INTRODUCTION

In view of the rapid development of nuclear power engineering and related industries, the contamination of environmental objects and, primarily, natural water is a severe ecological problem. The main contaminating component in mining of uranium ores and production and processing of nuclear fuel are uranium compounds. Being a typical heavy metal, uranium has a high toxicity, which can do an unacceptable damage to human health. In addition to exerting a general toxic influence, natural and artificial uranium radionuclides and products of its radioactive decay possess a high radiotoxicity.

All the aforesaid refers to a full extent to natural thorium and products of its decay. Therefore, the problem of extraction of uranium and thorium compounds from water media seems to be rather important and topical.

The methods of precipitation, liquid extraction, and ion exchange are widely employed to extract uranium and thorium from aqueous solutions [1]. At present, membrane methods based on micro- and ultrafiltration and reverse osmosis gain an increasingly wide acceptance [2].

In our preceding studies [3–5], we examined the extraction of nonferrous metal ions and cesium and strontium radionuclides by the nanofiltration (NF) method with a polymeric membrane manufactured by the Russia’s company RM Nanotekh (Vladimir, Russia). The membrane has a multilayer structure composed of a polyester, polysulfonamide, and poly(piperazine imide). The pore size in the NF membrane is 1–2 nm, the working pH range 1–12 (at 20–25°C), and the maximum working temperature 45°C The present study was concerned with the retention capacity of the RM Nanotekh NF membrane for uranium and thorium in their extraction from aqueous solutions with various pH values and concentrations of the metal being extracted.

EXPERIMENTAL

A membrane was placed in a standard filtration unit of 1812 type with length of 315 mm and diameter of 55 nm. The total filtration area was 0.3 m2, and the output capacity of the unit for the filtrate of about 25 dm3/h under a pressure of 7 atm.

The experiments were performed by preparing a model solution followed by its being passed through the NF unit. The NF process was performed under a constant membrane pressure of 6.5–7.0 at in the circulation mode when the concentrate and filtrate flows are returned into the same vessel. The results of analyses of filtrate samples taken after the membrane were used to calculate the retention capacity (R) of the NF membrane for the corresponding component by the formula R = (1 – Cf/C0) × 100%, where Cf, C0 are the concentrations of the component in the filtrate after the membrane and in the starting solution, respectively.

To determine the influence exerted by the initial concentration in solution of the component under study on the retention capacity of the NF membrane, filtrate samples were taken from the solution, and the concentrate was returned, with the initial concentration of the substance in solution thereby raised.

The concentrations of uranium and thorium were determined by the photometric method with arsenazo III at a wavelength of 656 nm [6] with an Ekotest photocolorimeter. In some of the experiments the content of calcium and magnesium was determined in solutions, in addition to the concentration of uranium and thorium, by the method of complexometric titration [7]. The solution pH values were determined with an Ekotest 2000 ion meter. The model solutions were prepared from reagents of analytically pure and chemically pure grades and distilled or tap water.

RESULTS AND DISCUSSION

A study of the retention capacity of an NF membrane for uranium. In a study of how the retention capacity (R) of an NF membrane depends on pH, we used a solution of UO2(NO3)2 with uranium concentration of 10 mg/dm3, which contained 0.1 mol/dm3 of NaNO3 as a supporting electrolyte. The solution pH was varied within the range 2–10 via addition of 0.1 mol/dm3 HNO3 and NaOH solutions. The results obtained are shown in Fig. 1.

Fig. 1.
figure 1

Dependence of the retention capacity (R) of an NF membrane for uranium on solution pH.

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The results obtained demonstrate that the retention capacity of an NF membrane for uranium depends on its state in aqueous solutions at various pH values [8]. The smallest values of R (82–83%) are observed id uranium exist in a solution as uranyl ions or oxyhydrate complexes at pH values in the range from 2 to 6. At pH exceeding 6 the retention sharply increases (to R ≈ 95%), which is due to the formation of the insoluble U(IV) hydroxide and also di- and tricarbonate complexes.

