Determination of low uranium concentration in carbonate-bicarbonate solutions by X-ray fluorescence

A method has been developed for the direct determination of uranium in carbonate solutions by X-ray fluorescence. Uranium, in the concentration range 0.2–50 ppm, is first absorbed as the carbonate complex on an anion exchange resin and then determined by X-ray fluorescence. A sensitivity corresponding to 20 ppb in 250 ml solution and a precision of ±6.5% was obtained. The sensitivity was 100-fold greater than that obtained by the direct determination in solution by X-ray fluorescence.     

Assessing colloidal stability of long term MWCNT based nanofluids

This report presents an assessment on colloidal stability of functionalized multiwalled carbon nanotubes based nanofluids. To this end, an innovative technique that allows for measurement of settling velocity during centrifugation is applied. This method also enables measurements without dilution, inferring further accuracy to the experimental study. The results suggest that functionalization techniques enable the production of highly stable nanofluids. It is also found, that the colloidal stabilities of these nanofluids are characterized by hindered settling. The settling velocity decreases when the nanoparticles volume fraction rises from 0.25% to 1.50% due to the increase of interparticle interaction. Furthermore, a high aspect ratio of nanoparticles directly contributed to an increase in colloidal stability. It is expected that these results may significantly contribute to proper tailor of nanofluids engineering, ensuring a long term stable dispersion enhancing industrial application suitability.     

The long term stability of the titanium/hydrogen system as the solid reference electrode in high-temperature proton conductor-based hydrogen sensors

The electrochemical hydrogen sensor comprised of the high temperature proton conductor and the solid reference material of Ti/TiH₂ mixture showed the good sensing characteristics at elevated temperatures. However, in the long run, the EMF drift of about 0.2-0.3 mV/h was observed at 973 K. It is thought that the raise of EMF is due to the increase of the reference hydrogen pressure and the lack of the stability of the solid reference material is very critical to cause this phenomenon. In the present study, the long term stability of the solid reference material is investigated by EMF measurement with time.     

Differential pulse voltammetric determination of uranium in low concentration streams

A differential pulse voltammetric method has been successfully used for the determination of uranium in low concentration streams of a uranium plant. The method gives a precision of about 13% to 7% in the range of 300 ppb to 15 ppm. The accuracy of the results was ascertained by comparing the values with those obtained by a spectrophotometric method. The method is simple, fast, sensitive, fairly accurate and does not require a preconcentration step.     

Short and long term stability of the elemental composition of human body fluid reference materials and their use as master lots

Summary Studies have been made of both short and long term stability of trace elements in lyophilized human body fluid reference materials, as well as the stability of mercury in reconstituted urine solutions. No detectable concentration changes for mercury, lead and aluminium occurred during the 5-year period. There are large differences in the amounts of mercury loss among different reconstituted materials. Addition of traces of gold to the solutions minimized the mercury loss and increased the useable time of the reconstituted material from hours to 8 days. A certification process based on direct determinations by reference laboratories and by comparison against master lots of the same material and against similar certified reference materials is presented. Values for calcium, copper and mercury obtained from reference laboratories and using the data transfer principle have been assigned in new batches of urine and serum.     

