Ozone
From Wikipedia, the free encyclopedia
Ozone // (systematically named
1λ1,3λ1-trioxidane and
μ-oxidodioxygen), or
trioxygen, is an inorganic molecule with the
chemical formula O
2(μ-O) (also written [O(μ-O)O] or
O
3). It is a pale blue gas with a distinctively pungent smell. It is an
allotrope of
oxygen that is much less stable than the
diatomic allotrope O
2, breaking down in the lower atmosphere to normal dioxygen. Ozone is formed from dioxygen by the action of
ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the
Earth's atmosphere. In total, ozone makes up only
0.6 ppm of the atmosphere.
Ozone's odor is sharp, reminiscent of chlorine, and detectable by many people at concentrations of as little as
10 ppb in air. Ozone's O
3 formula was determined in 1865. The molecule was later proven to have a bent structure and to be
diamagnetic. In
standard conditions, ozone is a pale blue gas that condenses at progressively cryogenic temperatures to a dark blue
liquid and finally a violet-black
solid.
Ozone's instability with regard to more common dioxygen is such that
both concentrated gas and liquid ozone may decompose explosively.
[1] It is therefore used commercially only in low concentrations.
Ozone is a powerful
oxidant
(far more so than dioxygen) and has many industrial and consumer
applications related to oxidation. This same high oxidizing potential,
however, causes ozone to damage mucus and respiratory tissues in
animals, and also tissues in plants, above concentrations of about
100 ppb. This makes ozone a potent respiratory hazard and pollutant near ground level. However, the so-called
ozone layer (a portion of the stratosphere with a higher concentration of ozone, from two to eight ppm) is beneficial, preventing damaging
ultraviolet light from reaching the
Earth's surface, to the benefit of both plants and animals.
Nomenclature
The
trivial name ozone is the most commonly used and
preferred IUPAC name. The systematic names
1λ1,3λ1-trioxidane and
μ-oxidodioxygen, valid
IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. The name
ozone derives from
ozein (ὄζειν), the
Greek word for smell (verb), referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as
trioxidane with two hydrogen atoms removed, and as such,
trioxidanylidene
may be used as a context-specific systematic name, according to
substitutive nomenclature. By default, these names pay no regard to the
radicality of the ozone molecule. In even more specific context, this
can also name the non-radical singlet ground state, whereas the
diradical state is named
trioxidanediyl.
Trioxidanediyl (or
ozonide) is used,
non-systematically, to refer to the substituent group (-OOO-). Care
should be taken to avoid confusing the name of the group for the
context-specific name for ozone given above.
History
ozonometer, 1865
Ozone, the first
allotrope of any
chemical element to be recognized, was proposed as a distinct
chemical substance by
Christian Friedrich Schönbein in 1840, who named it after the
Greek verb ozein (ὄζειν, "to smell"), from the peculiar odor in
lightning storms.
[2][3] The formula for ozone, O
3, was not determined until 1865 by
Jacques-Louis Soret[4] and confirmed by Schönbein in 1867.
[2][5]
For much of the second half of the nineteenth century and well into
the twentieth, ozone was considered a healthy component of the
environment by naturalists and health-seekers. The City of Beaumont in
California had as its official slogan "Beaumont: Zone of Ozone," as
evidenced on postcards and Chamber of Commerce letterhead.
[6]
Naturalists working outdoors often considered the higher elevations
beneficial because of their ozone content. "There is quite a different
atmosphere [at higher elevation] with enough ozone to sustain the
necessary energy [to work]," wrote naturalist Henry Henshaw, working in
Hawaii.
[7]
Seaside air was considered to be healthy because of its "ozone" content
but the smell giving rise to this belief is in reality that of rotting
seaweed.
[8]
Physical properties
Ozone is colourless or slightly bluish gas (blue when liquified),
slightly soluble in water and much more soluble in inert non-polar
solvents such as
carbon tetrachloride or fluorocarbons, where it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue
liquid.
It is dangerous to allow this liquid to warm to its boiling point,
because both concentrated gaseous ozone and liquid ozone can detonate.
At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a
violet-black
solid.
[9]
Most people can detect about 0.01 μmol/mol of ozone in air where it has a very specific sharp odor somewhat resembling
chlorine bleach. Exposure of 0.1 to 1 μmol/mol produces headaches, burning eyes and irritation to the respiratory passages.
[10]
Even low concentrations of ozone in air are very destructive to organic
materials such as latex, plastics and animal lung tissue.
Ozone is
diamagnetic, which means that its electrons are all paired. In contrast, O
2 is
paramagnetic, containing two unpaired electrons.
Structure
According to experimental evidence from
microwave spectroscopy, ozone is a bent molecule, with C
2v symmetry (similar to the
water molecule). The O – O distances are 127.2
pm (1.272
Å). The O – O – O angle is 116.78°.
[11] The central atom is
sp² hybridized with one lone pair. Ozone is a polar molecule with a
dipole moment of 0.53
D.
[12] The bonding can be expressed as a
resonance hybrid with a
single bond on one side and
double bond on the other producing an overall
bond order of 1.5 for each side.
