Characterization of macroporous carbonate-substituted hydroxyapatite bodies prepared in different phosphate solutions

Bone mineral of human is different in composition from the stoichiometric hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) in that it contains additional ions, of which CO₃²⁻ is the most abundant species. Carbonate-substituted hydroxyapatite (CHA) bodies were prepared by the hydrothermal treatment of highly porous calcium carbonate (CaCO₃) body at 120 °C in 1 M M₂HPO₄ and M₃PO₄ solutions (M = NH₄ or K). It was found that CaCO₃ body was almost transformed into CHA body after hydrothermal treatment for 24 h irrespective of type of phosphate solution. However, a small amount of CaCO₃ still remained after the treatment in K₃PO₄ for 48 h. Crystal shape of CHA bodies prepared in those solutions except for K₂HPO₄ was flake-like, which was different from that (stick-like) of original CaCO₃ body used for the preparation of CHA body. CHA prepared in the K₂HPO₄ showed globule-like crystal. Average pore size and hole size of the CHA bodies were 150, 70 μm and their porosities were about 89% irrespective of the solution. Carbonate content was slightly higher in the CHA bodies obtained from potassium phosphate solutions than in those obtained from ammonium phosphate solutions. Mostly B-type CHA was obtained after the hydrothermal treatment in the potassium phosphate solutions. On the other hand, mixed A- and B-type CHA (ca. 1–2 in molar ratio) was obtained in the ammonium phosphate solutions. The content of CO₃²⁻ in the CHA body depended on the type of phosphate solution and was slightly larger in the potassium phosphate solutions.     

Wet chemical etching of ZnO films using NH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-based (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>CO<Subscript>3</Subscript> and NH<Subscript>4</Subscript>OH alkaline solution

In this paper, we deposited ZnO thin films by RF magnetron sputtering at room temperature from un-doped targets. Wet chemical etching of ZnO films in (NH₄)₂CO₃ and NH₄OH solutions were examined. For comparison, hydrochloric acid was also used as an etchant. The NH _x -based alkaline solutions provide well-controlled etching rate, and smooth surface and sidewall profiles. Although NH _x -based alkaline solution etch rates for ZnO were relatively low, they were enhanced with the use of a H₃O stabilizer. In this case, the NH₄OH solution went from reaction-dominant mode to diffusion-dominant mode, which is beneficial for smooth surface morphology.     

Simultaneous uptake of ammonium and phosphate ions by composites of γ-alumina/potassium aluminosilicate gel

The simultaneous uptake of ammonium and phosphate ions from solution by composites of γ-alumina/potassium aluminosilicate (KAS) gel has been investigated. The composites were prepared by selective leaching of calcined kaolinite (Al₂Si₂O₅(OH)₄) using KOH solution, followed by neutralization of the leachate at pH 5.5 with nitric acid. The composites were reacted with solutions containing various concentrations of (NH₄)₂HPO₄ at pH 5, 7 and 10 at room temperature for 24 h to examine their uptake of ammonium and phosphate ions. Simultaneous uptake of ammonium and phosphate ions was found, the uptake of ammonium ions being greater than for phosphate ions, especially at pH=7. This observation is considered to result mainly from the porous properties of the composites, which should therefore be controlled to enhance the simultaneous uptake of both ions, especially phosphate ion.     

Extraction of U(VI), Th(IV), and REE(III) from nitrate solutions with diaryl(dialkylcarbamoylalkyl)phosphine oxides containing a dialkylcarbamoylmethyl substituent in the alkylene bridge

The extraction of microamounts of U(VI), Th(IV), and REE(III) from HNO₃ and NH₄NO₃ solutions with solutions of diaryl(dialkylcarbamoylalkyl)phosphine oxides containing a dialkylcarbamoylmethyl substituent in the alkylene bridge was studied. The stoichiometry of the complexes extracted from nitric acid solutions with N,N,N',N'-tetrabutyl-2-(di-p-anisylphosphinyl)butanedioic diamide I was determined. The influence of the extractant structure and aqueous phase composition on the efficiency and selectivity of the extraction of U(VI), Th(IV), and REE(III) into the organic phase was examined. Introduction of the–CH₂C(O)NAlk₂ fragment into the methylene bridge of the diaryl(dialkylcarbamoylmethyl)phosphine oxide molecule considerably enhances the extraction of REE(III) from neutral nitrate solutions. Such modification of the extractant molecule only slightly influences the extraction of REE(III) from nitric acid solutions, but leads to a considerable increase in the U(VI) extraction and to a decrease in the Th(IV) extraction. The selectivity of the extraction of U(VI) and REE(III) is thus considerably increased.     

