2010Adsorption of cesium and strontium on natrified bentonites

Adsorption of cesium and strontium on natri?ed bentonites

Michal Galambos

ˇ?Jana Kufc ˇa ′kova ′?

Ol’ga Rosskopfova

′?Pavol Rajec Received:22November 2009/Published online:30December 2009

óAkade

′miai Kiado ′,Budapest,Hungary 2009Abstract The in?uence of chemical activation–natri?ca-tion of bentonites on adsorption of Cs and Sr was studied with regards to utilization of bentonites for depositing high-level radioactive waste and spent nuclear fuel.Ben-tonite samples from three Slovak deposits in three different grain-size (15,45and 250l m),natural and natri?ed forms (Na-bentonites);under various experimental conditions,such as contact time,adsorbent and adsorbate concentra-tion have been studied.When comparing the Na-bentonites and their natural analogues,the highest adsorbed Cs and Sr amounts were reached on the natri?ed samples.After the Sr adsorption a drop in the pH equilibrium value was observed together with the increase of the initial Sr con-centration.A disadvantage of the natri?ed bentonite forms is formation of colloid particles.After 2h of phase mixing a gentle turbidity was observed as well as formation of a gel-like form.The above ?ndings were con?rmed by observing the particle distribution in dry and wet dispersion and centrifugation at two different speeds.Natri?cation as a technological process of bentonite quality improvement cannot be applied when constructing a long-term repository for high-level radioactive waste and spent nuclear fuel.The main problem of natri?cation is a technological process which leads to a signi?cant pH increase.Alkaline envi-ronment in combination with the K presence and increased temperature in the vicinity of radio-active waste can lead to a rapid illitization of smectite and loss of the original adsorption qualities.Moreover,sodium additions are a

signi?cant point of uncertainty since it is not possible to state what amount of Na enters the interlayer space and what amount stays in the inter-partition space.Keywords Chemical modi?cation áNatri?cation áActivation áSlovak Na-bentonite áSmectite á

Montmorillonite áAdsorption áCesium áStrontium áDeep geological repository

Introduction

Clay rocks—bentonites—have an extraordinary impor-tance in the waste and environmental management.Their properties are subject to their chemical and mineralogical composition.Bentonites consist of a mineral of the dioc-taedric smectite group—montmorillonite (50–85%)and accompanying materials such as clay minerals (illite,beidellite,nontronite,caolinite),quartz diatomite,calcite,organic materials and others [1–3].

The qualities of bentonites such as low permeability,large speci?c surface,high swelling ability,ability to interchange ions as well as their colloid-chemical prop-erties are planned to be taken advantage of in form of bentonite barriers in the deep geological repositories for high-level radioactive waste and spent nuclear fuel [4–14].

In connection with the above problematics the adsorp-tion properties of bentonites are studied and the effect of their modi?cation on adsorption processes,chemical and mineralogical stability are studied [15–25].The study of adsorption properties of bentonites is a step inevitable for working out a migration model for different eco-toxically signi?cant radionuclides such as Cs,Sr,Co,Tc,I,Rb,Pu,Am,U and others [26–35].

M.Galambos

ˇ(&)áJ.Kufc ˇa ′kova ′áO.Rosskopfova ′áP.Rajec Department of Nuclear Chemistry,Faculty of Natural Sciences,

Comenius University in Bratislava,Mlynska

′Dolina,84215Bratislava,Slovak Republic e-mail:galambos@fns.uniba.sk

J Radioanal Nucl Chem (2010)283:803–813DOI 10.1007/s10967-009-0424-9

The properties of bentonites are signi?cantly determined by the crystalline structure of smectites and are result of the following structure ratios[36–42]:

(1)Weak coupling forces among the basic triple-layers in

the direction of Z axis[43–48].The dimensions of the basic particles—the majority is smaller than2l m (natural nanomaterials).High swelling ability—enables them to increase their volume as much as 12times when in contact with water or polar liquids.

High speci?c surface of particles(incomparable with other clay minerals)—when in contact with water a high thixotropy is observed in dispersive state,high plasticity and denticity.

