Environmental Chemistry & Pollution

Introduction: Chemistry of the Biosphere

Chemistry is not confined to laboratories. Every breath you take, every raindrop, every seed sprouting is chemistry at work in the Earth's biosphere. Understanding environmental chemistry is essential for addressing the planet's greatest challenges: climate change, air and water pollution, ozone depletion, and soil degradation. This chapter explores the chemistry of pollution, its consequences, and potential solutions through the lens of "green chemistry"—designing sustainable, environmentally friendly processes.

Part 1: Air Pollution

Major Air Pollutants

Carbon Dioxide (CO₂)

Source:

• Combustion of fossil fuels (coal, oil, natural gas)
• Industrial processes
• Respiration by organisms

Greenhouse effect:

• CO₂ allows visible light to pass but absorbs infrared (heat) radiation
• Creates a "blanket" trapping heat in atmosphere
• Average global temperature rise: ~1.1°C since 1850

Consequences:

• Rising sea levels
• Changing precipitation patterns
• Ecosystem disruption

Concentration: Pre-industrial: 280 ppm; Current (2026): ~425 ppm

Nitrogen Oxides (NOₓ = NO + NO₂)

Sources:

• Vehicle exhausts
• Coal-fired power plants
• Industrial processes

Reactions:

• At high temperatures (in engines): N₂ + O₂ → 2NO
• In atmosphere: 2NO + O₂ → 2NO₂
• NO₂ is brown, toxic gas

Health effects:

• Respiratory damage
• Reduced oxygen-carrying capacity of blood
• Contributes to photochemical smog

Sulfur Dioxide (SO₂)

Source:

• Burning coal containing sulfur compounds
• Metal ore smelting
• Industrial processes

Reactions:

• S + O₂ → SO₂
• 2SO₂ + O₂ → 2SO₃ (in atmosphere)
• SO₃ + H₂O → H₂SO₄ (sulfuric acid)

Acid rain:

• SO₂ and NOₓ dissolve in rain to form acids
• pH of acid rain: 4–5 (normal rain pH: 5.6)
• Damages forests, corrodes buildings, leaches metals from soil

Carbon Monoxide (CO)

Source:

• Incomplete combustion of fossil fuels (especially in vehicles)
• Fire

Health effect:

• Binds to hemoglobin (Hb) more strongly than O₂
• Prevents oxygen transport
• Causes headaches, fatigue, death at high concentrations

Chemical reaction:

• 2C + O₂ → 2CO (incomplete combustion)

Particulate Matter (PM)

Types:

• PM₂.₅: Fine particles < 2.5 micrometers (penetrate deep into lungs)
• PM₁₀: Coarse particles < 10 micrometers

Sources:

• Diesel exhausts
• Industrial emissions
• Construction dust
• Saharan dust (sometimes blown to India)

Health effects:

• Respiratory diseases
• Lung cancer
• Cardiovascular problems

Ozone (O₃) - Tropospheric

Formation: Photochemical smog in troposphere

• NO₂ + light → NO + O
• O + O₂ → O₃

Health effects:

• Respiratory irritation
• Reduced lung function
• Most dangerous in summer afternoons

Note: Tropospheric ozone (ground-level) is harmful; stratospheric ozone (10–50 km altitude) protects from UV radiation

Photochemical Smog (Ozone Formation)

Process:

1. Vehicles emit NO and volatile organic compounds (VOCs)
2. Sunlight breaks down NO₂: NO₂ + light → NO + O
3. Oxygen atom attacks O₂: O + O₂ → O₃
4. More NO oxidation: NO is oxidized, regenerating O₃

Result: Cycle produces O₃ and other oxidants (PAN, formaldehyde)

Conditions: Worst in summer; sunny, stagnant air; during afternoon

Real example: Delhi air quality crisis; smog during winter (vehicle emissions + stubble burning + weather patterns)

[Memory Hook] NOₓ and VOCs + sunlight → photochemical smog (tropospheric ozone)

Part 2: Water Pollution

Types of Water Pollutants

Hardness

Cause: Dissolved Ca²⁺ and Mg²⁺ ions

Problems:

• Soap doesn't lather (forms insoluble precipitate)
• Scale deposits in kettles and pipes
• Industrial water treatment needed

Sources: Limestone regions; groundwater in mineral-rich areas

Removal: Boiling (temporary hardness), ion exchange (permanent hardness), addition of washing soda (Na₂CO₃)

Salinity

Cause: Dissolved salts (NaCl, MgCl₂, etc.)

