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+ >Ca2+ >Mg2+ > K+ > Na+
Also means that affinity is dependent on valence
Me3+ > Me2+ > Me+

11. Competing Cations
Increased Affinity with decrease in diameter of hydrated ion
Cs+ > Rb+ > K+ > Na+ > Li+
Selectivity Series for divalent cations (Deutsch, 1997)
Pb2+ > Ba2+ = Sr2+ > Cd2+ = Zn2+ = Ca2+ > Mg2+ = Mg2+ = Ni2+ = Cu2+ > Mn2+ > 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 Fine grained Large surface area
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 Quantity indirectly correlated to grain-size
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