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 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

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

Clay Mineral

Organic Matter

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

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

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

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