I am really grateful for the text you mentioned, but it did not change my opinion on this technology as a carbon removal technology, it only confirmed what I understood. However, it is very well written and I will probably reference others to it whenever I can. The only minor mistake I could detect was a "boron" when clearly carbon was intended. If you have a similar quality text detailing the relevant parameters of adding carbonated rocks to counter the acidification I would be interested.
I was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect...
In my analogy I stand corrected and should have stated that the bottle of water represented the surface seawater, since there is no free fast oceanic mixer. The surface water is equilibrating much faster with atmosphere than the whole water column of the ocean.
You defend the technology by mentioning that the average residence time is about 200.000 years, but that is for all inorganic carbon across the whole depth of the ocean, not just the surface layer! As you say there is not enough mixing, so dissolved inorganic carbon in surface seawater has a much shorter residence time. The long residence time is dominated by the slow movement of deep sea water...
So unless the proposal is augmented with either a huge oceanic mixer, or with dumping the carbonate over the mariana trench (if it dissolves there, it will take a long time before it reaches atmosphere again), lacking these augmentations we are dissolving carbonates in surface seawater, and the carbonate ions can equilibrate back to CO2 so it really is just emissions foisted of as capture...
If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...
The pH shift from adding more alkalinity via silicate weathering shifts the equilibrium to favor more CO2 dissolved as carbonate in the oceans. That is why I do not worry about captured CO2 going back to the atmosphere in less than geological time: the shifted chemical equilibrium will favor more dissolved carbonate.
As a small scale example, consider a beaker of distilled water freely exposed to the atmosphere. It dissolves a small amount of CO2 and becomes slightly acidic from carbonic acid. Now add sodium hydroxide -- it becomes strongly alkaline. Wait again and the pH falls again (though not to its original level), due to dissolving more carbon dioxide, which is deprotonated to form carbonate anions. You can keep adding hydroxide and absorbing more atmospheric CO2 for quite some time, until solubility limits come into play. Even though it is just a solution at equilibrium with the atmosphere, and not a precipitated solid, the sodium carbonate solution will not spontaneously separate back to sodium hydroxide solution in the beaker and CO2 in the surrounding atmosphere. It takes thermodynamic work to reverse the carbonate-heavy equilibrium.
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 was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect...
In my analogy I stand corrected and should have stated that the bottle of water represented the surface seawater, since there is no free fast oceanic mixer. The surface water is equilibrating much faster with atmosphere than the whole water column of the ocean.
You defend the technology by mentioning that the average residence time is about 200.000 years, but that is for all inorganic carbon across the whole depth of the ocean, not just the surface layer! As you say there is not enough mixing, so dissolved inorganic carbon in surface seawater has a much shorter residence time. The long residence time is dominated by the slow movement of deep sea water...
So unless the proposal is augmented with either a huge oceanic mixer, or with dumping the carbonate over the mariana trench (if it dissolves there, it will take a long time before it reaches atmosphere again), lacking these augmentations we are dissolving carbonates in surface seawater, and the carbonate ions can equilibrate back to CO2 so it really is just emissions foisted of as capture...
If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...