Environmental chemistry is the study of the origin, transport, reactions, effects, and fates of chemical species in the environment. It deals with the chemical aspects of our surroundings and the impact of human activities and natural processes on them. Environmental studies encompass broader social, economic, and biological interactions, but environmental chemistry focuses specifically on the chemical perspective.

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Environmental Pollution

Environmental pollution refers to the harmful effects caused by undesirable changes in our surroundings, negatively impacting living organisms and the environment. A substance causing pollution is called a pollutant.

Pollutants are substances present in concentrations higher than their natural abundance, often originating from human activities (industrial emissions, waste disposal, agriculture) or natural events (volcanic eruptions, forest fires). Pollutants can be in solid, liquid, or gaseous states.

Pollutants are categorized by their degradability:

• Degradable pollutants: Break down relatively quickly through natural processes (e.g., discarded vegetables).
• Slowly degradable/Non-degradable pollutants: Persist in the environment for long periods, sometimes decades, without significant breakdown by natural processes. These are often synthetic chemicals or heavy metals (e.g., DDT, plastics, nuclear wastes, heavy metals like lead, mercury). They pose long-term threats as they can accumulate in the environment and living organisms, potentially reaching toxic levels through bioaccumulation and biomagnification in food chains.

Pollutants originate from a source, are transported (via air, water), and deposited or accumulate in different parts of the environment (air, water bodies, soil).

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Atmospheric Pollution

Atmospheric pollution refers to the presence of undesirable substances in the Earth's atmosphere, causing harm. The atmosphere is divided into concentric layers.

• Troposphere: The lowest layer, extending up to $\sim 10$ km. Contains most of the air, water vapour, and clouds. Where weather occurs and where humans and most organisms live.
• Stratosphere: Above the troposphere, from $\sim 10$ to 50 km. Contains dinitrogen, dioxygen, ozone, and little water vapour.

Atmospheric pollution is typically discussed as tropospheric or stratospheric pollution.

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Tropospheric Pollution

Tropospheric pollution is caused by undesirable solid or gaseous particles in the troposphere. Major tropospheric pollutants are:

• Gaseous pollutants: * Oxides of Sulphur ($\text{SO}_x$): Primarily sulfur dioxide ($\text{SO}_2$), produced from burning sulfur-containing fossil fuels. $\text{SO}_2$ is poisonous, causing respiratory problems in humans and damage to plants. It can be oxidised to sulfur trioxide ($\text{SO}_3$) in the presence of catalysts or oxidants ($\text{O}_3$, $\text{H}_2\text{O}_2$), leading to acid rain.

  $\text{2SO}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow \text{2SO}_3\text{(g)}$

  $\text{SO}_2\text{(g)} + \text{O}_3\text{(g)} \rightarrow \text{SO}_3\text{(g)} + \text{O}_2\text{(g)}$

  $\text{SO}_2\text{(g)} + \text{H}_2\text{O}_2\text{(l)} \rightarrow \text{H}_2\text{SO}_4\text{(aq)}$

  * Oxides of Nitrogen ($\text{NO}_x$): Nitric oxide (NO) and nitrogen dioxide ($\text{NO}_2$). Formed from the reaction of N$_2$ and O$_2$ at high temperatures (e.g., in lightning or automobile engines). NO reacts with O$_2$ to form $\text{NO}_2$ (a brown irritant gas). $\text{NO}_2$ is harmful to plants and causes respiratory issues in children.

  $\text{N}_2\text{(g)} + \text{O}_2\text{(g)} \xrightarrow{\text{High T}} \text{2NO(g)}$

  $\text{2NO(g)} + \text{O}_2\text{(g)} \rightarrow \text{2NO}_2\text{(g)}$

  * Hydrocarbons: Unburnt fuels from automobiles and industries. Many are carcinogenic (cancer-causing) and harmful to plants (causing ageing, tissue breakdown). * Oxides of Carbon ($\text{CO}_x$): * Carbon monoxide (CO): Produced by incomplete combustion of carbon fuels (auto exhaust, burning coal/wood). Highly poisonous due to its ability to bind strongly to hemoglobin, reducing oxygen transport in blood (forming carboxyhemoglobin). Causes headaches, vision problems, and cardiovascular disorders. * Carbon dioxide (CO$_2$): Produced by complete combustion, respiration, volcanic eruptions, decomposition of limestone. A natural component of the atmosphere (0.03%). Increased levels from burning fossil fuels and deforestation are the main cause of global warming (enhanced greenhouse effect).
• Particulate pollutants: Fine solid particles or liquid droplets suspended in the air. * **Viable particulates:** Minute living organisms like bacteria, fungi, moulds, algae, etc. Can cause allergies and plant diseases. * **Non-viable particulates:** Non-living particles classified by size and nature: * Smoke: Solid or solid/liquid particles from combustion (e.g., cigarette smoke, fuel combustion smoke). * Dust: Fine solid particles from grinding, crushing, storms (e.g., fly ash, cement dust). * Mists: Droplets from sprays or vapour condensation (e.g., acid mist, pesticide sprays). * Fumes: Fine solid particles from vapour condensation (e.g., metal fumes). Particulate effects depend on size; smaller particles (<10 microns) can enter lungs. Lead from leaded petrol was a major particulate pollutant.