In the second stage of studies, the retention was examined of uranium from Moscow tap water of composition, mg/dm3: Na+ 6.7, K+ 4.1, Mg2+ 15.06, Ca2+ 75.35. Cl– 6.0, SO42– 36.8, HCO3– 201; total salt content 315–345; total hardness 3.76 mg-eq/dm3; pH 7.8. Before the start of the experiments, a UO2(NO3)2 solution was added to the water until the uranium concentration reached 11.6 mg/dm3; the water volume was about 20 dm3. The experiment was carried out in the concentration mode; therefore, the uranium concentration in the initial solution increased continuously. Figure 2 shows the dependence of the retention capacity of the NF membrane for uranium on its concentration in the initial solution.

Fig. 2.
figure 2

Dependence of the retention capacity (R) of an NF membrane for uranium on its concentration in the starting solution.

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It can be seen from the above graph that with an increase in the uranium concentration in the initial solution from 11.6 to 20 mg/dm3, the value of R increased from 98 to 99%. The observed effect is most likely associated with an increase in the diffusion component of the NF retention process. With an increase in concentration, the thickness of the electric double layer (EDL) on the membrane surface increases, which causes diffusion difficulties for ions migrating through the membranes and, accordingly, leads to an increase in the retention capacity of the membrane. After the final formation of EDL at the uranium concentration of more than 20 mg/dm3, the retention capacity is at a constant level. The authors of [9] came to similar conclusions regarding the mechanism of uranium retention. With a further increase in the uranium concentration, the value of R remains practically at the same level. After NP purification of 20 dm3 of water, 18.5 dm3 of filtrate and 0.76 dm3 of concentrate were produced. In addition to the uranium concentration in the filtrate and concentrate, the concentrations of calcium and magnesium ions were determined. The results are shown below:

Ion

UO22+

Ca2+

Mg2+

R, %

98.4

67.4

43.2

It can be seen that, in addition to uranium, hardness salts are also extracted from water during NF.

Study of the thorium retention capacity of NF membranes. To study the dependence of the retention capacity of the NF membrane by thorium on pH, we used a Th(NO3)4 solution with a thorium concentration of 100 mg/dm3, containing 10 g/dm3 NaNO3 as a background electrolyte. The pH of the solution was varied in the range 1–12 by adding solutions of 1 mol/dm3 HNO3 and NaOH (Fig. 3).

Fig. 3.
figure 3

Dependence of the retention capacity (R) of an NF membrane for thorium on solution pH.

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We also determined the influence exerted by the concentration of thorium in the starting solution on the degree of its retention. The experiment was performed with a solution containing 10 g/dm3 of NaNO3 with pH 1.0 as the background electrolyte. The thorium concentration was varied within the range 0.5–4.0 g/dm3. The results obtained are presented in Fig. 4.

Fig. 4.
figure 4

Dependence of the retention capacity (R) of an NF membrane for thorium on its concentration in the starting solution, pH 1.0.

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It can be seen in the figure that, as the initial concentration of thorium is raised from 0.5 to 4 g/dm3, the retention capacity of the NF membrane hardly changes, being 93 ± 1%, which indicates that the thorium concentration remains unchanged in the given range of concentrations at pH 1.0. The results obtained can be to a certain extent extended to the behavior of Pu(IV) in NF of aqueous solutions because the hydrolytic behavior of Th and Pu(IV) in aqueous solutions is about the same.

CONCLUSIONS

It was shown that the retention capacity (R) of a nanofiltration (NF) membrane for uranium depends on its state in aqueous solutions. The smallest values of R (82–83%) are observed if uranium exists in solution as uranyl ions or oxyhydrate complexes at pH in the rane from 2 to 6. In solutions with larger pH values, the retention of uranium increases (R ≈ 9%%) due to the formation of the insoluble U(IV) hydroxide and/or carbonate complexes.

In the NF of aqueous solutions of thorium, R grows from 94 to 99% with pH increasing from 1 to 3.5, which is due to the hydrolysis of thorium ions to give oxyhydrate complexes of various compositions. At pH 3–9, the retention of thorium remains constant due to the formation of the insoluble thorium hydroxide. In alkaline solutions (pH > 9), R decreases because of the formation of negatively charged hydroxo and/or carbonate complexes.

On the whole, the nanofiltration is an effective method for purification of wastewater and natural water to remove admixtures of uranium and thorium.