UNIT 14 ENVIRONMENTAL CHEMISTRY

Syllabus
• Environmental Pollution-air, water and soil pollution.
• Chemical reactions in atmosphere.
• Smogs
• Major atmospheric pollutants
• Acid rain
• Ozone and its reactions
• Effects of depletion of ozone layer
• Greenhouse effect and global warming
• Pollution due to industrial wastes
• Green chemistry as an alternative tool for reducing pollution
• Strategy for control of environmental pollution

Environmental chemistry is the branch of chemistry that deals with the study of various chemical processes taking place in various segments of environment. It is the study of the sources, transportation, reactions, effects and fates of the chemical species in the environment. In this unit we study some important topics in the field of environmental chemistry with special reference to pollution of air, water and land and some interesting chemical phenomena.
ENVIRONMENTAL POLLUTION
Environmental pollution may be described as the contamination of the environment with harmful wastes arising mainly from certain human activities. These activities release into environment certain materials which pollute the atmosphere, water and soil. The effects of pollution are increasing at a fast rate and if same are not checked and controlled, human existence and habitat would be in peril.
Any substance , which causes pollution , is known as a pollutant. A substance becomes a pollutant when it is present in concentrations harmful to the natural environment. Often only highly toxic substances are normally considered , but even substances that are normally considered harmless can cause pollution if they are present in undesirable concentrations and in the wrong place at wrong time. For example, nitrate is added to soil in order to increase plant growth, but an excessive concentration of nitrate present in drinking water can be toxic, specially to young children.
In an environment pollution, a pollutant originates from a source and gets transported by air or water or is dumped on land by man. Some of the pollutants may be absorbed (assimilated) or chemically changed by environment ; the rest build up to concentrations which are harmful to environment.



ATMOSPHERIC POLLUTION
Atmosphere which envelopes the earth , is divided into different layers or regions. The lowest region called troposphere , extends up to a height of  10 km from sea level. Above the troposphere , between 10 and 50 km above the sea level, lies the stratosphere . The troposphere contains about 80% of the total mass of air and almost all of the water vapours. The stratosphere on the other hand , contains nitrogen, oxygen and ozone. Atmospheric pollution is generally studied under two heads : Tropospheric pollution and Stratospheric pollution. Since only a few substances produced by human activity reach the stratosphere, its pollution results in the depletion of ozone layer which causes biologically harmful ultra violet radiation to reach the earth. Tropospheric pollution is much more wide spread.
Tropospheric pollution
Tropospheric pollutants may be gaseous or particulate in nature. The following are the major gaseous and particulate pollutants.
1. Gaseous air pollutants
Oxides of sulphur, nitrogen and carbon ; hydrogen sulphide ; hydrocarbons ; ozone and other oxidants.
2. Particulate pollutants
Dust , fumes, mist , spray, smoke etc.
Gaseous Air Pollutants
(a) Oxides of sulphur
These are probably the most harmful of the common gaseous pollutants. The combustion of any sulphur containing material results in the formation of sulphur dioxide (SO2) and sulphur trioxide (SO3).
Sulphur dioxide is the major constituent ( 97%) of the mixture. Sulphur dioxide is the most wide spread and serious air pollutant. It has been reported that the lower concentrations of sulphur dioxide causes temporary spasm of the smooth muscle of bronchioles. Sulphur dioxide is also considered to cause cough, shortness of breath and spasm of larynx. Sulphur dioxide can cause acute irritation to membranes of the eyes resulting in tears and redness. Moreover, it reacts in atmosphere to form another undesirable compound such as sulphuric acid. Sulphuric acid droplets attached to soot particles may pass through the nose and damage lungs.
Although sulphur dioxide is an irritant to living tissues, it is less harmful than sulphur trioxide to which it is converted according to the reaction :

