Recovery of Uranium by Se-Derivatives of Amidoximes and Composites Based on Them

An Se-derivative of amidoxime was synthesized for the first time as a result of the reaction of oxidative polycondensation of N’-hydroxy-1,2,5-oxadiazole-3-carboximidamide with SeO2: its elementary units are linked to each other due to the formation of strong diselenide bridges. The element composition of the material was established, and the structure of the elementary unit was suggested. The sorption-selective properties were evaluated, and it was found that the adsorbent can be used for the selective recovery of U (VI) from liquid media with a pH of 6–9. The effect of some anions and cations on the efficiency of recovery of U (VI) was estimated. Composite materials were fabricated, in which silica gel with a content of 35, 50, and 65 wt.% was used as a matrix to be applied in sorption columns. The maximum values of adsorption of U (VI) calculated using the Langmuir equation were 620–760 mg g−1 and 370 mg g−1 for the composite and non-composite adsorbents, respectively. The increase in the kinetic parameters of adsorption in comparison with those of the non-porous material was revealed, along with the increase in the specific surface area of the composite adsorbents. In particular, the maximum sorption capacity and the rate of absorption of uranium from the solution increased two-fold.


Introduction
The growing demand for uranium ore due to its widespread use in the nuclear industry is accompanied by the accumulation of solid and liquid radioactive waste containing uranium and its daughter decay products. The process of continuous release of uranium and its decay products to the biosphere occurs from rocks and parent rocks due to natural factors. This process is intensified during the mining and subsequent processing of uranium ore at nuclear industry facilities, thus creating risks of contamination of natural waters used in agriculture, as well as sources of drinking water [1]. Considering the highly toxic and radiotoxic hazards of uranium isotopes, characterized by a wide range of water-soluble forms in the biosphere, there is an urgent task to find methods for its selective recovery and concentration from natural and technological waters, including treating it as a valuable raw material.
A number of methods were developed over the past few decades for the recovery of uranium from aqueous solutions: extraction [2], membrane filtration [3], photocatalysis [4], chemical precipitation [5], electrocoagulation [6], adsorption [7], and others. However, most traditional methods have their disadvantages. In particular, despite the high efficiency of uranium extraction by membrane filtration, the main disadvantages of this method include the high energy costs required to restore the membranes and maintain high pressure [4]. The chemical deposition method is quite simple and economical, but it is often ineffective for reducing the concentration of uranium below the legally permissible levels and tends to form secondary wastes [5]. In addition, under conditions of working with high-salinity carbon tubes [30], chitosan [31], and cellulose [32]. However, despite high selectivity towards uranium and high sorption capacity, their use in real conditions is limited by low hydro-mechanical strength [30] and solubility in alkaline media [31]. The use of polyurethane foam to increase the specific surface area leads to partial or complete blockage of the pores in the process of chemical modification [28,29]. Besides, the multi-step synthetic modification leads to the decrease in the degree of inoculation of amidoxime and spatial separation of chelate groups in the sorbent, which adversely affects the ability to bind uranyl ions and decreases the affinity. The materials comprising ordered three-dimensional coordination polymers consisting of metal cations (Al, Zn) and amidoxime radicals [33] are promising-they exhibit high sorption capacity.
To sum up, sorption materials based on amidoxime compounds are actively studied from the practical point of view, where they have proven to be highly effective for recovering uranyl ions from anthropogenic and natural liquid media with high contents of carbonates and sulfates, as well as seawater.
Despite significant advances in the production of polymer organic materials based on amidoximes and oximes, methods of production and properties of organic sorbents with an inorganic component of the Se type were not earlier published. In view of this, the objective of the present work was to synthesize new derivatives of 4-aminofufazane-3carboxamidoxime, both in the pure form and as mechanically strong composites based on silica gel, through the introduction of heteroelements of high coordination capacity, which would facilitate the formation of polymer materials characterized with developed structure and high content of ion-exchange groups (amino and hydroxyl groups) and distinguished by an increased selectivity to uranium.

