Line 35 Use of CCS as the overarching term for point-source CO2 mitigation is inappropriate because CCS has come to mean a very specific form of that mitigation https://en.wikipedia.org/wiki/Carbon_capture_and_storage

Line 79-81 “The concept of AWL was first proposed by Rau and Caldeira more than two decades ago (Rau and Caldeira, 1999). It provides a geochemistry-based method for CCS in which the dissolution of carbonate minerals is artificially enhanced (Rau and Caldeira, 1999).”

You mean -

The concept of AWL was first proposed more than two decades ago by Rau and Caldeira (1999). It provides a geochemistry-based method for CO2 emissions mitigation in which the aqueous reaction of carbonate minerals with CO2  is enhanced due to the elevation of CO2 in typical waste combustion gases (Rau and Caldeira, 1999).  ?

Line 90 Cite Caserini et al (2021) in initially introducing/describing BAWL.

Table 1

Row 1 - The initial values here are very uncharacteristic of low latitude, surface SW. Chou et al 2015 are referenced as the source, and the values appear to be taken from their Table 1 (representing offshore and probably deep water samples) although I don’t see the specific At and DIC values used by the present authors. In any case, it is clear from Chou et al Table 1 that the starting solutions were not air equilibrated, pCO2>700 uatms, thus DIC is elevated and pH and Omega are depressed. The more realistic starting conditions are listed in Chou eta Table 2 where pH>8 and esp Omega(c) >4.5.  The choice of starting conditions will have a very significant effect on the modeling outcomes of the present study, so I ask the authors to carefully justify their initial choice of values here.

Row 2 The amount of DIC rise in equilibrium with 0.15atm CO2 will very much depend on the chemistry of the starting solution that I question above.

Row 3 Ditto. Why does Omega(c) only rise to 0.203? In a perfect world under full CO2 and CaCO3 equilibrium OmegaC = 1. Granted, the kinetics for reaching this equilibrium are too slow to be reached in a practical application, but why is CaCO3 dissolution stopped at OmegaC=0.203 when the solution is still significantly carbonate undersaturated?  The ratio of DeltaDICseq/DeltaDICcarb = 0.83/0.19 = 4.4. Shouldn’t this be closer to 1? Or is there a huge amount of excess, unreacted CO2aq in solution?

Rows 4 and 5 Values are highly dependent on the accuracy of the preceding conditions/modeling.

Equ 1 Only valid at low pH (<7). The stoichiometry changes as pH rises so as to accommodate the spontaneous formation of (alkalinity hog) CO3-- ; ACO2 + BH2O + CaCO3 ---> Ca++ CHCO3- + DCO3-- + ….. such that the total moles carbon added is A+1=C+D (<=2) and A<=1 (see eq 1 here https://bg.copernicus.org/articles/20/27/2023/)

Line 143-4  “However, one can easily show that equilibration followed by mixing, provides the same CO2 transfer as mixing followed by equilibration.” This assumes that discharing a supersaturated CO2 solution into seawater will in fact equilibrate with air (on human-relevant timescales). That is unlikely to happen due the the slow kinetics of air/sea gas exchange coupled with vertical SW mixing that will remove some of the supersaturated solution out of contact with air prior to equilibration. Gorey details here: https://www.nature.com/articles/s41558-024-02179-9   

Bottom line: Assuming air equilibration underestimates C storage because some excess CO2aq added in unbuffered AWL will not have a chance to degas to air.

Fig 2 Should be modified depending on the (new) outcomes listed in Table 1.

Line 178-80. “In a similar fashion, the final alkalinity value is the result of alkalinity addition during carbonate dissolution and possibly some extra addition during lime buffering”

Unclear. If you are adding lime you are adding alkalinity, no “possibly” about it. Or are you saying that adding lime is a possibility? In this region of the text the discussion seems to move from AWL with an option to lime to one where liming is now assumed/required. Please be clear from the start about how you are treating AWL +/-liming.

Line193-4 Full air equilibration after discharge is unlikely (https://www.nature.com/articles/s41558-024-02179-9)

Equ 8 Missing an operator between the 2^nd the 3^rd right hand terms?

Line 196-204. Assumes full air/sea CO2 equilibration, unlikely (https://www.nature.com/articles/s41558-024-02179-9)

Line 219-225 Revise depending on outcomes in (revised) Table 1?

