Chapter 14 — Environmental Chemistry

Fig 1 — Educational Diagram: Introduction to Environmental Chemistry

What is Environmental Chemistry?

Environmental chemistry is the study of chemical processes that occur in the environment (atmosphere, hydrosphere, lithosphere and biosphere) and the effects of human activities (pollution, resource use) on these systems. It links basic chemical principles to real-world environmental problems such as air and water pollution, soil contamination, and climate change.

Scope and Key Concepts

• Compartments: Atmosphere (gases, aerosols), Hydrosphere (rivers, lakes, oceans, groundwater), Lithosphere (soils, sediments), Biosphere (organisms).
• Pollutant types: Gaseous (SO2, NOx, CO2), particulate matter (PM), organic pollutants (pesticides, PCBs), nutrients (nitrate, phosphate), heavy metals (Pb, Hg), and persistent chemicals (CFCs).
• Fate and transport: Emission → transport (advection, diffusion) → transformation (photolysis, hydrolysis, oxidation, biodegradation) → removal (deposition, sedimentation, volatilization).
• Partitioning and bioaccumulation: Chemicals distribute between phases (air/water/soil/biota) according to partition coefficients (e.g., KOW). Some compounds bioaccumulate and biomagnify up food chains.

Important Processes

• Photochemical reactions: Sunlight drives formation of secondary pollutants (e.g., ground-level ozone, photochemical smog from VOCs + NOx).
• Oxidation-reduction reactions: Influence nutrient cycling (nitrate/nitrite/ammonium) and metal speciation.
• Hydrolysis and biodegradation: Breakdown of organics in water and soil; rates determine persistence.
• Equilibrium partitioning: Henry's law (air–water partitioning) and KOW (octanol–water partitioning) control where a pollutant preferentially resides.

Monitoring and Water Quality Indicators

• pH: Acidity/basicity of water or rain (acid rain has pH < 5.6).
• Dissolved oxygen (DO): Necessary for aquatic life; low DO indicates organic pollution.
• BOD (Biochemical Oxygen Demand): Amount of oxygen consumed by microbes decomposing organic matter—indicator of biodegradable pollution.
• COD (Chemical Oxygen Demand): Oxygen equivalent of organic matter that can be chemically oxidized—measures total oxidizable pollutants.
• TDS (Total Dissolved Solids), nutrients, heavy metals, and specific organics are also measured.

Why it matters (Impacts)

• Human health: air pollutants (PM, NO2, SO2, ozone) cause respiratory and cardiovascular diseases.
• Ecological effects: eutrophication from excess nutrients leads to algal blooms and fish kills (low DO).
• Global change: greenhouse gases (CO2, CH4) change climate; CFCs deplete stratospheric ozone.

Approach in Environmental Chemistry

Problems are studied quantitatively: measure concentrations, identify sources, understand reaction kinetics and equilibrium, and model transport and fate. Control strategies include emission reduction, treatment (physical, chemical, biological), regulation and monitoring.

Fig 2 — Educational Diagram: Atmosphere: Composition and Structure

Overview: The atmosphere is the envelope of gases surrounding Earth. It is essential for life, climate, and protection from solar radiation. Composition and structure can be described by (a) chemical composition (which gases are present and in what proportions) and (b) physical layers (how temperature and density change with altitude).

Composition (by volume, dry air):

• Nitrogen (N2): ~78.09%
• Oxygen (O2): ~20.95%
• Argon (Ar): ~0.93%
• Carbon dioxide (CO2): ~0.04% (≈400–420 ppm and rising)
• Trace gases: neon, helium, methane (CH4), krypton, hydrogen, nitrous oxide (N2O), ozone (O3) and others (total <0.05%)
• Water vapour: variable (0–4% by volume) depending on temperature and humidity — not included in "dry air" composition.

Partial pressure of a constituent: p_i = x_i · p_total (where x_i is mole fraction). For ideal gases, volume% ≈ mole%.

Mixing: The lower atmosphere (the homosphere, up to ≈80–100 km) is well mixed by turbulence and convection so the major gases have nearly uniform composition. Above this, in the heterosphere, gases separate by molecular weight (diffusion), so lighter gases (H, He) dominate at very high altitudes.

Vertical structure (temperature-based layers) — typical altitudes and features:

• Troposphere (~0–8 km at poles, ~0–15 km at equator; mean ~0–12 km): temperature decreases with height (lapse rate ≈6.5 K/km). Contains ~75–80% of atmosphere's mass and almost all water vapour and clouds. Weather and life occur here. Top is the tropopause.
• Stratosphere (~15–50 km): temperature increases with altitude because ozone (O3) absorbs UV radiation, heating the layer. This creates stable stratification (little vertical mixing). The ozone layer (peak ~20–30 km) filters harmful UV.
• Mesosphere (~50–85 km): temperature again falls with altitude; coldest region. Meteors burn up here. Top is the mesopause.
• Thermosphere (~85–600 km): temperature rises strongly due to absorption of high-energy solar radiation. Low density, large molecular energies; contains the ionosphere (important for radio propagation and auroras).
• Exosphere (~600 km to several thousand km): extremely low density; particles can escape to space. Gradual transition to interplanetary space.

Important physical ideas:

• Hydrostatic equilibrium: balance between pressure gradient and gravity — dp/dz = -ρg.
• Barometric (pressure) variation: pressure decreases approximately exponentially with altitude; most mass is near the surface.
• Scale height H = RT/(Mg): typical H ≈ 7–8 km for Earth (depends on temperature), gives the characteristic height over which pressure falls by factor e.

Environmental and chemical relevance:

• Greenhouse gases (CO2, CH4, H2O vapour) trap outgoing infrared radiation and warm the surface (greenhouse effect).
• Ozone in the stratosphere protects life by absorbing UV-B and UV-C; tropospheric ozone (a pollutant) harms health and vegetation.
• Human emissions (CO2, aerosols, NOx, SO2, VOCs) alter composition and lead to climate change, acid rain, smog, and ozone depletion.

Summary: The atmosphere is mainly N2 and O2 with small but important trace gases. Its layered structure (troposphere → stratosphere → mesosphere → thermosphere → exosphere) is defined by temperature gradients and by physical and chemical processes (mixing, photochemistry, diffusion). Understanding composition and structure explains weather, climate, radiation shielding, and many environmental issues.

Fig 3 — Educational Diagram: Composition and Structure of Atmosphere

Overview
The atmosphere is the gaseous envelope surrounding Earth. It contains a mixture of permanent gases (nearly constant in composition), variable gases (vary with time and place), and aerosols (solid/liquid particles). The atmosphere is vertically stratified into layers that differ in temperature, composition and physical behaviour.

Composition (by volume, near sea level)

Important chemical processes
Photochemical reactions driven by solar UV are crucial in the atmosphere. Example: ozone formation and destruction in the stratosphere:

• O2 + hv (<240 nm) → 2O
• O + O2 + M → O3 + M (M = third body)
• O3 + hv (<320 nm) → O2 + O

Vertical structure and key properties

• Troposphere (surface → ~8–15 km): Temperature decreases with altitude (average lapse rate ≈ 6–7 K/km; dry adiabatic ≈9.8 K/km). Weather, clouds and most water vapour are here. Pressure and density drop rapidly with altitude.
• Stratosphere (~15 → ~50 km): Temperature increases with altitude because of UV absorption by ozone (ozone layer peak ~20–30 km). Commercial jets often cruise in the lower stratosphere to avoid turbulence.
• Mesosphere (~50 → ~85 km): Temperature decreases again; meteors burn up here. Coldest layer (mesopause ~85 km).
• Thermosphere (~85 → 600 km): Temperature rises strongly with altitude due to absorption of high-energy solar radiation; contains ionized gases (ionosphere region important for radio propagation and auroras).
• Exosphere (above ~600 km): Very low density; atoms and molecules can escape to space; satellites and space debris orbit here.

Pressure and density variation
Atmospheric pressure decreases approximately exponentially with altitude. The hydrostatic balance and the barometric formula describe this behaviour. Temperature structure and composition determine the exact profile.

Environmental significance
The atmosphere protects life (ozone blocks much UV), regulates climate (greenhouse gases trap outgoing IR), and transports water and pollutants. Human activities (fossil fuel combustion, deforestation, emissions) change concentrations (e.g., rising CO2, methane) and cause problems such as global warming, ozone depletion (CFCs) and urban smog.

Summary
Understanding composition and vertical structure explains weather, climate, radiation shielding, communication (ionosphere) and where human-caused pollutants act.

Fig 4 — Educational Diagram: Air Pollutants: Types and Sources

Overview: Air pollutants are substances in the atmosphere that, in high enough concentrations, harm human health, ecosystems, or materials. They are classified by origin (natural vs anthropogenic), by form (gaseous vs particulate) and by whether they are emitted directly or formed in the atmosphere.

1. Primary and Secondary Pollutants

• Primary pollutants: emitted directly from sources. Common examples: CO, SO2, NO, NO2, VOCs (volatile organic compounds), particulate matter (PM10, PM2.5), lead (Pb), hydrocarbons, and black carbon.
• Secondary pollutants: formed by chemical reactions in the atmosphere from primary pollutants. Examples: ozone (O3) at ground level, peroxyacetyl nitrate (PAN), nitric acid (HNO3), sulfate aerosols (from SO2 oxidation), and secondary organic aerosols (SOA).

2. Important Chemical Processes (brief)

• Photochemical reactions (require sunlight): NO2 + hv (< ~420 nm) → NO + O; then O + O2 → O3 (ground-level ozone formation).
• Thermal oxidation forming nitrogen oxides (from high-temperature combustion): N2 + O2 → 2NO (then 2NO + O2 → 2NO2).
• Oxidation of SO2 to sulfate and sulfuric acid (acid rain precursors): 2SO2 + O2 → 2SO3; SO3 + H2O → H2SO4 (also via OH radical pathways).
• VOCs + NOx + sunlight → radicals → O3 and PAN (photochemical smog). PAN is a peroxyacyl nitrate (generalized formation from oxidized VOC fragments + NO2).

3. Common Pollutants, Characteristics and Effects

• Carbon monoxide (CO): colorless, odorless; from incomplete combustion (vehicles, poorly ventilated stoves); binds hemoglobin → reduces O2 transport.
• Nitrogen oxides (NOx = NO + NO2): from vehicle engines and power plants; contribute to O3 formation, respiratory irritation, and acid rain.
• Sulfur dioxide (SO2): from coal/oil combustion, smelting; causes respiratory problems and forms sulfate aerosols and acid rain.
• Ground-level ozone (O3): a secondary oxidant; causes lung inflammation, reduces crop yields; peaks in sunny afternoons in urban areas.
• Particulate matter (PM10, PM2.5): solid or liquid particles suspended in air; sources include combustion, dust, construction, biomass burning; penetrate lungs and bloodstream causing cardiovascular and respiratory disease.
• Volatile organic compounds (VOCs): from solvents, petrol, vegetation; react with NOx to form O3 and secondary organic aerosols.
• Lead (Pb): mainly from industrial emissions and historically from leaded petrol; neurotoxic, particularly harmful to children.
• CFCs and halogenated compounds: long-lived; reach stratosphere and cause ozone depletion (not a local urban pollutant but global concern).

4. Sources of Air Pollutants

• Anthropogenic (human-made):
  • Transport: vehicles emit CO, NOx, VOCs, PM.
  • Power plants and industry: SO2, NOx, CO2, PM, heavy metals.
  • Domestic heating and cooking: CO, PM, VOCs (especially with biomass fuel).
  • Agriculture: NH3 (ammonia), methane (CH4), pesticide volatilization, dust.
  • Solvent use and chemical industries: VOCs and specific organics.
  • Biomass and crop residue burning: large PM, CO, VOCs and NOx releases.
• Natural sources:
  • Volcanoes: SO2, ash (particulates).
  • Sea spray: sea-salt aerosols.
  • Soil and desert dust: mineral particulates.
  • Wildfires: PM, CO, VOCs.
  • Vegetation: biogenic VOCs (isoprene, terpenes) that can form ozone and SOA.
  • Lightning and microbial processes: small amounts of NOx and other gases.

