Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO₂ storage

Abstract

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.

Global‐Scale Evaluation of Coastal Ocean Alkalinity Enhancement in a Fully Coupled Earth System Model

“…As such, the quantity of olivine sand ultimately required to satisfy this dissolution is dynamic and varies in both space and time (and, additionally, with climate warming). Nonetheless, considerable uncertainty remains around actual TA addition, including from the precise minerals involved (as noted at OAE's inception; Kheshgi, 1995), the dissolution dynamics of particles (e.g., Feng et al., 2017), and the resulting chemical kinetics (e.g., Hartmann et al., 2023; Moras et al., 2022). Consideration also needs to be given to seafloor sediment dynamics in the event that OAE minerals are transported by tidal or current processes and then buried (Fuhr et al., 2022).…”

Techno-economic evaluation of buffered accelerated weathering of limestone as a CO2 capture and storage option

“…The location of the discharge should be far from enclosed coastal areas and possibly within off-shore currents in order to limit local alterations of the carbonate chemistry and to avoid negative impacts on the biota (Kirchner et al 2020). Dilution is also useful to avoid or minimize the risk of CO 2 degassing, allowing the enriched seawater to mix in a certain time with the surrounding seawater without overcoming the aragonite saturation state (Ω ar ) value of 5, identified by Hartmann et al (2022) as a threshold for abiotic precipitation of carbonate.…”

Life Cycle Assessment of Coastal Enhanced Weathering for Carbon Dioxide Removal from Air

“…Equation suggests that 1 mole of dissolved olivine can sequester 4 moles of CO 2 , i.e., 1.25 kg CO 2 per kg of olivine . However, the overall CO 2 sequestration potential is 20% lower when accounting for buffering within the carbonate system and may be lower still when considering inefficiencies in air–sea gas exchange . Furthermore, olivine-rich silicate rocks like dunite, an ultramafic intrusive igneous rock that belongs to the peridotite group and contains around 90% of olivine, will likely be used in CEW rather than pure olivine.…”

“… 28 However, the overall CO 2 sequestration potential is 20% lower when accounting for buffering within the carbonate system 13 and may be lower still when considering inefficiencies in air–sea gas exchange. 36 Furthermore, olivine-rich silicate rocks like dunite, 10 an ultramafic intrusive igneous rock that belongs to the peridotite group and contains around 90% of olivine, 37 will likely be used in CEW rather than pure olivine. Therefore, a conservative assumption was made and peridotite’s CO 2 sequestration potential was used (0.8 kg of CO 2 per kg of rock).…”