In an effort to develop new chelating agents for the decorporation of uranium and other actinides, the interaction of the
clinically used 1,2-dimethyl-3-hydroxypyrid-4-one (Deferiprone or L1) with hexavalent uranium was investigated by using UV-VIS
spectroscopy and solubility measurements. The complex stoichiometry estimation carried out by the Job plot method indicated
that under normal conditions up to pH 8.0 a 1[U(VI)]∶1[L1] complex is formed. The stability constant of the UO2L1+ complex was determined by spectroscopic and solubility experiments and found to be log β11=9.1±0.3. The molar extinction coefficient at pH 7.6 for the complex at 500 nm was estimated to be 650 l·mol−1·cm−1. At ligand concentrations higher than 6·10−4 mol·l−1 the formation of a precipitate was observed. The stoichiometry UO2(L1)2 was identified following FTIR measurements of the red precipitate and UV/VIS spectroscopy after dissolution.
The complexation of the uranyl ion with humic acid is investigated. The humic acid ligand concentration is described as the
concentration of reactive humic acid molecules based on the number of humic acid molecules, taking protonation of functional
groups into account. Excess amounts of U(VI) are used and the concentration of the humic acid complex is determined by the
solubility enhancement over the solid phase. pH is varied between 7.5 to 7.9 in 0.1M NaClO4 under normal atmosphere and room temperature. The solubility of U(VI) in absence of humic acid is determined over amorphous
solid phase between pH 4.45 and 8.62. With humic acid, only a limited range of data can be used for the determination of the
complexation constant because of flocculation or sorption of the humic acid upon progressive complexation. Analysis of the
complex formation dependency with pH shows that the dominant uranyl species in the concerned pH range are UO2(OH)+ and (UO2)3(OH)5+. The complexation constant is evaluated for the humate interaction with the to UO2(OH)+ ion. The stability constant is found to be logβ = 6.94±0.3 l/mol. The humate complexation constant of the uranyl mono-hydroxo species thus is significantly higher than that
of the nonhydrolyzed uranyl ion (6.2 l/mol). Published data on the Cm3+, CmOH2+ and Cm(OH)2+ humate complexation are reevaluated by the present approach. The higher stability of the hydrolysis complex is also found
for Cm(III) humate complexation.
The interaction of the clinically used 1,2-dimethyl-3-hydroxypyrid-4-one with trivalent europium Eu(III), was investigated
by using potentiometric and spectroscopic methods. The stability constants of the EuLn(3−n)+ complexes determined by spectroscopic and potentiometric measurements were found to be log β11 = 6.5±0.3 and log β12 = 12.0±0.5. However, at pH ≥ 5, hydrolysis of the Eu-L complexes starts, resulting in the formation of needle-type, yellow
crystals. The low solubility of the Eu-L complexes in the neutral pH range is disadvantageous with respect to the use of deferiprone
as chelating agent for decorporation of trivalent f-elements.
The effectiveness of the uranium removal by olive cake from aqueous solutions has been demonstrated in batch type experiments
under normal atmospheric conditions. The adsorption capacity has been evaluated by using both Langmuir and Freundlich isotherms.
The optimum pH regarding for uranium adsorption on olive cake is 7.5. The amount of adsorbed uranium is governed by the amount
of active sites on the biomass surface indicating an inner sphere complexation. The adsorption of uranium on olive cake is
an endothermic and entropy-driven process and does not depend on the ionic strength of the solution.
The stability and solubility of UO2(OH)2 has been studied as a function of the humic acid concentration in 0.1M NaClO4, in the pH range from 4 to 7 under normal atmospheric conditions. The solid phase under investigation has been prepared by
alkaline precipitation and characterized by TGA, ATR-FTIR, XRD, SEM and solubility measurements. According to the experimental
data UO2(OH)2 is stable and remains the solubility limiting solid phase even in the presence of increased humic acid concentration in the
solution. However, humic acid affects texture and particle size of the solid phase. Increasing humic acid concentration results
in decreasing crystallite size of the UO2(OH)2 solid phase. Based on the solubility data, the logKsp(UO2(OH)2) has been evaluated to be −22.0±0.3 and the stability constant for the UO2(OH)HA(I) species has been estimated to be logβ1101 = 15.3±0.5.
Preconcentration of uranium from natural water samples using Chelex-100 cation-exchange resin, its selective extraction by
tributylphosphate and electrodeposition on stainless steel discs is reported. The validity of the separation procedure and
the chemical recoveries were checked by addition of uranium standard solution as well as by tracing with 232U. The average uranium yield for the cation-exchange was (97±2)%, for the liquid-liquid extraction was (95±2)% and for the
electrodeposition was more than 99%. Employing high-resolution a-spectroscopy, the measured activity of 238U and 234U radioisotopes was found to be ~7 mBq.l-1 and ~35 mBq.l-1 for ground- and seawater samples, respectively. The energy resolution (FWHM) of the α-peaks was 22 keV, the minimum detectable
activity (MDA) was estimated to be 1 mBq.l-1 (at 95% confidence limit).
Liquid scintillation counting of the alpha-radionuclides after pre-concentration by cation-exchange represents a simple and
robust method for the determination of total alpha-radioactivity in seawater. The total efficiency and the minimum detectable
activity were calculated to be 95% and 30 mBq, respectively, for a liter sample and 1000-minute measuring time. The method
has been applied successfully for the determination of alpha-radioactivity in seawater from five different coastal areas in
Cyprus. The average alpha-radioactivity and uranium concentration were found to be 124±8 mBq . l-1 and 3.2±0.2 mg . l-1, respectively.
Authors:A. Ioannidou, I. Samaropoulos, M. Efstathiou, and I. Pashalidis
The activity concentrations of 238U and 234U have been determined in groundwater samples of hot springs and deep wells in Northern Greece. The analysis was performed
by alpha spectroscopy after pre-concentration and separation of uranium by cation exchange (Chelex 100 resin) and finally
its electro-deposition on stainless steel discs. The uranium concentration in deep wells and springs varies strongly between
0.15 and 7.66 μg L−1 and the corresponding 238U and 234U activity concentrations between 1.82–95.3 and 1.70–160.1 mBq L−1, respectively. The obtained isotopic ratio 234U/238U varies between 0.95 and 1.74 indicating a disturbed radioactive equilibrium between the two uranium isotopes. In the studied
waters uranium concentrations in solution decrease with increasing pH in the pH range between 7 and 9. This is attributed
to the fact that at lower pH dissolution of soil minerals occurs and uranium which is adsorbed or forms solid solution with
the geological matrix enters the aqueous phase. The strong dependence of the uranium concentration in the studied waters from
the dissolution of the geological matrix is corroborated by the strong correlation of the uranium concentration with the electrical
conductivity measured in the ground waters under investigation.
Authors:S. Antoniou, C. Kolokassidou, K. Polychronopoulou, and I. Pashalidis
The stability of UO2CO3 has been studied as a function of the humic acid concentration in 0.1M NaClO4, in the weak acidic pH range (4.5–5) under CO2 atmosphere. The solid phase under investigation has been prepared by alkaline precipitation and characterized by TGA, ATR-FTIR,
XRD, SEM and solubility measurements. According to the experimental data, UO2CO3 is stable and remains the solubility limiting solid phase even in the presence of increased humic acid concentration in solution.
However, humic acid affects texture and particle size of the solid phase. Increasing humic acid concentration results in decreasing
crystallite size of the UO2CO3 solid phase. Based on the solubility data, the logKsp(UO2CO3) has been evaluated to amount −13.7±0.2 for the humic acid-free system and −13.2±0.3 for the humic acid containing system.