ASS: How Does Pyrite Get There?
What causes ASS?
ASS are any material or sediment that contains iron sulfides that, when disturbed or drained, forms sulfuric acid. In most cases, the iron sulfide present is pyrite (FeS2, also known as Iron (IV) Sulfide). This pyrite is very reactive.
It is usually the smaller crystals of pyrite found in ASS. The pyrite reacts with oxygen to create sulphuric acid (therefore disturbing the soil creates the acid, as the pyrite is exposed to the air). The smaller pyrite framboids (raspberry shaped granules) are more reactive than the larger ones as there is more surface area exposed to the oxygen.
How does the Pyrite get there?
The production of pyrite is a very specific procedure, requiring certain conditions, facilitators and ingredients. Sulfate …show more content…
reducing bacteria can produce pyrite if: A supply of sulfur (usually found in seawater, although can be present in sulfur-rich groundwater) Wet, anaerobic conditions (the bacteria consume oxygen when breeding, making minimal amounts non-existent quickly) Organic matter for the bacteria to feed on and gain energy Iron (found abundantly in land sediments – does not pose an issue) Although not necessary, tidal movement is an excellent way to remove the products of the reaction taking place – if the products are not removed, the reaction slows considerably Warm temperatures also speed up the reaction
When this process occurs, the bacteria excrete hydrogen sulfide (H2S) and bicarbonate ions. The hydrogen sulfide reacts with the iron to form iron sulphides – pyrite (Several steps omitted)
Where does the pyrite usually form?
Anywhere that the conditions are correct, pyrite will form. The most concerning portion of pyrite was formed during the last significant sea level rise – the Holocene period. The high tides then carried sediments inland and deposited them, creating ideal conditions for pyrite formation. However, these sediments are often covered by other land matter, resulting in buried pyrite.
PASS and AASS
It is important to note that ASS may be referring to one of two things – PASS or AASS. The difference between Potential Acid Sulfate Soils (PASS) and Actual Acid Sulfate Soils (AASS) is that PASS has not been exposed to oxygen and has not begun to form sulfuric acid. AASS has a pH lower than 4 and can usually be identified by a more yellow-y colour whereas PASS is usually a dark grey. AASS can also be identified by iron staining in the surrounding environment.
Potential acid sulfate soils appear as a dark grey layer below the AASS, PASS are generally found below the water table (which excludes oxygen) and the permanent water table can be found around the 1m mark
What makes PASS into AASS? Along with this reaction, many heavier metals can be produced including iron (specifically ferrous iron, Fe2+, also known as the reduced form of iron). This iron reacts with oxygen to form Fe3+ (ferric iron form – rusted) and acid, but different types of reactions can occur depending on the pH.
These reactions give the surrounding vegetation and environment a rust-red colour
Overall the iron can be summarized to:
Fe2+ (Ferrous iron)
• Soluble; can be transported offsite in streams and drains (secondary oxidation)
• iron II + oxygen iron III + acid
• Liberates large amounts of acid and consumes oxygen
• Clear in colour
Fe3+ (Ferric iron)
• Highly reactive
• Can oxidise pyrite without the presence of O2
• Can liberate more acid
• Precipitates if pH >4 (rust-red colour)
Further Issues
The clay in soil contains insoluble aluminium. The sulfuric acid attacks the clay and makes the aluminium soluble. This soluble aluminium can easily make its way to a water source and flocculates the clay particles, creating extremely clear water – unfortunately the clear water becomes highly acidic.
The Natural State vs Disturbed State
In nature, PASS is found below the water table and is only exposed and oxygenated during periods of severe drought. The low levels of acid produced from this does not affect the environment in any negative way as seawater can naturally neutralize acids. This also means that the acid cannot group (and form ‘slugs’).
When the PASS is disturbed by non-natural sources, it is constantly exposed to oxygen, resulting in a higher acid production rate. This can cause: Ecological impacts: Fish kills and diseases; habitat destruction and/or modification; reduced plant growth and death; invasion of acid tolerant species; and death and decline of benthos/aquatic organisms Human health impacts: Acid tolerant mosquitoes and arboviruses; foul smells; dermatitis and eye inflammation; and blue green algae Engineering impacts: Subsidence; cracking; weakening and corrosion of concrete structures; and clogging of drains Economic impacts: $10 billion worth of development and investment affected in Australia and $ millions are spent each year in Queensland.
