Effects of Temperature and Sulfuric Acid and Iron (II) Concentrations on the Efficacy of Decontamination of Spent Ion-Exchange Resins Containing Hematite

Abstract

The effect of H₂SO₄ and FeSO₄ concentrations and temperature on the efficacy of decontamination of spent ion-exchange resins was estimated. The study was performed on model spent ion-exchange resins purposefully contaminated with hematite and Co-57 radionuclide. It was found that an increase in solution temperature up to 50 °C and the addition of FeSO₄ increases the efficacy of decontamination of spent ion-exchange resins by 1 M and 2 M H₂SO₄ solutions by 1–2 orders of magnitude, whereas the decontamination factor value here is >10³. Since under static conditions, the secondary adsorption of Co-57 was observed, the extra washing of ion-exchange resins by 3 M solution of NaNO₃ is required. Decontamination under dynamic conditions excludes the secondary adsorption of Co-57, so that the necessity of the extra stage of washing can be skipped. Under dynamic conditions, the consumption of a solution of the composition H₂SO₄ (1 mol/L) + FeSO₄ (0.2 mol/L) is 1.5-fold lower in comparison with the 2M solution of H₂SO₄ at compatible values of the decontamination factor. Such an approach enables reduction in the volume of secondary waste and the equipment corrosion due to the decrease in H₂SO₄ concentration.

1. Introduction

Ion-exchange resins are used at nuclear power plants to maintain the water-chemical conditions. During long-term operations, these resins’ sorption characteristics change, as they become contaminated with radionuclides. The problem of safe disposal of these spent ion-exchange resins (SIER) remains immensely relevant nowadays.

Cementing is widely used for SIER conditioning. However, it is only possible to introduce up to 12 wt. % of SIER to a cement matrix without losing its mechanical strength. Methods of increasing the resin content in a cement compound without changing its strength characteristics were suggested in the following works: Kononenko et al. [1] and Lin et al. [2]. Despite the success achieved in the cementing method, scientists keep searching for and developing recycling technologies that would lead to a decrease in the volume of SIER with a minimum amount of secondary waste.

Thermal methods allow an effective reduction in the volume of SIER by the destruction of a polymer matrix. The resulting solid residue after SIER combustion comprises hematite as well as other products, in the form of H₂O, CO₂, and H₂SO₄ (Korpiola et al. [3]). Pyrolysis at 300–350 °C allows reduction in the SIER volume by up to 50%, and the inductively coupled plasma can be used for post-combustion of the resulting gaseous products (Castro et al. [4]). The introduction of manganese oxide reduces the pyrolysis temperature down to 300 °C, at which the residual mass of the SIER is 34.14% (Luo et al. [5]). Decontamination of the carbonized SIER residue can be carried out by chlorine gas at high temperatures with the transition of radionuclides (Cs-134/137, Sr-90, Co-58/60, Fe-59, Mn-54, Zn-65, Zr-95) into volatile chlorides, which are concentrated in a scrubber (Yang et al. [6]).

Supercritical aqueous oxidation using H₂O₂ allows complete destruction of SIER [7]. The introduction of K₂CO₃ increases the efficiency of SIER decomposition in the temperature range 550–750 °C (Wang et al. [8]). In the work of Leybros et al. [9], which focused on comparing the processes of SIER decontamination under both supercritical and hydrothermal conditions, the authors indicated that treatment under hydrothermal conditions was more cost-efficient while maintaining an acceptable efficiency of SIER decontamination. However, in terms of recycling the secondary waste, supercritical oxidation is more advantageous (Leybros et al. [9]). An industrial plant for processing SIER with a capacity of 150 kg/h was proposed in Kim et al. [10]: it required preliminary separation into cation- and anion-exchange resins with subsequent mechanical transformation into a pulp. The widespread use of thermal methods is limited by high power consumption, the need to decontaminate secondary waste (including gaseous waste), and risks associated with high pressure and the corrosion of equipment.

