Ilona Mohai

Sorption of copper, zinc and lead on soil mineral phases

Soil mineral phases play a significant role in controlling heavy metal mobility in soils. The effective study of their relation needs the integrated use of several analytical methods. In this study, analytical electron microscopy analyses were combined with sequential chemical extractions on soils spiked with Cu, Zn and Pb. Our aims were to study the metal sorption capacity of soil mineral phases and the effect of presence of iron oxide and carbonate on this property of soil minerals. Copper and Pb were found to be characterized by higher and stronger sorption on the studied samples than Zn. Only the former two metals showed significant differences in their immobilized metal amounts on the studied samples and soil mineral particles. Highest metal amounts were sorbed on the swelling clay mineral particles (smectites and vermiculites), but iron-oxide phases may also have similar lead sorption capacity. Alkaline conditions due to the carbonate content of soils resulted both in increased sorption on the mineral particles for Cu and in enhanced role of precipitation for all the studied metals. On the other hand, the intimate association of phyllosilicates and iron resulted in significant increase in metal sorption capacity of the given particle. The results of sequential extractions could be successfully completed by the analytical electron microscopy analyses for studying the sorption capacity of discrete mineral particles. Their integrated use helps us in better understanding the heavy metal-mineral interactions in soils.

Particle size distribution and dislocation density determined by high resolution X-ray diffraction in nanocrystalline silicon nitride powders

Two silicon nitride powders were investigated by high resolution X-ray diffraction. The first sample was crystallized from the powder prepared by the vapour phase reaction of silicon tetrachloride and ammonia while the second was a commercial powder produced by the direct nitridation of silicon. Their particle size and dislocation density were obtained by the recently developed modified Williamson‐Hall and Warren‐Averbach procedures from X-ray diffraction profiles. Assuming that the particle size distribution is log-normal the size distribution function was calculated from the size parameters derived from X-ray diffraction profiles. The size distributions determined from TEM micrographs were in good correlation with the X-ray results. The area-weighted average particle size calculated from nitrogen adsorption isotherms was in good agreement with that obtained from X-rays. The powder produced by silicon nitridation has a wider size distribution with a smaller average size than the powder prepared by vapour phase reaction. The dislocation densities were found to be between about 10 14 and 10 15 m 2 . Published by Elsevier Science S.A. All rights reserved.

Thermal plasma synthesis of zinc ferrite nanopowders

Ž. Nanosized zinc ferrite spinel powders of various compositions were produced in a radiofrequency RF thermal plasma from oxide mixtures and from co-precipitated hydroxides. Bulk and surface chemical compositions of the products were measured by ICP-AES and XPS. Phase conditions were determined by XRD. Morphology was investigated by SEM and TEM. In spite of the short residence time of reagents in the plasma reactor, a high degree of spinel formation was achieved. The Zn enrichment on the surface determined by quantitative XPS is attributed to a thin ZnO layer on the surface of the zinc ferrite particles. In zinc ferrous ferrites, an inhomogeneous distribution of Zn inside the grains may also exist. q 2001 Elsevier Science B.V. All rights reserved.

Association of individual soil mineral constituents and heavy metals as studied by sorption experiments and analytical electron microscopy analyses.

Sorption characteristics of bulk soil samples and discrete soil mineral constituents were studied by Cu, Zn and Pb batch sorption experiments and analytical electron microscopy analyses. Copper and zinc sorbed mostly on soil mineral constituents, while lead was associated mainly to soil organic matter. Additionally, the competitive situation resulted in increase of the role of iron oxides in Pb sorption. Close association of iron oxides and silicates resulted in significant change in their sorption capacities for all the studied metals. The alkaline conditions due to the calcite content in one of the studied soil samples resulted in both increased role of precipitation for Pb and Cu and elevated sorption capacity for Cu by discrete mineral particles. Using the analytical electron microscopy analyses the sorption characteristics of metals were supported by particular data. When the methods used in this study are combined, they become an extremely powerful means of getting a deeper insight into the soil-metal interaction.

