Modelling the mobility in porous media of iron colloids for groundwater remediation: from micro- to macroscale

Colloidal suspensions of engineered nanoparticles have been studied in recent years for waste water and in-situ groundwater remediation. 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 viscous dispersions, characterized by complex rheological properties. Assessing the mobility of iron-based colloids is a key issue for field applications of these materials. In this work, co-funded by European Union project AQUAREHAB (FP7 - Grant Agreement Nr. 226565), micro- and macro-scale modelling approaches are proposed to determine the key factors that control the mobility of iron suspensions, and to simulate their transport at both laboratory and field scale. A micro-scale model can improve knowledge of pore-scale interaction mechanisms, that are lumped together in semi-empirical coefficients in macro-scale simulations. A macro-scale model can in turn help interpreting laboratory transport tests, and provide an estimation of the radius of influence for the injection points, for a correct dimensioning of full scale remediation and to predict short- and long-term mobility of the iron particles injected in the subsurface. In the micro-scale model, CFD was used to describe the flow of a non-Newtonian fluid carrying the particles through the pores of a two-dimension geometry representing a small portion of the porous medium. The Navier-Stokes and continuity equations were solved with Newtonian and non-Newtonian rheological models. In the range of superficial velocity (10-6–10 m/s), particle size (1-103 nm) and porosity investigated, the continuum hypothesis was always valid. In the macro-scale model, a numerical solution of the transport equations was developed coupling the advection/dispersion equation with kinetic expressions for dual-phase, non-equilibrium interactions between particles in the liquid (water) and solid (grains) phase. Rheological properties of the shear-thinning carrier fluid, hydrodynamic parameters of the porous medium (porosity, permeability), and colloid concentrations (both suspended and deposed) are strongly inter-dependent, thus resulting in a complex set of coupled partial differential equations and constitutive relationships.