Problems:

• Agricultural areas: Salt accumulation in soil reduces crop yield
• Coastal aquifers: Saltwater intrusion from overpumping groundwater

India-specific: High salinity in parts of Gujarat, Rajasthan; coastal areas

Eutrophication

Process:

1. Excess nutrients (N, P) from fertilizers and sewage enter water
2. Algae grow explosively (algal bloom)
3. Algae die and decompose
4. Decomposition consumes dissolved oxygen
5. Water becomes anoxic (oxygen-depleted); fish die

Real example: Lake eutrophication in Kerala; Ganga pollution in Northern India

Pesticides and Herbicides

Examples:

• DDT (dichlorodiphenyltrichloroethane): Persistent; banned in many countries but still used in India for malaria control
• Organophosphates: Insecticides; toxic to nervous system

Problems:

• Bioaccumulation: Concentration increases up food chain (small organism → fish → bird)
• Persistence: Don't degrade; remain in environment for decades

Heavy Metals

Examples: Mercury, Lead, Cadmium, Chromium

Sources:

• Industrial discharge
• Mining operations
• Corrosion of pipes (lead from old plumbing)

Health effects:

• Neurological damage (mercury, lead)
• Kidney failure (cadmium)
• Carcinogenic (chromium, arsenic)

Bioaccumulation: Fish from polluted waters accumulate toxic levels; eating contaminated fish causes poisoning

Plastic and Microplastics

Scale: ~8 million tons of plastic enter oceans annually

Fate:

• Large plastic items break into microplastics (< 5 mm)
• Microplastics consumed by fish and marine life
• Enter human food chain

Real example: India produces ~5–6 million tons of plastic waste annually; most ends up in landfills or oceans

[Memory Hook] Water pollutants: hardness, salinity, eutrophication, pesticides, heavy metals, plastics

Part 3: Ozone Depletion and Protection

Stratospheric Ozone (O₃)

Location: 10–50 km altitude

Function: Absorbs ultraviolet (UV) radiation from sun

• UV-A (320–400 nm): Less harmful; causes tanning
• UV-B (280–320 nm): More harmful; causes skin cancer, cataracts, immune suppression
• UV-C (100–280 nm): Most harmful; absorbed by ozone

Ozone Hole Mechanism

Culprit: Chlorofluorocarbons (CFCs)

Examples:

• CFC-11 (CFCl₃): Refrigerants
• CFC-12 (CF₂Cl₂): Air conditioning, foam blowing
• Halons: Fire extinguishers

Mechanism:

1. CFCs released from human sources
2. Rise to stratosphere (very stable, survive 50–100+ years)
3. UV light breaks C-Cl bond: CFCl₃ + light → CFCl₂ + Cl•
4. Chlorine radical attacks ozone: Cl• + O₃ → ClO• + O₂
5. ClO• regenerates Cl•: ClO• + O → Cl• + O₂ (catalytic cycle)
6. One Cl• can destroy 100,000+ O₃ molecules

Result: Ozone depletion, especially over Antarctica (ozone hole)

Montreal Protocol (1987)

Achievement: International treaty phasing out CFCs

Details:

• Developed countries: 1995 (complete phase-out)
• Developing countries (including India): 2010 (later deadline)

Replacement: HCFCs (hydrofluorochlorocarbons) and HFCs (hydrofluorocarbons)

• Less ozone-depleting but still contribute to climate change
• Now being phased out under Kigali Amendment (2016)

Result: Ozone hole stabilizing; expected recovery by 2070

[Memory Hook] CFCs destroy ozone (catalytic cycle); Montreal Protocol phased them out

Part 4: Greenhouse Gases and Climate Change

Major Greenhouse Gases

Gas	Source	Greenhouse Potential	Atmospheric Lifetime
CO₂	Fossil fuels, deforestation	1 (reference)	100–300 years
CH₄	Livestock, rice paddies, wetlands	25–28	12 years
N₂O	Agriculture (nitrogen fertilizers)	265–310	121 years
SF₆	Electronics manufacturing	23,500	3,200 years

