Modelling a basalt reactor for direct air CO2 capture

Ground basalt has been used as mineral fertilizer since the early thirties. Ground basalt captures CO2 from the atmosphere and the soil pore space, raises the soil pH and reduces ocean acidification. One tonne of basalt captures 0.153–0.165 tonne CO2, depending on infiltration rate (400–1200 mm/a), reactive surface area (3.7–15 m2/g) and CO2 partial pressure (41.1–3000 Pa). When the infiltration rate is high (1200 mm/a), the CO2 capture capacity of basalt is exhausted after 9.5–11.4 years. When the infiltration rate is low (400 mm/a), the capture capacity is exhausted after 28.2–33.1 years. With the exhaustion of the capture capacity, the newly formed carbonates that sequestered CO2 start dissolving. The dissolution is complete after 34.9–101.7 years, depending on infiltration rate, reactive surface area and CO2 partial pressure. The reaction products are transported to the ocean via surface waters. The degree to which the fugacity of CO2 controlled by the Henry constant exceeds the fugacity of atmospheric CO2 along the travel path depends on many unknowns. Thus, it is impossible to reliably predict to which degree the captured CO2 is recycled to the atmosphere, if it is recycled at all.