ENVIRONMENVIRONMENTAL ENTAL CHEMISTRYCHEMISTRY
(Air II)(Air II)
Chem. 3030Chem. 3030
Stratospheric chemistry and the ozone layer; principles of photochemistry, light absorption by molecules, noncatalytic and catalytic process of ozone distraction, free radicals, Cl and Br as X catalysts, the ozone hole and its consequences, chlorofluorocarbons (CFCs).
Ground-level (tropospheric) air chemistry; ground-level ozone and photochemical smog, oxidation of methane, hydrocarbons and atmospheric SO2, acid rain, ecological effects of outdoor air pollutants, indoor air pollution: formaldehyde, NO2, CO, tobacco smoke, asbestos, radioactivity from radon gas.
The greenhouse effect and global warming; energy absorption, the major and minor greenhouse gases: CO2, water vapour, methane, N2O, CFCs.
Environmental consequences of energy use: CO2 emissions, solar energy, conventional and alternative fuels, nuclear energy.
The chemistry of natural waters; acid-base chemistry, CO2/carbonate system, ion concentations, alkalinity, seawater, redox chemistry in natural waters, oxygen demand, the pE scale, sulphur and nitrogen compounds, ion complexes, stratification, precipitation.
Soil chemistry; soil components, weathering process, aerobic, anaerobic soils, water-sediment-soil system.
Photochemical smog oxidation reactions are
initiated by the hydroxyl radical (produced in large
part due to the presence of NO from combustion
emissions). Hydrocarbons and other volatile
organic compounds are the oxidisable substrates.
The products of the smog-producing reactions
include partially oxidised hydrocarbons, CO,
aldehydes and ketones, residual nitrogen oxides
and ozone
Reducing photochemical smog and ground-level ozone:Reducing photochemical smog and ground-level ozone:
VOCs (C=C)VOCs (C=C)
NOxNOx
ACID RAIN ACID RAIN (atmospheric precipitation of substantial acid, more acidic than natural)(atmospheric precipitation of substantial acid, more acidic than natural)
H2SO4 and HNO3 from SO2 and NOx(primary pollutants)
Sources:Sources:
• volcanos, e.g. Mount Pinatubo in the Phillipines is known to have volcanos, e.g. Mount Pinatubo in the Phillipines is known to have contributed a lot of sulphate aerosols to the Arcticcontributed a lot of sulphate aerosols to the Arctic
• crude oil – petroleum industrycrude oil – petroleum industry
Claus reaction – removal of sulphides: 2HClaus reaction – removal of sulphides: 2H22S + SOS + SO22 → 3S + 2H→ 3S + 2H22OO
• petrochemical processes (CHpetrochemical processes (CH33SH, (CHSH, (CH33))22S, CHS, CH33SSCHSSCH33))
TRS – total reduced sulphurTRS – total reduced sulphur
• smelting metal ores with sulphidessmelting metal ores with sulphides
The sulphuric acid in precipitation originates from a number of chemical precursors. Reduced sulphur compounds can be oxidised to SO2 with hydroxyl radical as the primary oxidising agent. The SO2 then dissolves in water droplets and is further oxidised to sulphuric acid via several homogeneous and heterogeneous processes
Environmental consequences of photochemical smog and acid rainEnvironmental consequences of photochemical smog and acid rain
• wet deposition
• dry deposition
• acidification depends strongly on the soil composition
limestone neutralises acid, granite or quartz are strongly affected
• acidified lakes
growth of water animals and plants
DOC decreased – higher penetration of UV light to the lower water levels
elevated conc. of dissolved Al and Fe
• deterioration of soils – washing out plant nutrients
• effect on trees
• agriculture crops
ozone reacts with ethylene forming free radicals – resulting in slowed photosynthesis
Air particulatesAir particulates
Solid or liquid particles suspended in air.
They are not visible to the naked eye, but form a haze that reduces visibility.
