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A promising superparamagnetic nanomagnetite dipped with Gd was synthesized for possible medical applications. Its size and morphology are independent of Gd content ranging from 1 to 5%. Gadolinium (III) replaced Fe(III) in the lattice. The sizes of Gd-doped nanoparticles ranged from 5 to 50 nm and exhibited a pure magnetite mineralogical phase., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The mobility of 79 Se, a fission product of 235 U and long-lived radioisotope, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Nonradioactive selenium is also an important contaminant of drainage waters from black shale mountains and coal mines. Highly mobile and soluble in its high oxidation states, selenate (Se( VI )O 4 2- ) and selenite (Se( IV )O 3 2- ) oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, thereby being reduced and consequently immobilized by forming low-solubility solids. Here, we investigated the sorption and reduction capacity of synthetic nanomagnetite toward Se(VI) at neutral and acidic pH, under reducing, oxygen-free conditions. The additional presence of Fe( II ) aq , released during magnetite dissolution at pH 5, has an effect on the reduction kinetics. X-ray absorption spectroscopy analyses revealed that, at pH 5, trigonal gray Se(0) formed and that sorbed Se(IV) complexes remained on the nanoparticle surface during longer reaction times. The Se(0) nanowires grew during the reaction, which points to a complex transport mechanism of reduced species or to active reduction sites at the tip of the Se(0) nanowires. The concomitant uptake of aqueous Fe(II) and Se(VI) ions is interpreted as a consequence of small pH oscillations that result from the Se(VI) reduction, leading to a re-adsorption of aqueous Fe(II) onto the magnetite, renewing its reducing capacity. This effect is not observed at pH 7, where we observed only the formation of Se(0) with slow kinetics due to the formation of an oxidized maghemite layer. This indicates that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO 3 2- ) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO 3 2- -intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO 3 , is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl 2 shows much stronger affinity and larger distribution ratio (R d ∼ 17800 L kg -1 ) toward SeO 3 2- than AFm-SO 4 (R d ∼ 705 L kg -1 ). At stoichiometric SeO 3 2- loading for anion exchange, the newly formed AFm-SeO 3 phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO 3 2- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl - , SO 4 2- , MoO 4 2- , and SeO 3 2- ). Contrary to AFm-SeO 3 with inner-sphere SeO 3 2- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO 3 2- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO 4 2- ) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO 4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms-edge sorption, interfacial dissolution-reprecipitation-are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.

The objective of this work was to investigate the fate of silver nanoparticles (Ag-NPs) in a sludge-amended soil cultivated with monocot (Wheat) and dicot (Rape) crop species. A pot experiment was performed with sludges produced in a pilot wastewater treatment plant containing realistic Ag concentrations (18 and 400 mg kg(-1), 14 mg kg(-1) for the control). Investigations focused on the highest dose treatment. X-ray absorption spectroscopy (XAS) showed that Ag2S was the main species in the sludge and amended soil before and after plant culture. The second most abundant species was an organic and/or amorphous Ag-S phase whose proportion slightly varied (from 24% to 36%) depending on the conditions. Micro and nano X-ray fluorescence (XRF) showed that Ag was preferentially associated with S-rich particles, including organic fragments, of the sludge and amended soils. Ag was distributed as heteroaggregates with soil components (size ranging from ≤0.5 to 1-3 μm) and as diffused zones likely corresponding to sorbed/complexed Ag species. Nano-XRF evidenced the presence of mixed metallic sulfides. Ag was weakly exchangeable and labile. However, micronutrient mobilization by plant roots and organic matter turnover may induce Ag species interconversion eventually leading to Ag release on longer time scales. Together, these data provide valuable information for risk assessment of sewage sludge application on agricultural soils.