Sesquioxides and Ion Exchange Flashcards
what are Fe and Al oxides and hydroxides (sesquioxides)?
weathering products of clay minerals that have lost all Si ions and other cations except Fe 3+ and Al 3+ -> consist of modified octahedral sheets of Fe3+ and Al3+
what are three characteristics of Fe and Al sesquioxides?
- little to no isomorphous substitution
- little cation exchange capacity
- posses covalent bound OH- ions which may cause strong absorption of certain anions like phosphate
what are 5 Fe and Al sesquioxides and their color?
- goethite Fe(OH)3 - brown
- limonite Fe(OH)3- yellow
- haematite Fe2O3- red
- gibbsite Al(OH)3- colourless
- alumina Al2O3- colourless
what do dark colors signify in soil?
high organic matter content
what does red signify in soil?
soil rich in iron oxides
what does blue-grey color signify in soil?
presence of iron in its reduced form
why does soil become blue/grey?
with prolonged water saturation, iron oxides are loss and aluminum oxides and hydroxides dominate
what is laterite?
cementation in soil when large oxide or hydroxide content -> tropics
which particles types of minerals provide anion exchange capacity?
sesquioxides
what are the three sources of negative charges for cation exchange capacity?
- isomorphous substitution within clay minerals
- broken bonds at edge of clay minerals
- dissociation of bonds at edge of humus (carboxylic or phenolic)
how do you compare primary and secondary minerals to a pile of books?
primary minerals: book covers
secondary minerals: pages of book
how does soil solution become acidic?
exchange of base cations by hydrogen until there are no more base cations
why is prolonged soil acidification worrisome?
because strong Al bonds may eventually break and leach into soil solution which is toxic to plants
how can acidification of soil be reversed?
through fertilization (Ca, K) or nutrient release from decomposing tissue
what is the process of adding calcium or magnesium to soil called?
liming ! adding base cations to soil to decrease acidity
what are three mechanisms of neutralization of acid rain?
- reaction with carbonates (Ca binds to sulfate)
- reaction with exchangeable cations or weatherable minerals such as feldspar (Cation binds to sulfate)
- anion adsorption of SO4 of Fe and Al hydroxides
why can soils rich in Fe and Al neutralize acid rain?
through anion adsorption
what solution is best for soil that has been affected by acidic rain?
calcium carbonate because adding calcium chloride would not help the problem long term. as soon as it would acid rain again, the equation would reverse. Calcium carbonate is irreversible!
how do you determine how tightly a cation will be bound to the surface of a colloid?
the higher the charge and the smaller the hydrated radius of the cation, the more strongly it will adsorb to the colloid
- ALSO likelihood that an adsorbed cation will be displaced is influenced by how strongly its neighboring cations are adsorbed
what cation has the strongest adsorbtion?
aluminum 3+, once it is mobilized, soil no longer has any buffering capacity
how are cations measured?
cations per mass of soil
how do you calculate cation exchange capacity?
total amount of charges per soil which is found by first finding moles per soil
why do kaolinites have low CEC?
because of the strong hydrogen bonds between sheets
what mineral has good CEC?
smectites!
what is the relationship between SOC and CEC?
the higher the SOC, the larger the CEC
what is the relationship between pH and CEC?
the lower the pH, the lower the cation exchange capacity because cations have already been exchanged in acidic soils
how does anion exchange capacity work?
anions and organic matter displace OH- groups from sesquioxides
why do we have to add a lot of P to tropical soils?
phosphate bonds strongly to iron/al oxides in soil, making it unavailable to plants
what is the relationship between pH and surface charge?
the higher the soil pH, the larger the negative charge
