Extrapolation studies on adsorption of thorium and uranium at different solution compositions on soil sediments

Study on adsorption of thorium and uranium radionuclides by a soil sediment as a function of ionic composition of Ca, Mg and Na has been carried out. Experimentally determined slopes represents an average of adsorption on soil sediments having different relative affinities for thorium, uranium, calcium and magnesium. Both thorium and uranium were found to be adsorbed to ion-exchange sites together with calcium and magnesium cations as effective competitors An extrapolated equation for the distribution coefficientK _d was formed for both radionuclides thorium and uranium at the specified site where the soil sediments were sampled. The combined cation concentration of both calcium and magnesium in solution correlates linearly with the measuredK _d ^Th,U values.     

Identification and characterization of resistate minerals containing uranium and thorium

A set of natural matrix Standard Reference Materials were developed by the National Bureau of Standards for analytical methods evaluation. These materials were analyzed using a KF fusion procedure and an acid dissolution procedure. The latter method yielded radioactive concentrations that were 15–20% lower then that of the former. This was thought to be due to a fraction of the sample, “resistates,” that did not dissolve during the dissolution. In this study, HF dissolutions were conducted on NIST natural matrix SRMs, in which ~0.08% of total sample mass remained after dissolution. The acid resistant residual materials were concentrated, then dissolved using a LiBO₂ fusion procedure and were found to contain a considerable fraction of the uranium and thorium.     

Quantification of thorium and uranium sorption to contaminated sediments

Desorption tests, using a sequential extraction method, were used to characterize and quantify thorium and uranium sorption to contaminated wetland sediments collected from the Department of Energy's Savannah River Site located in South Carolina. In situ distribution coefficients, or K _d values (K _d = C _solid/C _liquid), were determined. Sequential extraction data were used to assign solid-phase radionuclide concentrations (C _solid) that, by definition, should represent only the reversibly sorbed fraction. A series of selective and sequential extractions was used to determine desorption K _d values. Thorium K _d values ranged from 115 to 2255 ml/g. Uranium K _d values ranged from 170 to 6493 ml/g. Compared to sorption K _d values, these desorption K _d values were appreciably greater because they captured the aging process of the radionuclides with the sediment, making the radionuclide more refractory. Compared to nonsite-specific data, these in situ K _d values improved accuracy, were more defensible, and removed unnecessary conservatism for subsequent transport and risk calculations. Additional tests were conducted to provide geochemical information relevant for selecting appropriate remediation technologies for the contaminated site. Thorium and uranium were associated with labile fractions and were not concentrated with the smaller sediment particles. These findings suggest that phytoremediation or heap leaching, and not soil washing, are viable remediation approaches for this site.     

Dissolution of resistate minerals containing uranium and thorium: Environmental implications

Summary Minerals in the soil range from those that easily weather to those that are very resistant to the weathering processes. The minerals used in this study are referred to as “resistates” because of their resistance to natural weathering processes.¹ It is also known that there are some resistate minerals that have a tendency to contain uranium and thorium within their crystal structure. These resistates can contain as much as 15-20% of the total uranium and thorium present in the soil.⁹ Do resistates dissolve in acids, particularly in the HF/HNO₃ procedures, if not what can be done to the HF/HNO₃ process to dissolve more of the resistate minerals? How would these acid techniques compare to the fusion method used for mineral dissolution? Could the resistate minerals contain considerable amount of uranium and thorium? These were the questions addressed in this research. The comparative data indicate that the use of H₂SO₄ in the dissolution process resulted in ~25% overall increase in the minerals dissolving therefore resulting in a higher yield of extracted uranium and thorium.     

Thermodynamic and kinetic investigations of uranium adsorption on soil

In order to understand the mobility of uranium it is very important to know about its sorption kinetics and the thermodynamics behind the sorption process on soil. In the present study the sorption kinetics of uranium was studied in soil and the influence parameters to the sorption process, such as initial uranium concentration, pH, contact time and temperature were investigated. Distribution coefficient of uranium on soil was measured by laboratory batch method. Experimental isotherms evaluated from the distribution coefficients were fit to Langmuir, Freundlich and Dubinin?CRadushkevich (D?CR) models. The sorption energy for uranium from the D?CR adsorption isotherm was calculated to be 7.07?kJ?mol^?1.The values of ??H and ??S were calculated to be 37.33?kJ?mol^?1 and 162?J?K^?1?mol^?1, respectively. ??G at 30?°C was estimated to be ?11.76?kJ?mol^?1. From sorption kinetics of uranium the reaction rate was calculated to be 1.6?×?10^?3?min^?1.     