(2)Unlimited cation substitution in octahedral positions

and limited cation substitution in tetrahedral posi-tions[49–54].It has a principal in?uence on the presence of the negative charge on the surface of smectite particles immediately connected with the ability to accept and exchange in the interlayer area the so-called exchangeable cations(most often Na, Ca,Mg and K).This ability is the basic reason for the adsorption properties of smectites.

It is possible to use bentonites in natural,physically or chemically modi?ed state[55–59].Physical modi?cation comprises the basic mechanical processing of bentonites such as drying and grinding[60,61].By means of drying the original humidity of bentonite is decreased to desired 7–8%(when won on surface winning Na-bentonite about 30%,Ca-bentonite about25%).The temperature at which bentonites are being dried spans from100to200°C at entry and as much as800°C of exit temperature,according to the required application.The structural properties of smectites decrease considerably if this temperature is exceeded.A more demanding way of modi?cation of bentonites,nowadays rather common,is based on prepa-ration of chemically modi?ed forms[62–73].During chemical modi?cation the natural bentonite is processed by means of its?nal products being exposed to the in?uence of chemicals,which happens usually at the bentonite winning site(Mining Plant).Chemical modi?cation is possible thanks to the large speci?c surface with active sorption centres,presence of molecular water and exchangable cations in the smectite interlayer space. Monoionic forms are prepared of which protonisation is most commonly used(exchangeable positions in the smectite interlayer are saturated with H?cations)as well as natri?cation(exchangeable positions in the interlayer are saturated with the Na?cations).As natri?cation salt Na2CO3is used almost exclusively thanks to its?nancial accessibility and rather high speed of exchange.The amount of added salt changes according to the kind of bentonite and its?nal intended use.A more convenient way is the wet way of activation,which changes all the bentonite to Na-bentonite.Natri?cation best enables the division of the basic smectite layers which provides new space for cation exchange and leads increase in the overall adsorption capacity.Speci?c and more complicated way of chemical modi?cation is presented by intercalation and pillaring of the smectite structure.In the?rst case it is the process of intercalation by organic and organometallic cations during which the so-called organobentonites are prepared[74–77].Pillaring or strengthening of the smectite structure is done due to their low thermal stability[78–81]. Into the interlayer space a molecule of e.g.Al2O3is introduced,which functions as a pillar preventing the collapse of smectite structure.

Experimental

Solid phase

Samples of bentonite were taken from three Slovak deposits and technologically processed by Envigeo a.s. (Table1).Samples of bentonite were at disposal in three different grain sizes,samples ground under250,45and 15l m(what means,that these are fractions of bentonite with size of particles smaller than250,45and15l m).

The samples of bentonite were dried in Petri dish in drying oven at the temperature105°C approximately2–3h. Then they were taken into small plastic containers and lay aside into a desiccator,where they were stored till next use.

Chemical analyses

Chemical composition of bentonites from the deposits of Jelsˇovy′potok,Kopernica and Lieskovec is shown in Table2.The Jelsˇovy′potok and Kopernica bentonites are of a very similar chemical composition.The biggest dif-ference is in the higher content of Mg in Jelsˇovy′potok Table1Used samples of bentonites

Deposits Jelsˇovy′potok Kopernica Lieskovec

Type Smectic bentonite Smectic bentonite Smectic bentonite Locality Middle Slovakia Middle Slovakia Middle Slovakia

Natural forms

Sample designation J K L Grain size[l m]15,45,25015,45250

Natri?ed forms

Sample designation BJ BK BL Grain size[l m]15,45,25015,4545

804M.Galambosˇet al.

bentonite which apart from the exchangeable positions is also signi?cantly present in the octahedron of montmoril-lonite.The Lieskovec bentonite has a signi?cantly higher Fe content in comparison to the previous two bentonites.It is due to its other mother rock—andesite.When comparing the chemical composition of particular fractions(15,45 and250l m)from one deposit no signi?cant differences were observed.When comparing the Kopernica and Jelsˇovy′potok bentonites the differences are minimal.In the Lieskovec bentonites it is possible to spot slight differ-ences.In L250fraction the SiO2content is almost4% lower than in L45fraction and the difference is equalized by a higher Al and Fe content in L250sample.

