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Environmental Toxicology and Chemistry

Environmental Toxicology and Chemistry. Environmental Transport. Environmental Transport. Transport and Fate of Toxicants in the Environment. Dissipation. Dissipation. ln (C o /C) = k 1 t t 1/2 = ln 2/k 1 = 0.693/k 1. Environmental Transport. Advection. Advection.

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Environmental Toxicology and Chemistry

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  1. Environmental Toxicologyand Chemistry EnvironmentalTransport

  2. EnvironmentalTransport

  3. Transport and Fate of Toxicants in the Environment

  4. Dissipation

  5. Dissipation • ln (Co/C) = k1t • t1/2 = ln 2/k1 = 0.693/k1

  6. Environmental Transport

  7. Advection

  8. 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.

  9. 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.

  10. 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.

  11. Diffusion

  12. 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.

  13. 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.

  14. Intraphase Diffusion Fick’s law: • J = -D A(dC/dz) • J = -kM AC

  15. Interphase Diffusion • Diffusion between two phases can be described using the following formula: J = kA(C1-C2K12)

  16. Transport in and from Surface Water

  17. Transport in Solution • Advection • Molecular diffusion • Turbulent diffusion • Dispersion

  18. 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

  19. Water-Air Transport

  20. Transport between Water-Air • J (moles/cm2/s) = KL (Cw – Ca/H’) • C = Coe -KL t/Z • T1/2 = 0.69Z/KL

  21. Estimating Volatilization Rates from H’

  22. Transport on Particles • Particle size • Velocity • Viscosity of the medium

  23. Transport in Soil and Groundwater

  24. Transport through Soil • Vadose zone • Saturated zone

  25. 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

  26. Transport through Groundwater

  27. Atmospheric Transport • Volatilization • Advection • Deposition

  28. 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

  29. Advective Transport

  30. Deposition

  31. Deposition

  32. Deposition J g/cm2/s = vgCa where vg is the deposition velocity and Ca is the atmospheric concentration

  33. Global Transport

  34. 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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