Environmental pollution

ENVIRONMENTAL POLLUTION

Environment pollution

Environmental chemistry is the multidisciplinary science involving chemistry, physics, life science, agriculture, medical science, public health and sanitary engineering. In broader terms it is the study of the sources, reactions, transport effect and fate of chemical species in air, water, soil and the effect of human activity upon these.

The objective of environmental education is to enlighten the public, particularly students and social workers, about the importance of protection and conversation of our environment and the need to restrain human activities which lead to indiscriminate release of pollutants into the environment.


A wave of concern for the environmental swept across the developed countries in the sixties & reached its climax in 1970 with the celebration of earth day under the auspices of the United Nations. Then from 1972 onwards with the conclusion of United Nations conference on human environment of Stockholm this wave started sweeping across India & other developing nations. At present there are many environmental issues, which have grown in size & complexity day by day, threading the survival of mankind on earth. However several Japanese killed by eating fish from Minamata Bay in the sixties. Why did 3000-4000 people die in London in 1952? Why are historic marble status in Greece & Italy getting damaged by rain water? How did the Mediterranean Sea polluted river in India? Should plutonium breeder reactors be developed? Is saccharin a dangerous food additive? These are some typical chemical issues which can be best handled in their chemical perspective. Of course the issues are linked with ecological, social, political and economic questions which have also to be taken into account.


Nomenclature: Pollutant: a substance present in nature, in greater than abundance

due to human activity, which ultimately effect on the environment and therefore on living organisms and mankind. E.g. Pb, Hg, SO2, CO2 etc.

Contaminant: a material which does not occur in nature, but is introduced by human activity into the environment, affecting its composition. A contaminant is classified as a pollutants when it exerts a detrimental effect

Receptor : The medium which is affected by a pollutant. Man is the receptor of photochemical smog causing irritation of the eyes and respiratory tract.

Sinks: The medium which retain and interacts with a long-lived pollutant. A marble wall will act a sink for atmospheric. The ocean are sinks for atmospheric CO2 & H2SO4 and ultimately damaged H2SO4 + CaCO3 = CaSO4 + H2O + CO2

Pathways of pollutant : The mechanism by which the pollutant is distributed from its source into the environment segments.

Pb(C2H5)4 ( in gasoline / in automobile petrol) PbCl2 + PbBr2(Air)

PbCl2 + PbBr2(soil)

Speciation : The different chemical forms or species of inorganic, organic or organometallic compounds present in the environment is known as specification. It is important to identify the chemical species of a pollutant since some species are more toxic than others. Thus methyl mercury (CH3Hg) , (CH⁺ 3)2Hg are deadly poison compared to others species of Hg.

Dissolved oxygen : Oxygen is a vitally important species in water. It is consumed by oxidation of organic matter/ reducing agents. It is an important water quality parameter. The optimum value for good water quality is 4-6 mg/L of DO, which ensures healthy aquatic life in water body. Lower DO values indicate water pollution. In natural & waste waters DO levels depend on the physical, chemical & biological activities of the water body.


Chemical oxygen demand (COD): This is and index of the organic content of water (oxygen demanding substances in water ) and is important water quality parameter. It is a rapidly measurable parameter for stream, industrial waste studies & control of water treatment plants.

Bio-chemical oxygen demand (BOD): Based on oxidation of organic matter in water. This is also a water quality parameter for organic matter in water, which is empirical in nature. It is measured by the quality of oxygen utilized by suitable aquatic microorganisms during a five-day period.

Threshold limit value(TLV): This indicates the permissible level of a toxic pollutant in atmosphere to which a healthy industrial worker is exposed during an eight hour day without any adverse effect. TLV values for Be , Zn, are 0.002 and 1.000 mg/ m3 respectively.

Environmental segments :Environment consists of four segments- atmosphere, hydrosphere, lithosphere and

biosphere.Atmosphere : It is the protective blanket of gasses surrounding the earth, which

contains life on earth and saves it from the hostile environment of outer space. It absorbs most of the cosmic rays, major portion of the electromagnetic radiation from the sun. It transmits only neat ultraviolet, visible, infrared radiation (300-2500 nm) and radio waves (0.10-40µ) while filtering out tissue- damaging ultraviolet radiation below about 300 nm.


Hydrosphere : This means all types of water resources-oceans, seas, rivers, lakes, stream, reservoirs etc.

Lithosphere : This is the outer mantle of the solid earth, consisting minerals occurring in the earth crust & the soil.

