Transport of Strontium Through a Ca-bentonite(Almería, Spain) and Comparison with MX-80 Na-bentonite:Experimental and Modelling

César Valderrama & Javier Giménez &

Joan de Pablo & María Martínez

Received: 13 July 2010 /Accepted: 5 October 2010 /Published online: 24 November 2010# Springer Science+Business Media B.V. 2010

Abstract In this work, the sorption of strontium on aCa-bentonite (CGA) from Almería (Spain) in columnexperiments was studied, and the results obtainedwere compared with the sorption onto the Na-bentonite (MX-80). The code CTXFIT (two sitenon-equilibrium sorption model) was used in orderto fit the experimental data and to determine sorptionand transport parameters. The effect of inlet Sr(II)initial concentration and the ionic strength wereevaluated. The results obtained showed that thesorption capacities as well as the transport andsorption parameters of both bentonites were affectedby the initial metal concentration. In experimentswith higher inlet concentrations, columns weresaturated faster, leading to shorter breakthroughand exhaustion times. On the other hand, a decreaseof sorption and transport parameters was observedat higher ionic strengths, which would confirm ionexchange as the main mechanism of Sr(II) sorptiononto both bentonites. The sorption parameters

(sorption capacity and retardation factor) obtainedindicated that the Ca-bentonite from Almería(Spain) presented better sorption performance thanthe Na-bentonite, which was related to the physicalproperties of the Ca-bentonite.

Keywords Ca-bentonite . Na-bentonite . Strontium .

MX-80 . Breakthrough curve . Two-site non-equilibrium sorption model

1 Introduction

Bentonite is an altered volcanic ash deposit thatconsists essentially of smectite mineral of the mont-morillonite group and has been extensively used as acommercial adsorbent for different metals, due to itshigh sorption capacity, especially for cations. Inaddition to the good sorption capacity, bentonite hasa very low permeability, which provides a geologicalbarrier, and both a good swelling capacity and a highplasticity, that allow the sealing of rock fractures(Madsen and Müller-Vonmoos 1989).

Because of such properties, bentonite has beenselected as a potential buffer or backfill material in thedesign of a high-level nuclear waste (HLNW)repository in a number of disposal programmes (Boultet al. 1998; Bucher and Müller-Vonmoos 1989; Neallet al. 1995; Wieland et al. 1994). In this sense, thebehaviour of different commercial bentonites such asNa-bentonite MX-80 (Wyoming, USA) and Ca-

Water Air Soil Pollut (2011) 218:471–478DOI 10.1007/s11270-010-0660-1

C. Valderrama (*) : J. Giménez : J. de Pablo :M. MartínezDepartament d’Enginyeria Química,Universitat Politècnica de Catalunya,Av. Diagonal 647, Edifici H Planta 4ª,Barcelona 08028, Spaine-mail: cesar.alberto.valderrama@upc.edu

J. de PabloCTM Centre Tecnològic,Avda. Bases de Manresa 1,08240 Manresa, Spain

bentonite (Bavaria, Germany) has been tested underconditions similar to those expected in a finalrepository (Madsen 1998). The results showed thatthe protective role of the bentonite is based on the factthat no convective water flow occurs in the bentoniticlayer and radionuclide transport through the bentoniteis possible only through ion diffusion in stagnantpore-water (Madsen 1998).

The sorption of different radionuclides onto Na-bentonite has already been studied, e.g. uranium,plutonium, neptunium, americium, cesium, thorium,technetium and selenium (see, for example, referencesBaston et al. 1995; Missana et al. 2004; Hurel et al.2009; Nagasaki et al. 1999; Shibutani et al. 1994; Tsaiet al. 2001; Wang et al. 2005; Wanner et al. 1996; Xuet al. 2006). In particular, the sorption of strontiumonto bentonite has been the object of different studiesin Na- and Ca-bentonite (Choi and Oscarson 1996;Eriksen et al. 1999; Khan et al. 1995; Galamboš et al.2009; Muurinen et al. 1987; Wang et al. 2004). Inthe Spanish concept of high-level nuclear wasterepository, the reference bentonite is a bentonite fromAlmería (Spain), which has a relatively high Ca-content(CGA bentonite). The sorption of strontium onto ahomoionic Na-form of the CGA was carried out byMissana and García-Gutiérrez (2007) as a function ofinitial Sr concentration, ionic strength and equilibriumpH in batch experiments.

