Environmental Chemistry Complete Notes..

Environmental Chemistry

Environmental chemistry:

It is the branch of chemistry that deals with the study of various chemical processes taking place in the various segments of the environment. Broadly speaking, it is the study of the sources, transportation, reactions, effects and the ultimate of the chemical species occurring in one or more segments of the environment.

Components of Environment:

• Atmosphere: This comprises a blanket of gaseous layer around earth.

• Hydrosphere: This comprises about 96% of earth’s surface & includes all 
• sources of water like oceans rivers lakes, glaciers, ground water etc.

• Lithosphere: It refers to earth’s solid crust containing the outer mineral cover. It comprises soil, minerals, organic matter etc.

•  Biosphere: It refers to the domain of living organism in covalent with atmosphere hydrosphere as well as lithosphere.  

Environmental pollution:

It is the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings.

• Pollutant:  Any substance or species produced either by a natural source or by human activity, which produces adverse effect on the environment.

Pollutants can be natural or anthropogenic:

a)  Natural pollutants: These are produced due to natural happenings like volcano eruptions etc.

b)  Anthropogenic pollutants: These are produced due to human activities.

Pollutants can be biodegradable or non – biodegradable:

a)  Biodegradable pollutants:

These are the pollutants which rapidly break down by natural processes. Example: discarded vegetables.

b)  Non – biodegradable:

These are the pollutants which are slowly degradable, and remain in the environment in an unchanged form for many decades. For example: DDT, plastic materials, heavy metals, many chemicals, nuclear wastes etc.

Primary pollutants: 

These are the pollutants which are emitted directly from the sources. Some examples are:

Particulate Matter:

Such as ash, smoke, dust, fumes etc.

Inorganic gases: 

Such as sulphur dioxide, carbon monoxide etc.

Secondary Pollutants:

These are the pollutants which are formed in the atmosphere by chemical interaction among primary pollutants & normal atmospheric constituents. Some examples are sulphur trioxide, nitrogen dioxide, ozone, aldehyde, ketones, various sulphate & nitrate salts.

• Contaminant: A substance which does not occurs in nature but is introduced by human activity into the atmosphere affecting its composition.
• Source: The site from which the pollution or contaminants originate.
• Sink: The material or medium which consumes or interacts with a long lived pollutant is called sink. 
• Receptor : Anything that is affected by the pollutants

Tropospheric pollution or Air pollution:

It is the atmosphere condition in which the presence of certain concentration produce harmful effects on man and his environment. These substances include:

(i) Gases such as oxides of sulphur, CO, oxide of N₂ and hydrocarbons

(ii) Particulate matter such as dust, smoke, fumes etc.

(iii) Radioactive material & many others.

Particulate matter:

• Soot: produced by incomplete combustion of carbonaceous fossils fuels such as coal, fuel oil, natural gas, wood etc in insufficient supply of oxygen.

• Metal particles: These are released by various metal finishing operation. The micro particles of toxic metal & SO₂ gas present in the polluted atmosphere get absorbed on the particles rendering them highly toxic.

• Metal oxides : They are generated by combustion of fuels containing metallic compounds.

• Lead salts: Their source is lead tetraethyl (Pb(C₂H₅)₄) which is added to gasoline to improve its antiknock property. In order to avoid deposition of PbO suitable amounts of C₂H₄Cl₂ & C₂H₄Br₂ are added to gasoline along with Pb(C₂H₅)₄.

• Fly ash: It originates from the combustion of high ash fossil. It contains partially burnt particles of the fuels.

• Asbestos dust: It originates from industrial units manufacturing asbestos sheets, gaskets ropes etc. Asbestos flowing & asbestos insulations also contribute towards asbestos dust in the atmosphere.

• Solid Hydrocarbons: These are emitted from petroleum refineries & comprise of paraffin’s, olefins & aromatics.

• Dust Particulates: Originate from natural, domestic, industrial or agricultural sources. These are thrown into atmosphere by volcanic eruptions, blowing of dust by wind, mining operations etc.

