Lecture 9: Colloids

Question 1

Memorize the names, color, and chemical composition of
common Fe and Al oxides

Answer 1

Gibbsite (Al(OH)3)
Goethite (FeOOH) (bog iron, brown ochre)
Hematite (Fe2O3) reddish brown
Ferrihydrite ((Fe3+)O3*0.5H2O) (more orangey)
Lepidocrocite (FeO(OH)) (spotty more brown with some red and yellow)

Question 2

Describe how primary silicate minerals weather into Fe and Al oxides and other conditions that create Fe oxides

Answer 2

Fine-grained micas, vermiculite, and chlorite are formed through mild weathering of primary aluminosilicate minerals
Intermediate weather intensity favors smectite
More intense weather creates kaolinite

Question 3

Describe how cations and anions are attracted to colloids via
inner- and, or outer-sphere complexes

Answer 3

Cations are typically connected to clays as outer sphere complexes
Micas can bind some cations via inner-sphere complexes, which bind directly to charged colloid surface
Anions can connect as inner (PO4 3-) or outer sphere complexes (Cl-,NO3-,SO4 2-)

Question 4

What are the 6 plant macronutrients

Answer 4

Ammonium NH4+
Calcium Ca 2+
Magnesium Mg 2+
Potassium K+
Nitrate NO3 -
Phosphate HPO4 2-
Sulfate SO4 2-

Question 5

6 rules of cation exchange

Answer 5

Reversibility
Charge equivalence
Ratio Law
Anion effects on mass action
Cation selectivity
Complementary ions

Question 6

Describe how cation and anion retention capacity of soils change as a function of colloid type and weathering

Answer 6

the more weathered the soil is, the less cation exchange capacity is but the more the anion exchange capacity it.
highest to lowest cation exchange capacity:
2:1 types clays -> 1:1 type clays -> mostly Fe, Al oxide clays

Question 7

Why does sand have a low ability to retain cations?

Answer 7

Sand is not a colloid, it’s a particle size. They are larger in diameter and have less surface area than colloids.

Question 8

How is the ability of soils to retain organic contaminants based on their partitioning coefficients?

Answer 8

greater values indicate greater attraction by colloids

Question 9

Reversibility

Answer 9

you can change a cation for a cation

Question 10

Charge equivalence

Answer 10

the charges must be balanced on both sides

Question 11

Ratio law

Answer 11

the ratio that exists in the liquid exists in the cation exchange

Question 12

Anion effects on mass action

Answer 12

you can force more of a specific cation by making some of the anion like CO2 go away as a gas

Question 13

Cation selectivity

Answer 13

the higher the charge and the smaller the hydrated radius, the stronger the absorption
EXCEPT for H+ that one is the strongest