Monday, November 21, 2016

Chemistry of sequestration and carbon cycles.

There has been some recent discussion of carbon cycles. ATTP had a good series on ocean CO₂ uptake here. And in the context of sequestration, WUWT reported on some recent experiments with sequestering CO₂ in basic basalt rocks. The discussion showed that there is a lot people don't understand about the basic chemistry driving the carbon cycle, which this chemical sequestration tries to exploit.

Long term cycles and disruption

There is a finite amount of carbon in short term exchange with the atmosphere; it passes through forms where it is reduced by photosynthesis, and re-oxidised quite quickly, as it must be by the ubiquity of oxygen. And there is constant exchange with the upper layers of the ocean. Over millions of years, the amount of such carbon varied, reflected in atmospheric ppm. Carbonate rocks are unstable to heat; CO₂ is emitted by molten rock, most obviously appearing in volcanic eruptions. This would lead to indefinite accumulation, were it not for a process where basic rocks are weathered, exposing surfaces which can convert the CO₂ back to carbonate. This leads to a kind of balance.

Humans are disrupting this by digging and oxidising many gigatons of reduced carbon. The energy that enabled this reduction came from millions of years of photosynthesis and deposition, which prevented re-oxidation. Our burning is running far ahead of the long cycle, so the idea of the basalt absorption is accelerated weathering, probably through fracking etc. I have no strong views on whether this is feasible, but I'd like to talk about the driving chemiatry.

There are three kinds of reaction occurring - redox, acid-base and solubility.

Solubility

This really should go with acid-base, but I'll deal with it first, because there is least to say. In the sea there is a lot of dissolved carbonate in equilibrium with solid calcium carbonate:
Ca⁺⁺+CO₃⁻⁻⇌CaCO₃₍ₛ₎
In the acid-base context, CO₃⁻⁻ is in equilibrium with other more acid species, and so CaCO₃ provides a backup. You can sequester CO₂ this way, but only by first getting it to carbonate, which is the acid-base issue.

Redox

This is the driver - the energy we get from returning reduced C to CO₂ is why it is all being dug up. That oxidation is only reversible by returning the energy. True, plants can do that, and do reduce more carbon each year than we oxidise. That is a long existing cycle, and the reduced products cannot usually last long in the presence of oxygen. If we could sequester the reduced C before it can be reoxidised, that would work. But it would be a huge enterprise to collect and bury all that plant material; much larger than the original task of mining. I have wondered about the feasibility of sinking plant matter in the deep ocean. Environmentally very risky, though.

There is no other chemical way we have of reducing CO₂. The earth's surface has been exposed to oxygen for millions of years, and anything that could reduce CO₂ would long since have reduced oxygen. Carbon is the main reductant available for reducing metals. Even there, we are using the stored energy from ancient photosynthesis. There is no other prevalent source of reductant.

Acid-base

If CO₂ can't be reduced, the other mode of long-term storage (other than as gas) is as calcium carbonate. But that requires reaction with a base, and I want to explore the difficulty of that. Lewis acid theory says that the driver here is the completion of electron shells by sharing electron pairs. Acids want them, bases donate. And you can think of those labile pairs as being conserved, in the sense that you can do global accountancy. When we burn carbon, we create a Lewis acid, CO₂. To form carbonate, each CO₂ needs an electron pair from somewhere, and there aren't large sources of suitable base (stronger than carbonate). Again, any such base on the surface would have long since reacted with CO₂ in the air.

You should try this accountancy whenever you hear of some scheme for absorbing Gtons of CO₂. The lab way, for example is with NaOH. But where does that come from? The ancient method is Gossage's, where the donor electron pairs (Lewis) are created by roasting CaCO₃. But ultimately, that puts one CO₂ in the air for every one absorbed. The modern way is with electrolysis, using a lot of energy. But it also produces HCl as a by-product. Well, you could try to sequester that, but not much easier than CO₂. And otherwise, it's looking for an electron pair, and will ultimately et it, somewhere from a carbonate (the only real source), releasing CO₂. Again no progress.

That is where the basalt comes in. It is a mixture of oxides which do have a significant component of CaO and MgO - ie highly basic, even as silicates. But it has survived exposure to CO₂ basically by being impervious. Weathering counters this, slowly. But we can accelerate the process, as described in the paper cited, basically by creating new surfaces. Whether storing CO₂ in this way as carbonate is competitive with storage as gas, I don't know.



