purification of radioactively-contaminated waters of 90sr and u(vi) by a ferrite method

ISSN 1063�455X, Journal of Water Chemistry and Technology, 2007, Vol. 29, No. 5, pp. 246–253. © Allerton Press, Inc., 2007.Original Russian Text © T.G. Timoshenko, G.N. Pshinko, B.Yu. Kornilovich, V.A. Bagrii, A.L. Makovetskii, 2007, published in Khimiya i Tekhnologiya Vody, 2007, Vol. 29, No. 5, pp. 449–461.

WATER TREATMENT AND DEMINERALIZATION TECHNOLOGY

Purification of Radioactively�Contaminated Waters of 90Sr and U(VI) by a Ferrite Method

T. G. Timoshenko, G. N. Pshinko, B. Yu. Kornilovich, V. A. Bagrii, and A. L. MakovetskiiDumanskii Institute of Colloid and Water Chemistry, Kiev, Ukraine

National Academy of Sciences of Ukraine, Kiev, Ukraine

Received June 14, 2006

Abstract—We investigated the processes of water purification of 90Sr and U(VI) by the ferrite method using the salts FeSO4 and FeCl3. It was found that for the formation of a crystalline sediment with mag�

netic properties there is required a total concentration of iron in the solution ≥150 mg/dm3. Due to partial oxidation of Fe(II) with air oxygen the optimal ratio Fe(II)/Fe(III) in solution should be equal to 6. Alkali concentration for precipitating ferrite under these conditions constitutes 0.01 M. When the alkali concen�tration is increased further the volume of magnetite sediments does not change, while the values of satu�ration magnetic induction of ferrite remain maximum. By means of the analysis of phase composition it is shown that the resultant sediment possesses high crystalline property and saturation magnetic induction virtually right after precipitation and has a structure of ferrite. We carried out an assessment of a water puri�fication degree of 90Sr and U(VI) by the ferrite method.

DOI: 10.3103/S1063455X07050050

Development of new and improvement of known methods and materials for decontamination of large vol�umes of contaminated waters with a low level of activity is an important task. One of the most difficult to remove radionuclides is 90Sr [1, 2] whose specific feature is little sorbability with natural alumosilicates and U(VI). The latter one is characterized by high complexation ability and, as a result, high migration of its com�pounds in the environment.

Lately the development of the methods for removing radionuclides from water has drawn much attention [3–11]. Efficiency of the proposed methods in great measure depends on physico�chemical features of water media and, consequently, forms of detecting radionuclides in them, which in the first place are determined by the chemical nature of a radioactive element, the pH value, the content in the water of dissolved organic and inorganic complexing ligands including suspended mineral particles, etc. One of the promising methods for purifying large volumes of waters of ions of heavy metals and radionuclides is a ferrite one based on the ability of ion compounds to form in aqueous solutions (under certain conditions) particles with magnetic properties, i.e., ferrites [12, 13]. In the middle of the ferrite�forming process metal ions are incorporated into a crystalline lattice of magnetite, i.e., newly�formed compounds—ferrites, which by their properties (magnetic, crystal�line) are virtually identical to a magnet. Despite high efficiency of the ferrite method it has not been used on a wide scale in the practice of water treatment due to the difficulties linked with reproducing the process of making ferrite by various chemical methods with a required complex of physico�chemical properties [10, 13]. This is especially significant when oxidizing Fe(II) (for instance, with oxygen when sparging air), or reducing Fe(III) till Fe(II) (for instance, sodium sulfate). The simplest method of making ferrite is when using the blend of Fe(II) and Fe(III). However, in this case too an extremely important is the ratio Fe(II) : Fe(III)under certain concentration conditions of their salts, concentration, and nature of an alkaline reagent, the precipi�tation pH, etc.

