Mineral Colloids Continued
Cation Exchange
K+
K+
Na+
Na+
K+
Na+
Na+
Na+
Na+
Na+
K+
K+
Soil Solution
Colloid
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Factors Determining Cation Preference
1. Concentration
2. Charge (+1, +2,+3)
3. Size
Mineral Cation Exchange Capacity
The total quantity of cations a
clay can adsorb.
Related directly to the amount of
Isomorphous substitution
Equal to the amount of charge
Units are cmolc/kg soil
Range: 0 - 180 cmolc/kg
K
+
O
H
H
1. Kaolinite
1. Has low CEC
Reason: limited isomorphous substitution
2. Is non-expanding
Reason: layers are “glued” shut by hydrogen bonds
3. Cation adsorption is on the outer mineral surfaces
Reason: layers are “glued” shut by hydrogen bonds
2. Smectite
1. Has high CEC
Reason: abundant isomorphous substitution (Mg for Al)
2. Is highly expanding
Reason: layers are not “glued” shut by hydrogen bonds
Allowing easy movement of water and hydrated cations
to the interlayer regions
3. Cation adsorption is in the interlayers of mineral
Reason: layers are not “glued” shut by hydrogen bonds
Allowing easy movement of water and hydrated cations
to the interlayer regions
3. Vermiculite
1. Has high CEC
Reason: abundant isomorphous substitution (Al for Si)
2. Is moderately expanding
Reason: layers are not “glued” shut by hydrogen bonds,
but interlayer cations are very close to the source of the
negative charge (the tetrahedra) holding the layers together.
3. Cation adsorption is in the interlayers of mineral
Reason: layers are not “glued” shut by hydrogen bonds
Clay Layer
K
3. Illite
K
K
Clay Layer
1. Has low CEC
Reason: abundant isomorphous substitution (Al for Si),
but potassium ions fit well in cavities on the surface of
the interlayers, holding them tightly together.
2. Is non-expanding
Reason: potassium ions fit well in cavities on the surface of
the interlayers, holding them tightly together.
3. Cation adsorption is mostly on external mineral surfaces
Reason: potassium ions fit well in cavities on the surface of
the interlayers, holding them tightly together.
Minerals
Mineral
1:1
Expansion
None
CEC (cmol/kg) 2 – 5
tetrahedral
substitution
Smectite
2:1
High
80 - 120
Octahedral
substitution
Vermiculite illite
2:1
Limited
100 – 160
2:1
None
20-40
{
Kaolinite
tetrahedral
substitution
Soils and Clays
A horizon
E horizon
(Elluvial)
Bt horizon
B horizon
Bt horizon
(Illuvial)
t = accumulation of silicate clays formed
in place or translocated from above
Implications of Negative Charge sites.
Important Cations: H+, Ca2+, Mg2+, Zn2+, Mn2+, K+, NH4+, Cu2+
Zn2+
Ca2+
reserve
Mg2+
Cu2+
K+
Na+
H+
Ca2+
H+
Na+
K+
K+
H+
Ca2+
Ca2+
Na+
K+
K+
H+
Ca2+
Na+
K+
Na+
Flocculation and Dispersion
Particles with a Single Charge
Na+
Na+
Ca2+
Clay-Sized Particles
------
-----Na+ Na+ Na+ Na+
Na+ Na+ Na+
Al3+
Al3+
Al3+
Al3+
Na+
Na+ Na+ Na+ Na+
-----Dispersed
-----Flocculated
Ambient concentration
High cation concentration
---------------
---------------
--------------Ambient solution cation concentration
---------------
---------------
Cations satisfying charge on both negatively charged colloids
Flocculation
Aids in development of soil structure
Increases infiltration of water at the soil surface
Can increase hydraulic conductivity
Increases movement of gases (O2)
Aids in root penetration
A Practical Matter for Flocculation
Wastewater Constituents
Pathogenic organisms: bacteria, protozoa, viruses
Non-pathogenic bacteria (> 100,000 / ml for sewage)
Organic (carbon) particles: feces, food, plant material, humus
Soluble Organics (carbon): pesticides, poisons, paints, drugs
Soluble Inorganic Chemicals: nutrients (N and P), metals
Inorganic particles: sand, silts, clays
Turbidity
Organic and inorganic
particles
Higher turbidity levels are often
associated with higher levels of
viruses, parasites and bacteria.
