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

Na+ Mg Na+ 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 exchange Ca, Mg, Na, K, H, Al 1. Add index cation NH 4 + ‘Saturated’ with NH Displace index cation with K + 3. Collect and measure index cation ‘Saturated’ with K + Mixed ions NH 4 +

Measuring CEC or AEC Remove excess salts with dilute solution (important step in arid zone soils) 1. Saturate soil with index cation (NH 4 + ) 2. Displace index cation with another cation (K + ) 3. Measure the amount of index cation displaced (NH 4 + ) Mixed cations Saturated with index cation

Calculate CEC using equivalents of charge e.g. Ca +2 has two equivalents and satisfies two negative sites on exchange; Na +, NH 4 +, and K + all have one equivalent each and can satisfy or adsorb onto one negative site each. Units = cmol c /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 + > NH 4 + > 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)

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 coefficient Kp = concentration on solid(q) concentration in solution (Ceq) q (mol/kg) Ceq (mol/L) 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 Slope = rise/run K = [sorbed]/[solution]

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

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

Distribution coefficients, K d K d = 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 K oc K oc = mg chemical sorbed / kg organic carbon mg chemical / L solution Because K d varies so much, K oc is a better predictor of organic compound behavior in soils K oc = K d / f oc where f oc is the fraction of organic C in soil Higher K d or K oc 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