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ВестНик ИГ Коми НЦ УрО РАН, февраль, 2015 г., № 2
УДК 547.992.3
sûRBEHTs based on MiNERAL AND IHDUsTRIAL MATERiALs
for radioactive wastes immobilizatioh
O. B. Kotova1, I. L. Shabalin2, D. A. Shushkov1, A. V. Ponaryadov1
1IG Komi SC of UB of RAS, Russia; [email protected] 2The University of Salford, United Kingdom; [email protected]
Nuclear engineering development inevitably results in risks of radiation instability in regions. The creation of scientific basis of radiation safety of population and environment is the vital task, which requires search and study of sorbents that are able to deactivate radioactive sources. Sorption materials, based on mineral and technogenic raw, on the example of analcime-bearing rocks and red mud, have been observed within this task. The complex of modern study methods was used, physical and chemical features of the specified sorbents are presented, immobilization and disposal technologies of radioactive wastes are discussed.
Key-words: zeolites, red mud, radionuclides, sorption, radioactive wastes, immobilization, disposal.
СОРБЕНТЫ НА ОСНОВЕ МИНЕРАЛЬНОГО И ТЕХНОГЕННОГО СЫРЬЯ для ЗАХОРОНЕНИЯ РАДИОАКТИВНЫХ ОТХОДОВ
О. Б. Котова1, И. Л. Шабалин2, Д. А. Шушков1, A. В. Понарядов1
:ИГ Коми НЦ УрО РАН, Россия; [email protected] 2Университет Солфорда, Великобритания; [email protected]
Развитие ядерной энергетики неизбежно создает риски радиационной нестабильности в регионах. Создание научных основ обеспечения радиационной безопасности населения и окружающей среды является актуальной задачей, для успешного решения которой необходим поиск и изучение сорбентов, способных дезактивировать радиоактивные источники. В рамках этих задач рассматриваются сорбционные материалы на основе минерального и техногенного сырья на примере анальцимсодержащих пород и красного шлама. Использован комплекс современных исследовательских методов, представлены данные физико-химических свойств указанных сорбентов, обсуждаются технологии консервирования и утилизации радиоактивных отходов.
Ключевые слова: цеолиты, красный шлам, радионуклиды, сорбция, радиоактивные отходы, консервирование, утилизация.
Introduction
The safety of nuclear wastes is an urgent task for the sustainable development of the regions. Mineral and industrial raw materials are widely used to solve the problems of radiological protection of people and environment. The main strategy of development in this direction is to expand the range of effective sorption materials for advanced radiological safety (from the radioactive waste immobilization to sorption of radionuclides in air, water and soil) and study structures, properties, and immobilization mechanisms (transformation, disposal) of radiation-polluted objects. The research and development of methods for the radioactive waste disposal and modification of the properties of raw materials (sorption and ion exchange) have a vital importance.
Objects and methods
The sorbents are based on mineral raw materials (analcime-bearing rocks) and industrial materials (red mud) with the Russian origin.
The following advanced research methods were used: X-ray fluorescent analysis (Horiba MESA-500W); thermal analysis (derivatograph Shimadzu DTG-60A/60AH); X-ray diffractome-try (Shimadzu XRD-6000). Uranium, thorium and radium content were determined by luminescent, emanation and photocolorimetric methods, respectively. The desorption was evaluated by the content of radionu-clides in extracts made by consequent processing of the sorbent with distilled water, 1M ammonium acetate (CH3COONH4) and 1M hydrochloric acid (HCl).
Results and discussion
The sorption methods of radionu-clide immobilization are used to remove ion forms by organic, non-organic or bio-sorbents. Non-organic sorbents include natural and synthetic alumosili-cates, e. g. zeolites. The sorption is carried out due to the exchange of moving cations (Na+, K+ and others) in sorbents to some solution cations. By their crystalline structure alumosilicates are divided into framed (zeolites) and layered (clay minerals). The advantages of natural zeolites in comparison with synthetic sorbents include their low costs and sufficient sorption features, as well as their abundance. Besides it the zeolites show sorption ability even at low substance concentrations.
Clinoptilolite is widely used to immobilize radioactive pollutions; it is one
Vestnik IG Komi SC UB RAS, February, 2015, № 2
of the most common zeolites in the world with industrial accumulations with contents more than 60 %. Clinoptilolite (or clinoptilolite tuff) is characterized by a high selectivity to radionuclides especially to cesium and strontium; it also possesses such service features as chemical, thermal and radiation resistances jointly with high mechanical strength.
The alternative to clinoptilolite-based sorbents can be analcime-bearing rocks (Fig. 1 A), which are very common in the Komi Republic. It should be noted that analcime is related to narrow-porous zeolites with pore size 0.26 and 0.16^0.42 nm. The porosity of clinoptilolite, which is a medium-porous zeolite, is determined by two types of channels with pore sizes 0.40^0.55 and 0.44^0.72 nm. Though they are related to poor raw by zeolite contents (from 1 to 30 %), but the association of zeolites with clay minerals with content from 50 to 70 % and with an adsorption activity allow considering these rocks as a sorption raw with combined composition. Therefore it is very important to study sorption and desorption of radioactive elements by the zeolite rocks, which are originally from the Komi Republic, Russia.