Materials and Methods
Selenium dioxide, acetic acid, ethanol, and methylene chloride of the especially pure grade were used to prepare the sorption material, as well as silica gel with a particle size of 35-70 mesh. In order to prepare working solutions simulators, metal salts of the chemically pure grade were used without additional purification-they were purchased from Nevareaktiv LLC (Saint-Petersburg, Russia). The radionuclide U-235 in the form of UO 2 (NO 3 ) 2 of the especially pure grade was used as a sorption element.

Preparation of Materials
The 4-aminofurazane-3-carboxamidoxime (compound 1) was preliminarily synthesized by the following method [34]. An amount of 7.6 g of sodium nitrite (110 mmol) was gradually added to 140 mL of a solution of malononitrile (100 mmol) under intensive stirring at 20 • C, and then the mixture was cooled down to 5 • C. Thereafter, for 15-20 min, 3.8 mL of concentrated H 3 PO 4 solution was added to it drop by drop under intensive stirring; the mixture was again stirred for 1.5 h and cooled down to 15-20 • C. Finally, 21 g of hydroxylamine hydrochloride (0.3 mol) was added to the mixture, and the stirring continued until the latter was completely dissolved. Thereafter, the mixture was alkalized to pH 10 using a solution of KOH (0.45 mol) and boiled for 2 h. The resulting solution was slowly cooled down to room temperature and acidified to pH 7 using a 30% H 3 PO 4 solution. The resulting mixture was held at 0-5 • C for 12 h for crystallization, and the final compound was filtered and washing with cold water. The yield of compound 1 was 75-85%. Prior to use, compound 1 was recrystallized from an aqueous solution in the presence of activated carbon.

2.2.
Synthesis of the Se-Derivative N -hydroxy-1,2,5-oxadiazole-3-carboximidamide A mixture of 1.43 g (10 mmol) 4-aminofurazane-3-carboxamidoxime 1 and Se(IV) oxide with a weight of 1.11 g (10 mmol) was boiled in acetic acid (30 mL) with a reverse refrigerator for 80-100 min under intensive stirring, a dark purple precipitate was formed as a result. At the end of the reaction, the mixture was cooled down to room temperature, and then the precipitate was separated from the mother liquor by a blue ribbon filter (JSC "ECOS-1", Moscow, Russia). The precipitate was then washed sequentially with cold distilled water (three times), ethanol (once), and methylene chloride (once) to remove the unreacted components of the initial mixture. The resulting product was dried to a constant weight in a vacuum desiccator at 5-10 mm Hg above P 2 O 5 at a temperature of 20-25 • C for 24 h. The final material, denominated as Se-init had a yield of 80-90% and comprised dark purple granules of an irregular shape with a grain size of 0.05-0.2 mm [35].

Synthesis of Composite Sorbents
In order to prepare composite samples, silica gel was added to a mixture of 1.43 g (10 mmol) of 4-aminofurazane-3-carboxamidoxime, 1.11 g (10 mmol) of selenium dioxide, and 30 mL of acetic acid during boiling. Boiling of the mixture lasted 80-100 min under intensive stirring. After completely falling out of the dark purple precipitate, the boiling was stopped, and the mixture was gradually cooled to room temperature and filtered. The resulting material was washed with cold water (three times), ethanol (twice), and methylene chloride to remove unreacted organic compounds. The final product was dried to a constant weight in a vacuum oven at 50 • C. The prepared materials comprised dark purple powders with a grain size of 0.05-0.2 mm. Three types of composite materials were fabricated; they were denominated as Se-35, Se-50, and Se-65, with a silica gel content of 35, 50, and 65 wt.%, respectively.

Study of Sorption Characteristics under Static Conditions
In the sorption experiments, initially, the U(VI) salt (nitrate) was used. However, to simplify the description, hereinafter, U(VI) and its possible forms of existence will be marked as just uranium.
