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Tomsk Polytechnic University, Russia
Nina Osipova, associate professor, candidate of chemistry, associate professor
Marianna Chubik, associate professor, candidate of medicine, associate professor
Anna Moskalenko, student
Томский политехнический университет
Radioactive wastes are formed during the nuclear fuel cycle and the use of nuclear materials (use of radioactive isotopes in medicine, research and industry) as well. The radioactive and operating hazards (expected from radioactive wastes) vary from extremely low rate in case of short-lived low-active wastes to very high value for highly radioactive ones. As a result of nuclear power production about 200000 m3 of low-active and intermediate radioactive wastes and 10000 m3 of highly radioactive ones are formed every year all over the world . The waste production is increasing due to the introduction into service of new nuclear reactors, the dismantling of nuclear facilities and the increasing of radioactive isotopes using. Highly radioactive wastes contain about 99 percent of the radioactive isotopes and thus bring a significant radioactive hazard. Effective technologies are needed for the radioactive wastes disposal from the environment. One of the important problems is development of materials that can efficiently sorb radionuclides, primarily from water bodies .
Specially synthesized, high-priced polymers with specific functional groups are used usually for the selective retention of specified radionuclides . However the question of new sorbent materials using is open now because a multipurpose and cheap sorbent that can extract radionuclides from wastewater selectively, efficiently and in large volumes has not been developed yet .
The aim of research is to study the characteristics of the uranium sorption from water solutions using metal nanoparticles and to evaluate the possibility for applying of fungi mycelium that modified by these nanoparticles for potential using as a components of a new composite, effective and safe sorbent. The prerequisites for metal nanoparticles using are hypotheses that state - the metal nanoparticles can be used as matrices for immobilization of plutonium, technetium, uranium and transuranic elements due to the ability to absorb radioactive ions  while the deformation process of the nanomaterial is. As a result, the absorbed radionuclides are permanently enclosed in the structure of the sorbent .
For the research the titanium dioxide nanotubes (TiO2) were used that produced by low-temperature sintering of electroexplosive nanopowders. Nanotubes were visualized using scanning electronic microscope Hitachi S-3400N with the additional attachment for microanalysis which was used for fluorescent-spectral analysis of nanomaterials. The length of the nanotubes varied from 300 to 600 nm, the diameter was about 8nm, the thickness of the walls was about 2.5 nm.
The research of the uranium sorption process was carried out under static conditions at the indoor temperature. Uranium was sorbed from model uranyl nitrate solution with an initial concentration of (UO2) 2 + equal to 1.3 mg/l. Sorption in the static conditions was carried out at the ratio of solid and liquid phases of 1:1000. The volume of the test solution was 10 ml, and the mass of the dry sorbent sample was from 1 to 10 mg. The time of the sorbent contacting with the solution lasted from 1 to 14 days. At the end of the sorption solutions were separated from the solid phase by centrifugation or filtration.
Sorption ability of titanium dioxide nanotubes was evaluated by indicators: the relative adsorption (S%) and distribution coefficient (Kd).
The values of equilibrium coefficients of radionuclides distribution between the solid and liquid phases, calculated for achieved maximum of sorption degree ranged from 1400 ± 120 to 8302 ± 770 ml/g. Peak values of the relative adsorptiondegree and the distribution coefficient were achieved with the initial concentration of 0.4 mg / l uranyl nitrate in the model solution. The minimum values of the sorption degree and distribution coefficient were marked with the initial concentration of uranyl ion 1.2 mg/l.
According to the statistical exchange capacity and the concentration of uranium in the solution the sorption isotherm was graphed, which is identical to the sorption isotherms of known and approved sorbents that is indicator of the normal process on the specified level of sorption (Fig. 1).
Fig. 1. Sorption isotherm
Parameter estimation for the uranium extraction from the model solution with a starting concentration of uranyl ion 1.3 mg/l displayed the decrease of the Kd values from 9818 ± 810 ml/g to 756 ± 79 ml/g when the mass of dry sorbent, added into the sample solution, was changing from 10 mg to 1 mg. At the same time the indicators of relative adsorption degree were dropped from 91.5 % to 43.08 %.
During the analysis of the solid phase the deformation of the nanotubes was found while the uranium absorption was detected. They were fragmented and unfold. Also we made fluorescent-spectral analysis of the used sorbent, and we proved that uranium was detected in the sorbent.
Another component of the composite sorbent can be mold fungi as the main characteristics of metal nanoparticles deposited on the growing mycelium of fungi are not differed from the nanoparticles properties suspended in the solution. During the research we used pure culture of mold Penicillium glaucum and Aspergillus niger, cultivated on Sabouraud medium. Fragments of the mold mycelium was transferred to a colloidal solution containing nanoparticles of titanium dioxide. At the same time there was no any preliminary modification of the particles or mold for the adsorption of nanoparticles on the surface of the growing mycelium.
Due to visual inspection using a light microscope (40X increase) it was noted that the deposition of nanoparticles on the surface of the mycelium does not lead to deterioration of the mold growth or a change in its cultural properties. It proves that the formation of the hybrid matrix is not attended by the adhesion of the nanoparticles. The resulting hybrid material is a tubular mold mycelium, wrapped in several layers of nanoparticles.
Owing to the fact that the metal nanoparticles are separated, fungal matrixes preserve all the properties of the initial nanoparticles. The absence of aggregation contributes to the significant increasing of the modified mycelium activity in comparison with the nanoparticles suspended in the solution. Apart from, an important advantage of mold matrices using is the fact that the hybrid mycelium is separated easily from the reaction mixture.
Thus, we would like to note the high sorption characteristics of titanium oxide nanostructures found during of these experiments. Nanotubes of titanium oxides can retain the trapped radioactive substances permanently. Additionally, the synthesis of the chemically pure titanium oxide nanotubes is quite simple and cheap. The efficiency of the nanotubes increases using effect of mold fungi. The resulting hybrid could serve as a promising material for a new composite sorbent developed for environment treatment from natural and man-made radioactive nuclides.