Environmental Chemistry

Learning Objectives

• Identify major air pollutants (CO_x, NO_x, SO_x, O₃, PM) and their sources & health effects
• Explain photochemical smog formation, acid rain chemistry, ozone depletion mechanism, and greenhouse effect
• Analyse water quality parameters (BOD, COD, DO, TDS) and interpret their significance
• Describe soil pollution sources, pesticide classification, and bioremediation strategies
• Apply the 12 principles of green chemistry to real-world industrial processes

1A. Classification of Pollutants

Pollutants can be classified based on various criteria such as their origin (natural vs anthropogenic), state of matter (gaseous, liquid, solid), and whether they are emitted directly or formed in the atmosphere.

Primary vs Secondary Pollutants

Feature	Primary Pollutants	Secondary Pollutants
Definition	Emitted directly from a source into the atmosphere	Formed by chemical reactions between primary pollutants in the atmosphere
Examples	CO, SO2, NO, NO2, PM, unburned hydrocarbons	O3, PAN, HNO3, H2SO4, aldehydes, ketones
Formation	Combustion, industrial processes, natural emissions	Photochemical reactions, oxidation, hydrolysis in the atmosphere
Persistence	Variable (minutes to years)	Often short-lived but highly reactive and toxic
Control strategy	Source reduction, emission controls, filters, scrubbers	Control of primary precursors (NOx, VOCs, SO2)

Natural vs Anthropogenic Sources

Pollutant	Natural Sources	Anthropogenic (Human-made) Sources
SO2	Volcanic eruptions, hot springs, biological decay	Coal and oil combustion, smelters, paper mills, petroleum refineries
NOx	Lightning, forest fires, soil microbial activity, stratospheric intrusion	Vehicle engines, thermal power plants, industrial boilers, fertiliser plants
CO	Forest fires, oxidation of methane (CH4), oceanic emissions	Incomplete combustion of fossil fuels, biomass burning, cigarette smoke
Particulate matter	Dust storms, sea spray, volcanic ash, pollen, spores, forest fires	Industrial emissions, construction, vehicle exhaust, road dust
VOCs	Vegetation (isoprene, terpenes from trees), forest fires, microbial activity	Petrochemicals, solvents, paints, fuel evaporation, dry cleaning

Criteria Pollutants (USEPA & CPCB Standards)

The six criteria pollutants regulated by environmental agencies worldwide are: PM (PM₁₀ and PM_2.5), SO_x, NO_x, CO, O₃ (tropospheric), and Pb. These are routinely monitored to assess ambient air quality and enforce compliance.

Pollutant	Symbol	Primary Sources	Health & Environmental Effects	NAAQS Standard (India, 24-hr avg)
Particulate Matter	PM2.5 / PM10	Combustion, industrial processes, dust, construction, vehicles	Respiratory illness, cardiovascular disease, lung cancer, visibility reduction	PM2.5: 40 μg/m³; PM10: 60 μg/m³
Sulphur Dioxide	SO2	Coal and oil burning, smelters, paper mills, volcanic eruptions	Bronchoconstriction, respiratory irritation, acid rain precursor	50 μg/m³
Nitrogen Dioxide	NO2	Vehicle emissions, power plants, industrial combustion	Lung inflammation, increased asthma attacks, photochemical smog precursor	40 μg/m³
Carbon Monoxide	CO	Incomplete combustion of fossil fuels, biomass burning, smoking	Carboxyhaemoglobin formation, reduced O2 delivery, headaches, dizziness, death at high levels	2 mg/m³ (8-hr)
Ozone (tropospheric)	O3	Secondary pollutant formed photochemically from NOx + VOCs	Eye and throat irritation, coughing, reduced lung function, crop damage, rubber cracking	100 μg/m³ (8-hr)
Lead	Pb	Leaded petrol (phased out), battery manufacturing, paints, smelters	Neurological and developmental damage, anaemia, kidney damage, hypertension	0.5 μg/m³

1B. Atmospheric Pollution

The atmosphere is a thin gaseous envelope surrounding the Earth. Any undesirable change in its composition that adversely affects living organisms is called atmospheric pollution. Pollutants can be classified as primary (emitted directly from a source) and secondary (formed by chemical reactions in the atmosphere).

Tropospheric Pollution

The troposphere extends from Earth's surface up to ~12 km. It contains ~75% of the total air mass and is where all weather phenomena occur. Tropospheric pollutants are broadly divided into gaseous and particulate categories.

Gaseous Pollutants (CO_x, NO_x, SO_x, O₃)

Carbon monoxide (CO): Colourless, odourless gas produced by incomplete combustion of fossil fuels. It binds to haemoglobin 200× more strongly than O₂, forming carboxyhaemoglobin, reducing oxygen-carrying capacity of blood. Permissible limit in ambient air: 9 ppm (8-hour average).

Oxides of Nitrogen (NO_x): NO and NO₂ are produced from high-temperature combustion in vehicles and power plants. NO₂ is a reddish-brown, toxic gas that causes respiratory problems and contributes to photochemical smog. NO reacts with O₃ in the stratosphere contributing to ozone depletion.

Oxides of Sulphur (SO_x): SO₂ is a colourless, pungent gas produced by burning sulphur-containing fossil fuels (coal, oil). It causes respiratory diseases, forms acid rain (H₂SO₄), and damages buildings and monuments (marble cancer).

Tropospheric Ozone (O₃): Unlike stratospheric ozone, tropospheric ozone is a harmful secondary pollutant formed by photochemical reactions of NO_x and VOCs in sunlight. It causes respiratory irritation, reduces crop yields, and damages rubber and plastics.

Particulate Pollutants

Particulate matter (PM) consists of tiny solid or liquid particles suspended in air. PM₁₀ (diameter ≤10 μm) can reach the lungs; PM_2.5 (≤2.5 μm) can enter the bloodstream. Sources include dust, smoke, vehicle emissions, and industrial processes. Health effects: asthma, bronchitis, lung cancer, cardiovascular diseases.

