Chemosphere II

Question 1

What is the colloidal fraction?

Answer 1

Clay and organic matter particles in a soil.

Question 2

Why are clay and OM referred to as the colloidal fraction?

Answer 2

Because they are small with a high surface area, and because they are highly reactive with electrically charged surfaces.

Question 3

Describe the importance of surface area for soil colloids.

Answer 3

The larger the surface area, the more sites are available for adsorption, catalysis, and microbial colonization. Overall, the amount of chemical reactivity increases with surface area.

Question 4

Describe the importance of surface electric charge for soil colloids.

Answer 4

The internal and external surfaces carry a (usually) negative charge, which attracts water molecules and anions in solution as well as other colloids.

Question 5

Where do water and ions adsorb on colloids?

Answer 5

Between clay layers in the interlayer space

Question 6

What type of water is held in the interlayer space in colloids?

Answer 6

Hygroscopic water

Question 7

What are the main 2 types of colloids?

Answer 7

Mineral colloids and organic colloids (organic matter)

Question 8

What are the main types of mineral colloids?

Answer 8

Crystalline silicate clays (1:1 clays, 2:1 clays) and iron and aluminum oxides

Question 9

What are the building blocks of silicate clays?

Answer 9

Each silicate clay has silicon tetrahedron and aluminum (or magnesium) octahedron units.

Question 10

Describe the structure of silicon tetrahedrons.

Answer 10

They are 4-sided building blocks composed of a Si ion surrounded by 4 O atoms.

Question 11

Describe the structure of an aluminum or magnesium octahedron.

Answer 11

They are 8-sided building blocks composed of an Al or Mg ion surrounded by 6 OH groups or O atoms.

Question 12

Describe the structure of clays, beginning with the fundamental building blocks.

Answer 12

The single tetrahedron and octahedron units link together to form sheets. The sheets then connect to one another to form layers.

Question 13

What are the 2 major types of silicate clays? Give an example of each.

Answer 13

1. 1:1 clay, ex: Kaolinite
2. 2:1 clay, ex: montmorillonite

Question 14

Describe the structure of 1:1 clays.

Answer 14

Each layer consists of 1 tetrahedral sheet + 1 octahedral sheet, which are connected via a shared O. The octahedral sheet has an OH plane exposed, and the different layers are bound together by the hydrogen bonding between the exposed OH groups

Question 15

Describe the properties of 1:1 clays in terms of water retention and expandability.

Answer 15

They cannot hold very much water because of the tight H bonds between the layers. Therefore, they cannot expand very much when wet.

Question 16

Describe the structure of 2:1 clays.

Answer 16

Each layer consists of 2 tetrahedral sheets + 1 octahedral sheet. The sheets are weakly bound together by cations or oxygen.

Question 17

Describe the properties of 2:1 clays in terms of water retention and expandability.

Answer 17

Since the sheets are weakly bound together, the internal surface area is high and accessible to water and ions. 2:1 clays are therefore prone to expansion when wet.

Question 18

Is clay used in pottery 1:1 or 2:1? Why?

Answer 18

It has to be 1:1, because the water can’t expand and shrink with water, otherwise it would crack.

Question 19

Is cracked, dry clay 1:1 or 2:1? Why?

Answer 19

It has to be 2:1, because in order for cracks to form, the clay had to have expanded when wet, which is a characteristic of 2:1.

Question 20

What are metal oxides and where are they found?

Answer 20

They are another type of soil colloid. They are soils that contain an abundance of clay-sized minerals that lack Si but have some properties of silicate clays. They are often found in weathered soils.

Question 21

Describe the key characteristics of metal oxides.

Answer 21

They can have a positively charged surface under certain pH conditions, so they can adsorb anions. They are therefore important drivers of phosphorus availability in tropical soils.

Question 22

What type of organic matter is considered colloidal?

Answer 22

Only highly decomposed organic matter is considered colloidal.

Question 23

Does organic matter have internal surface area?

Answer 23

No.

Question 24

What are the main 3 groups found on the edge of organic molecules?

Answer 24

Carboxylic groups, phenolic hydroxyl groups, and alcoholic hydroxyl groups.

Question 25

Is organic matter positively or negatively charged? Why?

Answer 25

It is negatively charged. This is because of all the functional groups on its edge which can have a negative charge if they are deprotonated.

Question 26

How does the charge of 2:1 clays compare to the charge of 1:1 clays?

Answer 26

2:1 clays have a much higher charge than 1:1 clays.

Question 27

How does the charge of organic matter colloids compare to the rest of the colloids?

