Environmental chemistry deals with the study of the origin, transport, reactions, effects and fates of chemical species in the environment.
- The atmosphere that surrounds the earth is not of the same thickness at all heights.
- There are concentric layers of air or regions and each layer has different density.
- The lowest region of atmosphere in which the human beings along with other organisms live is called troposphere. It extends up to the height of about 10 km from sea level.
- Above the troposphere, between 10 and 50 km above sea level lies stratosphere.
- Troposphere is a turbulent, dusty zone containing air, much water vapour and clouds. This is the region of strong air movement and cloud formation. The stratosphere, on the other hand, contains dinitrogen, dioxygen, ozone and little water vapour.
- Atmospheric pollution is generally studied as tropospheric and stratospheric pollution.
- The presence of ozone in the stratosphere prevents about 99.5 per cent of the sun’s harmful ultraviolet (UV) radiations from reaching the earth’s surface and thereby protecting humans and other animals from its effect.
TROPOSPHERIC POLLUTION | ENVIRONMENTAL CHEMISTRY
Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air.
The following are the major gaseous and particulate pollutants present in the troposphere:
- Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.
- Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.
- Gaseous Air Pollutants
(a) Oxides of Sulphur:
- Oxides of sulphur are produced when sulphur containing fossil fuel is burnt. The most common species, sulphur dioxide, is a gas that is poisonous to both animals and plants.
- It has been reported that even a low concentration of sulphur dioxide causes respiratory diseases e.g., asthma, bronchitis, emphysema in human beings.
- Sulphur dioxide causes irritation to the eyes, resulting in tears and redness.
- High concentration of SO2 leads to stiffness of flower buds which eventually fall off from plants.
- Uncatalysed oxidation of sulphur dioxide is slow. However, the presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.
(b) Oxides of Nitrogen:
- Dinitrogen and dioxygen are the main constituents of air. These gases do not react with each other at a normal temperature.
- At high altitudes when lightning strikes, they combine to form oxides of nitrogen. NO2 is oxidised to nitrate ion, NO3– which is washed into soil, where it serves as a fertilizer.
- In an automobile engine, (at high temperature) when fossil fuel is burnt, dinitrogen and dioxygen combine to yield significant quantities of nitric oxide (NO) and nitrogen dioxide (NO2).
- The irritant red haze in the traffic and congested places is due to oxides of nitrogen.
- Higher concentrations of NO2 damage the leaves of plants and retard the rate of photosynthesis.
- Nitrogen dioxide is a lung irritant that can lead to an acute respiratory disease in children. It is toxic to living tissues also.
- Nitrogen dioxide is also harmful to various textile fibres and metals.
- Hydrocarbons are composed of hydrogen and carbon only and are formed by incomplete combustion of fuel used in automobiles.
- Hydrocarbons are carcinogenic, i.e., they cause cancer.
- They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs.
(d) Oxides of Carbon:
- Carbon monoxide:
- Carbon monoxide (CO) is one of the most serious air pollutants. It is a colourless and odourless gas, highly poisonous to living beings because of its ability to block the delivery of oxygen to the organs and tissues.
- It is produced as a result of incomplete combustion of carbon. Carbon monoxide is mainly released into the air by automobile exhaust.
- Other sources, which produce CO, involve incomplete combustion of coal, firewood, petrol, etc.
- The number of vehicles has been increasing over the years all over the world. Many vehicles are poorly maintained and several have inadequate pollution control equipments resulting in the release of greater amount of carbon monoxide and other polluting gases.
- Carbon monoxide binds to haemoglobin to form carboxyhemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex.
- In blood, when the concentration of carboxyhemoglobin reaches about 3-4 per cent, the oxygen carrying capacity of blood is greatly reduced. This oxygen deficiency, results into headache, weak eyesight, nervousness and cardiovascular disorder.
- This is the reason why people are advised not to smoke. In pregnant women who have the habit of smoking the increased CO level in blood may induce premature birth, spontaneous abortions and deformed babies.