Particulate Type	Size Range	Source	Health Effect
Fly ash	1–100 μm	Thermal power plants	Fibrosis, lung damage
Lead particles	<1 μm	Vehicle exhaust (leaded petrol)	Neurological damage, anaemia
Asbestos fibres	0.1–10 μm	Construction, insulation	Asbestosis, mesothelioma
Soot (carbon)	0.01–1 μm	Diesel engines, biomass burning	Respiratory irritation, cancer

Photochemical Smog

Photochemical smog is a brownish-grey haze formed in warm, sunny urban areas when primary pollutants (NO_x, VOCs) react under sunlight. The key reactions are:

• NO₂ ^hv → NO + O(³P)
• O(³P) + O₂ → O₃
• O₃ + NO → NO₂ + O₂ (normal sink)
• VOCs + OH• → Radicals that oxidise NO to NO₂, shifting equilibrium → O₃ accumulates
• NO₂ + OH• → HNO₃ (nitric acid contributes to acid rain)
• Peroxyacyl nitrates (PAN) are formed as by-products

Effects: Eye irritation, respiratory distress, damage to plants (leaf bleaching), cracking of rubber. Control: Use of catalytic converters, reducing VOCs, promoting public transport.

Classical Smog vs Photochemical Smog

Comparison of Classical Smog (London-type) and Photochemical Smog (Los Angeles-type)

Property	Classical Smog (Reducing Smog)	Photochemical Smog (Oxidising Smog)
Also known as	London smog, Reducing smog, Sulphurous smog	Los Angeles smog, Oxidising smog, Summer smog
Nature	Reducing (high SO2, soot, smoke)	Oxidising (high O3, NO2, PAN)
Formation conditions	Cold (<10°C), humid (>80%), foggy, temperature inversion	Warm (>25°C), sunny (intense sunlight), dry, low wind
Time of occurrence	Early morning and winter months	Afternoon (peak sunlight) and summer months
Major pollutants	SO2, soot (carbon), smoke, fog droplets	O3, PAN, NO2, VOCs, aldehydes, ketones, HNO3
Chemical reactions	SO2 → H2SO4 (oxidation in fog droplets)	NO2 hv → NO + O; O + O2 → O3; VOCs + ·OH → PAN
Visibility effect	Greatly reduced (thick fog + smoke reduces visibility to <100 m)	Moderately reduced (brownish haze, visibility 1–5 km)
Health effects	Respiratory diseases, bronchitis, lung damage, asphyxiation	Eye and throat irritation, asthma, chest tightness, cough
Damage to materials	Corrosion of metals, damage to buildings (acid attack)	Cracking of rubber, fading of dyes, damage to paints and plastics
Control measures	Smokeless fuels, flue gas desulphurisation, scrubbers	Catalytic converters, reducing VOCs and NOx, promoting public transport
Historic example	London 1952 (Great Smog: ~12,000 deaths)	Los Angeles 1940s–present; Delhi, Beijing

Acid Rain

Normal rainwater has pH ~5.6 due to dissolved CO₂ (H₂CO₃). When pH drops below 5.6, it is termed acid rain. Primary causes: SO₂ and NO_x emissions that form H₂SO₄ and HNO₃ in the atmosphere.

Reactions:
2SO₂ + O₂ → 2SO₃ → SO₃ + H₂O → H₂SO₄
4NO₂ + 2H₂O + O₂ → 4HNO₃

Effects: Acidification of lakes (fish kills), leaching of toxic metals (Al, Hg) from soil, damage to Taj Mahal (marble cancer — CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), forest decline.

Chemistry of Acid Rain Formation

Sulphur dioxide and nitrogen oxides undergo atmospheric oxidation to form strong acids:

• SO₂ + H₂O → H₂SO₃ (sulphurous acid, weak acid, pH ~4–5)
• 2SO₂ + O₂ → 2SO₃ (catalysed by metal ions, sunlight, or O₃)
• SO₃ + H₂O → H₂SO₄ (sulphuric acid, strong acid, pH <3)
• NO + O₃ → NO₂ + O₂
• 2NO₂ + H₂O → HNO₃ + HNO₂
• 4NO₂ + 2H₂O + O₂ → 4HNO₃ (overall reaction for nitric acid formation)

The combination of H₂SO₄ and HNO₃ in rainwater can result in pH as low as 2–3 (comparable to lemon juice or vinegar).

Detailed Effects of Acid Rain

• Aquatic ecosystems: Lakes become acidic (pH <5), killing fish eggs and aquatic insects. Aluminium leached from soil clogs fish gills and is toxic.
• Forests and vegetation: Leaches essential nutrients (Ca²⁺, Mg²⁺, K⁺) from soil, damages leaf cuticles, reduces photosynthesis, and increases susceptibility to disease.
• Buildings and monuments (marble cancer): Marble (CaCO₃) reacts with H₂SO₄: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. The gypsum (CaSO₄ · 2H₂O) formed is water-soluble and gets washed away, causing surface erosion. The Taj Mahal, Parthenon, and many heritage structures are severely affected.
• Human health: Inhalation of acid aerosols (H₂SO₄ mist, HNO₃ vapour) aggravates asthma, bronchitis, and cardiovascular conditions.
• Materials: Corrodes metals (iron, steel, copper, bronze), degrades paint, paper, leather, synthetic fibres, and causes weathering of stone and concrete.

Greenhouse Effect & Global Warming

Greenhouse gases (GHGs) absorb infrared radiation emitted by Earth's surface and re-radiate it back, warming the lower atmosphere. Major GHGs:

Greenhouse Gas	Source	Relative Potency (CO2 = 1)	Contribution to Warming (%)
CO2	Fossil fuel combustion, deforestation	1	~60
CH4	Rice paddies, livestock, landfills	25	~16
N2O	Fertilisers, industrial processes	298	~6
CFCs	Refrigerants, aerosol propellants	10,000+	~12
H2O vapour	Natural (evaporation)	—	~36 (natural)

Consequences of global warming: Rising sea levels (thermal expansion + melting glaciers), extreme weather events, shifting agricultural zones, loss of biodiversity, spread of tropical diseases.

