Download Environmental Chemistry dissolved oxygen

E 5. Dissolved oxygen
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Outline biochemical oxygen demand (BOD) as a
measure of oxygen- demanding wastes in water.
Distinguish between aerobic and anaerobic
decomposition of organic material in water.
Describe the process of eutrophication and its
effects.
Describe the source and effects of thermal
pollution in water.
Dissolved oxygen in water
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BOD = Biochemical Oxygen Demand
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can be used as an indicator of the amount of
organic waste (=oxygen-demanding waste) in
water
impure water has more oxygen-demanding waste
and therefore a higher BOD than pure water
BOD = amount of oxygen used (in ppm) by
aerobic bacteria to decompose organic matter in a
fixed volume of water at 293K over 5 days.
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Dissolved oxygen in water
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solubility of oxygen at 1 atm and 293K is
9 ppm
solubility decreases as temperature rises
needed for aerobic respiration of aquatic
plants and animals, e.g.
fish need 3 ppm or 3 mg dm-3 (kg of water)
 large biodiversity needs 6 ppm
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Dissolved oxygen in water
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organic waste = faeces, dead organisms,
urea, untreated sewage, waste from food
production
organic waste is first broken down by
aerobic respiration by bacteria (for energy)
dissolved oxygen used up in this oxidation
products of aerobic respiration: water,
carbon dioxide, inorganic ions like nitrates,
nitrites, phosphates
Dissolved oxygen: measurement
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water sample is collected and added to
oxygen saturated water ([O (aq)] is known)
water is left for 5 days
new [O (aq)] is measured using an oxygen
electrode or titration
the BOD = [O (aq)] - [O (aq)]
pure water has BOD of less than 1 ppm
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measuring dissolved oxygen
Winkler method:
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2Mn2+(aq) + 4OH-(aq) + O2(aq)  2 MnO2 (s) + 2H2O (l)
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MnO2 (aq) + 2I-(aq) + 4H+(aq)  Mn2+(aq) + I2(aq) + 2H2O(l)
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2S2O32- (aq) + I2 (aq)  S4O62- (s) + 2I- (aq)
Eutrophication
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large amounts of organic matter produce large amounts of
inorganic ions such as nitrates (fertilizers) and phosphates
(detergents) in water
these ions are nutrients to algae
an algal bloom (or ‘red tide’) occurs
this bloom removes a lot of dissolved oxygen from the water
below a level many aerobic organisms cannot survive
including aerobic bacteria – they die
more organic waste is added reducing oxygen further
anaerobic bacteria take over decomposition of organic waste
products of anaerobic respiration: methane, ammonia,
amines, hydrogen sulphide – foul-smelling gases
Eutrophication: causes
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untreated sewage
run-off animal waste
from farms
nitrates from fertilizers
phosphates from
detergents
Eutrophication is apparent as increased turbidity in
the northern part of the Caspian Sea, imaged from
orbit.
http://en.wikipedia.org/wiki/Eutrophication
Eutrophication
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A process in which water
bodies (rivers, lakes, ..)
receive excessive amount of
nutrients which causes
excessive plant growth e.g.
algae which in turn reduces
the amount of dissolved
oxygen and prevents
sunlight from entering the
water
http://www.umanitoba.ca/institutes/fisheries/eutro.html
Eutrophication
Animation:
http://www.mp-docker.demon.co.uk/environmental_chemistry/topic_4b/movie.html
Dissolved oxygen: temperature
Higher water temperatures:
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decrease amount of dissolved oxygen in
water
increase rate of metabolism of aquatic life
requiring them to need more oxygen