treatment of radioactively contaminated waters with an increased content of salts

ISSN 1063�455X, Journal of Water Chemistry and Technology, 2009, Vol. 31, No. 1, pp. 46–52. © Allerton Press, Inc., 2009.Original Russian Text © T.G. Timoshenko, A.A. Bogolepov, G.N. Pshinko, 2009, published in Khimiya i Tekhnologiya Vody, 2009, Vol. 31, No. 1, pp. 78–88.

WATER TREATMENT AND DEMINERALIZATION TECHNOLOGY

Treatment of Radioactively Contaminated Waters with an Increased Content of Salts

T. G. Timoshenko, A. A. Bogolepov, and G. N. PshinkoDumanskii Institute of Colloid and Water Chemistry,

National Academy of Sciences of Ukraine, Kiev, UkraineReceived November 27, 2007

Abstract—The article gives a comparison characteristic of purifying highly mineralized radioactively con�taminated mine waters by different methods. It is shown that the use of the coagulation method does not ensure purification to the MAC standards, while the sorption–coagulation method requires additional conditioning of spent sludge. For complex purification of mine waters (of the uranium compounds and partially of hardness salts) the reagent�magnetic method was proposed with the use of iron(II) and (III) salts, alkaline reagent CaO and natural magnetite.

DOI: 10.3103/S1063455X0901007X

A negative outcome of the operation of uranium�extracting and production enterprises is inevitable pollu�tion of the environment. At such enterprises in the case of the extraction and processing of ore much volumes of water are spent and as a result liquid radioactive wastes (LRW) are formed, the so�called mine waters char�acterized with high concentration of mineral and organic compounds in the form of true solutions, colloids, or suspensions and containing natural radionuclides U, 226Ra, 230Th, 210Po, 210Pb. Among them the most complex for removal is U(VI). It is U(VI) that makes the main contribution to the total α�activity of LRW, where its share constitutes up to 90%. A specific feature of U(VI) is high capacity to complexation and, as a result, its large mobility in the environment determining the propagation of radioactive contamination at a large distance. Therefore an important and priority task is the necessity of developing effective and rational methods of purifying waste highly mineralized mine waters.

At present for purification of radioactively contaminated waters of uranium compounds sorption, coagu�lation, coagulation–sorption, membrane methods and liming ones are used on a wide scale [1–4]. Taking into account contamination features of mining waters, for their purification we need an effective method of com�plex removal of impurities ensuring simultaneous reduction of not only radioactivity, but also mineralization of these water (hardness).

The objective of the present paper is the development of the methods of purifying mine waters contami�nated with natural radionuclides having an increased salt content allowing for their physico�chemical fea�tures.

EXPERIMENTAL

The investigations were carried out on mine water of the Eastern Mine–Dressing Works (Zhovti Vody, Ukraine) having the following composition:

Na+,mg/dm3…………………………………….............241.0

K+ , mg/dm3………………………………………………..9.0

Ca2+ , mg/dm3……………………………………………..170.0

Mg2+, mg/dm3……………………………………………..53.4

Fetot mg/dm3…………………………………………..……1.75

Al3+ , mg/dm3……………………………………………...0.12

NH4+ , mg/dm3…………………………………………….0.45

HCO3–,mg/dm3……………………………………………234.5

46

TREATMENT OF RADIOACTIVELY CONTAMINATED WATERS 47

CO32–mg/dm3……………………………………………....28.8

Cl–, mg/dm3……………………………………………......277.5

SO42–, mg/dm3……………………………………………..467.0

Hardness, mg�eq/dm3………………………………….....15.6

COD, mg O/ dm3…………………………………………..58.8pH…………………………………………………………...... 7.95

Total content, g/dm3…………………………..…………..1.5

Uranium, mg/dm3…………………………………………..0.85

Radium 226, Cu /dm3 × 10–11 (Bq/dm3)……………..1.485 (0.55)

Thorium 230, Cu /dm3 × 10–11 (Bq/dm3) ……………0.745 (0.28)

Polonium 210, Cu /dm3 × 10�11 (Bq/dm3) ………….1.36 (0.50 )

Lead 210, Cu /dm3 × 10–11 (Bq/dm3) ………………....2.50 (0.93)

Uranium in the amount ≈10 mg/dm3, which corresponded to the composition of industrial waters of bulk and unit leaching and, in addition, made it possible to reliably determine by spectrophotometric method the uranium concentration after cleansing.

