Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental chemistry is the branch of chemistry that studies the chemical and biochemical processes occurring in the environment. It focuses on pollutants — their sources, reactions, transport, and effects on living organisms and ecosystems. For NEET, the key areas are atmospheric pollution, water pollution, and green chemistry.

The Atmosphere — Layered Structure:

Layer	Altitude	Temperature Trend	Key Features
Troposphere	0–12 km	Decreases with altitude (~6.5°C/km)	Weather, clouds, most air pollution
Stratosphere	12–50 km	Increases (due to ozone)	Ozone layer; jet aircraft fly here
Mesosphere	50–80 km	Decreases	Coldest layer (~–90°C)
Thermosphere	80–700 km	Increases dramatically	Aurora; ISS orbits here
Exosphere	700+ km	–	Gradual transition to space

Greenhouse Effect — The Earth’s Blanket:

The greenhouse effect is a natural phenomenon essential for life on Earth. Without it, Earth’s average temperature would be ~–18°C instead of the actual ~+15°C. The gases that trap heat are:

• Water vapour (H₂O): Most important greenhouse gas (~60% of natural greenhouse effect)
• Carbon dioxide (CO₂): ~26% of enhanced greenhouse effect; level ~420 ppm (2024)
• Methane (CH₄): ~80× more potent per molecule than CO₂; level ~1.9 ppm
• Nitrous oxide (N₂O): ~273× more potent per molecule than CO₂
• Ozone (O₃): Both greenhouse gas and UV shield

⚡ Exam Tips:

• CO₂ is the primary greenhouse gas responsible for climate change — know its sources and sinks
• The ozone hole is in the stratosphere (different from greenhouse effect!) — caused by CFCs
• BOD measures organic water pollution — higher BOD = more organic waste = more bacterial decomposition = lower dissolved oxygen

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Air Pollution — Pollutants and Their Effects:

Air pollutants are classified as primary (emitted directly) or secondary (formed in the atmosphere).

Primary Pollutants:

• Carbon monoxide (CO): Colourless, odourless; binds to haemoglobin (Hb) with 200× affinity of O₂ → forms carboxyhaemoglobin (COHb) → reduces O₂-carrying capacity → headaches, confusion, death at >50% COHb
• Sulphur dioxide (SO₂): Irritates respiratory tract; dissolves in rain → acid rain; major sources: coal combustion, metal smelting
• Nitrogen oxides (NO, NO₂): NO is colourless; NO₂ is brownish-red with pungent odour; cause respiratory irritation, smog formation
• Particulate matter (PM2.5, PM10): Fine particles penetrate deep into lungs; PM2.5 causes cardiovascular disease, lung cancer

Secondary Pollutants:

• Photochemical smog (summer smog): Formed by UV-driven reactions between NOₓ and VOCs → ozone, PAN (peroxyacetyl nitrate), aldehydes
• Acid rain: SO₂ + O₂ → SO₃ → H₂SO₄; NO₂ → HNO₃; pH of normal rain ~5.6 (slightly acidic due to CO₂); acid rain pH can fall to 3–4

Smog Types:

Smog Type	Conditions	Primary Pollutants	Effects
Classical/London	Winter, fog, cold	Smoke + SO₂	Bronchitis, “pea-souper” fog
Photochemical/LA-type	Summer, sunny, warm	NOₓ + VOCs + sunlight	Ozone, eye irritation, crop damage

Ozone Layer Depletion:

The ozone layer (15–30 km altitude in stratosphere) absorbs 98% of incoming UV-B (280–315 nm) and UV-C (100–280 nm). UV radiation damages:

• DNA (thymine dimers → skin cancer)
• Proteins and lipids
• Immune system

Ozone Depletion Mechanism: CFCs (chlorofluorocarbons, e.g., CFCl₃ — freon-11, CCl₂F₂ — freon-12) are stable in troposphere but broken down by UV in stratosphere:

1. CFCl₃ → UV → Cl· + CFCl₂
2. Cl· + O₃ → ClO· + O₂
3. ClO· + O· → Cl· + O₂

One chlorine atom can destroy ~100,000 ozone molecules before being sequestered. This chain reaction makes CFCs extremely potent ozone depleters.

