Biogeochemsitry: lecture 3 - Sulphur and Phosphorus Cycles

What are the oxidation states of elemental sulphur?

\-2 to +6

Why is elemental sulphur reactive?

It ignites and burns easily, making it useful for promoting ignition.

How is sulphur utilized in biological systems?

proteins, enzymes, and amino acids like methionine and cysteine.

What are some common sulphur minerals?

Sulphides: iron pyrite (FeS2), copper chalcocite (Cu2S) and covellite (CuS), and nickel millerite (NiS)

Sulphates: gypsum (CaSO4) and selenite

Where is elemental sulphur (S8) formed?

when another sulphur mineral is incompletely oxidized or reduced, usually in the presence of volcanoes or thermal vents

sulphur reservoirs

Atmosphere (gas and particulates), hydrosphere (oceans, waterways, water table), lithosphere (earth's crust), and biosphere

What is the original pool of sulphur in the global sulphur cycle?

Pyrite (FeS2)

How do volcanoes contribute to the sulphur cycle?

By releasing sulphur dioxide (SO2) or hydrogen sulphide (H2S) from the lithosphere, gradually or through vents. Underwater volcanoes release sulphur species directly into the hydrosphere

abiotic processes involved in the sulphur cycle

Release of sulphur from volcanoes, sea spray action, wind-blown dust, chemical weathering, biomass burning, and the burial/fossilization of biomass

How is sulphur assimilated in biological systems?

Producers can absorb sulphate (SO42-), reduce it to sulphite (SO32-), and use it to produce amino acids.

How do some bacteria contribute to the sulphur cycle under anaerobic conditions?

Bacteria like Desulfovibrio sp. or Desulfotomaculum sp. can reduce sulphate (SO42-) during respiration, producing H2S

What is the role of green sulphur bacteria in the sulphur cycle?

utilizes sulphur-based anoxygenic photosynthesis, which does not produce oxygen as a byproduct

How do microbes oxidize reduced sulphur compounds in anoxic environments?

while reducing CO2, producing CH2O, H2O, and S

role of chemosynthesis in hydrothermal vent ecosystems

acts as a form of primary production in the absence of light, using reduced sulphur compounds

What is the precursor compound for the production of dimethylsulphide (DMS) in phytoplankton?

DMSP: Dimethylsulphoniopropionate

biogenically produced sulphur compounds

DMDS, carbonyl sulphide (OCS or COS), carbon disulphide (CS2), and methyl mercaptan

What are the main forms of sulphur in sediments?

Elemental sulphur (S) and sulphides

How does sulphate get reduced to hydrogen sulphide (H2S) or mineralized into sulphate by microorganisms in soils?

On decay, microorganisms mineralize the organic sulphur into H2S or SO42-

What happens to H2S in sediments?

reacts with metals to form metal sulphides or oxidized to elemental sulphur or sulphur dioxide (SO2), depending on local conditions or biological processes

How does sulphate in the atmosphere return to the surface?

oxidized either in the gas phase to form sulphuric acid or in clouds to form SO42-, which is then returned to the surface in rain

What reactions lead to the formation of pyrite in sediments?

Sulphate reduction and the subsequent reactions of H2S with Fe2+ or reactive Fe-minerals.

How is sulphur transported in natural rivers?

Weathering of pyrite, gypsum and rainfall

How do human activities affect the transport of sulphur in rivers?

air pollution, mining (acid mine drainage), erosion, and other factors, contribute to 28% of the current sulphur content in rivers

Requirements for pyrite formation

Oxygen and water (4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO42- + 16H+)

What can aqueous ferric iron (Fe(III)) do to pyrite?

can oxidize pyrite: FeS2 + 14Fe3+ + 8H2O = 15Fe2+ + 2SO42- + 16H+

How is Fe(III) formed?

oxidation of ferrous iron (Fe(II)) by oxygen: Fe2+ + ¼O2 + H+ = Fe3+ + ½H2O

What can happen to Fe(II) in the presence of oxygen and water?

