D. Martínez-Blanco

Optimisation of magnetic separation: a case study for soil washing at a heavy metals polluted site.

Sandy loam soil polluted with heavy metals (As, Cu, Pb and Zn) from an ancient Mediterranean Pb mining and metallurgy site was treated by means of wet high-intensity magnetic separation to remove some of the pollutants therein. The treated fractions were chemically analysed and then subjected to magnetic characterisation, which determined the high-field specific (mass), magnetic susceptibility (κ) and the specific (mass) saturation magnetisation (σS), through isothermal remanent magnetisation (IRM) curves. From the specific values of κ and σS, a new expression to assess the performance of the magnetic separation operation was formulated and verified by comparison with the results obtained by traditional chemical analysis. The magnetic study provided valuable information for the exhaustive explanation of the operation, and the deduced mathematical expression was found to be appropriate to estimate the performance of the separation operation. From these results we determined that magnetic soil washing was effective for the treatment of the contaminated soil, concentrating the majority of the heavy metals and peaking its separation capacity at 60% of the maximum output voltage.

Scrutinizing the role of size reduction on the exchange bias and dynamic magnetic behavior in NiO nanoparticles.

NiO nanoparticles (NPs) with a nominal size range of 2-10 nm, synthesized via high-temperature pyrolysis of a nickel nitrate, have been extensively investigated using neutron diffraction and magnetic (ac and dc) measurements. The magnetic behavior of the NPs changes noticeably when their diameter decreases below 4 nm. For NPs larger than or equal to this size, Rietveld analysis of the room temperature neutron diffraction patterns reveals that there is a reduction in the expected magnetic moment per [Formula: see text] ion with respect to bulk NiO, which is linked to the existence of a magnetically disordered shell at the NP surface. The presence of two peaks in the temperature dependence of both the dc magnetization after zero-field-cooling and the real part of the ac magnetic susceptibility is explained in terms of a core (antiferromagnetic, AFM)/shell (spin glass, SG) morphology. The high-temperature peak ([Formula: see text] K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak ([Formula: see text] K) is a signature of a SG-like freezing of the surface [Formula: see text] spins. In addition, an exchange bias (EB) effect emerges due to the core/shell magnetic coupling. The cooling field and temperature dependences of the EB effect and the coercive field are discussed in terms of the core size and the effective magnetic anisotropy of the NPs. However, NiO NPs of 2 nm in size no longer show AFM order and the [Formula: see text] magnetic moments freeze into a SG-like state below [Formula: see text] K, with no evidence of EB effect.

Unravelling the onset of the exchange bias effect in Ni(core)@NiO(shell) nanoparticles embedded in a mesoporous carbon matrix

Ni(core)@NiO(shell) nanoparticles (NPs) were synthesized through the pyrolysis of an inorganic precursor taking place within the pores of an active carbon matrix at different temperatures between 673 and 1173 K, and a subsequent oxidation in air. For the lowest temperature (673 K), the smallest average size of the NPs (9 nm) and the largest percentage of NiO (82%) are observed. Upon increasing the temperature up to 1173 K, an average diameter of 23 nm is observed while the NiO percentage decreases below 20%. We found that each NP consists of a Ni core surrounded by a structurally disordered NiO shell with a constant thickness of ∼2 nm, regardless of the core size. The spins inside the NiO shell freeze into a spin glass (SG)-like state below Tf ∼ 40 K. The magnetic exchange coupling between the Ni core and the NiO shell spins gives rise to the occurrence of the exchange bias (EB) effect, whose temperature dependence follows a universal exponential trend in all samples. The SG nature of the shell spins yields a vanishing EB above Tf. This is far below the Neel temperature of bulk antiferromagnetic NiO (TN ∼ 523 K) that usually determines the onset of the EB effect in Ni/NiO interfaces.