The uncatalysed oxidation of SO2 is slow, but polluted air often contain particulate matter that catalyses the oxidation. The reaction can be promoted by the presence of NO2 or H2O2. The SO3 reacts with water in air (or in lungs) to form H2SO4.
Oxides of nitrogen
The main oxides of nitrogen are nitric oxide(NO) and NO2. Nitric oxide is formed as a result of lightning discharge taking place in the atmosphere when nitrogen and oxygen react as follows:
N2+ O2  2 NO
The major sources of oxides of nitrogen are combustion of gasoline in automobiles, burning of hydrocarbons , coal etc. Supersonic jet also release oxides of nitrogen into the atmosphere. Nitric oxide itself is not considered to have significant adverse effect on human health, even at elevated concentrations. However , it is oxidised by oxygen , and even more rapidly by ozone , to nitrogen dioxide.
2 NO(g) + O3 (g)  NO2 (g) + O2 (g)
Eventually, NO2 is oxidised to nitrate NO3 ion and washed into the soil , where it serves as a fertiliser.
Harmful effects
Oxides of nitrogen are injurious in a number of ways as listed below :
(i) High concentration of NO2 in atmosphere is harmful to plants resulting in leaf spotting, retardation of photosynthetic activity and also suppresses the vegetation growth.
(ii) Nitrogen dioxide (NO2) results in respiratory problems in human beings and leads to bronchitis.
(iii) Oxides of nitrogen have harmful effects on the nylon, rayon and cotton yarns and also cause cracks in rubber.
(iv) They also react with ozone (O3) present in atmosphere and thus decrease the density of ozone.
Oxides of carbon
(i) Carbon monoxide
Carbon monoxide a lethal gas, is produced as result of incomplete combustion of carbon. Its main unnatural source is the incomplete combustion of fossil fuels and its largest portion comes from the exhausts of motor vehicles. The complete combustion of butane , for example requires 6.5 moles of oxygen per mole of hydrocarbon. If only 6.0 moles of oxygen are present, then 1 mole of CO and 3 moles of CO2 are produced.
C4H10(g) + 6.5 O2(g)  4 CO2(g) + 5 H2O(ℓ)
C4H10(g) + 6O2(g)  3 CO2(g) +CO(g) + 5 H2O(ℓ)
The operation of engines with a lean fuel mixture , that is with less fuel and more air , reduces the emission of CO.
Harmful effects
1. Carbon monoxide is poisonous because it binds to haemoglobin much more strongly (about 200 times) than oxygen. The presence of CO , therefore reduces the amount of haemoglobin available in the blood for transport of oxygen to the body cells and therefore , with less O2 levels, normal metabolism is inpaired. Depending upon the concentration, CO in the blood can cause mental impairment, laboured respiration, muscular weakness, dizziness and even death.
2. There is also a strong evidence which links cigarette smoking to reduced mental performance and to heart diseases.
Carbon dioxide
Carbon dioxide is a natural constituent of the atmosphere and is vital to all forms of plant life. Normally it forms about 0.03% by volume of the atmosphere. With increased use of fossil fuels (coal, oil) , a large amount of carbon dioxide gets released into the atmosphere. In a balanced ecosystem, the CO2 additionally released into the atmosphere is removed regularly by green plants (because they require CO2 for photosynthesis and they in turn emit oxygen, thus maintaining the cycle). Forests on the earth account for about 70% of all photosynthesis occurring at any time but the extent of deforestration that has taken place to date, there is an increased build-up of CO2 in the atmosphere.
Carbon dioxide causes mild narcotic effects, stimulation of the respiratory centre and leads to asphyxiation. The increasing concentration of CO2 also changes the climatic conditions, especially by raising the global temperature. This phenomenon is known as Greenhouse effect.
GLOBAL WARMING AND GREENHOUSE EFFECT
Greenhouse effect is the phenomenon in which earth’s atmosphere traps the heat from the sun and prevents it from escaping into the outer space. The greenhouse effect received its name because the earth’s atmosphere acts much like the glass or plastic roof and walls of a greenhouse. Sunlight enters a greenhouse through the transparent glass or plastic panes and heats the plants but the heat emitted by the plants in the form of infra red radiation cannot pass through the glass or plastic panes. As a result, the inside temperature increases. Similarly, the earth's atmosphere allows most of the sunlight that falls on it to pass through and heat the surface. But the heat radiated by the heated surface cannot pass freely into space, because certain gases called greenhouse gases such as carbon dioxide, methane, ozone, chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere absorb it. Thus, they add to the heating of the atmosphere.
The concentration of water vapour in the atmosphere has not changed appreciably in recent years. There has , however, been a marked increase in the levels of other greenhouse gases. Deforestation and a large scale burning of fossil fuels , have contributed to the steady rise in the atmospheric concentration of CO2 . If the trends continue in the future, there would be an increase in the average temperature of the atmosphere resulting in climatic changes. For example, there may be less rainfall in temperature zones and more rain fall in drier areas of the world. The other severe outcome of the global warming would be the rise in sea level due to increased rate of melting of glaciers. It has been estimated that the sea level may rise by 0.5 m to 1.5 m in the next 50 to 100 years. Higher sea levels would increase the frequency and severity of floods, damage coastal areas, cause loss of soil replenishment and sea water intrusion into rivers and other aquatic systems near the ocean.
Increased carbon dioxide levels in the atmosphere also expected to cause plants, undergoing photosynthesis , to take up the gas at a greater rate, so that plants in warmer climates with adequate rainfall would grow faster. An increase in average global temperature is likely to increase the incidence of infectious diseases such malaria, sleeping sickness, dengue and yellow fever.
ACID RAIN
Normal rain is slightly acidic due to the dissolution of atmospheric carbon dioxide in water. But as the gas is only slightly soluble in water under ordinary conditions of temperature and pressure, its effect is very little and the pH of the normal rain water
varies from 5.6 to 6.0.


