CHEM108 Chemistry of the Environment PDF

CHEMISTRY OF THE ENVIRONMENT I. EARTH’S ATMOSPHERE AND HUMAN ACTIVITIES As technology has advanced and the world human population has increased, humans have put new and greater stresses on the environment. Paradoxically, the very technology that can cause pollution also provides the tools t...

CHEMISTRY OF THE ENVIRONMENT I. EARTH’S ATMOSPHERE AND HUMAN ACTIVITIES As technology has advanced and the world human population has increased, humans have put new and greater stresses on the environment. Paradoxically, the very technology that can cause pollution also provides the tools to help understand and manage the environment in a beneficial way. Chemistry is often at the heart of environmental issues. The economic growth of both developed and developing nations depends critically on chemical processes that range from treatment of water supplies to industrial processes. Some of these processes produce products or by-products that are harmful to the environment. Ozone Depletion Figure 7.1 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. Harmful Gases 1. Chlorofluorocarbon CFCs (CFCl3, CF2Cl2) 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) The C—Cl bond is easily broken, though, when the molecule absorbs radiation with a wavelength between 190 and 225 nm. 1  The chlorine atoms formed react with ozone: 2. Sulfur compounds and acid rain Sulfur dioxide (SO2) is a by- product of the burning of coal or oil. It reacts with moisture in the air to form sulfuric acid. It is primarily responsible for acid rain. Although its concentration is low, SO2 is regarded as the most serious health hazard Figure 7.2 Water pH values from freshwater sites across the United States, 2008. The numbered dots indicate the locations of monitoring stations. High acidity in rainfall causes corrosion in building materials. Marble and limestone (calcium carbonate) react with the acid; structures made from them erode. 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 (a) (b) sulfite Figure 7.3 Damage from acid rain. The right photograph, recently taken, shows how the statue has lost detail in its carvings. 2  Figure 7.4 One method for removing SO2 from combusted fuel. 3. Carbon Monoxide Formed by the incomplete combustion of carbon containing material such as fossil fuels. Carbon monoxide binds preferentially to the iron in red blood cells. CO binds to hemoglobin over 200 times stronger than O2 does. Exposure to significant amount of CO can lower O2 levels to the point that loss of consciousness and death can result. Only 0.1% CO can convert more than half of Hb into COHb (Carboxyhemoglobin) Products that can produce carbon monoxide must contain warning labels. Carbon monoxide is colorless and odorless. 4. Nitrogen oxides Nitrogen oxides are primary components of smog. The majority of nitrogen oxide emissions comes from cars, buses, and other forms of transportation. 5. Photochemical smog A photochemical smog is the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere, which leaves airborne particles (called particulate matter) and ground-level ozone. 3  Ozone, carbon monoxide, and hydrocarbons also contribute to air pollution that causes severe respiratory problems in many people. Greenhouse Gases 1. Water Vapor 2. Carbon Dioxide 3. Methane 4. Nitrous oxide 5. HFC’s 6. CFC’s The influence of H2O, CO2, and certain other atmospheric gases on Earth's temperature is called the greenhouse effect because in trapping infrared radiation these gases act much like the glass of a greenhouse. The gases themselves are called greenhouse gases. Water vapor and carbon dioxide The average surface temperature of the Earth would be 254 K, without gases in the atmosphere. The 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 vapor. This blanketing effect is known as the “greenhouse effect.” Water vapor, 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. A liter of gasoline produces about 2 kg of CO2. 4  Figure 7.5 Rising CO2 levels. The sawtooth shape of the graph is due to regular seasonal variations in CO2 concentration for each year. Other Greenhouse Gases Although CO2 receives most of the attention, other gases contribute to greenhouse effect, including methane, CH4, hydrofluorocarbons (HFCs), and chlorofluorocarbons (CFCs). Hydrofluorocarbons (HFCs) have replaced CFCs in a host of applications, including refrigerants and air-conditioner gases. Although they do not contribute to the depletion of the ozone layer, HFCs are nevertheless potent greenhouse gases. For example, one of the byproduct molecules from production of HFCs that are used in commerce is HCF3, which is estimated to have a global warming potential, gram for gram, more than 14,000 times that of CO2. Methane is formed in biological processes that occur in low-oxygen environments. Anaerobic bacteria, which flourish in swamps and landfills, near the roots of rice plants, and in the digestive systems of cows and other ruminant animals. It also leaks into the atmosphere during natural-gas extraction