Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO2 storage
Authors: Hartmann Jens , Suitner Niels , Lim Carl , Schneider Julieta , Marín-Samper Laura , Arístegui Javier , Renforth Phil , Taucher Jan , Riebesell Ulf 
Published: February 2023

Abstract. According to modelling studies, ocean alkalinity enhancement (OAE)
is one of the proposed carbon dioxide removal (CDR) approaches with large
potential, with the beneficial side effect of counteracting ocean
acidification. The real-world application of OAE, however, remains unclear
as most basic assumptions are untested. Before large-scale deployment can be
considered, safe and sustainable procedures for the addition of alkalinity
to seawater must be identified and governance established. One of the
concerns is the stability of alkalinity when added to seawater. The surface
ocean is already supersaturated with respect to calcite and aragonite, and
an increase in total alkalinity (TA) together with a corresponding shift in
carbonate chemistry towards higher carbonate ion concentrations would result
in a further increase in supersaturation, and potentially to solid carbonate
precipitation. Precipitation of carbonate minerals consumes alkalinity and
increases dissolved CO2 in seawater, thereby reducing the efficiency of
OAE for CO2 removal. In order to address the application of alkaline
solution as well as fine particulate alkaline solids, a set of six
experiments was performed using natural seawater with alkalinity of around
2400 µmol kgsw−1. The application of CO2-equilibrated alkaline
solution bears the lowest risk of losing alkalinity due to carbonate phase
formation if added total alkalinity (ΔTA) is less than 2400 µmol kgsw−1. The addition of reactive alkaline solids can cause a net loss of
alkalinity if added ΔTA > 600 µmol kgsw−1 (e.g. for
Mg(OH)2). Commercially available (ultrafine) Ca(OH)2 causes, in
general, a net loss in TA for the tested amounts of TA addition, which has
consequences for suggested use of slurries with alkaline solids supplied
from ships. The rapid application of excessive amounts of Ca(OH)2,
exceeding a threshold for alkalinity loss, resulted in a massive increase in
TA (> 20 000 µmol kgsw−1) at the cost of lower efficiency and
resultant high pH values > 9.5. Analysis of precipitates
indicates formation of aragonite. However, unstable carbonate phases formed
can partially redissolve, indicating that net loss of a fraction of
alkalinity may not be permanent, which has important implications for real-world OAE application. Our results indicate that using an alkaline solution instead of reactive
alkaline particles can avoid carbonate formation, unless alkalinity addition
via solutions shifts the system beyond critical supersaturation levels. To
avoid the loss of alkalinity and dissolved inorganic carbon (DIC) from
seawater, the application of reactor techniques can be considered. These
techniques produce an equilibrated solution from alkaline solids and
CO2 prior to application. Differing behaviours of tested materials
suggest that standardized engineered materials for OAE need to be developed
to achieve safe and sustainable OAE with solids, if reactors technologies
should be avoided.