Disentangling magnetic core/shell morphologies in Co-based nanoparticles

Co-based nanoparticles (NPs) have been extensively explored due to their prospective applications in areas as diverse as efficient water treatment (Co NPs), hydrogen generation (CoO NPs) and combustion catalysis (Co3O4 NPs). In recent years, the emergence of Co-based entities as bi-magnetic core/shell NPs has opened new avenues for their innovative use in fields ranging from energy storage and magnetic recording to biomedicine. The control and characterization of these nanomaterials thus becomes of paramount importance for targeting their foreseen applications. Here, we show that the intentional oxidation of metallic Co NPs with different sizes (3–50 nm) gives rise to a wide variety of core/shell morphologies including Co, CoO and Co3O4 phases. Bridging the information coming from high-resolution transmission electron microscopy, X-ray absorption spectroscopy and magnetic measurements gives us a self-consistent picture that describes the role played by the morphology and microstructure in the magnetism of Co and its oxides at the nanoscale.

Size effects on the Néel temperature of antiferromagnetic NiO nanoparticles

Among all antiferromagnetic transition metal monoxides, NiO presents the highest Neel temperature (TN ∼ 525 K). In this work, the size-dependent reduction of TN in NiO nanoparticles with average diameters (D) ranging from 4 to 9 nm is investigated by neutron diffraction. The scaling law followed by TN(D) is in agreement with the Binder theory of critical phenomena in low-dimensional systems. X-ray absorption fine structure measurements link the decrease of TN to the occurrence of size effects (average undercoordination, bond relaxation and static disorder) in the nearest and next-nearest Ni coordination shells that hold the key for the maintenance of the antiferromagnetic order.

Nanoscale zero-valent iron-assisted soil washing for the removal of potentially toxic elements

The present study focuses on soil washing enhancement via soil pretreatment with nanoscale zero-valent iron (nZVI) for the remediation of potentially toxic elements. To this end, soil polluted with As, Cu, Hg, Pb and Sb was partitioned into various grain sizes (500-2000, 125-500 and <125 μm). The fractions were pretreated with nZVI and subsequently subjected, according to grain size, to Wet-High Intensity Magnetic Separation (WHIMS) or hydrocycloning. The results were compared with those obtained in the absence of nanoparticles. An exhaustive characterization of the magnetic signal of the nanoparticles was done. This provided valuable information regarding potentially toxic elements (PTEs) fate, and allowed a metallurgical accounting correction considering the dilution effects caused by nanoparticle addition. As a result, remarkable recovery yields were obtained for Cu, Pb and Sb, which concentrated with the nZVI in the magnetically separated fraction (WHIMS tests) and underflow (hydrocyclone tests). In contrast, Hg, concentrated in the non-magnetic fraction and overflow respectively, while the behavior of As was unaltered by the nZVI pretreatment. All things considered, the addition of nZVI enhanced the efficiency of soil washing, particularly for larger fractions (125-2000 μm). The proposed methodology lays the foundations for nanoparticle utilization in soil washing operations.

Disclosure of Double Exchange Bias Effect in Chromium (III) Oxide Nanoparticles

In the last decade, the renewed interest in antiferromagnetic (AF) magnetoelectric (ME) materials has been driven by the challenging multifunctionality of spintronic devices. One of the most ambitious goals is to build exchange-biased ferromagnetic/AF junctions with electric field-controlled properties. In this context, the understanding of the modifications that size reduction induces in the magnetic properties of a material being both AF and ME holds the key to control the magnetic coupling at the interface. Here, we show that the spin arrangement in chromium (III) oxide core/shell nanoparticles changes significantly as a function of the radial distance from the nanoparticle center. While the nanoparticle core retains an AF structure, magnetic moments located on a thin surface shell are in a disordered spin-glass (SG)-like state. In addition, canted spins develop at the boundary of the ME AF core. These spins, which mediate a moderate AF/SG exchange coupling at low temperature, are exchange coupled to the AF core, thus giving rise to a lower yet more robust exchange bias effect, which persists up to the Néel temperature of the AF core.

On the exchange bias effect in NiO nanoparticles with a core(antiferromagnetic)/shell (spin glass) morphology

The unexpected appearance of magnetic hysteresis and exchange bias effects in nominally antiferromagnetic NiO nanoparticles is usually explained in terms of a core/shell morphology, where a spin glass-like shell is exchange coupled to an antiferromagnetic core. However, recent studies have challenged the validity of this assumption for small enough NiO nanoparticles. In this work we present proof of the core/shell model for NiO nanoparticles with sizes below 10 nm by combining neutron powder diffraction and magnetic measurements. In addition, we have verified that the exchange bias effect persists even when the particle size is reduced down to 4 nm.