Carbon dioxide

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Carbon dioxide
Other names	Carbonic acid gas, Carbonic anhydride, dry ice (solid)
Molecular formula	CO2
Molar mass	44.0095(14) g/mol
Solid state	Dry ice, carbonia
Appearance	colorless gas
CAS number	[124-38-9]
Properties
Density and phase	1600 kg/m³, solid approx. 1.98 kg/m³, gas at STP
Solubility in water	1.45 kg/m³
Latent heat of sublimation	25.13 kJ/mol
Melting point	−57 °C (216 K), pressurized
Boiling point	−78 °C (195 K), sublimes
Acidity (pKa)	6.35 and 10.33
Viscosity	0.07 cP at −78 °C
Structure
Molecular shape	linear
Crystal structure	quartz-like
Dipole moment	zero
Hazards
MSDS	External MSDS
Main hazards	asphyxiant, irritant
NFPA 704	0 0 0   (liquid)
R-phrases	R: As, Fb
S-phrases	S9, S23, S36(liquid)
RTECS number	FF6400000
Supplementary data page
Structure & properties	n, εr, etc.
Spectral data	UV, IR, NMR, MS
Related compounds
Related oxides	carbon monoxide carbon suboxide dicarbon monoxide carbon trioxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. It is often referred to by its formula CO₂. It is present in the Earth's atmosphere at a low concentration of approximately 0.03%-0.06% and is an important greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle.

[edit] Origins

Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, and the respiration processes of living aerobic organisms; man-made sources of carbon dioxide come mainly from the burning of fossil fuels for heating, power generation and transport. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to carbohydrates during a process called photosynthesis. They produce the energy needed for this reaction through the photolysis of water. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms, forming a cycle.

[edit] Chemical and physical properties

Carbon dioxide is a colorless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. One may notice this sensation if one attempts to stifle a burp after drinking a carbonated beverage.

Its density at standard temperature and pressure is around 1.98 kg/m³, about 1.53 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole. As it is fully oxidized, it is not very reactive and is non-flammable.

Carbon dioxide pressure-temperature phase diagram

At temperatures below −78 °C, carbon dioxide changes directly from a gas to a white solid called dry ice through a process called deposition. Liquid carbon dioxide forms only at pressures above 5.1 atm; at atmospheric pressure, it passes directly between the solid phase and the gaseous phase in a process called sublimation.

[edit] Synthesis and chemistry

Carbon dioxide may be obtained from air distillation, however this yields only very small quantities of CO₂. A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. As an example, the reaction between sulfuric acid and calcium carbonate (limestone or chalk) is depicted below:

H₂SO₄ + CaCO₃ → CaSO₄ + H₂CO₃

The H₂CO₃ then decomposes to water and CO₂. Such reactions are accompanied by foaming and/or bubbling. In industry such reactions are widespread because it can be used to neutralize waste acid streams.

The production of quicklime (CaO) a chemical that has widespread use, from limestone by heating at about 850 ^oC also produces CO₂:

CaCO₃ → CaO + CO₂

The combustion of all carbon containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), but also of coal and wood, will yield carbon dioxide, and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.

CH₄ + 2 O₂ → CO₂ + 2 H₂O

Carbon dioxide can be used in chemistry to create a carboxylic acid from a Grignard reagent.

R-MgX + CO₂ → R-COOH

Yeast produces carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits:

Glucose → 2 CO₂ + 2 C₂H₅OH

All aerobic organisms produce CO₂ when they burn carbohydrates, fatty acids and proteins; it is the prime energy source and the main metabolic pathway in heterotroph organisms such as animals, and also a secondary energy source in phototroph organisms such as plants when not enough light is available for photosynthesis. The large amount of reactions involved are exceedingly complex and not described easily. Photoautotrophs (i.e. plants, cyanobacteria) utilize another modus operandi: They absorb the CO₂ from the air, and, together with water, react it to form carbohydrates:

nCO₂ + nH₂O → (CH₂O)_n + nO₂

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO₂ and H₂CO₃ (carbonic acid). The relative concentrations of CO₂, H₂CO₃, and the deprotonated forms HCO₃^- (bicarbonate) and CO₃^2-(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

[edit] Industrial production

Carbon dioxide is manufactured mainly from six processes:^[1]

1. As a byproduct in ammonia and hydrogen plants, where methane is converted to CO₂;
2. From combustion of carbonaceous fuels;
3. As a byproduct of fermentation;
4. From thermal decomposition of CaCO₃;
5. As a byproduct of sodium phosphate manufacture;
6. Directly from natural carbon dioxide gas wells.

