Preprints
https://doi.org/10.5194/egusphere-2024-645
https://doi.org/10.5194/egusphere-2024-645
12 Mar 2024
 | 12 Mar 2024
Status: this preprint is open for discussion.

Effects of grain size and seawater salinity on magnesium hydroxide dissolution and secondary calcium carbonate precipitation kinetics: implications for ocean alkalinity enhancement

Charly Andre Moras, Tyler Cyronak, Lennart Thomas Bach, Renaud Joannes-Boyau, and Kai Georg Schulz

Abstract. Understanding the impact that mineral grain size and seawater salinity have on magnesium hydroxide (Mg(OH)2) dissolution and secondary calcium carbonate (CaCO3) precipitation is critical for the success of ocean alkalinity enhancement. We tested the Mg(OH)2 dissolution kinetics in seawater using three Mg(OH)2 grain sizes (<63, 63–180 and >180 µm) and at three salinities (~36, ~28 and ~20). While Mg(OH)2 dissolution occurred quicker the smaller the grain size, salinity did not significantly impact measured rates. Our results also demonstrate that grain size can impact secondary CaCO3 precipitation, suggesting that an optimum grain size exists for ocean alkalinity enhancement (OAE) using solid Mg(OH)2. Of the three grain sizes tested, the medium grain size (63–180 µm) was optimal in terms of delaying secondary CaCO3 precipitation. We hypothesize that in the lowest grain size experiments, the higher surface area provided numerous CaCO3 precipitation nuclei, while the slower dissolution of bigger grain size maintained a higher alkalinity/pH at the surface of particles, increasing CaCO3 precipitation rates and making it observable much quicker than for the intermediate grain size. Salinity also played a role in CaCO3 precipitation where the decrease in magnesium (Mg) allowed for secondary precipitation to occur more quickly, similar in effect size to another known inhibitor, i.e., dissolved organic carbon (DOC). In summary, our results suggest that OAE efficiency as influenced by CaCO3 precipitation not only depends on seawater composition but also on the physical properties of the alkaline feedstock used.

Charly Andre Moras, Tyler Cyronak, Lennart Thomas Bach, Renaud Joannes-Boyau, and Kai Georg Schulz

Status: open (until 23 Apr 2024)

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Charly Andre Moras, Tyler Cyronak, Lennart Thomas Bach, Renaud Joannes-Boyau, and Kai Georg Schulz
Charly Andre Moras, Tyler Cyronak, Lennart Thomas Bach, Renaud Joannes-Boyau, and Kai Georg Schulz

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Short summary
We investigate the effects of mineral grain size and seawater salinity on magnesium hydroxide dissolution and calcium carbonate precipitation kinetics for ocean alkalinity enhancement. Salinity did not affect the dissolution, but calcium carbonate formed earlier at lower salinities due to the lower magnesium and dissolved organic carbon concentrations. Smaller grain size dissolved faster, but calcium carbonate precipitated earlier, suggesting that medium grain size is optimum for both kinetics.