A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels

Abstract

Underground repository in crystalline bedrock is a widely accepted solution for long-term disposal of spent nuclear fuels. During future deglaciations, meltwater will intrude via bedrock fractures to the depths of future repositories where O2 left in the meltwater could corrode metal canisters and enhance the migration of redox-sensitive radionuclides. Since glacial meltwater is poor in reduced phases, the quantity and (bio)­accessibility of minerogenic Fe­(II) in bedrock fractures determine to what extent O2 in future meltwater can be consumed. Here, we determined Fe valence and mineralogy in secondary mineral assemblages sampled throughout the upper kilometer of fractured crystalline bedrock at two sites on the Baltic Shield, using X-ray absorption and Mössbauer spectroscopic techniques that were found to deliver matching results. The data point to extensive O2-consuming capacity of the bedrock fractures, because Fe­(II)-rich phyllosilicates were abundant and secondary pyrite was dispersed deep into the bedrock with no overall increase in Fe­(II) concentrations and Fe­(II)/Fe­(III) proportions with depth. The results imply that repeated Pleistocene deglaciations did not cause a measurable decrease in the Fe­(II) pool. In surficial fractures, largely opened during glacial unloading, ferrihydrite and illite have formed abundantly via oxidative transformation of Fe­(II)-rich phyllosilicates and recently exposed primary biotite/hornblende.

Iron‑sulfur geochemistry and acidity retention in hydrologically active macropores of boreal acid sulfate soils: Effects of mitigation suspensions of fine-grained calcite and peat

“…To aid interpretations of the experimental XAS data, we compiled a total of 19 Fe reference spectra, representing a broad range of Fe oxidation states and coordination environments that could possibly occur in our samples, for details see Table S1 in the Supporting Information (SI). These reference spectra were either (i) collected during our previous experiments at the Balder beamline (Shahabi-Ghahfarokhi et al, 2022) and the beamline I811 of the MAX-lab (Yu et al, 2015;Yu et al, 2020); or (ii) provided directly by the corresponding author of Burton and Johnston (2012). All XAS spectra were analyzed using Demeter and SIXpack software packages, following standard procedures including energy calibration, averaging of multiple scans, background removal and normalization.…”

Decoupling of particles and dissolved iron downstream of Greenlandic glacier outflows

“…In addition to Fe K-edge XAS data of experimental samples, we measured a suite of Fe mineral reference spectra as part of the current study and in our previous work (van Genuchten et al, , 2014. Our study also includes Fe mineral reference spectra obtained from an online database provided by Shoenfelt et al, (2017) and, which was also collected at beam line 4-1 at SSRL, and received directly from the corresponding author of Yu et al, (2020), which was collected at MAX IV.…”