The objective of the present paper lies in optimization of conditions of using the ferrite method for purifi�cation of waters containing strontium and uranium using the salts FeSO4 and FeCl3. In particular, it is neces�sary to determine: minimal necessary total concentration of iron in solution for formation of a sediment with magnetic properties; the optimal ratio Fe(II) and Fe(III) in solution since during the stirring of the latter part of Fe(II) oxidizes by dissolved oxygen of the air; the precipitation pH; concentration and nature of the alkaline reagent for the formation of a sediment with the best magnetic properties and, accordingly, with its minimal volume; to assess the degree of water purification of 90Sr and U(VI) under optimal conditions of precipitation.

246

PURIFICATION OF RADIOACTIVELY�CONTAMINATED 247

EXPERIMENTAL

High disperse sediments of magnetite were obtained by the Elmore method [14]. For this purpose the solu�tions of the salts FeSO4 ⋅ 7H2O and FeCl3 ⋅ 6H2O with initial concentrations 10.0 g/dm3 by metal. Precipita�tion was carried out by the 1M solutions of NaOH and NH4OH concurrently with the constant stirring of the sample; the set pH value was measured on a universal ionometer EV�74 with an ESL�43�07 glass electrode. For determining magnetic properties dried�out crystalline remainders with a fixed mass of 60, 120, and 180 mg were charged into a copper capsule, were covered with paraffin, and measured their magnetization by means of a magnetometer having a Hall’s sensor. The volume of a humid sediment (Vsed) was measured after

its separation from the solution; remaining contents of 90Sr in the filtrate were measured by the radiometric method, that of a stable isotope by the atomic�adsorption one, and U(VI)—by the photometric one with arsenazo III. X�ray investigations of sediments obtained under optimal conditions of ferrite–formation were carried out using a DRON�2 diffractometer on filtered CoKα radiation at the rate of the meter movement 1 deg/min. Iron salts were precipitated from distilled water, ionic force—0.01 was maintained with solutions of sodium perchlorate. Initial concentrations of U(VI) and stable isotope—constituted 1 × 10–4 M, while the activity of the initial solutions of 90Sr—5 × 103 Bq/dm3.

RESULTS AND DISCUSSION

As is known complete precipitation of iron compounds with a different degree of oxidation occurs at dif�ferent pH values: for Fe(II)—9.5–9.7, for Fe(III)—3.2–4.1. When using a blend of iron salts having an increased OH– in solution the potential of the system varies and respectively the rate of oxidation rate of Fe(II) → Fe(III), while at pH > 11.5 the oxidation rate exceeds the rate of formation of ferromagnetic parti�cles; the most complete precipitation of magnetite occurs within the pH range 9.5 ± 1.5. In most technologies when purifying water by the ferrite method an alkaline reagent is added for reaching pH 9–10, which is related to an increase of the total content of salts and the pH value is higher than MAC. These are pH values that are used by us for choosing the optimal total concentration ΣFe in solution and the values K (Fe(II) : (III)).

Based on the experimental data we found that with the total content of iron ions in solution 150 mg/dm3

at K = 4–6 one can observe the formation of a dark sediment with magnetic properties (Fig. 1a). With small values K ≈ 1–3 (CFe(III)

> CFe(II)) one can observe the formation of yellow�brown sediments characteristic of iron hydroxides, i.e., there occurs mainly a predominant process of coagulation of ions of Fe(II) ad Fe(III). At K > 4 there forms a black suspension followed by sedimentation of crystalline sediments, which are char�acterized by a small volume and good magnetic properties. The settlement process of such a suspension pro�

(a)

0

20

40

60

3 5 7 9 K 0

20

40

60

V sed , cm 3/dm 3 σ s , m2 /kg

0

20

40

60

80

110 130 150 170 ΣFe , mg/dm 3

0

20

40

60

Vsed, m3/dm3 σ s , A m 2/kg

(b)

Fig. 1. Influence of the value K = Fe(II)/Fe(III) (ΣFe = 150 mg/dm3) (a) and the total iron dose (ΣFe) (K = 6.0) (b) on the sediment volume (Vsed) and its magnetic properties (σs) when precipitating with the NaOH solution to pH 10.5.

JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY Vol. 29 No. 5 2007

248 TIMOSHENKO et al.

ceeds much faster than the settlement of the suspension during a normal reagent (coagulation) purification. But with a further increase of the value K > 6 during settlement one can observe an increase of the volume, which is determined by insufficient oxidation of Fe(II) ions with dissolved oxygen of the air and the formation in this case of a brown sediment characteristic of Fe(II) oxides and not taking part in the ferrite–formation reaction. Based on the data of magnetic properties and the volume of a crystalline sediment it was found that the optimal value K = 6 at ΣFe = 150 mg/dm3.

When choosing necessary conditions of water purification and the formation of magnetite a substantial role is played not only by a correctly selected ratio Fe(II) : Fe(III) in solution at a certain total share of iron, but also its minimally necessary value for the formation of the ferrite crystalline sediment with magnetic proper�ties. In this case the quantity of iron in the solution should be sufficient for effective purification of water of radionuclides.

As can be seen from Fig. 1b with the content of the total amount of iron > 120 mg/dm3 there occurs a grad�ual reduction of the sediment volume having implicitly expressed magnetic properties—it is predominately the coagulation process. Concurrently with an increase of the total dose of iron in the solution up to ΣFe ≥140 mg/dm3 there forms a sediment of magnetite of black color with magnetic properties, while the volume of the formed sediment will be minimal with the total iron dose 150 mg/dm3, which completely in line with the data of the authors of paper [13].

One of the important requirements when choosing the method of deactivation of waters is the reduction of the volume of the radioactively contaminated sediments. Therefore, we investigated the kinetics of changes of the sediment volume at ΣFe = 150 mg/dm3 and K = 6.0. It was found that under optimal conditions the for�mation of a ferrite sediment occurs virtually instantaneously, while a complete precipitation, formation of a dense crystalline structure, and reduction of its volume is observed within the first 10–15 min (Fig. 2). It should be noted that the mechanism and the kinetics for the formation of magnetite in solutions were studied insufficiently. This is associated with the complexity of the development of the process itself and difficulty of observation over the composition of intermediate products formed.

In addition, as can be seen from the data obtained the nature of studied alkaline reagents under optimal conditions of the formation of a sediment does not affect the volume of a sediment formed. Experiments car�ried out demonstrated (Fig. 3) that the formation of the black sediment with magnetic properties having a minimal volume is observed at the concentration of alkaline reagents ≥0.01 mol/dm3, i.e., virtually stoichio�metric quantity. Deviation of the alkaline reagent concentration in either direction from the stoichiometric one in a different way affects the properties of the sediments. In the case when both NaOH and NH4OH are used (given their quantities are less than stoichiometric ones) the sediments do not have magnetic properties and their volume is much greater. In contrast, the volume of magnetite sediments obtained at the concentra�tion of the alkaline reagent ≥0.01 mol/dm3 does not vary with further change of the concentration of the latter, while magnetic properties remain maximum.

0

250

500

0 10 20 30 40 50 60min

V sed, cm 3/dm 3

2

1

Fig. 2. Precipitation kinetics (decrease of the magnetite sedimentation volumes) when precipitating with the solutions of NaOH (1) and NH4OH (2), ΣFe = 150 mg/dm3; K = 6.0; pH 10.5.



Based on the results obtained we studied the efficiency of the given method for the removal from a solution of U(VI) and 90Sr compounds. As can be seen from Fig. 4 the employment of this method under optimal con�ditions of precipitation makes it possible to achieve high degrees of water purification of U(VI) and 90Sr com�pounds within the range of concentrations 100–500 μmol/dm3. The volume and magnetic properties of ferrite sediments containing strontium virtually do not change within the whole range of metal concentrations, while for solutions containing uranium (in the case its concentration increases to > 200 μmol/dm3 magnetic prop�erties of sediments vary and concurrently their volume increases as well. Variation of magnetic properties of sediments with an increase of uranium concentration perhaps is determined by predominant formation of negatively charged anionic and colloid�disperse particles of uranium. Based on this the content of uranium in wastewaters of uranium�processing enterprises is much less than 100 μmol/dm3 the given method may be suc�cessfully used for water purification of U(VI) compounds.

