Environmental Chemistry — NCERT Class 11 Chemistry

Environmental pollution is the undesirable change in the physical, chemical, or biological characteristics of our air, water, or land that can harmfully affect human life, the life of other living organisms, or industrial processes. A pollutant is any substance that causes pollution. Pollutants can be solid, liquid, or gaseous, and they may originate from natural sources (volcanic eruptions, forest fires) or human activities (industrial emissions, vehicle exhaust).

The three major types are: 1. Atmospheric (air) pollution: caused by gaseous pollutants (CO, SO₂, NOₓ, hydrocarbons) and particulate matter (dust, smoke, fly ash) in the troposphere and stratosphere 2. Water pollution: caused by discharge of domestic sewage, industrial effluents, agricultural chemicals, and heavy metals into water bodies 3. Soil (land) pollution: caused by pesticides, fertilisers, industrial waste, radioactive waste, and improper solid waste disposal

The major tropospheric pollutants are:

• Gaseous pollutants: CO, SO₂, NOₓ (NO and NO₂), hydrocarbons, oxidants (O₃, PAN — peroxyacyl nitrates)
• Particulate pollutants: smoke, soot, fly ash, dust, fumes, mist, aerosols, lead compounds

Carbon monoxide (CO) is produced by:

• Incomplete combustion of fossil fuels (petrol, diesel, coal) in vehicles and industries
• Incomplete combustion of biomass (wood, agricultural waste)
• Industrial processes (steel mills, petroleum refineries)

CO is dangerous because it binds to haemoglobin in red blood cells about 200 times more strongly than oxygen, forming carboxyhaemoglobin. This reduces the oxygen-carrying capacity of blood, leading to oxygen deprivation. Symptoms include headache, dizziness, nausea; high concentrations can cause death by asphyxiation.

Acid rain is rain with a pH below 5.6 (normal rain has pH ≈ 5.6 due to dissolved CO₂ forming carbonic acid). It is formed when atmospheric sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) react with water vapour and oxygen to form sulphuric acid (H₂SO₄) and nitric acid (HNO₃):

SO₂ + ½O₂ → SO₃ SO₃ + H₂O → H₂SO₄ 2NO₂ + H₂O → HNO₃ + HNO₂ (or NO + NO₂ + H₂O → 2HNO₃)

These acids dissolve in rainwater and fall to Earth as acid rain.

1. On buildings and monuments: Corrodes limestone, marble, and metal structures (e.g., damage to the Taj Mahal) 2. On aquatic life: Lowers pH of lakes and rivers, killing fish and other aquatic organisms; leaches toxic aluminium from soil into water bodies 3. On vegetation: Damages leaves, reduces chlorophyll, stunts growth, makes plants susceptible to disease 4. On soil: Leaches essential nutrients (Ca²⁺, Mg²⁺, K⁺) and increases toxic aluminium levels 5. On human health: Causes respiratory problems, eye irritation, and skin problems

The greenhouse effect is the process by which certain gases in the atmosphere (greenhouse gases) absorb and re-emit infrared radiation (heat) radiated by the Earth's surface, trapping heat and warming the lower atmosphere. Sunlight passes through the atmosphere and warms the Earth. The Earth radiates this energy as infrared radiation. Greenhouse gases (CO₂, CH₄, N₂O, CFCs, water vapour) absorb this infrared radiation and re-radiate it in all directions, including back towards the Earth's surface. This natural process keeps the Earth about 33°C warmer than it would be without greenhouse gases. However, increased greenhouse gas concentrations from human activities are enhancing this effect, leading to global warming.

Global warming is the increase in the average temperature of the Earth's surface due to the enhanced greenhouse effect caused by increased concentrations of greenhouse gases (primarily CO₂ from fossil fuel combustion and deforestation). Consequences include:

• Rise in sea level due to melting of polar ice caps and glaciers
• Changes in weather patterns (more extreme weather events — droughts, floods, hurricanes)
• Shift in agricultural zones and reduced crop yields
• Loss of biodiversity and habitat destruction
• Increased spread of tropical diseases
• Ocean acidification (CO₂ dissolved in seawater)

Water pollution sources are classified as: 1. Point sources: Specific, identifiable sources — industrial effluent pipes, sewage treatment plant outlets, oil spills 2. Non-point sources: Diffuse sources — agricultural runoff (fertilisers, pesticides), urban stormwater drainage, atmospheric deposition

Specific pollutants include:

• Domestic sewage (organic matter, pathogens)
• Industrial waste (heavy metals — Hg, Pb, Cd; chemicals, acids, alkalis)
• Agricultural chemicals (fertilisers, pesticides, herbicides)
• Thermal pollution (hot water from power plants)
• Oil spills and petroleum products

BOD (Biochemical Oxygen Demand) is the amount of dissolved oxygen required by aerobic microorganisms to decompose the organic matter in a water sample over 5 days at 20°C. It is measured in mg/L (or ppm).