Smog: A common type of air pollution. * **Classical smog:** Occurs in cool, humid climates. Mixture of smoke, fog, and $\text{SO}_2$. Reducing smog. * **Photochemical smog:** Occurs in warm, dry, sunny climates. Main components are formed by sunlight reacting with $\text{NO}_x$ and hydrocarbons from vehicles/factories. Oxidising smog. Components: ozone, NO, acrolein, formaldehyde, peroxyacetyl nitrate (PAN). Causes eye/respiratory irritation, headaches, chest pain, damage to plants and materials.

Formation of photochemical smog: $\text{NO}_x$ and hydrocarbons react under sunlight. $\text{NO}_2$ photolyses to NO and O atoms. O atoms react with O$_2$ to form O$_3$. O$_3$ reacts with NO to reform $\text{NO}_2$. O$_3$ and $\text{NO}_2$ react with hydrocarbons to form acrolein, formaldehyde, PAN. $\text{NO}_2\text{(g)} \xrightarrow{hv} \text{NO(g)} + \text{O(g)}$ $\text{O(g)} + \text{O}_2\text{(g)} \rightarrow \text{O}_3\text{(g)}$ $\text{NO(g)} + \text{O}_3\text{(g)} \rightarrow \text{NO}_2\text{(g)} + \text{O}_2\text{(g)}$ $\text{Hydrocarbons} + \text{NO}_x + \text{O}_3 \xrightarrow{hv} \text{Acrolein} + \text{Formaldehyde} + \text{PAN}$

Controlling photochemical smog involves reducing precursor emissions ($\text{NO}_x$ and hydrocarbons) using catalytic converters in vehicles and planting specific trees that can metabolise $\text{NO}_x$.

Global Warming and Greenhouse Effect: Certain gases in the atmosphere ($\text{CO}_2$, $\text{CH}_4$, $\text{O}_3$, $\text{CFCs}$, water vapour) trap heat radiated from the Earth's surface, warming the atmosphere. This is the greenhouse effect. A natural greenhouse effect is essential for life, maintaining Earth's temperature. However, increased concentrations of greenhouse gases from human activities enhance this effect, leading to **global warming** (increase in average global temperature). $\text{CO}_2$ is the largest contributor. Global warming can cause melting polar ice, sea level rise, flooding, and increase infectious diseases.

Acid Rain: Rainwater is naturally slightly acidic (pH 5.6) due to dissolved $\text{CO}_2$. Acid rain occurs when rainwater pH drops below 5.6, primarily due to dissolution of $\text{SO}_x$ and $\text{NO}_x$ from atmospheric pollution. These oxides are formed from burning fossil fuels and industrial processes. They are converted to sulfuric and nitric acids in the atmosphere, which are deposited as wet deposition (in rain, snow, fog) or dry deposition (as particles). $\text{2SO}_2\text{(g)} + \text{O}_2\text{(g)} + \text{2H}_2\text{O(l)} \rightarrow \text{2H}_2\text{SO}_4\text{(aq)}$ $\text{4NO}_2\text{(g)} + \text{O}_2\text{(g)} + \text{2H}_2\text{O(l)} \rightarrow \text{4HNO}_3\text{(aq)}$ Acid rain is harmful to ecosystems (acidifies lakes/rivers, damages forests/crops), human health (respiratory issues), and damages buildings/monuments made of stone or metal (e.g., Taj Mahal).

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Stratospheric Pollution

Stratospheric pollution primarily concerns the **depletion of the ozone layer**. The ozone layer in the upper stratosphere absorbs harmful UV radiation from the sun, protecting life on Earth.

