Environmental Chemistry & Fertilizers — Pollution Chemistry, Pesticides & Green Solutions

Overview

Environmental chemistry deals with the chemical processes occurring in the environment -- the composition, reactions, and effects of chemical species in air, water, and soil. For UPSC, this topic bridges General Science (Prelims) and GS-3 (Environment, Pollution, Agriculture) by connecting fundamental chemistry with real-world pollution, fertilizer use, and remediation.

India faces severe environmental challenges: air quality in several cities ranks among the worst globally, river pollution affects over 350 stretches across major rivers, and excessive fertilizer use has degraded soils in the Indo-Gangetic plains. Understanding the chemistry behind these problems is essential for answering questions on pollution control, agricultural productivity, and green technology.

This chapter covers: atmospheric pollutants and their reactions, water chemistry and contamination, ozone layer depletion, acid rain, greenhouse gases, fertilizers and pesticides, food additives, water purification, soil chemistry, and the emerging field of green chemistry.

Air Pollutants — Types and Chemistry

Primary vs Secondary Pollutants

• Primary pollutants are emitted directly from sources -- SO2, NO, CO, particulate matter, hydrocarbons
• Secondary pollutants form through atmospheric reactions -- O3 (ground-level), NO2, PAN (peroxyacetyl nitrate), smog

Sulphur Oxides (SOx)

• SO2 is produced by burning fossil fuels (coal, petroleum) containing sulphur
• In the atmosphere, SO2 oxidises to SO3, which reacts with water to form sulphuric acid (H2SO4) -- a major component of acid rain
• India's thermal power plants are the largest source of SO2; the Ministry of Environment mandated flue gas desulphurisation (FGD) for all coal plants

Nitrogen Oxides (NOx)

• NO forms at high temperatures in automobile engines and industrial furnaces (N2 + O2 -> 2NO)
• NO is rapidly oxidised to NO2 (brown gas), which contributes to photochemical smog
• NO2 + UV light -> NO + O (atomic oxygen); O + O2 -> O3 (ground-level ozone -- harmful)
• NOx reacts with VOCs (volatile organic compounds) in sunlight to produce photochemical smog, first identified in Los Angeles

Carbon Monoxide (CO)

• Produced by incomplete combustion of fossil fuels, biomass burning, and vehicular exhaust
• CO binds to haemoglobin (~240 times more strongly than O2) to form carboxyhaemoglobin, reducing oxygen-carrying capacity
• Indoor CO poisoning from poorly ventilated coal/wood stoves is a significant health risk in rural India

Particulate Matter (PM2.5 and PM10)

PM2.5 is the single largest environmental health risk in India; the NCAP targets a 40% reduction in PM levels from 2017 baseline by 2026 across 131 non-attainment cities.

Volatile Organic Compounds (VOCs) and Hydrocarbons

• Released from vehicle exhausts, industrial solvents, paints, and petrochemical processing
• Methane (CH4), benzene, toluene, xylene are common VOCs
• Benzene is a known carcinogen (Group 1, IARC); BS-VI norms limit benzene in petrol to 1%

Water Pollutants — Chemistry of Contamination

Biochemical Oxygen Demand (BOD)

• BOD measures the amount of dissolved oxygen consumed by microorganisms to decompose organic matter in water over 5 days at 20 degrees Celsius
• Higher BOD indicates greater organic pollution (untreated sewage has BOD of 200-600 mg/L)
• Clean water: BOD less than 1 mg/L; moderately polluted: 3-8 mg/L; heavily polluted: more than 8 mg/L

Chemical Oxygen Demand (COD)

• COD measures the total oxygen required to chemically oxidise both organic and inorganic matter
• COD is always greater than or equal to BOD because it includes non-biodegradable substances
• Important for assessing industrial effluent quality

Dissolved Oxygen (DO)

• Healthy aquatic ecosystems require DO of 6-8 mg/L or above
• When BOD is high, microorganisms consume DO rapidly, causing oxygen depletion and fish kills
• Many stretches of the Yamuna in Delhi have near-zero DO levels during lean flow months

Heavy Metal Contamination

Heavy metals undergo bioaccumulation and biomagnification through the food chain. The concept of biological magnification is a Prelims favourite.

Eutrophication

• Excess nitrogen and phosphorus (from fertilizer runoff, detergents, sewage) enrich water bodies
• This triggers algal blooms, which block sunlight and consume oxygen upon decomposition
• Results in hypoxic (dead) zones where aquatic life cannot survive
• Major concern in Indian lakes (Bellandur Lake, Bengaluru) and coastal zones

Ozone Chemistry — Stratospheric Ozone and Depletion

The Ozone Layer

• Ozone (O3) is concentrated in the stratosphere at 15-35 km altitude
• Absorbs UV-B radiation (280-315 nm), protecting life from DNA damage and skin cancer
• Total ozone in a column is measured in Dobson Units (DU); normal levels are 300-350 DU
• The ozone hole is defined as the region (primarily over Antarctica) with total ozone below 220 DU

Chapman Cycle (Natural Ozone Formation and Destruction)

1. O2 + UV (less than 240 nm) -> 2O (photodissociation)
2. O + O2 + M -> O3 + M (ozone formation; M is a third body)
3. O3 + UV (200-320 nm) -> O2 + O (ozone photolysis)
4. O + O3 -> 2O2 (natural destruction)

This natural cycle maintains a steady-state ozone concentration in the stratosphere.