• Acid mist: Sulphuric acid mist is produced when SO₃ present in the atmosphere comes in contact with moisture. Nitric acid mist is produced when oxides of nitrogen, viz, NO & NO₂, undergo the series of reactions in the atmosphere.

 Harmful effects of particulates

• Effect on human beings: Affect the human respiratory system & cause several respiratory illnesses. The particles with small size are more harmful in this context. The particulates in fact, become the carriers of the toxic substances from the atmosphere to the human & cause big health hazards.

• Effect on visibility: Particulates in the atmosphere cause scattering & absorption of sunlight & reduce the visibility.

• Effect on Materials : The adverse effect of particulates on materials include corrosion of metals (when the atmosphere is humid), erosion & soiling of building, sculptures & painted surfaces & soiling of clothes & draperies.

Environmental pollution is of three types: a. Atmospheric pollution i. Tropospheric pollution ii. Stratospheric pollution b. Water pollution c. Soil and land pollution

Atmospheric pollution occurs when the concentration of a normal component of the air or a new chemical substance added or formed in air builds up to undesirable proportions causing harm to humans, other animals, vegetation and materials.

Troposphere: 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 ~ 10 km from sea level.

Stratosphere: Above the troposphere, between 10 and 50 km above sea level lies stratosphere.

Tropospheric pollution: Is because of two types of pollutants:

a)  Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.

b)  Particulate pollutants: Particulate 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. Examples of particulate pollutants are dust, mist, fumes, smoke, smog etc.

Oxides of sulphur as pollutant:

Sources: Burning of fossil fuels containing sulphur Harmful effects:

·         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 sulphur dioxide leads to stiffness of flower buds which eventually fall off from plants.

Oxides of nitrogen as pollutant:

Sources:

·         At high altitudes when lightning strikes, dinitrogen and dioxygen combine to form oxides of nitrogen.

·         Burning of fossil fuel in an automobile engine, at high temperature, dinitrogen and dioxygen combine to yield significant quantities of nitric oxide (NO) and nitrogen dioxide (NO2).

N2 (g) + O2 (g) → 2 NO (g)

2 NO (g) + O2 (g) → 2 NO2 (g)

Harmful effects:

·         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 as pollutant:

Source:

Incomplete combustion of fuel used in automobiles

Harmful effects:

·         Hydrocarbons are carcinogenic, i.e., they cause cancer

·         They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs

Oxides of carbon as pollutant:

a)  Carbon monoxide:

Source:

·         Incomplete combustion of carbon of coal, firewood, petrol, etc

·         By automobile exhaust

Harmful effects:

It is highly poisonous to living beings because of its ability to block the delivery of oxygen to the organs and tissues. It binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. In blood, when the concentration of carboxyhaemoglobin 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.

b)  Carbon dioxide:

Source:

·         Respiration

·          Burning of fossil fuels for energy

·         By decomposition of limestone during the manufacture of cement

·         By volcanic eruptions

·         Deforestation

Harmful effects:

·         Causes global warming

Green house effect:

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. This trapping of the sun’s heat near the earth’s surface and keeping it warm is called natural greenhouse effect. It maintains the temperature and makes the earth perfect for life.

If the amount of carbon dioxide crosses the delicate proportion of 0.03 per cent, the natural greenhouse balance may get disturbed. This may lead to global warming.

Green house: 

In a greenhouse, visible light passes through the transparent glass and heats up the soil and the plants. The warm soil and plants emit infrared radiations. Since glass is opaque to infrared (heat) radiations, it partly reflects and partly absorbs these radiations. This mechanism keeps the energy of the sun trapped in the greenhouse.

Global warming: 

An increase in the average temperature of the earth's atmosphere (especially a sustained increase that causes climatic changes) which may be caused by additional heat being trapped by greenhouse gases.

Acid rain: 

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.