Reactions
Ozone is a powerful
oxidizing agent, far stronger than O
2.
It is also unstable at high concentrations, decaying to ordinary
diatomic oxygen. It has a varying length half-life (meaning half as
concentrated, or half-depleted), depending upon atmospheric conditions
(temperature, humidity, and air movement). In a sealed chamber, with fan
moving the gas, ozone has a half-life of approximately a day at room
temperature
[13]
Some claims have been stated that ozone can have a half life as short
as a half an hour in atmospheric conditions, although this claim is not
verified by this reference:
[14]
- 2 O
3 → 3 O
2
This reaction proceeds more rapidly with increasing temperature and increased pressure.
Deflagration of ozone can be triggered by a spark, and can occur in ozone concentrations of 10
wt% or higher.
[15]
With metals
Ozone will oxidize most
metals (except
gold,
platinum, and
iridium) to
oxides of the metals in their highest
oxidation state. For example:
- 2 Cu+ + 2 H
3O+ + O
3 → 2 Cu2+ + 3 H
2O + O
2
With nitrogen and carbon compounds
Ozone also oxidizes
nitric oxide to
nitrogen dioxide:
- NO + O
3 → NO
2 + O
2
This reaction is accompanied by
chemiluminescence. The
NO
2 can be further oxidized:
- NO
2 + O
3 → NO3 + O
2
The
NO
3 formed can react with
NO
2 to form
N
2O
5:
Solid
nitronium perchlorate can be made from NO
2, ClO
2, and
O
3 gases:
- 2 NO
2 + 2 ClO2 + 2 O
3 → 2 NO2ClO4 + O
2
Ozone does not react with ammonium
salts, but it oxidizes
ammonia to
ammonium nitrate:
- 2 NH
3 + 4 O
3 → NH
4NO
3 + 4 O
2 + H
2O
Ozone reacts with
carbon to form
carbon dioxide, even at room temperature:
- C + 2 O3 → CO2 + 2 O2
With sulfur compounds
Ozone oxidizes
sulfides to
sulfates. For example,
lead(II) sulfide is oxidised to
lead(II) sulfate:
- PbS + 4 O3 → PbSO4 + 4 O2
Sulfuric acid can be produced from ozone, water and either elemental
sulfur or
sulfur dioxide:
- S + H2O + O3 → H2SO4
- 3 SO2 + 3 H2O + O3 → 3 H2SO4
In the
gas phase, ozone reacts with
hydrogen sulfide to form sulfur dioxide:
- H2S + O3 → SO2 + H2O
In an
aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce
sulfuric acid:
- H2S + O3 → S + O2 + H2O
- 3 H2S + 4 O3 → 3 H2SO4
With alkenes and alkynes
Alkenes can be oxidatively cleaved by ozone, in a process called
ozonolysis, giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.
Usually ozonolysis is carried out in a solution of
dichloromethane, at a temperature of −78
oC. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g.
zinc in
acetic acid or
dimethyl sulfide), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic
hydrogen peroxide), carboxylic acids will be formed.
[16]
Other substrates
All three
atoms of ozone may also react, as in the reaction of
tin(II) chloride with
hydrochloric acid and ozone:
- 3 SnCl2 + 6 HCl + O
3 → 3 SnCl4 + 3 H2O
Iodine perchlorate can be made by treating
iodine dissolved in cold
anhydrous perchloric acid with ozone:
- I2 + 6 HClO4 + O3 → 2 I(ClO4)3 + 3 H2O
Combustion
Ozone can be used for
combustion reactions and combusting gases; ozone provides higher temperatures than combusting in
dioxygen (O
2). The following is a reaction for the combustion of
carbon subnitride which can also cause higher temperatures:
- 3 C
4N
2 + 4 O
3 → 12 CO + 3 N
2
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic
hydrogen reacts with liquid ozone to form a hydrogen
superoxide radical, which
dimerizes:
[17]
- H + O
3 → HO2 + O - 2 HO2 → H
2O
4
Reduction to ozonides
Reduction of ozone gives the
ozonide anion, O
−
3. Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the
alkali metals are known. KO
3, RbO
3, and CsO
3 can be prepared from their respective superoxides:
- KO2 + O3 → KO3 + O2
Although KO
3 can be formed as above, it can also be formed from
potassium hydroxide and ozone:
[18]
- 2 KOH + 5 O3 → 2 KO3 + 5 O2 + H2O
NaO
3 and LiO
3 must be prepared by action of CsO
3 in liquid NH
3 on an
ion exchange resin containing Na
+ or Li
+ ions:
[19]
- CsO3 + Na+ → Cs+ + NaO3
A solution of
calcium in ammonia reacts with ozone to give to ammonium ozonide and not calcium ozonide:
[17]
- 3 Ca + 10 NH3 + 6 O
3 → Ca·6NH3 + Ca(OH)2 + Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2
Applications
Ozone can be used to remove
manganese from
water, forming a
precipitate which can be filtered:
- 2 Mn2+ + 2 O
3 + 4 H2O → 2 MnO(OH)2 (s) + 2 O2 + 4 H+
Ozone will also detoxify
cyanides by converting them to
cyanates, which are a thousand times less toxic.