5. Impacts and brief control measures: Health (respiratory, cardiovascular, neurological), environmental (acidification, eutrophication, crop yield losses, visibility reduction, climate forcing). Control measures include fuel quality improvement, emission standards, catalytic converters, flue-gas desulfurization, dust control, and promoting public transport.

Fig 5 — Educational Diagram: Chemistry of Major Air Pollutants

Overview: Air pollution arises from natural and anthropogenic sources. Major pollutants include carbon monoxide (CO), sulfur oxides (SOx, mainly SO₂), nitrogen oxides (NO and NO₂, collectively NOx), volatile organic compounds (VOCs, including hydrocarbons), ozone (O₃ in the troposphere), particulate matter (PM₁₀, PM_2.5), lead (Pb), and greenhouse gases (CO₂, CH₄, N₂O).

1. Carbon monoxide (CO)
Chemistry: Produced by incomplete combustion of carbon-containing fuels: C + 1/2 O₂ → CO (or from hydrocarbon combustion with limited O₂). CO oxidises slowly in the atmosphere to CO₂ (OH radical mediated). Lifetime: weeks–months in troposphere. Concentration units: ppm or ppb.

Health/environmental effects: CO binds to haemoglobin to form carboxyhaemoglobin (COHb), reducing O₂ transport and causing hypoxia. Common in vehicle exhaust, poorly ventilated combustion appliances.

2. Sulfur dioxide (SO₂) and SOx
Chemistry: Formed by combustion of sulfur-containing fuels and smelting: S + O₂ → SO₂. SO₂ can be oxidised in air (gas phase by OH or heterogeneous/aqueous oxidation) to SO₃ and then to sulfuric acid: SO₂ + 1/2 O₂ → SO₃; SO₃ + H₂O → H₂SO₄. In cloud droplets, SO₂ → H₂SO₄ leads to acid rain. Lifetime: days.

Effects: Respiratory irritation, acidity damage to buildings, aquatic ecosystems acidification. Major sources: coal power plants, smelters.

3. Nitrogen oxides (NOx = NO + NO₂)
Chemistry: High-temperature oxidation of atmospheric N₂ during combustion produces NO: N₂ + O₂ → 2NO. NO is oxidised to NO₂: 2NO + O₂ → 2NO₂. NOx participates in photochemistry producing ozone and nitric acid (HNO₃): NO₂ + hν → NO + O(3P); O + O₂ + M → O₃ + M. 2NO₂ + H₂O → HNO₂ + HNO₃ (simplified in aqueous phase).

Effects: Respiratory problems, acidification, visibility reduction, and a key role in photochemical smog and ozone formation. Sources: vehicle engines, power plants.

4. Volatile organic compounds (VOCs) and hydrocarbons
Chemistry: VOCs are emitted from fuels, solvents, vegetation and industry. In sunlight they react with OH to form radicals and oxidised products. VOC + OH → R• → RO₂•. Peroxy radicals (RO₂•) oxidise NO to NO₂ without consuming O₃, allowing net O₃ buildup. VOCs also include hazardous air pollutants (benzene, formaldehyde).

5. Tropospheric ozone (O₃) and photochemical smog
Chemistry: Ozone in the lower atmosphere is a secondary pollutant formed by photochemical reactions involving NOx and VOCs as above. Simplified net (in presence of VOCs): VOC + NO_x + sunlight → O₃ + oxidised organics. Ozone is a strong oxidant and damages lungs and crops. Photochemical smog examples: Los Angeles, Delhi.

6. Particulate Matter (PM)
Definition: Suspended solid or liquid particles with aerodynamic diameter ≤ 10 μm (PM₁₀) or ≤ 2.5 μm (PM_2.5). Composition: soot (carbonaceous), sulfates, nitrates, organic aerosols, metals, dust. Sources: combustion, industrial processes, dust storms, biomass burning.

Effects: PM_2.5 penetrates deep into lungs and bloodstream causing cardiopulmonary disease, reduced life expectancy, and visibility impairment.

7. Lead (Pb)
Chemistry/Source: Historically from leaded petrol (tetraethyl lead), also smelting and industrial emissions. Lead is persistent and accumulates in soils and organisms.

Effects: Neurotoxin, especially harmful to children (cognitive deficits). Leaded petrol bans dramatically reduced atmospheric Pb.

8. Greenhouse gases (CO₂, CH₄, N₂O)
Chemistry & role: These gases absorb outgoing longwave IR radiation, causing radiative forcing and global warming. Key sources: CO₂ from fossil fuel combustion (C + O₂ → CO₂), CH₄ from anaerobic decomposition and energy production (oxidises in troposphere to CO₂ and H₂O via OH), N₂O from agriculture/fertilizer use (long atmospheric lifetime, also O₃-depleting in the stratosphere).

Control measures (brief): catalytic converters (reduce CO, NOx), flue gas desulfurization (remove SO₂), electrostatic precipitators and bag filters (remove particulates), VOC capture/abatement (carbon adsorption, thermal oxidation), fuel switching to low-sulfur fuels and regulations (emission standards).

Units and concentrations: CO in ppm, NOx/NO₂ in ppb/μg·m⁻³, SO₂ in ppb/μg·m⁻³, PM in μg·m⁻³. Typical lifetimes: CO ~ weeks–months, NOx ~ hours–days, SO₂ ~ 1–4 days, CH₄ ~ 12 years, CO₂ decades–centuries.

Summary: Major air pollutants arise mainly from combustion and industrial processes. Their atmospheric chemistry (oxidation, photolysis, radical cycles) determines whether they are primary or secondary pollutants (e.g., O₃, HNO₃, H₂SO₄). Understanding reaction pathways allows targeted control technologies to reduce health and environmental impacts.

Fig 6 — Educational Diagram: Greenhouse Gases and Greenhouse Effect

Overview
The greenhouse effect is the process by which certain gases in Earth’s atmosphere trap outgoing longwave (infrared) radiation emitted by the Earth’s surface, raising the planet’s average temperature. Greenhouse gases (GHGs) are atmospheric constituents that absorb and emit infrared radiation. The natural greenhouse effect is essential for life (without it Earth's mean surface temperature would be about -18°C instead of about +15°C). Human activities have increased concentrations of several GHGs, enhancing the greenhouse effect and causing global warming.

Major greenhouse gases

• Carbon dioxide (CO2) – from fossil fuel burning, deforestation, cement production.
• Methane (CH4) – from wetlands, rice paddies, livestock (enteric fermentation), landfills, fossil fuel extraction.
• Nitrous oxide (N2O) – from agricultural soils (fertiliser use), industrial processes, combustion.
• Ozone (O3) – tropospheric ozone produced by photochemical reactions of pollutants; stratospheric ozone shields UV.
• Fluorinated gases (CFCs, HCFCs, HFCs, SF6, PFCs) – industrial refrigerants and insulators with very high warming potentials.
• Water vapour (H2O) – the most abundant greenhouse gas; acts as a feedback (not a primary anthropogenic forcing).

How the greenhouse effect works (stepwise)

1. Incoming solar radiation (mostly shortwave visible and UV) passes through the atmosphere and is absorbed by Earth’s surface.
2. The surface warms and emits longwave infrared (IR) radiation (peak emission determined by surface temperature).
3. Greenhouse gases absorb specific IR wavelengths and then re-emit IR in all directions; some of that re-emitted energy returns to the surface, warming it further.
4. The planet reaches a radiative balance where the incoming solar energy equals outgoing longwave radiation (modified by greenhouse trapping).

Natural vs. enhanced greenhouse effect
Natural greenhouse effect maintains habitable temperatures. Enhanced greenhouse effect refers to additional warming resulting from increased concentrations of GHGs due to human activities (anthropogenic emissions), producing a positive radiative forcing and a rise in global mean temperature.

Feedbacks

• Water vapour feedback (positive): warming increases atmospheric water vapour, which amplifies warming because water vapour is a strong GHG.
• Ice–albedo feedback (positive): melting ice reduces surface albedo, so more solar energy is absorbed, causing further warming.
• Cloud feedbacks: complex—clouds can cool (by reflecting sunlight) or warm (by trapping IR); net effect depends on cloud type/altitude.

Consequences of enhanced greenhouse effect
Global temperature rise, sea-level rise (thermal expansion and ice melt), changes in precipitation patterns, more frequent extreme weather events, ocean acidification (from increased CO2 uptake), ecosystem and biodiversity impacts, and socio-economic effects (agriculture, health, infrastructure).

Important quantities and concepts

• Radiative forcing (ΔF): change in net (down minus up) radiative flux at the tropopause due to a perturbation (e.g., increased CO2). Positive ΔF warms the climate.
• Global Warming Potential (GWP): a comparative metric of integrated radiative forcing of a unit mass of a GHG relative to CO2 over a chosen time horizon (commonly 20, 100 years).
• Atmospheric lifetime: characteristic time a gas remains in the atmosphere (CO2 is long-lived with complex sinks; CH4 ~12 years; N2O ~100+ years).

Typical concentration history
Pre-industrial CO2 was ~280 ppm; modern measurements (Keeling Curve) show a steady increase to ~410–420 ppm (and rising). Methane and nitrous oxide have also risen substantially since pre-industrial times.

Fig 7 — Educational Diagram: Ozone Layer and Ozone Depletion

What is the ozone layer?
Ozone (O3) is a triatomic form of oxygen concentrated in the stratosphere roughly between 15 and 35 km above Earth's surface. The stratospheric ozone layer absorbs most of the Sun's biologically harmful ultraviolet-B (UV-B, 280–315 nm) radiation, protecting life on Earth.

Formation and natural balance (Chapman mechanism)

• Oxygen photo-dissociation: O2 + hv (<240 nm) → 2 O
• Ozone formation (three-body): O + O2 + M → O3 + M (M = any third body to carry off excess energy)
• Ozone photolysis: O3 + hv (<320 nm) → O2 + O
• Recombination (destruction): O + O3 → 2 O2

These reactions produce a dynamic steady-state concentration of ozone in the stratosphere under natural conditions.

Ozone depletion — causes and mechanism

Human-made stable halogenated compounds—chiefly chlorofluorocarbons (CFCs), halons, carbon tetrachloride, methyl bromide and HCFCs—are major ozone-depleting substances (ODS). They are stable in the troposphere, so they reach the stratosphere where UV radiation breaks them down, releasing reactive chlorine (Cl) and bromine (Br) atoms which catalytically destroy ozone.

Key catalytic cycles (examples):

• Chlorine catalytic cycle: Cl + O3 → ClO + O2
  ClO + O → Cl + O2
  Net: O + O3 → 2 O2 (one Cl regenerates)
• ClO dimer cycle (important in polar ozone holes): 2 ClO + M → Cl2O2 + M; Cl2O2 + hv → Cl + ClOO → products that destroy O3
• Bromine and nitrogen oxides (NO, NO2) also participate in catalytic cycles that destroy ozone.

Special case: Antarctic ozone hole

During Southern Hemisphere spring (Aug–Oct) strong polar vortex winds isolate the Antarctic stratosphere and extremely cold temperatures form polar stratospheric clouds (PSCs). Heterogeneous reactions on PSC surfaces convert reservoir species (like HCl, ClONO₂) into reactive Cl₂, which photolyzes to Cl atoms when sunlight returns. This produces rapid, large ozone loss — the seasonal 'ozone hole'.

Consequences of ozone depletion

• Increased UV-B at surface → higher skin cancer rates, cataracts, immune suppression in humans.
• Damage to terrestrial and aquatic ecosystems, reduced crop yields, altered biogeochemical cycles.
• Material degradation (polymers, paints) due to increased UV exposure.

