Leaching Behavior of Cs and Radionuclides from Stabilized ... · PDF file*Professor,...

* Professor, Department of Radioactive Waste Management, Hot Laboratories and Waste Management Center,Inshas, Atomic Energy Authority, Egypt, P.O. 13759, (Corresponding author: E-mail: [email protected])

** Assistant Lecturer, Department of Radioactive Waste Management, Hot Laboratories and Waste ManagementCenter, Inshas, Atomic Energy Authority, Egypt, P.O. 13759, (E-mail: [email protected])

*** Professor, Department of Radioactive Waste Management, Hot Laboratories and Waste Management Center,Inshas, Atomic Energy Authority, Egypt, P.O. 13759, (E-mail: [email protected])

I J E E S© International Science Press

International Journal of ENVIRONMENTAL ENGINEERING SCIENCEVOLUME 2 F NUMBER 2 F JULY-DECEMBER, 2011 F PP. 199-211

Leaching Behavior of 137Cs and 60CoRadionuclides from Stabilized Waste Matrices

Ahmed-Mohamed El-Kamash*, Nesreen Mohamed Sami** &Mohamed-Ibrahim El-Dessouky ***

ABSTRACT Leach characteristics of 137Cs and 60Co radionuclides from both ordinary Portlandcement and cement mixed with two different ratios of blast furnace slag andceramic sludge have been studied using IAEA’s standard leach method.A mathematical model has been simulated to predict the release rate of each nuclidefrom cylindrical geometry waste matrix, and the predicted values are discussedin relation to experimentally observed leach rates to confirm the validity of theproposed mechanism in the model. The net fractional release of the tworadionuclides from different waste forms showed a decreasing pattern as, 137Cs >60Co indicating the largest diffusion coefficient for cesium in all waste matrices.

Keywords Leaching kinetics, Mathematical modeling, Radionuclide leach, Cement-basedmaterials.

1. INTRODUCTION

Cement represents the most known material for solidification of low and intermediatelevel radwastes. The solidified blocks should have adequate characteristics to withstandthe operating conditions. Some additives such as blast furnace slag, silica fume , andilmenite (Mehat 1982; Tallent et al, 1987; Mcdaniel et al, 1989; Manaseer, and Kiel 1992;Johnston, and Wilmot 1992) were found to be a very effective ingredients of cement-based grout to improve the characteristics of the waste matrix towards the safetyrequirements.

The disposal of low and intermediate level radwaste in shallow land burial is basedon encapsulation of these wastes with cement and cement based materials.Radionuclides can escape from the waste matrix through diffusion migration underwater-saturated conditions. Hence, there is considerable interest in understanding thediffusive transport of radionuclides through cement matrix. Several leaching studies(Atkinson et al, 1986; Oblath, 1989; Pescatore, 1991) had reported using different

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INTERNATIONAL JOURNAL OF ENVIRONMENTAL ENGINEERING SCIENCE [IJEES]200

methods to study the temporal distribution of radionuclides in the leachant medium.However, leaching tests are conducted over short periods of time and the extrapolationof these measured values is therefore subject to large uncertainties. The mathematicalmodeling is then used to assess the long term behavior of embedded radioactive wastesthrough mechanistic studies and annually radionuclide fraction calculations.(Krishnamoorthy et al, 1993) proposed an iterative model to simulate the release ratesof radionuclides from cylindrical cement blocks. (Pescatore 1991) derived two leachrate expressions for the diffusive release of radioactive constituents from bothcylindrical and rectangular-shaped waste forms. In the present study, the IAEA’sstandard leach method proposed by (Hespe 1991) has been employed to study theleach pattern of 137Cs and 60Co radionuclides encountered in low and intermediatelevel radwastes solidified in cement and cement mixed with two different ratios oflocally produced blast furnace slag and ceramic sludge. The leaching of theseradioactive materials can be related to the diffusion transport from the cementiousmaterials. Also, the study is directed to achieve a simple mathematical model, basedon diffusion mechanism for cylindrical geometry waste matrix, to predict the releaserate of each nuclide from the cement and cement based materials waste forms.

