Radionuclide separation processes that have been used successfully in the past as well as recent developments in separati on techniques are discussed with emphasis on their applications in the treatment of radioactive wastes. Both aqueous and nonaqueous processes are evaluated. Separation procedures proposed for use in systems involving transmutation of nuclear waste products are also discussed.
Authors:P. N. Pathak, P. N. Pathak, P. N. Pathak, G. R. Choppin, G. R. Choppin, and G. R. Choppin
The complexation behavior of Ni2+with ortho-silicic acid (o-SA) has been studied as a function of ionic strength (I) from 0.20 to 1.00M (NaClO4) at pcH 4.55±0.05 and 25 °C by a solvent extraction technique with bis(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant.
The stoichiometry of the extracted species was determined to be Ni(DEHP)2(HDEHP)2. Ni2+forms a 1:1 complex, Ni(OSi(OH)3)+, as the predominant species withconcentrations of 1.00. 10-3to 1.00. 10-2M o-SA. The stability constant (logb1) values for Ni(OSi(OH)3)+complex formation decrease with increased ionic strength. These values have been fitted with the extended Debye-Huckel expression
to obtain the value of logb1=6.34±0.03 at I=0.00M. The data allowed the calculation of speciation of the Ni2+- silicate system as a function of ionic strength.
Sorption of nickel, Ni2+, on SiO2 . xH2O (silica gel) has been studied as a function of time conditions: amount of silica gel: 0.10-1.00 g, nickel concentration:
5.00 . 10-5-1.20 . 10-3M, ionic strength: 0.20-1.40M NaClO4, pH 6.50 to 8.50, and temperature 273-318 K. From the kinetic data, the diffusion coefficient of Ni2+ ion was calculated to be 1.28(±0.07) . 10-11 m2 . s-1 under particle diffusion-controlled conditions. The sorption rate was determined as 3.79(±0.35) . 10-3 s-1 at 298 K, pH 8.40 in 0.20M (NaClO4). The sorption data were fitted to Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Nickel sorption on 0.20
g silica gel decreased with ionic strength from 77.70±0.70% (0.20M NaClO4) to 16.12±0.37% (1.40M NaClO4) at intial pH of 8.50±0.05. A gradual decrease in pH with increasing ionic strength suggests an ion exchange mechanism and
the sorption of Ni2+ on silica gel increasing with temperature indicates an endothermic enthalpy. The effect of different ligands such as fluoride,
carbonate, phosphate and oxalate on Ni2+ sorption on silica gel was studied.
The complexation behavior of Co2+with ortho-silicic acid (o-SA) has been studied as a function of ionic strength (I) from 0.20 to 1.00M (NaClO4) at pH 4.96±0.03 and 25 °C by solvent extraction with bis(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant. The stoichiometry
of the extracted species was determined to be Co(DEHP)2(HDEHP)2. Co2+forms a 1:1 complex, CoOSi(OH)3+, as the predominant species witho-SA concentrations of 3.00. 10-4to 4.00. 10-3M. The stability constant (logb1) values for CoOSi(OH)3+complex decrease with the increase in ionic strength. These values were fitted with the extended Debye-Huckel expression to
obtain the value of logb1at I=0.00M. The effect of aging time of the o-SA solution on logb1values for CoOSi(OH)3+complex was investigated and compared with those of the UO2OSi(OH)3+complex.
Sorption behavior of europium, Eu3+, on SiO2 . xH2O (silica gel) has been investigated as a function of time, the amount of silica gel, Eu3+ concentration, the ionic strength, and pH (in absence and in presence of carbonate). The sorption data were fitted to Freundlich,
Langmuir and Dubinin-Radushkevich (D-R) isotherms. The sorption capacity of silica gel was determined to be in the range of
(2.62-8.00) . 10-7 mol/g at pH 5.30±0.05 and 0.20M NaClO4. The mean energy of sorption was calculated to be 13.50±0.05 kJ/mol from the D-R isotherm, suggesting the involvement of
ion-exchange reactions in the sorption process. Sorption of Eu3+ decreased with increased ionic strength. A gradual decrease in pH with increased ionic strength supports the involvement
of an ion-exchange mechanism in the sorption process. The diffusion coefficient of Eu3+ ion on silica gel was calculated as (3.98±0.12) . 10-13 m2 . s-1 under the particle diffusion-controlled conditions.
Authors:J. Schramke, Dhanpat Rai, R. Fulton, and G. Choppin
The reliability of two solvent extraction techniques for the determination of Pu oxidation states in solution was tested with low-ionic-strength solutions and with high-Na and high-Mg brines that contained Pu concentrations sufficient for spectrophotometric analysis. One procedure only differentiates between reduced Pu [Pu(III) and Pu(IV)] and oxidized Pu [Pu(V) and Pu(VI)], whereas the second procedure was designed to differentiate between Pu(IV), Pu(V), and Pu(VI) in solution. Both procedures successfully differentiated between oxidized and reduced Pu in both dilute solutions and brines when tested with samples that contained only the Pu(IV), Pu(V), or Pu(VI) oxidation states. However, when the second solvent extraction procedure, which differentiates between Pu(V) and Pu(VI), was employed for solutions that did not contain a strong oxidant to maintain the Pu(VI) oxidation state, significant quantities of Pu(VI) were reduced to Pu(V) during extraction, indicating that accurate quantification of Pu(V) and Pu(VI) is not possible with this procedure.
Authors:M. Borkowski, R. Moore, M. Bronikowski, Jianfeng Chen, O. Pokrovsky, Yuanxian Xia, and G. Choppin
A geochemical model describing the solubility of actinides in underground water at the Waste Isolation Pilot Plant (WIPP) Project is under development. The database for this model consists of standard chemical potentials and Pitzer model parameters for hundreds of species that may be present in the WIPP disposal room. Organic ligands used in separation and decontamination processes may be present in the nuclear wastes placed in the WIPP site and could have a significant impact on mobile actinide concentrations. In this work the
2 stability constants of NpO2+, UO22+, Am3+ and Th4+ with the oxalate anion have been measured in 0.3–5.0M NaCl media at 25 °C by a solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO2+<Am3+<UO22+<Th4+, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction model was used to model the data. Because the data were collected mainly in the high ionic strength region, values of
(1) were estimated from these plus literature values. The Pitzer model gives a good representation of the data using three interaction parameters
(1), and c.
Authors:R. Zeisler, S. B. Clark, S. J. Parry, G. R. Choppin, S. B. Clark, P. R. Danesi, S. J. Parry, M. Rossbach, C. Williamson, Zhifang Chai, R. Zeisler, S. J. Parry, and S. F. Heller- Zeisler
Since the 1970's, concern about the training of radiochemists in the US has been increasing along with the age of nuclear work force, while the number of academic institutions offering undergraduate courses and the number of radiochemistry faculty has been steadily decreasing. To address this problem, a summer program has been offered through the American Chemical Society to undergraduates since 1984. Students compete nationally for this opportunity, and those selected receive a 3,000 stipend. In addition, all other expenses are paid. In this manuscript, the curriculum is described, along with the need for additional training programs and their impact.