(455a) Effective Removal of Pertechnetate from Groundwater By Bimetallic Porous Iron

⁹⁹Tc is a major long-lived fission product generated during nuclear power generation. Over the years, Tc has been inadvertently introduced into the environment from leaks at waste storage facilities and currently is one of the key risk drivers at the US DOE sites. The most common chemical form of Tc in liquid nuclear wastes or in the environment is pertechnetate (TcO₄^-). TcO₄^- displays limited adsorption to common sediment minerals and is highly mobile making it difficult to be immobilized. As the stockpile of ⁹⁹Tc-bearing nuclear waste continues to increase rapidly, novel sequestration technologies are needed to reduce its potential contamination of the environment and living organisms.

We developed highly efficient and cost-effective bimetallic porous iron materials that were investigated for TcO₄^- sequestration from groundwater. Catalytic bimetallic nanoparticles were grown onto porous iron by chemical reduction methods. The bimetallic porous iron materials were characterized by several analytical techniques to elucidate their morphology, size, composition, and porosity. Specifically, we used X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and BET surface area using Kr as the adsorption probe. The bimetallic porous iron materials were evaluated for removal of TcO₄^- and its surrogate (ReO₄^-) from artificial groundwater under ambient conditions using batch experiments. Tc and Re chemical speciation and molecular removal mechanisms were investigated using synchrotron radiation X-ray near-edge structure spectroscopy.

The results show that porous iron is >92% α-iron with small amounts of Fe and Al oxides. It has a sponge-like porous structure with a BET surface area of 0.95 m²/g, pore volume of 0.0068 mL/g, and average pore diameter of 364 Ã…. When evaluated for contaminant removal, the porous iron successfully removed >97% present in an artificial groundwater solution containing 3.2×10^-6 M TcO₄^-. In contrast, conventional granular zero valent iron, tested as a control, was entirely ineffective with 0% Tc removal. The removal capacity of the porous iron was demonstrated to be 27 mg Tc/g iron. Further, Tc K-edge X-ray absorption spectra of the porous iron samples retrieved from Tc batch experiments showed that both Tc(IV) and Tc(VII) species are present. Linear combination fitting indicated that 50-70% of the Tc was reduced to Tc(IV) that was incorporated into the secondary Fe oxides. In conclusion, these results indicate that porous irons are highly efficient materials for the immobilization of TcO₄^- present in groundwater through both adsorption and chemical reduction mechanisms. They might also be novel materials for effectively immobilizing a wide array of other radioactive contaminants in the environment.