Contaminant Fate Processes with focus on the subsurface

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Contaminant Fate Processes(with focus on the
subsurface)

• SORPTION-DESORPTION
• HYDROLYSIS
• Oxidation-Reduction (Redox)
• Elimination
• VOLATILIZATION
• BIODEGRADATION

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SORPTION-DESORPTION

• Generally accepted nomenclature
• Absorption the dissolution of gas molecules
  into bulk liquid.
•  
• Adsorption physical, reversible attachment of
  gas (or liquid) molecules to a solid surface
•  
• Chemisorption the attractive forces between gas
  molecule and surface are closer to chemical
  bonding than physical attraction.
• The term Sorption is used to avoid having to
  make the distinction,

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SORPTION-DESORPTION

• BRN subsurface contaminant nomenclature
• Absorption the dissolution of contaminant into
  bulk solid.
•  
• Adsorption attachment of contaminant to the
  solid surface
•  
• Sorption is used for the sum of the two
  processes
• Desorption - return of the contaminant to the
  bulk fluid phase
• The significance of sorption is that it causes
  the contaminant to be retarded relative to the
  movement of the bulk fluid phase

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Abiotic processes

• Chemical reaction mechanisms that do not include
  the direct involvement of living organisms

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HYDROLYSIS

• Chemical interaction between contaminant and
  water
• Do not include direct involvement of living
  organisms, thus abiotic
• Some contaminants are more susceptible to
  hydrolysis than others, Tables 7.3 and 7.4
• Strongly dependent on temperature and pH

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Oxidation-reduction (another abiotic process)

• Transfer of electrons between contaminant and
  other species (Fig 7.4)
• LEO-GER (Lost electrons oxidized, gained
  electrons reduced).
• Electron donors, acceptors (I.e.reducing or
  oxidizing agents)
• Halogenated hydrocarbons and nitroaromatics known
  to undergo abiotic oxidation-reduction reactions
  in anaerobic subsurface environments

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Elimination(another abiotic process)

• Release of a halogen group and a proton from an
  adjacent carbon in halogenated ethanes and
  propanes, followed by the formation of a CC
  bond.
• 1,1,1 trichloroethane to 1,1 - dichloroethene
• Correct the figure to show CC bond instead of C-C

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VOLATILIZATION

• Transfer of contaminant from aqueous, organic, or
  adsorbed phase to the gas phase
• (Transfer from adsorbed phase to gas phase also
  called desorption)
• Aqueous-gas phase equilibrium described by
  Henrys law coefficient seen earlier, e..g.

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BIODEGRADATION

• Conversion of contaminant to mineralized end
  products (CO2, H2O, salts) through metabolism by
  living organisms (bacteria)
• Biotransformation conversion of contaminant to
  another chemical form, but not mineralized end
  products

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Metabolism anabolism catabolism

• Anabolism the biochemical processes involved in
  the synthesis of cell constituents from simpler
  molecules, usually requiring energy
• Catabolism the biochemical processes leading to
  the production of energy

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Catabolism and the source of the energy
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Catabolism and the source of the energy

• The extraction of energy from organic chemical
  forms involves their oxidation (I.e. losing
  electrons)
• Thus chemotropic substrates (food for bacteria)
  are primary electron donors
• There must also be terminal electron acceptors at
  the end of complex oxidation-reduction reactions
• O2 aerobic,
• NO3 Fe3, SO4-2, CO2 anaerobic

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Dissolved oxygen effects in the subsurface

• Presence or absence of oxygen determines the type
  of bacteria that can survive
• Direct participation of oxygen in reactions
  catalyzed by oxygenase enzymes (Fig 7.7)
• Examples, benzene, fuel constituents, PAHs, some
  chlorinated solvents etc.

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Contaminant biodegradation

• Evaluate the availability of
• Primary electron donors
• Terminal electron acceptors
• Definitions
• Primary substrate primary electron donor
  providing the energy for cell growth
• Secondary substrate provides energy but not
  sufficient for all energy needs
• Cometabolite a compound metabolized as a side
  process while the cell is deriving energy from
  another primary substrate

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Common subsurface contaminants

• Monoaromatic hydrocarbons Benzene, toluene,
  ethylbenzene, xylenes (BTEX)
• Polynuclear Aromatic Hydrocarbons (PAH)
• Phenolic compounds
• MTBE
• Chlorinated solvents
• Nitroaromatic compounds
• Biodegradation pathways for these compounds is an
  active area of research.