Elemental iron Fe0 often called zero valent iron (ZVI) is a promising reductant for removal of Tc from complex aqueous waste streams that contain sulfate, halides, and other inorganic anions generated during processing of legacy radioactive waste. However, the impact of sulfate on the kinetics of oxidation and reduction capacity of Fe0 in the presence of Tc has not been elaborated. We investigated the oxidative transformation of Fe0 and reductive removal of TcO4- in 0.1 M Na2SO4 as a function of initial pH (i.e., pHi 4, 7, and 10) under aerobic conditions up to 30 days. Tc reduction was the fastest at pHi 7 and slowest at pHi 10 (Tc reduction rate for pHi 7 > 4 > 10). Aqueous Tc was below detection limits (0.005 µg/L) at pHi 7 within 6 hours, whereas = 97% of Tc was removed from solution at pHi of 4 and 10 within 24 hours. Solid phase characterization showed that magnetite was the only oxidized crystalline phase for the first 6 hours for all pH experiments. Lepidocrocite was the most abundant oxidized product for the pHi 10 after 5 days, but was not observed at pH 4 and 7. Fe0 oxidation was also pH dependent with the following trend 4 > 7 > 10.
Published: August 18, 2023
Citation
Kandel S., Y. Katsenovich, D. Boglaienko, H.P. Emerson, and T.G. Levitskaia. 2022.Time dependent zero valent iron oxidation and the reductive removal of pertechnetate at variable pH.Journal of Hazardous Materials 424, no. Part B:Art. No. 127400.PNNL-SA-164032.doi:10.1016/j.jhazmat.2021.127400