Environmental Chemistry- Comprehensive Notes

Environmental Chemistry – Comprehensive Notes

Environmental Chemistry deals with the study of the origin, transport, reactions, effects, and fates of chemical species in the environment. It is a multidisciplinary science that involves the study of environmental pollution and its impact on living organisms and natural phenomena. This chapter is vital for understanding environmental issues from a chemical perspective for NEET and JEE Main.

1. Introduction to Environmental Pollution

  • Environmental Pollution: The undesirable change in physical, chemical, or biological characteristics of our land, air, and water that may or will adversely affect human life, industrial process, living conditions, and cultural assets.
  • Pollutant: A substance (solid, liquid, or gas) present in concentrations higher than the natural abundance and which causes harmful effects on the environment.
  • Contaminant: A substance that does not naturally occur in the environment but is introduced by human activity, becoming a pollutant if it has harmful effects.
  • Receptor: The medium affected by a pollutant.
  • Sink: The medium or process that removes pollutants from the environment.
  • Threshold Limit Value (TLV): Permissible level of a pollutant in the atmosphere to which a worker is exposed in an 8-hour day without adverse effects.

2. Atmospheric Pollution

Atmospheric pollution is categorized based on the layers of the atmosphere where it occurs.

2.1. Tropospheric Pollution

The troposphere is the lowest layer of atmosphere (up to 10 km from sea level). It consists of gaseous pollutants and particulate pollutants.

  • a. Gaseous Pollutants:
    • Oxides of Sulfur (SO2, SO3):
      • Sources: Burning of fossil fuels (coal, petroleum) in power plants, industrial units (smelters, sulfuric acid plants).
      • Effects:
        • Acid Rain: SO2 and SO3 react with water vapor to form H2SO4.
        • Respiratory diseases (asthma, bronchitis, emphysema), irritation to eyes.
        • Damage to plants (chlorosis, necrosis, stunted growth).
        • Corrosion of metals, deterioration of buildings (e.g., Taj Mahal by acid rain).
    • Oxides of Nitrogen (NO, NO2, N2O):
      • Sources: High-temperature combustion in automobiles, power plants, forest fires, lightning.
      • Effects:
        • Acid Rain: NO2 reacts with water to form HNO3.
        • Respiratory problems, irritation to eyes.
        • Damage to plants.
        • Photochemical Smog: NO2 is a key component.
        • Reddish-brown haze in traffic.
    • Carbon Monoxide (CO):
      • Sources: Incomplete combustion of carbonaceous fuels (automobiles, furnaces).
      • Effects: Highly poisonous to humans. Binds to hemoglobin 200-300 times more strongly than oxygen, forming carboxyhemoglobin, which reduces oxygen carrying capacity of blood, leading to headaches, blurred vision, cardiovascular disorders, and even death.
    • Carbon Dioxide (CO2):
      • Sources: Complete combustion of fossil fuels, deforestation, volcanic eruptions.
      • Effects: Primary greenhouse gas, contributes to global warming. (Less directly toxic at ambient levels compared to CO).
    • Hydrogen Sulfide (H2S):
      • Sources: Decomposition of organic matter, volcanic eruptions, industrial emissions.
      • Effects: Foul smell, tarnishes silver, causes headaches.
    • Hydrocarbons:
      • Sources: Incomplete combustion of fuels, industrial emissions (volatile organic compounds, VOCs).
      • Effects: Carcinogenic (e.g., Benzene). Contribute to photochemical smog.
  • b. Particulate Pollutants: Tiny solid particles or liquid droplets suspended in air.
    • Viable Particulates: Living organisms like bacteria, fungi, moulds, algae. Cause allergies, diseases.
    • Non-Viable Particulates: Fine particles like dust, mists, fumes, smoke, smog.
      • Smoke: Solid particles from combustion of organic matter (e.g., coal, wood, diesel).
      • Dust: Fine solid particles formed by crushing, grinding, etc.
      • Mists: Liquid droplets from spray operations.
      • Fumes: Condensed vapours, usually from metals.
      • Smog: Mixture of smoke and fog.
        • Classical (London/Sulfurous) Smog: Occurs in cool, humid climate. Mixture of smoke, fog, and sulfur dioxide. Reducing in nature. (Historically associated with London).
        • Photochemical (Los Angeles) Smog: Occurs in warm, dry, sunny climate. Formed by the action of sunlight on nitrogen oxides and unsaturated hydrocarbons (volatile organic compounds). It is oxidizing in nature.
          • Components: Ozone (O3), Nitric Oxide (NO), Acrolein (CH2=CHCHO), Formaldehyde (HCHO), Peroxyacetyl Nitrate (PAN) (CH3COOONO2).
          • Formation: NO2hv​NO+O. O+O2→O3. NO+O3→NO2+O2. Hydrocarbons + NO + O2 + Sunlight -> PAN, Aldehydes, Ketones.
          • Effects: Eye irritation, respiratory problems, damage to plants, cracking of rubber.
          • Control: Using catalytic converters in vehicles (to reduce NOx and hydrocarbons), planting trees (e.g., Pinus, Quercus, Pyrus which can metabolize NOx).