and transport. It is oxidized in the stratosphere, producing water vapor, a powerful greenhouse gas that is otherwise virtually absent from the stratosphere. II. EARTH’S WATER AND HUMAN ACTIVITIES All life on Earth depends on the availability of suitable water. Many human activities entail waste disposal into natural waters without any treatment. These practices result in contaminated water that is detrimental to both plant and animal aquatic life. Dissolved Oxygen and Water Quality The amount of O2 dissolved in water is an important indicator of water quality. o At 1 atm, 20 °C, water fully saturated with air has 9 ppm oxygen. o Cold-water fish require at least 5 ppm oxygen. Aerobic bacteria consume dissolved oxygen to oxidize organic materials for energy. The organic material the bacteria are able to oxidize is said to be biodegradable. 5  Organic materials that bacteria can oxidize reduce oxygen content. Plant nutrients contribute to water pollution by stimulating excessive growth of aquatic plants (floating algae). Figure 7.6 Eutrophication. This rapid accumulation of dead and decaying plant matter in a body of water uses up the water’s oxygen supply, making the water unsuitable for aquatic animals. Distillation ▪ An energy-intensive process. ▪ As water is distilled from seawater, for example, the salts become more and more concentrated and eventually precipitate out. Desalination ▪ The removal of salts from seawater or brackish water to make the water usable. Seawater has too high concentration of NaCl for human consumption. For drinkable water, NaCl content should be less than about 0.05%. Seawater can be desalinated through distillation or reverse osmosis. 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. 6  Water Purification ▪ Water goes through several filtration steps. ▪ CaO and Al2(SO4)3 are added to aid in the removal of very small particles. ▪ 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 7.7 Common steps in treating water for a public water system. Figure 7.8 A LifeStraw purifies water as it is drunk. Water Disinfection ▪ One of the greatest public health innovations in human history. ▪ It has dramatically decreased the incidences of waterborne bacterial diseases such as cholera and typhus. Trihalomethanes (THMs) ▪ In 1974 scientists in Europe and the United States discovered that chlorination of water produces a group of by-products previously undetected. ▪ All have a single carbon atom and three halogen atoms: CHCl3, CHCl2Br, CHClBr2 and CHBr3. III. EARTH’S SOIL AND HUMAN ACTIVITIES Soil is a mixture of sand, silt, and clay. All three of these components are ground-up rock, and the differences are in how finely the particles are ground. Sand particles are the largest, and clay particles are the smallest. Fertile topsoil is a mixture of at least four components – mineral particles, water, air, and organic matter. But as soil loses its plant nutrients to harvested crops and to leaching, it loses its fertility. Farmers amend soil by adding fertilizers, which are replacement sources for these lost nutrients. Naturally occurring fertilizers are mined minerals and compost which is decayed organic matter from animal manure, food scraps, or plant material. 7 Straight Fertilizer ▪ Ammonium nitrate, NH4NO3, is an example of a straight fertilizer which contains only one nutrient – nitrogen. Complete Fertilizer ▪ Any fertilizer containing a mixture of the three most essential nutrients (nitrogen, phosphorus, and potassium) is called either a complete fertilizer or a mixed fertilizer. ▪ All mixed fertilizers are graded by the N-P-K system, which lists the percent of nitrogen (N), phosphorus (P), and potassium (K) they contain. A high-yield crop needs more than adequate nutrition. It also needs defense against a host of natural enemies like pests. To control these pests, farmers can apply substances known as pesticides. There are several kinds of pesticides, including insect-killing insecticides, weed-killing herbicides, and fungus-killing fungicides. Insecticides ▪ The most widely used insecticides are chlorinated hydrocarbons, organophosphorus compounds, and carbamates. The chlorinated hydrocarbons have a remarkable persistence, killing insects for months and years on treated surfaces. There are at least two reasons for this persistence. First, chlorinated hydrocarbons tend to be nonbiodegradable, which means there are no natural pathways to break them down chemically. Second, they are nonpolar compounds, which means they are insoluble in water and so are not washed away by rainwater. An example of chlorinated hydrocarbon is DDT (dichlorodiphenyltrichloroethane) Organophosphorus compounds and carbamates, in contrast to chlorinated hydrocarbons, readily decompose to water-soluble components and so do not act over extended periods of time. Their immediate toxicity to both insects and animals is much greater than that of chlorinated hydrocarbons, however. Added safety precautions are required during the application of both organophosphates and carbamates, especially because of their toxicity to honeybees. Two important examples are malathion, an organophosphorus compound, and carbaryl, a carbamate. Malathion kills a variety of insects, such as aphids, leafhoppers, beetles, and spider mites. Carbaryl, like many other carbamates, is relatively selective in the types of insects it kills. 