The sorption properties of the fabricated materials were investigated using solutions of pH of 2-10 containing uranium with the ratio V/m = 1000 mL g −1 (V is the volume of the model solution, m is the weight of the sorbent). The phase contact time was 48 h under continuous stirring on an orbital shaker with an amplitude of 10 mm and a rotation speed of 150 rpm. Prior to the start of the uranium recovery experiment, the sorbent samples were held for 24 h in the model solutions that did not contain uranium.
At the end of the experiment, the model solution was separated from the sorbent by a blue ribbon filter, and the residual uranium + content was analyzed by the spectrophotometric method using Arsenazo III at a wavelength of 656 nm [36]. The results obtained were used to further calculate the extraction efficiency (S%) and the uranium distribution coefficients (K d , mL g −1 ): The extraction efficiency was calculated according to Equation (1): The value of the uranium distribution coefficient (mL g −1 ) was calculated using Equation (2): where C i is the initial concentration of uranium in the solution (mg L −1 ), C e is the equilibrium residual concentration of uranium in the solution (mg L −1 ), V is the volume of the solution (mL), and m is the weight of the sorbent sample (g). Additionally, at the end of the experiment, the equilibrium pH of the solution was measured, followed by the determination of the zero charge point (pHpzc) by a graphical method. The effect of the presence of cations and anions in the solution on the static exchange capacity was evaluated in the presence of solutions of metal chlorides (K + , Na + , Mg 2+ , Ca 2+ , Co 2+ , and Ni 2+ ) or sodium salts ( used: 0.001; 0.001; 0.01; 0.1 mol L −1 . The static exchange capacity (SEC, mg/g) was calculated according to the following formula: where V is the volume of the solution (L). The nature of the adsorption process was estimated from the uranium adsorption isotherms, which were obtained by contacting a series of samples of a given weight with the model solutions of pH 6 and 8 containing a certain amount of uranium + . The standard Langmuir Equation (4) was used to describe the sorption isotherm: where G max is the maximum sorption value (mg g −1 ), K l are the constants of the adsorption equilibrium characterizing the energy of the adsorbent-adsorbate bond (L g −1 ). The experimental data were approximated by means of the specified equations using the SciDAVis software (version 1.23, scidavis.sourceforge.net, accessed on 15 September 2021).
The kinetic characteristics of the sorption process were evaluated using a model solution with pH 6 containing 0.1 mol L −1 of NaNO 3 and 30 mol L −1 of uranium. The adsorbents were brought into contact with the model solution, V/m = 200 mL g −1 , and, after a certain period, aliquots of the solution were taken, and the residual content of uranyl ions in the solution was determined.
where A t is the amount of adsorbed uranium at time t (mmol g −1 ), A e is the equilibrium uranium adsorption (mmol g −1 ), t-time (min), and k 1 and k 2 are the pseudo-first-order and pseudo-second-order constants, respectively. The kinetic parameters of the adsorption process in solutions with pH 6 and 8 were comparable; therefore, this work presents the results obtained using solutions with pH 6.
The assessment of the hydro-mechanical strength (HMS) of the prepared sorbents was carried out as follows: a suspension of the sorbent with a weight of 1 g and grain size of 0.1-0.2 mm was mixed with 50 mL of distilled water for 24 h. Thereafter, the sorbent was separated from the solution, dried to a constant weight at 105 • C, sieved again, and weighed on an analytical balance with an accuracy of 0.001 g. The degree of destruction of the sorbent sample with a grain size of 0.1-0.2 mm was estimated by the ratio of the weight remaining after the mixing to the initial weight of the sample.
where m 0 is the initial weight of the sorbent with a grain size of 0.1-0.2 mm, m 1 is the weight of the sorbent that retained the grain size of 0.1-0.2 mm after mixing and sieving.

