Yong-Gui Chen

Effect of shaking time, ionic strength, temperature and pH value on desorption of Cr(III) adsorbed onto GMZ bentonite

The Cr(III) desorption experiments of Gaomiaozi (GMZ) bentonite in aqueous solutions were performed. The variables affecting the desorption behaviors, such as contact time, concentration of the desorbent, pH value of the solution, temperature and desorption isotherms, were investigated by the batch experiments. The results show that the adsorbed Cr(III) on GMZ bentonite can be easily extracted by the desorbent. Kinetics examination shows that desorption is slower than adsorption, and the desorption rate increases with time and reaches the equilibrium after 3 h. The final desorption ratios of Cr(III) are 89.4%, 56.5% and 77.2% in the desorption solution with 0.1 mol/L HCl, 1 mol/L NaCl, and 1 mol/L CaCl2, respectively, and the concentration can promote the desorption progress. Furthermore, the results of successive regeneration cycles indicate that the bentonite has a good regeneration ability and reusability. The pH value is an important factor in the Cr(III) desorption from the GMZ bentonite. The results of adsorption and desorption isotherms show that both adsorption and desorption isotherms are consistent with the Freundlich equation. The comparison of adsorption and desorption isotherms implies that the adsorption/desorption hysteresis is negligible and the transport of Cr(III) in bentonite can be described by a reversible adsorption process.

Effect of salt concentration on desiccation cracking behavior of GMZ bentonite

Bentonite has been proposed for use as an engineering barrier and buffer in nuclear waste repositories and has been used frequently in municipal waste landfills. The cracking behavior and deformation properties of this material can be influenced by the chemistry of pore water. In the present work, the influence of salt concentration on the desiccation cracking behavior of GMZ bentonite was investigated with laboratory experiments. Image processing techniques and SEM tests were performed on the specimens which had undergone the desiccation testing in order to analyze the cracking mechanisms. Results show that the water evaporation process can be identified by a steady rate stage, a falling rate stage and a residual stage. The water evaporation rate is strongly affected by the salt concentration of the pore water; higher salt concentrations result in lower evaporation rates; the final water content is strongly impacted by a high initial salinity; otherwise the water contents are very similar for the residual stage. During desiccating, most of cracks appeared at the steady evaporation stage. The cracking morphology and patterns were greatly affected by the salt concentration of the pore water; and larger crack lengths and lower crack densities were obtained as the initial salinity was increased.

Advances in experimental investigation on hydraulic fracturing behavior of bentonite-based materials used for HLW disposal

Abstract Bentonite-based materials (BBM) in the form of blocks or rings have been commonly proposed as buffer materials for high-level radioactive waste disposal. In a real repository, hydraulic fracturing is likely to be present when the ground water pressure exceeds to hydraulic resistance, and the existence of technological gaps increases the risk of hydraulic fracturing. Based on the previous studies of hydraulic fracturing of BBM and technological interfaces, the present work systematically analyzes the experimental achievements. Results show that the fracturing pressure of BBM and interfaces increases with hydration time, initial dry density, bentonite content and overburden pressure, while it decreases with initial water content and gap thickness. The temperature effect on hydraulic fracturing depends on the predominant cation of bentonite. For Na-bentonite, the higher temperature leads to the larger swelling pressure, and thus the stronger hydraulic resistance. Tensile failure, shear failure and synthesis failures are increasingly accepted for hydraulic fracturing mechanism. The hydraulic fracturing of highly compacted BBM is mainly controlled by tensile failure, while the initial shear failure can be found for BBM with a low enough density. Since hydraulic fracturing is expected to occur several times during the repository operation, the long-time monitoring of hydro-mechanical behavior is of vital importance until the technological gaps are completely sealed. Furthermore, the failure mechanism of hydraulic fracturing of BBM should be further researched, as well as the sealing properties of interfaces under thermo-hydro-mechanically coupled conditions are proposed to be studied thoroughly.

Investigation on swelling-shrinkage behavior of unsaturated compacted GMZ bentonite on wetting-drying cycles

It is generally recognized that swelling/shrinkage deformation and accumulative deformation of compacted expansive soil on wetting-drying cycles are strongly influenced by the specimen’s dry density and the net stress applied. However, investigations on the coupling effects of dry density and vertical net stress on the swelling/shrinkage deformation and accumulative deformation are limited. Cyclical wetting-drying tests were conducted on unsaturated compacted GMZ bentonite specimens with three dry densities under five vertical net stresses. The influence of vertical stress on accumulative deformation was analyzed. Then, the critical vertical stress was proposed for specimens with different dry densities. The effect of dry density on accumulative deformation was studied and the critical dry density was determined for different vertical stresses. Based on these, the concept of critical swelling-shrinkage line is proposed in the e-p space. According to the relationship between the initial state of specimen and the critical swelling-shrinkage line, a state parameter is proposed for determination of the accumulative strain on wetting-drying cycles.

