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Chapter 18
Chemistry of the Environment
Chapter 18
Introduction
Human activities impact our environment.
Economic growth depends of chemical processes
– e.g. clean water, energy usage, chemical synthesis, etc.
Earth Summit – Brazil 1992
1997 – Kyoto meeting
2001 – Bonn – Signing of “Kyoto Protocols”
Protocols designed to internationally address environmental
concerns and establish regulations.
2007 – UN meeting to establish further regulations and discuss
monetary help for developing countries.
Chapter 18
Topics
Earth’s Atmosphere
– Ozone
– Acid rain
– Greenhouse effect
The Ocean
Freshwater
Soils and Weathering: A little basic geochemistry
Green Chemistry (self-study)
Find the topic interesting? Some easy reading…
“An Introduction to Environmental Chemistry” by JE Andrews,
P Brimblecombe, TD Jickells and PS Liss.
“A Short History of Nearly Everything” by B Bryson.
Chapter 18
Earth’s Atmosphere
Four regions.
Remember region and
boundary names
Notice relationship
between altitude
and T and P.
(Colder in JNB than KNP AND
why jets are pressurized.)
Jets fly at this level.
Chapter 18
Earth’s Atmosphere
Earth’s atmosphere is affected by temperature and pressure
and gravity.
Lighter molecules and atoms are found at higher altitudes.
Less O2, air is “thinner”.
There is slow mixing of gases between regions in the
atmosphere (important for ozone and pollution).
Troposphere and stratosphere account for 99.9% of the
atmosphere’s mass, with about 75% of that in the
troposphere.
Two major components of the atmosphere are nitrogen, N 2,
Chapter 18
and oxygen, O2.
A Note on N2 and O2 Reactivity
N2 has a triple bond (bond energy of 941kJ/mol)
O2 double bond (bond energy of 495kJ/mol)
Oxygen is more reactive than nitrogen.
Generally, oxides of non-metals form acidic solutions
with water and oxides of metals form basic solutions in
water.
Chapter 18
Chapter 18
Parts Per Million
1 part per million (ppm) refers to 1 part in 1 million units of
the whole. (One red pencil in a million yellow pencils)
If considering PV = nRT, volume fraction which is usually
used for ppm can be interchanged with mole fraction.
So 1ppm = 1 mole in 1 million moles of total gas.
concentration in ppm = mole fraction x 106
Other common ppm designations: solids in mg kg-1, liquids in mg L-1, and
most engineers use mg dm-3 (remember that 1L = 1dm3).
Chapter 18
Example
Calculate the concentration of methane in ppm if mole fraction
= 0.000002.
ppm = mole fraction x 106
= 0.000002 x 106 = 2 ppm
Calculate the concentration of xenon in ppb (parts per billion) if
the mole fraction = 0.000000087.
ppm = mole fraction x 106
ppb = mole fraction x 109
= 0.000000087 x 109 = 87 ppb
Chapter 18
Outer Regions of the Atmosphere
Contains only a small percentage of the atmospheric mass.
Low pressure!
Forms the outer defense against radiation and high-energy
particles from space.
(If Earth was the size of a desk globe, our atmosphere’s
thickness would be ~ two layers of varnish. Very thin, but
important, layer!)
Outer atmospheric reactions: photodissociation and
photoionization
Chapter 18
Photodissociation
Wave equation: The higher the frequency, the shorter the
wavelength and the higher the energy of radiation.
hc
E  h 

For photodissociation reactions to occur, photons must have
sufficient energy to break the required bonds and the
molecules must also absorb these photons.
Defn: the rupture of a chemical bond induced by radiation.
Chapter 18
Photodissociation
In the upper atmosphere (above 120km), photodissociation
causes the formation of oxygen atoms:
O2(g) + h  2O(g)
Minimum energy required determined by the bond
dissociation energy of O2 (495kJ/mol).
Dissociation of O2 is very extensive at high elevations:
at 400km only 1% of oxygen is O2 while at 130km, about
50% is O2.
Chapter 18
Photoionization
Defn: ionization of molecules (and atoms) caused by radiation.
1924: electrons discovered in the upper atmosphere.
