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

Minerals commonly incorporate foreign elements into their crystal lattices, attenuating metal and radionuclide mobility. Prior work showed Fe oxides/oxyhydroxides can host various cations and that Tc immobilization in Fe minerals is enhanced when Tc(IV) is structurally incorporated. However, most technetium studies have been performed under carefully controlled anoxic conditions using Fe(II) precursors, leaving mechanisms of Tc incorporation during aerobic iron oxidation poorly understood. Zero-valent iron (ZVI, Fe0) offers a practical, robust reductant for in situ mineral transformation and potential concurrent Tc(VII) reduction under ambient oxic conditions, but the redox pathways, mineral phases formed, and Tc loading limits and speciation remain unclear. This study investigates a heterogeneous oxic system of granular ZVI in NaCl electrolyte at elevated TcO4− concentration (17 mM; Fe:Tc = 53:1) to elucidate (i) reductive removal of Tc(VII) and its speciation, (ii) structural incorporation of Tc(IV) into iron oxides formed in situ, and (iii) the effect of Tc on iron mineral transformation, particularly ferrihydrite-to-magnetite pathways, under spontaneous aerobic redox conditions.

• Natural and synthetic Fe oxides/oxyhydroxides (magnetite, maghemite, hematite, goethite, lepidocrocite, ferrihydrite) can incorporate metal cations; computational studies predict Tc(IV) incorporation into magnetite up to ~5 wt.% with higher stability around 1–3 wt.%.
• Tc-99 is a long-lived, mobile radionuclide as TcO4− under oxic conditions; under anoxic conditions Tc precipitates as hydrous TcO2 and can persist in reduced form even after exposure to air in Fe-rich systems.
• Prior Tc immobilization studies largely employed Fe(II)-mediated in situ synthesis of iron minerals under anoxic, controlled pH/redox, demonstrating that Tc(IV) incorporation into magnetite, hematite, and goethite yields durable waste forms with low Tc release and that incorporated Tc(IV) can remain reduced even as magnetite oxidizes.
• Tc(IV)-doped magnetite has been synthesized with ~2.5 wt.% Tc by dissolving Fe in Tc solutions; incorporation into pre-formed magnetite is sensitive to Tc concentration. Anoxic vs. oxic iron oxidation follows different pathways and products.
• ZVI has been proposed for reductive remediation of Tc and U in groundwater; TcO4− reduction by ZVI can be effective, but Tc anticorrosive properties can modulate ZVI behavior; effects at high Tc loading and on mineral transformation are understudied.
• Transformation of ferrihydrite depends on pH, temperature, and co-ions; high Fe(II) can drive ferrihydrite to magnetite. Adsorption or incorporation of foreign species can suppress or redirect transformation pathways.

Batch experiments: Granular ZVI (Alfa Aesar, electrolytic, 1–2 mm, 99.98% purity) at 50 g/L was contacted with 80 mM NaCl (pH 6.9–7.3, DI water) under ambient temperature and pressure in air for up to one month on a shaker. A 99Tc stock (40 g/L Tc as NH4TcO4) was used to prepare 17 mM Tc in 80 mM NaCl; added to ZVI at Fe:Tc molar ratio 53:1 (≈3.3 wt.% Tc). Parallel controls without Tc were run. Short-term (up to 1 week) and long-term (up to 1 month) contacts were investigated. Tc(VII) in solution was quantified by liquid scintillation counting (background-corrected; 94% efficiency) after centrifugation. Solids were separated by centrifugation, rinsed with DI water, dried and stored under N2.