Other depleters: Halons, methyl bromide, CCl₄, CH₃CCl₃.

Water Pollution — Types and Sources:

Pollutant	Source	Effect
Organic waste (sewage, agricultural runoff)	Microbial decomposition consumes DO	Fish kills; anaerobic conditions produce H₂S, CH₄
Heavy metals (Hg, Pb, Cd, Cr)	Industrial discharge	Bioaccumulation; Minamata disease (Hg), lead poisoning
Pesticides (DDT, endosulfan)	Agriculture	Biomagnification; DDT → eggshell thinning in birds
Nutrients (N, P)	Fertiliser runoff	Eutrophication → algal bloom → oxygen depletion
Thermal pollution	Power plant cooling water	Reduces dissolved O₂; affects aquatic life
Radioactive waste	Nuclear industry	Genetic mutations; bioaccumulation in food chain

BOD, COD, and DO — Key Definitions:

• DO (Dissolved Oxygen): O₂ dissolved in water; 8–10 ppm at 25°C for healthy water; measured by Winkler method
• BOD (Biochemical Oxygen Demand): Amount of O₂ consumed by bacteria decomposing organic matter in 5 days at 20°C; indicator of organic pollution
• COD (Chemical Oxygen Demand): Total O₂ required to chemically oxidise all organic matter (including non-biodegradable); always ≥ BOD

Eutrophication: Excess nutrients (especially phosphate from detergents and nitrate from fertilisers) cause:

1. Algal bloom on water surface
2. Sunlight blocked → submerged plants die
3. Algae die → organic matter increases
4. Bacteria consume O₂ during decomposition
5. DO drops to near zero → fish kills
6. Anaerobic conditions → H₂S production → foul smell

Green Chemistry — The 12 Principles:

Green chemistry aims to design chemical products and processes that reduce or eliminate hazardous substances. The 12 principles include:

1. Prevention (waste is easier to prevent than treat)
2. Atom economy (maximise incorporation of all atoms into final product)
3. Less hazardous synthesis
4. Designing safer chemicals
5. Safer solvents and auxiliaries
6. Energy efficiency (minimise energy requirements)
7. Use of renewable feedstocks
8. Reduce derivatives
9. Catalysis (catalytic reagents superior to stoichiometric)
10. Design for degradation
11. Real-time analysis for pollution prevention
12. Inherently safer chemistry for accident prevention

⚡ Common Mistakes:

• Students confuse the ozone hole (stratosphere) with greenhouse effect (troposphere) — they are different layers with different chemistries
• London smog is classical (SO₂ + particulate) while Los Angeles smog is photochemical (ozone + PAN)
• BOD is not the same as DO — BOD is the demand for O₂; DO is the supply
• Ozone in stratosphere is good (UV shield); ozone in troposphere is a pollutant (respiratory irritant)

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🔴 Extended — Deep Dive (exam-level mastery)

For students preparing for top-rank selection.

Atmospheric Chemistry — Detailed Reaction Mechanisms:

Chapman Mechanism (Ozone Formation in Stratosphere):

1. O₂ + UV (λ < 240 nm) → 2 O·
2. O· + O₂ + M → O₃ + M (M = third body, absorbs excess energy)
3. O₃ + UV (λ < 1180 nm) → O₂ + O·
4. O· + O₃ → 2 O₂

Net: 2 O₃ → 3 O₂ (ozone is continuously formed and destroyed in dynamic equilibrium)

Catalytic Cycles Accelerating Ozone Destruction: The simple Cl· cycle above is supplemented by:

• NOₓ cycle: NO + O₃ → NO₂ + O₂; NO₂ + O → NO + O₂ — net: O₃ + O → 2 O₂ (one NO destroys many O₃)
• HOₓ cycle: OH· + O₃ → HO₂· + O₂; HO₂· + O → OH· + O₂ — net same
• BrOₓ cycle: Similar to HOₓ but with bromine; Br is ~40× more efficient per atom than Cl

These cycles explain why relatively small amounts of CFCs can cause significant ozone depletion.