- Carbon dioxide:
- Carbon dioxide (CO2) is released into the atmosphere by respiration, burning of fossil fuels for energy, and by decomposition of limestone during the manufacture of cement.
- It is also emitted during volcanic eruptions. Carbon dioxide gas is confined to troposphere only.
- Normally it forms about 0.03 per cent by volume of the atmosphere. With the increased use of fossil fuels, a large amount of carbon dioxide gets released into the atmosphere.
- Excess of CO2 in the air is removed by green plants and this maintains an appropriate level of CO2 in the atmosphere.
- Green plants require CO2 for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance.
- Deforestation and burning of fossil fuel increases the CO2 level and disturb the balance in the atmosphere.
- The increased amount of CO2 in the air is mainly responsible for global warming.
- Particulate Air Pollutants
- Particulates pollutants are the minute solid particles or liquid droplets in air. These are present in vehicle emissions, smoke particles from fires, dust particles and ash from industries.
- Particulates in the atmosphere may be viable or non-viable.
- The viable particulates e.g., bacteria, fungi, moulds, algae etc., are minute living organisms that are dispersed in the atmosphere.
- Human beings are allergic to some of the fungi found in air. They can also cause plant diseases.
- Non-viable particulates may be classified according to their nature and size as follows:
- Smoke particulates consist of solid or mixture of solid and liquid particles formed during combustion of organic matter. Examples are cigarette smoke, smoke from burning of fossil fuel, garbage and dry leaves, oil smoke etc.
- Dust is composed of fine solid particles (over 1 µm in diameter), produced during crushing, grinding and attribution of solid materials. Sand from sand blasting, saw dust from wood works, pulverized coal, cement and fly ash from factories, dust storms etc., are some typical examples of this type of particulate emission.
- Mists are produced by particles of spray liquids and by condensation of vapours in air. Examples are sulphuric acid mist and herbicides and insecticides that miss their targets and travel through air and form mists.
- Fumes are generally obtained by the condensation of vapours during sublimation, distillation, boiling and several other chemical reactions. Generally, organic solvents, metals and metallic oxides form fume particles.
- The effect of particulate pollutants are largely dependent on the particle size. Air-borne particles such as dust, fumes, mist etc., are dangerous for human health.
- Particulate pollutants bigger than 5 microns are likely to lodge in the nasal passage, whereas particles of about 10 micron enter into lungs easily.
- Lead used to be a major air pollutant emitted by vehicles. Leaded petrol used to be the primary source of air-borne lead emission in Indian cities.
- This problem has now been overcome by using unleaded petrol in most of the cities in India. Lead interferes with the development and maturation of red blood cells.
Smog | ENVIRONMENTAL CHEMISTRY
The word smog is derived from smoke and fog. This is the most common example of air pollution that occurs in many cities throughout the world. There are two types of smog:
- Classical smog occurs in cool humid climate. It is a mixture of smoke, fog and sulphur dioxide. Chemically it is a reducing mixture and so it is also called as reducing smog.
- Photochemical smog occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories.
- Photochemical smog has high concentration of oxidising agents and is, therefore, called as oxidising smog.
Formation of photochemical smog
- When fossil fuels are burnt, a variety of pollutants are emitted into the earth’s troposphere.
- Two of the pollutants that are emitted are hydrocarbons (unburnt fuels) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which NO is converted into nitrogen dioxide (NO2).
- This NO2 in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.
- NO2 (g) -> NO(g) + O(g)
- Oxygen atoms are very reactive and combine with the O2 in air to produce ozone. The ozone formed, reacts rapidly with the NO (g) formed initially to regenerate NO2. NO2 is a brown gas and at sufficiently high levels can contribute to haze.
- Ozone is a toxic gas and both NO2 and O3 are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).
Effects of photochemical smog
- The common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN). Photochemical smog causes serious health problems. Both ozone and PAN act as powerful eye irritants.