The batch experiments may be criticised becausetheir solid/liquid ratio is often different from thatobserved in the field; since the contaminant travels inthe inter-particle pores of packed geological substan-ces in the column approach, then, this methodologyoffers a better simulation of the real environment thanbatch experiments. Accordingly, column experimentswould be relatively suitable to determine the transportproperties of relevant contaminants of concern inlaboratory scale (Wang et al. 2009).

Sorption is an important process that determinesthe rate of movement and the persistence of solutes insolid. Mathematical models that consider sorption areneeded for a complete evaluation of the chemical fateof contaminants in the environment. One of theseapproaches is the two-site non-equilibrium modelwhich has been properly used to predict the sorptionand transport of metals, heavy metals and organicpollutants on different sorbents (Florido et al. 2010;Mao and Ren 2004; Tsang et al. 2007; Valderramaet al. 2010).

The main objective of this study is to quantify Sr(II) transport and sorption from aqueous solutiononto the Ca-bentonite in a simplified fixed-bedconfiguration and to compare its behaviour with thetransport and sorption onto the Na-bentonite. Thesorption capacities, the initial Sr(II) concentrationand ion strength effects are evaluated. The CXTFITcode (Toride et al. 1995) has been used to estimatethe transport and sorption parameters of the convective–dispersive equation (CDE) and the two-site deter-ministic non-equilibrium (TSM/CDE) models byadjusting the models to the experimental breakthroughcurves.

2 Material and Methods

2.1 Solids

The CGA Ca-bentonite was properly characterised byMissana and García-Gutiérrez (2007) this clayshowed high smectite content (93%) together withquartz (2%), plagioclase (3%) and cristobalite (2%),the structural formula of this bentonite contains morethan 99% of smectite (fraction less 2 μm). The cationexchange capacity was 102±4 meq·100 g−1. Themajor exchangeable cations were: Ca (42%), Mg(33%), Na (23%) and K (2%). A comprehensivecharacterisation of the clay can be found elsewhere(Huertas et al. 2000).The surface area of bothbentonites was calculated in this work by using theBET methodology; the values obtained for bothbentonites are collected in Table 1.

The composition of the MX-80 bentonite has beenreported by Kruse (1993), being montmorillonite67.2%, kaolinite 7.1%, illite 3.7%, feldspars 8.8%,quartz 10% and carbonate 2%. The whole claycontent (2 μm particle size) is approximately 85%.

2.2 Experimental Methodology

The experiments were carried out by using glasscolumns uniformly packed with 10 g of a mixture of10% bentonite and 90% quartz (particle size 0.25–0.50 mm). Two experiments were performed inparallel, with two columns each one filled with oneof the bentonites. The columns (KONTES CHX)dimensions were: diameter 10 mm, longitude100 mm, section 78.54 mm2.

472 Water Air Soil Pollut (2011) 218:471–478

Sr(NO3)2 (Merck) was used to prepare the solutions.NaCl (Merck) was used as ionic media (0.001, 0.05and 0.1 molL−1). During the column sorptionoperation, the aqueous Sr(II) solution was pumpedupwards through the column at a constant flow rate(11±0.6 mh−1) by a peristaltic pump (GILSONMinipuls 3). Samples were collected from the outletof the column by a fraction collector (Gilson FC204)at pre-set time intervals. The pH of the solution wasmonitored by using a glass electrode, the pHmeasured from the starting point of the experimentuntil the column was exhausted ranged from 5 to 6.Sr(II) concentration in solution was determined byICP (Spectroflama Modula SPECTRO AnalyticalInstrument GmbH).