Long term stability of organic selenium species in aqueous solutions

Long term stability of organic selenium compounds (selenocystine, selenomethionine, trimethylselenonium ion) has been studied over a one year period for 2 analyte concentrations: 25 and 150 μg/L Se, at pH 4.5 in the dark, under different storage conditions: temperature of –20°C, 4°C, 20°C, 40°C; in Pyrex, Teflon, or polyethylene containers; in an aqueous matrix or in the presence of a chromatographic counter ion (pentyl sulfonate at 10^–4 mol/L concentration). Light effects have also been tested. The stability of the selenium species was monitored by HPLC-ICP/MS. Storage conditions can drastically alter the stability of organic selenium species. Organoselenium compounds were shown to be stable in the dark over a one year period in an aqueous matrix at pH 4.5 in Pyrex containers at both 4°C and 20°C. Pyrex vials exposed to natural sunlight at room temperature resulted in a steady decrease of the selenoamino acid concentration. Teflon containers caused losses of less than 25% at both 4° C and 20° C in the dark. However, polyethylene vials presented, at all temperatures tested, a rapid decrease of the TMSe⁺ concentration. The stability of the Se species studied did not show significant differences between 4° C and 20° C in any container material used. Storage of solutions at 40° C led to slight differences between the Pyrex and Teflon containers. However, polyethylene presented a drastic decrease of the three species over time at this higher temperature. Solutions frozen at –20° C in polyethylene vials did not stabilize the TMSe⁺ signal. Finally, concentrations and matrices of the samples did not significantly affect the stability of the species. Received: 15 July 1996 / Revised: 14 July 1997 / Accepted: 18 July 1997     

Hydrous oxides in the concentration and removal of fission products from alkaline solutions of uranium

A great variety in retention properties occurs as a result of different methods of preparation of the sorbents. Specific surfaces and porosities, which are mostly connected with the sorption activities, may vary widely. The activation of Al₂O₃, SnO₂ and silica gel, to produce highly active sorbents occurs only if the oxide is contacted with acid immediately after thermal treatment. The efficiency of the separation scheme has been tested using uranium and fission products under static conditions from strongly alkaline aqueous solutions.     

Talc-filled PP: A new concept to maintain long term heat stability

Talc as filler improves the mechanical properties of polypropylene. However, talc also reduces the efficiency of many stabilizers and so the life-time of a final product.In a study to find new modifiers to improve the heat stability of talc-filled PP formulations it was observed that selected blends of modifiers were more efficient than the single modifiers at the same total loading.     

Influence of the postreaction time in the concentration of141Ce anionic species in uranium tricarbonated solutions

An effective method to minimize the presence of¹⁴¹Ce in the final hexahydrated uranyl nitrate recovery product has been obtained. This condition is considered as one prepurification stage in the recovery process of residual nonfissioned uranium in the production of⁹⁹Mo of fission.     

Determination of low levels of uranium in solutions obtained by acid attack on phosphate rock

The uranium present in the leach liquors obtained by attack on phosphate rock with sulphuric acid can be extracted with di(2-ethylhexyl)phosphoric acid and TBP after oxidation of any iron(II), and then stripped at 65 degrees with iron(II) in 8.6M phosphoric acid. The uranium is finally determined with arsenazo III.     

Quality control of uranium concentration measurements

Quality assurance and quality control are necessary to ascertain the output of any instrument with a certain level of confidence and maintain the measurement process in a state of statistical control. Data generated by three laser fluorimeters, used for analysing mass concentration of total uranium in liquid samples, were used in the present study. Erroneous data values, called outliers, were detected by Dixon test, Discordance outlier test, Rosner outlier test and Box plot. The distribution fitted to the data, was tested by Kolmogorov–Smirnov test, Chi-square test and Shapiro–Wilks test. From Shewart Control Charts it was concluded that the instruments were not in statistical control and an instrumental problem existed. The instrumental problem was solved and the instruments were subjected to quality control exercise again. Statistical parameters of the data from the detectors were compared before and after servicing. The improvement of the quality of detector output was evident.     