Mineralogical analyses

The use of bentonites is signi?cantly subject to the content of smectites,their crystal-chemical composition and addi-tions of clay and other minerals.Other clay minerals are mostly represented by kaolinite especially in deposits of hydro-thermal origin.Halloysite,even in the case of ben-tonite,belongs to the accessory present clay minerals and has no signi?cant impact on its properties and use.Illite is likely to be found in the sediment deposits in which case it can have a negative impact.Cristobalite is one of the other minerals occurring as an addition in bentonite and belongs usually to hyalite.It occurs on all genetic bentonite types, but mainly in volcanic-sedimentary where it forms during volcanic glass alteration.As an addition its volume reaches sometimes as much as40%.It can not be effectively removed by means of any modi?cation,therefore its use is considerably limited,especially with its more demanding applications.

Lieskovec,Kopernica and Jelsˇovy′potok have similar qualitative mineral composition(Table3).Kopernica and Jelsˇovy′potok are similar quantitative as well.Lieskovec has signi?cantly lower content of smectite(about20% less),its representation is substitute mainly by kaolinite and cristobalite.

Aqueous phase and radiotracer

Adsorption experiments were realized in the Cs and Sr concentration range1910-5–1910-1mol dm-3solu-tions prepared from CsCl and Sr(NO)3(Table4).

Table2Chemical analysis of samples from Jelsˇovy′potok,Kopernica and Lieskovec deposits

Sample designation SiO2[%]Al2O3[%]CaO[%]Fe2O3[%]MgO[%]MnO[%]Na2O[%]K2O[%]TiO2[%]P2O5[%]SO3[%]

J1567.9021.12 1.61 2.88 3.440.080.64 1.670.160.320.21

J4567.4121.32 1.78 2.81 4.220.090.40 1.010.090.710.21

J25068.3320.50 2.03 2.52 4.370.070.64 1.230.180.060.07

BJ1565.2521.51 2.62 2.65 2.840.07 3.09 1.000.080.730.16

BJ4566.0022.67 1.55 2.62 2.840.07 3.02 1.040.120.010.07

K1566.0923.96 1.90 2.69 2.930.050.43 1.680.150.010.11

K4566.0523.93 2.00 2.61 2.780.050.26 1.210.090.980.06

L1565.2221.00 1.437.02 1.820.080.59 1.870.700.080.20

L4567.2619.54 1.31 6.98 1.680.090.51 1.700.750.090.11

L25063.4022.29 1.178.19 1.590.040.46 1.730.870.200.07 Table3Mineralogical

composition

Sample Quartz Albite Biotite Kaolinite Smectite K-felspar Cristobalite

J159.5– 1.53768.5 1.5

J456S1 2.586 3.51

J250 2.50.522829.5 1.5

K156S 2.52808 1.5

K45 6.5S 2.518262

L15 5.51 1.5 6.567.5126

L4512 1.50.56649.5 6.5

L2509.50.529.5649.55

BJ152S0.5–94 3.5–

BJ45111 1.5–7411 1.5

BJ2506S1–84.58.5–Adsorption of cesium and strontium on natri?ed bentonites805

In experiments chemicals of ordinary laboratory purity from supplies companies Slavus,s.r.o.,Bratislava SR,or Lachema n.p.,Brno CZ were used.Radionuclides 137Cs with the volume activity 1MBq mL -1and 85Sr with the volume activity 3.7MBq mL -1was used as a radiotracer.Batch method and measuring of radioactivity

Adsorption of Cs and Sr on samples of bentonites was studied through radioisotope indication using radioisotopes of 137Cs and 85Sr in static arrangement of experiment,in aerobic conditions at laboratory temperature.Adsorption parameters were determined after mixing in 0.05g of adsorbent with 5mL of water phase in plastic test tubes with internal diameter 14mm and capacity 10mL in lab-oratory extractor with constant speed of mixing both pha-ses.After realization of adsorption and subsequent centrifugation (t =15min,3500rpm or t =10min,10000rpm)1mL was extracted from supernatant to measure radioactivity.

Radioactivity determination of 137Cs and 85Sr solutions was done with spectrometer Modumatic using NaI(Tl)detector.

Results and discussion

Adsorption properties of bentonite were calculated by following equations:

Distribution coefficient K d ?a 0àa a ?V m

;mL g à1

?

