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 104
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