Structural and spectroscopic characterization of air-stable zerovalent iron nanoparticles after an activation process

A. SALEMME; J. CRESPI; E. B. HALAC; C. P. RAMOS; N. QUICI

Congreso; WCCE11 - 11th World Congress of Chemical Engineering; 2023

Zerovalent iron nanoparticles (nZVI) are used for the removal of a wide range of pollutants. nZVI have a core-shell structure, with a core of zerovalent iron and a thin layer of iron oxides shell and are very reactive and pyrogenic. Commercial nZVI NANOFER STAR (NSTAR) manufactured by NANOIRON s.r.o., supplied as a dry air-stable powder is provided with a stabilized surface. According to the user manual, in order to recover the nanoparticles reactivity, NSTAR should be activated by preparing a 20% NSTAR aqueous suspension kept at room temperature (RT) for 48 hours. [1]. In this work, the aim was to study the changes in structure and composition experimented by the nZVI due to the activation process.The study was performed using three types of samples: (1) NSTAR powder; (2) fresh NSTAR aqueous suspension; (3) NSTAR aqueous suspension activated for 48 hours. Suspensions were prepared using Milli-Q water (OSMOION water purification equipment) and the homogenization was carried out with a ProScientific homogenizer. Sample (1) was taken directly from the bottle. Sample (2) was prepared by suspending NSTAR in water, homogenized for 2 minutes and then filtered and dried. Sample (3) was prepared as sample (2), left 48 h for activation at RT in a sealed flask and then homogenized again for 2 minutes before filtration. Both samples (2 and 3) were filtered and dried for 48 h in anoxic environment. The dried solid was placed in a nitrogen-purged Eppendorf tube and sealed to prevent further oxidation. The samples were analyzed by SEM, TEM/EDAX, XRD, Raman and Mössbauer spectroscopy.XRD and Mössbauer results confirmed the presence of Fe(0) in the three samples. Taking sample (1) as reference, Mössbauer analysis showed a change in composition and in dispersion in particle size. These changes are associated with the increase in the proportion of oxides in samples (2) and (3), being sample (3) the one with higher amount of oxides. TEM images showed the preservation of the spherical shape and core-shell structure of nZVI together with a higher aggregation in sample (3) compared to (1) and (2). Sample (3) showed a higher percentage of oxygen according to the EDAX measurements, related to the increase in the percentage of oxides. Also needle-shaped structures were present in (3), which could be associated with different iron oxides (ferrihydrite, goethite or lepidocrocite). SEM images showed nZVI clusters. In Raman spectra, hematite and magnetite were identified in samples (2) and (3); lepidocrocite was also found in (3).In conclusion, it was found that the activation process modifies the composition and structure of the nZVI, increasing the presence of iron oxides and hydroxides and varying the crystalline phases. A higher activating time of the suspension could cause excessive oxidation, decreasing the Fe(0) content, and therefore their ability to remove pollutants.