Environmental Toxicology
and Chemistry
Environmental Transport
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Environmental Transport
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Transport and Fate of Toxicants in
the Environment
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Dissipation
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Dissipation
 ln (Co/C) = k1t
 t1/2 = ln 2/k1 = 0.693/k1
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Environmental Transport
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Advection
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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.
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Homogeneous Advection
J = Cvw
where vw = flow rate and 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.
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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.
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Diffusion
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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.
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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.
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Intraphase Diffusion
Fick’s law:
 J = -D A(dC/dz)
 J = -kM AC
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Interphase Diffusion
 Diffusion between two phases can be
described using the following formula:
J = kA(C1-C2K12)
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Transport in and from Surface
Water
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Transport in Solution
 Advection
 Molecular diffusion
 Turbulent diffusion
 Dispersion
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Transport in Solution
 Advection
 Molecular diffusion
 J = -Dm dC/dz
 Dm = 2.7 x 10-4/M0.71 cm2/s
 Dm1/Dm2 = M20.5/M10.5
 Turbulent diffusion
 Dispersion
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Water-Air Transport
Transport between Water-Air
 J (moles/cm2/s) = KL (Cw – Ca/H’)
 C = Coe -KL t/Z
 T1/2 = 0.69Z/KL
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Estimating Volatilization Rates
from H’
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Transport on Particles
 Particle size
 Velocity
 Viscosity of the medium
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Transport in Soil and Groundwater
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Transport through Soil
 Vadose zone
 Saturated zone
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Transport in the Vadose Zone
 Chemicals are able to migrate
through the vadose zone by three
main mechanisms:
 adsorbed to particles
 as gases (vapor)
 dissolved in solution
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Transport through Groundwater
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Atmospheric Transport
 Volatilization
 Advection
 Deposition
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Volatilization
 J=k1(n/V)Dv/l
 J = k2(n/V)(1/M0.5)(M)/l
= k3PM0.5/ lRT
J mg/cm2/min= 0.029PM-2
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Advective Transport
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Deposition
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Deposition
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Deposition
J g/cm2/s = vgCa
where vg is the deposition velocity and Ca is the
atmospheric concentration
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Global Transport
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References
 1-Crosby, D.G. Environmental Toxicology and Chemistry.
2000. Oxford University Press, New York.
 2- Connell, D.W. Basic Concepts of Environmental
Chemistry. 1997. Lewis Publishers, New York.
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