Transcript Handout
TOX 715: Environmental
Toxicology
Environmental Transport
Transport and Fate of
Toxicants in the Environment
Transport and fate model
Environmental factors that may
modify exposure
Toxicant Source(s)
Exposure-Response Model
Toxicant Exposure
Toxicant Effects
Environmental Compartments
Environmental
Chemodynamics
Fugacity
“Tendency of a
compound to escape
from one
environmental
compartment into
another one”
Fugacity Basics
Fugacity is to mass diffusion what
temperature is to heat transfer.
Fugacity is linearly proportional to
concentration.
Chemicals move from compartments in
which they have high fugacities to those
of low fugacity.
Fugacity Basics
When the fugacities of a compound in
two adjacent phases are equal, the
system is in equilibrium.
Fugacity is expressed in units of
pressure.
Fugacity
C1
C0
Partitioning
Partitioning
C1
K1, 2
C2
Solvent Partitioning
Csolvent
KP
Cwater
S solvent
KP
S water
Other Partition Coefficients:
Kow
Koc, Kb and Kpw
Partitioning Theory and the
Environment
Partitioning can be used to model the
distribution of an organic compound in
the environment.
Examples:
▫ Sorption
▫ Bioconcentration.
Bioaccumulation
Bioaccumulation
▫ general term used to describe a series of
processes by which chemicals found in the
environment are accumulated and
concentrated in living organisms
Bioconcentration
Biomagnification
Bioconcentration Factors
Calculating BCFs
log BCF = 0.76 log Kow – 0.23
log BCF = log Kow – 1.32
log BCF = 0.50 log kow – 3.457
Bioavailability
Vapor Pressure
The pressure that the vapor of a
substance exerts on its own liquid or
solid state at equilibrium
1 atm = 760 mmHg = 760 torr = 1.013
x 105 Pa.
Measuring Vapor Pressures
PV = nRT
P = nRT/V = 244.4 (n/V) atm
▫ n/V = vapor density in moles/L
▫ R = universal gas constant = 0.082 L
atm/°K/mole
▫ T = temperature in the generating column
in °K
Henry’s Law
At equilibrium and at a determined temperature a
constant relationship exists between the
concentration of a chemical in air and water.
Henry’s Law Constant
CA P
H
CW S
'
Estimating Volatilization Rates
from H’
Adsorption
General Characteristics
Physico-chemical properties of the
sorbent and the adsorptive
Area of the sorbent
The lower the aqueous solubility of the
adsorptive (solute) the higher the
binding potential
Heat has the potential to reduce
adsorption
Dissipation
C0
ln k1t
C
t1/ 2
ln 2 0.693
k1
k1
Environmental Transport
Advection
Refers to the passive
movement of a chemical as
part of its presence in a
medium that is in
movement itself.
It can happen in the same
compartment or between
different compartments.
Homogeneous Advection
J CvW
vW flow rate
C Concentration
Example, consider water in a stream flowing
at 1000 m3/h and carrying a chemical at 0.5
μg/m3. The chemical is being advected in
water at a rate of 500 μg/h.
Heterogeneous Advection
Refers to the case where
there is a secondary phase
present inside the main
advective medium.
Examples: particulate matter
present in advecting river
water, particles carried by
wind.
Diffusion
Random movement of chemical
molecules due to the presence of a
state of disequilibrium.
It will transport chemicals from one
place to the other one within the same
compartment as well as between
compartments until equilibrium is
reached.
Intraphase Diffusion
Two types of diffusional intraphase transport:
molecular and turbulent diffusion.
Molecular diffusion: movement of particles
because of a concentration gradient.
Turbulent diffusion: happens because of the
turbulent mixing of the bulk medium.
Intraphase Diffusion
Fick’s law:
C
J DA
z
J kMAC
Interphase Diffusion
Diffusion between two phases can be
described using the following formula:
J kA(C1 C2 K12 )
Transport in Solution
Advection
Molecular diffusion
Turbulent diffusion
Dispersion
Transport in Solution
Advection
Molecular diffusion
C
J DM
z
2.7 104
DM 0.71 2
M cm /s
Turbulent diffusion
Dispersion
DM 1 M 20.5
0.5
DM 2 M1
Water-Air Transport
Transport between Water-Air
Ca
J K L CW
H'
C C0e
K L t / z
t1/ 2
0.69Z
KL
Transport through Soil
Vadose zone
Saturated zone
Aquaclude (basal rock)
Transport in the Vadose Zone
Chemicals are able to migrate
through the vadose zone by
three main mechanisms:
▫ dissolved in solution
▫ as gases (vapor)
▫ adsorbed to particles
Transport through
Groundwater
Atmospheric Transport
Volatilization
Advection
Deposition
Advective Transport
Deposition
Deposition