?e1TAdsorption percentage R ?

100?K d

K d tV m ;%? e2TAdsorption capacity C ?K d ?c eq ;mmol g à1

??

e3T

where c eq ,equilibrium concentration [mol dm -3];V ,aqueous phase volume [mL];m ,adsorbent mass [g];a 0,

volume activity of initial solution [mL s -1];a ,equilibrium

volume activity of solution [mL s -1].

Speci?c surface area and cation exchange capacity Speci?c surface area is characterized by the active surface of the adsorbent,which enables high reactivity with the environment in which the bentonite becomes adsorbent.When stating the speci?c surface we used the method of EGME (ethylengklykolmonoethyleter)sorption and its gradual releasing.EGME is a medium used to state the total speci?c surface.The method is based on construction of curve of process of releasing of EGME through the sample weight,from which we obtain the retention EGME (R )necessary for calculation of the measurable surface (SA)[40]:eSA T?

R

M

á0:326e4T

where R ,retention EGME;M ,sample weight.

The natri?ed samples from the Jels

ˇovy ′potok deposit have the largest speci?c surface (Fig.1).Partly lower values were reached in Kopernica samples and the natural

samples of Jels

ˇovy ′potok.Lieskovec bentonites show the lowest values.Natri?cation enables the most effective division of the basic smectite layers which provides new space for EGME sorption.The drop in speci?c surface from the ?nest to the thickest fraction was gentle and equal for all samples.

Cation Exchange Capacity (CEC)is de?ned by the ability of bentonites to adsorb the cations and keep them in the exchangeable positions—in smectites in the interlayer.During the characterisation of technological types of bentonites three methods of cation exchange determination were used.These are the Ba-method,Mg-method and Cu-trietylentetramin method.The qualitative analysis of particular exchangeable cations con?rmed the dominion of

Table 4Cesium and strontium ions concentration used in adsorption studies

c [mol dm -3

]1910-55910-51910-4

5910-4

1910

-3

5910-31910-25910-21910-1

Fraction of bentonites [μm]

J 15BJ 15K 15J 45BJ 45K 45J 250BJ 250L 250

S p e c i f i c s u r f a c e [m 2.g -1]

200

400

600

800

1000

Fig.1Speci?c surface values in different grinding fractions

806

M.Galambos

ˇet al.

Ca and Mg over Na and K elements.The distribution of original cations indicates a match between Kopernica, Lieskovec and Jelsˇovy′potok bentonite samples,the only difference being that Lieskovec samples show a slightly higher content of Na.The CEC values for particular deposits drop in the following order:Jelsˇovy′potok[ Kopernica[Lieskovec.

Adsorption kinetics

The adsorption of cesium and strontium was investigated as a function of shaking time in previous experiments and the results show that the adsorption is very fast and equi-librium is reached almost instantaneously after mixing[23, 24,39,40,51].The adsorption process was fast,maximum K d was nearly reached within1min from the beginning of contact of solid and liquid phase.The comparable values of K d were reached in a time interval of1–480min(Fig.2).

A period of2h was chosen for the further adsorption experiments.Almost‘‘instantaneous’’capture of the cesium ions on the bentonite can be explained by adsorp-tion and ions exchange on the surface for the cesium ions.

Cs-adsorption experiments

The results reached in Cs adsorption on the Jelsˇovy′potok and Kopernica benotnite samples were modelled by means of Langmuir isotherm(Fig.3).The amounts sorbed in natri?ed samples were compared to those sorbed in their

natural analogues.Higher values of sorbed Cs were reached on the natri?ed samples.Cation exchange is used as the basic sorption mechanism.The highest values of Cs sorbed were reached on the Kopernica bentonite samples—in natri?ed as well as natural forms.

Natri?cation causes a signi?cant growth of the total speci?c surface and enables best division of the basic smectite layers which leads to new space for adsorption of ions.Speci?c surface is the active surface of the observed row material,surface which eventually can take part in the interaction with surrounding agents.

A disadvantage of the natri?ed bentonite forms is the formation of colloid particles.After2h of mixing the phases turbidity was observed as well as formation of a gel-like form.