Greenhouse Effect Mechanism

Process:

1. Sunlight reaches Earth; some reflects (albedo), most absorbed
2. Absorbed energy re-radiates as infrared (heat)
3. Greenhouse gases (CO₂, CH₄, etc.) absorb this infrared
4. Heat is trapped; temperature rises
5. Positive feedbacks amplify warming

Real example: Global temperature rise ~1.1°C since 1850; accelerating

India-Specific Concerns

Sources of greenhouse gases:

• Agriculture: 50% of CH₄ from rice paddies and livestock
• Coal mining and combustion: Major CO₂ source
• Deforestation: Reduces CO₂ sink

Consequences:

• Monsoon pattern changes
• Glacier retreat (Himalayan glaciers melting; water shortage downstream)
• Increased extreme weather events

India's commitment: Paris Agreement targets; renewable energy expansion

[Memory Hook] Greenhouse gases trap heat; CO₂ longest-lived; CH₄ most potent per molecule

Part 5: Green Chemistry

Green Chemistry: Designing chemical processes to reduce or eliminate harmful substances.

Principles of Green Chemistry

1. Prevention: Prevent waste rather than treat it after formation
2. Atom economy: Use maximum atoms of starting materials in final product
3. Less hazardous synthesis: Use and generate least toxic chemicals
4. Design safer chemicals: Design chemicals that degrade after use
5. Safer solvents: Avoid toxic solvents; use water when possible
6. Energy efficiency: Minimize energy requirements
7. Renewable feedstocks: Use renewable resources (not fossil fuels)
8. Reduce derivatives: Avoid protection/deprotection steps
9. Catalysis: Use catalysts; reduce byproducts
10. Biodegradable products: Design chemicals that break down harmlessly
11. Pollution prevention: Monitor processes; prevent pollutant formation
12. Reduce accidents: Choose chemicals with low hazard potential

Examples of Green Chemistry

Traditional bleaching: Cl₂ → HCl (toxic gas), environmental hazard

Green alternative: H₂O₂ (hydrogen peroxide) → H₂O + O₂ (benign byproducts)

Benefit: Safer for workers, reduces waste, biodegradable

Part 6: Remediation Technologies

Air Remediation

Electrostatic precipitators: Remove particulate matter

• Particles charged; attracted to oppositely-charged plates
• Particles collect; easily removed

Scrubbers: Remove SO₂, NOₓ

• Exhaust passed through alkaline solution
• SO₂ + Ca(OH)₂ → CaSO₃ (calcium sulfite precipitates)

Catalytic converters: Vehicles

• Convert CO, NOₓ to CO₂, N₂ (less harmful)

Water Remediation

Activated charcoal: Removes organic pollutants, colors, odors

• Porous structure absorbs contaminants

Reverse osmosis: Desalination; removes dissolved salts

• Force water through semipermeable membrane
• Salts left behind

Bioremediation: Use microorganisms to degrade pollutants

• Bacteria consume pesticides, heavy metals (chelation)

Soil Remediation

Phytoremediation: Use plants to absorb pollutants

• Example: Sunflowers accumulate heavy metals; remove soil contamination

Chemical remediation: Add lime (CaCO₃) to acidic soils; neutralizes

Conclusion

Environmental chemistry reveals the hidden cost of modern industrialization. Air pollution, water contamination, ozone depletion, and climate change are all chemical processes. Understanding them opens the door to solutions: green chemistry, renewable energy, and sustainable practices. The future depends on applying chemistry not to exploit the environment but to heal and sustain it.

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23 MCQ Questions

Q1: Acid rain forms when which gases dissolve in atmospheric water?

