Iron Induced Alteration of Bentonite to Serpentine Minerals at 90°C

In deep geological repositories for nuclear waste, iron container-bentonite interactions may impact the long-term performance of engineered clay barrier. Anaerobic canister corrosion, leads to Fe(II) release. Reactions of Fe(II) with bentonite, may modify its properties over time. Accurate performance predictions for high-level radioactive waste (HLW) storage systems require detailed insights into these interactions under relevant thermal conditions. This study explores the secondary phase formation during the reaction of iron powder with GMZ bentonite, MX-80 bentonite, or KGa-2 kaolinite under anoxic conditions. Two series of batch experiments are presented: a continuous high-temperature experiment with mixed bentonite iron suspensions stored at 90°C for up to seven months, and a high-plus-low temperature experiment, with mixed iron clay suspensions stored for four months at 90°C and five months at ambient temperature.
In the presented experiments, montmorillonite - the main component of bentonite - undergoes complete delamination within one week. Iron is taken up by the clay matrix, reaching Fe-contents of ~ 30 atom %, which then remain constant. We suggest that the Fe uptake is driven by the formation of Fe(II/III) containing hydroxide sheets near/at delaminated montmorillonite sheets, inducing the formation of a serpentine like 1:1 phyllosilicate. Crystalline iron oxides (i.e. magnetite) form only at later stages, suggesting the kinetic preference of silicate-bound iron retention. At ambient temperature, the crystallinity of the Fe-phyllosilicate improves. All clay educts, bentonites as well as kaolinite are transformed into chemically and structurally similar phyllosilicates, emphasizing the relative stability and preferential formation of the resulting serpentine phases.
In this data publication we publish the spectroscopic and diffraction data as well as geochemical modelling files, which are explained in detail in the associated manuscript.