Therefore, cations must be present in the upper atmosphere
(for charge balance).
Photoionization occurs when a molecule absorbs a photon of
sufficient energy to remove an electron.
Wavelengths of light that cause photoionization and
photodissociation are filtered by the atmosphere (occur more
readily further from Earth).
Chapter 18
Ozone and the Upper Atmosphere
Ozone (O3) absorbs photons with a wavelength between 240
and 310 nm. (N2, O2 and O absorb wavelengths shorter than
240nm.)
Most of the ozone is present in the
stratosphere, 12-50 km altitude.
Maximum ozone concentration at an
altitude of ~20 km.
Between 30-90km photodissociation of oxygen is possible:
O2(g) + h  2O(g)
Chapter 18
Ozone and the Upper Atmosphere
Oxygen atoms can collide with oxygen molecules to form
ozone with excess energy, O3*:
O(g) + O2(g)  O3*(g) (releases 105kJ/mol)
The excited ozone loses energy by decomposing to oxygen
atoms and oxygen molecules (the reverse reaction) or by
transferring the energy to M (usually N2 or O2):
O(g) + O2(g)
O3*(g)
O3*(g) + M(g)  O3(g) + M*(g)
Chapter 18
Ozone and the Stratosphere
The formation of ozone in the atmosphere depends on the
presence of O(g):
At low altitudes, most radiation with sufficient energy to
form O(g) has been absorbed by upper atmosphere.
Release of energy from O3* depends on collisions,
which generally occur at lower altitudes (more gas
molecules).
Combining altitude and O(g) concentration means
maximum ozone formation in the stratosphere.
Chapter 18
Ozone Natural Cycle
A combination of photoionization and photodecomposition
reactions – makes and breaks ozone naturally.
O2(g) + hv  O(g) + O(g)
O(g) + O2(g)  O3(g) + M*(g) (heat released)
O3(g) + hv  O2(g) + O(g)
O(g) + O(g)  O2(g) (heat released)
O3 also formed by lightning strikes (can split O 2) – can detect
metallic smell of ozone during thunderstorms.
Chapter 18
Ozone Depletion: CFCs
Crutzen 1970: naturally occurring nitrogen oxides
catalytically destroy ozone. In 1974, Rowland and
Molina showed that chlorine from chlorofluorocarbons
(CFCs) deplete the ozone layer by catalyzing the
formation of ClO and O2.
CFC Characteristics
CFCl3 (Freon-11TM) and CF2Cl2 (Freon-12TM) used as
propellants in spray cans, as refrigerant gases, and foaming
agents for plastics.
Unreactive in lower atmosphere, insoluble in water.
Diffuse slowly into stratosphere.
Several million tons now present in the atmosphere.
Chapter 18
CFC Depletion of Ozone
In the stratosphere, CFCs undergo photodissociation of C-Cl:
CF2Cl2(g) + h  CF2Cl(g) + Cl(g) (optimal at 30km)
Think about this…
Subsequently: Cl(g) + O3(g)  ClO(g) + O2(g)
rate = k[Cl][O3], k = 7.2  109 M-1s-1 at 298K
In addition, the ClO generated produces Cl as well:
2ClO(g)  O2(g) + 2Cl(g)
The overall reaction: 2O3(g)  3O2(g).
The rate at which ozone is destroyed increases with the amount of Cl, thus the
greater the amount of CFCs that diffuse into the stratosphere, the faster the
destruction of the ozone layer.
Chapter 18
The Ozone Hole and CFC Replacements
1992: ~100 nations agreed to ban
production and use of CFCs by 1996.
October 1994: Ozone map in the
Southern hemisphere showed a
hole over Antarctica.
In 1995 the Nobel Prize for chemistry was awarded to F. Sherwood Rowland,
Mario Molina, and Paul Crutzen for their studies of ozone depletion.
Replacement for CFCs are HFCs (e.g. CH2FCF3), where a
C-H bond is broken instead of a C-Cl.
Chapter 18
Down to Earth:
Chemistry of the Troposphere
The troposphere consists mostly of O2 and N2 (~99%), but also
CO, CO2, H2O, CH4, NOx, and SOx.
Even low to trace concentrations of other gases can have
profound effects on the environment.
