Zeolites have uses in advanced nuclear waste reprocessing methods, where their micro-porous ability to capture some ions while allowing others to pass freely allow many fission products to be efficiently removed from nuclear waste and permanently trapped. Equally important are the mineral properties of zeolites. Their alumino-silicate construction is extremely durable and resistant to radiation even in porous form. Additionally, once they are loaded with trapped fission products, the zeolite-waste combination can be hot pressed into an extremely durable ceramic form, closing the pores and trapping the waste in a solid stone block. This is a waste form factor that greatly reduces its hazard compared to conventional reprocessing systems. Zeolites are also used in the management of leaks of radioactive materials. For example, in the aftermath of the Fukushima Daiichi nuclear disaster, sandbags of zeolite were dropped into the seawater near the power plant to adsorb radioactive cesium which was present in high levels.The chemical formula for clinoptilolite, a common natural zeolite is: (Na3K3)(Al6Si40)O96 24H2O Atoms or cations (i.e., charged metal atoms) within the second set of parentheses are known as structural atoms, because with oxygen they make up the rigid framework of the structure. Those within the first set of parentheses are known as exchangeable ions, because they can be replaced (exchanged) more or less easily with other cations in aqueous solution, without affecting the aluminosilicate framework. This phenomenon is known as ion exchange, or more commonly cation exchange. The exchange process involves replacing one singly-charged exchangeable atom in the zeolite by one singly-charged airborne atom. The magnitude of such cation exchange in a given zeolite is known as its cation-exchange capacity (CEC) and is commonly measured in terms of moles of exchangeable cation per gram (or 100 grams) of zeolite or in terms of equivalents of exchangeable cations per gram (or 100 grams) of zeolite. (GSA: cation exchange) The zeolite traps small, highly-charged particles that fit through the pores and channels of the zeolite cage. The SiO4 units are electrically neutral, but each AlO4 unit carries a negative charge, creating fixed, negatively charged sites throughout the crystal structure. The negative charges of the AlO4 units are balanced by the presence of exchangeable, positively charged metals known as cations. These positively charged cations usually consist of calcium, magnesium, sodium, potassium and iron. These ions are only loosely held and can be readily displaced by other substances, such as toxic heavy metals, microorganisms and free radicals. This phenomenon is known as cationic exchange, and it is this very high cationic exchange capacity of zeolites which provides for many of their useful properties.In their chemical makeup, zeolites are a lot like clay, in that they are both made up of aluminum, silica and oxygen. However, there is an important difference in their structure. Many types of clay have a layered crystalline structure (similar to a deck of cards) and are subject to shrinking and swelling as water is absorbed and removed between the layers. In contrast, zeolites have a rigid, 3-dimensional crystalline structure (similar to a honeycomb) consisting of a network of interconnected tunnels and cages. Water moves freely in and out of these pores but the zeolites framework remains rigid. Another special aspect of this structure is that the pore and channel sizes are nearly uniform, allowing the crystal to act as a molecular sieve. Research has shown that the smaller the diameter of the metal and the higher the charge of the metal, the greater the affinity it has for the zeolite. Higher charges simply increase the strength of binding with higher binding characteristics. The small size allows for deeper access into the zeolite pores with more points of coordination (attachment). Larger atoms do not fit into the zeolite cage as well and so are more easily exchanged for higher-affinity metals.Zeolites ability to attract and bind different substances is based on the size, shape, and electrical configuration of its molecular cages. Zeolites greatest affinity is thought to be with heavy metals, especially mercury, cadmium, lead, and arsenic. Lighter metals such as calcium, magnesium, and phosphorous have smaller ions, and so their affinity to zeolites negative charge in order of magnitude is much less. As an example of this phenomenon, arsenic has a charge of +3 and an atomic radius of approximately 1.8 angstroms, while potassium has a charge of only +1 and an atomic radius of approximately 2.8 angstroms. The arsenic binds with very high affinity for the zeolite while the potassium has no affinity whatsoever. It just so happens that the most toxic metals are those with a small radius and high ionic charges. The healthy minerals and electrolytes tend to have larger size with smaller ionic charges. Our zeolites binds a variety of toxins. This includes heavy metals (Lead, Cadmium, Mercury, etc..) and radioactive particles e.g. 137Cs and 90Sr and others. Cationic exchange is an entirely passive process when the zeolite is in close proximity to these high-affinity compounds, they will be drawn to the zeolite and either absorbed into the cage or adsorbed onto the surface of the zeolite. There is no chemical activity in this process. The higher the negative charge the higher the CEC. The higher the CEC the stronger the attraction and the more cations and other positively charged molecules that can be held in the cages1. 1 Zeolite Review 2011 October, 2011.