The biogeochemistry of technetium: A review of the behavior of an artificial element in the natural environment

Abstract

Interest in the chemistry of technetium has only increased since its discovery in 1937, mainly because of the large and growing inventory of ⁹⁹Tc generated during fission of ²³⁵U, its environmental mobility in oxidizing conditions, and its potential radiotoxicity. For every ton of enriched uranium fuel (3% ²³⁵U) that is consumed at a typical burn-up rate, nearly 1 kg of ⁹⁹Tc is generated. Thus, the mass of ⁹⁹Tc produced since 1993 has nearly quadrupled, and the pace of generation will likely increase if more emphasis is placed on nuclear power to slow the accumulation of atmospheric greenhouse gases. In order to gain a comprehensive understanding of the interaction of ⁹⁹Tc and the natural environment, we review the sources of ⁹⁹Tc in the nuclear fuel cycle and its biogeochemical behavior. We include an evaluation of the use of Re as a chemical analog of Tc, as well as a summary of the redox potential, sorption, colloidal behavior, and interaction of humic substances with Tc, and the potential for re-oxidation and remobilization of Tc(IV). What emerges is a more complicated picture of Tc behavior than that of an easily tractable transition of Tc(VII) to Tc(IV) with consequent immobilization. Reducing conditions (+200 to +100 mVE_h) and the presence of Fe(II) sorbed onto Fe(III) (oxy)hydroxides will bring the mobile Tc(VII) species to a lower oxidation state and will form the relatively insoluble Tc(IV)O₂ · nH₂O, but even as a solid, equilibrium concentrations of aqueous Tc are nearly a factor of 20× above the EPA set drinking water standards. However, sequestration of Tc(IV) into Fe(III)-bearing phases, such as goethite, iron-bearing phyllosilicates and, perhaps, siderite, may ameliorate concerns over the mobility of Tc. A key factor, elucidated through experiment, in retarding the mobility of Tc in the environment is isolation from exposure to oxygen. One way to achieve isolation from oxygen occurs when Tc is locked in a crystallographic position in a solid phase.