Environmental pollution and control

What you'll learn

Environmental chemistry applies chemical understanding to the problems of pollution and its control. Small island developing states in the Caribbean are especially vulnerable: their economies depend on clean seas, healthy reefs and tourism, and they face sea-level rise, coral bleaching and ocean acidification driven by global emissions. This guide covers the major air pollutants — carbon monoxide, oxides of nitrogen and sulfur, particulates, ground-level ozone and volatile organic compounds — and their sources, effects and control; acid rain; the greenhouse effect and climate change; ozone depletion in the stratosphere; water and land pollution from fertilisers, pesticides, heavy metals, sewage, oil and solid waste; eutrophication; and methods of monitoring and controlling pollution, including catalytic converters, scrubbing, recycling and international agreements.

CAPE questions ask you to identify pollutants and their sources, write equations for their formation and removal, explain environmental effects using chemistry, and evaluate control strategies. Specific chemical detail distinguishes strong answers.

Key terms and definitions

Pollutant — A substance released into the environment that causes harm to living things or the environment.

Primary pollutant — A pollutant released directly from a source.

Secondary pollutant — A pollutant formed by reactions of primary pollutants in the atmosphere (e.g. ground-level ozone).

Acid rain — Rain with a pH below about 5.6, caused by dissolved sulfur and nitrogen oxides.

Greenhouse effect — The warming of the Earth's surface due to greenhouse gases absorbing and re-emitting infrared radiation.

Enhanced greenhouse effect — Additional warming caused by human emissions of greenhouse gases.

Ozone layer — The region of the stratosphere containing ozone, which absorbs harmful ultraviolet radiation.

Photochemical smog — Air pollution formed when sunlight drives reactions between nitrogen oxides and volatile organic compounds.

Eutrophication — Excessive nutrient enrichment of water, leading to algal blooms and oxygen depletion.

Biomagnification — The increase in concentration of a persistent pollutant at higher levels of a food chain.

Particulates — Tiny solid or liquid particles suspended in the air.

Core concepts

Carbon monoxide

Source: incomplete combustion of fuels in vehicle engines, generators and stoves: 2C₈H₁₈ + 17O₂ → 16CO + 18H₂O. Effect: CO binds to haemoglobin much more strongly than oxygen, forming carboxyhaemoglobin and reducing oxygen transport; it causes headaches, dizziness and, at high levels, death. Control: adequate ventilation, engine tuning, and catalytic converters: 2CO + O₂ → 2CO₂.

Oxides of nitrogen (NOₓ)

Source: at the high temperatures in engines and power stations, nitrogen and oxygen from the air combine: N₂ + O₂ → 2NO; 2NO + O₂ → 2NO₂. Effects: respiratory problems; formation of acid rain (NO₂ dissolves to form nitric acid); formation of photochemical smog and ground-level ozone. Control: catalytic converters reduce NO using CO or unburnt hydrocarbons: 2NO + 2CO → N₂ + 2CO₂.

Sulfur dioxide

Source: burning fossil fuels containing sulfur impurities (especially coal and heavy fuel oil); smelting of sulfide ores; volcanic eruptions (such as those of Soufrière Hills in Montserrat and La Soufrière in St Vincent). Effects: respiratory irritation; acid rain: 2SO₂ + O₂ → 2SO₃; SO₃ + H₂O → H₂SO₄. Control: desulfurisation of fuels before burning; flue-gas desulfurisation — passing waste gases through a slurry of limestone or lime: CaCO₃ + SO₂ → CaSO₃ + CO₂, and CaSO₃ is oxidised to calcium sulfate (gypsum).

Acid rain

Natural rain is slightly acidic (about pH 5.6) because of dissolved carbon dioxide. Acid rain is more acidic because of dissolved sulfuric and nitric acids.

Effects:

• damages buildings and statues made of limestone and marble: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂;
• corrodes metal structures;
• acidifies lakes and rivers, harming fish and aquatic life;
• leaches nutrients and releases toxic aluminium ions from soils, damaging forests.

Control: reduce SO₂ and NOₓ emissions; add lime (liming) to acidified lakes and soils.

Particulates

Sources: diesel engines, burning of waste and sugar-cane fields, construction, volcanic ash and Saharan dust, which regularly blows across the Atlantic to the Caribbean. Effects: respiratory and heart disease; reduced visibility; soiling of buildings. Control: particulate filters on diesel engines, electrostatic precipitators in factories, reducing open burning.

