Analytical Characterisation of Nanoscale Zero-Valent Iron: A Methodological Review
Chekli, L., B. Bayatsarmadi, R. Sekine, B. Sarkar, A. Maoz Shen, K. Scheckel, W. Skinner, R. Naidu, H. Shon, E. Lombi, AND E. Donner. Analytical Characterisation of Nanoscale Zero-Valent Iron: A Methodological Review. Richard P. Baldwin (ed.), ANALYTICA CHIMICA ACTA. Elsevier Science Ltd, New York, NY, 903:13-35, (2016).
In recent years, manufactured nanoparticles (MNPs) have attracted increasing interest for their potential applications in the treatment of contaminated soil and water. In comparison to traditional macro- and micro-scale materials, MNPs possess significantly higher surface-to-volume ratios, which frequently translate into unexpected surface effects and unique beneficial properties. In addition, by virtue of their small size, MNPs can potentially be used directly in the field for in situ treatment via injection at almost any location and depth in soil and groundwater systems . Ideally, for this type of application, MNPs are expected to feature several key properties including: (i) high reactivity for the removal of targeted contaminants; (ii) high mobility in porous media (i.e. aquifers, soil); (iii) high reactive longevity after injection and (iv) low toxicity to the biota in the surrounding environment . These properties are the main drivers when designing MNPs for the purpose of soil and groundwater remediation. However, it is quite clear that in practice some of these properties are difficult if not impossible to achieve simultaneously. For instance, high reactivity usually translates to short longevity, and potentially greater ecotoxicological impact. In addition, these nanoparticles must also be produced and delivered at a cost that remains sufficiently low to compete with other conventional technologies. Due to their purportedly low cost, highly reactive surface sites and high in-situ reactivity, the most widely studied MNPs for soil and environmental remediation are nanoscale zero-valent iron (nZVI) [2-4]. Many studies suggested a core-shell structure for nZVI [14,15] as illustrated in Fig. 1. The core consists primarily of zero-valent iron while the mixed valent oxide shell resulted from the oxidation of the core metallic iron . nZVI materials have been shown to exhibit high reactivity in remediating aquifers contaminated by non-aqueous phase liquids (NAPL), and many other hazardous compounds [3,17,18].
Zero-valent iron nanoparticles (nZVI) have been widely tested as they are showing significant promise for environmental remediation. However, many recent studies have demonstrated that their mobility and reactivity in subsurface environments are significantly affected by their tendency to aggregate. Both the mobility and reactivity of nZVI mainly depends on properties such as particle size, surface chemistry and bulk composition. In order to ensure efficient remediation, it is crucial to accurately assess and understand the implications of these properties before deploying these materials into contaminated environments. Many analytical techniques are now available to determine these parameters and this paper provides a critical review of their usefulness and limitations for nZVI characterisation. These analytical techniques include microscopy and light scattering techniques for the determination of particle size, size distribution and aggregation state, and X-ray techniques for the characterisation of surface chemistry and bulk composition. Example characterisation data derived from commercial nZVI materials is used to further illustrate method strengths and limitations. Finally, some important challenges with respect to the characterisation of nZVI in groundwater samples are discussed.