[citation needed]
- CN− + O3 → CNO− + O2
Ozone will also completely decompose
urea:
[20]
- (NH2)2CO + O3 → N2 + CO2 + 2 H2O
Ozone in Earth's atmosphere
The distribution of atmospheric ozone in partial pressure as a function of altitude
Concentration of ozone as measured by the
Nimbus-7 satellite
Total ozone concentration in June 2000 as measured by EP-TOMS satellite instrument
The standard way to express total ozone levels (the amount of ozone in a vertical column) in the atmosphere is by using
Dobson units. Point measurements are reported as
mole fractions in nmol/mol (parts per billion, ppb) or as
concentrations in μg/m
3. The study of ozone concentration in the atmosphere started in the 1920s.
[21]
Ozone layer
Location and production
The highest levels of ozone in the atmosphere are in the
stratosphere, in a region also known as the
ozone layer
between about 10 km and 50 km above the surface (or between about 6 and
31 miles). However, even in this "layer" the ozone concentrations are
only two to eight parts per million, so most of the oxygen there remains
of the dioxygen type.
Ozone in the stratosphere is mostly produced from short-wave
ultraviolet rays (in the UVC band) but it can be also produced from
x-rays reacting with oxygen:
- O
2 + photon (radiation λ < 240 nm) → 2 O
- O + O
2 + M → O
3 + M
- α + β− + O
2 → He + O
3
where "M" denotes the third body that carries off the excess energy
of the reaction. The thus produced ozone is destroyed by the reaction
with
atomic oxygen:
- O
3 + O → 2 O
2
The latter reaction is
catalysed
by the presence of certain free radicals, of which the most important
are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and
bromine (Br). In recent decades the amount of ozone in the stratosphere
has been declining mostly because of emissions of
chlorofluorocarbons (CFC) and similar
chlorinated and brominated organic molecules, which have increased the concentration of ozone-depleting catalysts above the natural background.
Importance to surface-dwelling life on Earth
Levels of ozone at various altitudes and blocking of different bands of
ultraviolet radiation. Essentially all UVC (100–280 nm) is blocked by
dioxygen (at 100–200 nm) or by ozone (at 200–280 nm) in the atmosphere.
The shorter portion of this band and even more energetic UV causes the
formation of the ozone layer, when single oxygen atoms produced by UV
photolysis
of dioxygen (below 240 nm) react with more dioxygen. The ozone layer
itself then blocks most, but not quite all, sunburn-producing UVB
(280–315 nm). The band of UV closest to visible light, UVA (315–400 nm),
is hardly affected by ozone, and most of it reaches the ground.
Ozone in the ozone layer filters out sunlight wavelengths from about
200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm.
[22]
This ozone UV absorption is important to life, since it extends the
absorption of UV by ordinary oxygen and nitrogen in air (which absorb
all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the
entire UV-B band (280–315 nm). The small unabsorbed part that remains
of UV-B after passage through ozone causes sunburn in humans, and direct
DNA damage in living tissues in both plants and animals. Ozone's effect
on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm,
which has a radiation intensity 350 million times as powerful at the top
of the atmosphere as at the surface. Nevertheless, enough of UV-B
radiation at similar frequency reaches the ground to cause some sunburn,
and these same wavelengths are also among those responsible for the
production of
vitamin D in humans.
The ozone layer has little effect on the longer UV wavelengths called
UV-A (315–400 nm), but this radiation does not cause sunburn or direct
DNA damage, and while it probably does cause long-term skin damage in
certain humans, it is not as dangerous to plants and to the health of
surface-dwelling organisms on Earth in general (see
ultraviolet for more information on near ultraviolet).
Low level ozone
Low level ozone (or tropospheric ozone) is an atmospheric pollutant.
[23] It is not emitted directly by
car engines or by industrial operations, but formed by the reaction of sunlight on air containing
hydrocarbons and
nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind.
Ozone reacts directly with some hydrocarbons such as
aldehydes and thus begins their removal from the air, but the products are themselves key components of
smog. Ozone
photolysis by UV light leads to production of the
hydroxyl radical
HO• and this plays a part in the removal of hydrocarbons from the air,
but is also the first step in the creation of components of smog such as
peroxyacyl nitrates,
which can be powerful eye irritants. The atmospheric lifetime of
tropospheric ozone is about 22 days; its main removal mechanisms are
being deposited to the ground, the above-mentioned reaction giving HO•,
and by reactions with OH and the peroxy radical HO
2•.
[24]
There is evidence of significant reduction in agricultural yields
because of increased ground-level ozone and pollution which interferes
with
photosynthesis and stunts overall growth of some plant species.
[25][26] The
United States Environmental Protection Agency
is proposing a secondary regulation to reduce crop damage, in addition
to the primary regulation designed for the protection of human health.
Certain examples of cities with elevated ozone readings are
Houston, Texas, and
Mexico City,
Mexico. Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol.