The mineral composition of anal-cime-bearing rocks was studied, and their physical and chemical properties as well as the methods of modification were revealed [1]. The results of radio-nuclide sorption tests revealed that anal-cime-bearing rocks have high adsorption features to all the studied radionu-clides: the removal efficiency of thorium was 100 %, radium — from 99.2 to 99.8 %, uranium — from 98.5 to 99.7 %. Analcime effectively removes only thorium (the removal efficiency was 98 %), while radium and uranium removal efficiency were much less — 64.2 and 55.4 % respectively.
The experiments on the desorption of radionuclides showed that thorium was retained most strongly by analcime-
bearing rocks, insignificant extraction of thorium to solution by distilled water (1.0—2.7 %) and ammonium acetate processing (0—0.7 %) were observed. Only in acid medium a significant thorium extraction was determined from 38.0 to 68.0 %. Analcime retains thorium less strongly during distilled water and ammonium acetate processing: 6.1 and 20.4 % of radionuclide was extracted respectively. Acid processing led to the desorption of 48.3 % of radionuclide.
Among the studied radioactive elements radium is mostly tending to ion exchange; that results in its significant extraction to solutions from analcime-bearing rocks (41.4—49.2 %) and from analcime (more than 60 %) by processing. Water processing of analcime-bear-ing rocks and analcime results in desorption of less 1.0 and 3.6 % of radium, respectively, however acid processing also contributes to the radionuclide extraction, though it is less than that in the case of ammonium acetate.
Analcime retains uranium rather strongly: 2.3 % were desorbed by distilled water processing and 12.1% — by ammonium acetate and acid processing. Analcime-bearing rocks also retain ura-
nium strongly during the distilled water processing: only 0.1—0.3 % of radio-nuclide were extracted; however, 17.6— 25.4 and 22.4—34.7 % of radionuclide were extracted to solution by ammonium acetate and acid processing, respectively [1, 2].
Red mud (Fig. 1 B) — wastes of alumina production, which are characterized by high contents of fine Fe, Al and Ti oxyhydrates. The mineral composition of red mud from the Ural Aluminum Plant (Vezhayu-Vorykvinskoe deposit, Middle Timan and Northern Ural bauxite mines) and technological features were discussed, and the variation of aluminium minerals, increased content of carbonate rocks and alumosilicates were determined [3]. Iron-containing miner-
als are well separated by magnetic and gravitation methods. It was determined that hematite was concentrated in smaller fractions. The specific surface area of red mud is 23—25 m2/g, density — 3.3— 3.4 g/cm3. It was found that the maximum sorption capacity of red mud on strontium is 420±24 mg-eq/100 g [4, 5]. Due to these physical and chemical features red mud should be an effective sorption material to be used in the technologies of radioactive waste immobilization. The use of various flocculants and coagulants considerably increases sorption characteristic of particle surface.
Thus, the results of radionuclides sorption showed that analcime-bearing rocks demonstrated high removal efficiency of thorium, uranium and radium (with 100 % thorium absorption rate), while analcime was efficient in removing of thorium only; radium and uranium removal efficiency was 64.2 and 55.4 %, respectively. According to the results of radionuclides extraction, thorium was retained most strongly, its insignificant extraction to solution by distilled water and ammonium acetate processing was observed. Analcime retained uranium quite strongly, while analcime-bear-ing rocks retained it strongly only during distilled water processing. The high radium extraction (about 50 % from anal-cime-bearing rocks and more than 60 % from analcime) was probably caused by its inclination to ion exchange. Due to these physical and chemical features red mud should be an effective sorption material to be used in technologies of radioactive waste immobilization.
Another problem is the disposal (immobilization) of radioactive wastes. A variety of technologies are being developed to solve it. Many works have been devoted to the placement of radioactive wastes in the geological formations taking into account that the potential danger exists for hundreds thousand years. The demand of reliable protective technologies against pollution of environment by natural and industrial radioactive wastes remains a vital problem.
Methods of transition of radionu-clides to lower degrees of oxidation under reducing conditions in bentonite porous waters are developed [5]. Such ra-dionuclide compositions are low-soluble, have limited mobility and can be used for migration barriers.
The latest works in the area of natural materials application (arenaceous-argillacious rocks) showed the influence of iron films, formed on the quartz
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ВестНик ИГ Коми НЦ УрО РАН, февраль, 2015 г., № 2
Fig. 2. Iron films formed on the quartz grains
grains (and other minerals), on sorption process as antimigration barriers [7, 8]. The images of surface of fine particles of quartz sands from weathering crusts of Sub-Polar Urals (Fig. 2), made by scanning electron microscope (JSM 6400), revealed that quartz grains have strongly pitted surface covered by a film of particles, which size are several orders less, but mineralogically have the same composition. The chemical analysis showed the presence of large amounts of iron oxides.
Vitrification is a mature technology and has been used for high level radioactive waste immobilization for more than 40 years in France, Germany and Belgium, Russia, UK, India, Japan, and the United States; it includes melting of wastes with addition of glass bits of different compositon (e. g. wastes at macro- and microlevels). The advantages of vitrification are connected with pos-
sibility of application of this process for a large number of radioactive elements, high resistance to corrosion and relatively small volume of final product [9]. Nevertheless, active research and development of new methods of immobilization, including glasses, ceramics and studies of their properties, are being performed.
The work was done under financial support of UB RAS programs (project 1511-5-33).
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Reviewer PhD I. I. Shuktomova