2. MERIAL AND METHODS

2.1. Materials

All materials used in this study were of locally produced. Their chemical compositionswere given in (Table 1).

Table 1Chemical Compositions of Cement and Additives

MaterialChemical Ordinary Blast Ceramiccomposition Portland Cement Furnace Slag SludgeSiO2 21.0 38.9 61.50Al2O3 4.74 8.96 18.20Fe2O3 4.00 1.66 3.15CaO 61.0 32.5 2.6MgO 2.5 6.88 0.17K2O 0.6 0.91 2.22Na2O - 1.49 4.31SO3 2.43 1.54 0.41CI - — —MnO - 3.82 0.02L.O.I - 3.34 7.42L.O.I. : Loss On Ignition.

2.2. Mechanical Strength

The mechanical strength test, given as applied maximum load divided by a cross-sectional area of a specimen, was performed to confirm the integrity of waste forms.

LEACHING BEHAVIOR OF 137CS AND 60CO RADIONUCLIDES FROM STABILIZED WASTE MATRICES 201

The specimen of a cubic shape with the dimension 7x7x7 cm would be desirable forthe test because the restriction effect, the increase effect of apparent strength by virtueof friction between loading plate and specimen, could be reduced to a minimum. Thecriterion of mechanical strength for cement waste form is recommended by the USNuclear Regulatory Commission (NRC) Standard (Johnston and Wilmot 1992).

2.3. Static Leaching Test

Static leaching tests were preformed using ground water solution, obtained from AbuZaabal well no. 202 which is one of the nearest ground water well to Inshas nuclearsite, to study the desorption of 137Cs and 60Co radionuclides from hardened blocks ofcement and cement mixed with additives. The chemical analysis of the ground waterwas given in (Table 2).

Table 2Chemical Analysis of the Ground Water

Soluble cations, ppm Soluble anions, ppm

T.D.S, mg/l PH K+ Na+ Mg+ Ca++ Cl- SO4– HCO3–

1.05 7.2 23 149 13 74 137 317 272

T.D.S. = Total Dissolved Salts.

The hard plain cement sample and mixed samples is prepared as a cylindricalsamples of diameter 2.0 cm and 1.0 cm height and 6.5 gm weight, where theInternational Atomic Energy (IAEA) standard test proposed by (Hespe, 1991) wasapplied. to All samples used in leaching test were prepared using cylindrical mouldswith the height/diameter ratio of 2.0 dimensions, were stored in laboratory at ambienttemperature (25 ± 3°C) for 28 days curing time. Each sample was immersed in beakercontaining 50 ml ground water for certain time intervals and 1 ml of leachant wastaken, dried, and counted. The gamma spectra of studied nuclides were measuredusing a gamma spectrometer with 4 “x 4 “ NaI crystal activated with Thallium. Thecrystal is connected to a multichannel analyzer. The cumulative leach fraction wascalculated according to the following equation,

0

( )= A t VCumulative leach fractionA S

Σ

(1)

where, Σ A(t) = Cumulative radioactivity leachedA0 = Initial radioactivity present in specimenV = Volume of specimen, cm3

S = Exposed surface area of specimen, cm2.

3. MATHEMATICAL MODEL

Radionuclides from the cement block are transferred to the surrounding water bydissolution to the interstitial water, diffusion through interstitial water to the surface


of the cement block, and eventual release to the water. Since the geometry of the cementblock is well defined, the following equation is used to represent the migration ofnuclides from the cubic cement block to the surrounding water (El-Kamash et al, 2002):

2

2

C CDt x

∂ ∂= ∂ ∂ (2)

Where, C = Concentration of radionuclide in cement block, mol/cm3

D = Diffusion coefficient, cm2/s.x = Spatial coordinate in the direction of flow, cm.t = Time, s.

