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 CO 2 to form weak carbonic acid – CO 2 + H 2 O ⇔ H 2 CO 3

acid rain – pH < 5.6 – primarily formed from sulfur dioxide (SO 2 ) and nitrogen oxides (NO or NO 2 ) these oxides dissolve in the rain to make – strong nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 )

Causes industrialization – increasing emissions of nitrogen and sulfur oxides – coal burning releases sulfur dioxide (SO 2 ) which becomes sulfuric acid (H 2 SO 4 ) 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 (CaCO 3 ) reacting with sulfuric acid rain (H 2 SO 4 ) o CaCO 3 (s) + H 2 SO 4 (aq) → CaSO 4 (aq) + H 2 O(l) + CO 2 (g) marble (CaCO 3 ) reacting with nitric acid rain (HNO 3 ) o CaCO 3 (s) + 2HNO 3 (aq) → Ca(NO 3 ) 2 (aq) + H 2 O(l) + CO 2 (g)

most metals contain iron – iron reacting with sulfuric acid rain (H 2 SO 4 ) Fe + H 2 SO 4 -> H 2 + FeSO4 [If Fe is +2 ] or 2Fe + 3H 2 SO 4 -> 3H 2 + Fe 2 (SO 4 ) 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) + SO 2 (g) ⇌ CaSO 3