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Carbon Dioxide Sequestering Chemistry

To fight global warming, try silicon
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Obviously, the more carbon dioxide is taken out of the atmosphere, the less is present to cause Global Warming.

I am not the first to propose some method of getting carbon dioxide out of the atmosphere, and locking it away. Some have literally proposed extracting it from the atmosphere and pumping it down into deep wells, but I have doubts about how long it will stay down there.

I'm going to suggest taking a few hints from Nature, and then expanding upon it. As necessary background information, here is a list of the commonest elements in the Earth's crust:
Oxygen: 46.6% by weight
Silicon - - - 27.7%
Aluminum - - - 8.1%
Iron - - - - - 5.0%
Calcium- - - - 3.6%
Sodium - - - - 2.8%
Potassium- - - 2.6%
Magnesium- - - 2.1%
All others - - 1.5%

The early Earth was well-supplied with carbon dioxide. This caused a lot of global warming, but was also good because the early Sun was 30% dimmer than it is today. Well, as both Sun and Earth aged, various lifeforms made use of the ready availability of carbon dioxide. The stuff naturally dissolves easily in water, and actually seems to undergo a chemical reaction with water, that so far as I know is not well understood. Perhaps it should be researched. The reaction is:
H2O + CO2 -> H2C03, which is "carbonic acid".

The reason I say this reaction is not well understood is because it happens so easily, yet carbon dioxide and water are both molecules that have quite-strong chemical bonds; it takes a fair amount of energy to break those bonds! Most molecules having strong bonds just sit there as a mixture, and don't chemically react without a lot of help (example: nitrogen and hydrogen gasses can combine to make ammonia in an energy-releasing reaction, but this doesn't happens without the assistance of a lot of heat and pressure, to break the N2 and the H2 bonds). So how/why do carbon dioxide and water combine so easily? I don't know if anyone knows the answer to that....

When carbon dioxide is forced under modest pressure into water, quite a lot of it dissolves. Every can of soda pop has this. Carbonic acid provides a tangy taste to soda pop. But to get an idea of just how much carbon dioxide I'm talking about, try opening a transparent bottle of pop, and measuring the liquid level immediately, then letting it sit for a while, let it "go flat" and measuring the liquid level again. If the soda is truly flat, there will be a quite notable difference in the liquid levels.

Well, even without pressurization, a fair amount of carbon dioxide can dissolve in water, partly because of the reaction that makes carbonic acid. That is, the more acid, the less gas AS GAS is in the water, and so it can dissolve more CO2 gas. (An "equilibrium" will be reached; carbonic acid breaks down again just-as-easily into carbon dioxide and water.)

Back to the early Earth. Carbonic acid is a fairly weak acid, but when it reacts with something, it tends to result in an insoluable compound. The compound drops to the bottom of the sea and stays there. Well, the most well-known example involves calcium. Animals the world over found that calcium carbonate was a useful biological building material, many shells and skeletons are chock-full of it. The animals eventually died and their insoluable shells and bones accumulated (coral reefs, for example); limestone deposits across the world today represent untold numbers of such dead animals --and also represents a truly vast sequestering of carbon dioxide from the early Earth's atmosphere!

So. Can we find some way of doing more-of-the same? I have my doubts about sources of calcium today, NOT already tied up calcium carbonate deposits, being as plentiful as they were in the early days of the Earth. Likely we will have to find some other element, to combine with carbonic acid and thereby sequester excess atmospheric carbon dioxide.

And so that's why I presented that list of elements earlier. As it happens, the list doesn't look too promising at first. This is because most of those elements are desired by Civilization in such quantities that it would be difficult to, for example, essentially "throw away iron" to make iron carbonate sequester-deposits. Not to mention that we MAKE a lot of carbon dioxide in the process of producing iron in the first place! It would be simpler to just stop smelting iron ore (than to make iron and then to use it to sequester carbon dioxide)!

Aluminum as a sequestering element has possibilities because it is generally made by electrolysis using electricity from hydroelectric dams, and so we don't add much CO2 to the atmosphere when we make aluminum. But it also is valued too much to throw away in big piles of aluminum carbonate.

Other elements like magnesium and sodium and potassium are often found in salty minerals combined with chlorine. It would be a bad thing to let that chlorine loose, while trying to make sodium or magnesium carbonate (chlorine destroys the ozone layer).

