Environmental Chemistry
Subject: Chemistry
Topic: 10
Cambridge Code: 0620 / 0971 / 5070
Air Pollution
Greenhouse Gases
Carbon Dioxide (CO₂):
- Major contributor to climate change
- Produced: Combustion, respiration
- Concentration increasing steadily
- Atmospheric lifetime: 100+ years
Methane (CH₄):
- 25× more effective at trapping heat than CO₂
- Sources: Livestock, landfills, wetlands
- Atmospheric lifetime: ~12 years
Nitrous Oxide (N₂O):
- 300× more effective than CO₂
- Sources: Agriculture, industry
- Ozone-depleting
Acid Rain
Formation:
- SO₂ + O₂ → SO₃ (in atmosphere)
- SO₃ + H₂O → H₂SO₄
- Similar for NOₓ → HNO₃
Causes:
- Fossil fuel combustion
- Industrial emissions
Effects:
- Lowers pH of rain (< 5.6)
- Corrodes buildings
- Damages forests
- Kills aquatic life
Solutions:
- Remove SO₂ from fuel (desulfurization)
- Use catalytic converters (reduce NOₓ)
- Lime neutralization
Particulate Matter
Sources:
- Vehicle emissions
- Industrial processes
- Burning biomass
Effects:
- Respiratory problems
- Reduced visibility
- Climate effects (reflects sunlight)
Ozone Layer Depletion
Ozone (O₃):
- In stratosphere (10-50 km)
- Absorbs UV radiation
- Protects life on Earth
CFCs (Chlorofluorocarbons)
Sources:
- Refrigerants (historically)
- Aerosol propellants
- Cleaning agents
Mechanism:
- CFC rises to stratosphere
- UV breaks C-Cl bond
- Cl radical attacks O₃: Cl + O₃ → ClO + O₂
- Cl regenerated (catalytic destruction)
- One Cl destroys thousands of O₃
Effects:
- Thinning ozone layer
- Increased UV-B radiation
- Skin cancer, cataracts, immune damage
Solutions:
- Montreal Protocol bans CFCs
- HCFCs replacement (less damaging)
- HFCs phase-out underway
Climate Change
Greenhouse Effect
Natural:
- Radiation trapped by atmosphere
- Maintains Earth temperature (~15°C)
- Essential for life
Enhanced:
- Excess CO₂ from human activities
- Temperature rising (1.5°C in 200 years)
- Consequences: Melting ice, rising seas, extreme weather
Mitigation Strategies
Reducing emissions:
- Renewable energy (solar, wind, hydro)
- Energy efficiency
- Carbon capture and storage
Adaptation:
- Building flood defenses
- Developing drought-resistant crops
- Relocating settlements
Water Treatment
Purification Methods
1. Filtration
- Removes particles > 1 μm
- Sand/gravel beds
- Mechanical straining
2. Sedimentation
- Allows particles to settle
- Gravity separation
- Reduces turbidity
3. pH Adjustment
- Add lime to raise pH
- Prevents corrosion
4. Chlorination
- Kills bacteria
- Disinfects water
- Excess chlorine removed
5. Reverse Osmosis
- Semipermeable membrane
- Desalination
- High pressure required
6. Ion Exchange
- Removes dissolved ions
- Softens hard water
- Replaces Ca²⁺/Mg²⁺ with Na⁺
Hard Water
Types:
- Temporary: Ca(HCO₃)₂, Mg(HCO₃)₂ (soluble bicarbonates)
- Removed by boiling (decomposes to insoluble)
- Permanent: CaSO₄, MgSO₄ (soluble sulfates)
- Requires treatment (ion exchange, salt addition)
Problems:
- Scum with soap (doesn't lather well)
- Scale in pipes (reduces flow)
- Wasted soap (hardness consumes soap)
Softening:
- Ion exchange plants
- Add Na₂CO₃ (precipitates Ca²⁺, Mg²⁺)
Green Chemistry
Green chemistry - Design of chemicals/processes to minimize environmental impact
Twelve Principles
- Prevention - Prevent waste rather than treat
- Atom economy - Use maximum atoms in final product
- Safer synthesis - Less hazardous chemicals
- Design benign chemicals - Non-toxic yet effective
- Reduce solvents - Safer or eliminate altogether
- Energy efficiency - Lower temperature/pressure
- Use renewable feedstocks - Not fossil fuels
- Reduce derivatives - Fewer processing steps
- Catalytic - Use catalysts over stoichiometric
- Degradable products - Break down safely
- Real-time analysis - Prevent pollution
- Safer chemistry - Reduce accident potential
Examples
Green solvents:
- Water instead of organic solvents
- CO₂ supercritical fluid
Alternative energy:
- Photochemistry (sunlight)
- Microwaves instead of conventional heating
Sustainable Chemistry
Raw Materials
Renewable sources:
- Plant-based chemicals
- Biomass as feedstock
- Reduce fossil fuel dependence
Waste Reduction
- Develop biodegradable plastics
- Reduce single-use items
- Improve recycling rates
Energy Sources
- Solar-powered synthesis
- Wind power for heat
- Biomass for fuels
Pollution Indicators
Biodiversity
Declining species - Sign of environmental stress
- Polar bears (ice loss)
- Frogs (habitat/pollution)
- Bees (pesticides)
Bioindicators
Organisms sensitive to pollution:
- Lichen indicates air quality
- Aquatic invertebrates indicate water quality
- Bird populations reflect ecosystem health
Recycling
Benefits
- Conserves resources
- Reduces waste
- Saves energy
- Prevents landfill
Challenges
- Contamination
- Economic viability
- Infrastructure needed
- Sorting complexity
Hierarchy
- Reduce - Use less
- Reuse - Use again
- Recycle - Process for new materials
- Recover - Energy recovery
- Dispose - Last resort
Key Points
- Greenhouse gases trap heat (CO₂, CH₄, N₂O)
- Acid rain from SO₂ and NOₓ
- CFCs destroy ozone (Cl radicals)
- Climate change from enhanced greenhouse effect
- Water treatment: Filtration, sedimentation, disinfection
- Hard water reduced by ion exchange
- Green chemistry minimizes environmental impact
- Sustainability requires renewable sources and waste reduction
Practice Questions
- Explain formation of acid rain
- Describe ozone depletion mechanism
- Outline water treatment steps
- Explain hard water and softening
- Apply green chemistry principles
- Design sustainable process
- Calculate carbon footprint reduction
Revision Tips
- Know causes of air pollution
- Understand climate change mechanism
- Learn water treatment methods
- Know hard/soft water difference
- Understand green chemistry principles
- Know ozone layer importance
- Sustainability concept
- Pollution indicators