Incorporation of pertechnetate and perrhenate into corroded steel surfaces studied by X-ray absorption fine structure spectroscopy

Abstract

Batch reaction experiments and solid-phase characterization analyses were completed to examine the uptake of dissolved perrhenate [Re(VII)] or pertechnetate [Tc(VII)] by A-516 steel coupons that corroded in simulated groundwater solutions or dilute water. The goal was to identify the mechanism(s) that control the uptake of ⁹⁹Tc by corrosion products on carbon steel in the presence of dilute solutions. X-ray absorption fine structure spectroscopy (XAFS) was used to study the oxidation states of Re and Tc incorporated into the corroded steel coupon surfaces. X-ray fluorescence maps showed that the corroded coupons contain localized regions enriched in Re or Tc. The Re L₃ near edge XAFS results for the coupons reacted with Re-spiked waters were consistent with nearly all of the sorbed Re being present as perrhenate and not significantly reduced to Re(IV). Linear combination fits of the extended XAFS signals for the perrhenate and Re^IVO₂ standards indicate that Re sorbed to the steel coupons corroded in simulated J-13 (a relatively dilute Na-HCO₃-CO₃ groundwater) and even more dilute waters consists of a maximum of 5 and 10% Re(IV), respectively. The fluorescence results also showed that the Re concentrations increased with increasing time of exposure to the X-ray beam, which suggests that the perrhenate ions are only weakly bonded to the matrix of the corrosion product. In contrast to the Re results, the Tc K edge XAFS results for the coupons reacted in ⁹⁹Tc-spiked waters indicate that most of the sorbed Tc had been reduced to Tc(IV). The shape of the near edge and extended fine structure is similar to the Tc(IV)-hydrous ferric oxide (HFO) and not the TcO₂·nH₂O standard. Differences were noted in the XAFS results for steel coupons reacted with waters spiked with 0.001 vs. 0.1 mmol/L ⁹⁹Tc in that much more of the sorbed Tc from 0.001 mmol/L ⁹⁹Tc experiments was in the form of pertechnetate. Comparison of the XAFS results for coupons reacted with 0.001 mmol/L ⁹⁹Tc-spiked dilute simulated Na-HCO₃-CO₃ groundwater vs. 0.001 mmol/L ⁹⁹Tc-spiked dilute water also suggest that there are likely differences in the sorption mechanism for the pertechnetate fraction in the corrosion product which formed in these two test solutions. The cause for these differences is not known, but is likely due to differences in the compositions of the dilute simulated Na-HCO₃-CO₃ groundwater and more dilute waters, such as the dissolved carbonate concentrations.