Global Warming Potential (GWP) Explained

GWP is a measure of how much heat a greenhouse gas traps in the atmosphere relative to CO₂ over a specific time horizon (usually 100 years). CO₂ has GWP = 1 by definition. GWP depends on two factors:
(a) The efficiency of the gas in absorbing infrared (IR) radiation
(b) The atmospheric lifetime of the gas

For example, CH₄ has a GWP of 25 over 100 years because it traps 25× more heat per kilogram than CO₂. However, its atmospheric lifetime is only ~12 years, whereas CO₂ persists for 100–500 years. N₂O has GWP = 298 and a lifetime of ~121 years. CFCs have GWPs in the thousands and lifetimes of 50–100+ years.

Carbon Footprint

Carbon footprint is the total amount of CO₂ and other greenhouse gases emitted directly or indirectly by an individual, organisation, event, or product, expressed as CO₂ equivalent (CO₂e). Major components include: energy use (electricity, heating/cooling), transportation (vehicles, air travel), food consumption (meat production has high footprint), and manufacturing of goods. The global average carbon footprint is ~4.8 tonnes CO₂e per person per year; in developed countries it is 10–20 tonnes per person.

Paris Agreement (2015) and International Climate Policy

The Paris Agreement under the UNFCCC (United Nations Framework Convention on Climate Change) aims to limit global warming to well below 2°C (preferably 1.5°C) above pre-industrial levels. Key elements include:

• Nationally Determined Contributions (NDCs): Each country sets its own emission reduction targets and updates them every 5 years
• Net-zero emissions goal: Global net-zero CO₂ emissions by 2050–2070
• Climate finance: Developed countries committed to provide $100 billion per year to developing nations for mitigation and adaptation
• India's NDC targets: Reduce emissions intensity of GDP by 33–35% by 2030 (from 2005 levels); achieve 40% cumulative installed capacity from non-fossil fuel sources; create an additional carbon sink of 2.5–3 billion tonnes CO₂ equivalent through forestry
• Panchamrit strategy (COP26, 2021): India committed to 500 GW non-fossil energy capacity by 2030, 50% energy from renewables, 1 billion tonnes reduction in carbon emissions, reduce emissions intensity by 45%, and net-zero by 2070

Climate Change Impacts in Detail

• Global temperature rise: Average surface temperature has already increased by ~1.1°C since the pre-industrial era (1850–1900). The last decade (2014–2024) was the warmest on record.
• Sea level rise: Global mean sea level has risen by ~20 cm since 1901, accelerating to ~3.7 mm/yr in recent decades. This threatens coastal cities like Mumbai, Shanghai, New York, and entire island nations (Maldives, Tuvalu).
• Extreme weather: Frequency and intensity of heatwaves, floods, droughts, cyclones, and wildfires have increased significantly. Severe floods in Pakistan (2022), heatwaves in Europe (2023), and wildfires in Australia (2019–2020) are linked to climate change.
• Agricultural impact: Shifts in growing seasons, reduced crop yields (wheat, rice, maize decline by 5–10% per °C warming), increased pest pressure, and reduced nutritional quality of staple crops (lower protein and mineral content at elevated CO₂ levels).
• Biodiversity loss: 20–30% of plant and animal species are at increased risk of extinction if warming exceeds 1.5–2.5°C. Coral bleaching has severely damaged the Great Barrier Reef and other tropical reefs.
• Human health: Spread of vector-borne diseases (malaria, dengue, chikungunya) to higher latitudes; increased heat-related mortality; food and water insecurity leading to malnutrition and displacement.

Ozone Layer Depletion

The ozone layer is present in the stratosphere (15–35 km altitude). It absorbs 97–99% of harmful UV-B (280–315 nm) and UV-C (100–280 nm) radiation.

Depletion mechanism (Chapman cycle + catalytic destruction):

• O₂ + hv (λ ≤ 242 nm) → 2 O(³P)
• O(³P) + O₂ + M → O₃ + M (M = N₂ or O₂)
• O₃ + hv (λ ≤ 320 nm) → O₂ + O(³P)
• CFC destruction: CFCl₃ + hv → CFCl₂ + Cl• (at ~25 km altitude)
• Cl• + O₃ → ClO• + O₂
• ClO• + O(³P) → Cl• + O₂ (net: O + O₃ → 2O₂)

Each chlorine atom destroys ~10⁵ ozone molecules before being removed (by reaction with CH₄ to form HCl, which is washed out by rain). Ozone hole (thinning) is most severe over Antarctica due to polar stratospheric clouds (PSCs) that activate chlorine reservoirs.

Substance	Ozone Depletion Potential (ODP)	Source
CFC-11 (CFCl3)	1.0	Refrigerants, foams
CFC-12 (CF2Cl2)	1.0	Aerosols, A/C
Halons (fire extinguishers)	3–10	Fire suppression
N2O	0.017	Fertilisers, industry
HCFCs (transitional)	0.02–0.1	CFC substitutes

The Chapman Cycle (Ozone Formation and Natural Destruction)

The natural balance of stratospheric ozone was first described by Sydney Chapman in 1930. The cycle involves four key reactions:

• O₂ + hv (λ ≤ 242 nm) → 2O(³P)   (photolysis of molecular oxygen)
• O(³P) + O₂ + M → O₃ + M   (ozone formation; M = N₂ or O₂ acts as a stabilising third body)
• O₃ + hv (λ = 240–320 nm) → O₂ + O(³P)   (ozone photolysis in stratosphere)
• O(³P) + O₃ → 2O₂   (net natural destruction)

Under normal conditions, ozone production and destruction are balanced, maintaining a steady-state ozone concentration of ~10 ppm at 30 km altitude with a total column abundance of ~300 Dobson Units (DU). One Dobson Unit = 0.01 mm thickness of ozone at STP, and 300 DU corresponds to an ozone layer ~3 mm thick if compressed at sea level.

