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.

Cation Exchange

• Mechanisms are poorly understood and a topic of debate, but is driven by net negative surface charge on mineral surfaces;

– May involve ions adsorbed to mineral surface

– 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

Clay Mineral

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

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).

CEC VS Grain Composition

From Horowitz, 1991

CEC VS Grain Size

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.

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+

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+

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

Clay Minerals

• Role as trace metal collectors through adsorption is unclear

• Jenne (1976) suggests they from 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

Collector Characteristics

• 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

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