Size: 2nm – 100 µm
Coarse particles > 2.5 µm < fine particles
Soot or inorganic soot or sulphate, nitrate aerosols
Basic due to a soil content acidic due to unneutralised acids
PM index – amount of particulate matter per volume (µg/m3)PM10 = 10 µm
PM10 = all fine particles <10 µm – inhalablePM2.5 – respirablePM0.05 - ultrafine
Distribution of numbers of aerosol particles
vs. sizein a typical urban environment
Aerosol particles cover a size range from 1 nm to Aerosol particles cover a size range from 1 nm to
100 100 μμm in diameter, but it is particles in the m in diameter, but it is particles in the
range 0.01 – 10 range 0.01 – 10 μμm that are most stable in m that are most stable in
suspension. Particles smaller than 0.01 suspension. Particles smaller than 0.01 μμm in m in
diameter tend to coagulate into larger units, while diameter tend to coagulate into larger units, while
those larger than 10 those larger than 10 μμm readily settle out. The m readily settle out. The
aerosol fraction that consists of very fine particles is aerosol fraction that consists of very fine particles is
of concern to human health because of its of concern to human health because of its
association with respiratory problems.association with respiratory problems.
Chemistry of the troposphere – free radicals
Prediction of fate of gases emitted
into the air
Prediction of fate of airborne free radicals
Oxidation of methane in the troposphere
(the rep.of other alkenesand VOCs with single bonds)
CH4 – no water soluble, does not absorb sunlight, no multiple
bonds – abstraction of H by OH*
CH3* – addition of O2 to produce
peroxy radical
Peroxy radical oxidize by transfer of O atom
O2 abstracts H to produce formaldehyde
Formaldehyde decompose photochemically and H adds to O2
O2 abstracts the H to produce CO2 and OOH* radical
O2 addition to double bond H-C=O
OH* addition to triple bond C≡O
CH4 + 5O2 + NO* →(UV-A) CO2 + H2O + NO2* + 4HOO*
Intermediate stable species of partially oxidized materialsIntermediate stable species of partially oxidized materials
Variation of the concentration of gases during a day in urban areaVariation of the concentration of gases during a day in urban area
The oxidation of most reactive VOCs The oxidation of most reactive VOCs – hydrocarbons with C=C bond– hydrocarbons with C=C bond
Indoor air pollutionIndoor air pollution
• Formaldehyde, H2C=O – stable intermediate in the oxidation of methane and VOCs
sources: cigarette smoke, synthetic materials, glues, dyes, resins
CARCINOGEN
• NO2 and CO
Sources: combustion processes
NO2 dissolves in biological tissues, OXIDANT
CO bonds to haemoglobin and stops oxygen bonding
Tobacco smoke
Very complex toxic action – solids (tar), liquids and VOCs
• Asbestos
CARCINOGEN
• Radon –Radioactivity
THE GREEN HOUSE EFFECT THE GREEN HOUSE EFFECT
AND GLOBAL WARMINGAND GLOBAL WARMING
Average air temp. increases as a result of the build-up of carbon dioxide and other “greenhouse” gases in the atmosphere.
The mechanism of greenhouse effect: The mechanism of greenhouse effect:
Incoming energy Incoming energy to the Earth is mostly visible light from Sunto the Earth is mostly visible light from Sun
Outgoing energy Outgoing energy is in a form of infrared radiation (heat) is in a form of infrared radiation (heat)
Balance of energy:Balance of energy:
From the total light coming to Earth – 50% absorbed by surface
- 20% absorbed by gases (O3, O2, CO2, H2O and water droplets)
- 30% reflected back into space (by clouds, snow, ice, sand)
Some gases in air can temporarily absorb IR light of specific wavelengths Some gases in air can temporarily absorb IR light of specific wavelengths and later re-emit in all directions (randomly); some are redirected to the and later re-emit in all directions (randomly); some are redirected to the earth’s surface – earth’s surface – GREENHOUSE EFFECTGREENHOUSE EFFECT
Normally gases in the atmosphere operate as a ‘blanket’ for the Earth Normally gases in the atmosphere operate as a ‘blanket’ for the Earth keeping the same level of coming in and going out radiation.keeping the same level of coming in and going out radiation.