Previously to the Sr column experiments, anacetone solution (15% volume) was used as a tracerin order to determine the transport parameters ofthe column (because acetone has no retention onbentonites). The tracer was pumped through thecolumn at the same flow rate as in the experimentsof strontium uptake and it was quantified in theoutflow solution by UV-vis spectrophotometry(Hewlett-Packard 8453). For nonreactive tracerssuch as acetone, the convective–dispersive equationcan be reduced to (van Genuchten 1993):

@C

@t¼ D

@2C

@x2� n

@C

@xð1Þ

2.3 Breakthrough Capacity

The breakthrough point is chosen arbitrarily at somelow value, namely Cb (mg·L−1). When the effluentconcentration, namely Cx (mg·L−1), is closelyapproaching to 90% of the initial concentration ofsorbate, C0 (mg·L−1), then the sorbent is considered tobe essentially exhausted (Goud et al. 2005; Guptaet al. 2000).

The capacity at exhaustion q0 (mg·g−1) is determinedby calculating the total area below the breakthrough

curve. This area represents the amount of solute sorbedby mass of solid m (g) in the sorption zone thatgoes from the breakpoint Vb (L) to exhaustion Vx

(L) (Al-Degs et al. 2009; Goud et al. 2005; Guptaet al. 2000).

q0 ¼R Vx

VbC0 � Cð ÞdVm

ð2Þ

where, C is the outlet metal concentration (mg·L−1)and m is the mass of sorbent (g).

The column sorption process requires prediction ofthe concentration–time profile or breakthrough andunderstanding of transport mechanisms that occur insoils and the capability to represent these mechanismsproperly by mathematical models.

2.4 Two-Site Non-Equilibrium Model

CXTFIT (Toride et al. 1995) is a programmepresenting a number of analytical solutions for one-dimensional transport models based on the CDE. Itassumes steady-state flow in homogeneous sorbentand first-order transformation kinetics. Both trans-port and sorption parameters were estimated byusing the CXTFIT code under flux-type boundaryconditions.

The conceptual model of chemical (two-site) non-equilibrium transport (Cameron and Klute 1977)regards the sorption as two types, occurring either inseries or parallel. One is instantaneous and atequilibrium; the other is rate-limited and describedby first-order kinetics. They could be related toheterogeneity among different soil components (e.g.clay minerals, Fe oxides and organic matter) orheterogeneity inside a single soil component (Tsanget al. 2007).

According to this model, the sorbent is assumed toconsist of two different types of sorption site and thesorption is instantaneous for a fraction of the sorbent

Sorbent Solid fraction(μm)

BET Surfacearea 2(m2·g−1)

CEC(meq 100 g−1)

Na-bentonite (MX-80) 150–300 30.29±0.22 76.4

Ca-bentonite (CGA) 300–900 53.28±0.16 102±4

Table 1 Physical propertiesof the sorbent bentonites

BET Brunauer, Emmet,Teller method; CEC cationexchange capacity

Water Air Soil Pollut (2011) 218:471–478 473

(Eq. 3) and rate-limited for the remainder (Eq. 4)(Bajracharya and Barry 1995; Kamra et al. 2001):

S1 ¼ k1C ¼ FkLC ð3Þ

S2 ¼ k2C ¼ 1� Fð ÞkLC ð4Þ

where the subscripts 1 and 2 refer to Types 1 and 2sites, respectively, and C is the flux-averaged orresident concentration (mg·L−1), S1 is the solid phaseconcentration on equilibrium sites (mg·kg−1), S2 is thesolid phase concentration on kinetic non-equilibriumsites (mg·kg−1), F represents the fraction of the sitesavailable for instantaneous sorption; thus, total sorption,S, is given by S ¼ S1 þ S2.

For linear sorption, which is assumed for bothsorption domains, the governing equations for thetwo-site model are (Tsang et al. 2007):

@C

@tþ r

q

� � @S1@t

þ @S2@t

� �¼ D

@2C

@x2� n

@C

@xð5Þ

@S2@t

¼ a 1� Fð ÞkLC � S2½ � ð6Þ

where ρ is the sorbent bulk density (kg·m−3), θ is thevolumetric water content (m3m−3), t is time (h), D isthe dispersion coefficient (cm2·h−1), x is distance (cm)v is the average pore water velocity (cm·h−1), α is afirst-order rate coefficient (h−1) and kL (m3·kg−1) isthe linear isotherm sorption coefficient.