• Clean water has BOD < 5 mg/L
• Moderately polluted water has BOD 5–15 mg/L
• Highly polluted water has BOD > 15 mg/L

Higher BOD means more organic matter in the water, which means more oxygen is consumed by bacteria, leaving less dissolved oxygen (DO) for fish and other aquatic organisms. This leads to fish kills and degradation of water quality.

Eutrophication is the excessive growth of algae and other aquatic plants in water bodies due to the enrichment of nutrients (especially nitrates and phosphates from agricultural runoff and sewage). This leads to: 1. Algal blooms that cover the water surface, blocking sunlight 2. Death and decomposition of algae, consuming dissolved oxygen 3. Depletion of dissolved oxygen (DO), killing fish and other aquatic organisms 4. Release of foul-smelling gases (H₂S, NH₃, CH₄) 5. Conversion of the water body into a marsh or swamp

Heavy metals (Hg, Pb, Cd, As) are toxic even at low concentrations and are non-biodegradable. They enter water bodies through industrial effluents, mining, and agricultural runoff. Effects include:

• Mercury: Minamata disease (neurological disorder, paralysis); biomagnification through food chain
• Lead: Damage to nervous system, kidneys; anaemia; mental retardation in children
• Cadmium: Itai-itai disease (painful bone fractures, kidney damage)
• Arsenic: Skin lesions, cancer, cardiovascular disease

Biomagnification: heavy metals concentrate at each trophic level, so organisms at the top of the food chain (including humans) receive the highest doses.

Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It focuses on preventing pollution at the molecular level rather than cleaning up waste after it is formed. The 12 principles of green chemistry (Paul Anastas and John Warner, 1998) guide the development of environmentally benign chemical processes.

1. Prevention: Prevent waste rather than treat or clean up waste after it is formed 2. Atom economy: Synthetic methods should maximise the incorporation of all materials into the final product 3. Less hazardous synthesis: Use methods that generate less toxic intermediates and products 4. Designing safer chemicals: Chemical products should be effective but have minimal toxicity 5. Safer solvents and auxiliaries: Avoid using solvents, separation agents, or other auxiliary substances wherever possible 6. Design for energy efficiency: Minimise energy requirements; conduct reactions at ambient temperature and pressure 7. Use renewable feedstocks: Use renewable raw materials rather than depletable ones 8. Reduce derivatives: Minimise use of blocking groups, protecting groups, and temporary modifications 9. Catalysis: Catalytic reagents are superior to stoichiometric reagents 10. Design for degradation: Products should be designed to break down into harmless substances 11. Real-time analysis for pollution prevention: Enable in-process monitoring and control 12. Inherently safer chemistry for accident prevention: Minimise the potential for chemical accidents (releases, explosions, fires)

1. Synthesis of ibuprofen: The traditional Boots process used 6 steps with an atom economy of 40%. The green BHC process uses only 3 steps with an atom economy of 99%, producing much less waste. 2. Dry cleaning with liquid CO₂: Replacing tetrachloroethene (PERC, a suspected carcinogen) with liquid CO₂ for dry cleaning is a green chemistry alternative. 3. Manufacture of polystyrene: Using CO₂ as a blowing agent instead of CFCs prevents ozone depletion. 4. Bleaching paper with H₂O₂: Replacing chlorine (which produces toxic dioxins) with hydrogen peroxide for paper bleaching.

Environmental chemistry is the branch of chemistry that deals with the study of the origin, transport, reactions, effects, and fates of chemical species in the environment (air, water, and soil). It also includes the study of the impact of human activities on the environment and strategies for controlling environmental pollution.

Tropospheric gaseous pollutants include:

Carbon monoxide (CO): Produced by incomplete combustion of fossil fuels. Binds to haemoglobin, reducing oxygen transport in blood. Causes headache, dizziness, and in high concentrations, death.

Sulphur dioxide (SO₂): Produced by burning sulphur-containing fossil fuels (coal, petroleum). Causes respiratory problems, forms sulphuric acid (H₂SO₄) in the atmosphere contributing to acid rain. Also causes corrosion of buildings and damage to vegetation.