When preparing initial solutions of uranium the salt UO2SO4 ⋅ 3H2O was used, while for initial salts of coagulants—FeSO4 ⋅ 7H2O and FeCl3 ⋅ 6H2O. Coagulation was carried out according to the following tech�

nique: cylinders with the volume 250 cm3 filled with mine water a coagulant was introduced at different con�centrations, stirred, settled to the formation of flocks and settlement of the suspension. After complete clari�fication of the solution and decantation the effluent was analyzed for the content of uranium by the photo�metric method using arsenazo III at λ = 656 nm [5]. The amount of the coagulant was calculated by waterless salts. In the coagulation–sorption experiment concurrently to the introduction of the coagulant a weighted amount of a dry sorbent was added (montmorillonite of the Cherkassy deposit, fraction ≤ 0.25 mm).

The degree of purification (DP), % was calculated by the formula

,

where C0 and Ceq are initial and equilibrium concentrations of uranium, mg/dm3.

For settling uranium using a ferrite method a technique of obtaining a highly disperse deposition of ferrite with magnetic properties from a blend of iron salts (II) and (III) was used at the room temperature [6]. The initial concentration of the solution of iron constituted 10.0 g/dm3, precipitation was conducted by a 1 M solution of NaOH and suspension CaO at constant stirring of the sample on a magnetic stirrer; the pH value was corrected by means of an EV�74 universal ionometer with a glass electrode ESL�43�07.

To reduce the salt content of mine water lime was used whose amount was calculated theoretically and was confirmed experimentally based on water hardness. A suspension of lime milk was added to the water sample after introduction of the coagulant and sorbent. The sample was stirred, settled until complete sedimentation of the suspension, decanted and analyzed for the content of U(VI). Determination of some indicators of water quality after purification (alkalinity, hardness, etc.) was conducted according to [7].

RESULTS AND DISCUSSION

Purification of Mine Waters by Coagulation

Paper [3] demonstrated a fundamental possibility of purifying natural river water of uranium ions by the coagulation method. Aluminum sulfate was used as a coagulant. However, it is known that nearly all radioac�tive isotopes of the actinoid series are removed by iron hydroxide more effectively than by aluminum hydrox�ide, more so, that the pH value of initial mine water constitutes 7.95, which constitutes the pH of coagulation of iron salts (III). In addition, the indicated pH value is maximum for precipitation of uranium hydroxide therefore for purification of mine waters by the coagulation method it is more expedient to use iron salts (III) [2].

COC0 Ceq–

C0

�� 100×=

JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY Vol. 31 No. 1 2009

48 TIMOSHENKO et al.

As can be seen from Table 1 during purification of mine waters using iron (III) chloride the maximum degree of purification (98.7%) at the coagulant dose 100 mg/dm3 was achieved. This purification degree is much higher than the one obtained by the authors of [3] using aluminum sulfate as a coagulant, however in the given case too the residual content of uranium in water (140 μg/dm3) exceeds MAC (1 Bq/dm3), which corresponds to the mass concentration 40 μg/dm3 at a typical natural ratio of uranium isotopes [8]. It should also be taken into account that in the course of coagulation apart from the formation of hardly soluble hydrox�ides alkalinity of the water decreases; this alkalinity is determined mainly by hydrocarbonate–ions. Thus, the coagulant FeCl in the amount > 100 mg/dm3 results in a decrease of the decree of water purification, which is determined by a decrease of water alkalinity reserve (see Table 1).

Thus, the use of the coagulation method of purifying by means of FeCl3 does not allow us to achieve the required water quality benchmarks.

Purification of Mine Waters by the Combination Coagulation–Sorption Method

As is known, the efficiency of the coagulation purification method may be raised by the introduction of mineral natural sorbents–alumosilicates [3]. When treating water by the given method we used the following order of introducing reagents: at the first stage Fe(III) salts were introduced, at the second one—montmoril�lonite. During the introduction of the coagulant the hydrolysis process of Fe (III) and U (VI) occurs, then—their coprecipitation. Residual equilibrium cationic form of uranium effectively sorb by the introduced mont�morillonite and after coagulation the optimal value of the pH sorption of uranyl–ions [3] sets in, however, it should still be noted that the bulk of uranium coprecipitates with the coagulant.

