EFFECTS OF HIGH PH AND NITRATE CONCENTRATIONS ON PERTECHNETATE REDUCTION KINETICS AT THE ZEROVALEN IRON-WATER INTERFACE

Technetium-99 (⁹⁹Tc) is a low level radioactive waste product resulting from uranium fission processes in nuclear reactors. Currently, the U.S. Department of Energy (DOE) has been investigating solidification and stabilization remediation methods via reductive precipitation of mobile Tc(VII)O₄^- to Tc(IV) oxides and sulfides for long term waste storage purposes. They are seeking effective reducing agents for the current saltstone cement formulation to optimize the Tc immobilization technology. In this study, the reducing capacity of synthetic zerovalent iron (ZVI) was evaluated using a chemical analog of ⁹⁹TcO₄^- , perrhenate (ReO₄^-). The objective was to evaluate the kinetics Re(VII) reduction under simulated conditions (i.e., high pH and high nitrate concentrations) in low level radioactive waste storage tanks at DOE sites.

The synthetic ZVI [1] was used to conduct the Re(VII) sorption experiments at various pH and [NaNO₃]. Perrhenate concentration was analyzed using a spectrophotometric method [2]. Batch methods were used for the reduction kinetic experiments using various carbonate buffer solutions (pH 8.3-10.2) under varying [nitrate]: 0, 0.001, 0.01 and 0.1M. Kinetic samples were taken at 10,30 min, 1, 3, 6, 12, and 24 hrs. The extent of Re(VII) removal and kinetic rate data were evaluated using pseudo-first and -second order kinetic models [3,4] The preliminary results showed that the extent of Re(VI) removal decreased with increasing pH and [nitrate].The rates were also affected by these variables. Implications of the research findings will be discussed with solid state characterization data of reaction products.

References:

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2. Lenell B.A. and Arai Y. (2016) Talanta. In Press, DOI: 10.1016/j.talanta.2015.10.063.

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4. Zhou et al. (2016) J. Colloid Interface Sci. 462, 200-207.