SWS3022 Exam #2

(T/F) Isomorphous substitution is pH dependent.

False

What kind of net charge is developed on aluminosilicate minerals? What are the implications in terms of soil solution cations?

From isomorphous substitution, the remaining charge on the soil is either a result of a deficit of positive charge or a surplus of negative charge in the mineral structure. The negative charge developed is satisfied by cations in soil solution, also known as the cation exchange.

What soil characteristic dramatically effects the strength of hydraulic conductivity?

Pore space plays a major factor in the degree of hydraulic conductivity. Anything that increases pore space increases hydraulic conductivity.

What are the two properties that make certain types of organic chemicals highly water soluble?

Polar and electrically charged

Kaolinite

-1:1 Mineral -Non-expansible due to extensive hydrogen bonds -CEC=2-5 -Limited isomorphous substitution in the tetrahedra -Adsorption occurs on the outer mineral surface -pH dependent

Smectite

-2:1 Mineral -Includes montmorillonite -Significant substitution in the octahedra -CEC=80-120 -Highly expansible -A large distance between the source of charge and adsorbed cation (contributes to expansibility)

Illite

-2:1 Mineral -Significant substitution in both layers -Source of negative charge very close to absorbed cations (non-expansible) -CEC=20-40 -Potassium ions fit perfectly in the tetrahedra; direct contact creates a strong attraction, which limits expansibility

Vermiculite

-2:1 Mineral -Significant substitution in the tetrahedra -CEC=120-180 -Source of negative charge close to the adsorbed cations -Layers tightly bound; moderate expansibility

What are the three factors that determine cation preference at a negative site?

1. Concentration in solution 2. Charge (Higher charge more attracted to the surface) 3. Size (smaller cations preferred at negative sites)

What are the three basic classes of organisms and which is the one primarily responsible for OM decomposition?

1. Herbivores: Consume live plants (ie-Insects, mammals, reptiles) 2. Detritivores: Consume dead tissues (ie-Fungi, bacteria) 3. Predators: Consume other organisms (ie-Insects, mammals, reptiles, birds) Detritivores primarily responsible for OM decomposition/humic material.

What are some characteristics of humus?

1. Highly resistant to breakdown 2. Amorphous (no structure/shape) 3. High surface area 4. Can develop significant levels of charge

What are the three fundamental features of soil mineral colloids?

1. Small (<1 micron) 2. Large Surface Area 3. Highly Reactive

What is the CEC range for organic colloids?

150-500 cmolc/kg (mineral colloids range from 0-180 cmolc/kg)

What is the main difference between a 1:1 mineral and a 2:1 mineral?

A 1:1 mineral is made up of two crystalline layers of silica tetrahedra and aluminum octahedra. A 2:1 mineral has a sandwich-like structure of octahedra between two tetrahedra.

Under what conditions relative to pH is negative or positive charge developed on Fe and Al oxides?

A low pH indicates a more acidic solution, which also means there is a greater positive charge on iron aluminum oxides (protonation). A high pH indicates a more basic solution, which also means there is a greater negative charge on iron aluminum oxides (deprotonation).

Anions

A result of a gain of electrons; negative charge

Cations

A result of a loss of electrons; positive charge

What are the dominant ions comprising aluminosilicate clays?

Al3+ Si4+ OH- O2-

What is cation charge density?

An indication of how tightly cations are held at exchange sites. charge divided by size

What is an octanol-water partitioning coefficient?

An organic carbon partitioning coefficient is a measurement of how much carbon is in a chemical. It is found by mixing the chemical in a solution of octanol and water. Kow=concentration of chemical in octanol/in water Chemicals preferring octanol have high Kow, which corresponds to low water solubility and strong attraction to carbon matrices.

How do hydrophobic organic chemicals interact with organic colloids in soils?

An organic colloid has a non-polar, uncharged center, in which it is surrounded by carboxylic and phenolic molecules (charged). They provide a carbon-based media/domain for hydrophobic organic chemicals to dissolve in.

What are the important soil solution constituents in inorganic soils?

Based on metals, chemical nutrients, and acids (pesticides) -Sometimes based on the carbon atom

What are the principal elements in organic chemical compounds?

Carbon and oxygen

What do organic and inorganic soil solution constituents have in common?

Dissolved ions that originate in soil solution from the breakdown of organic matter in mineral weathering.

What are the important soil solution constituents in organic soils?

Dominated by carbon, hydrogen, and oxygen (mostly carbon and oxygen)

What is the relationship between pH and H+ ion concentration?

H+ ions are common in acids, while OH- ions are common in bases. pH is the measure of the amount of hydrogen ions in water. pH=-log(H+) -Log indicates that for each 1-unit change in pH, the H+ concentration changes by 10-fold. -pH of 7=10^-14

What are the impediments to water solubility for non-ionic hydrophobic organic compounds?

Have strong interactions with the soil, and pose a chemical leaching hazard in soils. These chemicals prefer carbon-based environments.

How does density impact hydraulic conductivity?