Control measures and recovery

The Montreal Protocol (1987) and its amendments phased out many ODS (CFCs, halons). As a result, atmospheric concentrations of many controlled CFCs have declined and models indicate gradual recovery of the ozone layer (full recovery of global ozone expected mid-21st century to late-21st century depending on species and region). Long atmospheric lifetimes of some ODS (decades to a century) mean recovery is slow.

Units and typical values

Ozone column is expressed in Dobson Units (DU): 1 DU = 0.01 mm thickness of pure O3 at STP. Typical global mean total column ozone is about 300 DU. The Antarctic ozone hole is typically defined when column ozone falls below about 220 DU.

Monitoring and indicators

Ozone is monitored by ground-based Dobson/ Brewer spectrophotometers, balloons, aircraft and satellites. Key indicators include total column ozone (DU), vertical profiles (ozone vs altitude), and area of ozone depletion (e.g., ozone hole area below threshold).

Summary: The ozone layer is formed and maintained by photochemical reactions but is vulnerable to catalytic destruction by halogen radicals released from anthropogenic ODS. International policy (Montreal Protocol) has successfully reduced emissions, and recovery is underway but slow.

Fig 8 — Educational Diagram: Photochemical Smog and Industrial Smog

Overview: Smog is a harmful air-pollution phenomenon caused by chemical reactions of pollutants in the atmosphere. Two classical types are photochemical smog (typical of sunny urban areas with vehicle exhaust) and industrial (sulfurous or classical/London) smog (associated with coal/industrial combustion and cool, foggy conditions).

Photochemical smog — cause and mechanism

• Requires sunlight, nitrogen oxides (NO and NO₂, collectively NO_x) and volatile organic compounds (VOCs, i.e., hydrocarbons from petrol, solvents, vegetation).
• Key photochemical sequence (simplified):
• Sunlight (hν) photolyzes NO₂ to give atomic oxygen, which forms ozone (O₃): NO₂ → NO + O (by hν); O + O₂ → O₃.
• Ozone and reactive radicals (OH, RO₂) oxidize VOCs to give secondary oxidants such as peroxyacetyl nitrate (PAN) and more ozone. PAN and ozone are major toxic components of photochemical smog.
• Features: brownish haze (NO₂ gives brown color), high midday ozone peak, strong photochemical activity, often in warm, sunny, stagnant-air cities (e.g., Los Angeles, Mexico City).

Industrial (London) smog — cause and mechanism

• Associated with combustion of sulfur-containing fuels (coal, heavy oil). Requires cool, humid conditions and often fog or temperature inversion that traps pollutants near ground.
• Major chemistry: sulfur (or sulfur-containing fuel) combustion yields SO₂, which is oxidized to SO₃ and then forms sulfuric acid aerosols (H₂SO₄) and sulfates. Particulate soot and ash combine with these acidic aerosols producing the dense, grayish smog.
• Features: gray/black haze, heavy particulate load (soot, sulfate aerosols), pronounced respiratory/corrosive effects, historically exemplified by the Great London Smog of 1952.

Conditions favoring each type

• Photochemical smog: strong sunlight, warm temperature, abundant NO_x and VOCs, light winds/stagnation.
• Industrial smog: high SO₂ emissions, cold/humid weather, fog or inversion layers, coal/biomass burning.

Impacts: Respiratory and eye irritation, aggravated asthma and bronchitis, crop yield losses (ozone is phytotoxic), corrosion of metals and building materials, reduced visibility, formation of acid rain (from SO₂/SO₃).

Control and prevention

• Photochemical smog: reduce NO_x (catalytic converters, low-NOx burners), control VOC emissions (solvent limits, leak detection), traffic management, alternative fuels.
• Industrial smog: switch to low-sulfur fuels, flue gas desulfurization (FGD), electrostatic precipitators and bag filters for particulates, stricter emission standards, improved combustion efficiency.

Differences (brief)

• Photochemical smog: sunny, ozone-rich, brownish; primary precursors NO_x + VOCs + sunlight.
• Industrial smog: foggy/cold, sulfur-rich, gray/black; primary precursors SO₂ + particulates + moisture.

Fig 9 — Educational Diagram: Air Pollution: Overview and Classification

Overview
Air pollution is the presence of substances in the atmosphere, above natural levels, that are harmful to living organisms, materials or disturb climate. Pollutants can be gases, vapours, aerosols (particulate matter) or biological materials and may originate from natural or anthropogenic (human) sources.

Key definitions

• Primary pollutants: Emitted directly from sources (e.g., SO2, NO, CO, PM, VOCs).
• Secondary pollutants: Formed in the atmosphere by chemical reactions among primary pollutants and natural components (e.g., O3, HNO3, H2SO4, PAN, secondary organic aerosols).

Classification of air pollutants

• By phase: Gaseous pollutants (SO2, NOx, CO, CO2, NH3, O3, VOCs) and Particulate matter (PM10, PM2.5, soot, dust, mists).
• By origin: Natural (volcanoes, wildfires, dust storms, biogenic VOCs) vs Anthropogenic (vehicular emissions, industry, power plants, residential burning).
• By effect/importance: Criteria pollutants (SO2, NO2, CO, PM, O3, Pb, CO2) often regulated; greenhouse gases (CO2, CH4, N2O) affecting climate; toxic pollutants (Hg, benzene, formaldehyde).

Important pollutants & brief chemistry

• Sulfur oxides (SO2, SO3): Emitted from coal/oil combustion. Oxidation gives SO3 which hydrolyses to H2SO4 causing acid rain.
• Nitrogen oxides (NO, NO2): From high-temperature combustion (vehicles, power plants). Photochemistry of NO2 + hv ⇌ NO + O(3P) followed by O + O2 → O3 creates ground-level ozone.
• Carbon monoxide (CO): Incomplete combustion; binds to haemoglobin reducing O2 transport.
• Particulate matter (PM10, PM2.5): Solid/liquid particles; PM2.5 penetrates deep into lungs causing severe health effects.
• Ozone (O3, tropospheric): Secondary pollutant from VOCs + NOx + sunlight (photochemical smog). Harmful to lungs and plants.

Mechanisms & phenomena

• Photochemical smog: VOCs + NOx + sunlight → O3, PAN (peroxyacetyl nitrates) and other oxidants (typical in sunny urban areas).
• Acid rain: SO2/NOx → oxidised to H2SO4/HNO3 in atmosphere → falls as acidic precipitation, damaging ecosystems and buildings.
• Temperature inversion: A layer of warm air above cooler air traps pollutants near ground, increasing concentrations and health risks.
• Long-range transport: Some pollutants (e.g., sulfate aerosols, ozone precursors) travel hundreds to thousands of km.

Effects

• Health: Respiratory and cardiovascular diseases, reduced lung function, aggravated asthma, carcinogenic risks (benzene, formaldehyde).
• Environment: Acidification of soils and waters, eutrophication, damage to crops & forests, reduced visibility (haze).
• Materials: Corrosion of metals, deterioration of buildings, cultural monuments (acid attack).
• Climate: Greenhouse gases (CO2, CH4, N2O) enhance global warming; aerosols can cool or warm depending on properties.

Measurement & standards
Pollutant concentrations are measured in ppm, ppb or mass per volume units (µg m-3). Many countries use Air Quality Index (AQI) to communicate health risk levels based on pollutant concentrations.

Control measures (brief)

• Source control: cleaner fuels, emission standards, catalytic converters, flue gas desulfurization, electrostatic precipitators.
• Urban planning and transport management: public transport, vehicle emission norms.
• Regulation & monitoring: ambient air quality standards, industrial permits.
• Behavioural: reduced burning of biomass and waste, energy efficiency.

Summary
Air pollution is a complex mix of gases and particles from natural and human sources. Understanding classification (primary vs secondary, gaseous vs particulate, natural vs anthropogenic) and basic chemistry (e.g., formation of acid rain and photochemical smog) helps in designing mitigation strategies and protecting health and environment.

Fig 10 — Educational Diagram: Greenhouse Effect and Global Warming

Overview
The greenhouse effect is a natural process in which certain atmospheric gases absorb outgoing longwave (infrared) radiation from the Earth’s surface and re-radiate part of it back toward the surface, raising the planet’s average temperature. Global warming refers to the observed long-term rise in Earth’s average surface temperature, primarily driven by an enhanced greenhouse effect due to increased concentrations of greenhouse gases (GHGs) from human activities.

Mechanism (step-by-step)

• Incoming solar radiation (shortwave, mostly visible and UV) reaches the Earth; some is reflected by clouds, atmosphere and surface (albedo), the rest is absorbed and heats the surface.
• The warmed surface emits energy as longwave (infrared) radiation.
• Certain gases in the atmosphere (water vapour, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, etc.) absorb specific IR wavelengths because their molecular vibrational modes match those wavelengths.
• After absorption, these gases re-emit IR radiation in all directions; some is sent back toward the surface, reducing the net outgoing energy and warming the lower atmosphere and surface (the greenhouse effect).
• An increase in GHG concentrations increases the atmospheric absorption of outgoing IR, producing a positive radiative forcing and a warming tendency (global warming). Feedbacks such as increased atmospheric water vapour and ice-albedo changes can amplify or modulate the warming.

Key greenhouse gases and properties

• Water vapour (H2O) — strongest natural greenhouse gas, variable; acts as a feedback.
• Carbon dioxide (CO2) — long-lived, main anthropogenic contributor from fossil fuel combustion and deforestation; strong absorption near 15 μm.
• Methane (CH4) — more efficient per molecule than CO2 at absorbing IR, shorter atmospheric lifetime, large agricultural and waste sources.
• Nitrous oxide (N2O) — potent, from fertilizers and industrial processes.
• Ozone (O3) — in troposphere acts as a greenhouse gas; in stratosphere protects from UV.
• Chlorofluorocarbons (CFCs) and similar halogenated gases — very effective GHGs and also ozone-depleting.

Consequences of enhanced greenhouse effect / global warming

• Global temperature rise and greater frequency of heatwaves.
• Melting of glaciers and polar ice, contributing to sea level rise.
• Changes in precipitation patterns, more intense storms and droughts in some regions.
• Ocean warming and acidification, coral bleaching, ecosystem disruption.
• Socioeconomic impacts: agriculture, health (vector-borne diseases), displacement.

Mitigation and adaptation (short)
Mitigation: reduce GHG emissions (switch to low-carbon energy, energy efficiency, reforestation), capture and store CO2, reduce CH4 emissions from waste and agriculture, phase out potent fluorinated gases. Adaptation: infrastructure adjustments, water management, disaster preparedness.

Fig 11 — Educational Diagram: Major Air Pollutants and Chemistry

Air pollutants are substances in the atmosphere, natural or anthropogenic, that harm human health, ecosystems or climate. In Class 11 Environmental Chemistry, we study the identity, sources, chemical behaviour, transformations and effects of major air pollutants. Key categories: particulate matter (PM), gaseous primary pollutants (SO₂, NO_x, CO, VOCs, Pb), secondary pollutants formed in the atmosphere (O₃, HNO₃, H₂SO₄, PAN), and greenhouse gases (CO₂, CH₄, N₂O).

