Environmental Chemistry and Pollution

Environmental Chemistry and Pollution

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental chemistry studies the chemical composition, reactions, and behaviour of substances in air, water, and soil, with emphasis on pollution sources, effects, and control. A pollutant is any substance released into the environment in quantities that harm living organisms or disrupt ecosystems, while a contaminant is simply a substance present where it should not be (not all contaminants are pollutants).

The must-know equations for WAEC are the acid rain formation steps: SO₂ + H₂O → H₂SO₃, 2SO₂ + O₂ → 2SO₃, SO₃ + H₂O → H₂SO₄, and 4NO₂ + 2H₂O + O₂ → 4HNO₃. For water analysis, memorise pH = -log[H⁺] and the BOD formula: BOD (mg/L) = (D₁ – D₂) / V, where D₁ and D₂ are initial and final dissolved oxygen, and V is the sample volume. High-yield pointers: greenhouse gases are CO₂, CH₄, N₂O, and CFCs; ozone depletion is caused by CFCs (not CO₂); eutrophication is driven by excess nitrates and phosphates from fertilisers, not by pesticides.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Air Pollution

Major air pollutants include SO₂ (from burning fossil fuels and smelting sulphide ores), NOₓ (from vehicle engines and lightning), CO (incomplete combustion), CO₂, particulates, and CFCs (from old refrigerators and aerosols). Acid rain forms when SO₂ and NO₂ react with atmospheric water and oxygen:

• 2SO₂ + O₂ → 2SO₃
• SO₃ + H₂O → H₂SO₄ (strong acid, pH often 3–4)
• 4NO₂ + 2H₂O + O₂ → 4HNO₃

Acid rain corrodes iron roofing sheets, damages limestone buildings (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), lowers soil pH, and acidifies lakes, killing fish.

Greenhouse Effect vs Global Warming

The greenhouse effect is the natural trapping of outgoing infrared radiation by CO₂, H₂O vapour, CH₄, N₂O, and CFCs. Global warming is the enhanced effect caused by human activities (burning fossil fuels, deforestation, livestock farming) increasing these gases. This traps more heat, melts polar ice, and raises sea levels.

Ozone Layer Depletion

CFCs (e.g. CCl₂F₂) released from aerosols and refrigerants diffuse into the stratosphere, where UV light liberates Cl• radicals that catalytically destroy ozone: Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂. One chlorine atom can destroy up to 100,000 ozone molecules.

Water and Soil Pollution

BOD (Biochemical Oxygen Demand) measures the oxygen consumed by bacteria degrading organic matter in 5 days at 20 °C; COD measures the total oxidisable matter (using K₂Cr₂O₇). High BOD/COD = polluted water. Eutrophication occurs when nitrate/phosphate fertiliser runoff causes algal blooms; the dead algae are decomposed by aerobic bacteria, depleting oxygen and killing aquatic life.

Sewage Treatment

Stage	Process	Removes
Primary	Screening, sedimentation	Solids, floating debris
Secondary	Biological (activated sludge, trickling filter)	Dissolved organic matter
Tertiary	Chlorination, filtration, nutrient removal	Pathogens, nitrates, phosphates

Common WAEC Traps

• Confusing BOD with COD: BOD = biodegradable only; COD = biodegradable + non-biodegradable.
• Saying ozone depletion is caused by CO₂ — it is CFCs.
• Saying eutrophication is caused by pesticides — it is excess fertiliser nitrates/phosphates.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Bioaccumulation and Biomagnification

Bioaccumulation is the build-up of a substance (e.g. Hg, DDT) in an organism over time because it is excreted slowly. Biomagnification is the increase in concentration along a food chain: plankton (0.003 ppm) → small fish (0.04 ppm) → large fish (2 ppm) → fish-eating birds (25 ppm of DDT), which then suffer eggshell thinning.

Photochemical Smog

Formed when NOₓ and unburnt hydrocarbons react under sunlight to produce ozone (O₃), peroxyacetyl nitrate (PAN), aldehydes, and acrolein. It causes eye irritation, damages rubber, and reduces visibility. Reducing emissions from vehicles and using catalytic converters (Pt, Pd, Rh catalysts that oxidise CO and unburnt HC, and reduce NOₓ) are key controls.

Soil Pollution and Control

Heavy metals (Pb from leaded petrol, Hg from industrial waste, Cd from batteries) are non-biodegradable, persist for decades, and enter food chains. Pesticides like DDT are persistent organic pollutants (POPs). Control measures include: scrubbers (CaCO₃ slurry absorbs SO₂), electrostatic precipitators for particulates, unleaded petrol, recycling, afforestation, and use of biodegradable detergents.

Green Chemistry Principles

The 12 principles include prevention of waste over treatment, maximising atom economy, designing safer chemicals and products, using renewable feedstocks, and designing for degradation so products break down into harmless substances at end-of-life.

Worked Example

A 250 cm³ water sample had initial dissolved oxygen D₁ = 8.0 mg/L and after 5 days D₂ = 4.0 mg/L. Calculate the BOD.

BOD = (D₁ – D₂)/V = (8.0 – 4.0) / 0.250 = 16 mg/L

This exceeds the WHO safe limit of 5 mg/L — the water is heavily polluted.

Practice Prompts

1. Write balanced equations showing how NO₂ contributes to both acid rain and photochemical smog formation.
2. Explain why DDT concentrations increase from 0.003 ppm in water to 25 ppm in fish-eating birds, and state two environmental consequences.

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