8 Herbicides ▪ Weeds compete with crop plants for valuable nutrients. The traditional method for controlling weeds is to plow them under the soil, where in decomposing they release the nutrients they absorbed while they were alive. Plowing also aerates the soil, but it is either labor-intensive or energy- intensive and can lead to topsoil erosion. Two selective herbicides are the carboxylic acids 2,4- dichlorophenoxyacetic acid (2,4-D) and 2,4,5- trichlorophenoxyacetic acid (2,4,5-T). Both mimic the action of plant growth hormones and are selective in killing broad-leafed plants but not grass-like crops such as corn and wheat. ▪ Three other commonly used herbicides are atrazine, paraquat, and glyphosate. Atrazine is toxic to common weeds but not to many grass-like crops, which can rapidly detoxify this herbicide through metabolism. Paraquat kills weeds in their sprouting phase. Paraquat residues made their way into the illicit drug products, however, causing lung damage in users. So, for ethical reasons, the spraying of paraquat on drug-producing plants is no longer common practice. Glyphosate is a nonselective herbicide that affects a biochemical process common to all plants-the biosynthesis of the amino acids’ tyrosine and phenylalanine. Glyphosate has low toxicity in animals because most animals do not synthesize these amino acids, obtaining them from food instead. Fungicides ▪ As decomposers, fungi play an important role in soil formation, but they can also harm crops. Most of the harm they cause occurs during a plant's early growth stages. Fungi can also spoil stored food and are particularly devastating to the world's fruit harvest. An example of a fungicide is thiram, widely used on fruits and vegetables. Organic Farming ▪ For controlling pests and maintaining fertile soil, the conventional agricultural industry is now looking at the efforts of many small-scale farmers who have demonstrated that significant crop yields can be obtained without pesticides and synthetic fertilizers. ▪ This method of farming is known as organic farming, where the term organic is used to indicate a concern for the environment and a commitment to using only chemicals that occur in nature. ▪ To protect against pests, organic farmers alternate the crops planted on a particular plot of land. Such crop rotation works fairly well because different crops are damaged by different pests. ▪ For fertilizer, organic farmers rely on compost. They also include nitrogen- fixing plants in their crop-rotation schedules. 9  Integrated Crop Management (ICM) ▪ To meet concerns about sustaining agricultural resources over the long term, groups from industry, government, and academia have identified a whole-farm strategy called integrated crop management (ICM). ▪ This method of farming involves managing crops profitably and with respect for the environment in ways that suit local soil, climatic, and economic conditions. ▪ Its aim is to safeguard a farm's natural assets over the long term through the use of practices that avoid waste, enhance energy efficiency, and minimize pollution. ▪ ICM is not a rigidly defined form of crop production but rather a dynamic system that adapts and makes sensible use of the latest research, technology, advice, and experience. ▪ One of the more significant aspects of ICM is its emphasis on multi- cropping, which means growing different crops on the same area of land either simultaneously, or in rotation from season to season. IV. GREEN CHEMISTRY The planet on which we live is, to a large extent, a closed system, one that exchanges energy but not matter with its surroundings. If humankind is to thrive in the future, all the processes we carry out should be in balance with Earth’s natural processes and physical resources. This goal requires that no toxic materials be released to the environment, that our needs be met with renewable resources, and that we consume the least possible amount of energy. Green chemistry is an initiative that promotes the design and application of chemical products and processes that are compatible with human health and that preserve the environment. Green chemistry rests on a set of 12 principles: 1. Prevention. It is better to prevent waste than to clean it up after it has been created. 2. Atom Economy. Methods to make chemical compounds should be designed to maximize the incorporation of all starting atoms into the final product. 3. Less Hazardous Chemical Syntheses. Wherever practical, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment. 4. Design of Safer Chemicals. Chemical products should be designed to minimize toxicity and yet maintain their desired function. 