ENVIRONMENTAL CHEMISTRY

1 CHEMISTRY398 UNIT 14 ENVIRONMENTAL CHEMISTRYA fter studying this unit, you will beable to understand the meaning ofenvironmental CHEMISTRY ; define atmospheric pollution, listreasons for global warming. greenhouse effect and acid rain; identify causes for ozone layerdepletion and its effects; give reasons for water pollutionand know about internationalstandards for drinking water; describe causes of soil pollution; suggest and adopt strategiesfor control of environmentalpollution; appreciate the importance of greenchemistry in day to day have already studied about environment in your earlierclasses. ENVIRONMENTAL studies deal with the sum of allsocial, economical, biological, physical and chemicalinterrelations with our surroundings.

2 In this unit the focuswill be on ENVIRONMENTAL CHEMISTRY . Environmentalchemistry deals with the study of the origin, transport,reactions, effects and fates of chemical species in theenvironment. Let us discuss some important aspects ofenvironmental POLLUTIONE nvironmental pollution is the effect of undesirable changesin our surroundings that have harmful effects on plants,animals and human beings. A substance, which causespollution, is known as pollutant. Pollutants can be solid,liquid or gaseous substances present in greaterconcentration than in natural abundance and areproduced due to human activities or due to naturalhappenings. Do you know, an average human beingrequires nearly 12-15 times more air than the food.

3 So,even small amounts of pollutants in the air becomesignificant compared to similar levels present in the can be degradable, like discarded vegetableswhich rapidly break down by natural processes. On theother hand, pollutants which are slowly degradable, remainin the environment in an unchanged form for manydecades. For example, substances such as dichlorodi-phenyltrichloroethane (DDT), plastic materials, heavymetals, many chemicals, nuclear wastes etc., once releasedinto the environment are difficult to remove. TheseThe world has achieved brilliance without wisdom, powerwithout conscience. Ours is a world of nuclear giants andethical infants. NCERTnot to be republishedENVIRONMENTAL CHEMISTRY 399pollutants cannot be degraded by naturalprocesses and are harmful to living the process of ENVIRONMENTAL pollution,pollutants originate from a source and gettransported by air or water or are dumped intothe soil by human ATMOSPHERIC POLLUTIONThe atmosphere that surrounds the earth isnot of the same thickness at all heights.

4 Thereare concentric layers of air or regions and eachlayer has different density. The lowest regionof atmosphere in which the human beingsalong with other organisms live is calledtroposphere. It extends up to the height of~ 10 km from sea level. Above the troposphere,between 10 and 50 km above sea level liesstratosphere. Troposphere is a turbulent,dusty zone containing air, much water vapourand clouds. This is the region of strong airmovement and cloud formation. Thestratosphere, on the other hand, containsdinitrogen, dioxygen, ozone and little pollution is generally studiedas tropospheric and stratospheric presence of ozone in the stratosphereprevents about per cent of the sun sharmful ultraviolet (UV) radiations fromreaching the earth s surface and therebyprotecting humans and other animals from PollutionTropospheric pollution occurs due to thepresence of undesirable solid or gaseousparticles in the air.

5 The following are the majorgaseous and particulate pollutants present inthe troposphere:1. Gaseous air pollutants: These are oxidesof sulphur, nitrogen and carbon, hydrogensulphide, hydrocarbons, ozone and Particulate pollutants: These are dust,mist, fumes, smoke, smog Gaseous air pollutants(a) Oxides of Sulphur: Oxides of sulphurare produced when sulphur containing fossilfuel is burnt. The most common species,sulphur dioxide, is a gas that is poisonous toboth animals and plants. It has been reportedthat even a low concentration of sulphurdioxide causes respiratory diseases ,asthma, bronchitis, emphysema in humanbeings. Sulphur dioxide causes irritation tothe eyes, resulting in tears and redness.