CFC-Induced Ozone Depletion

• Chlorofluorocarbons (CFCs) -- such as CFC-11 (CFCl3) and CFC-12 (CF2Cl2) -- are chemically inert in the troposphere
• In the stratosphere, UV light breaks the C-Cl bond, releasing free chlorine radicals
• Cl + O3 -> ClO + O2
• ClO + O -> Cl + O2
• Net reaction: O3 + O -> 2O2
• One chlorine atom can destroy approximately 100,000 ozone molecules before being deactivated
• Polar stratospheric clouds (PSCs) in Antarctica provide surfaces that accelerate ozone destruction, explaining why the ozone hole is most severe there

Montreal Protocol (1987)

• Signed on 16 September 1987; entered into force on 1 January 1989
• Phased out production of CFCs, halons, carbon tetrachloride, and methyl chloroform
• First universally ratified treaty in UN history (198 parties)
• Phased out over 98% of ODS (ozone-depleting substances) globally
• Kigali Amendment (2016) extended the Protocol to phase down HFCs (hydrofluorocarbons), which are potent greenhouse gases though they do not deplete ozone
• India committed to an 85% HFC reduction by 2047 under the Kigali Amendment

Acid Rain — Mechanism and Effects

Formation

Normal rainwater has a pH of about 5.6 due to dissolved CO2 forming weak carbonic acid (H2CO3). Acid rain is defined as precipitation with pH below 5.6.

Chemical mechanism:

• SO2 + H2O -> H2SO3 (sulphurous acid)
• 2SO2 + O2 -> 2SO3 (catalytic oxidation in atmosphere)
• SO3 + H2O -> H2SO4 (sulphuric acid)
• 2NO2 + H2O -> HNO3 + HNO2 (nitric acid)

About two-thirds of acid rain acidity comes from sulphuric acid and one-third from nitric acid.

Effects of Acid Rain

The Taj Trapezium Zone (TTZ) -- a 10,400 sq km area around Agra -- was established by Supreme Court order to control SO2 emissions and protect the Taj Mahal from acid rain damage.

Greenhouse Gases and Global Warming Potential

Major Greenhouse Gases

GWP values are from the IPCC Sixth Assessment Report (AR6, 2021-2023).

Key UPSC note: Methane has a much higher GWP per molecule than CO2 but shorter atmospheric lifetime. Over a 20-year horizon, methane's GWP is 81-83 -- making short-term methane reduction highly effective for near-term climate goals.

India's Greenhouse Gas Profile

• India is the third-largest emitter of GHGs (after China and the USA)
• Energy sector contributes approximately 73% of India's emissions
• Agriculture sector is the largest source of CH4 (rice cultivation, cattle) and N2O (fertilizer use)
• India's updated NDC (2023) targets 45% reduction in emissions intensity of GDP by 2030 (from 2005 levels) and 50% cumulative non-fossil electric power capacity

Fertilizers — NPK, Urea, and Neem-Coated Urea

Essential Plant Nutrients

• Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K) -- the "NPK" trio
• Secondary nutrients: Calcium, Magnesium, Sulphur
• Micronutrients: Iron, Zinc, Manganese, Boron, Copper, Molybdenum

Major Fertilizer Types in India

Neem-Coated Urea

• Government mandated that a minimum of 75% of domestically produced urea must be neem-coated
• Neem oil coating slows nitrification (conversion of ammonium to nitrate), improving nitrogen use efficiency
• Benefits: reduced nitrogen leaching into groundwater, lower N2O emissions, discourages diversion of subsidised urea for industrial use
• Urea Gold -- a new variant containing sulphur along with neem coating -- was introduced in 2023

Fertilizer Subsidy and Policy

• India's fertilizer subsidy exceeded Rs 1.8 lakh crore in recent years
• Nutrient-Based Subsidy (NBS) scheme (2010) applies to P and K fertilizers; urea remains under statutory price control
• PM-PRANAM (PM Programme for Restoration, Awareness, Nourishment and Amelioration of Mother Earth) incentivises states to promote balanced fertilizer use and reduce chemical fertilizer consumption
• Imbalanced N:P:K ratio (ideal 4:2:1, actual skewed towards nitrogen) degrades soil health

Pesticides — Types, Risks, and Biopesticides

Classification of Pesticides

Environmental and Health Concerns

• Bioaccumulation: Fat-soluble pesticides (DDT, endosulfan) accumulate in fatty tissues and magnify up the food chain
• Endosulfan disaster (Kerala): Aerial spraying on cashew plantations in Kasaragod led to severe health effects; Supreme Court banned endosulfan in India in 2011; listed under Stockholm Convention in 2011
• Pesticide residues in food monitored by FSSAI under Food Safety and Standards (Contaminants, Toxins, and Residues) Regulations

