Strategies for Control of Environmental Pollution

• The 12 principles of Green Chemistry (Anastas-Warner) and how to apply atom economy
• Green chemistry examples: H₂O₂ bleaching and liquid CO₂ dry cleaning
• How catalytic converters work (Pt/Pd/Rh) to control vehicle emissions
• Electrostatic precipitator operation for removing SPM from stack gases
• Primary, secondary, and tertiary sewage treatment processes
• Bioremediation using microorganisms to clean polluted sites

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Green Chemistry — Principles

Green Chemistry (also called sustainable chemistry) designs chemical processes that reduce or eliminate the use and generation of hazardous substances.

Developed by Paul Anastas and John Warner (1998) — 12 principles:

Atom Economy:

Atom Economy (%) = (Molecular mass of desired product / Total molecular mass of all products) × 100

• Addition reactions: AE = 100% (no by-products)
• Substitution reactions: AE < 100% (leaving group is waste)
• Rearrangement reactions: AE = 100%
• Elimination reactions: AE < 100%

Example: Hydrogenation of ethylene:

CH₂=CH₂ + H₂ → CH₃CH₃
MM of desired product (ethane) = 30
Total MM of products = 30
Atom Economy = (30/30) × 100 = 100%

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Green Chemistry in Practice

H₂O₂ as Bleaching Agent (replacing Cl₂)

Old process — Cl₂ bleaching:

Cl₂ + H₂O → HCl + HOCl
HOCl → bleaching action
By-products: organochlorine compounds (AOX) → persistent, toxic, carcinogenic

Green alternative — H₂O₂ bleaching:

H₂O₂ → [O] + H₂O    (under mild conditions)
[O] → bleaching action
By-product: H₂O (harmless)

Advantages:

• No toxic by-products
• Atom economy = higher
• Biodegradable
• Used in paper pulp bleaching and textile industries

Liquid CO₂ as Dry Cleaning Solvent (replacing PERC)

Old process — Perchloroethylene (PERC, tetrachloroethylene, CCl₂=CCl₂):

• Effective degreaser/solvent
• Problem: carcinogenic, ozone-depleting, persistent groundwater contaminant

Green alternative — Liquid CO₂:

• CO₂ at ~60 bar, 31°C → supercritical CO₂ → excellent solvent
• After cleaning, pressure released → CO₂ gas escapes (recyclable), clothes are dry
• No toxic residue, no solvent waste
• Atom economy concern not applicable here (solvent, not reagent) — advantage is non-toxic, recyclable solvent

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Catalytic Converters

Fitted in vehicle exhaust system to convert toxic emissions into harmless gases before release.

Catalyst: Platinum (Pt), Palladium (Pd), and Rhodium (Rh) on honeycomb alumina support

Three reactions:

Reaction 1: CO oxidation
2CO + O₂ → 2CO₂          (catalysed by Pt/Pd)

Reaction 2: Hydrocarbon combustion  
CₓHᵧ + O₂ → CO₂ + H₂O   (catalysed by Pt/Pd)

Reaction 3: NOₓ reduction
2NO → N₂ + O₂  }         (catalysed by Rh)
2NO₂ → N₂ + 2O₂}

Overall: CO, unburned hydrocarbons, NOₓ → CO₂, H₂O, N₂

Design features:

• Honeycomb structure → maximises catalyst surface area
• Three-way catalyst (TWC) → handles all three reaction types simultaneously
• Requires lead-free petrol (Pb poisons/deactivates the catalyst — "catalytic converter poisoning")

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Electrostatic Precipitator (ESP)

Removes suspended particulate matter (SPM, dust, fly ash) from industrial stack gases.

Working principle:

1. Stack gas passes between two electrodes (collecting plates + discharge wires)
2. High voltage (~50,000 V) applied → corona discharge → ionises gas molecules → produces ions
3. Ions attach to dust/SPM particles → particles become charged
4. Charged particles migrate to oppositely charged collecting plate
5. Particles accumulate on plates → periodic rapping dislodges them → collected in hoppers

Efficiency: >99% removal of particulates
Used in: thermal power plants, cement kilns, smelters, paper mills

Does NOT remove gaseous pollutants (SO₂, NOₓ, CO) — only solid/liquid particles.

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Sewage Treatment

Municipal wastewater (sewage) treatment occurs in three stages.

