Dissolved+Oxygen+in+Water

= Dissolved Oxygen in W a t e r = Lin Rouxiu

Before starting on this topic, it is important to understand the learning outcomes demanded by IB: 5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen demanding wastes in water. 5.2 Distinguish between //aerobic// and //anaerobic// decomposition of organic material in water. 5.3 Describe the process of eutrophication and its effects. 5.4 Describe the source and effects of thermal pollution in water.

Introduction to Dissolved Oxygen in Water
The amount of oxygen dissolved in water is essential to the survival of aquatic plants and animals, as most require oxygen for aerobic respiration. The maximum solubility of oxygen in water is approximately 9 ppm (0.0003gdm -3 ) at one atm pressure and 20°C. Fish requires the highest level while bacteria requires the lowest level. Although fish requires at least 3ppm for survival, oxygen content should not be less than 6 ppm to maintain a balanced and diversified aquatic community.

Oxygen enters the water through diffusion from the surrounding air, as a by-product of photosynthesis or through aeration. Aeration is rapid movement of water, and rarely occurs in lakes, reservoirs and estuaries as the water is more stagnant, and as a result stratification of various temperatures of water occurs. The level of dissolved oxygen also varies with the seasons and time of day. This is because as temperature increases, the solubility of gases decrease.

As warmer water results in unfavorably low levels of dissolved oxygen, fish stocks are moving to greater ocean depths where the denser cold temperature waters in response to global warming. Water is said to be polluted, when the dissolved oxygen level falls below the required value for life.

Biochemical Oxygen Demand
**Biochemical Oxygen Demand (BOD)** is the amount of dissolved oxygen needed by aerobic decomposers to break down the organic materials in a given volume of water at a certain temperature over a specified time period. Bacteria make use of dissolved oxygen while decomposing plant and animal matter and human waste. Pure water has a BOD of approximately 1 ppm, and water with BOD level above 5 ppm is regarded as polluted. Water with a high level of BOD while being unable to replenish dissolved oxygen will result in insufficient oxygen level, and thus rendering aquatic life unsustainable. A faster flowing river would therefore be be able to recover its water quality through aeration.

The **Winkler method** may be used to determine the BOD of a sample of water. The steps are as follow: 1. A sample of water is collected, while ensuring sample bottle is completely full to prevent contamination through an outside air source. 2. Manganese sulfate and an alkaline-iodide reagent is used to fix amount of oxygen in the water sample. 2Mn 2+ (aq) + 4OH - (aq) + O 2 (g) → 2MnO 2 (s) + 2H 2 O(l) 3. Concentrated sulfuric acid is added, to remove the brownish floc of MnO2 formed, and form iodine. MnO 2 (s) + 2I - (aq) + 4H - (aq) → Mn 2+ (aq) + I 2 (aq) + 2H 2 O(l) 4. Titration is performed using standardized solution of sodium thiosulfate and starch as an indicator, and is stopped when the blue color disappears. I 2 (aq) + 2S 2 O 3 2- (aq) → S 4 O 6 2- (aq) + 2I - (aq) 5. Knowing the number of moles of iodine produced, allows for calculation of amount of oxygen present, and hence its concentration.

Aerobic and Anaerobic Decomposition
**Aerobic decomposition** is when decomposition of organic matter uses oxygen in the process to produce oxides or oxyanions. While **anaerobic decomposition** is carried out by organisms that do not require oxygen. Aerobic decomposition results in elements oxidizing or loosing electrons. An example would be the decomposition of carbon-containing waste, where the oxidation number of carbon changes from 0 to +4. C(s) + O 2 (g) → CO 2 (g) Another process is the aerobic decomposition of phosphorus: P 4 (aq) + 8O 2 (g) + 12e - → 4PO 4 3- (aq)

Anaerobic decomposition on the other hand, results in elements reduction, or the gaining of electrons. The anaerobic decomposition of carbon-containing waste results in the oxidation number changing from 0 to -4. C(s) + 2H 2 (g) → CH 4 (g) The anaerobic decomposition of sulfur: S 8 (s) + 8H 2 (g) → 8H 2 S(g) Results in the formation of dihydrogen sulfide that has a distinctive rotten egg smell, associated with foul smelling marshlands. Products in the reduced form are often foul smelling and toxic.

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**Eutrophication** is the process by which lakes, estuaries and other still bodies of water receive higher than normal levels of nutrients, which results in an excessive growth of plants - an algal bloom. This process occurs naturally over thousands of years, however anthropogenic influences causes eutrophication in an unnatural, negative manner. It begins when nitrates from fertilized agricultural fields and lawns accumulate in lakes, and phosphates from untreated household waste water with high levels of detergent enter the water system too.