Equipment
The infrared spectra (IRS) of the samples were recorded using a Spectrum 1000 spectrometer (Perkin Elmer, Waltham, MA, USA) in KBr tablets. X-ray diffraction patterns were recorded using an Advance D8 (Bruker, Billerica, MA, USA) device, with CuK α -radiation in the angle range of 2 • < 2θ < 90 • in the point-by-point scanning mode. The maximum deviation of the position of the reflexes, as determined through NIST SRM 1976, was less than 0.01 • 2θ.
Nuclear magnetic resonance ( 13 C NMR) spectra of the polymers in the solid phase were recorded using a Bruker Advance AV-300 device (Billerica, MA, USA) with a proton resonance frequency of 300 MHz. The method of magic-angle spinning (MAS) was used to record the spectra. Tetramethylsilane was used as a standard for carbon nuclei, whereas the chemical shift (CS) zero was set in a separate experiment. The error in determining the CS did not exceed 1-2 ppm, depending on the resolution of the peak. The spectra were recorded at a temperature of 300 K.
The X-ray photoelectron spectroscopy (XPS) spectra were obtained using an ultrahigh vacuum photoelectron spectrometer manufactured by Specs (Berlin, Germany) with a hemispherical electrostatic energy analyzer PHOIBOS (Specs, Berlin, Germany). An X-ray tube with a magnesium anode (MgKa-1253.6 eV, Specs, Berlin, Germany) was used as the radiation source. The pressure in the chamber during the measurement was 5 × 10 −7 mbar. The energy scale was calibrated according to the carbon level C 1s, the energy of which was assumed to be 285.0 eV. The transmission energy of the analyzer was 50 eV; the step at scanning the survey spectra was 1.0 eV, while for the detailed spectra, it was equal to 0.1 eV. At the sample preparation stage, the investigated materials were pressed onto a 1 cm × 1 cm sticky conductive tape.
The high-resolution scanning electron microscopy study was performed using a HITACHI TM 3000 device (Hitachi, Tokyo, Japan) at accelerating voltages of 5-15 kV and a beam current of I ≈ 100 pA. The device was equipped with an accessory for energy dispersion analysis (EDA) by Bruker (Berlin, Germany).
The specific surface area and the pore size of the material were determined by the method of low-temperature nitrogen adsorption using an Autosorb IQ device by Quantachrome Instruments (Boynton Beach, FL, USA). The calculation was performed according to the BET method (the theory of Brunauer-Emmett-Teller).
The element content in the model solution was evaluated by the atomic absorption flame spectroscopy using a Thermo Solar AA M6 device (Thermo Electron Corporation, Waltham, MA, USA). The surface morphology was studied by scanning electron microscopy (SEM) using a Carl Zeiss CrossBeam 1540-XB instrument (ZEISS Microscopy, Munich, Germany).
The content of uranyl ions in the solution was determined by the spectrophotometry method using a Shimadzu UV-1800 spectrophotometer (Kyoto, Japan).

