Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO 2 storage

[摘要] According to modelling studies, ocean alkalinity enhancement (OAE)is one of the proposed carbon dioxide removal (CDR) approaches with largepotential, with the beneficial side effect of counteracting oceanacidification. The real-world application of OAE, however, remains unclearas most basic assumptions are untested. Before large-scale deployment can beconsidered, safe and sustainable procedures for the addition of alkalinityto seawater must be identified and governance established. One of theconcerns is the stability of alkalinity when added to seawater. The surfaceocean is already supersaturated with respect to calcite and aragonite, andan increase in total alkalinity (TA) together with a corresponding shift incarbonate chemistry towards higher carbonate ion concentrations would resultin a further increase in supersaturation, and potentially to solid carbonateprecipitation. Precipitation of carbonate minerals consumes alkalinity andincreases dissolved CO 2 in seawater, thereby reducing the efficiency ofOAE for CO 2 removal. In order to address the application of alkalinesolution as well as fine particulate alkaline solids, a set of sixexperiments was performed using natural seawater with alkalinity of around2400  µ mol kgsw −1 . The application of CO 2 -equilibrated alkalinesolution bears the lowest risk of losing alkalinity due to carbonate phaseformation if added total alkalinity ( Δ TA) is less than 2400  µ mol kgsw −1 . The addition of reactive alkaline solids can cause a net loss ofalkalinity if added Δ TA  >  600  µ mol kgsw −1 (e.g. forMg(OH) 2 ). Commercially available (ultrafine) Ca(OH) 2 causes, ingeneral, a net loss in TA for the tested amounts of TA addition, which hasconsequences for suggested use of slurries with alkaline solids suppliedfrom ships. The rapid application of excessive amounts of Ca(OH) 2 ,exceeding a threshold for alkalinity loss, resulted in a massive increase inTA ( >  20 000  µ mol kgsw −1 ) at the cost of lower efficiency andresultant high pH values >  9.5. Analysis of precipitatesindicates formation of aragonite. However, unstable carbonate phases formedcan partially redissolve, indicating that net loss of a fraction ofalkalinity may not be permanent, which has important implications for real-world OAE application. Our results indicate that using an alkaline solution instead of reactivealkaline particles can avoid carbonate formation, unless alkalinity additionvia solutions shifts the system beyond critical supersaturation levels. Toavoid the loss of alkalinity and dissolved inorganic carbon (DIC) fromseawater, the application of reactor techniques can be considered. Thesetechniques produce an equilibrated solution from alkaline solids andCO 2 prior to application. Differing behaviours of tested materialssuggest that standardized engineered materials for OAE need to be developedto achieve safe and sustainable OAE with solids, if reactors technologiesshould be avoided.

[学科分类] 大气科学