Increasing the conc. of the gases in air which absorb the IR radiation will Increasing the conc. of the gases in air which absorb the IR radiation will increase re-direction of IR and keep more heat on the Earth - increase re-direction of IR and keep more heat on the Earth -
ENHANCED GREENHOUSE EFFECTENHANCED GREENHOUSE EFFECT
(distinguish from naturally operated for millennia)(distinguish from naturally operated for millennia)
Energy absorption by greenhouse gasesEnergy absorption by greenhouse gases
COCO2 2 O=C=OO=C=O4.26 µm – antisymmetric
stretch vibration
15.0 15.0 µmµm – – bond-angle bond-angle bending vibrationbending vibration
The COThe CO2 2 molecules absorb now molecules absorb now
about half of the outgoing thermal about half of the outgoing thermal IR radiation.IR radiation.
HH22O H-O-HO H-O-H
Emission of COEmission of CO22 – 0.4% annually – 0.4% annually
Higher temp. causes increase of Higher temp. causes increase of water vapour water vapour → more absorption of → more absorption of IR radiation = IR radiation = double effectdouble effect
from 18.0 from 18.0 µmµm – rotation – rotation without vibrationwithout vibration
6.3 6.3 µmµm – –bending vibrationbending vibration
Other IR absorbing gases:Other IR absorbing gases:CHCH44
NN22OO
OO33
CFCs CFCs
Experimentally Experimentally measured intensity of measured intensity of
thermal IR light leaving thermal IR light leaving the Earththe Earth
Theoretical intensity of Theoretical intensity of thermal IR light that thermal IR light that would be leaving the would be leaving the
Earth without Earth without absorption by absorption by
greenhouse gasesgreenhouse gases8-13 µm - window
GREENHOUSE GASESGREENHOUSE GASES
ATMOSPHERIC RESIDENCE TIME, TATMOSPHERIC RESIDENCE TIME, Tavgavg
TTavgavg = C / R = C / R
C – total atmospheric amountC – total atmospheric amount
R – average rate of input or output per unit timeR – average rate of input or output per unit time
Effects of aerosolsEffects of aerosols
All solids and liquids (atmospheric particles) have the ability to reflect light.All solids and liquids (atmospheric particles) have the ability to reflect light.
Airborne particle
Incoming sunlight
Reflection
Absorption
The common aerosols are: clouds; ammonium sulphate and other sulphate-based solid aerosols; biomass aerosols such as black carbon or soot.Sulphate aerosols can be from natural oceanic sulphide (e.g. dimethyl sulphide) and anthropogenic sources of SO2. While sulphate aerosols tend to backscatter incoming light, soot adds to positive radiative forcing.
The amount of sunlight reflected into space by anthropogenic aerosols The amount of sunlight reflected into space by anthropogenic aerosols (Watts per m(Watts per m2 2 of the Earth surface) of the Earth surface)
Effects of global warmingEffects of global warming
If COIf CO22 is not reduced is not reduced,,according to UN Environmental Plan and according to UN Environmental Plan and
computer modelling, by the year 2040 the average global temp. will computer modelling, by the year 2040 the average global temp. will
be 1 degree higher than at present. This will lead to:be 1 degree higher than at present. This will lead to:
•Rainfall increaseRainfall increase
• Sea level rise by about 18 cm (due to the melting of ice)- less of landSea level rise by about 18 cm (due to the melting of ice)- less of land
• temp. and moisture changes – some ecosystems destabilisedtemp. and moisture changes – some ecosystems destabilised
• Antarctic and Greenland ice sheets will meltAntarctic and Greenland ice sheets will melt
• monsoon rains more heavymonsoon rains more heavy
• some areas fully drysome areas fully dry
• and……more unknown consequences!and……more unknown consequences!
