2.1. Effect of the Resorcinol/Formaldehyde Molar Ratio and the Solidification Temperature on Chemical Stability and Sorption Characteristics of the Resulting Resins
The results of microscopic studies of the RFR 3-1 sorbent are presented in
Figure 1. The ion-exchanger surface is monolithic and does not contain cavities or channels. In the process of grinding the solid ion-exchanger, it cracks into irregularly shaped fragments with characteristic faces. The images obtained for RFR 1-1 and RFR 2-1 are the same as for RFR 3-1 and, therefore, are not shown here.
Figure 2 shows diagrams of changes in the RFR mechanical strength depending on the solidification temperature and the molar ratio of resorcinol/formaldehyde. With an increase in the proportion of resorcinol added, the mechanical strength RFR increases, which is indicated by a decrease in the abrasion degree value (
Figure 2a). Sample RFR 3-1 has the highest mechanical strength, which was also noted by us during grinding at the sample preparation stage. The effect of solidification temperature on mechanical strength was evaluated using sample RFR 3-1 (
Figure 2b). According to the figure, the ion-exchangers mechanical strength increases with rising in the solidification temperature up to 210 °C. At a temperature of 250 °C, thermo-oxidative degradation of the polymer already begins, which negatively affects the RFR mechanical strength.
A series of the RFR samples with a different resorcinol/formaldehyde ratio and solidification temperature were tested under static conditions in model solutions of various compositions. The experimental results are shown in
Figure 3.
The presented diagrams indicate a direct relationship between the sorption-selective characteristics of the RFR and the synthesis conditions. For the RFR 1-1 and RFR 3-1 samples, along with the increase in the solidification temperature, the sorption-selective characteristics increase, which is associated with the course of the solidification process and the further formation of the polymer lattice. The maximum values of the distribution coefficient of RFR 1-1 and RFR 3-1 were observed for the samples solidified at 210 °C. Above 210 °C, the polymer undergoes thermal oxidative degradation, which results in the decrease in the efficiency of radionuclide removal [
19]. For RFR 2-1, the dependence of sorption-selective characteristics on the solidification temperature is of a different nature, which is probably related to the peculiar features of the formation of the polymer lattice. In highly alkaline solutions, the model solutions No. 2 and 3 demonstrate the tendency to increase the distribution coefficient along with the increase in the resorcinol/formaldehyde ratio. This effect is most clearly visible in the presence of potassium ions having a greater negative impact on the efficiency of cesium extraction, decreasing the value of the distribution coefficient by one or two orders of magnitude in comparison with sodium ions. Based on the results obtained, further work was carried out using the RFR samples cured at a temperature of 210 °C.
Figure 4 shows the C-13 NMR spectra of the RFR samples solidified at 210 °C. The spectra correspond to those described in the published works and contain the following signals: 11.5 CS, 27.5 ppm—methyl and methylene, CS 53.8, 75.5 ppm—oxygen-containing groups (oxydiethylene bridges, etc.), CS 120, 1 ppm—C2, C4, C6, CS 130.8 ppm—S5, CS 138.3 ppm—vinyl, CS 153.5, 155.5 ppm—C1, C3, CS 181.9 ppm—quinones, CS 205.6—aldehydes, ketones [
20].
Table 1 shows the values of chemical shifts and the corresponding peak areas. The determination of the relative integral intensities of the spectrum components was carried out by applying an experimental resonance line to a theoretical curve using the least squares method, which was carried out in an independently developed software. The error in determining CS was 0.3 ppm; the intensity of the spectral line was 5% of its area.
Based on the results given in
Table 1, it can be concluded that the increase in the resorcinol/formaldehyde molar ratio does not lead to the formation of additional bridge structures connecting resorcinol rings (CS signals at 119, 153, 160), the areas of the peaks corresponding to oxygen-containing groups are of 75.8 and 53.8 ppm (variations within the error). However, it is noteworthy that the samples RFR 1-1 and RFR 2-1 are characterized by the presence of a CS peak at 10 ppm, which probably indicates the break of bridges during polymerization. As for the RFR 3-1 sample, there is no CS peak at 10 ppm, while the CS of 28 ppm is amplified, which indicates the formation of complex bridge structures of the -CH
2-(CH
2)
n type during the thermal destruction of methylol groups. These bridge structures, probably, contribute to the formation of a strong polymer lattice with a large molecular cell size, which improves the availability of exchange centers.
