Assessment of solid ikaite release into seawater – implications for ocean alkalinity enhancement

Alkaline feedstocks for ocean alkalinity enhancement (OAE) must guarantee efficient alkalinity release while having limited impact on marine ecosystems and carbonate mineral saturation levels (Ω CaCO3 ). When considering mineral powder addition as a deployment option, currently considered feedstocks either exhibit slow dissolution kinetics or may require additional water treatment to limit rapid pH changes. Carbonate minerals, on the other hand, feature fast dissolution when undersaturated, accompanied with a reduced impact on pH and Ω CaCO3 . However, traditional (non-hydrated) carbonate minerals such as calcite and aragonite are insoluble in seawater and therefore impractical as direct-to-use feedstocks for OAE. Here, we examine the dissolution kinetics and alkalinity release efficiency of ikaite (CaCO 3 ·6H 2 O) — a hydrated carbonate mineral producible from limestone and dissolvable in seawater — across a range of global sea surface temperatures through a series of controlled laboratory-scale experiments. The main focus lay on determining the alkalinity release efficiency at temperatures where ikaite usually becomes thermodynamically unstable (> 6° C). The conducted experiments showed that ikaite dissolved at temperatures of 10 – 26 °C with initial dissolution rates comparable to calcite at highly undersaturated conditions (7.62 x 10 -6 - 2.17 x 10 -5 mol m -2 s -1 ) but exhibited a limited alkalinity release efficiency (50 – 80 % of potential alkalinity added). The reduced alkalinity release was likely caused by partial transformation of ikaite into non-soluble carbonate minerals before complete alkalinity release could be reached. An additional experiment at 3 °C resulted in a higher alkalinity release efficiency (85 – 100 %) but was accompanied by a two magnitudes lower initial dissolution rate (1.43 – 1.63 x 10 -7 mol m -2 s -1 ) revealing a tradeoff between the dissolution rate and alkalinity release efficiency as a function of temperature. The results emphasize that efficient ikaite dissolution is limited to cold waters ( 10 °C) the alkalinity disperses within 10 m below the sea surface, but due to reduced alkalinity release efficiency, CO 2 would be lowered to ∼0.7 – 1.3 mole per mole of added Ca 2+ . At 3 °C, on the other hand, the CO 2 uptake efficiency is less affected by incomplete dissolution of ikaite but likely varies more with mean particle size, emphasizing its critical role in cold waters. It remains to be tested whether these results also apply on a larger scale, for example by conducting tank or mesocosm experiments. • Ikaite dissolves fast in seawater over a wide temperature gradient • addition of ikaite to seawater likely comes with a reduced alkalinity release efficiency at water temperatures above 10 °C • Particle size distributions with a mean size <100 μm facilitate releasing major fractions of alkalinity within the ocean mixed layer which constitute CO 2 uptake