Physiochemical Properties
According to the objective of the present work, the materials based on amidoxime functional groups, in particular, the Se-derivative of N -hydroxy-1,2,5-oxadiazole-3-carboximidamide (Se-init), were studied as highly effective sorbents for uranium recovery. The choice of Se was determined by the mild selective oxidizing properties of SeO 2 towards N -hydroxy-1,2,5oxadiazole-3-carboximidamide. The method of preparation is based on the reaction of oxidative polycondensation of N -hydroxy-1,2,5-oxadiazole-3-carboximidamide with SeO 2 in the presence of polar organic solvents followed by the formation of a polymer structure, whose elementary units are linked to each other due to the formation of strong diselenide bridges. This original method offers a very simple implementation by a single-stage reaction, as well as the use of an organic radical with a large number of reactive groups (hydroxyl and oxime) that remain in the final product. A number of physical-chemical methods were used to determine the molecular structure of the resulting compound. In particular, Figure 1 shows the IR spectrum of Se-init. The X-ray diffraction analysis showed the presence of peaks corresponding to the initial N -hydroxy-1,2,5-oxadiazole-3-carboximidamide, which indicated the complete binding of the functional organic part with partial preservation of the original structure (27 Å- Figure 2, X-ray Pattern b). The syngony and the spatial group P 3(1)21 were established-a structure similar to gray selenium (β-Se) but different from it ( Figure 2, X-ray pattern b). In addition, the Se-init X-ray pattern shows signals corresponding to the Se phases that were not earlier found in the published data. The presence of the crystal structure can be indirectly corroborated by the images of scanning electron microscopy ( Figure 3). One can see that the morphology of the Se-init surface is represented by clearly expressed crystallites of irregular structures and sizes. The material was analyzed by the method of solid-state NMR on 13 C and 1 H nuclei. In the 13 C spectrum (Figure 4a), there is a clearly expressed signal at 138 ppm corresponding to carbon at the oxime group-C=NOH, as well as signals of 146 ppm corresponding to the -HN-C= and 154 ppm typical for =C-C= (Table 1). In the PMR spectrum (Figure 4b), there are signals of 1.5 ppm corresponding to the proton at aliphatic C-NH, and 7.1 ppm corresponding to H-N(C)-Se.  The additional PMR analysis in the liquid phase was performed: it showed the presence of an additional signal with an intensity of 3 at 2.73 ppm, which probably corresponded to three acetylene protons. Figure 5 shows the XPS spectra of the prepared material. In order to obtain a more accurate understanding of the structure of the compound, the material was pre-washed with distilled water and ethanol alcohol to remove the synthesis by-products.
The XPS spectrum shows the presence of a C 1s peak (Figure 5b (Figure 5e) of the binding energy of 164.0 eV is typical for tetravalent selenium, probably bound to the second selenium atom by a double bond (>Se=Se<). The spectrum also contains a signal at 169.3 eV corresponding to a selenium atom with the C-NH-Se surrounding. Based on the performed physical and chemical analysis, we suggested the mechanism of binding of compound 1 ( Figure 6) to selenium dioxide and suggested the molecular structure of the resulting material ( Figure 6). binding energies of 533.6 eV and 531.2 eV correspond to an oxygen atom with surroundings of NH2-C=N-O-N and N-O-H, respectively. The position of the Se 3p peak ( Figure  5e) of the binding energy of 164.0 eV is typical for tetravalent selenium, probably bound to the second selenium atom by a double bond (>Se=Se<). The spectrum also contains a signal at 169.3 eV corresponding to a selenium atom with the C-NH-Se surrounding. Based on the performed physical and chemical analysis, we suggested the mechanism of binding of compound 1 ( Figure 6) to selenium dioxide and suggested the molecular structure of the resulting material ( Figure 6). It was found that, as a result of the polycondensation reaction, the degree of oxidation of Se (IV) is preserved with the formation of a strong double bond >Se=Se< bound by functional amidoxime radicals.
