Picosecond Pulse Radiolysis of Highly Concentrated Carbonate Solutions.

Highly concentrated potassium carbonate aqueous solutions are studied by picosecond pulse radiolysis with the purpose of exploring the formation processes of carbonate radical CO3(•-). The transient absorption band of solvated electron produced by ionizing is markedly shifted from 715 to 600 nm when the solute concentration of K2CO3 is 5 mol L(-1). This spectral shift is even more important than that observed for the solvated electron in 10 mol L(-1) KOH solutions. The broad absorption band of solvated electron in K2CO3 solutions overlaps with that of carbonate radical CO3(•-) formed at ultrashort time. Nitrate ion is used to scavenge the solvated electron and to observe the contribution of carbonate radical CO3(•-). The analysis of the amplitude and the kinetics of carbonate radical formation in highly concentrated solutions shows that CO3(•-) is formed within the electron pulse (7 ps) by two parallel mechanisms: a direct effect on the solute and the oxidation of the solute by water radical hole H2O(•+). These two mechanisms are followed by an additional one, by reaction between the solute and OH(•) radical especially in lower concentration. The radiolytic yield of each process is discussed.

Reactivity of HTcO(4) with methanol in sulfuric acid: Tc-sulfate complexes revealed by XAFS spectroscopy and first principles calculations.

The reaction between HTcO(4) and MeOH in 13 M H(2)SO(4) was investigated by (99)Tc NMR, UV-visible and X-ray absorption fine structure (XAFS) spectroscopy. Experimental results and first principles calculations show the formation of Tc(+5) sulfate complexes. The results expand the fundamental understanding of Tc in high acid solutions.

Speciation of technetium(IV) in bicarbonate media.

The technetium isotope (99)Tc is a major fission product from nuclear reactors. Ultimately it is disposed of as radioactive waste since it has few applications outside of scientific research. Geochemical modeling of the dissolution of nuclear waste and of the solubility and speciation of the dissolved radionuclides in groundwater is an important part of the Performance Assessment for the safety of nuclear waste repositories. It relies on the availability of a critically assessed thermodynamic database. The potential of the Tc(VII)/Tc(IV) redox couple is measured here under various chemical conditions to verify the stoichiometries of Tc complexes and determine their stabilities: (i) -log(10)[H(+)] in the range 7.0-10.0, for 0.3, 0.6, and 0.7 M [CO(3)](total); (ii) [CO(3)](total) in the range 0.01-0.6 M at -log(10)[H(+)] approximately 8.6; and (iii) [Tc(VII)]/[Tc(IV] ratios of (6.02 10(-5) M)/(10(-6) M) and (6.02 10(-5) M)/(6.02 10(-5) M) at -log(10)[H(+)] = -9.1 and [CO(3)](total) = 1 M. Assuming that Tc(VII), TcO(4)(-) is the only species which exists under all the above chemical conditions, the potentiometric results can be interpreted by considering the presence of two hydroxide-carbonate monomeric complexes. The hydrolysis equilibrium between these two complexes is Tc(CO(3))(OH)(2) + H(2)O <--> Tc(CO(3))(OH)(3)(-) + H(+) with -log(10)[H(+)](1/2) = 8.69 +/- 0.20, which is consistent with the -8.3 +/- 0.6 corresponding hydrolysis constant of the NEA TDB review. 733 +/- 44 mV/SHE and 575 +/- 60 mV/SHE are measured for the standard potentials of the TcO(4)(-)/Tc(CO(3))(OH)(2), and for the TcO(4)(-)/Tc(CO(3))(OH)(3)(-) redox couples respectively. The corresponding formation constants from TcO(OH)(2) are log(10)K(1,2) = 19.8 +/- 0.5 and log(10)K(1,3) = 10.5 +/- 0.5, to be compared with the 19.3 +/- 0.3 and 11.0 +/- 0.6 values proposed by the NEA TDB review. Note that these values have been converted for the formation reactions described here, thus the given values are not those of the NEA TDB review. However, Tc(CO(3))(OH)(2) is predicted to dominate over a surprisingly large range of chemical conditions. The monomeric character of the Tc(IV) complexes is verified in this study.