The authors present experimental results on the reactivity of kiln dust in seawater. Kiln dust is a waste product of the cement and lime industry that is either recycled or disposed in e.g. landfills. The authors show that reactive phases in kiln dust (CaO, Ca(OH)₂) are rapidly dissolved in seawater such that most of these phases are consumed within a few minutes after exposure to seawater. Kiln dust added to marine surface waters would, hence, add alkalinity and promote CO2 uptake in the surface ocean. The remaining material (mostly calcite) may settle to the seabed where it might produce additional alkalinity if sediment porewaters are undersaturated with respect to calcite.

The experimental results are very solid and clearly show the potential of kiln dust for ocean alkalinity enhancement (OAE). The paper is well written and I would suggest to publish the paper after a few revisions that are outlined below:

The authors present the mineral composition of kiln dusts in Table 2. It would be good to add data on Ca-Si-phases that are formed during cement production and should be present in cement kiln dust (CKD). These cement phases should have a high reactivity in seawater. They could explain the release of excess alkalinity observed in CKD experiments (line 331). The authors should consider these Ca-Si-phases in the discussion of their experimental results and add information on these phases in Table 2 if possible.

The authors propose that kiln dust should be applied to the surface ocean in shelf regions where the seabed is covered by permeable (sandy) sediments to allow for calcite dissolution in sediments that would add further alkalinity to the ocean (line 420). It is, however, likely that porewaters of these permeable sediments have a composition that is close seawater due to the rapid exchange with ambient bottom waters driven by fast tidal currents. Since shelf waters are usually  oversaturated with respect to calcite, calcite dissolution may not proceed in these permeable deposits. Muddy sediments with restricted advective porewater exchange might offer a better environment for respiration-driven calcite dissolution as discussed in Dale et al., 2024 and Fuhr et al., 2025. I would suggest to update the text accordingly considering that muddy deposits are at least as favorable for calcite dissolution as permeable (sandy) sediments.

Kiln dust disposed in landfills reacts with CO2-bearing rain waters which may lead to a substantial uptake of atmospheric CO2. It is not clear to me whether the total CO2 uptake is enhanced when this material is added to the ocean instead of being disposed on land. The authors should add a paragraph on cement and kiln dust weathering under terrestrial conditions which has been intensively studied over the past decades. They should also try to compare the net CO2 balance of their approach (using kiln dust for OAE) with alternative kiln dust uses (disposal on land, recycling).  

Dear authors,

Please find attached my reviewer comments.

Best,

Charly

General comments

The manuscript investigates the potential for atmospheric CO₂ removal via ocean alkalinity enhancement using waste products from cement and lime kilns. The study is based on laboratory dissolution experiments and evaluates the dissolution kinetics, CO₂ sequestration potential, and ecological risks associated with cement kiln dust (CKD) and lime kiln dust (LKD). In my opinion, the manuscript offers a valuable contribution to scientific progress within the scope of Biogeosciences, presenting new concepts and ideas for CO₂ removal that worth further testing and investigation.

Overall, the manuscript is well structured and clearly written. The language is fluent, the figures are clear and easy to interpret, and the amount and quality of the supplementary material are appropriate.

The conclusions are generally supported by the experimental results. However, I have reservations about the representativeness of the reported values, given that the experiments were conducted over relatively short timescales (8 hours and 15 days) in 200-mL polystyrene vials filled with filtered seawater under controlled laboratory conditions. These constraints limit the extent to which the findings can be extrapolated to real-world applications. In my view, the discussion, conclusions, and abstract should adopt a more cautious tone regarding the scalability and environmental impacts of the results.

The manuscript also addresses the potential ecological impacts of kiln dust dissolution. However, the assessment focuses primarily on turbidity and trace metal concentrations, without sufficiently considering biological responses or broader ecological consequences of kiln dust deployment in marine environments.

Finally, the Methods section could be strengthened by providing additional detail to facilitate reproducibility by other researchers.

I hope the comments below help the authors improve the clarity, rigor, and reproducibility of the study.

Specific comments

Material and methods

- Please specify the source of all materials used in the experiments (e.g., supplier, kiln type, facility, geographical origin). This information is essential for assessing the broader applicability of the results and ensuring reproducibility.

- Please clarify which analytical methods were applied in each of the experiments. For Experiment II, indicate how total alkalinity (AT) was measured.

- It appears that only two sampling times (after 1 day and after 15 days) were analyzed for Experiment II. Please clarify in the manuscript.

