Rapid remobilisation of plutonium from estuarine sediments

J. Environ. Radioactivity 5 (1987) 409-423

Rapid Remobilisation of Plutonium from Estuarine Sediments

J. Hamilton-Taylor, M. Kelly, S. Mudge and K. Bradshaw Department of Environmental Science, University of Lancaster, Bailrigg, Lancaster, LA 1 4YQ, UK (Received 8 August 1986: accepted 19 December 1986)

A B S T R A CT Plutonium remobilisation experiments, using contaminated Esk Estuary sediments" and uncontaminated River Esk and North Sea waters', have successfully reproduced the observed non-conservative behaviour of dissolved Pu in the Esk Estuarv, Cumbria, UK. Remobilisation is greatest at low salinites (<4O/oo) and, under these conditions, Pu(lll, IV) and Pu(V, VI) are released into solution in similar absolute amounts. The remobilisation process appears" to be associated with a rapid (equilibrium achieved in 15min) exchange reaction involving competition between Pu species, protons and major cations for the available surface sites. The experimental results show a systematic variation in distribution coefficients (i.e. K,i) ]br total Pu (7.2 × 104-4.1 × 10°), Pu(lll, IV) (1.0 × 105-5.8 × 10 ~) and Pu(V, VI) (6.4 × 103-5.5 × 105).

INTRODUCTION

Plutonium in the Irish Sea, originating in low-level waste discharges from the Sellafield nuclear fuel reprocessing plant, is transported into the estuaries of the area by tidal inflow. A much smaller quantity of Pu, derived from atmospheric fallout, is brought in by rivers. In the Esk Estuary (Cumbria, UK), Pu enters from the marine source both in particulate form (i.e. retained by a 0.22 ~m membrane filter) and in solution (Assinder etal., 1985). Dissolved Pu behaves non-conservatively in the estuary, with the low-salinity waters being substantially enriched in Pu relative to the 409

J. Environ. Radioactivity 0265-93 IX/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

1 007.6 (44) 946.7 (28) 3-8 (0.1) 0.03 (0.01) 0.07 (0.01)

38.6 (2.2) 48-8 (2-0) 12-9 (0.2) 0-02 (0-01) 0-13 (0-02)

1 046.2 (44-1) 995-5 (27.9) 16.7 (0-2) 0.05 (0.01) 0.20 (0.02)

Pu (V, VI) Total Pu (mBqg -1 o r m B q l 1)

Ptt a

7.74 6.4(I 8.10

pH

+ 184 -

Eh (mV)

21-5 32.0

S (%0)

20 2-2 1.(I

DOC (mgl I)

a Values in p a r e n t h e s e s are 1 o- propagated counting errors. b M o i s t u r e c o n t e n t of s e d i m e n t = 37.57 _+ 1-09% (n = 6). c S e d i m e n t p o r e w a t e r s p r o v i d e a negligible direct contribution to the dissolved activities observed in the e x p e r i m e n t s ( m a x i m u m c o n t r i b u tion = 2-3 × 10 -3 m B q P u ( l l l , IV) a n d 7.7 × 10 -3 m B q P u ( V , VI)).

Porewater c River water Sea w a t e r

Sediment b

Pu (111, IV)

239, 240

TABLE I Analytical Data tor E n v i r o n m e n t a l Materials

i

z:

Rapid remobilisation of plutonium from estuarine sediments

4 11

theoretical dilution curve (Assinder et al., 1984; Eakins et al., 1987). Both groups of workers suggested that the excess Pu is remobilised from suspended sediment. This phenomenon is important since it represents a potential mechanism for the remobilisation of Pu from the extensive sediment stores in the Irish Sea, via their onshore transport. In order to investigate the mechanisms involved, a series of desorption experiments was carried out in the laboratory. The environmental materials employed were kept in their initial field conditions as far as possible and the experiments were intended to reproduce realistic estuarine conditions.

M A T E R I A L S AND M E T H O D S The materials used were (a) Esk Estuary sediment, comprising the surface 1 cm of an intertidal mud from Newbiggin maintained in its oxidised and field moist state, (b) filtered River Esk waters from above the tidal limit and (c) filtered seawater from the North Sea at Bridlington. Their initial characteristics are shown in Table 1. Both the River Esk water and the North Sea water are effectively uncontaminated relative to the Pu concentrations in the estuary. The Pu(V, VI):Pu(III, IV) ratio in the North Sea (ca. 2: 1) is low compared to that in the Irish Sea (Table 2). This agrees with the unpublished data of Nelson, who found that the ratio decreased with increasing distance away from the Irish Sea (Edgington & Nelson, 1984). In our experiments, the sediment was suspended in the natural waters at various concentrations (Figs 1-4) intended to cover the range observed in TABLE 2 Oxidation State Ratios for 239,240pu

Sample type Newbiggin sediment Newbiggin sediment porewater ( < 0.22/xm) Desorption experiments ( < 0.22~m) River Esk water North Sea water North Sea/River Esk mixes Irish Sea C u m b r i a n coastal suspended sediment ( > 0.22/,tm) C u m b r i a n coastal waters ( < 0.22/~m)

Source

Pu ( V, VI) : Pu (111, IV)

a "

1:24 3:1

'~ '~ '~

b b

1:2"4--2-6:1 6:1 1'1:1--18:1

1: 10--1:77 4:1--11:1

This study. b Nelson & Lovett, 1978. The four 'shoreline' values are used in the comparison.

412

J. Hamilton-Taylor, M. Kelly, S. Mudge, K. Bradshaw

10.

1000 mg 1-1

f

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100 mg 1-1 ~/'

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0.01

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,

Time

Fig. !.

