Soil Colloids, the final frontier

1. Soil Colloids, the final frontier Measuring CEC; sorption concepts; environmental implications

2. 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)

3. Ion exchange measurement 1. Add index cation NH4+ 2. Displace index cation with K+ 3. Collect and measure index cation Soil with mixed ions on the exchange Ca, Mg, Na, K, H, Al ‘Saturated’ with NH4+ ‘Saturated’ with K+ NH4+ Mixed ions

5. 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+) Saturated with index cation Mixed cations

6. Calculate CEC using equivalents of charge 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 • Long, tedious process – labor consuming, thus expensive in analytical labs • Why we use SOM and clay % to estimate CEC

7. 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+

8. Sorption* processes in soil *general term referring to the retention of material on solid surfaces – includes cation exchange, adsorption, surface precipitation, and polymerization

9. sorbate (not sorbet) sorptive sorbent

10. Ion Exchange (electrostatic complex) http://www.mpi-muelheim.mpg.de/kofo/ institut/arbeitsbereiche/schueth/grafik/z_ion_exchange.gif

11. 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).

13. 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)

14. 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

15. 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)

16. 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)

17. Kp, partitioning coefficient 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 q (mol/kg) Low Kp (weak sorption) e.g., Water soluble compound (hydrophilic) that prefers to stay in solution Ceq (mol/L)

18. Partitioning sorption processes • Linear relationship between solid and solution phases up to relatively high concentrations • Sorption is highly correlated to OM or OC Kp 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

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

20. 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)

21. 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

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

23. Swelling clays

24. 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