Country-level climate change impacts for 2080 based on a) Max-Plank model, b) Hadley model, c) Canadian model (Fischer at al. 2001)
FLOODSFLOODS
WATERWATER
ECOSYSTEMS DESTABILIZEDECOSYSTEMS DESTABILIZED
COCO22 uptake uptake
TEMPERATURE CHANGESTEMPERATURE CHANGES
Water and CO2 absorb large amounts of IR radiation and have the potential
to contribute to a warmer climate than would otherwise exist on the Earth.
Several other trace gases – methane, ozone, NO and CFCs – also absorb IR
radiation, mostly in the “window” region. All these are called greenhouse
gases. Both natural and anthropogenic aerosols contribute an additional
warming effect.
ENERGY PRODUCTIONENERGY PRODUCTION
Global quantities of energy measured with parameter QGlobal quantities of energy measured with parameter Q
1Q = 101Q = 102121Joules = heat energy from burning 20 kmJoules = heat energy from burning 20 km33 block of coal block of coal
Total amount of commercial energy consumed by humans is Total amount of commercial energy consumed by humans is about 0.5Q per year.about 0.5Q per year.
COCO22 emissionemission
Currently, emission of COCurrently, emission of CO22 = 4 tones / person / year = 4 tones / person / year
COCO22 emission scenariosemission scenarios
Minimising future emission of COMinimising future emission of CO22
Energy production from:Energy production from:
Carbon C + OCarbon C + O22 = CO = CO22
Polymers CHPolymers CH2 2 + 1.5O + 1.5O22 = CO = CO22 + H + H22OO
Natural gas CHNatural gas CH44 + 2O + 2O22 = CO = CO22 + 2H + 2H22OO
Ratio 1 : 1.33 : 2 of energy per mole of ORatio 1 : 1.33 : 2 of energy per mole of O2 2 consumed consumedCOCO2 2 Sequestered Sequestered
For ex. at the bottom of deep sea;For ex. at the bottom of deep sea;
COCO2 2 + H + H22O + CaCOO + CaCO33 = Ca(HCO = Ca(HCO33))2 2 (aq)(aq)
Injection of COInjection of CO22 in the ocean depth - CO in the ocean depth - CO22 trapped in a liquid state trapped in a liquid state
(supercritical fluid under big pressures below 3500m – stable lake of (supercritical fluid under big pressures below 3500m – stable lake of carbon dioxide)carbon dioxide)
Or COOr CO22 + H + H22O + CaSiOO + CaSiO33 = Ca(HCO = Ca(HCO33))22 (aq) + SiO (aq) + SiO22
Plants absorb a lot of COPlants absorb a lot of CO2 2 - growing more forests - growing more forests
Or ……..Or ……..
Carbon taxesCarbon taxes
Solar energySolar energy
Available 3000Q Used 0.5QAvailable 3000Q Used 0.5Q
Note that Note that 1Q = 101Q = 102121JoulesJoules
Indirect use of solar energy: Indirect use of solar energy:
Hydroelectric power - av. 0.1Q/year, used 0.02QHydroelectric power - av. 0.1Q/year, used 0.02Q
Wind power - av. 0.5Q, used 0.00025QWind power - av. 0.5Q, used 0.00025Q
Biomass - av. 3Q, used Biomass - av. 3Q, used → 0→ 0
Geothermal power - av. 0.01Q, used → 0Geothermal power - av. 0.01Q, used → 0
Direct absorption of solar energy:Direct absorption of solar energy:
Thermal conversion (passive) – heat exchangersThermal conversion (passive) – heat exchangers
Photo-conversion (active) – solar thermal electricityPhoto-conversion (active) – solar thermal electricity
Solar thermal energy conversion – 2Solar thermal energy conversion – 2ndnd law of law of thermodynamicsthermodynamics
2nd law of thermodynamics is an expression of the universal law of increasing entropy. It means that over time, differences in temperature, pressure and density tend to even out in a physical system that is isolated from the outside world. Entropy is the measure of how far along this evening-out process has progressed.