Line 247-and after Flows and efficiencies are calculated from data in Table 2 with the implication that these values will be characteristic of AWL at scale, yet what is the evidence that the data in Table 2 represent optimized systems?

Line 258 You mean 150,000 m^3, yet eq 20 is in units of tonnes/tonne and the assumes that 1L SW = 1kg?

Line 293-6 What is the evidence that the efficiencies stated are representative of optimized systems?

Line 301-2 You likely mean Rau (2011) rather than Caldeira and Rau (2000)? The former pub offers numerous results/data for a one step reactor. Compare/contrast with Chou et al 2015 and you subsequent calcs?

Line 324-5 This does not jibe with Rau (2011) which states “Comparing resulting DIC and alkalinity to that of the original solutions and to ambient seawater demonstrates that 61-85% of the carbon originally added to the seawater remained in solution (Figure 2c), with little change in alkalinity and with no visual evidence of carbonate precipitation after aeration.”

Line 327-8 “Consequently, the overall CO2 sequestration efficiency of a one-step reactor remains low due the lack of conversion from hydrated CO2 to HCO3-.” Hydrated CO2 is  HCO3- + H2CO3. What is apparently meant here is lack of conversion of hydrated CO2 balanced by Ca++ rather than by H+? Or do you mean lack of conversion of CO2 to carbonic acid? Anyway, how does this square with the 61-85% of the initially captured C shown to be air stable by Rau (2011)?

Line 417-19 If the now alkalized and carbonated SW is discharged at the same pH as ambient SW the pCO2 must be higher than ambient? Don’t you need to discharge at higher pH to avoid this? And wouldn’t higher discharge pH beneficially help counter ongoing ocean acidification?

Line 435-332 Check out Langer et al for further discussion of limestone sources (in the US): https://www.researchgate.net/publication/283868780_Accelerated_weathering_of_limestone_for_CO2_mitigation_Opportunities_for_the_stone_and_cement_industries

Line 443-6 Here and elsewhere “high water demand” is implied to be an AWL showstopper, yet the global supply of seawater seems rather limitless. What is apparently meant here is that the pumping costs of seawater can become prohibitive, yet so far no discussion of exactly what these costs are, especially relative to the (high) cost of the industry darling, CCS – capturing, concentrating and storing molecular CO2 underground.

Line 447-8 Who has proposed the use of anything but seawater for AWL? The only places AWL will work are near the ocean, eps powerplants that use SW for cooling(?)

Line 458-9 “The BAWL reactor setup proposed by Caserini et al. consumes 0.4 tons of Ca(OH)2 to store 1 ton of CO2.”  Or 1/0.4 = 2.5 t CO2/t Ca(OH)2(?) Yet the delta DIC/deltaAlk in the surface ocean is about 0.85. Since 1 mole of Ca(OH)2=2 moles Alk, then the mole CO2 captured and stored per mol Ca(OH)2 should be 2x0.85/1 = 1.7 moles/mole. CO2= 44g/mol, Ca(OH)2= 74g/mol, Thus ,1 tonne of Ca(OH)2 is able to capture and store about 1.7x44/74 = 1 t CO2/tCa(OH)2 in seawater @pCO2= 420 uatms?  Or does 2.5 t/t only apply to deep ocean, high pressures?

Line 466-8 Seems pretty obvious from the previously published lit. Why even hint at the use of other water sources?

Line 499-500 “..the increased alkalinity and pH could potentially limit ocean acidification..” You mean  “…the increased alkalinity and pH would help counter ocean acidification and its effect on marine biota, see for example Albright et al (2016)” https://www.nature.com/articles/nature17155

Line 514 How about inserting “All of the preceding argue for the use of relatively clean waste gas streams (such as from the combustion of natural gas) in (B)AWL applications.” ?

Line 515-20 Bach (2024) specifically discusses the application of alkaline solids to marine sediments and the effect of alkalinity generation there. Discharge of dissolved alkalinity  into surface waters some distance from sediments and with rapid dilution, as characteristic of (B)AWL, would seem to pose much less risk to benthic/sediment processes.  

The paper is a useful summary of the chemistry and the applicability of accelerated weathering of limestone or buffered accelerated weathering of limestone, and it deserves publication. Minor comments below.

