Soil Colloids, the final frontier

Measuring CEC; sorption concepts; environmental

implications

Na+ Na+

Mg Mg

Na+K+ K+

Cation exchange reaction: [Soil Colloid]:Na+ + K+

(aq) ↔ [Soil Colloid]:K+ + Na+(aq)

NaX + K+(aq) ↔ KX + Na+(aq)

↔

Ion exchange measurement

Soil with mixed ions on the exchangeCa, Mg, Na, K, H, Al

1. Add index cation NH4+

‘Saturated’ with NH4+

2. Displace index cation with K+

3. Collect and measure index cation

‘Saturated’ with K+

Mixed ions NH4+

Measuring CEC or AEC

• Remove excess salts with dilute solution (important step in arid zone soils)

1. Saturate soil with index cation (NH4+)

2. Displace index cation with another cation (K+)

3. Measure the amount of index cation displaced (NH4

+)

Mixed cations Saturated with index cation

• Calculate CEC using equivalents of chargee.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

• Long, tedious process – labor consuming, thus expensive in analytical labs – Why we use SOM and clay % to estimate CEC

Selectivity

• Ions with small hydrated radius are preferred over larger ions.(ions in most soil environments are usually hydrated)

Cs+ > Rb+ > K+ > Na+ > Li+

• Higher valence preferred over lower valence

Al+3 > Ca+2 > Mg+2 > K+ > NH4+ > Na+

Sorption* processes in soil

*general term referring to the retention of material on solid surfaces

– includes cation exchange, adsorption,

surface precipitation, and polymerization

sorbent

sorbate

sorptive

(not sorbet)

http://www.mpi-muelheim.mpg.de/kofo/institut/arbeitsbereiche/schueth/grafik/z_ion_exchange.gif

Ion Exchange (electrostatic complex)

Surface Complexes: 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).

Outer Sphere Complex

• weak (held by H-bonding)

• electrostatic interaction, thus surface must be charged

• rapid

• reversible (= exchangeable)

• affected by effective concentration of the solution (ionic strength)

• E.g., ion exchange (CEC or AEC)

Inner-Sphere Complex

• Strong (held by covalent and/or ionic bonding)• Mono- or polydentate (held by one or more

bonds) • Slower than outer sphere complexation• Irreversible or “fixed” (permanently held or

unavailable to plants, leaching, etc)• Surface charge can be changed by

complexation• E.g., phosphate fixation by Al or Fe oxides

Sorption of Organic Compounds• Soil colloids help control the movement of

pesticides and other organic compounds into groundwater

• Some compounds are charged (+ or -) and can be held by ion exchange processes

• Most organic molecules are hydrophobic (hate water) and are attracted to organic matter in the soil (“like dissolves like”)– Partitioning into soil organic colloids (and out

of aqueous solution)

Partitioning

• Hydrophobic compounds dissolve into the SOM

• Sorbed organic compound permeates into the network of SOM and is held by weak, physical forces

• Analogous to the extraction of an organic compound from water into an immiscible organic phase (called partitioning)

Kp, partitioning coefficientKp = concentration on solid (q) concentration in solution (Ceq)

q (m

ol/k

g)

Ceq (mol/L)

High Kp (strong sorption)e.g., hydrophobic compoundson organic matter

Low Kp (weak sorption)e.g., Water soluble compound (hydrophilic) that prefers to stay in solution

Slope = rise/runK = [sorbed]/[solution]

Partitioning sorption processes

• Linear relationship between solid and solution phases up to relatively high concentrations

• Sorption is highly correlated to OM or OCKp increases with increasing SOM or SOC

• Organic compounds with low water solubility (hydrophobic) have higher Kp values

• % SOM or OC has more effect that % clay, pH, Fe and Al oxides.Soils high in SOM will retain more pesticides

Soil organic matter (SOM) is 50-65% C

Distribution coefficients, Kd

Kd = mg chemical sorbed / kg soil mg chemical / L solution

• The ratio of chemical sorbed to the soil compared to what remains in solution (units are L/kg or mL/g)

• Useful for predicting compound behavior and movement in the soil

• Varies widely depending on soil properties (especially SOM or OC, clay content, etc)

Organic C distribution coefficient Koc

Koc = mg chemical sorbed / kg organic carbon mg chemical / L solution

• Because Kd varies so much, Koc is a better predictor of organic compound behavior in soils

• Koc = Kd / foc where foc is the fraction of organic C in soil

• Higher Kd or Koc values = more sorption and retention by soils and less leaching

Montmorillonite (2:1 expansive clay) adsorbs more biomolecules than kaolinite (1:1 clay), but much less than organic matter (not shown)

Swelling clays

Expansive Clays (smectites)

• Water incorporation into the clay structure swells the soil by 25%

• Bad for building (use deep pilings to support structure on bedrock or nonexpansive strata)

• Useful for clay linings of lagoons, ponds, well caps, etc (as long as they stay wet)– E.g., bentonite-grout mixtures used to prevent

preferential flow down the walls of monitoring wells

• when dry, these clays crack and are very hard; difficult to work with