Soil pollution is a major environmental issue. The treatment of polluted soils relies on molecular or colloidal transport processes in confined geometries, coupled with adsorption/desorption phenomena at solid/liquid interfaces. Clays and silts (silica), as major constituents of soil, play a crucial role in this regard.
In this context, the research project we are developing within LCR CARMEN and the 80 Prime CNRS initiative focuses on assemblies of plate-like clay and spherical silica (acting as silts) nanoparticles. Its goal is to combine in-depth, multi-scale characterization of the pore network in such assemblies with in situ monitoring of the molecular and/or colloidal diffusive transport. This is the core of the PhD of Sivagen Vydelingum.
The strategy is three-fold. First, elaborate on the different preparation methods leading to mixed porous deposits made of clay and silica/alumina colloids; second, implement a multi-scale characterization approach by joining several experimental techniques to yield 3D structural reconstructions with different spatial resolutions; third, combine the structural/geometrical information on the pore network with in situ monitoring of molecular and/or colloidal diffusive transport.
For the 3D imaging of such a hierarchical porous material, two classes of microscopies are traditionally used: transmission electron microscopy, able to reach an outstanding spatial resolution, well below the nm, but on very small samples (thickness lower than 100 nm); and X-ray tomography, with a lower spatial resolution, above 100 nm, applicable to larger samples (thickness above 10 μm). Probing intermediate length scales on relatively extended samples remains a challenge.
Using Transmission X-ray microscopy in the so-called water window (512 eV), we have recently succeeded in providing structural information in the “missing” intermediate length scale. We have imaged an extended aggregate (10 μm) of clay platelets mixed with spherical silica particles (diameter of 300 nm), using a voxel size of about 11 nm. The measurement was performed on the MISTRAL beamline at the Spanish synchrotron ALBA – one of the few existing X-ray microscopes suitable for studies at the “water window” (soft) X-ray energies. A 3D reconstruction of the aggregate is shown in the following movie. Spherical silica particles are observed mixed with several flat stackings of clay sheets.
This first trial needs to be consolidated with further experiments. However, it is an important step in filling the gap in structural information at intermediate length scales, a crucial component of the multi-scale strategy currently developed in the PHENIX laboratory (UMR8234).