Factors governing the efficiency of dissolution of UO2 ceramic pellets in aqueous solutions of iron nitrate

Dissolution of ceramic UO₂ in aqueous Fe(NO₃)₃ solutions at different temperatures under the conditions of limited contact with air and in the autoclave mode was studied. In the course of UO₂ dissolution at 60–90°C, the U/Fe molar ratio appears to be ～1, whereas at room temperature (25°C) this value is ～0.5. By varying the acidity of Fe nitrate solutions at these temperatures, it is possible to increase the U/Fe molar ratio to ～4 and to obtain uranyl nitrate solutions with simultaneous removal of Fe from the solution in the form of a precipitate of the basic salt, or to perform quantitative dissolution of UO₂ under the conditions excluding the formation of such precipitate. In the course of dissolution of ceramic UO₂ in Fe(NO₃)₃ solutions, the appearance or absence of Fe(II) ions, the formation or absence of the precipitate of the Fe basic salt, and variation of solution pH are interrelated and are determined by the process temperature.     

Effects of pH and H2O2 upon coprecipitated PbTiO3 powders

The properties of precipitated materials are highly dependent upon the complex ionic equilibria of the species in the solutions used for precipitation. Concentration, temperature, and pH dictate the complex species present within aqueous systems, and therefore affect the final precipitate properties. This paper discusses the effect of pH on the properties of PbTiO₃ precursor powders prepared by adding stoichiometric mixtures of TiCI₄ and Pb(NO₃)₂, in aqueous solution, to NH₄OH solutions. Several powders were prepared between pH 8.00 and 10.50. The pH does not affect the amorphous structure, but does have a pronounced effect upon the specific surface area and growth mechanisms of the precipitates.Since previous studies indicated that hydrogen peroxide (H₂O₂) affects the hydroxylation of the precipitated powders, the effect of (H₂O₂) concentration on the precipitate properties was also studied. Several precipitates were prepared from solutions containing (H₂O₂): PbTiO₃ ratios between 0:1 and 6:1. When (H₂O₂) was not added to the solutions used for precipitation, atmospheric CO₂ dissolved in solution caused precipitation of carbonate species. Thus, addition of the (H₂O₂) to the solutions inhibited precipitation of the carbonates.     

New approaches to reprocessing of oxide nuclear fuel

Dissolution of UO₂, U₃O₈, and solid solutions of actinides in UO₂, including those containing Cs, Sr, and Tc, in weakly acidic (pH 0.9–1.4) aqueous solutions of Fe(III) nitrate or chloride was studied. Complete dissolution of the oxides is attained at a molar ratio of Fe(III) nitrate or chloride to uranium of 1.6 or 2.0, respectively. In the process, actinides pass into the solution in the form of U(VI), Np(V), Pu(III), and Am(III). At 60°C, actinide oxides dissolve in these media faster than at room temperature. In the solutions obtained, U(VI) and Pu(III) are stable both at room temperature and at elevated temperatures (60°C), and also at high U concentrations (up to 300 mg ml⁻¹) typical of process solutions (6–8 M HNO₃, ∼60–80°C). After the oxide fuel dissolution, U and Pu are recovered from the solution by peroxide precipitation. In so doing, the content of Fe, Tc, Cs, and Sr in the precipitate does not exceed ∼0.05 wt %. From the solution after the U and Pu separation, the fission lanthanides, Tc, Cs, and Sr can be recovered by precipitation of Fe hydroxides in the presence of ferrocyanide ions and can be immobilized in appropriate matrices suitable for long-term and environmentally safe storage.     

