INVESTIGATION OF RADIATION RESISTANCE OF ADSORBENTS USING THE 90 Sr – SOURCE

Purifying aqueous solutions from radioactive contamination is an extremely relevant scientific topic today. Many organic and inorganic adsorbents can be recommended for the adsorption of heavy metal ions and radionuclides from aqueous solutions, or as carriers for storage and disposal of radioactive waste. Since radionuclides are sources of ionizing radiation, the radiation resistance of the adsorbent is an important characteristic. These studies aim to investigate the titanium silicate behavior and its adsorption properties' changes or their invariability in the field of intense β -radiation. Experimental techniques describe the synthesis of titanium silicate adsorbent by sol-gel method and the study of its adsorption capacity toward Ba 2+ cations. The adsorption of Ba 2+ cations was investigated under batch conditions with neutral pH of the solution. Initial and residual concentrations of Ba 2+ cations were controlled by direct complexometric titration with Na-EDTA with Eriochrom Black T as an indicator. The study of the radiation resistance of the adsorbent to high-energy β -radiation was performed using a 90 Sr-90 Y β - - source “Sirius” installed in the Microtron Laboratory of the

Purifying aqueous solutions from radioactive contamination is an extremely relevant scientific topic today.Many organic and inorganic adsorbents can be recommended for the adsorption of heavy metal ions and radionuclides from aqueous solutions, or as carriers for storage and disposal of radioactive waste.Among them are ion exchange resins, adsorbents based on titanium dioxide or titanium silicate, zeolites, metal-organic frameworks, etc.However, only a small part of scientific papers is devoted to the investigation of the radiation resistance of adsorbents.Since radionuclides are sources of ionizing radiation, the radiation resistance of the adsorbent is an important characteristic [1][2][3][4].
According to publications [5,6], the radiation resistance of adsorbents is manifested in the invariability of their adsorption properties.The adsorbent is radiation-resistant to a certain radiation dose if the ionizing radiation of a certain type and a given dose does not decrease its adsorption capacity.In our work, the adsorption capacity of titanium silicate toward barium cations (which was measured before and after irradiation) was also chosen as an indicator of TiSi radiation resistance.
These studies aim to investigate the adsorbent's behavior and its adsorption properties' changes or their invariability in the field of intense β-radiation.2+ cations.Titanium silicate (TiSi) was synthesized in ISPE, NAS of Ukraine, according to the technique described in [1,7].For the synthesis of titanium silicates, titanyl sulfate TiOSO 4 solution was used.TiSi adsorbents were obtained by mixing two initial solutions: number one was prepared by mixing pure (3.4 M) TiOSO 4 and ligand, a mixture with D-sorbitol and L-lactic acid; and the second solution was obtained by mixing a technical solution of liquid glass (3.81 M Si) with 5 M NaOH.This process was performed at room temperature using a magnetic agitator.The sol-gel synthesis of this adsorbent can be described as follows:

Synthesis of adsorbent and study of its adsorption capacity toward Ba
This solution was taken from the technological sulfate line of production of the rutile white titanium pigment.The heating temperature was not higher than 150 °C, heating was carried out for 48 hours.
The adsorption of Ba 2+ cations was investigated under batch conditions with neutral pH of the solution.Barium was chosen as the object of investigation because it is an alkali earth element.It is a heavy metal cation with chemical properties close to radium.In addition, barium isotopes, for example 141 Ba, can formed as fission radionuclides, like 90 Sr or 137 Cs.The mass of the adsorbent was 0.05 g; the solution volume (V) was 5 mL in all adsorption experiments.Initial and residual concentrations of Ba 2+ cations were controlled by direct complexometric titration with Na-EDTA with Eriochrom Black T as an indicator (in addition to Raman analysis of the TiSi surface).Kinetic studies of Ba 2+ adsorption by irradiated and nonirradiated TiSi were performed using the 0.1 M BaCl 2 aqueous solution with the duration of adsorption 5, 10, 15, …, and 60 minutes.The mass of adsorbent and the solution volume were the same, as was mentioned before.
The adsorption values were calculated by equations (1): where q e -the amount of adsorbate uptake at equilibrium, mg‧g -1 ; C o and С е -adsorbate initial and residual (equilibrium) concentration, mg‧L -1 ; V -the volume of the solution, L; m -is the mass of the adsorbent (g).