With low Cs cation concentration in the solution low K d values(on BJ250)were reached—an evidence of the presence of colloids.With a gradual increase of Cs cation concentration in the solution the K d value increased as well.The value kept increasing until the concentration of 5910-3mol dm-3,when a lack of bentonite sorption centers started to be apparent(Table5).

When comparing the natural and natri?ed forms,the highest value of the distribution ratio was reached with the natri?ed bentonite sample of the Jelsˇovy′potok location BJ250.

The in?uence of adsorbent concentration

The in?uence of ratio change of solid and liquid phase on distribution coef?cients for the bentonite sample BJ250 was studied(Fig.4).A system was studied in which the

Adsorption of cesium and strontium on natri?ed bentonites807

weight of bentonite was changing from50to500mg at the constant volume(of5mL)of CsCl water phase in con-centration of c=1910-4mol dm-3.

When increasing the weight of adsorbent a change in the properties of the system was observed.A gel-like forms of phase appeared in the solution which could not be sepa-rated by centrifugation or by?ltration using micro-?lters in pore size of0.4l m.This is the result of the presence of colloid particles in the solution.

The particle distribution in the dry and wet dispersion The particle distribution for bentonite samples BJ45and BJ15was measured in dry and wet dispersion.The pres-ence of small particles in the studied samples can be observed in Figs.5,6and7.The middle values measured for BJ15in dry form are approximately7l m,but it is also possible to see fractions with smaller particle size(Fig.5). It is likely that if a measuring was carried out in the 0.1mol dm-3Na2P2O7solution,a presence of smaller particles would be discovered,just like in the case of BJ45bentonite(Fig.6).For BJ45values of23–27l m were measured(Fig.7).

Sr-adsorption experiments

The Sr adsorption on the Jelsˇovy′potok,Kopernica and Lieskovec bentonite samples was modeled by means of the Langmuir isotherm(Figs.8,9).The sorbed amounts for natri?ed samples and their natural analogues were com-pared,where higher values of sorbed Sr were reached with the natri?ed samples.

Cation exchange is taking place as the basic sorption mechanism.The highest sorbed amounts of Sr,similarly as

Table5Cs distribution coef?cients for Jelsˇovy′potok and Kopernica bentonites

c(Cs)[mol dm-3]K d[mL g-1]

BJ250BJ45BJ15BK45BK15

1910-4460231263255295 5910-4619315401327362 1910-3513297352249261 5910-3366369552318537 1910-2135143182192166 5910-21919191918 1910-1551399

Fig.5Particle distribution BJ15dry dispersion

Fig.6Particle distribution BJ45dispersant0.1M Na4P2O7

Fig.7Particle distribution BJ45dry dispersion

808M.Galambosˇet al.

with the Cs adsorption,were reached with the bentonite samples of Kopernica on natri?ed as well as natural forms.

In the following part values of K d were compared which were obtained in Sr2?cation adsorption on natri?ed BJ45, BK15,BK45and BL45samples.Values of K d were compared after the?rst and the second centrifugation (Figs.10,11,12,13).After the second centrifugation at 10000rotations/min higher values of K d were reached which is a result of partial separation of colloid forms from the solution.K d values kept increasing gradually from the lowest concentration of Sr2?in the solution.A pronounced drop of K d in BK15,BK45and BL45samples was observed from concentration of5910-3mol dm-3.In case of BJ45sample this was true from concentration of 1910-3mol dm-3.The highest K d values were reached in bentonite samples BK15(K d=1686mL g-1in con-centration Sr2?1910-3mol dm-3).The lowest values of K d were reached at the highest Sr2?ion concentration in the solution.In solutions,where multivalent cations are present,such as Sr2?,coagulation of colloid particles occurs.

Adsorption of cesium and strontium on natri?ed bentonites809

pH after adsorption

A drop of the pH value has been observed in the span of Sr concentrations1910-4–5910-2mol dm-3with increas-ing of Sr adsorption on the bentonite samples.The Fig.14 shows the dependency of the solution pH value on the initial concentration of strontium after the adsorption equilibrium.