Quasielastic light-scattering studies of micellar sodium dodecyl sulfate solutions at the low concentration limit

Correlation functions of scattered light intensity of carefully purified sodium dodecyl sulfate (SDS) solutions were measured as a function of tenside concentration and NaCl concentration of the aqueous phase. The correlation functions were analyzed by taking into account the influence of the Coulomb interaction between the micelle (macroion) and small electrolyte ions on the diffusion coefficient. Values of the hydrodynamic radius, the aggregation number, and the effective surface charges were obtained. The aggregation number increases from N = 27 to N = 95 upon increasing the NaCl concentration from 0 to 0.05 mole per liter, while it remains constant when the salt concentration increases further up to 0.2 mole per liter. The effective charge of the micelles decreases with increasing NaCl content in the whole concentration region studied. These results could be interpreted qualitatively in terms of a model which relates the existence of an equilibrium size of the micelles to the balance between hydrophobic and Coulomb interactions. Our results lead to the conclusion that at least up to an NaCl concentration of 0.2 mole per liter the SDS-micelles exhibit an oblate spherical shape rather than a cylindrical form.     

The influence of ultrasonic waves on the hydrogen ion concentration and the stability of colloidal solutions

Summary The ultrasonic waves causes the cerie hydroxide sol to set to a gel. This change has been followed by measuring viscosity, conductivity and hydrogen ion concentration. It has been shown that the extent of hydrogen peroxide formed under the action of ultrasonic waves on water is not alone sufficient to explain for the increase in viscosity, nor at such concentrations it would set the sol to a gel.

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Zusammenfassung Ultraschall verursacht in Cerhydroxyd-Solen Gelierung. Dieses Ver?nderung wurde durch Messen der Viskosit?t, Leitf?higkeit und der Wasserstoffkonzentration verfolgt. Es hat sich gezeigt, da? der Betrag an Wasserstoffperoxydbildung unter Einwirkung des Ultraschalles auf Wasser nicht allein ausreichend ist, den Anstieg in der Viskosit?t oder hinsichtlich der Konzentration zu erkl?ren, um die Gelierung zu deuten.

     

超高稳定性的等级孔碳负载高分散铜基选择性加氢催化剂

负载型Pd,Pt,Au等贵金属催化剂由于具有较高活性而被广泛应用于选择性加氢催化领域,但资源稀缺、价格昂贵等问题严重制约了其在催化领域的长远发展.目前大量研究结果表明,非贵金属催化剂也具有较高的选择性催化加氢能力,在已被报道的非贵金属加氢催化剂中,铜基催化剂由于在选择性加氢反应中表现出较高加氢选择性和活性引起了人们的广泛关注.然而,早期研究的负载型铜基催化剂普遍存在催化稳定性较低的问题,所以提高铜基催化剂的使用寿命成为了问题关键.本文以铜基有机金属框架HKUST-1作为合成目标催化剂的前驱体,首先探究了水热合成条件对HKUST-1合成结构完整性及结晶度的影响,再通过精确调控HKUST-1的原位碳化过程,利用金属有机框架高温分解自还原行为,成功制备出了等级孔碳负载的高分散铜基催化剂,并将所制备的催化剂应用于1,3-丁二烯选择性加氢反应中.扫描电子显微镜、高分辨透射电子显微镜、X射线衍射、氮气吸脱附、傅里叶红外吸收光谱、X射线光电子能谱等技术用来表征了碳化前后催化剂载体结构的变化,铜粒子尺寸、价态及其在载体中分布的变化.文中也深入探究了以上因素对催化剂选择性催化加氢性能的影响.研究表明:120℃水热合成18 h能获得尺寸在15μm左右,结晶度高且形貌规整的HKUST-1前驱体.随后通过合理地控制金属有机框架分解过程,可实现对碳载体的等级孔结构和活性铜纳米粒子的分散程度的精确调控,获得高效等级孔载体结构和高分散铜位点的催化剂.不仅如此,通过一步碳化自还原HKUST-1制备的等级孔碳负载Cu的催化性能表现出对碳化温度高度的关联性.其催化活性随碳化处理温度的升高呈现先增强后减弱的趋势,但所有获得的催化剂对单烯烃都具有很高的选择性(>98%).特别地,本文发现在600℃碳化合成的催化剂在低温75℃反应可实现对1,3-丁二烯的100%转化,对丁烯的选择性为100%.同时,该催化剂在恒温75℃下持续反应120 h以上,其对丁二烯转化率和对丁烯选择性依然保持100%,表现出了超高的催化稳定性和潜在的商用价值.本文展示了通过简单地调控金属有机骨架的碳化过程是获得具有优异选择性催化加氢性能的铜基催化剂的有效途径.     

