Environmental Chemistry

Environmental Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Environmental Chemistry studies chemical changes in air, water, and soil caused by natural processes and human activity. Three pollutant families dominate the syllabus: SOx/NOx (acid rain precursors), CO2/CH4/CFCs (radiatively active greenhouse gases), and non-biodegradable organics like DDT (bioaccumulative). Acid rain forms when 2SO2 + O2 → 2SO3 then SO3 + H2O → H2SO4, with 3NO2 + H2O → 2HNO3 + NO contributing nitric acid. Water quality is measured by BOD (oxygen microbes consume in 5 days, mg/L) and COD (oxygen consumed by chemical oxidation, mg/L) — high BOD means oxygen-starved, lifeless water. The greenhouse effect traps outgoing IR via C=O and C–F vibrational modes; the ozone hole arises when UV photolyses CFCs into Cl• radicals that catalytically destroy stratospheric O3. NABTEB tests short reactions, definitions, and one or two structured questions on sewage treatment.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Air Pollution

Primary gaseous pollutants include CO (incomplete combustion, binds haemoglobin as carboxyhaemoglobin), SO2 (sulphur in coal/oil, irritates lungs), NO2 (from vehicle engines, brown gas), and CO2 (greenhouse). Particulates (PM10, PM2.5) carry adsorbed heavy metals deep into alveoli. Photochemical smog forms when NO2 photolyses to NO + O, the O reacts with O2 to give O3, and ozone + hydrocarbons produce peroxyacetyl nitrate (PAN) and aldehydes — visible as a brown haze over cities like Lagos in dry season.

Water and Soil Pollution

Water pollutants fall into domestic (sewage, detergents), industrial (heavy metals, cyanide, phenols), and agricultural (nitrates, phosphates, pesticides) categories. Excess nitrates and phosphates trigger eutrophication: algal bloom → light blockage → plant death → bacterial decomposition → dissolved O2 crash → fish kill. Persistent pollutants such as DDT, mercury, and lead undergo bioaccumulation (build-up in one organism) and biomagnification (concentration rising up food chains) — DDT at the top of an aquatic food chain can be 10⁶× the water concentration.

Sewage Treatment

Treatment is staged. Primary (physical): screening, grit removal, sedimentation of suspended solids. Secondary (biological): aerobic bacteria in activated sludge or trickling filters oxidise dissolved organics, lowering BOD by 80–90%. Tertiary (chemical): chlorination, ozone, ion exchange, or reverse osmosis to remove residual nutrients, pathogens, and heavy metals before discharge.

Green Chemistry

The 12 principles (Anastas & Warner) prioritise waste prevention, atom economy, safer solvents, renewable feedstocks, and design for degradation — replacing stoichiometric reagents with catalytic ones to cut downstream pollution.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Stratospheric Chemistry of Ozone Depletion

Ozone (O3) is generated by O2 + hv (λ < 240 nm) → 2O• followed by O• + O2 → O3. Destruction in the Antarctic spring is driven by CFC-11 (CFCl3) and CFC-12 (CF2Cl2), which are stable in the troposphere but photolyse in the stratosphere: CFCl3 + hv → CFCl2• + Cl•. A single Cl• catalytically destroys ~10⁵ O3 molecules via Cl• + O3 → ClO• + O2 then ClO• + O → Cl• + O2, regenerating the radical. The 1987 Montreal Protocol phased out CFCs, replaced now by HFCs (zero ODP) and HCFCs (intermediate ODP). Note: tropospheric O3 is a pollutant; stratospheric O3 is protective — the same molecule, opposite roles.

BOD vs COD — The Classic Confusion

BOD measures biodegradable organic load: the dissolved oxygen consumed by aerobic microbes over 5 days at 20 °C. COD measures total oxidisable matter, determined by refluxing with K2Cr2O7/H2SO4 (silver sulphate catalyst) and titrating excess dichromate with Fe(NH4)2(SO4)2. COD is always ≥ BOD; if COD ≫ BOD, the water contains a large fraction of non-biodegradable toxins (e.g., phenolic effluent) that microbes cannot attack.

Exam Strategy and Common Traps

NABTEB questions usually test (i) balancing acid-rain equations including the NO recycle step, (ii) labelling the greenhouse vs ozone-depletion gases correctly, and (iii) sequencing sewage-treatment stages. Pitfalls: writing acid rain as H2SO4 only (forget HNO3); stating that ozone depletion causes global warming (separate mechanisms, separate layers); saying primary treatment uses chlorine (it does not — chlorination is tertiary); and confusing bioaccumulation (within one organism) with biomagnification (across trophic levels).

Practice Prompts

1. Write balanced equations showing how NO2 contributes to acid rain, identifying the gas recycled in the process.
2. A factory discharges effluent with BOD 850 mg/L and COD 1800 mg/L. Comment on its biodegradability and the likely class of pollutant responsible.

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