Allowing for high tendency to complexation it was necessary to investigate the impact of main inorganic and organic complexation agents on the degree of water purification of uranium (VI) compounds and the value of a ferrite sediment.

As is known perchlorate ions virtually do not form complex compounds with metals including U(VI) com�pounds therefore sufficiently high concentrations of ClO4

– in solution do not affect the degree of water puri�

Vsed, cm3/dm

3

0

20

40

60

0 0.01 0.02 0.030

4

8

12

pH

2

1

1'

2'

σs, A m2/kg

0

20

40

60

0 0.01 0.02 Calk.reag., mol/dm3

1, 2

(a) (b)

Fig. 3. Influence of the nature and concentration of the alkaline reagent on the values of magnetite sediment value (1, 2) and the pH (1′, 2′ ) (a) and its magnetic properties (b) during precipitation with the solutions of NaOH (1) and NH4OH (2), ΣFe = 150 mg/dm3; K = 6.0.

0

20

40

60

0 100 200 300 400 500 CU(VI), μmol/dm

3

0

20

40

60

80

1001

2

Vsed, cm3/dm

3 CO, %

0

20

40

0 100 200 300 400 500 CSr(II), μmol/dm

3

0

20

40

60

80

100

1'

2'

Vsed, cm3/dm

3 CO, %

(a) (b)Fig. 4. Influence of the concentration of U(VI) (a, b) and Sr(II) (c, d) on the purification degree (CO) of water (1, 1′ ), volume of the sediment formed (2, 2′ ) (a, b) and its magnetic properties (c, d). UFe = 150 mg/dm3; K = 6.0; pH 10.5; CNaOH = 0.015 mol/dm3.



fication of U(VI) compounds using the given method (Fig. 5) and in the presence of SO4– and CO4

– the degree of purification insufficiently decreases only at rather high concentrations of the latter in solution (> 3 × 10–2 M) owing to the formation of more stable anionic rather than hydroxide complexes with uranium in the region of the pH of ferrite formation. The impact of Na2EDTA is most substantial: with the content of

CEDTA > 1 × 10–2 M in solution (pH < 11) iron salts are completely bound into complex compounds with

EDTA and under such conditions the ferrite–formation reaction does not occur (Fig. 6).

Low concentrations of an organic matter of natural origin, e.g. humic acids (HA), virtually do not affect the degree of purification of U(VI) compounds since the U–HA complexes are formed only within an acid region of the pH and at the pH ≥ 7 they are completely destroyed (Fig. 7). At an increased concentration of HA (> 20 mg/dm3) in a solution a coprecipitation of them occurs an iron hydroxides concurrently with an increase of a sediment volume, however magnetic properties of sediments are retained. This is an indication, perhaps, of an occurrence of two parallel precipitation processes: coagulation and ferritization.

Fig. 4. (Contd.)

0

20

40

60

0 100 200 300 400 500

СU(VI), μmol/dm3

σs, A m2/kg

0

20

40

60

0 100 200 300 400 500CSr(II), μmol/dm

3

σs, A m2/kg

(c) (d)

20

40

60

80

100

0 0.02 0.04 0.06 0.08 Can, mol/dm3

CO, %

4

1

2

3

Fig. 5. Influence of the concentration of anions: Cl4– (1), SO4

2–(2), CO32–(3), EDTA (4) on the water purification degree of

U(VI) when precipitating with the NaOH solutions.



