High-resolution transmission electron microscopic evidences of stacking faults in zeolitic minerals formed in Hanford sediments reacted with simulated tank waste solutions.
EMSL Project ID
3387
Abstract
High-level radioactive waste solutions stored in the underground tanks at Hanford sites have leaked into the vadose zone. The waste tank supernatants are known to consist of solutions with high pH (9-14), high ionic strength (sodium nitrate, sodium nitrite and other electrolytes), and high aluminate concentration. In simulated experiment, allophane, Linde Type A, cancrinite and sodalite formed when Hanford sediment contacted the waste tank solutions. These zeolite-like materials have high negative charge sites in their structures and, therefore, they are capable of incorporating and adsorbing cationic radionuclides such as Cs in their framework and external surfaces. In addition to the high charge properties, the zeolite-like minerals have unique cages and channels within their structures, the radionuclides might be trapped in the cages or channels. Theoretically, stacking faults in the zeolitic minerals may block the channels or cages, inhabit diffusion of the radionuclides in the channels, and increase their stability in the minerals. A fully investigation of adsorption, incorporation, stability, and the location of the radionuclides on or in the minerals is crucial to understand the environmental fate of the radionuclides in the vadose zone. Our preliminary experiments have shown the particle size and crystallinity of cancrinite and sodalite depends mostly on the NaOH concentration and the relative ratio of Si to Al in the supernatant. For the high crystalline cancrinite and sodalite, the individual particles can grow to 5-10 micron in diameter. For the lower crystalline minerals, the sizes of the particles are normally less than 2 micron. SEM and TEM images of the particles also showed that intergrowth or twinning is very common in both the large and small particles, implying several crystal domains exist in a single particle and it is very likely that there are structure disordering in the joint planes of the domains. Cesium adsorption, incorporation and desorption experiments have also indicated not all of the incorporated cesium can be replaced even after extensive washing with other cations such as K+ or Ca2+. The difficulty of the desorption is partly because of the slow diffusion of the large Cs+ cation and is also likely the result of the discontinuity of the channels caused by the stacking faults.
This proposed experiment aimed to check if the stacking faults exist and their occurring frequencies in different crystalline cancrinite, sodalite and Linde Type A zeolite minerals. The analysis will be preformed on JEOL 2010 TEM at EMSL. We expect that the TEM will enable us to check the atom arrangements of the crystals based on their diffraction fringe patterns. One problem associated with the samples is the stability of the zeolite to the electron beam in the instrument. The minerals tend to dehydrate and become amorphous when observed under high magnification. We have found that K+ treatment can enhance the stability of the samples and we are also going to coat the samples with carbon before the analysis. The cancrinite, sodalite, Linde type A zeolite samples have been synthesized at Washington State University (WSU) and washed with Cs, K, and Ca respectively. The cesium used in this experiment is the stable cesium-133, a surrogate of radioactive Cs-137. No radiation will be generated from the samples. The samples have been characterized with X-ray diffraction, Fourier transform IR, and SEM in our previous experiments. The small particles will be mounted on grids directly. The high crystalline particles will be embedded in resin and thin sectioned at WSU before mounting. Lacey supported grids will be used and the grids will be coated with carbon at EMSL. A three-day instrument time will be requested for the analysis.
Project Details
Project type
Exploratory Research
Start Date
2003-03-04
End Date
2006-03-05
Status
Closed
Released Data Link
Team
Principal Investigator