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Applied Radiation and Isotopes 65 (2007) 1095–1100

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Preparation and characterization of manganese dioxide impregnatedresin for radionuclide pre-concentration

Zsolt Varga�

Radiation Safety Department, Institute of Isotopes, Hungarian Academy of Sciences, Konkoly-Thege utca 29-33, H-1121, Budapest, Hungary

Received 15 February 2007; received in revised form 3 May 2007; accepted 10 May 2007

Abstract

An easy and reproducible preparation of manganese dioxide impregnated resin of homogeneous particles has been described. The

characteristics of radium, thorium, uranium and plutonium uptake (pH dependency, kinetic studies and matrix dependency) have been

determined in batch mode. The resin due to its high efficiency for radium, uranium and thorium at neutral pH values can be an effective

tool for radionuclide pre-concentration from liquid samples even with high dissolved solid content.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Manganese dioxide; Pre-concentration; Liquid samples

1. Introduction

Manganese dioxide (MnO2) is widely used for the pre-concentration of 234Th, radium and plutonium isotopesfrom water samples in oceanographic studies or radiolo-gical protection measurements (Nour et al., 2004; Sidhuand Hoff, 1999; Koulouris et al., 2000). Impregnatedcellulose or acrilamide cartridges and filters havebeen applied in-field to adsorb the analytes from large-volume fresh or seawater samples (Dulaiova and Burnett,2004; Moore and Reid, 1973). Moreover, freshly preparedmanganese –dioxide-coated membranes (Zoriy et al.,2005; Eikenberg et al., 2001; Surbeck, 2000) or resins(Moon et al., 2003; Maxwell, 2006) are used in laboratoriesto pre-concentrate the analytes of interest from smallervolume (up to 1 l) liquid samples. The advantages ofimpregnated cartridges or filters over co-precipitationare ease of use and rapidity, as it enables the rapidpre-concentration of radionuclides with high efficiency(in the range of 70–90%) even from hundreds of litresof sample. Moreover, as adsorption coefficients of mostmatrix components are low, this characteristic allows

e front matter r 2007 Elsevier Ltd. All rights reserved.

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392 2222/1220; fax: +36 1 392 2529.

ess: varga@iki.kfki.hu

a rapid separation step easing the following chemicalsample preparation. However, as the adsorption efficiencydepends on several parameters (e.g. flow rate, MnO2

preparation method, surface, pH or kinetics) MnO2

impregnated sorbents and separation procedure have tobe properly characterized and optimized, otherwise ad-sorption efficiency can be low or its estimation can beincorrect. Moreover, as several preparation techniquesare described in the literature without comprehensivecharacterization, method development for pre-concentra-tion may be tedious and misleading due to lack ofimportant data. Furthermore, for most laboratory applica-tions use of resins with small particle size is favourableover membrane or filters due to its better reproducibilityand easy handling. Recently, study on MnO2 impregnatedresins with well-defined particle size is in progressat Eichrom Inc. (marketed as MnO2 ResinTM) (Moonet al., 2003; Maxwell, 2006), though lack of completecharacterization and high price hinder its application forroutine use.The aim of this study is to describe an easy, cheap and

reproducible procedure for the preparation of MnO2

impregnated resin. The uptake characteristics of radium,thorium, uranium and plutonium are described togetherwith kinetic properties and matrix dependency.

ARTICLE IN PRESSZ. Varga / Applied Radiation and Isotopes 65 (2007) 1095–11001096

2. Experimental

2.1. Instrumentation

The mass spectrometric analysis of the radionuclideswas carried out using a double-focusing magneticsector inductively coupled plasma mass spectrometer(ICP-SFMS) equipped with a single electron multiplier(ELEMENT, Thermo Electron Corp., Bremen, Germany).All measurements were carried out in low-resolution mode(R ¼ 300) using a low-flow nebulizer operated in a self-aspirating mode (flow rate 100 ml min�1) in combinationwith a desolvation unit (MCN-6000, CETAC TechnologiesInc., Omaha, NE, USA). Optimized operating parametersare summarized in Table 1. Prior to analysis of samplesthe instrument was tuned using a 1 ng g�1 uraniumstandard solution. Sensitivity was about 2� 106 cps for1 ng g�1 238U. Light microscopic images of the impregnatedresin were taken using the CCD camera and digitalimagining software associated with a UP-213 LaserAblation system (New Wave, Freemont, USA).