High density suggests low porosity, which indicates smaller pores. Smaller pores mean weaker hydraulic conductivity.

What conditions relative to pH favor charge development in soil organic colloids?

High pH (High levels of hydroxide ions-negative charge development)

Actual water movement is strongly dependent on:

Hydraulic conductivity

How do hydrophobic organic compounds accumulate in surface soils?

Hydrophobic organics become stored in these organic domains that naturally occur in aquatic and terrestrial systems. It protects them from degradation and keeps them in the environment for decades. Hydrophobic chemicals prefer the environment of fatty lipid tissue in organisms because of their high levels of carbon, which attributes to bioaccumulation in organisms.

How does hydraulic conductivity and gradient determine the overall flow of water in soils?

In Darcy's equation, multiplying the hydraulic conductivity by the gradient yields the total flow of water in cm/hr.

Mineral Colloids

Inorganic and organic matter with very small particle size and a correspondingly large surface area per unit of mass, resulting in high reactivity. Its layered structure is a result of a 1:1 or 2:1 mineral layer.

How does structure impact hydraulic conductivity?

Inter-aggregate macro pores improve hydraulic conductivity.

How do ions differ from elements in the Perioidic table?

Ions are stable forms of elements that acquire an electrical charge from the gain or loss of electrons, while elements have equal numbers of protons and electrons and therefore have no charge.

Why are interlayer spaces in Kaolinite unavailable for exchange reactions or water entry?

Its layers are glued together by hydrogen bonds and are non-expansible, which prevents the interlayer spaces from being available for exchange reactions or water entry. Therefore, there is limited isomorphous substitution in the tetrahedra (Al3+ for Si4+), which limits surface area and reactivity.

How are crystalline aluminosilicate clay minerals derived?

Made from the chemical weathering of primary minerals. These secondary minerals are dominated by aluminum, silicon, and oxygen.

Why is NH4+ less mobile in the environment than NO3-?

Nitrate ions are more common in the environment than the ammonium ion because nitrates are negatively charged and are therefore repelled, moving along with water. Ammonium, in contrast, is positively charged, so it is attracted to the negative sites in soil solutions (adsorption).

What is the difference in charge development in organic colloids compared to that of aluminosilicate minerals?

Organic colloids develop charge from deprotonation of hydrogen ions from acidic functional groups on humic organic materials. Aluminosilicate minerals generally get their electrical charges from isomorphous substitution.

How do iron and aluminum oxides form?

Products of advanced weathering of primary silicate minerals in which silicon is lost during the weathering process.

How is negative charge developed on soil organic colloids? What are the functional groups involved?

Protonation and deprotonation develops charge on soil organic colloids. The functional groups involved include carboxylic and phenolic.

What are the building blocks of aluminosilicate clays?

Silicon tetrahedron and aluminum octahedron.

How does texture impact hydraulic conductivity?

Small particles yield small pores, which creates poor conductivtiy.

Partitioning

The distribution of organic chemicals into a portion that dissolves in the soil organic matter and a portion that remains undissolved in the soil solution.

Hydraulic Conductivity

The ease with which water moves through soils.

What is Cation Exchange Capacity?

The exchanging of cations in soil for one another in negative sites found within a soil. For every negative charge, there must be an equal number of positive charges. Any disruption in soil equilibrium will force cations to exchange and create a new equilibrium.

Gradient

The likelihood water will flow; found by calculating the difference in total potential energy divided by the distance between the two points.

What is the relationship between the water solubility of organic chemicals and the strength of interaction with organic colloids?

The more soluble an ion is, the greater the tendency not to interact strongly with humic materials. Hydrophobic solutions tend to favor hydrophobic environments, and therefore move into the organic phase. (The greater the water solubility of organic compounds, the lesser the strength of its interaction with organic colloids).

Humus

The organic component of soil; formed by the decomposition of leaves and other plant materials by detritivores. Usually very reactive due to carbon content, surface area, and charge.

Isomorphous Substitution

The substitution of lower-charge cations for higher-charge cations during mineral formation; creates a greater negative charge on minerals, which dramatically increases the reactivity of clays. -Al3+ for Si4+ in tetrahedra -Mg2+ for Al3+ in octahedra

What is the cation exchange capacity units and range for the various mineral colloids?

The units are cmolc/kg, with the range for mineral colloids being anywhere from 0-180 cmolc/kg

How does one determine the direction in which water will flow?

To find the total potential energy, you add all the energy values together. By comparing this value with the total potential energy for the other point, you can find the direction of flow. Water ALWAYS flows from areas of high total potential to low total potential energy.

Does water move in response to a single potential energy or total potential energy?

Total potential energy

(T/F) Charged ions are well accommodated in water and are prominent constituents in soil solution.

True

(T/F) The charge on iron and aluminum oxides is variable and pH dependent.

True

(T/F) The stronger the gradient, the greater the driving force of water movement.

True

Ion Hydration

Water's ability to orient itself relative to charged ions and therefore remain in solution.