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

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.