- Ozone and nitric oxide irritate the nose and throat and their high concentration causes headache, chest pain, dryness of the throat, cough and difficulty in breathing.
- Photochemical smog leads to cracking of rubber and extensive damage to plant life. It also causes corrosion of metals, stones, building materials, rubber and painted surfaces.
How can photochemical smog be controlled?
- Many techniques are used to control or reduce the formation of photochemical smog.
- If we control the primary precursors of photochemical smog, such as NO2 and hydrocarbons, the secondary precursors such as ozone and PAN, the photochemical smog will automatically be reduced. ENVIRONMENTAL CHEMISTRY
- Usually catalytic converters are used in the automobiles, which prevent the release of nitrogen oxide and hydrocarbons to the atmosphere.
- Certain plants e.g., Pinus, Juniparus, Quercus, Pyrus and Vitis can metabolise nitrogen oxide and therefore, their plantation could help in this matter.
STRATOSPHERIC POLLUTION | ENVIRONMENTAL CHEMISTRY
Formation and Breakdown of Ozone
- The upper stratosphere Consists of considerable amount of ozone (O3), which protects us from the harmful ultraviolet (UV) radiations (λ 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 stratosphere 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 the molecular oxygen to form ozone.
- Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus, a dynamic equilibrium exists between the production and decomposition of ozone molecules.
- In recent years, there have been reports of the 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, non flammable, non toxic organic molecules and therefore used in refrigerators, air conditioners, in the production of plastic foam and by the electronic industry for cleaning computer parts etc. Once CFCs 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 and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer. ENVIRONMENTAL CHEMISTRY
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 and chlorine atoms, forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica.
- These polar stratospheric clouds provide surface on which some reactions occur which give molecular chlorine.
- When sunlight returns to the Antarctica in the spring, the sun’s warmth breaks up the clouds and decomposes molecular chlorine to chlorine radicals which cause ozone depletion as described earlier. ENVIRONMENTAL CHEMISTRY
Effects of Depletion of the Ozone Layer
- With the depletion of ozone layer, more UV radiation filters into troposphere.
- UV radiations lead to ageing of skin, cataract, sunburn; skin cancer, killing of many phytoplanktons, damage to fish productivity etc.
- It has also been reported that plant proteins get easily affected by UV radiations which leads to the harmful mutation of cells.
- It also increases evaporation of surface water through the stomata of the leaves and decreases the moisture content of the soil. Increase in UV radiations damage paints and fibres, causing them to fade faster.
Global Warming and Greenhouse Effect | ENVIRONMENTAL CHEMISTRY
- About 75 % of the solar energy reaching the earth is absorbed by the earth’s surface, which increases its temperature. The rest of the heat radiates back to the atmosphere. Some of the heat is trapped by gases such as carbon dioxide, methane, ozone, chlorofluorocarbon compounds (CFCs) and water vapour in the atmosphere.
- Thus, they add to the heating of the atmosphere. This causes global warming.
- We are not surrounded by a blanket of air called the atmosphere, which has kept the temperature on earth constant for centuries.
- But it is now undergoing change, though slowly. Just as the glass in a greenhouse holds the sun’s warmth inside, atmosphere traps the sun’s heat near the earth’s surface and keeps it warm.
- This is called natural greenhouse effect because it maintains the temperature and makes the earth perfect for life.
- In a greenhouse, solar radiations pass through the transparent glass and heat up the soil and the plants. The warm soil and plants emit infrared radiations. Since glass is opaque to infrared radiations (thermal region), it partly reflects and partly absorbs these radiations.
- This mechanism keeps the energy of the sun trapped in the greenhouse. Similarly, carbon dioxide molecules also trap heat as they are transparent to sunlight but not to the heat radiation.
- If the amount of carbon dioxide crosses the delicate proportion of 0.03 per cent, the natural greenhouse balance may get disturbed. Carbon „dioxide is the major contributor to global warming. Besides carbon dioxide, other greenhouse gases are methane, water vapour, nitrous oxide, CFCs and ozone.