That seems to leave us with silicon. Is there such a thing as silicon carbonate? I had to look it up -- the answer appears to be "yes", although so far in my looking I've only seen very complex molecules in which silicon carbonate formed only a part! (Will have to look in the Handbook of Chemistry and Physics.) At the moment, though, I'd say that Si(CO3)2 appears to be nice candidate for long-term sequestering of carbon dioxide. And not only is silicon so common that the entire silicon-using electronics industry will take millenia to make a noticeable dent in the worldwide supply, but there are possibilites for being sneaky and efficient, about doing the sequestering. See, silicon is commonly found combined with oxygen as silicon dioxide (SiO2), and if we could directly combine that with CO2 the way the gas reacts with water, then we would have this reaction:
SiO2 + 2(CO2) -> Si(CO3)2

If we have to do it the hard way, then we would need to use electrolysis to separate the silicon from the oxygen, and then react that with carbonic acid. Carefully, lest SiO2 form again!!!

Yet there is one other possibility for silicon carbonate, which once again involves taking a cue from Nature. There are life-forms out there (diatoms) that make their skeletons out of silicon dioxide, and not calcium carbonate. That means that Biology has found ways to manipulating individual SiO2 molecules (silicon dioxide is normally a long-long-LONG chain/array, in three dimensions, of connected molecules). Thus we may be able to find and use various genes for, among other things, breaking the silicon-oxygen bond, just as plants everywhere already break the stronger carbon-oxygen bond. (I'm sure the electronics industry would love a less expensive way to obtain silicon.) Then we would seek more genes able to carefully control a reaction between silicon and carbonic acid. The resulting genetically engineered organisms would finally yeild the silicon carbonate that we would eventually want to throw away, probably into the deep ocean, by the millions of tons.

Vernon, Sep 11 2006

Carbonic Acid http://www.newton.d...hem99/chem99661.htm
Some info. [jhomrighaus, Sep 11 2006]


       Pick up a copy of last months Scientific American. The whole issue is devoted to Global warming, and one large section is on CO2 Sequestration.
Galbinus_Caeli, Sep 11 2006

       [phlish], most carbonates are insoluable. This should be the equivalent to dumping plain rocks into the ocean. Perhaps there might be some worry about rising ocean levels from the dumping, instead of from melting ice-caps. In that case we should find uses for the stuff, building-bricks, maybe.
Vernon, Sep 11 2006

       Vernon, change the title, please. "Silicon Carbonate Carbon Sequestering" or something more descriptive and less general. There are many ideas around for CO2 sequestering, but this one is new and different. [+]
baconbrain, Sep 11 2006

       Encouraging plankton to sequester carbon dioxide can hardly be considered a novel idea. Sheesh, what a waste of words!
DrCurry, Sep 11 2006

       I doubt that the plankton which make silica skeletons have anything to do with extant SiO2. No doubt they use dissolved Si that they take from the water and react it with oxygen they generate or take from the water.   

       I liked Poly-CO2 better.
bungston, Sep 11 2006

       I think diatoms and plankton are two very different types of organism. And while I didn't say what sort of organism we might genetically engineer to make silicon carbonate, I was thinking just about any single-celled organism that has chloroplasts could be a workable starting point. See, the chloroplasts are the structures that trap sunlight and use that energy to break both water apart (so the organism gets hydrogen) and carbon dioxide apart (to get carbon). The biology of those chloroplasts we would want to modify to break silicon dioxide apart, instead. (And I already wrote that this should be easier to do, than breaking apart either water or carbon dioxide.)
Vernon, Sep 11 2006

       Im not so impressed with your chemistry, See link to address your considered understanding of Carbonic Acid reactions.   

       You need to do a bit of research on chemical equalibrium at which point you will gain an understanding as to why carbon Sequestration is such a challenging technology to do in a cost effective and permanant way.   

jhomrighaus, Sep 11 2006

       //try silicon// better still, try a silly con
xenzag, Sep 11 2006

       [jhomrighaus], thanks for the link. I admit my chemical knowledge is somewhat out of date (from 1970s), and don't mind a bit now learning, according to link, only "very recently" has the answer been found, with water as a catalyst. Logically, a catalyst that works both ways, not only to decompose carbonic acid, but also to make it from carbon dioxide dissolved in water.   

       Now all we need is an equivalent catalyst to get SiO2 to combine with CO2, heh!
Vernon, Sep 12 2006

       Um.. -small point, but aluminIUM production produces a shiteload of carbon dioxide. Carbon anodes are used, which are consumed (in effect we are using the carbon as a sacrificial anode, and driving the substitution reaction with massive ammounts of current. The oxygen is shifted from the Al2O3 to go to CO2).-   

       oh and what Jhomrighaus said, without the bone. My only real objection is the whole "I don't really understand it, and couldn't easily find a good summary and therefore no-one in science knows" assumption I keep seeing.   

