Transcript 8. Acid Rain

Acid Precipitation
Definitions
+
Acid: substance that releases H in water
-
Base: substance that produces OH in water
Molarity (M): the number of moles of solute
present in 1 liter of solution
Some Acids and Bases
HCl

NaOH 
NH3
H+ + Cl-
Na+ + OH-
+ H 2O 
NH4+ + OH-
Neutralization:
HCl + NaOH  H2O + Na+ + Cl-
?
pH = - log10 [H+]
?
acidic
basic
neutral
MH+ > MOH-
acidic
MH+ < MOH-
basic
MH+ = MOH-
neutral
Dissociation of Water
• Pure water naturally undergoes a slight
dissociation reaction:
H2O → H+ + OH• The pH of pure water, which is neutral, is 7, so
pH = 7 = -log([H+])
-7 = log([H+])
10-7 = [H+]
• Ditto for the OH- in water:
pOH = 7 = -log([OH-])
-7 = log([OH-])
10-7 = [OH-]
pH of Common Substances
CO2 as an Acid?
• exists in air at 370 ppm
• from natural processes
• an acid anhydride
CO2 + H2O  H2CO3  H+ + HCO3carbonic acid
pH of unpolluted air
• So when rain water falls through the air a certain
amount of CO2 is dissolved
• A certain about of this becomes carbonic acid
• As a result, unpolluted rain has a pH of about 5.6
• But human activities release acid precursors so
that in many places the pH of rain is much lower
Acid Precursors
• When gas pollutants e.g. sulphur dioxide,
nitrogen dioxide dissolve in rain water,
various acids are formed.
• CO2 + H2O  H2CO3 (carbonic acid)
• SO2 + H2O
 H2SO4 (sulphuric acid)
• NO2 + H2O  HNO2 (nitrous acid) &
HNO3 (nitric acid)
Detailed Reactions
• Sulphuric Acid:
2SO2(g) + O2(g)  2SO3(g)
SO3(g) + H2O(l)  H2SO4(aq)
• Nitric Acid:
2NO(g) + O2(g)  2NO2(g)
4NO2(g) + 2H2O(l) + O2(g)  4HNO3(aq)
Causes of Acid Precipitation
SO2: 70%
NOx: 30%
Acid Rain
• Reactions to convert
to acid take place in
~2 days - travel 1000
miles
• Down wind - Acid
rain
• Dry Dep. vs Wet Dep.
• Dry Deposition 50 %
of total
A Typical NADP/NTN* Site
• Precipitation sampler
• Recording rain gage
• Consistent sampling
protocols
• Central laboratory
• Internal and external
QA/QC
*National Atmospheric
Deposition Program/ National Trends Network
NADP/ NTN Site Locations
Check out http://nadp.sws.uiuc.edu/nadpdata/state.asp?state=VA
Sources of SOx and NOx
C135H96O9NS
coal
SO2 Emissions across the United States
NO2 Emissions across the United States
Overview: Acid Precipitation
Affected Areas
• Canada
– Acid rain is a problem in Canada
– Water and soil systems lack natural alkalinity
such as lime base
• Cannot neutralize acid
– Canada consists largely of hard rock such as
granite
• Do not have the capacity to effectively neutralize
acid rain
Affected Areas
• Industrial acid rain is a substantial problem in
China, Eastern Europe and Russia and areas
down-wind from them.
• Acid rain from power plants in the Midwest
United States has also harmed the forests of
upstate New York and New England.
• This shows that the effects of acid rain can spread
over a large area, far from the source of the
pollution
• The net acid
deposition flux
=
the total acid
deposition flux
–
deposition flux of
base cations.
Effects of Acid Rain
• Harmful to aquatic life
– Increased acidity in water bodies
– Causes eggs of certain organisms (e.g. fish) to
stop hatching
• Changes population ratios
• Affects the ecosystem
• Fish usually die only when the acid level of a lake is
high
• When the acid level is lower, they can become sick,
suffer stunted growth, or lose their ability to
reproduce.
• Aluminum liberated from soil can interfere with gills
• Birds can die from eating "toxic" fish and insects.
Effects of Acid Rain
• Harmful to vegetation
– Increased acidity in soil
– Leeches nutrients from soil, slowing plant
growth
– Leeches toxins from soil, poisoning plants
– Creates brown spots in leaves of trees,
impeding photosynthesis
– Allows organisms to infect through broken
leaves
• The needles and leaves
of trees turn brown and
fall off.
• Trees can also suffer
from stunted growth
• Damaged bark and
leaves can make them
vulnerable to weather,
disease, and insects
Effects of Acid Rain
Effects of Acid Rain
• Accelerates weathering in metal
and stone structures
– Eg. Parthenon in Athens,
Greece; Taj Mahal in Agra,
India
Effects of Acid Rain
• Impacts human health
– Respiratory problems, asthma, dry coughs,
headaches and throat irritations
– Leeching of toxins from the soil by acid rain
can be absorbed by plants and animals. When
consumed, these toxins affect humans severely.
– Brain damage, kidney problems, and
Alzheimer's disease has been linked to people
eating "toxic" animals/plants.
There are natural sources…
• Terrestial, tidal, and nutrient-rich oceanic areas
• Volcanic eruptions
• Forest fires
• Outgassing from anaerobic wetlands
• Breakdown of amino acids in organic wastes
Preventative Measures
• Reduce amount of sulphur dioxide and oxides of
nitrogen released into the atomosphere
– Use less energy (hence less fuel burnt)
– Use cleaner fuels
– Remove oxides of sulphur and oxides of
nitrogen before releasing
• Flue gas desulphurization
• Catalytic Converters
Preventative Measures
• Use cleaner fuels
– Coal that contains less sulphur
– "Washing" the coal to reduce sulphur content
– Natural Gas
Preventative Measures
• Flue Gas Desulphurization (FGD)
– Removes sulphur dioxide from flue gas (waste
gases)
– Consists of a wet scrubber and a reaction
tower equipped with a fan that extracts hot
smoky stack gases from a power plant into the
tower
– Lime or limestone (calcium carbonate) in
slurry form is injected into the tower to mix
with the stack gases and reacts with the
sulphur dioxide present
Preventive Measures
• (continued)
– Produces pH-neutral
calcium sulphate that is
physically removed from
the scrubber
– Sulphates can be used for
industrial purposes
Preventive Measures
• Use other sources of electricity (i.e. nuclear power,
hydro-electricity, wind energy, geothermal energy,
and solar energy)
– Issue of cost
Reducing the effects of Acid Rain
• Liming
– Powdered limestone/limewater
added to water and soil to
neutralize acid
– Used extensively in Norway and
Sweden
– Expensive, short-term remedy
Drawbacks of liming bodies of acidified water
•
•
•
•
•
Very expensive
Treatment needs to be repeated every 3-6 years
Remote lakes are inaccessible
Large pieces of lime dissolve slowly
Doesn’t solve the problem for soil acidification
Policy
Many policies and programs were established in the
past couple of decades once governments started to
become concerned with the harmful effects of air
pollution.
• 1979 - Geneva Convention on Long-Range Transboundary
Air Pollution took place with the idea to limit sulfur and
nitrogen emissions was successfully achieved
• 1995-1997 Title IV of the Clean Air Act was established to
reduce the adverse effects of acid deposition through
reductions in annual emissions of sulfur dioxide of ten
million tons from 1980 emission levels.