During the synthesis, compounds 2 and 3 remained in the dissolved phase and were then completely removed by repeated washing with distilled water and organic solvents, as evidenced by the results of the analysis of the material (Table 1). Table 1 shows the data calculated for an elementary unit of Se-init ( Figure 6). The experimental percent values of the elements match the theoretically calculated ones with insignificant deviations, which is probably related to the presence of non-removable monomeric or oligomeric structures. Figure 7 shows the experimental results of the determination of the zero charge point (pH pzc ) using a NaNO 3 solution of a concentration of 0.01 mol L −1 . The results are shown in the form of a dependence of the equilibrium pH of solutions after contact with a sorbent on the initial pH of these solutions. The bend point on curve 3 corresponds to the zero charge point that is the value of the solution pH, at which the sorbent surface is uncharged. In our case, the pH pzc value is equal to 4.3. Below this value of solution pH, the sorbent surface is charged positively, which prevents uranium adsorption due to electrostatic repulsion. Above pH pzc , the sorbent surface is charged negatively, which positively affects the efficiency of recovery of uranium and its hydrated cationic forms. The results of the determination of pH pzc are in good agreement with the dependence of the efficiency of uranium recovery on the solution pH ( Figure 8). The solution pH correction was carried out using solutions of HNO 3 and NaOH. In acidic solutions, low recovery efficiency is related to positively charged sorbent surface as, probably, to the predominant reaction of protonation. The form of uranium presence in a solution depends significantly on the solution pH. In solutions with pH above 4, one observes the increase in the uranium recovery due to strong binding by the adsorbent negatively charged surface and predominant cationic hydrate forms of uranium. The dependence of the composition of the hydrated uranium cation with the pH increase (from 4 to 8) is expressed in the row UO 2 OH + → (UO 2 ) 2 (OH) 2 2+ → (UO 2 ) 3 (OH) 5+ [37]. The recovery efficiency maximum is attained at pH 8, with a subsequent decrease at the transition to the alkaline range. The decrease in the efficiency of uranium recovery in solutions with pH above 8 is probably related to the increased formation of the anionic form of uranium UO 2 (OH) 3− [37] and the uranium adsorption on the flask walls, which was found in the control experiment without the sorbent (curve 3, Figure 8). The high efficiency of the uranium recovery is explained by the presence of a large number of sorption sites forming at polymer formation as a result of polycondensation of cyclic molecules through the >Se=Se< bond. Table 2 shows the values of the sorption-selective properties of the sorbent, depending on the initial pH of the solution. According to the obtained data, these materials can be recommended for the recovery of uranium from neutral and slightly alkaline solutions. A change in the value of pH of the model solution as compared to the initial value was observed. This was probably the result of deprotonation of the =N-O-H group during the adsorption.  Figure 9 shows a diagram of the dependence of the SEC uranium values on the concentration of cations in the solution. It was found that double-charged cations reduce the SEC value to a greater extent than single-charged cations, which is explained by the electrostatic interaction, as well as due to the larger number of occupied adsorption sites, which is associated with an increase in the equivalence factor of multiply charged elements. The sequence of cations by the degree of increasing negative impact on the value of SEC is as follows: Na + ≤ K + < Mg 2+ ≤ Ni 2+ ≤ Ca 2+ ≤ Co 2+ . However, it is worth mentioning that despite the decrease in SEC values at increasing the concentration of competing cations, the efficiency of the uranium recovery exceeds 75%, regardless of the competing ions. The uranium extraction efficiency in the presence of other (competing) cations at pH 6 and 8 is generally comparable to each other. For this reason, this experiment was carried out only at pH6. The most negative effect on the uranium extraction efficiency is exerted by anions, the effect of which strongly depends on the solution pH.

Sorption Selective Properties
Uranium undergoes hydrolysis and has the affinity to complexation: this effect is clearly expressed in neutral and slightly alkaline media, which significantly complicates its recovery by sorption. The presence of various anions in the solutions, such as bicarbonates or sulfates, can significantly reduce the efficiency of uranium extraction. Experiments were carried out to estimate the negative effect of anions. Figure 10 shows the curves of the dependence of the SEC uranium values on the type and concentration of some anions that may be present in natural and technological liquid media.  [38], the probability of their formation increases with the pH growing. However, when the anion concentration is equal to 0.01 mol L −1 , one observes less than a two-fold decrease in the adsorption of uranium, which is also associated with the formation of complex ions such as [UO 2 NO 3 ] + , [UO 2 PO 4 ] -, [UO 2 Cl] + UO 2 Cl 2 0 , etc. [39]. The weakening of the sorption properties in alkaline solutions is related to the partial oxidation of the organic radical, accompanied by the destruction of the sorbent.