Ways to stop ASS
Lime-assisted [Ca(OH)2] tidal exchange can control both existing and potential acidity: On-going hydrated lime [Ca(OH)2] assisted tidal flushing will neutralize the existing water acidity and the acidified actual acid sulfate soil in the upper soil layers Once the existing acidity is treated, oxidation of the deeper potential ASS will eventually be halted by regular tidal inundation as this denies oxygen to the sulfides
Acidified water is actively treated by pumping water from the creek and then mixing it with hydrated lime [Ca(OH)2] to form a slurry. This slurry has a high pH and can neutralise some of the acidity in the creek when it is pumped back into the waterway.
Hydrated lime [Ca(OH)2] is one of the more soluble forms of ‘lime’, but it is still not very soluble. It has a pH around 12-12.5 which is very alkaline. Adding too much hydrated lime (overliming) can cause problems as both extremes of the pH range are problematic
Agricultural lime (CaCO3) is spread on acidified soils above the level of consistent tidal exchange. It is used for treating soils as it has a pH around 8.5. It cannot excessively increase the soil pH (both extremes of the pH range are problematic). It also has low solubility and will therefore stay where you put it rather than leaching out through the soil when it rains.
BUT BEWARE! These beneficial effects are not permanent. Tidal exchange must continue indefinitely or the site will drain down and revert to an acid-producing condition again
The pH Scale
The pH scale goes from 0 to 14, 0 being the most acidic and 14 being the most alkaline.
It is a log scale meaning that every number (e.g. 7) is ten times more acidic than the number after it (e.g. 8).
This means that something with a pH of 3 would be 10000x more acidic than distilled water, which has a pH of 7.
How does temperature/the presence of salinity affect the oxidation rate?
Pyrite oxidation generates O2 and produces heat. Due to this, the acidity and temperature of the surrounding soil/solution will affect the overall reaction rates (4). Biological oxidation only occurs between 0oC and 55oC with an optimum range of 24oC – 45oC (4), but chemical oxidation can happen at a much higher temperature. It is predicted that the PASS will oxidize at a much faster rate if the temperatures are at the higher end of the scale as it is known that higher temperatures increase the speed of reactions (reference).
It is also known that as the salinity of water decreases, oxygen solubility rises. Since the slurry of ASS will be made using water with the pre-set amount of salt already dissolved, the ASS will come into contact with more dissolved oxygen (in water with less salt dissolved in it), and it will oxidize
faster.
ASS are any material or sediment that contains iron sulfides that, when disturbed or drained, forms sulfuric acid. In most cases, the iron sulfide present is pyrite (FeS2, also known as Iron (IV) Sulfide). This pyrite is very reactive.
It is usually the smaller crystals of pyrite found in ASS. The pyrite reacts with oxygen to create sulphuric acid (therefore disturbing the soil creates the acid, as the pyrite is exposed to the air). The smaller pyrite framboids (raspberry shaped granules) are more reactive than the larger ones as there is more surface area exposed to the oxygen.
How does the Pyrite get there?
The production of pyrite is a very specific procedure, requiring certain conditions, facilitators and ingredients. Sulfate …show more content…
reducing bacteria can produce pyrite if: A supply of sulfur (usually found in seawater, although can be present in sulfur-rich groundwater) Wet, anaerobic conditions (the bacteria consume oxygen when breeding, making minimal amounts non-existent quickly) Organic matter for the bacteria to feed on and gain energy Iron (found abundantly in land sediments – does not pose an issue) Although not necessary, tidal movement is an excellent way to remove the products of the reaction taking place – if the products are not removed, the reaction slows considerably Warm temperatures also speed up the reaction
When this process occurs, the bacteria excrete hydrogen sulfide (H2S) and bicarbonate ions. The hydrogen sulfide reacts with the iron to form iron sulphides – pyrite (Several steps omitted)
Where does the pyrite usually form?
Anywhere that the conditions are correct, pyrite will form. The most concerning portion of pyrite was formed during the last significant sea level rise – the Holocene period. The high tides then carried sediments inland and deposited them, creating ideal conditions for pyrite formation. However, these sediments are often covered by other land matter, resulting in buried pyrite.
PASS and AASS
It is important to note that ASS may be referring to one of two things – PASS or AASS. The difference between Potential Acid Sulfate Soils (PASS) and Actual Acid Sulfate Soils (AASS) is that PASS has not been exposed to oxygen and has not begun to form sulfuric acid. AASS has a pH lower than 4 and can usually be identified by a more yellow-y colour whereas PASS is usually a dark grey. AASS can also be identified by iron staining in the surrounding environment.