Entropy , S = q / TEntropy , S = q / T
Entropy change, Entropy change, ΔΔS, for energy conversion from initial high temp. TS, for energy conversion from initial high temp. Th h to to
final lower temp. Tfinal lower temp. Tcc could be positive or zero: could be positive or zero:
ΔΔS = entropy after conversion – entropy before conversion = S = entropy after conversion – entropy before conversion =
qqcc/T/Tcc – q – qhh / T / Thh
For the most conversion of energy to electricity is possible when For the most conversion of energy to electricity is possible when
ΔΔS = 0S = 0
qqcc/T/Tcc = q = qhh / T / Th h andand qqc c = q= qh h TTcc/T/Thh
Heat converted = qHeat converted = qhh – q – qc c = q = qhh – q – qhh T Tcc/T/Th h = q = qhh (T (Thh –T –Tcc)/T)/Thh
Heat converted / Initial heat = (THeat converted / Initial heat = (Thh –T –Tcc)/T)/Thh
Solar cellSolar cell
Photovoltaic effect – Photovoltaic effect – photogeneration of charge photogeneration of charge carriers (ecarriers (e-- and holes) and holes)
Semiconductor Semiconductor → ‘band theory’ → ‘band theory’
→ → ,band gap’ between bonding ,band gap’ between bonding and antibonding levels small →and antibonding levels small →
If it’s too large, then the If it’s too large, then the material is a good resistormaterial is a good resistor
Si – 124 kJ/mol → IR light Si – 124 kJ/mol → IR light energyenergy
Only 28% of energy converted into DC currentOnly 28% of energy converted into DC current
The theory is that:
• In a crystal, the atoms are packed so tightly that the energy states of
individual atoms are modified, thus forming bands or energy levels
• Photons in sunlight hit the solar panel and are absorbed by semi-
conducting materials, such as silicon
• Electrons are knocked loose from their atoms, allowing them to flow
through the material to produce electricity. The complementary positive
holes “move” in the opposite direction
Advantages and disadvantages of solar energyAdvantages and disadvantages of solar energy
AdvantagesAdvantages
Free and abundantFree and abundant
Environmentally friendlyEnvironmentally friendly
Low operating costsLow operating costs
Does not require distribution network – transportDoes not require distribution network – transport
‘‘natural’ form of energynatural’ form of energy
DisadvantagesDisadvantages
Requires sun – problems with cumulation and storageRequires sun – problems with cumulation and storage
Diffuse – low density of energy/unit of collection area – large solar collectors Diffuse – low density of energy/unit of collection area – large solar collectors (1kW requires about 1m(1kW requires about 1m22))
High costs of construction of collectorsHigh costs of construction of collectors
Till now no economic regulatory credits from goverments (like tax incentives)Till now no economic regulatory credits from goverments (like tax incentives)
Fuels and the environmentFuels and the environment
Gasoline – PetrolGasoline – Petrol
Mixture of alkanes, cycloalkanes, aromatic hydrocarbons (benzene and its Mixture of alkanes, cycloalkanes, aromatic hydrocarbons (benzene and its derivatives)derivatives)
BTX – benzene + toluene + xyleneBTX – benzene + toluene + xylene
Regular petrol C7 – C8Regular petrol C7 – C8
diesel C9 - C11diesel C9 - C11
‘‘Knocking” – Long chains and cycloalkanes ignite spontaneously before Knocking” – Long chains and cycloalkanes ignite spontaneously before complete compression in an engine complete compression in an engine
Isooctane and other highly branched alkanes – very good combustionIsooctane and other highly branched alkanes – very good combustion
Octane number – scale: isooctane 100Octane number – scale: isooctane 100