If dimensionless parameters are used, the two-sitenon-equilibrium model reduces to the followingdimensionless form:

bR@C1

@T¼ 1

P

� �@2C1

@X 2

� �� @C1

@X� w C1 � C2ð Þ ð7Þ

1� bð ÞR @2C2

@T¼ w C1 � C2ð Þ ð8Þ

where the subscripts 1 and 2 refer to equilibrium andnon-equilibrium sites, respectively; the dimensionlessparameters are defined as follows (Tsang et al. 2007):

C1 and C2 (mg·L−1) are the relative concentrationof metal in equilibrium and kinetic sites, respectively,with respect to the input concentration C0 (mg·L−1):

C1 ¼ C

C0ð9Þ

C2 ¼ S21� Fð ÞkLC0½ � ð10Þ

T is the dimensionless time and L is column length :

T ¼ ntL

ð11Þ

X is the dimensionless distance : X ¼ x

Lð12Þ

R is the retardation factor : R ¼ 1þ rkLq

ð13Þ

P is the Peclet number : P ¼ nLD

ð14Þ

b is the fraction of instantaneous metal retardation :

b ¼ q þ FrkLð Þq þ rkLð Þ

ð15Þ

and w is a dimensionless mass transfer coefficient :

w ¼ a 1� bð ÞRLn

ð16Þ

The CXTFIT code adjusts the dimensionless formof the two-site non-equilibrium model (Eq. 7) to thebreakthrough curves in order to estimate the transportand sorption parameters. The Marquardt’s percentstandard deviation (MPSD) indicates an estimation oferror between experimental and theoretical values ofC/C0 used for predicted breakthrough curves, andthey were calculated by using the following equation(Srivastava et al. 2009):

MPSD ¼ 100

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N � P

Xn

i¼1

ðC=C0Þexp � ðC=C0ÞtheoðC=C0Þexp

" #2

i

vuutð17Þ

474 Water Air Soil Pollut (2011) 218:471–478

where N is the number of data points and P is thenumber of parameters (or the degrees of freedom ofthe system).

3 Results and Discussion

3.1 Tracer Experiments

The breakthrough curve obtained for the nonreactivetracer (acetone) was fitted by using Eq. 1, togetherwith the CXTFIT code. The dispersion coefficient (D)and the average pore water velocity (v) obtained arepresented in the following sections as the physicalparameters (R=1). Those two parameters were keptconstant for the fitting of the sorption breakthroughcurves. The pore water velocity for both bentonitecolumns used in the dynamic test at different inletconcentrations was from 0.25 to 0.5 cmh−1. This

deviation can be related to the slight variation of theexperimental flow-rate at different inlet concentrationcolumns. In the case of dispersion coefficients, thevalues obtained ranged from 5.5 to 6.3 cm2h−1 forboth bentonite solids. This variation was not surprisingconsidering the heterogeneity of the sorbent materialinside each column.

3.2 Effect of Initial Sr(II) Concentration

Figures 1 and 2 show the variation of the [Sr]/[Sr]0ratio as a function of volume for the experiments withboth bentonites. As it can be seen, in both cases anincrease of the initial Sr(II) concentration significantlyaffected the breakthrough curve when other experi-mental parameters are kept constant. The values of theparameters calculated are shown in Table 2.