Biosphere : This denotes the realm of the living organisms & there interactions with environment.

Environmental pollution

Air pollution water pollution Land pollution

Surface water Underground water Marine water

pollution pollution pollution


Air pollutionThere are five primary pollutants which together contribute more than 20% of global air

pollution . These are (i) CO, (ii) NOx (Nitrogen oxides), (iii) HC (Hydrocarbon), (iv) Sox (Sulphur oxides), (v) particulates, part.

CO : It is a colorless, odorless and tasteless gas , It is 96.5% as heavy as air and is not soluble in water.

The basic chemical reactions yielding CO are-Incomplete combustion of fuel or C- containing compounds. 2C + O2 2CO2

Reaction between CO2 and carbon–containing materials at elevated temps. in industrial process e.g. in blast furnace.

CO2 + C 2CO

Dissociations of CO2 at high temperatures.

CO2 CO + O

Natural process, e .g. volcanic action , Natural gas emission , electrical discharge during

storm & seed germination , marsh-gas production etc. contribute CO in the atmosphere. The significant contribution is from human activities.

a) Transportations contribution about 64%b) Misc sources (Agricultural burning , forest fires etc) 16.9 %c) Industrial processes . 9.6 %

Control of CO pollution :

Modification of internal contribution engines to reduce the amount of pollutants formed during fuel combustion.

Development of exhaust system reactors which will complete the combustion process.

Development substitute fuels for gasoline. Development of pollution – free power sources to replace the

internal engine. Use of catalytic converters in two stages helps in eliminating

pollutants for exhaust gases before they are discharged into the atmosphere.

Gasoline containing pb(C2H5)4 is the major source of Pb pollution in the environment. Pb–free gasoline should be introduced.

The possible approach to CO pollution is a substitute fuel for gasoline . May be compressed natural gas (CNG) & Liquefied natural gas (LNG).

Nitrogen oxides NOx :

NOx represents composite atmospheric gases, nitric oxides & nitrogen dioxide , which are Primarily involved in air pollution . NO is colorless , odorless gas but NO2 has a reddish – Brown-color and pungent suffocating odor .The basic chemical reaction leading to Formation of NO & NO2 are

N2 + O2 1210-1765 º C 2NO

2NO + O2 2NO2

The 2nd reaction Is also favored at temps. About 1100 C⁰

uv NO Air

NO2 sun light (O2) O3

O

O2

The distribution of NOx from natural sources is more or less uniform on a global basic lent man- made sources varies depending Urban / rural areas . In Urban atmospheres , NOx is 10-100 times greater than in rural areas.

Nitrogen oxides NOx

The end product in HNO3. A possible mechanism for the formation of HNO3 is show below in which O3 plays an important role.

O3 + NO2 NO3 + O2

NO3 + NO2 N2O5

N2O5 + H20 2HNO3

1. Before day light NO & NO2 levels remain fruity stable at concentration slightly higher than the daily minimum .

2. As the traffic rush begins & increases (6-8 am) , the level of NO increases & becomes maximum.

3. At mid-morning with increased ultraviolet light , the NO2 level increases due to conversion of NO into NO2.

4. O3 build up as NO level drops below 0.1 ppm.

5. In the evening the No level again goes up during the evening traffic rush.

6. O3 accumulated during day times , reacts with NO during night with the result that NO2

concentration goes slightly up & O3 level drops. Busy cities heavy automobile traffic has revealed that the maximum values of NO & NO2 levels are 1-2 and 0.5 ppm, respectively.

Sources of NOx pollution : Natural bacteria action discharges. Man made source release 50*107 tones of NOx .man made some in combustion of coal, oil,

natural gas & gasoline. By photo chemical reaction.

Control of NOx pollution :a) The use of catalytic converts for control of automobile emissions provides for

removals of ( NO)x.

b) Power plants emit about 50-1000 ppm of (NO)x. Such emission can be reduced to 90% by a two stage combustion process.

(i) The fuel (coal/gas/oil) is fired at a relatively high temp .whit a sub-stoichiometric amount of air, say 90- 90 % of the stoichiometric requirement. This yield of NO is limited in the absence of excess of O2.

(ii) Fuel burnout is completed at a relatively low temperature in excess air,Under this condition NO is not favored. A possible approach to NOx removed from stack gas is the chemical sorption process using H2SO4 solutions or alkaline scrubbing solution containing Ca(OH)2 & Mg(OH)2.Simultaneously SO2 is also removed. NO is converted into N2O3 which is easily absorbed.

