Denisa Kubániová

By adjusting the Co/Ni ratio in Co _{1− x} Ni _x Fe ₂ O ₄ nanoparticles, the magnetic anisotropy is easily tuned, whereby either Brownian or Néel relaxation is selected as the dominant mechanism that determines the performance in MPI.

To date, the generation of natural hydrogen (H ₂ ) from the alteration of Fe ^II ‐bearing minerals has mainly been studied through serpentinization in (ultra)mafic rocks. This study explores Banded Iron Formations (BIF), which are rich in Fe ^II minerals, as a potential source for H ₂ . We conducted a hydrothermal experiment at 200°C with a combined magnetite‐siderite assemblage, two major components of BIF. The experiment, designed with a high water‐to‐rock ratio and a gas phase (W‐R = 300) enabled to assess mineral transformations during the alteration. Thermodynamic simulations were finally conducted, to explore H ₂ ‐generating yields in more realistic geological scenarios (no gas phase, lower W‐R). Our experimental findings show that H ₂ is produced through complete siderite dissolution and magnetite precipitation. Concomitantly, non‐stoichiometric primary magnetite (Fe ^II /Fe ^III < 0.5) did not enhance H ₂ yield; instead, it acted as a sink for dissolved Fe ²⁺ , sequestrating about 10% of the iron from siderite without oxidation to recover a more ideal stoichiometry. This suggests that abundant, non‐stoichiometric magnetite in natural settings may reduce H ₂ generation yields. Mass balance calculations indicate that 83% of the expected H ₂ generated was unaccounted for, consistent with suspected CO ₂ reduction and formation of dissolved organic compounds in the fluids. Thermodynamic simulations at varying W‐R ratios (from 300 to 1) reveal that H ₂ yield ranges widely (39 mmol–73 μmol H ₂ per kg of siderite), since lower W‐R prevent from siderite dissolution and enhance H ₂ consumption via carbon reduction. These findings imply that low W‐R ratios in BIF ‐and other siderite‐bearing lithologies, may limit H ₂ resources.

Mn-Zn ferrite nanoparticles of the composition Mn0.61Zn0.42Fe1.97O4 and mean size of crystallites dXRD = 11 nm are synthesized under hydrothermal conditions as a single-phase product. Subsequently, two coated samples are prepared by encapsulation of the ferrite particles into silica and titania. Transmission electron microscopy confirms the core-shell structure of the products and shows that the cores are actually formed by small clusters of ferrite crystallites. Powder X-ray diffraction combined with experimental hydrothermal treatment of the titania-coated product demonstrates that the titania coating is amorphous but can be easily transformed to anatase. The colloidal stability of nanoparticles in water is evidenced by dynamic light scattering, and the respective hydrodynamic sizes are dZ = 87 nm and 157 nm for silica-coated and titania-coated particles. The colloidal behavior is confirmed based on the measurements of zeta-potential, whose negative values lead to strong Coulombic repulsion among coated particles. Magnetic measurements on bare and coated particles show high magnetization of Mn0.61Zn0.42Fe1.97O4 cores and superparamagnetic state at room temperature. The relaxometric study on aqueous suspensions in magnetic fields of 0.5 T and 11.75 T reveals high transverse relaxivity of the samples and two distinct forms of its temperature dependence, which are analyzed with respect to the role of temperature-dependent parameters, i.e., the diffusion of water and the magnetization of ferrite cores. Finally, careful evaluation of cytotoxicity of coated particles is carried out by using two different methods, namely the determination of viability and proliferation of Jurkat cells and the real-time monitoring of attachment and proliferation of A549 cells. In the studied range of concentrations, the viability and proliferation of suspension cells are not affected, and only negligible effects are detected in the cell index of adherent cells.