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

Charly A. Moras; Tyler Cyronak; Lennart T. Bach; Renaud Joannes-Boyau; Kai G. Schulz

doi:10.5194/bg-21-3463-2024

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

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

Charly A. Moras, Tyler Cyronak, Lennart T. Bach, Renaud Joannes-Boyau and Kai G. Schulz

Biogeosciences, Vol.21(14), pp.3463-3475

30/07/2024

DOI: https://doi.org/10.5194/bg-21-3463-2024

Appears in Recent Faculty of Science and Engineering Publications

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Abstract

Understanding the impacts 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 Mg(OH)2 dissolution kinetics in seawater using three Mg(OH)2 grain sizes (<63, 63–180 and >180 µm) at three salinities (∼36, ∼28 and ∼20). While Mg(OH)2 dissolution occurred more quickly 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 hypothesise that in the lowest-grain-size experiments, the higher surface area provided numerous CaCO3 precipitation nuclei, while the slower dissolution of bigger grain sizes maintained a higher alkalinity and pH at the surface of particles, increasing CaCO3 precipitation rates and making them observable much more quickly than for the intermediate grain size. Salinity also played a role in CaCO3 precipitation, where the decrease in magnesium (Mg) allowed 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 depends not only on seawater composition but also on the physical properties of the alkaline feedstock used.

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Title: Effects of grain size and seawater salinity on magnesium hydroxide dissolution and secondary calcium carbonate precipitation kinetics: implications for ocean alkalinity enhancement
Creators: Charly A. Moras - Southern Cross University
Tyler Cyronak - Georgia Southern University
Lennart T. Bach - University of Tasmania
Renaud Joannes-Boyau - Southern Cross University, Science
Kai G. Schulz - Southern Cross University
Publication Details: Biogeosciences, Vol.21(14), pp.3463-3475
Publisher: Copernicus GmbH
Grant note: Australian Research Council: LE200100022, LE120100201
This research is part of the PhD project of Charly A. Moras that is funded by a Cat. 5 - SCU graduate school scholarship from Southern Cross University, Lismore, Australia. The ICP-MS analyses were made possible by Australian Research Council grants to Renaud Joannes-Boyau and Kai G. Schulz (grant no. LE200100022) and to Renaud Joannes-Boyau (grant no.LE120100201).
Identifiers: 991013211413002368
Academic Unit: Faculty of Science and Engineering; Science
Language: English
Resource Type: Journal article

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

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