Removal of thorium and uranium from aqueous solution by adsorption on hydrated manganese dioxide

Journal of Radioanalytical and Nuclear Chemistry - The hydrated manganese dioxide (HMO) was synthesized by hydrothermal process using MnSO4 and KMnO4. The adsorption of HMO for Th(IV) and U(VI)...     

Successive uranium and thorium adsorption from Egyptian monazite by solvent impregnated foam

The present work deals with uranium and thorium recovery from the Egyptian monazite sulfate leach liquor using the extraction chromatography technique (solvent impregnated material), where tributylamine (TBA) and di-n-octylamine (DOA) solvents were impregnated onto foam uranium and thorium separate recovery. The calculated theoretical capacities of the latter solvents were about 1.4 gU/g foam and 1.6 gTh/g foam, respectively. The attained uranium and thorium adsorption efficiencies (using ion-exchange columnar technique) were about 75 and 70% of its theoretical capacities, respectively. Using 1 M NaCl–0.1 M H₂SO₄ and 2 M H₂SO₄ as eluent solutions for uranium and thorium from the loaded solvents impregnated foam gave 95.8 and 98.7% elution efficiencies, respectively.     

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.     

Studies on thorium and uranium disequilibrium on the surface layers of the coastal sediments and their absolute concentrations in the sediments

Measurements of thorium and uranium isotopic activity ratios have been carried out with top layers of the sediments of the West coast of India in order to understand the behaviour and distribution of these isotopes in the coastal region. The southern part of the West coast contains a large amount of placer deposits while the northern region is free from such deposits.     

Electrodeposition of uranium and thorium

Cathodic depositions of uranium and thorium were carried out from a number of baths containing the metal salts, and complexing agents. A reducing agent was also present to prevent oxidation of the element. The deposition was also carried out at controlled pH. The current density ranged from 50 to 200 mA cm^–2. The purity of the deposited metals was better than 99.7%. The mechanism of formation of uranium and thorium is proposed and discussed.     

Sorption of radioiodine on organic rich soil,clay minerals and alumina

Batch method was used to investigate the sorption behavior of radioiodine on organic rich soil, alumina, chlorite-illite clay mixture and bentonite.¹³¹I was used as tracer. The grain sizes of the samples used were all below 38 m. A rather slow kinetics was observed for the adsorption of radioiodine on organic rich soil. The distribution ratio increased with increasing solution/solid (V/m) ratio, and the contact time. The pH of the synthetic groundwater did not change the distribution ratio appreciably. The soil biomass however, showed a striking effect on the adsorption of radioiodine. Among the clay minerals, the highest distribution ratio value was found for chlorite-illite clay mixture. All the values were however well below those of the organic rich soil. The sorption data were fitted to Freundlich and Dubinin-Radushkevich types isotherms. Means energies of adsorption, as well as the affinity ratios of the sorption sites to iodine and chlorine were calculated.     

Preferential adsorption of extracellular polymeric substances from bacteria on clay minerals and iron oxide

The adsorption of extracellular polymeric substances (EPS) from Bacillus subtilis on montmorillonite, kaolinite and goethite was investigated as a function of pH and ionic strength using batch studies coupled with Fourier transform infrared (FTIR) spectroscopy. The adsorption isotherms of EPS on minerals conformed to the Langmuir equation. The amount of EPS-C and -N adsorbed followed the sequence of montmorillonite>goethite>kaolinite. However, EPS-P adsorption was in the order of goethite>montmorillonite>kaolinite. A marked decrease in the mass fraction of EPS adsorption on minerals was observed with the increase of final pH from 3.1 to 8.3. Calcium ion was more efficient than sodium ion in promoting EPS adsorption on minerals. At various pH values and ionic strength, the mass fraction of EPS-N was higher than those of EPS-C and -P on montmorillonite and kaolinite, while the mass fraction of EPS-P was the highest on goethite. These results suggest that proteinaceous constituents were adsorbed preferentially on montmorillonite and kaolinite, and phosphorylated macromolecules were absorbed preferentially on goethite. Adsorption of EPS on clay minerals resulted in obvious shifts of infrared absorption bands of adsorbed water molecules, showing the importance of hydrogen bonding in EPS adsorption. The highest K values in equilibrium adsorption and FTIR are consistent with ligand exchange of EPS phosphate groups for goethite surface. The information obtained is of fundamental significance for understanding interfacial reactions between microorganisms and minerals.     

Determination of uranium and thorium concentrations in some West Malaysian limestones using neutron activation and delayed neutron analysis

Concentrations of uranium and thorium in some West Malaysian limestones have been determined using neutron activation and delayed neutron analyses. These limestones are mainly calcium carbonates and contain uranium and thorium in concentrations of about a few parts per million.     

