Radiochemical analysis of uranium isotopes in soil and sediment samples with extraction chromatography

An accurate and simple analytical technique for uranium isotopes in highly contaminated soil samples was developed and validated by application to IAEA-Reference samples and environmental samples. For overcoming the demerits of the TBP extraction method, sample materials were decomposited with HNO(3) and HF and uranium isotopes were purified with an anion exchange resin and a TRU Spec resin. With the extraction chromatography method, hindrance elements were completely removed from the uranium fraction. The chemical yields with the extraction chromatography method were <10% higher than those with the TBP extraction method. The concentrations of uranium isotopes using the extraction chromatography method were consistent with the reference values reported by the IAEA.     

Determination of uranium and thorium isotopes in soil samples by coprecipitation

Summary The paper presents a procedure to prepare soil samples for U and Th isotope measurement by alpha-spectrometry after coprecipitation with LaF₃. In this procedure the reduction of U(VI) to U(IV) was performed by Zn metal in 4M HCl solution. The recoveries of chemical separation equal to e_U-chemistry = 78±4% for uranium and e_Th-chemistry = 82±4% for thorium. Canberra alpha-spectrometer was used with PIPS detectors of A-1200-37-AM Model of 1200 mm² active area. The counting efficiency of the measuring system equals to e_counting = 18% and the total efficiencies were e_U = e_counting ^.e_U-chemistry = 14.0±0.7% for uranium and e_Th = e_counting ^.e_Th-chemistry = 14.7±0.7% for thorium. The recoveries of chemical separation were rather high (about 80%), that leads to the use of a small weight of soil sample (about 0.5 g). The efficiencies were also stable, that allows analyzing the soil sample without using radiotracers. They are advantages of the sample preparation procedure of this work.     

Chemical and radiochemical characterization of depleted uranium in contaminated soils

The main results of chemical and radiochemical characterization and fractionation of depleted uranium in soils contaminated during the Balkan conflict in 1999 are presented in the paper. Alpha-spectrometric analysis of used depleted uranium material has shown the presence of man-made radioisotopes ²³⁶U, ²³⁷Np, and ^{239, 240}Pu traces. The fractionation in different soil types was examined by the application of a modified Tessier’s five-step sequential chemical extraction procedure, specifically selective to certain physical/chemical associations. After ion-exchange-based radiochemical separation of uranium, depleted uranium is distinguished from naturally occurring uranium in extracts on the basis of the isotopic activity ratios ²³⁴U/²³⁸U and ²³⁵U/²³⁸U and particular substrates for recently present uranium material in soils are indicated. The text was submitted by the authors in English.     

Determination of uranium isotopes in environmental samples

The determination of isotopes of uranium by alpha spectrometry in different environmental components (sediments, soil, water, plants and phosphogypsum) is presented and discussed in this paper. The alpha spectrometry is a very convenient and good technique for activity concentration of natural uranium isotopes (²³⁴U, ²³⁵U, ²³⁸U) in environmental samples and provides the most accurate determination of isotopic activity ratios between ²³⁴U and ²³⁸U. The analysis were provided information about possible sources of high concentrations of uranium in the examined sites determined by anthropogenic sources. The calculation of values ²³⁴U/²³⁸U in all analyzed samples was applied to identifying natural or anthropogenic uranium origin. Activity concentration of uranium isotopes in analyzed environmental samples shows that measurement of uranium levels is of great importance for environmental and safety assessment especially in contaminated areas (phosphogypsum waste heap).     

A sequential radiochemical procedure for isotopic analysis of uranium and thorium in soil

A sequential radiochemical procedure for isotopic analysis of uranium and thorium in soil has been developed. Analysis involves total dissolution of the samples to allow equilibration of the natural isotopes with added tracers, followed by radiochemical separation using anion exchange chromatography (BioRad AG 1–X8). Further separation and purification is performed employing solvent extraction techniques. Finally, the U and Th fractions are co-precipitated with lanthanum and cerium fluoride, respectively, and quantified by alpha-particle spectrometry. Overall chemical yields range from 60 to 90%. Under normal operating conditions and present counting set up, the minimum detectable concentration (MDC) is approximately 2 Bq/kg for soil samples. This is based on one gram aliquot of sample, 80% chemical yield, and 1000 minute counting with a detector having about 15% counting efficiency. The procedure has been successfully tested with Standard Reference Materials. Various soil samples were analyzed with high chemical yields and fine quality of alpha-spectra. Decontamination factor studies were performed to determine the extent of the carry over of²¹⁰Po,²²⁵Ac,²²⁶Ra, and²²⁹Th into U fraction and²¹⁰Po,²²⁵Ac,²²⁶Ra, and²³²U into Th fraction.     

