Ion Exchange

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Ion Exchange
Colloids are small particles in soil that act
like banks managing the exchange of nutrient
currency in the soil
Different soils, like checking accounts, have
different capacities to hold nutrient currency
cations and anions
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Types of Adsorption
A Outer-sphere comlpex B Inner-sphere
complex C Isomorphous substitution DE Surface
Polymers F Polymers Embeded G Desorption
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Ion Exchange Mechanisms
outer-sphere and inner-sphere complexes
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Principles of Ionic Exchange
Reversible Reactions Charge Balance Ratio
Law Mass Action Ion Selectivity Complementary
Cations
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Reversible Reactions
Can go forwards or backwards Example
2K
2H
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Balanced by Charge
Charge for Charge.. NOT ion for ion
Ca
micelle
Ca
2K
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The Ratio of Ions on Exchange Site is Equal to
the Ratio of Ions in the Soil Solution
4 H 2 Na After on colloid
2H 1Na After in soln.
6 H 3 Na before
H
H
H
H
Na and 2H
micelle
3Na
H
H
H
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Mass Action
CO2 is a gas and escapes from the soil
easily. This drives the reaction to the right.
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Ion Selectivity
Al3 gt Ca2 gt Mg2 gt K NH4 gt Na
Held tightly ----------------------------------
Held loosely
Based on Valence Charge and Hydrated Ionic Radius
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The Effects of Neighboring Cations
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Field Estimates of CEC
Uses Soil Texture and Organic Matter Content to
predict the CEC of a soil
How much of a Soil Colloid () ? What type or
types of Colloids present ?
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CEC of selected colloids
Humus 200 in cmolc/kg Vermiculite 150 at
pH 7 Montmorillonite 110 Illite 30 Chlorite
30 Kaolinite 10 Gibbsite 5 Hematite 5 Allo
phane 80
Mixed Clays 60
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Calculating CEC
A soil contains 20 smectite, 5 Fe/Al oxides,
and 4 humus. Calculate its CEC. (5 0.05 kg
per 1 kg soil) Visit Table 8.3 pH of 7 is
neutral smectite CEC 100 cmolc/kg Organic
Matter CEC 200 cmolc/kg Gibbsite/Goethite
(Fe/Al oxide) CEC 4 cmolc/kg From the clays
0.2 kg x 100 cmolc/kg 20 cmolc From O.M. .04
kg x 200 cmolc/kg 8 cmolc From oxides 0.05 kg
x 4 cmolc/kg 0.2 cmolc so Total CEC of the
soil 20 8 0.2 28.2 cmolc/kg soil
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Typical CEC Values
Figure 8.13  Ranges in the cation exchange
capacities (at pH 7) that are typical of a
variety of soils and soil materials. The high CEC
of humus shows why this colloid plays such a
prominent role in most soils, and especially
those high in kaolinite and Fe, Al oxides, clays
that have low CECs.
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What About Anion Exchange ? First we need to
know about Soil pH And Variable Charge
Cl- chlorine NO3- nitrate SO4-2
sulfate PO4-3 phosphate
Essential Plant Nutrients
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pH and pOH
pH -logH
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Typical pH Values
Figure 9.2  Some pH values for familiar
substances (left) compared to ranges of pH
typical for various types of soils (right).
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Plants and Soil pH
Figure 9.13   Ranges of pH in mineral soils
optmal for growth of selected plants.
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Nutrient availability and pH
Figure 9.11  Relationships existing in mineral
soils between pH and the availability of plant
nutrients. The relationship with activity of
certain microorganisms is also indicated. The
width of the bands indicates the relative
microbial activity or nutrient availability. The
jagged lines between the P band and the bands for
Ca, Al, and Fe represent the effect of these
metals in restraining the availability of P. When
the correlations are considered as a whole, a pH
range of about 5.5 to perhaps 7.0 seems to be
best to promote the availability of plant
nutrients. In short, if the soil pH is suitably
adjusted for phosphorus, the other plant
nutrients, if present in adequate amounts, will
be satisfactorily available in most cases.
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3 pools of Soil Acidity
Total Residual Exchangeable Active All 3
pools are effected by each other
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Variable Charge
Greatly effects Kaolinite Oxides Humus
Zoomed In
Si
H
O
(or Al)
Si O H2
Si O - H
Si - O
Charge 1 0
-1 pKa
for Silica 1.99
3.0
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Sources of Chargeand their influence on CEC
Figure 8.14  Influence of pH on the cation
exchange capacity of smectite and humus. Below pH
6.0 the charge for the clay mineral is relatively
constant. This charge is considered permanent and
is due to ionic substitution in the crystal unit.
Above pH 6.0 the charge on the mineral colloid
increases slightly because of ionization of
hydrogen from exposed hydroxyl groups at crystal
edges. In contrast to the clay, essentially all
of the charges on the organic colloid are
considered pH dependent. Smectite data from
Coleman and Mehlich (1957) organic colloid data
from Helling et al. (1964)
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Weathering, Clay Minerals and CEC
Figure 8.17  The effect of weathering intensity
on the charges on clay minerals and, in turn, on
their cation and anion exchange capacities (CECs
and AECs). Note the high CEC and very low AEC
associated with mild weathering, which has
encouraged the formation of 21-type clays such
as fine-grained micas, vermiculites, and
smectites. More intense weathering destroys the
21-type clays and leads to the formation of
first kaolinite and then oxides of Fe and Al.
These have much lower CECs and considerably
higher AECs. Such changes in clay dominance
account for the curves shown.
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CEC vs AEC
Figure 8.16  (Left) Effect of increasing the pH
of subsoil material from an Ultisol from Georgia
on the cation and anion exchange capacities. Note
the significant decrease in anion exchange
capacity associated with the increased soil pH.
When a column of the low-pH material (pH 4.6)
was leached with Ca(NO3)2 (right), little sulfate
was removed from the soil. In contrast, similar
leaching of a column of the soil with the highest
pH (6.56), where the anion exchange capacity had
been reduced by half, resulted in anion exchange
of NO32 ions for SO42 ions and significant
leaching of sulfate from the soil. The importance
of anion adsorption in retarding movement of
specific anions or other negatively charged
substances is illustrated. Data from Bellini et
al. (1996)
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SUMMARY
Mechanisms and Principles of CEC / AEC
Reactions Field Estimates of CEC Review of pH
concepts Variable Charge Anion Exchange
KNOW HOW TO DO THE CALCULATION!