Figure 5 shows the IR spectra of the RFR 3-1 samples solidified at various temperatures and containing all the characteristic bands described earlier [
21]. In particular, the bands in the range 3700–3300 cm
−1 correspond to the intra- and intermolecular N-bonds in dimers and polymers. The bands in the range 3200–2800 cm
−1 correspond to weak (average) stretching vibrations of C-H bonds in the aromatic components, 1608–1400 cm
−1 correspond to stretching vibrations of the aromatic ring in arenas and acid with H- bonds (formic acid), 1295–1005 cm
−1 correspond to plane bending vibrations of 1,2-, 1,4-, and 1,2,4-substituted C-H bonds, 1440–1300 cm
−1 correspond to bending vibrations of OH- groups, 1000–550 cm
−1 correspond to out-of-plane bending vibrations of C-H bonds in arenes, vibrations of hydroxyl groups, and C-O bonds in carboxylic acids. A distinctive feature for the RFR samples heated at 105, 130, and 150 °C is a band in the vibration range 2400–2000 cm
−1 corresponding to the bound OH group and stretching vibrations of C-H in carboxylic acids. This fact confirms the presence of a certain number of methylol groups, which indicates incomplete crosslinking of the polymer. The oxidation of such methylol groups to carboxyl groups can occur with the rupture of bridging –CH
2-groups, which also leads to the destruction of the polymer [
22].
The increase in the resorcinol/formaldehyde molar ratio contributes to chemical stability to alkaline media, as evidenced by the data obtained during the study of the kinetics of dissolution of sorption materials in contact with a model solution No. 2 under static conditions.
Figure 6 shows dissolution diagrams of the RFR, which demonstrate a two-fold increase in the dissolution rate of RFR 1-1 compared to RFR 3-1.
Since the total content of Cs (stable isotope Cs-133) in the clarified part of heterogeneous LRW can reach several tens of milligrams per liter [
4], an important task is to determine the value of the maximum adsorption, which is calculated using the known equations.
Figure 7 shows the isotherms of sorption of stable Cs obtained using model solution No. 1 and No. 2.
Table 2 shows the corresponding values of the parameters of the Sips equation. Taking into account the fact that the experimental values are well described by the Sips equation (
R2 > 0.99), it can be concluded that the heterogeneity of exchange centers has a significant effect on the monolayer filling process. Along with the increase in pH, the increase in the theoretical value of the limiting sorption is observed, which is associated with the deprotonation of functional groups; however, the value of
KSIPS has similar values for both model solutions.
Figure 8 shows the kinetic sorption curves of the macro-concentrations of the stable isotope Cs-133 (
Figure 8a) and the micro-concentrations of the radionuclide Cs-137 (
Figure 8b) from various solutions. A common feature here is the increase in the SEC value and the effectiveness of the Cs-137 extraction during the transition from pH 9 to pH 13, which is associated with the increased dissociation of the functional –OH groups of resorcinol. However, in a model solution with pH 13, the curves pass through a maximum in the case of extraction of both the stable isotope Cs and the radionuclide Cs-137. The presence of the maximum is explained by the gradual destruction of the polymer. This process is accompanied by the model solution turning yellow.
Along with the increase in the resorcinol/formaldehyde ratio, the stability of ion-exchangers in solutions with a pH ≥ 13 increases; in addition, the SEC value and the efficiency of Cs-137 extraction increases. It is worth mentioning that in comparison with the RFR 2-1 and RFR 3-1, the kinetic curves for the RFR 1-1 in a solution with pH 13 are characterized by a sharp decrease after reaching a maximum, which indicates the low chemical stability of the ion-exchanger.
Figure 9 shows the sorption and desorption curves of Cs-137 micro-concentrations from the model solution No. 2 under dynamic conditions. Along with the increase in the number of sorption cycles, the efficiency of Cs-137 extraction from the model solution gradually increases, which is probably related to the gradual release of the ion-exchanger to the operating mode. The value of the sorption of radionuclide from the model solution in all six sorption cycles exceeds 98%, indicating high efficiency and chemical stability of the RFR 3-1 ion-exchanger.
The desorption curves shown in
Figure 9b indicate that, at feeding 35 mL of 1 M HNO
3 solution, the desorption efficiency exceeds 95% from one to four desorption cycles. In cycles 5 and 6, there is a gradual accumulation of Cs-137 in the resin matrix, which, however, accounts for 3.3% of the total sorbed activity. Based on the data presented, the volume of 1 M HNO
3 solution can be reduced to 40 mL that enables one to reduce the formation of secondary waste.