       You certainly do a lot of thinking Vernon, and post some huge essays. Maybe summarise? Like have an executive summary at the beginning?
Custardguts, Sep 13 2006

       [Custardguts], in the USA it is perfectly acceptable to use "aluminum" as the spelling (and associated pronunciation, one less syllable) for the name of the metal that is, in England, spelled/pronounced "aluminium".   

       I knew they used carbon electrodes in making aluminum, but I thought it was because ordinary metal electrodes would either melt or dissolve or be too expensive.   

       The synopsis of this Idea is the subtitle. Most of the posting tries to explain the rationale for it.
Vernon, Sep 13 2006

       Ummm... Why not just break the carbon off of the CO2. You'll then end up with less CO2, and more oxygen, which can be used for many things, from breathing to rocket fuel.   

       The carbon, for it's part can be converted into graphite for lubricating various moving parts, or into diamonds, which are great for grinding, and, being harder than glass, would make an excellent glazing material if we could construct them cheaply enough...   


       Is sequestering CO2 into Silicon going to be cheaper than stripping Carbon off of CO2 particles for some other purpose? Well... considering the fact that the finished product has known uses...
ye_river_xiv, Sep 13 2006

       how about we sequester CO2 with carbon based life forms rather than silicon based lifeforms?   

       Pump the CO2 into green-houses next to coal-fired power plants. Genetically engineer fast growing plants that enjoy high CO2 levels and plant in green-houses. When plants are fully grown, chop down and make into furniture etc.
xaviergisz, Sep 14 2006

       a tangetially related question (which will reveal my ignorance):   

       atmospheric levels of CO2 have fluctuated over the last 800,000 years and this has been correlated with change in the Earth's average temperature (more CO2 = warmer; less CO2 = cooler). but where does the CO2 actually go? e.g. Do forests grow denser/bigger in cooler temperature and suck up more CO2, then release the CO2 in warmer temperatures?
xaviergisz, Sep 14 2006

       Plant matter, animal matter, ocean, peat bogs, ocean floor, rocks. Vernon is talking about speeding up the animal matter, ocean floor, rock cycle.
DrCurry, Sep 14 2006

       [ye river xiv], the main reason the carbon is attached to the oxygen in CO2 is because we burned it to get energy. It would take the same amount of energy to separate the carbon out again.   

       [saviergisz], After plants finish growing, they die and rot, and then the carbon they sequestered is again released (by bacteria) as CO2. The dead plants would have to be gathered up and buried, perhaps with some sort of bacteriacide. Also, remember that those plants will also have sequestered other stuff, such as nutrients from the soil. So if instead we could get a modified plant to separate oxygen from silicon, and to attach carbon dioxide to the silicon (as carbonate), then we only need to extract the silicon carbonate from the dead plants, and dispose of that (and recycle the rest of the plant matter).   

       Regarding where the CO2 went in past glaciation cycles, much of it probably went into the oceans (cold water can absorb more gas than warm water. Note that some of the first glaciations on Earth were global in scope (ice at low altitude at the Equator), and at that time probably a lot of carbon was being sequestered either as calcium carbonate or as coal. Then lots of species died so the rate of biological sequestering went way down, but volcanoes kept (fairly steadily) pumping new CO2 into the atmosphere, starting the next melting cycle.
Vernon, Sep 14 2006

       //Pump the CO2 into green-houses next to coal-fired power plants. Genetically engineer fast growing plants that enjoy high CO2 levels and plant in green-houses. When plants are fully grown, chop down and make into furniture etc.//   

       Yes. The Cannabis plant (AKA Hemp) is great for this purpose.
webfishrune, Sep 14 2006

       //After plants finish growing, they die and rot, and then the carbon they sequestered is again released (by bacteria) as CO2.//   

       True, but the plants offspring then take up CO2 and so balance the cycle. As long as the plants are not wiped out completely (i.e allowed to reproduce without destruction) then a balance is maintained.   

       This is why rainforest destruction is such an issue. As one plant dies, it is replaced by another, keeping balance. If they are all destroyed, the CO2 is released with no corresponding plant to absorb the CO2 again, thus disturbing equilibrium.
webfishrune, Sep 14 2006

       /Likely we will have to find some other element, to combine with carbonic acid and thereby sequester excess atmospheric carbon dioxide./   

       How about sodium? Sodium bicarbonate is friendly stuff, and the oceans are full of sodium that is a difficult waste product of desalinators.
bungston, Sep 21 2006

       [bungston], the main text mentions sodium, but points out that to obtain it, you usually have to separate it from other stuff like chlorine, which is worse to release in to the air, than using sodium to take out carbon dioxide.
Vernon, Sep 22 2006

       The chlorine could be tied up with iron (FeCl2), and sequestered in the same underground repository. I think those two should be stable together.
bungston, Sep 22 2006


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