Most of these environmentally unfriendly gases are produced
by the combustion of fossil fuels, industry and everyday life.
Chapter 18
Sulfur Compounds and Acid Rain
Sulfur dioxide, SO2, is largely produced by the combustion of
oil and coal and is a serious health hazard especially to
people with respiratory difficulties.
SO2 is oxidized to SO3 by reacting with O2 or O3 which then
reacts with water to produce sulfuric acid (acid rain):
SO3(g) + H2O(l)  H2SO4(aq)
Note: nitrogen oxides also contribute to acid rain (nitric acid).
Normal rainwater has a pH of about 5.6 (due to dissolved
H2CO3 from CO2 in the air).
Chapter 18
Sulfur Compounds and Acid Rain
Acid rain has a pH around 4 (pH of natural waters containing
living organisms is 6.5 to 8.5). Natural waters with pH below
4 cannot sustain life.
Acid rain: corrosive to metals and stone building materials (will
slowly dissolve limestone).
Too expensive to remove sulfur from oil and coal prior to its
use. Therefore, the SO2 is removed from fuel upon
combustion.
More than 30MT per year of SO2 are released into the
atmosphere in the USA. Even more in developing countries.
Chapter 18
Sulfur Removal from Coal Combustion
SO2 is commonly removed from fuel (oil and coal) as follows:
2. CaSO3 and unreacted SO2 are passed into a
scrubber (purification chamber) where the excess
SO2 is converted to CaSO3 by jets of CaO.
1. Powdered
limestone
decomposes
into CaO
which reacts
with SO2 to
form CaSO3
in a furnace.
3. CaSO3 is precipitated into a watery slurry.
Chapter 18
Carbon Monoxide
Produced by incomplete combustion of carbon-containing
materials, e.g. fossil fuels.
About 1014 g of CO is produced in the United States per year
(mostly from automobile exhaust).
CO binds irreversibly to the Fe in hemoglobin (about 210 times
more strongly than oxygen).
Extremely dangerous and potentially fatal if inhaled in large
quantities.
Chapter 18
Nitrogen Oxides
Photochemical smog (“the brown cloud”) is the result of
photochemical reactions on pollutants.
In car engines, NO forms as follows:
N2(g) + O2(g)
2NO(g)
H = 180.8 kJ
In air, rapid oxidation of NO takes place:
2NO(g) + O2(g)
2NO2(g)
H = -113.1 kJ
At 393nm (sunlight), NO2 decomposes
NO2(g) + h  NO(g) + O(g)
Chapter 18
Nitrogen Oxides
The O produced by photodissociation of NO 2 can react with
O2 to form O3, which is the key component of smog.
O(g) + O2(g)  O3(g) + M*(g)
Ozone is undesirable in the troposphere because it’s toxic
and reactive.
Ozone problem…too much of it in smog, not enough in the
stratosphere.
Chapter 18
Water Vapor, CO2, and Climate
Thermal balance between Earth and its surroundings.
Ideally, radiation is emitted from Earth at the same rate as it
is absorbed.
The troposphere is transparent to visible light (comes through),
but not to IR radiation (heat).
CO2 and H2O absorb IR radiation escaping from Earth’s surface
at night keeping us warm. Called the Greenhouse Effect.
Chapter 18
Water Vapor, CO2, and IR Radiation
Chapter 18
Historical CO2 Profile
The carbon dioxide level on Earth has been increasing over the years.
Current CO2 levels = 380 ppm (a 35% increase from pre-industrial
levels).
www.physorg.com
Majority of the increase is from burning of fossil fuels.
Chapter 18
No Quick Solution for CO2
Between 2050 and 2100, the CO 2 concentration is expected to
double, possibly resulting in a global temperature increase of
1 to 3˚C.
International Energy Agency reports China will surpass US in
CO2 emissions by 2009 (heavy reliance on coal).
NY Times, 7 Nov 2006
CO2 levels higher now than in the past 650,000 years based on
comparision to tiny bubbles trapped in Antarctic ice cores.
25 Nov 2005, Science
Could result in melting of glaciers and a subsequent sea level
rise.
