Iron oxide nanoparticles (FeONPs)
prepared with plant extracts
have been emerging as green and sustainable materials. FeONPs are
usually amorphous due to the chelation of the tea polyphenols (TPs)
to the iron, and the real nature of the iron compounds is not completely
understood. The main goal of this study was to investigate the behavior
of the green FeONPs synthesized from an Fe3+ salt and Cammelia sinensis (black tea) extract upon thermal
treatment, in order to remove TPs and enable the formation of crystalline
materials suitable for a thorough characterization and with the potential
for diverse applications. The as-prepared FeONPs were assigned as
mixed-valence Fe(III) oxyhydroxides and Fe(II)/Fe(III) ions bound
to TPs. A detailed description of the phase transformation upon heating
revealed the formation of the rare nano β-Fe2O3 phase at 400 °C, followed by a transformation to α-Fe2O3 as the temperature increased. Above 600 °C,
the unprecedented formation of FePO4 and Fe3PO7 was observed, produced from the reaction of Fe2O3 and free phosphate ions present in the black
tea leaves, Fe3PO7 being the major phase obtained
at 900 °C. Finally, the catalytic potential of the FeONPs to
treat the azo dye methyl orange through a heterogeneous Fenton-like
system was investigated.
The
reaction of Fe(CO)5 and Pt2(dba)3 in 1-n-butyl-methylimidazolium tetrafluoroborate
(BMIm.BF4), hexafluorophosphate (BMIm.PF6),
and bis(trifluoromethanesulfonyl)imide (BMIm.NTf2) under
hydrogen affords stable magnetic colloidal core–shell nanoparticles
(NPs). The thickness of the Pt shell layer has a direct correlation
with the water stability of the anion and increases in the order of
PF6 > BF4 > NTf2, yielding
the metal
compositions Pt4Fe1, Pt3Fe2, and Pt1Fe1, respectively. Magnetic measurements
give evidence of a strongly enhanced Pauli paramagnetism of the Pt
shell and a partially disordered iron-oxide core with diminished saturation
magnetization. The obtained Pauli paramagnetism of the Pt shell is
2 orders of magnitude higher than that of bulk Pt, owing to symmetry
breaking at the surface and interface, resulting in a strong increase
in the density of states at the Fermi level, and thus to enhanced
Pauli susceptibility. Moreover, these ultrasmall NPs showed efficient
catalytic activity for the direct production of selective short-chain
hydrocarbons (C1–C6) by the Fischer–Tropsch
synthesis with efficient conversion (18–34%) and selectivity
(69–90%, C2–C4). The selectivity
and activity were dependent on the Fe-oxides@Pt particle size. The
catalytic activity decreased from 34 to 18% as the NP size increased
from 1.7 to 2.5 nm at 15 bar and 300 °C.
On November 5, 2015, a large tailing deposit failed in Brazil, releasing an estimated 32.6 to 62 million m3 of iron mining tailings into the environment. Tailings from the Fundão Dam flowed down through the Gualaxo do Norte and Carmo riverbeds and floodplains and reached the Doce River. Since then, bottom sediments have become enriched in Fe(III) oxyhydroxides. Dissimilatory iron-reducing microorganisms (DIRMs) are anaerobes able to couple organic matter oxidation to Fe(III) reduction, producing CO2 and Fe(II), which can precipitate as magnetite (FeO·Fe2O3) and other Fe(II) minerals. In this work, we investigated the presence of DIRMs in affected and non-affected bottom sediments of the Gualaxo do Norte and Doce Rivers. The increase in Fe(II) concentrations in culture media over time indicated the presence of Fe(III)-reducing microorganisms in all sediments tested, which could reduce Fe(III) from both tailings and amorphous ferric oxyhydroxide. Half of our enrichment cultures converted amorphous Fe(III) oxyhydroxide into magnetite, which was characterized by X-ray diffraction, transmission electron microscopy, and magnetic measurements. The conversion of solid Fe(III) phases to soluble Fe(II) and/or magnetite is characteristic of DIRM cultures. The presence of DIRMs in the sediments of the Doce River and tributaries points to the possibility of reductive dissolution of goethite (a-FeOOH) and/or hematite (a-Fe2O3) from sediments, along with the consumption of organics, release of trace elements, and impairment of water quality.
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