The Se-init after the sorption of uranyl ions was analyzed by the XPS spectroscopy: the results showed the presence of U 4f signals with binding energies of 392.8 and 382.0 eV, which indicated the binding of uranyl ions in the sorbent due to complexation and partial ion exchange. Figure 11 illustrates the structure of the binding of uranyl ions by Se-init. After the deprotonation reaction of the -OH group, the binding of the radionuclide can proceed according to the M1 mechanism in acidic or neutral media, as well as by the M2 mechanism ( Figure 11) in slightly alkaline and alkaline media due to binding of the oxygen and nitrogen atoms at the oxime group.
The preservation of mechanical stability is an important requirement for sorption materials created for long-term use in sorption columns. This requirement can be fulfilled by developing composite materials by depositing them on a mechanically stable matrix. In the present work, silica gel was used as such a matrix: it was modified with a polymer-based on the Se-derivative of amidoxime. Table 3 shows the surface characteristics of the resulting materials: the data were obtained using nitrogen adsorption and subsequent calculation according to the BET equation. It was shown that, unlike the initial material (Se-init), the composite materials have a more developed surface, which increases proportionally to the increase in the weight content of silica gel.  Figure 12 shows the adsorption isotherms obtained in the solutions with pH 6 ( Figure 12a) and pH 8 (Figure 12b), which can be attributed to the L-type [40], thus indicating a high affinity of the adsorbents to uranium. Besides, the highest SEC is attained at pH 8, which is probably related to peculiarities of the ionic form of uranium in solution: in particular, at pH 8, the predominant forms are UO 2 (OH) 3 and (UO2) 3 (OH) 5 + , while at pH 6-UO 2 (OH) + , (UO 2 ) 2 (OH) 2 2+ , UO 2 (OH) 2 0 , and (UO2) 3 (OH) 5 + , respectively [37].  Table 4 shows the values of G max and the adsorption equilibrium constant K l obtained by approximating the experimental points. The coefficient of determination exceeds 0.95, which indicates that the chosen model is correct. For the composite sorbents, the values of the adsorption equilibrium constant are quite close, which indicates the preservation of the adsorption mechanism. The adsorption equilibrium constants obtained for the composites have lower values in comparison with Se-init due to the reduced content of the sorption-active component (Se-derivative of 4-aminofurazane-3-carboxamidoxime). The highest values of the maximum adsorption were obtained for Se-50 and Se-65, which could be related to the greater availability of adsorption sites; however, we do not exclude the adsorption of uranyl ions on silica gel. The highest values of the maximum adsorption were obtained for Se-50 and Se-65, which could be related to the greater availability of adsorption sites and also with partial adsorption of uranium on silica gel. In order to evaluate the silica gel effect on the composite's sorption characteristics, a control experiment was carried out. In the control experiment, the efficiency of uranium adsorption with unmodified silica gel was evaluated. Figure 13 shows the diagram of uranium sorption from model solutions with pH 6 and 8 on synthesized materials and pure silica gel used in the fabrication of composite materials. It is known that various types of silica gels are capable of adsorbing heavy metal ions from liquid media [41,42]. It should be noted that unmodified silica gel cannot efficiently extract uranium since ion-exchange hydroxyl groups in silica gel do not exhibit high selectivity to uranium in the presence of other metal ions and anions. In addition, the binding of uranium by embedding it in the crystal lattice of silica gel is significantly limited due to the low sorption capacity of the synthetic mineral. The efficiency of uranium extraction from the model solution does not exceed 50%; therefore, unmodified silica gel is not an effective sorbent. However, its use as a matrix for composite materials (along with Se-init) promotes the improvement of sorption properties, which is reflected in the increase in the efficiency of recovery ( Figure 13) and SEC (Table 4), relatively to Se-init. We evaluated the kinetic characteristics of the adsorption process on the composite sorbents. Figure 14 shows the kinetic curves of the sorption of uranium from the model solution in semi-logarithmic coordinates (Figure 14a). The kinetic curves indicate that the adsorption process on the composite sorbents proceeds at a higher rate, which is also associated with greater availability of the adsorption sites as compared with the noncomposite sorbent. An increase in the specific surface area of composite materials leads to a decrease in the time to reach equilibrium adsorption, which is 10-20 min, while the adsorption efficiency exceeds 95%. A significant increase in the time to reach adsorption equilibrium on the Se-init sample is probably associated with the specific features of the adsorption process in the presence of uranium in various ionic forms. In addition, the diffusion process is important in the uranium-binding rate on a dense surface of a non-porous material, which in this case is likely to be limiting.  