These nitrates and phosphates serve as nutrients for the plants and algae, thus resulting in an increase of plant growth. When these plants and algae die, aerobic decomposition occurs and oxygen is used in forming of carbon dioxide and water. However, due to the increased amounts of plants and algae the rate at which the dissolved oxygen is being consumed is faster than it can be replaced. Furthermore, light is unable to penetrate into the water as excessive amounts of algae covers the water surface. This prevents the photosynthetic plants under the water surface from photosynthesizing and replenishing the dissolved oxygen within the water.

Therefore resulting in anaerobic decomposition occurring, and releasing foul smelling and toxic hydride such as NH 3 and H 2 S. The level of dissolved oxygen is thus reduced dramatically, and aquatic life is suffocated. The diversity of the aquatic community is reduced, and the food web of the entire habitat is impacted.

T h e r m a l Pollution


Thermal pollution refers to an increase in temperature caused by anthropogenic means. A common cause is when water is used and heated in industrial process before being released back into the environment, or when large amounts of cold water is released into the environment. The cutting down of trees also removes natural shade from the lake, resulting in increased exposure to direct sunlight and raising of temperature.

The level of dissolved oxygen in water is dependent on the water temperature, therefore a change in temperature is bound to impact the environment. It is an inverse relationship where an increase in temperature results in a decrease in solubility of gases. As seen in the graph beside, the level of dissolved oxygen is highest during the colder temperatures of January and February, while lowest during the warmer months of August and September. As temperature increases, the metabolic rate of fish and other aquatic organisms also increases due to increased demand for oxygen, and so they would have to consume more food and possibly leading to an instance of food shortage. Species of aquatic life that are unable to relocate such as rooted plants and shellfish will die, while fish and other mobile species will move to seek a cooler environment. At the same time, other species that favor warmer conditions may move into the area. This upsets the food web dramatically, as the adapted organisms moving in are likely to have an advantage over the native organisms, thus resulting in a problem of compromising food chains. Therefore a change as small as 1°C can have drastic effects on the entire ecosystem as the food web is altered.

A "fish-chase" procedure where the temperature is increased gradually may be used to drive fish away from the pipes, before the temperature reaches lethal levels may be used. However, this method will still be impacting the food web as the aquatic organisms are being redistributed. In serious cases, this may even result in the dramatic decrease of a certain species of inhabitants.

So, did you accomplish the learning outcomes? 5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen demanding wastes in water. 5.2 Distinguish between //aerobic// and //anaerobic// decomposition of organic material in water. 5.3 Describe the process of eutrophication and its effects. 5.4 Describe the source and effects of thermal pollution in water.

Sources Acknowledgment:


 * blackwood, p. (2008, July 20). //Marshland, south- west of Buchal Farm:: OS grid NO2751 :; Geograph Britian and Ireland - photograph every gird square!// Retrieved October 17, 2010, from Geograph Britain and Ireland: http://www.geograph.org.uk/photo/890634
 * Center for Earth and Environmental Science. (n.d.). //Temperature//. Retrieved October 17, 2010, from EUPUI Center for Earth and Environmental Science: http://www.cees.iupui.edu/education/Education_Outreach/Water_Resources_Ed/Temperature.htm
 * Coad, B. W. (2010, September 24). //Fishes of Canada's National Capital Region//. Retrieved October 17, 2010, from Salmonidae: http://www.briancoad.com/ncr/Salmonidae.htm
 * Environmental Chemistry. In //Chemistry: For use with the IB Diploma Programme// (pp. 280-283).
 * Muir, P. (2009, October 13). //1.Eutrophication//. Retrieved October 7, 2010, from Oregon State University: BI301 HUMAN IMPACTS ON ECOSYSTEMS: http://people.oregonstate.edu/~muirp/eutrophi.htm
 * Neuss, G. (2007). IB Study Guides Chemistry for the IB Diploma Standard and Higher Level. New York: Oxford University Press.
 * //Thermal Pollution//. (n.d.). Retrieved October 7, 2010, from Pollution Issues: http://www.pollutionissues.com/Te-Un/Thermal-Pollution.html
 * //What Can I Do to Help? Lake-Friendly Fertilizers & Their Applications//. (n.d.). Retrieved October 17, 2010, from InterLinc: Low/No Phosphorus Fertilizer: http://lincoln.ne.gov/city/pworks/watrshed/educate/fertiliz/index.htm