Lines 90-92 – In the sentence “Based on preliminary tests, three different masses of CKD and LKD were added targeting a specific aragonite saturation state (ΩArg) at the end ...” To facilitate reproducibility, please indicate the correspondence masses values (in grams) added to the 200 mL of FSW.

Lines 103- 104- Referring to the 15-day experiment as a “long-term dissolution experiment” may be misleading in the context of OAE. I suggest referring simply to a 15-day experiment, unless additional justification is provided.

Lines 105-109 – For reproducibility, please provide details on the number of replicates used and the number of vials per treatment.

Line 104 – Are 200 mL plastic vials representatives of sea water collum? Please explain the rationale for choosing this container size.

Line 109 – Please clarify why different rotation speeds were used in the two experiments (700 rpm in Experiment I vs. 14 rpm in Experiment II). What was the intended effect of this difference?

Lines 109-110 – In the sentence “The one-day incubation reflects the time needed for complete dissolution of the reactive phases in the kiln dusts.”, please specify the exact duration in hours from the start of the experiment until dissolution was considered complete.

Lines 237-238 – In the sentence “…while for LKD, the ΔAT curve showed a maximum at higher concentrations (Fig. 2B)” does not seem to match the figure: the maximum appears to occur before the highest concentration. Please revise accordingly.

Lines 318-319 – Regarding this sentence “The compositional complexity of kiln dusts underscores the need for detailed mineralogical and chemical characterization to properly assess the CDR potential and environmental risks in OAE applications.”, more details about the provenance of the kiln dust should be added to the methods section.

Line 363 – In the sentence “… leading to a net AT loss, as seen at the highest LKD concentration after 15 days (Fig. 2A)”. the reference appears to be to Figure 2B rather than Figure 2A, please check.

Line 371 – In the sentence “… several minutes during ship-based ocean liming Caserini et al. (2021), which may have an impact on marine life if pH exceeds”. It seems that the reference should be placed between brackets, please check.

Line 373-374 – “To avoid temporary exceedances of pH 9, CKD concentrations should stay below 343–502 mg kg-1, and LKD below 102–149 mg kg-1, depending on local seawater conditions (AT = 2350 μmol kg-1, DIC = 2100 μmol kg-1, salinity = 35, temperature = 10–25 °C).”

Your experiment was conducted at a salinity level of 32.3 ± 0.5 and a temperature level of 17.5–22.7 °C. Are these values comparable to a salinity of 35 and a temperature of 10–25 °C? Would these variations in salinity and temperature influence the concentrations of CKD and LKD? What impact would they have on the pH? These points need to be clarified in the manuscript.

Line 385-387 – “In real applications, kiln dust will be rapidly mixed into much larger volumes of surface water, ….”

How representative is your experiment, that used 200 ml sea-water bottle, to be extrapolated for real application conditions? How would be guarantee a concentration below the referenced concentrations at the discharge point? In my opinion, without stronger justification, the scalability of the results remains uncertain.

Line 407-409 – “Assuming full mixing in the top 10 cm of the sediment, up to 1.4 kg CKD or 74.8 kg LKD per m² could be applied …..”

Please clarify what would be the resulting thickness of kiln dust deposited on the seafloor? Even if the 10-cm surface layer is assumed to be fully mixed over time, the initial deposition could create a substantial layer of fine material. Benthic infauna and epifauna cannot survive rapid burial under more than ~1–2 cm of sediment, and a thick layer of fine particles would also strongly reduce oxygen exchange at the sediment–water interface. This physical disturbance should also be addressed in the discussion, as compliance with chemical SQGs alone does not ensure ecological safety.

Line 420-424 – The manuscript notes that large-scale fining of permeable sediments by kiln dust could reduce oxygen penetration depth and thereby limit the zone of metabolic CaCO₃ dissolution. However, the biological implications of such physical and geochemical changes are not discussed. What would be the expected impact on benthic organisms living within these sediments, particularly those that produce or maintain calcium carbonate shells (e.g., molluscs, foraminifera, small crustaceans)? Reduced permeability, shallower oxic layers, and burial by fine material could harm calcifiers through smothering, reduced oxygen availability, and altered porewater chemistry. I recommend addressing these ecological consequences alongside the geochemical considerations

Line 470-471 – “Overall, LKD and, to a lesser extent, CKD show promise for OAE, with a CDR potential of up to 13.4 Mt year-1 for LKD and 57 Mt year-1 for CKD at current production levels”

In my opinion, given the significant uncertainties surrounding ecological impacts, including water-column responses, sediment interactions, and potential biological effects, the Conclusions should acknowledge these uncertainties and reflect them in the overall assessment of kiln dust as a viable CDR strategy.