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Hydrothermal treatment of Zr, Ti, Sn and Ge hydrogenphosphates

Characterization of the derived compounds by thermal methods

Journal of Thermal Analysis and Calorimetry
Authors: P. Patrono, A. La Ginestra, C. Ferragina, M. A. Massucci, A. Frezza, and S. Vecchio

The Zr, Ti, Sn and Ge hydrogenphosphates, generally prepared in a crystalline form by the refluxing method, have been submitted to hydrothermal treatment at 180° and 300°C in order to observe if the preparation time can be shortened maintaining their chemical composition and their α-structure. Simultaneous TG and DTA together with XRD revealed to be very suitable techniques for the characterization of the obtained products.

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The prepared amorphous γ-ZrP\SiO2 composite had a complicated composition, since a part of γ-ZrP is converted to α-form during the exfoliation of it. The γ-ZrP\SiO2 composite have specific surface area of 421 m2g–1. The acidic P–OH groups of the lamellae species placed on the surface (it is ≈1.0 meq g–1), do not destroy until the temperature of 1030 K. During the thermal treatment the total mass loss of 7.79% was found. This value corresponds to 0.42 mole of H2O per molecule unit. The water loss process was found very slow, because of the placing of bilamellar species in the composite.

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Phosphogypsum (PG) is a residue of the phosphate fertilizer industry that has relatively high concentrations of harmful radioactive materials. The reduction in concentration of the radionuclides from PG was investigated. The removal process is based on leaching of radionuclides using suitable organic extractants. The studied radionuclides were 226Ra, 210Pb, 238U and 40K. The factors affect the leaching process such as type of leaching materials, contact time, concentration of the desired solvent, liquid to solid ratio, and temperature were studied. Based on the experimental results, about 71.1, 76.4, 62.4, and 75.7% of 226Ra, 210Pb, 238U and 40K respectively were successfully removed from the PG. The reduction in the concentration of radionuclides was accompanied by reduction in the concentration of rare earth elements (∑REE) equals to 69.8%. Using the desired organic extractant under optimum conditions for treatment of the PG waste leads to obtain a decontaminated product that can be safely used in many industrial applications.

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Natural fluorapatite samples were contacted with uranyl nitrate solutions (from 10−2 to 10−6M), adjusted to pH 6.0, then, shaken for times varying between 15 minutes to 72 hours, at room temperature. After that, the solid and liquid phases were separated by centrifugation and the solid was dried at 80°C overnight. The uranium analysis of the solid samples and solutions revealed that uranium was incorporated over fluorapatite. Selected solid samples produced by contacting treatments were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD patterns showed the growth of uranyl species in the fluorapatite. Imaging by SEM at 20000x showed the location of uranyl compounds in a crystalline layer in the surface of fluorapatite grains. This layer was well defined for the 10−2 M of U-contacting solution, but a saturation value was attained at 64% of uranium uptake yield. In the case of 10−4 M and lower U-contacting solution, the uranium uptake yield was near of 90% after 45 minutes. This fact suggests that natural fluorapatite has excellent properties to immobilize uranium compounds in a solution. Afterwards, the pregnant fluorapatite mineral was regenerated using an alkaline-leaching process. The uranium separated in this way is concentrated and can be handled to a final disposition.

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The photoluminescence (PL) of barite is a noncharacteristic property and cannot be used for the investigation of its structure. After thermal treatment of barite at 600°C several luminescent centers were observed, providing information about different impurities. UO 2 2+ was determined from the vibrational structure and the long decay time of the luminescence band. Two different types of uranyl were detected, thin films of uranyl mineral (most probably, reserfordin) and a solid solution of uranyl ion in barite crystal. Characteristic green luminescence of UO 2 2+ may be used as indicative feature for the prospecting of uranium deposits and for the sorting of barite ores with the aim of cleaning from harmful U impurities. Eu2+ was determined from the spectral position, the half-width and the characteristic decay time of the luminescence band. Mn2+ and Ag+ were determined by comparing luminescence bands spectral parameters to those of synthesized BaSO4−Mn and BaSO4−Ag. Fe3+ or Mn4+ were determined from the spectral-kinetic parameters of the luminescence bands.

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Phosphate treatment of carbonaceous clays increased the uptake capacity for uranium and thorium. Heat treatment up to 400°C improved enormously the uptake property of the treated clay. These treatments failed to improve the uptake quality of argileous clays. Strontium exchanges weakly with clays of high Ca and Mg contents but strongly with a silicoaluminium clay of low Ca and Mg contents. A mixed bed of these two clays was found adequate for decontamination of actinides and fission products.

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The thermal behaviour of wool, untreated and chlorinated to various extents, was investigated. The kinetic parameters of the water loss and pyrolysis processes were computed and, based on their values, the way chlorination treatment affects wool fibre is discussed. As it appears, the chlorination process affects the fibre only superficially and not its internal chemical composition, as the values of the kinetic parameters of the thermal decomposition of wool fibre do not seem to be influenced at all by the treatment.

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In biomaterial field, the introduction of new types of composites presents a great interest for orthopaedic surgeons. In this work, geopolymers which are a family of aluminosilicates were synthesised and mixed with biphasic mixture (hydroxyapatite and of tricalcic phosphate). The optimised thermal treatment causes the reduction of pH to 7 units and favours the expansion of composites. Consequently, the increasing of porosity percentage was induced. These properties offer a good opportunity for applied composite as potential osseous biomaterial. To study the consequences of thermal treatment in the initial amorphous structure of composites some physico-chemical techniques like SEM, MAS-NMR and FTIR were employed. These methods permitted to evaluate the porosity, different links in composites and contributions of different groups of Si and Al before and after thermal treatment.

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Energy dispersive X-ray fluorescence (EDXRF) was employed for the identification of pigments decorating Hellenistic figurines, and the assessment of the efficiency of a treatment with barium hydroxide applied to stone. Elements present in the colored areas of the figurines, as well as the treated stone was identified by EDXRF. These data together with complementary information obtained by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analysis (XRD) led to the identification of several precious pigments. As far as the treatment efficiency is concerned, EDXRF analysis revealed that barium is unevenly distributed on the treated surface and reaches a maximum depth of 2.5 mm.

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Treatment of an aqueous radioactive waste solution to remove radium prior to discharge was conducted at a laboratory scale. The actual solution is mainly composed of combined radium (226Ra and228Ra) with high concentrations of manganese, iron and calcium, which are present as chlorides in dilute hydrochloric acid. Direct precipitation by sulfate anions was selected to be the more viable treatment technique. Sulfate anion concentration, free acidity, temperature and aging of the sulfate precipitate in the supernate prior to filtration are factors that were investigated for their effect on the separation efficiency. The data obtained are discussed in detail.

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