Chapter 14 Environmental Chemistry of Colloids and Surfaces

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Chapter 14Environmental Chemistry of Colloids
and Surfaces

• CH350/EV350
• Spring 2008

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Colloids

• Interface between water and sediment
• Interface between soil and pore water

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Colloids

• Colloids are in the size fraction between soluble
  and precipitated.
• Colloids have a defined size of 10 nm to 10 mm
• Experimentally, materials that pass through a
  0.45 mm filter are considered dissolved

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Colloids and surface area

• Small particles have large surface areas
• Humic and Fulvic acids have large surface areas

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Colloid surface properties

• Adsorption of molecules or ions to surface
• Reversible adsorption temporarily removes
  molecule from surrounding solution
• Irreversible adsorption permanently removes
  molecules from surrounding solution
• Electrostatic attraction is one type of
  adsorption mechanism
• Depends on surface charge
• Clay minerals have relatively constant surface
  charge
• Iron and aluminum oxides have variable surface
  charge depending on solution pH

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Colloid surface charge

• Humic material
• Negative charge if carboxylic acid groups are
  deprotonated
• Positive charge (unlikely at normal pH) if amine
  groups are protonated
• Protonation/Deprotonation equilibrium is a unique
  property of each material
• pH where there is nosurface charge is knownas
  the point of zerocharge (pH0)
• If pH lt pH0 than surfaceis positively charged
• If pH gt pH0 than surfaceis negatively charged

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Colloid surface charge

• Natural waters have pH near 7 so most colloids
  exist with negative surface charge
• Iron and aluminum oxides may have positive
  surface charge under some conditions

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Colloid electric double layer

• Surface of colloid attracts ions of opposite
  charge so that the distribution of ions at the
  surface is different than that of bulk water

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Colloid electric double layer

• High salt concentrations decrease thickness of
  double layer
• Colloids become destabilized and aggregate
  together to form larger particles which then
  settle out
• This occurs where rivers discharge colloids into
  sea water causing sedimentation near the mouth of
  the river

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Ion exchange

• Colloid-Na K ? colloid-K Na
• Ca2 gt Mg2 gt K gt Na
• Ca2 binds more strongly than Na
• Ca2 will displace Na
• At low pH Al3 and H may also be important
  cations in colloid surface chemistry
• The measure of the amount of ions a colloid can
  bind is known as the cation exchange capacity
  (CEC)
• Units of centimoles of positive charge per
  kilogram of solid (cmol ()/kg)

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Specific adsorption

• Covalent bonds are formed during specific
  adsorption
• Not related to surface charge but to specific
  combinations of interactions between surface and
  molecule

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Langmuir relation

• Assumptions of Langmuir binding
• specific number of binding sites
• All sites are the same
• Once filled no more binding can occur

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Langmuir relation

• Cs quantity adsorbed (mol/g)
• Caq quantity in aqueous phase (mol/L)
• Csm maximum quantity adsorbable (mol/g)
• b binding constant (L/mol) depends on the
  physical and chemical nature of the solid material

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Langmuir relation rearranged to linear form

• Cs quantity adsorbed (mol/g)
• Caq quantity in aqueous phase (mol/L)
• Csm maximum quantity adsorbable (mol/g)
• b binding constant (L/mol) depends on the
  physical and chemical nature of the solid material

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Freundlich relation

• Deviations from Langmuir binding
• Not all sites are the same, adsorption becomes
  more difficult as adsorbate accumulates on the
  surface
• Once surface is filled additional species can be
  adsorbed multilayered adsorption
• Completely empirical no defined binding
  mechanism

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Freundlich adsorption

• Cs quantity adsorbed (mol/g)
• Caq quantity in aqueous phase
• KF (L/g) and nF (dimensionless number usuallylt
  1) are empirical Freundlich constants

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Partitioning between water and soil/sediment

• Kd depends on
• Organic solute
• Chemical nature of solid phase
• Temperature
• Ionic strength
• Sorption can occur on
• Mineral surfaces (MM) usually small
• Organic components (OM)

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Octanol/water partition coefficient

• Octanol has hydrophobic and hydrophilic
  characteristics
• Used to approximate humic material binding

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Octanol/water partition coefficient

• Larger Kow values when compound is hydrophobic
• Compounds will bind with humic materials rather
  than be in the freely dissolved state

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Uptake of chemicals by organisms

• Bioconcentration higher concentration of a
  chemical in an organism than in the surrounding
  environment
• 1989 measurements in Scotland showed DDT
  concentrations in water to be 1 ng/L while the
  concentration in mussels was 300 mg/kg
• Bioconcentration factor (BCF) ratio of chemical
  in organism to that in the environment
• BCF for mussels (300 x 10-6 g DDT/kg)/(1 x 10-9
  ng DDT/kg) 300,000

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Uptake of chemicals by organisms

• Bioaccumulation process by which an organism
  takes up and retains a chemical
• Biomagnification process that results in a
  chemical being increasingly concentrated at
  higher levels of the food chain

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Uptake of chemicals by organisms

• Compounds with larger Kow values will likely
  associate with lipid regions of organisms
• BCF is usually correlated with Kow

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Behavior of compounds in the aquatic environment

• Compounds can exist in freely dissolved state or
  complexed with minerals, organic matter
  particles, dissolved organic matter, black
  carbon, or organisms
• All these areas need to be considered to
  accurately account for the physical state of a
  compound in water