In both experiments, the sorption bed capacityincreases when the initial Sr(II) concentration isincreased. The difference in the breakthrough andexhaustion times between higher and lower inlet Sr(II) concentrations was larger in the Ca-bentonitecompared to the observed for Na-bentonite. Adecrease in the inlet Sr(II) concentrations resulted indelayed breakthrough curves leading to higher break-through and exhaustion volumes, since the lower

0

0,2

0,4

0,6

0,8

1

0 300 600 900 1200 1500

C/C

0

V (mL)

6 mg/L95 mg/L

Fig. 1 Experimental and predicted breakthrough curves for theSr(II) sorption onto Ca-bentonite CGA by using the non-equilibrium sorption model

0

0,2

0,4

0,6

0,8

1

0 20 40 60 80 100 120 140 160 180 200

C/C

0

V (mL)

36 mg/L83.4 mg/L

Fig. 2 Experimental and predicted breakthrough curves for theSr(II) sorption onto Na-bentonite MX-80 by using the two-sitenon-equilibrium sorption model

Table 2 Physical parameters obtained by fitting the equilibriumCDE to the breakthrough of tracer (R=1) and sorption bedcapacity calculated by Eq. 1

Bentonite [NaCl] C0 D v q0

Ca-bentonite CGA 0.001 6 6.3 0.52 4.34

95 5.5 0.22 18.0

Na-bentonite MX-80 0.001 36 6.0 0.35 1.42

83 6.0 0.32 3.74

C0 (mg·L−1 ); D (cm2 ·h−1 ); v (cm·h−1 ); q0 (mg·g−1 )

Table 3 Model parameters obtained by fitting Sr(II) experi-mental breakthrough data to the non-equilibrium sorptionmodel

Bentonite C0 R kL α MPSD

Ca-bentonite CGA 6 7.13 9.1×10−4 4.3×10−2 47.4

95 6.7 2.8×10−4 7.6×10−2 15.0

Na-bentonite MX-80 36 2.02 1.6×10−4 1.4×10−1 2.9

83 1.97 2.4×10−4 1.6×10−1 4.1

C0 (mg·L−1 ); kL (m3 ·kg−1 ); α (h−1 )

Water Air Soil Pollut (2011) 218:471–478 475

concentration gradient caused slower transport due todecreased diffusion coefficient (Florido et al. 2010;Malkoc and Nuhoglu 2006).

The simulation of the breakthrough curves atdifferent initial concentration was performed by theCTXFIT code. The transport and sorption parametersare reported in Table 3. Figures 1 and 2 show theexperimental breakthrough and the calculated valuesobtained by the two sites non-equilibrium sorptionmodel. The fitting of the model to the experimentaldata denotes that this model describes properly theexperimental data for both bentonites. On the otherhand, the MPSD indicates that a small deviation wasobserved for Ca-bentonite, as shows values reportedin Table 3. As was expected from the breakthroughcalculations, the Sr(II) transport was more retarded(large R values) at lower inlet concentration (Tsang etal. 2007). The retardation factors for the Ca-betoniteswere 7.13 and 6.7 for 6 and 95 mgL−1, respectively,while the Na-bentonite observed no significant differ-ences for the retardation factor at different inletconcentration. The linear isotherm sorption coeffi-

cients kL as well as the first order-rate coefficient αwere in the same order of magnitude for both inlet Sr(II) concentrations.

A comparison of the sorption performance ofboth bentonites at low inlet Sr(II) concentrationsindicates that Ca-bentonite reports larges sorptioncapacities and retardation factor values than Na-bentonite. This can be related to the differences inexchangeable cations of both bentonites, resultingin better physical properties of Ca-bentonite (highersurface area and higher cation exchange capacity).Additionally, the linear isotherm coefficients observedfor Na-bentonite were slightly lower than those reportedfor the Ca-bentonite. The linear isotherm coefficientfor Ca-bentonite is lower than the determined inbatch experiments (Missana and García-Gutiérrez2007), which indicates that sorption of Sr(II) ontoCa-bentonite in a column system is also affected bytransport and kinetics parameters.