15 comments:

  1. I wouldn't attach too much importance to the chemistry of sequestration. This is really a non-equilibrium diffusional problem. The CO2 will go into the ocean for a time, randomly walking around and then it will randomly walk out. And this is all depending on the temperature.

    You are way too focussed on the details on the chemistry. It's much more important to come up with a diffusional model, unless you are really that interested in determining the pH.

    The analogy is of dopant diffusion into semiconductors. We treat dopants as inert yet they find their way into a solid and remain without truly chemically reacting with the solid. And for CO2 entering the ocean, with the assist of wave action and vertical eddies, we have a really interesting diffusional model that we can explore.

    Advice for when you meet up with the lunatics that will nitpick the chemistry to no end.

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    1. I'm focussing here on the processes that might sequester CO2 as solid. I agree that much of ocean chemistry is transient. The key issues there are dissolution and reformation of CaCO₃, which has lasting effect. But here is is mainly ideas of artificially accelerated CaCO₃ formation, and what is required.

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    2. Yet whatever sequesters as solid also unsequesters at approximately the same rate. The only interesting aspect of this proceeds differentially at a glacial pace. And I am not interested at glacial pace because that is the trick box that the lunatic deniers want you to enter.

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  2. "The only interesting aspect of this proceeds differentially at a glacial pace."
    Respectable scientists are looking at speeding it up (and not the back reaction). A paper I linked describes experiments. I don't know (doubt) whether it is feasible at the necessary scale, but it will have to be considered. So I think the chemistry is important.

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    1. The explanations of the physics work at all scales, from the micro to the macro. However, the solutions do not scale. For example, on the small scale, one would use the process of quenching in place to trap a diffusing material. Unfortunately, that would obviously never work as a feasible engineering solution on a large scale.

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  3. Nick, very nice look at the macro-picture of planetary surface carbon. I agree with @whut that for the next 10 decades the equilibrium shift of ocean diffusion will be the main concerning factor. I am also not sure that we want to sequester the CO2 since it has the benefits of increasing crop yields, greening deserts and perhaps rising winter lows, thus expanding habitable lands. If we can be forward looking for technological solutions I would rather endeavor to create am albedo control knob that we could turn both ways to moderate GMST, either in reflective bio-engineered plankton or low Earth orbit reflective colloidal suspension layer. Considering the precarious current Milankovitch metrics the precautionary principle should include preventing any possible global winter causing event that could resume the normal glacial cycle. Humanity should never allow CO2 to fall below 350ppm until the planet is no longer significant to its domicile.

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    1. Graf, You think anybody pays any attention to your spew?

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    2. Web, I would like to retain basic courtesy here. I don't agree with the post you criticise, but you can ignore it if you wish.

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    3. He's talkin plankton! We might as well invite some krill to comment -- they've got bigger brains.

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    4. One idea would be to engineer plankton with reflective husks, another would be to have them secrete a surfactant that would promote ocean spume (foam) from white caps that has a high reflectivity. My other idea would be to suspend a colloid just below low Earth orbit. Nano-particles stay dispersed when containing a spherically repellent charge. When warmth knob is needed the colloid can be easily disrupted by introducing charged particles that would cause clumping and precipitation.

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    5. Yes, all you need is some protomatter. I don't understand why people are so afraid of the stuff.

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    6. LOL. Why doesn't Graf just use Brawndo? You know it's got electrolytes.

      Typical of these duh-niers is they say that AGW is not happening but then they misdirect with discussions on mitigations, as if we weren't paying attention to what they have said in the past. We're not the ones living out this Trump idiocracy, we're just observing it.

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  4. "protomatter" -- Protomatter is the stuff that was used in the Genesis Device in Star Trek II: The Wrath of Kahn. The Genesis Planet was eventually destroyed because of the unstable nature of Protomatter.

    I loved the original Star Trek as a child but had to look up this later genre reference. It's interesting to realize that activists climate scientists have more in common with Amish farmers then future-looking intellectuals. The Brexit vote and US elections show that a growing number of people are looking for solvers rather than whiners and blamers. Progress is not always a straight line but it always has only one direction.

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    1. No ethical scientist uses protomatter

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  5. I didn't grow up around Amish farmers; I grew up around Hutterites. My Dad used to control thousands of acres acres of ranch land. Most of it is now part of a flourishing Hutterite Colony. They're very productive farmers and ranchers. My father was a professional who worked for farmers and ranchers of all kinds. In the early 1960s he concluded the Hutterites were going to be winners because of their model.

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