- Methane is produced naturally when vegetation is burnt, digested or rotten in the absence of oxygen. Large amounts of methane are released in paddy fields, coal mines, from rotting garbage dumps and by fossil fuels.
- Chlorofluorocarbons (CFCs) are man-made industrial chemicals used in air conditioning etc. CFCs are also damaging the ozone layer.
- Nitrous oxide occurs naturally in the environment. In recent years, their quantities have increased significantly due to the use of chemical fertilizers and the burning of fossil fuels.
- If these trends continue, the average global temperature will increase to a level which may lead to melting of polar ice caps and flooding of low lying areas all over the earth.
- Increase in the global temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc. ENVIRONMENTAL CHEMISTRY
- Normally rain water has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rain water with carbon dioxide present in the atmosphere. When the pH of the rain water drops below 5.6, it is called acid rain.
- Acid rain refers to the ways in which acid from the atmosphere is deposited on the earth’s surface.
- Oxides of nitrogen and sulphur which are acidic in nature can be blown by wind along with solid particles in the atmosphere and finally settle down either on the ground as dry deposition or in water, fog and snow as wet deposition.
- Acid rain is a byproduct of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere.
- As mentioned earlier, burning of fossil fuels (which contain sulphur and nitrogenous matter) such as coal and oil in power stations and furnaces or petrol and diesel in motor engines produce sulphur dioxide and nitrogen oxides. SO2 and NO2 after oxidation and reaction with water are major contributors to acid rain, because polluted air usually contains particulate matter that catalyse the oxidation.
- Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.
- It causes respiratory ailments in human beings and animals. When acid rain falls and flows as ground water to reach rivers, lakes etc. it affects plants and animal life in aquatic ecosystem.
- It corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into the drinking water.
- Acid rain damages buildings and other structures made of stone or metal. The Taj Mahal in India has been affected by acid rain.
- Pollution of water originates from human activities. Through different paths, pollution reaches surface or ground water.
- Easily identified source or place of pollution is called as point source. e.g., municipal and industrial discharge pipes where pollutants enter the water-source.
- Non point Sources of pollution are those where a source of pollution cannot be easily identified, e.g., agricultural runoff (from farm, anima end crop-lands), acid rain, storm-water drainage (from streets, parking lots and lawns), etc.
CAUSES OF WATER POLLUTION | ENVIRONMENTAL CHEMISTRY
- The most serious water pollutants are the disease causing agents called pathogens.
- Pathogens include battens and other organisms that enter water from domestic sewage and animal excreta.
- Human excreta contain bacteria such as Escherichia coli and Streptococcus feacalis which cause gastrointestinal diseases.
- The other major water pollutant is organic matter such as leaves, grass, trash etc.
- They pollute water as a consequence of runoff. Excessive phytoplankton growth within water is also a cause of water pollution.
- These wastes are biodegradable.
- The large population of bacteria decomposes organic matter present in water. They consume oxygen dissolved in water.
- The amount of oxygen that water can hold in the solution is limited. In cold water, dissolved oxygen (DO) can reach a concentration up to 10 ppm (parts per million), whereas oxygen in air is about 200,000 ppm. That is why even a moderate amount of organic matter when decomposes in water can deplete the water of its dissolved oxygen.
- The concentration of dissolved oxygen in water is very important for aquatic life. If the concentration of dissolved oxygen of water is below 6 ppm, the growth of fish gets inhibited.
- Oxygen reaches water either through atmosphere or from the process of photosynthesis carried out by many aquatic green plants during day light. However, during night, photosynthesis stops but the plants continue to respire, resulting in reduction of dissolved oxygen.
- The dissolved oxygen is also used by microorganisms to oxidise organic matter. If too much of organic matter is added to water, all the available oxygen is used up.