Ozone formation and breakdown in the stratosphere: UV radiation splits O$_2$ molecules into O atoms, which combine with O$_2$ to form O$_3$. O$_3$ is also naturally decomposed. An equilibrium exists. $\text{O}_2\text{(g)} \xrightarrow{UV} \text{2O(g)}$ $\text{O(g)} + \text{O}_2\text{(g)} \rightarrow \text{O}_3\text{(g)}$ $\text{O}_3\text{(g)} \xrightarrow{UV} \text{O}_2\text{(g)} + \text{O(g)}$ (Natural decomposition)

Ozone depletion: Certain chemicals, particularly **chlorofluorocarbon compounds (CFCs)**, released into the atmosphere reach the stratosphere and are broken down by UV radiation to release chlorine free radicals (C•l). These radicals catalyse the breakdown of ozone in a chain reaction. $\text{CF}_2\text{Cl}_2\text{(g)} \xrightarrow{UV} \text{C}•\text{l(g)} + \text{C}•\text{F}_2\text{Cl(g)}$ $\text{C}•\text{l(g)} + \text{O}_3\text{(g)} \rightarrow \text{ClO}•\text{(g)} + \text{O}_2\text{(g)}$ $\text{ClO}•\text{(g)} + \text{O(g)} \rightarrow \text{C}•\text{l(g)} + \text{O}_2\text{(g)}$ The regenerated C•l radical continues to destroy O$_3$ molecules.

The **ozone hole** is a region of severe ozone depletion over Antarctica during spring, facilitated by polar stratospheric clouds in winter that convert chlorine compounds into active chlorine radicals under spring sunlight. The thinning of the ozone layer increases the amount of UV radiation reaching the Earth's surface.

Effects of ozone depletion: Increased UV radiation causes health problems (skin cancer, cataracts, premature skin ageing), damages plant life (mutation, reduced growth), harms aquatic ecosystems (killing phytoplankton), and degrades materials (paints, fabrics).

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Water Pollution

Water pollution is the contamination of water bodies (rivers, lakes, oceans, groundwater) by substances that are harmful to living organisms and the environment. Pollution originates from point sources (identifiable locations like discharge pipes) and non point sources (diffuse sources like agricultural runoff, acid rain).

Pollutant	Source
Micro-organisms (Pathogens)	Domestic sewage, animal excreta
Organic wastes	Domestic sewage, animal waste, plant decay, food processing industries
Plant nutrients	Chemical fertilizers
Toxic heavy metals	Industries, chemical factories
Sediments	Soil erosion from agriculture, mining
Pesticides	Chemicals used in agriculture/homes
Radioactive substances	Mining radioactive minerals
Heat	Water used for cooling in industries

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Causes Of Water Pollution

1. **Pathogens:** Disease-causing microorganisms (bacteria, viruses, protozoa) from sewage and animal waste. Cause gastrointestinal diseases.
2. **Organic wastes:** Biodegradable matter (leaves, grass, sewage, food waste) from runoff or decomposition. Decomposed by bacteria, consuming dissolved oxygen (DO) in water. Depletion of DO (< 6 ppm) harms aquatic life. **Biochemical Oxygen Demand (BOD)** is the amount of oxygen consumed by bacteria to decompose organic matter in a water sample; higher BOD indicates more organic pollution.
3. **Chemical Pollutants:** * Inorganic chemicals: Heavy metals (Cd, Hg, Ni) are toxic and accumulate in the body, damaging organs. Acids (from mine drainage), salts (from snow melting, industrial discharge) are also pollutants. * Organic chemicals: Pesticides, herbicides, PCBs, detergents, petroleum products. Many are toxic, persistent, and can biomagnify in food chains. Detergents can cause oxygen depletion due to rapid bacterial growth (algal blooms, eutrophication).

Eutrophication: Excessive nutrient enrichment of water bodies (especially with phosphates and nitrates), causing dense growth of algae (blooms). When algae die and decompose, bacteria consume vast amounts of DO, leading to anaerobic conditions, death of fish and other aquatic life, and loss of biodiversity.

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International Standards For Drinking Water

Standards define maximum allowable concentrations of various substances in drinking water to ensure safety.

• Fluoride: Essential for preventing tooth decay (1 ppm). Excess (>2 ppm mottling, >10 ppm bone/teeth damage).
• Lead: Max limit 50 ppb. Damages kidneys, CNS, reproductive system.
• Sulphate: (>500 ppm laxative effect).
• Nitrate: Max limit 50 ppm. Can cause methemoglobinemia ('blue baby' syndrome).
• Other metals: Maximum concentrations specified for Fe, Mn, Al, Cu, Zn, Cd.

