Micro-Scale Modelling of Iron Particles Transport in Saturated Porous Media

Zero-valent iron micro- and nano-particles represent a promising technology for groundwater remediation. Due to their large surface area, reactivity and mobility, iron micro- and nano-particles are extremely effective in contaminants degradation, allowing source treatment, as they can be injected directly in the subsurface in the form of aqueous dispersions. Some promising results have shown that some mobility limitations of iron particles, due to aggregation, can be overtaken by modification of the suspensions using biodegradable hydrocolloids, namely guar gum and xanthan gum, thus forming non-Newtonian fluids. The design of a remediation project is thus strictly connected to the determination of the radius of influence of an injection well (ROI). This can be estimated using a macro-scale modelling approach, able to predict particle transport and accounting for particle-particle and particle-grain interactions. The aim of this work (co-funded by European Union project AQUAREHAB (FP7 - Grant Agreement Nr. 226565)), is to use Computation Fluid Dynamics (CFD) and Population Balance Models (PBM) at the micro-scale to derive constitutive equations for the model at the macro-scale. Several two-dimensional computational grids, created in Gambit and obtained by Scanning Electron Microscopy (SEM) micrographs, are here employed to describe the real geometry of a small portion of the porous medium. Fluent 12.1 was used to model the flow field at the microscale, using different rheological models (Newtonian and non-Newtonian), effective velocities, as well as sand size and porosity values. Preliminary results show good agreement between the permeabilities predicted by the CFD model and by the empirical Ergun equation. An example is shown in the figure, where the velocity magnitude contour plot for water flowing through sand particles of 300 micrometers is reported. The CFD code is then coupled with PBM to simulate the presence and the evolution of the particles, including for example aggregation, in order to extract attachment-detachment kinetics to be used in macroscale approach.