cation exchange definition: substitution of ions in solution for those held by a mineral grain....
TRANSCRIPT
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