MNM3D: a modelling tool for simulation of nanoparticle injection and transport in 3D geometries

The design of a field-scale injection of engineered nanoparticle (NP) suspensions for the remediation of a polluted site requires the development of quantitative predictive models for the system design and implementation. In general, micro- and nanoparticle transport in porous media is controlled by particle-particle and particle-porous media interactions, which are in turn affected by flow velocity and pore water chemistry. Particle transport is modelled by a modified advection-dispersion equation, which accounts for particle deposition and release phenomena. During the injection, a strong perturbation of the flow field is induced around the well, and the particle transport is mainly controlled by the consequent sharp variation of pore-water velocity, and by the hydro-chemical properties of the injected fluid. Conversely, when the injection is stopped, the particles are transported solely due to the natural flow, and the influence of groundwater geochemistry (ionic strength, IS, in particular) on the particle behaviour becomes predominant. Pore-water velocity and IS are therefore important parameters influencing particle transport in groundwater, and have to be taken into account by the numerical codes used to support nanoremediation design. Several analytical and numerical tools have been developed in recent years to model the transport of colloidal particles in simplified geometry and boundary conditions. For instance, the numerical tool MNMs was developed by the authors of this work to simulate colloidal transport in 1D Cartesian and radial coordinates. Only few simulation tools are instead available for 3D colloid transport, and none of them implements direct correlations accounting for variations of groundwater IS and flow velocity. In this work a new modelling tool, MNM3D (Micro and Nanoparticles transport Model in 3D geometries), is proposed for the simulation of injection and transport of nanoparticle suspensions in generic complex scenarios. MNM3D implements a new formulation to account for the simultaneous dependency of the attachment and detachment kinetic coefficients on groundwater IS and velocity. To the authors' knowledge, MNM3D is the first 3D numerical code embedding a specific routine for the continuous calculation, cell by cell and at the time-step level (for transient flow simulations), of the attachment and detachment kinetics. The software was developed in the framework of the FP7 European research project NanoRem and can be used to predict the NP mobility at different stages of a nanoremediation application, both in the planning and design stage (i.e. support the design of the injection plan), and later to predict the long-term particle mobility after injection (i.e. support the monitoring, final fate of the injected particles). In this work MNM3D is used to model a 2D pilot scale injection of CARBO-IRON® in a small scale flume carried out at the VEGAS facilities in the framework of the NanoRem project. Moreover, the long term fate of an hypothetical release of nanoparticles into the environment from a landfill is simulated. The work is co-funded by the FP7 EU project NANOREM (GA 309517).