2.2. Stratospheric Pollution (Ozone Layer Depletion)

The stratosphere is the upper part of the atmosphere (10-50 km above sea level). It contains the ozone layer, which protects Earth from harmful UV radiation.

  • Ozone Formation and Depletion:
    • Formation: O2UV​O+O. O+O2UV​O3
    • Depletion: The balance between ozone formation and decomposition is disturbed by pollutants, primarily Chlorofluorocarbons (CFCs).
      • CFCs (Freons): Stable, non-flammable, non-toxic organic compounds like CCl2F2, CCl3F.
      • Mechanism: CFCs rise to the stratosphere. UV radiation breaks them down to release chlorine free radicals.
        • CCl2F2(g)UV​Cl⋅(g)+⋅CClF2(g)
        • Cl⋅(g)+O3(g)→ClO⋅(g)+O2(g)
        • ClO⋅(g)+O(g)→Cl⋅(g)+O2(g)
        • The chlorine radical is regenerated, so a single chlorine atom can destroy thousands of ozone molecules.
      • Ozone Hole: Region of dramatically thinned ozone layer, particularly over Antarctica (polar stratospheric clouds (PSCs) play a role).
  • Effects of Ozone Depletion:
    • Increased UV radiation reaching Earth.
    • Skin cancer (melanoma).
    • Cataracts (eye damage).
    • Damage to immune system.
    • Harm to marine phytoplankton (base of food web).
    • Reduced crop yields.

3. Acid Rain

  • Definition: Rainwater having a pH less than 5.6 (normal rainwater is slightly acidic due to dissolved CO2 forming carbonic acid, H2CO3, pH ~ 5.6).
  • Cause: Primarily due to oxides of sulfur (SOx) and oxides of nitrogen (NOx) released into the atmosphere from industrial emissions and combustion of fossil fuels.
  • Reactions:
    • SO2(g)+O2(g)particulates/catalyst​SO3(g)
    • SO3(g)+H2O(l)→H2SO4(aq)
    • NO2(g)+O2(g)​NO3(g) (from atmospheric oxidation of NO)
    • 2NO2(g)+H2O(l)→HNO3(aq)+HNO2(aq)
  • Effects:
    • Damage to historical monuments and buildings (e.g., marble corrosion of Taj Mahal: CaCO3+H2SO4→CaSO4+H2O+CO2).
    • Harm to aquatic life (acidification of lakes, affecting fish and other organisms).
    • Damage to forests (dissolves nutrients, releases toxic heavy metals from soil).
    • Corrosion of metals.
    • Damage to crops.

4. Greenhouse Effect and Global Warming

  • Greenhouse Effect: The natural process by which certain gases in Earth’s atmosphere trap heat, warming the planet to a temperature that supports life.
  • Greenhouse Gases (GHGs): Gases that absorb and emit infrared radiation, trapping heat.
    • Major GHGs: Carbon Dioxide (CO2 – largest contributor), Methane (CH4), Nitrous Oxide (N2O), Chlorofluorocarbons (CFCs), Ozone (O3), Water Vapor (H2O).
  • Global Warming: The long-term heating of Earth’s climate system observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere.
  • Consequences of Global Warming:
    • Melting of polar ice caps and glaciers, leading to rising sea levels and coastal flooding.
    • Changes in global weather patterns (more extreme events: droughts, floods, heatwaves, storms).
    • Impact on agriculture (reduced crop yields).
    • Threat to biodiversity (species extinction).
    • Increased spread of diseases.
  • Control Measures:
    • Reduce consumption of fossil fuels.
    • Increase use of renewable energy sources (solar, wind, hydro).
    • Improve energy efficiency.
    • Afforestation (planting trees) and prevent deforestation.
    • Reduce emissions of other GHGs (e.g., methane from agriculture, N2O from fertilizers).

5. Water Pollution

Contamination of water bodies by substances that make water unfit for use.