Catalytic Ozone Destruction Cycles

In addition to natural destruction, several catalytic cycles accelerate ozone depletion. These cycles are initiated by radicals (X = Cl, Br, NO, OH):

• Chlorine cycle (from CFCs): Cl + O₃ → ClO + O₂; ClO + O → Cl + O₂. Net: O₃ + O → 2O₂. Each Cl atom destroys ~10⁵ O₃ molecules.
• Bromine cycle (from halons): Br + O₃ → BrO + O₂; BrO + O → Br + O₂. Br is 40× more efficient than Cl per atom.
• Nitrogen cycle (from N₂O): NO + O₃ → NO₂ + O₂; NO₂ + O → NO + O₂. N₂O from fertilisers is a natural source of NO in the stratosphere.
• Hydrogen cycle (from CH₄, H₂O): OH + O₃ → HO₂ + O₂; HO₂ + O → OH + O₂.

The Ozone Hole over Antarctica

The Antarctic ozone hole was first discovered in 1985 by British scientists (Farman, Gardiner, and Shanklin) working at Halley Bay. It forms annually during the Austral spring (September–November). The key factors are:

• Polar Stratospheric Clouds (PSCs): Form at temperatures below −78°C in the Antarctic winter. PSCs provide surfaces for heterogeneous chemical reactions that convert inactive chlorine reservoirs (HCl, ClONO₂) into active Cl₂.
• Spring sunlight: Cl₂ photolyses rapidly in August–September to release Cl radicals that catalytically destroy ozone.
• Polar vortex: A strong circumpolar wind pattern isolates the Antarctic air mass, preventing mixing with ozone-rich mid-latitude air and allowing the depletion to concentrate.

The ozone hole reached a record area of ~28.4 million km² in 2015. As of 2023–2024, the hole is slowly recovering thanks to the Montreal Protocol. Full recovery of the Antarctic ozone layer is projected by ~2065, and the Arctic ozone layer (which experiences less severe depletion) is expected to recover by ~2045.

Effects of UV-B Radiation on Humans and Ecosystems

Stratospheric ozone absorbs 97–99% of UV-B (280–315 nm) and all UV-C (100–280 nm) radiation. Increased UV-B reaching Earth’s surface due to ozone depletion has several harmful effects:

Effect	Mechanism	Preventive Measure
Skin cancer (melanoma, basal cell carcinoma, squamous cell carcinoma)	UV-B causes DNA damage (formation of cyclobutane pyrimidine dimers) in skin cells	Use sunscreen with SPF 30+, wear protective clothing, avoid peak sun hours (10 AM – 4 PM)
Cataracts (clouding of the eye lens)	UV-B induces oxidative damage to lens proteins (crystallins), leading to protein aggregation	Wear UV-blocking sunglasses and wide-brimmed hats
Immune suppression	UV-B suppresses the activity of Langerhans cells (antigen-presenting cells) in the skin, reducing immune response	Avoid prolonged sun exposure; maintain adequate vitamin D levels through diet
Premature skin ageing (photoageing)	UV-B degrades collagen and elastin fibres in the dermis	Daily use of broad-spectrum sunscreen, antioxidants (vitamin C, E)
Plant damage	UV-B reduces photosynthetic efficiency, damages DNA, and alters plant morphology	Developing UV-resistant crop varieties
Marine ecosystems	UV-B inhibits photosynthesis in phytoplankton (base of the marine food web), affecting the entire aquatic ecosystem	Reducing ozone-depleting substances globally

Substitutes for CFCs: HCFCs and HFCs

As a result of the Montreal Protocol, CFCs have been progressively replaced by transitional and long-term substitutes with lower ozone depletion potential.

Substitute	Formula	ODP	GWP (100-yr)	Application	Status / Phase-out
HCFC-22	CHClF2	0.05	1,810	Refrigerants, air conditioning	Phase-out by 2030 (developed) / 2040 (developing)
HCFC-141b	CH3CCl2F	0.11	725	Foam blowing (polyurethane)	Phase-out by 2030
HCFC-142b	CH3CClF2	0.07	2,310	Foam blowing, refrigerants	Phase-out by 2030
HFC-134a	CH2FCF3	0	1,430	Car AC, domestic refrigerators	Widely used (Kigali phase-down)
HFC-125	CHF2CF3	0	3,500	Fire extinguishers (blend component)	Being phased down (Kigali Amendment)
HFC-32	CH2F2	0	675	Air conditioning systems (split ACs)	Low-GWP alternative gaining adoption
HFO-1234yf	CF3CF=CH2	0	4	Mobile air conditioning (car AC)	Ultra-low GWP, next-generation refrigerant
Ammonia	NH3	0	0	Industrial refrigeration	Natural refrigerant, toxic but efficient
CO2 (R-744)	CO2	0	1	Commercial refrigeration, heat pumps	Emerging natural refrigerant technology

Montreal Protocol (1987): International treaty to phase out ozone-depleting substances. One of the most successful environmental treaties — 99% of ODSs have been phased out; the ozone layer is expected to recover by 2060–2070.

2. Water Pollution

Water pollution is the contamination of water bodies by harmful substances that degrade water quality and harm aquatic life or human health. The WHO estimates that 80% of diseases in developing countries are waterborne.

Sources of Pollution

Domestic, Industrial, Agricultural

Domestic: Sewage, detergents (phosphates cause eutrophication), food waste, pathogens (E. coli, Vibrio cholerae). Industrial: Heavy metals (Hg, Pb, Cd, Cr), acids, cyanides, dyes, thermal pollution from cooling water. Agricultural: Fertiliser runoff (nitrates, phosphates), pesticides (DDT, organophosphates), animal waste.

Pollutant Type	Example	Source	Major Effect
Pathogens	E. coli, Salmonella, V. cholerae	Sewage, animal waste	Cholera, typhoid, diarrhoea
Oxygen-demanding waste	Sewage, food processing waste	Domestic, industry	DO depletion, fish kills
Nutrients	Nitrates, phosphates	Fertilisers, detergents	Eutrophication, algal blooms
Heavy metals	Hg, Pb, Cd, Cr, As	Industry, mining	Bioaccumulation, toxicity
Organic chemicals	Pesticides, PCBs, PAHs	Agriculture, industry	Carcinogenic, endocrine disruption
Thermal	Heated water	Power plant cooling	Reduced DO, thermal shock

Biomagnification

Biomagnification (or biological magnification) is the progressive increase in the concentration of a persistent, non-biodegradable substance (e.g., DDT, Hg, PCBs) at successively higher trophic levels in a food chain. This differs from bioaccumulation, which refers to the build-up of a substance within a single organism over its lifetime.

