Environmental Chemistry Chapter 20 Environmental Chemistry Edited by Dr. Tark Hamilton, Camosun College

Atmosphere Temperature varies greatly with altitude. However, there is not a linear relationship between altitude and temperature. Figure 20.1 (a)

Atmosphere Although the relationship between altitude and pressure is not linear, pressure does decrease with an increase in altitude. Figure 20.1 (b)

Composition of the Atmosphere Because of the great variation in atmospheric conditions, the composition of gases in the atmosphere is not uniform. Lighter gases tend to rise to the top. Table 20.1

Outer Atmosphere The Sun emits a wide range of wavelengths of radiation. Remember that light in the ultraviolet region has enough energy to break chemical bonds. Figure 20.2

Photodissociation O=O + h  2 O When these bonds break, they do so homolytically. Oxygen in the upper atmosphere absorbs much of this radiation before it reaches the lower atmosphere. .. : O=O + h  2 O

Photoionisation Table 20.2 Shorter wavelength radiation causes electrons to be knocked out of molecules in the upper atmosphere; very little of this radiation reaches the Earth’s surface. The presence of these ions makes long-range radio communication possible.

Ozone Ozone absorbs much of the radiation between 240 and 310 nm. It is a result of the reaction of molecular oxygen with the oxygen atoms produced in the upper atmosphere by photodissociation. O + O2  O3

Ozone Depletion In 1974, Rowland and Molina discovered that chlorine from chlorofluorocarbons (CFCs) may be depleting the supply of ozone in the upper atmosphere by reacting with it. CFCs were used for years as aerosol propellants and refrigerants. They are not water soluble (so they do not get washed out of the atmosphere by rain) and are quite unreactive (so they are not degraded naturally).

Chlorofluorocarbons The C—Cl bond is easily broken though when the molecule absorbs radiation with a wavelength between 190 and 225 nm. The chlorine atoms formed react with ozone. Cl + O3  ClO + O2

Chlorofluorocarbons In spite of the fact that the use of CFCs is now banned in over 100 countries, ozone depletion will continue for some time because of the tremendously unreactive nature of CFCs.

Troposphere Although the troposphere is made up almost entirely of nitrogen and oxygen, other gases present in relatively small amounts still have a profound effect on the troposphere. Table 20.3

Sulfur Sulfur dioxide is a by-product of the burning of coal or oil and upon oxidation to SO3, reacts with moisture in the air to form sulfuric acid. It is primarily responsible for acid rain. High acidity in rainfall causes corrosion in building materials. SO3(g) + H2O(l)  H2SO4(aq)

Sulfur SO2 can be removed by injecting powdered limestone which is converted to calcium oxide. The CaO reacts with SO2 to form a precipitate of calcium sulfite. Figure 20.4

Carbon Monoxide Carbon monoxide binds preferentially to the iron in red blood cells. Exposure to significant amounts of CO can lower O2 levels to the point that loss of consciousness and death can result. Figure 20.5

Carbon Monoxide Products that can produce carbon monoxide must contain warning labels. Carbon monoxide is colourless and odourless. Figure 20.6

Nitrogen Oxides and Photochemical Smog What we recognise as smog, that brownish gas that hangs above large cities, is primarily nitrogen dioxide, NO2. It is the result of the oxidation of nitric oxide, NO, a component of car exhaust. Figure 20.7

Nitrogen Oxides and Photochemical Smog These nitrogen oxides are just some components of photochemical smog. Ozone, carbon monoxide, and hydrocarbons also contribute to air pollution that causes severe respiratory problems in many people. As a result, government emission standards for automobile exhaust have become continually more stringent.

Water Vapour and Carbon Dioxide Gases in the atmosphere form an insulating blanket that causes the Earth’s thermal consistency. Two of the most important such gases are carbon dioxide and water vapour. Figure 20.8

Water Vapour and Carbon Dioxide This blanketing effect is known as the “greenhouse effect.” Water vapour, with its high specific heat, is a major factor in this moderating effect. But increasing levels of CO2 in the atmosphere may be causing an unnatural increase in atmospheric temperatures. Figure 20.9

The World Ocean The vast ocean contains many important compounds and minerals. However, the ocean is only a commercial source of sodium chloride, bromine, and magnesium.

Desalination “Water, water everywhere, and not a drop to drink.” Seawater has too high a concentration of NaCl for human consumption. It can be desalinated through reverse osmosis. Figure 20.12

Reverse Osmosis Water naturally flows through a semipermeable membrane from regions of higher water concentration to regions of lower water concentration. If pressure is applied, the water can be forced through a membrane in the opposite direction, concentrating the pure water. Figure 20.12

Dissolved Oxygen and Water Quality Biodegradable is the term used to refer to organic material that bacteria are able to oxidise. However, excessive quantities of this material leads to oxygen depletion. Nitrogen and phosphorus from fertilisers and detergents are plant nutrients that contribute to water pollution by stimulating excessive growth which leads to increased dead and decaying matter and oxygen depletion, a process called eutrophication. Figure 20.14

Treatment of Water Supplies Water goes through several filtration steps. CaO and Al2(SO4)3 are added to aid in the removal of very small particles. Figure 20.15

Treatment of Water Supplies The water is aerated to increase the amount of dissolved oxygen and promote oxidation of organic impurities. Ozone or chlorine is used to disinfect the water before it is sent out to consumers. Figure 20.15

The Challenges of Water Purification We have become increasingly aware over the past decades that modern processes are not always compatible with maintaining a sustainable environment. For example, when purifying water through chlorination, trihalomethanes (suspected carcinogens) are formed. HClO(aq) and HBrO(aq) (formed from Cl2(g), H2O(l) and Br-) will oxidise organics to CHCl3, CHCl2Br, etc. However, the risks of cancer from these materials in treated water are very low compared with the risks of cholera, typhus and gastrointestinal disorders from untreated water.