Worksheet 16.2 Chapter 16: Environmental chemistry fast facts E.1 Air pollution E.10 Smog Photochemical smog is caused by interaction between sunlight and the primary pollutants nitrogen monoxide and volatile organics produced from car exhausts. Geographical conditions, such as surrounding high mountains and a windlessness help create a temperature inversion which traps the pollutants close to the ground. The brown colour of photochemical smog is due to the presence of nitrogen dioxide formed by the reaction between NO and O 2 : 2NO O 2 2NO 2. The reactive oxygen atom reacts with molecular oxygen to produce ozone: O 2 + O O 3. Volatile organics, primary pollutants, are oxidised to produce OH radicals: RH + O R + OH. The OH radicals react with alkane molecules to produce more alkyl radicals, which react with oxygen molecules to produce peroxy radicals: 1
RCH 3 + OH RCH 2 + H 2 O RCH 2 + O 2 RCH 2 O 2 The RCH 2 O radicals then react with oxygen to form aldehydes: RCH 2 O + O 2 RCHO + HO 2. Peroxyacylnitrate (PAN), a secondary pollutant produced by a further sequence of free radical reactions, causes respiratory problems. RH + OH R + H 2 O R + O 2 ROO ROO + NO 2 ROONO 2 (PAN) E.2 and E.11 Acid deposition Acid deposition refers to the process by which acidic particles, gases and precipitation leave the atmosphere. Wet deposition includes acid rain, fog and snow and dry deposition includes acidic gases and particles. Rain is naturally acidic because of dissolved CO 2 ; but acid rain has a ph of less than 5.6. It is caused by oxides of sulfur and nitrogen. The burning of sulfur containing fossil fuels produces sulfur dioxide: S (s) + O 2 (g) SO 2 (g) Sulfur trioxide is a secondary pollutant formed in the atmosphere by the oxidation of the primary pollutant sulfur dioxide. The sulfur trioxide dissolves in water to form sulfuric acid. 2SO 2 (g) + O 2 (g) 2SO 3 (g) H 2 O (l) + SO 3 (g) H 2 SO 4 (aq) The overall oxidation reaction: 2H 2 O (l) + 2SO 2 (g) + O 2 (g) 2H 2 SO 4 (aq) Under conditions of high temperatures in the combustion engine, nitrogen and oxygen in the air react to form nitrogen monoxide: High temp N 2 (g) + O 2 (g) 2NO (g) Nitrogen dioxide, a secondary pollutant, is produced from the oxidation of NO. 2NO (g) + O 2 (g) 2NO 2 (g) The nitrogen dioxide dissolves in water to form a mixture of nitrous and nitric acid: H 2 O (l) + 2NO 2 (g) HNO 2 (aq) + HNO 3 (aq) Alternatively nitrogen dioxide can be oxidized to form nitric acid: 2H 2 O (l) + 4NO 2 (g) + O 2 (g) 4HNO 3 (aq) 2
The effects of acid rain include: Erosion of stone and metal objects; CaCO 3 (s) + 2H + (aq) Ca 2 + (aq) + H 2 O (l) + CO 2 (g) Fe (s) + H 2 SO 4 (aq) FeSO 4 (aq) + H 2 (g) Damage to aquatic life, either directly or because of increased levels of Al 3 + ; Damage to plant life; Possible respiratory problems. Sulfur dioxide emissions can be reduced by removing the sulfur either before or after combustion. Pre-combustion methods: The sulfur present in coal as metal sulphides can be removed by crushing the coal and washing with water. Hydrogen sulphide can be removed from crude oil by mixing with basic potassium carbonate solution: H 2 S (g) + CO 3 2 (aq) HS (aq) + HCO 3 (aq) The hydrogen sulfide can be recovered from solution by reversing the equilibrium. Hydroxyl radicals, produced in the reactions discussed earlier can react with the oxides of sulfur and nitrogen to form sulfuric and nitric acids. Post combustion methods remove SO 2 from combustion products and include alkaline scrubbing and fluidized-bed combustion. In the alkaline scrubbing method an alkaline mixture is sprayed downwards into the exhaust gases. CaO (s) + SO 2 (g) CaSO 3 (s) CaCO 3 (s) + SO 2 (g) CaSO 3 (s) + CO 2 (g) 2CaSO 3 (s) + O 2 (g) 2CaSO 4 (s) In the fluidized combustion method the coal is mixed with powdered limestone. The heat produced from combustion of the coal breaks up the calcium carbonate to calcium oxide and carbon dioxide: CaCO 3 (s) CaO (s) + CO 2 (g) The calcium oxide reacts with sulfur dioxide to form calcium sulfate: CaO (s) + SO 2 (g) CaSO 3 (s) 2CaO (s) + 2SO 2 (g) + O 2 (g) 2CaSO 4 (s) Formation of hydroxyl radicals: H 2 O + O 3 2HO + O 2 3
