Water deactivation by reverse osmosis

Water deactivation by reverse osmosis

DESALINATION Desalination 157 (2003) 403~407 ELSEVIER www.elsevier.com/locate/desal Water deactivation by reverse osmosis Antonina R Kryvoruchko*, ...

341KB Sizes 0 Downloads 43 Views

DESALINATION Desalination 157 (2003) 403~407

ELSEVIER

www.elsevier.com/locate/desal

Water deactivation by reverse osmosis Antonina R Kryvoruchko*, Boris Yu. Kornilovich Institute of Colloid and Water Chemistry, National Academy of Sciences, pr. Vernadskogo, 42, 03142 Kyiv, Ukraine Tel. + 38 (044) 452 55 49; Fax +38 (044) 452 02 76, email: [email protected]~net

Received 20 December 2002; accepted 30 December 2002

Abstract

The method of reverse osmosis was used to investigate the effects of inorganic and organic additives on the treatment of waters contaminated with cesium-137 and strontium-90. Sodium chloride was used as an inorganic additive while polyacrylamide was used as an organic additive. Treatment was carried out using an OFAM composite reverse osmosis membrane. It has been shown that the introduction of sodium chloride into the system treated results in worsening of the purification index (the retention factor for ions of cesium-137 and strontium-90) used by the membrane while the introduction of polyacrylamide improves the treatment of water contaminated with radionuclides. In addition, as polyacrylamide concentration increases the OFAM membrane coefficient of retaining radionuclides of cesium-137 and strontium-90 increases as well. The value of the transmembrane flux with the increase in the concentration of the polyelectrolyte decreases in the case of treatment of waters contaminated with strontium-90. Keyu~ords: Membrane process; Reverse osmosis; Radionuclides; Sodium chloride; Polyacrylamide

1. I n t r o d u c t i o n

Operation o f atomic power plants, a number o f other factors lead to radioactive contamination o f the water basin and soils [1,2]. Depending on the source o f contalnination, isotopes o f various elements (cesium, strontium, zirconium, rareearth e l e m e n t s , etc.) may penetrate into the *Corresponding author.

environment. Based on the available data on the physico-chemical state o f radioactive isotopes o f the e l e l n e n t s in the w a t e r m e d i u m it was established that cesium within a wide range o f the pH exists in a c a t i o n i c f o r m w h i l e for strontium, in addition to the ion and ion-disperse form, occurrence in the form o f complex compounds is very probable [3,4]. Zirconium, cerium, niobium and other elements may be in water both

Presented at the European Conference on Desalination and the Environment: Fresh Water for All, Malta, 4-8 May 2003. European Desalination Socie~, International Water Association.

0011-9164/03/$- See front matter © 2003 Elsevier Science B.V. All rights reserved PII: S0011-9164(03)00423-5

404

A.t~ Kryvoruchko, B. Yu. Kornilovich /.Desalination 157 (2003) 403-407

in the ionic and molecular states and in hydrolyzed and colloid forms and in addition in the form of various complexes while ruthenium substantial complexing ability determines even a greater variety of possible forms [5]. Experimental consideration of distribution of radionuclides among different possible states in surface waters shows that the bulk of cesium- 137 is transferred in the form of positive ions although its substantial amounts may firmly sorb on solid particles and accordingly transferred by them. The main quantities of strontium-90, however, in fresh waters most likely are transported in the ion form, while in waters of seas and oceans - - i n the iondisperse state. Transport of cerium is brought about in the form of true colloids, which represent hydrolysis products of this element in different degree of polymerization. Trace quantities of zirconium, niobium and ruthenium are characterized by a migration in the form of negatively-charged highly-dispersed colloids of hydroxides [6]. The nature of radioactive contamination, volumes of waters requiring treatment, needed concentration of radionuclides in the effluent determines the choice of the required methods of water treatment ensuring achievement of the above goals. As is known the bulk of radiocontaminations that are in the colloid state may be removed by coagulation followed by filtration [7,8]. Complete purification of waters from radionuclides found in the ionic state is achieved using ion-exchange filters [9]. However the use of ion exchange has a number of substantial shortcomings: large amounts of regeneration and rinse w a s t e w a t e r s f o r m e d and m a i n t e n a n c e and management of large reagent facilities, etc. The above setbacks in the known methods of water deactivation may be eliminated by the use of membrane technologies of water treatment concurrently with concentration of radiocontaminations in limited volumes [10-13]. Membrane (electro- and baromembrane) technologies of removing radionuclides may be used for tertiary water treatment of natural ponds, cooling ponds

of nuclear power plants, wastewaters of nuclearwaste processing plants, deactivation shops. Therefore the objective of the present paper is the investigation of the influence of an inorganic component (sodium chloride) and an organic component (polyacrylamide (PAA)) of different concentrations on deactivation using reverse osmosis of water containing 137Cs or 9°Sr.