Primary Water Treatment
Primary Treatment
Screening, Grit Removal, Primary Settling
particles larger than ¼”
Bar Screen
Grit Chamber
Large particulates
Sand and gravel
esp. storm water
Short time period
V = KD2
Primary Settling Tank
Smaller Particles
Suspended solids
Longer residence time
Chemicals to promote flocculation/coagulation
Solids settle to the tank bottom as “primary sludge”
Some remains as suspended solids
Chemical Flocculation
Suspended silts, clays and organic matter are
Generally small and negatively charged.
Al3+ Al3+
Al3+ 3+
Al
Al3+ Al3+
Primary Treatment
Suspended solids
Sludge
Iron and Aluminum
Oxides/hydroxides
Weathering of Rocks and Minerals
Rocks
Granite
Basalt
physical
chemical
Primary Minerals
Quartz
Muscovite
Feldspars
biotite
Secondary Minerals
Silicate clays
Fe oxides
Al oxides
chemical
soluble constituents (Ca2+, Mg2+, K+, Fe2+, Al3+, Si4+,CO3 2 -, SO42-)
Formation
Smectites
Increased
Weathering
Kaolinite
Fe, Al oxides
Hot, wet climates
Iron Oxides
Release of metal ions from minerals
Chemical weathering
-SiO-Fe2+ + 2H+ <--> Fe2+ + 2OH-SI
Iron bearing silicate
Free iron
Fe2+ (Mobile)
oxygen
Fe3+
water
Fe(OH)3
Lack of oxygen
Redoximorphic Features
Fe2+
Fe3+
Iron Oxides
Color
Goethite
most frequently occurring Fe-oxide
in soil and has a characteristic
yellowish brown color
Bo Horizon
Hematite
High soil temperature, better drained
soils, rapid biomass turnover, high
Fe-release rate from rocks
Bo is an oxic subsurface horizon
Water and Temperature
Less weathered, poorer drainage
More weathered, better drained
Goethite
Hematite
Temperate
Tropical
Iron Oxides
Can possess negative, positive, zero charge
Potential interaction with cations and anions
Cl-, F-, Br-, SO42-, NO3-, CO32-, PO4-3
Anion Exchange
Aluminum Oxides
Products of Extreme Weathering
Breakdown of Al-O-Si linkages
Si, Al
kaolinite
Removal
by leaching
water
Al3+
Al3+
Water, crystallization
Al(OH)3 Gibbsite (grayish-white color)
The principal aluminum hydroxide in soil
Products of Extreme Weathering
Al(OH)3
Gibbsite
crystalline and octahedral
Products of Extreme Weathering
kaolinite
Gibbsite
Al(OH)3
Environments
Less weathered
Highly weathered
Kaolinite Dominated
Gibbsite Dominated
Aluminum Oxides
Can possess negative, positive, zero charge
Potential interaction with cations and anions
Cl-, F-, Br-, SO42-, NO3-, CO32-, PO4-3
Anion Exchange
Aluminum has a high affinity for phosphorus
Chemical Flocculation
Suspended silts, clays and organic matter are
Generally small and negatively charged.
Al3+ Al3+
Al3+ 3+
Al
Al3+ Al3+
Primary Treatment
Suspended solids
High in Al
Aluminum has a high affinity for phosphorus
Both iron and aluminum oxides are
prominent in highly weathered environments
Can possess negative, positive, zero charge
Both are capable of cation and anion exchange
Next: Organic Colloids