H₂O (l) + CO₂ (g) → H₂CO₃ (aq)

H₂CO₃ (aq) → 2H⁺ +CO₃^2-

Source:

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 catalyses the oxidation.

2SO₂ (g) + O₂ (g) + H₂O (l) → 2H₂SO₄ (aq)

4NO₂ (g) + O₂ (g) + H₂O (l) → 4HNO₃ (aq)

Harmful effects:

·         Harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.

·         Causes respiratory ailments in human beings and animals.

·         Affects plant and animal life in aquatic ecosystem when acid rain falls and flows as ground water to reach rivers, lakes etc.

·         Corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into the drinking water.

·         Damages buildings and other structures made of stone or metal. The Taj Mahal in India has been affected by acid rain.

Particulates in the atmosphere may be viable or non-viable:

a)  Viable are minute living organisms that are dispersed in the atmosphere. Example: bacteria, fungi, moulds, algae etc.

b)  Non-viable particulates may be classified as:

·         Smoke particulates: consist of solid or mixture of solid and liquid particles formed during combustion of organic matter. Example: cigarette smoke, smoke from burning of fossil fuel, garbage and dry leaves, oil smoke etc.

·         Dust: composed of fine solid particles (over 1m 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. Example: 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.

Smog: 

Smoke is a mixture of smoke, dust particles and small drops of fog.

Smog is of two types:

Classical Smog	Photochemical Smog
1. It occurs in cool humid climate. 2. It is a mixture of smoke, fog & sulphur dioxide. 3. It is also called reducing smog.	1. It occurs in warm, dry and sunny climate. 2. Components of photochemical smog result from the action of sunlight on unsaturated hydrocarbons & oxides of nitrogen produced by automobiles & factories. 3. It is also called oxidizing smog.

Formation of photochemical smog:

Ø  Burning of fossil fuels.

Ø  Emission of a variety of pollutants (hydrocarbons and nitric oxide to troposphere).

Ø  At high levels, leads to Chain reaction between pollutants and sunlight:

2NO(g) + O₂(g) → 2NO₂

NO₂(g) → NO(g) + O˚(g)

O˚(g) + O₂(g) → O₃(g)

O₃(g) + NO(g) → NO₂(g) + O₂(g)

 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:

 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, and 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.

Control of photochemical smog:

·         Use of catalytic converters in 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 is basically due to ozone layer depletion.

Formation of ozone in stratosphere:

O₂(g) → O˚(g) + O˚(g)

O˚(g) + O₂(g) → O₃(g)

Depletion of ozone layer:

Release of chlorofluorocarbon compounds (CFCs), also known as freons lead to their mixing with the normal atmospheric gases and eventually reach the stratosphere.

 In stratosphere,

CF₂Cl₂(g) → Cl^˚(g) + C^˚F₂Cl(g)

Cl^˚(g) + O₃(g) → Cl^˚O(g) + O(g)

Cl^˚O(g) + O(g) → Cl^˚(g) + O₂(g)

 This way, the chlorine radicals are continuously regenerated and cause the breakdown of ozone layer.

 Ozone hole over Antarctica:

 In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion.

ClO^˚(g) + NO₂(g) → ClONO(g)

Cl^˚(g) + CH₄(g) → C^˚H₃(g) + HCl(g)

 In winter, special type of clouds called polar stratospheric clouds are formed over Antarctica. These polar stratospheric clouds provide surface on which chlorine nitrate formed gets hydrolysed to form hypochlorous acid.

ClONO₂(g) + H₂O(g) → HOCl(g) + HNO₃(g)

It also reacts with hydrogen chloride produced to give molecular chlorine.

ClONO₂(g) + HCl(g) → Cl₂(g) + HNO₃(g)

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.

HOCl(g) → O^˚H(g) + Cl^˚(g)

Cl₂(g) → 2Cl^˚(g)

The chlorine radicals thus formed, initiate the chain reaction for ozone depletion.