[26]
Ozone cracking
Ozone gas attacks any
polymer possessing olefinic or
double bonds within its chain structure, such as
natural rubber,
nitrile rubber, and
styrene-butadiene
rubber. Products made using these polymers are especially susceptible
to attack, which causes cracks to grow longer and deeper with time, the
rate of crack growth depending on the load carried by the rubber
component and the concentration of ozone in the atmosphere. Such
materials can be protected by adding
antiozonants,
such as waxes, which bond to the surface to create a protective film or
blend with the material and provide long term protection.
Ozone cracking used to be a serious problem in car tires for example, but the problem is now seen only in very old tires.
[clarification needed][citation needed] On the other hand, many critical products like
gaskets and
O-rings may be attacked by ozone produced within compressed air systems.
Fuel lines
made of reinforced rubber are also susceptible to attack, especially
within the engine compartment, where some ozone is produced by
electrical components. Storing rubber products in close proximity to a
DC electric motor can accelerate ozone cracking. The
commutator of the motor creates sparks which in turn produce ozone.
Ozone as a greenhouse gas
Although ozone was present at ground level before the
Industrial Revolution,
peak concentrations are now far higher than the pre-industrial levels,
and even background concentrations well away from sources of pollution
are substantially higher.
[27][28] Ozone acts as a
greenhouse gas, absorbing some of the
infrared
energy emitted by the earth. Quantifying the greenhouse gas potency of
ozone is difficult because it is not present in uniform concentrations
across the globe. However, the most widely accepted scientific
assessments relating to
climate change (e.g. the
Intergovernmental Panel on Climate Change Third Assessment Report)
[29] suggest that the
radiative forcing of tropospheric ozone is about 25% that of
carbon dioxide.
The annual
global warming potential of tropospheric ozone is between 918–1022 tons
carbon dioxide equivalent/tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a
radiative forcing effect roughly 1,000 times as strong as
carbon dioxide. However, tropospheric ozone is a short-lived
greenhouse gas, which decays in the atmosphere much more quickly than
carbon dioxide. This means that over a 20 year horizon, the
global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons
carbon dioxide equivalent / tons tropospheric ozone.
[30]
Because of its short-lived nature, tropospheric ozone does not have
strong global effects, but has very strong radiative forcing effects on
regional scales. In fact, there are regions of the world where
tropospheric ozone has a
radiative forcing up to 150% of
carbon dioxide.
[31]
Health effects
Ozone air pollution
Red Alder leaf, showing discolouration caused by ozone pollution
[32]
Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of
fossil fuels. Ground-level ozone pollution (
tropospheric ozone) is created near the Earth's surface by the action of daylight
UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but
methane
is a natural precursor, and the very low natural background level of
ozone at ground level is considered safe. This section examines health
impacts of fossil fuel burning, which raises ground level ozone far
above background levels.
There is a great deal of evidence to show that ground level ozone can harm lung function and irritate the
respiratory system.
[23][33] Exposure to ozone and the pollutants that produce it is linked to premature death,
asthma,
bronchitis,
heart attack, and other cardiopulmonary problems.
[34][35]
Long-term exposure to ozone has been shown to increase risk of death from
respiratory illness.
A study of 450,000 people living in United States cities showed a
significant correlation between ozone levels and respiratory illness
over the 18-year follow-up period. The study revealed that people living
in cities with high ozone levels such as Houston or Los Angeles had an
over 30% increased risk of dying from lung disease.
[36][37]
Air quality guidelines such as those from the
World Health Organization, the
United States Environmental Protection Agency (EPA) and the
European Union are based on detailed studies designed to identify the levels that can cause measurable ill
health effects.
According to scientists with the US EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol.
[35][38][39]
In the EU, the current target value for ozone concentrations is
120 µg/m³ which is about 60 nmol/mol. This target applies to all member
states in accordance with Directive 2008/50/EC.
[40]
Ozone concentration is measured as a maximum daily mean of 8 hour
averages and the target should not be exceeded on more than 25 calendar
days per year, starting from January 2010. Whilst the directive requires
in the future a strict compliance with 120 µg/m³ limit (i.e. mean ozone
concentration not to be exceeded on any day of the year), there is no
date set for this requirement and this is treated as a long-term
objective.
[41]
In the USA, the
Clean Air Act directs the EPA to set
National Ambient Air Quality Standards
for several pollutants, including ground-level ozone, and counties out
of compliance with these standards are required to take steps to reduce
their levels. In May 2008, under a court order, the EPA lowered its
ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved
controversial, since the Agency's own scientists and advisory board had
recommended lowering the standard to 60 nmol/mol.
[35] Many public health and environmental groups also supported the 60 nmol/mol standard,
[42] and the
World Health Organization recommends 51 nmol/mol.
On January 7, 2010, the U.S. Environmental Protection Agency (EPA)
announced proposed revisions to the National Ambient Air Quality
Standard (NAAQS) for the pollutant ozone, the principal component of
smog:
- ... EPA proposes that the level of the 8-hour primary standard,
which was set at 0.075 μmol/mol in the 2008 final rule, should instead
be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to
provide increased protection for children and other ‘‘at risk’’
populations against an array of O
3 - related adverse health effects that range from
decreased lung function and increased respiratory symptoms to serious
indicators of respiratory morbidity including emergency department
visits and hospital admissions for respiratory causes, and possibly
cardiovascular-related morbidity as well as total non- accidental and
cardiopulmonary mortality....[43]
The EPA has developed an
Air Quality Index
(AQI) to help explain air pollution levels to the general public. Under
the current standards, eight-hour average ozone mole fractions of 85 to
104 nmol/mol are described as "unhealthy for sensitive groups," 105
nmol/mol to 124 nmol/mol as "unhealthy," and 125 nmol/mol to 404
nmol/mol as "very unhealthy."