Physical�chemical processes taking place during precipitation of salts of Fe(II) and Fe(III) in an alkaline medium sufficiently fully can be characterized based on the data of an X�ray analysis. An analysis of powder diffractograms of ferrite sediments obtained under optimal conditions of water purification of U(VI) com�pounds using NH4OH and NaOH showed virtually complete identity of diffractions bands for various alkaline reagents. On diffractograms one can observe the most characteristic peaks in the region of the angles 2Θ from 30 to 50� corresponding to ferrite, which may be identified by the most intensive diffraction reflection with the interplanar distance equal to 0.25 nm and less intensive—0.29; 0.21 nm.

Fe (II)

0 20 40 60 80

100

2 4 6

Fe2+

Fe L2–

mol, %

pH0

20

40

60

80

100

2 4 6 8 10 pH

mol, %

Fe(OH)

FeL–

Fe (III)

(a) (b)

Fig. 6. Distribution of the Fe(II) (a) and Fe(III) (b) forms in the presence of EDTA at various values of the pH (CEDTA = 0.01 M; logKFe(II)�EDTA = 14.33; logKFe(III)�EDTA = 25.1; log β(H4EDTA) = 21.40).

0

10

20

30

40

50

60

70

0 20 40 60 80 100 CHA, mg/dm

3

Vsed, cm3/dm

3

0

20

40

60

80

100

CO, %

1

2

Fig. 7. Influence of the HA concentration of the water purification degree of uranium (1) and the ferrite sedimentation volume (2) when precipitating with the NaOH solution. UFe = 150 mg/dm3; K = 6.0; pH 10.5.

75 70 65 60 55 50 45 40 35 30 25 20 15 10 5

1

2

2Θ

1.4839 1.6153

1.7083 2.098

2.532

2.977 4.912

2.098

2.532

2.969 4.860

1.4754

1.7107

1.610 1.0629

Fig. 8. Diffractograms of sediments obtained when precipitating iron salts with the NH4OH (1) NaOH (2).



*Purification results were obtained and reproduced by the authors of the present paper.

Thus, the analysis conducted of the phase composition of sediments showed that under steady conditions the sediment being formed is ferrite (Fe3O4). It possesses magnetic properties virtually right after precipita�tion; the formation of FeO(OH) is not observed. Obtained diffraction reflections have characteristically expressed narrow peaks, which indicates high crystallinity of obtained samples.

For comparative assessment of the efficiency of the ferrite method with other ones proposed for purifica�tion of large volumes of radioactively contaminated natural and waste waters we provide main characteristics of some methods for water purification of the U(VI) and 90Sr compounds described in the literature and ver�ified experimentally (see the table).

From the data quoted above one can see that some methods are not reasonable to be used for purification of large volumes of radioactively contaminated waters due to their low capacity and high costs [8], formation of large volumes of water�bearing sediments complicating the technology of their separation and processing [3, 4, 5, 9]. At the same time the research conducted demonstrated that the formation of ferrite sediments using Fe(II) and Fe(III) salts occurs at the concentration slightly exceeding such during coagulation treat�ment, however, the purification degree, which is achieved, is much higher. In addition, the sediment volume is small and compact, it easily lends itself to separation and subsequent processing; the salt content of the water does not substantially change since the bulk of the salts introduced is removed in the form of a sludge with magnetic properties, which is conducive to the increase of strength, fall velocity, decrease of the possibility of desorption with the change of the pH. It should be taken into account that the given method may be used as an independent reagent method for natural water treatment and not only for wastewaters of hydraulic produc�tions combining it with utilization of liquid wastes of production.

REFERENCES

1. Bronic, J. and Subotic, B., Radioanal. And Nucl. Chem. Art., 1992, vol. 162, no. 2, pp. 339–350.2. Defilipps, I., Yates, S., Sedoth, R, and Straszewski, Separ. Sci. and Technol., 1997, vol. 32, no. 1/4, pp. 93–113.

