Effects of water-induced aging on iron oxide nanoparticles: Linking crystal structure, iron ion release, and toxicity

Abstract The effects of aging of colloidal dispersions of iron (Fe) oxy(hydr)oxides have practical implications for a variety of fields, including medicine, biology, chemistry, and environmental science. Aging affects the stability of these materials under different environmental conditions, thereby affecting their reactivity and applicability in remediation. However, only a limited number of studies have focused on aging-induced changes in the phase composition, surface properties, and toxicological effects of nanoparticles (NPs). In this study, a variety of Fe oxides were synthesized, including the closely related Fe oxides magnetite and maghemite, intermediate phases (Fe3-δO4: Fe3O4, γ-Fe2O3, 5Fe2O3∙9H2O), and δ-FeOOH. Fe3O4 was synthesized by precipitation, γ-Fe2O3 by direct oxidation of Fe3O4, while 5Fe2O3∙9H2O and δ-FeOOH were prepared by precipitation with slow and fast oxidation, respectively. The crystal structure, surface charge, and leaching of Fe ions of these materials were measured. All synthesized materials were then tested in bioassays with ciliates and higher plants at circumneutral pH, both upon preparation and after aqueous aging. Quantitative analysis of the XRD data using the Rietveld method showed that the crystal structure of the magnetite nanoparticles changed to γ-Fe2O3. The evaluation of biological activity in Sinapis alba (white mustard) showed that NPs of different compositions, stored at a maximum concentration of 10 g L-1, inhibited root growth by 50%. In the case of δ-FeOOH and Fe3O4, however, concentrations of 1 g L-1 caused only minor inhibition. The toxic effects of Fe-NPs, attributed to the release of Fe2+ and Fe3+ ions by oxidation, were found to be consistent with the redox behavior of NPs. The study of the properties of magnetic nanoparticles, both in their initial state and after aqueous aging, enhances our understanding of their performance in magnetic nanofluids.