A new radiochemical procedure for uranium assay in environmental samples

A new and economical method for assay of environmental samples for uranium isotopes is proposed. Separation and radiochemical purification of uranium isotopes (²³⁴U,²³⁵U and²³⁸U) from other elements is achieved on a single anion exchange column by washing with various concentrations of hydrochloric acid. Iron, the principal interfering element is removed from the colum by washing with 4.5M hydrochloric acid with a combination of reducing agents under the conditions described. Weightless samples of uranium are prepared by either evaporation in a polished stainless steel dish or electroplated on a stainless steel planchet. This method is applicable for air particulates, soils, sediments, coal, water, vegetation, and biologicals. Text of the paper presented in the symposium on Practical Applications of Nuclear and Radiochemistry, at Las Vegas, Nevada, August 25–29, 1980. Submitted for publication in Advances in Chemistry Series.     

Simultaneous determination of trace uranium and vanadium in biological samples by radiochemical neutron activation analysis

Summary A radiochemical neutron activation analysis (RNAA) for simultaneous determination of uranium and vanadium in a single sample at trace levels is described. The method is based on post-irradiation wet-ashing and solvent extraction of vanadium with N-benzoyl-N-phenyl-hydroxylamine reagent. From the remaining aqueous phase, uranium is extracted into a toluene solution of tri-n-butyl phosphate. The chemical yields are determined spectrophotometrically for vanadium and by gamma-counting of the added natural uranium carrier for uranium. The method was evaluated by the analysis of reference materials and the results showed a good agreement with the certified values. The method was applied to the determination of vanadium and uranium in five military total diet samples in Slovenia.     

Determination of uranium isotopes in environmental samples by alpha-spectrometry

A new and accurate method for the determination of uranium isotopes (²³⁸U, ²³⁴U and ²³⁵U) in environmental samples by alpha-spectrometry has been developed. Uranium is preconcentrated from filtered water samples by coprecipitation with iron(III) hydroxide at pH 9-10 using an ammonia solution and the precipitate is dissolved in HNO₃ and mineralized with H₂O₂ and HF; uranium in biological samples is ashed at 600 °C, leached with Na₂CO₃ solution and mineralised with HNO₃, HF and H₂O₂; uranium in soil samples is fused with Na₂CO₃ and Na₂O₂ at 600 °C and leached with HCl, HNO₃ and HF. The mineralized or leaching solution in 2M HNO₃ is passed through a Microthene-TOPO (tri-octyl-phosphine oxide) column; after washing, uranium is directly eluted into a cell with ammonium oxalate solution, electrodeposited on a stainless steel disk and measured by alpha-spectrometry. The lower limits of detection of the method is 0.37 Bq^.kg^-1 (soil) and 0.22 mBq^.l^-1 (water) for ²³⁸U and ²³⁴U and 0.038 Bq^.kg^-1 (soil) and 0.022 mBq^.l^-1 (water) for ²³⁵U if 0.5 g of soil and 1 litre of water are analyzed. Five reference materials supplied by the IAEA have been analyzed and reliable results are obtained. Sample analyses show that, the ²³⁸U, ²³⁴U and ²³⁵U concentrations are in the ranges of 0.30-103, 0.49-135 and 0.02-4.82 mBq^.l^-1 in waters, of 1.01-7.14, 0.85-7.69 and 0.04-0.32 Bq^.kg^-1 in mosses and lichens, and of 25.6-53.1, 26.4-53.8 and 1.18-2.48 Bq^.kg^-1 in sediments. The average uranium yields for waters, mosses, lichens and sediments are 74.5±9.0%, 80.5±8.3%, 77.8±4.9% and 89.4±9.7%, respectively.     