Table 2 gives the data of coagulation–sorption purification of water with an increased salt content.

Note: Initial concentration U(VI)—10 mg/dm3, concentration of FeCl3—100 mg/dm3.

Thus, the introduction of the sorbent after the coagulant ensures a high degree of purification (the residual content of uranium is below MAC for the water of household�drinking purpose) due to the removal of residual cationic forms of uranium after coagulation.

Table 1. Impact of the concentration of the coagulant FeCl3 on the purification degree of water with a increased salt content of the U(VI) compounds

FeCl3,

mg/dm3 pHAlkalinity,

mg�eq/dm3CU(VI) in solution after

purification, mg/dm3Purification degree,

%

0 7.85 4.24 10.4 –

20 7.45 3.24 9.99 3.9

40 7.25 2.79 4.76 54.2

80 6.62 1.51 0.77 92.6

100 5.80 0.59 0.14 98.7

150 3.60 0 7.48 28.1

Table 2. Impact of montmorillonite amount on coagulation–sorption water purification of U(VI)

Montmorillonite, mg/dm3 Equilibrium uranium concentration, mg/dm3 Purification degree, %

0 1.18 88.2

100 0.77 92.3

200 0.68 93.2

300 0.55 94.5

500 0.13 98.7

700 0.01 99.9

1000 0.01 99.9



Similar results were achieved by the authors of [3] during the joint use of aluminum sulfate and montmo�rillonite for natural river water. An increase of the quantity of the sorbent in the given case (up to 700 mg/dm3

compared with 100 mg/dm3) [3] is determined by a high salt content of mine water. It should be noted that even at an achieved degree of purifying radioactive sludge which form as a result of both coagulation and coag�ulation–sorption purification have a high moisture content and require additional conditioning for subse�quent processing prior to utilization. This is especially important during the purification of mine waters in which due to a high salt content the volumes of precipitations will be much higher than when purifying natural waters.

Reduction of Salt Content of Mine Waters by Liming

The presence in mine water of anions and cations of inorganic and organic origin of a high concentration affects the state of radionuclides and, consequently, the degree of purification and the choice of its methods.

Earlier Goncharuk, et al. [3] studied the effect of anions on the sorption of U (VI) and showed that chlo�ride– and nitrate–ions at low concentrations virtually do not affect, and at elevated concentrations, are even conducive to the formation of well sorbed cationic complexes. At the same time the presence of sulfate– and acetate–ions (a representative of low�molecular organic acids) reduces the sorption value due to the forma�tion of neutral and anionic complexes. According to [9] for natural water coagulation is intensified mainly by the ions SO4

2– and HCO3– and for aluminum sulfate the impact of the anionic composition of the water is

stronger than for chloride iron. The presence of basic cations to a lesser degree affects the coagulation process.Based on the above for complex purification of mine waters after clarification and water purification of ura�

nium by means of mineral sorbents and coagulants an important task is reduction of the salt content. Taking into account a rather high value of water hardness the possibility of partial removal of calcium salts by the lim�ing method often used in practice was investigated [1, 10]. The most spread technology of purifying mining water is based on extraction from the water of calcium carbonate and magnesium hydroxide by reagent soft�ening with lime milk (one of the cheapest reagents), in this case radionuclides are effectively removed mainly due to their coprecipitation with crystalline and amorphous sediments of hardness salts. However, in such technology the removal of uranium may be negatively affected by the complexation processes with carbonate–ions and organic components of the water since as a result there appear dissolved anionic carbonate forms of uranium. Organic matter of natural origin (humic and fulvic acids) substantially delay the process of forma�tion of crystals of calcium carbonate, while at a very high content of these substances (COD: 75–100 mgO/dm3) the formation of the sediment may terminate. In this case as a result of the so�called stabilizing effect of organic colloids hardness decreases only by 15–20%. To prevent similar processes it is necessary to carry out preliminary coagulation with salts of iron or aluminum. That is why, taking into consideration a high value of mine water COD its coagulation was carried out before its softening.