Ozone Hole Formation: Antarctica’s ozone hole (discovered 1985 by British Antarctic Survey) forms because:

1. Winter polar vortex isolates air above Antarctica
2. Temperatures drop to –80°C → polar stratospheric clouds (PSCs) form
3. PSC surfaces provide heterogeneous reactions: ClONO₂ + HCl → Cl₂ + HNO₃ (solid); ClONO₂ + H₂O → HOCl + HNO₃
4. Spring (September) — sunlight returns → Cl₂, HOCl, ClO photolyse → massive Cl· and ClO·
5. Ozone destruction rates become catastrophic

London Smog (1952) — Chemistry: The Great Smog of London (December 1952) killed ~4,000 people initially and ~12,000 total:

• Cold weather + high pressure → temperature inversion (pollution trapped below inversion layer)
• SO₂ emitted from coal burning dissolved in fog droplets → H₂SO₃ aerosol
• Particulates (soot) from domestic coal combustion
• Combined effect: respiratory infection rates exploded; bronchitis, asthma deaths

Photochemical Smog — Peroxyacyl Nitrate (PAN): PAN (CH₃COOOONO₂) is a powerful lachrymator (causes eye watering) and phytotoxin (kills plants):

1. NO₂ (from vehicle exhaust) + hν → NO + O· (O ¹D state)
2. O ¹D + H₂O → 2 OH·
3. OH· + RCHO (aldehydes from VOC oxidation) → RCO· + H₂O
4. RCO· + O₂ → RCOOO· (peroxyacyl radical)
5. RCOOO· + NO₂ → RCOOOONO₂ (PAN)

Greenhouse Gas Radiative Forcing: The enhanced greenhouse effect from human activities:

• CO₂: +2.16 W/m² (IPCC AR6, 2021)
• CH₄: +0.54 W/m²
• N₂O: +0.21 W/m²
• CFCs: +0.32 W/m²
• Total anthropogenic: ~3 W/m²

The radiative forcing from human activities has increased Earth’s energy budget by ~3 W/m² — this extra energy drives global warming.

Carbon Cycle — Natural and Perturbed:

Natural carbon cycle:

• Photosynthesis: ~120 Gt C/year absorbed by land + ocean
• Respiration: ~119 Gt C/year released
• Net natural balance ≈ 0

Human perturbation: Fossil fuel combustion (~10 Gt C/year) + deforestation (~4 Gt C/year) → total ~11 Gt C/year emitted. Ocean absorbs ~2.5 Gt C/year; land absorbs ~3 Gt C/year; remaining ~5.5 Gt C/year accumulates in atmosphere → atmospheric CO₂ rising ~2.4 ppm/year.

Soil and Water Chemistry:

Hard Water:

• Temporary hardness: Ca(HCO₃)₂, Mg(HCO₃)₂ — removed by boiling or adding Ca(OH)₂
• Permanent hardness: CaSO₄, MgSO₄, CaCl₂ — removed by ion exchange (zeolite process) or washing soda (Na₂CO₃)

Treatment equations:

• Zeolite process: Ca²⁺ + Na₂Z → 2 Na⁺ + CaZ (regenerate with NaCl)
• Soda-lime process: Ca(HCO₃)₂ + Ca(OH)₂ → 2 CaCO₃↓ + 2 H₂O

Heavy Metal Pollution:

• Mercury: Minamata disease (methylmercury in fish) — neurological damage; bioaccumulates (trophic magnification)
• Lead: Inhibits δ-aminolevulinic acid dehydratase (ALA dehydratase) in haemoglobin synthesis → anaemia; neurological damage in children
• Arsenic: Groundwater contamination in West Bengal, Bangladesh; causes skin lesions, cancer
• Cadmium: Itai-itai disease (Japan) — bone demineralisation; accumulates in kidneys

NEET High-Yield Pattern:

• Ozone hole is in the stratosphere; greenhouse effect is in the troposphere
• CFCs cause ozone depletion; CO₂ causes global warming
• London smog: classical (SO₂ + smoke + fog); LA smog: photochemical (ozone + PAN)
• BOD measures organic pollution; COD measures total oxidisable matter
• Temporary hardness removed by boiling; permanent hardness requires chemical treatment
• Green chemistry principle 2 is atom economy: maximise atoms in final product