Modeling PCDD and PCDF adsorption

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Modeling PCDD and PCDF adsorption

• The atomistic view

Arnaud Marmier
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Outline of Talk

• Environmental Problem
• Atomistic Modelling
• Energy optimisation
• Free Energy diagram
• ab initio surface thermodynamics

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The Environmental Problem

• Persistent Organic Pollutants
• Pesticides/by product
• Soil pollution
• Remediation strategies informed by physical
  properties
• Many pollutants, many components of soil !

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Simulation
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Simulation

• Atomistic Model
• Atom/ions as
• interaction centres
• Bonding interactions (intra-molecule)
• Electrostatic interactions
• Dispersion-repulsion (van der Walls)
• Empirical !
• No electron redistribution

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Simulation, background

• Extracting information
• Energy optimisation
• Thermodynamic (Monte Carlo, Molecular Dynamics
  -newtons law-)
• Many technical details
• periodicity, choice of potential, ensemble
• Still, a powerful approach, relatively
  inexpensive
• 1000000 atoms affordable

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Simulation, details
Short Range Interaction, extended for covalent
materials
Long Range Coulombic Interaction
Spring Constant, k
Shell Model to account for electronic
polarisation
(Dick Overhauser, 1958)
Core Charge
Shell Charge At Centre
Oxygen ion is represented as a core plus a shell
coupled by a harmonic spring.
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PCDD/Pyrophillite
76 conformers
polychloro-dibenzo-dioxin (PCDDs)
Clay (Pyrophylite)
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PCDD/Pyrophillite, condor
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PCDD/Pyrophillite
?E.25eV
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PCDF/Pyrophillite
polychloro-dibenzo-furans (PCDFs)
Clay (Pyrophylite)
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PCDF/Pyrophillite
?E.12eV
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PCDD/Quartz
polychloro-dibenzo-dioxin (PCDDs)
Quartz (001)
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PCDD/Quartz
Minimisation in progress !!
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Water, direct
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Salt Distribution - Calcite
1M NaCl solution Build up of negative charge
near interface due to chlorine adsorption Charge
distribution oscillates EVEN if surface is
charge neutral
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Metal Ion Adsorption - Magnesium
Inner-sphere complex much lower in free energy 1
Inner-sphere complex 1 Outer-sphere complex Free
energy barrier similar to that of calcium and
strontium
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Metal Ion Adsorption - Strontium
Profile very similar to that of calcium 1
Inner-sphere complex 2 Outer-sphere
complexes Position and size of free energy
barrier is the same
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Water, Free Energies of Adsorption
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ab initio surface Thermodynamics
Calcite (0114)
Alumina (0112)
Kerisit, S. Marmier, A. Parker, S. C. Journal
of Physical Chemistry B 2005, 109, 18211.
Marmier, A. Parker, S. C. Physical Review B
2004, 69, 115409.
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Conclusion

• Can get results
• Other substrates (Al2O3, FeOOH, CaCO3)
• Which results (for PHREEQ etc ) ?
• Coarse graining the method ?
• - cellular automata
• - other ?