envirochem test prep - soils

how do soils become more or less enriched in their soil fractions

overall: Primary minerals then dissolve into secondary minerals. All of these reactions require acidity and to release cations which then become available for plants or organisms. acidity, land use, weathering resistance (Therefore, minerals with isolated Si tetrahedra that are bonded through other cations have low molar Si-O ratios, **low numbers of bridging oxygens per Si tetrahedron, and weather most rapidly.)**, and the presence of microorganisms can lead to the enrichment of soil fractions. things such as land use and crops can remove nutrients from soil and make it less enriched

**Describe and categorise the** ***principal weathering reactions*** **that occur in soils.**

* Hydrolysis - the breakdown of rock by acidic water to produce clays and soluble salts. 
* Solution (acid-base)  - removal of rock in solution by acidic rainwater. In particular, limestone is weathered by rainwater containing dissolved CO2 , (this process is sometimes called carbonation)
* Oxidation - the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-coloured weathered surface
* Chelation: complexation and transport of ions by chelating agents. Primarily organic ligands derived from decay of plant materials.

**Discuss how weathering reactions at the Earth's surface are able to modulate** ***atmospheric temperature over geologic timescales.***

* something in the atmosphere causes warming (natural variations in incoming solar radiation as shown by the top squiggly line but also co2 levels) (when there is increased solar radiation there is increased co2 because of more respiration). The result of this is a warmer climate
* This will lead to an increased temperature which results in more precipitation and in turn more vegetation growth. The plants respire by taking in co2 so that can increase the amount of chemical weathering that occurs which increase co2 transport (if we take it in and incorporate into rocks it can be buried and taken out of the system)
* This then leads to cooling due to removal of co2
* Therefore, the earth is able to modulate climate at a global scale, but happens over a long period of time. This is an example of negative feedback

**Predict the effect of pH and specific adsorption on the** ***surface charge of soil minerals*** **of different types.**

 

* Surface charges are the main driver for attraction between molecules and in soil it's no different 

at low pH (acidic conditions), the surface of most soil minerals will have a net positive charge due to the presence of hydrogen ions (H+). At high pH (alkaline conditions), the surface charge of soil minerals will have a net negative charge due to the presence of hydroxide ions (OH-). The magnitude of the surface charge will depend on the specific mineral type and the pH of the soil.

**Describe the principal features of** ***gley soils***

gley soils form where soils are saturated for long periods due to high or perched water tables, or slow drainage. they have a distinctive pale blue-grey colouring due to reducing conditions (Fe2+/Mn2+) and Mottles form where cations oxidise -> along root channels/profile texture changes (Fe3+/Mn4+)

**Describe the principal features of** ***allophanic soils***

allophanic soils are rich in allophane clay minerals formed from the weathering of volcanic glass in tephra. They strongly adsorb anions such as phosphorus (P), sulphur (S) & carbon and have Excellent texture for plant root growth. But requires careful management as natural fertility is low. They are pH dependent and their negative charge increases with pH

**Describe the principal features of** ***podzol soils***

podzol soils occur in areas of high rainfall and are usually associated with forest trees with an acid litter. they have a Pale “E” horizon beneath topsoil -> Leaching. Fe, Al & Si-bearing minerals moved down the profile. They have an organic-rich, red Fe-rich layer underlying leached horizon and are extremely acidic (pH \~4)

pH vs alkalinity

Alkalinity: a measure of the ability of a water body to neutralise acidity whereas pH is a measure if acidity \[H+\] ions

describe the reactions that contribute to alkalinity

Dissolving co2 in water gives us the acidity CO2 (g) + H2O ( H2CO3 ) → H+ + HCO3- and how much of this occurs depends on whether we are in an open or closed system. Open systems allows more exchange with the atmosphere and so can produce more acid than a closed system. the bicarbonate acts as a buffer consuming h+ ions / binding therefore reducing conc of H+ and acidity

**Describe how** ***waters evolve along groundwater*** **flow paths.**

water may run straight across the land and into the ocean or it may go underground and move into the groundwater. It can then stay in the shallow regime or can percolate further into the deep regime where it encounters crystalline rock. Because this groundwater moves slowly its in contact with the rock for long periods of time meaning that the groundwater can evolve chemically while it moves

**Discuss the causes and chemical mechanisms underlying the** ***arsenic problem in SE Asia.***

* arsenic is naturally produced in geothermal systems and is a byproduct of mining operations so there are areas more prevalent with it than others
* This is because of reduction
* When reducing iron from iron 3 to iron 2 it kicks out the arsenic that is adsorbed to the iron oxides and the arsenic is in a soluble form and so can go into water and be carried around as a result of the iron being reduced and as a result can no longer hold onto the arsenic ions which then go into solution
* This is a particular problem when there are high levels of ion in groundwater. In some countries the cost of treating the water is too expensive
* You dont have to have just reduction to have this effect
* If you are in an oxic environment, the iron state depends on the function of pH so once at alkaline conditions the arsenate is no longer adsorbed to the iron even though it is oxidised, so it can then go into solution.
* In some cities, the groundwater they rely on is heavily reduced so are at risk of high quantities of soluble arsenic in the water. This is more a problem for poorer cities who cannot afford to treat the water