Comparative characteristic of some methods of water purification of 90Sr and U(VI)

MethodPurification degree, %

Note Ref.90Sr U(VI)

Sorption 26.7 43.8 Montmorillonite (1 g/dm3) [3]

– 19.8 Water of the river Dnieper; montmorillonite (1 g/dm3)

– 15.0 Magnetic concentrate (1 g/dm3)* –

– ≤ 95.0 Active sludge [4]

60.0 – Carbonates (4 g/dm3) [5]

90.0 – Red sludge

72.2 – Glaukonite concentrate (50 g/dm3) [7]

88.5 – Kaolinite – white alluvial clay (50 g/dm3) [8]

≤ 20.0 5–7 Synthetic magnetite (>200 mg/dm); pH 6.6 [11]

Coagulation 3.0 82.1–85.2 Water of the river Dnieper; Al2(SO4)3 (100 mg/dm) [3]

3.0 89.0 Mine water; FeCl3 (20.0 mg/dm)* –

Sorption–coagulation 22.4 99.6–99.8 Water of the river Dnieper; Al2(SO4)3, montmorillo�nite (by 100 mg/dm of every component)

[3]

Soda–lime – 98.3 CaO (800 mg/dm3)*; saturation of model mine water with CO2 and alkalization of NaOH to pH 7

–

Ultrafiltration–complex�ation

– 99.8 Polyethylene imine (50 mg/dm3); pH 5 [8]

Chemical coprecipitation with CaCO3, CaC2O4and Ca3(PO4)2

60.0 – CaCO3; = 0,01 mol/dm3, pH 10 [9]

Ferrite method 80.0 100 *K = 6,0; pH 10.5 –

CCa2+



3. Goncharuk, V.V., Kornilovich, B.Yu., Pavlenko, V.M., Babak, M.I., Pshinko, G.N., Pismenyi, B.V., Koval’chuk, I.A.,and Safronova, V.G., Khimiya i Tekhnologiya Vody, 2001, vol. 23, no. 4, pp. 410–418.

4. Kornilovich, B.Yu., Gvozdyak, O.I., Pshinko, G.N., Spasenova, L.N., Koval’chuk, I.A., and Safronova, V.G., ibid., 2001, vol. 23, no. 5, pp. 545–551.

5. Kornilovich, B.Yu., Spasenova, L.N., Kosorukov, A.A., Pshinko, G.N., and Mas’ko, ibid., 1992, vol. 14, no. 1, pp. 48–52.

6. Ryzhov, B.I., Bogatyrev, B.A., and Shikina, Geoekologiya, 1996, no. 4, pp. 50–57.7. Tret’yakov, S.Ya., Radiokhimiya, 2002, vol. 44, no. 1, pp. 89–91.8. Kornilovich, B.Yu., Koval’chuk, I.A., Pshinko, G.N., Tsapyuk, E.A., and Krivoruchko, A.P., Khimiya i Tekhnologiya

Vody, 2000, vol. 22, no. 1, pp. 66–73.9. Zabrodskii, V.N. and Prokshin, N.E., ibid., 1998, vol. 20, pp. 317–324.

10. Goncharuk, V.V., Radovenchik, V.M., and Gomelya, M.D., Otrymannya ta vykorystanna vysokodispersnykh sorbentiv z magnitnymy vlastyvostyamy (Obtaining and Use of High�Disperse Sorbents with Magnetic Properties), Kiev: 2003.

11. Shut’ko, A.N., Radovenchik, and Gomekya, N.D., Khimiya i Tekhnologiya Vody, 1994, vol. 16, no. 1, pp. 58–61.12. Topkin, Yu.V., Roda, I.G., Anfinogenov, N.V., and Prishchep, N.N., ibid., 1990, vol. 12, no. 10, pp. 895–897.13. Filinovskii, V.Yu., Nikol’skaya, T.Yu., and Shevchenko, V.K., Ekologiya i Promyshlenost’ Rossii, 1998, June, pp. 4–8.14. Elmore, W.C., J. Phys. Rev., 1938, vol. 54, no. 4, pp. 309–310.


purification of radioactively-contaminated waters of 90sr and u(vi) by a ferrite method

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