[44]
Ozone can also be present in
indoor air pollution,
partly as a result of electronic equipment such as photocopiers. A
connection has also been known to exist between the increased pollen,
fungal spores, and ozone caused by thunderstorms and hospital admissions
of
asthma sufferers.
[45]
In the
Victorian era,
one British folk myth held that the smell of the sea was caused by
ozone. In fact, the characteristic "smell of the sea" is caused by
dimethyl sulfide a chemical generated by
phytoplankton. Victorian British folk considered the resulting smell "bracing," but in high concentrations,
dimethyl sulfide is actually toxic.
[46]
Heat waves
Ozone production rises during heat waves, because plants absorb less
ozone. It is estimated that curtailed ozone absorption by plants is
responsible for the loss of 460 lives in the UK in the hot summer of
2006.
[47]
A similar investigation to assess the joint effects of ozone and heat
during the European heat waves in 2003, concluded that these appear to
be additive.
[48]
Physiology
Ozone, along with reactive forms of oxygen such as
superoxide,
singlet oxygen,
hydrogen peroxide, and
hypochlorite ions, is naturally produced by
white blood cells and other biological systems (such as the roots of
marigolds)
as a means of destroying foreign bodies. Ozone reacts directly with
organic double bonds. Also, when ozone breaks down to dioxygen it gives
rise to oxygen
free radicals, which are highly reactive and capable of damaging many
organic molecules. Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of
inflammation.
The cause-and-effect relationship of how the ozone is created in the
body and what it does is still under consideration and still subject to
various interpretations, since other body chemical processes can trigger
some of the same reactions. A team headed by Dr. Paul Wentworth Jr. of
the Department of Chemistry at the
Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidation pathway of the human
immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of
trioxidane from water and neutrophil-produced singlet oxygen.
[49]
When inhaled, ozone reacts with compounds lining the lungs to form
specific, cholesterol-derived metabolites that are thought to facilitate
the build-up and pathogenesis of
atherosclerotic plaques (a form of
heart disease).
These metabolites have been confirmed as naturally occurring in human
atherosclerotic arteries and are categorized into a class of secosterols
termed
atheronals, generated by
ozonolysis of cholesterol's double bond to form a 5,6 secosterol
[50] as well as a secondary condensation product via aldolization.
[51]
Ozone has been implicated to have an adverse effect on plant growth:
"... ozone reduced total chlorophylls, carotenoid and carbohydrate
concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC)
content and ethylene production. In treated plants, the ascorbate leaf
pool was decreased, while lipid peroxidation and solute leakage were
significantly higher than in ozone-free controls. The data indicated
that ozone triggered protective mechanisms against oxidative stress in
citrus."
[52]
Safety regulations
Due to the strongly oxidizing properties of ozone, ozone is a primary
irritant, affecting especially the eyes and respiratory systems and can
be hazardous at even low concentrations. The Canadian Center for
Occupation Safety and Health reports that:
"Even very low concentrations of ozone can be harmful to the upper
respiratory tract and the lungs. The severity of injury depends on both
by the concentration of ozone and the duration of exposure. Severe and
permanent lung injury or death could result from even a very short-term
exposure to relatively low concentrations." [53]
To protect workers potentially exposed to ozone,
U.S. Occupational Safety and Health Administration
has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29
CFR 1910.1000 table Z-1), calculated as an 8 hour time weighted average.
Higher concentrations are especially hazardous and
NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol.
[54]
Work environments where ozone is used or where it is likely to be
produced should have adequate ventilation and it is prudent to have a
monitor for ozone that will alarm if the concentration exceeds the OSHA
PEL. Continuous monitors for ozone are available from several suppliers.
Elevated ozone exposure can occur on
passenger aircraft, with levels depending on altitude and atmospheric turbulence.
[55] United States
Federal Aviation Authority regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol.
[56] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure.
[55]
Production
Ozone often forms in nature under conditions where O
2 will not react.
[10] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m
3,
mg/h (milligrams per hour) or weight percent. The regime of applied
concentrations ranges from 1 to 5% in air and from 6 to 14% in oxygen
for older generation methods. New electrolytic methods can achieve up 20
to 30% dissolved ozone concentrations in output water.
Temperature and humidity plays a large role in how much ozone is
being produced using traditional generation methods such as corona
discharge and ultraviolet light. Old generation methods will produce
less than 50% its nominal capacity if operated with humid ambient air
than when it operates in very dry air. New generators using electrolytic
methods can achieve higher purity and dissolution through using water
molecules as the source of ozone production.