Chapter 18
Increased CO2 Implications
Glacial recession in Glacier
National Park, Montana (USA).
Wikipedia
Adapted from Gormitz and Lebedeff,
1967 by UNFCCC.
Submerged Florida peninsula (USA)
shown with 1 meter sea level rise
(in red). NASA
Chapter 18
Chapter 18
The World Ocean
72% of Earth’s surface is covered with water.
97.2% of Earth’s water is seawater, with a volume of 1.35 
109 km3. Only 0.6% of Earth’s total water is in rivers, lakes,
and groundwater!
Salinity: mass in grams of dry salts in 1 kg of seawater.
Seawater salinity averages about 35.
Most elements in seawater are only present in small quantities
(trace).
Commercially, NaCl, Br- and Mg2+ are obtained from seawater.
Chapter 18
Chapter 18
Desalination
Water used for drinking should contain less than 500 ppm
dissolved salts (United States water regulation). Joburg
water is at ~350 ppm.
Desalination: removal of salts from seawater.
Common method: reverse osmosis (energy intensive).
– Osmosis: transport across a semipermeable membrane,
where solvent moves from dilute to concentrated.
No good for desalination.
– Reverse osmosis: under applied pressure, solvent moves
from more concentrated solution to more dilute solution.
Chapter 18
Desalination
Seawater is introduced
under pressure and water
passes through the fiber
walls and is separated from
the ions.
Largest desalination plant in Saudi Arabia responsible for 50% of the
country’s drinking water by reverse osmosis from Persian Gulf. Small scale
reverse osmosis desalinators used for camping, traveling, and at sea.
Chapter 18
Freshwater
An adult needs about 2 L a day for drinking.
The average person uses about 300 L of freshwater per day.
Industry uses about 105 L of water are used to make enough
steel for one car!
As water flows over the terrain it dissolves many substances.
Freshwater usually contains some ions (Na +, K+, Mg2+, Ca2+,
Fe2+, Cl-, SO42-, and HCO3-) and dissolved gases (O2, N2,
and CO2).
Chapter 18
Dissolved O2 and Water Quality
Water fully saturated with air at 1 atm and 20C has 9 ppm of
O2 dissolved in it.
Cold water fish require about 5 ppm of dissolved oxygen for
life.
Chapter 18
Bacterial Activity in Groundwater
Aerobic bacteria require oxygen to biodegrade organic
material, e.g., sewage, industrial waste from food-processing
plants and paper mills, and effluent from meat packing
plants.
Oxidize organic material into CO2, HCO3-, H2, NO3-, SO42-,
and phosphates.
Once the oxygen level has been depleted, aerobic bacteria
cannot survive.
Anaerobic bacteria complete the decomposition process
forming CH4, NH3, H2S, PH3, and other smelly products.
Chapter 18
Municipal Water Treatment
There are five steps
5.Sterilization.
Chlorine used,
forms HClO(aq) in
solution which kills
bacteria.
1.Coarse filtration.
Occurs as water is
taken up from lake,
river or reservoir.
2.Sedimentation. Water is
allowed to stand so that
solid particles (e.g. sand)
can settle out. Gelatinous
precipitate of Al(OH)3
settles out slowly.
4.Aeration. Air oxidizes any
organic material.
3.Sand Filtration. Filteration
through a sand bed to remove
Al(OH)3 and anything it trapped.
Chapter 18
Hard Water
Hard water contains relatively high amounts of Ca2+,
Mg2+ and other divalent cations. Unsuitable for most
household and industrial uses.
-Forms insoluble soap scum and water “stains”
Mineral deposits form when hard water is heated
Ca2+(aq) + 2HCO3-(aq)
CaCO3(s) + CO2(g) + H2O(l)
“scale”
Chapter 18
Water Softening
Water from underground sources with considerable contact
with CaCO3 and other minerals containing Ca2+, Mg2+ and
Fe3+ requires softening.
Lime-soda process used for large scale municipal watersoftening operations. Water treated with lime, CaO (or
slaked lime, Ca(OH)2), and soda ash, Na2CO3.
Ca2+(aq) + CO32-(aq)
Mg2+(aq) + 2OH-(aq)
CaCO3(s)
Mg(OH)2(s)
precipitates
Chapter 18
Water Softening
Ion exchange used for household water softening.