But in thickly populated industrial areas, oxides of nitrogen and sulphur as a result of combustion of coal and oil are released to the atmosphere. Both of them dissolve in water to form nitric acid and sulphuric acid.

The presence of soot (carbon particles) in air speeds up the oxidation process. The acids are highly soluble in water. In addition, vapours of hydrogen chloride gas also being released into the atmosphere as a result of the chemical reactions that are undergoing in our factories and industrial establishments. All these are likely to make the rain water sufficiently acidic with pH varying from 3 to 4.5. This rain has been given a special name called acid rain.
Harmful effects of Acid rain
Acid rain is never desirable because it adversely affects human beings, animals , plants and materials. Some of the harmful effects of acid rain are as follows.
(i) Acid rain decreases the pH of rain water which damages the digestive, respiratory and nervous systems in human beings and cause many diseases. The heavy metals released by acid rain may cause potential threat to human health.
(ii) Due to increased acidity of rain water, the aquatic animals like fishes etc. are unable to survive. Just because of acid rains , several lakes in Swedan and U.S.,A have become fishless.
(iii) The increased acidity of rain water also kills micro-organisms such as bacteria and damages blue green algae. This disturbs the ecological balance of the nature.
(iv) Acid rain is harmful to plants, trees and bushes. It damages their leaves and retards their growth. This is because acid rains leach away the nutrients like potassium, magnesium, calcium etc from soils and makes them short of essential elements.
(v) Acid rain reduces the activity of nitrogen fixing bacteria due to loss of fertility of soil.
(vi) Acid rain has also been reported to reduce the rate of photosynthesis in plants. This affects the growth of plants.
(vii) Acid rain causes substantial damage to buildings made of marble, lime stone, slate etc. Attack of acid rain on marble is termed as stone leprosy.
(viii) Acid rain causes damage to steel, zinc and automobile coatings. It accelerates the corrosion of metals.
Measures to reduce Acid rains
Acid rains have wide spread harmful effects and are a major concern to scientists. Keeping in view the potential damages caused by acid rain, it has become almost essential to adopt the necessary measures to control the pollutants causing acid rains at source. The thermal power plants which are largely responsible for acid rains must be replaced by suitable alternatives such as hydroelectric stations. The automobiles should be fuelled by LPG or CNG in place of gasoline or diesel. Every effort must be made to reduce the concentration of SO2 and nitrogen oxides in the atmosphere. Then only it will be possible to protect our environment from the ill effects of acid rains.
Pariculates in Atmospheric Pollution
The black smoke released into the air by a diesel truck is the common form of pollution of the atmosphere. The smoke is composed of pariculate matter. Particulates are the tiny solid or liquid particles suspended in the air. These particles are usually individually invisible to the naked eye. Collectively, these small particles often form a haze that restricts visibility. Particulates in atmosphere may be viable or non-viable.
The viable paticulates are the minute living organisms that are dispersed in atmosphere. These include bacteria, fungi, moulds, algae etc. Human beings are allergic to some of the fungi found in air. Fungi can also cause plant diseases.
Non-viable paticulates are formed either by breakdown of larger materials or by the condensation of minute particles and droplets. There are four types of non-viable particulates in the atmosphere : mists, smoke, fumes and dust.
(i) Mists
Mists are produced by particles of spray liquids and the condensation of vapours in air. Examples are portions of herbicides and incecticides that miss their targets and travel through the air to form mists.
(ii) Smoke
Smoke denotes very small soot particles produced by burning and combustion of organic matter. Oil smoke, tobacco smoke and carbon smoke are typical examples of this type of particulate emission.
(iii) Fumes
Fumes are condensed vapours ; fumes of metals are the well known particulates of this type. Examples of this category also include metallurgical fumes and alkali fumes.
(iv) Dust
Dust consists of fine particles produced during crushing, grinding and attribution of solid materials. Non-viable dust particulates in the atmosphere consists of ground lime stone, sand tailings from flotation, pulverised coal, cement , fly ash and silica dust.