2.2. Reagents and materials

Radium (SRM 4967, NIST, USA), thorium (UltraScientific, Kingston, USA), uranium (Merck, Darmstadt,Germany) and 242Pu (NIST 4334F, USA) standards wereused to prepare the test samples. Multielement standardsolution purchased from Merck (Darmstadt, Germany)was used for the optimization of the ICP-SFMS instru-ment. The anion exchange resin (AG 1-X4, particle size:100–200 mesh, chloride form) was supplied by EichromTechnologies Inc. (Darien, Illinois, USA). All reagentsused were of analytical grade. For dilution high-puritywater was used (Milli-Q Element, Millipore, USA). Nitricacid used for sample preparation and source acidificationwas of Suprapure grade (Merck, Darmstadt, Germany).

Table 1

Optimized ICP-SFMS operating condition and data acquisition para-

meters

Element ICP-SFMS

RF power (W) 1245

Cooling gas flow rate (lmin�1) 15.6

Auxiliary gas flow rate (lmin�1) 0.85

Nebulizer gas flow rate (lmin�1) 1.095

Resolution 300

Runs and passes 4� 5

Mass window (%) 20

Samples per peak 250

Search window (%) 40

Integration window (%) 80

Scan type E-Scan

MCN-6000 in combination with low-flow nebulizer

Solution uptake rate (ml min�1) 60

Spray chamber temperature (1C) 70

Membrane temperature (1C) 160

Sweep gas flow rate (lmin�1) 5.20

3. Chemical procedures

3.1. Preparation of MnO2 impregnated resin

To prepare MnO2 impregnated resin 40 g of AG 1-X4anion exchange resin (chloride form) was placed in a 250mlcentrifuge tube with twice the volume of high-purity waterand mixed thoroughly. About 40ml of saturated KMnO4

solution was added to the resin and mixed vigorously.After 15min the sample was centrifuged and the super-natant was discarded. The resin was rinsed three timeswith 80ml of high-purity water with successive centrifuga-tion discarding the rinsing solution. The resin obtainedwas filtered gravimetrically through a Whatman 2-typefilter and quantitatively transferred to 250ml plasticcontainer with water. The resin was dried at 70 1Covernight. After drying the resin was thoroughly homo-genized with a plastic rod.

3.2. Batch experiments on pH dependency

Fifty millilitre of high-purity water acidified with 500 mlof ultrapure nitric acid in polyethylene containers wereused as test solutions. The samples were spiked with10 pg of 226Ra, 15 ng of 232Th and 238U and 30 pg of 242Pu.Addition of each spike was carried out by weight. Thesamples were placed on stirrer plate and continuouslystirred with Teflon covered magnetic stirrer bar. The pH ofthe samples was adjusted with 10m/m% NH3 solutionchecked with Merck Universalindikator (pH 0–14) pHpaper. Right afterwards 100mg of MnO2 impregnatedresin was added to the samples. The pH of the samples wasverified again. In either case, no noticeable change in thepH was caused by the resin. After 15min of stirring 1mlaliquots of the supernatant were taken carefully from eachsample and 50 ml of ultrapure HNO3 was added. The

226Ra,232Th, 238U and 242Pu content of the samples weremeasured by ICP-SFMS with external calibration.

3.3. Kinetic studies

About 100ml of high-purity water acidified with 1ml ofultrapure nitric acid in polyethylene containers were usedas test solutions for kinetic studies. The samples similarlyto the pH dependency experiments were spiked by weightwith 20 pg of 226Ra, 30 ng of 232Th and 238U and 60 pg of242Pu. The samples were placed on stirrer plates andcontinuously stirred with Teflon covered magnetic stirrerbar. The pH of the samples was adjusted with 10m/m%NH3 solution checked with Merck Universalindikator(pH 0–14) pH paper. For the kinetic studies pH ¼ 6 and8 were chosen as these values are above the reported pKvalue of MnO2 at zero point charge (pK(MnO2) ¼ 5.5)(Koulouris, 1995). Right afterwards 200mg of MnO2

impregnated resin was added to the samples. After theaddition of resin small aliquots of supernatant (approxi-mately 0.5ml) were taken carefully from the samples at

ARTICLE IN PRESSZ. Varga / Applied Radiation and Isotopes 65 (2007) 1095–1100 1097

given times while stopping the stirring for a short period.To the aliquots 50 ml of ultrapure HNO3 was added. The226Ra, 232Th, 238U and 242Pu content of the samples weremeasured by ICP-SFMS with external calibration.

