SWS3022 Exam 2
Know the 3 fundamental features of soil mineral colloids
1. small (< 1 micron) 2. large surface area 3. very reactive compounds
Know the difference between a 1:1 and 2:1 mineral
A 1:1 mineral has 1 tetrahedra sheet for every octahedra sheet while a 2:1 mineral has 1 octahedra sheet that is sandwiched between 2 tetrahedra sheets.
Understand what is meant by acidic functional groups on organic matter and be able to identify the 2 dominant types
Acidic function groups have an OH group that can be deprotonated at a high pH. - carboxylic and phenolic are the 2 dominant types
Which of the following would be most preferred at charged silicate clay surfaces all else being equal
Al3+
The three chemical elements that dominate the structure of aluminosilicate minerals are
Aluminum, silicon and oxygen
Define humus
An amorphous colloidal organic substance that posses no plant cellular organization - highly resistant to breakdown - highly reactive due to carbon content, surface area, and charge - can contribute up to 80% of reactivity of surface soils
Cations
Are positively charged and are attracted to charged soil silicate clay mineral surfaces
Understand fundamentally how cation exchange capacity is measured
CEC is measured by saturating a soil with one specific type of ion then displacing those ions into an empty beaker and measuring how many were extracted from the soil per kg.
Identify the elements common to organic chemicals
Carbon, hydrogen, oxygen
Cation A has a +3 charge and is small. Cation B has a +1 charge and is large. Which is more likely to occupy negatively charged sites on a clay mineral given equal concentrations in solution?
Cation A
Understand the concept of cation charge density and how it impacts preference of various cations for negatively charged sites on mineral colloids
Cation charge density is the charge / size of the ion. This indicates how tightly cations are held at exchange sites. A higher cation charge density will be adsorbed better to the surface.
Understand cation exchange and the three factors that determine cation preference at negative sites
Cation exchange is when cations that are adsorbed to the soil surface exchange for other cations taht are floating around the soil solution. 1. concentration in solution 2. size of cation 3. charge of cation (magnitude)
Know the difference between cations and anions. Understand particularly how charge is acquired
Cations are positively charged ions that are formed through losing an electron anions are negatively charged ions that are formed through gaining an electron
Describe the two factors that render some organic chemicals to be highly soluble in water
Charged and polar molecules
Know the fundamental building blocks for aluminosilicate clays and the dominant ions comprising them.
Common building blocks: silicon tetrahedron, aluminum octahedron Ions: Al3+, Si4+, O2-, OH-
Understand that charge on iron and aluminum oxides is variable and pH-dependent
Crystal edges are very pH dependent and undergo protonation/deprotonation
Understand how electrical charge is developed on organic matter, and the role of soil pH in developing charge
Electrical charge is developed on organic matter through deprotonation of acidic functional groups. At a high pH, the molecule will be deprotonated which causes a negative charge to occur. **organic matter can participate in cation exchange**
Illite has a relatively low CEC because there is a very little isomorphous substitution
False
Isomorphous substitution in smectites takes place in the tetrahedral sheet
False
Negative charge on soil minerals is satisfied by anions from soil solution
False
The charged on soil clays is satisfied by anions from soil solution
False
Vermiculites are highly expanding minerals
False
Know how ions differ from elements in the periodic table
Ions are stable forms of elements that have been formed through either gaining or losing an electron
Understand ion hydration and how it can impact the size of cations in soil solution.
Ions from OM breakdown and weathering generally are compatible with water. Because of waters ability to orient relative to charged ions, they remain in solution. A hydration sphere forms around the charged ion and makes cations bigger than without the sphere.
Understand isomorphous substitution in silicate clay minerals.
Isomorphous substitution is the substitution of a lower charged cation for a higher charged cation in the formation of aluminosilicate clays. - Mg2+ for Al3+ in aluminum octahedra - Al3+ for Si4+ in silicon tetrahedra
Understand the features of the 4 clay minerals discussed: location of isomorphous substitution, CEC, expansibility, interlayer accessibility, and the basic reasons for differences in the fundamental properties of these clay minerals.
Kaolinite: - CEC: 2-5 cmolc/kg (low due to limited isomorphous substitution) - limited isomorphous substitution in the tetrahedra - nonexpansible - Lack of interlayer access and expansibility due to interlayer hydrogen bonding. - adsorption is on external surfaces Smectite: - CEC: 80-120 cmol/kg (high due to significant substitution) - significant substitution in the octahedra - Highly expansible - layers weakly held together by cations Vermiculite: - CEC: 100-180 cmolc/kg (due to significant substitution) - significant substitution in the tetrahedra - moderately expansible layers are tightly bounded since the source of engative charge is very close to the adsorbed cations Illite: - CEC: 20-40 cmolc/kg (K+ ions are nonexchangeable) - significant substitution in the tetrahedra and octahedra - nonexpansible - K+ ions are non exchangeable and fit perfectly within the cavities on the surface of the interlayers which holds them together
Know why NH4+ is less mobile in the environment than is NO3-.