The effects of particulate pollutants are largely dependent on the particle size. The coarser particles of size greater than 5 microns are likely to lodge in the nasal passages, the smaller ones are more likely to penetrate into the lungs – the rate of penetration being inversely proportional to the size of the particles. A number of these fine particulates are carcinogens . Inhalation of small particles irritates the lungs and exposure to such particles for long periods of time causes ‘scarring’ or ‘fibrosis’ of the lung lining. This type of disease is well known in industrial settings and is termed as ‘pneumocosis’.
The suspended particulate matter in the atmosphere effectively reduces the amount of light rays reaching the surface of the earth and thus lowers the temperature of the earth. By blocking the sunlight, they contribute to a drop in the earth’s temperature and by providing condensation nuclei ; they contribute to increased fog and rain in cities. Suspended particles in the city atmosphere also reduce the visibility. The visibility decreases as the atmospheric particulate concentrations increase.
SMOG
Smog is a mixture of smoke and fog . It is the extreme form of atmospheric pollution resulting from the combustion of fossil fuels in internal combustion engines.
Types of Smog
1. Classical smogs which occur in cool humid climate and are the result of build up of sulphur oxides and particulate matter from fuel combustion. Classical smog is chemically reducing smog with high concentrations of SO2.
2. Photochemical smogs which occur in warm, dry and sunny climates and results from the action of sunlight on the nitrogen oxides and hydrocarbons produced by automobiles and factories. Photochemical smog is an oxidising smog having a high concentration of oxidising agents. Photochemical smog occurs generally in those cities which have very large populations and high vehicular density.
Formation of smog
The formation of smog involves a series of photochemical branched chain reactions. The major stages are as follows.
(i) The hydrocarbons and oxides of nitrogen released by automobiles get accumulated in atmosphere.
(ii) Nitrogen dioxide(NO2) thus accumulated in the atmosphere absorbs sunlight in the blue and near ultraviolet region of the spectrum and decomposes into nitric oxide (NO) and atomic oxygen (O).
(iii) The decomposition of NO2 into NO and O• is followed by a series of reactions in which NO is converted into NO2 with simultaneous formation of ozone and aldehydes.
(iv) The hydrocarbons present in the atmosphere undergo oxidation to form a number of secondary pollutants such as formaldehyde, acrolein, peroxyacyl nitrates (PAN). The secondary pollutants form haze like aerosols which constitute a photochemical smog known as Los Angeles smog.
The chemical reactions involved can be described in a simplified manner as follows :
NO2 + h  NO + O•
O• + O2  O3
O3 + NO  NO2 + O2
RH + O•  RO•
(Hydrocarbon)
RO• + O2  RO•3
RO3• + RH  Aldehydes + Ketones
RO3• + NO  RO2• + NO2
RO2• + O2  RO3• + O2
RO2• + NO2  Peroxyacyl nitrate (PAN)
Photochemical smog increases with increase in concentration of NO2 and hydrocarbons in atmosphere and also with the intensity of sun. In the night , when there is no sunlight , it dissipates.
Effects of photochemical smog
The three main components of photochemical smog are nitrogen oxides, ozone and organic derivatives ( such as acrolein, formaldehyde, PAN etc.) Each contributes to the hazardous effects of the smog. Pungent smelling , smog-produced ozone is known to be toxic. It can cause coughing, wheezing, bronchial constriction and irritation to the respiratory mucous system. Peroxyacyl nitrates and aldehydes found in smog are eye irritants. Materials are also adversely affected by some components. Rubber has a high affinity for ozone and it is cracked and aged by it. Smog also affects plants in an adverse way. Ozone is also known to cause damage to vegitation and reduction in plant growth and crop productivity. Of all components of smog, PAN has the highest toxicity to plants, attacking younger leaves and causing ‘bronzing’ and ‘glazing ‘ of their surfaces.
Measures to reduce smog
The formation of smog can be reduced by decreasing the concentrations of nitrogen oxides and hydrocarbons in the atmosphere. This could be achieved by using catalytic converters in automobiles. The catalytic converters reduce the quatity of hydrocarbons and nitrogen oxides in the smoke coming out of the exhaust pipes of automobiles.
Photochemical smog can also be suppressed by certain compounds, which act as free radical traps. When these compounds are sprayed in the atmosphere, they generate free radicals which readily combines with free radical precursors of photochemical smog.
STRATOSPHERIC POLLUTION
The upper strastosphere consists of considerable amount of ozone (O3) which protects us from the harmful ultraviolet (UV) radiations (  255 nm) coming from the sun. These radiations cause skin cancer (melanoma) in humans. Therefore , it is important to maintain the ozone shield.
Ozone in the atmosphere is a product of UV radiations acting on dioxygen(O2) molecules. The UV radiations split apart molecular oxygen into free oxygen(O) atoms. These oxygen atoms combine with molecular oxygen to form ozone.

Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus a dynamic equilibrium exists between production and decomposition of ozone molecules. In recent years , there have been reports of depletion of this protective ozone layer because of the presence of certain chemicals in the stratosphere. The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs) , also known as freons. These compounds are nonreactive , noninflammable, non toxic organic molecules and therefore used in refrigerators , air conditioners, in the production of plastic foam and by electronic industry for cleaning computer parts etc. Once CFSs are released in the atmosphere , they mix with the normal atmospheric gases and eventually reach the stratosphere. In stratosphere , they get broken down by powerful UV radiations , releasing chlorine free radical.

The chlorine radical then react with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.

Reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals.

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus , CFCs are transporting agents for continuously generating chlorine radicals into the strastosphere and damaging the ozone layer.
The Ozone Hole
In 1980s atmospheric scientists working in Antarctica reported about depletion of ozone layer commonly known as ozone hole over the South Pole. It was found that a unique set of conditions was responsible for the ozone hole. In summer season, nitrogen dioxide and methane react with chlorine monoxide(reaction iv) and chlorine atoms (reaction v) forming chlorine sinks , preventing much ozone depletion, whereas in winter , special type of clouds called polar stratosphere clouds are formed over antartica. These polar stratospheric clouds provide surface on which chlorine nitrate formed (reaction iv) gets hydrolysed to form hypochlorous acid (reaction vi) . It also reacts with hydrogen chloride as per reaction (v) to give molecular chlorine.

When sunlight returns to the Antarctica in the spring, the sun’s warmth breaks up the clouds and HOCl and Cl2 are photolysed by sunlight , as given in reactions (viii) and (ix).

The chlorine radicals thus formed , initiate the chain reaction for ozone depletion as described earlier.
Effects of Ozone Layer Depletion
The depletion of ozone layer leading to the ozone hole is hazardous to human health. This is because ozone layer absorbs harmful ultraviolet radiations of the sun. Depletion of ozone layer thus increases the extent of ultraviolet radiations coming on the earth. Ultra violet radiations are very harmful to human beings. They cause sun burns, skin ageing, leukemia, contract of eyes, skin cancer, lung cancer, breast cancer, premature ageing etc. Ultra violet radiations also cause DNA breakage, alteration of DNA replication and even death.
WATER POLLUTION
The quality of drinking water is very important for human welfare. The pollution of water by sewage has been linked to the spread of diseases such as cholera and typhoid fever. The following TABLE lists the major water pollutants and their sources.
Major Water Pollutants
Pollutant Source
Microorganisms Domestic sewage
Organic wastes Domestic sewage, animal waste, decaying animals and plants and discharge from food processing factories.
Toxic heavy metals Industries and chemical factories
Sediments Erosion of soil by agriculture and strip mining.
Pesticides Chemicals used for killing insects, fungi and weeds.
Radioactive substances Mining of uranium containing minerals.
Heat Cooling water used by industrial plants (which is discharged as hot water)
In addition , industrial wastes also contaminate water.
(i) Heavy metals : Metals such as Cd, Pb and Hg may be present in industrial or mining waste. These metals can prove poisonous to humans – Cadmium and mercury can cause kidney damage, and lead poisoning can cause damage to the kidneys, liver brain and central nervous system. All of these metals are cumulative poisons – the body does not excrete them and their concentration builds up.
(ii) Detergents and Fertilisers : These may contain phosphorus as additives. The addition of phosphorus to water, in the form of the phosphate anion PO43, encourages the formation of algae, which reduces the dissolved oxygen concentration of water. This process is known as eutrophication , impedes the development of higher life forms, such as fish.
(iii) Acid–polluted water ( pH < 3) : This is deadly to the most forms of aquatic life. Water downstream from a mine may be contaminated acid mine drainage, the result of microbial oxidation of the discarded waste material at the mine site. Acid mine water principally contains sulphuric acid produced by the oxidation of iron pyrites (FeS2) . Industrial wastes and acid rain may also contribute to the acidity of natural waters. (iv) Polychlorinated biphenyls (PCBs) : These chemicals are relatively recent additions to the list of contaminants of water. Having high stabilities, PCBs find many applications, for example they are used as fluids in transformers and capicitors. PCBs are resistant to oxidation and their release into the environment causes skin disorders in humans. They are reported as carcinogenics. INTERNATIONAL STANDARDS OF DRINKING WATER The quality of water is of vital concern for mankind since it is directly linked with human welfare. There are some international standards for drinking water, which must always be obeyed if water is used for drinking purposes. These are : Fluoride Soluble fluoride is often added to the drinking water to bring it up to a concentration of 1 ppm or 1 mg dm3. This concentration is within agreed safety limits and has been shown to protect teeth against decay. High concentrations of fluoride are poisonous and are harmful to bones and teeth at levels over 10 ppm (mg dm3). Lead The limit for the concentration of lead ions in drinking water is 50 ppb (g dm3). If water is relatively acidic and lead pipes are used for water transport, then water is liable to contaminated with lead. pH The pH of drinking water should be between 5.5 and 9.5. A decrease in the pH of the water increases the solubility of metal ions. Other metals The maximum recommended levels of common metals in drinking water are as follows : Metal Maximum concentration (ppm or mg dm3) Zn 5 Fe 0.2 Mn 0.05 Cu 3 Cd 0.005 Al 0.2 Sulphate Sulphate is harmless at moderate levels, but excess sulphate (> 500 ppm) is thought to have laxative effect.
Nitrate
Excess nitrate in drinking water can lead to methemoglobinemia (‘blue baby syndrome’). It also may be linked to stomach cancer , although this link has not been proved. A maximum limit of 50 ppm for nitrate ion in drinking water has been set.
Importance of Dissolved Oxygen in Water
The concentration of dissolved oxygen in water is of vital importance for the support of aquatic life. The fish growth is inhibited if the dissolved concentration of oxygen in water is below 6 ppm. The lower the concentration of dissolved oxygen, the more polluted is the water sample. Oxygen reaches water through two sources. The first is that oxygen dissolves at the surface of the water from the atmosphere. Still water takes up oxygen slowly whereas the turbulent water takes it up more rapidly since bubbles are often submerged. The second source of oxygen in water is from photosynthesis. Where there are many aquatic green plants present, the water often becomes supersaturated with oxygen during hours of day light. However, after dark, photosynthesis stops but the plants continue to respire and actually reduce the amount of dissolved oxygen. Therefore , during a 24 hour period, some water samples have a considerable range of dissolved oxygen levels.
Various processes contribute to the deoxygenation of water. The dissolved oxygen in water is consumed rapidly by microorganisms to oxidise organic matter of the sewage.

