Environmental Science (3110007)

• When the pH of rain water is less than 5.6 it is called as acid rain.
• The natural rain water has pH of 5.6 at 20°C due to formation of carbonic acid due to dissolution of CO₂ in water.
  CO₂ + H₂O → H₂CO₃
• Oxygen, nitrogen and sulphur originating from industrial operation and fossil fuel combustion are the major causes of acid rain formation.
  2SO₂ + O₂ → 2SO₃
  SO₃+ H₂O → H₂SO₄
  2NO + O₂ → 2NO₂
  4NO₂ + 2H₂O + O₂ → 4HNO₃
• Sulphuric acid and Nitric acid are the major acid found in the rain water. The proportion of Sulphuric acid ranges from 60 to 70% and that of Nitric acid is 30 to 40%.
• In the absence of rain, dry, deposition of acid may occur. Acid forming gases like oxides of sulphur and nitrogen and acid aerosols get deposited on the surface of water bodies, vegetation, soil and other materials.

 

Causes of Acid rain

• SO₂ and NOx are mainly responsible for forming of acid rain. Main sources of this pollutant are
• Fossil fuel based power plants
• Smelting of sulphide ore
• Automobiles exhaust
• Industrial plant using sulphuric acid and nitric acid
• Environmental effects of acid rain

 

Effects on building material

• Acid rain will cause damage to common building materials (such as lime stone and marble), statues monuments.
  CaCO₃ + 2H⁺ → Ca²⁺ + CO₂ + H₂O
• Many metals get oxidized. Iron corrodes with the presence of acid rain to form rust. The cost of maintenance of iron structures is high in highly polluted areas.
  Fe + 2H⁺ → Fe²⁺ + H₂

 

Effects on Aquatic life

• Aquatic life especially fish are badly affected by lake acidification.
• Acid rain mobilizes heavy metals such as calcium and mercury in soils, rock and sediments, which are then leached out by rain and enter the surface water.
• Acidified lakes have high levels of cadmium, lead, aluminium, manganese, zinc, copper and nickel. All these can kill living organisms if present in sufficient quantities. It disturbs the food chain of aquatic ecosystem.

 

Effects on fertility of soil

• Due to high solubility of acidic rain water, plant nutrients like nitrogen, phosphorous and potassium gets leached away, which reduces fertility of soil.
• It damages foliage and weakens trees.
• It also make atmospheric hazy.

 

Control of Acid Rain

• Emission of SO₂ and NO₂ from industries and power plants should be reduced by using pollution control equipment.
• Liming of lakes and soils should be done to correct the adverse effects of acid rain.