  • 1. Causes of Water Pollution:
    • Pathogens: Disease-causing microorganisms (bacteria, viruses) from domestic sewage, animal excreta. (e.g., Typhoid, cholera).
    • Organic Wastes: Biodegradable organic matter from domestic sewage, food processing plants, animal waste.
      • Biochemical Oxygen Demand (BOD): The amount of oxygen consumed by microorganisms in decomposing organic matter in a given water sample over 5 days at 20°C.
        • High BOD (e.g., > 17 ppm): Indicates highly polluted water.
        • Low BOD (e.g., < 5 ppm): Indicates clean water.
      • Eutrophication: The process by which a body of water becomes overly enriched with nutrients (e.g., phosphates, nitrates from fertilizers, detergents), leading to excessive growth of algae and aquatic plants (algal bloom). This consumes dissolved oxygen (DO) as algae die and decompose, harming aquatic life.
    • Chemical Pollutants:
      • Heavy Metals: Lead (Pb), Mercury (Hg), Cadmium (Cd), Arsenic (As). Non-biodegradable, accumulate in food chains (biomagnification).
        • Mercury pollution: Causes Minamata disease.
        • Cadmium pollution: Causes Itai-Itai disease.
      • Pesticides and Herbicides: Non-biodegradable, persistent organic pollutants (POPs).
      • Polychlorinated Biphenyls (PCBs): Carcinogenic, endocrine disruptors.
      • Acids (from industries), Alkalis, Oil spills.
    • Sediments: Silt, clay from soil erosion.
    • Radioactive Wastes.
    • Thermal Pollution: Discharge of hot water from power plants, decreasing dissolved oxygen.
  • 2. Effects of Water Pollution:
    • Diseases (typhoid, cholera, dysentery).
    • Loss of aquatic life due to low DO.
    • Accumulation of toxins in food chain.
    • Damage to ecosystems.
  • 3. Control Measures:
    • Treatment of industrial effluents and domestic sewage before discharge.
    • Minimize use of chemical fertilizers and pesticides.
    • Promote organic farming.
    • Afforestation to reduce soil erosion.

6. Soil Pollution

  • Definition: Alteration in soil composition or properties due to the accumulation of undesirable substances.
  • Causes:
    • Pesticides and Insecticides: DDT (Dichlorodiphenyltrichloroethane), Aldrin, Dieldrin (persistent, non-biodegradable).
    • Fertilizers: Excess use leads to eutrophication and soil degradation.
    • Industrial Wastes: Heavy metals, toxic chemicals.
    • Solid Wastes: Municipal solid waste, plastics, e-waste.
    • Deforestation: Leads to soil erosion.
  • Effects: Loss of soil fertility, biomagnification, groundwater contamination.
  • Control: Sustainable agricultural practices, proper waste management, recycling, bioremediation.

7. Green Chemistry

  • Definition: The design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It is a philosophy for preventing pollution at the molecular level, rather than just treating it after formation.
  • Principles of Green Chemistry (12 Principles, key ones for NEET/JEE):
    • Prevention: Better to prevent waste than to treat or clean up waste after it has been created.
    • Atom Economy: Maximize the incorporation of all materials used in the process into the final product (minimize waste).
      • % Atom Economy = (Formula mass of desired product / Sum of formula masses of all reactants) x 100
    • Less Hazardous Chemical Syntheses: Design synthetic methods to use and generate substances that possess little or no toxicity to human health and the environment.
    • Designing Safer Chemicals: Design chemical products that are fully effective yet have little or no toxicity.
    • Safer Solvents and Auxiliaries: Avoid the use of auxiliary substances (solvents, separating agents, etc.) wherever possible and make them innocuous when used. Water, supercritical CO2 are green solvents.
    • Design for Energy Efficiency: Minimize the energy requirements of chemical processes. Conduct reactions at ambient temperature and pressure.
    • Use of Renewable Feedstocks: Use raw materials and feedstocks that are renewable rather than depleting.
    • Reduce Derivatives: Avoid unnecessary derivatization (e.g., use of protecting groups, temporary modification of physical/chemical processes).
    • Catalysis: Catalytic reagents are superior to stoichiometric reagents.
    • Design for Degradation: Design chemical products so that at the end of their function, they break down into innocuous degradation products and do not persist in the environment.
    • Real-time Analysis for Pollution Prevention: Develop analytical methodologies to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
    • Inherently Safer Chemistry for Accident Prevention: Choose substances and the form of a substance used in a chemical process to minimize the potential for chemical accidents, including releases, explosions, and fires.
  • Examples of Green Chemistry in practice:
    • Replacing traditional dry cleaning solvent (tetrachloroethene) with liquid CO2.
    • Bleaching of paper using H2O2 instead of Cl2.
    • Synthesis of ethene (polyethylene) by using Ziegler-Natta catalysts at low pressure and temperature.

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