Study on radiation resistance of TiSi adsorbent.
The study of the radiation resistance of the adsorbent in relation to high-energy β - radiation was performed using 90 Sr-90 Y β --source "Sirius" installed at the Microtron Laboratory of the Uzhhorod National University.This source is a radionuclide of 90 Sr in solid form on a ceramic carrier. 90Sr is in secular equilibrium with its daughter radionuclide 90 Y.The decay chain of these radionuclides is shown in Diagram 1.

Diagram 1. Radioactive decay of 90 Sr. Adapted from resource [8]
The radioactive source of 90 Sr -"Sirius" was manufactured in 1980.It consists of 16 cassettes of 90 Sr, the initial activity of which on the surface at the time of manufacture was 5.55•10 9 Bq.In other words, the initial activity of each of these sources was close to 1 Ci.
After about 1.5 half-lives (present time), the electron flux at a distance of 20 cm from the source core is 1.10 8 electrons/cm 2 per second.The dose was calculated according to the data (initial activity) indicated in the data sheet and was also controlled by a clinical dosimeter, which is usually used to control the radiation dose at accelerators.
These sours are called 90 Sr-90 Y because (a) they were made by isolating 90 Sr from a mixture of fission radionuclides by the oxalate technique, i.e. precipitation with yttrium oxalate; (2) 90 Y is a daughter radionuclide of strontium (Diagram 1). 90Y is always present in the vicinity of 90 Sr and if there is a lot of 90 Sr, then 90 Y radiation cannot be neglected.Since the half-life of 90 Y is much shorter than that of 90 Sr, they are in a state of secular equilibrium, and the 90 Y amount can be calculated using the formula (2) below: where N Y and N Sr are the number of 90 Y and 90 Sr nuclei, respectively; λ Sr and λ Y are decay constants of 90 Sr and 90 Y, respectively.
90 Y is always the same amount, while 90 Sr is constantly decreasing.The use of such a source for the study of radiation resistance was due to two profits: (1) in this way, the situation of the influence of 90 Sr on the adsorbent was simulated since some authors recommend this adsorbent specifically for the adsorption of 90 Sr from aqueous solutions [4,7].(2) The use of source "Sirius" radiation compared with accelerator radiation saves a lot of electricity.
Thus, in our experiment, the distance from the source to the adsorbent samples was 20 cm.The flux of electrons at this distance was 10 8 electrons/cm 2 ‧per second.The maximum energy of beta particles was 0.456 MeV for 90 Sr and 2.28 MeV for 90 Y (see in Fig. 1 a).The maximum duration of exposure was 21 days.The radioactive source "Sirius" creates a dose of 4 Roentgen per minute, which is equal to 5760 Roentgen per day.Thus, the adsorbent samples were irradiated with doses of 5.76‧10 3 ; 11.52‧10 3 , 12.10‧10 4 , and 13.That is, the adsorbent samples were exposed to high-energy electrons with a maximum energy of up to 2.28 MeV (Fig. 1 a, as well as bremsstrahlung gamma rays, the energy distribution of which is given in Fig. 1 b and in Table 1.After irradiation, TiSi was used for a study of the adsorption of barium ions from an aqueous solution of BaCl 2 in a neutral medium.Part of the irradiated adsorbent was left for Raman spectroscopy.
Raman spectroscopy.Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified, therefore this type of spectroscopy was chosen for analysis of TiSi.The technical characteristics of the Raman spectrometer XploRA PLUS are given in Table 2.