The initial and boundary conditions for Eq. (1) are given byC (x,0) = C0 (3)C (0, t) = 0 (4)C (a, t) = 0 (5)

where, C0 is the initial concentration of radionuclide in the cement block and a is thedimension of the cement block.

Using initial and boundary conditions, the solution for equation (1) can be written as:

2 21 22 1 ( 1)0( , ) sin exp

1

Dn tnC n aC x t xn an

π−+∞ + − π= ∑ π=

(6)

The leaching flux F(t) from the surface of the cement block can be evaluated from

the relation ( ) * CF t S Dx

∂=∂

at x = a as:

2 22 2( ) exp

1,3,..

Dn tC SD aF ta n

π−∞°= ∑=

(7)

where, S is the surface area of the cement block.The cumulative radioactivity, A(Bq) in the leachant can be evaluated by integrating

Eq. (7) with respect to t. Since C0 = A0/V, where A0 is the initial radioactivity added tothe cement block, and V is the volume of the cement block, the fraction of the activityreleased to the leachant can be obtained as:

2 2( )( ) 2 1 21 exp2 20 1

Dn tA t aA nn

π−∞= −

π =∑ (8)


Where, the following equation is used to represent the migration of nuclides from thecylindrical cement block to the surrounding water (El-Kamash et al, 2006):

3. RESULTS AND DISCUSSION

3.1. Mechanical Characteristics

The incorporation of radioactive waste into cement is the most widely used techniquefor its practical, technological and economical advantages. Mechanical properties areone of the important factors to be taken into consideration in evaluating the finalsolidified products. It is worth mentioning that certain minimum values of mechanicalproperties are required for safe handling of the immobilized waste forms duringtransportation and final disposal (Eskander and Ghatas, 2002).

The results of the mechanical strength of the hardened cement pastes and cementpastes mixed with different percentages of blast furnace slag (BFS) and ceramic sludge(CS), at 0.35 W/C ratio, are given in Table 3. Replacement of cement by blast furnaceslag and ceramic sludge led to improve the compressive strength where the maximumcompressive strength was achieved after addition of 50%BFS and 15% CS. This can beattributed to that they formation of additional amount of calcium silicate whichaccelerate the rate of hydration,. Any. additions beyond these percentages decreasedthe compressive strength which may be attributed to the decrease in the clinker content(Heikal, and Didamony, 1999).

The higher compressive strength of OPC mixed with BFS which is more than theCS waste forms may be due to the reaction between BFS and Ca(OH)2 which producemore cementing materials (El-Dakroury and Nowier,2003) and the compressivestrength of cement samples increase.

Table 3The Mechanical Strength of Cement and Cement Mixed with Different Weight Per cent of

Blast Furnace Slag and Ceramic Sludge at 0.35 W/C RatioComposition Additives Per cent Mechanical Strength (Kg/cm2)Plain OPC 0 159.4(B F S) 5 179.6

10 186.215 195.020 201.125 209.630 216.040 225.250 232.960 219.170 199.0

(C S) 5 169.910 186.215 194.220 175.825 154.230 139.7


3.2. Static Leaching Test

3.2.1. Leaching Characteristic of 137Cs and 60Co Radionuclides

The shallow land burial waste forms contain fairly high concentrations of short-livedradionuclides and very low concentrations of long-lived radionuclides. Variousradionuclides and salts make chemical bondage with cement components, or theyexist dispersively in the state of sole crystals in concrete. When the cemented wasteforms come in contact with water, the movement of soluble materials from the wasteto the surrounding water is caused by dissolution or chemical reaction with chemicalcomponents of water. In this study, the radionuclides chosen for the leach test areintended to represent the desorption (leaching) behavior of some of the typicalradionuclides encountered in low-level solid waste forms.

