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Soil of the Environment > Colliods > Flashcards

Colliods Flashcards

1
Q

layer silicate clays

A
  • mostly permanent (except Kaolinite), negative charge

- cation absorption and exchange

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2
Q

colloid characteristics

A
  • size
  • surface area
  • surface charge
  • absorption of plant nutrients (& water). the nutrient ions are held on colloidal surfaces where they are available for plant uptake by roots
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3
Q

layer silicate clays formation

A
  • soil crumb, 1mm
  • clay skin, 100um
  • clay domain, 10um
  • clay particle <2um
  • clay layer (100x100x1nm)
  • sheets
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4
Q

Kaolonite

A
  • width 10um
  • 1:1 type silicate clay
  • layers are held together by hydrogen bonding. cations and water do not enter between layers. therefore, low surface area and net negative charge
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5
Q

tetrahedral silicon

A

4 faces

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6
Q

octahedral magnesium or aluminium

A

8 faces

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7
Q

building blocks of layer silicate clays

A

the specific composition, number and combination of sheets within the layers vary from one type of clay to another, and largely control the physical and chemical properties of the clay

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8
Q

isomorphous substitution

A

the replacement of one atom by another of similar size in a tetrahedral or octrahedral sheet without disrupting or changing the basic structure of the crystal unit

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9
Q

2 basic types of silicate clays

A

on the basis of the number and arrangement of tetrahedral and octahedral sheets

1: 1 type
2: 1 type

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10
Q

1:1 type

A
  • 1 tetrahedral (Si) to 1 octahedral (Al) sheet
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11
Q

2:1 type

A
  • 2 tetrahedral (Si) to 1 octahedral (Al) sheet

- Smectite:

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12
Q

Expanding 2:1 type

A

layers held together by weak oxygen to oxygen and cation to oxygen bonds. water and cations can enter between layers, therefore, high surface are and net negative charge

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13
Q

Sectite

A
  • weak bond, water entry causes swelling, exchangeable cations on both external and internal faces
  • isomophous substitution of Mg2+ for some Al3+ in dioctahedral sheet accounts for most of negative charge
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14
Q

Vermiculite

A

isomorphous substitution of Al3+ for some Si4+ in tetrahedral sheet accounts for most negative charge

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15
Q

Illite

A
  • non expanding: strong bond prevents inter layer expansion, exchangeable cations on external faces
  • large extent of isomorphous substituion of Al3+ for Si4+ in tetrahedral sheet accounts for most of negative charge
  • consequent attraction of K+ forms tight bridge between layers, preventing interlayer expansion. therefore, surface are & negative charge > kaolinite but < smectitie and vermiculite
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16
Q

Fe and Al oxide clays

A
  • pH dependent (variable) charge

- cation and anion absorption and exchange

17
Q

Humus

A
  • pH dependent (variable)

- cation and anion absorption and exchange

18
Q

sources of charges in soil

A
  • permanent (constant) charge or pH dependent (variable) charge
19
Q

permanent (constant) charge

A
  • usually negative charge due to isomorphous substitution in layer siliacte clays
  • replacement of one atom by another of similar size without disruption or changing the basic structure of the crystal unit
  • charge is not affected by the chemistry of the surrounding soil suction
20
Q

pH dependent (variable) charge

A
  • at the ends and edges of sheets, the charge is not exactly balanced
  • surface charge is controlled by the chemistry of the surrounding soil solution
  • can be negative or positive
21
Q

Cation exchange

A
  • is the interchange between a cation in solution and another cation absorbed on the surface of any negatively charged colloid
  • allows essential cations fro plant growth, Ca, Mg, K to be released from the colloid surface to the soil solution
  • vital because the amount of nutrients in soil solution at any one time is much less than is needed to produce mature plants and prevents leaching losses of nutrients
22
Q

cation exchange capacity (CEC)

A
  • the sum total of exchanagble cations that a soil can absorb or the capacity of the soil to hold cations (measures as moles of charge/kg)
  • roughly proportional to the amount of clay and organic matter in soil
  • but remember pH dependent change and effect of colloid type
23
Q

units for exchangeable cations and CEC

A
  • do not exchange in terms of equal masses
  • do not exchange in terms of equal moles
  • do exchange in terms of equal charge
24
Q

determination of CEC

A
  • displace soil cations by ammonium
  • displace ammonium by potassium
  • measure ammonium: the ammonium concentration represents the total CEC
25
Q

anion exchange

A
  • is the interchange between a anion in solution and another cation absorbed on the surface of any positively charged colloid
  • overall electrical neutrality is maintained as equivalent moles of charge are exchanged
  • allows essential anions for plant growth, Ca, Mg, K to be released from the colloid surface to the soil solution
  • vital because the amount of nutrients in soil solution at any one time is much less than is needed to produce mature plants and prevents leaching losses of nutrients
26
Q

anion exchange capacity

A
  • the sum total of exchangeable anions that a soil ca absorb in freely exchangeable form
  • normally determined at the H (and concentration) of the soil solution, because of the pH dependence of the charge at the surfaces of Fe and Al oxides
  • expressed as moles or centimoles of charge/kg of soil
27
Q

Cation exchange continued

A
  • any of the cations in soil solution can substitute for the cations on the colloids
  • but overall electrical neutrality is maintained
  • cations present in solution in high concentrations will be favoured in the exchange reaction
28
Q

% base saturation

A

(ca+Mg+K)/CEC x 100

29
Q

Anion exchange continued

A
  • the affinity of anions at a colloid surface varies greatly, from anion exchange to anion binding
  • C1- and NO3- are attracted to positively charged surfaced only by electrostatic forces (freely exchangeable)
  • bind more strongly owing to the formation of a chemical bond between the anion and the surface, especially variable charge

Soil of the Environment (8 decks)

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