Major pollutants and their chemistry

• Particulate matter (PM10, PM2.5): solid or liquid particles suspended in air (dust, soot, aerosols). Sources: combustion, industrial processes, construction, biomass burning. PM2.5 penetrates deep into lungs, carries adsorbed toxic species (heavy metals, organics).
• Sulfur oxides (SO₂, SO₃): SO₂ is produced by combustion of sulfur-containing fuels (coal, oil) and from ore roasting. In air, oxidation yields SO₃, which reacts with water to form sulfuric acid: SO₂ + ½O₂ → SO₃ (catalysed by particulates/metal oxides); SO₃ + H₂O → H₂SO₄. Results in acid rain (lowers pH of rainwater), leathering of buildings and ecosystem damage.
• Nitrogen oxides (NO, NO₂, N₂O): Formed at high temperatures in engines/combustion: N₂ + O₂ → 2NO. NO is oxidised in air: 2NO + O₂ → 2NO₂. NO₂ leads to nitric acid formation (acid rain) and participates in photochemistry producing ozone (see below).
• Carbon monoxide (CO) and carbon dioxide (CO₂): CO results from incomplete combustion (car engines, indoor stoves). It binds to haemoglobin (CO + HbO₂ → carboxyhaemoglobin), reducing O₂ transport. CO is oxidised to CO₂: 2CO + O₂ → 2CO₂. CO₂ is the main anthropogenic greenhouse gas from fossil fuel burning.
• Volatile organic compounds (VOCs): Hydrocarbons and oxygenated organics from solvent use, petrol evaporation, vegetation. VOCs undergo oxidation to form ozone and secondary organic aerosols. Example: RH + OH → R· → RO₂ → reactions with NO_x produce O₃ and PANs.
• Ground-level ozone (O₃) — a secondary pollutant: Formed photochemically from NO₂ photolysis and VOC/NO_x chemistry: NO₂ + hν (λ < 420 nm) → NO + O; O + O₂ + M → O₃ + M. Ozone causes respiratory irritation, damages crops and materials. Peak formation occurs on sunny, stagnant days.
• Peroxyacyl nitrates (PANs): Formed from VOC oxidation and NO₂. PANs are strong eye irritants and act as reservoirs for NO_x, transporting them long distances.
• Lead (Pb) and other heavy metals: Emitted from industrial sources and legacy leaded petrol. They adsorb onto particulates and are toxic.

Photochemical smog and acid rain (process overview)

• Photochemical smog (typical in sunlight + hydrocarbons + NO_x): NO₂ + hν → NO + O; O + O₂ → O₃; VOC oxidation produces radicals that shift NO → NO₂ without destroying O₃, allowing O₃ to accumulate.
• Acid rain: SO₂ and NO_x oxidize (in gas phase and aqueous droplets) to H₂SO₄ and HNO₃, lowering precipitation pH and mobilising toxic metals.

Health and environmental impacts

• Respiratory/cardiovascular diseases (PM, O₃, SO₂, NO₂, CO).
• Crop yield reduction (O₃ damage), ecosystem acidification (H₂SO₄, HNO₃), visibility reduction (smog, aerosols).
• Climate forcing (CO₂, CH₄, N₂O) and radiative effects of aerosols.

Control and mitigation

• Fuel switching and desulfurization (remove S from fuel, flue-gas desulfurization).
• Catalytic converters to reduce NO_x and CO; particle filters and electrostatic precipitators for PM.
• Reduce VOC emissions (solvent controls, evaporative controls), promote clean energy to cut CO₂.

This summary links chemical reactions to real atmospherics behaviour important for Class 11 Environmental Chemistry.

Fig 12 — Educational Diagram: Ozone Layer Depletion

Definition: The ozone layer depletion is the thinning and reduction in concentration of ozone (O₃) in Earth’s stratosphere, especially in the lower stratosphere (15–35 km). The stratospheric ozone layer absorbs the Sun’s harmful ultraviolet-B (UV-B, 280–315 nm) radiation and protects life on Earth.

Where ozone is and why it matters: Ozone concentration is highest in the stratosphere (the ‘ozone layer’). Ozone absorbs incoming UV-B; without it, UV intensity at the surface would rise, causing increased skin cancer, cataracts, immune suppression, crop losses and damage to marine ecosystems.

Natural formation & Chapman cycle (basic stratospheric chemistry):

• Photodissociation of molecular oxygen: O₂ + hν (< 242 nm) → 2 O
• Ozone formation by third-body association: O + O₂ + M → O₃ + M
• Ozone photolysis: O₃ + hν (≤ 320 nm) → O₂ + O
• Recombination: O + O₃ → 2 O₂

These reactions maintain a steady-state ozone concentration in the absence of catalytic loss processes.

Catalytic destruction by radicals (main cause of depletion):

• Halogen radicals (especially chlorine and bromine) catalyse rapid O₃ destruction. Example cycle for chlorine (from CFCs):

Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂
Net: O + O₃ → 2 O₂

One Cl atom can destroy many O₃ molecules. Similar cycles operate for Br and for NO_x and OH radicals.

Anthropogenic sources:

• Chlorofluorocarbons (CFCs), halons, carbon tetrachloride, methyl bromide — release Cl or Br radicals after photolysis in the stratosphere.
• Nitrous oxide (N₂O) from agriculture is now a major ozone-depleting emission (produces NO in stratosphere).

Special case — Antarctic ozone hole: In polar winter/spring, polar stratospheric clouds (PSCs) convert reservoir species (e.g., HCl, ClONO₂) into active Cl₂. On return of sunlight, Cl₂ photolyzes to Cl radicals, initiating rapid, large-scale O₃ loss and an ‘‘ozone hole’’ over Antarctica.

Measurement & units: Total column ozone is commonly reported in Dobson Units (DU). 1 DU = 2.69 × 10¹⁶ molecules cm⁻². Typical mid-latitude column is ≈ 300 DU; Antarctic hole values can drop below 100 DU.

Consequences:

• Biological: increased skin cancers, cataracts, weakened immunity; reduced crop yields; reduced phytoplankton productivity affecting marine food webs.
• Materials: faster degradation of polymers, paints and rubber from extra UV.
• Climate interactions: ozone changes affect radiative balance and circulation, with complex feedbacks.

Control and recovery: The Montreal Protocol (1987) and amendments phased out major ozone-depleting substances (ODSs). As a result, stratospheric chlorine and bromine levels have been declining slowly and models indicate gradual recovery of the ozone layer toward pre-1980 values during the 21st century (full recovery timing depends on greenhouse gases and N₂O trends).

Prevention and good practices: Avoid use of banned CFCs/halons, prefer non-ozone-depleting refrigerants/foams, control agricultural N₂O emissions, support proper disposal and recovery of refrigerants.

Summary: Ozone layer depletion is mainly driven by catalytic cycles initiated by human-emitted halogen and nitrogen species. International policy reduced the worst ODSs, and recovery is ongoing but slow; continued vigilance is required.

Fig 13 — Educational Diagram: Acid Rain

Definition: Acid rain (acid deposition) is precipitation (rain, snow, fog, dew) with a pH lower than that of natural, unpolluted rain (~5.6). It is caused mainly by atmospheric oxidation of sulfur dioxide (SO2) and nitrogen oxides (NO and NO2, collectively NOx) to form strong acids (sulfuric and nitric acid) that dissolve in water droplets.

Why natural rain is slightly acidic: Carbon dioxide in air dissolves in rainwater and forms carbonic acid: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−. This gives unpolluted rain a typical pH ≈ 5.6.

Main formation processes (overview):

• Emission: SO2 and NOx are emitted from combustion of fossil fuels (power plants, industry), vehicles, some natural sources (volcanoes, lightning).
• Atmospheric oxidation: gaseous SO2 and NO are oxidized (in gas and aqueous phases) to form acids.
• Deposition: Acids are removed from the atmosphere by wet deposition (rain, snow, fog) or dry deposition (acidic gases/particles settling on surfaces).

Key chemical reactions (simplified):

• Formation of sulfuric acid: SO2 + 1/2 O2 → SO3 (gas-phase oxidation)
  SO3 + H2O → H2SO4 (aq)
• Atmospheric oxidation pathway (radical/aqueous): SO2 + OH → HOSO2 → … → H2SO4
• Formation of nitric acid: 2 NO + O2 → 2 NO2
  NO2 + OH → HNO3
• Acid–carbonate reaction (damage to carbonate materials): CaCO3 + H2SO4 → CaSO4 + CO2 + H2O
• Definition of acidity: pH = −log10[H+]

Typical pH range: Unpolluted rain ≈ 5.6 (due to CO2). Acid rain often has pH 4.0–5.5; very acidic rain can be <4.

Transport: SO2 and NOx can be transported hundreds to thousands of kilometres before converting to acids, so emissions in one region can cause acid deposition far away.

Environmental and material effects:

• Aquatic ecosystems: Lowered pH dissolves toxic metals (e.g., Al3+) from soils, kills fish and aquatic organisms, and alters biodiversity.
• Soils and forests: Leaching of essential base cations (Ca2+, Mg2+, K+) and mobilization of aluminum damages tree roots and reduces soil fertility.
• Buildings and cultural heritage: Acid attack dissolves carbonate stones (limestone, marble) and corrodes metals; sulphation blackens and erodes monuments.
• Human health: Acid rain itself is not directly harmful by contact, but the precursor pollutants (SO2, NOx, particulate matter) affect respiratory health and visibility.

Control and prevention measures: Reduce emissions of SO2 and NOx by using low-sulfur fuels, flue-gas desulfurization (scrubbers), catalytic converters, switching to cleaner energy (natural gas, renewables), energy efficiency, and regulatory policies (emission limits, cap-and-trade).

Measurement: Precipitation pH and conductivity are measured with field collectors and pH meters; chemical analysis identifies sulfate, nitrate, ammonium and other ions.

CBSE tip: Remember the important reactions, the pH concept, causes (SO2, NOx), types of deposition (wet/dry), major effects on living and non-living systems, and common control technologies.

Fig 14 — Educational Diagram: Smog

What is smog?
Smog is a type of air pollution that reduces visibility and harms health, formed when primary pollutants react in the atmosphere to produce secondary pollutants. Two classic types are:

• Industrial (London or sulfurous) smog: associated with coal/biomass burning under cool, humid, stagnant conditions. Rich in SO2, particulate matter and sulfuric acid (acid smog).
• Photochemical (Los Angeles) smog: associated with vehicle exhaust and volatile organic compounds (VOCs) under strong sunlight and stagnant air. Rich in ozone (O3), peroxyacyl nitrates (PAN), aldehydes and other oxidants.

Formation mechanisms (brief)

• Industrial/sulfurous smog: combustion of sulfur-containing fuels releases SO2 which is oxidized to SO3 and hydrolysed to form H2SO4; particulate soot + sulfate aerosols + fog create dense, sulfurous smog. Typical steps:
  

  S + O2 → SO2
  2 SO2 + O2 → 2 SO3
  SO3 + H2O → H2SO4 (aerosol)

• Photochemical smog: emitted NOx and VOCs react under sunlight. A simple chain:
  

  NO2 + hv → NO + O
  O + O2 + M → O3 + M
  VOCs (RH) + OH → R· + H2O → RO2· (peroxy radicals)
  RO2· + NO → RO· + NO2 (regenerates NO2 and promotes O3 formation)
  Peroxyacetyl radical + NO2 → PAN (peroxyacetyl nitrate)

  Net effect: VOC + NOx + sunlight → O3 + PAN + aldehydes + radicals

Factors favouring smog formation

• High emissions of SO2, NOx, VOCs and particulate matter (vehicles, industry, biomass/coal burning)
• Strong sunlight (for photochemical smog)
• Temperature inversion and stagnant air (trap pollutants near surface)
• Low wind speeds and topographical basins

Impacts

• Human health: irritation of eyes, nose and throat, respiratory problems, aggravation of asthma, long-term lung damage.
• Vegetation/crops: ozone and oxidants reduce photosynthesis and yields; acid smog causes acid deposition and leaf damage.
• Materials and visibility: corrosion of metals, decay of building materials, reduced visibility.

Control and prevention

• Reduce emissions: cleaner fuels, desulfurization, catalytic converters, control VOCs.
• Improve dispersion: urban planning, reduce temperature inversions by altering emissions timing.
• Regulation and monitoring: air quality standards, real-time alerts and restrictions on high-pollution activities.

Class 11 exam focus: understand the two types (sulfurous vs photochemical), key chemical steps leading to O3 and H2SO4, meteorological conditions that favour smog, major health/environmental effects, and basic mitigation measures.

