Takashi Itakura

The diffusion and sorption of volatile organic compounds through kaolinitic clayey soils

Laboratory experiments to estimate the effective molecular diffusion coefficient (D(e)) and sorption coefficient (K(d)) for volatile organic compounds through natural clayey soils were conducted using diffusion testing apparatus. The compounds tested were methyl ethyl ketone (MEK), toluene and trichloroethylene (TCE). The D(e) and K(d) values were determined by a curve fitting procedure. The compound losses, and the effects of porous disks used in the apparatus were significant. The transport of MEK was faster than that of TCE and toluene because of the lower sorption to the soils. The D(e) values of all the compounds were of the order of 10(-10) m(2)/s and smaller than the diffusion coefficient in pure aqueous solution at infinite dilution (D(0)), due to the tortuosity of the samples. The effects of the sample thickness on the parameter determination were not significant. Comparison to the K(d) values estimated from batch sorption tests and from organic carbon content (f(oc))-based predictions showed that the diffusion test results were intermediate between those from the other two methods. The diffusion tests use compacted soil samples and should be more relevant to in situ conditions, but the reliability of the tests is affected by large compound losses that cause uncertainties in their interpretation. It is recommended that more than one method be used to assess K(d) values.

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

Summary 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 (Kd 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 Kd 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 Kd. However, the Kd was related to the smectite content of the materials. There was no correlation between the kaolinite content and the Kd 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 Kd 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.

Tests using reconstituted samples have been performed to assess the diffusive transport of (137)Cs and (60)Co through natural regolith materials from a region in South Australia being considered for a radioactive waste repository. A double diffusion cell apparatus made of polycarbonate resin was developed to estimate the effective diffusion (D(e)) and sorption coefficients (K(d)) that allowed large withdrawals from the source and collector cells and has enabled tests with low concentrations of radioactivity. An alternative to porous stainless steel filter plates has also been used to reduce uncertainty in test interpretation. Analysis of the transient data used a staged method of the Laplace transform to take into consideration the volume of the samples withdrawn from the apparatus during testing. At test completion samples were cut into slices and analysed for radionuclide concentration. Data obtained from the sliced samples confirmed that both numerical and experimental data produced acceptable mass balance. The D(e) values obtained in this study were of the order of 10(-6) cm(2) s(-1) for both species, higher than previously published data. The K(d) values from the diffusion and batch sorption tests were in reasonable agreement for (137)Cs, but an order of magnitude different for (60)Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.

Migration of Cs-137 and Co-60 in the Australian Arid Zone

Batch adsorption experiments with Cs-137 and Co-60 were undertaken using representative samples of geologic materials from the arid region that has been selected for an Australian low-level waste repository. The results indicate that the pH is the main factor affecting the adsorption of Co-60 but has little influence on the sorption of Cs-137. The ionic strength affects Cs sorption, with a decrease in K d associated with higher ionic strength. Selective sorption sites on mica and illite control uptake of trace Cs, whereas the high total site availability of smectite is significant when the total Cs is higher (1 mmol/L). The effects of mineralogy on Cs sorption which were observed for these complex materials confirmed previous results reported for pure minerals.