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As btilly has mentioned a few times, ocean acidification is a major problem because atmospheric CO2 has risen so quickly. It takes a few thousand years for the

As btilly has mentioned a few times, ocean acidification is a major problem because atmospheric CO2 has risen so quickly. It takes a few thousand years for the atmosphere, the upper portion of the ocean, and the deep ocean to reach equilibrium. There is an especially elevated CO2 concentration and corresponding lowered pH in the upper layers because there hasn't been time to reach equilibrium with the larger volumes of water in the deep ocean. These upper layers of the ocean, sadly, are also where the vast majority of ocean life and photosynthetic activity exist. Adding alkalinity to the oceans via accelerated silicate weathering can protect sea life against acidification. More alkalinity in the ocean also increases its absorption capacity for CO2, which reduces warming feedback effects caused by radiative forcing from CO2 in the atmosphere. The ocean contains ~60x as much inorganic carbon as the pre-industrial atmosphere; to draw down the content of the present atmosphere to pre-industrial levels, it would require increasing the dissolved inorganic carbon content of the oceans by less than 1%. Converting some of it to solid carbonate minerals is a nice-to-have but not especially critical, because average residence time of dissolved inorganic carbon is about 200,000 years, which will take us well beyond the age of fossil fuels. https://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf https://www.soest.hawaii.edu/oceanography/faculty/zeebe_file...

DoctorOetker 8y ago
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...
- philipkglass 8y ago
  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.
  - DoctorOetker 8y ago
    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...
    - philipkglass 8y ago
      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. https://www.osti.gov/servlets/purl/1208898 https://www.osti.gov/servlets/purl/1208898 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.

DoctorOetker 8y ago

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...

philipkglass 8y ago
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.
- DoctorOetker 8y ago
  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...
  - philipkglass 8y ago
    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. https://www.osti.gov/servlets/purl/1208898 https://www.osti.gov/servlets/purl/1208898 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.