Fig 15 — Educational Diagram: Water Pollution: Types and Sources

Definition: Water pollution is the contamination of water bodies (rivers, lakes, oceans, groundwater) by substances that reduce water quality and harm organisms and ecosystems. Pollutants may be chemical, physical or biological.

1. Classification of Water Pollution (Types)

• By origin
  • Point-source pollution: contaminants discharged from a single identifiable location (e.g., a factory outlet, sewage pipe).
  • Non-point-source pollution: diffuse sources spread over large areas (e.g., agricultural runoff, urban stormwater).
• By chemical/biological nature
  • Organic pollution: biodegradable matter (sewage, food wastes) that increases biological oxygen demand (BOD).
  • Inorganic pollution: salts, acids, alkalis, heavy metals (Pb, Hg, Cd), nutrients (nitrate, phosphate).
  • Microbial pollution: pathogenic bacteria, viruses and parasites (e.g., faecal coliforms) causing water-borne disease.
  • Toxic/ persistent pollutants: non-biodegradable organics (PCBs, DDT), heavy metals and synthetic chemicals that accumulate in food chains.
• Other types
  • Thermal pollution: elevated temperature from industrial cooling water reducing dissolved oxygen (DO).
  • Radioactive pollution: radionuclide release from nuclear facilities or mining.
  • Plastic/microplastic pollution: physical hazards and chemical leaching.

2. Major Sources of Water Pollution

• Domestic sewage and municipal waste: raw or partially treated sewage increases BOD, introduces pathogens and nutrients.
• Agricultural runoff: fertilizers (nitrates, phosphates) cause eutrophication; pesticides contaminate surface and groundwater.
• Industrial effluents: heavy metals, toxic organics, acids/alkalis and suspended solids from tanneries, electroplating, chemical plants.
• Mining and quarrying: acid mine drainage (sulphuric acid + dissolved metals), suspended solids.
• Oil spills and marine dumping: hydrocarbons, long persistence, damage to marine life and coastlines.
• Urban runoff and stormwater: oil, grease, heavy metals from roads, sediments.
• Atmospheric deposition: airborne pollutants (SOx, NOx, particulates) that dissolve in rain and enter water bodies (acid rain).
• Groundwater contamination: leaching of nitrates, solvents, heavy metals from landfills and improper disposal.

3. Important Indicators and Impacts

• Dissolved oxygen (DO): low DO (< 3 mg L-1) stresses aquatic life.
• Biochemical Oxygen Demand (BOD): measure of biodegradable organic load.
• Chemical Oxygen Demand (COD): total oxygen equivalent of oxidizable matter (both biodegradable and non-biodegradable).
• Nutrients (N, P): cause algal blooms and eutrophication leading to oxygen depletion.
• Heavy metals and persistent organics: toxic, bioaccumulative and may cause chronic health effects.

4. Short note on impacts

Water pollution leads to loss of biodiversity, unsafe drinking water, fisheries collapse, disease outbreaks (cholera, dysentery), and economic losses. Thermal and chemical pollution alter species composition and reduce dissolved oxygen.

5. Prevention overview

Key controls: wastewater treatment (primary/secondary/tertiary), agricultural best practices (buffer strips, controlled fertilizer use), industrial effluent treatment, safe disposal, oil-spill response, regulations and monitoring.

Fig 16 — Educational Diagram: Noise Pollution

Definition: Noise pollution is unwanted or harmful sound that interferes with normal activities, causes annoyance, health issues or environmental harm. It is an important component of environmental chemistry because sound energy (not a chemical pollutant) affects human health and ecosystems and is regulated to protect public welfare.

Sources: Road traffic, railways, aircraft, industrial machinery, construction work, loudspeakers, household appliances, urban nightlife, and marine noise from ships and sonar.

Characteristics of noise:

• Intensity (I): energy flux per unit area (W m⁻²).
• Sound pressure (p): local pressure variation (Pa).
• Frequency (f): cycles per second (Hz) — high-frequency sounds are perceived as shriller, low-frequency as rumble.
• Duration and temporal pattern: continuous, intermittent or impulsive noise — health impact depends on total exposure.

Measuring sound — the decibel (dB) scale: Human ear responds over many orders of magnitude of intensity, so sound levels are reported on a logarithmic decibel scale. Typical reference intensity I₀ = 10⁻¹² W m⁻² and reference RMS sound pressure p₀ = 20 μPa.

Health and environmental effects:

• Auditory: temporary or permanent hearing loss, tinnitus (ringing), ear pain.
• Non-auditory: sleep disturbance, increased stress and cortisol, elevated blood pressure, cardiovascular risk, reduced concentration and learning in children, annoyance and reduced quality of life.
• Ecological: disruption of animal communication, altered predator–prey behavior, displacement of wildlife, effects on marine mammals from shipping and sonar.

Regulation (India example — CPCB ambient noise standards): permissible ambient noise levels (A-weighted dB(A))

• Day (06:00–22:00): Industrial 75, Commercial 65, Residential 55, Silence zone 50
• Night (22:00–06:00): Industrial 70, Commercial 55, Residential 45, Silence zone 40

Control measures: engineering (sound barriers, mufflers, quieter machinery, building insulation), urban planning (zoning, green belts), administrative (time restrictions, quieter road surfaces, traffic management), personal protective equipment (earplugs), public awareness and enforcement of standards.

Notes for students: In chemistry/environment units, emphasize how noise is quantified, the logarithmic nature of the decibel scale, how multiple sources combine, and the public-health rationale for standards and mitigation.

Fig 17 — Educational Diagram: Water Quality Parameters and Concepts

Overview
Water quality parameters are physical, chemical and biological measurements that describe the suitability of water for drinking, irrigation, aquatic life and industrial uses. Typical parameters: pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), hardness, alkalinity, turbidity, total dissolved solids (TDS), conductivity, nutrients (nitrate, phosphate) and microbiological indicators (coliforms).

1. pH
pH measures H+ activity (acidity/alkalinity). Safe drinking-water range (BIS/WHO) is about 6.5–8.5. pH affects solubility and toxicity of many substances (e.g., metals more soluble and toxic at low pH).

2. Dissolved Oxygen (DO)
DO = amount of gaseous O2 dissolved in water (mg·L⁻¹). Essential for aerobic organisms. DO saturation depends strongly on temperature and atmospheric pressure (colder water holds more O2). Low DO indicates organic pollution or eutrophication.

3. Biochemical Oxygen Demand (BOD)
BOD measures the amount of dissolved O2 required by microorganisms to biologically decompose organic matter under aerobic conditions over a specified time (commonly 5 days at 20 °C → BOD5). High BOD means large biodegradable organic pollution and can cause DO depletion and fish kills.

4. Chemical Oxygen Demand (COD)
COD measures the oxygen equivalent of organic matter oxidizable by a strong chemical oxidant (commonly dichromate in acidic medium). COD typically ≥ BOD; useful for total organic load including non-biodegradable organics.

5. Hardness
Hardness is mainly due to Ca²⁺ and Mg²⁺. Temporary hardness is due to bicarbonates (removed by boiling), permanent hardness due to sulphates and chlorides (not removed by boiling). Hard water affects soap efficiency and causes scale in boilers.

6. Alkalinity
Alkalinity is the capacity of water to neutralize acid; it is mainly from HCO₃⁻, CO₃²⁻ and OH⁻ and is reported as mg·L⁻¹ CaCO₃. Alkalinity buffers pH changes; too low alkalinity makes water susceptible to pH swings.

7. Turbidity, TDS and Conductivity
Turbidity (measured in NTU) is caused by suspended particles and reduces light penetration. TDS (mg·L⁻¹) represents dissolved solids (salts, organics). Electrical conductivity (µS·cm⁻¹) correlates with dissolved ionic content (TDS). High TDS affects taste, irrigation and industrial processes.

8. Nutrients and Microbial Indicators
Excess nitrate and phosphate cause eutrophication (algal blooms). Nitrate limit for drinking ~50 mg·L⁻¹ as NO3⁻ (WHO) or 45 mg·L⁻¹ in some standards. Coliform bacteria (esp. E. coli) indicate faecal contamination; drinking water should have zero coliforms per 100 mL.

Relationships and causes
- Organic pollution ↑ → BOD and COD ↑ → microbial respiration ↑ → DO ↓.
- Nutrient enrichment → algal growth → diurnal DO swings (photosynthesis/day ↑DO, respiration/night ↓DO).
- Temperature ↑ → DO solubility ↓; metabolic rates ↑ (can worsen DO depletion).

Monitoring & Interpretation
Regular field/ laboratory measurements and standards (BIS/WHO) are used to judge suitability. Many parameters are interlinked; a single parameter rarely gives a full picture.

Fig 18 — Educational Diagram: Sewage and Wastewater Treatment

Overview: Sewage (domestic wastewater) and wastewater (domestic + industrial + storm) contain organic matter, suspended solids, nutrients (N, P), pathogens and sometimes toxic chemicals. The objective of treatment is to protect public health and the environment by reducing solids, organic load, nutrients and pathogens so the effluent can be safely discharged or reused.

Qualitative characteristics: Key parameters are BOD (biochemical oxygen demand), COD (chemical oxygen demand), TSS (total suspended solids), pH, nutrients (N, P), pathogens, and toxic contaminants. Typical raw municipal sewage: BOD 200–400 mg/L, TSS 150–350 mg/L (ranges are indicative).

Treatment stages (conceptual flow):

• Preliminary treatment: Screening (removes rags, plastics), grit removal (sand, grit) to protect downstream equipment.
• Primary (physical) treatment: Sedimentation (primary clarifier) removes settleable solids and floating matter. Typical removal: 30–50% BOD, 50–70% TSS (varies).
• Secondary (biological) treatment: Biological oxidation of dissolved organic matter using microbes.
  • Activated sludge process: Aeration tank + secondary clarifier; microbes (flocs) biodegrade organics; part of biomass is wasted, some recycled. Key design quantities: HRT, F/M ratio, SRT (sludge age).
  • Trickling filters / biofilm reactors: Wastewater passes over media colonized by biofilm which oxidizes organics.
  • Stabilization ponds (oxidation ponds): Shallow basins using algal photosynthesis and bacterial action for treatment (low-cost, land-intensive).
• Tertiary (advanced) treatment: Nutrient removal (nitrogen and phosphorus), advanced filtration, adsorption (activated carbon), membrane processes, and final polishing to meet discharge or reuse standards.
• Disinfection: Chlorination, ozonation or UV to reduce pathogens. Chlorine is most common in municipal plants; ozone and UV are options when chlorine by-products are a concern.
• Sludge (biosolids) treatment: Thickening, anaerobic or aerobic digestion (reduces pathogens and volatile solids), dewatering, stabilization and disposal or beneficial use (composting, land application, biogas from anaerobic digestion).

Important operational concepts:

• Hydraulic retention time (HRT): time wastewater spends in a tank; affects contact and treatment.
• Solids retention time / Sludge age (SRT): average time biomass remains in system—affects microbial community and nitrification capability.
• Food to microorganism ratio (F/M): influent organic load per unit biomass; controls process stability.

Simple plant example (municipal): Screening → Grit chamber → Primary clarifier → Aeration basin (activated sludge) → Secondary clarifier → Disinfection → Discharge. Sludge: primary sludge + waste activated sludge → thickening → digestion → dewatering → disposal or reuse.

Environmental and health considerations: Proper treatment prevents oxygen depletion in receiving waters (lowers risk of fish kills), reduces disease transmission, and enables safe reuse (irrigation, industrial cooling) when standards are met. Industrial wastewaters often need pretreatment to remove toxic compounds before being combined with municipal sewage.

Design & monitoring: Plants are designed using mass balances and empirical/kinetic models (e.g., first-order BOD decay). Regular monitoring of BOD, COD, TSS, pH, and chlorine residual (for disinfection) ensures compliance with standards.

