Research PaperLayer size polydispersity in hydrated montmorillonite creates multiscale porosity networks
Introduction
Swelling clays such as smectites are among the most abundant inorganic nanomaterials in the lithosphere(Hochella Jr. et al., 2019), and play an outsize role in controlling the transport and retention of water, CO2, nutrients and pollutants in both natural and engineered settings due to their exceptionally high surface areas(Bourg and Ajo-Franklin, 2017; Charlet et al., 2017). SWy is an archetypical smectite whose clay fraction is almost exclusively montmorillonite (Mt) that formed via devitrification of volcanic ash and tuffs through submarine weathering, with no significant post-formational recrystallization(Cadrin et al., 1995). Ion binding selectivities(Whittaker et al., 2019), permeabilities(Tournassat et al., 2016b), shear strength(Ikari et al., 2015), and myriad other properties of Mt. are highly dependent on the shape, size, and the specifics of how smectite particles are arranged.
Smectite 2:1 layers are less than one nanometer thick and up to several microns in lateral dimensions, and stack to form particles in various ways depending on the relative concentrations of water, clay, and electrolyte(Tournassat and Steefel, 2015). While the average separation between layers generally decreases with increasing concentration of clay or salt(Norrish, 1954) there is no consensus about which of the many potential microscopic clay colloid arrangements are expected to form under specific conditions(Bergaya and Lagaly, 2013). Fluid and solute transport rates through natural SWy depend on the pore structures defined by layer stacking motifs(Wenk et al., 2008), and are generally classified into two broad categories based on the separation distance between adjacent Mt. layers: nanopores and macropores. Nanopores include clay interlayers that, because of the effective negative charge on smectite layers, generally exclude anions and therefore facilitate ion-selective transport(Tournassat et al., 2016a). Macropores are the larger and less well-defined spaces between particles through which anions, cations, and even larger solutes like macromolecules and nanoparticles can diffuse(Tournassat et al., 2016b). In both cases, experimental descriptions of pore geometries and connectivity that are required for accurate transport models are lacking(Churakov and Gimmi, 2011; Tournassat et al., 2016b).
High-resolution transmission electron microscopy (HR-TEM) has been utilized for decades to reveal atomic- and nanoscale structures in non-hydrous clay particles(Veblen, 1985; Vali and Köster, 1986), including stacking order between layers in smectite and illite/smectite (Veblen 1990; Guthrie and Veblen, 1989). However, there is evidence that the native structure of smectites is disrupted(Dudek et al., 2002) during conventional sample preparation(Drummy et al., 2005). Low-dose transmission electron microscopy of cryogenically frozen samples (cryo-TEM) is uniquely capable of characterizing hydrated clay structures over spatial scales ranging from near-atomic resolution(Whittaker et al., 2019) to whole particle aggregates(Gilbert et al., 2015; Segad et al., 2012; Whittaker et al., 2019). Water vitrifies without crystallization upon rapid freezing for sample thicknesses(Deirieh et al., 2018) that are electron-transparent at the accelerating voltages commonly employed for cryo-TEM (200–300 kV), preserving structures with minimal perturbation from their native-state(Cheng, 2018). Increased electron-dose robustness of cryo-frozen samples(Henderson and Glaeser, 1985) allows for 3D images to be reconstructed using cryo electron tomography (cryo-ET) from a series of images taken at different tilt angles with minimal beam-induced damage.
Here, we use cryo-TEM to show that native pore structures in minimally altered, hydrated SWy arise from disparately-sized layers that stack defectively and introduce pervasive layer curvature. We employ cryo-ET to characterize the 3D structure of SWy pore networks that cannot be resolved from 2D images alone and are not accounted for in commonly used models based on x-ray diffraction and simulations that assume perfectly planar layers. We observe rotational crystallographic ordering between adjacent layers separated by over 1 nm of interlayer water, in disagreement with the common assertion that hydrated smectites are fully turbostratic.
Section snippets
Methods
Wyoming bentonite (SWy-1/SWy-2) from the Clay Minerals Society was suspended in deionized water or an aqueous solution of NaCl (200 mM) or MgCl2 (100 mM) by manual shaking for 2 min with no filtration, washing or prior separation of non-clay minerals. Suspensions were incubated overnight before cryo-TEM analysis. No significant differences in clay structure were observed for samples suspended in NaCl versus MgCl2 and images from both electrolyte solutions are presented.
Imaging was performed on
Rotational ordering in Hydrated Mt
Mt. particles adopted a range of orientations that reflected their structure in suspension immediately prior to plunge-freezing. Cryo-TEM images of these suspensions exhibited two dominant modes of contrast that varied in relative intensity depending on the orientation of the layers with respect to the electron-beam axis. In a face-on orientation, parallel to the TEM grid supports, the particles were much thinner than they were wide, and contrast was generated primarily by phase interference (
Discussion
Using cryo-preparation methods to minimize sample preparation artifacts(Deirieh et al., 2018) and beam-induced damage during imaging(Henderson and Glaeser, 1985), in analogy to cryoEM of biological samples(Cheng, 2018), provides confidence that structures observed by cryo-TEM faithfully represent the native state of hydrated SWy. The observed structures are therefore likely the result of authigenic processes, because SWy does not appear to have recrystallized after formation(Cadrin et al., 1995
Conclusions
Microstructures of minimally altered, hydrated Wyoming smectite were imaged in two and three dimensions, revealing a panoply of defects that govern clay layer arrangements. The dominant feature of Mt. particles was the polydispersity in layer dimensions, which gives rise to defects via incommensurate stacking. Layers curve to accommodate stacking defects, creating hierarchical pore networks that can vary greatly in size distribution and can be highly interconnected. Some defects appear to be
Acknowledgements
This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, through its Geoscience program at LBNL under Contract DE-AC02-05CH11231.
Author contributions
M. L. W. analyzed and interpreted data, and wrote the manuscript. L. C. collected and analyzed data. B. G. and J. F. B. conceived the idea and wrote the manuscript.
Competing financial interests
The authors declare no competing financial interests.
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These authors contributed equally to this work