REACTIVITY OF FE(II) ASSOCIATED WITH BIOLOGICALLY AND CHEMICALLY REDUCED NONTRONITE TO TECHNETIUM REDUCTION

⁹⁹Tc is formed during nuclear reactions and has been released into the environment during weapon testing and waste disposal. The long half-life, high environmental mobility (as TcO4-) and subsequent uptake into the food chain cause ⁹⁹Tc to be a significant contaminant in effluents. This study was performed to evaluate the role of Fe(II) in bioreduced and chemically reduced clay mineral, nontronite, in reducing and thus immobilizing the soluble form of Tc [Tc(VII)]. Biologically and chemically reduced nontronite was washed with anaerobic buffer until all the media and reductant were removed. Tc(VII) reduction experiments were performed as a function of pH at different ratio of Tc(VII):Fe(II) in PIPES buffer. Reactivity of three types of Fe(II) species in different chemical environments in reduced nontronite [i.e. Fe(II) complexed by surface hydroxyl groups, Fe(II) bound by ion exchange at basal siloxane surfaces and interlayer, and structural Fe(II)] in Tc reduction was also investigated. The results showed that the reactivity of chemically reduced nontronite was very sensitive to pH. For a comparable Fe(II) concentration and nontronite density, chemically reduced nontronite was less reactive at pH 5.5, equally reactive at 7.0 and equal to slightly more reactive at pH 8.5, than biologically reduced nontronite at the corresponding pH values. When the total amount of Fe(II) was the same, nontronite with a lower extent of Fe(III) reduction was far less reactive than nontronite with a higher extent of reduction. The spike experiments performed to understand the ultimate Tc reduction capacity showed that Fe(II) complexed by surface hydroxyl groups are most reactive to reduce Tc followed by the Fe(II) in the basal and interlayer sites. The structural Fe(II) was also able to reduce Tc, but at slow rate. These results suggest that reduction and immobilization of Tc(VII) by multifaceted Fe(II) in reduced nontronite may be an environmentally safe and feasible pathway.