Biomagnification occurs when a substance is:
(a) Persistent (resists degradation in the environment)
(b) Fat-soluble (lipophilic) — stored in adipose tissue rather than excreted
(c) Mobile in the environment (can travel through water or air)

DDT Biomagnification: A Classic Example

Biomagnification of DDT in an aquatic food chain (Clear Lake, California study)

Trophic Level	Organism	DDT Concentration (ppm)	Magnification Factor vs Water
Water	—	0.000003 (3 × 10−6)	1×
Producer	Phytoplankton (algae)	0.04	~13,000×
Primary consumer	Zooplankton	0.5	~167,000×
Secondary consumer	Small fish (minnows)	2.0	~667,000×
Tertiary consumer	Large fish (predatory)	10–15	~3–5 million×
Top predator	Fish-eating birds (grebes, pelicans)	25–50	~8–16 million×

DDT causes eggshell thinning in birds (due to inhibition of Ca²⁺-ATPase in the shell gland), leading to reproductive failure. This was famously documented in Rachel Carson’s book Silent Spring (1962), which led to the ban of DDT in the USA in 1972.

Mercury Biomagnification

Inorganic mercury (Hg²⁺) discharged from industrial effluents is converted by anaerobic bacteria in sediments into methylmercury (CH₃Hg⁺). Methylmercury is highly lipophilic and bioaccumulates in fish muscle. Predatory fish (tuna, swordfish, shark, king mackerel) can have mercury levels of 1–10 ppm, exceeding the WHO safe limit of 0.5 ppm. Chronic exposure causes neurotoxicity, especially in developing foetuses and infants (impaired cognitive development).

Minamata Disease (Japan, 1956)

Caused by methylmercury poisoning from industrial wastewater discharged into Minamata Bay by Chisso Corporation (1932–1968). The mercury bioaccumulated in fish and shellfish, and local fishing communities who consumed contaminated seafood developed severe neurological symptoms: numbness in limbs, muscle weakness, loss of peripheral vision, impaired hearing and speech, paralysis, and congenital Minamata disease in infants. Over 2,000 victims were officially certified; the actual number affected is estimated at much higher.

Itai-itai Disease (Japan, 1912)

Caused by cadmium (Cd) poisoning from mining wastewater discharged into the Jinzū River in Toyama Prefecture by the Mitsui Mining and Smelting Company. Cadmium accumulates in bones, causing severe pain (itai = “ouch” or “it hurts” in Japanese), osteoporosis (bones become brittle and fracture easily), and kidney dysfunction (proteinuria). The disease was named for the Japanese exclamation of pain.

Groundwater contamination: In India, ~60% of districts have groundwater contaminated with fluoride, arsenic, or nitrates. Arsenic contamination in West Bengal and Bihar affects over 10 million people. Fluoride (>1.5 ppm) causes fluorosis (dental and skeletal).

Water Quality Parameters

BOD, COD, DO, TDS

Dissolved Oxygen (DO): Amount of O₂ dissolved in water. Clean water: DO ~8–10 mg/L. Below 4 mg/L is harmful to aquatic life. DO decreases with increasing temperature and organic pollution.

Biochemical Oxygen Demand (BOD): Amount of O₂ consumed by microorganisms to decompose organic matter in water at 20°C over 5 days. Higher BOD = more organic pollution. Potable water: BOD < 5 mg/L; polluted water: BOD > 20 mg/L.

Chemical Oxygen Demand (COD): Amount of O₂ equivalent consumed by chemical oxidation of pollutants using K₂Cr₂O₇ in acidic medium. COD ≥ BOD always, because COD includes non-biodegradable organics.

Total Dissolved Solids (TDS): Inorganic salts (Ca, Mg, Na, bicarbonates, chlorides, sulfates) dissolved in water. Potable water: TDS < 500 ppm; permissible: up to 2000 ppm.

Parameter	Definition	Acceptable Limit	Interpretation
DO	O2 dissolved in water	≥4 mg/L (aquatic life)	Low DO = pollution, high temp
BOD	O2 consumed by microbes in 5 days at 20°C	<5 mg/L (clean water)	High BOD = high organic waste
COD	O2 equivalent by chemical oxidation	<10 mg/L (clean)	COD ≥ BOD always
TDS	Dissolved inorganic salts	<500 ppm (drinking)	High TDS = salinity, hardness

BOD vs COD: Key Differences

Comparison of Biochemical Oxygen Demand and Chemical Oxygen Demand

Parameter	BOD (Biochemical Oxygen Demand)	COD (Chemical Oxygen Demand)
Full form	Biochemical (or Biological) Oxygen Demand	Chemical Oxygen Demand
Definition	Amount of O2 consumed by microorganisms to biodegrade organic matter at 20°C over 5 days (BOD5)	Amount of O2 equivalent consumed by chemical oxidation of pollutants using K2Cr2O7 in strong H2SO4 medium
Types of pollutants detected	Only biodegradable organic matter	Both biodegradable and non-biodegradable organic matter
Time required for test	5 days (at 20°C in dark)	2–3 hours (reflux method)
Numerical value	Lower (typically 10–50 mg/L for domestic sewage)	Higher (BOD/COD ratio ~0.4–0.8 for biodegradable waste)
Purpose	Assesses organic pollution load for aquatic ecosystem health	Assesses total pollution load; used for industrial effluent treatment design
Oxidising agent	Dissolved O2 (natural, biological)	K2Cr2O7 in acidic medium (chemical)
Significance	High BOD → high organic pollution → low DO → fish kills	COD ≥ BOD always; large gap indicates non-biodegradable industrial pollutants

