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Journal of Environmental Radioactivity 116 (2013) 193e196

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate/ jenvrad

Short communication

The distribution of tritium between water and suspended matter in a laboratoryexperiment exposing sediment to tritiated water

Philippe Jean-Baptiste*, Elise FourréLSCE, CEA-Saclay, Centre de Saclay, 91191 Gif sur Yvette, France

a r t i c l e i n f o

Article history:Received 20 June 2012Received in revised form5 November 2012Accepted 5 November 2012Available online 29 November 2012

Keywords:TritiumOBTBio-accumulation

* Corresponding author.E-mail address: Philippe.Jean-Baptiste@cea.fr (P. Je

0265-931X/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jenvrad.2012.11.004

a b s t r a c t

Following recent suggestions regarding the strong affinity of tritiated water for organic matter in sus-pended particulates and sediments, two equilibration experiments between sediment organic matter(dry and fresh) and tritiated water were performed to look for potential tritium bio-concentration. The T/H ratios measured at the end of both experiments are lower in the sediment organic matter than in thewater, indicating that only a fraction of the hydrogen pool (between 14% and 20%) within the sedimentequilibrated with the tritiated water. These results are consistent with the widely used concept ofexchangeable and non-exchangeable tritium pools in organic matter and show no sign of tritium bio-accumulation in the sediment relative to water.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Anthropogenic tritium is released in various amounts byindustrial activities, including nuclear power stations, spent fuelreprocessing plants, radiochemical plants, nuclear weaponproduction and research facilities. Even if, at the global scale,tritium levels have declined by several orders of magnitude sincethe 1963 ban on atmospheric nuclear tests, these tritium sourcesmay be quite significant locally or even regionally. Although formost radionuclides technological improvements have led tosubstantial reductions in their discharge to the environment,detritiation is not an industrial option with currently availabletechnologies. As a consequence, tritium released by the nuclearindustry has increased, and even larger discharge figures may beexpected in the future with the possible commissioning of fusionreactors.

Doses of tritium to the public are low, due to the strong dilutioneffect by the hydrosphere and the low energy of the tritium betaradiation. Nevertheless, this situation of increasing discharge,coupled with several intriguing observations with respect totritium behaviour, has led to a renewed interest in the radioecologyof tritium:

an-Baptiste).

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- From a health protection perspective, recent studies havehighlighted the heterogeneous dose distribution delivered bytritium and the uncertainties in quantifying the effects oftritium exposure. These factors raise concerns that currentenvironmental and human risks of tritium may be under-estimated (Bridges, 2008; Little and Lambert, 2008; Little andWakeford, 2008).

- From an environmental perspective, observations of locallyenriched tritium levels in marine biota have raised the issueof tritium bio-accumulation. In the Bristol Channel (UK),concentration factors up to 104 between seawater and somemarine species have been observed (McCubbin et al., 2001;Williams et al., 2001). These high tritium levels have beenattributed to marine biota and sediments incorporatingtritium-labelled organic molecules released to the sea asradioactive waste by the Amersham radiochemical plant atCardiff (McCubbin et al., 2001), in agreement with earlierexperiments showing that marine organisms can selectivelyaccumulate tritiated molecules (Strack et al., 1983). Smallerenrichments relative to tritium concentration in seawaterhave also been reported near Sellafield (UK) and at variousplaces along the English Channel (Masson et al., 2005;Hunt et al., 2010). Some consider these to be cases ofbio-concentration. Along those lines, Turner et al. (2009)reported substantial enrichment of sediment organic matterequilibrated with water spiked with tritium, suggestinga strong affinity of tritiated water for organic matter in sus-pended particulates and sediments. However, in this latter

P. Jean-Baptiste, E. Fourré / Journal of Environmental Radioactivity 116 (2013) 193e196194

study the estimated amount of isotopic enrichment was notthe result of a direct measurement but was deduced from thedifference in activity of the spiked water before and after theaddition of sediment.

Since any biochemical or biophysical isotopic process capable ofconcentrating tritium collected from water molecules wouldindeed be quite remarkable, it seems essential to us that theseexperimental results be confirmed by a direct determination oftritium enrichment. The purpose of this note is to report such anequilibration experiment between tritiated water and sediment, inwhich the initial and final tritium activities in both liquid and solidphases were determined. This procedure, inwhich the componentsof the tritium budget are separately and independently measured,ensures a direct determination of any tritium isotopic effect. Asshown below, our results show no tritium enrichment above theisotopic equilibrium, thus failing to reproduce the Turner et al. data.Possible reasons for this conflicting result are reviewed anddiscussed.