2. Experimental For clarification of the possibility of water purification from radionuclides by reverse osmosis and determining the impact on the system of the introduction into the system of inorganic and organic components a number of experiments have been performed for deactivation of water contaminated with radionuclides ~37Cs and 9°St in a mixture with sodium chloride and PAA. A reverse osmosis membrane OFAM manufactured by Vladipor, Russia was used as a separating partition. The initial activity of the water under investigation created by cesium-137 was 1.56x t 0-7 CtdL and that by strontium-90 - - 6.71x10 -7 Cu/L. Sodium chloride with concentration 1 g/L (in all experiments) and PAA with concentration 5 and 25 mg/L were introduced into the water being purified. The experiments were conducted in a noncirculating cell 1 L in volume with the membrane area of 95 cm 2 and the rotation speed of the magnetic agitator of 300 rpm and operating pressure 200 kPa. The volume of the solution studied was 0,5 L, the samples of 10 mL were taken. Ion concentration (activity) was determined by [3-radiometer. Based on the results obtained the retention factor (R) for 137Caand 9°Sr by the OFAM membrane and the value of the transinembrane flux through it ( J ) were determined. The impact of sodium chloride and polyacrylmnide on retention ability of the membrane under study during the deactivation process of water using the method of reverse osmosis was determined.

405

,4. P. Kryvoruchko, B. Yu. Kornilovich / Desalination 157 (2003) 403-407

3. Results and discussion

Fig. la shows the results wlfich indicate the impact of sodium chloride (curve 1), sodium chloride and polyacrylamide with concentration 5 mg/L (curve 2) and sodium chloride and polyacrylamide with concentration 25 mg/L on the process of purifying the solution containing ions of cesium-137. The retention factor for '~7Cs ions by the OFAM membrane was 0.68 and in the case the system was separated by reverse osmosis containing cesium-137 and sodium chloride. In the second case when the ~37Cs-NaC1 system was topped up with polyacrilamide with concentration 5 rag/L, the degree of solution purification increased which is confirmed by R = 0.74. A further increase of the concentration of PAA (up to 25 mg/L) introduced into the system did not lead to substantial improvement of the purification process since the retention factor increased by 0.3% compared with the previous result and equaled 0.78. It should be noted that the purification degree of the solution containing cesium-137 without any additives was characterized by the retention factor o f 0.81 previously determined experimentally. Obviously, ionic strength o f the solution created by the introduction of sodium chloride into the system being treated rather strongly affects the process (a)

of membrane separation, which is indicated by the given results. The introduction o f a PAA complexing agent into the system in sufficient amounts did riot rule out the impact o f sodium chloride. At the same time the introduction of PAA into the system affects the change of the transmembrane flux (Fig. 2b). The increase of polyacrylamide concentration does not lead to a further decrease of the transmembrane flux (Fig. 1, curves 2 and 3). These phenomena, most likely, are related with the formation of the OFAM membrane o f a dynamic membrane from PAA, which leads to the decrease of the transmembrane flux (from 1.51 to 0.90 gm/s) and simultaneously to the increase (though small) of the retention factor (Fig. 1). As for the results obtained when purifying an aqueous solution containing 9°Sr (Fig. 2), they indicate that the introduction of sodium chloride into the system being treated does not worsen the treatment process using reverse osmosis. Compared with the result obtained earlier (R = 0.86) when purifying the aqueous solution containing only strontium-90, the result after the introduction of sodium chloride into the system was somewhat better (Fig. 2a), The retention factor when introducing sodium chloride (Fig. 2a, curve 1) was 0.86, addition of 5 mg/L of PAA into the previous 3

J 3

(b)

0.8

1

2

0.7

I~ 0.6 0.5 0.4

-

-

-

-

-

i

20

40

t, rain

60

80

-

a

100

0

20

40

60

80

100

t, rain

Fig. 1. Variationof the retention coefficientof 137Csby the OFAM membrane(a) and the volume flux (b) during deactivation process in the absence of the PAA (1) and its presenceat its differentconcentrations,5 ppm (2) and 25 ppm (3).

406

A.t~ Kryvoruchko, B, Yu. Kornilovich / Desalination 157 (2003)403 407

11 (a)

2

(b)

3

i

i' 0.7 0.6 0.5,

3

0.9 0.8

0

20

40

60

80

"~

~

2

0

100

-

0

-

20

t, min

i

r

r

40

60

80

-

-

~

100

t, min

Fig. 2. Variation of the retention coefficient ofg°Sr by the OFAM membrane (a) and the volume flux (b) during deactivation process in the absence of the PAA (1) and its presence at different concentrations: 5 ppm (2) and 25 ppm (3).

system increases the retention factor to 0.90 (Fig. 2a, curve 2). A further increase o f the concentration o f PAA introduced (up to 25 mg/L) does not lead to the change in the OFAM membrane retention factor for 9°St ions since this indicator, as in the previous case, also equals 0.90 (Fig. 2, curve 3). The transmembrane flux in this case varies from 1.39 lam/s to 0.93 iam/s (Fig, 2b, curves 1-3). The introduction o f PAA into the system being treated leads to a pronounced decrease in the value o f the transmembrane flux. An increase ofpolyelectrolyte concentration leads to a further decrease o f the transmembrane flux.