Diffusion of La3+ in Compacted GMZ Bentonite Used as Buffer Material in HLW Disposal

It has been studied that the GMZ bentonite which has a chemical barrier property can play an important role in retarding the migration of radioactive nuclide. In order to understand the diffusion properties of nuclide in GMZ bentonite, a series of experiments were conducted to obtain the apparent diffusion coefficient of La3+ in GMZ bentonite under different dry density and pH value conditions using a autonomous ion diffusion instrument. The results show that the apparent diffusion coefficient of La3+ decreases significantly with the increasing dry density or pH value. The influence of dry density on the diffusion is very apparent for a low dry density, and the influence gradually slow down with the increase of dry density. At last the influence mechanism of the dry density and pH value on diffusion coefficient was also analyzed.

Adsorption of Heavy Metals onto Bentonites Used as Buffer/Backfilling Materials

Based on the former studies, experimental results of heavy metals adsorption on bentonites used as buffer/backfilling materials, such as MX-80, FEBEX and GMZ, are reviewed. It is observed that the high smectite content makes these materials an effective barrier. The adsorption of heavy metals onto bentonites is affected by contacting time, pH value, bentonite content, and ionic strength, etc. The adsorption of heavy metals on bentonites can be fitted by Langmuir and Freundlich models well.

Advances on Buffer/Backfill Properties of Heavily Compacted Gaomiaozi Bentonite

Compacted bentonite is considered as buffer/ backfill materials for geological disposal of high-level radioactive wastes in many countries. Through a national screening and comparison, GMZ bentonite is selected as the first choice of Chinese buffer materials for high-level radioactive waste repository. Many studies were conducted on GMZ bentonite, however, its still in the primary stage compared with other candidate bentonites. In order to better orientate the future research work, the progress on researches of GMZ bentonite was summarized, which consist of studies on mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior. Based on analysis of the current studies, the key issues to be explored in the future were also proposed, that is the influence of temperature on behavior of the GMZ bentonite; the migration law of nuclide in compacted bentonite and sealing properties of GMZ bentonite based materials under coupled T-H-M-C conditions..

Gas Breakthrough of Compacted Bentonite/Sand-Argillite Interfaces

This paper focuses on gas breakthrough tests through bentonite-argillite interface. The main contribution of our study is to provide insights into the swelling kinetics, gas breakthrough pressure of these materials. During the tests, bentonite/sand plugs and bentonite-argillite plugs are used to perform swelling tests and gas breakthrough tests. Experimental results show that continuous gas passage through fully saturated bentonite/sand is not obtained until 10.5 MPa gas pressure. When a bentonite/sand plug swells inside a smooth metal tube, gas passage occurs at 7–8 MPa, which is similar to the effective swelling pressure of bentonite, as measured by independent swelling experiments. For argillite alone, saturated with water until sealing (water permeability on the order of 10−20–10−21 m2), gas breakthrough may occur at as low a pressure as 0.2 MPa and up to 6 MPa for undisturbed argillite. Finally, also after full water saturation, continuous gas breakthrough through the bentonite-argillite interface is detected at between 7 to 8 MPa gas pressure. It is interpreted that the argillite rock or the bentonite-argillite interface are both preferential pathways for gas migration when all materials become fully saturated.

Thermal Volumetric Behaviour of Compacted GMZ Bentonite Saturated with Salt Solution

Gaomiaozi (GMZ) bentonite has been confirmed as the buffer material in the Chinese disposal program of high-level waste. To investigate the thermal volumetric behaviour of compacted GMZ bentonite, several stress paths were performed on the specimens presaturated with NaCl solutions to obtain the scheduled OCR and verticle stress. Then multi-step heating-cooling tests were conducted. Test results show that thermal volume change strongly depends on the vertical pressure, OCR and concentration of the presaturation solution. For the specimens presaturation with deionized water, heating induces contraction below the transition temperature and followed by dilation at normal consolidation state, while only induces dilation at overconsolidation state. A larger vertical pressure leads to a larger transition temperature. For the specimens presaturated with NaCl solutions, heating basically induces dilation. The specimens with large vertical pressure accumulate a total contraction after a thermal cycle. For a given concentration and verticle pressure, the specimens cumulate a larger irreversible dilation at higher OCR. The influence of salt solution is non-monotonic. The dilute solution and high saline solution significantly promote the thermal expansion of saturated GMZ bentonite during heating.

Removal of europium on GMZ bentonite corroded by young cement water at different temperatures

After construction of the high-level radioactive waste repository, the bentonite undergoes corrosion of the alkaline solution produced by cement degradation. Young cement water-YCW was employed to carry out the corrosion experiments on compacted GMZ bentonite specimens. Then the removal of europium (Eu(III)) onto corroded GMZ bentonite was studied by batch experiments. The results show the YCW corroded GMZ bentonite is still an effective adsorbent. The adsorption reaction of Eu(III) on corroded GMZ bentonite is endothermic. It can be described by Langmuir isotherm model well. The bentonite corroded at higher temperature shows more stable sorption property.