Corona discharge method
This is the most common type of ozone generator for most industrial
and personal uses. While variations of the "hot spark" coronal discharge
method of ozone production exist, including medical grade and
industrial grade ozone generators, these units usually work by means of a
corona discharge tube.
[57]
They are typically cost-effective and do not require an oxygen source
other than the ambient air to produce ozone concentrations of 3–6%.
Fluctuations in ambient air, due to weather or other environmental
conditions, cause variability in ozone production. However, they also
produce
nitrogen oxides as a by-product. Use of an
air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. Use of an
oxygen concentrator
can further increase the ozone production and further reduce the risk
of nitric acid formation by removing not only the water vapor, but also
the bulk of the nitrogen.
Ultraviolet light
UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators,
employ a light source that generates a narrow-band ultraviolet light, a
subset of that produced by the Sun. The Sun's UV sustains the ozone
layer in the stratosphere of Earth.
[58]
While standard UV ozone generators tend to be less expensive,
[clarification needed]
they usually produce ozone with a concentration of about 0.5% or lower.
Another disadvantage of this method is that it requires the air
(oxygen) to be exposed to the UV source for a longer amount of time, and
any gas that is not exposed to the UV source will not be treated. This
makes UV generators impractical for use in situations that deal with
rapidly moving air or water streams (in-duct air
sterilization, for example). Production of ozone is one of the
potential dangers of
ultraviolet germicidal irradiation. VUV ozone generators are used in swimming pool and
spa
applications ranging to millions of gallons of water. VUV ozone
generators, unlike corona discharge generators, do not produce harmful
nitrogen by-products and also unlike corona discharge systems, VUV ozone
generators work extremely well in humid air environments. There is also
not normally a need for expensive off-gas mechanisms, and no need for
air driers or oxygen concentrators which require extra costs and
maintenance.
Cold plasma
In the cold plasma method, pure oxygen gas is exposed to a
plasma created by
dielectric barrier discharge. The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.
Cold plasma machines utilize pure oxygen as the input source and
produce a maximum concentration of about 5% ozone. They produce far
greater quantities of ozone in a given space of time compared to
ultraviolet production. However, because cold plasma ozone generators
are very expensive, they are found less frequently than the previous two
types.
The discharges manifest as filamentary transfer of electrons (micro
discharges) in a gap between two electrodes. In order to evenly
distribute the micro discharges, a dielectric
insulator must be used to separate the metallic electrodes and to prevent arcing.
Some cold plasma units also have the capability of producing short-lived allotropes of oxygen which include O
4, O
5, O
6, O
7, etc. These species are even more reactive than ordinary
O
3.
[citation needed]
Electrolytic
Electrolytic ozone generation (EOG) splits water molecules into H
2, O
2, and O
3.
In most EOG methods, the hydrogen gas will be removed to leave oxygen
and ozone as the only reaction products. Therefore, EOG can achieve
higher
dissolution
in water without other competing gases found in corona discharge
method, such as nitrogen gases present in ambient air. This method of
generation can achieve concentrations of 20–30% and is independent of
air quality because water is used as the source material. Production of
ozone electrolytically is typically unfavorable because of the high
overpotential
required to produce ozone as compared to oxygen. This is why ozone is
not produced during typical water electrolysis. However, it is possible
to increase the overpotential of oxygen by careful catalyst selection
such that ozone is preferentially produced under electrolysis. Catalysts
typically chosen for this approach are
lead dioxide[59] or boron-doped diamond.
[60]
Special considerations
Ozone cannot be stored and transported like other industrial gases
(because it quickly decays into diatomic oxygen) and must therefore be
produced on site. Available ozone generators vary in the arrangement and
design of the high-voltage electrodes. At production capacities higher
than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as
ground electrode and assembled with tubular high-voltage electrodes on
the gas-side. The regime of typical gas pressures is around 2
bars (200
kPa) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of
electrical power may be installed in large facilities, applied as one phase AC
current at 50 to 8000 Hz and peak
voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.
The dominating parameter influencing ozone generation efficiency is
the gas temperature, which is controlled by cooling water temperature
and/or gas velocity. The cooler the water, the better the ozone
synthesis. The lower the gas velocity, the higher the concentration (but
the lower the net ozone produced). At typical industrial conditions,
almost 90% of the effective power is dissipated as heat and needs to be
removed by a sufficient cooling water flow.
Because of the high reactivity of ozone, only a few materials may be used like
stainless steel (quality 316L),
titanium,
aluminium (as long as no moisture is present),
glass,
polytetrafluorethylene, or
polyvinylidene fluoride.
Viton
may be used with the restriction of constant mechanical forces and
absence of humidity (humidity limitations apply depending on the
formulation).
Hypalon may be used with the restriction that no water come in contact with it, except for normal atmospheric levels.
Embrittlement
or shrinkage is the common mode of failure of elastomers with exposure
to ozone.
Ozone cracking is the common mode of failure of elastomer
seals like
O-rings.
Silicone rubbers are usually adequate for use as
gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.
Incidental production
Ozone may be formed from
O
2 by electrical discharges and by action of high energy
electromagnetic radiation.
Unsuppressed arcing breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [
O
2 → 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [
O
3].
