The dissertation is submitted to the degree of Doctor of Philosophy in Technical Sciences by spatiality 21.06.01 – Ecological Safety. – State Ecological Academy of Postgraduate Education and Management, Kyiv, 2021.
The dissertation is devoted to the study of the ecological safety of processes of removing radionuclides from multicomponent solutions. It is established that in the process of the destruction of organic substances – components of liquid radioactive waste (LRW) by ozone, a significant drop in pH value takes place as a result of a decrease in the destruction efficiency, and the process of the destruction of organic components occurs in two stages: fast and slow. It is determined that the highest efficiency of the destruction of organic components of LRW is achieved with constant alkalization of the solution and keeping the pH values at 11-12 units. It is shown that in the process of ozonation, precipitates of metal hydroxides and oxides are formed, on the surface of which radionuclides are fixed due to co-precipitation and adsorption. Thus, in the result of ozonation, the concentration of Mn2+ decreases by 94.3%, Co2+ – by 6%, the activity of 137Cs reduces by 26%, 90Sr – by 15.7%. It is found that in all parameters (degree of sorption, rate of establishment of sorption equilibrium, the prevalence of strongly fixed forms of sorption – acid-soluble and residual) Na-modified sorbents are the most effective among the studied sorbents: relative to 90Sr, Co2+, Mn2+ – Na-modified bentonite of the Cherkas’ke deposit PBA-20 (maximum sorption degree of 90Sr – 85%, Co2+ – 99%, Mn2+ – 99%); relative to 137Cs – an alkali-modified zeolite of the Sokyrnyts’ke deposit (the degree of 137Cs sorption – 99%). I.e. industrial modification of bentonites with soda and laboratory modification of zeolite with alkali (NaOH), significantly increases the degree of sorption of the studied radionuclides in comparison with their natural Ca-forms. It is determined that the sorption equilibrium upon adsorption of Co, Mn, Cs from the model LRW is established within from one to seven days, Sr – from two to fourteen days, the degree of sorption is practically independent of the initial pH of the solution, and the sorbents have a high buffering capacity. It is determined that it is more appropriate to use an alkali-modified zeolite as a sorbent under dynamic conditions. It is found that zeolites are characterized by high selectivity for 137Cs concerning its chemical analogue – 40К. It is shown that preliminary ozonation of the solution for 2 hours effectively destroys the organic components of LRW in such a way that the degree of sorption of the studied radionuclides is only 2 - 3% lower from the solution after ozonation in comparison with the control solution, which did not contain organic components. The adequacy parameters (MAPE, MPE, R2, R2adj, χ2, F, BIC, AIC) of mathematical models are calculated, and it is determined that the sorption model of the kinetics of complex chemical processes of all studied radionuclides (Co, Mn, Cs, Sr) with the most effective sorbents (PBA-20 bentonite and alkali-modified zeolite) is the most adequate and a universal model among those presented. Recommendations on the development of a scheme for LRW treatment by extracting radionuclides are presented, a conceptual block diagram of LRW treatment is given.
Key words: ecological safety of processes, liquid radioactive waste, technogenic radionuclides, sorption of radionuclides, physicochemical forms of radionuclides, ozonation, destruction of complexing agents by ozone.