Table 3 presents the data for softening mine water by means of lime Ca(OH)2. As can be seen, when intro�

ducing ≥ 4 mg�eq/dm3 of slaked lime water hardness increases due to the appearance of the excess of calcium ions in the mine water being studied. Therefore water softening is expedient to be conducted without an excess of the reagent or even with its shortage. The degree of mine water purification of calcium salts using only slaked lime is rather low, which is explained by high noncarbonated hardness. Further additional softening of the water may be carried out by additional introduction of soda. When introducing 3 mg�eq/dm3 of lime and 10 mg�eq/dm3 of soda to the solution of model water the degree of water softening constitutes 77.8%, with an increase of a soda dose to 15.0 mg�eq/dm3—86.6%; residual hardness of water is achieved at the level 1.2 mg�eq/dm3.

Thus, the use of the soda–lime method of settling makes it possible to lower the total salt content of mine water. In this case radionuclides 226Ra and 210Pb will precipitate, these radionuclides form insoluble carbon�ates. However, the use of such a method for purifying mine waters containing uranium is not sufficiently effec�tive due to partial formation of soluble complex compounds of uranium with carbonate–ions when using soda.

Purification of Mine Waters by the Combination Precipitation Method of Purification

As was noted above an important feature of purifying mine waters is the necessity of removing not only radionuclides, but hardness salts. For solving this issue the possibility of complex purification of mine waters by the ferritization method was used; this method was widely used in purification of wastewaters of nonferrous metallurgy of the compounds of heavy metals [11, 12] as well by the reagent�magnetic method for removing



hard salts [13–15]. For the formation of sediments with magnetic properties normally NaOH and NH4OH [6, 13] are used as alkaline reagent. In our paper for achieving the pH optimal value of ferrite formation CaO as a cheaper reagent. The formation of ferrite is observed at the ratio KFe(II)/Fe(III) equal to 6–7,

ΣFe=150 mg/dm3 and pH 9.5 [6]. For model solutions on distilled water it was shown that the replacement of alkaline reagent by CaO does not affect the set ratio, magnetic properties of the sediment and its volume. However, when similar conditions of precipitation are observed in experiments with mine waters the formation of sediments with magnetic properties were not observed. For clarifying the reasons of such a phenomenon the impact of components of mine waters (anions and cations) on the process of forming the ferrite sediment within the limits of concentration corresponding to the composition of mine waters (Fig. 1) was used.

Note: *Concentration was chosen with the account of carbonate hardness of mine water.

As can be seen from the said figure the formation of ferrite sediments in optimal conditions of their forma�tion chloride– and sulfite–ions virtually is not affected even in large concentrations. At the same time carbon�ate–ions and calcium ions even in low concentrations negatively affect the formation of ferrite sediments low�ering their magnetic properties to zero. In addition, in the given area of the pH in the presence of carbonate–ions the formation of insoluble iron carbonate is possible, which prevents the formation of ferrite sediment (Fig. 2). Therefore the reagent�magnetic method was used: fine�crystalline magnetite in the amount 70 mg/dm3 [13] was added to mine water prior too the introduction of the reagent (a mixture of iron salts and CaO). Thanks to this the sediment in precipitation acquires magnetic properties.

Table 4 gives comparative characteristic of different methods of purifying highly mineralized radioactive mine waters under optimal conditions.

Table 3. Results of decarbonization of highly mineralized water by a solution of slaked lime

Ca(OH)2*, mg�eq/dm3 Total hardness, mg�eq/dm3 pH Softening degree, %

0 13.2 8.95 –

1.0 10.0 9.0 24.2

2.0 8.8 11.4 33.3

3.0 8.4 10.2 36.4

4.0 9.6 11.8 27.3

0

20

40

60

80

100

0 500 1000

Mag

neti

c pr

oper

ties

, rel

. uni

ts

1

4

3

2

Cions

, mg/dm3

Fig. 1. Impact of the concentration of ions on the formation of ferrite: 1— Cl–, 2—SO42–, 3—CO3

2–, 4—Ca2+ (pH 9.5; alkaline reagent—CaO.)