[edit] Dry Ice

Dry ice pellets subliming in air.

Solid carbon dioxide, often known by the genericized trademark "dry ice", is a versatile cooling agent. Unlike water ice at atmospheric pressure it sublimes, changing from a solid directly to a gas. Its sublimation point is -78.5°C (-109.3ºF). A combination of its low temperature, solid phase and direct sublimation to gas makes it a simple and effective coolant. Dry ice is also inexpensive; it costs about US$2 per Kg (US$1 per lb).

[edit] History

Dry ice was first observed in 1825 by the French chemist Charles Thilorier. Upon opening the lid of a large cylinder of liquid carbon dioxide he noted much of the carbon dioxide rapidly evaporated leaving solid dry ice in container. Throughout the next 60 years, dry ice was observed and tested by many scientists.

[edit] Production

Dry ice is readily manufactured;

1. Carbon dioxide is obtained in any of the ways listed above.
2. It is pressurized and refrigerated, until it changes into its liquid form.
3. The pressure is reduced. When this occurs some liquid carbon dioxide vaporises, and this causes a rapid lowering of temperature of the remaining liquid carbon dioxide. The extreme cold makes the liquid solidify into a snow-like consistency.
4. The snow-like solid carbon dioxide is compressed into either small pellets or larger blocks of dry ice.

Dry ice is typically produced in two standard sizes; solid blocks and cylindrical pellets. A standard block is most common and will normally weigh about 30 kg (60 lb). These are largely used in the shipping industry because they sublime slowly due to a relatively small surface area. The pellets are around 1 cm (½ inch) in diameter and can be bagged easily. This form of dry ice is more suited to small scale use, for example at grocery stores and laboratories.

[edit] Safety

Dry ice can be a dangerous substance. It must be handled using protective insulated gloves. Direct contact with the skin can freeze it in seconds, causing a burn-like injury. Dry ice must not be stored in a sealed container, since its sublimation produces massive volumes of gaseous carbon dioxide. A sealed container can fail explosively from the pressure, sufficient to cause shrapnel injuries and hearing loss. Furthermore, dry ice should never be stored in a standard freezer or refrigerator. The dry ice is so cold that it can freeze and disable the thermostat of the unit.^{[citation needed]} It can also cause problems through thermal contraction. Dry ice should never be left on brittle surfaces or in glass containers. The contraction caused by cooling can result in cracking.

[edit] Uses

Aside from obvious uses in cooling and shipping, dry ice has many other applications:

• In medicine, dry ice can freeze warts and other similar skin conditions, easing their removal.
• Dry ice blast cleaning, see below.
• Removal of floor tiles - the low temperature makes the tiles shrink and crack. This will loosen them so that they can be removed.
• Carbonation of water (and other liquids). This is widely used in the carbonated drinks industry.
• Reduces the effect of dents on cars, the temperature puts pressure on the dent to align with the rest of the metal.
• Dry ice can repel mosquitoes and other insects due to its low temperature.
• Rapid sublimation of dry ice caused by putting it in water produces a dense fog of water vapour. This is a popular dramatic effect, common as a stage effect or in recreation at Halloween. The fog produced, being largely dense carbon dioxide, sinks to the floor.

[edit] Dry ice blast cleaning

One of the most important alternative uses of dry ice around the world is dry ice blast cleaning. Dry ice pellets are shot out of a jet nozzle with compressed air. This can remove residues from industrial equipment, for example ink, glue, oil, paint, mould and rubber, replacing sandblasting, steam blasting, water blasting or other (potentially environmentally damaging) solvent blasting.

Dry ice blasting involves three factors:

1. kinetic energy
2. thermal shock
3. thermal kinetic energy.

The kinetic energy of the dry ice pellets is transferred when it hits the surface, directly dislodging residues, as in other blasting methods. The thermal shock effect occurs when the cold dry ice hits a much warmer surface and rapid sublimation occurs. The thermal kinetic effect is the result of the rapid sublimation of the dry ice hitting the surface. These factors combine cause small "micro-explosions" of gaseous carbon dioxide where each pellet of dry ice impacts, dislodging the residue.

