Understanding molecular and/or colloidal transport in complex and multi-scale porous systems is a prerequisite in many areas where environmental, cultural or industrial issues are involved.
From pollution control and decontamination to geothermal energy: numerous applications
Regarding the soil management, we could mention, in particular:
- improving the soil remediation processes to meet the foreseeable needs linked to increasing urban expansion in the future. Cleaning up polluted soils involves implementing confined transport processes for molecules and/or colloids coupled with adsorption/desorption processes at solid/liquid interfaces. Clays and silts play an important role in this area;
- the transport and possible controlled blocking of porous horizons by colloids, whether clay-based or not, such as in geothermal applications.
The two topics will contribute to the general research field of the soil remediation process by using an approach based on nanoparticle injection (Engineering nano material).
An ambitious research project
The aim of the research project we are developing through LCR CARMEN – involving PHENIX (Sorbonne University-CNRS), IPCMS (Strasbourg University -CNRS) and IFPEN – is to choose several reconstituted porous systems (clay and silica colloids) and to combine in-depth, multi-scale knowledge of the pore network with in situ monitoring of molecular and/or colloidal diffusive transport.
The aim of the research project we are developing through LCR CARMEN is to combine in-depth, multi-scale knowledge of the pore network with in situ monitoring of the molecular and/or colloidal diffusive transport in the ternary diagram of the clay-water-silica particle (acting as silts).
Choices and strategies for multiphase porous media
Within this framework, defining the type and the nature of porous systems is still a matter of debate, given that we do not intent to study soils of natural origin, which are too complex and have a high degree of soil variability.
Our multiphase granular porous media will be reconstituted from mixtures of clays (with well-controlled size and chemistry) and silica particles of varying sizes (ranging from few hundreds of nm to several tens of µm).
Several mixing strategies will be tested (PHENIX). These will either combine clay grains with silica grains in bulk, or generate silica covered with a clay coating. Adding polyelectrolytes to control the flocculation of clays will be also considered.
Controlling humidity to monitor the structure of the clays
The chemical potential of the water (relative humidity and/or degree of water saturation) will be controlled. A systematic change of this parameter may be used to assess the hydric history of the samples which can be at the origin of a structural change in the porous materials.
In this regard, IFPEN's expertise in humidity control will be crucial in order to study the filling states before saturation with water. The local evolution of the clays will be monitored by XRD (X-ray diffraction), in particular by studying the changes in pseudo-ray 001.
Three stages of research work
The plan for this research project is divided into three complementary parts. In parallel and independently of the thesis work carried out by Syvagen Vydelingum, a prospective analysis will be conducted concerning colloidal transport.
What techniques can be used to establish the structure of the porous network?
First, the geometrical study of the porous media will be carried out by combining several techniques:
- small-angle X-ray or neutron scattering (SAXS, SANS), available at IFPEN, on a synchrotron (SWING line) and at ILL;
- X-ray tomography (IFPEN-SOLEIL-PHENIX);
- high and low energy X-ray microscopy (SOLEIL, ALBA, SLS) and high-resolution electron tomography as well as in-situ transmission electron microscopy in a partially water-saturated environment (IPCMS).
The project also aims to implement a multi-scale characterization approach by combining 3D reconstructions with different spatial resolutions.
Comparing the findings obtained by using these complementary techniques will enable us to monitor the variations of structural organization in the direct space at scales ranging from 0.5 nm to several hundred µm.
What approaches will be used to study molecular transport?
The second part of the project will allow to study the molecular transport of water within these porous materials. The monitoring process will draw on the tools used by PHENIX as part of the experimental and theoretical development of low field NMR.
The interaction of water and its dynamics with the interfaces of the porous systems (which could be termed "nanowettability") will be monitored using variable field-cycling NMR relaxometry (NMRD, Nuclear Magnetic Relaxation Dispersion, operated by PHENIX) and quasi-elastic neutron scattering (time-of-flight or spin echo/PHENIX-ILL).
The combination of these experimental approaches will enable us to monitor the dynamics on a time scale ranging from a fraction of a ns to 10 µs and for lengths ranging from 0.1 nm to 50 nm.
For larger scales (µm or more), we will need to qualify the diffusive transport properties in relation to the architecture and topology of the porous system.
This second study will essentially use the nuclear magnetic resonance techniques available at PHENIX and IFPEN, including pulsed magnetic field gradients. Finally, recently developed NMR techniques known as "2D T2-D-T2" will enable us to conduct a mesoscopic transport analysis focusing on the exchange between pores with different sizes.
Theoretical approaches and digital simulation
Finally, this research work will be completed by comprehensive theoretical studies using numeric and modelling tools. These theoretical approaches will be based primarily on molecular dynamics simulations (Molecular dynamics, coarse grain, Random Walk).
The simulations will specifically use as input the 3D experimental reconstructions obtained by X-ray and electron tomography. We will incorporate a statistical physics approach using the first-pass statistics.