The use of Fenton and Fenton-like processes to reduce SIER volume is currently under intensive investigation. Huang et al. [11] suggested a method for the destruction of SIER in the Fe(II) form, in a pseudo-fluidized bed. Despite highly efficient decomposition of resin (91.6–96.7%), the authors indicated the need for additional decontamination of the resulting gaseous products and secondary liquid waste. The Fenton process modified by replacing the Fe(II) catalyst by the Cu(II) one (Wan et al. [12]), the use of UV radiation (photo-Fenton process) (Feng et al. [13]) or ozone (Feng et al. [14]) enables an increase in the efficiency of decomposition of SIER and oligomeric products and its dissolution into low molecular-weight acids (Feng et al. [13,14]). Shen et al. [15] suggested using nanoscale bimetallic catalysts based on Fe⁰/Cu⁰ to increase the efficiency of SIER decomposition. The use of the electro-Fenton process increases the efficiency of SIER destruction due to the continuous reduction of Fe(III) to Fe(II) at the cathode. The volume of secondary waste can be reduced by reusing the solutions (Cheng el al. [16]). Despite the ongoing research in this field, the oxidation of SIER using the Fenton process is limited by the substantial consumption of a concentrated solution of hydrogen peroxide, the formation of secondary waste, and incomplete dissolution of ion exchange resins.

SIER can be transformed into non-radioactive waste by means of chemical and electrochemical decontamination methods, whereas the volume of secondary radioactive waste can be reduced by reusing decontamination solutions. Korchagin et al. [17] suggested a method for decontaminating SIER with a solution of HNO₃ and NaNO₃ at 60–80 °C. After decontaminating from Cs-137 and Co-60 and adjusting pH, the decontamination solution was reused, which reduced the secondary waste volume more than 100-fold.

Solutions of NaF (0.01 mol/L) and EDTA (0.1 mol/L) + Na₂CO₃ (0.5 mol/L) can be used to decontaminate SIER from plutonium and fission products (Ru-106, Zr-95, Nb-95) (Gupta el al. [18]). The efficiency was demonstrated by using a solution of the composition EDTA (1 mol/L) + Na₂CO ₃ (0.01 mol/L) for decontamination of model SIER from Co, Eu, and Sr (Leybros et al. [9]). However, the authors noted that the use of EDTA could complicate the process of further decontamination of secondary waste.

Radionuclides are concentrated from an acidic decontamination solution by means of a cell, inside which a cathode and anode are separated by an ion-exchange membrane. The reduction occurs at the cathode and is accompanied by the formation of radioactive particles collected in a separate container (Wood el al. [19]). Cs-134/137 radionuclides are removed from SIER via electrodialysis and then concentrated on selective sorbents (Nott et al. [20]). Decontaminating SIER via different methods can be complicated by the presence of insoluble inorganic deposits, which are responsible for a significant portion of activity. Aluminosilicate deposits can form on the anion-exchange resin and become a substrate for Cs-134/137 radionuclides adsorbed on them (Palamarchuk et al. [21]). Iron oxide deposits (crudes) accumulate on cation exchange resins, mainly in the form of hematite, which, in turn, accumulates radionuclides of the corrosive group (Tsai et al. [22], Sawicki et al. [23]).

In the patent of Miyamoto et al. [24], decontamination of SIER contaminated with iron oxide deposits was carried out with solutions of sulfuric or oxalic acid at 80–120 °C. The acidic decontamination solution was then decontaminated from radionuclides by electrodeposition. Ultrasound treatment was also used to remove iron oxide deposits from SIER, which were then separated from the solution by magnetic separation (Jo et al. [25]). However, Sawicki et al. [26] reported that hematite, which is one of a group of iron oxide deposits that are the most difficult to dissolve (Sidhu et al. [27]), was not removed by ultrasound, and even simple washing with water provided better results. This divergence in reports is probably related to the specifics of using ion-exchange resins.

The problem of dissolving hematite on SIER can be solved by its transformation into more soluble oxides. Here, in the presence of Fe(II), hematite can be transformed into magnetite as a result of non-redox reactions, during which pseudomorphic replacements occur [28]. In this regard, an effective solution to the problem of dissolution of hematite is the reductive dissolution of hematite using ascorbic (Echigo [29]) or oxalic acid (Siffert et al. [30]; Salmimies et al. [31]). The reduction of Fe(III) to Fe(II), with subsequent transfer into the solution and adsorption on the surface of hematite, results in transformation into more soluble magnetite due to autocatalytic oxidation. Here, in acidic media, the rate of oxidation of Fe(II) decreases noticeably, which leads to an increase in the rate of dissolution of hematite (Taxiarchou et al. [32]). For this reason, the addition of oxalic acid to the sulfuric acid solution increases the rate of dissolution of hematite, while the increase of the solution temperature from 15 to 50 °C reduces the processing time six-fold (Vehmaanperä et al. [33]). In the presence of hydrazine (pH 9.8), the dissolution of hematite under the conditions of the second PWR circuit proceeds in accordance with a similar mechanism of Fe(III) reduction followed by the formation of a well-crystallized octahedral magnetite (Li et al. [34]).

