Geochemical Negative Emissions Technologies: Part I. Review

Over the previous two decades, a diverse array of geochemical negative emissions technologies (NETs) have been proposed, which use alkaline minerals for removing and permanently storing atmospheric carbon dioxide (CO₂). Geochemical NETs include CO₂ mineralization (methods which react alkaline minerals with CO₂, producing solid carbonate minerals), enhanced weathering (dispersing alkaline minerals in the environment for CO₂ drawdown) and ocean alkalinity enhancement (manipulation of ocean chemistry to remove CO₂ from air as dissolved inorganic carbon). CO₂ mineralization approaches include in situ (CO₂ reacts with alkaline minerals in the Earth's subsurface), surficial (high surface area alkaline minerals found at the Earth's surface are reacted with air or CO₂-bearing fluids), and ex situ (high surface area alkaline minerals are transported to sites of concentrated CO₂ production). Geochemical NETS may also include an approach to direct air capture (DAC) that harnesses surficial mineralization reactions to remove CO₂ from air, and produce concentrated CO₂. Overall, these technologies are at an early stage of development with just a few subjected to field trials. In Part I of this work we have reviewed the current state of geochemical NETs, highlighting key features (mineral resources; processes; kinetics; storage durability; synergies with other NETs such as DAC, risks; limitations; co-benefits, environmental impacts and life-cycle assessment). The role of organisms and biological mechanisms in enhancing geochemical NETs is also explored. In Part II, a roadmap is presented to help catalyze the research, development, and deployment of geochemical NETs at the gigaton scale over the coming decades.