Implementation of alpha-spectrometry for uranium isotopes in excreta samples

Summary The measurement of radioactivity concentrations in excreta is an important tool for the monitoring of possible radionuclide intakes by occupationally exposed workers. For this purpose, a radiochemical procedure for the determination of alpha-emitting isotopes of uranium in excreta has been optimized. The main steps involved in this procedure are pre-concentration, dissolution of sample, separation by ion-exchange resin, electrodeposition and alpha-spectroscopy. ²³²U tracer is used to monitor chemical recoveries and correct the results to improve precision and accuracy. The quality control of radiochemical analysis in urine and faecal samples has been performed with participation in intercomparison exercises. The results obtained from these samples, with chemical recoveries (80-95%), are shown to be highly consistent. The method offers good prospects to be applied in routine monitoring programme of workers.     

Simultaneous radiochemical neutron activation analysis of iodine, uranium and mercury in biological and environmental samples

The determination of medium and long-lived nuclides can be combined with short-lived ones if a medium or long irradiation is made prior to the short irradiation and radiochemical processing. Thus, an RNAA method previously developed for determination of iodine based on the reaction¹²⁷I(n,)¹²⁸I (T _1/2=25 m) using oxygen flask ignition of the irradiated sample, followed by solvent extraction with an iodine-iodide redox cycle, was combined with an overnight preirradiation to induce the²³⁵U fission product¹³³I (T _1/2=20.8 h). By reactivating the sample, cooled 1–2 days after the first irradiation, for few minutes both¹²⁸I and¹³³I could be quantified in the separated iodine fraction. Non-combustible inorganic materials (e.g., sediment, soil, etc.) can be successfully ignited after mixing with excess cellulose powder. Chemical yields for iodine were determined spectrophotometrically in the organic phase, while homogeneously spiked Whatman cellulose powder was used as uranium standard. Mercury is also released on ignition and collected in the absorbing solution, from where it was separated by toluene extraction. Its chemical yield was determined for each aliquot using²⁰³Hg tracer and counting on an LEPD. Results for some suitable SRMs are presented, and the general features of the double irradiation technique discussed.     

Leaching dynamics of uranium in a contaminated soil site

Samples from a potentially contaminated industrial area were analyzed for uranium using neutron activation analysis (NAA). Uranium concentration values had a typical uncertainty of 2 % and a detection limit of 1 Bq/kg. To investigate the potential leaching dynamics into ground water two techniques were employed. The US EPA Toxicity Characterization Leaching Procedure (TCLP) and the Sequential Extraction Procedure (SEP) were used to determine the concentration of uranium in the leachates. TCLP and SEP showed that very little of the uranium leached into solution under different chemical conditions. Values of uranium leachates ranged from 0.05 to 3.5 Bq/L; a concentration much lower than the results found in the soil concentrations which ranged from 29 to 155 Bq/kg. NAA showed an 8 % uncertainty for leachates with a detection limit of 0.13 Bq/L. To mimic environmental conditions and acid rain, pH 4.3 water was used as the extraction solvent instead of the acetic acid routinely used in TCLP. Results confirmed that very low amounts of uranium leached with values ranging from 0.0002 to 0.0122 Bq/L. These values represent 0.01–1 % of the uranium in the soil samples. The distribution of uranium in soil according to particle size was also investigated to evaluate its potential movement and possible contamination of the water table. Particles below 250 μm in diameter showed a linear increase in uranium concentration whereas those with a larger diameter had constant uranium content.     

Determination of thorium,uranium and plutonium isotopes in atmospheric samples

A new procedure for the radiochemical measurements of thorium, uranium and plutonium in atmospheric samples is described. Analysis involves coprecipitation of these actinides with iron hydroxide from a 40-to 50-dm³ sample of rainwater, followed by radiochemical separation and purification procedures by the use of ion exchange chromatography (Dowex AG1×8) and solvent extraction. The new procedure enables one to determine the isotopes of thorium, uranium and plutonium, which are found in rainwater at extremely low concentrations, with a chemical yield ranging from 60 to 80%.     

Sequential separation and determination of uranium and thorium isotopes in soil samples with Microthene-TOPO chromatographic column and alpha-spectrometry

In the method, soil was fused together with Na₂CO₃ and Na₂O₂ at 600 °C, uranium and thorium were leached out with HCl, HNO₃ and HF, and HClO₄ was used to eliminate the residual HF through evaporation. The leaching solution (2 M HNO₃) was passed through a Microthene-TOPO column to adsorb uranium and thorium. Thorium was first eluted with 2 M HCl and electrodeposited in 0.025 M H₂C₂O₄ + 0.15 M HNO₃ on a stainless steel disc. Uranium was eluted with a 0.025 M ammonium oxalate solution and also electrodeposited. Both thorium and uranium isotopes on the discs were measured separately by α-spectrometry.     

