Gaomiaozi Research Articles

Influence of dry density on HTO diffusion in GMZ bentonite

With the low permeability and high swelling property, Gaomiaozi (GMZ) bentonite is regarded as the favorable candidate backfilling material for a potential repository. The diffusion behaviors of HTO in GMZ bentonite were studied to obtain effective diffusion coefficient (D e) and accessible porosity (e) by through- and out-diffusion experiments. A computer code named Fitting for diffusion coefficient (FDP) was used for the experimental data processing and theoretical modeling. The D e and e values were (5.2–11.2) × 10−11 m2/s and 0.35–0.50 at dry density from 1,800 to 2,000 kg/m3, respectively. The D e values at 1,800 kg/m3 was a little higher than that of at 2,000 kg/m3, whereas the D e value at 1,600 kg/m3 was significantly higher (approximately twice) than that of at 1,800 and 2,000 kg/m3. It may be explained that the diffusion of HTO mainly occurred in the interlayer space for the highly compacted clay (dry density exceeding 1,300 kg/m3). 1,800 and 2,000 kg/m3 probably had similar interlayer space, whereas 1,600 kg/m3 had more. Both D e and e values decreased with increasing dry density. For compacted bentonite, the relationship of D e and e could be described by Archie’s law with exponent n = 4.5 ± 1.0.

Effect of contact time, pH, and ionic strength on Cd(II) adsorption from aqueous solution onto bentonite from Gaomiaozi, China

Bentonite has been studied extensively because of its strong adsorption capacity. The Gaomiaozi (GMZ) bentonite is selected as the first choice of Chinese buffer materials for the high-level radioactive waste repository. The adsorption of Cd(II) on the GMZ bentonite as a function of contact time, pH, ionic strength, bentonite content, and Cd(II) concentration was studied by using batch technique. The adsorption process achieved equilibrium within 50 min, and the adsorption capacity of Cd(II) by the raw GMZ bentonite was about 3.16 mg/g under the given experimental conditions. The adsorption ability of the GMZ bentonite increases with increasing pH at 2–12 but decreases with increasing ionic strength from 0.01 to 0.1 M KNO3. The uptake of Cd(II) on this bentonite increases in response to an increase of the bentonite content. Chemisorption was considered to be a key of process and adsorption mechanism could be concluded to be complexation, ion exchange, and electrostatic interaction. The equilibrium adsorption data were fitted to the second-order kinetic equation. Furthermore, the Langmuir adsorption isotherm model was used for the description of the adsorption process very well.

Effect of temperature on soil-water characteristics and hysteresis of compacted Gaomiaozi bentonite

Laboratory tests under different constraint conditions were carried out to obtain the soil-water retention curves (SWRCs) of highly-compacted confined/unconfined Gaomiaozi (GMZ) bentonite at 20, 40 and 80 °C, respectively. The effect of temperature on the soil-water characteristics of the highly-compacted GMZ bentonite was analyzed. The results show that the water retention capacity of the highly-compacted GMZ bentonite decreases as the temperature increases under unconfined and confined conditions. At a certain temperature, the constraint conditions have little influence on the water retention capacity of the compacted bentonite at high suction, but the water retention capacity of the confined specimen is lower than that of the unconfined specimen at low suction. Under unconfined conditions, the hysteretic behaviour of the compacted bentonite decreases with increasing temperature. At high suction (>4 MPa), the hysteretic behaviour of the unconfined bentonite tends to increase with the decrease of the suction. In summary, the hysteretic behaviour of the compacted bentonite is not significant.