Soil Colloids, the final frontier. Measuring CEC; sorption concepts; environmental implications. Mg. Mg. ↔. Na+. Na+. Na+. K+. K+. Cation exchange reaction: [Soil Colloid]: Na + + K + ( aq) ↔ [Soil Colloid]: K + + Na + ( aq) NaX + K + ( aq) ↔ KX + Na + (aq).
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Measuring CEC; sorption concepts; environmental implications
Cation exchange reaction:
[Soil Colloid]:Na+ + K+(aq)↔ [Soil Colloid]:K+ + Na+(aq)
NaX + K+(aq) ↔KX + Na+(aq)
1. Add index cation NH4+
2. Displace index cation with K+
3. Collect and measure
Soil with mixed ions
on the exchange
Ca, Mg, Na, K, H, Al
‘Saturated’ with NH4+
‘Saturated’ with K+
1. Saturate soil with index cation (NH4+)
2. Displace index cation with another cation (K+)
3. Measure the amount of index cation displaced (NH4+)
Saturated with index cation
e.g. Ca+2 has two equivalents and satisfies two negative sites on exchange;
Na+, NH4+, and K+ all have one equivalent each and can satisfy or adsorb onto one negative site each.
Units = cmolc/kg soil or meq/100 g soil
(ions in most soil environments are usually hydrated)
Cs+ > Rb+ > K+ > Na+ > Li+
Al+3 > Ca+2 > Mg+2 > K+ > NH4+ > Na+
*general term referring to the retention of material on solid surfaces
includes cation exchange, adsorption, surface precipitation, and polymerization
colloid + ion or molecule in solution = “surface complex”
Outer-sphere complex - water molecule forms a bridge between the colloid and adsorbed ion or molecule.
Inner-sphere complex - no water molecule present between the colloid and sorbed ion or molecule.
Inner and outer-sphere complexation occurs simultaneously (i.e. not mutually exclusive).
Kp = concentration on solid (q)
concentration in solution (Ceq)
Slope = rise/run
K = [sorbed]/[solution]
High Kp (strong sorption)
e.g., hydrophobic compounds
on organic matter
Low Kp (weak sorption)
e.g., Water soluble compound (hydrophilic) that prefers to stay in solution
Kp increases with increasing SOM or SOC
Soils high in SOM will retain more pesticides
Kd = mg chemical sorbed / kg soil
mg chemical / L solution
Koc = mg chemical sorbed / kg organic carbon
mg chemical / L solution