Learning Objectives
- Understand environmental pollution: air, water, and soil
- Learn about major air pollutants and their effects
- Study ozone layer depletion and greenhouse effect
- Understand water pollution parameters (BOD, COD) and treatment
- Learn about green chemistry principles
Key Concepts
Air Pollution
Major pollutants: CO, CO₂, SO₂, NO₂, particulate matter (PM), hydrocarbons, lead compounds.
Particulate pollutants: Smoke, dust, mist, fumes, smog. PM₂.₅ and PM₁₀ are health hazards.
Photochemical smog: Formed by reaction of NO₂ and hydrocarbons in sunlight. Contains O₃, PAN (peroxyacetyl nitrate), formaldehyde. Causes eye irritation, respiratory problems, plant damage. Favoured by warm, sunny, calm conditions.
Classical smog: Formed in cold, humid conditions. Contains SO₂ and smoke (reducing smog). Example: London smog.
Acid rain: pH < 5.6. Caused by SO₂ and NO₂: SO₂ + H₂O → H₂SO₃ → H₂SO₄; 2NO₂ + H₂O → HNO₃ + HNO₂. Damages buildings (marble: CaCO₃ + H₂SO₄ → CaSO₄), kills aquatic life, harms vegetation.
Ozone Layer Depletion
Ozone layer in the stratosphere (15-35 km) absorbs harmful UV radiation.
Depletion by CFCs: CFCl₃ → CFCl₂ + Cl· (UV light); Cl· + O₃ → ClO· + O₂; ClO· + O → Cl· + O₂. One Cl atom can destroy ~100,000 O₃ molecules.
Effects: increased UV → skin cancer, cataracts, immune suppression, crop damage.
Montreal Protocol: International agreement to phase out CFCs and other ozone-depleting substances.
Greenhouse Effect and Global Warming
Greenhouse gases: CO₂, CH₄, N₂O, CFCs, water vapour. They trap infrared radiation from Earth's surface.
Natural greenhouse effect keeps Earth warm (average 15°C instead of -18°C). Enhanced greenhouse effect from human activities causes global warming.
Effects: rising sea levels, climate change, extreme weather events, glacier melting.
Water Pollution
Causes: Industrial effluents, sewage, agricultural runoff (pesticides, fertilisers), heavy metals.
BOD (Biochemical Oxygen Demand): Amount of O₂ consumed by bacteria to decompose organic matter. Higher BOD = more pollution.
COD (Chemical Oxygen Demand): Total O₂ needed to oxidise all organic matter. COD > BOD always.
Eutrophication: Excess nutrients (N, P) cause algal blooms → oxygen depletion → death of aquatic life.
Heavy metal pollution: Hg, Pb, Cd, As. Biomagnification increases concentration up the food chain. Minamata disease (Hg), Itai-Itai disease (Cd).
Soil Pollution
Pesticides: Insecticides (DDT, malathion), herbicides, fungicides. DDT is non-biodegradable and bioaccumulates.
Industrial waste: Heavy metals, radioactive waste.
Green Chemistry
Design of chemical processes to reduce/eliminate hazardous substances.
Principles: Prevent waste, atom economy, use safer solvents, use renewable feedstocks, design for degradation.
Examples: Use of H₂O₂ for bleaching instead of Cl₂, use of supercritical CO₂ as solvent.
Summary
Environmental chemistry addresses the chemical aspects of pollution. Air pollution involves gaseous and particulate pollutants causing smog, acid rain, and ozone depletion. The greenhouse effect is intensified by human activities. Water pollution is measured by BOD and COD. Soil pollution from pesticides and industrial waste threatens ecosystems. Green chemistry offers sustainable solutions by designing cleaner processes.
Important Terms
- BOD: Oxygen needed by bacteria to decompose organic matter
- Eutrophication: Nutrient enrichment causing algal bloom and oxygen depletion
- Photochemical Smog: Smog from NO₂ and VOC in sunlight (O₃, PAN)
- Acid Rain: Rain with pH < 5.6 due to SO₂ and NOₓ
- Biomagnification: Increasing concentration of toxins up the food chain
- Green Chemistry: Designing processes to minimise pollution and waste
Quick Revision
- Air pollutants: CO, SO₂, NOₓ, O₃, PM, hydrocarbons
- Photochemical smog: O₃ + PAN (warm, sunny); Classical smog: SO₂ + smoke (cold, humid)
- Acid rain: pH < 5.6; caused by SO₂ and NO₂
- Ozone depletion: CFCs → Cl radicals → O₃ destruction
- BOD: biological O₂ demand; COD: chemical O₂ demand; COD > BOD
- Green chemistry: prevent waste, atom economy, safer solvents