Mineralogical investigations of the interaction between iron corrosion products and bentonite form the NF-PRO experiments, (Phase 1)

This report summarises the findings of a programme of work under taken by the British Geological Survey (BGS) on behalf of Svensk Kärnbränslehantering AB (SKB), to characterise the mineralogical alteration of compacted bentonite from experiments designed to study the interaction between iron corrosion and bentonite. The experiments were undertaken by Serco Assurance (Culham Laboratory, Oxfordshire, United Kingdom), and were co-funded by SKB within the EU Framework 6 NF-PRO Project (Smart et al., 2006). Reacted bentonite residues from three NF-PRO Experiments – NFC12, NFC16 and NFC17 were examined by BGS using; X-ray diffraction analysis (XRD); petrographical analysis with backscattered scanning electron microscopy (BSEM) and energy-dispersive X-ray microanalysis (EDXA) techniques, cation exchange capacity (CEC) and exchangeable cation analysis; and sequential chemical extraction. Bentonite immediately adjacent to corroding steel was found to have interacted with Fe released from the corroding metal. This resulted in the formation of narrow haloes of altered bentonite around the corroding steel wires, in which the clay matrix was significantly enriched in Fe. Detailed petrographical observation found no evidence for the formation of discrete iron oxide or iron oxyhydroxide phases within the clay matrix but appeared to show that the clay particles themselves had become enriched in Fe. XRD observations indicated a slight increase in d002/d003 peak ratio, which could possibly be accounted for by a small amount of substitution of Fe into the octahedral layers of the montmorillonite. If correct, then this alteration might represent the early stages of conversion of the dioctahedral montmorillonite to an iron-rich dioctahedral smectite such as nontronite. Alternatively, the same effect may have been produced as a result of the displacement of exchangeable interlayer cations by Fe and subsequent conversion to form additional Fe-rich octahedral layers. In either case, the XRD results are consistent with the petrographical observations, potentially indicating the early stages of conversion of montmorillonite towards an iron-rich clay mineral as a result of interaction with Fe released by the corrosion of iron or steel. The cation exchange capacity (CEC) and exchangeable cation chemistry of the bentonite was also seen to be subtly affected by interaction with Fe. Bentonite from within the zones where corroded steel wires were present displayed a slightly reduced CEC, and depletion in exchangeable Ca and Na, and an increase in exchangeable Fe. This is consistent with at least partial displacement of the interlayer cations in montmorillonite by Fe. The loss in CEC might also correlate with a partial conversion of the montmorillonite to chlorite, which is tentatively suggested from the XRD analyses. Fe released from the corroding steel was also observed to displace Ca2+ from the interlayer cation sites in the montmorillonite component. This was manifested by the marked concentration of Ca at the interface with the corroding metal and along the leading edges of ‘fronts’ of Fe diffusing into the bentonite matrix. The displaced Ca was seen to have reprecipitated as aragonite. The petrographical observations show that the bentonite within the alteration zone, that has reacted with and is enriched by Fe, has a tendency to show significantly reduced shrinkage on sample drying in comparison to the background unaltered bentonite. Conversely, this would suggest that the reacted and altered clay will also have less ability to swell on hydration with water. This behaviour might be consistent with the partial conversion of the montmorillonite to an iron rich dioctahedral smectite such as nontronite, or to non-swelling clay mineral such as chlorite (or chlorite-smectite mixed-layer clay). If this is the case, then this may have important implications for the long-term behaviour of bentonite seals around radioactive waste canisters made of iron or steel.