Nanoscale encapsulation: the structure of cations in hydrophobic microporous aluminosilicates

Hydrophobic microporous aluminosilicates, created by organic surface modification of inherently hydrophilic materials such as zeolites and clays, are currently being investigated as storage media for hazardous cations. Use of organic monolayers to modify the surface of an aluminosilicate after introducing an ion into the zeolite/clay reduces the interaction of water with the material. Resulting systems are about 20 times more resistant to leaching of stored ion. XAS spectra from the encapsulated ion demonstrate that byproducts from the organic modifier can complex with the stored cation. This complexation can result in a decreased affinity of the cation for the aluminosilicate matrix. Changing the organic modifier eliminates this problem. XAS spectra also indicate that the reactivity and speciation of the encapsulated ion may change upon application of the hydrophobic layer

INOR-037: Encapsulation of hazardous metals with organic modified minerals

The authors studies have focused on the development of new materials for the control, treatment, and long term storage of hazardous metals. The process involves the introduction of hazardous cations into the matrix of clays through aqueous ion-exchange methods. These cations are subsequently encapsulated within the clay by treating the material with a variety of organic silanes. This treatment results in the formation of organic coatings which are chemically bonded to the surface of the clay. The coatings are hydrophobic in nature, and may restrict the diffusion of water into and out of the pores contained within the clay. The goal of this process is to reduce the undesirable migration of hazardous metals from the ion-exchanged clays into the environment. A smectic type clay, bentonite, has been the primary inorganic matrix for this study. Bentonite, which is a form of montmorillonite, consists of two-dimensional sheets of aluminosilicates. Like other smectite clays, these sheets are separated by an interlayer which contains cations and water. The reactive groups within the alkyl silanes react with hydroxyl groups on the clay surface, as well as water contained on and within the clay. The authors results show that there is little difference in the metal content of the coated and noncoated clays. The cations are not removed from the clay by exposure to the silane. The clays also maintain their general structure and cystallinity upon surface modification. The organic coatings are stable to 500{degrees}C when heated under nitrogen. The ability of these systems to encapsulate the cations and prevent their migration into the environment is currently being evaluated

Speciation of uranium in surface-modified, hydrothermally treated, (UO{sub 2}){sup 2+}-exchanged smectite clays

A successful solution to the problem of disposal and permanent storage of water soluble radioactive species must address two issues: exclusion of the radionuclides from the environment and the prevention of leaching from the storage media into the environment. Immobilization of radionuclides in clay minerals has been studied. In addition to the use of clays as potential waste forms, information about the interactions of radionuclides with clays and how such interactions affect their speciations is crucial for successful modeling of actinide-migration. X-ray absorption spectroscopy (XAS) is used to determine the uranium speciation in exchanged and surface-modified clays. The XAS data from uranyl-loaded bentonite clay are compared with those obtained after the particle surfaces have been coated with alkylsilanes. These silane films, which render the surface of the clay hydrophobic, are added in order to minimize the ability of external water to exchange with the water in the clay interlayer, thereby decreasing the release rate of the exchanged-uranium species. Mild hydrothermal conditions are used in an effort to mimic potential geologic conditions that may occur during long-term radioactive waste storage. The XAS spectra indicate that the uranyl monomer species remain unchanged in most samples, except in those samples that were both coated with an alkylsilane and hydrothermally treated. When the clay was coated with an organic film, formed by the acidic deposition of octadecyltrimethoxysilane, hydrothermal treatment results in the formation of aggregated uranium species in which the uranium is reduced from U{sup VI} to U{sup IV}

The effects of surface modification on the speciation of metal ions intercalated into aluminosilicates

Microporous aluminosilicates, including clay minerals and zeolites, are ion-exchange materials. In their most common forms, they have the ability to incorporate cationic species within their matrices. Because of this property, microporous aluminosilicates have been proposed as storage media for hazardous waste. In this paper the authors use X-ray absorption spectroscopy (XAS) to examine the structure of cations held within smectite clay minerals and to determine how modification of the surface of the clay using an organic monolayer affects the coordination of the stored cation. The effects of hydrothermal and thermal processing on the coordination of the ions contained within these systems are also investigated. The presence of the monolayer changes the surface of the clay from hydrophilic to hydrophobic. It inhibits the interlayer ions from exchanging freely into environmental water and reduces the leach rate of cations out of the clay by approximately a factor of 20. Significant changes are observed when these coated samples are treated under hydrothermal and thermal conditions. Reductions of uranium (VI), in the form of uranyl, and cupric ions occur. In addition, the uranium aggregates, forming small particles that appear similar to UO{sub 2}. Comparable conglomeration occurs with lead cations and with the reduced copper species

Speciation of uranium in surface-modified, hydrothermally treated, (UO{sub 2}){sup 2+}-exchanged smectite clays

A successful solution to the problem of disposal and permanent storage of water soluble radioactive species must address two issues: exclusion of the radionuclides from the environment and the prevention of leaching from the storage media into the environment. Immobilization of radionuclides in clay minerals has been studied. In addition to the use of clays as potential waste forms, information about the interactions of radionuclides with clays and how such interactions affect their speciations is crucial for successful modeling of actinide-migration. X-ray absorption spectroscopy (XAS) is used to determine the uranium speciation in exchanged and surface-modified clays. The XAS data from uranyl-loaded bentonite clay are compared with those obtained after the particle surfaces have been coated with alkylsilanes. These silane films, which render the surface of the clay hydrophobic, are added in order to minimize the ability of external water to exchange with the water in the clay interlayer, thereby decreasing the release rate of the exchanged-uranium species. Mild hydrothermal conditions are used in an effort to mimic potential geologic conditions that may occur during long-term radioactive waste storage. The XAS spectra indicate that the uranyl monomer species remain unchanged in most samples, except in those samples that were both coated with an alkylsilane and hydrothermally treated. When the clay was coated with an organic film, formed by the acidic deposition of octadecyltrimethoxysilane, hydrothermal treatment results in the formation of aggregated uranium species in which the uranium is reduced from U{sup VI} to U{sup IV}

Nanoscale Encapsulation : The Structure of Cations in Hydrophobic Microporous Aluminosilicates

Hydrophobic microporous aluminosilicates created by the modification of zeolites and clays are currently being investigated as storage media for hazardous cations. Addition of an organic monolayer to the surface of an aluminosilicate after introduction of an ion into the zeolite or clay reduces the interaction of water with the material. The resultant systems are approximately 20 times more resistant to leaching of the stored ion than the unmodified ion-exchanged materials. XAS spectra demonstrate that byproducts from the organic modifier can complex with the encapsulated cation. This complexation can result in a decreased affinity of the cation for the aluminosilicate matrix. Changing the organic modifier eliminates this problem. XAS spectra also indicate that the reactivity and subsequent speciation of the encapsulated ion alters upon application of the hydrophobic layer