Nitrogen oxides NOx

Control of NOx pollution :

Then NO2 + NO N2O3

NO2 is recycled. Four steps are involved in scrubbing process . Flue gas and NO2

and introduced into an oxidizer.

NO2 +SO2 +H2O H2SO4 +NO

NO & +NO2 react to form N2O3 which is scrubbed by H2SO4 in a scrubber.

The cleaned flue gas is released into the atmosphere.

NO2 + NO N2O3

N2O3 + 2H2SO4 2NOHSO4 + H2O

The reaction product is decomposed & the resulting H2SO4 recycled to the scrubber.

2NOHSO4 + ½ O2 +H2O 2H2SO4 + 2NO2

NO2 produces HNO3 in HNO3 reactor.

3NO2 + H2O 2HNO3 +NO

Excess NO2 & NO are recirculated through the oxidizer in first step.

Nitrogen oxides NOx

Sulphur dioxide so2 :

It is colorless gas with a pungent odour. It is produced from the combination of sulphur-bearing material.

S + O SO2

2SO2 + O2 2S03

SO3 + H2O H2SO4

Natural process e.g. volcanoes. Provide 67% SOx pollution distributed all over the globe. Men sources 33%. Fuel combustion, transportation, coal fired power station.

+H2O

SO2 + O2 SO3 + O2 H2SO4 (H2SO4)n aerosol droplet.

SO2 + 1/2 O2 Soot dust SO3 + H2O H2SO4 (H2SO4)n aerosol droplet

Or metal oxide

This gives rise to the phenomenon of acid rain.

Control :

4 possible ways to the removal & control of SOX emissions. Removal of Sox from flue gases. Removal of S from fuel before burning. Uses of low sulphur fuels Substitution of other energy sources of fuel combustion.

Sox from flue gases can be conveniently eliminated by using chemical scrubbers. The flue stack are passed through a slurry of limestone, CaCO3, which absorbs SO2 quite efficiently.

CaCO3 + SO2 + O2 CaSO4 + CO2

An alternate process is based on a reaction between HSO3 ions (from SO⁻ 2) and citrate ions. The flue gas is cooled to 50 C or lower and freed from particulates and traces of H⁰ 2SO4. Then pass to a Absorption tower into a solution containing citrate ions (H2Cit )⁻

SO2 + H2O HSO3 + H⁻ ⁺

HSO3 + H2Cit¯ (HSO3.H2Cit) ²⁻

Passed into,

closed vessel where

H2S is bubbled

H2S + H⁺

2S + H2Cit + 3H⁻ 2O

Acid rain :

Much of the NOx and SOx entering into the atmosphere are converted into HNO3 & H2SO4 respectively. The detailed photochemical reactions in the atmosphere are summarized:

NO + O3 NO2 + O2 NO2 + O2 NO3 + O2 NO2 + NO3 N2O5 N2O5 + H2O HNO3

HNO3 in removed as precipited or as particular nitrates after reaction with bases (NH3, particulate lime)

HC, NOX SO2 +1/2 O2 + H2o Soot particles (metal oxide) H2SO4

HNO3 & H2SO4 Combine with HCl emission (both by natural and anthropogenic sources) to generate acidic precipitation which is widely known as acid rain. Acid rain now a major pollution problem in some areas.

Acid rains causes extensive damage to buildings & sculptural materials of marble, limestone, slate, mortar etc. These materials become pitted and weakened mechanically as the soluble sulphates are leached out by rainwater.

Particulates : Small, solid particles and liquid droplets are collectively termed particulates. These are presents in the atmosphere in fairly large numbers and sometimes pose a serious air pollution problem.

Sources: Numerous natural processes injecting particulate matter in the atmosphere (800-2000 million tones each year) volcanic eruption, blowing dust & soil by the wind, spraying salt & other solid particles by the seas & oceans.

Man-made: Fly ash from power plants smelters from mining operation smoke from incomplete combustion process

Effects of particulates on materials: Air borne particles including soot, fumes and mist are potentially harmful for a variety of

materials. The extent & type of damage depend upon chemical composition & physical state of the pollutant. Extensive damage occurs when the particulates themselves are corrosive or when they carry toxic substances along with them.

Control: There are 4 types of equipment used for the control of particulates. Gravity setting chamber cyclone collector wet scrubbers Electro static precipitators.

Inorganic particulate matter: Metal oxide comprise a major class of inorganic particles in the atmosphere. They are produced whenever fuels containing metals are burnt. Thus particulate Fe3O4 in formed during the combustion of pyrite containing coal.

