Acid
Deposition
Topic 8.5
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 oceans and fresh water
– uptake of toxic minerals by plants or sea life
– corrosion of marble, limestone, metals,
etc...
• precipitation (mostly rain) is the most
common 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 water to make
HNO3, HNO2, H2SO4, and H2SO3
• acid rain is composed mostly of 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
Formation of nitric acid, HNO3
• all oxides of nitrogen eventually produce nitric
acid under certain conditions
• forming NO2 first
– nitrogen dioxide causes the brown color
of smog
• at high temperatures and pressure(e.g.
in a car engine), nitrogen gas reacts with
oxygen gas
–N2(g) + O2(g) → 2NO(g)
–2NO(g) + O2(g) → 2NO2(g)
• also be formed in one reaction
– N2(g) + 2O2(g) → 2NO2(g)
• nitrogen (IV) oxide (NO2)can react with
water to form nitric acid and nitrous acid
– 2NO2 (g) + H2O (l) HNO3 (aq) + HNO2 (aq)
– 2HNO2 (aq) + O2 (g)  2HNO3 (g)
Formation of sulfuric acid, H2SO4
• humans burning coal and oil releases the
sulfur they contain
• the combustion of sulfur
– S (s) + O2 (g)  SO2 (g)
• reacts with oxygen in the atmosphere
– 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
• reacts with rain
– SO3 (g) + H2O (l)  H2SO4 (aq)
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 inorganic 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