Owing to the significant climate change caused by the steady increase of CO₂ in the atmosphere along with industrial activity, the technology of CO₂ capture and storage with respect to less-energy intensiveness, cost effectiveness, and environmental friendliness has been long-awaited. In power plants, electricity is generated by burning fossil fuels such as coal and natural gas, which could be a large point source through the combustion process. An approach to reduce CO₂ emission is the efficient capture of CO₂ and its storage in a stable manner before release to the atmosphere. An important factor of CO₂ capture and storage is the long-term sequestration of CO₂, where CO₂ storage in geological reservoirs has been often considered. In this approach, CO₂ is injected into a deep geological formation to be physically confined below an impermeable or very low permeability caprock, such as a shale, allowing for a sequence of possible trapping mechanisms. A fraction of injected CO₂ is fixed as thermodynamically stable mineral carbonates, as e.g., CaCO₃ or MgCO₃ in the geological formation via the reaction with alkaline minerals there. The formation of stable carbonates in the deep underground geology, known as in situ mineral carbonation, could be suitable for long-term storage of CO₂. On the contrary, ex situ mineral carbonation is the above-ground process involving the reaction of CO₂ with alkaline earth metals extracted from basic rock.

 Saponite, a silicate clay mineral abundantly and ubiquitously available in nature, is structured through stacks of two dimensional (2D) nanosheets with thicknesses of a few nanometers, which are the minimum structural unit. The 2D nanosheets have a variety of sizes and cannot be perfectly stacked, resulting in partial overlapping. This results in the formation of nanoscale open spaces. Here, CO₂ adsorption in the open spaces originated from overlapped nanosheets in saponite clay minerals is explored by solid-state nuclear magnetic resonance coupled with open space analysis using positronium. The following scenario of instantaneous ex situ mineral carbonation under ambient conditions was found. In saponite, the O atoms are weakly bound to the octahedron by ionic bonding, in contrast to the strong semi-covalent bonding of O and Si atoms in the tetrahedron. CO₂ gas molecules thus easily pick up the O atoms from the octahedra at the nanosheet edges in the interior of the above open spaces to be activated as the carbonate ions of CO₃²⁻ type (see right-hand side). The CO₃²⁻ ions are then chemisorbed at the alkali metal cations on the surface of 2D nanosheets (see left-hand side). This instantaneous mineral carbonation and CO₂ physisorption occurs in the absence of an energy-consumption process or chemical solution enhancement. The present finding is of potential significance for CO₂ capture and storage and presents an approach of environmentally friendly recycling of low contaminated soil in Fukushima.

Kiminori Sato & Michael Hunger

Carbon dioxide adsorption in open nanospaces formed by overlap of saponite clay nanosheets, Kiminori Sato and Michael Hunger, Communications Chemistry 3, 91 (2020).

DOI: https://doi.org/10.1038/s42004-020-00346-5