Short term and long term variations of Ca-concentration in beard

The Ca-concentration has been measured in daily shaven beard samples over a seven-year period. The Ca-concentration range in beard of single individuals is the same as that in the head hair of different groups. Inviduals belonging to the group with low Ca-level (Ca<700 ppm) in hair (endangered group from ischaemic heart diseases) may increase the Ca-level in their hair and get into the group with high Ca-level (Ca>700 ppm; defended group).     

Decontamination treatment of uranium in tricarbonated solutions

It has been established that the coexistence of chemical species structurally different from cerium, is a direct consequence of the time taken for its preparation. Its practical applications, within the scope of purifying uranium, may constitute the most important technological aspect in the process of ionic exchange, to separate¹⁴¹Ce from uranium.     

Spectroscopy of solutions of uranium(IV) chlorides at low temperatures between 330 and 2500 nm

The solubilities, stabilities and absorption spectra of solutions of UCl₄, UCl₄, + HCl and compounds containing HCl²⁻₆ were investigated in a large number of non-aqueous solvents at temperatures ranging from 25°C to the freezing points of the solutions. Narrow spectral features, having band widths of 4–50cm⁻¹ and attributed to vibronic transitions of the UCl₆²⁻ ion, were formed from UCl₄ with excess Cl⁻ in solvents having low donor strength and containing CO, SO, CN or NO₂ groups. Bands corresponding to both increasing and decreasing vibrational excitation were observed, and vibrational spacings characteristic of UCl bonds were measured. Mixtures of UCl₄ and HCl in ketone and other solvents exhibit the narrow UCl²⁻_6- spectral features at low temperatures, but at high temperatures, the most prominent features were broad. These broad features were attributed to the species UCl₄·2HCl in which normally forbidden purely electronic transitions become allowed because hydrogen bonding distorts the octahedral field around U⁴⁺. In certain solvents, e.g. alcohols, very broad features were observed and ascribed to solvated UCl₄. Oxidation of U(IV)to U(V)was seen, and a spectrum of UCl₆⁻ dissolved in SOCl₂ was measured.     

Adsorption of U(VI) ions from low concentration uranium solution by thermally activated sodium feldspar

The thermally activated sodium feldspar (TASF), a novel material, was prepared by calcinating the micron sodium feldspar (SF) at 450 °C for 45 min. The TASF and the SF were characterized using X-ray diffraction and mercury injection porosimetry, and the TASF was found to have larger specific surface area and larger porosity than SF. The effects of the initial pH, temperature, contact time and initial U(VI) ions concentration on the adsorption of U(VI) ions from the low concentration uranium solution by TASF were investigated. The maximum adsorption efficiency of TASF for 0.5 mg/L uranium solution amounted to 95.49 % when the initial pH was 5.0, the temperature, 318 K, and and the contact time, 600 min. The relationship between the adsorption capacity q _e and equilibrium concentration C _e can well be described by Freundlich equation. Adsorption isotherm and the analysis by FT-IR coupled with SEM revealed that U(VI) ions were adsorbed onto the surface of TASF in multimolecular or cluster states, and that the intraparticle diffusion occurred in spontaneous process. The basic thermodynamic parameters including free energy change, entropy change and enthalpy change were calculated to comprehend the intrinsic features and spontaneous nature of the adsorption process. The FT-IR spectroscopic characterization for the TASF indicated that many groups were involved in the adsorption process of U(VI) ions, and the –OH group played a more important role. The experimental results suggested that TASF had great adsorption efficiency and strong potentiality of enrichment and recovery for the low concentration U(VI).