Oxides of nitrogen (NOₓ): NO and NO₂ produced at high temperatures in vehicle engines and power plants. NO₂ is a reddish-brown gas that causes respiratory problems, forms nitric acid (HNO₃) contributing to acid rain, and is a precursor to photochemical smog and ground-level ozone.

Hydrocarbons and volatile organic compounds (VOCs): Released from petrol, solvents, and industrial processes. Contribute to photochemical smog formation (PAN, ozone at ground level). Some are carcinogenic (e.g., benzene).

Photochemical smog is a mixture of pollutants formed when NOₓ, hydrocarbons, and other VOCs react in the presence of sunlight to form secondary pollutants like ozone (O₃), peroxyacyl nitrates (PAN), and aldehydes.

Formation mechanism: 1. NO₂ →ˣʰᵛ NO + O• (photodissociation) 2. O• + O₂ → O₃ (ozone formation at ground level) 3. Hydrocarbons + O₃ + NOₓ → PAN + other oxidants

Photochemical smog causes eye irritation, respiratory problems, damage to vegetation, and reduced visibility. It is common in cities with high traffic and warm, sunny climates (e.g., Los Angeles, Delhi).

The ozone hole is the thinning of the ozone layer (a region in the stratosphere 15–35 km above Earth where O₃ concentration is highest) observed primarily over Antarctica during spring (September–October).

Formation mechanism: 1. CFCs (chlorofluorocarbons) released from refrigerants, aerosols, and foam-blowing agents reach the stratosphere 2. UV radiation breaks CFCs: CFCl₃ →ˣʰᵛ CFCl₂ + Cl• (chlorine radical) 3. The chlorine radical catalyses ozone destruction: Cl• + O₃ → ClO• + O₂; ClO• + O• → Cl• + O₂ 4. One Cl• atom can destroy up to 10⁵ ozone molecules before being removed 5. Polar stratospheric clouds (PSCs) in Antarctica provide surfaces for heterogeneous reactions that convert inactive chlorine reservoirs (HCl, ClONO₂) into active Cl₂, which is photolysed to release Cl•

Consequences: increased UV-B radiation reaching Earth, causing skin cancer, cataracts, immune system suppression, and damage to phytoplankton and crops.

The major greenhouse gases are: 1. CO₂ (60% contribution): from fossil fuel combustion, deforestation, cement production 2. CH₄ (20% contribution): from rice paddies, cattle (enteric fermentation), landfills, natural gas leaks 3. N₂O (6% contribution): from fertilisers, fossil fuel combustion, industrial processes 4. CFCs (14% contribution): from refrigerants, aerosol propellants, foam-blowing agents 5. Water vapour: the most abundant greenhouse gas, but mainly natural

These gases are transparent to incoming short-wavelength solar radiation but absorb and re-emit outgoing long-wavelength infrared radiation from the Earth's surface. This trapped heat warms the lower atmosphere and the Earth's surface. The enhanced greenhouse effect (due to increased anthropogenic greenhouse gas emissions) is causing global average temperatures to rise — leading to melting ice caps, rising sea levels, changing weather patterns, and loss of biodiversity.

Formation of sulphuric acid: SO₂ + ½O₂ → SO₃ (oxidation of SO₂) SO₃ + H₂O → H₂SO₄ (sulphuric acid)

Also: SO₂ + H₂O → H₂SO₃ (sulphurous acid, which is further oxidised)

Formation of nitric acid: N₂ + O₂ →⁽ʰⁱᵍʰ ᵗᵉᵐᵖ⁾ 2NO (at high temperatures) 2NO + O₂ → 2NO₂ 3NO₂ + H₂O → 2HNO₃ + NO (nitric acid)

Acid rain = H₂SO₄ + HNO₃ dissolved in rainwater, lowering pH below 5.6.

COD is preferred for industrial wastewater monitoring because it is faster and measures all organic content, including compounds that are toxic to bacteria.

Eutrophication is the excessive enrichment of water bodies with nutrients (nitrates and phosphates) from agricultural runoff, sewage, and detergents, leading to: 1. Rapid growth of algae and aquatic plants (algal blooms) 2. Dense algal cover blocks sunlight from reaching submerged plants 3. When algae die, decomposing bacteria consume large amounts of dissolved oxygen 4. Oxygen depletion (hypoxia/anoxia) kills fish and other aquatic organisms 5. Dead organic matter produces foul-smelling gases (H₂S, NH₃, CH₄) 6. The water body becomes a stinking marsh (lake → marsh → dry land)

Effects on aquatic life: fish kills due to oxygen depletion, loss of biodiversity, disappearance of clear-water species, and dominance of tolerant species.