Solid characterization: PXRD (Rigaku Ultima IV; and Bruker D8 Venture single-crystal mode for pin-mounted powders) with Rietveld refinement (TOPAS v6) quantified phases; instrument broadening corrected using NIST SRM 660c. Mössbauer spectroscopy collected at room temperature for all samples and at 77 K for one-month Tc sample (57Co/Rh source; WISSEL MVT-1000; Recoil software; Voigt-based fits) to assess Fe0, Fe2+/Fe3+ distributions, and iron oxide stoichiometry. XPS (Kratos AXIS Ultra DLD; Al Kα) analyzed Tc oxidation states and Fe 2p; CasaXPS used for fitting. XANES/EXAFS at Tc K-edge were collected in fluorescence mode at APS beamline 20-ID-C; thin sections were prepared by epoxy embedding and polishing; data analyzed with ATHENA/ARTEMIS; XANES linear combination fitting used standards (Tc4+ in magnetite, TcO2·nH2O, Tc7+ adsorbed); EXAFS FEFF6 models based on Fe3O4 with Tc substitution at octahedral Fe. SEM/EDS (Quanta 250FEG) imaged morphologies and mapped elements. STEM/EDS (JEOL ARM 200F, aberration-corrected; HAADF and bright-field) on FIB-prepared lamellae examined nanoscale structure and Tc distribution. All sample handling post-reaction minimized oxidation (N2 drying/storage); aqueous pH and ORP behavior referenced from prior work; pH measured after 25 days.

• Reductive removal: 99.8% of aqueous Tc removed after 25 days with granular ZVI in 80 mM NaCl containing 17 mM TcO4− (remaining aqueous fraction 0.0010 ± 0.0004). Post-centrifugation supernatant had ~0.01 mM TcO4−; wash water showed 0.08 mM, suggesting minor re-oxidation/resuspension.
• System chemistry: After ~25 days, pH reached 10.3, consistent with Fe dissolution/oxidation reactions.
• Tc speciation and incorporation (XANES/EXAFS): Linear combination fits required three components at all examined locations on granules; approximately 45–55% Tc as TcO2·nH2O, ~32–34% Tc4+ associated with iron oxides (magnetite-like environment), and 10–21% as Tc7+ (likely adsorbed). Principal component analysis supported three components. XPS Tc 3d deconvolution showed 35.3% Tc4+ (256.4 eV), 32.8% Tc7+ (259.6 eV), and ~31.9% Tc4+ in a distinct local environment (257.4 eV), corroborating multiple Tc4+ environments.
• Quantitative incorporation: The structurally incorporated Tc4+ fraction (~32% of total Tc retained) corresponds to ~1.86 wt.% Tc in magnetite (based on Mössbauer-estimated magnetite content at one month).
• Iron mineralogy and transformation: PXRD detected magnetite in all samples; magnetite reflections were more prominent without Tc. Mössbauer confirmed non-stoichiometric (partially oxidized) magnetite and showed time-dependent maghematization in Tc-free samples (Fe2+/Fe3+ ratio decreased from 0.47 at 1 week to 0.39 at 1 month). In Tc-present samples, room temperature Fe2+/Fe3+ was 0.33, indicating a higher degree of oxidation (non-stoichiometric magnetite). The magnetite fraction was suppressed by Tc: Rietveld estimates 2–7% magnetite with Tc vs. 15–26% without Tc. Ferrihydrite content was approximately four times higher with Tc; more unreacted Fe0 persisted with Tc, consistent with Tc’s anticorrosion effect.
• Microscopy: SEM showed spinel-like (cubo-octahedral) morphologies. EDS mapping indicated Tc associated with the iron oxide both at surface and within particles. STEM on FIB lamellae showed Tc distributed throughout the iron oxide with heterogeneous, locally enriched regions (clusters). HR-STEM lattice imaging was consistent with magnetite/maghemite; other phases (e.g., goethite/lepidocrocite) were not dominant in the examined lamellae.
• EXAFS structure: Tc–O distances ~2.00–2.01 Å with reduced first-shell coordination numbers (CN ~2.7–4.4) consistent with some surface Tc and mixed Tc7+/Tc4+. Tc–Fe shells at ~3.09–3.14 Å (Fe1) and ~3.48–3.51 Å (Fe2) matched octahedral Tc substitution in magnetite-like structure. Tc–Tc paths (R ~2.61–2.64 Å; CN ~0.5–0.8) indicated the presence of TcO2 monomers/dimers, supporting clustered Tc within/at iron oxides.
• Overall: Elevated Tc loading (17 mM; Fe:Tc 53:1) under ambient oxic conditions led to near-quantitative Tc(VII) reduction and significant structural incorporation of Tc(IV) into non-stoichiometric magnetite formed in situ. Presence of Tc retarded ferrihydrite-to-magnetite transformation and reduced the extent of ZVI oxidation.

