Importance of complexes:
(1) Complexing increases solubility of minerals

• total concentration of element high when in equilibrium with mineral

e.g. Assume S[Ca²⁺] controlled by calice solubility:
 
 

But what if contains CO₃^2-, and HCO₃^-?

Complexes form:
 
(2) Some elements more common as complexes

 • Particularly true of metals:, e.g Cu²⁺. Hg²⁺, Pb²⁺, Fe³⁺, U⁴⁺
 • The chemical behavior (ie. Mobility, toxicity) etc. are properties of complex, not ion.
 
 

 
(3) Adsoption affected by complex

e.g. Hydroxide complexes of uranyl (UO₂²⁺) readily adsorbed by oxide and hydroxides
 Carbonate complexes rarely adxorbed.

In general:

 Carbonate, sulfate, and fluoride complexes rarely adsorbed.
 
(4) Toxicity and bioavailability depends on complexes

Toxicity:
 • e.g. Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺, Hg²⁺, Pb²⁺
• Toxicity depends on activity and complexes, not total concentrations
• e.g. CH₃Hg⁺ and Cu₂⁺ are toxic to fish, but other complexes are not (e.g. CuCO₃)

Bioavailability:
 • some metals essential nutrients: Fe, Mn, Zn, Cu
 • uptake depends on complexes
 
General Observations of complexes:

• Complex stability increases with: Increasing charge and/or decreasing radius of cations,
 e.g. inner sphere complexes have strong covalent bonding
• Cations and ligands with strong complexes form minerals with low solubilities
• Complexing increases solubility of minerals that contain species being complexed
• Complexing enhance or inhibit adsorption and affect toxicity and bioavailability
• Higher salinity increases complexeing
• Higher salinity water increases solubility because of complexing
 
Metal- cation relations in complexes

• metal cations generally smaller than ligands
• charge of metal cations exceeds coordinating ligands
• complexes thus are cations surrounded by one or more coordinating ligands
 - number of ligands depends on coordinations number (CN)
 - most common CN are 4 and 6, although 2, 3, 5, 6, 8, and 12 possible
 
All possible coordination sites rarely filled with ligands (except for aquocation complexes):
 • reason is that coordination number is proportional to concentration of ligand
 • H2O concentration ~ 55.6 mol/kg
 • ligand concentrations 0.001 to 0.00001 mol/kg

Possible to write complex reaction: