Sure but in your example you are adding sodiumhydroxide.
I agree that after adding say CaCO3 to distilled water containing inorganic carbon (CO2, carbonate ions, ...) the carbon content will have increased after equilibrating with the atmosphere, but not with the claim that the eventual carbon content of the water will be the sum of the original carbon content plus added CaCO3 carbon content... some undisclosed part of the added carbon content will be released as CO2 to atmosphere...
The pH will change only very slightly if you add CaCO3 to distilled water, because CaCO3 is very poorly soluble. I also agree that adding CaCO3 to seawater would not sequester carbon dioxide. But releasing basic metal cations via weathering silicates like olivine will sequester carbon dioxide. The difference is that the starting olivine does not contain carbonate, whereas in your example there is already carbonate in the starting CaCO3.
Schematically:
A) H2O + CO2 <=> H2CO3
Equilibrium favors left hand side, but water exposed to atmosphere becomes slightly acidic from right hand side.
B) Mg2SiO4 + 2 H2CO3 => 2 MgCO3 + SiO2 + 2H2O
Equilibrium strongly favors the right hand side. But the reaction is strongly kinetically hindered with naturally occurring large lumps of rock. This is why it will take a very long time for natural silicate weathering processes to absorb the extra CO2 that humans have recently added to the atmosphere.
C) CaCO3 + H2CO3 <=> 2 CaHCO3
Equilibrium favors left hand side, but limestone can be solubilized from right hand side reaction at a low rate (or faster in presence of high CO2/water concentration).
Note that the metal in the silicate of the left hand side of B can be various alkali and alkaline earth metals, but magnesium dominates in olivine.
EDIT: "CO2 Mineral Sequestration Studies in US" by Golberg et al appears to be the best reference to the thermodynamic and kinetic aspects of magnesium silicate weathering that I can easily find outside of a paywall.
This paper is focusing on a different way to accelerate weathering: apply wet, concentrated, hot CO2 to crushed silicates. The olivine-crushing proposal discussed here on HN takes a different approach to accelerated weathering: crush and disperse larger quantities of silicates, but do not try to heat or pre-concentrate the CO2. Just let the ambient conditions of the atmosphere and oceans work on crushed rock (this is still far faster than natural weathering).
The key takeaway from this paper is on pages 3 and 4: magnesium silicate carbonation is exothermic (thermodynamically favored). Once magnesium silicate reacts with CO2, it would take more energy to undo the reaction and put that CO2 back in the atmosphere.
Following back on your comments from the Mars colony thread...
Olivine weathering is so energetically favorable from that paper that, if you put enough of it into a sphere, and feed it enough pure CO2, it's actually a usable thermal energy source.
You can "burn" it like coal, except that it "burns" CO2 instead of oxygen.
To relate back to Mars, you can probably do similarly absurd things with the perchlorates in the soil there. You can "burn" perchlorates in a reducing atmosphere of e.g. methane from the sabatier process, and end up with salt and an explosion.
You can "burn" it like coal, except that it "burns" CO2 instead of oxygen.
That is a bit optimistic :-)
The potential energy per gram of mass is much lower than for coal burning in Earth's atmosphere -- worse, the kinetics are so sluggish that you would need a very large vessel with good insulation to build up a useful temperature differential.
You'd also need to concentrate perchlorates from the Martian soil before they would sustain combustion with methane. Assuming that was done, though, perchlorates plus hydrocarbons will combust with vigor.
I think the analysis I saw was that it's energetic enough that the entire mining + grinding + "burning" process is energetically favorable. Which I found pretty astounding, but I think that points more to the incredible efficiency of mining and industrial processes than anything.
I think that was also at elevated temperature in a carbonic acid solution, so basically the fastest possible "weathering".
Do you know where I can find reaction rate constants? I tried the NIST reaction kinetics database, but H2O + CO2 -> H2CO3 is not even listed... I have implemented chemical reaction simulations before (gillespie and normal differential equations), the hard part is not the theory of simulating reactions but knowing how to determine the needed reaction rates for small inorganic reactions...
I read the paper you referenced, but it does not really add much? The key takeaway you refer to is probably the exothermic reaction enthalpy... we were discussing equilibria before this, so while a profound one, it is still a plattitude to point just at the exothermic nature as if at equilibrium all matter will be in the lowest energy state. It's still ~300K out there...
Somewhat less of a plattitude is to look at such a reaction and pretend we have a 2 level system (i.e. no other reactions occuring, no substep reactions). Let's take reaction number 2 on page 4 you mention:
So the right hand side does indeed look very much preferred
But this calculation assumes not dissolving in water.
This paper does not propose dissolving the resulting mineral carbonate in water, they propose burying it in the same mine the igneous rock was found!
I am still worried that simply dissolving it in surface water of the oceans means the CO2 can be released, or at the very least the CO2 in one of the dissolved species CO2, HCO3- or CO3(2-) are too bio-available... this may sound good, but if it is captured back into the biosphere it will be exhaled again by the organism (or its predator) pretty soon... grass clippings can be considered carbon sequestration, until you feed it to the organisms in your composting heap!
I would love to see numerical simulations of the chemical reactions, it would help sway those of us who understand how to simulate a set of reactions but have insufficient domain knowledge to know which reactions should be kept in mind.
The different competing entities that wish to get sponsored for such activities have a common interest to produce such a model or at least a list of relevant chemical reactions in the ocean and their kinetic rate constants. They could pool their resources to build this model.
I see that I could have skipped some of my previous explaining :-)
If you are interested in modeling rate constants and mechanisms, the most interesting work I have come across is the Reaction Mechanism Generator developed at MIT and Northeastern University:
As you may be aware, determining rate constants from calculations is quite difficult even for gas-phase reactions. It's much harder for condensed-phase reactions. I do not have any hope of applying these techniques to olivine weathering at present. There have been quite a few small scale laboratory experiments on olivine weathering. There will be more factors at work in a real near-shore environment: abrasion by sand and wave action, biological activity, varying temperatures depending on the locale. I think that questions of rates need to be answered by field trials now; theory is inadequate and small lab experiments have already been done. But I still contend that this is not "simply dissolving" CO2 in ocean surface waters -- it is an acid-base reaction, with magnesium providing alkalinity.
I agree that after adding say CaCO3 to distilled water containing inorganic carbon (CO2, carbonate ions, ...) the carbon content will have increased after equilibrating with the atmosphere, but not with the claim that the eventual carbon content of the water will be the sum of the original carbon content plus added CaCO3 carbon content... some undisclosed part of the added carbon content will be released as CO2 to atmosphere...