The mechanism of acid deposition caused by the nitrogen oxides: The mechanism of acid deposition caused by the sulphur oxides: H 2 O + O 2HO HO + NO 2 HNO 3 HO + NO HNO 2 HO + SO 2 HOSO 2 HOSO 2 + O 2 HO 2 + SO 3 SO 3 + H 2 O H 2 SO 4 Ammonia, in the atmosphere, neutralizes the acids formed and forms ammonium salts. Weakly acidic ammonium salts, (NH 4 ) 2 SO 4 and NH 4 NO 3, formed in the atmosphere sink to the ground or are washed out of the atmosphere with rain. The NH 4 + is deposited in the soil where nitrification and acidification occur to produce nitric acid: NH 4 + + 2O 2 2H + + NO 3 + H 2 O. E.3 Greenhouse effect Greenhouse gases allow the passage of incoming solar short-wavelength radiation but absorb the longer-wavelength radiation from the Earth. Some of the absorbed radiation is re-radiated back to Earth. Greenhouse gases include CH 4, H 2 O, CO 2, N 2 O and chlorofluorocarbons (CFCs). Their effects depend on their abundance and their ability to absorb heat radiation. 4
The effects of increasing levels of greenhouse gases on the atmosphere include: thermal expansion of the oceans; melting of the polar ice-caps and floods; droughts and changes in precipitation and temperature; changes in the yield and distribution of commercial crops; changes in the distribution of pests and disease-carrying organisms. E.4, E.9 Ozone depletion Formation of ozone: In the stratosphere the strong covalent double bond in normal oxygen, O 2, is broken by high energy UV radiation to form two oxygen atoms. UV light O 2 (g) O (g) + O (g) (atomic oxygen) The oxygen atoms have unpaired electrons. They are reactive free radicals and so react with another oxygen molecule to form ozone. O (g) + O 2 (g) O 3 (g) Depletion of ozone: The bonds in ozone are weaker than the double bond in oxygen; ultraviolet light of lower energy is needed to break them. UV light O 3 (g) O (g) + O 2 (g) The oxygen atoms then react with another ozone molecule to form two oxygen molecules. O 3 (g) + O (g) 2O 2 (g) This reaction produces heat, which maintains the relatively high temperature of the stratosphere. The major ozone depleting pollutants include: chlorofluorocarbons (CFCs), used as a refrigerant and a propellant in aerosol sprays; oxides of nitrogen (NO x ) from high temperature combustion with engines or high flying aircraft. The main problem with CFCs is the C Cl bond, so replacements generally have fewer C Cl bonds. A number of alternatives to CFCs have been proposed: Hydrocarbons such as propane, C 3 H 8 and methylpropane (CH 3 CH (CH 3 )CH 3 ). They have no C Cl bonds but are flammable. Fluorocarbons; these are not flammable and have very strong C F bond which makes them stable under ultraviolet radiation. Hydrochlorofluorocarbons (HCFCs); although they have C Cl bonds, most molecules are destroyed in the lower atmosphere before reaching the stratospheric ozone layer. They are 20 times less destructive than CFCs. Hydrofluorocarbons (HFCs), which have no chlorine atoms and so are considered the best alternative, as they are not flammable. 5
The structures of the different forms of oxygen: The bond order in O 2 (O = O) is 2. It has a σ and π bond. The bond order in O 3 is 1.5. It has a σ bond plus a delocalized π bond. The weaker bond in ozone can be broken by UV light of longer wavelength. λ = 242 nm O 2 2O CCl 2 F 2 CClF 2 + Cl λ = 330 nm O 3 O 2 + O The mechanism for the catalysis of O 3 depletion by CFCs. Cl + O 3 ClO + O 2 ClO + O O 2 + Cl The mechanism for the catalysis of O 3 depletion by NO x. Overall reaction: O 3 + O 2O 2 NO + O 3 NO 2 + O 2 NO 2 + O NO + O 2 Overall reaction: O 3 + O 2O 2 Ozone depletion is greater in polar regions. Ice particles act as heterogeneous catalysts and provide a surface area for the pollutants to react. The crystals also contain small amounts of hydrogen chloride, HCl and chlorine nitrate, ClONO 2 which react to produce chlorine: HCl + ClONO 2 HNO 3 + Cl 2. The chlorine photo dissociates, Cl 2 Cl + Cl, to produce Cl atoms which catalyze ozone depletion. E.5 Dissolved oxygen in water The biochemical oxygen demand (BOD) is the amount of oxygen needed to oxidize organic material in a specified time (5 days) at a specified temperature (20 C). The elements in the organic compound are reduced or oxidized by anaerobic processes. 6