Table 4. Indicators of purifying river and mine waters of U(VI) (C0 = 100 μmol/dm3) by various methods

Purification methodPurification degree, % Sediment volume, cm3/dm3

river water mine water river water mine water

Sorption–montmorillonite (1.0 g/dm3)

19.8 [3] 2.7 2.0 2.0

Coagulation–Al2(SO4)3 (100 mg/dm3)

82.1[3] 75.7 48.0 60.0

Coagulation–FeCl3 (100 mg/dm3)

95.2 94.7 30.0 40.0

Sorption–coagulation–Al2(SO4)3,

montmorillonite (100 mg/dm3

of every reagent)

99.6* 86.7 36.0 40.0

Sorption–coagulation–FeCl3 (100 mg/ dm3), montmorillonite (700 mg/dm3)

100.0 99.9 22.0 28.0

Precipitation of CaO to pH 9.5 – 25.2 – 12.0

Coagulation of Fe(II) (150 mg/dm3)–precipitation of CaO to pH = 9.5

– 96.3 – 52.0

Coagulation of Fe(III) (150 mg/dm3)–precipitation of CaO to pH = 9.5

– 96.3 – 53.0

Coagulation of Fe(II) + Fe(III) (ΣCFe =150 mg/dm3, KFe(II)/ Fe(III) = 6)–precipitation of CaO to pH 9.5

– 96.5 – 53.0

0

20

40

60

80

100

3 5 7 9 11 pH

CO3

2–, mol %

FeCO3(sed.)

H2CO

3

HCO3

–

CO3

2–

CaCO3(sed.)

Fig. 2. Distribution of carbonate forms during purification of mine waters by the ferrite method, CFe(II) = 135 mg/dm3, con�centrations of Ca2+ and HCO3

– correspond to the composition of mine waters



CONCLUSIONS

As can be seen from the obtained data the highest degree of purification of uranium compounds may be achieved by the coagulation–sorption method using iron salts (II) and montmorillonite and also by the ferrite method with additional introduction to the solution of fine crystalline particles of natural magnetite—as cen�ters of sedimentation and crystallization of sediments of calcium carbonate and magnesium carbonate with acquired magnetic properties [11]. However, the use of the second method is more expedient since the vol�umes that form in the course of purification of sediments are substantially smaller as a result of the decrease of their humid content, in addition, the sediments possess magnetic properties and may be easily removed on magnetic filters. The use as a alkaline reagent of slaked lime makes it possible to simultaneously reduce salt content and thus to solve the problem of their complex purification.

REFERENCES

1. Pilipenko, A.T., Kompleksnaya pererabotka shakhtnykh vod (Complex Treatment of Mine Waters), Kiev: Tekhnika, 1985.

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7. Novikov, Yu.V., Lastochkina, K.O., and Boldina, Z.N., Metody opredeleniya vrednykh veshchestv v vode vodoemov (Methods of Determination of Harmful Substances in Water of Water Bodies), Moscow: Meditsina, 1981.

8. Normy radiotsionoi bezpeky Ukrainy (NRBU�97); Derzhavni gigienichni normatyvy (Standards of Radiation Safety in Ukraine (NRBU�97); State Hygienic Standards, Kiev: Viddil Poligrafii Ukr. Tsentru Derzhsanepidednaglyadu MOZ Ukrainy, 1997.

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10. Mongaite, I.L., Tekinidi, K.D., and Nikoladze, G.I., Ochistka shaktnykh vod (Treatment of Mine Waters), Moscow: Nedra, 1978.

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12. Filinovskii, V.Yu., Nikol’skaya, T.Yu., and Shevchenko, V.K., Ekologiya i Promyshlennost’ Rossii, 1998.13. Goncharuk, V.V., Radovenchik, V.M., and Gomelya, M.D., Otrymannya ta vykorystannya vysokodispersnykh sorben�

tov z magnitnymy vlastyvostyamy (Production and Use of Highly Disperse Sorbents with Magnetic Properties), Kiev: 2003.

14. Shut’ko, A.P., Radovenchik, V.M., and Gomelya, N.D., Khimiya i Tekhologiya Vody, 1990, vol. 12, no. 10, pp. 895–897.

15. Topkin, Yu.V., Roda, I.G., Anfinogenov, N.V., and Prishchep, N.N., ibid., 190, vol. 12, no 10, pp. 895–897.

Coagulation Fe(II) + Fe(III) (ΣFe = 150 mg/dm3, KFe(II)/ Fe(III)= 6)–precipitation of CaO to pH 9.5–natural magnetite (70 mg/dm3)

– 99.9 – 4.5

Table 4. Indicators of purifying river and mine waters of U(VI) (C0 = 100 μmol/dm3) by various methodsTable 4. (Contd.)


treatment of radioactively contaminated waters with an increased content of salts

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