Lecture 8 - Sulphur and Trace Element Cycling Flashcards
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.
How is mercury cycled?
Methylation occurs in the sediments by bacteria and these exchange with the water column above. The mercury is then consumed by animals and bioaccumulation occurs.
What are residence times for mercury?
Atmosphere - 11 days
Soil - 1000 years
Oceans - 3200 years
Ocean sediments - 2.3 x 10⁸ years
What is the case study for Mercury?
Sulphur Bank Mine, Clear Lake California
What area do mine tailings cover?
120 acres and extend a quarter of a mile of the shoreline.
How do pollutants get to Clear Lake?
Herman Pit is uphill of Clear Lake so water can leach from the pit into the lake.
How many people’s water supply is from Clear Lake?
4,700.
These people have likely accidentally ingested contaminated surface waters and suffered adverse health effects.
How can sulphur be found in the atmosphere?
In the gas phase.
As an aerosol compound.
What are three gas phase compounds of sulphur?
Sulphur dioxide
Hydrogen sulphide
Dimethyl sulphide (SO(CH₃)₂
What are three aerosol compounds of sulphur?
Ammonium sulphate
Sulphuric acid
Sulphurous acid
How much do natural and anthropogenic sources of sulphur account for?
Natural - 26 Mt
Anthropogenic - 73 Mt
What are natural fluxes of sulphur into the environment?
Volcanoes release SO₂ and H₂S.
Decaying vegetation releases H₂S and SO₂.
Phytoplankton release DMS. DMS is converted to SO₂ which converts to SO₄ aerosol.
What are anthropogenic fluxes of sulphur into the atmosphere?
Coal and oil contain up to 4% sulphur.
Natural gas contains 25% sulphur. To reduce the effects desulphurisation occurs before distribution.
What are the sulphur levels in ULSD (Ultra low sulphur diesel fuel) and LSD (Low sulphur diesel fuel)?
USLD - <15ppm
LSD - <50ppm
What were the effects of the Great London Smog?
Caused by calm conditions and a temperature inversion which trapped pollutants in.
12,000 died over 4 days.
Caused the Clean Air Act to be made.
What are the fluxes of sulphur out of the atmosphere?
Wet and dry deposition.
What is a critical load?
The maximum tolerable rate of acidic deposition by a terrestrial environment.
The range is typically 5-10 kg S ha⁻¹ yr⁻¹.
How is DMS formed?
Phytoplankton release DMSP which is the precursor to DMS.
How do aerosols affect cloud formation?
Clean air produces small numbers of large droplets which causes dark clouds with a low albedo.
When there are high concentrations of aerosol, water can easily condense on particles forming large numbers of small droplets. These clouds are bright white and have a high albedo.
How do aerosols affect climate change?
They increase the albedo of clouds which results in the cooling of the planet.
What is CLAW theory?
A negative feedback loop of increased sea temperature. This increases phytoplankton growth which increases DMS production which causes higher SO₂ concentrations. This causes whiter clouds and so cools the oceans.
What is anti-CLAW theory?
A positive feedback loop of increased sea temperature. This decreases phytoplankton growth which decreases DMS production which causes lower SO₂ concentrations. This causes darker clouds and so warms the oceans further.