Determination of radium isotopes in soil samples by alpha-spectrometry

A sensitive and accurate method for determination of radium isotopes in soil samples by α-spectrometry has been developed ²²⁵Ra, which is in equilibrium with its mother ²²⁹Th, was used as a yield tracer. Radium in soil samples was fused together with Na₂CO₃ and Na₂O₂ at 600 °C, leached with HNO₃, HCl and HF, preconcentrated by coprecipitation with BaSO₄, separated from uranium, thorium and iron using a Microthene-TOPO chromatographic column, isolated from barium in a cation-exchange resin column using 0.05M 1,2-cyclohexylene-dinitrilo-tetraacetic acid monohydrate as an eluant, electrodeposited on a stainless steel disc, and counted by α-spectrometry. The detection limit of the method is 0.43 Bq·kg⁻¹ for ²²⁶Ra, ²²⁸Ra and ²²⁴Ra if 0.50 g of soil sample are analyzed. The method was checked with two certified reference materials supplied by the IAEA, and reliable results were obtained Fourteen soil samples collected from the refractory industry in Italy were also analyzed. The mean radiochemical yields for radium were 85.7±4.3%, and the obtained radium concentrations in the soil samples were in the range of 8.08–3878 Bq·kg⁻¹ for ²²⁶Ra, of 1.60–678 Bq·kg⁻¹ for ²²⁸Ra and 1.25–550 Bq·kg⁻¹ for ²²⁴Ra, with ²²⁸Ra/²²⁶Ra and ²²⁴Ra/²²⁶Ra ratios ranged from 0.159–0.821 and from 0.142 to 0.525, respectively.     

Geostatistical and multivariate statistical analysis of heavily and manifoldly contaminated soil samples

The surroundings of the former Kremikovtzi steel mill near Sofia (Bulgaria) are influenced by various emissions from the factory. In addition to steel and alloys, they produce different products based on inorganic compounds in different smelters. Soil in this region is multiply contaminated. We collected 65 soil samples and analyzed 15 elements by different methods of atomic spectroscopy for a survey of this field site. Here we present a novel hybrid approach for environmental risk assessment of polluted soil combining geostatistical methods and source apportionment modeling. We could distinguish areas with heavily and slightly polluted soils in the vicinity of the iron smelter by applying unsupervised pattern recognition methods. This result was supported by geostatistical methods such as semivariogram analysis and kriging. The modes of action of the metals examined differ significantly in such a way that iron and lead account for the main pollutants of the iron smelter, whereas, e.g., arsenic shows a haphazard distribution. The application of factor analysis and source-apportionment modeling on absolute principal component scores revealed novel information about the composition of the emissions from the different stacks. It is possible to estimate the impact of every element examined on the pollution due to their emission source. This investigation allows an objective assessment of the different spatial distributions of the elements examined in the soil of the Kremikovtzi region. The geostatistical analysis illustrates this distribution and is supported by multivariate statistical analysis revealing relations between the elements.     

Resolution of uranium isotopes with kinetic phosphorescence analysis

This study was conducted to test the ability of the Chemchek? Kinetic Phosphorescence Analyzer Model KPA-11 with an auto-sampler to resolve the difference in phosphorescent decay rates of several different uranium isotopes, and therefore identify the uranium isotope ratios present in a sample. Kinetic phosphorescence analysis (KPA) is a technique that provides rapid, accurate, and precise determination of uranium concentration in aqueous solutions. Utilizing a pulsed-laser source to excite an aqueous solution of uranium, this technique measures the phosphorescent emission intensity over time to determine the phosphorescence decay profile. The phosphorescence intensity at the onset of decay is proportional to the uranium concentration in the sample. Calibration with uranium standards results in the accurate determination of actual concentration of the sample. Different isotopes of uranium, however, have unique properties which should result in different phosphorescence decay rates seen via KPA. Results show that a KPA is capable of resolving uranium isotopes.