• A) CO and O₂
• B) SO₂ and NOₓ
• C) CH₄ and N₂
• D) O₃ and CO₂

Q2: The primary component of photochemical smog is:

• A) Carbon monoxide
• B) Sulfur dioxide
• C) Ozone (O₃)
• D) Nitrogen gas

Q3: Photochemical smog formation requires all of the following EXCEPT:

• A) Nitrogen oxides (NOₓ)
• B) Sunlight
• C) Rain
• D) Volatile organic compounds (VOCs)

Q4: Water hardness is caused by:

• A) Dissolved oxygen
• B) Dissolved Ca²⁺ and Mg²⁺ ions
• C) Suspended particles
• D) Dissolved iron

Q5: Eutrophication is a process where:

• A) Water becomes more acidic
• B) Water becomes more basic
• C) Excess nutrients cause algal blooms and oxygen depletion
• D) Salts accumulate in water

Q6: Heavy metals in water can accumulate in organisms through:

• A) Dilution
• B) Bioaccumulation (concentration increases up the food chain)
• C) Evaporation
• D) Photosynthesis

Q7: CFCs (chlorofluorocarbons) deplete stratospheric ozone through:

• A) Reacting with water
• B) Reacting with CO₂
• C) A catalytic cycle where Cl• radicals destroy O₃ molecules
• D) Blocking UV radiation

Q8: The Montreal Protocol (1987) addressed which environmental issue?

• A) Climate change
• B) Water pollution
• C) Ozone depletion
• D) Soil degradation

Q9: One chlorine radical from a CFC can destroy approximately _____ ozone molecules.

• A) 10
• B) 100
• C) 1,000
• D) 100,000+

Q10: Which greenhouse gas has the shortest atmospheric lifetime?

• A) CO₂
• B) CH₄ (methane)
• C) N₂O
• D) SF₆

Q11: The primary source of methane in agriculture is:

• A) Nitrogen fertilizers
• B) Rice paddies and livestock
• C) Soil erosion
• D) Water evaporation

Q12: Global average temperature has risen approximately _____ since 1850.

• A) 0.5°C
• B) 1.1°C
• C) 2.5°C
• D) 5°C

Q13: Which of the following is a principle of green chemistry?

• A) Use maximum hazardous chemicals for efficiency
• B) Prevent waste formation rather than treat waste
• C) Maximize fossil fuel use
• D) Ignore environmental impact

Q14: In green chemistry, H₂O₂ is preferred over Cl₂ for bleaching because:

• A) H₂O₂ is cheaper
• B) H₂O₂ produces benign byproducts (H₂O + O₂) while Cl₂ produces toxic HCl
• C) H₂O₂ is more effective
• D) Cl₂ is not used in bleaching

Q15: Electrostatic precipitators used in air pollution control work by:

• A) Burning pollutants
• B) Charging particles and attracting them to oppositely-charged plates
• C) Dissolving pollutants in water
• D) Converting pollutants to CO₂

Q16: Activated charcoal is used in water treatment to:

• A) Kill bacteria
• B) Remove dissolved salts
• C) Absorb organic pollutants, colors, and odors
• D) Add minerals to water

Q17: Reverse osmosis removes dissolved salts by:

• A) Evaporation
• B) Forcing water through a semipermeable membrane under pressure
• C) Chemical reaction
• D) Bacterial degradation

Q18: Phytoremediation is a technique that uses:

• A) Chemicals to break down pollutants
• B) Bacteria to absorb pollutants
• C) Plants to absorb pollutants from soil
• D) Heat to decompose pollutants

Q19: The pH of acid rain is typically:

• A) 7–8 (neutral to basic)
• B) 5.6–6.5 (slightly acidic)
• C) 4–5 (acidic)
• D) 2–3 (very acidic)

Q20: India's plastic waste generation is approximately _____ million tons annually.

• A) 1–2
• B) 3–4
• C) 5–6
• D) 10+

Q21: Catalytic converters in vehicles primarily convert which harmful gases to less harmful forms?

• A) CO, NOₓ to CO₂, N₂
• B) O₃ to O₂
• C) CH₄ to CO₂
• D) SO₂ to S

Q22: The atom economy of a chemical process measures:

• A) Cost efficiency
• B) Energy efficiency
• C) What percentage of starting material atoms end up in the desired product
• D) Waste reduction percentage

Q23: Which human activity is the primary cause of rising atmospheric CO₂ levels?

• A) Respiration by organisms
• B) Natural volcanic eruptions
• C) Combustion of fossil fuels and deforestation
• D) Ocean evaporation

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Answer Key: 1-B, 2-C, 3-C, 4-B, 5-C, 6-B, 7-C, 8-C, 9-D, 10-B, 11-B, 12-B, 13-B, 14-B, 15-B, 16-C, 17-B, 18-C, 19-C, 20-C, 21-A, 22-C, 23-C