The BOD of water increases as a result of the addition of nitrates and phosphates to the water. These ions cause eutrophication: the excessive growth of plants and algae. The algae die owing to the limited oxygen available. The subsequent decay leads to a further increase in the amount of nutrients. Anaerobic bacteria take over and produce gases, such as ammonia and hydrogen sulfide, which poison the water. Deaths occur and the process continues until there is no life remaining in the water. Water is used as a cooling agent in industry as it has a relatively high specific heat capacity. Thermal pollution occurs when the water is returned with increased temperatures. The concentration of dissolved oxygen decreases with rising temperature and this has a number of harmful effects. The oxygen concentration of the water may be insufficient for fish to survive. The metabolic rate of organisms increases with temperature and this places an additional demand on the oxygen in the water. Biochemical processes may be so upset that organisms die. The spawning, fertilization and hatching of eggs is very sensitive to temperature and unseasonable temperatures will upset life cycles. E.6 Water treatment Waste water contains a wide variety of toxic chemicals including: Heavy metals from mining and industrial processes, like electroplating. The heavy metal ions of mercury, lead and cadmium interfere with the behaviour of other essential ions in the body, such as Ca 2 +, Mg 2 + or Zn 2 +. Pesticides. Dioxins found as impurities in weed-killers and pesticides. Polychlorinated biphenyls (PCBs) from capacitors and transformers used in power supplies. Organic matter from food processing and water treatment. Nitrates and phosphates from overuse of fertilizers. Water treatment removes hazardous materials, reduces the BOD (biological oxygen demand) and kills microorganisms. Different types of treatment with different levels of effectiveness are carried out depending on the availability of resources. 7
Primary treatment includes filtration and sedimentation and removes suspended solids. Secondary treatment uses aerobic bacterial to oxidize organic matter. Oxygen is passed through a suspension of activated sludge. Tertiary treatment removes dissolved ionic pollutants, such as heavy metals, nitrates and phosphates. Heavy metals may be removed by ion exchange resins or precipitated as sulfides. Nitrates can be reduced to nitrogen gas by denitrifying bacteria under anaerobic conditions and phosphates may be precipitated. Sea water is not fit for human consumption and has limited agricultural and industrial uses. Desalination processes remove salts and produce fresh water. Two common methods are multistage distillation and reverse osmosis. Distillation involves heating the solution and collecting the condensed water vapour. Nonvolatile salts are left in solution. A multistage process maximizes efficiency as the heat produced in condensation can be reused for further boiling. Osmosis is the movement of water from a dilute to a concentrated solution by passing through a semi-permeable membrane. Reverse osmosis occurs when a pressure of 70 atm. (the osmotic pressure) is applied to the more concentrated salt solution. Water passes through the semi-permeable membrane and leaves the dissolved salts behind. As reverse osmosis does not involve a phase change, it uses less energy and is less expensive than distillation. There are costs, however, in maintaining the osmotic pressure. E.7 Soil All plants and land organisms depend on soil for their existence. It is formed by the biological, chemical and physical weathering of rock, and is a mixture of inorganic and organic matter, including air and water. Soil degradation occurs where human activity (either directly or indirectly) reduces the capacity of the soil to support life. Salinization is the result of continually irrigating soils. Dissolved salts are left behind after the water evaporates. Plants cannot grow in salty soil. Agriculture disrupts nutrients cycles when crops are harvested. These nutrients can be replaced by adding compost or artificial fertilizers to the soil. Soil pollution occurs with pesticides and fertilizers, as they disrupt the food chains and the soil s biodiversity. The chemicals cause further pollution if they are allowed to run off into water systems. 8