The study demonstrates that under ambient aerobic conditions, granular ZVI can drive a spontaneous redox continuum: Fe0 oxidation and dissolution elevate pH and supply Fe(II), enabling ferrihydrite formation and transformation toward magnetite, while simultaneously reducing Tc(VII) to Tc(IV). A substantial fraction of Tc(IV) becomes structurally associated with magnetite-like phases, persisting even as magnetite partially oxidizes (maghematizes). Spectroscopic evidence (XANES/EXAFS/XPS) shows multiple Tc(IV) environments: (i) hydrous TcO2-like species (precipitates and surface complexes) and (ii) Tc(IV) in octahedral sites of the spinel structure, with Tc–Fe distances characteristic of magnetite substitution and Tc–Tc signatures indicating monomer/dimer clusters. Microscopy confirms that Tc is distributed throughout iron-oxide particles with localized enrichment, consistent with clustered incorporation or co-precipitation during growth.

The presence of Tc significantly alters iron mineral transformation, retarding the ferrihydrite-to-magnetite pathway and yielding higher ferrihydrite and unreacted Fe0 fractions; this aligns with known anticorrosive behavior of TcO4−/Tc species. Despite this retardation, the system still achieves near-complete Tc removal and notable structural incorporation (~1.86 wt.% Tc in magnetite), providing experimental validation for computational predictions of Tc(IV) stability in magnetite at ~1–3 wt.% and indicating feasibility under uncontrolled oxic conditions. These findings are relevant for remediation strategies and development of iron-oxide-based waste forms, as Tc structurally incorporated into magnetite is known to be resistant to reoxidation and release during subsequent magnetite oxidation.

This work establishes that a spontaneous aerobic ZVI system can: (i) reduce Tc(VII) with 99.8% removal at elevated loading (17 mM TcO4−), (ii) structurally incorporate a significant fraction of Tc(IV) into non-stoichiometric magnetite formed in situ (~32% of retained Tc; ~1.86 wt.% Tc in magnetite), and (iii) alter iron mineral transformation by retarding ferrihydrite-to-magnetite progression and preserving more Fe0. Spectroscopy and microscopy corroborate Tc(IV) incorporation with clustered environments within magnetite-like nanostructures. These results experimentally support theoretical predictions regarding Tc(IV) stability in spinel ferrites and highlight the potential of granular ZVI for practical Tc remediation and waste form generation under ambient conditions.

Future work should evaluate long-term stability and leach resistance of Tc(IV) in partially oxidized magnetite/maghemite, delineate kinetics and mechanisms of Tc-induced transformation retardation, quantify scalability and performance in complex waste matrices, and optimize conditions (electrolytes, Fe:Tc ratios, particle sizes) to maximize structural incorporation and durability.

• Partial oxidation of Tc(IV) during sample preparation, storage, or X-ray analysis cannot be ruled out; some Tc(VII) detected may reflect handling/beam effects.
• PXRD could not distinguish magnetite from maghemite; interpretations rely on combined Mössbauer and microscopy data.
• EXAFS cannot unambiguously differentiate sorbed vs. incorporated Tc due to similar Fe–Fe distances across iron oxides; conclusions are drawn in conjunction with XANES/XPS and STEM/EDS.
• FIB lamellae examine limited regions and may not represent the bulk; amorphous powder-like fractions were unsuitable for FIB, so additional phases cannot be excluded.
• Heterogeneity in Tc distribution (localized enrichment) introduces variability; some scan locations showed higher adsorbed Tc(VII) contributions.
• Results pertain to a specific electrolyte (80 mM NaCl), ZVI type (granular, 1–2 mm), and loading (Fe:Tc 53:1); generalization to other conditions/materials requires further study.