6 Highconcentration of SO2 leads to stiffness of flowerbuds which eventually fall off from oxidation of sulphur dioxide isslow. However, the presence of particulatematter in polluted air catalyses the oxidationof sulphur dioxide to sulphur (g) +O2 (g) 2SO3(g)The reaction can also be promoted byozone and hydrogen (g) +O3 (g) SO3(g) + O2 (g)SO2(g) + H2O2(l) H2SO4(aq)(b) Oxides of Nitrogen: Dinitrogen anddioxygen are the main constituents of gases do not react with each other at anormal temperature. At high altitudes whenlightning strikes, they combine to form oxidesof nitrogen. NO2 is oxidised to nitrate ion, 3NO which is washed into soil, where it serves as afertilizer. In an automobile engine, (at hightemperature) when fossil fuel is burnt,dinitrogen and dioxygen combine to yieldsignificant quantities of nitric oxide (NO) andnitrogen dioxide ( NO2 ) as given below:N2 (g) + O2 (g) 1483K 2NO(g)NO reacts instantly with oxygen to give NO22NO (g) + O2 (g) 2NO2 (g)Rate of production of NO2 is faster whennitric oxide reacts with ozone in (g) + O3 (g) NO2 (g) + O2 (g)The irritant red haze in the traffic andcongested places is due to oxides of concentrations of NO2 damage theleaves of plants and retard the rate ofphotosynthesis.

7 Nitrogen dioxide is a lungirritant that can lead to an acute respiratorydisease in children. It is toxic to living tissuesalso. Nitrogen dioxide is also harmful tovarious textile fibres and metals. NCERTnot to be republishedCHEMISTRY400(c) Hydrocarbons: Hydrocarbons arecomposed of hydrogen and carbon only andare formed by incomplete combustion of fuelused in automobiles. Hydrocarbons arecarcinogenic, , they cause cancer. Theyharm plants by causing ageing, breakdown oftissues and shedding of leaves, flowers andtwigs.(d) Oxides of Carbon(i ) Carbon monoxide: Carbon monoxide (CO)is one of the most serious air pollutants. It is acolourless and odourless gas, highlypoisonous to living beings because of its abilityto block the delivery of oxygen to the organsand tissues.

8 It is produced as a result ofincomplete combustion of carbon. Carbonmonoxide is mainly released into the air byautomobile exhaust. Other sources, whichproduce CO, involve incomplete combustionof coal, firewood, petrol, etc. The number ofvehicles has been increasing over the years allover the world. Many vehicles are poorlymaintained and several have inadequatepollution control equipments resulting in therelease of greater amount of carbon monoxideand other polluting gases. Do you know whycarbon monoxide is poisonous? It binds tohaemoglobin to form carboxyhaemoglobin,which is about 300 times more stable than theoxygen-haemoglobin complex. In blood, whenthe concentration of carboxyhaemoglobinreaches about 3 4 per cent, the oxygencarrying capacity of blood is greatlyreduced.

9 This oxygen deficiency, results intoheadache, weak eyesight, nervousness andcardiovascular disorder. This is the reason whypeople are advised not to smoke. In pregnantwomen who have the habit of smoking theincreased CO level in blood may inducepremature birth, spontaneous abortions anddeformed babies.(ii) Carbon dioxide: Carbon dioxide (CO2) isreleased into the atmosphere by respiration,burning of fossil fuels for energy, and bydecomposition of limestone during themanufacture of cement. It is also emittedduring volcanic eruptions. Carbon dioxide gasis confined to troposphere only. Normally itforms about per cent by volume of theatmosphere. With the increased use of fossilfuels, a large amount of carbon dioxide getsreleased into the atmosphere.

10 Excess of CO2in the air is removed by green plants and thismaintains an appropriate level of CO2 in theatmosphere. Green plants require CO2 forphotosynthesis and they, in turn, emit oxygen,thus maintaining the delicate balance. As youknow, deforestation and burning of fossil fuelincreases the CO2 level and disturb the balancein the atmosphere. The increased amount ofCO2 in the air is mainly responsible for Warming and Greenhouse EffectAbout 75 % of the solar energy reaching theearth is absorbed by the earth s surface, whichincreases its temperature. The rest of the heatradiates back to the atmosphere. Some of theheat is trapped by gases such as carbondioxide, methane, ozone, chlorofluorocarboncompounds (CFCs) and water vapour in theatmosphere.