Water Quality Standards (WHO & BIS IS 10500:2012)

Drinking Water Quality Standards — WHO Guidelines and Indian Standards

Parameter	WHO Guideline Value	BIS Acceptable Limit	BIS Permissible Limit (in absence of alternate source)
pH	6.5–8.5	6.5–8.5	No relaxation
TDS (mg/L)	<500	500	2,000
Hardness as CaCO3 (mg/L)	<200	200	600
Chloride (mg/L)	<250	250	1,000
Sulphate (mg/L)	<250	200	400
Nitrate (mg/L)	<50	45	No relaxation
Fluoride (mg/L)	0.5–1.5	1.0	1.5
Arsenic (mg/L)	0.01	0.01	0.05
Lead (mg/L)	0.01	0.01	No relaxation
Mercury (mg/L)	0.001	0.001	No relaxation
Cadmium (mg/L)	0.003	0.003	No relaxation
Total Coliform (CFU/100 mL)	0	0	0
E. coli (CFU/100 mL)	0	0	0
Iron (mg/L)	<0.3	0.3	1.0
Aluminium (mg/L)	0.1–0.2	0.03	0.2

Eutrophication

Eutrophication is the enrichment of water bodies with nutrients (especially phosphates and nitrates) leading to excessive growth of algae (algal bloom). When algae die, their decomposition consumes DO, causing fish kills and release of toxins.

Causes: Fertiliser runoff, untreated sewage, detergents (Na₃PO₄), industrial effluents. Natural eutrophication occurs over centuries; cultural (accelerated) eutrophication is human-caused and occurs rapidly.

Water Treatment

Drinking water treatment involves: (1) Screening → (2) Coagulation (alum, Al₂(SO₄)₃) → (3) Sedimentation → (4) Filtration (sand/gravel) → (5) Disinfection (chlorination, ozonation, UV).

Wastewater treatment: (1) Primary (physical — sedimentation, skimming) → (2) Secondary (biological — activated sludge process, trickling filters) → (3) Tertiary (advanced — reverse osmosis, UV, chemical precipitation).

Treatment Stage	Process	Removes
Primary (Physical)	Screening, grit removal, sedimentation (primary clarifier), floatation, skimming	Suspended solids (50–70%), oils, grease, grit, floating debris
Secondary (Biological)	Activated sludge process (aeration + biological floc), trickling filter, oxidation ponds, rotating biological contactors (RBC), sequencing batch reactors (SBR)	Biodegradable organic matter (85–95% BOD reduction), pathogens (partial)
Tertiary (Advanced)	Reverse osmosis (RO), nanofiltration, UV disinfection, ozonation, chlorination, activated carbon adsorption, ion exchange, chemical precipitation	Dissolved salts, trace contaminants, residual pathogens (99.9% removal), colour, odour, heavy metals, nutrients (N, P)

Activated Sludge Process (ASP) — Most Common Secondary Treatment

In the activated sludge process, primary-treated effluent is aerated in a tank where aerobic microorganisms (bacteria, protozoa) form flocs that consume organic pollutants. The mixed liquor (wastewater + microbes) then flows to a secondary clarifier where sludge settles. A portion of the settled sludge is returned to the aeration tank (return activated sludge, RAS) to maintain microbial population, and the excess is disposed of (waste activated sludge, WAS). The clear supernatant (treated effluent) is discharged or sent to tertiary treatment.

Overall treatment efficiency: Primary + Secondary treatment can achieve ~90–95% BOD removal, ~85–90% COD removal, and ~99% pathogen removal when combined with disinfection. Tertiary treatment can produce water suitable for industrial reuse or groundwater recharge.

3. Soil Pollution

Soil pollution is the contamination of soil with harmful substances that alter its chemical, physical, or biological properties, reducing its fertility and posing health risks.

Sources & Effects

Sources: Industrial waste dumping, pesticide/fertiliser overuse, mining, oil spills, landfill leachate, acid rain, radioactive fallout. Effects: Loss of soil fertility, contamination of groundwater, bioaccumulation of toxins in food chains, desertification.

Heavy Metal Contamination

Metal	Source	Health Effect	Permissible Limit (drinking water)
Lead (Pb)	Batteries, paints, petrol	Neurological damage, anaemia	0.05 ppm
Mercury (Hg)	Chlor-alkali industry, thermometers	Minamata disease, kidney damage	0.001 ppm
Cadmium (Cd)	Battery manufacturing, fertilisers	Itai-itai disease, bone softening	0.01 ppm
Arsenic (As)	Pesticides, natural groundwater	Arsenicosis, skin cancer	0.05 ppm
Chromium (Cr-VI)	Electroplating, tanning	Lung cancer, skin ulcers	0.05 ppm

Pesticides & Herbicides

Organochlorines: DDT, BHC, endosulfan — persistent, bioaccumulative, now banned in many countries. DDT is stored in adipose tissue and causes egg-shell thinning in birds. Organophosphates: Malathion, parathion — less persistent, inhibit acetylcholinesterase (nerve agents). Carbamates: Carbaryl — similar mechanism to organophosphates. Herbicides: 2,4-D, atrazine — weed control, some are endocrine disruptors.

Soil Pollution Control Measures

• Phytoremediation: Using hyperaccumulator plants (Thlaspi caerulescens for Zn, Pteris vittata for As, Brassica juncea for Pb) to extract heavy metals from soil
• Soil washing: Using chelating agents (EDTA) to mobilise and remove metal contaminants
• Thermal desorption: Heating soil to volatilise organic contaminants (e.g., PAHs, PCBs)
• Land farming: Aerobic biodegradation of organic pollutants by tilling and adding nutrients to contaminated soil

Bioremediation

Bioremediation uses living organisms (bacteria, fungi, plants) to degrade or detoxify environmental pollutants. Types:

• In situ: Bioventing, biosparging, phytoremediation (plants absorb heavy metals — e.g., Thlaspi for Zn, Cd; Pteris vittata for As)
• Ex situ: Land farming, biopiles, bioreactors
• Composting: Aerobic degradation of organic waste

Microorganisms used: Pseudomonas putida (degrades hydrocarbons), Deinococcus radiodurans (radioactive waste), Geobacter (reduces U(VI) to U(IV)).

