River alkalinity enhancement for scalable and energy-efficient geochemical carbon dioxide removal: potential, costs and risks?

Gigaton-scale carbon dioxide removal (CDR) from the atmosphere will be necessary to limit global warming to below 2 °C. Current approaches, such as afforestation or biochar, are constrained by land and biomass availability, whereas direct-air carbon capture technologies are energy-intensive and costly. Here, we discuss river alkalinity enhancement (RAE), a geochemical carbon dioxide removal strategy that uses readily available limestone (CaCO₃) and derived slaked lime (Ca(OH)₂) to increase alkalinity in rivers. Unlike the direct addition of these minerals to the ocean, which is currently restricted by the London protocol, addition of alkalinity to rivers is already commercially applied in some regions and alkalinity dosing can be monitored in controlled systems. Controlled rock dissolution in ponds allows for direct measurement of CO2 removal prior to water release back into rivers, enhancing traceability and verification. We simulate CO2 uptake in 149 major rivers, considering river flows and chemistry and life-cycle emissions from mining, grinding and transporting rock and pumping water in reactor ponds. Our results indicate that RAE using CaCO₃ in these rivers could remove an upper limit of over 0.2 but below 0.4 gigatons of CO2 per year, yet over 1 gigaton CO2 per year using Ca(OH)₂, even after subtraction of major process emissions. Estimated costs may be below $200 per ton of carbon dioxide removed, substantially lower than conventional energy-intensive direct-air capture approaches. Nonetheless, the ecological risks and potential alteration of photosynthesis by aquatic plants after RAE-induced pH and alkalinity increases should be thoroughly investigated prior to widescale RAE adoption. If ecological risks would be minor, RAE may offer a scalable, relatively energy-efficient and economically viable pathway for geochemical CDR, while also delivering the co-benefit of mitigating ocean acidification downstream.