[edit] Solid amorphous CO₂

An alternative form of solid carbon dioxide, an amorphous glass-like form, is possible, although not at atmospheric pressure.^[2] This form of glass, called carbonia, was produced by supercooling heated CO₂ at extreme pressure (40 – 48 GPa or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium. Unlike silica and germanium oxide glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

[edit] Biology

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

[edit] Human physiology

See also: Arterial blood gas

CO₂ is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood.)

• Most of it (about 80% – 90%) is converted to bicarbonate ions HCO₃⁻ by the enzyme carbonic anhydrase in the red blood cells.^{[citation needed]}
• 5% – 10% is dissolved in the plasma^{[citation needed]}
• 5% – 10% is bound to hemoglobin as carbamino compounds.^{[citation needed]}

The CO₂ bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO₂ does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.

Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO₂ or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect.

Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue.^{[citation needed]}

Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO₂ in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.

It is interesting to note that although it is oxygen that the body requires for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (e.g., pure nitrogen) leads to loss of consciousness without subjective breathing problems. This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others — otherwise one risks going unconscious without being aware of the imminent peril.

According to a study by the USDA,^[3] an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.

[edit] Use in plants

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration. Carbon dioxide gas must be introduced into greenhouses to maintain plant growth, as even in vented greenhouses the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm, at which level photosynthesis is significantly retarded. Venting can help offset the drop in carbon dioxide, but will never raise it back to ambient levels of 340ppm. Carbon dioxide supplementation is the only known method to overcome this deficiency. Direct introduction of pure carbon dioxide is ideal, but rarely done because of cost constraints. Most greenhouses burn methane or propane to supply the additional CO₂, but care must be taken to have a clean burning system as increased levels of NO₂ result in reduced plant growth. Sensors for SO₂ and NO₂ are expensive and difficult to maintain, accordingly most systems come with a carbon monoxide (CO) sensor under the assumption that high levels of carbon monoxide mean that significant amounts of NO₂ are being produced. Plants can potentially grow up to 50 percent faster in concentrations of 1000ppm CO₂ when compared with ambient conditions.^[4]

Plants also emit CO₂ during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tonnes of CO₂ each year, however a mature forest will produce as much CO₂ from respiration and decomposition of dead specimens (e.g. fallen branches) as used in biosynthesis in growing plants.^{[citation needed]} Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere.

[edit] Pollution and toxicity

Carbon dioxide content in fresh air varies and is between 0.03% (300 ppm) and 0.06% (600 ppm), depending on location and in exhaled air approximately 4.5%. When inhaled in high concentrations (greater than 5% by volume), it is immediately dangerous to the life and health of humans and other animals. The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an 8-hour work day is 0.5% (5000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues would be significantly less.

These figures are valid for carbon dioxide supplied in "pure" form. In indoor spaces occupied by humans the carbon dioxide concentration will also reach a level higher than in pure outdoor air. Concentrations higher than 1000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO₂ concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO₂ itself. At 2000 ppm will the majority of occupants feel a significant degree of discomfort, and many will develop nausea and headache. The CO₂ concentration between 300 and 2500 ppm is used as an indicator of indoor air quality in spaces polluted by human occupation.

Acute carbon dioxide toxicity is sometimes known as Choke damp, an old mining industry term, and was the cause of death at Lake Nyos in Cameroon, where an upwelling of CO₂-laden lake water in 1986 covered a wide area in a blanket of the gas, killing nearly 2000. The lowering of carbon dioxide in the atmosphere is largely due to absorption by plants, which convert it to sugars through photosynthesis. Phytoplankton photosynthesis absorbs dissolved CO₂ in the upper ocean and thereby promotes the absorption of CO₂ from the atmosphere.^[5]

Carbon dioxide is a surrogate for indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor carbon dioxide must be reduced to below 600 ppm. NIOSH considers that indoor air concentrations of carbon dioxide that exceed 1000 ppm are a marker suggesting inadequate ventilation (1,000 ppm equals 0.1%). ASHRAE recommends that CO₂ levels not exceed 1000 ppm inside a space. OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 3% and considers concentrations exceeding 4% as "immediately dangerous to life and health." People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7 – 10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.

[edit] Atmospheric concentration

Atmospheric CO₂ concentrations measured at Mauna Loa Observatory.