3.3 Effect of Ionic Strength

Because the main sorption mechanism for strontiumon bentonite has been considered to be ion-exchange(Wang et al. 2004), the effect of ion strength effecthas been studied in this work. In Fig. 3, the variationof the [Sr]/[Sr]0 ratio as a function of the volumeintroduced in the columns is shown for the experi-ments with Ca-bentonite and Na-bentonite at twodifferent ionic media (0.1 and 0.001 molL−1 for Na-bentonite and 0.05 and 0.001 molL−1 for Ca-bentonite). As can be seen in Fig. 3, the sorptionperformance of both bentonites is quite different; forinstance at ionic media of 0.001 mol·L−1, thebentonite Na-bentonite reaches the exhaustion volumebefore 200 mL, while the relative concentration for

0

0,2

0,4

0,6

0,8

1

0 200 400 600 800 1000 1200

C/C

0

V (mL)

6 mg/L 0.001 NaCL CGA9 mg/L, 0.05 NaCl CGA8 mg/L 0.001 NaCl MX-8013 mg/L 0.1 NaCl MX-80

Fig. 3 Comparison of the experimental and predicted break-through curves at different ion strength for the Sr(II) sorptiononto Ca-bentonite CGA and Na-bentonite MX-80

Bentonite [NaCl] C0 q0 R kL α r2 MPSD

Ca-bentonite CGA 0.05 9 4.52 5.41 4.6×10−4 2.9×10−2 0.93 45.7

120 26.2 4.80 7.7×10−4 6.0×10−2 0.98 18.0

0.001 6 4.34 7.13 9.1×10−4 4.3×10−2 0.94 47.4

95 18.0 6.70 2.8×10−4 7.6×10−2 0.99 15.0

Na-bentonite MX-80 0.1 4 0.25 1.30 9.3×10−5 1.2×10−1 0.99 2.1

13 0.41 1.10 3.3×10−5 9.2×10−2 0.98 2.2

0.001 8 0.37 2.20 2.7×10−4 1.5×10−1 0.99 1.9

36 1.42 2.02 1.6×10−4 1.4×10−1 0.99 2.9

Table 4 Calculated sorptionbed capacity and modelparameters obtained byfitting the experimentalbreakthrough data to thenon-equilibrium sorptionmodel at different ionstrength for the sorption ofMX-80 and CGA bentonite

C0 (mg·L−1 ); q0 (mg·g−1 );kL (m3 ·kg−1 ); α (h−1 )

476 Water Air Soil Pollut (2011) 218:471–478

the Ca-bentonite at the same volume is around of30% of the inlet concentration. This fact can beconfirmed by checking the sorption bed capacityand the retardation factor reported by both materials(see Table 4).

The Ca-bentonite reported small differences insorption capacities for 0.05 and 0.001 molL−1 ionicmedia (4.52 and 4.34 mgg−1, respectively); while nosignificant differences were observed among thesorption capacity values obtained for Na-bentonitefor 0.1 and 0.001 molL−1 ionic media.

Furthermore, the retardation factor increases bydecreasing the ionic strength (1.3 times and two timeshigher at the lowest ionic strength for Ca- and Na-bentonites, respectively). These results may beexplained considering that there is a competitionbetween Sr2+ and Na+ from the ionic medium forthe sorption sites of the bentonite.

4 Conclusions

The Ca-bentonite (CGA) reported higher sorptioncapacity and retardation factor than Na-bentonite(MX-80) under the same ionic strength. This mightbe related to the difference in physical properties ofboth materials which results in better sorptionproperties for the Ca-bentonite. This sorption capacityreinforces the potential role of the Ca-bentonite asbuffer or backfill material in a HLNW repository.

The results show that the two-site sorption modeldescribes satisfactorily the mobility of Sr(II) ontoboth bentonites. The sorption capacity is dependingon the inlet Sr(II) ion concentration, the higher theinitial metal concentration the quicker the bentonitecolumn saturation. Transport and sorption parametersare affected by the ionic strength, retardation anddistribution factors determined with the two site non-equilibrium model decreasing with ionic strength inboth bentonites.

The linear sorption coefficients obtained arelower than the ones reported in literature for batchexperiments, indicating that sorption of Sr(II) ontoCa-bentonite is affected by the transport andkinetics as well as by equilibrium parameters.

Acknowledgments This work has been financially supportedby the Spanish ‘Ministerio de Ciencia e Innovación’ by meansof the PAIS Project (CGL2008-06373-C03-02).

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