- This causes oxygen dependent aquatic life to die. Thus, anaerobic bacteria (Which do, not require oxygen) begin to break down the organic waste and produce chemicals that have a foul smell and are harmful to human health.
- Aerobic (oxygen requiring) bacteria degrade these organic wastes and keep the water depleted in dissolved oxygen.
- Thus, the amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water, is called Biochemical Oxygen Demand (BOD).
- The amount of BOD in the water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically.
- Clean water would have BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17 ppm or more.
- Water soluble inorganic chemicals that include heavy metals such as cadmium, mercury, nickel etc constitute an important class of pollutants.
- All these metals are dangerous to humans because our body cannot excrete them. Over the time, it crosses the tolerance limit.
- These metals then can damage kidneys, central nervous system, liver etc.
- Acids (like sulphuric acid) from mine drainage and salts from many different sources including raw salt used to melt snow and ice in the colder climates (sodium and calcium chloride) are water soluble chemical pollutants.
- The organic chemicals are another group of substances that are found in polluted water. Petroleum products pollute many sources of water e.g., major oil spills in oceans. Other organic substances with serious impacts are the pesticides that drift down from sprays or runoff from lands.
- Various industrial chemicals like polychlorinated biphenyls, (PCBs) which are used as cleansing solvent, detergents and fertilizers add to the list of water pollutants. PCBs are suspected to be carcinogenic. Nowadays most of the detergents available are biodegradable.
- However, their use can create other problems. The bacteria responsible for degrading biodegradable detergent feed on it and grow rapidly. While growing, they may use up all the oxygen dissolved in water. The lack of oxygen kills all other forms of aquatic life such as fish and plants.
- Fertilizers contain. phosphates as additives. The addition of phosphates in water enhances algae growth. Such profuse growth of algae, covers the water surface and reduces the oxygen concentration in water.
- This leads to anaerobic conditions, commonly with accumulation of abnoxious decay and animal death. Thus, bloom-infested water inhibits the growth of other living organisms in the water body.
- This process in which nutrient enriched water bodies support a dense plant population, which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as Eutrophication.
International Standards for Drinking Water | ENVIRONMENTAL CHEMISTRY
The International Standards for drinking water are given below and they must be followed.
- For drinking purposes, water should be tested for fluoride ion concentration. Its deficiency in drinking water is harmful to man and causes diseases such as tooth decay ‘etc. Soluble fluoride is often added to drinking water to bring its concentration upto 1 ppm.
- The F ions make the enamel on teeth much harder by converting hydroxyapatite, the enamel on the surface of the teeth, into much harder fluorapatite.
- However, F– ion concentration above 2 ppm causes brown mottling of teeth. At the same time, excess fluoride (over 10 ppm) causes harmful effect to bones and teeth, as reported from some parts of Rajasthan. ENVIRONMENTAL CHEMISTRY
- Drinking water gets contaminated with lead when lead pipes are used for transportation of water.
- The prescribed upper limit concentration of lead in drinking water is about 50 ppb. Lead can damage kidney, liver, reproductive system etc. ENVIRONMENTAL CHEMISTRY
- Excessive sulphate (>500 ppm) in drinking water causes laxative effect, otherwise at moderate levels it is K6rmless.
- The maximum limit of nitrate in drinking water is 50 ppm. Excess nitrate in drinking water can cause disease such as methemoglobinemia (blue baby’ syndrome).
Green chemistry is a way of thinking and is about utilising the existing knowledge and principles of chemistry and other sciences to reduce the adverse impact on environment. Green chemistry is a production process that would bring about minimum pollution or deterioration to the environment. The byproducts generated during a process, if not used gainfully, add to the environmental pollution. Such processes are not only environmental unfriendly but also cost-ineffective. The waste generation and its disposal both are economically unsound. Utilisation of existing knowledge base for reducing the chemical hazards along with the developmental activities is the foundation of green chemistry. ENVIRONMENTAL CHEMISTRY