Redox Behavior of Am(IV) and Methods of Its Preparation in Alkaline Media

A scheme was suggested for Am(OH)₄ isolation by treatment of Am(OH)₃ suspension in 0.1–1.0 M NaOH with ozone (3.5–5 vol %)-oxygen mixture (4–5 l h⁻¹ flow rate) at 20°C for 40 min, followed by ultrasonic treatment of the resulting Am(VI) (44 kHz, 1 W cm⁻³) for 45 min. The separated precipitate of Am(III) hydroxy peroxide was treated with 1–2 mlof 7–10 M NaOH to form Am(OH)₄. Mixing suspensions of equivalent amounts of Am(III) and Am(V) hydroxides in NaOH also gives Am(IV) hydroxide in a >98% yield. The reproportionation Am(III) + Am(V) = 2Am(IV) in 1 M NaOH starts on heating above 70°C, whereas at NaOH concentration higher than 7 M it is completed even at room temperature. The reaction of Am(III) with Am(VI) in alkaline solutions, Am(VI) + 2Am(III) → 3Am(IV), occurs during mixing the reactants. The equilibrium reaction of Am(OH)3 with [Fe(CN)₆]³⁻ in alkaline solutions was studied. It was shown that increasing the alkali concentration to 2 M NaOH promotes formation of Am(OH)₄. At further increase in the alkali concentration, Am(V) is formed.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 234–238.Original Russian Text Copyright © 2005 by Nikonov, Gogolev, Tananaev, Myasoedov, Clark.     

Dissolution of WWER-1000 spent nuclear fuel in a weakly acidic solution of iron nitrate and recovery of actinides and rare earth elements with TBP solutions

It is demonstrated on real solutions of samples of spent nuclear fuel (SNF) from WWER-1000 reactors (1000-MW_el water-cooled water-moderated energy reactors) that weakly acidic solutions of iron(III) nitrate at the molar ratio Fe(III): U ≥ 2.0 dissolve SNF with quantitative transfer of U and Pu into the solution. In the process, Fe partially precipitates in the form of a basic salt precipitate together with a part of the fission products (>90% of Ru, ~90% of Мо, >60% of Tc, and 40% of Zr) already in the step of the fuel dissolution. Cs, Eu, and Am pass into the solution together with U and Pu. With the required conditions followed, U and Pu can be separated from the solution by precipitation of their peroxides or quantitatively extracted from this solution with 30% TBP in Isopar L. The presence of ≥1 M Fe(NO₃)₃ in the solution considerably increases the distribution ratios of TPE and REE, which allows their recovery from a weakly acidic nitrate solution to be also performed with 30% TBP in a diluent. This process can serve in the future as a basis for the development of a new integrated technology combining the PUREX process with TPE partitioning using a common extractant.     

Supercritical fluid extraction of uranium and fission products in reprocessing of simulated spent nuclear fuel in weakly acidic solutions of Fe(III) nitrate in the presence of tributyl phosphate

The kinetics of dissolution of ceramic UO₂ in aqueous solutions of Fe(III) nitrate in the presence of TBP in supercritical CO₂ was studied. Quantitative recovery of U into solutions of Fe nitrate in combination with its extraction from the solution into the fluid occurs under these conditions within ～2 h, which is by more than an order of magnitude faster than in dissolution in a Fe(III) nitrate solution without TBP under common conditions. The behavior of uranium and simulated fission products (FPs) in reprocessing of simulated spent nuclear fuel (SSNF) in weakly acidic Fe(III) nitrate solutions using supercritical CO₂ containing TBP was studied. Under these conditions, SSNF dissolves quantitatively with the simultaneous recovery of the U nitrate complex into the TBP-containing fluid phase. Uranium is recovered from the fluid phase by back extraction with an H₂O₂ solution with simultaneous precipitation of U in the form of the peroxide UO₄·2H₂O. In the process, high degree of uranium decontamination from all the FPs is reached, which allows repeated use of U in the nuclear fuel cycle.     