2. Material and methods

The experiment consisted of equilibrating known amounts oftritiated water and natural sediments in tightly closed bottles.This allowed us to determine the change in the tritium distribu-tion between the solid and liquid phases due to sediment expo-sure to the tritiated water. Slightly tritiated water was obtained byspiking tap water with a tritium standard supplied by the Labo-ratoire de Radioanalyse et de Chimie de l’Environnement (CEA/Saclay e France). Two experiments were carried out: in the firstexperiment (Exp. A), we used dry (lyophilized) sediment,collected in 2010 in the mouth of the River Aulne (western Brit-tany) and stored in sealed plastic boxes. In order to investigate thepossible role of living organisms, a similar experiment (Exp. B)was carried out with fresh sediment collected in the same area in2011 immediately prior to the equilibration experiment. Theinitial tritium contents of the interstitial water and the organicmatter of the collected sediments were measured, as well as theinitial tritium content of the spiked water. In both experiments,water and sediment were equilibrated for three weeks. Thesamples were shaken every 12 h to ensure a good mixing of bothphases. At the end of the experiment, solid and liquid phases wereseparated using a centrifuge. The solid phase was dried ina lyophilizer and the final tritium contents of the water and drysediment were measured. All tritium measurements were carriedout using the 3He spectrometric technique (Jean-Baptiste et al.,1992, 2010).

3. Results

3.1. Activity budget

The results of experiments A and B are shown in Tables 1 and 2,respectively. For both experiments, the initial and final total tritiumactivities are identical within analytical uncertainties; this confirmsthat there was no detectable tritium loss from the system during

Table 1Activity transfer for a lyophilized sediment (experiment A) exposed to tritiated water (a

Dry sediment Spiked water

Mass (kg) OBT activity (Bq/kg) Mass (kg)

Initial Final

0.10032 1.15 � 0.05 1.75 � 0.04 0.20955

the experiment. In Exp. A, the spiked water lost 0.82% of its initialactivity, decreasing by 0.6 Bq kg�1, from 73.4 Bq kg�1 to72.8 Bq kg�1. This loss was taken up by the sediment, the activity ofwhich increased from 1.15 Bq kg�1 to 1.75 Bq kg�1 (þ52.2%). In Exp.B, the spiked water was mixed with the water naturally present inthe fresh sediment. The total activity of the water (i.e. the sum ofthe activity of the spiked water and the sediment water) decreasedby 0.4 Bq kg�1 from 36.1 Bq kg�1 to 35.7 Bq kg�1 (�1.11%). Thisactivity was transferred to the sediment, with an activity gain of89.1% (from 0.55 Bq kg�1 to 1.04 Bq kg�1).

3.2. T/H ratios

For both experiments, the initial and final T/H ratios of thesediment can be calculated from the measured activities AT (inBq kg�1) and hydrogen content of the sediment using the equation:

T=H ¼ AT

l� MH

N½H� (1)

where l is the tritium radioactive decay constant, N is Avogadro’snumber, [H] is the hydrogen mass fraction of the analysed drymaterial and MH is the mass of 1 mol of hydrogen (MH ¼ 10�3 kg).

The results are shown in Table 3. The final T/H ratios are lower inthe sediment than in thewater, indicating that only a fraction of thehydrogen pool of the sediment equilibrated with the spiked water.This result is consistent with the widely used concept ofexchangeable and non-exchangeable tritium pools in organicmatter (Diabate and Strack,1993). The exchangeable fraction, a, canbe calculated for each experiment using the equations:

�T=H

�i ¼ aðT=HÞexi þ �

1� a�ðT=HÞnexi (2)

�T=H

�f¼ aðT=HÞexf þ

�1� a

�ðT=HÞnexf (3)

where subscripts i and f stand for initial and final, and superscriptsex and nex for exchangeable and non-exchangeable. Here, theinitial state corresponds to the sediment equilibrated with itsinterstitial water in the natural environment and the final statecorresponds to the sediment equilibrated with the spiked water.Since by definition ðT=HÞnexi ¼ ðT=HÞnexf , a can be obtained bysubstracting equations (3) and (2) :

a ¼hðT=HÞf � ðT=HÞi

i.hðT=HÞexf � ðT=HÞexi

i(4)

where (T/H)ex is assumed to be equal to the T/H value of the waterin equilibrium with the sediment.

The a values calculated for experiments A and B are 14% � 7 and20% � 8 respectively (Table 3), indicating that about 80e86% of thehydrogen in the studied sediments is non-exchangeable. Thiscompares well with literature data, which show that a values forvarious types of organic matter are in the range 6.3%e26.8%(Schimmelmann, 1991).

ll uncertainties are at 2-sigma).

Tritium budget

Tritium activity (Bq/kg) Total activity (Bq)

Initial Final Initial Final

73.4 � 0.4 72.8 � 0.4 15.50 � 0.09 15.43 � 0.09

Table 2Activity transfer for a fresh sediment (experiment B) exposed to tritiated water (all uncertainties are at 2-sigma).

Dry sediment Interstitial water Spiked water Total water Tritium budget

Mass (kg) OBT activity (Bq/kg) Mass (kg) Tritium activity (Bq/kg) Mass (kg) Tritium activity (Bq/kg) Mass (kg) Tritium activity (Bq/kg) Total activity (Bq)

Initial Final Initial Initial Initial Final Initial Final

0.07194 0.55 � 0.04 1.04 � 0.05 0.14606 0.31 � 0.02 0.11205 82.7 � 0.4 0.25811 36.1 � 0.2 35.7 � 0.4 9.35 � 0.05 9.29 � 0.11

Table 3Hydrogen isotopic exchangeability (in percent of total hydrogen) in the organic matter of the studied sediments (experiments A and B) deduced from initial and final T/Hisotopic ratios for exchangeable and total OBT (Organically-Bound Tritium). The T/H value for exchangeable OBT is assumed to be that measured of the interstitial water of thesediment (initial state) or of the spiked water at the end of the equilibration experiment (final state). Hydrogen mass fractionwas measured with a CHN at ICSN (CNRS/Gif-sur-Yvette). All uncertainties are at 2-sigma.