3.4. Matrix effects on analyte uptake

To check matrix effects, synthetic seawater containing0.25M NaCl, 2.1� 10�2M Na2SO4, 2.7� 10�3M CaCl2,6.7� 10�3M KCl and 5.7� 10�3M MgSO4 was used as amodel solution. This test sample was spiked with 40 pg of226Ra, 60 ng of 232Th and 238U and 120 pg of 242Pu. Thesample was placed on stirrer plate and continuously stirredwith Teflon covered magnetic stirrer bar. The pH ofthe sample was adjusted to pH ¼ 8 with 10m/m%NH3 solution checked with Merck Universalindikator(pH 0–14) pH paper. Right afterwards 200mg of MnO2

impregnated resin was added to the samples. After theaddition of resin small aliquots of supernatant (approxi-mately 0.5ml) were taken carefully from the samples atgiven times while stopping the stirring for a short time. Tothe aliquots 50 ml of ultrapure HNO3 was added. Thesample was diluted to 10ml with 2m/m% ultrapure nitricacid solution prior to the ICP-MS analysis in order todecrease the total dissolved solid concentration. Alldilutions were carried out by weight measurements. The226Ra, 232Th, 238U and 242Pu content of the samples weremeasured by ICP-SFMS with external calibration.

3.5. ICP-SFMS measurements and calculation of results

The determination of radionuclide concentrations wascarried out with external calibration using In as an internalstandard. Optimized measurement parameters are sum-marized in Table 1. The detection limits (on the basis ofthree times the standard deviation of method blank) of226Ra, 232Th, 238U and 242Pu were 2.2 fg g�1 (0.079mBq),95 fg g�1 (3.8� 10�7mBq), 670 fg g�1 (8.3� 10�6mBq) and0.5 fg g�1 (7.3� 10�5mBq), respectively. The precision ofthe measurements ranged between 2–25% at 95% con-fidence level (k ¼ 2) depending on concentration level.The overall uncertainty was calculated with error propaga-tion rules taking into account the uncertainty of weightmeasurements, standard concentrations and measuredintensities.

4. Results and discussions

4.1. General characteristics of MnO2 impregnated resin

The approximate manganese dioxide content of the resinestimated by the difference of the resin weight before andafter impregnation is about 20% assuming that manganeseis in MnO2 form. The formation of MnO2 is expected to bethe result of the reaction of KMnO4 with the chloridecounter-ion of the quaternary-amine of AG 1-X4 resin. Asthis reaction takes place on the high surface of the resin it

results in high surface (high capacity) and strong binding tothe styrene–divinylbenzene support. The impregnated resin(Fig. 1a and b) consists of very fine grained, homogeneousaggregates. The average particle size estimated as anaverage of 20 randomly chosen particles is 99722 mm,which agrees with that of the support AG 1-X4 resin(100–200 mesh, equivalent to 74–140 mm), thus impregna-tion does not cause significant change in particle size. Thedensity of the resin is high enough to enable the easyseparation from the liquid phase as it settles easily.

4.2. Effects of pH on analyte uptake

The uptake of the analysed radionuclides as a functionof pH is shown in Fig. 2. This behaviour is similar to thoseobtained previously for the different MnO2 sorbents(Koulouris et al., 2000; Moon et al., 2003), however,deviation may arise from the different experimentalconditions (e.g. contact time or slightly different resin tosample ratio). In case of natural waters having pH between7.5 and 8.5 and above, quantitative pre-concentration canbe achieved without pH adjustment for radium, uraniumand thorium, though absorption is highly pH sensitive forradium and uranium. During sample preparation optimi-zation this feature has to be taken into account. In alkalimedium colloid formation and mineral precipitation of theradionuclide cannot be excluded, thus the mechanism ofuptake is presumably not only surface adsorption. Forplutonium, the uptake also depends on the differentsorption properties of different oxidation states. SinceU(VI) and Th(IV) can be considered the non-isotopicanalogue of Pu(VI) and Pu(IV), respectively, the differenceof plutonium uptake from both U(VI) and Th(IV) ispossibly is the consequence of the low affinity of Pu(V) tothe resin. As plutonium in seawater is primarily present insolution as Pu(V), probably as PuO2

+, this fact has to betaken into consideration during sample preparation andefficiency determination.

4.3. Kinetic behaviour investigations

Uptake of different radionuclides is usually highly timedependent, thus contact time in batch mode or flow rate incolumn arrangement have to be checked and verifiedduring the sample preparation optimization. The uptake ofthe investigated radionuclides at two different pH values isshown in Figs. 3 and 4. There is significant differencebetween the uptake curves indicating that the primaryparameter that affects adsorption is pH. At pH ¼ 8 uptakeof radium and uranium is very rapid, while for plutoniumand thorium the adsorption rate is slightly lower. In case ofplutonium this is presumably the result of the presence ofPu(V)-form. Using batch mode at pH ¼ 8 (the ratio of theliquid volume in ml and the resin weight in g isapproximately 500) more than 80% of radium, uraniumand thorium can be pre-concentrated on the resin within30min in the absence of matrix effects. Assuming that the

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Fig. 1. Light microscopical images of the prepared MnO2 impregnated resin at 45� (a) and 180� magnification (b).

pH Dependance

0

20

40

60

80

100

1 2 4 6 8 10 12 14

pH

Ad

so

rbed

, %

Radium

Uranium

Plutonium

Thorium

Fig. 2. Uptake of the investigated radionuclides as a function of pH. Liquid phase: 50ml, resin added: 100mg. Contact time: 15min. (Number of

replicates: 2.)

