## 4 Discussion
Fe\({}^{3+}\) predominated over Fe\({}^{2+}\), which increased slightly by approximately 0.2 wt.% over the first 1.5 mm of bentonite from the steel interface. This was in agreement with the presence of tri-(Fe\({}^{2+}\)),di-(Al\({}^{3+}\))-chlorite mineral in minor amounts, coexisting with Fe(III) oxides, evidenced by the presence of hematite on the bentonite side and Fe\({}^{3+}\)/Fe\({}^{2+}\) oxides, mainly magnetite, within the steel corrosion products. The decline in CEC observed in the present study was attributed to the formation of mixed iron oxides in the material rather than montmorillonite dissolution, as no peak broadening XRD evidence supported such dissolution (Figure S1). After being exposed to a carbon-steel heater at 100 \({}^{\circ}\)C for 18 years in the in situ FEBEX experiment, a reduction in CEC was also observed in FEBEX bentonite [43]. That decrease was attributed to one of the following factors: an increase in the smectic layer charge, the reduction of Fe\({}^{\text{III}}\)-bearing smectite, which induced the collapse of clay particles (not observed in the present research), or the shrinkage of lamellar particles induced by high salinity. In FEBEX bentonite, the salinity was nearly constant at each reaction time, except in the section closest to the hydration zone. Therefore, the decrease in CEC could not be ascribed to higher salinity resulting from direct contact with the saline solution. [PERSON] et al. [68] proposed that the potential decrease in CEC could be attributed to the increased experimental temperature and/or the development of corrosion products within the bentonite matrix. In the current set of experiments, it was noted that smectite aggregates at the steel-bentonite interface were coated with iron oxides, as illustrated in Figure 5, in agreement with the interpretation by [PERSON] et al. [68]. The characterization of aqueous extracts from the bentonite porewater showed a decrease in K\({}^{+}\) concentrations after 22 months. It could have occurred due to a fixation of K\({}^{+}\) in the interlayer during the initial stages of illitization, as suggested by [PERSON] et al. [68; 69], although the low general K\({}^{+}\) concentration provided no visible evidence for this, extracted from the XRD patterns. Another explanation for the general salinity reduction involving the remaining ions over the long term could be attributed to the formation of iron oxides, wherein ions subsequently associated to form complex minerals, such as green rust, producing an overall reduction of ions (chloride) in solution, as observed after 22 months of reaction. Fe-silicates and Fe-Cl corrosion products were detected, evidencing chloritization processes, as confirmed by XRD after 22 months of reaction. The formation of Fe-spicules or fibrous Fe-silicates was reported by [PERSON] et al. [70] in their SEM study on Fe-bentonite interactions. Previous experiments with MX-80 and FEBEX bentonite conducted for 1 and 6 months under the same experimental conditions showed similar precipitation of iron oxides in the corrosion layer at the steel-bentonite contact. The authors reported the formation of magnetite, maghemite and goethite after 1 and 6 months [53]. The increase in Fe content within the montmorillonite structure could be elucidated through various mechanisms, such as adsorption within the interlayer via cation exchange, adsorption at the edge sites of montmorillonite or association with corrosion products [36]. Additionally, redox reactions may have occurred, leading to the precipitation of Fe\({}^{\text{III}}\)-oxides or mixed Fe\({}^{\text{II}}\)/Fe\({}^{\text{III}}\)-oxides and Fe\({}^{\text{II}}\)-carbonates like ankerite and siderite. Soluble Fe\({}^{2+}\) migrated into the bentonite as oxygen from the bentonite pores diffused to the steel surface. The interaction of iron with oxygen could have resulted in oxidation and subsequent precipitation on montmorillonite surfaces, forming iron (hydr)oxides like lepidocrocite.