3FeS2 + 8O2 Fe3O4 + 6SO2 & V converted to V2O5, CaCO3 CaO + CO2

Pb(C2H5)4 + O2 + C2H4Cl2 + C2H4Br2 CO2 + H2O + PbCl2 + PbBrCl + PbBr2

dichloroethane dibromoethane

then Pb-halides which are volatile & emerge through exhaust system which condenses to form particles. This is basic of Pb pollutions in the atmosphere. Due to presence of H2SO4 (SO2 + O2 + 2H20 H2SO4) & basis air pollutant, such as NH3 OR CaO, Salts are formed

H2SO4 +NH3 (NH4)2SO4

H2SO4 + CaO CaSO4 + H2O

Organic particulate matter : These are benzene extracted substances poly cyclic aromatic hydrocarbon

(PAH) are important components of organic particulate matter because of their carcinogenic nature. Some typical PAH compounds are Banzo pyrene, chrysene etc.

Most of PAH compounds are absorbed into soot particles.

MonitoringAnalysis of CO : (i)Infrared spectrosphotometry. It is based on the

principle that CO strongly absorbs infra-red radiation at certain wavelengths. When IR radiation is passed through a long cell containing a trace of CO, part of the energy is absorbed by the gas. At higher CO level more of the IR energy is absorbed.

(ii)Gas chromatography : 10 ppm or lower levels of CO can be conveniently measured by gas chromatography.

Hydrocarbon and photochemical smog:

Natural source : Natural sources, particularly trees, emit large quantities

of hydrocarbons in the atmosphere. Methane is the major naturally occurring hydrocarbon emitted into the atmosphere. It is produced in considerable quantities by bacteria in the anaerobic decomposition of organic matter in water, sediments and soil.

2{CH20} bacteria CO2 + CH4

Man-made source: Automobiles are the major sources of hydrocarbon

emitted to the atmosphere.

Control:

Hydrocarbons are removed from the atmosphere by several chemical & photochemical reactions. The end product are CO2, solid organic particulate matter that settles from the atmosphere or water- soluble products e.g. acids & aldehydes which are washed by rain .The majority of the harmful effects of hydrocarbon pollution are not due to the hydrocarbon themselves, but the products of photochemical reactions. An important characteristics of atmosphere which are loaded with large quantities of automobiles exhausts, trapped by an inversion layer (stagnant air masses) and at the same time exposed to intense sunlight, is the formation of photochemical oxidants in the atmosphere. This gives rise to the phenomenon of photochemical smog. Photochemical smog is characterized by brown, hazy fumes which irritate in eyes & lungs, lead to the cracking of rubber and extensive damage of plant life.

Control:

The probable mechanism of smog forming reactions as follows.

a) Reactive hydrocarbon ( those with C=C group ) from auto exhaust interact with O3 to from a hydrocarbon free radical RCH2•

b) RCH2• rapidly reacts with O2 to from another free radical RCH2O2•

c) RCH2O2• reacts with NO to produce NO2 & the free radical RCH2O•

d) This new free radical next interacts with O2 to yield a stable aldehyde, RCHO, and hydroperoxyl radical, HO2•

e) HO2• then reactive with another molecule of NO to give NO2 & HO•

f) HO• is extremely reactive & rapidly reacts with a stable hydrocarbon RCH3 to yield H2O & regenerate the hydrocarbon free radical RCH2• , thereby completing cycle.

Reactive hydrocarbon

O3

RCH2• O2 RCH202•

RCH3 NO

NO2 + HO• RCH2O• + NO2

NO O2

HO•2

+

RCHO

Stable aldehyde

Route of PAN formation

RCHO

HO•

RC• = C acyl radical

O2 peroxyacyl radical

RC = O

OO•

NO2

RC = O

OONO2 peroxyacyl nitrate (PAN)

The aldehyde RCHO may initiate another route by interaction with the HO• Radical, leading to the formation of an acyl radical RC=O, peroxyacyl radical RCOO2 ( by reaction with O2) & finally peroxyacyl nitrate PAN ( by reaction with NO2). PAN is one of the most potent eye irritants found smog.

Control of hydrocarbon & photochemical pollutants.

O3 & PAN secondary pollutants, so their control ultimately depends on the control of their primary precursors, hydrocarbons & nitrogen oxides. The control of NOx has been described earlier & that of HC in connection with the control of auto exhaust emissions.

Environmental pollution

Environment

Transcript of Environmental pollution