5. Safer Solvents and Auxiliaries. Auxiliary substances (for example, solvents, separation agents, etc.) should be used as little as possible. Those that are used should be as nontoxic as possible. 6. Design for Energy Efficiency. Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, chemical reactions should be conducted at room temperature and pressure. 7. Use of Renewable Feedstocks. A raw material or feedstock should be renewable whenever technically and economically practical. 10 8. Reduction of Derivatives. Unnecessary derivatization (intermediate compound formation, temporary modification of physical/chemical processes) should be minimized or avoided, if possible, because such steps require additional reagents and can generate waste. 9. Catalysis. Catalytic reagents (as selective as possible) improve product yields within a given time and with a lower energy cost compared to non-catalytic processes and are, therefore, preferred to noncatalytic alternatives. 10. Design for Degradation. The end products of chemical processing should break down at the end of their useful lives into innocuous degradation products that do not persist in the environment. 11. Real-Time Analysis for Pollution Prevention. Analytical methods need to be developed that allow for real-time, in-process monitoring and control prior to the formation of hazardous substances. 12. Inherently Safer Chemistry for Accident Prevention. Reagents and solvents used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires. Styrene An important building block for many polymers, including the expanded polystyrene packages used to pack eggs and restaurant takeout food. For many years, styrene has been produced in a two-step process: Benzene and ethylene react to form ethyl benzene, followed by the ethyl benzene being mixed with high-temperature steam and passed over an iron oxide catalyst to form styrene: This process has several shortcomings. One is that both benzene, which is formed from crude oil, and ethylene, formed from natural gas, are high- priced starting materials for a product that should be a low-priced commodity, and another is that benzene is a known carcinogen. In a recently-developed process that bypasses some of these shortcomings, the two-step process is replaced by a one-step process in which toluene is reacted with methanol at over a special catalyst: The one-step process saves money both because toluene and methanol are less expensive than benzene and ethylene, and because the reaction requires less energy input. Additional benefits are that the methanol could be produced from biomass and that benzene is replaced by less-toxic toluene. The hydrogen formed in the reaction can be recycled as a source of energy. 11  Supercritical Solvents ▪ A major area of concern in chemical processes is the use of volatile organic compounds as solvents. ▪ The solvent may be toxic or may decompose to some extent during the reaction, thus creating waste products. ▪ The use of supercritical fluids represents a way to replace conventional solvents. ▪ Supercritical fluid is an unusual state of matter that has properties of both a gas and a liquid. ▪ Water and carbon dioxide are the two most popular choices as supercritical fluid solvents. ▪ Replaces chlorofluorocarbon solvents with liquid or supercritical CO2 in the production of polytetrafluoroethylene ([CF2CF2] n, sold as Teflon®). ▪ Para-xylene is oxidized to form terephthalic acid, which is used to make polyethylene terephthalate (PET) plastic and polyester fiber. ▪ This commercial process requires pressurization and a relatively high temperature. Oxygen is the oxidizing agent, and acetic acid (CH3COOH) is the solvent. ▪ An alternative route employs supercritical water as the solvent and hydrogen peroxide as the oxidant. This alternative process has several potential advantages, most particularly the elimination of acetic acid as solvent. Greener Reagents and Processes Let us examine more examples of green chemistry in action. Hydroquinone, HO-C6H4-OH, is a common intermediate used to make polymers. The standard industrial route to hydroquinone, used until recently, yields many by-products that are treated as waste: 12 Using the principles of green chemistry, researchers have improved this process. The new process for hydroquinone production uses a new starting material. Two of the byproducts of the new reaction (shown in green) can be isolated and used to make the new starting material. The new process is an example of “atom economy,” a phrase that means that a high percentage of the atoms from the starting materials end up in the product. Another example of atom economy is a reaction in which, at room temperature and in the presence of a copper(I) catalyst, an organic azide and an alkyne form one product molecule: This reaction is informally called a “click reaction”. The yield—actual, not just theoretical—is close to 100%, and there are no by-products. Depending on the type of azide and type of alkyne we start with, this very efficient click reaction can be used to create any number of valuable product molecules. 13