Ground-level ozone and photochemical smog

In sunlight, NO₂ splits: NO₂ → NO + O; the oxygen atom reacts with O₂ to form ozone: O + O₂ → O₃. Ozone and volatile organic compounds form photochemical smog, irritating eyes and lungs and damaging plants. Ground-level ozone is a harmful secondary pollutant, unlike protective stratospheric ozone.

The greenhouse effect and climate change

The Sun's radiation passes through the atmosphere and warms the Earth's surface, which emits infrared radiation. Greenhouse gases — carbon dioxide, methane, water vapour, nitrous oxide and CFCs — absorb this infrared radiation because their bond vibrations change their dipole moments, and re-emit it in all directions, warming the lower atmosphere.

Human activities — burning fossil fuels, deforestation, agriculture (methane from cattle and rice), landfill — have increased greenhouse gas concentrations, causing global warming.

Effects relevant to the Caribbean:

• sea-level rise threatening low-lying coastal communities and infrastructure;
• more intense hurricanes;
• coral bleaching as sea temperatures rise;
• ocean acidification as the sea absorbs CO₂: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, reducing the carbonate available for corals and shellfish to build their skeletons;
• changes in rainfall affecting agriculture and water supplies.

Mitigation: renewable energy, energy efficiency, reforestation, carbon capture and storage, and international agreements such as the Paris Agreement.

Depletion of the ozone layer

Stratospheric ozone absorbs harmful UV radiation. CFCs, once used in refrigerators, air conditioners and aerosols, are stable in the lower atmosphere but break down in the stratosphere under UV light to release chlorine radicals:

CCl₂F₂ → CClF₂• + Cl• Cl• + O₃ → ClO• + O₂ ClO• + O → Cl• + O₂

The chlorine radical is regenerated, so one radical can destroy thousands of ozone molecules. Thinning of the ozone layer increases UV radiation, causing skin cancer, cataracts and damage to crops and plankton. The Montreal Protocol (1987) phased out CFCs, and the ozone layer is slowly recovering. Replacements such as HFCs do not damage ozone but are powerful greenhouse gases.

Water and land pollution

• Fertilisers: nitrates and phosphates washed into rivers and coastal waters cause eutrophication: algae grow rapidly, block light and die; bacteria decomposing the dead algae use up dissolved oxygen, killing fish. Nitrates in drinking water also pose health risks to infants.
• Pesticides: persistent pesticides such as DDT accumulate in fatty tissues and biomagnify up food chains; in the Caribbean, the past use of chlordecone on banana plantations in Martinique and Guadeloupe has left long-lasting contamination of soils and waters.
• Heavy metals (lead, mercury, cadmium) from industry, batteries and mining are toxic and accumulate in organisms.
• Sewage introduces pathogens and nutrients into coastal waters, threatening health and reefs.
• Oil spills smother coastlines and harm marine life.
• Solid waste and plastics pollute beaches and harm wildlife; open dumping can contaminate groundwater with leachate.
• Thermal pollution from power station cooling water reduces dissolved oxygen.

Monitoring and controlling pollution

Monitoring: air quality sensors, measuring pH and dissolved oxygen in water, biochemical oxygen demand (BOD), heavy metal analysis by atomic absorption spectroscopy, GLC for pesticide residues.

Control strategies: legislation and emission limits; catalytic converters and scrubbers; sewage treatment; integrated pest management; proper landfill design with liners and leachate collection; recycling and reduction of single-use plastics; public education.

Worked examples

Example 1: Catalytic converter reactions

Write equations for two reactions in a catalytic converter.

2CO + 2NO → 2CO₂ + N₂ and 2CO + O₂ → 2CO₂ (also: hydrocarbons + O₂ → CO₂ + H₂O).

Example 2: Flue-gas desulfurisation

Explain how limestone removes SO₂ from power station emissions.

Flue gases pass through a slurry of limestone; SO₂ reacts with calcium carbonate: CaCO₃ + SO₂ → CaSO₃ + CO₂. The calcium sulfite is oxidised to calcium sulfate (gypsum), which can be sold for making plasterboard.

Example 3: Eutrophication

Describe how fertiliser run-off can lead to the death of fish in a lagoon. (4 marks)

Nitrates and phosphates from fertilisers enter the water and cause rapid algal growth (1). The algae block sunlight, so plants below die (1). Bacteria decompose the dead plants and algae, using up dissolved oxygen (1). The oxygen level falls so low that fish die (1).

Example 4: Ozone depletion

Explain why one chlorine radical can destroy many ozone molecules.