Unless water restores its dissolved oxygen concentration for example, by turbulent flow of shallow waters or by a reaction, it is not able to support many organisms.
In addition to the microorganism- mediated oxidation of organic matter described above, oxygen in water may be consumed by bio-oxidation of the nitrogeneous material.
NH4+(aq) + 2 O2(g)  2 H+(aq) + NO3 (aq) + H2O(l)
And by the chemical or biochemical oxidation of chemical reducing agents :
4 Fe2+(aq) + O2 (aq) + 10 H2O(l)  4 Fe(OH)3(s) + 8 H+(aq)
2 SO32 (aq) + O2 (aq)  2 SO42 (aq)
BIOCHEMICAL OXYGEN DEMAND (BOD) AND CHEMICAL OXYGEN DEMAND(COD)
The polluted water may contain large amounts of inorganic and organic compounds. Some of these can be oxidised by dissolved oxygen in the presence of microorganisms. Biochemical oxygen demand (BOD) is a measure of dissolved oxygen that would be needed by microorganisms to oxidise these compounds. BOD therefore is a measure of the contamination caused by the totality of those compounds which can be oxidised in the presence of microorganisms. A large number of organic and inorganic compounds , however, are resistant to microbial oxidation. They therefore don’t contribute to the BOD , though their presence makes water unfit for consumption.The BOD is taken as a realistic measure of water quality – ‘clear water’ would have a BOD value less than 5 ppm whereas highly polluted river would have a BOD value of 17 ppm or more.
Determination of BOD
In order to find , the water sample is first saturated with oxygen. It is then incubated at constant temperature, usually 20C, for 5 days. This allows time for microorganisms in water sample to oxidise pollutants. The remaining amount of the dissolved oxygen is determined and BOD is is obtained by subtraction.
CHEMICAL OXYGEN DEMAND (COD)
The measurement of BOD takes a long time- five days. So another parameter called chemical oxygen demand (COD) is sometimes measured.
In COD determination, the water sample is treated with a known quantity of an oxidising agent, usually K2Cr2O7 in acidic medium. This reagent oxidises most of the polluting substances, including those which are resistant to microbial oxidation. The remaining K2Cr2O7 is determined by back titration with a suitable reducing agent like Mohr’s salt. From the concentration of K2Cr2O7 consumed, the amount of oxygen used in the oxidation may be calculated using the following chemical equation :
K2Cr2O7 (aq) + 4 H2SO4(aq)  K2SO4(aq)) + Cr2 (SO4) 3(aq)
+ 4 H2O(ℓ) + 3 (O) (aq)
The results are expressed in terms of oxygen , in ppm, that would be required to oxidise the contaminants. This is COD.
LAND POLLUTION
Most of the land pollution is caused by pesticides and other chemicals which are added to the soil to grow better crops. Often , a pesticide poisons many more organisms than those intended. Some of these poisons pass through food chains and eventually reach harmful proportions. Solid wastes are another cause of land pollution.
Pesticides
Pesticides are substances that are used to kill or block the reproductive processes of unwanted organisms. Synthetic pesticides are of concern to us, because of the possible effect upon human health through eating of food, or drinking water, contaminated with these chemicals. Most of the pesticides can be put into one of the three categories.
(i) Insecticides : Control of insects by insecticides helps to curb disease ( for example malaria and yellow fever) and protect crops. Organochlorines are a group of compounds which have been developed and used as insecticides since 1950s. The best known organo chlorine compound is DDT. Organochlorines are stable in the environment, toxic to insects in small amounts, but much less so to humans ; and because they are organic compounds, not very soluble in water. The advantage of these insecticide is that , being persistent, they show their biological activity for long periods of time. On the negative side, these insecticides by accumulating in the environment affect many non-target organisms, not just the target pests. It is primarily because of their persistent nature that many of these insecticides phased out of use.
(ii) Herbicides : Herbicides are used to kill weeds. Sodium chlorate NaClO3 and sodium arsenite Na3AsO3 were commonly used as weed killers, but inorganic arsenic compounds , are toxic to animals. Organic herbicides are now used. They are more toxic to certain types of plants than to others. Organic herbicides such as triazines are presently widely used to kill weeds in cornfields.
iii) Fungicides : Fungicides are used to check the growth of fungi. Fungi, are the plants without chlorophyll. They therefore cannot use solar energy to convert carbon dioxide and water into energy-rich carbohydrates. They live as saprophytes on decaying organic matter or as parasites at the expense of living organisms. Hence they are deemed as a threat to human interests. Fungicides are important because they counter the growth of fungi. Organic compounds of mercury have been used as fungicides. These compounds break down in soil and this has had disastrous consequences – many human deaths in Iraq (1971-1972) resulted from people eating bread made from grain that had been treated with the fungicide methylmercury.
STRATEGY FOR CONTROL OF ENVIRONMENTAL POLLUTION - THE MANAGEMENT OF WASTE
The production and improper disposal of waste are causes for a great deal of environmental pollution. In addition, the household waste, which includes sewage and municipal garbage, many toxic industrial waste from manufacturing processes require treatment/ or safe disposal.
RECYCLING
When materials are recycled, there are several benefits, apart from saving on the cost of raw materials, waste disposal costs are reduced. Examples of recycling by industry are :
1. The collection and recycling of glass ( in bottle banks)
2. The use of scrap metal in the manufacture of steel.
3. The recovery of energy from burning combustible waste.
SEWAGE TREATMENT
The main stages in the treatment of sewage are as follows.
1. The removal of large solids that get into the system by filtering the waste water through screens. The solids that are removed are disposed of in land fill sites.
2. Settlement in tanks to allow the removal of solids that settle out (called sludge) . This process also allows removal of grease, which floats to the surface and can be skimmed off.
3. The degradation of the organic content of waste water by microbial oxidation.
4. Application of various physical and chemical processes for improving the quality of waste water. The processes include chemical removal of phosphate, coagulation, filtration and disinfection using chlorine.
The safe disposal of sludge produced during water treatment is a problem. The sludge is dried and then may be incinerated , digested or dumped.
Incineration
Incineration converts organic materials to CO2 and water. It may serve to destroy household waste, chemical waste and biological waste. A high temperature is required, usually in excess of 1000C and a plentiful supply of oxygen. Exhausted gases may be filtered. The process greatly reduces the volume of waste – an inorganic ash is left behind, which is disposed of as landfill. Incineration provides a mean to dispose of the relatively inert PCBs and the high temperatures generated allow endothermic reactions, such as distruction of C¬–Cl bonds in organochlorine compounds, to take place. The chief disadvantage of incineration is that it leads to air pollution. Further the ash from the municipal incinerators is very finely divided and can be ingested into the lungs. Incomplete combustion of PCBs can cause formation of highly toxic polychlorodibenzodioxins (PCDDs) and polychlorodibenzofurans (PCDFs) . This is most likely to happen in older municipal waste incinerators.