1. Lines 39-59. Please revise this section because it could lead to confusion among “enhanced weathering”, “enhanced rock weathering” , “mineralization”, and “carbonation” (in the case the mineral obtained is a carbonate mineral). The studies by Rau and Caldeira, 1999, Renforth and Kruger, 2013, Caserini et al., 2021, cited as “enhanced rock weathering” processes, could be better identified as accelerated weathering of limestone, to avoid confusion with enhanced weathering (that is a CDR approach that removes atmospheric carbon).
2. Line 52-55 Please specify that what is called “ex situ mineral carbonation” (methods where alkaline minerals react with CO₂, producing solid carbonate minerals) is also called “mineralization”, as in Campbell et al (2022) https://doi.org/10.3389/fclim.2022.879133.
3. line 62: please specify that the CO2 removed by ocean alkalinization is atmospheric CO2
4. line 63. I don’t see the need to add “chemical” between natural and weathering, since all the weathering processes are chemical processes.
5. Lines 91, 93, 99, and others: It is not clear what “upon discharge” means: just before the discharge of the process water or after the discharge? Sometimes, it seems just before (i.e.: … buffering with Ca(OH)2 upon discharge into the sea), in other cases, just after the discharge in seawater (upon re-exposure to atmospheric conditions, aqueous CO2 which is not stabilized by the increased AT will degas back to the atmosphere)
6. Line 99 “(4a-b) the unbuffered or buffered”. Please clarify that 4a is unbuffered and 4b is buffered.
7. Lines 119-125 (table 1). It should be stated in the title what (1) (2) (3) (4a) and (4b) in the first column means. Since just before figure 1 there is (i) (ii) (iii) and (iv), there could be some misunderstanding.
8. Line 124: the pH for 4a, unbuffered process water upon discharge, is 8.16, quite high, very close to the 8.27 for the buffered case. The pH is quite higher than in Caldeira and Rau 2000 https://doi.org/10.1029/1999GL002364. Please add some comments on this point.
9. Lines145-149. Add more recent experimental studies:
   
   Hartmann, J., Suitner, N., Lim, C., Schneider, J., Marín-Samper, L., Arístegui, J., Renforth, P., Taucher, J., & Riebesell, U. (2023). Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO₂ storage. Biogeosciences, 20(4), 781–802. https://doi.org/10.5194/bg-20-781-2023
   
   Moras, C. A., Bach, L. T., Cyronak, T., Joannes-Boyau, R., & Schulz, K. G. (2022). Ocean alkalinity enhancement – avoiding runaway CaCO₃ precipitation during quick and hydrated lime dissolution. Biogeosciences, 19(15), 3537–3557. https://doi.org/10.5194/bg-19-3537-2022

1. Lines 221-224. I would further clarify the reason behind the additional CO2 removal through liming. This represents a novelty of this study that was not addressed in Caserini et al. (2021), because buffered AWL is a carbon dioxide storage process. In contrast, ocean liming is a carbon dioxide removal process.
2. Line 258. I think the exponent of the unit of measurement is 3, not 2.
3. Line 427-428. I would provide more details about this calcination-free process as a method for Ca(OH)₂ recovery, since Ca(OH)₂ recovered from steel slag is obtained through calcination, then used in the steel industry, and ultimately ends up in the steel slag. Furthermore, I would elaborate on whether this process has other potential environmental side effects and provide more insights into its availability, as it depends on the residuals of an industrial process.
4. Lines 461-462. Please provide a reference for the value of 1 ton of CO2 produced per ton of Ca(OH)2.
5. Lines 504-514. It’s worth adding that the problems of trace metals or other pollutants are much lower if AWL or BAWL are used just for the storage of the CO2 produced by calcination, i.e. in the case of electric calcination
6. Lines 515-521. Regarding potential impacts on marine biota, I would also cite the recent study by Sánchez et al. (2024).
   
   Sánchez, N., Goldenberg, S. U., Brüggemann, D., Jaspers, C., Taucher, J., & Riebesell, U. (2024). Plankton food web structure and productivity under ocean alkalinity enhancement. Science Advances, 10(49), eado0264. https://doi.org/10.1126/sciadv.ado0264