Comparative analysis of ZVI materials for reductive separation of ⁹⁹Tc(VII) from aqueous waste streams

Technetium-99 (Tc) is a long-lived radioactive contaminant present in legacy nuclear waste streams and contaminated plumes of the nuclear waste storage sites worldwide that poses risks for human health and the environment. Pertechnetate (TcO₄^-), the most common chemical form of Tc under oxidative conditions, is of particular concern due to its high aqueous solubility and mobility in the subsurface. One approach to treatment and remediation of TcO₄^- is reduction of Tc⁷⁺ to less soluble and mobile Tc⁴⁺ and its removal from the contaminated streams such as liquid secondary waste generated during vitrification of the Hanford low activity tank waste. Zero valent iron (ZVI) is a common reactive agent for reductive treatment of environmental contaminants, including reducible heavy metal ions, which can offer a potential solution to this challenge. Here, we present a comparative study of eleven commercial ZVI materials manufactured by different methods that were evaluated for the reductive removal of TcO₄^- from an aqueous 80 mM NaCl solution at near neutral pH representing low activity waste off-gas condensate. Performance of ZVI materials was analyzed in relation to time-dependent Fe²⁺ dissolution as well as pH and ORP profiles of the contact solution. Large variability in the efficiency and kinetics of Tc⁷⁺ reduction by different ZVI materials was contingent on their origin. ZVI materials manufactured by electrolytic method exhibited superior performance, and the kinetics of the Tc⁷⁺ reduction correlated to particle size. ZVI materials manufactured by iron pentacarbonyl reduction with hydrogen were ineffective for Tc⁷⁺ reduction. In general, our results highlight the need for thorough performance analysis of commercial ZVI materials for any contaminant of interest.