Transport of highly concentrated slurries of iron colloids for groundwater remediation: experimental and modeling

Colloidal suspensions of engineered nanoparticles have been studied in recent years for waste water and in-situ groundwater remediation. In particular, injectable Fe-based materials (nano- and micro-sized zero-valent iron particles, NZVI and MZVI) are an improvement on commonly used granular iron: they can be suspended in a slurry and directly injected into the source of contamination, overcoming most of the limitations of zerovalent iron permeable reactive barriers (PRBs). Moreover, colloidal particles show increased reactivity, thanks to their high specific surface area. To improve colloidal stability and transportability of MZVI and NZVI, the use of highly viscous shear-thinning carrier fluids have been suggested, and proved to be effective. This work, co-funded by European Union project AQUAREHAB (FP7 - Grant Agreement Nr. 226565), presents experimental and modeling results for the transport of highly concentrated (up to 20 g/l) non-Newtonian (namely, shear-thinning) slurries of MZVI and NZVI in sand packed columns. Assessing the mobility of iron-based colloids is a key issue for field applications of these materials. A transport model is required to provide a reliable estimation of the radius of influence for the injection points, for a correct dimensioning of full scale remediations and to predict short- and long-term mobility of the iron particles injected in the subsurface. If compared to transport models developed for natural colloids and synthetic model particles (namely latex spheres), the model here presented incorporates more complex phenomena. 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. The model results highlight that deposition of relevant amounts of iron particles onto the soil grains significantly reduce porosity and permeability, thus resulting in clogging phenomena, especially for nanoscale particles. Moreover, mechanical filtration is shown to play a role under certain hydrochemical conditions, thus influencing the particle transport.