IS SOLUBILITY THE ONLY CONTROL ON SOLUTE CONCENTRATIONS?

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IS SOLUBILITY THE ONLY CONTROL ON SOLUTE
CONCENTRATIONS?

• The answer is NO! Solubility often controls the
  concentrations of major solutes such as Si, Ca,
  and Mg, and some minor or trace solutes such as
  Al and Fe.
• However, for many trace elements, sorption
  processes maintain concentrations below
  saturation with respect to minerals.
• In other words, sorption is a means to remove
  solutes even when the solution is undersaturated
  with any relevant solids.

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Mineral Surfaces

• Minerals which are precipitated can also interact
  with other molecules and ions at the surface
• Attraction between a particular mineral surface
  and an ion or molecule due to
• Electrostatic interaction (unlike charges
  attract)
• Hydrophobic/hydrophilic interactions
• Specific bonding reactions at the surface

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DEFINITIONS

• Sorption - removal of solutes from solution onto
  mineral surfaces.
• Sorbate - the species removed from solution.
• Sorbent - the solid onto which solution species
  are sorbed.
• Three types of sorption
• Adsorption - solutes held at the mineral surface
  as a hydrated species.
• Absorption - solute incorporated into the mineral
  structure at the surface.
• Ion exchange - when an ion becomes sorbed to a
  surface by changing places with a similarly
  charged ion previously residing on the sorbent.

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Charged Surfaces

• Mineral surface has exposed ions that have an
  unsatisfied bond ? in water, they bond to H2O,
  many of which rearrange and shed a H
• S- H2O ? SH2O ? S-OH H

OH
OH
OH2
H
OH
OH
OH
H
OH
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Surfaces as acid-base reactants

• The surface SITE acts as an amphoteric
  substance ? it can take on an extra H or lose
  the one it has to develop charge
• S-O- H ? S-OH ? S-OH2
• The of sites on a surface that are , -, or 0
  charge is a function of pH
• pHzpc is the pH where the sites - sites 0
  sites and the surface charge is nil

OH
OH2
O-
OH
O-
OH
OH2
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GOUY-CHAPMAN DOUBLE-LAYER MODEL
STERN-GRAHAME TRIPLE-LAYER MODEL
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Sorption to S-OH sites

• S-OH M2 ? S-OM H
• S-OH L2- ? S-L- OH-
• In addition, can also have bi-dendate sorption
  reactions

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pHzpc

• Zero Point of Charge, A.k.a Zero Point of Net
  Proton Charge (pHZPNPC) or the Isoelectric Point
  (IEP)
• Measured by titration curves (pHzpc similar to
  pKa) or electrophoretic mobility (tendency of
  the solids to migrate towards a positively
  charged plate)
• Below pHzpc ? more sites are protonated ? net
  charge
• Above pHzpc ? more sites are unprotonated ? net -
  charge

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POINT OF ZERO CHARGE CAUSED BY BINDING OR
DISSOCIATION OF PROTONS
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From Stumm and Morgan, Aquatic Chemistry
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Anion-Cation sorption

• Equilibrium description for sorption of
• S-OH M2 ? S-OM H
• Where Dz is the stoichiometric net change in
  surface charge due to the sorption reaction (1
  here), F is Faradays constant (96485 Coulombs
  per mole), ? is the electrical potential at the
  surface, R is the gas constant, and T is
  temperature in Kelvins, the whole right term is
  called the coulombic term

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Inner Sphere and Outer Sphere

• Outer Sphere surface complex ? ion remains
  bounded to the hydration shell so it does not
  bind directly to the surface, attraction is
  purely electrostatic
• Inner Sphere surface complex ? ion bonds to a
  specific site on the surface, this ignores
  overall electrostatic interaction with bulk
  surface (i.e. a cation could bind to a mineral
  below the mineral pHzpc)

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ADSORPTION OF METAL CATIONS - I

• In a natural solution, many metal cations compete
  for the available sorption sites.
• Experiments show some metals have greater
  adsorption affinities than others. What factors
  determine this selectivity?
• Ionic potential defined as the charge over the
  radius (Z/r).
• Cations with low Z/r release their waters of
  hydration more easily and can form inner-sphere
  surface complexes.

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ADSORPTION OF METAL CATIONS - II

• Many isovalent series cations exhibit decreasing
  sorption affinity with decreasing ionic radius
• Cs gt Rb gt K gt Na gt Li
• Ba2 gt Sr2 gt Ca2 gt Mg2
• Hg2 gt Cd2 gt Zn2
• For transition metals, electron configuration
  becomes more important than ionic radius
• Cu2 gt Ni2 gt Co2 gt Fe2 gt Mn2

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ADSORPTION OF METAL CATIONS - III

• For variable-charge sorbents, the fraction of
  cations sorbed increases with increasing pH.
• For each individual ion, the degree of sorption
  increases rapidly over a narrow pH range (the
  adsorption edge).

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SORPTION ISOTHERMS - I

• The capacity for a soil or mineral to adsorb a
  solute from solution can be determined by an
  experiment called a batch test.
• In a batch test, a known mass of solid (S m) is
  mixed and allowed to equilibrate with a known
  volume of solution (V ) containing a known
  initial concentration of a solute (C i). The
  solid and solution are then separated and the
  concentration (C ) of the solute remaining is
  measured. The difference C i - C is the
  concentration of solute adsorbed.

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SORPTION ISOTHERMS - II

• The mass of solute adsorbed per mass of dry solid
  is given by
• where S m is the mass of the solid.
• The test is repeated at constant temperature but
  varying values of C i. A relationship between C
  and S can be graphed. Such a graph is known as an
  isotherm and is usually non-linear.
• Two common equations describing isotherms are the
  Freundlich and Langmuir isotherms.

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FREUNDLICH ISOTHERM

• The Freundlich isotherm is described by
• where K is the partition coefficient and n ? 1.
  

When n lt 1, the plot is concave with respect to
the C axis. When n 1, the plot is linear. In
this case, K is called the distribution
coefficient (Kd ).
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LANGMUIR ISOTHERM

• The Langmuir isotherm describes the situation
  where the number of sorption sites is limited, so
  a maximum sorptive capacity (S max) is reached.

The governing equation for Langmuir isotherms is
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ION EXCHANGE REACTIONS

• Ions adsorbed by outer-sphere complexation and
  diffuse-ion adsorption are readily exchangeable
  with similar ions in solution.
• Cation exchange capacity The concentration of
  ions, in meq/100 g soil, that can be displaced
  from the soil by ions in solution.

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ION EXCHANGE REACTIONS

• Exchange reactions involving common, major
  cations are treated as equilibrium processes.
• The general form of a cation exchange reaction
  is
• nAm mBX ? mBn nAX
• The equilibrium constant for this reaction is
  given by

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Sorption of organic contaminants

• Organic contaminants in water are often sorbed to
  the solid organic fractions present in soils and
  sediments
• Natural dissolved organics (primarily humic and
  fulvic acids) are ionic and have a Koc close to
  zero
• Solubility is correlated to Koc for most organics

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Measuring organic sorption properties

• Kow, the octanol-water partition coefficient is
  measured in batches with ½ water and ½ octanol
  measures proportion of added organic which
  partitions to the hydrophobic organic material
• Empirical relation back to Koc
• log Koc 1.377 0.544 log Kow