Primary Treatment (Physical)

• Screening: coarse bar screens remove large solids (rags, plastic)
• Grit chamber: settling removes sand, gravel
• Primary sedimentation: 1–2 hour settling → removes ~50% suspended solids, ~30% BOD
• Output: primary sludge (settled solids) + primary effluent

Secondary Treatment (Biological)

• Aerobic treatment: effluent aerated → bacteria consume dissolved organic matter → BOD reduced by ~90%
  • Methods: trickling filters (bacteria on gravel), activated sludge process (aerated tanks), rotating biological contactors
• Anaerobic treatment: primary sludge digested in anaerobic digesters → produces biogas (CH₄ + CO₂) — energy recovery
• Secondary effluent has BOD < 30 ppm (from ~200 ppm raw sewage)

Tertiary Treatment (Chemical/Physical)

For removal of residual pollutants before discharge into sensitive water bodies:

• Reverse osmosis (RO): removes dissolved salts, heavy metals, residual BOD
• Chemical precipitation: add alum [Al₂(SO₄)₃] or lime [Ca(OH)₂] → flocculate phosphates, turbidity
• Activated carbon adsorption: removes trace organics, colour, odour
• Chlorination/UV: disinfection to kill pathogens
• Produces: near-drinking quality water (used for industrial cooling, irrigation, or groundwater recharge)

Sewage Treatment Efficiency Summary:
Raw sewage: BOD ~ 200 ppm
After primary: BOD ~ 130 ppm (35% reduction)
After secondary: BOD < 30 ppm (>85% reduction)
After tertiary: BOD < 5 ppm (>97% reduction)

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Bioremediation

Use of living organisms (mainly microorganisms) to break down or neutralise pollutants in a contaminated environment.

Pseudomonas and oil spills:

• Pseudomonas putida was the first patented genetically engineered organism (Ananda Chakraborty, 1980)
• Capable of degrading multiple hydrocarbon fractions in crude oil
• Converts hydrocarbons → CO₂ + H₂O (complete mineralisation)

In situ bioremediation: treating contaminated soil/groundwater without excavation
Ex situ bioremediation: excavating contaminated material and treating in bioreactors

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Atom Economy — Calculation Practice

Reaction A (substitution): CH₄ + Cl₂ → CH₃Cl + HCl

Desired product = CH₃Cl (MM = 50.5)
All products = CH₃Cl + HCl (50.5 + 36.5 = 87)
Atom Economy = 50.5/87 × 100 = 58%
HCl is "waste" in terms of atom economy

Reaction B (addition): CH₂=CH₂ + H₂ → CH₃CH₃

Atom Economy = 30/30 × 100 = 100%
All atoms from reactants appear in desired product

Addition reactions always have 100% AE — one of the reasons catalytic hydrogenation is preferred in green chemistry.

Catalytic Converter — Why Rh for NOₓ?

Rh is specifically chosen for NOₓ reduction because:

• Rh adsorbs NO strongly on its surface
• Adsorbed NO dissociates: 2NO(ads) → N₂ + O₂ (surface reaction)
• N₂ is released (harmless); O₂ used by Pt/Pd for CO oxidation
• Pt/Pd are more effective oxidation catalysts; Rh is more effective for reduction

Reverse Osmosis — Osmotic Pressure

π = MRT (van't Hoff equation for dilute solutions)

For seawater (0.6 M NaCl): π ≈ 27 atm
RO requires applied pressure > π (typically 55–80 atm for seawater desalination)
Freshwater forced through semipermeable membrane; dissolved salts rejected

Biogas Composition and Anaerobic Digestion

Primary sludge (cellulose, proteins, fats) → anaerobic bacteria (acetogenic + methanogenic) → biogas

Step 1 (hydrolysis + acidogenesis):
C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂  (acetogens)

Step 2 (methanogenesis):
CH₃COO⁻ + H₂O → CH₄ + HCO₃⁻  (methanogens, e.g., Methanosaeta)
4H₂ + CO₂ → CH₄ + 2H₂O

Biogas composition: ~65% CH₄, ~35% CO₂ (trace H₂S)
Calorific value: ~22 MJ/m³

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Example 1: Atom Economy calculation

Problem: Calculate the atom economy for the manufacture of ethanol from
ethylene by (a) direct hydration and (b) indirect hydration via ethyl sulphate.

Case (a): Direct hydration
  CH₂=CH₂ + H₂O → CH₃CH₂OH
  
  MM desired product (ethanol) = 46
  Total MM of products = 46 (only ethanol)
  Atom Economy = 46/46 × 100 = 100%
  
  This is an ADDITION reaction → 100% atom economy

Case (b): Indirect hydration via sulphuric acid
  Step 1: CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H  (ethyl sulphate)
  Step 2: CH₃CH₂OSO₃H + H₂O → CH₃CH₂OH + H₂SO₄
  Net:    CH₂=CH₂ + H₂O → CH₃CH₂OH
  
  Same net reaction, but H₂SO₄ is used and regenerated → not a by-product
  For the OVERALL net equation, AE is still 100%.
  
  However, the indirect route has additional considerations:
  - Uses corrosive H₂SO₄ (Principle 3: less hazardous synthesis violated)
  - Higher energy requirement (Principle 6 violated)
  - Requires additional processing steps
  → Direct hydration (catalytic, steam process) is the GREEN choice

Answer: Both routes: AE = 100% for net equation.
Direct hydration preferred on green chemistry principles 3, 5, 6.