4. Solid & Radioactive Waste

Municipal & Industrial Waste

Municipal solid waste (MSW): Household garbage, food waste, paper, plastics, glass, metals. India generates ~62 million tonnes annually, of which only ~20% is processed. Industrial waste: Fly ash (thermal plants generate ~200 million tonnes/yr), slag, chemical sludge, scrap metal, e-waste.

E-waste (printed circuit boards, batteries, CRTs) contains toxic metals (Pb, Hg, Cd, Be) and requires special handling. India is the third-largest e-waste generator globally (~3.2 million tonnes/yr). The E-Waste (Management) Rules, 2016 mandate extended producer responsibility (EPR).

Plastic waste: India generates ~9.4 million tonnes of plastic waste annually, of which only ~60% is recycled. Microplastics (<5 mm) from degradation of larger plastics contaminate water, soil, and even air, entering the food chain and causing endocrine disruption.

Radioactive Pollution

Radioactive pollution results from nuclear power plants, weapons testing, medical waste (^99mTc, ¹³¹I), and mining of uranium/thorium. Key isotopes: ⁹⁰Sr (bone cancer), ¹³⁷Cs (muscle tissue), ²³⁹Pu (lung cancer, half-life 24,100 yr). Effects: DNA damage, mutation, radiation sickness, cancer (leukaemia, thyroid). Disposal: Deep geological repositories (e.g., Yucca Mountain, USA), vitrification (immobilisation in glass).

Waste Management Strategies

• 3Rs: Reduce, Reuse, Recycle — most preferred hierarchy
• Incineration: Reduces volume by 80–90%, but releases toxic gases (dioxins, furans)
• Sanitary landfill: Engineered disposal with leachate collection and methane recovery
• Composting: Aerobic conversion of organic waste into manure
• Pyrolysis/Gasification: Thermal treatment in oxygen-limited environment to produce syngas
• Plasma gasification: High-temperature (>4000°C) vitrification of waste into inert slag

Solid Waste Management Methods

Method	Description	Advantages	Disadvantages
Sanitary Landfill	Waste is compacted and buried in engineered pits with liner, leachate collection system, and methane gas recovery	Low operating cost; generates methane for energy recovery; can handle all types of waste	Requires large land area; risk of groundwater contamination if liner fails; methane is a potent GHG
Composting (Aerobic)	Microbial decomposition of organic waste (food, yard waste) in presence of O2 to produce humus (fertiliser)	Reduces landfill volume by 30–40%; produces nutrient-rich compost for agriculture; low cost	Requires segregation of organic waste; odour and pest problems; cannot handle non-biodegradable or hazardous waste
Incineration (Waste-to-Energy)	Combustion at 800–1000°C; waste volume reduced by 80–90%; energy recovered as electricity or heat	Significant volume reduction; energy recovery; destroys pathogens and toxic organics	Air pollution (dioxins, furans, SO2, NOx, heavy metals in fly ash); high capital and operating costs; ash disposal problem
Recycling	Processing waste materials (paper, glass, metals, plastics, electronics) into new products	Conserves natural resources; reduces energy consumption (e.g., recycling Al saves 95% energy vs virgin production); reduces landfill burden	Requires effective segregation and collection systems; market demand for recycled materials fluctuates; downcycling limits infinite recyclability
Anaerobic Digestion	Microbial decomposition of organic waste in absence of O2 to produce biogas (CH4 + CO2) and digestate	Produces renewable energy (biogas); digestate can be used as fertiliser; reduces GHG emissions from landfills	Requires controlled conditions (temperature, pH); slower than aerobic composting; only suitable for organic waste
Pyrolysis / Gasification	Thermal decomposition in oxygen-limited environment at 300–700°C to produce bio-oil, syngas, and char	Produces valuable fuels and chemicals; lower emissions than incineration; can process mixed plastics and biomass	High energy input; technical complexity; tar formation can cause operational problems

Biodegradable vs Non-Biodegradable Waste

Feature	Biodegradable Waste	Non-Biodegradable Waste
Decomposition by microorganisms	Yes (broken down by bacteria, fungi, actinomycetes)	No (cannot be decomposed by natural biological processes)
Time required for degradation	Days to months (e.g., vegetable peels: 1–2 weeks; paper: 2–5 months)	Years to centuries (e.g., plastic bag: 450 years; glass: 1 million+ years; Styrofoam: never)
Examples	Food waste, fruit/vegetable peels, paper, cotton, jute, wood, leaves, grass, animal manure, sewage sludge	Plastics (PE, PP, PET, PS), glass, metals (Al, Fe, Cu), synthetic fibres (polyester, nylon), e-waste, rubber tyres
Environmental impact	Produces methane (GHG) in landfills if not composted; can cause odour and leachate	Accumulates in environment; causes microplastic pollution in oceans and soil; toxic metals can leach from e-waste
Preferred management	Composting, anaerobic digestion, vermicomposting, land farming	Recycling, reuse, incineration (with energy recovery), landfilling as last resort
Effect on soil	Improves soil fertility, structure, and water-holding capacity (as compost)	Inert fragments reduce soil aeration and water infiltration; microplastics affect soil organisms and nutrient cycling

Plastic Waste and Microplastics

India generates ~9.4 million tonnes of plastic waste annually, of which only ~60% is recycled. The remaining 40% ends up in landfills or the environment. Microplastics (<5 mm) are formed by fragmentation of larger plastic items through UV radiation, mechanical abrasion, and biodegradation. They contaminate water bodies, soil, air, and even Arctic ice. Microplastics enter the food chain through ingestion by aquatic organisms and have been found in human blood, lungs, and placentas. They act as vectors for toxic additives (phthalates, BPA) and adsorbed pollutants (PCBs, heavy metals). The government banned single-use plastics (SUP) items in India from July 1, 2022.

