Time dependent zero valent iron oxidation and the reductive removal of pertechnetate at variable pH

Area of Science:

• Environmental Science
• Radiochemistry
• Materials Science

Background:

• Legacy radioactive waste contains complex aqueous streams with technetium-99 (Tc).
• Elemental iron (Fe⁰) is a potential reductant for Tc removal.
• The influence of sulfate on Fe⁰ performance in Tc remediation is not well understood.

Purpose of the Study:

• To investigate the oxidative transformation of Fe⁰.
• To evaluate the reductive removal of pertechnetate (TcO₄^-) by Fe⁰.
• To determine the effect of initial pH on these processes in a sulfate-rich environment.

Main Methods:

• Experiments were conducted in 0.1 M Na₂SO₄ solutions.
• Initial pH was varied (4, 7, and 10) under aerobic conditions.
• Fe⁰ oxidation and Tc removal were monitored for up to 30 days, with solid phase characterization.

Main Results:

• Tc reduction was fastest at pH_i 7 (0.4% aqueous Tc within 6 h) and slowest at pH_i 10.
• Over 97% Tc removal was achieved within 24 h at pH_i 4 and 10.
• Magnetite was the initial oxidized Fe phase; lepidocrocite formed at pH_i 10 after 5 days, but not at pH_i 4 or 7.

Conclusions:

• Sulfate-containing environments can be challenging for Fe⁰-mediated Tc removal.
• Initial pH significantly influences both the rate of Tc reduction and the Fe⁰ oxidation pathways.
• Optimizing pH is crucial for efficient radioactive technetium remediation using elemental iron.