Spontaneous redox continuum reveals sequestered technetium clusters and retarded mineral transformation of iron

Abstract

The sequestration of metal ions into the crystal structure of minerals is common in nature. To date, the incorporation of technetium(IV) into iron minerals has been studied predominantly for systems under carefully controlled anaerobic conditions. Mechanisms of the transformation of iron phases leading to incorporation of technetium(IV) under aerobic conditions remain poorly understood. Here we investigate granular metallic iron for reductive sequestration of technetium(VII) at elevated concentrations under ambient conditions. We report the retarded transformation of ferrihydrite to magnetite in the presence of technetium. We observe that quantitative reduction of pertechnetate with a fraction of technetium(IV) structurally incorporated into non-stoichiometric magnetite benefits from concomitant zero valent iron oxidative transformation. An in-depth profile of iron oxide reveals clusters of the incorporated technetium(IV), which account for 32% of the total retained technetium estimated via X-ray absorption and X-ray photoelectron spectroscopies. This corresponds to 1.86 wt.% technetium in magnetite, providing the experimental evidence to theoretical postulations on thermodynamically stable technetium(IV) being incorporated into magnetite under spontaneous aerobic redox conditions.

Competitive Reduction of Pertechnetate and Chromate in Radioactive Waste Streams via Zero-Valent Iron

“…However, as equilibrium conditions were reached, the changes slowed, which is consistent with previous research where pristine iron has been shown to initiate relatively fast reductive removal, but as iron corrosion occurs on the particle surface resulting in the formation of iron oxide phases, electron transfer from the core of the iron is inhibited, leading to a slowdown in the removal process . The anticorrosive properties of both Cr­(VI) and Tc­(VII) minimize this effect as they passivate the surface, reducing corrosion reactions while also impacting the ability of ZVI to reduce contaminants on the surface. , …”

“…The XRD and SEM-EDS analysis indicated the incorporation of Tc into iron oxides, such as magnetite and goethite, which could potentially impede the reoxidation of Tc(IV). 14,33 Solid characterization reveals inhibited iron oxide formation in Tc(VII)-bearing samples, and larger concentrations of Cr(VI) lead to slower treatment rates. Although the concentration of Cr(VI) did not significantly influence the Tc(VII) reoxidation in this study, alterations in iron oxide formation could have implications for large-scale treatment processes.…”

“…Despite concerns about reoxidation of the reduced Tc­(IV), the reoxidation did not exceed 30% for all samples with Cr­(VI) concentrations of 25 mg/L (0.481 mM) or higher. The XRD and SEM-EDS analysis indicated the incorporation of Tc into iron oxides, such as magnetite and goethite, which could potentially impede the reoxidation of Tc­(IV). , …”

“…7 The anticorrosive properties of both Cr(VI) and Tc(VII) minimize this effect as they passivate the surface, reducing corrosion reactions while also impacting the ability of ZVI to reduce contaminants on the surface. 14,29 Increasing the concentration of Cr(VI) affected the reduction efficiency of Tc(VII). Increasing from an initial Cr(VI) concentration of 10 mg/L, the reductive removal of Tc(VII) was delayed, leaving 0.85 of Tc(VII) remaining aqueous fraction after 1 h from the start of the experiment.…”

“…Cr­(VI) possesses corrosion-resistant properties that can create passivating layers on surfaces of ZVI, leading to a slowdown in iron oxidation . Tc­(VII) in the aqueous phase exists as TcO 4 – and also exhibits corrosion-resistant properties, which can further hinder iron oxidation. − Moreover, Cr­(VI) may compete with Tc­(VII) removal because reduction to Cr­(III) is more favorable in their redox couples. − …”