The variation of cumulative leach fractions of 137Cs and 60Co radionuclide incorporatedin cement and cement mixed with 50% blast furnace slag and 15% ceramic sludge aredepicted (Figure 1a-b), respectively. The results showed that, all hardened cement

Figure 1: Cumulative Leach Fractions of (a) Cesium and (b) Cobalt Radionuclides from PlainCement and Cement Mixed with 50% Blast Furnace Slag and 15% Ceramic


samples have cumulative fraction for 137Cs and 60Co lower than plain OPC withoutany addition. This may be attributed to that all additives added contain silica andalumina which react with calcium hydroxide produced during the hydration of cementto form more calcium silicates which fill the pores present in the cement since thepores decrease then prevent the movement and diffusion of the radionuclides (Sami,2006, Sami et al 2006). Cement mixed with 15% ceramic sludge has the lowestcumulative fraction which may be due to that fineness of CS relative to ordinaryportland cement (OPC) that capable of reducing the volume of large pores andcapillaries that are normally found in cement pastes and refinement in pore structure(El-Dakroury, 2003). The magnitude of the cumulative leach fraction is an index forthe release of the nuclide from the cement matrix. In all studied cases, 137Cs has highestvalues of cumulative leach fraction than 60Co.

3.2.2. Incremental Fraction

Data of incremental fractions for plain OPC and OPC mixed with 50% blast furnaceslag and 5% ceramic sludge loaded with cesium and cobalt radionuclides, expressedas cm/day on log scale against leach time in days, are graphically depicted in(Figure 2a-b) respectively. The entire leaching period can be broadly divided intothree regions. Region I from 1 to 7 days, region II from 8 to 50 days and region IIIfrom 51 to 90 days. Region I shows the initial leaching within the first 7 days, i.e.leaching from superficial surfaces of the block. Region II shows that after initialleaching within first seven days, there is a drastic reduction in release of bothradionuclides from the blocks which is maintained over a longer period of time (50days). In region III the leach rates are further lowered to less than 10 -5 cm/day. Thistrend continues up to 90 days. The leach behavior of the blocks can be explained asa combination of the two processes: (i) surface wash-off mechanism and (ii) diffusionstage mechanism. Region I shows the maximum leaching of the ions because of thesurface wash-off process. In this process a rapid equilibrium is established betweenionic species present in the surface pores of the Portland cement block and ions inthe leachant solution. When each ion has been leached out from the surface of theblock, it migrates by longer pathways from the bulk through diffusion-controlledstage, which determines the long term leaching behavior of the block. (Shrivastavaand Shrivastava, 2002).

3.2.3. Effective Diffusion Coefficient

Leaching is assumed to be diffusion controlled process. The mechanism of this diffusioncould not be completely studied due to complex microstructure of composite and thepresence of multi variables which affect the rate of leaching such as matrix composition,temperature, chemical nature of leaching solution, chemical nature of the elementdiffused out, and radiation effects. Several methods are used to measure leaching dataand IAEA suggested that diffusion coefficients may be used to compare leaching data(Atkinson, and Nickerson, 1988) where the quantity of radionuclide leached out froma unit surface area during time, tn is given by:


2 0DtnA An =

π (9)

where; An = activity leached out after time tn, CiA0 = initial activity in the composite, CiD = diffusion coefficient, m2 s-1.

From the above equation, the cumulative fraction leached out from the compositecan be expressed as;

Figure 2: Variation of Incremental Fraction of (a) Cesium and (b) Cobalt Radionuclidesfrom Solidified Cement Pastes as a Function of Time


( )0.52 00 0

S A D tA nSnA V A V

ΣΣ= (10)

0.52

0

A D tSn nA V

Σ Σ=

π(11)

where; Σ An = cumulative amount of radioactivity leached during cumulative time tn.

Thus a plot of [ΣAn/A0] versus nt should give a straight line if D remains constant.The value of D can be calculated from the slope m of the line, i.e.