[61] Certain
electrical equipment generate significant levels of ozone. This is especially true of devices using
high voltages, such as
ionic air purifiers,
laser printers,
photocopiers,
tasers and
arc welders.
Electric motors using
brushes can generate ozone from repeated
sparking
inside the unit. Large motors that use brushes, such as those used by
elevators or hydraulic pumps, will generate more ozone than smaller
motors. Ozone is similarly formed in the
Catatumbo lightning storms phenomenon on the
Catatumbo River in
Venezuela, which helps to replenish ozone in the upper
troposphere. It is the world's largest single natural generator of ozone, lending calls for it to be designated a
UNESCO World Heritage Site.
[62]
Laboratory production
In the laboratory, ozone can be produced by
electrolysis using a
9 volt battery, a pencil graphite rod
cathode, a
platinum wire
anode and a 3
molar sulfuric acid electrolyte.
[63] The
half cell reactions taking place are:
- 3 H2O → O3 + 6 H+ + 6 e− (ΔEo = −1.53 V)
- 6 H+ + 6 e− → 3 H2 (ΔEo = 0 V)
- 2 H2O → O2 + 4 H+ + 4 e− (ΔEo = −1.23 V)
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of
hydrogen. Oxygen formation is a competing reaction.
It can also be "prepared" by
high voltage arc.
This can be done with an apparatus consisting of two concentric glass
tubes sealed together at the top, with in and out spigots at the top and
bottom of the outer tube. The inner core should have a length of metal
foil inserted into it connected to one side of the power source. The
other side of the power source should be connected to another piece of
foil wrapped around the outer tube. Dry
O
2 should be run through the tube in one spigot. As the
O
2 is run through one spigot into the apparatus and high voltage is applied to the foil leads,
electricity will discharge between the dry dioxygen in the middle and form
O
3 and
O
2 out the other spigot. The reaction can be summarized as follows:
[10]
- 3 O
2 — electricity → 2 O
3
Applications
Industry
The largest use of ozone is in the preparation of
pharmaceuticals,
synthetic lubricants, and many other commercially useful
organic compounds, where it is used to sever
carbon-carbon bonds.
[10] It can also be used for
bleaching substances and for killing microorganisms in air and water sources.
[64] Many municipal drinking water systems kill bacteria with ozone instead of the more common
chlorine.
[65] Ozone has a very high
oxidation potential.
[66] Ozone does not form
organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen
bromate
in source water with high bromide concentrations. The Safe Drinking
Water Act mandates that these systems introduce an amount of chlorine to
maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes,
based on results of regular testing. Where
electrical power
is abundant, ozone is a cost-effective method of treating water, since
it is produced on demand and does not require transportation and storage
of hazardous chemicals. Once it has decayed, it leaves no taste or odor
in drinking water.
Although low levels of ozone have been advertised to be of some
disinfectant use in residential homes, the concentration of ozone in dry
air required to have a rapid, substantial effect on airborne pathogens
exceeds safe levels recommended by the U.S.
Occupational Safety and Health Administration and
Environmental Protection Agency.
Humidity control can vastly improve both the killing power of the ozone
and the rate at which it decays back to oxygen (more humidity allows
more effectiveness).
Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations where asthma patients start to have issues.
Industrially, ozone is used to:
- Disinfect laundry in hospitals, food factories, care homes etc.;[67]
- Disinfect water in place of chlorine[10]
- Deodorize air and objects, such as after a fire. This process is extensively used in fabric restoration
- Kill bacteria on food or on contact surfaces;[68]
- Sanitize swimming pools and spas
- Kill insects in stored grain[69]
- Scrub yeast and mold spores from the air in food processing plants;
- Wash fresh fruits and vegetables to kill yeast, mold and bacteria;[68]
- Chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "colour");
- Provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic are removed);
- Manufacture chemical compounds via chemical synthesis[70]
- Clean and bleach fabrics (the former use is utilized in fabric restoration; the latter use is patented);[citation needed]
- Assist in processing plastics to allow adhesion of inks;
- Age rubber samples to determine the useful life of a batch of rubber;
- Eradicate water borne parasites such as Giardia lamblia and Cryptosporidium in surface water treatment plants.
Ozone is a
reagent in many
organic reactions in the laboratory and in industry.
Ozonolysis is the cleavage of an
alkene to
carbonyl compounds.
Many hospitals around the world use large ozone generators to
decontaminate operating rooms between surgeries. The rooms are cleaned
and then sealed airtight before being filled with ozone which
effectively kills or neutralizes all remaining bacteria.
[71]
Ozone is used as an alternative to
chlorine or
chlorine dioxide in the
bleaching of wood pulp.
[72]
It is often used in conjunction with oxygen and hydrogen peroxide to
eliminate the need for chlorine-containing compounds in the manufacture
of high-quality, white
paper.
[73]
Ozone can be used to detoxify
cyanide wastes (for example from
gold and
silver mining) by oxidizing cyanide to
cyanate and eventually to
carbon dioxide.
[74]
Consumers
Devices generating high levels of ozone, some of which use
ionization, are used to sanitize and deodorize uninhabited buildings,
rooms, ductwork, woodsheds, and boats and other vehicles.