The available approaches to decontamination of SIER containing iron oxide deposits, including the addition of organic acids and complexing agents, could complicate treatment of secondary waste due to the formation of strong complexes with the radionuclides of the corrosion group. Such complexes cannot be virtually removed from liquid radioactive waste streams by sorption and precipitation methods. The addition of Fe(II) salts into the decontaminating solution could promote the increase in efficacy of SIER decontamination at the expense of the dissolving intensification of acid-resistant hematite deposits, Further disposal of secondary waste consists of pH correction to precipitate radionuclides with Fe(OH)₃. As a result, the addition of Fe(II) salts to the decontaminating solution in order to increase the efficacy of SIER decontamination could be preferable to treatment with organic acids and complexing agents in view of the further management of secondary waste. Thereafter, decontaminated SIER could be placed in the household waste category, which removes the necessity to destruct them by thermal or chemical methods.

The available literature does not contain information related to estimation of the effects of Fe(II) on the efficacy of decontamination of SIER containing hematite by H₂SO₄ solutions at heating. Accordingly, the objective of the present work is to estimate the effects of temperature and concentrations of sulfuric acid and Fe(II) on the efficacy of decontamination of model SIER containing hematite.

2. Materials and Methods

2.1. Materials and Reagents

Iron(III) chloride (FeCl₃ × 6H₂O), iron(II) sulfate (FeSO₄ × 7H₂O), ammonium hydroxide (NH₄OH), sodium hydroxide (NaOH), sodium nitrate (NaNO₃), hydrochloric acid (HCl) solution with a concentration of 11.3 mol/L, sulfuric acid (H₂SO₄) solution (17.5 mol/L), and nitric acid (HNO₃) solution (13.3 mol/L) of the chemically pure grade were purchased from Nevareactive LLC, Russia and used without additional purification. Radionuclide Co-57 in 1 M HCl solution was purchased at the Leipunskiy Institute of Physics and Power Engineering. KU 2-8 cation-exchanger was purchased from TOKEM, Russia. Prior to the start of the work, the cation-exchanger was washed with 1M HNO₃ solution under dynamic conditions, after which it was washed from the acid residues with distilled water and stored in a flask with a glass stopper.

2.2. Decontamination under Static Conditions

The model SIER used in the present work was synthesized as described in Egorin et al. [35] (Figure 1,

Table 1. Characteristics of the model SIER per 1 g of dry resin.

Mass of swollen resin (g)	2.6 ± 0.1
Volume of swollen resin (ml)	2.4 ± 0.1
Iron content (mg)	47 ± 5

). The initial activity of the model SIER was 5 × 10⁴ Bq/kg.

The effect of the temperature and concentration of Fe(II) (FeSO₄) was evaluated under static conditions with continuous contact of the resin with the decontamination solution in a conical flask placed in a thermostatically controlled shaker bath. The ratio of the volumes of the decontamination solution and swollen resin was 50:1, and the volume of the solution was 100 mL. To reduce secondary adsorption of Co-57 as a result of ion exchange on the functional groups of the resin, NaNO₃ (3 mol/L) was additionally added to the model solutions. After the specified time intervals, mixing was stopped, an aliquot of the solution was taken from the flask, and its activity was measured. After this, the solution was placed back in the flask and stirred again. The decontamination efficiency was additionally controlled by direct measurement of the decrease in activity of the resin itself.

The decontamination factor (DF) under static conditions was calculated by the Equation (1).

$$D{F}_s=\frac{A_0}{A_t}$$

(1)

where A_o is the initial activity of the resin sample (DPM) and A_t is the activity of the resin sample after decontamination under static conditions (DPM).

