**6. Zeolites as bio-detoxifiers of radionuclides**

Natural zeolites are one of the most interesting groups among minerals, some of which (clinoptilolite, mordenite, chabazite) have enormous potential in science and technology due to their high sorption capacity and the presence of deposits with huge reserves in many countries, including Bulgaria. In the early years of zeolite research, Ames (1960) found that clinoptilolite from the Hector deposit, California, is highly selective for Sr2+ and Cs+ [46]. Other heavy metals, especially monovalent ones, were also well adsorbed—respectively ion-exchanged by this natural zeolite. The author introduced an order of selectivity of clinoptilolite, which is:

$$\text{Cs}^\* > \text{Rb}^\* > \text{K} + \text{NH}\_4^\* > \text{Ba}^\* > \text{Sr}^{2\*} > \text{Na}^\* > \text{Ca}^{2\*} > \text{Fe}^{2\*} > \text{Al}^{3\*} > \text{Mg}^{2\*} > \text{Li}^\*$$

*Radionuclide Contamination as a Risk Factor in Terrestrial Ecosystems: Occurrence, Biological… DOI: http://dx.doi.org/10.5772/intechopen.104468*

The ion exchange properties of clinoptilolite and its selective sorption are especially valuable in the control of radioactive waste from nuclear energy production. The mineral has been successfully used as a sorbent of radionuclides from water and contaminated soils, as well as a food additive to limit 137Cs absorption in livestock [39, 41, 45].

Very significant research on zeolites has been conducted in Bulgaria for the past five decades, with two deposits of clinoptilolite in the Eastern Rhodope Mountains— Beli Plast and Beliya Bair-Zhelezni Vrata—being particularly suitable for bio-sorbents of heavy metals and radionuclides in the form of additives to food and livestock feed [47]. Recently, it was demonstrated that modified natural clinoptilolite from the Golobradovo deposit in the Eastern Rhodopes was practically non-toxic to laboratory mice and facilitated significantly the excretion of Pb2+ions from the gastro-intestinal tract of the experimental animals, thus protecting them against lead toxicity [48, 49]. In parallel, other Bulgarian researchers validated the use of zeolites from the Eastern Rhodopes in decontamination procedures and as soil fertilizer and even developed a clinoptilolite-based artificial soil ("Balkanin") that was used for growing vegetables in space onboard the Mir station [50]. In the early 1990s, researchers from the Bulgarian Academy of Sciences developed a specially modified natural clinoptilolite (CLS-5) as a bio-sorbent for radiocesium (134Cs and 137Cs) and radiostrontium (89Sr and 90Sr) [51]. In a modified form and labeled KS-3, CLS-5 was used in the production of over 55,000 personal radiation protection emergency kits, most of which were distributed among the personnel of the Kozloduy Nuclear Power Plant and the people from the surrounding areas (**Figure 2**).

Two plastic vials containing CLS-5 with a quantity of 7 grams each have been integrated in the radiation emergency kit. The other components of the radiation protection kit are a painkiller syrette, a syrette with an antiemetic, a broad-spectrum antibiotic, potassium iodide (KI) tablets, and CBT (a radioprotector for abating acute exposure to radiation), bandages, and ethanol for disinfection [51].

As evident from the material presented, research into zeolites as bio-sorbents of radionuclides and heavy metals is fairly advanced in Bulgaria. The past achievements in developing modified clinoptilolite derivatives as 90Sr and 137Cs sorbents, and current and ongoing basic research in clinoptilolites as a countermeasure to Pb2+ and Cd2+ intoxication in mammalian species promise to yield the interesting results.

#### **Figure 2.**

*Modified natural zeolites as part of a radiation protection emergency kit: Plastic vials containing CLS-5 (left), and the entire emergency kit (right) [51].*