Fig 19 — Educational Diagram: Soil Pollution and Degradation

Definition: Soil pollution is the presence of human-made chemicals or other alteration in the natural soil environment that has harmful effects. Soil degradation is the decline in soil quality caused by natural or anthropogenic processes that reduce its capacity to support plant life and ecosystem services.

Main sources of soil pollution

• Agricultural inputs: excessive use of chemical fertilizers, pesticides (organophosphates, organochlorines like DDT), herbicides and irrigation with poor-quality water (saline or sodic).
• Industrial activities: deposition of heavy metals (Pb, Cd, Hg, Cr, As), disposal of industrial effluents, sludge and fly ash.
• Urban and municipal wastes: dumping of solid wastes, landfill leachates, sewage sludge containing organic contaminants and heavy metals.
• Mining and accidental spills: acid mine drainage, release of metals and metalloids.
• Atmospheric deposition: acid rain (from SO2, NOx) and airborne particulates.

Key processes of soil degradation

• Erosion: removal of topsoil by water or wind, reducing nutrient-rich layer.
• Salinization: accumulation of soluble salts in the root zone (often from improper irrigation and high water tables).
• Alkalinization/sodication: high exchangeable sodium percentage (ESP) causes poor structure and reduced infiltration.
• Acidification: from acid rain, excessive N-fertilizers, or leaching of basic cations (Ca2+, Mg2+, K+).
• Loss of organic matter and biodiversity: intensive cultivation, burning, lack of crop rotations reduce soil organic carbon and microbial activity.
• Compaction: heavy machinery increases bulk density, reducing porosity and root growth.

Environmental and health impacts

• Reduced agricultural productivity and crop quality.
• Bioaccumulation and biomagnification of persistent pollutants (e.g., organochlorine pesticides, heavy metals) in food chains causing human and animal health risks.
• Contamination of groundwater via leaching (nitrates, pesticides, heavy metals) causing drinking-water hazards.
• Loss of ecosystem services: reduced water infiltration, carbon sequestration, and biodiversity.

Chemical reactions and important processes (examples)

• Acid rain formation: SO2/NOx oxidation to acids: NO2 + OH → HNO3; SO2 + 1/2 O2 → SO3; SO3 + H2O → H2SO4.
• Neutralization of acid in soil by lime: CaCO3 + H2SO4 → CaSO4 + CO2 + H2O.
• First-order decay of pesticide in soil: C(t) = C0 e^−kt, t1/2 = ln2 / k.

Remediation and management

• Preventive measures: integrated pest management (IPM), precision fertilizer application, use of organic amendments (compost), controlled irrigation to avoid waterlogging and salinization.
• Physical/chemical remediation: soil washing, stabilization/solidification, excavation and safe disposal for hotspots.
• Biological remediation: bioremediation (microbial degradation), phytoremediation (hyperaccumulator plants), phytostabilization.
• Soil conservation practices: contour ploughing, terracing, cover crops, crop rotation, reduced tillage to prevent erosion and maintain organic matter.

Monitoring indicators: soil pH, electrical conductivity (EC), sodium adsorption ratio (SAR), bulk density, organic carbon content, cation exchange capacity (CEC), concentrations of specific contaminants (nitrates, pesticides, heavy metals).

Fig 20 — Educational Diagram: Water Quality Parameters

Overview: Water quality parameters are measurable physical, chemical and biological characteristics used to assess the suitability of water for drinking, irrigation, aquatic life and industrial uses. These parameters indicate pollution, nutrient load, hardness, salinity and biological contamination.

Key physical parameters

• pH — measure of acidity/alkalinity (scale 0–14). Affects solubility and toxicity of chemicals. Typical desirable range for drinking water: ~6.5–8.5.
• Turbidity — cloudiness due to suspended particles, measured in NTU (nephelometric turbidity units). High turbidity reduces light penetration and can protect pathogens from disinfection.
• Total Dissolved Solids (TDS) — dissolved salts and organic matter, given in mg/L. TDS affects taste, scaling and conductivity.
• Temperature — affects dissolved oxygen (DO) and reaction rates; warmer water holds less oxygen.

Key chemical parameters

• Dissolved Oxygen (DO) — concentration of free oxygen in water (mg/L). Essential for aquatic life; saturation depends on temperature and pressure. Low DO indicates organic pollution or eutrophication.
• Biochemical Oxygen Demand (BOD) — amount of oxygen consumed by microorganisms to decompose organic matter over a specified period (commonly BOD5, mg/L). High BOD → high organic pollution.
• Chemical Oxygen Demand (COD) — oxygen equivalent of organic and oxidizable inorganic matter determined by chemical oxidation. COD ≥ BOD and is faster to measure.
• Hardness — concentration of multivalent cations, mainly Ca2+ and Mg2+, expressed as mg/L as CaCO3. Affects soap use, scaling in boilers and industrial equipment.
• Alkalinity — water's buffering capacity (mainly HCO3-, CO32-, OH-), expressed as mg/L as CaCO3. Determines how pH changes on addition of acid; important for aquatic systems and treatment processes.
• Conductivity / Electrical Conductance (EC) — indicates ionic content; useful proxy for salinity and TDS. Units: µS/cm.
• Nutrients (nitrate, nitrite, phosphate) — excess leads to eutrophication. Nitrate in drinking water is a health risk (infant methemoglobinemia).
• Chloride, sulfate, heavy metals — affect taste, corrosion and toxicity. Heavy metals (Pb, Cd, Hg, As) are toxic even at low concentrations.

Key biological parameters

• Total coliforms / E. coli — indicators of fecal contamination and pathogen risk. Presence in potable water indicates need for disinfection.
• Algal indicators — bloom presence signals eutrophication and possible toxin production.

Interpretation and relationships

• High BOD/COD and low DO indicate organic pollution (sewage, effluent). BOD5 >5 mg/L often suggests pollution; unpolluted waters usually have BOD <3 mg/L.
• Hardness as CaCO3 is sum of calcium and magnesium contributions; temporary hardness (bicarbonates) can be removed by boiling, permanent hardness (sulfates/chlorides) cannot.
• pH influences the carbonate system: at pH >8.3 CO32- predominates, between ~6.3–8.3 HCO3- predominates, below ~6.3 CO2(aq)/H2CO3 predominates (speciation diagram useful).
• Conductivity correlates with TDS (TDS ≈ k × EC, k ≈ 0.5–0.9, often taken ≈0.64–0.67 for rough estimates).

Why these parameters matter (real-world effects)

• Drinking water: taste (TDS, chloride), corrosivity (low alkalinity/pH), health risks (nitrate, pathogens, heavy metals).
• Aquatic life: DO and temperature determine survival; eutrophication (excess nitrate/phosphate) can cause fish kills.
• Industry: hardness causes scaling in boilers; high TDS and sulfate affect cooling systems and process water quality.
• Agriculture: salinity (EC/TDS) and specific ion toxicity (Na+, Cl-) affect crop yield and soil structure.

Measurement methods (brief)

• pH: pH meter
• DO: electrochemical probes (Winkler titration as reference)
• BOD: incubation at 20°C for 5 days (BOD5)
• COD: dichromate oxidation and titration
• Hardness/Alkalinity: titrations with EDTA (complexometric) or acids
• Turbidity: nephelometer (NTU)
• Microbial indicators: membrane filtration or multiple-tube fermentation

Note: Numerical guideline values differ by country and application (drinking, aquatic life, irrigation). Always consult local regulatory standards (WHO, BIS, EPA) for limits.

Fig 21 — Educational Diagram: Eutrophication

Definition: Eutrophication is the process by which a water body (lake, river, estuary, coastal sea) becomes enriched with nutrients — primarily nitrogen (N) and phosphorus (P) — causing excessive growth of algae and aquatic plants. This often leads to oxygen depletion, loss of biodiversity and deterioration of water quality.

How it happens (stepwise):

• Input of nutrients (mainly phosphate and nitrate) from natural sources and human activities (agricultural runoff, sewage, detergents, industrial effluents).
• Rapid growth of phytoplankton and algae (algal bloom). Some blooms are harmful algal blooms (HABs) that produce toxins.
• Increased biomass eventually dies and settles; aerobic bacteria decompose the organic matter.
• Microbial decomposition consumes dissolved oxygen (DO), causing hypoxia (low DO) or anoxia (no DO).
• Oxygen depletion causes fish kills, loss of aerobic organisms, shifts to anaerobic processes (sulfide, methane production), foul smells and poor water quality.

Types: Natural (slow, geological timescale) and cultural/accelerated eutrophication (caused by human activities; faster and more severe).

Key ecological points:

• Limiting nutrient: In freshwater systems phosphorus is usually the limiting nutrient; in many marine systems nitrogen can be limiting.
• Redfield C:N:P ratio (average stoichiometry for marine phytoplankton): 106:16:1 — used to assess nutrient limitation.
• Algal blooms reduce light penetration (increased turbidity) affecting submerged vegetation.
• Some algal blooms produce toxins (e.g., microcystins from cyanobacteria) that threaten human and animal health.

Consequences: fish and invertebrate kills, reduced biodiversity, production of toxic compounds, bad odor and taste in drinking water, formation of ‘dead zones’ (large hypoxic areas in coastal seas).

Prevention and control:

• Reduce nutrient inputs: improved sewage treatment (tertiary nutrient removal), limit phosphate in detergents, better fertilizer management and buffer strips along waterways.
• Constructed wetlands and vegetated strips to trap nutrients.
• In-lake measures: aeration, dredging, chemical P inactivation (e.g., alum) and biomanipulation in specific cases.
• Policy and land-use practices: integrated watershed management and monitoring.

Short note for students: Eutrophication is a clear example of how human activity alters biogeochemical cycles (N and P) and can rapidly change ecosystem structure and function. Understanding indicators like BOD, COD and DO is important for water quality assessment.

Fig 22 — Educational Diagram: Solid Waste Management

What is solid waste? Solid waste (or municipal solid waste, MSW) is any discarded solid material from households, institutions, industries and commercial activities that is no longer useful. It includes organic kitchen waste, paper, plastics, glass, metals, textiles, construction debris, e-waste and hazardous wastes.

Why manage it? Improper disposal pollutes soil, water and air; causes greenhouse gas emissions (methane from anaerobic decomposition); spreads disease; and wastes recoverable resources and energy. Good management protects public health, conserves resources and recovers energy.

Sources and composition

• Household/municipal (organic waste, paper, plastic, glass, metal, textile, inert)
• Industrial (process wastes, sludges, hazardous wastes)
• Biomedical (infectious, pathological, sharps)
• Construction and demolition debris
• E-waste (discarded electronic equipment)

Key principles (3Rs and hierarchy)

• Reduce (minimise generation)
• Reuse (use again without major processing)
• Recycle (material recovery and reprocessing)
• Energy recovery (waste-to-energy by combustion, biogas)
• Safe disposal (sanitary landfill, engineered containment)

Management steps

• Segregation at source: separate organic, recyclable, hazardous and inert fractions.
• Collection & transport: scheduled picking, transfer stations.
• Treatment: biological (composting, vermicomposting, anaerobic digestion), thermal (incineration, pyrolysis), mechanical (shredding, baling) and chemical for certain wastes.
• Recovery: recycling of paper, metal, glass, plastics; material recovery facilities (MRFs).
• Disposal: sanitary landfills with liners, leachate collection and gas management.

Major treatment methods (brief)

• Composting (aerobic): microbes convert organic matter to stable humus-like compost. Needs C:N balance (~30:1), aeration, moisture control.
• Vermicomposting: earthworms accelerate decomposition to high-quality compost.
• Anaerobic digestion: microbes produce biogas (CH4 + CO2) and digestate. Used for high-moisture organics.
• Incineration: high-temperature combustion reduces volume, destroys pathogens; produces ash and flue gases — requires emission control.
• Sanitary landfills: engineered disposal with liners, leachate treatment and landfill gas capture to reduce pollution.
• Recycling: sorting, cleaning and reprocessing materials into new products.
• Hazardous & biomedical waste: special handling, sterilization (autoclave), secure disposal or incineration.

