Soil Science Exam 2- Soil Colloids

What are soil colloids?

Highly active soil particles

- Extremely small in size (colloidal clays are less than 1 um in diameter)

- High surface area (may have both internal and external SA)

- Usually carry a net negative charge (Attract cations)

- Highly absorptive (water, pesticides, etc)

Clays are almost always formed via a 2-step process...

1) Dissolution of other minerals

2) Recrystallization to form secondary clay minerals

3 general classes of soil colloids

1) silicate clays

2) Oxide clays

3) Organic Colloids

What are the 2 general units of construction in silicate clay formation and what do they typically contain

1) Tetrahedra- one silicon atom (SI4+) surrounded by four oxygens (Typically contains silicon, sometimes aluminum)

2) Octahedra- One aluminum atom (Al3+) surrounded by six hydroxyl (-OH) groups (typically contain aluminum, sometimes iron and/or magnesium)

Isomorphous Substitution

Tetrahedral layers in which all of the tetrahedra contain Si4+ and octahedral layers in which all of the octahedra contain Al3+ ALLLLL have an overall charge of 0.

HOWEVER, sometimes, a portion of the tetrahedra or octahedra contain the Al3+ or Fe2+ and/or Mg2+ respectively instead. This leads to a permanent net negative charge to the tetrahedral or octahedral sheet.

T/F: Not all negative charges on colloids is permanent

TRUE: All clays can have a small amount of pH-dependent charge along their edges

On silicate clays, negative charges develop at greater than what pH?

Greater than 6 pH

T/F: Tetrahedral and Octahedral sheets have nothing to do with each other

FALSE: Tetrahedral and octahedral layers are covalently bound together to form larger sheet-like structures. As a result, these layered sub structures do not separate from each other.

- they are the buildings blocks for making silicate clays

Kaolinite

- 1:1 silicate clay

- small negative charge

- non-expanding due to strong hydrogen bonding between sheets

- No isomorphous substitution (no permanent charge)

- The only potential negative charges are pH dependent and occur at the edges of sheets

- Can actually develope an overall positive charge at low pHs

- Comparatively very low surface area- external surface area only plus fairly large particle size

Smectities

- 2:1 silicate clay

- exs. Montmorillonite and bentonite

- Large negative charge

- isomorphous substitution in octahedral layer

- also has potential for pH dependent negative charge at the edges of sheets

- expanding clay - very high shrink swell capacity due to weak bonding between sheets which allows water and large ions (Na+) to enter

- High surface area- has both internal and external surface area

Vermiculite

- 2:1 silicate clay

- Very large negative charge

- Iso. Sub. in both tet and oct layers

- Also has potential for small pH dependent negative charge at edges of sheets

- Expanding clay- has moderate shrink swell capacity (moisture dependent); divalent cations between sheets help to hold layers close together

- High surface area- has both interal and external surface area

Fine Micas

- Class of 2:1 silicate clays

- illite is a common member

- 2:1 is 2 tetrahedral 1 octahedral

-Fairly low negative charge

- moderate iso sub in tet layers

- Also has potential for pH dependent negative charge at the edges of sheets

- Non-expanding due to strong bonding between sheets provided by K+ which can pit into holes in tet. layers

- Moderate surface area- Has only external surface area but small particle size

Chlorite

- 2:1:1 silicate clay

- Like 2:1 with extra octahedral layer

- Moderate negative charge

- Moderate Iso. Sub. in both tet. and oct. layers

- Also has potential for small pH dependent negative charge at edges of sheets

- Non- expanding due to strong ability of extra Mg octahedral layer to bind sheets together

- Moderate surface area- has only external surface area but small particle size

Gibbsite

- An Al-oxide clay

- Non-silicate clay- Al Octahedra only

- Little to no negative charge

- Can develop net positive charge at low pHs

- Non expanding due to H bonding

- Moderate surface area b/c has only external surface area but small particle size

- Some Al oxides are less structured than gibbsite

Geothite

- An Fe oxide clay

- Non silicate- often poorly organized octahedra or amorphous; variable shapes

- little to no negative charge

- can develop positive charge at low pHs

- Non-expanding

- Moderate surface area due to only having external surface area but small particle size

- Give rise to yellow/red (hematite) colors in soils

Humus

- An Organic colloid

- Very large negative charge

- essentially all pH dependent

- Exhibited over a wide range of pHs, including less than pH 6

- Iso. Sub. does not apply

- Non-expanding

- Surface area difficult to measure- estimates can vary widely

- overall dark color

- The different types of hydrogens are removed at different soil pHs

Cation affinities are determined by what 2 things

1) Charge (primary)

2) Hydrated radius (secondary)

Cation exchange capacity

- The sum of the cation exchange sites per unit weight of dry soil

- One cmole of one cation can be replaced by one cmole of any other cation

What is the range of cation exchange capacity of average soil

5-30 cmol/kg exchanger

Anion exchange capacity

Sum total of exchangeable anions that a soil can absorb

- increases as soil acidity increases (pH decreases)

- Caused by "extra" positively charged protons (H+) attaching to the edges of silicate clays or surfaces of metal (Fe/Al) oxides

Leaching of cations is primarily affected by

- Rate and duration of percolation

- strength with which a cation is absorbed

What can expanding clays be used for

- Sealing water table monitoring well

- drinking water wells