Iodide interaction with natural pyrite

Abstract

Abstract129 I is one of the major dose-determining nuclides in the safety analysis of deep storage of radioactive waste. Iodine forms anionic species that hardly sorb on the surfaces of common host-rock minerals. Recently, interest has arisen on the role of pyrite, an accessory mineral capable of binding anionic selenium. Whereas the interaction of selenium with pyrite is well documented, corresponding results on iodine sorption are still scarce and controversial. Pyrite is present in argicilleous rocks which are being considered in many countries as potential host rocks for a radioactive waste repository. The uptake of iodide (I -) on natural pyrite was investigated under nearly anoxic conditions (O 2 \ 5 ppm) over a wide concentration range (10 -11 -10 -3 M total I -) using 125 I as the radioactive tracer. Weak but measurable sorption was observed; distribution coefficients (R d ) were less than 0.002 m 3 kg -1and decreased with increasing total iodide concentration. Iodide sorption was connected to the presence of oxidized clusters on the pyrite surface, which were presumably formed by reaction with limited amounts of dissolved oxygen. The results obtained indicated that pyrite cannot be considered as an effective scavenger of 129 I under the geochemical conditions prevailing in underground radioactive waste geologic storage.

How mobile is iodide in the Callovo–Oxfordian claystones under experimental conditions close to the in situ ones?

“…Aimoz et al (2011) showed a concentrationdependent uptake of iodide by this altered mineral even Through-diffusion Out-diffusion from upstream reservoir Out-diffusion from downstream reservoir RD, mL g -1 Fig. 6.…”

“…The concentration-dependent uptake of iodide was often mentioned in literature with sorption evidenced only for iodide concentrations less than or equal to about 10 −4 mol L −1 (Aimoz et al, 2011;Bazer-Bachi et al, 2006;Descostes et al, 2008;Savoye et al, 2006;Wittebroodt et al, 2008Wittebroodt et al, , 2012Zhang et al, 2011). Moreover, when desorption was investigated in some of these studies, the iodide uptake was generally found to be largely irreversible.…”

“…Among the mechanisms behind the kinetically-controlled iodide retention, it can be proposed one based on the study of Aimoz et al (2011). This mechanism can be related to a slow process of pyrite oxidation that could occur under our nearly anoxic conditions, and could progressively increase the amount of the new reactive phases capable of sorbing iodide, until the iodide desorption.…”

“…This means that, if the iodide sorption had been related to the oxidation of pyrite, the oxidation could not have occurred throughout the duration of the experiments but would have taken place before the addition of the reductive agent. Nevertheless, this last statement can also be excluded because of (i) the care paid to limit oxygen entrance (argon-drilled borehole; nearly anoxic conditions throughout the experiments) and (ii) the relative large size of our pristine rock samples (~1 cm) compared to the powdered pyrite (grain sizeb 63 μm) used by Aimoz et al (2011), which reduces the oxygen contamination to the sample surface only. Therefore, excluding the slow oxidation of pyrite as a possible candidate, led us to assume that the kinetic control could be very likely related to the iodide retention mechanism itself, as previously suggested by Bazer-Bachi et al (2006) from their batch experiments performed under oxic conditions but with addition of S 2 O 3 2− .…”

“…This should limit the oxidation of the rock sample, and especially of the pyrite whose oxidation products, i.e. ferric oxyhydroxide, are more reactive towards iodide (Aimoz et al, 2011). This operation was previously performed by Bazer-Bachi et al (2006) and Descostes et al (2008) on COx samples but under oxic conditions.…”