DIGESTION
Anaerobic digestion occurs when microorganisms degrade wastes in the absence of oxygen. It may be used to treat sewage sludge , but the process can be used to degrade a variety of toxic organic wastes. Carbon dioxide and methane , which may be used as a fuel, are the products. The overall process is the conversion of the organic material into carbon dioxide and methane, i.e.,
2 [CH2O]  CO2 (g) + CH4(g)
DUMPING
Ocean dumping of sewage sludge has been widely practiced in seas around the world. However, the practice of application to sludge to land is increasing. The sludge contains nitrogen and phosphorus which make it useful as a fertiliser. Urban areas produces sludge with high toxic metal content, so the amount of such sludge dumped in this way must be carefully controlled.
GREEN CHEMISTRY
A NEW ROUTE TO PROTECTION OF ENVIRONMENT
In order to combat the ever increasing gigantic problem of pollution, a new approach called green chemistry was introduced in early 1990s. This approach aims at evolving such methods which would allow zero discharge of toxic and persistent substances into the environment. Green chemistry may be defined as follows.
Green chemistry is the strategy of designing chemical products and processes in a way that eliminates or reduces the use and generation of substances which are hazardous to health and environment.
The green chemistry may be regarded as an alternative tool for reducing pollution. It not only aims to eliminate toxic pollutants but also intends to change our habits and life style suitable to make a pleasant and healthy environment. The basic objectives of green chemistry are as follows.
i) It intends to reformulate the synthetic routes in such a way that harmful pollutants are either not produced or do not enter into the atmosphere.
ii) It aims to design isolating methods for obtaining raw materials in such a way that the environment is least affected.
iii) It aims to modify the existing industrial processes in such a way that they use or produce materials of low or no toxicity.
iv) It aims to produce green chemicals and materials which would be friendly to the environment.
v) It aims to use the renewable sources in place of depletable sources as far as possible.
vi) It aims to use new methods and tools to evaluate environmental impact.
vii) It aims to create a general awareness among common people to save environment from being polluted and to encourage them to use green products as far as possible.
Green Chemistry in day-to-day Life
(i) Dry Cleaning of Clothes
Tetrachloroethene (Cl2C=CCl2 ) was earlier used as solvent for dry cleaning. The compound contaminates the ground water and is also a suspected carcinogen. The process using this compound is now being replaced by a process , where liquefied carbon dioxide , with a suitable detergent is used. Replacement of halogenated halogenated solvent by liquid carbon dioxide will result in less harm to ground water.
These days hydrogen peroxide (H2O2) is used for the purpose of bleaching clothes in the process of laundary, which gives better results and makes use of lesser amount of water.
(ii) Bleaching paper
Chlorine gas is used earlier for bleaching paper. These days , hydrogen peroxide (H2O2) with suitable catalyst , which promotes the bleaching action of hydrogen peroxide, is used.
(iii) Synthesis of Chemicals : Ethanal (CH3CHO) is now commercially prepared by one step oxidation of ethane in the presence of ionic catalyst in aqueous medium with an yield of 90%.