Environmental concerns & controls

• Leachate: polluted liquid from waste percolation — treat by collection, biological/chemical processes.
• Landfill gas: mainly CH4 and CO2 — capture and use as fuel or flare to reduce greenhouse effect.
• Air emissions from incinerators: particulate matter, NOx, SOx, dioxins — controlled by filtration and scrubbers.
• Illegal open dumping & burning: causes air pollution, contaminates groundwater and promotes disease vectors.

Performance indicators & planning

• Per-capita waste generation (kg/person/day).
• Segregation/recycling rates and diversion from landfill (%).
• Calorific value of refuse (important for WtE plants).
• Life-cycle thinking: energy, emissions and resource recovery across the system.

Note: Effective solid waste management combines technical measures, public participation (source segregation), economic instruments and regulations.

Fig 23 — Educational Diagram: Agricultural Chemicals: Fertilizers and Pesticides

Overview
Agricultural chemicals are substances applied to crops and soils to increase yield and protect plants. The two main classes are fertilizers (supply nutrients) and pesticides (control pests, weeds and diseases). Proper use raises productivity; misuse causes environmental and health problems.

Fertilizers
Fertilizers supply essential plant nutrients, mainly nitrogen (N), phosphorus (P) and potassium (K) — the NPK nutrients. An NPK label like '10-20-10' gives percentages of available N, P2O5 and K2O respectively. Types:

• Natural/organic: manure, compost, bone meal, rock phosphate.
• Inorganic/chemical: nitrogenous (urea, ammonium sulfate), phosphatic (single/triple superphosphate), potassic (muriate of potash KCl).
• Special: micronutrient fertilizers (Fe, Zn, Mn), slow-release and controlled-release fertilizers, and biofertilizers (Rhizobium, Azotobacter, blue-green algae).

Key industrial processes

• Haber process (ammonia synthesis): N2 + 3H2 ⇌ 2NH3 (Fe catalyst; high pressure/temperature) — base for most N-fertilizers.
• Ostwald process (nitric acid from NH3) — used to make nitrates and ammonium nitrate.
• Urea synthesis: 2NH3 + CO2 → NH2CONH2 + H2O — urea is high in N (~46% N).
• Phosphate fertilizers by treating rock phosphate with acids to make water-soluble phosphates (single/triple superphosphate).

Environmental issues from fertilizers

• Eutrophication: runoff carrying nitrates and phosphates causes algal blooms, oxygen depletion and fish kills in lakes/estuaries.
• Groundwater contamination: nitrates leach causing methemoglobinemia (''blue baby'' syndrome); WHO guideline for nitrate (NO3-) in drinking water is 50 mg/L.
• Soil salinization and acidification from overuse or inappropriate fertilizer types.

Pesticides
Pesticides are chemicals that kill or repel pests. Categories by target: insecticides, herbicides, fungicides, rodenticides, nematicides. Categories by chemistry: organochlorines (DDT, BHC), organophosphates (malathion, parathion), carbamates (carbaryl), pyrethroids (permethrin), and biologicals (Bt, neem-based).

Modes of action

• Contact vs systemic (absorbed and translocated in plant).
• Stomach poisons (ingested by pests), fumigants (gaseous action), repellents.
• Biochemical modes: neurotoxins (organophosphates inhibit acetylcholinesterase), metabolic inhibitors, growth regulators.

Problems with pesticides

• Persistence and bioaccumulation: organochlorines (DDT, aldrin) persist, biomagnify up food chains and harm birds, mammals and humans.
• Resistance: repeated use selects resistant pest populations.
• Non-target effects: beneficial insects (pollinators), aquatic life and soil microbes are harmed.
• Human health risks: acute poisoning, chronic effects (carcinogenicity, endocrine disruption) and residues in food.

Safe and sustainable approaches

• Integrated Nutrient Management (balanced NPK, use of organic matter and biofertilizers).
• Integrated Pest Management (IPM): monitoring, biological control, cultural practices, selective and minimal pesticide use.
• Use of biopesticides (Bt, neem) and modern low-residue molecules; proper storage, application and personal protective equipment (PPE).

Summary
Fertilizers and pesticides are essential for modern agriculture but must be managed to maximize crop benefit while minimizing pollution, health risks and ecosystem damage. Education, regulations (maximum residue limits, safe handling) and adoption of sustainable alternatives reduce negative impacts.

Fig 24 — Educational Diagram: Heavy Metal and Toxic Contamination

Definition: Heavy metals are metallic elements with relatively high density and toxicity at low concentrations (commonly: lead, mercury, cadmium, arsenic, chromium, copper, zinc). "Toxic contamination" refers to their introduction into air, water, or soil such that ecosystem and human health are harmed.

Sources:

• Natural: weathering of rocks, volcanic emissions.
• Anthropogenic: mining, smelting, battery and electronics manufacturing, tanneries, pesticides, coal combustion, artisanal gold mining, improper disposal and e‑waste recycling.

Environmental chemistry & speciation: Toxicity and mobility depend on chemical form (oxidation state, complexes, organometallic species). Examples: Cr(VI) (chromate/dichromate) is highly mobile and carcinogenic, while Cr(III) forms less soluble hydroxides and is less toxic. Mercury can be converted by microbes to methylmercury (CH3Hg+), which biomagnifies in food chains.

Key behaviour: Solubility, redox chemistry, pH and complexation control transport. Many heavy metals precipitate as (hydr)oxides or sulfides under suitable conditions (e.g., MS(s) for many M2+), or adsorb to sediments and organic matter. Adsorption/desorption and partitioning between phases determine exposure.

Bioaccumulation & biomagnification: Bioaccumulation factor (BAF) and biomagnification lead to higher concentrations in organisms and top predators (including humans). Lipophilic or strongly binding organometallic forms (e.g., methylmercury) are especially prone to biomagnification.

Health effects: Neurotoxicity (Hg, Pb), kidney damage (Cd), bone demineralization (Cd), developmental delays (Pb), carcinogenicity (Cr(VI), As), gastrointestinal and skin problems (As). Chronic, low‑level exposure is often most harmful.

Monitoring & remediation: Analytical monitoring uses AAS, ICP‑MS, voltammetry. Remediation options: chemical precipitation, coagulation/flocculation, adsorption (activated carbon, biochar), ion exchange, membrane filtration (RO/NF), phytoremediation (hyperaccumulator plants), stabilization/solidification, electrochemical treatments. Choice depends on metal speciation, concentration and matrix.

Prevention & policy: Source control, proper waste treatment, safe e‑waste handling, emission controls, regular monitoring and adherence to drinking water standards are essential.

Fig 25 — Educational Diagram: Pesticides, Persistent Organic Pollutants (POPs) and Biomagnification

Overview

Pesticides are chemicals used to kill or control pests (insects, weeds, fungi, rodents). Persistent Organic Pollutants (POPs) are a subset of pollutants that are chemically stable, persist in the environment, are toxic, bioaccumulate in organisms and can travel long distances. Biomagnification is the increase in concentration of a pollutant as it moves up the food chain.

Types of pesticides (brief)

• By target: Insecticides, Herbicides, Fungicides, Rodenticides.
• By chemistry: Organochlorines (e.g., DDT, aldrin), Organophosphates (e.g., malathion), Carbamates, Pyrethroids.

Why some pesticides become POPs

• Chemical stability (resistance to hydrolysis, photolysis, biodegradation).
• Low water solubility but high lipid solubility (lipophilic) → accumulate in fat.
• Low volatility/gas exchange or, for some, sufficient volatility for long-range atmospheric transport.
• High octanol–water partition coefficient (Kow) often indicates potential to bioaccumulate.

Characteristics of POPs

• Persistence: long environmental half-lives (months to decades).
• Bioaccumulation: build-up in individual organisms over time.
• Biomagnification: increasing concentration at higher trophic levels.
• Toxicity: affects wildlife and human health (endocrine disruption, neurological damage, cancers).
• Long-range transport: found far from sources (polar regions, remote ecosystems).

Bioaccumulation vs Bioconcentration vs Biomagnification

• Bioconcentration: uptake and accumulation of a chemical from the surrounding medium (usually water) into an organism (expressed by BCF).
• Bioaccumulation: net accumulation from all sources (water, food, air) in an individual over time (expressed by BAF).
• Biomagnification: increase in pollutant concentration from one trophic level to the next (measured by BMF).

Environmental fate and effects

Pesticides/POPs applied to fields can adsorb to soil, dissolve or partition into water, volatilize to the atmosphere, be degraded slowly or taken up by plants and animals. Because many POPs are lipophilic, they concentrate in fatty tissues and persist, causing chronic effects (reproductive failure, developmental problems, immune suppression). Notable effects include DDT-induced eggshell thinning in birds and mercury-related neurological damage in humans.

Regulation and control

The Stockholm Convention (2001) targeted an initial list of problematic POPs (the "dirty dozen", e.g., DDT, aldrin, dieldrin, chlordane, heptachlor, hexachlorobenzene, mirex, toxaphene, PCBs, dioxins and furans). Countries now restrict, phase out or manage many POPs; safer pest management practices (IPM) and alternative chemicals are promoted.

Practical implications for humans and ecosystems

• POPs enter food chains — the major human exposure route is animal fats in diet (fish, meat, dairy).
• High trophic-level predators (birds of prey, marine mammals, humans) show highest concentrations.
• Monitoring uses indicators like fish tissue concentrations, eggs, adipose samples and environmental sampling.

Simple kinetic behaviour (useful in problems)

Many degradation processes follow first-order kinetics: concentration decreases exponentially with time. The octanol–water partition coefficient (Kow) is used to predict bioaccumulation potential; higher log Kow → greater lipophilicity and likely accumulation.

Key classroom points: know definitions (POPs, bioaccumulation, biomagnification, bioconcentration), examples (DDT, PCBs, dioxins), and the direction of concentration change in food chains (increases up trophic levels).

Fig 26 — Educational Diagram: Radioactive Pollution

Definition: Radioactive pollution is contamination of the environment by radioactive substances (radionuclides) or ionizing radiation, causing harmful effects to living organisms and ecosystems.

Sources

• Natural: cosmic rays, terrestrial radionuclides (uranium, thorium series), and radon gas from ground.
• Anthropogenic: nuclear power plant accidents and releases, atmospheric nuclear weapons testing, medical and industrial radioactive waste, uranium mining, improper disposal (e.g., orphan sources), and radioactive effluents from reprocessing plants.

Types of ionizing radiation

• Alpha (α): heavy, low penetration (stopped by paper/skin) but highly damaging if inhaled/ingested.
• Beta (β): lighter, greater penetration (stopped by plastic/metal), can damage tissue and contaminate food/water.
• Gamma (γ) / X-rays: highly penetrating electromagnetic radiation; external exposure hazard and difficult to shield.

Pathways and fate: Radionuclides enter air, water and soil. They can be absorbed by plants, bioaccumulate and biomagnify along the food chain, or persist in sediments and groundwater depending on chemical form and half-life.

Effects on health and environment

• Acute effects: radiation sickness from high doses (nausea, hair loss, bleeding).
• Chronic effects: increased cancer risk, genetic mutations, developmental defects, ecological damage to populations and biodiversity.
• Long-term contamination: long-lived radionuclides (e.g., Pu-239, U-238) can render areas unsafe for many generations.

Measurement units and monitoring: Activity is measured in becquerels (Bq = 1 disintegration/s) or curies (Ci; 1 Ci = 3.7×10^10 Bq). Absorbed dose = gray (Gy = J/kg). Equivalent/effective dose (biological impact) = sievert (Sv), which weights by radiation type and tissue sensitivity. Environmental monitoring uses detectors (Geiger-Mueller, scintillation, gamma spectrometers) and sampling of air, water, soil and biota.

