Environ. Sci. Technol. 1996, 30, 2259-2262
Reduction of Pertechnetate by Ferrous Iron in Solution: Influence of Sorbed and Precipitated Fe(II) DAQING CUI* AND TRYGVE E. ERIKSEN Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden
The reduction of TcO4- to TcO2‚nH2O by Fe(II) in slightly acid to basic solution has been investigated in an all-glass reaction vessel with a hydrophobic inner surface. The three-electron reduction process, although thermodynamically feasible, was found to proceed very slowly if at all. Fe(II) sorbed on the wall of untreated reaction vessels or precipitated as Fe(OH)2(s) or FeCO3(s) was found to reduce TcO4-, the observed rates being proportional to the amount of sorbed or precipitated Fe(II). The experimental results are discussed in light of the standard redox potentials and possible reaction paths leading from TcO4- to TcO2‚nH2O.
Introduction The fission product 99Tc constitutes a potential long-term environmental hazard due to its long half-life (2.13 × 105 yr) and abundance in nuclear wastes. The speciation of the redox-sensitive element in aqueous solution is determined by oxidation-reduction, hydrolytic and complexation equilibria. In oxic groundwaters, technetium exists as the weakly sorbing anionic species TcO4-, whereas under the reducing conditions expected to prevail in a deep bedrock repository, the solubility is controlled by the precipitated tetravalent hydrous oxide TcO2‚nH2O ()Tc(OH)4(s)) (1). Under reducing conditions or in the absence of oxygen, the sorption of technetium from a solution initially containing TcO4- onto granite has been reported by several researchers (2-6). Vandergraaf et al. (7) demonstrated that sorption takes place on the surfaces of iron oxides present as inclusion in Fe(II)-containing minerals and fracturefilling material. Haines et al. (8) showed by a Fourier transform infrared (FTIR) study that the reaction between TcO4- and Fe3O4 occurs via surface-mediated reduction of TcO4- to Tc(IV) and by precipitation of TcO2 on the Fe3O4 surface. Reduction of TcO4- by Fe(II) in aqueous solution has been reported by several research groups (6, 9, 10). However, the presence of, added or unintentionally formed, Fe(II)-containing solid phases in the systems studied does not make the interpretation of the experimental data straightforward. The aim of the present work is to study the reduction of TcO4- by Fe(II) and to demonstrate the
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1996 American Chemical Society
importance of surface-mediated three-electron reduction of TcO4- to TcO2‚nH2O on Fe(II) precipitates.
Experimental Section The experiments were carried out at ambient temperature (22 ( 2 °C) in a controlled atmosphere glovebox flushed with argon to minimize the intrusion of oxygen into the reactions vessels. All chemicals used were of analytical grade. The water used was deionized and triple distilled in quartz The pH measurements were made with a Radiometer Model pH M84 pH meter and a GK 2321 combined glass/reference electrode, and the redox potentials were monitored with a Metrohm Pt electrode and a Yokagawa SR20/NP24 (Ag/AgCl) reference electrode connected to a Metrohm 632 pH meter. The electrodes were calibrating using standard pH and saturated quinhydrone buffers at pH 4 and 7. 99Tc was purchased from Amersham in 0.1 mol dm-3 NH4OH aqueous solution. TcO4- solutions were prepared by diluting aliquots of the stock solution. Solution samples were analyzed for 99Tc in a Beckman Model 5801 liquid scintillation system using Ready Safe Liquid Scintillation Cocktail (Beckman). The counting was calibrated with standard solutions containing known 99Tc concentrations. The counting error (2σ) was kept