2.3. Decontamination under Dynamic Conditions

A glass column with an internal diameter of 9 mm was used for the research. The temperature was maintained at a given level using a heating element and a controller with a thermocouple located directly inside the column. The heating area was additionally insulated with non-combustible mineral fiber and asbestos cloth. The temperature measurement error was 1–3 °C. The thermocouple was pre-coated with a shell of chemically resistant plastic, which excluded its destruction in a solution of sulfuric acid. The decontamination solution was fed into the column from below, after which it was collected using a fraction collector for subsequent activity measurement. The flow rate of the solution into the column was 3 mL/h, and the resin volume was 2 mL. Figure 2 shows the appearance of the installation and its conditional scheme.

After feeding a given volume of the decontamination solution through the column, its activity and the residual activity of the resin were measured. Using the obtained results, the dependence of the decontamination coefficient of the model resin on the volume of the passed solution was built in the form of integral curves.

The decontamination factor under dynamic conditions was calculated by Equation (2).

$$D{F}_d=\frac{A_0}{A_v}$$

(2)

The decontamination efficiency under dynamic conditions was calculated using Equation (3).

$$DE{S}_{\%}=\frac{A_v}{A_0}\times 100$$

(3)

A_v variable shows the amount of desorbed Co-57 radionuclide when fed a given volume of the solution; it was calculated for each experimental point by Equation (4).

$$A_V=A_0-{{\displaystyle \int }}_0^{n}f{\left(A\right)}_{n}d{V}_n$$

(4)

where A₀ is the initial activity of the resin measured in decays per minute (DPM), A_n is the activity of the resin after feeding the solution of a given volume (DPM), A_n is the activity of the passed decontamination solution (DPM/mL), and V_n is the volume of the fed decontamination solution (ml).

The maximum value of DF_d was within the limit of quantification (LOQ) of the Co-57 radionuclide for the spectrometric route used during the experiment. Decontamination under dynamic conditions was stopped when the level of activity decreased to LOQ.

In cases when the total activity of the resin upon decontamination approached the limit of quantitative determination of the Co-57 radionuclide by the gamma-spectrometer (DF > 10³), the experiments were carried out repeatedly in order to confirm the accuracy of the obtained results. In other cases, when the resin activity was high, we limited ourselves to a single experiment. In addition, experiments under dynamic conditions were carried out just once due to their special features and complex character of performance.

2.4. Equipment and Software

The specific activity of Co-57 (photopeak energy: 122 keV (85.6%) was determined by direct radiometric method using a MKS-AT1315 gamma-spectrometer equipped with NaI(Tl) detector measuring 63 × 63 mm (RPE Atomtech, Belarus, Minsk, st. Gikalo, 5), the measured energy range was 50–3000 keV, and the energy resolution of Cs-137 (662 keV) was 8%. The results were processed using the Veusz software (ver. 3.4, GNU Public License).

3. Results and Discussion

Figure 3 shows a diagram of the DF dependence on the concentration of sulfuric acid and the temperature of the solution under static conditions. Only 5 mol/L H₂SO₄ solution demonstrated efficacy in the entire temperature range under study. However, according to the presented results, the increase in temperature up to 50 °C leads to a noticeable increase in the DF value when using 1 M and 2 M H₂SO₄ solutions. Further increase in the temperature of the solution does not lead to a noticeable increase in DF.

Figure 4 shows a diagram of the activity of the H₂SO₄ solution in decays per minute (DPM), at continuous contact with the model SIER, and its change depending on concentration, temperature, and time. When using 0.01 M and 0.1 M H₂SO₄ solutions, the values of the counting number of pulses remained below the limit of statistical significance considering background values and are not shown in the figure. At 25 °C, as time passed, there was an increase in the activity in the 2 and 5 M solutions of H₂SO₄, which indicated the gradual dissolution of iron oxide deposits. However, we found that in the temperature range of 50–90 °C, the activity of 2 and 5 M H₂SO₄ solutions could decrease. This fact is associated with the process of secondary adsorption of Co-57, which is transferred into the solution in the ionic form during the dissolution of iron oxide deposits. Along with the increase in the concentration and temperature of the solution, the efficacy and rate of dissolution of iron oxide deposits increase, which is then followed by secondary adsorption of the radionuclide.

To evaluate the effect of the secondary adsorption process on the efficacy of SIER decontamination, we built kinetic curves of the dependence of the H₂SO₄ solution activity on the contact time (Figure 5). The effectiveness of decontamination at 90 °C was not evaluated due to the proximity of the results obtained at 50 and 75 °C. The activity of the H₂SO₄ solution is also given in decays per minute.

