Magnetic characterization of water suspensions of iron nanoparticles for groundwater remediation

Iron nanoparticles are a potential answer to the need for effective in-situ groundwater remediation technologies. Nanoscale iron slurries can be injected in the subsurface allowing to target directly the source of contamination reducing times and costs of remediation. Unfortunatelly the use of this material is strongly hindered by magnetic interactions. Several studies [1] have shown that this system is unstable and tends to separate into water and solid phases after a relatively short time. The colloidal instability is due to the aggregation of the nanoparticles into micrometric dendritic structures which tend to sediment. In this study a magnetic characterization was performed on some samples of iron nanopowders with and without the addition of hydrocolloids to prevent the aggregation and settling. Hysteresis loops have been measured on water dispersed Fe nanoparticles by means of an alternating gradient field magnetometer, at room temperature. The studied systems are characterized by a soft, isotropic magnetic behaviour. However, even though the samples are constituted by nanometer-sized Fe particles, the magnetization processes do not follow a Langevin-type curve, typical of superparamagnetic systems. On the contrary it has been shown that the particles form multi-domain aggregates [2]. An extension of the Stoner-Wohlfarth model can be applied, in which a new switching rule for the local magnetization is postulated, accounting for a nucleation of a new magnetic domain and consequent domain wall displacement within any agglomerate of particles. A so-called cut coefficient r (0 _ r _ 1) parametrizes the nucleation field, being r = 1 the case of magnetization reversal due only to reversible or irreversible rotations (thus corresponding to the “classical” Stoner-Wohlfarth case), whereas when r tends to 0 the domain wall movements become increasingly important in inverting the sample magnetization. Samples with different concentrations of Fe nanoparticles, dispersed in water bare or after coating with hydrocolloids, have been studied through their room temperature hysteresis loops. Proper application of the “extended” Stoner-Wohlfarth model allows an estimation of the effectiveness of the coating to prevent the formation of large aggregates; in fact, if the particles are more separated, their magnetic behaviour should progressively tend towards that of smaller clusters (with a cut coefficient r closer to 1) or even non-interacting particles (with a superparamagnetic behaviour).