E 5. Dissolved oxygen

E 5. Dissolved oxygen 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 • BOD = Biochemical Oxygen Demand • 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.

Dissolved oxygen in water • 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

Dissolved oxygen in water • 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 • water sample is collected and added to oxygen saturated water ([O2 (aq)] is known) • water is left for 5 days • new [O2 (aq)] is measured using an oxygen electrode or titration • the BOD = [O2 (aq)] -[O2 (aq)] • pure water has BOD of less than 1 ppm

measuring dissolved oxygen Winkler method: 2Mn2+(aq) + 4OH-(aq) + O2(aq)  2 MnO2 (s) + 2H2O (l) MnO2 (aq) + 2I-(aq) + 4H+(aq)  Mn2+(aq) + I2(aq) + 2H2O(l) 2S2O32- (aq) + I2 (aq)  S4O62- (s) + 2I- (aq)

Eutrophication • 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 • 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 • 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: • decrease amount of dissolved oxygen in water • increase rate of metabolism of aquatic life requiring them to need more oxygen

E 5. Dissolved oxygen