Metal	Maximum concentration (ppm or mg dm$^{-3}$ )
Fe	0.2
Mn	0.05
Al	0.2
Cu	3.0
Zn	5.0
Cd	0.005

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Soil Pollution

Soil pollution is caused by the build-up of toxic compounds in soil at levels harmful to humans and other organisms. Major sources include agricultural chemicals, industrial wastes, and improper waste disposal.

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Pesticides

Chemicals used to control pests (insects, rodents, weeds, fungi). Post-WWII, synthetic pesticides (like DDT) became widely used in agriculture and public health. However, their persistence (non-biodegradable) leads to accumulation in soil and water and **biomagnification** in food chains, reaching high concentrations in higher organisms, causing health problems.

Less persistent organophosphates and carbamates were developed but are often more acutely toxic nerve toxins. Pesticides also lead to resistant pests. Herbicides (weed killers) are also toxic, though often less persistent than organo-chlorides, but they can also be concentrated in the food web and cause adverse effects.

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Industrial Waste

Industrial activities generate large quantities of solid waste. These wastes are classified as biodegradable (e.g., from textile, paper, food industries) and non-biodegradable (e.g., fly ash from power plants, slag from metal industries, toxic wastes like heavy metals, chemicals, drugs, dyes). Non-biodegradable and hazardous wastes pose significant environmental threats if not managed properly.

Proper waste disposal methods are crucial, including controlled incineration for toxic wastes and recycling of materials like fly ash, slag, plastics, glass, and metal scraps. Innovative technologies can convert some waste into valuable products or energy (e.g., producing cement from fly ash/slag, generating electricity from garbage via biogas production, recycling plastics into fuel or textiles).

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Strategies To Control Environmental Pollution

Controlling environmental pollution requires a combination of waste management strategies, technological solutions, and changes in practices.

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Waste Management

Effective waste management is crucial for reducing pollution. This involves proper collection, sorting (separating biodegradable from non-biodegradable), and disposal of waste.

• Biodegradable wastes can be composted or used for biogas production.
• Non-biodegradable wastes should be sent for recycling or disposed of in properly managed landfills (for less hazardous types) or by controlled incineration (for hazardous types).

Improper disposal can lead to soil and water contamination, health hazards for waste handlers and the public, and environmental degradation. Initiatives like 'Swachh Bharat Abhiyan' promote cleanliness and proper waste management.

Recycling waste materials into useful products or energy sources is a key strategy for sustainable waste management (e.g., using plastic waste for fuel or textiles, generating electricity from garbage).

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Green Chemistry

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Introduction

Green chemistry is an approach to chemistry and chemical engineering that aims to design products and processes that reduce or eliminate the use or generation of hazardous substances. It applies existing chemical knowledge to minimise the adverse environmental impact of chemical activities throughout the life cycle of a chemical product (design, manufacture, use, and disposal).

Key principles of green chemistry focus on:

• Reducing waste generation and promoting atom economy (incorporating most atoms of reactants into the product).
• Designing safer chemicals and processes (using less toxic reactants, solvents, and conditions).
• Using renewable feedstocks.
• Designing for degradation (products should break down into harmless substances).
• Using catalysts (more efficient than stoichiometric reagents).
• Preventing pollution at the source rather than cleaning it up afterwards.

Green chemistry is a cost-effective approach that reduces material and energy consumption and waste generation, benefiting both the environment and the economy.

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Green Chemistry In Day-To-Day Life

Examples of applying green chemistry principles in daily life and industry:

1. **Dry Cleaning:** Replacing hazardous chlorinated solvents (like tetrachloroethene, a suspected carcinogen) with liquefied carbon dioxide and suitable detergents.
2. **Bleaching:** Using hydrogen peroxide (H$_2$O$_2$) with catalysts for bleaching paper and textiles instead of chlorine gas (which can produce toxic chlorinated byproducts).
3. **Synthesis of Chemicals:** Developing more environmentally friendly synthesis routes, such as the one-step oxidation of ethene to ethanal in aqueous medium using an ionic catalyst, replacing older processes that generated more waste.

   $\text{CH}_2\text{=CH}_2\text{(aq)} + \text{O}_2\text{(g)} \xrightarrow{\text{Catalyst (Pd(II)/Cu(II)), water}} \text{CH}_3\text{CHO(aq)}$

4. **Cleaning Turbid Water:** Using natural, biodegradable, and cost-effective materials like the powder from tamarind seeds (agricultural waste) to clean municipal and industrial wastewater, replacing chemicals like alum that can increase toxic ions in treated water.

Green chemistry offers a path towards sustainable development by reducing chemical hazards and environmental pollution associated with chemical production and consumption.

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