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.