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Tonight PowerPoint Presentation

Tonight

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Tonight

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  1. Tonight • Module 1 exam • Mineralogy • Soil colloids

  2. Sand • May be rounded or irregular • Not sticky, even when wet • Water holding capacity is low • Quite inactive chemically

  3. Silt • Irregularly fragmental • Diverse in shape • Seldom smooth or flat • Really is microsand • Has an adhering film of clay

  4. Physical properties of silt • By itself, acts like sand • Quite inactive chemically • Coated with clay: • some plasticity • some cohesion • some adsorption

  5. Soil Colloids • Phyllosilicate clays • Hydrous oxides of Al and Fe • Amorphous minerals • Organic soil colloids

  6. Importance of Soil Colloids • Shrinking • Swelling • Plasticity (Stickiness) • Cohesion • Dispersion • Flocculation

  7. Properties of Soil Colloids • Small size • High specific surface area • Surface charge

  8. What is clay? • Size of <2 m • Layered structure, thin plates • Tetrahedral and octahedral layers • Covalent bonding

  9. Clay • High surface area • High adsorption power • Highly plastic when wet • High heat of wetting • Chemically active

  10. Permanent charge • Due to charge imbalance in the crystal structure • Charge imbalance due to isomorphous substitution • Charge due to broken edges of crystals

  11. Cation exchange capacity • A cation has a positive charge • Once cation can be exchanged for another • CEC = cation exchange capacity • CEC is pH dependant

  12. Cation exchange • Cations attracted to the negatively charged clay • Cations concentrated next to the clay surface • Less concentration of cations as move away from the clay surface

  13. Adsorbed cations • H+, Al3+ and Ca2+ • Mg2+ • K+ and Na+

  14. Lyotrophic Series • How close a cation can get to the surface of a clay depends on the hydrated size and charge of the cation • Al3+>Ca2+>Mg2+>NH4+>K+>Na+

  15. 1:1 and 2:1 layers • 1:1 One sheet of tetrahedral and one of octahedral • 1:1 (open faced sandwich) • 2:1 two tetrahedral sheets (bread) with an octahedral sheet in the middle (filling) • (closed sandwich)

  16. Swelling clay • Swelling in the interlayer space is due to water. • Not all clays swell • Some have their layers bonded too tightly to swell

  17. Surface area of clay • Clays have a high surface area • The area depends on their structure • 2:1 clays have more surface area than 1:1 clays • More area = more opportunity for reactions

  18. Effect of surface area • Adsorbing power • Swelling • Plasticity and cohesion • Heat of wetting Brady 1974 SAND SILT CLAY COLLOIDAL CLAY

  19. Iron and aluminum oxides • pH dependant charge • Al or Fe bonded to OH2+ at low pH • Al or Fe bonded to OH at neutral pH • Al or Fe bonded to O- at high pH

  20. Organic matter • Starts off as litter • Decomposed to humus • Humus has colloidal properties