Going submicron in the precise analysis of soil structure: A FIB-SEM imaging study at nanoscale

Soil structure is defined by many physical properties and functions. If detailed information about the structure is known, a number of modelling techniques can be used to perform simulations to assess the physical properties of soil and evaluate them at different scales that cannot be measured otherwise or to predict changes due to variation in boundary conditions. With the widespread accessibility of X-ray computed tomography (XCT), it is possible to describe soil structure at µm scale. However, it is now evident that such characterization is not enough to describe all the necessary processes or properties of soils since detailed information about the soil structure at the nano-scale is still lacking. This paper presents the initial results of the nanostructure characterization of soils using focused ion beam plus scanning electron microscopy (FIB-SEM) imaging technique. We prepared two ~50 × 50 µm2 FIB milled and polished surfaces, each for Retisol and Chernozem soil aggregates. In addition to the FIB-SEM imaging with a resolution of up to 4 nm, we performed regular SEM surface imaging and high-resolution XCT scanning with a resolution of 1.01 µm. The images obtained using the three approaches showed their varying capabilities of distinguishing structural features at different scales. The FIB-SEM images were useful in the characterization of pore size distributions and we observed pore inscribed radii ranging from 2.5 nm to ~1 µm. We also observed three general types of nanoporosity: one is between the mineral grains and the organomineral assemblages, another within the organic matter and then within the minerals. We highlighted all the advantages and shortcomings of the FIB-SEM imaging and proposed practical solutions to addressing the disadvantages as well; In addition, future directions for research using FIB-SEM to improve the characterization of soil structure at the nano-scale were outlined.