Water adsorption on clay minerals as a function of relative humidity: application of BET and Freundlich adsorption models

Water adsorption on kaolinite, illite, and montmorillonite clays was studied as a function of relative humidity (RH) at room temperature (298 K) using horizontal attenuated total reflectance (HATR) Fourier transform infrared (FTIR) spectroscopy equipped with a flow cell. The water content as a function of RH was modeled using the Brunauer, Emmett, and Teller (BET) and Freundlich adsorption isotherm models to provide complementary multilayer adsorption analysis of water uptake on the clays. A detailed analysis of model fit integrity is reported. From the BET fit to the experimental data, the water content on each of the three clays at monolayer (ML) water coverage was determined and found to agree with previously reported gravimetric data. However, BET analysis failed to adequately describe adsorption phenomena at RH values greater than 80%, 50%, and 70% RH for kaolinite, illite, and montmorillonite clays, respectively. The Freundlich adsorption model was found to fit the data well over the entire range of RH values studied and revealed two distinct water adsorption regimes. Data obtained from the Freundlich model showed that montmorillonite has the highest water adsorption strength and highest adsorption capacity at RH values greater than 19% (i.e., above ML water adsorption) relative to the kaolinite and illite clays. The difference in the observed water adsorption behavior between the three clays was attributed to different water uptake mechanisms based on a distribution of available adsorption sites. It is suggested that different properties drive water adsorption under different adsorption regimes resulting in the broad variability of water uptake mechanisms.     

Modeling the adsorption of citric acid onto Muloorina illite and related clay minerals

The adsorption of citric acid onto goethite, kaolinite, and illite was measured as a function of pH (adsorption edges) and concentration (adsorption isotherms) at 25 degrees C. The greatest adsorption was onto goethite and the least onto illite. Adsorption onto goethite was at a maximum below pH 5 and decreased as the pH was increased to pH 9. For kaolinite, maximum adsorption occurred between pH 4.5 and pH 7, decreasing below and above this pH region, while for illite maximum adsorption occurred between about pH 5 and pH 7, decreasing at both lower and higher pH. ATR-FTIR spectra of citrate adsorbed to goethite at pH 4.6, pH 7.0, and pH 8.8 were compared with those of citrate solutions between pH 3.5 and pH 9.1. While the spectra of adsorbed citrate resembled those of the fully deprotonated solution species, there were significant differences. In particular the C[bond]O symmetric stretching band of the adsorbed species at pH 4.6 and 7.0 changed shape and was shifted to higher wave number. Further spectral analysis suggested that citrate adsorbed as an inner-sphere complex at pH 4.6 and pH 7.0 with coordination to the surface most probably via one or more carboxyl groups. At pH 8.8 the intensity of the adsorbed bands was much smaller but their shape was similar to those from the deprotonated citrate solution species, suggesting outer-sphere adsorption. Insufficient citric acid adsorbed onto illite or kaolinite to provide spectroscopic information about the mode of adsorption onto these minerals. Data from adsorption experiments, and from potentiometric titrations of suspensions of the minerals in the presence of citric acid, were fitted by extended constant-capacitance surface complexation models. On the goethite surface a monodentate inner-sphere complex dominated adsorption below pH 7.9, with a bidentate outer-sphere complex required at higher pH values. On kaolinite, citric acid adsorption was modeled with a bidentate outer-sphere complex at low pH and a monodentate outer-sphere complex at higher pH. There is evidence of dissolution of kaolinite in the presence of citric acid. For illite two bidentate outer-sphere complexes provided a good fit to all data.     

Leaching of uranium,radium and thorium from vertisol soil by ground water

Shallow land burial is routinely used for the disposal of low-level radioactive waste. Natural processes causing leaching of radionuclides can lead to contamination of surrounding ground water and soil by the radionuclides. The comparative leachability of radionuclides U_(nat), ²²⁶Ra, ²²⁸Ra and Th_(nat) from the soil of a radioactive waste disposal site, by ground water was evaluated. The probability of leaching was obtained in the following order Ra (≈77%) > U (≈40%) > Th (≈20%). Observed ratios (OR) were calculated to correlate leachability of radionuclides to that of major cations Ca²⁺ and Mg²⁺. The leaching of the radionuclides was seen to be dependent on Ca²⁺ and SO₄²⁻ leached from the soil. This study provides sitespecific leachability of radionuclides, that can be used as indicator of the tendency for migration or retention in soil. It can play an important role during an unforeseen accident like breach of containment at the waste disposal site leading to contamination of soil and ground water and causing hazard to public via drinking water route.     

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.