Soil pollution is the contamination of soil with harmful substances that affect its productivity and the health of organisms. Sources include:

1. Pesticides and insecticides (DDT, BHC, malathion): accumulate in soil, biomagnify through food chains, kill beneficial soil organisms 2. Fertilisers (NPK): excess nitrates and phosphates leach into groundwater (blue baby syndrome from nitrates), alter soil pH and microbial balance 3. Industrial waste: heavy metals (Hg, Pb, Cd, As), toxic chemicals, acids, alkalis 4. Municipal solid waste: plastics, glass, metals, organic waste 5. Radioactive waste: from nuclear power plants and weapons testing 6. Agricultural waste: crop residues, animal waste

Effects: reduced soil fertility, contamination of groundwater, bioaccumulation in food chains, health problems in humans and animals.

Air pollution control:

• Use of cleaner fuels (CNG, LPG) and renewable energy (solar, wind)
• Catalytic converters in vehicles to reduce CO, NOₓ, and hydrocarbon emissions
• Scrubbers and electrostatic precipitators in industries
• Afforestation and reducing deforestation

Water pollution control:

• Treatment of industrial effluents before discharge
• Sewage treatment plants (primary, secondary, tertiary treatment)
• Reducing use of chemical fertilisers and pesticides
• Rainwater harvesting and watershed management

Soil pollution control:

• Use of biofertilisers and biopesticides instead of chemicals
• Composting and vermicomposting organic waste
• Proper disposal and recycling of industrial and municipal waste
• Bioremediation of contaminated soil

General strategies:

• Implementation and enforcement of environmental laws (Air Act, Water Act, Environmental Protection Act)
• Public awareness and education
• Adoption of green chemistry principles in industry

Green chemistry plays a crucial role in environmental protection by: 1. Preventing pollution at the source: Designing chemical processes that generate minimal or no waste, rather than treating waste after generation 2. Atom economy: Maximising the use of all raw materials in the final product, reducing waste 3. Safer chemicals: Designing chemicals that are effective but have minimal toxicity to humans and the environment 4. Renewable feedstocks: Using biomass-based raw materials instead of petroleum 5. Energy efficiency: Conducting reactions at ambient conditions, reducing energy consumption 6. Catalysis: Using catalysts to reduce the amount of reagents and energy needed 7. Biodegradable products: Designing products that decompose harmlessly in the environment

Examples: Ibuprofen synthesis (99% atom economy vs 40% in traditional process), CO₂ as a blowing agent replacing CFCs, H₂O₂ bleaching replacing chlorine.

CFCs (chlorofluorocarbons) are stable, non-toxic, non-flammable compounds used as refrigerants, aerosol propellants, and foam-blowing agents. Their stability allows them to reach the stratosphere intact, where they are photolysed by UV radiation:

Step 1: CFCl₃ →ˣʰᵛ CFCl₂• + Cl• (chlorine radical released) Step 2: Cl• + O₃ → ClO• + O₂ (ozone destroyed) Step 3: ClO• + O• → Cl• + O₂ (chlorine radical regenerated)

Net reaction: O₃ + O• → 2O₂ (ozone destroyed, chlorine acts as catalyst)

One Cl• atom can destroy up to 10⁵ ozone molecules before being removed (as HCl or ClONO₂). In Antarctica, polar stratospheric clouds (PSCs) enhance ozone depletion by converting inactive chlorine reservoirs back into active Cl₂. The Montreal Protocol (1987) phased out CFC production to protect the ozone layer.

(a) O₂ →ˣʰᵛ (< 242 nm) 2O• (photodissociation of O₂) O• + O₂ + M → O₃ + M (third body M absorbs energy) Net: 3O₂ →ˣʰᵛ 2O₃

(b) CFCl₃ →ˣʰᵛ CFCl₂• + Cl• (CFC photolysis) Cl• + O₃ → ClO• + O₂ (ozone destruction) ClO• + O• → Cl• + O₂ (chlorine regenerated) Net: O₃ + O• → 2O₂ (catalysed by Cl•)

(c) SO₂ + ½O₂ → SO₃; SO₃ + H₂O → H₂SO₄ 2NO₂ + H₂O → HNO₃ + HNO₂ (or: 4NO₂ + 2H₂O + O₂ → 4HNO₃)

