Lecture 8 - Sulphur and Trace Element Cycling Flashcards
(45 cards)
Why are trace elements important but careful management needs to be done?
They are essential for biological processes.
They can be very hazardous to human and animal health.
What are some examples of trace elements?
Nickel - Used in stainless steel.
Copper - Used in fungicides.
Cadmium - Used in rechargeable batteries.
Arsenic - In insecticides and electronics.
Mercury - In dentistry.
What two things do humans change about the natural cycling of trace elements?
The rate at which they are transferred between reservoirs.
The chemical form of the metal it was deposited as.
What does smelting typically do to trace elements?
It converts the metals and metalloids from a compound to their elemental form.
In terms of trace elements, what does lithophile mean?
Oxygen or silicon loving.
How abundant are trace elements?
Very low.
Their availability depends on their relative abundance and the stability of minerals in which they occur.
Minerals that weather more easily release trace elements into the environment which increases their availability.
How are trace elements mobilised?
Through weathering.
In volcanic gases.
Mining and smelting.
What six factors control the behaviour of a trace element?
Complexation
Redox Reactions
Volatilisation
Precipitation
Adsorption
Reactions Involving Organisms
What is complexation?
If an element forms an ion, it may be able to form a complex ion which would allow it to be mobilised in water.
How do redox reactions control the behaviour of the trace element?
Iron is soluble in the 2+ form but not in the 3+ form.
Organic arsenic forms and As³⁺ are more mobile than As⁵⁺ which leads to higher concentrations.
Cr³⁺ is important to human diets whereas Cr⁶⁺ is a carcinogen.
How does volatilisation control the behaviour of trace elements?
Volatile trace elements can be mobilised by high temperatures such as in smelting, power station chimneys or volcanic activity.
How does precipitation control the behaviour of trace elements?
Precipitation can occur as a result to changes in redox or pH. If a trace element precipitates, this can be exploited and they can be cleaned up from water.
How does adsorption control the behaviour of trace elements?
Trace elements can also adsorb to mineral surfaces.
How does pH affect the adsorption of trace elements in iron oxide surfaces?
At a low pH, iron oxide surfaces are protonated (Fe-OH₂⁺). This will repel positively charged trace elements.
At higher pHs, the surface will be neutral (Fe-OH) or negatively charged (Fe-O⁻). When the surface is negatively charged, trace elements can adsorb onto the surface.
How do reactions involving organisms control the behaviour of trace elements?
Some bacteria obtain energy to grow by oxidising S²⁻ to SO₄²⁻.
The trace elements bonded to sulphide will be released into the environment.
What two main forms does mercury exist in?
Elemental mercury
Divalent mercury ion (Hg²⁺)
Why is elemental mercury unusual?
It is a liquid at room temperature.
Where is mercury found in the environment?
As the mineral ore cinnabar (mercury sulphide).
Bonded to organic species.
What are the features of mercury?
It has a high vapour pressure.
It is the most volatile metal on the periodic table.
Its global cycling is dominated by atmospheric transport.
What are sources of mercury?
Volcanic gases
Flue gases
Gold mining/smelting of ore
Burning fossil fuels as mercury readily forms bonds with organic matter.
Volatilisation from land or ocean surfaces.
How has gold been extracted in the past?
By forming an amalgam with mercury and heating it.
For every one gram of gold produced, two grams of mercury are released into the environment.
What are several environmentally important forms of mercury?
Lithosphere - Cinnabar (HgS) is insoluble but reduction and weathering in soils can generate elemental mercury.
Seawater - Mercury is present as the Hg²⁺ ion which can be complexed with a chloride ion to form HgCl⁺. It can also form complexes with hydroxide ions.
Natural Waters - Mercury can bond to carboxyl groups of organic ligands and therefore from humic and fulvic acids.
Sediments - Hg²⁺ undergoes biomethylation to form (CH₃)₂Hg. This process is undertaken by bacteria in the surface sediments of fresh and saltwater ecosystems.
What are the properties of mercury organic compounds?
They are highly volatile
They can be concentrated in the organic matter in sediments.
They can undergo bioaccumulation (increased concentration up the food chain).
How does mercury move?
Primarily from the land surface to the atmosphere.
Smelting, mining and burning of fossil fuels have resulted in a significant increase.