Kinetic Behaviour at pH = 6

0

20

40

60

80

100

0 2 5 10 15 20 30

Ad

so

rbed

, %

Radium

Uranium

Plutonium

Thorium

Time, min

Fig. 3. Kinetic behaviour of the investigated radionuclides at pH ¼ 6. Liquid phase: 100ml, resin added: 200mg. (Number of replicates: 2.)

Z. Varga / Applied Radiation and Isotopes 65 (2007) 1095–11001098

adsorption of radionuclides follows first-order kinetics,plotting the logarithm of the ratio of the remaining activityand total activity as a function of exposure time the

sorption uptake rate can be calculated (Eikenberg et al.,2001). The sorption uptake constant (lsorb) or sorptionhalf-life can be extracted from this regression analysis as

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Kinetic Behaviour at pH = 8

0

20

40

60

80

100

0 2 5 10 15 20 30

Ad

so

rbed

, %

Radium

Uranium

Plutonium

Thorium

Time, min

Fig. 4. Kinetic behaviour of the investigated radionuclides at pH ¼ 8. Liquid phase: 100ml, resin added: 200mg. (Number of replicates: 2.)

Simulated Seawater at pH = 8

0

20

40

60

80

100

0 20 25 10 15 20 30

Time, min

Ad

so

rbed

, %

Radium

Uranium

Plutonium

Thorium

Fig. 5. Kinetic behaviour of the investigated radionuclides in simulated seawater at pH ¼ 8. Liquid phase: 100ml, resin added: 200mg. (Number of

replicates: 2.)

Z. Varga / Applied Radiation and Isotopes 65 (2007) 1095–1100 1099

the negative slope of the fitted linear curve. The calculatedsorption uptake constants at pH ¼ 6 are 0.004, 0.009, 0.047and 0.44min�1 for radium, uranium, plutonium andthorium, respectively. At pH ¼ 8 the sorption uptakeconstants are 0.54, 0.71, 0.16 and 0.27min�1 for radium,uranium, plutonium and thorium, respectively.

4.4. Matrix effects on the radionuclide uptake

The uptake of the radionuclides from the syntheticseawater sample is shown in Fig. 5. The calculated sorptionuptake constants are 0.46, 0.22, 0.077 and 0.066min�1 forradium, uranium, plutonium and thorium, respectively.Radium uptake kinetic is not different from that of measuredin high-purity water, while uranium and plutonium sorption

kinetic is slightly slower from the synthetic seawater sample.Thorium uptake is strongly inhibited possibly as a conse-quence of complex formation. Thus, attention has to be paidto avoid this problem or it has to be controlled during thepre-concentration step (i.e. longer contact time in batchmode, use of higher amount of resin or higher pH).Furthermore, as natural waters usually contain carbonateion in highly variable concentration that may affect especiallyuranium uptake, radionuclide recoveries are advisable to bechecked analyzing a sample having similar chemical composi-tion (i.e. with standard addition method). For example, thehigh carbonate content of most natural waters will precludeuranyl retention on the resin. However, it offers an easypossibility for the separation uranium from the otherradionuclides if necessary (e.g. 234Th/238U disequilibrium

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measurement (Rose et al., 1994) or 239Pu/238U for ICP-SFMSanalysis (Varga et al., 2007)).

5. Conclusion

A simple and reproducible procedure for the preparationof MnO2 impregnated resin was developed that can be usedin either batch mode or in a column arrangement forradionuclide pre-concentration. Applying the resin thelabour intensive and cumbersome co-precipitation can beomitted. The proposed pre-concentration can be easilycoupled to the subsequent clean up steps (e.g. ion exchange,liquid–liquid exchange or extraction chromatography) as itavoids the use of elevated amount of reagents and highmanganese loading due to the high pre-concentration factor.However, during the optimization of sample preparationprocedure care has to be taken to control and check samplepH and some matrix interferences. With the thoroughcharacterization of the resin it can be a useful and versatiletool for radionuclide analysis in liquid samples.

Acknowledgements

Part of this study was carried out within a TechnicalCooperation with the International Atomic Energy Agency(IAEA) at the Marine Environment Laboratory, Monacoand financially supported by the Hungarian AtomicEnergy Authority (OAH-ANI-ABA-05/06).

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