Biopesticides and Integrated Pest Management (IPM)

• Biopesticides include microbial agents (Bacillus thuringiensis/Bt, Trichoderma), plant-derived pesticides (neem-based Azadirachtin, pyrethrum), and biochemicals (pheromones)
• IPM combines biological control, cultural practices, resistant varieties, and minimal chemical use
• India's Central Insecticides Board and Registration Committee (CIB&RC) regulates pesticide registration under the Insecticides Act, 1968

Food Additives and Preservatives

Common Food Additives

• Regulated in India by FSSAI under the Food Safety and Standards Act, 2006
• The "E-number" system (European origin) is widely used for classification
• Adulteration (mixing inferior/harmful substances) is a criminal offence under FSSAI regulations

Water Purification Chemistry

Physical Methods

• Sedimentation and filtration: Removal of suspended solids by gravity settling and sand/membrane filters
• Reverse Osmosis (RO): Uses semi-permeable membranes under pressure to remove dissolved salts, heavy metals, and microorganisms; effective for desalination and arsenic/fluoride removal

Chemical Methods

UV Disinfection

• UV light (254 nm wavelength) damages microbial DNA, preventing replication
• Does not add chemicals; no residual disinfection capacity
• Effective against bacteria, viruses, and protozoa (including Cryptosporidium, which is chlorine-resistant)

Arsenic and Fluoride Removal

• Arsenic in groundwater affects 230 districts across 25 states; West Bengal, Bihar, Jharkhand, UP most affected
• Fluoride contamination found in 469 districts across 27 states; Rajasthan, Haryana, Karnataka, Telangana worst affected
• Removal methods: activated alumina adsorption, ion exchange, coagulation with ferric salts, membrane filtration (RO/NF)
• Nalgonda technique (developed by CSIR-NEERI): uses alum and lime for community-level fluoride removal

Soil Chemistry

Soil pH and Nutrient Availability

• Acidic soils (pH below 6.5): Common in high-rainfall areas (Northeast India, Western Ghats); aluminium toxicity limits crop growth; treated with liming (CaCO3)
• Alkaline soils (pH above 7.5): Common in arid/semi-arid regions (Rajasthan, Haryana); excess sodium causes sodicity; treated with gypsum (CaSO4.2H2O)
• Most nutrients are optimally available at pH 6.0-7.0

Soil Degradation

• Salinisation: Accumulation of soluble salts (NaCl, Na2SO4) from irrigation with brackish water or poor drainage
• Waterlogging: Raises water table, reduces soil aeration, kills roots
• Nutrient depletion: Intensive cropping without adequate replenishment; India's soils are deficient in secondary and micronutrients in many regions
• Soil organic carbon (SOC): Indian soils have low SOC (typically 0.3-0.5%) compared to the global average; practices like zero tillage, crop residue retention, and vermicomposting can improve SOC

Soil Health Card Scheme

• Launched in 2015; provides farmers with information on nutrient status of their soil and recommendations for appropriate fertilizer dosage
• Tests for 12 parameters: pH, EC, organic carbon, N, P, K, S, Zn, Fe, Cu, Mn, B
• Over 23 crore soil health cards distributed across India

Green Chemistry — 12 Principles and Applications

What Is Green Chemistry?

Green chemistry, also called sustainable chemistry, is the design of chemical products and processes that reduce or eliminate hazardous substances. The concept was formalised by Paul Anastas and John Warner in their 1998 book Green Chemistry: Theory and Practice.

The 12 Principles of Green Chemistry

Applications of Green Chemistry

• Pharmaceutical industry: Pfizer's green synthesis of sertraline (Zoloft) reduced waste by 65% and eliminated hazardous solvents
• Supercritical CO2 as solvent: Used in decaffeination of coffee, dry cleaning (replacing toxic perchloroethylene)
• Biodegradable plastics: Polylactic acid (PLA) from corn starch; polyhydroxybutyrate (PHB) from bacteria
• Biocatalysis: Enzymes replace harsh chemical catalysts in food processing, detergent manufacturing
• Green hydrogen: Produced via electrolysis of water using renewable energy; India's National Green Hydrogen Mission (2023) targets 5 MMT annual production by 2030

Key Comparisons for UPSC

BOD vs COD

Ozone Layer vs Ground-Level Ozone

Exam Strategy and Previous Year Relevance

Environmental chemistry is a high-frequency Prelims topic. UPSC frequently tests:

• BOD/COD definitions and comparisons
• Ozone depletion chemistry and Montreal Protocol
• Greenhouse gases and their GWP
• Bioaccumulation and biomagnification
• Pesticide classification (organochlorines vs organophosphates)
• Green chemistry principles
• Water purification methods

For Mains GS-3, questions integrate chemistry with policy: fertilizer subsidy reform, soil health management, pollution control technology, and green chemistry for sustainable development.

Key tip: Remember that environmental chemistry questions often appear disguised under the "Environment" section in Prelims. Understanding the underlying chemistry gives you the edge to eliminate wrong options confidently.

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