Elemental Iron: Reduction Of Pertechnetate In The Presence Of Silica And Periodicity Of Precipitated Nano-Structures

Nano-structural transformation of iron minerals in the natural environment is altered and often retarded in the presence of silica (e.g., impeded transformation of ferrihydrite) resulting in a modulated interaction with constituents or contaminants present in groundwater. This phenomenon can significantly affect molecular mechanisms of reduction, precipitation, and sequestration of pertechnetate (TcO4-), the most prevalent chemical form of radioactive contaminant technetium-99 in the environment, by elemental iron Fe-0 often referred to as zero valent iron (ZVI). Understanding the role of silica in moderating the reactivity of Fe-0 toward reduction of TcO4- to Tc-4(+) and its interaction with in situ formed iron minerals (ferrihydrite, magnetite) is crucial for successful design of a practical separation system and can be related to similar environmental systems. This study was designed to evaluate silica-modified ZVI systems with two commercially available iron materials. The results revealed that the efficiency of TcO4- reduction by Fe-0 increased in the presence of silica due to inhibited transformation of iron oxyhydroxide into non-stoichiometric magnetite. Moreover, microscopic evaluation of the newly formed iron mineral phases, both in the presence and absence of silica, revealed unique morphologies related to geological phenomena, such as orbicular rocks and Liesegang rings, suggesting that iron dissolution/re-precipitation is a rhythmical reaction-diffusion process, which occurs in both micro-scaled and macro-geological environments resulting in layered structures of iron oxidation products. Environmental significance Radioactive contaminants pose risks upon their spread in the environment. Pertechnetate, studied here, is a highly soluble anion of technetium-99, found in subsurface environments near nuclear waste storage and reprocessing sites. Application of metallic iron for reductive removal of pertechnetate is a promising method due to its availability, efficiency, low toxicity, and low cost; however, its performance and oxidative transformations are altered in the presence of silica species, common in natural environments. Our results demonstrate the enhanced effect of silica on the pertechnetate reduction with in situ formed iron minerals. Additionally, evaluation of iron minerals revealed rhythmical formations common in geological structures, relating phenomena such as orbicular rocks and Liesegang rings to both macro- and micro-scaled and macro-geological environments resulting in layered structures of iron oxidation products.