Controlling phosphorus doping concentration in ZnO nanorods by low temperature hydrothermal method

Phosphorous-doped ZnO nanorods (NRs) were synthesized with controllable morphology and doping concentration on top of the array of undoped natural n-type ZnO NRs by hydrothermal process. By changing the concentration of NH₄OH to adjust the pH of the reactant solution containing zinc acetate, hexamethylenetetramine, and ammonium phosphate, it was possible to control the morphology, doping concentration, and band structure of the ZnO nanostructures formed on the top side of the undoped NRs array. When the NH₄OH concentration was 1 vol% substitutional dopant P formed shallow acceptor levels in the ZnO NRs, while higher NH₄OH concentration resulted in the formation of donor levels as well as the compensation of the acceptors. A model p–n homojunction device based on the P-doped ZnO NRs consecutively attached to the initially synthesized n-type NRs showed a rectifying junction behavior confirming the formation of the p-type ZnO NRs with low doping concentration of P.     

Absorption of nitrogen hemioxide in aqueous solutions of gas treatment systems upon dissolution of UN in nitric acid

The absorption of N₂O from an air flow in various aqueous solutions at 293–298 K was studied. The maximal N₂O absorption under the experimental conditions is reached for water (~22–24%) and saturated solution of K₂Cr₂O₇ in concentrated H₂SO₄ in the presence of Al₂O₃ and without it (~34 and ~30%, respectively). In concentrated HNO₃ and NH₄OH solutions and in 1.0 M NaOH and N₂H₄·nH₂O solutions, the degree of the N₂O absorption varied from ~7.5 to ~11.5%. Similar degree of absorption was obtained with 0.5 M (NH₂)₂CO (~11%). In the other solutions tested, the degree of the N₂O absorption did not exceed ~4.0%.     

Localization of U(VI) on nickel hydroxide from aqueous solution in the presence of complexing anions at 25°C

Sorption and coprecipitation of U(VI) from aqueous solution containing various complexing anions (CO₃^25-, SO₄²⁻, H₂EDTA²⁻) with the Ni(OH)₂ solid phase at 25°C was studied. Uranium(VI) is not noticeably sorbed on the Ni(OH)₂ solid phase from aqueous solutions containing CO₃²⁻ and SO₄²⁻. The distribution coefficients K _d are less than 1.0 ml g⁻¹ throughout the examined range of [U(VI)]: [L] ratios (L = CO₃²⁻, SO₄²⁻) at V/m ≥ 100 ml g⁻¹ and contact time of the solid and liquid phases of 60 min. In the presence and in the absence of H₂EDTA²⁻, the degree of the U(VI) sorption is essentially the same (K _d ∼90–140 ml g⁻¹ at V/m ≥ 100 ml g⁻¹). Uranium(VI) does not coprecipitate with Ni(OH)₂ from aqueous solutions containing SO₄²⁻ and H₂EDTA²⁻. The distribution coefficients K _d are less than 0.001 ml g⁻¹ at V/m ≥ 200 ml g⁻¹ and contact time of the solid and liquid phases of 60 min. In solutions containing CO₃²⁻, the U(VI) capture by the Ni(OH)₂ precipitate depends on the [CO₃²⁻]: [U(VI)] ratio. The higher the [CO₃²⁻]: [U(VI)] ratio, the more strongly U(VI) coprecipitates with Ni(OH)₂.     

Extraction of metal ions from aqueous solutions of mineral acids with crown ether-ionic liquid systems

