Waste Encapsulation with Zeolite
Natural zeolite is often used to limit the environmental impact of radioactive waste. It supports cement matrix stability and can significantly reduce leaching rates.
One of the largest challenges facing the nuclear industry is managing waste that has the potential to pollute the environment and transfer toxins to soil-plant-animal food chains. For years, cement has been used for the encapsulation of radioactive wastes because it is inexpensive, can be easily prepared in remote locations, and can penetrate complex waste shapes.
Encapsulation is a waste disposal method that packs hazardous materials in containers made of an impervious and non-reactive material. The containers are sealed with concrete, plastic, or steel for burial or storage. In the United States, Portland cement is commonly used in the encapsulation process because it is inexpensive, highly alkali, and can incorporate wet waste.
Cement assists in the immobilization of radionuclides by acting as a diffusion barrier, providing sorption and reaction sites, and maintaining a high pH, which in turn decreases radionuclide solubility. However, reactions that occur between radioactive waste and cement used for encapsulation can compromise the integrity of the waste form.
Zeolite to Reduce Environmental Impact
Natural zeolite is often used to limit the environmental impact of radioactive waste. Historical applications include limiting the impact of the Three Miles Island and Chernobyl nuclear accidents, removing radioactive cesium and strontium isotopes from nuclear industry effluents, and decontaminating water. Since 1985, the Sellafield Ion Effluent Plan has used clinoptilolite to reduce the levels of cesium and strontium discharged into the Irish Sea.
Leaching is a large challenge linked to radioactive waste encapsulation. Studies have found that improving pore structures is one method to contain waste in a solid structure. For example, Bagosi conducted an experiment with a cement and natural zeolite blend to prevent cesium leaching. The results indicated that the addition of zeolite decreased cesium release by up to 75 percent over the course of a three-year period.
Li & Wang studied the effects of zeolite on the strength, hydration heat, and leaching rates of the resin's cementation matrix. After a simulated leaching test, they determined that the inclusion of zeolite in cement reduced the leaching rates of radionuclides significantly. They concluded that zeolite is a supplement that supports cement matrix stability and can significantly reduce leaching rates.
Zeolite for the Waste Encapsulation Stabilization
Hogg and Koop stated that the use of natural zeolite as a binding material prior to encapsulation capitalizes on the unique ability to absorb, trap, and immobilize the contaminants in the zeolite structure. When contaminants are confined, they do not interfere with the concrete development process. Lab results also indicate that there is a reduction in the amount of leachable contaminant when the zeolite is included in concrete formation.
In the process of waste encapsulation, zeolite stabilizes contaminants and contributes to the integrity and corrosion resistance of concrete. Stabilization occurs in five ways: volatile organic compounds are captured by molecular sieving; non-volatile organics are adsorbed onto the surface area of zeolite grains; heavy metals are trapped by ion exchange; radioactive cations are trapped by ion exchange; metals are immobilized by the hydrogen bonding of oxyanions.