The chlorine radical reacts with ozone to form ClO• and O₂, then ClO• reacts with an oxygen atom to regenerate Cl•. Because the radical is regenerated, the process is a chain reaction, with the chlorine acting as a catalyst.

Example 5: Sulfur dioxide from coal

A power station burns 1000 kg of coal containing 2.0% sulfur by mass. Calculate the mass of sulfur dioxide produced. (Aᵣ S = 32; Mᵣ SO₂ = 64)

Mass of sulfur = 2.0% of 1000 kg = 20 kg. S + O₂ → SO₂, and 32 g of sulfur forms 64 g of SO₂. Mass of SO₂ = 20 × 64 ÷ 32 = 40 kg.

This shows why removing sulfur from fuels, or scrubbing flue gases, makes such a large difference to acid rain.

Example 6: Limitations of catalytic converters

Evaluate the role of catalytic converters in reducing vehicle pollution.

Catalytic converters greatly reduce emissions of carbon monoxide, nitrogen oxides and unburnt hydrocarbons, improving air quality in busy cities such as Kingston, Port of Spain and Bridgetown. However, they do not reduce carbon dioxide (they actually convert CO to CO₂), they only work efficiently once warmed up (so short journeys produce more pollution), they are poisoned by lead and damaged by sulfur, and they contain expensive, scarce metals such as platinum and rhodium. They are therefore an important but partial solution, alongside cleaner fuels, efficient engines and public transport.

Example 7: Ocean acidification and coral reefs

Explain, with equations, how increasing atmospheric CO₂ threatens Caribbean coral reefs.

More CO₂ dissolves in seawater, forming carbonic acid, which releases H⁺: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ The H⁺ ions react with carbonate ions: H⁺ + CO₃²⁻ → HCO₃⁻, reducing the concentration of CO₃²⁻ available for corals and shellfish to build their calcium carbonate skeletons. Combined with coral bleaching caused by warmer water, this weakens reefs that protect coastlines from storms and support fisheries and tourism.

Example 8: Biomagnification

Explain why a persistent pesticide can harm fish-eating birds even when its concentration in seawater is very low.

The pesticide is not broken down and is stored in fatty tissue rather than excreted. Small organisms absorb it from the water; fish eat large numbers of these organisms, and birds eat many fish. At each step, the pesticide accumulates, so its concentration becomes much higher at the top of the food chain than in the water, causing harm such as thin eggshells or poisoning.

Pollutants at a glance

Common mistakes and how to avoid them

• Confusing the greenhouse effect with ozone depletion — different gases, different mechanisms and effects.
• Calling ground-level ozone beneficial — only stratospheric ozone is protective.
• Saying CO₂ causes acid rain — SO₂ and NOₓ are the main causes.
• Describing eutrophication without the role of bacteria using up oxygen.
• Writing unbalanced equations for pollution reactions.

How this links to your Internal Assessment

Environmental SBAs are highly relevant to Caribbean students. Possible investigations include measuring the pH and nitrate/phosphate levels of local rivers or coastal waters upstream and downstream of farms or settlements, comparing dissolved oxygen in different water bodies, investigating the effect of acid solutions on limestone and marble chips, or surveying particulate deposition using sticky slides around a school. Strong reports use reliable sampling methods, repeat measurements, compare results with national or WHO standards, and evaluate limitations such as weather variations and sampling times.

Exam technique for Environmental pollution and control

• For each pollutant, give source, equation for formation, effect and control with an equation where possible.
• Distinguish stratospheric and ground-level ozone and the greenhouse effect from ozone depletion.
• Use Caribbean examples — Saharan dust, coral bleaching, sea-level rise, chlordecone, volcanic SO₂ — to strengthen evaluations.
• In evaluation questions, weigh effectiveness, cost and feasibility of control measures.

Quick revision summary

• CO (incomplete combustion; binds haemoglobin), NOₓ (high-temperature combustion; acid rain, smog), SO₂ (sulfur in fuels; acid rain), particulates (diesel, burning, Saharan dust).
• Catalytic converters remove CO, NOₓ and hydrocarbons; flue-gas desulfurisation uses limestone.
• Acid rain damages limestone, metals, lakes and forests.
• Greenhouse gases absorb IR; climate change causes sea-level rise, stronger hurricanes, coral bleaching and ocean acidification.
• CFCs release Cl• radicals that catalytically destroy stratospheric ozone; Montreal Protocol.
• Water/land pollution: fertilisers (eutrophication), pesticides (biomagnification), heavy metals, sewage, oil, plastics.
• Control through legislation, technology, treatment, recycling and international cooperation.