Reduction and Simultaneous Removal of ⁹⁹Tc and Cr by Fe(OH)₂(s) Mineral Transformation

Abstract

Technetium (Tc) remains a priority remediation concern due to persistent challenges, including mobilization due to rapid reoxidation of immobilized Tc, and competing comingled contaminants, e.g., Cr(VI), that inhibit Tc(VII) reduction and incorporation into stable mineral phases. Here Fe(OH)(s) is investigated as a comprehensive solution for overcoming these challenges, by serving as both the reductant, (Fe(II)), and the immobilization agent to form Tc-incorporated magnetite (FeO). Trace metal analysis suggests removal of Tc(VII) and Cr(VI) from solution occurs simultaneously; however, complete removal and reduction of Cr(VI) is achieved earlier than the removal/reduction of comingled Tc(VII). Bulk oxidation state analysis of the final magnetite solid phase by XANES shows that the majority of Tc is Tc(IV), which is corroborated by XPS measurements. Furthermore, EXAFS results show successful, albeit partial, Tc(IV) incorporation into magnetite octahedral sites. Cr XPS analysis indicates reduction to Cr(III) and the formation of a Cr-incorporated spinel, CrO, and Cr(OH) phases. Spinel (modeled as FeO), goethite (α-FeOOH), and feroxyhyte (δ-FeOOH) are detected in all reacted final solid phase samples analyzed by XRD. Incorporation of Tc(IV) has little effect on the spinel lattice structure. Reaction of Fe(OH)(s) in the presence of Cr(III) results in the formation of a spinel phase that is a solid solution between magnetite (FeO) and chromite (FeCrO).

Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide

“…23 Tc XANES spectra were energy calibrated using a Tc(VII) reference (pertechnetate (TcO 4 -) adsorbed on Reillex-HPQ polymer resin) and fit using a linear combination of Tc(IV) and Tc(VII) standards. 14,[24][25] For EXAFS fitting, a Tc-substituted goethite (α-FeOOH) structure was used in addition to models for The formation of goethite instead of magnetite, 18,[34][35] or hercynite (as predicted by Pourbaix diagrams presented in the SI), suggests that the solid product is also heavily influenced by co-mingled constituents. As previously mentioned, an Fe(OH) 2 (s):simulant ratio of ~200 g/L removes ~45% ( ~12,200 mg/L) of the simulant nitrite and ~45% of Al(III) (SI Figure S1).…”

“…In a preliminary study, ferrous hydroxide solid (Fe(OH) 2 (s)) was successfully used to reduce Tc(VII) in the presence of Cr(VI) and incorporate Tc(IV) into magnetite via Fe(OH) 2 (s) mineral transformation under oxic conditions. 18 This work was performed under the high ionic strength and pH conditions expected for Hanford low-activity waste (LAW) streams, but used a simple solution chemistry that only considered Tc(VII) and Cr(VI) in 1 M NaOH. Here, Fe(OH) 2 (s) is used to treat Tc in a simulated LAW solution, to assess (i) how complex and realistic waste streams affect the efficacy of this approach and (ii) provide mechanistic evidence for Cr (VI) reduction and solid formation and how this impacts the Fe(OH) 2 (s) treatment mechanism for Tc.…”

Facile incorporation of technetium into magnetite, magnesioferrite, and hematite by formation of ferrous nitratein situ: precursors to iron oxide nuclear waste forms

“…This project is somewhat analogous to the work of Um and coworkers who have used Fe(OH)2 to remove TcO4from alkaline solutions, especially simulants of low activity waste streams at the Hanford Site, and incorporate it into a variety of iron oxides including goethite, magnetite, cobalt ferrite, and nickel ferrite. [37][38][39][40] The primary goal in that work is stabilizing Tc during vitrification to decrease its volatility. The primary goal of the work presented here is removing Tc from acid waste streams during reprocessing of spent nuclear fuel and stabilizing the Tc in a form that may be easily handled and ultimately converted to a durable nuclear waste form.…”

“…28 Furthermore, treatment of alkaline solutions containing TcO4 -and CrO4 2-with white rust, Fe(OH)2, results in incorporation of both transition metals into the magnetite structure. 38 To address the aforementioned problem of Tc(VII) volatility during glass vitrification, transition metal doped magnetite has been studied and Ni and Co-doped magnetite were found to reduce the extent of Tc(IV) oxidation to volatile Tc(VII) during high temperature treatment. 19 Finally, Tc-doped goethite has been investigated both experimentally and computationally.…”

Successful Decontamination of 99TcO4− in Groundwater at Legacy Nuclear Sites by a Cationic Metal‐Organic Framework with Hydrophobic Pockets

“…The most important experiment showing a real utility is the batch experiment based on simulated Hanford Site groundwater that is contaminated with 1 ppm of 99 Tc . This groundwater sample contains SO 4 2− CO 3 2− , SiO 3 2− , and Cl − at concentrations 4–5 orders of magnitude higher than that of TcO 4 − (Supporting Information, Table S2), representing a huge decontamination challenge.…”