Studies of the Extent of Surface Diffusion in the Migration of Radionuclides through Geological Materials

Hydraulic conductivity study of compacted clay soils used as landfill liners for an acidic waste

Three natural clayey soils from Tunisia were studied to assess their suitability for use as a liner for an acid waste disposal site. An investigation of the effect of the mineral composition and mechanical compaction on the hydraulic conductivity and fluoride and phosphate removal of three different soils is presented. The hydraulic conductivity of these three natural soils are 8.5 × 10(-10), 2.08 × 10(-9) and 6.8 × 10(-10)m/s for soil-1, soil-2 and soil-3, respectively. Soil specimens were compacted under various compaction strains in order to obtain three wet densities (1850, 1950 and 2050 kg/m(3)). In this condition, the hydraulic conductivity (k) was reduced with increasing density of sample for all soils. The test results of hydraulic conductivity at long-term (>200 days) using acidic waste solution (pH=2.7, charged with fluoride and phosphate ions) shows a decrease in k with time only for natural soil-1 and soil-2. However, the specimens of soil-2 compressed to the two highest densities (1950 and 2050 kg/m(3)) are cracked after 60 and 20 days, respectively, of hydraulic conductivity testing. This damage is the result of a continued increase in the internal stress due to the swelling and to the effect of aggressive wastewater. The analysis of anions shows that the retention of fluoride is higher compared to phosphate and soil-1 has the highest sorption capacity.

How mobile are sorbed cations in clays and clay rocks?

Diffusion of cations and other contaminants through clays is of central interest, because clays and clay rocks are widely considered as barrier materials for waste disposal sites. An intriguing experimental observation has been made in this context: Often, the diffusive flux of cations at trace concentrations is much larger and the retardation smaller than expected based on their sorption coefficients. So-called surface diffusion of sorbed cations has been invoked to explain the observations but remains a controversial issue. Moreover, the corresponding surface diffusion coefficients are largely unknown. Here we show that, by an appropriate scaling, published diffusion data covering a broad range of cations, clays, and chemical conditions can all be modeled satisfactorily by a surface diffusion model. The average mobility of sorbed cations seems to be primarily an intrinsic property of each cation that follows inversely its sorption affinity. With these surface mobilities, cation diffusion coefficients can now be estimated from those of water tracers. In pure clays at low salinities, surface diffusion can reduce the cation retardation by a factor of more than 1000.

Effective diffusivity of neptunium and plutonium in granite from Inada, Ibaraki, Japan under anaerobic conditions

The effective diffusivities of neptunium and plutonium in Inada granite have been determined using the through–diffusion method. Experiments were performed under anaerobic conditions in the presence of carbonate. The actinides are expected to be present as carbonato or carbonatohydroxo complexes. Effective diffusivity (De) obtained were (4.0 ± 1.5) × 10−13m2s−1for neptunium and (2.0 ± 1.6) × 10−13m2s−1for plutonium. Effective diffusivities of neptunium and plutonium in granite were estimated based on the pore diffusion model and compared with the experimentally obtained values.

Sorption kinetics of strontium in porous hydrous ferric oxide aggregates II. Comparison of experimental results and model predictions

In a previous paper, we introduced the Donnan diffusion model to describe cation diffusion into microporous solids with variably charged surfaces, such as hydrous ferric oxides (HFO). Here, we present experiments investigating slow diffusion and sorption of strontium by HFO aggregates with well-characterized porosity. Adsorption of protons and strontium at the HFO surface was evaluated by acid-base titration and batch adsorption experiments with dispersed HFO. The experimental data were fitted with a 1-pK basic Stern model including surface ion pair formation of Na(+) and NO(3)(-) and charge distribution for Sr surface complexes. Sorption-diffusion experiments were conducted in flow-through columns at controlled flow rates and at two different pH values, pH 4 and 7. Wet HFO aggregates, which were synthesized using a freezing and thawing method, were packed into chromatographic columns, pre-equilibrated to reach a constant pH, and then Sr breakthrough curves for adsorption and desorption of Sr were recorded. Strong retardation of Sr indicated that diffusion was sufficiently fast in a fraction of pores, so that sorption sites in these pores were rapidly accessible. Based on the analysis of NaNO(3) breakthrough curves, this rapidly accessible pore fraction was estimated to be 37% of the total aggregate pore volume at pH 4.0 and 72% at pH 7.0, respectively. Taking this into account, the Donnan diffusion model gave a good description of the experimental Sr breakthrough curves. Cation exclusion was correctly predicted at pH 4.0. At pH 7, the strong tailing of Sr breakthrough curves due to Sr diffusion into the smallest pores was very well simulated. The Donnan diffusion model proved adequate for pore sizes between approximately 2 and 5 nm, depending on pH and ionic strength. This category of pores was dominant in the HFO aggregates used in this work.

Diffusion mechanism of chloride ions in sodium montmorillonite

For safety assessment of geological disposal of HLW, it is necessary to understand the diffusion mechanism of radionuclides in compacted bentonite. In this study, the diffusion behavior of chloride ions in compacted montmorillonite was studied from the viewpoints of the activation energy for apparent diffusion and the basal spacing of the compacted montmorillonite. A unique change in the activation energy as a function of the dry density of the montmorillonite was found. The activation energy decreased from 17.4 to 13.5 kJ mol-1 as the dry density increased from 0.7 to 1.0 Mg m-3 and increased to 25.1 kJ mol-1 at dry densities above 1.0 Mg m-3. The basal spacing of 1.88 nm, corresponding to the three-water layer hydrate state, was not observed by X-ray diffraction (XRD) until the dry density increased to 1.0 Mg m-3, where the minimum activation energy was obtained. On the other hand, a basal spacing of 1.56 nm, corresponding to the two-water layer hydrate state, was observed at the dry densities of 1.4 Mg m-3 and above, where the activation energies were more than 22 kJ mol-1. These experimental results suggest that there are at least two additional diffusion processes that can raise or reduce the activation energy and are affected by water in the region adjacent to the montmorillonite surfaces. If the "Grahame model" can be introduced to describe the electrical double layer, surface diffusion will be considered the possible predominant diffusion process, even for anions like chloride ions.