Lost through oxidation and hydrolysis to form the relatively insoluble iron hydroxide Fe(OH)3

Main sources of atmospheric sulphur

Volcanic eruptions, soil dust, biogenic gases (e.g., hydrogen sulphide, dimethyl sulphide, carbonyl sulphide), and anthropogenic emissions

How are biogenic sulphur compounds converted to sulphur dioxide (SO2)?

through OH or NO3 initiated photochemistry

role of OH in converting sulphur dioxide (SO2) to sulphuric acid

OH reacts with SO2 to form HSO3, which further reacts with O2 to produce HO2 and SO3. Finally, SO3 reacts with H2O to form H2SO4

typical lifetime of most sulphur species in the atmosphere

due to chemical oxidation, is on the order of days

What is the chemical structure of carbonyl sulphide (OCS or COS)?

very similar to CO2

sinks for carbonyl sulphide

tropospheric oxidation (very slow), hydrolysis in the sea surface, oxidation in the stratosphere, and mistaken uptake by photosynthetic organisms in place of CO2

atmospheric lifetime of carbonyl sulphide

5 years, which is exceptionally long for a natural sulphur species

primary loss mechanism for atmospheric sulphur

production of sulphuric acid and subsequent deposition

lifetime of atmospheric sulphur in the troposphere

1-10 days.

How does sulphuric acid condense in the atmosphere?

It has a low vapour pressure & can form liquid particles on its own, and react with bases, such as ammonia, to produce bisulphate & sulphate salts.

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What is nucleation or new particle formation?

spontaneous formation of new particles directly from the gas phase, rather than the condensation of material onto pre-existing particles.

main mechanism for particle nucleation in the atmosphere

Sulphuric acid

Where is nucleation often observed in the atmosphere?

locations with high production of sulphuric acid vapour (e.g., power plant and volcano plumes) or in the remote atmosphere (e.g., polar regions, coastal locations, free troposphere, boreal forests)

At what size do particles need to grow before they can participate in important processes like cloud formation?

around 50-100 nm

aerosol size for sulphuric acid and MSA

less than 1 µm

Where is MSA production from DMS mainly important?

Marine environments at high latitudes

dominant loss mechanism for sulphate-based particles in the atmosphere

Wet deposition - transfers sulphur directly to the hydrosphere

How does sulphur dioxide (SO2) react in cloud droplets?

dissolves into cloud droplets and forms bisulphite (HSO3-) and sulphite ions (SO32-) in solution, which are S(IV) oxides

main reaction that converts S(IV) to S(VI) in cloud processing

Hydrogen peroxide (H2O2)

How do cloud-based S(IV) to S(VI) reactions compare to gas-phase reactions in terms of speed?

much faster than gas-phase reactions, reducing the lifetime of SO2 from days to minutes in the presence of clouds

What dictates the relative concentrations of sulphuric acid, bisulphate, and sulphate in liquid particles?

solution pH (i.e., alkali availability)

What are the main anthropogenic sources of sulphur emissions?

Burning fossil fuels, particularly coal, is the biggest anthropogenic source of sulphur emissions. Smelting of sulphide-based minerals and biomass burning are also significant sources

What is a significant source of H2S emissions in the atmosphere?

Landfill and sewage emissions from decaying biological waste or high-sulphur wastes, such as gypsum (drywall) in rubble, are significant sources of H2S emissions

Is synthetic SF6 considered part of the chemical sulphur cycle?

No, but it is a strong greenhouse gas and is addressed in climate science and the Montreal Protocol alongside CFCs.

What are the net effects of increasing atmospheric sulphur?

More atmospheric particles, larger atmospheric particles, increased solubility of pre-existing particles, more acidic cloud water, and enhanced weathering of carbonate minerals

What are some measures to mitigate sulphur emissions?

Using cleaner fuels, chemical scrubbers, segregating waste types, limiting water, treating waste, and trapping emissions

PM2.5

Particulate matter of a size smaller than 2.5 microns

Great Dun Fell experiments

demonstrated the significance of SO2 oxidation in the atmosphere and the role of cloud processing

What is the importance of low level stratocumulus clouds on the Earth’s radiation budget?

significant impact on shortwave absorbed solar radiation due to their high albedo

How do thicker hazes affect the atmospheric albedo?

increase the atmospheric albedo

How does a higher Cloud Droplet Number Concentration (CDNC) affect cloud whiteness?

makes the cloud whiter, for a given Liqiud water content (LWC)

How does drizzle formation relate to CDNC?