The pH value of the natri?ed forms of bentonite(J250, J45,BJ250,BJ45,K45,BK15)is already in the alkaline area and during the Sr adsorption moves to the lower values.The pH values for the natural bentonites(J15,J45, K45)occurs in a neutral or in an acidic area.

The tendency of pH value drop with the increase of the initial Sr cation concentration in the concentration span 1910-4–5910-2mol.dm-3has its cause most probably in the mechanism of Sr adsorption,where apart from the exchange of Sr for the cations present also the exchange of protons for Sr ions on OH groups happens.

Conclusion

Jelsˇovy′potok and Kopernica bentonites have a very similar chemical composition.The most pronounced difference is in the higher Mg content in the Jelsˇovy′potok bentonite, which apart from the exchangeable positions is signi?-cantly present in the montmorillonite octahedron.The Lieskovec bentonite has a signi?cantly higher Fe content in comparison to the Jelsˇovy′potok and Kopernica samples. Natri?ed bentonite samples in comparison to their natural analogues contain signi?cantly higher amounts of Na2O, which comes as a result of the nature of their chemical activation.The Jelsˇovy′potok,Kopernica and Lieskovec samples have similar qualitative mineral composition.The bentonites from the Jelsˇovy′potok and Kopernica deposit are similar quantitative as well.The sample of Lieskovec bentonite has signi?cantly lower content of smectite(about 20%less),its representation is substitute mainly by kao-linite and cristobalite.The natri?ed bentonite samples from the Jelsˇovy′potok deposit have the biggest speci?c surface. Partially lower are the values reached with the Kopernica deposit samples and the natural samples from the Jelsˇovy′potok deposit.The Lieskovec deposit samples show the lowest values.The drop in speci?c surface from the?nest to the thickest fraction was gentle and common for all the samples.The qualitative analysis of particular exchange-able cations con?rmed the dominion of Ca and Mg over Na and K elements.The distribution of original cations indi-cates a match among the Jelsˇovy′potok,Kopernica and Lieskovec bentonite samples,the only difference being that

810M.Galambosˇet al.

Lieskovec samples show a slightly higher content of Na. The CEC values for particular deposits drop in the fol-lowing order:Jelsˇovy′potok[Kopernica[Lieskovec.

Natri?ed bentonite forms come with a disadvantage, which is formation of the colloid particles.After2h of mixing the phases a slight turbidity was observed as well as a formation of a gel-like form.The mentioned?ndings were con?rmed by observation of the particle distribution in dry and wet dispersion and centrifugation at two dif-ferent speeds.When comparing the natri?ed bentonites and their natural analogues the highest values of Cs and Sr amount sorbed were reached on samples which had been chemically activated by natri?cation.Looking at all sam-ples studied,the highest values of amount sorbed were reached with the Kopernica deposit samples,which is true for both—natri?ed as well as natural forms.The results show a signi?cant in?uence of natri?cation on adsorption process of both cations.The in?uence of adsorbent con-centration on Cs adsorption on natri?ed form has been con?rmed.With the increasing weight of adsorbent the distribution coef?cient values of Cs were dropping.With the increase of initial Sr concentration,after Sr adsorption a drop in equilibrium pH value was observed.The pH value of the natri?ed forms of the studied bentonites is already in the alkaline area and during the Sr adsorption moves to the lower values.The pH values for the natural bentonites occurs in a neutral or in a slightly acidic area.

Chemical activation–natri?cation as a technological process of bentonite quality increase,where a part of the natural exchangeable cations is substituted for a Na?cation can not be applied in the construction of a long-term radio-active waste and spent nuclear fuel repository.The main problem with natri?cation is a technological process during which a signi?cant increase in pH value occurs.Alkaline environment in combination with the K presence and increased temperature in the vicinity of radio-active waste can lead to a rapid illitization of smectite and loss of the original adsorption properties.Moreover,the sodium additions create a signi?cant area of uncertainty in the system since it is not possible to state what amount of Na is that gets into the interlayer and what amount remains in the inter-particle space.Sodium as well as carbonate cation are at the same time free for further reactions,which in the long run,are dif?cult to predict or simulate.It is also possible to assume,that natri?cation will cause a too much pressure during swelling of the bentonite barrier. References

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Adsorption of cesium and strontium on natri?ed bentonites813