Hard water is passed through a bed of ion exchange resin:
plastic beads with covalently bound anion groups (–COO- or
–SO3-). These anion groups have Na+ attached to counter
their charges. The Ca2+ and other cations in the hard water
are exchanged with the Na+.
2Na+(R-COO-)(s) + Ca2+(aq)
Ca2+(R-COO-)2(s) + 2Na+(aq)
Chapter 18
Soils and Weathering
Soils are complex, largely variable, made up of any number of
inorganic and organic components, generally a product of
mineral or rock weathering.
Weathering: physical and chemical processes
• Water (good solvent for ions)
• Oxidation reactions (most metals can change oxidation state
when exposed to the environment)
• Freeze-thaw sequences
Chapter 18
Clays
THESE are South African soils!
Sheet silicates – continuous Si-O bonds in tetrahedral shape.
Each Si is partially + and each O is partially -.
Cations (+) in water interact with clays like ion exchange
resins.
+
+
O
Si
+
O
Si
O
Si
Si
+
O
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
O
+
1:1 clay
“open-faced sandwich”
Next sheet looks the
same, just right on top.
Si
+
Si
+
Si
+
Si
Si
2:1 clay
“closed sandwich”
Next sheet looks the
same.
Chapter 18
Clays
Cation exchange depends on:
+
+
O
Si
+
O
Si
1. Ionic Potential = charge / radius
measure of charge density on ion surface.
+
O
Si
O
Si
Si
Consider Na+ vs Cs+. Both have charge = +1, but Na
is much smaller than Cs. Na will have a higher ionic
potential and will interact more strongly with negative
surfaces than Cs.
2. Concentration of ions
Chapter 18
Mine Dumps and Wetlands
Mine dumps – extreme heavy metal concentrations in waste
materials which will leach out with rain (usually very acidic
conditions from sulfur-containing mineral weathering).
Water table extends upwards.
Leach plume can be extensive, damaging, hazardous, and
gross!
Wetlands (slow moving water, lots of vegetation) = good
“natural” treatment due to deposition and usage of metals
(uptake by plants, bacterial degradation, adsorption by
clays).
Chapter 18
Green Chemistry
Chemical industry has recognized that it is important to use
environmentally friendly chemicals and processes.
There are several goals for green chemistry:
• Prevent waste rather than subsequently cleaning it.
• Synthesize new products minimizing waste.
• Design energy efficient processes.
• Use catalysts as much as possible.
• Raw materials should be renewable feedstocks.
• Eliminate solvents as much as possible.
Chapter 18
Green Solvents and Reagents
Many organic molecules that are used as solvents are volatile
and can be environmentally damaging.
Many solvents are also toxic (Please don’t use benzene!).
Liquid or supercritical CO2 is a non-toxic solvent that has many
potential applications.
DuPont uses supercritical CO2 to make Teflon™ instead of the
environmentally damaging chlorofluorocarbon solvents.
Supercritical: substance has higher than usual T (bp) and P.
Chapter 18
Green Solvents and Reagents
Supercritical water can be used to make the plastic and
polyester fiber PET in place of acetic acid.
Lexan™ polycarbonate, the coatings on CDs, could be made
from dimethyl carbonate instead of the very toxic phosgene,
which is currently used.
Liquid CO2 is currently used in the dry cleaning industry as an
alternative to Cl2C=CCl2, which is toxic.
Ionic liquids can be used in place of acidic solvents.
Chapter 18
Green Water Purification
When Cl2(g) is used to treat water, some trihalomethanes
(THM) are often produced, which can go undetected.
THMs (CHCl3, CHClBr2) are suspected carcinogens.
Ozone and ClO2 could be used as alternatives, but they are
not completely safe.
Green water purification is an open problem.
Chapter 18
Suggested Problems
Earth’s Atmosphere: 2
Upper Atmosphere: 10, 11, 13
Troposphere: 17, 20, 22a, 25
The World Ocean: 29
Freshwater: 41
Additional Problems: 57
Chapter 18
End of Chapter 18:
Chemistry of the Environment
Chapter 18