Partitioning of contaminants in the environment

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Partitioning of contaminants in the environment

• Examples of interaction between different
  environmental media (air, surface waters, soil)
  encountered in contaminant transport analysis
• Reflection off the ground in atmospheric
  dispersion
• Source/load terms from the atmosphere for surface
  waters
• Source/load terms from sediments in lakes
• Source/sink terms due to biota in rivers
• The distribution of a pollutant among different
  phases at equilibrium is of interest because the
  difference from this equilibrium value is the
  driving force for inter-phase transport, thus
  determining the ultimate fate of a pollutant
  released into the environment.

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The unit world (Fig. 4.1 4.2 Samirullah)

• Air
• Water
• Suspended solids
• Sediment
• Soil
• Fish

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Partitioning (equilibrium) coefficients
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Dimensional partitioning coefficients

• Henrys law constant, H
• Great care is required to ensure that the
  constant has the appropriate units

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Dimensional partitioning coefficients

• Distribution coefficient in adsorption

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Partitioning coefficients

• Specific to the compound and the two phases
• Function of temperature
• Typically need to be determined experimentally
• Empirical relations may express coefficient in
  terms of other easily measured properties

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Aquatic ecosystems

• Most organic compounds are lipophilic (lipid, or
  fat loving) and hydrophobic (water hating)
• Kfw (fish/water) 105 for things like PCB, DDT
• This is called bioconcentration
• If we drink 20 L water per week and eat 200 g
  fish per week, mass consumption of water is 100
  times mass consumption of fish
• But, our exposure to contaminant from the fish
  is 1000 times exposure from water!

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The octanol-water partitioning coefficient Kow

• A simple experiment gives Kow (Fig.5.2
  Samirullah)
• Kow can be used as a surrogate for the
  lipid-water partitioning coefficient.
• Most organics have low solubility in water, the
  Coctanol vs Cwater relationship ends at the
  solubility limit.
• Kow may then be estimated as

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Partitioning among multiple phases

• How does a mass M of pollutant distribute among
  the 5 phases in figure 5.1 (Samirullah), Air,
  water, fish, particles, sediment
• Vi volume of phase i
• Ci concentration in phase I

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Details of one phase

• Say fish contains 5 lipid
• Then 95 of Vfish contains almost no contaminant
• We should really re-adjust the fish-water
  partitioning coefficient Kfw as follows

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Details of phases

• Say particles contain 20 organic carbon, and
  sediment contains 5 organic carbon
• We can define
• Koc can be expected to correlate with Kow and has
  been suggested as Koc0.41 Kow

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Partitioning of Naphtalene at 25 C

• C10H8 M128.19 (molecular weight)
• Water solubility 31 g/m3
• Vapour pressure 10.4 Pa
• Kow1580

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Partitioning of Naphtalene at 25 C
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Partitioning of 100 mols Naphtalene
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Partitioning of 100 mols Naphtalene