Adsorption of Ba 2+ cations by irradiated and non-irradiated titanium silicate.
According to publications [9][10][11], beta radiation is incapable of generating radiation defects of the classical type due to the small mass of beta particles.For example, to shift the atom of the cell into the interstitial space, or to form a Frenkel pair.However, beta radiation with mega-electron-volt Energy is capable of breaking the chemical bonds and changing the oxidation state of the elements.According to [12], this can change the properties of the adsorbent, and even improve them in some cases.For example, the rupture of certain found bonds can lead to the emergence of new adsorption centers.Therefore, the kinetic studies of Ba 2+ cations of irradiated samples of TiSi and nonirradiated samples were performed simultaneously in parallel experiments.
The results of the adsorption of barium cations by irradiated titanium silicate and an unirradiated sample are shown in Fig. 2. The figure shows that the value of the maximum adsorption is 140.5±9.2mg/g (6.55 %) confidence level of 95 % [1,13].The values of adsorption of barium ions by irradiated and non-irradiated titanium silicate coincide.This indicates that the adsorption properties of this adsorbent do not change under the influence of such a radiation dose.There is also no reason to believe that the adsorption mechanism changes.Earlier we showed the adsorption of barium cations to be better described by the theory of Freundlich and Dubinin-Radushkevich, compared to the Langmuir theory [1].
This increases the probability that the surface of titanium silicate cannot be considered homogeneous, i.e. there are different surface groups on the surface of titanium silicate, e.g.≡Ti-OH and ≡Si-OH, which can be adsorption centers.It is necessary, however, to note that not every =Ti-OH-surface group is an adsorption center [14].
Most likely, the adsorption of Ba 2+ at the pedestrian stage occurs due to the electrostatic interaction of Ba 2+ cations with the entire surface of titanium silicate.The Dubinin-Radushkevich adsorption theory, which is based on the potential Polanyi theory, is the theory of non-localized adsorption.As we can see, the experimental results of the adsorption of barium ions by this adsorbent are well-described by D-R theory.The oscillation of 380 cm -1 refers to the oscillations of the Si-OH group.The oscillations, the maximum of which lies in the region of 430-500 cm -1 , refer to the oscillations of SiO 2 (Si-O-Si).There are no maxima at 520-620 cm -1 that could be attributed to oscillations of radical Si-O* groups on the spectrum.The oscillation of 700-800 cm -1 also refers to the oscillations of SiO 2 surrounded by oxygen atoms [15].Oscillations of 144 and 195 cm -1 refer to TiO 2 oscillations, according to the literature [16,17].
The peak at 300, or rather 274 cm -1 , is absent in the Raman spectra of pure silicates and TiO 2 but is manifested in composite materials, for example, TiO 2 nanotubes modified with silicon dioxide [18].This maximum can belong to Ti-O-Si bonds.However, some authors attribute the maxima of 270 and 304 cm -1 to the oscillations of the TiO 6 groups [19].A small maximum at the 620-650 cm -1 position may belong to the oscillations of adsorbed barium.At the same time, this maximum is diagnosed only on the spectrum of titanium silicate (irradiated) after the adsorption of barium cations.According to the authors [20], the Raman spectra of bariumcontaining glasses and alloys always contain a maximum of about 600-720 cm -1 at low, medium, and high barium content in the structure [21,22].The oscillation of 810-830 cm -1 may refer to the vibration of the Si in the oxygen cage, according to the publication [15].The small peaks in regions 870-910 cm -1 may correspond to the groups Si 2 O 7 6- [15].The Raman spectra of irradiated and nonirradiated titanium silicate coincide, while they do not identify free radicals, or ionic formations,

CONCLUSIONS
The studied sample of titanium silicate is radiation-resistant.It can withstand a radiation dose of 1310 Gy without changing its adsorption properties.
The Raman scattering spectra of irradiated and unirradiated titanium silicate coincide with high accuracy and do not identify maxima that would belong to ion formations or free radicals.
Titanium silicate can be used for the adsorption of strontium radionuclides, it can be a carrier for the disposal of radioactive waste. 1‧10

Fig. 1 .
Fig. 1.Total irradiation from the 90 Sr-90 Y beta source: (a) Distribution of β --particles of the source "Sirius" by the energy; (b) Modelling spectrum of bremsstrahlung gamma-ray, which were generated by beta particles of the source "Sirius".This spectrum was constructed using the program NPMA Bremmstraglung

Table 1 .
Qualitative composition of radioactive radiation, which was used to study the radiation resistance of the adsorbent[8]

Table 2 .
Specifications of Raman spectrometerXploRA PLUS