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Cation Exchange

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.

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Cation Exchange

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  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 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

  3. Clay Mineral

  4. 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

  5. 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).

  6. CEC VS Grain Composition From Horowitz, 1991

  7. CEC VS Grain Size

  8. 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.

  9. 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+

  10. 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+

  11. 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 Geochemical Substrates that Service as Significant Trace Metal Collectors

  12. 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 Clay Minerals From Horowitz, 1991

  13. 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 Collector Characteristics From Horowitz, 1991

  14. 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 Organic Matter From Horowitz, 1991

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