Cation Exchange

1
Cation Exchange

• Definition substitution of ions in solution for
  those held by a mineral grain.
• Associated with many different types of materials
  found in alluvial sediments including clay
  minerals, Fe and Mn oxides, and organic matter.
• Most trace metals behave as cations and are
  sorbed to materials with net negative charge
  thus, generally interested in cation exchange
  processes anion exchange occurs, but is not very
  prevalent in aquatic systems with normal pH
  values.

2
Cation Exchange

• Mechanisms are poorly understood and a topic of
  debate, but is driven by net negative surface
  charge of on mineral surfaces
• May involve ions adsorbed to mineral surface as
  inner- or out-sphere complexes.
• Not limited to ions adsorbed to mineral surface
  may also involve the substitution of one ion for
  another within the crystalline structure of the
  mineral

3
Na-Zeolite Example

• Na-zeolite Ca2 ? Ca-zeolite 2Na
• To regenerate, pass a solution containing high
  concentration of Na back through Zeolite
  Indicates that concentration of the ions in
  solution is has an important influence on the
  exchange process

4
Clay Mineral
5
Organic Matter
6
Cation Exchange Capacity

• CEC is operationally defined determine the
  amount of a cation that can be removed by a
  specific substance once the material and solution
  have come to equil.
• Generally reported as milliequivalents per 100
  grams of sold (meq/100 g).

7
CEC VS Grain Composition
From Horowitz, 1991
8
CEC VS Grain Size
9
Competing Cations

• When one or more cations are present in the
  solution, the different cations compete for the
  anion adsorption sites on the mineral surface
• In general, as concentration of a cation in
  solution goes up, the amount of it exchanged and
  sorbed to the surface of the mineral goes up
• However, even when the concentrations of the ions
  are the same, some cations have a stronger
  affinity for the mineral surface.

10
Influences on Cation Affinities

• Charge on cation more highly charged solution
  species are preferentially adsorbed.
• Al3 gtCa2 gtMg2 gt K gt Na
• Also means that affinity is dependent on valence
• Me3 gt Me2 gt Me

11
Competing Cations

• Increased Affinity with decrease in diameter of
  hydrated ion
• Cs gt Rb gt K gt Na gt Li
• Selectivity Series for divalent cations (Deutsch,
  1997)
• Pb2 gt Ba2 Sr2 gt Cd2 Zn2 Ca2 gt
  Mg2 Mg2 Ni2 Cu2 gt Mn2 gt Fe2 Co2

12
Geochemical Substrates that Service as
Significant Trace Metal Collectors

• Mn oxides/hydroxides
• Fe oxides/hydroxides
• Organic Matter
• Clay Minerals
• Relative importance varies with environment

• Small particle size and high surface area
• High Cation Exchange Capacity
• High Surface Charge
• Amorphous or Cryptocrystalline
• Thermodynamically unstable

13
Oxides

• Fe Mn Oxides
• Excellent scavengers of trace elements from
  solution
• Commonly occur as coatings on grains and as
  finely dispersed particles
• Very important in rivers

• Fine grained
• Large surface area
• High cation exchange capacity
• High negative surface charge
• Amorphous or poorly crystalline

From Horowitz, 1991
14
Organic Matter

• Ability to concentrate trace metals varies with
  constituent and type of organic matter four
  types exist
• Humins
• Humic acids
• Fulvic acids
• Yellow organic acids
• Occurs as coatings (fine sed. fraction) and
  separate particles (coarse fraction)

• Quantity indirectly correlated to grain-size
• Large surface area
• High CEC
• High negative charge

From Horowitz, 1991
15
Clay Minerals

• Role as trace metal collectors through adsorption
  is unclear
• Jenne (1976) suggests they form a substrate upon
  which Fe, Mn, or Organic matter coatings can
  form Thus, adsorption is not that significant

• Depending on clay mineral type, exhibit moderate
  to high CEC
• Large Surface Area small grain size
• High negative surface charge

From Horowitz, 1991