Efficient synthesis of fullerenes in RF thermal plasma reactor

Abstract Formation of fullerene soot was studied in an inductively coupled, radiofrequency (RF) thermal plasma reactor. A previously developed kinetic model of fullerene formation was applied to determine synthesis conditions leading to high fullerene yield. The experimental results verified the kinetic model. Maximum yield of 4.1% was obtained in particular conditions. It corresponded to a fullerene production rate of 6.4 g h −1 .

The effect of heat-treatment on the grain-size of nanodisperse plasmathermal silicon nitride powder

Nanodisperse silicon nitride has been synthesized by vapor phase reaction of silicon tetrachloride and ammonia in a thermal plasma reactor and crystallized at temperatures 1250, 1350, 1450 and 1500°C. The average grain-size and the dislocation density of the samples were determined by the recently developed modified Williamson-Hall and Warren-Averbach procedures from X-ray diffraction profiles. A new numerical method provided log-normal grain-size distributions from the size parameters derived from X-ray diffraction profiles. It has been shown that the average grain-size in the amorphous phase is lower than that observed in the crystalline fraction. On the other hand, the average grain-size in the crystalline fraction decreases up to 1450°C while it increases during heat-treatment at 1500°C. The size distribution and the area-weighted average grain-size obtained by X-rays were in good agreement with those determined by TEM and from the specific surface area, respectively. The dislocation density was found to be of the order of 1014 and 1015 m−2.

Composition and microstructure of nanosized, amorphous and crystalline silicon nitride powders before, during and after densification

The composition, microstructure and densification behaviour of two amorphous and three crystalline silicon nitride powders containing an MgO–Y2O3 sintering aid have been investigated. The powders were characterised in terms of bulk and surface chemical compositions, crystallinity and morphology. The hot-pressing behaviours of the powders were assessed by considering the mass losses on densification, relative densities and the chemical and phase compositions and microstructure of the dense materials. Densification kinetics were analysed by the computer evaluation of hot-pressing data.

Densification of nanosized amorphous and crystalline silicon nitride powders

The hot-pressing behavior of two amorphous and three crystalline silicon nitride powders, including both experimental and commercial samples has been investigated in the presence of MgO-Y2O3 sintering aid. The powders were characterized in terms of bulk and surface chemistry, phase composition and morphology. The sintering behavior was assessed on the basis of green and final densities, weight loss on densification and chemical and phase compositions of the dense material. ©

RF thermal plasma processing of fullerenes

This paper presents results on fullerene formation during the processing of different graphite powders in an RF thermal plasma reactor. Graphite powders of different particle size and purity were fed into the reactor (Aldrich, KS4). Optical emission spectroscopy of atomic and molecular species is used as a diagnostic tool of the RF plasma flame. It was found that rotational and vibrational temperatures of C2 radicals depend on the feed rate of the precursor used. The volumes of RF plasma flame during processing of graphite powders have been calculated. By analysing scanning electron microscopy micrographs of soot, the evaporation rate of the precursors used has been evaluated as well. Based on the obtained values of volumes of plasma flame and evaporation rate of starting powders, the concentration of C2 radicals has been calculated.

RF Thermal Plasma Synthesis of Ferrite Nanopowders from Metallurgical Wastes

RF thermal plasma synthesis of zinc-ferrite nanopowders has been studied from metallurgical wastes of high iron oxide and zinc oxide and/or hydroxide content. XRD measurements of the products revealed that an extensive spinel formation took place on RF plasma treatment. However, instead of expectable ZnFe 2O4, formation of zinc ferrous ferrites (Zn 0,7 Fe 2,3 O4 and Zn 0,4 Fe 2,6 O4) was concluded from the XRD investigations. Addition of some nickel salt to the waste mixture resulted in the formation of Ni-Zn ferrites of different composition. Measurements of magnetic properties revealed that the saturation magnetization of ferrite powders produced by thermal plasma treatment was much higher than that of ferrites synthesized by the conventional ceramic route.