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Acid Deposition


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Acid Deposition

  1. 1. Acid Deposition
  2. 2. What is acid deposition? <ul><li>Scientific term for Acid Rain </li></ul><ul><li>Acid occurs naturally even without the impact from the extra air pollutants. As Pure water has a pH of 7.0, Rain Water has a pH of 5.5. This is due to the CO 2 in the atmosphere. </li></ul><ul><li>H 2 O (l) + CO 2 (g) -> H 2 CO 3 (aq) </li></ul>
  3. 3. Types of Deposition <ul><li>Wet Deposition </li></ul><ul><ul><li>Pollutants in air (eg. sulphur dioxide and nitrogen oxides ) mix with water vapour in the air </li></ul></ul><ul><ul><li>Water vapour  sulphuric and nitric acids therefore making the rain acidic </li></ul></ul><ul><li>Dry Deposition </li></ul><ul><ul><li>dry deposition occurs when the acids are first transformed chemically into gases and salts, before falling under the influence of gravity back to Earth. </li></ul></ul>
  4. 4. The chemistry: <ul><li>Sulphur dioxide is oxidised by the following reaction: </li></ul><ul><li>SO 2 + OH -> HOSO 2 </li></ul><ul><li>Then: </li></ul><ul><li>HOSO 2 + O 2 -> HO 2 + SO 3 </li></ul><ul><li>When water is present: </li></ul><ul><li>SO 3 + H 2 O(l) -> H 2 SO 4 </li></ul><ul><li>Nitric acid is formed by the reaction of OH with nitrogen dioxide: </li></ul><ul><li>NO 2 + OH -> HNO 3 </li></ul>
  5. 5. Reduction of SO 2 <ul><li>As the major source of the acid is Sulphuric Acid ( H 2 SO 4 ) the USA has required all Coal Power Plants to use Flue gas desulfurization to remove the sulphur gas. </li></ul>
  6. 6. Flue gas desulfurization <ul><li>These FGDs can remove up to 95% of the SO 2 </li></ul>
  7. 7. Flue gas desulfurization <ul><li>The reaction takes place in wet scrubbing using a CaCO 3 (limestone) slurry produces CaSO3 (calcium sulfite): </li></ul><ul><ul><li>CaCO3 (solid) + SO2 (gas) -> CaSO3 (solid) + CO2 (gas) </li></ul></ul><ul><li>With Ca(OH)2 (lime) slurry also produces CaSO3 (calcium sulfite): </li></ul><ul><ul><li>Ca(OH)2 (solid) + SO2 (gas) -> CaSO3 (solid) + H2O (liquid) </li></ul></ul><ul><li>With Mg(OH)2 (magnesium hydroxide) slurry, reaction produces </li></ul><ul><li>MgSO3 (magnesium sulfite) : </li></ul><ul><ul><li>Mg(OH)2 (solid) + SO2 (gas) -> MgSO3 (solid) + H2O (liquid) </li></ul></ul>