Example 2: Catalytic converter reactions

Problem: A catalytic converter processes exhaust gas containing CO (500 ppm),
NO (300 ppm), and C₈H₁₈ (50 ppm). Write the balanced reactions for each
pollutant and identify which metal catalyst is needed.

Pollutant 1: CO → CO₂
  2CO + O₂ → 2CO₂
  Catalyst: Platinum (Pt) or Palladium (Pd) — oxidation catalyst
  
  Result: CO (toxic, binds Hb) → CO₂ (harmless greenhouse gas)

Pollutant 2: NO → N₂
  2NO → N₂ + O₂
  Catalyst: Rhodium (Rh) — reduction catalyst (Rh most effective for N–O bond breaking)
  
  Result: NO (toxic, irritant, acid rain precursor) → N₂ (harmless)

Pollutant 3: Octane (C₈H₁₈) → CO₂ + H₂O
  2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
  Catalyst: Pt/Pd — oxidation catalyst
  
  Result: Unburned hydrocarbon (smog precursor) → CO₂ + H₂O

Overall converter function:
  Input:  CO, NOₓ, unburned HC  (toxic)
  Output: CO₂, N₂, H₂O         (harmless or greatly reduced harm)
  
Important notes:
  - Must use unleaded petrol (Pb deactivates Pt/Pd/Rh)
  - Operating temperature: 400–800°C (converter heats up during driving)
  - Cold-start problem: first 1–2 minutes, converter not yet hot → significant raw emissions

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Q1. Atom economy for the reaction: CH₂=CH₂ + HBr → CH₃CH₂Br is:

A) 50%
B) 75%
C) 100%
D) 25%

Q2. In a three-way catalytic converter, which metal primarily catalyses the reduction of NOₓ to N₂?

A) Platinum
B) Palladium
C) Rhodium
D) Gold

Q3. The BOD of raw municipal sewage before treatment is approximately:

A) 3–5 ppm
B) 10–20 ppm
C) 50–100 ppm
D) 150–250 ppm

Q4. H₂O₂ is preferred over Cl₂ for bleaching in green chemistry because:

A) H₂O₂ is a stronger bleaching agent than Cl₂
B) H₂O₂ produces water as the only by-product, not toxic organochlorines
C) H₂O₂ is less expensive than Cl₂ at industrial scale
D) H₂O₂ has a higher atom economy than Cl₂ in all contexts

Q5. Pseudomonas bacteria are used in bioremediation of:

A) Heavy metal contamination (Hg, Pb)
B) Radioactive waste
C) Petroleum hydrocarbon oil spills
D) Excess nitrates in agricultural runoff

Answer Key: 1-C | 2-C | 3-D | 4-B | 5-C

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• NCERT Class 12 Chemistry — Chapter 14: Environmental Chemistry
• Pages 424–432 (strategies for pollution control)
• Key sections: 14.5 Strategies to control environmental pollution
• Green Chemistry section (14.5.1): Anastas-Warner principles, examples
• Cross-reference: NCERT Class 12 Chemistry Chapter 5 (Surface Chemistry) for catalyst mechanism
• Also see NCERT Exemplar, Chapter 14 for assertion-reason questions on sewage treatment

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🎯 Exam Tips

• Atom Economy = 100% for addition reactions ALWAYS — this is a guaranteed point if you remember it
• Three-way catalytic converter metals: Pt + Pd (oxidation of CO, HC) and Rh (reduction of NOₓ) — Rh is the unique one
• Sewage treatment order: Primary (physical) → Secondary (biological, 90% BOD removal) → Tertiary (chemical/RO) — don't mix the order
• Green chemistry H₂O₂ vs Cl₂ bleaching and liquid CO₂ vs PERC dry cleaning are classic examples frequently asked

📊 Weightage

• Strategies for pollution control: 0–1 question per JEE Main paper
• Green Chemistry increasingly featured in recent papers (2022–2026 trend)
• Atom economy calculation is a high-confidence point scorer

🔗 Related Topics

• Surface Chemistry (catalyst mechanism — adsorption on Pt/Pd/Rh surface)
• Electrochemistry (electrostatic precipitator — high-voltage corona discharge)
• Biomolecules (microbial digestion in secondary sewage treatment)
• Chemical Kinetics (catalytic reaction rates in converter)

📝 Revision Checklist

• State the atom economy formula and calculate for addition vs substitution reactions
• Name 3 Anastas-Warner principles with examples
• Explain H₂O₂ bleaching advantage over Cl₂ with chemical equations
• Write all 3 catalytic converter reactions with correct metal catalyst
• Describe ESP working principle in 4 steps
• Distinguish primary, secondary, and tertiary sewage treatment with BOD reduction
• Name Pseudomonas sp. as bioremediation organism for oil spills
• Define bioremediation and give one in situ and one ex situ example