type 1)
Z values
fugacity of the system
concentrations and amounts for
each compartment
a summary diagram
Z = “fugacity capacity”
C=Zf
concentration = Z  fugacity
units: mol/m3 = mol/m3Pa  Pa
like Henry’s law, Z could also be dimensionless
At eqbm, f is equal in all phases:
f1 = f2= C1/Z1 = C2/Z2 so K12 = C1/C2 or Z2/Z1
all these parameters (C, Z, K) depend on T, P, properties of
solute/solvent, etc.
Z is like C (heat capacity)
• At equilibrium, all phases will have same
fugacity (temperature).
• C (heat capacity) = amount of heat (energy
in J/unit volume)/Temperature
• Z = amount of chemical (moles per unit
volume)/fugacity
Types of chemicals
• Type 1: chemicals that partition into all media
– input VP, solubility, program calculates Kh
– Z factor of air is one (1/RT to convert units)
• Type 2: involatile chemicals
– VP not available, so input everything as partition coefficients
– Z factor of water is one
• Type 3: chemicals with zero, or near-zero, solubility
– Solubility not available, so input everything as K’s
– Z factor of air is one (1/RT to convert units)
All K’s must be internally consistent if everything is at
equilibrium
KH = PoL/Csatw
Kow = Csato/Csatw
Koa = Csato/PoL
Air
A gas is a gas is a gas
T, P
Koa
KH
Octanol
Po L
Water
Fresh, salt, ground, pore
T, salinity
Csat
w
NOM, biological lipids,
other solvents
Kow
Pure Phase
(l) or (s)
Ideal behavior
Csato
INPUTS:
MW (g/mol)
Data T (C)
Log Kow
water sol. (g/m3)
VP (Pa)
Melting pt. (C)
CALCULATED:
Kh (Pa m3/mol)
Sub-cooled liquid VP
Fugacity (Pa)
Total VZ Products
Amt of chem (mol)
pyrene
PCB 52
202.3
292
25
25
5.18
6.10
0.132
0.03
0.00060 0.00490
156
87
0.92
0.0119
47.7
0.0201
1.64E-08 7.09E-08
3.01E+13 4.83E+12
4.94E+05 3.42E+05
% in each compartment
air
0.13
water
0.72
soil
97
sedmt.
2.15
susp. Sedmt.
0.067
fish
0.0055
aerosol
0.0014
0.84
0.087
97
2.15
0.067
0.0055
0.005
The fact that these are the same is an
artifact, arising because the ratio of
Kow to VP is the same for both PCB 52
and pyrene