Nuclear Science and Technology, Vol,5, No, 3 (2015). pp, 3 8 ^ 3
Using bentonite for NPP liquid waste treatment
Bui Dang Hanh
Vietnam .Atomic Energy Institute. 59 Ly Thuang Kiet. Hanoi.
Email bdhanhiu@gmail com (Recen ed 26 October 2015, accepled 15 December 2015)
Abstract: Dunng operation, nuclear power plants (NPPs) release a large quantity of water waste containing radionuclides required treatment for protection of the radiation workers and the environment This paper introduces processes used lo treat water waste from Paks NPP in Hungary and It also presents the results ofa study on the use of Vietnamese bentonite to remove radioactive Caesium from a simulated waier waste containing Cs
Keywords: lu/uidwa. e Cs. Vielnamese Benlonile. Pah NPP
I. INTRODUCTION Water waste from a NPP includes several sources, waste leaked from the primary cooling loop of a pressurized water reactor, aqueous solution used to decontaminate equipment installed on the primary loop and other sources.
Thiswaste usually is collected in storage system located at the NPP,
Radionuclides existed in the liquid waste include fission products like Cs-137, and activation products such as Co-60. Fe-59, Cr- 51, Mn-54, etc, [I A] In NPP with WWER-400 type - reactor m Paks (Hungary), the aqueous solutions used to decontaminate equipments contain chemicals such as alkaline, KMnO^.
EDTA, gluconic acid, etc [4] to form complexes with the radionuclides which easily
Destroying Complexes
Precipitation, filtration
solubilize into the decontamination solution.
The water waste with radionuclides collected has been stored for a certam time to decay Dunng this time period, the short lived radionuclides like Fe-59. Cr-51, and Mn-54 will decay completely, however the longer lived radionuclides, e.g, Co-60, Cs-137 in the solution still have a concentration well above the allowable discharge limit of 1 kBq/l and are not allowed to discharge into the environment even after several years of storage [4], Therefore, the waste is required to remove and concentrate Co-60, Cs-137 radionuclides in a small volume before discharging the waste into the environment
The technology for liquid waste treatment which applied t the Paks NPP comprises four stages as descnbed in fig 1
Cs Removal Crystallization of Borate Fig 1. Four stages of the technology of liquid waste trealment used at the P In the first stage, the underwater plasma
and H : 0 , techniques are used to destroy complex of radionuclides The pH of the liquid
^^aste IS adjusted to 13 by NaOH to keep boric acid in the solution Bone acid will be removed m the last slasc
Plasma m the system will degrade complexes of radionuclides with lagans in the decontamination solution. The radionuclides released from the complexes in the form of ions and it IS easily to remove by ion exchange and adsorption
1201 ? Vietnam Atomic Energy Society and Vietnam Atomic Energy insiituic
BUT DANG H.-WH After the destruction of eomplexes.
radionuclides such as Co-60. Fe-59. Cr-51, and Mn-54. etc at high pH condition will preeipitate/co precipitate with suspended matters m the solution and it will be removed by filtration. The filtration system mcludes coarse and fme filter to remove suspended particles with size of 0.1 - 1.0 (im as well as colloidal clusters.
The third stage is the Cs removal. The plant uses » specific ion exehange-rcsm to remove Cs radionuclide. The name of the resm is Fortum, BME, Termoxid-35 which capable of selective exchange for Cs ion.
The last stage is crystallization of borate.
Nirtc acid is added to the solution to decrease pH fi-om 13 to 9,2 - 9,3. When the pH of the solution decreases boron in the solution will precipitate as borate sodium (NajBjO,) Tbe borate precipitation is separated by filtration
Waste water, after removal of radionuclides and boric acid, could be released mto the environment (Fig.2)
hi this paper, Vietnamese bentomte was tested as an adsorption matenal lo replace the ion exchange resm used in the stage of the Cs removal, i.e. the third stage of the lechnology applied at the Pak NPP as descibed above
Commonly, Bentonile is type of naniral clay and the main componem is montmorillonite (MMT). Chemieal composition of MMT is AI,03.4SiO;.nH,0, where n ranges from to 8 However, composition of MMT is differeni fi-om its theoretical composition because Si'" ion could be replaced by ions Al'', Fe'", Fe'", M g " in SiO. and ion Al'" could be replaced by ions Fe'-, Fe'", Mg'" in AIO,. In literamres. it is eommonh agreed ihat MMT consists of nvo stmcmres one octahedral A f O , is in benveen nvo tenahedral SiO,. Bentonite has a senes of
39
outstanding physicochemical properties such as large specific area, high cation exchange capacity, and strong adsorptive affinity for organic and inorganic pollutants, low pemieabihty-. low cost, accessibility and ubiqmtous presence m most soils. With these advantages, bentonite is considered as one of the most promismg candidate as adsorption matenal for radioactive hquid waste tieatment.
