9: Sources of Charges in Colloids Flashcards
Crystalline clays (phyllosilicates or silicate clays)
Dominant source of charge in most soils
Layered structure of mostly Si, Al, and O atoms
Differ widely
Phyllosilicates
A type of silicate clay with a leaflike or planar structure
Building blocks for silicate clays
Silica tetrahedrons and alumina octahedrons. These building blocks form sheets, and these sheets form layers.
1:1 silicate clays
Clays in which each layer contains one tetrahedral and one octahedral sheet
Variable charge
2:1 silicate clays
Clays in which each layer contains one octahedral sheet sandwiched between two tetrahedral sheets
Always have negative net charge
What are the 2 types of soil colloids?
The two types of soil colloids are crystalline and non-crystalline
What are the 2:1 non expanding silicate clays?
Micas and chlorites
What are the 2:1 expanding silicate clays?
Vermiculites and smectites
Micas
What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area
2:1 layer mineral
Relatively stable
Low shrink-swell potential
Moderate-low CEC 10-40 cmolc/kg
Intermediate surface area 70-100m2/g
K resides in interlayer spaces which strongly bonds adjoining sheets
Isomorphous substitution in tetrahedral sheets of Al3+ for Si4+ in every fourth tetrahedral site
Chlorites
What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area
2:1 layer mineral
Stable
No shrink-swell
Moderate-low CEC 15-40 cmolc/kg
Surface area is 25-150 m2/g
Vermiculites
What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area
2:1 layer mineral
Relatively stable
Low shrink-swell potential
High CEC 100-150 cmolc/kg
Surface area is 600-800 m2/g
Potential issue is K fixation
Substitution of Al3+ for Si4+
Smectites
What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area
2:1 layer mineral
Not stable
High shrink-swell potential
Moderate CEC 60-100 cmolc/kg clay
Surface area is 600-800 m2/g
Pros and cons of kaolinite
Not optimal for soil fertility (it has a low charge and CEC) but pretty stable for construction
Uses for smectite
Once it is saturated with water, it becomes highly impermeable to water so it’s useful for some types of construction
Kaolinite
What layer
What stability
What shrink-swell potential
What level and range of CEC
What level surface area
1:1 layer mineral
Stable
No shrink-swell
Low CEC: 4-10 cmolc/kg
Surface area: 7-30 m2/g
Layers held together by hydrogen bonding, very low isomorphic substitution
Crystalline Fe and Al oxides (Gibbsite, Goethite)
Where are they found
What atoms are they made up of
Structure
High or low charge, variable or not
Colloids found in highly weathered soils of warm, humid regions
Made up of Fe or Al and O atoms
Modified octahedral sheets with Al3+ (no tetrahedral sheets)
Low charge (low stickiness and plasticity, low water-holding capacity)
Can have variable charge (+, neutral, or -)
Amorphous or non-crystalline silicate clays
What are they a mixture of?
Does it have high/low amounts of positive/negative/both charge and why?
Extra note about nutrients
Allophane-Imogolite mixture common in soils weathered from volcanic ash
High amounts of both positive and negative charge derived from the presence of OH- groups in the molecular structure
Tend to “fix” anions such as phosphates PO42- and sulfate SO42-, fixed nutrients are not plant available
Organic matter (humus)
What is it made up of
Is it crystalline or not
How is its plasticity and shrink-swell capabilities?
Highly decomposed organic matter that is made up of large molecules of C chains and rings
Non-crystalline
No plasticity or shrink-swell
Patterns of colloid charges and pH
What characteristics of colloids result in them having charges which are more pH-dependent or permanent?
Colloids with isomorphous substitution are not really affected by pH changes (like smectite and vermiculite)
For crystalline silica colloids, simpler structure means lower percentage of permanent charge and higher percentage of pH-dependent charge. More complex crystalline colloids are the opposite
Anion exchange
Same as cation adsorption but in positively charged colloid surfaces
Higher anion exchange capacity in acid soils
Contributes to retention of important plant nutrients
