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Fluid–solid interactions in nanoporous materials underlie processes fundamental to natural and engineered processes, including the thermochemical transformation of argillaceous materials during high-level nuclear waste disposal. Operando fluid–solid resolution at the nanoscale, however, is still not possible with existing optical and electron microscopy approaches that are constrained by the diffraction limit of light and by vacuum-fluid incompatibility, respectively. In this work, we develop an operando scanning electron microscopy (SEM) platform that enables the first direct in situ imaging of dynamic fluid–solid interactions in nanoporous materials with spatio-temporal-chemical resolutions of ∼2.5 nm per pixel and 10 fps, along with elemental distributions. Using this platform, we reveal necessary conditions for thermochemical pore and fracture generation in shales and measure their surface wetting characteristics that constrain the feasibility of high-level nuclear waste containment. Notably, we show that low heating-rate conditions typical of radioactive decay produce hydrocarbon liquids that wet fracture and pore surfaces in a self-sealing manner to impede aqueous radionuclide advection. 

Reclamation of coal fly ash, a legacy waste material, provides an alternative pathway for the recovery of rare earth elements (REEs) while reducing the environmental stresses that stem from traditional mining. The reactive transport processes underlying the recovery of REEs from ash wastes, however, are yet to be fully elucidated owing to the physicochemical complexity of the micro/nanoscale fly ash particles, including the crystallinity of the particulate matrix. In this work, we use transmission electron microscopy to characterize the material properties of ash particles and reveal the impact of crystallinity on the reactive transport processes governing access to and recovery of the encapsulated REEs. Our results show, for the first time, two distinct crystalline structures of REE-bearing aluminosilicate particles: dense amorphous matrices that facilitate the exchange of chemical species through their lattice interstices and porous polycrystalline matrices characterized by connected intraparticle pores and chemical inertness to leaching solutions. Notably, the presence of matrix crystallinity, or the lack thereof, governs the extent of reagents consumed parasitically by secondary reactions with the aluminosilicate matrix. Our work reveals how the variability of crystalline structures of the ash matrices hosting REEs defines the pathways for the recovery of REEs, providing key insights required for the development of targeted recovery processes.