[palmer]

I would start with the fact that capture of CO2 generally makes no sense. The earth was in CO2 starvation mode which caused (and is also the result of) C4 monocot dominance: corn, sorghum, sugarcane and their natural relatives. The other plants would be outcompeted. They still are, step into the middle of a corn field in full sun and you will measure CO2 100ppm or more lower than average. In other words at the preindustrial 280 or less instead of 400. So I fail to see any reason whatsoever to make that situation worse.

Volcanoes generate a total of about 1/100 of mankind, so they don't really matter. As I have often pointed out here, the CO2 generated by Pinatubo was about 42 Mt or about 1/2 day of manmade emissions. Capture of CO2 from diffuse and dangerous sources like volcanoes is not feasible.

Humans exhale up to 40,000 ppm after breathing in 400ppm so 100 times more than ambient, seemingly a good target for capture, but as you point out, quite impractical. The typical power plant exhaust vent is 100,000 ppm (10% CO2) or more. Obviously that would be the only practical target. Is it practical? Not yet. Does it make sense ever? probably not.

Methane production is probably a good target, but only for sources like landfills, coal mines, and gas wells, not animals or human beings.

[Parmy]

To encapsulate what you are saying, if I understand what you have said, that the whole idea is an exercise in futility and not worth the effort that has been put into it.