Zerovalent iron nanoparticles for groundwater remediation: Surface and magnetic properties, colloidal stability, and perspectives for field application

In the context of groundwater remediation, the use of zero-valent metals is known to be successful for the degradation of a wide range of contaminants. In particular, granular iron filings in permeable reactive barriers (PRBs) are a consolidated technology applied on a number of sites. Nevertheless, the realization and construction limitations restrain, in some cases, the applicability of this technology. In order to overcome some of these restrictions, the use of nanoscale zerovalent iron (nZVI) was proposed. NZVI particles, thanks to their reduced size, can be dispersed in aqueous slurries, and directly injected in the subsoil slurry, thus allowing to directly target the contaminant close to the source of contamination. However, critical points for successfully full-scale applications are stability against aggregation, mobility in subsurface environments, and longevity under subsurface conditions. Iron nanoparticles should remain in suspension for a time sufficient to allow slurry preparation, handling and injection in the subsurface. Also, they should have a sufficient mobility in the subsurface to be transported for some extent around the injection point. However, several studies have shown nZVI to be scarcely mobile and stable in both laboratory studies and field-scale tests, due to the strong tendency of nZVI particles to aggregate when dispersed in water, forming big dendritic flocs and subsequently network structures, which may widely exceed the micron. Such aggregates could also significantly limit transport by plugging the pores of the aquifer, and exhibit reduced specific surface area, and consequently reactivity. Reasons for nZVI aggregation are to be found in the balance of repulsive and attractive forces acting between particles. Iron nanoparticles, being mainly composed by Fe0 and iron oxides are subject to long-ranged attractive magnetic forces. Magnetic interactions have been stated to be the cause for the abatement of the energy barrier in the inter-particle interaction potential, promoting aggregation. Therefore, strong long-ranged repulsive forces among particles are needed to overcome this magnetic attraction. Coating of the iron nanoparticles with hydrophilic polymers and increasing the viscosity of the nZVI slurry were found successful approaches for improving both colloidal stability and mobility in lab-scale experiments. The particles can be delivered into the subsurface using both permeation and fracturing technologies. When injected into natural aquifers, highly concentrated nZVI slurries (usually between 10 - 20 g/L) will need to move from the wells to the contaminated zone, coming in contact with the contaminants. At the same time, they should not disperse in the environment or leave the contaminated area with a residual reactivity. As a consequence, the mobility of the particles is to be predicted with a sufficient precision before designing any injection at the field scale. Although it is unfeasible to draw absolute conclusions for nZVI mobility in natural subsurface environments from results of laboratory tests, numerical transport models can be developed from column tests, and used to simulate nZVI transport in porous media at the field scale. Such models can represent a tool in the development of an efficient injection technology for field-scale applications of nZVI slurries.