Figure 15 shows high-resolution SEM images for Se-init and the composite sorbents. Unlike Se-init, whose structure is represented by pronounced crystallites, the surface of the composite sorbents is relatively homogeneous and contains small particles of presumably the sorption-active component. The surface distribution of the sorption-active component indirectly corroborates the fact that the adsorption sites of the composite sorbent can have greater accessibility in comparison with the Se-init material, some of the sorption sites of which are located in the bulk of dense crystallites and therefore do not participate in the exchange.  Table 5 shows the parameters calculated for the pseudo-first and pseudo-second order equations (Figure 14b). According to the determination coefficients, the kinetic adsorption curve of uranium for Se-init is better described by the pseudo-first-order equation, as opposed to the composite adsorbents. The increase in the reaction order for the composite sorbents could also indicate the increase in the availability of the adsorption sites and the predominance of the chemisorption process of uranium binding in comparison with the non-composite sorbent. It is also worth mentioning that the pseudo-second-order equation constant k 2 differs for different composite sorbents. For the sorbent Se-65, the calculated k 2 has the lowest value, which is associated with a reduced content of the sorption-active component affecting the kinetic characteristics.  Table 6 shows the results of testing the hydro-mechanical strength of the fabricated materials. One can note that at a transfer to Se-35, the material loses stability rather sharply, which is probably related to an excess of the sorption-active component washed off from the surface of the matrix during the stirring. The latter is corroborated by the fact that Se-init is mostly peptized in the model solutions, which complicates working with it.

Conclusions
To sum up, a new approach to the synthesis of sorption materials containing amidoxime functional groups in their structure was developed. In particular, the Se-derivative 4-aminofurazane-3-carboxamidoxime was synthesized by polycondensation of individual substances in the presence of polar organic solvents. The molecular structure of the elementary unit of the initial Se-derived amidoxime was established, which indicates the formation of strong bridging bonds: >Se=Se<.
The sorption properties of the material under static conditions in the presence of both various cations and anions were characterized. The materials manifest mechanical and chemical stability, as well as high sorption-selective characteristics towards uranium in the pH range of 6-9.
In order to increase the kinetic parameters of ion exchange, the composite materials were fabricated based on the initial Se-derivative of amidoxime using silica gel as a matrix. The values of the maximum static exchange capacity in neutral and slightly alkaline model solutions were established: they equaled to 370 mg g −1 (pH-6) and 270 mg g −1 (pH-8) for Se-init and to 620 mg g −1 (pH-6) and 760 mg g −1 (pH-8) for Se-50.
It was found that, along with the increase in the specific surface area of sorbents, the kinetic parameters of the uranium sorption increase by order of magnitude as compared to the non-porous material.
Thus, the fabricated materials can be recommended for application in the recovery and concentration of uranium from technological and natural slightly alkaline waters with high mineralization.

Funding:
The work was financially supported by the Russian Foundation for Basic Research (RFBR) (Project No. 19-33-90148). In addition, the study of sorption characteristics was carried out was partially supported within the frames of the State Order of the Institute of Chemistry of the Far Eastern Branch of the Russian Academy of Sciences (Project No. 0205-2021-0002).
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data available on request due to restrictions eg privacy or ethical.