Mitigation and control

• Containment and secure storage of radioactive waste, engineered barriers and deep geological repositories.
• Control releases, decontamination of land and water, removal/disposal of contaminated materials.
• Monitoring, emergency preparedness, public protection (evacuation, sheltering, iodine tablets for radioiodine), and regulation of use and transport.

Importance of half-life: Half-life determines how long a radionuclide remains hazardous. Short-lived isotopes cause high short-term activity; long-lived isotopes are long-term contamination hazards.

Fig 27 — Educational Diagram: Environmental Monitoring and Analysis

What it is: Environmental monitoring and analysis is the systematic measurement of physical, chemical and biological parameters in air, water, soil and biota to assess environmental quality, detect pollution, and guide remediation and policy.

Main objectives: detect contaminants, determine concentrations and trends, check compliance with standards, identify pollution sources, and provide data for risk assessment.

Key types of parameters:

• Physical: temperature, turbidity, total suspended solids (TSS), conductivity, colour.
• Chemical: pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), nutrients (nitrate, phosphate), heavy metals (Pb, Cd, Hg), gases (CO, SO2, NOx, O3), volatile organic compounds (VOCs).
• Biological: coliform bacteria, algal counts, bioindicators, species diversity.

Sampling and QA/QC: Representative sampling (grab vs composite), correct containers, preservation (refrigeration, acidification), chain of custody and quality controls (blanks, duplicates, calibration standards) are essential to get reliable data.

Common analytical methods and instruments: pH meters and electrodes; DO meters (electrochemical or optical); spectrophotometry (for nitrate, phosphate, colorimetric tests); titrations (alkalinity, hardness, COD back-titration); gravimetric methods (TSS, particulate matter); gas analyzers and electrochemical sensors (CO, SO2, NOx); atomic absorption spectrometry (AAS) or ICP-OES for metals; gas chromatography (GC) for VOCs; biological assays for BOD and microbial tests.

Data interpretation: Compare measured values with regulatory standards (national or WHO guidelines), plot time series to detect trends, use spatial maps to find pollution hotspots, and apply simple models (e.g., first-order decay of BOD) to estimate pollutant fate.

Why it matters: Monitoring provides early warning of environmental degradation, helps protect public health, supports enforcement of environmental law, and informs pollution control strategies.

Fig 28 — Educational Diagram: Wastewater and Sewage Treatment

Introduction

Wastewater (sewage) is water that has been used in households, industries, agriculture or stormwater runoff and contains dissolved and suspended impurities. Treatment is required to protect public health and the environment and to enable reuse or safe discharge.

Key characteristics

• BOD (Biochemical Oxygen Demand): amount of dissolved oxygen needed by aerobic microorganisms to decompose organic matter (usually measured as BOD5, oxygen consumed in 5 days).
• COD (Chemical Oxygen Demand): oxygen equivalent of organic (and some inorganic) matter oxidized chemically (faster test than BOD).
• DO (Dissolved Oxygen): O2 available in water for aquatic life; low DO indicates pollution.
• TSS (Total Suspended Solids): particulate matter that can be removed by sedimentation/filtration.
• Nutrients (N, P): cause eutrophication if not removed.
• Pathogens: bacteria, viruses, protozoa—must be reduced before reuse or discharge.

Overall treatment stages

1. Preliminary treatment: screening to remove large debris (rags, plastics), grit removal to remove sand/grit.
2. Primary treatment: physical settling (primary clarifiers); removes large suspended solids and some BOD (typically 25–50%).
3. Secondary (biological) treatment: removes dissolved/colloidal organics using microorganisms.
   • Aerobic processes: activated sludge process (air/oxygen supplied to mixed liquor), trickling filters (biofilms on media), oxidation ponds.
   • Anaerobic processes: UASB, anaerobic digesters—useful for high-strength industrial waste (produces biogas).
4. Tertiary (advanced) treatment: nutrient removal (nitrification/denitrification for nitrogen, chemical or biological phosphorus removal), filtration (sand, membrane), adsorption (activated carbon), advanced oxidation (ozone, UV), and disinfection (chlorination, UV) to remove pathogens and residual contaminants.
5. Sludge (biosolids) treatment: thickening, anaerobic/aerobic digestion (stabilisation; reduces pathogens, produces biogas), dewatering and disposal or beneficial reuse (composting, land application).

Important biological processes

• Nitrification: oxidation of ammonia (NH3/NH4+) to nitrite (NO2-) then nitrate (NO3-) by aerobic chemoautotrophs. Requires oxygen.
• Denitrification: reduction of nitrate to N2 gas by facultative bacteria under anoxic conditions (removes total nitrogen).
• Sludge settling: formation of flocs in activated sludge; good settling characteristics (low SVI) are important for separation in secondary clarifiers.

Disinfection

Final step to kill pathogens. Common methods: chlorination (common, residual disinfectant), UV irradiation (no residual), ozonation (strong oxidant). Choice depends on required log-reduction, water quality and downstream use.

Design/operation considerations

• Hydraulic loading and hydraulic retention time (HRT) determine contact time in tanks.
• Organic loading (BOD/COD) and F/M ratio influence activated sludge behaviour.
• Oxygen supply (aeration energy) is often the largest operational cost in aerobic processes.
• Effluent standards and reuse objectives determine level of tertiary treatment.

Environmental impacts of inadequate treatment

Untreated or poorly treated sewage causes oxygen depletion (fish kills), eutrophication (algal blooms), spread of waterborne diseases, and accumulation of toxic substances in ecosystems.

Fig 29 — Educational Diagram: Control Measures and Pollution Prevention

Introduction: Pollution control and prevention aim to reduce or eliminate the release of harmful substances into air, water and soil, protecting human health and ecosystems. Strategies act at three levels: source reduction (prevent pollution), process modification (reduce formation), and end‑of‑pipe treatment (remove pollutants before discharge).

Levels of control:

• Source control — change raw materials, cleaner production, improved maintenance, process redesign to avoid pollutant formation (best option).
• Process control — optimize combustion, use catalysts, filtration within the process to lower emissions.
• End‑of‑pipe control — capture/treat pollutants before discharge (e.g., scrubbers, filters, wastewater plants).

Water pollution control (wastewater treatment):

• Primary treatment — physical removal: screening, sedimentation (reduces suspended solids).
• Secondary treatment — biological processes: activated sludge, trickling filters to remove BOD (biological oxygen demand).
• Tertiary treatment — advanced chemical/physical removal: nutrient (N, P) removal, filtration, disinfection (chlorination/UV), adsorption, reverse osmosis for micropollutants.
• Sludge handling — digestion (anaerobic/ aerobic), dewatering, safe disposal or use (compost, biogas).

Air pollution control:

• Particulate controls — cyclones (centrifugal), fabric filters (baghouses), electrostatic precipitators (ESP) to capture particles.
• Gas controls — wet/dry scrubbers (acid/base gas removal), adsorption (activated carbon), catalytic converters (NOx, CO, hydrocarbons in vehicles).
• Combustion control — low‑NOx burners, fuel switching (e.g., to natural gas or CNG), improved combustion efficiency.

Soil and solid waste management:

• Reduce generation (packaging, product redesign), reuse, recycling (metal, paper, plastic), composting organic waste, sanitary landfilling with leachate collection and gas recovery.
• Remediation of contaminated sites: bioremediation (microbial degradation), phytoremediation (plants uptake), excavation and safe disposal for heavily contaminated soils.

Noise and radiation control:

• Noise—source control (quieter machinery), engineering controls (enclosures, barriers), administrative (time limits), personal protection (earplugs).
• Radioactive—containment, shielding, controlled storage, strict regulation of disposal and transport.

Preventive approaches (policy & practice):

• Regulations and standards (e.g., Water (Prevention & Control) Act, Air Act, environment rules), emission limits, effluent standards, environmental impact assessment (EIA).
• Cleaner production, industrial symbiosis (waste of one as input to another), extended producer responsibility (EPR), economic instruments (pollution taxes, tradable permits), public awareness and green procurement.

Monitoring and indicators: continuous emission monitoring systems (CEMS), ambient air/water quality monitoring, biological indicators, and parameters such as BOD, COD, TSS, pH, heavy metal concentrations are used to assess control effectiveness.

Summary: Effective pollution control uses a mix of prevention at source, process improvements, end‑of‑pipe treatment, legal frameworks and community participation. The preferred sequence is: avoid → reduce → recycle/reuse → treat → dispose.

Fig 30 — Educational Diagram: Drinking Water Treatment and Purification

What it is: Drinking water treatment is the set of physical, chemical and biological processes used to remove or reduce suspended solids, turbidity, colour, odour, disease-causing microorganisms, dissolved salts and other contaminants so that raw water becomes safe and palatable.

Common impurities in raw water

• Suspended solids and colloids (silt, clay, organic matter) → cause turbidity
• Dissolved inorganic salts (Ca2+, Mg2+ causing hardness; Na+, Cl−, SO4²⁻, NO3−, F−)
• Pathogenic microorganisms (bacteria, viruses, protozoa)
• Organic pollutants (humic substances, pesticides)
• Dissolved gases and metals (Fe, Mn)

Objectives and standards (typical)

• Remove turbidity (ideally <1 NTU), control microbial contamination (no detectable E. coli), control chlorine residual (≈0.2–0.5 mg/L), maintain acceptable taste and pH (≈6.5–8.5), and limit TDS (palatable <500 mg/L).

Stepwise treatment process (municipal plants)

1. Pretreatment / Screening & Aeration

Large debris is removed by screens. Aeration vents dissolved gases (H₂S, CO₂) and oxidizes Fe(II)/Mn(II) to insoluble oxides that can be removed.

2. Coagulation and Flocculation

Coagulants (alum Al₂(SO₄)₃·18H₂O, ferric chloride) neutralize charges on colloids. Gentle stirring (flocculation) encourages particles to collide and form larger flocs (Al(OH)₃ gelatinous precipitate) that settle faster.

3. Sedimentation (Clarification)

Flocs settle by gravity in clarifiers; sludge is removed from the bottom. Effective removal reduces turbidity and particulate load before filtration.

4. Filtration

Filtered through sand filters (rapid or slow sand), sometimes with activated carbon. Filters remove remaining suspended solids, some bacteria, organic molecules and improve taste and colour.

5. Disinfection

Chlorination (Cl₂ or hypochlorite) is widely used: chlorine forms hypochlorous acid (HOCl), a strong disinfectant. Alternatives: chloramines (stable residual), ozonation (powerful oxidant, no long residual), UV radiation (inactivates microbes but no residual).

6. Softening and Chemical Treatment (if required)

Hard water (Ca2+, Mg2+) is softened by lime-soda ash treatment (precipitation) or ion-exchange (zeolite resins) to protect plumbing and improve soap action. Fluoride may be adjusted, iron and manganese removed by oxidation/filtration.

7. Storage and Distribution

Treated water stored in tanks and distributed; maintenance of a small disinfectant residual is important to prevent microbial regrowth.

Household purification methods

• Boiling: kills pathogens (simple, no chemical residues).
• Chlorination tablets: low-cost disinfection.
• Household RO (reverse osmosis): removes dissolved salts, heavy metals and many organics (wastes water).
• Activated carbon filters: improve taste/odor, remove some organics and chlorine.
• UV purifiers: inactivate microbes without chemicals.

Important practical notes

• Jar test: small-scale coagulation tests to determine optimum coagulant dose.
• Excessive chlorination can form harmful disinfection by-products (DBPs) like trihalomethanes—control dose and contact time.
• RO gives very low TDS; remineralization is sometimes needed for taste and health.

Summary: A combination of physical settling, chemical coagulation, filtration, disinfection and, when necessary, softening or desalination produces water that meets health and aesthetic standards. Choice of processes depends on raw water quality, cost and local infrastructure.