Potential acid sulfate soils appear as a dark grey layer below the AASS, PASS are generally found below the water table (which excludes oxygen) and the permanent water table can be found around the 1m mark
What makes PASS into AASS? Along with this reaction, many heavier metals can be produced including iron (specifically ferrous iron, Fe2+, also known as the reduced form of iron). This iron reacts with oxygen to form Fe3+ (ferric iron form – rusted) and acid, but different types of reactions can occur depending on the pH.
These reactions give the surrounding vegetation and environment a rust-red colour
Overall the iron can be summarized to:
Fe2+ (Ferrous iron)
• Soluble; can be transported offsite in streams and drains (secondary oxidation)
• iron II + oxygen iron III + acid
• Liberates large amounts of acid and consumes oxygen
• Clear in colour
Fe3+ (Ferric iron)
• Highly reactive
• Can oxidise pyrite without the presence of O2
• Can liberate more acid
• Precipitates if pH >4 (rust-red colour)
Further Issues
The clay in soil contains insoluble aluminium. The sulfuric acid attacks the clay and makes the aluminium soluble. This soluble aluminium can easily make its way to a water source and flocculates the clay particles, creating extremely clear water – unfortunately the clear water becomes highly acidic.
The Natural State vs Disturbed State
In nature, PASS is found below the water table and is only exposed and oxygenated during periods of severe drought. The low levels of acid produced from this does not affect the environment in any negative way as seawater can naturally neutralize acids. This also means that the acid cannot group (and form ‘slugs’).
When the PASS is disturbed by non-natural sources, it is constantly exposed to oxygen, resulting in a higher acid production rate. This can cause: Ecological impacts: Fish kills and diseases; habitat destruction and/or modification; reduced plant growth and death; invasion of acid tolerant species; and death and decline of benthos/aquatic organisms Human health impacts: Acid tolerant mosquitoes and arboviruses; foul smells; dermatitis and eye inflammation; and blue green algae Engineering impacts: Subsidence; cracking; weakening and corrosion of concrete structures; and clogging of drains Economic impacts: $10 billion worth of development and investment affected in Australia and $ millions are spent each year in Queensland.
Ways to stop ASS
Lime-assisted [Ca(OH)2] tidal exchange can control both existing and potential acidity: On-going hydrated lime [Ca(OH)2] assisted tidal flushing will neutralize the existing water acidity and the acidified actual acid sulfate soil in the upper soil layers Once the existing acidity is treated, oxidation of the deeper potential ASS will eventually be halted by regular tidal inundation as this denies oxygen to the sulfides
Acidified water is actively treated by pumping water from the creek and then mixing it with hydrated lime [Ca(OH)2] to form a slurry. This slurry has a high pH and can neutralise some of the acidity in the creek when it is pumped back into the waterway.
Hydrated lime [Ca(OH)2] is one of the more soluble forms of ‘lime’, but it is still not very soluble. It has a pH around 12-12.5 which is very alkaline. Adding too much hydrated lime (overliming) can cause problems as both extremes of the pH range are problematic
Agricultural lime (CaCO3) is spread on acidified soils above the level of consistent tidal exchange. It is used for treating soils as it has a pH around 8.5. It cannot excessively increase the soil pH (both extremes of the pH range are problematic). It also has low solubility and will therefore stay where you put it rather than leaching out through the soil when it rains.
BUT BEWARE! These beneficial effects are not permanent. Tidal exchange must continue indefinitely or the site will drain down and revert to an acid-producing condition again
The pH Scale
The pH scale goes from 0 to 14, 0 being the most acidic and 14 being the most alkaline.
It is a log scale meaning that every number (e.g. 7) is ten times more acidic than the number after it (e.g. 8).
This means that something with a pH of 3 would be 10000x more acidic than distilled water, which has a pH of 7.
How does temperature/the presence of salinity affect the oxidation rate?
Pyrite oxidation generates O2 and produces heat. Due to this, the acidity and temperature of the surrounding soil/solution will affect the overall reaction rates (4). Biological oxidation only occurs between 0oC and 55oC with an optimum range of 24oC – 45oC (4), but chemical oxidation can happen at a much higher temperature. It is predicted that the PASS will oxidize at a much faster rate if the temperatures are at the higher end of the scale as it is known that higher temperatures increase the speed of reactions (reference).
It is also known that as the salinity of water decreases, oxygen solubility rises. Since the slurry of ASS will be made using water with the pre-set amount of salt already dissolved, the ASS will come into contact with more dissolved oxygen (in water with less salt dissolved in it), and it will oxidize
faster.