Perhaps, this takes place due to the fact that PAA macromolecules formed by the dynamic membrane on the surface o f the OFAM substrate membrane slightly cover the opening o f the pores. Therefore, the transmembrane flux diminishes too although it does not affect the OFAM membrane retention ability. Table 1 gives generalized results characterizing the logic o f the treatment process o f waters contaminated with radionuclides ~37Cs and 9°Sr using reverse osmosis. The data are also given on the influence exerted on this process by sodium chloride and polyacrylamide.

Table l Principal characteristics of the reverse osmosis process of water deactivation from 137Csand 9°Sr by membrane OFAM Initial solution activity, Cu/L

NaC1 content, g/L

90St

137Cs 1.56×10 7 1.56x 10_7 1 . 5 6 : < 1 0 -v 1.56×

10 -v

6.71xl0 6.71×10 6.71×10 6.71×10

7 7 v v

PAA content, mg/L

Retention coefficient

Volume flux,

--

--

1

--

1 1 --

5 25 --

1

--

1 1

5 25

0.81 0.68 0.74 0.78 0.86 0.86 0.90 0.90

1.51 1.48 1.09 0.90 1.39 1.33 1.23 0.93

~I,1TI/S

.4.I~ Kl"yvorztchko. B. )'7t. Kornilovich / Desalination 157 (2003) 403-407

4. Conclusions It has been shown that the method o f reverse osmosis is effective for purification o f wastewaters from radionuclides. The impact o f sodium chloride and polyacrylamide on trausmembrane transport o f radiocontaminants ~-~VCsand 9°St has been determined. Obtained relationships in variations in the O F A M m e m b r a n e r e t e n t i o n f a c t o r f o r ioncontaminants and variations in the volume flux through it over time and at different values o f PAA concentration and at NaCI fixed concentration indicate that the ionic strength o f a solution created by sodium chloride worsens the treatment process. The introduction o f the polyelectrolyte leads to the increase o f the retention factor for radionuclides. The transmembrane flux decreases when p o l y a c u l a m i d e is added.

References [1] J.A. MacDonald, Evaluating natural attenuation for groundwater cleanup, Environ. Sci. Technol., News, 34 (2000) 346A-353A. [2] A. Rigol, M. Roig, M. Vidal and G. Rauret, Sequential extractions for the study of radiopcesium and radiostrontium dynanlics in mineral and organic soils from Western Europe and Chernobyl Areas, Enviro. Sci. Technol., 33 (1999) 887-895. [3] E. Broune and R.B. Firestone, Tables of Radioactive Isotopes, Wiley lnterscience, New York, 1986.

407

[4] K.H. Lieser, A. Amend, R. Hill, R.N. Singho U. Stingl and B. Thybusch, Colloids in groundwater and their influence on migration of trace elements and radionuclides, R~/diochim. Acta, 49 (1990) 83-100. [5] J. Buffie, Complexation Reaction in Aquatic System: an Analytical Approach, Wiley lnterseience, New York, 1988. [6] P.N. Linnik and B.I. Nabivanets, Fonns of Metals Migration in Natural Waters, Gidrometeoizdat, Leningrad, 1988 [in Russian]. [7] L.A. Kulsky, Basics of Water Chemistry and Technology, Nauk. Dumka, Kiev, 1991 [in Russian]. [8] R.G.Castle, Radioactivity in water supplies, J. Water Environ. Manag., 2 (1988) 275-284. [9] V.D. Grebenyuk and A.A. Mazo, Desalination of Water by Ionites, Khimiya, Moscow, 1980 [in Russian]. [10] A.P. Kryvoruchko, M.1. Ponomarev, B.Yu. Kornilovich and A.N. Masko, Decontamination of lowmineralized radioactively contaminated water by the electrodialysis method, J. Water Chem. Technol., 18 (1996) 36-38. [11] R Huikuri, L. Salonen and O. Raft, Removal of natural radionuclides from drinking water by point of entry reverse osmosis, Desalination, 119 (1998) 235-239. [12] B.F. Smith, R.R. Gibson, G.D. Jarvinen, T.W. Robison, N.C. Schroeder and N.D. Stalnaker, Preconcentration of low levels of americium and plutonium from wastewater by synthetic water soluble metal-binding polymers with ultrafiltration, J. Radioanal. Nuclear Chem., 234 (1998) 225-229. [13] E. Gaubert, H. Barnier and A. Maurel, Selective strontium removal from sodium nitrate aqueous medium by nanofiltration-complexation, Separ. Sci. Technol., 32 (1997) 535-557.