Green chemistry , in a nutshell is cost effective approach which involves reduction in material, energy consumption and waste generation.

QUESTIONS
1. What is environmental chemistry ?
2. Discuss the social relevance of environmetal chemistry.
3. Define environmetal pollutant ?
4. What do you understand by environmetal pollution model ?
5. Why does the rain water normally have a pH of about 5.6 ? When does it become acid rain ?
6. What is the cause of acid rain ? How is it harmful to the environmet ?
7. Why is acid rain considered a threat to Taj Mahal ?
8. Explain giving reasons ‘’ The percentage of CO reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells’’.
9. What do you understand by ‘Greenhouse effect’ ?
10. What are the major Greenhouse gases ?
11. Why does Greenhouse effect lead to the global warming ?
12. What would be the consequences of global warming ?
13. What do you understand by :
(i) mists (ii) smoke (iii) fumes and (iv) dust ?
14. What is Pneumoconiosis ? How does it occur ?
15. What are smogs ?
16. Distinguish between classical and photochemical smogs.
17. How is photochemical smog formed ? What are its effects ? How can it be controlled ?
18. State briefly the reactions causing ozone layer depletion in the stratosphere.
19. What do you understand by ozone hole ? Why does it occur mainly over Antartica ?
20. The depletion of ozone layer occurs over Antartica during spring time and it gets replenished after spring time. Explain.
21. What do you understand by (i) Polar stratospheric clouds (ii) polar vortex.
22. Fish do not always grow as well in warm as in cold water . Why ?
23. List five major water pollutants giving their sources.
24. What do you understand by the term eutrophication ? How does it threaten the development of fish ?
25. Discuss the importance of dissolved oxygen in water.
26. What do you understand by BOD and COD ? How are they determined ?
27. ‘Oxygen plays a key role in the troposphere while ozone , in the stratosphere ‘. Explain.
28. For your aricultural field or garden you have developed a compost producing pit. Discuss the process in light of bad odour, flies and recycling of wastes for good produce.
29. How can domestic waste be used as manure ?
30. A large number of fish are suddenly found dead on a lake. There is no evidence toxic dumping but you find an abundance of phytoplankton. Suggest a reason for the fish kill.