NH4+ participates in cation exchange which causes it to adsorb to the soil which reduces mobility. NO3- is repelled from the negative charged soil and flows freely.
Be able to identify important soil solution constituents, both organic and inorganic
Organic - compounds generally based on the carbon atom (either synthetic or natural. - dominant elements (carbon, hydrogen, oxygen) - usually are covalently bonded Inorganic - may contain carbon atoms but are not generally based on it - metals, chemical nutrients, acids - most are dissolved ions (Na+, K+, Fe3+, Mg2+, NO3-)
Identify the types of liquids in which hydrophobic organic chemicals will dissolve easily
Organic solvents (ex. hexane, octane, oils) **like dissolves like**
Know what is meant by protonation and deprotonation and the role each plays with respect to charge development on Fe and Al oxides.
Protonation is the addition of protons in acidic conditions that causes a positive charge while deprotation is the removal of a proton in basic conditions which causes a negative charge.
Describe why hydrophobic organic chemicals like DDT and dioxins are insoluble in water
Since they are uncharged and nonpolar, hydration spheres do not form around the chemical which make them not miscible with water. They cant form significant interaction with very polar solvents like water.
Define water solubility
The ease with which substances dissolve in water. The degree to which organic contaminants impact soil and water quality often is related to their water solubility.
Define bioaccumulation and how this occurs in organisms, particularly regarding hydrophobic organic chemicals
The gradual accumulation of substances, such as pesticides or other chemicals in an organism. Hydrophobic organic chemicals accumulate within the fatty tissue/lipids of animals since this is an organic solvent. Once in the lipids, the small amounts of contaminants in the water accumulates within the lipids of an animal over time.
Discuss how water solubility can influence movement and retention of organic chemicals (e.g. pesticides) in soils and aquatic systems.
The lower the water solubility, the stronger the interaction with the organic colloid and the slower the chemical movement throughout the soil. More chemicals are retained within the soil if there is a lower water solubility.
Define organic chemical partitioning relative to soils
The organic chemical will distribute between the organic and aqueous phases. The organic colloid behaves like an organic solvent and takes up most of the chemical while a small amount remains in the aqueous phase.
Know the types of net charged developed on aluminosilicate minerals and the implications in terms of soil solution cations
The result of isomorphous substitution is a deficit of positive charge or a surplus of negative charge in the mineral structure. The more negative charge that is in the structure, the more it participates with cation exchange in the soil.
Know what cation exchange capacity is, the units, and the range for various mineral colloids.
The total quantity of cations a clay can adsorb. Related directly to the amount of isomorphous substitution. Units: cmolc/kg Range: 0-180 cmolc/kg (for aluminosilicates)
Understand why interlayer spaces in Kaolinite are unavailable for exchange reactions or water entry
There are hydrogen bonds within the colloid that are "gluing: the layers together. This causes an area with no interlayer accessibility.
An example of an uncharged aluminosilicate mineral is talc
True
Anions are negatively charged
True
Charged is developed on silicate clay minerals during mineral formation
True
Highly charged cations are attracted to soil colloid surfaces more strongly than weakly charged cations
True
Illite layers are held together tightly by potassium ions
True
Isomorphous substitution typically results in a net negative charge on soil clay minerals
True
Isomorphous substitution typically results in a net negative charged on soil clay minerals
True
Kaolinite layers are bound together by hydrogen bonds
True
Secondary silicate clay minerals are dominated by the elements aluminum, oxygen and silicon
True
Small cations are attracted to colloid surfaces more strongly than large cations
True
Smectites are 2:1 minerals
True
Under what conditions relative to pH is negative or positive charge developed on Fe and Al oxides?
Under the conditions of high pH, negative charges are developed on the colloids through deprotonation under the conditions of low pH, positive charged are developed on the colloids through protonation.
A chemical element that has acquired an electrical charged by losing an electron during a reaction is called a/an
cation and ion
The units used to express cation exchange capacity are
cmolc/kg
Know the relationship between pH and H+ ion concentration in solution
low pH --> high H+ ion concentration high pH --> low H+ ion concentration **ion concentration is out of 10^-14 between OH- and H+ ions**
A 2:1 mineral has
one octahedral sheet and 2 tetrahedral sheets
A 1:1 mineral has
one tetrahedral and octahedral sheet
Which of the following has an influence over the preference of cations at charged clay surfaces?
size, charge, and concentration in solution