2 224

m VDS

π= (12)

Figure 3a-b represent the plotting of the fraction leached of cesium and cobaltradionuclides from both studied samples versus square root of leaching time,respectively. As can seen from these plots, for all studied leaching tests, the resultsindicated an initial fast leaching during the first period followed by slow leaching inthe subsequent periods. This behavior suggests the presence of two different valuesof diffusion coefficient for the fast and slow components. So, the calculated diffusioncoefficients for both nuclides, as presented in (Table 4) are the main average values oftheir diffusion coefficients. These data indicated that 137Cs have largest values ofdiffusion coefficients in both waste matrices compared to 60Co nuclide.

The American National Standards Institute (ANSI) defines a material parameterof the leachability of diffusing species. This parameter is called the leachability index,L, and is defined as (American Nuclear Society Standards committee [23]):

logLDβ =

(13)

where, β is a defined constant (1.0 cm2/s) and D is the diffusion coefficient of thespecies (cm2/s) assumed to be independent of time during the course of leaching.

The corresponding values of the leachability index (L) for all studied samplescalculated using Eq. (20), as listed in Table 4, exceeded the value of 6, which is theminimum value for acceptance as a low level waste form (American Nuclear SocietyStandards committee 1986).

3.2.4. Prediction of Time-Profile Leach Fraction

Temporal profiles of leach fractions of 137Cs and 60Co from Portland cement wasteform measured under laboratory condition are shown in (Figure 4). The predictedcurves by the simulated model (Eq. 8) using the evaluated diffusion coefficients are


Table 4Diffusion Coefficient and Leachability Index Values of Caesium and Cobalt Radionuclides

Leached from Solidified Cement MatricesDiffusion Coefficient (D)(cm2. s-1) Leachability Index (L)

Mix Symbol 137 Cs 60Co 137Cs 60CoPlain OPC 3.42E-7 1.56E-7 6.46 6.80OPC+50%BFS 9.06E-8 3.56E-8 7.04 7.44OPC+15%CS 4.04E-8 1.50E-8 7.39 7.82

Figure 3: Variation of Fraction Leached of (a) Cesium and (b) Cobalt Radionuclides from PlainCement and Cement Mixed with 50% Blast Furnace Slag and 15% Ceramic Sludge Versus

Square Root of Time


also presented in this figure. The simulation gives an acceptable representation ofexperimental data; thereby demonstrating that the proposed mechanism in the modelcan represent the leaching kinetics of studied radionuclides from cylindrical geometrycement matrix. The observed difference in data can be related to the discrepancies incalculating the diffusion coefficients. The obtained results, in all studied leaching tests,indicated an initial fast leaching during the first period followed by slow leaching inthe subsequent periods. This behavior suggests the presence of two different valuesof diffusion coefficient for the fast and slow components. Further work, is underinvestigation to rectify this model.

4. CONCLUSION

This paper presented results of immobilization of cesium and cobalt radionuclidesloaded blast furnace slag and ceramic sludge in a mixture with Ordinary PortlandCement. The results showed that addition of (0-50%) blast furnace slag and (0-15%)ceramic sludge leads to increase the compressive strength of the final matrix and behindthis per cent the compressive strength is decreased. Also, The addition of blast furnaceslag and ceramic sludge to cement affects the characteristics of waste form towardsthe safety requirements. The leaching tests of 137Cs and 60Co radionuclides representthe leaching behavior of some of the typical radionuclides encountered in low - levelsolid waste forms. The obtained results showed that the addition of (0-50%) blastfurnace slag and (0-15%) ceramic sludge to cement decrease the leaching rate of eachnuclide at different studied temperatures and suggested a rapid release of radioactivityin the beginning (fast component) followed by slow release for long periods of time

Figure 4: Experimental and Predicted Values of the Fraction Leached of 137Cs and 60CoRadionuclides from Portland Cement Matrix with 15% Ceramic Sludge Versus Leaching Time


(slow component). The simulation model gives an acceptable representation of theexperimental data. The leachability index measured for each of the samples wasexceeded the required value of 6, the minimum value for acceptance as a low levelwaste form.

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