Current Status of the Thermodynamic Data for Technetium and Its Compounds and Aqueous Species Current Status of the Thermodynamic Data for Technetium and Its Compounds and Aqueous Species

Abstract 99 Tc is a major fission product from nuclear reactors. Because 99 Tc has few applications outside of scientific research, most of this technetium will ultimately be disposed of as nuclear waste. The radioactive decay of 99 Tc to 99 Ru produces a low energy β -particle, but because of its fairly long half-life of t 1/2 = 2.13 × 10 5 years, 99 Tc is a major source of radiation in low level waste. Technetium forms the soluble 4 -TcO anion under oxic conditions and this ion is very mobile in groundwater, but technetium is reduced to less soluble Tc(IV) hydrolyzed species under anoxic conditions. Geochemical modeling of the dissolution of nuclear waste, and of the solubility and speciation of the dissolved radionuclides in groundwaters, is an integral part of the Performance Assessment of the safety of a nuclear waste repository, and this modeling requires a criticallyassessed thermodynamic database. Such a database for technetium was published in the book Chemical Thermodynamics of Technetium, with literature coverage through 1998. This database is described herein, along with more recent relevant studies. Gaps in the knowledge of the chemical and thermodynamic properties of technetium are pointed out, and recommendations are made for measurements that are needed to eliminate these gaps

Isopiestic determination of the osmotic and activity coefficients of {zH2SO4 + (1 - Z)MgSO4} (aq) at T = 298.15 K. II. Results for z = (0.43040, 0.28758, and 0.14399) and analysis with Pitzer's model

Isopiestic vapor pressure measurements were made for {zH2SO4 + (1 - z)MgSO4}(aq) solutions with H2SO4 molality fractions of z = (0.43040, 0.28758, and 0.14399) at the temperature 298.15 K. Measurements extend from total molalities of mT = (0.29999, 0.32948, and 0.36485) mol·kg-1 to mT = (4.84740, 4.28433, and 3.96206)mol·kg-1, respectively. These highest molalities correspond to the greatest degrees of supersaturation that could be achieved during isothermal removal of solvent. The corresponding water activity ranges are 0.9924 ≥ aw ≥ 0.7469, 0.9924 ≥ aw ≥ 0.7983, and 0.9924 ≥ aw ≥ 0.8291 for z = (0.43040, 0.28758, and 0.14399), respectively. Isopiestic results for this system with z = (0.85811, 0.71539, and 0.57353) were reported previously (J. Chem. Thermodynamics 1997, 29, 533-555). The combined results to mT ≤ 3.5 mol·kg-1 are represented essentially to experimental accuracy with Pitzer's model, using ionic strength-dependent third virial coefficients for the constituent binary solutions. A second set of parameters is also presented for the more extended molality range of mT ≤ 6.0 mol·kg-1, where this model gives a slightly less accurate representation of the experimental information. The nature and extent of ionic association in MgSO4(aq) is discussed, but it was not possible to select a value of the association constant owing to large discrepancies among the various studies