[H] mass fraction (%) Initial state Final state Exchangeable fraction (a) %

Exchangeable OBT Total OBT Exchangeable OBT Total OBT

T/H (TU) T/H (TU) T/H (TU) T/H (TU)

Exp. A 0.67 � 0.05 2.5 � 0.1 160 � 19 612 � 3 244 � 24 14 � 7Exp. B 0.75 � 0.05 2.6 � 0.1 68 � 10 300 � 3 129 � 15 20 � 8

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4. Discussion

Our results show no sign whatsoever of tritium enrichment ofthe sediment relative to the water. This observation is in line withthe prevailing view that organic matter behaves as a two-component system, where one exchangeable hydrogen poolequilibrates isotopically with the ambient water, and one non-exchangeable pool keeps the tritium content acquired by theorganic matter when it was formed. This is in stark contrast withthe recent results of Turner et al. (2009), who claimed strongtritium enrichments in the same type of experiment. Onestraightforward explanation for this disagreement may be thatthese strong enrichments were not the result of direct measure-ment of the activity of the sediment, but were calculated from theactivity change in the water. These changes were assumed to beentirely transferred to and taken up by the sediment. In theirexperiment, the sediment-water distribution coefficient Kd isdefined as the sorbed activity per gram of dry sediment (in Bq g�1)divided by the activity of the water (in Bq mL�1). Eq. (5) belowexpresses the transfer of activity from the water to the sediment:

MsedAsed ¼ MwbAw (5)

whereMsed is themass of dry sediment (in g) andMw the volume ofwater (in mL). Ased is the tritium activity gained by the sediment (inBq g�1) and b Aw is thewater activity lost (in BqmL�1), expressed asa fraction b of the initial tritium activity of the water Aw (NB, in theTurner et al. experiment for instance, b is in the range 0.03e0.08).

Combining the definition of Kd (mL g�1), Kd ¼ Ased/(1 � b) Aw,and Eq. (5), it follows that:

Kd ¼ ðMw=MsedÞ � b=ð1� bÞor

Kd ¼ b=�1� b

�� 106=SPM (6)

where SPM (Suspended Particulate Matter) is the sediment loadMsed/Mw expressed in mg of dry sediment per L of water (Turneret al., 2009).

Given that SPM values can be as low as 10 mg L�1 (Turner et al.,2009), Eq. (6) shows that even a very small water activity loss b, ifwrongly attributed to sediment uptake, directly translates intoa substantial enrichment factor (Kd). Eq. (6) also implies that Kd is

not constant but follows an inverse relationship with the sedimentload. This is indeed the case in the Turner et al. results. Asacknowledged by the authors, the precise cause for this “particleconcentration effect” is “unclear” (Turner et al., 2009); but in ourview, this observation further supports our impression that at leastsome of the water-tritium decrease was actually not due to sedi-ment uptake, but rather due to some uncontrolled losses such asevaporation or adsorption on container walls. The best way toclarify the situation would be to repeat the experiment, but thistime with a separate, independent measurement of the finalactivity of the particulate matter.

5. Concluding remarks

We carried out equilibration experiments between sedimentmatter (dry and fresh) and tritiated water in order to reproduce theintriguing, but potentially very important, findings of Turner et al.(2009) concerning tritium enrichment of particulate matterexposed to tritiated water. Our results fail to reproduce their data.We suggest that this disagreement is due to a misinterpretation oftheir data due to the lack of separate measurement of the finaltritium activity of the particulate matter. On the other hand, ourresults are consistent with the prevailing view that hydrogen inorganic matter behaves as a two component system: oneexchangeable hydrogen pool which equilibrates isotopically withthe ambient water and one non-exchangeable pool that keeps thetritium content acquired by the organic matter when it wasformed.

This does not mean that tritium enrichments relative to waterdo not exist. As observed in some marine biota, tritium enrichmentdoes indeed occur; however, this enrichment is not due to some asyet unknown isotopic effect but to the uptake of tritiatedcompounds already present in the medium. These compounds mayeither be artificially tritiated organic molecules dumped into theenvironment as in the Amersham example, or organic matterformed in a tritiated water plume close to a discharge area andsubsequently dispersed by currents, as may be occurring along thecoast of the English Channel. Thus, the concentration factor can bequite a misleading concept when applied to tritium, since it seemsto imply that tritium concentrated in aquatic organisms is derivedfrom the tritiated water molecules themselves. It is thus importantto remember that whereas there is good scientific evidence for thebio-accumulation of tritium from tritiated compounds or tritiatedsuspended matter present in the aquatic environment, there is no

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evidence so far for tritium bio-accumulation from the tritiatedwater itself.

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