According to the kinetic curves, the increase in temperature and H₂SO₄ concentration increases the rate of Co-57 entering the solution. The presence of an extremum point on the curves obtained at 50 and 75 °C, after which the solution activity decreased, confirms the secondary adsorption of the radionuclide on the model SIER as the result of ion exchange.

The preliminary introduction of NaNO₃ (3 mol/L) into the H₂SO₄ solution does not exclude secondary adsorption. Figure 6 shows the diagrams of adsorption extraction of the Co-57 radionuclide by the ion-exchange resin KU 2-8 in the solutions of various compositions.

In the NaNO₃ solution (3 mol/L), the value of sorption removal of radionuclide gradually increases from 20.5% in 24 h up to 22.2% in 72 h. In H₂SO₄ solutions, the value of sorption removal of radionuclide is noticeably lower due to the competing action of H⁺. However, the results obtained showed that even in concentrated H₂SO₄ solutions, a noticeable adsorption of radionuclide (up to 10%) was observed, which could negatively affect the effectiveness of the ion-exchanger decontamination under static conditions.

To estimate the actual value of the decontamination coefficient, the resins were additionally washed with a 3 M NaNO₃ solution on Schott filters with an internal diameter of 3 cm in portions of 5–10 mL. The total volume of the NaNO₃ solution was 30 mL per 2 mL of the swollen resin.

A diagram in Figure 7 shows the effect of additional washing with NaNO₃ solution on the efficiency of SIER decontamination. This procedure increases the DF value due to the desorption of residual Co-57: this effect is clearly manifested in 1M and 2M H₂SO₄ solutions. After decontaminating the SIER at a temperature of 50 °C followed by washing with a NaNO₃ solution, the DF values obtained in 1M and 2M H₂SO₄ solutions have a comparable value.

Table 2. Effects of H₂SO₄ and FeSO₄ concentrations and temperature on the efficacy of decontamination of the model SIER under static conditions.

Temperature (°C)	FeSO4 (mol/L)	Molar Ratio Fe(S)/Fe(H) **	DF
			0.1 M H2SO4	0.2 M H2SO4	1 M H2SO4	2 M H2SO4
25	0.01	1.5	nm *		20 ± 2	20 ± 2
	0.05	7.4			150 ± 20	>103
	0.1	15			750 ± 200	>103
	0.2	30			>103	>103
50	0.01	1.5	1.20 ± 0.04	30 ± 2	120 ± 15	320 ± 60
	0.05	7.4	1.30 ± 0.04	30 ± 2	380 ± 80	>103
	0.1	15	1.30 ± 0.04	40 ± 4	>103	>103
	0.2	30	1.40 ± 0.04	60 ± 7	>103	>103
75	0.01	1.5	nm *		50 ± 8	180 ± 40
	0.05	7.4			150 ± 30	870 ± 360
	0.1	15			350 ± 100	>103
	0.2	30			670 ± 250	>103

* Not measured. ** Fe(H)—iron content in the form of hematite in SIER, Fe(S)—iron content in solution.

shows the DF values as to dependence on H₂SO₄ and FeSO₄ concentrations and temperature. According to the obtained results, the increase in the FeSO₄ concentration in H₂SO₄ solution is accompanied with an increase in the efficacy of SIER decontamination over the whole temperature range. However, this effect becomes noticeable only when using H₂SO₄ solutions of concentrations of 0.2 mol/L and higher. The increase of the FeSO₄ concentration from 0.01 up to 0.2 mol/L increases the DF value by 1–2 orders of magnitude, which is especially noticeable when using 1 M solution of H₂SO₄. Thus, the addition in the form of FeSO₄ enables the use of H₂SO₄ solutions of lower concentrations without a loss of decontamination efficacy, which, in theoretical terms, decreases potential costs and equipment corrosion.

The increase in the solution temperature up to 50 °C further increased the efficacy of SIER decontamination at FeSO₄ addition to 1 M solution of H₂SO₄. Here, heating up to 50 °C allows reducing the FeSO₄ concentration in the solution with preservation of the DF, which also reduction in the reagents’ consumption and the secondary waste volume. However, the increase in the solution temperature up to 75 °C has the opposite effect and decreases the DF value. This could be related to Fe²⁺ oxidation until Fe³⁺ by the dissolved air oxygen at an elevated temperature. For this reason, further experiments were carried out at a temperature of 50 °C.

