Soils 205

Lecture 9 - Cation exchange

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13,14                                                    339-360

A. Source of charge on clays-  permanent or variable (pH dependent charge)

1. Permanent or constant charge

(a) isomorphous substitution

for example: Zn⁺² for Al⁺³ in octahedral sheet

no substitution

isomorphous substitution = negative charge

(b) both octahedral and tetrahedral sheets

Mg⁺², Zn⁺², Cu⁺², Fe⁺² for Al⁺³

Al⁺³ for Si⁺⁴

(c) can form + charge

- Al⁺³ for Mg⁺²

- negative charge dominates

2. Variable or pH-dependent charge

(a) Dissociation of exposed OH groups

]-OH ¬¾¾® ]-O^- + H⁺

uncharged

negative charge

<-----------(+ H⁺)---------

-------(- H⁺)--------->

(b) Occurs with humus, hydrous oxides,

and broken edges of silicate clays

(c) Increased pH values = more negative charge

acid = less negative charge

(d) Protonation of O and OH groups

]-O^- + H⁺ ¬¾® ]-OH + H⁺ ¬¾® ]-OH₂⁺

negative charge	no charge	positive charge
high pH	intermediate pH	low pH

(e) Depends on soil colloids present

Colloid	Negative charge	Positive charge	% constant	% variable
Humus	200	0	10	90
Vermiculite	120	0	95	5
Smectite	100	0	95	5
Illite	40	0	80	20
Kaolinite	12	4	5	95
Fe & Al Oxides	5	5	0	100

(f) Positive charge << negative charge in most temperate zone soils

acid tropical soils = net + charge

 

B. Cation Exchange

1. Process of cation replacement

 

 

Example:

Ca⁺²-colloid + 2 H⁺ ¬¾® 2 H⁺-colloid + Ca⁺²

= H⁺ replaces Ca⁺² adsorbed to soil colloids

or

Ca-x + 2 H⁺ ¬¾® 2 H-x + Ca⁺²

x = the soil solid phase

or

Ca_(ad) + 2 H⁺ ¬¾® 2 H_(ad) + Ca⁺²

X_(ad) = "adsorbed" cation X

2. Maintain electroneutrality

2 Na⁺ for 1 Ca⁺²

1 Mg⁺² for 2 K⁺

3 H⁺ for 1 Al⁺³

 

 

C. Cation Exchange Capacity (CEC)

1. Total charge on soil

= sum of adsorbed cations (charge)

- (soil) = + (cations)

2. Determination

 

 

1.

2.

3.

Cation Exchange Capacity =

 

 

3. Units of charge per unit weight

(a) centimoles of charge per kilogram of soil

= cmol_c/kg

= old units of milliequivalents per 100 grams of soil

(meq/100 g)

(b) all cations are equal on a charge basis

1 cmol_c/kg Na⁺

= 1 cmol_c/kg Ca⁺²

= 1 cmol_c/kg Al⁺³

(c) conversion of charge to weight

5 cmol_c/kg of Ca⁺²

= ? g Ca⁺² (1)

= ? g CaCl₂ (2)

= ? lbs/A-ft (3)

molecular weights: Ca⁺² = 40 g/mol

Cl^- = 35.5 g/mol

(from periodic table)

(1) (5 cmol_c/kg soil)(mol_c/100 cmol_c)(mol Ca⁺²/2 mol_c)(40 g Ca⁺²/mol Ca⁺²)

= 1 g Ca⁺²/kg soil

(2) (5 cmol_c/kg soil)(mol_c/100 cmol_c)(mol CaCl₂/2 mol_c)(111 g CaCl₂/mol CaCl₂)

= 2.775 g CaCl₂/kg soil

(3) (2.775 g CaCl₂/kg soil)(kg soil/1000 g soil)(454 g soil/lb soil)(lb CaCl₂/454 g CaCl₂)

(4 x 10⁶ lb soil/A-ft) = 11,100 lb CaCl₂/ A-ft

 

4. Cation exchange capacity of soils

(a) related to components

humus » 200 cmol_c/kg
smectites » 100 cmol_c/kg
illite » 25 cmol_c/kg
kaolinite » 10 cmol_c/kg
Fe and Al oxides » 4 cmol_c/kg

(b) estimate soil cation exchange capacity from composition:

5 % O.M. & 20 % smectite clay

200 x 0.05 = 10

100 x 0.20 = 20

total = 30 cmol_c/kg

vs

2 % O.M. & 30 % kaolinite clay

200 x 0.02 = 4

10 x 0.30 = 3

total = 7 cmol_c/kg

5. Cation exchange capacity and pH

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