E-Waste Management in India

India is the third-largest generator of e-waste globally (~3.2 million tonnes per year), after China and the USA. E-waste contains valuable metals (Au, Ag, Cu, Pd, Pt) as well as toxic substances (Pb, Hg, Cd, Be, Cr-VI, brominated flame retardants). The E-Waste (Management) Rules, 2016 (amended 2018) in India follow the principle of Extended Producer Responsibility (EPR), making manufacturers responsible for collecting and channelising end-of-life products to authorised recyclers. The rules also mandate reduction of hazardous substances (RoHS compliance) and target 30–70% collection efficiency. Despite regulations, ~90% of e-waste in India is still handled by the informal sector (scrap dealers, backyard recyclers) using unsafe practices (open burning, acid leaching), causing severe occupational and environmental hazards.

5. Green Chemistry

Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It applies across the entire lifecycle of a chemical product.

12 Principles of Green Chemistry

#	Principle	Description
1	Prevention	Prevent waste rather than treat it after formation
2	Atom Economy	Maximise incorporation of reactants into final product
3	Less Hazardous Synthesis	Use and generate substances with minimal toxicity
4	Safer Solvents/ auxiliaries	Minimise use of auxiliary substances; use innocuous solvents
5	Energy Efficiency	Conduct reactions at ambient temperature/pressure
6	Renewable Feedstocks	Use renewable raw materials instead of depleting ones
7	Reduce Derivatives	Avoid protection/deprotection steps
8	Catalysis	Use catalytic reagents (vs. stoichiometric)
9	Biodegradable Products	Design products that degrade after use
10	Real-time Analysis	Monitor reactions to prevent hazardous by-products
11	Inherent Safety	Minimise potential for accidents (releases, explosions, fires)
12	Atom Economy (expanded)	% Atom Economy = (Formula weight of desired product ÷ Sum of formula weights of all reactants) × 100

Atom Economy Calculation

Atom economy is a measure of how many atoms of reactants end up in the desired product. High atom economy = less waste.

Comparison: Traditional vs Green Chemistry

Process	Traditional Method	Green Method	Benefit
Ibuprofen synthesis	6 steps, atom economy ~40%	3 steps, catalytic (Pd), atom economy ~99%	8 million kg/yr waste reduction
Coffee decaffeination	CH2Cl2 solvent extraction	Supercritical CO2 extraction	Non-toxic, CO2 is recycled
Paper bleaching	Cl2/ClO2 bleaching	H2O2 or O3 bleaching	No dioxin formation
Polyethylene production	High-pressure (3000 atm) radical process	Metallocene catalysts at low pressure	Energy saving, controlled polymer structure
Adipic acid (nylon precursor)	HNO3 oxidation (emits N2O)	Bio-catalytic route using E. coli	Eliminates N2O greenhouse gas

Examples & Applications

1. Ibuprofen synthesis (BHC process): Traditional synthesis had atom economy ~40%; the green process uses a catalytic method with atom economy ~99% and eliminates 8 million kg of waste annually.

2. Supercritical CO₂ as solvent: Used in decaffeination of coffee (replaces CH₂Cl₂), dry cleaning (replaces perchloroethylene), and as a reaction medium. scCO₂ is non-toxic, non-flammable, and easily recycled.

3. Hydrogen peroxide bleaching: Replaces chlorine-based bleaching in paper and textile industries, eliminating formation of dioxins and chlorinated organics.

4. Microwave-assisted synthesis: Reduces reaction time from hours to minutes, saving energy. Used in esterification, Diels-Alder, and cross-coupling reactions.

5. Biocatalysis: Enzymes (lipases, acylases) perform highly selective transformations under mild conditions. Example: production of acrylamide using nitrile hydratase (Rhodococcus rhodochrous).

6. Aqueous-phase reactions: Use water as solvent instead of organic solvents. Example: Pericyclic reactions like Claisen rearrangement in water.

6. Types of Environmental Pollution — Comparative Overview

Comparison of Major Types of Environmental Pollution

Type of Pollution	Primary Sources	Major Pollutants	Key Effects	Control Measures
Air Pollution	Vehicle emissions, industrial stacks, power plants, biomass burning, construction	CO, NOx, SO2, PM, O3, Pb, VOCs	Respiratory diseases, acid rain, smog, global warming, ozone depletion	Catalytic converters, scrubbers, electrostatic precipitators, clean fuels (CNG, EVs)
Water Pollution	Industrial effluents, domestic sewage, agricultural runoff, oil spills	Pathogens, organic waste, heavy metals, nitrates, phosphates, pesticides	Waterborne diseases, eutrophication, biomagnification, DO depletion, fish kills	Wastewater treatment (primary/secondary/tertiary), nutrient removal, constructed wetlands
Soil Pollution	Pesticide/fertiliser overuse, industrial dumping, mining, landfill leachate, e-waste	Heavy metals (Pb, Hg, Cd, As), PCBs, PAHs, pesticides, plastics	Loss of fertility, groundwater contamination, bioaccumulation in food chain, desertification	Phytoremediation, bioremediation, soil washing, composting, regulated landfilling
Noise Pollution	Traffic, construction, industries, loudspeakers, aircraft, railways	Sound >85 dB (continuous) or >120 dB (impulse)	Hearing loss, hypertension, sleep disturbance, cognitive impairment in children	Sound barriers, silencers, zoning regulations, green belts, noise limits (CPCB: 55 dB day / 45 dB night in residential areas)
Thermal Pollution	Power plant cooling water discharge, industrial processes, deforestation	Heated water (5–10°C above ambient)	Reduced DO, thermal shock to aquatic life, altered species composition, increased metabolic rate in fish	Cooling towers, cooling ponds, heat exchangers, artificial wetlands for heat dissipation
Radioactive Pollution	Nuclear power plants, weapons testing, medical isotopes, uranium mining	90Sr, 137Cs, 239Pu, 131I, 99mTc	DNA damage, cancer (leukaemia, thyroid), mutation, radiation sickness, genetic disorders	Deep geological repositories, vitrification, containment shielding, strict regulatory protocols