II. EXPERIMENT.AL SECTION A. Preparation of Bentonite
Bentomte used in these expenments is supplied from mine m the Lam Dong Province The matenal was refined by dispersing hi distillation water with Bentomte- water ratio of 1-10. The mix was stined for 10 minutes then let setile for 2 hours. The suspended sluny was separated from the sediment in the bonom, and the sediment was discarded. The obtained sluny was centrifiiged at the speed of 10000 rounds /min for 15 minutes to separate solid from liquid. The solid fraction was dried at 80"C and then pulverized for funher use
B, Preparation of solution for the experiment Solution for the expenment was prepared based on the charactenstics of the real solution The most important characteristics of the real solution are its high concentration of boric and alkaline The conceno-ation of boric is 120 g/l and NaOH concenu-ation is 90 g/l. Chlonde caesium (CsCI. PA grade, Merck) was used to make solution with Cs ion.
C. Experiment for adsorption process Expenments on the adsorption of Cs on bentomte » e r e perfonned in batch. An exact amount of bentonite (from 0 1 to 0 5 grams) was put into Erienmeyer flasks of 250 ml capacity Then an exact volume of the solufion prepared was filled into the flasks (usually with
USIN'G BENTONITE FOR NPP LIQUID WASTE TREATMENT 100 ml). The mixture was shaken at a speed of
100 cycles/minute for a certain penod to reach equilibnum. After shaking, the mixture was centnfiiged at a speed of 10,000 rounds/min for 15 minutes to separate clear solution for Cs analysis on an atomic absorption spectrometer (AAS) to determine die Cs concentration remaining m the solution. The volume of solution was assumed to be unchanged after the adsorption.
The content of Cs adsorbed on the solid, q^. (mg/g) was calculated from the concentration of the solution as follows:
q. = ( C o - C ) V / G (1) Where
Co IS initial the solution concentration, mg/l;
C IS the solution concentration after adsorption, mg/l
V IS the voiume of the solution, I, G is the weight of Bentonite, g.
To determine the equilibrium time, the Erlenmeyers contaimng the mixture of bentomte with solutions were shaken for different interval of time (10 minutes, 30 minutes, 60 mmutes, 120 minutes and 180 mmutes and 6 hours). After each inter\'al of time the mixture was centrifuged to get solution for AAS analysis to determine the Cs concentration. The sorption was considered to be at the equilibnum if the Cs concentration in solution was unchanged.
III. RESULTS A N D DISCUSSIONS A. The Equilibrium time
Figure 2 depicted the change of Cs concentration in solution with time.
O"
Flg.2. The c
\ \
\ \
\
r—,"""'";^^-^
hange of Cs concentrahon in solution in different periods of lime
As seen from figure 2, the penod of 120 mmutes can be considered as the time to reach the adsorption equilibrium. This mterval of time IS apphed for the expenments of the adsorption of Cs on bentonite
B. Adsorption isotherms
To evaluate the adsorption ability of Cs on bentonite, Langmuir isothenn mode! and Where:
Freundlich isotherm model were used in the study Based on experimental data, the typical parameters of adsorption process were detennined by using linear regression method.
The Langmuir equation is given by
q, = q„KCA\+KC,) (2)
Linear form is C^q, = (l/q^)Ce + ] / ( M ^ ) (3)
BUI D.A.XG HANH q, IS the contentof ion adsorbed ( m e g ) conesponding to C,:
Ce IS the equilibrium concentration (mg I):
qm IS max adsorption capacity ( m g g ) : k. is adsoiption equilibrium constant.
The Freundlich equation is g n e n b\
<I. = K . C , " (4) Linear form is.