Industrial waste introduces a wide range of pollutants into water bodies:

• Paper mills: Release organic matter (high BOD), chlorine compounds, and dioxins
• Textile industry: Discharge dyes, heavy metals (Cr, Cu), and organic solvents
• Tanneries: Release chromium (Cr⁶⁺), sulphides, and organic waste
• Fertiliser plants: Discharge nitrates, phosphates, and ammonia
• Metal processing: Release heavy metals (Hg, Pb, Cd, As) and acids
• Thermal power plants: Release fly ash and raise water temperature (thermal pollution)

Example: The Minamata disaster (1950s, Japan) — a chemical factory released methylmercury into Minamata Bay. Mercury bioaccumulated in fish and shellfish, which were consumed by local residents, causing Minamata disease (severe neurological damage, paralysis, and death).

• Pathogens (from sewage): cholera, typhoid, hepatitis, diarrhoeal diseases
• Heavy metals: Mercury → Minamata disease (neurological damage); Lead → anaemia, kidney damage, mental retardation; Cadmium → Itai-itai disease (bone fractures, kidney damage); Arsenic → skin cancer, cardiovascular disease
• Nitrates: Blue baby syndrome (methemoglobinaemia) in infants — nitrates converted to nitrites, which bind to haemoglobin reducing oxygen transport
• Fluoride: Dental and skeletal fluorosis (mottling of teeth, stiff bones)
• Pesticides: Carcinogenic, neurotoxic, reproductive effects, endocrine disruption
• Organic pollutants: PCBs, dioxins — cancer, immune suppression, hormonal disruption

Biomagnification (or bioaccumulation) is the increasing concentration of a persistent, non-biodegradable substance (like DDT, mercury, or PCBs) at successive trophic levels in a food chain.

Example: DDT in an aquatic food chain:

• Water: 0.003 ppb DDT
• Zooplankton: 0.04 ppm (13× concentration)
• Small fish: 0.5 ppm (500×)
• Large fish: 2.0 ppm (2000×)
• Fish-eating birds: 25 ppm (850,000×)

DDT is fat-soluble and not excreted, so it accumulates in fatty tissues. At each trophic level, the predator consumes many prey organisms, accumulating the DDT from all of them. Top predators (eagle, humans) receive the highest concentrations, leading to reproductive failure, neurological damage, and death.

Catalytic converters are devices fitted in vehicle exhaust systems that reduce emissions of CO, NOₓ, and unburnt hydrocarbons. They contain a catalyst (platinum, palladium, rhodium) coated on a ceramic honeycomb structure that provides a large surface area.

Reactions:

• Oxidation of CO: 2CO + O₂ → 2CO₂
• Oxidation of unburnt hydrocarbons: CₙHₘ + (n + m/4)O₂ → nCO₂ + m/2 H₂O
• Reduction of NOₓ: 2NO → N₂ + O₂; 2NO₂ → N₂ + 2O₂

Catalytic converters work best with unleaded petrol (lead poisons the catalyst). They reduce CO, hydrocarbon, and NOₓ emissions by up to 90%.

1. Reduce fossil fuel consumption by switching to renewable energy (solar, wind, hydro) 2. Increase energy efficiency in transportation, industry, and buildings 3. Afforestation and reforestation (trees absorb CO₂) 4. Reduce deforestation and promote sustainable forestry 5. Use public transport, electric vehicles, and carpooling 6. Carbon capture and storage (CCS) technologies 7. Reduce methane emissions from agriculture (rice cultivation, cattle) 8. Phase out CFCs and HFCs (Montreal Protocol and Kigali Amendment) 9. International agreements (Paris Agreement, Kyoto Protocol) 10. Individual actions: reduce, reuse, recycle; energy conservation; plant trees

The ozone layer (in the stratosphere, 15–35 km altitude) absorbs 97–99% of harmful UV-B (280–315 nm) and UV-C radiation from the Sun, preventing it from reaching the Earth's surface.

Consequences of ozone depletion: 1. Human health: Increased skin cancer (melanoma and non-melanoma), cataracts, immune system suppression 2. Marine life: Damage to phytoplankton (base of marine food chain), reduced fish populations 3. Terrestrial plants: Reduced crop yields, damage to forest trees 4. Materials: Degradation of plastics, rubber, and paints 5. Climate: Changes in stratospheric temperature and atmospheric circulation patterns

The Montreal Protocol (1987) and its amendments have successfully phased out CFC production, and the ozone layer is expected to recover by 2050–2070.