Acid
Deposition
Acid Deposition
• process by which acid-forming pollutants
are deposited on Earth’s surface by…
– wet- rain, snow, fog
– dry- smoke or dust particles
• impacts on environment include:
– deforestation
– increased acidity in lakes and rivers
– uptake of toxic minerals by plants or sea life
– corrosion of marble, limestone, metals,
etc...
• precipitation (mostly rain) is the most
prevalent form of deposition
• rainwater has natural pH of 5.6
– reacts with CO2 to form weak carbonic acid
– CO2 + H2O ⇔ H2CO3
• acid rain
–pH < 5.6
–primarily formed from sulfur dioxide
(SO2) and nitrogen oxides (NO or NO2)
• these oxides dissolve in the rain to make
–strong nitric acid (HNO3) and sulfuric acid
(H2SO4)
Causes
• industrialization
– increasing emissions of nitrogen and sulfur
oxides
– coal burning
• releases sulfur dioxide (SO2) which
becomes sulfuric acid (H2SO4) in the
atmosphere
• volcanic emissions and biological
processes
Effects of acid rain on structures
• limestone and marble contain calcium
carbonate
• when exposed to acid rain, a neutralization
reaction occurs and the building is gradually
eroded
• marble (CaCO3) reacting with sulfuric acid rain
(H2SO4)
o CaCO3(s) + H2SO4(aq) → CaSO4(aq) + H2O(l) + CO2(g)
• marble (CaCO3) reacting with nitric acid rain
(HNO3)
o CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(l) + CO2(g)
• most metals contain iron
– iron reacting with sulfuric acid rain (H2SO4)
• Fe + H2SO4 -> H2 + FeSO4 [If Fe is +2]
or
2Fe + 3H2SO4 -> 3H2 + Fe2(SO4)3 [If Fe is +3]
Prevention
Pre-combustion methods:
• techniques used on fuel before combustion
• can reduce 80-90% of sulfur before
combustion even occurs
• physical cleaning
– coal is crushed, followed by flotation to reduce
sulfur
Post-combustion methods:
• techniques used on gases after combustion
• remove sulfur dioxide, nitrogen oxides, and
heavy metals from the gasses before they are
released into the air
• EX: calcium oxide or lime will react with
sulfur dioxide and remove it from the gasses
– CaO (s) + SO2 (g) ⇌ CaSO3