Inq, = (I nllnC.. - InK, A plot of C , q , versus C, should mdicate
a straight line of slope I /q„ and an intercept of \">here K.-and n are constants l'(k.q.,)
Table 1 presented Ihe equilibnum data of Cs in solution and on rhe adsorbent- bentomte Table I. Equilibnam da,a of Cs adsoq,ti„„ f™„ ,„ „ , „ „ o „ c „ „ a i „ , „ , H3B03 , | : 0 ,i,e I, ,,„d NaOH ,90
mg I) onto bentonile
Tie,}, The isotherm of Cs adsoqilici, oMoljenlonile from a solunon ol H-BO (120ragL)andNaOHl90mgL)msolidpha.se ' '
Figure 3 depicted the isothemi of the adsoiption of Cs from solution of H,BO, (120 mf'L) and NaOH (90 nig'L) onto bentonile.
Table II presented the results of the Table.l but was modified lo elucidate whether
lhe Cs adsorpnon follows the Langmuir or Freundlich models. Figure 3 and 4 showed Ihe relationship of C, and q, by the Langmuir (eq 3) and Freundlich (eq 5). respeclnely.
USING BENTONITE FOR NPP LIQUID WASTE TREATMENT Table IL The value of C^,qe, Inqe, InC,. that were derived liom the results of table I C,libr,.n,(C„mgn
Mci(mgCs/gBentoniie) (q„ mgCs/g)
C./qc Lnqe LnCe
9.13 0,87 10,49 -0,14 2,21
18,36 1,64 11,19 0,49 2,91
27,67 2,33 11,87 0,84 3,32
36,97 3,03 12.20 1,10 3,61
46,37 3,63 12.77 1,2S 3.84
56,34 3,66 15,39 1.29 4.03
N&ng dg can b^ng, Ce
Fig.4. Langmuir isotherm in linear form Fig.5. Freudhch isotherm in linear fonn
As seen from Fig,3 and Fig,4 the correlation coefficients (R") for the Freundlich model was higher (R-=0,988) than that for the Langmuir model ( R ' =0 871) suggesting that adsorption followed the former model.
Therefore the equation describing the equilibnum according to Freundlich model is given by.
Lnqe = 0,821 I n C , - 1,916
And the constants Kr and n can be calculated as follows*
InK, =-1,916 ^ K , = 0.147 l/n = 0,82l ^ n = 1,218 And the adsorpnon model is q = K r C ' " = 0.147C''"-'
IV. CONCLUSIONS The equilibrium time for the adsorption of Cs from the solution of boric acid 120g/l and NaOH 90g/l on the Lam Dong bentonite was examined and found to be 120 mm Tiie adsorption follows the Freundlich model wilh a maximum adsorption capacity of 4 mg/g,
REFERENCES
I K REKAB, C LEPEYTRE, M DUNAND, H2O2 and/or photocatalysis under UV-C irradiation for llie removal of EDTA, a chelating agent present in nuclear waste waters, Applied Catalysis A, GeneraL 488, pp.103-110,2014 H. OMAR, H , ARIDA, A , DAIFULLAH, Ad,sorpiion
of''"Co radionuclides from aqueous solution by raw and modified bentonite. Applied Clay Science, 44, pp 21 -26,2009,
S . K J I E G A M E D I E V , D . A NURBAEVA, N CTASHTEMIROVA. Modified benionite as adsorbents for radionulides: adsorption ii
BUI DANG HANH carrier-free radiocobalI-57 on acid modified and calcined bentomte, Intemational Conference "Nuclear Science and its Apphcanon", Samarkand, Uzbekistan, September 25-28, pp.345-346,2012.
IAEA, Modified Combined methods for liquid radioactive waste treatment, Fmal Report IAFA-TECDOC-1336, 1997-2001.
J DALE ORTEGO, M KOWALSKA, D . COCKE, Interactions of montmorillonite with orgamc compounds-adsorpnve and catalytic properties, Chemosphere, VOL 22, No 8, pp 769-798' 1991.
V.V. MiLVUIN. OA. KONOSENKO. Sorption of Cesium on finely dispersed composite ferrocyanide sorbenis. Radioehemisfry, VOL 52, No.3,pp 281-283,2010
Do Quy Son et al, Final Report on Scienhfic Topic of Instimte for Teclmology of Radioactive and Rare Elements, 2004 F.I Said. M B Hafez, Fixanon of radioactive isotopes on bentonile as radioactive waste tteatment step. Journal of Radioanalytical and Nuclear Cheimstry-, Vol. 241. No. 3, 643-645 1999
