Relationship of quantitative X-ray diffraction measurements of geologic materials to cesium sorption

Abstract

Quantitative XRD / Rietveld analysis / Cesium / Sorption / Illite / SmectiteSummary. The amounts of clay minerals in geologic samples from the Australian arid zone were measured using the SIROQUANT program, which quantifies mineral abundances from X-ray diffraction patterns using the Rietveld method. The sorption of trace cesium on the same samples was investigated in batch experiments. The statistical relationships between the Cs distribution coefficients (K d values) and the smectite, illite and kaolinite contents were assessed. Other characterisation data such as the BET surface area and total clay content were included in the statistical analysis. The correlation of the illite content with the K d for trace Cs was stronger than any other correlation that was tested. This can be attributed to the presence of frayed edge sites on illite which are highly selective for the adsorption of trace Cs. Sorption experiments were also carried out with higher Cs concentrations, which were expected to saturate the selective Cs sorption sites on illite. Under these conditions, there was no significant correlation between the illite content and the cesium K d . However, the K d was related to the smectite content of the materials. There was no correlation between the kaolinite content and the K d for Cs sorption under any of the experimental conditions. The sorption of Cs on the samples was influenced by the BET surface areas, but normalisation of the K d data by surface area did not fully explain the sorption results. Only the quantitative XRD data were able to explain the shapes of Cs sorption isotherms. The isotherm was distinctly different for an illite-rich sample and did not conform to the Freundlich isotherm exhibited by other samples. This study demonstrates that quantitative XRD measurements are valuable in predicting Cs sorption properties of complex geologic samples, particularly sorption data obtained with trace Cs levels where bulk measurements such as the BET surface area are less useful.

Laboratory studies of the diffusive transport of 137Cs and 60Co through potential waste repository soils

“…The accuracy of the K d values from the diffusion tests is adversely affected by the strong sorption of 137 Cs, which limited its travel through the clay. Nevertheless the K d values are similar to those from the batch tests on sub-specimens, however, this may reflect specimen variability as Payne et al (2002) reported values for the clay from 4300 L kg À1 to 5060 L kg À1 and for the quartzite between 1040 L kg À1 and 6970 L kg À1 . These values are sensitive to clay, and particularly illite content.…”

“…The batch K d values (Payne et al, 2002(Payne et al, , 2009 were also measured under a saturated CaSO 4 solution, similar to that in the diffusion tests. The K d value estimated for 137 Cs from the diffusion tests was 5000 L kg À1 for both soils.…”

“…However, it is clear from the data available for caesium (e.g. Payne et al, 2002) that the sorption is non-linear and a Freundlich isotherm is required. For example, Melkior et al (2007) show that K d drops by two orders of magnitude as the caesium concentration increases from 10 À7 mol L À1 to 10 À2 mol L À1 .…”

“…Results from this apparatus are presented for 137 Cs and 60 Co diffusing through reconstituted geologic materials (Quaternary clay and quartzite) obtained from the central northern region of South Australia. Related studies investigating the sorption of 137 Cs and 60 Co to soils from this region have been reported by Payne et al ( , 2002Payne et al ( , 2009). The through-diffusion, or double-diffusion cell, method is a well established procedure that has been used to estimate the diffusion coefficients of a wide range of chemicals in geologic media including various radionuclides.…”

Radionuclide applications in laboratory studies of environmental surface reactions

“…One recent example is the use of isotopes at trace levels to identify preferential sorbing materials in mixed environmental sorbents (Payne et al, 2002). This application is significant because highly specific surface sites probably play dominant roles in environmental reactions of trace metals.…”