Soil organic matter (SOM) includes undecayed plant and animal tissues, their partial decomposition products and the soil biomass. It includes: high-molecular-weight organic materials (for example, polysaccharides and proteins); sugars, amino acids and other small molecules; humic substances. The functions of SOM are: Biological: provides source of nutrients (P, N, S) and contributes to the resilience of the soil/plant system. Physical: improves structural stability, influences water-retention properties, and alters the soil s thermal properties. Chemical: contributes to the cation exchange capacity (CEC), enhances the ability of soil to buffer changes in ph, complexes cations, reduces concentrations of toxic cations, promotes the binding of SOM to soil minerals. Common organic soil pollutants include: petroleum hydrocarbons from lubricants, such as engine oil and volatile organic compounds (VOCs); agrichemicals from overuse of fertilizers, pesticides and herbicide; solvents used in many industrial products and present in paints, adhesives, etc.; polyaromatic hydrocarbons (PAHs) from the incomplete combustion of fossil fuels; polychlorinated biphenyls (PCBs) from capacitors and transformers used in power supplies; organotin compounds from anti-fouling marine paints; semi-volatile organic compounds (SVOCs) from industrial solvents, hydraulic fluids, etc. E.12 Water and soil Metal ions can be removed from water by chemical precipitation. Given the equilibrium formed by a metal M and a non-metal X: MX (s) M + (aq) + X (aq). K sp = [M + ][X ] is called the solubility product constant. It is a measure of the solubility of an ionic compound. Increasing the concentration of X (aq) shifts the equilibrium to the left. This is called the common ion effect. The amount of exchangeable cations in a clay; called its cation-exchange capacity. 9
The cation-exchange capacity (CEC) equals the number of moles of single charged positive ions which can be held in 1 kg of soil. The CEC is an indicator of soil fertility. The larger the CEC value, the more cations the soil can absorb and make available to plants. As hydrogen ions can replace these cations their availability is affected by ph. The more strongly the cation bonds to the clay the lower the ph required to release it. Ions of high charge density such as Al 3 + and Fe 3 + are only released at low ph. Metals, which form insoluble hydroxides, are not available in alkaline conditions: M n + (aq) + noh (aq) M (OH) n (s) The soil organic matter (SOM): Contributes to cation-exchange capacity; Enhances the ability of soil to buffer changes in ph; Binds to organic and inorganic compounds in soil; Reduces the negative environmental effects of pesticides, heavy metals and other pollutants by binding to contaminants; Forms stable complexes with cations. E.8 Waste An increase in the world population and consumption has led to a rapid growth in the quantity of solid waste. Landfill is simple and cheap but there are issues in finding suitable sites, and concerns over the leaching of toxic substances, such as heavy metals, into water systems and the release of flammable gases, such as methane, from anaerobic decomposition. Incineration greatly reduces the waste volume but the process is expensive due to fuel and plant costs. Toxic combustion products are released into the air. 10
Recycling has many benefits, but the waste needs to be efficiently separated. Commonly recycled materials include: Aluminium: the extraction uses large quantities of electricity, so recycling leads to significant energy savings. Glass: broken glass can be melted and moulded into new products. This saves raw materials and energy but it must first be sorted into different colours. Paper: waste paper is pulped, the ink removed and bleached before being transformed into paper. This conserves raw materials, but the recycled product is of lower quality. Plastic: thermoplastics can be recycled but they must be separated into different polymer types. Steel: scrap steel can be added to molten iron from the blast furnace. This saves energy and recycles the alloying substances. Radioactive waste can be classified as low-level or high-level waste. Low-level waste has low levels of activity and short half-lives. High-level waste has high activity and a longer half-life. The radioisotopes which are used in research laboratories and hospitals are low-level waste. The spent fuel rods from nuclear power stations are high-level waste. Low-level waste is stored in cooling ponds of water until the activity has fallen to safe levels. The water is then passed through an ion exchange resin which removes the isotopes responsible for the activity, and then diluted before being released into the sea. Other methods of disposal include keeping the waste in steel containers inside concrete-lined vaults. High-level waste is cased in ceramic or glass and then packed in metal containers before being buried deep in granite rock, in remote places that are geologically stable. 11