n-heptane 0n-heptane 0 -c-c-c-c-c--c-c-c-c-c-
-c-c-c-c-c--c-c-c-c-c-
CC CC
CC
Leaded petrol – addition of Pb(CHLeaded petrol – addition of Pb(CH33))44
Unleaded – addition of organometallic MgUnleaded – addition of organometallic Mg
• The octane rating is a measure of the autoignition
resistance of petrol (% by volume of isooctane in a
mixture of isooctane and normal heptane)
• A 90-octane gasoline has the same anti-knock rating
as a mixture of 90% iso-octane and 10% heptane
GasGas
Natural gas CHNatural gas CH4 4 + C+ C22HH6 6 + C+ C33HH88
Propane CPropane C33HH88
Methanol, ethanol – Methanol, ethanol – as petrol additives or replacementas petrol additives or replacement
EthersEthers
Fuel cellsFuel cells
• A FC is an electrochemical energy conversion device, producing electricity from
external supplies of fuel (on the anode) and oxidant (on the cathode). These rxt
in an electrolyte
• FCs differ from batteries in that they consume the reactants while batteries
store electrical energy chemically in a closed system. Electrodes within a battery
rxt and change as a battery is charged, an FC’s electrodes are catalytic and
relatively stable
Fuel cellsFuel cells Anode CathodeAnode Cathode
(Porous graphite + catalyst Pt)(Porous graphite + catalyst Pt)
HH2 2 (g) + ½ O(g) + ½ O22 (g) (g) → → ΔΔH = -242 kJ/molH = -242 kJ/mol
T ~ 80T ~ 8000C C ~ 0.8 V ~ 0.8 V
Most fuel cells use H2 and O2
Passing electricity in water produces H2 and O2. Combining these 2 should give H2O) and electricity!
Problems with fuel cellsProblems with fuel cells
• hydrogen sources – hydrogen sources –
• transport of hydrogen or transport of hydrogen or
• continuous production from liquid fuelscontinuous production from liquid fuels
CHCH33OH (Cu/Zn oxide catalyst, 275OH (Cu/Zn oxide catalyst, 27500C) C) → 2H→ 2H22 + CO (H + CO (H22O) O) → 2H→ 2H22 + COCO22
• hydrogen storagehydrogen storage
•temp. hydrides with metals, H diffusion into metals (containers)temp. hydrides with metals, H diffusion into metals (containers)
• combustion of Hcombustion of H2 2 - formation of NOx and H- formation of NOx and H22OO22
• production of Hproduction of H22
•Electrolysis of water (use power from photovoltaic cells)Electrolysis of water (use power from photovoltaic cells)
requires ‘solar tower’ of mirrors to collect sun heat →requires ‘solar tower’ of mirrors to collect sun heat → T 2200T 220000C is C is splitting only 25% of water steamsplitting only 25% of water steam
•Fossil fuels + gas + water steamFossil fuels + gas + water steam
C + 2HC + 2H22O → 2HO → 2H22 + CO + CO2 2 and and CHCH44 + 2H + 2H22O → 4HO → 4H22 + CO + CO22
•Biofuels : Methane from plants and alcoholsBiofuels : Methane from plants and alcohols
Nuclear energyNuclear energy
FissionFission
FusionFusion
10n
heavy nuclei
2 nuclei
+ n + energy
2 smaller nuclei
combine
heavy nuclei
+ energy
Nuclear energyNuclear energy
The energy is used to produce water steam to run turbines to produce electricityThe energy is used to produce water steam to run turbines to produce electricity
235235U 0.7% abundance,U 0.7% abundance,
238238U 99.3%U 99.3%
Side reactions produce highly radioactive byproducts and Side reactions produce highly radioactive byproducts and ββ radiation radiation
Used fuel rods are more radioactive – storage problem, e.g., in USA the Yucca Used fuel rods are more radioactive – storage problem, e.g., in USA the Yucca Mountain project seeks to deal with nuclear wasteMountain project seeks to deal with nuclear waste
High energy required to start fusion reactionHigh energy required to start fusion reaction
Nuclear energyNuclear energy