Decontamination of SIER under dynamic conditions is more preferable due to the simplicity of the process, since there is no need to separate the resin from the decontamination solution. Moreover, secondary adsorption of Co-57 is eliminated and there is no need for additional washing with NaNO₃ solution.

Table 3. Results of decontamination of the model SIER under dynamic conditions by the 0.1 M solution of H₂SO₄.

Fe(II) (mol/L)	Temperature (°C)	DF *
0.1	25	1.1 ± 0.3
	50	1.2 ± 0.2
	75	1.1 ± 0.4
0.2	25	1.1 ± 0.5
	50	1.2 ± 0.4

* After feeding 250 mL of solution.

shows the results of resin decontamination under dynamic conditions when using 0.1 M H₂SO₄ in the presence of FeSO₄. When using 0.1M and 0.2M FeSO₄ solutions, the DF values do not exceed 2, while the decontamination efficacy does not exceed 20%. We did not achieve a noticeable increase in the decontamination coefficient by introducing FeSO₄, and increasing the temperature. Based on the experiment results, we concluded that application of the 0.1 M H₂SO₄ solution was inefficient, even when heating in the presence of FeSO₄, for decontamination of SIER containing hematite.

When heating up to 50 °C in the absence of FeSO₄, only the 2 M solution of H₂SO₄ was efficient at decontamination of the model SIER (DF > 10²) (Figure 8). Low DF values when using the 1 M solution of H₂SO₄ can be associated with a low rate of hematite dissolution. The addition of FeSO₄ promoted the increase in the efficacy of dissolution of hematite on SIER and, as a result, increased the DF value and decreased the volume of the solution spent for decontamination. When using the solution of the composition H₂SO₄—1 mol/L, FeSO₄—0.2 mol/L, the rate of decontamination of the model SIER was notably somewhat higher than when using the 2 M solution of H₂SO₄.

Therefore, under dynamic conditions, at 50 °C, the addition of Fe(II) enables an increase in the efficiency of dissolution of iron oxide deposits, while the DF value increases by more than one order of magnitude. This provides the possibility of using less concentrated H₂SO₄ solutions without other organic additives, while maintaining high decontamination efficiency. The spent solution can be then disposed of by correcting the pH to the alkaline region with the formation of Fe(OH)₃ precipitate, which adsorbs radionuclides of the corrosive group.

4. Conclusions

It has been shown that, during the dissolution of hematite, secondary adsorption of Co-57 on the ion-exchanger occurs due to ion exchange, which requires the use of additional efforts to increase the efficiency of decontamination of SIER. Adding NaNO₃ (3 mol/L) directly to the H₂SO₄ solution does not exclude secondary adsorption. However, washing SIER additionally with NaNO₃ solution (3 mol/L) immediately after treatment with H₂SO₄ makes it possible to elute a significant part of the exchange Co-57 and thereby increase the decontamination efficiency.

The effects of FeSO₄ and temperature of the efficacy of decontamination of the model SIER containing hematite by H₂SO₄ solutions were estimated. The increase in the temperature of the decontaminating solution up to 50 °C increases the DF value by 1–2 orders of magnitude (10²–10³) when using 1M and 2M solutions of H₂SO₄. In the presence of FeSO₄, the DF value increases, presumably due to transformation of the hematite crystal lattice as a result of redistribution of the electron density between Fe²⁺ in solution (Fe_(S)) and Fe³⁺ in the composition of hematite (Fe_(H)). At a temperature of 25 °C, the increase of the molar ratio Fe_(S)/Fe_(H) from 1.5 up to 30 yields a DF increase by 2 orders of magnitude (>10³). Thus, the FeSO₄ addition enables use of less concentrated H₂SO₄ solutions with preservation of the decontamination efficacy. The solution heating up to 50 °C further intensifies the decontamination process in the presence of FeSO₄. Heating of the solution containing FeSO₄ up to 75 °C, on the contrary, decreases the DF value, presumably due to Fe²⁺ oxidation. Under dynamic conditions, at 50 °C, the consumption of the solution of the composition H₂SO₄ (1 mol/L) + FeSO₄ (0.2 mol/L) is 1.5-fold lower in comparison with the 2M solution of H₂SO₄, at compatible values of DF (>10³). The volume of secondary waste can be further decreased by pH correction into the alkaline range and co-precipitation of radionuclides with Fe(OH)₃.