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

Soil Colloids, the final frontier
Measuring CEC; sorption
concepts; environmental
implications
Mg
Mg
Na+
K+
↔
Na+
K+ Na+
Cation exchange reaction:
[Soil Colloid]:Na+ + K+(aq) ↔ [Soil Colloid]:K+ + Na+(aq)
NaX + K+(aq) ↔ KX + Na+(aq)
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+
Mixed ions
‘Saturated’ with K+
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 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
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
sorbate
sorptive
sorbent
(not sorbet)
Ion Exchange
(electrostatic complex)
http://www.mpi-muelheim.mpg.de/kofo/
institut/arbeitsbereiche/schueth/grafik/z_ion_exchange.gif
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
q (mol/kg)
Kp =
concentration on solid (q)
concentration in solution (Ceq)
High Kp (strong sorption)
e.g., hydrophobic compounds
on organic matter
Slope = rise/run
K = [sorbed]/[solution]
Low Kp (weak sorption)
e.g., Water soluble
compound (hydrophilic) that
prefers to stay in solution
Ceq (mol/L)
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, 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