The extraction of U(VI), Sr, and Cs from solutions of mineral acids (HNO₃, HCl) with a crown ether, cis-syn-cis-dicyclohexyl-18-crown-6 (DCH18C6-A), dissolved in ionic liquids (ILs), 1-butyl-3-methyl-imidazolium derivatives (bmimPF₆, bmimBF₄, bmimTf₂N), was studied. The best physicochemical characteristics (solubility in the aqueous phase, viscosity, hydrophobicity, etc.) are exhibited by a solution of DCH18C6-A in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (bmimTf₂N). The metal distribution ratios (D _M) in the extraction with a 0.01 M DCH18C6-A solution are 80 for CsNO₃, 78 for CsCl, and 162 for Sr(NO₃)₂. With an increase in the HNO₃ and HCl concentrations, D _M decreases, and with 1 M acids it does not exceed 1. The U(VI) extraction from nitric acid solutions with a 1 M solution of DCH18C6-A in bmimTf₂N initially increases with an increase in the aqueous phase acidity, with D _U(VI) reaching 5.4 in 4 M HNO₃, but then decreases in the interval 4–8 M HNO₃, whereas in the extraction with a 1 M solution of tributyl phosphate (TBP) in bmimTf₂N D _U(VI) monotonically increases with an increase in the HNO₃ concentration to 8 M. From hydrochloric acid solutions, U(VI) is extracted with solutions of DCH18C6-A in bmimTf₂N with the D _U(VI) values characteristic of solutions of DCH18C6-A in nonpolar organic diluents. On the whole, ILs as solvents exhibit unusual properties in the extraction of alkali and alkaline-earth elements from neutral solutions and of U(VI) with TBP from concentrated nitric acid solutions.     

Determination of small amounts of uranium by redox potentiometric titration

The redox titration of U in H₃PO₄ solution with an automatic titrator for determination of small amounts of U (0.1–0.2 mg) was studied. The choice of the oxidant appreciably affects the accuracy of the U determination, which decreases in the order KMnO₄ > K₂Cr₂O₇ > NH₄VO₃ > Ce(SO₄)₂. Irrespective of the oxidant used, the metrological characteristics of the method (repeatability r, laboratory precision R, and accuracy Δ) are significantly improved when the amount of U in the sample is increased from 0.1 to 0.2 mg, and high accuracy of the U determination is attained. The relationships obtained can be used in the development of a method for quantitative determination of small amounts of U in solutions.     

Reactive Atmosphere Synthesis of Polymer‐Derived Si–O–C–N Aerogels and Their Cr Adsorption from Aqueous Solutions

Polymer derived ceramic (PDCs) aerogels belonging to the Si–O–C–N system are synthesized by crosslinking a preceramic polymer in a diluted solution followed by supercritical or atmospheric drying and pyrolysis in inert (N₂) or reactive (NH₃/CO₂) atmosphere. Accordingly, aerogels with hierarchical porosity ranging from some microns to few nanometers together with high specific surface area in the range 30–400 m² g^?1 have been obtained. Moreover, their surface contains a broad range of moieties (Si–OH, Si–NH, C=O, etc.) that can interact and bind metal ions thanks to electrostatic interactions. This combination of hierarchical porosity, high SSA, and broad range of chemical functionalities makes these PDCs aerogels interesting candidates for water purification. In this work, SiOC and SiCN aerogels have been tested as adsorbents for Cr(III)/(VI) ions from aqueous solutions with promising results for the SiOC aerogel pyrolyzed in N₂ and the SiCN treated in NH₃. Correlations and similarities among the Cr(VI)/(III) adsorption capacity with the main features of the porous substrates (SSA, N₂ TPV, amount of free C, bulk density, isoelectric point, main IR peaks (Si–OH, OH, NH, C=O, C=C, Si–O, C–N, Si–N) have been investigated by applying the Principal Component Analysis (PCA).

     

Coprecipitation of Pu(IV) with Fe(III) and Cr(III) hydroxides from nitrate-acetate solutions under hydrothermal conditions simulating deep disposal of liquid radioactive wastes

Precipitation of Fe(III), Cr(III), Ni(II), and Mn(II) from nitrate-acetate solutions and coprecipitation of Pu(IV) with Fe(III) and Cr(III) were studied. The degree of precipitation of 80–95% is attained for Fe(III) at 95–200°C and pH>0.5–0.6, and for Cr(III), at T=95°C and pH≥4.0 or T=200°C and pH≥1.0. The phase composition of the precipitates formed by thermal hydrolysis of iron nitrate in model solutions was analyzed. Depending on pH and temperature, the solid phase contains various modifications of Fe₂O₃, FeOOH, and amorphous phases. Noticeable coprecipitation of plutonium from nitrate-acetate solutions is observed at pH≥4, and it is incorporated in the precipitate only at formation of FeOOH. No coprecipitation of Pu(IV) with Fe₂O₃ was found. Under the given experimental conditions, plutonium in aqueous solutions occurs in the oxidation state +4 forming monoacetate (or, probably, hydroxo acetate) complexes.     