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 and skin cancer etc in human beings

·         Killing of many phytoplanktons

·         Damage to fish productivity

·         Affect the plant proteins which lead to the harmful mutation of cells

·         Increases the 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

Water pollution:

Major water pollutants	Sources	Harmful effects
Pathogens (Micro-organisms)	Domestic sewage	Cause gastrointestinal diseases.
Organic wastes (leaves, grass, trash)	Domestic sewage, animal excreta and waste, decaying animals and plants, discharge from food processing factories	Lead to decrease in concentration of dissolved oxygen in water and lead to death of aquatic life
Plant nutrients	Chemical fertilizers
Toxic heavy metals (cadmium, mercury, nickel)	Industries and chemical factories	Can damage kidneys, central nervous system, liver etc
Sediments	Erosion of soil by agriculture and strip mining
Pesticides (insecticides, herbicides , fungicides)	Chemicals used for killing insects, fungi and weeds	Lead to eutrophication
Radioactive substances	Mining of uranium containing minerals
Heat	Water used for cooling in industries

Dissolved Oxygen of water (DO):

The oxygen needed to saturate a given sample of water at any specific temperature is called the dissolved oxygen (DO) of that sample of water.

It is generally expressed in mg/L or (ppm).

NOTE:  DO of pure water at 20˚C is 9 ppm.

Biochemical Oxygen Demand (BOD):

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 Biological Oxygen demand (BOD).

NOTE: BOD is a measure of organic pollutant present in the sample of water. Higher the value of BOD, higher is the level of organic pollution in water. Drinking water must contain BOD less than 5 ppm.

Chemical Oxygen demand (COD):

The amount of Oxygen (in mg/L) consumed for oxidising all organic and oxidisable inorganic materials in a sample of water is called Chemical oxygen demand (COD).

NOTE:  COD is always higher than BOD.

Eutrophication:

The 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.

Some constituents of drinking water:

Constituent	Maximum concentration	Harmful effects of higher concentration
Fluoride	1 ppm or 1 mg dm–3	Causes brown mottling of teeth
Lead	50 ppb	Can damage kidney, liver, reproductive system etc
Sulphate	500 ppm	Causes laxative effect
Nitrate	50 ppm	Causes disease such as methemoglobinemia (‘blue baby’ syndrome)
Metals
Fe	0.2 ppm
Al	0.05 ppm
Mn	0.2 ppm
Cu	3.0 ppm
Zn	5.0 ppm
Cd	0.005 ppm

Pesticides: 

They are basically synthetic toxic chemicals with ecological repercussions.

Herbicides: 

They are used to kill weeds or undesirable vegetation. Example: sodium chlorate (NaClO₃), sodium arsinite (Na₃AsO₃)

Strategies to control environmental pollution:

a)  Water management

·         Segregate the water as biodegradable and non- biodegradable waste:

 Ø  Biodegradable waste:

a) Generated by cotton mills, food processing units, paper mills, and textile factories.

b) Management: are deposited in landfills and are converted into compost

     Non – biodegradable water:

  Generated by thermal power plants which produce fly ash; integrated iron and steel plants which produce blast furnace slag and steel melting slag

  Management:

      - Recycling

      -Toxic wastes are usually destroyed by controlled incineration

Green chemistry:

The branch of chemistry that emphasises on the process and products that reduce or eliminate the use and generation of toxic/hazardous substances is called green chemistry.

Green chemistry is a strategy to design chemical processes and products that reduces or eliminates the use and generation of hazardous substances. The chemical reactions should be such that the reactants are fully converted into useful environmental friendly products by using an environment friendly medium so that there would be no chemical pollutants introduced in the environment.

Green chemistry in daily life:

Purpose	Earlier	Now
Dry cleaning of clothes	Tetrachloroethene (Cl2C=CCl2) which contaminates the ground water	Liquefied carbon dioxide, with a suitable detergent
Bleaching of paper	Chlorine gas	Hydrogen peroxide (H2O2) with suitable catalyst

By:

Anjani Kumar Singh