A higher CDNC for a given LWC leads to smaller particles and less drizzle formation

Largest negative contributor to anthropogenic climate forcing

Aerosol Sulphur

Impact of mitigation of sulphur in the developing world

short term warming

CLAW hypothesis

proposed negative climate feedback mechanism related to changes in marine biogenic activity and DMS fluxes

HPMTF

a new S molecule formed from the oxidation of DMS in the marine atmosphere

potential significance of HPMTF in the atmosphere

If it breaks down rapidly to sulphuric acid, it could contribute to new particle formation or add S to pre-existing aerosols

Does phosphorus have a gaseous form in the environment?

No

How is phosphorus primarily transported through the atmosphere?

airborne dust and in clouds and rain

What is the oxidation state of elemental phosphorus?

\+V

Who isolated phosphorus and how?

Hennig Brand by heating boiled-down urine in 1669

In what forms does phosphorus naturally exist?

Inorganic phosphate ion (PO43-), phosphoric acid (H3PO4), and organic phosphate (R-OPO(OH)2)

How does phosphoric acid dissociate in solution?

dissociates between different states depending on the pH of the solution: H3PO4 ↔ H2PO4- ↔ HPO42- ↔ PO43-.

What are some examples of complexes formed by phosphate with metal cations?

MgPO4-, CaPO4-, NaHPO4-, and CaHPO4

Does phosphate bind to solid materials?

Yes, such as soils or mineral particals, removing it from solutinon

What are polyphosphates and how are they formed?

polymers of phosphate, such as diphosphate and triphosphate. They can be formed biologically or manufactured artificially as food additives and chelating agents.

How can polyphosphates return to orthophosphate?

hydrolysis

How do animals contribute to the movement of phosphorus?

move or reverse the flow of phosphorus through roaming and excretion

What has the extinction of megafauna resulted in?

reduced the efficiency of phosphorus redistribution

How has human activity further affected phosphorus redistribution?

further reduced the number and range of animals, limiting the redistribution of phosphorus

Where does most production of phosphorus in lakes occur?

near the surface

What happens to dead organic matter in lakes?

sinks and can be remineralized, consuming oxygen

What can cause a suppression in phosphorus concentrations at the surface of lakes?

The formation of a thermocline, typically in summer in mid-latitudes, can cause a suppression in phosphorus concentrations at the surface.

How is phosphorus returned to the surface water in mid- and higher latitudes?

wintertime cooling causes the breakdown of the thermocline, allowing enhanced mixing to return phosphorus to the surface water

What are sediments in lakes important for in relation to phosphorus?

act as a reservoir, binding phosphorus

major processes involved in the phosphorus cycle in the ocean

upwelling, biological productivity, recycling of inorganic reactive phosphorus, remineralization, slow exchange between surface and deep waters, and incorporation into oceanic sediments

Where is phosphorus almost absent in the ocean?

oceanic euphotic zone (surface layer interacting with sunlight).

What drives the ratio of C:P in the surface ocean?

biological phosphorus in microorganisms and is around 106:1

What happens to detrital material from ocean biology?

In the form of particles and is readily decomposed within the euphotic zone and reused biologically, with only a small fraction sinking to the ocean bottom and being incorporated into sediment

Why are nitrogen and phosphorus often considered together in river management?

because they both contribute to nutrient pollution and have similar fractional contributions from agriculture

What are some anthropogenic uses of phosphate?

food additives, detergents, treatment of drinking water, and toothpaste

chelation

process where polyphosphates bind to metal cations by wrapping around the metal ion, forming a more effective complex

two main technologies for enhancing phosphate removal from effluent

inorganic precipitation using iron or aluminium and enhanced biological phosphorus removal (EBPR)

How is phosphate removed from sediments?

dredging