One company has been successfully selling a CPAP sanitizer for the
CPAP gear used by sleep apnea patients. This sanitizer works by pumping
high concentration levels of electrically-generated ozone into the
unit's humidification water tank (with or without water in it) and out
through the hose into the mask, which is enclosed and sealed in an
ozone-capturing receptacle (that also contains the ozone generator and
pump that pushes it into the water tank), which completes a closed-loop
system. This closed-loop system prevents the high levels of ozone from
escaping while effectively sanitizing the CPAP equipment, as the CPAP
equipment is prone to developing bacterial infestations and harboring
viruses and other pathogens because of the constant moisture generated
by the CPAP system's humidifier. The sanitizing unit has a two-hour
cycle, it pumps the ozone for 6–10 minutes (user-designated) and then
resting for two hours while maintaining the sealed closed-circuit loop
as the ozone decays back into oxygen and finishes the sanitizing effect.
[75]
In the U.S.,
air purifiers
emitting low levels of ozone have been sold. This kind of air purifier
is sometimes claimed to imitate nature's way of purifying the air
without filters and to sanitize both it and household surfaces. The
United States Environmental Protection Agency
(EPA) has declared that there is "evidence to show that at
concentrations that do not exceed public health standards, ozone is not
effective at removing many odor-causing chemicals" or "viruses,
bacteria, mold, or other biological pollutants." Furthermore, its report
states that "results of some controlled studies show that
concentrations of ozone considerably higher than these [human safety]
standards are possible even when a user follows the manufacturer’s
operating instructions."
[76]
A couple kept repeating health claims for the generator they sold,
without supporting scientific studies. In 1998 a federal jury convicted
them, among others things, of illegally distributing an ozone generator
and of
wire fraud.
[77]
Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the
U.S. Food and Drug Administration (FDA), it is "amending the
food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an
antimicrobial agent on food, including meat and poultry." Studies at
California Polytechnic University
demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered
tapwater can produce a reduction of more than 99.99% in such food-borne
microorganisms as salmonella,
E. coli 0157:H7 and
Campylobacter. This quantity is 20,000 times the
WHO-recommended limits stated above.
[68][78] Ozone can be used to remove
pesticide residues from
fruits and
vegetables.
[79][80]
Ozone is used in homes and
hot tubs
to kill bacteria in the water and to reduce the amount of chlorine or
bromine required by reactivating them to their free state. Since ozone
does not remain in the water long enough, ozone by itself is ineffective
at preventing cross-contamination among bathers and must be used in
conjunction with
halogens. Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water.
[81]
Ozone is also widely used in treatment of water in aquariums and fish
ponds. Its use can minimize bacterial growth, control parasites,
eliminate transmission of some diseases, and reduce or eliminate
"yellowing" of the water. Ozone must not come in contact with fish's
gill structures. Natural salt water (with life forms) provides enough
"instantaneous demand" that controlled amounts of ozone activate bromide
ion to
hypobromous acid,
and the ozone entirely decays in a few seconds to minutes. If oxygen
fed ozone is used, the water will be higher in dissolved oxygen, fish's
gill structures will atrophy and they will become dependent on higher
dissolved oxygen levels.
Aquaculture
Ozonation - a process of infusing water with ozone - can be used in
aquaculture to facilitate organic breakdown. Ozone is also added to
recirculating systems to reduce
nitrite levels
[82] through conversion into
nitrate.
If nitrite levels in the water are high, nitrites will also accumulate
in the blood and tissues of fish, where it interferes with oxygen
transport (it causes oxidation of the heme-group of
haemoglobin from ferrous (
Fe2+) to ferric (
Fe3+), making haemoglobin unable to bind
O
2).
[83]
Despite these apparent positive effects, ozone use in recirculation
systems has been linked to reducing the level of bioavailable iodine in
salt water systems, resulting in iodine deficiency symptoms such as
goitre and decreased growth in Senegalese sole (Solea senegalensis)
larvae.
[84]
Ozonate seawater is used for surface disinfection of
haddock and
Atlantic halibut
eggs against nodavirus. Nodavirus is a lethal and vertically
transmitted virus which causes severe mortality in fish. Haddock eggs
should not be treated with high ozone level as eggs so treated did not
hatch and died after 3–4 days.
[85]
Agriculture
Ozone application on freshly cut pineapple and banana shows increase
in flavonoids and total phenol contents when exposure is up to 20
minutes. Decrease in
ascorbic acid (one form of
vitamin C) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect.
[86] Tomatoes upon treatment with ozone shows an increase in β-carotene, lutein and lycopene.
[87] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content.
[88]
Ozone facilitates the extraction of some heavy metals from soil using
EDTA. EDTA forms strong, water-soluble coordination compounds with some heavy metals (
Pb,
Zn)
thereby making it possible to dissolve them out from contaminated soil.
If contaminated soil is pre-treated with ozone, the extraction efficacy
of
Pb,
Am and
Pu increases by 11.0–28.9%,
[89] 43.5%
[90] and 50.7%
[90] respectively.
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