Answer 27

Organic matter is extremely negative (-100 to -500), moreso than mineral colloids.

Question 28

Name 4 major plant macronutrient cations and their plant available form(s).

Answer 28

Ammonium: NH4+
Potassium: K+
Calcium: Ca2+
Magnesium: Mg2+

Question 29

Name 3 major plant macronutrient anions and their plant available form(s).

Answer 29

Nitrate: NO3-
Phosphorus: PO43-, HPO4-2, H2PO4-
Sulfur: SO2, SO42-

Question 30

Name 4 plant micronutrient cations and their plant available form(s).

Answer 30

Iron: Fe2+, Fe3+
Manganese: Mn2+
Zinc: Zn2+
Copper: Cu+, Cu2+

Question 31

Name 3 plant micronutrient anions and their plant available form(s).

Answer 31

Boron: H3BO3, B4O72-
Molybdenum: MoO42-
Chlorine: Cl-

Question 32

If you add nitrate vs ammonium into soil, which is more likely to stay in the soil and why?

Answer 32

Ammonium because it is a cation, so it will be held by the colloids.

Question 33

If you ammonium into soil in a 1:1 clay vs a 2:1 clay, where will it be held in greater quantities and why?

Answer 33

There were be more in a 2:1 clay because the surface area and charge are higher.

Question 34

An example of a soil amendment for managing pesticide mobility is […]

Answer 34

The addition of biochar

Question 35

Explain how biochar changes the movement of pesticides in soil.

Answer 35

Without biochar, pesticides are taken up by plants and reduce microbial populations, which limits the biodegradations of pesticides. With biochar, adsortion is increased in the soil, reducing uptake by plants. It also supports the microbial population that partakes in pesticide degradation.

Question 36

What type of soil is biochar most useful in and why?

Answer 36

It is most useful in a sandy soil with poor adsorption.

Question 37

What are the two possible sources of electrical charge in colloids?

Answer 37

1. Permanent (isomorphic substitution)
2. pH-dependent (variable)

Question 38

What is isomorphic substitution?

Answer 38

it is a charge imbalance brought about by the isomorphous substitution in some clay structures. Occurs during the development of the clay and is a fixed feature of the clay structure.

Question 39

How does isomorphic substitution create a charge imbalance?

Answer 39

This happens when there is replacement of an ion with another ion of similar size but with a different charge, leaving an unsatisfied negative net charge.

Question 40

What type of colloids typically undergo isomorphic substitution?

Answer 40

2:1 clays, not 1:1 clays or OM.

Question 41

Explain how (in general) pH can affect the charge of mineral colloids.

Answer 41

Depending on the pH, hydroxyls on the surface of colloids deprotonate or accept H+ ions, resulting in different net charges.

Question 42

Explain how lower pH can affect the charge of mineral colloids.

Answer 42

At a lower pH, the surface charge will be less negative. Surface OH groups will tend to keep or gain their H+.

Question 43

Why can some clays be positively charged in very acidic conditions?

Answer 43

At very low pHs, if enough surface OH groups keep or gain their H+, the charge could go positive.

Question 44

Explain how higher pH can affect the charge of mineral colloids.

Answer 44

At higher pHs, H+ will be released from hydroxyl groups into solution, leaving behind a higher negative charge.

Question 45

What type of clay typically experiences pH-dependent charge? Why?

Answer 45

1:1 clays, because they have an octahedral sheet with an OH plane exposed, allowing for easy H+ gain and loss. 2:1 clays don’t have this exposed octahedral sheet, so there’s less of an effect on pH.

Question 46

Is the charge of organic matter mainly permanent or pH-dependent? Explain.

Answer 46

It has almost no permanent charge and is highly pH dependent.

Question 47

Which of the following types of colloids are or aren’t pH dependent?
Non-mineral, 2:1 clays, 1:1 clays, Al and Fe oxides, noncrystalline clays

Answer 47

Non-mineral: pH dependent
2:1 clays: not pH dependent
1:1 clays: pH dependent
Al and Fe oxides: pH dependent
Noncrystalline clays: pH dependent

Question 48

How does weathering affect the makeup of 2:1 clays vs 1:1 clays vs Fe-Al oxides?

Answer 48

As weathering increases, it generally goes from 2:1 to 1:1 to Fe-Al oxides.

Question 49

What are 2:1 non-expandable silicate clays?

Answer 49

They are similar to typical 2:1 expandable clays except that the sheets are closer together, as they are connected with a K+. Therefore, there is low internal surface area and it is non-expansive.