Natural iodine in a clay formation: Implications for iodine fate in geological disposals

Abstract

Iodine is one of the most problematic radioisotopes in the context of nuclear waste geological disposal due to its high mobility. Considerable effort has been dedicated to the measurement of its potential retardation during diffusive transport leading to conflicting results, from no retardation to significant retardation, leading in turn to considerable debate. The present study aims at providing new insights into this aspect of the iodine problem by careful quantification of iodine reservoirs in the Callovian-Oxfordian (COx) clay rock taken here as model material for these studies. The present study confirmed the ubiquitous presence of iodine at 1-5 mg kg -1 level in the COx clayey formation. The iodide concentration level in the porewater is also confirmed at a value in the range ~20-40 µmol L -1 , i.e. higher than the expected range of radio-iodine concentration in the far-field of the storage. Surprisingly, most of the iodine was found not to be associated with organic matter but rather in an inorganic form associated with carbonate minerals. This result has potentially significant implications for the fate of radio-iodine. In undisturbed far-field conditions, most natural iodine would not be accessible for isotopic exchange with radioactive iodine, reducing the effective Kd to negligible values. During laboratory experiments, good monitoring of the geochemical parameters (at least the Eh, pH, P CO2 , [Ca] and [Mg]) is mandatory to avoid iodine-bearing carbonate precipitation and to enable rigorous interpretation of the iodide diffusion/retention experiments.2

Is soil natural organic matter a sink or source for mobile radioiodine (129I) at the Savannah River Site?

“…Hu et al (2005) suggested that some inorganic iodine (I À or IO 3 À ) could be present by sorption to the mineral phase, which possesses positively charged surfaces. While it is possible that iodine (likely as iodate) can be trapped in the mineral pool such as carbonate (Claret et al, 2010), this is not the case for the soils of this study, since barely any inorganic carbon was detected based on the difference between the total carbon and total organic carbon of the whole soils (Electronic Annex EA-1). Other minerals (quartz, kaolinite, goethite, etc.)…”

“…Recently, Yamaguchi et al (2010) using iodine K-edge X-ray absorption near-edge structure (XANES) reported soil organic matter (SOM) is capable of oxidizing I À or reducing IO 3 À and subsequently binding iodine to reactive carbon sites. Even though it was apparently observed that a main reservoir for iodine can be the carbonate fraction in a Callovian-Oxfordian (COx) clay rock, one needs to consider the fact that organic matter contents in these clay materials under investigation were mostly below 1% (Claret et al, 2010). Li et al (2011) provided indirect evidence that iodide was enzymatically transported into the soil bacterial cell and incorporated into cellular components via covalent binding.…”

Benchmarks for multicomponent reactive transport across a cement/clay interface

“…The mass ratios of different phases were extracted from the BRGM mineralogical database [40] considering the C2b2 lithostratigraphic level (see Fig. 2 in [41]). Illite and illite/smectite (I/S) were identified as the main clay phases of the COx claystone.…”

Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian–Oxfordian clayey formation of Bure (France)

“…Prevailing fossils are bivalves from the upper part of the benthos, but pelagic bivalves (posidonomya), sponges, foraminifers, brachiopods, and echinoderms from the deep benthos are also present. Previous observations of carbonate bioclasts were performed in the framework of the study of iodine in carbonates (Claret et al, 2010;Lerouge et al, 2010b). The diagenetic carbonates are virtually all micritic calcite forming the cement of the rock but minor 10 lmsized euhedral calcite and dolomite grains are also dispersed in the sediment (Buschaert et al, 2004;Gaucher et al, 2004;Clauer et al, 2007).…”