It has been suggested that this section be split into a new article entitled Atmospheric carbon dioxide. (Discuss)

As of January 2007, the earth's atmospheric CO₂ concentration is about 0.0383% by volume (383 ppmv) or 0.0582% by weight.^[6] This represents about 2.996×10¹² tonnes, and is estimated to be 105 ppm (37.77%) above the pre-industrial average.^[7]

Because of the greater land area, and therefore greater plant life, in the northern hemisphere as compared with the southern hemisphere, there is an annual fluctuation of up to 6 ppmv (± 3 ppmv), peaking in May and reaching a minimum in October at the end of the northern hemisphere growing season, when the quantity of biomass on the planet is greatest.^{[citation needed]}

Despite its small concentration, CO₂ is a very important component of Earth's atmosphere, because it absorbs infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode) and enhances the greenhouse effect.^{[citation needed]} See also "Carbon dioxide equivalent".

The three vibrational modes of carbon dioxide: (a) symmetric, (b) asymmetric stretching; (c) bending. In (a), there is no change in dipole moment, thus interaction with photons is impossible, while in (b) and (c) there is optical activity.

The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity; this was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year.^[8] Volcanic releases are about 1% of the amount which is released by human activities.

Global fossil carbon emissions 1800 – 2000.

Since the start of the Industrial Revolution, the atmospheric CO₂ concentration has increased by approximately 110 µL/L or about 40%, most of it released since 1945. Monthly measurements taken at Mauna Loa^[9] since 1958 show an increase from 316 µL/L in that year to 376 µL/L in 2003, an overall increase of 60 µL/L during the 44-year history of the measurements. Burning fossil fuels such as coal and petroleum is the leading cause of increased man-made CO₂; deforestation is the second major cause. Around 24 billion tonnes of CO₂ are released from fossil fuels per year worldwide, equivalent to about 6 billion tonnes of carbon. (See List of countries by carbon dioxide emissions.)

Smoke and ozone pollution from Indonesian fires, 1997.

In 1997, Indonesian peat fires may have released 13% – 40% as much carbon as fossil fuel burning does.^[10][11] Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO₂ remains in the atmosphere; some is absorbed in the oceans or biosphere. The ratio of the emitted CO₂ to the increase in atmospheric CO₂ is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically 57% over longer (5 year) periods.

Increased amounts of CO₂ in the atmosphere tend to enhance the greenhouse effect and thus contribute to global warming. The effect of combustion-produced carbon dioxide on climate is called the Callendar effect.

[edit] Variation in the past

CO₂ concentrations over the last 400,000 years

The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO₂ levels were about 260 – 280µL/L immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.

The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years before the present.^[12] During this time, the atmospheric carbon dioxide concentration has varied between 180 – 210 µL/L during ice ages, increasing to 280 – 300 µL/L during warmer interglacials.^[13] The data can be accessed here.

Some studies have disputed the claim of stable CO₂ levels during the present interglacial (the last 10 kyr). Based on an analysis of fossil leaves, Wagner et al.^[14] argued that CO₂ levels during the period 7 – 10 kyr ago were significantly higher (~300 µL/L) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO₂.^[15] Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO₂ values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.^[16]) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO₂ measurements.

Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The recent period is located on the left-hand side of the plot, and it appears that much of the last 550 million years has experienced carbon dioxide concentrations significantly higher than the present day.

On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO₂ concentrations (>3,000 µL/L) between 600 and 400 Myr BP and between 200 and 150 Myr BP.^[17] On long timescales, atmospheric CO₂ content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO₂. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO₂ response to emissions over the next hundred years. In more recent times, atmospheric CO₂ concentration continued to fall after about 60 Myr BP, and there is geochemical evidence that concentrations were <300 µL/L by about 20 Myr BP. Low CO₂ concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Myr BP. Present carbon dioxide levels are likely higher now than at any time during the past 20 million years.^[18] During this period however atmospheric CO₂ concentration has been lower than in preceding history.

[edit] Capturing/Extracting CO₂

Methods of CO₂ extraction/separation include:

1. Aqueous solutions
   • Amine extraction
   • High pH solutions
2.  :For example, Carbon dioxide reacts with dissolved CaO, to form Calcite (CaCO₃)^[19]
3. Adsorption
   • Molecular Sieves
   • Activated Carbon^[20][21]
   • Metal-organic frameworks(MOF's)^[22]
4. Solid reactants
   • Serpentine, Olivine, Quicklime^[23][24]
5. Membrane gas separation^[25][26]
6. Regenerative Carbon Dioxide Removal System (RCRS)
7.  :The RCRS on the space shuttle Orbiter uses a two-bed system that provides continuous removal of CO₂ without expendable products. Regenerable systems allow a shuttle mission a longer stay in space without having to replenish its sorbent canisters. Older lithium hydroxide (LiOH)-based systems, which are non-regenerable, are being replaced by regenerable metal-oxide-based systems. A metal-oxide-based system primarily consists of a metal oxide sorbent canister and a regenerator assembly. This system works by removing carbon dioxide using a sorbent material and then regenerating the sorbent material. The metal-oxide sorbent is regenerated by pumping air heated to around 200 °C at 7.5 standard cubic feet per minute through its canister for 10 hours.^[27]
8. Algae Bioreactor Technology
9.  :Originally developed at MIT using power plant flue gas to support bio diesel feed stock, they use algae to process out the C02. Commercial studies have been performed on over 2000 MW of power plants in the United States since 2001. As of March 2007, this is the only commercially installed technology for C02 mitigation on active power plants. The largest test site for an Algae bioreactor system is connected directly to smokestack of Arizona Public Service Redhawk 1,040 megawatt power plant, producing renewable biofuels as a process by product. At commercial scale, this organic process holds the potential to “scrub’ C02 without the considerable solid and fluid waste issues associated with other technologies^[28]
10. Underground geological storage.

[edit] Oceans

Air-sea exchange of CO₂

The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions — much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:

CaCO₃ + CO₂ + H₂O ⇌ Ca²⁺ + 2 HCO₃^-

Reactions like this tend to buffer changes in atmospheric CO₂. However, since it produces an acidic compound, the pH of sea water is thought to go down with increasing carbon dioxide levels. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO₂. Over hundreds of millions of years this has produced huge quantities of carbonate rocks.

The vast majority of CO₂ added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.

As the oceans warm, carbon dioxide solubility in the surface waters decreases markedly. However, the overall system is quite complex, as indicated above, and further details may be found in the article on the carbon solubility pump.

[edit] History

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre).

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he termed "fixed air." He observed that the fixed air was denser than air and did not support either flame or animal life. He also found that it would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) onto chalk in order to produce carbon dioxide and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.^[29]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.^[30] The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO₂.

[edit] See also

• Air pollution
• Amorphous carbonia
• Bosch reaction
• Carbon audit regime
• Carbon capture and storage
• Carbon dioxide (data page)
• Carbon dioxide sink
• Carbon offset
• Cellular respiration
• CO₂ sensor
• CO₂ dragster
• Emission standards
• Greenhouse gas
• List of countries by carbon dioxide emissions per capita
• Low-carbon economy
• Massachusetts v. Environmental Protection Agency
• Natural gas
• Ocean acidification
• Paintball
• Sabatier process
• Dry ice bomb

[edit] References

[edit] External links

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Wikimedia Commons has media related to:

Carbon dioxide

• Trends in Atmospheric Carbon Dioxide (NOAA)
• Carbon cycle science (NOAA)
• International Chemical Safety Card 0021
• CID 280 from PubChem
• Dry Ice information
• CO₂ as a natural refrigerant - CO₂ technology
• Bassam Z. Shakhashiri: Chemical of the Week: Carbon Dioxide
• Keeling, C.D. and T.P. Whorf: Atmospheric carbon dioxide record from Mauna Loa, 2002
• Mauna Loa 2004 update
• CO₂ Carbon Dioxide Properties, Uses, Applications
• Flash animation at McGraw-Hill biochemistry-Oxygen%20Carbon%20Dioxide
• Computational Chemistry Wiki
• Pressure-Temperature phase diagram for carbon dioxide
• North American Carbon Program
• Molview from bluerhinos.co.uk See Carbon dioxide in 3D
• Sharp rise in CO₂ levels recorded - 381ppm
• Algae-Like a Breath Mint for Smokestacks dicusses the use of waste CO₂ from power plants to grow oil-rich algae which can be processed to obtain biodiesel
• Carbon Dioxide Production Detailed description of environmental production of CO₂ and its effects on Earth
• Carbon dioxide glass created in the lab - New Scientist
• 12,000 CO₂ emitting facilities (EU25) interactive map
• CO2 Wells outside of Jackson Mississippi interactive map of CO2 wells]
• CO2 Calculator.
• Carbon Dioxide is Good for the Environment paper by The National Center for Public Policy Research
• Environmental Effects of Increased Atmospheric Carbon Dioxide paper by The Oregon Institute of Science and Medicine

• Links to external chemical sources