Interaction of Pu(VI) with Aluminosilicate in Alkaline Solution

Behavior of Pu(VI) in the course of crystallization of aluminosilicate in 2 and 3 M NaOH was studied. Plutonium(VI) inhibits aluminosilicate crystallization. At the Al : Si : Pu molar ratio of 10 : 40 : 2 in the initial mixture, only a minor amount of the X-ray amorphous phase is formed. Partial sorption of Pu(VI) on the aluminosilicate precipitate depends on the alkali concentration in the solution. As determined by spectrophotometry, only neutral and low-charged Pu(VI) hydroxo complexes are sorbed on the aluminosilicate; anionic complexes like [PuO₂(OH)₄]^{2 -} formed in more alkaline solutions are not sorbed. Plutonium(IV) formed by reduction of Pu(VI) is sorbed on aluminosilicate from 3 M NaOH.     

Mechanism of cerium(III) oxidation with ozone in sulfuric acid solutions

The kinetics of Ce(III) oxidation with ozone in 0.1–3.2 M H₂SO₄ solutions was studied by spectrophotometry. The reaction follows the first-order rate law with respect to each reactant. The rate constant k slightly increases with an increase in the acid concentration, which is associated with an increase in the O₃/O ₃ ^? oxidation potential. The activation energy in the range 17–35°C is 46 kJ mol^?1. With excess Ce(III), the stoichiometric coefficient Δ[Ce(IV)]/Δ[O₃] increases from 1.6 to 2 in going from 0.1 to 1–3.2 M H₂SO₄. The extent of the Ce(III) oxidation is 78% in 0.1 M H₂SO₄ and reaches 82% in 1 M H₂SO₄. The ozonation involves the reactions Ce(III) + O₃ → Ce(IV) + O ₃ ^? , O ₃ ^? + H⁺ → HO₃, HO₃ → OH + O₂, OH + HSO ₄ ^? → H₂O + SO ₄ ^? , OH + Ce(III) → OH^? + Ce(IV), and SO ₄ ^? + Ce(III) → SO_4/2? + Ce(IV). Low stoichiometric coefficient of the Ce(III) oxidation is associated with the hydrolysis of Ce(IV). The excited Ce(IV) ion arising from oxidation of Ce(III) with OH radical forms with the hydrolyzed Ce(IV) ion a dimer whose decomposition yields Ce(III) and H₂O₂. After the ozonation termination, Ce(IV) is relatively stable in sulfuric acid solution, with only 5–7% of Ce(IV) disappearing in 24 h.     

Calcium metal to synthesize amorphous or cryptocrystalline calcium phosphates

Metallic calcium was never used before as the calcium source in synthesizing bioceramics. Amorphous calcium phosphate (ACP) powders were synthesized at room temperature, in synthetic mineralization solutions which contained Na⁺, K⁺, Mg^2 +, Cl^?, HCO₃^? and HPO₄^2 ? ions at concentrations similar to those found in human blood plasma, by using calcium (Ca) metal as the only calcium source. The experimental conditions leading to the formation of PCA (cryptocrystalline or poorly crystallized apatite) or CaCO₃ powders were also determined when using metallic Ca in aqueous synthesis in the mineralization solutions. The formation of calcium phosphate (CaP) in synthesis solutions was immediately initiated by the addition of calcium metal granules (or shots), at appropriate amounts, into the solutions while the solutions were being continuously stirred in glass bottles at room temperature (22 ± 1 °C). The synthesis reactions were reaching completion in less than 30 min with the final solution pH values ranging from 9 to 12, without a necessity for any external pH adjustment in the form of any strong base (such as NH₄OH, LiOH, NaOH or KOH) additions. ACP or PCA powders are useful for dentin and enamel re-mineralization applications or orthopedic (bone) defect-filling applications.