[philipkglass]

Accelerated silicate weathering seems like the best bet. Naturally occurring silicate compounds of magnesium and calcium react with ambient CO2 in the presence of water to form stable carbonates and silicon dioxide:

CaSiO3 + CO2 -> CaCaO3 + SiO2

The thermodynamics are much more favorable than "combustion-in-reverse" (trying to reform hydrocarbons from CO2). The products are stable and the process can deal with point CO2 sources, distributed sources, atmospheric CO2, and ocean acidification -- the whole shebang.

But the natural kinetics of silicate weathering are very slow. Kinetics can be improved by orders of magnitude if you grind the silicate minerals to dust; it's a surface area limited reaction. Kinetics get better yet if you apply the mafic rock dust to e.g. acid sulfate soils used for agriculture in the tropics. Those soils already need a pH boost to avoid crop stunting by soluble aluminum. Doing the adjustment with silicates instead of limestone means a two-for-one benefit (crop productivity plus drawing down CO2). But I think that to see widespread adoption there will have to be some targeted donor aid to poorer countries; ground limestone is cheaper than ground silicates if you don't care about CO2 abatement, and the rich world's farmers don't have a lot of acid sulfate soils in need of treatment within their home countries.

[protomyth]

That sounds really interesting. If a government said "yep, we are doing that" what is the mechanism they would use?

[philipkglass]

I'm not an expert, but here are some ideas:

0) Donor government identifies a country where acid soils are already cultivated for agriculture. The recipient country should also be reasonably stable so you don't have to plan around armed conflict. Palm oil production in Malaysia looks like a good candidate. It's important to get buy-in from the (potential) recipient country's government too.

1) Within the chosen country, identify a region that has a lot of acid soil cultivation and that has easily accessible deposits of ultramafic rocks nearby. (Shouldn't be too hard, as such rocks are very common.)

2) Provide electrically powered crushers and/or grinders for size reduction, plus power sources if remote from the grid (ideally solar, maybe diesel generators as are common for mining sites; even if it's fossil powered the net CO2 is deeply negative). There may be some necessary R&D optimizing said crushers for low-maintenance conditions, since ultramafic rocks are among the hardest and toughest.

3) Pay locals to run the crushers and transport discounted-or-free rock dust to farmers in place of limestone. There might be some gaming of the system if e.g. drivers are really just dumping rock dust on the nearest empty land instead of finding farmers to take it, but that's tolerable. The project is already 75% done if you just get the rock crushed to dust in the first place. So key metric is ensuring that the crushers are still operable and are actually being used to crush rocks as intended.

Another idea, after I've written this out: there is less but still some acid soil under cultivation in the richer nations of the world, e.g. parts of Australia. Might want to try it first in the developed world where monitoring, language barriers, and infrastructure are all less challenging.

EDIT: a somewhat less effective but still potentially interesting idea is to put the ground rock in near-shore ocean environments where wave action will help abrade/weather the particles. Probably not as good as acidic warm soil in the tropics, but still a speedup over the natural way.