Transport of iron micro and nanoparticles in saturated porous media

A broad range of natural and anthropogenic colloids is found in natural subsurface systems. On the one hand, properties and mobility of natural colloids are studied as they can act as vehicles for strongly sorbing contaminants (e.g. clays, metal oxides), or as contaminants themselves (e.g. microorganisms). On the other hand, enhanced transport of engineered colloids for groundwater remediation is a major topic extensively addressed in recent years. In this work the mechanisms controlling the transport of colloidal suspensions in saturated porous media, the improvement of stability of highly concentrated micro- and nanosized iron suspensions, and the assessment of their transportability via addiction of “green” polymers are studied. Beside advection-dispersion phenomena, colloid transport in saturated porous media is controlled by particle-particle and particle-collector interactions. While in the presence of unfavourable deposition conditions, the amount of retained particles do not affect significantly the hydrodynamic parameters of the porous matrix and the properties of the pore fluid, when dealing with iron particles the hydrodynamic parameters and fluid properties cannot be considered independent on the concentration of deposed and suspended colloids. In this latter case, particle-particle interactions are strongly attractive, and larger aggregates are found rather than single primary particles, that are filtered by the porous medium. Moreover, attractive forces can lead to ripening phenomena (i.e. deposed particles attract the suspended ones, increasing deposition rates with time) Green polymers can be used to prevent aggregation of primary particles, to enhance stability against sedimentation but also to increase the mobility of slurries in saturated porous media. Sand-packed column experiments were performed injecting iron suspensions at a concentration of 20 g/L, amended with xanthan gum (3g/L), at different ionic strength values (6•10-3 mM or 12.5 mM) in 0.46 m long columns. Breakthrough curves of iron, obtained by in-line continuous measurement of magnetic susceptibility, under each experimental condition showed that normalized elution concentration at the end of the injection (i.e. after 7 or 26 pore volumes) is higher for MZVI (>0.94) than for NZVI (>0.88). Additional susceptibility measurements along the column and pressure drop also confirmed that MZVI is more easily eluted than NZVI. Moreover, water flushing after the iron injection phase lead to recoveries of over 95% for MZVI, and over 92% for NZVI of the total injected iron mass. The tests proved that xanthan gum is an excellent stabilizing agent and delivery vehicle of ZVI particles and has a high potential for use in real scale remediation interventions. Mathematical modelling was carried out as well. A numerical model was developed for the description of colloidal transport under unfavourable deposition conditions, coupling the advection/dispersion partial differential equation with kinetic expressions for dual-phase, non-equilibrium interactions between particles in the liquid (water) and solid (grains) phase. Influence of hydrochemical parameters (namely, ionic strength) was also include into the governing equations.