Environmental Chemistry

Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Environmental Chemistry studies chemical species in air, water, and soil — their sources, reactions, transport, effects, and fates.

Must-know formulas:

• pH = −log[H⁺] — defines acidity of rain, water bodies
• ppm = (mass of solute ÷ mass of solution) × 10⁶ — concentration unit for pollutants
• BOD = (DOᵢ − DOₓ) × dilution factor — measures biodegradable organic pollution (5-day test)

Key definitions to memorise:

• Pollutant: harmful chemical introduced above natural levels (SO₂, NO₂, CO, Pb compounds)
• Contaminant: any unwanted substance regardless of harm level
• Eutrophication: excess nutrients causing algal bloom → oxygen depletion → aquatic death

JAMB high-yield pointers:

1. Primary pollutants (CO, SO₂) come directly from sources; secondary pollutants (O₃, H₂SO₄) form via reactions
2. Acid rain pH < 5.6; causes: SO₂ → SO₃ → H₂SO₄ and NOₓ → HNO₃
3. CFCs destroy ozone via Cl· radicals; stratospheric O₃ is protective, ground-level O₃ is harmful
4. Greenhouse gases: CO₂, CH₄, N₂O trap infrared radiation → global warming

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🟡 Standard — Regular Study (2d–2mo)

Pollution Classification

Air pollutants split into two categories:

Type	Examples	Source
Primary	SO₂, NO₂, CO, particulates, Pb	Directly emitted
Secondary	O₃, H₂SO₄, HNO₃	Formed by atmospheric reactions

CO and SO₂ are invisible gases — many students assume all pollutants are visible.

Acid Rain Mechanism

Sulphur dioxide dissolves in rainwater: $$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3$$

Then oxidised: $$\text{2H}_2\text{SO}_3 + \text{O}_2 \rightarrow \text{2H}_2\text{SO}_4$$

Similarly, NOₓ produces HNO₃. Result: pH drops below 5.6 (normal rain ~5.6 due to dissolved CO₂).

Effects: lake acidification kills fish; limestone buildings corrode; vegetation damage.

Ozone Layer Depletion

CFCs (chlorofluorocarbons, e.g. CCl₂F₂) are chemically stable enough to reach the stratosphere. UV radiation causes: $$\text{CCl}_2\text{F}_2 \xrightarrow{UV} \text{Cl} \cdot + \text{CClF}_2$$ $$\text{Cl} \cdot + \text{O}_3 \rightarrow \text{ClO} \cdot + \text{O}_2$$ $$\text{ClO} \cdot + \text{O} \rightarrow \text{Cl} \cdot + \text{O}_2$$

One chlorine atom catalytically destroys ~1000 O₃ molecules. This is the “ozone hole” mechanism.

Water Pollution

Sources: sewage, industrial effluent, agricultural runoff (nitrates/phosphates), oil spills.

Eutrophication sequence: nutrient runoff → algal bloom → algae die → bacterial decomposition consumes O₂ → aquatic organisms die.

Sewage treatment has three stages:

1. Preliminary: screening and grit removal
2. Primary: sedimentation of suspended solids
3. Secondary: biological degradation by aerobic microbes (reduces BOD)

Secondary treatment reduces pathogens but does not eliminate all pathogens — a common exam misconception.

BOD vs COD

• BOD: biodegradable organic matter, measured over 5 days using microbes; indicates organic pollution strength
• COD: total organic + inorganic reducible matter, measured chemically; higher value than BOD for the same sample

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🔴 Extended — Deep Study (3mo+)

Greenhouse Effect Mechanism

Solar radiation (short wavelength) passes through greenhouse gases in the atmosphere. Earth’s surface absorbs it and emits infrared radiation (long wavelength). Greenhouse gases absorb and re-emit infrared radiation in all directions (including back toward Earth), trapping heat.

Key gases ranked by warming potential (GWP): CO₂ (most abundant, ~400 ppm), CH₄ (80× more potent per molecule than CO₂), N₂O, CFCs.

This differs from the ozone layer function: O₃ absorbs harmful UV radiation, while greenhouse gases trap infrared heat.

Lead Pollution

Petrol exhaust historically contained lead (tetraethyl lead as anti-knock additive). Lead compounds accumulate in the body, causing neurological damage, especially in children. Regulations phased lead out of petrol — this is a classic environmental improvement case study used in JAMB.

Modern particulate matter (PM₂.₅ and PM₁₀) remains a major urban pollutant concern, penetrating deep into lungs.

Lime Softening

Hard water (high Ca²⁺, Mg²⁺) is treated with slaked lime (Ca(OH)₂):

$$\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow \text{2CaCO}_3 \downarrow + \text{2H}_2\text{O}$$

Removes carbonate hardness. Combined with soda ash (Na₂CO₃), it removes non-carbonate hardness too. Relevant to sewage treatment and industrial water management.

Common Exam Traps

1. Students equate O₃ at different altitudes — stratospheric O₃ (UV shield) is essential life-protecting; ground-level O₃ (formed from vehicle NOₓ + VOCs in sunlight) is a harmful pollutant causing respiratory problems
2. Confusing BOD/COD — BOD uses microbes over 5 days; COD uses potassium dichromate — know which gives higher values and why
3. Forgetting CFC stability — their chemical inertness is precisely what allows them to reach the stratosphere intact; they wouldn’t damage if they reacted at ground level
4. Thinking clean air = absence of O₂ — atmospheric CO at 1–2 ppm is immediately lethal; knowing safe limits matters

Practice Prompts

1. A river sample has initial DO of 9 mg/L and DO after 5 days of 2 mg/L with a dilution factor of 50. Calculate BOD and comment on whether the water is safe for aquatic life. (Hint: apply BOD formula, then interpret against safe discharge limits)
2. Write the stepwise reactions showing how CFC-12 (CCl₂F₂) reaching the stratosphere leads to ozone depletion. Identify the catalyst in the cycle. (Hint: Cl radicals are regenerated)

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