~

'

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/h

desorption in river water: distribution with time of dissolved phase Pu(ill, IV), Pu(V, VI) and total Pu activities, for three sediment concentrations.

239'24°pu

the estuary. Suspended loads are generally less than 100 mg litre 1but peak values in excess of 500 mg litre-I have been reported (Assinder et al., 1985). Pu remobilization was determined at various salinities, obtained by mixing River Esk and North Sea waters. Particular attention was given to lowsalinity and pure river waters, which come into contact with the con-

Rapid remobilisation of plutonium from estuarinesediments

413

.0

10, i

.1

"7 "3 E

6

nr, w ~,~I

I

% 2

0

1'0

2'0

3~0

4'0

Ti me / mi n Fig. 2. 239"24°pudesorption in river water: distribution with time of dissolved Pu activities and hydrogen ion uptake, with a sediment concentration of 860mgl ~ (with lo- counting errors; A[H +] is the difference between the hydrogen ion concentration at a given time and the initial hydrogen ion concentration).

taminated Esk sediments at certain states of the tide and have the highest concentrations of excess dissolved Pu (i.e. that above the dilution curve) (Assinder et al., 1984; Eakins et al., 1987). The desorption experiments were conducted under conditions of continuous mixing (i) as batch experiments using either sets of 1-1itre of solution or sequentially sampled 15-1itre bulk samples and (ii) in a specially designed 320-ml capacity continuous-flow cell in which the sediment is retained by membrane filters. Millipore 0.22 p,m GSWP membrane filters were used throughout. One-litre filtered samples from the experiments were analysed by c~-spectrometry for 238puand 239'24°pu in each of the two oxidation-state categories, (III, IV) and (V, VI), after separation according to the method of Lovett and Nelson (1981). Analysis is based on the use of a holding oxidant solution to maintain the Pu(V, VI) state whilst Pu(III, IV) is co-precipitated with NdF3. Pu(V, VI) is then reduced to Pu(III, IV) and precipitated. Lovett and Nelson (1981) found only a ca. 0.25% per week reduction of Pu(V, VI) to Pu(III, IV) in seawater and 1% per day in distilled water. All samples were analysed within 18 h of experimentation.

J. Hamilton-Taylor, M. Kelly, S. Mudge, K. Bradshaw

414

(b)

4

7: A

'7

'7

0"

E 4

E

n o

"1"

d

<1 I

OI

%

i

i

i

2

i

I

I

T

4

Volume discharged

I

6 /I

Fig. 3. 239"24°pu desorption in river water: variation of dissolved phase Pu activities and hydrogen ion uptake in continuous flow experiment, with a sediment concentration of 430 mg 1-1 (A[H +] as defined in Fig. 2).

RESULTS Only data for 239'24°pu a r e presented here. The trends for 238pu a r e similar but of smaller magnitude. The mean 238pu:239"24°pu ratio in the dissolved fraction is 0.23 - 0.01 (n -- 40).

Desorption into freshwater Figure 1, derived from sequentially sampled, large volume batch experiments, shows the following: (i) Pu desorption by river water yields total dissolved Pu activities similar to the in situ excess levels in the freshwaters of the estuary (up to ca. 15 mBq litre-], Assinder et al., 1984). (ii) Initial Pu desorption is rapid with dissolved activities varying relatively little after 15 min.

Rapid remobilisation of plutonium from estuarine sediments

415

10.

10 ~ S a l i n i t y

20 / %o

30 b

14

E 10

~

a.

~

6 ]ll,lg

16o

2ao

360

NaCI / m M Fig. 4. 239'24°pu desorption in (a) seawater/river water and (b) NaCl/river water mixtures: dissolved phase Pu activities with a sediment concentration of 840 and 870 mg 1-1 respectively and 1 h desorption time (with 1 o- counting errors). E = total Pu.

(iii) Total amounts of Pu desorbed increase non-linearly with sediment concentration. A hundred-fold increase in sediment concentration from 10 to 1000 mg/l produces a ten-fold increase in dissolved Pu activity. (iv) The two oxidation-state categories appear to undergo independent initial desorption reactions. There is no clear evidence of any rapid interconversion once in solution, although dissolved Pu(V, VI) consistently shows a small increase in activity throughout the experimental period.

416

J. Hamilton-Taylor, M. Kelly, S. Mudge, K. Bradshaw

(v) During the initial desorption step, the two oxidation-state categories are of similar magnitude, with Pu(III, IV) generally being the greater (dissolved Pu(V, VI):dissolved Pu(III, IV) of 1:2). This is in marked contrast to local coastal waters, where Pu(V, VI) predominates in the dissolved phase (Table

2). A batch experiment with greater time resolution (Fig. 2) confirms the short equilibation period of ca. 15 min and shows a similarity between the rates of Pu remobilisation and decreasing hydrogen ion concentration (i.e. proton uptake by the sediment). A desorption experiment with the continuous flow cell was conducted so that the River Esk water had a mean residence time of 1 h. A good relationship is again evident between the changes in hydrogen ion concentration [H *] and Pu activity in water leaving the cell (Fig. 3). Bearing in mind the equilibration period of ca. 15 min, the continuous pH record is interpreted as follows. Prior to the experiment, the sediment is probably at equilibrium with its high pH, saline porewater. The rapid initial increase in pH (see A[H +] in Fig. 3) represents a period of large-scale proton adsorption by the sediment under non-equilibrium conditions. The subsequent phase of slowly decreasing pH reflects an equilibrium condition with respect to the sediments, whose surface hydrogen ion activity is gradually increasing. Finally, the sediment is protonated to the point where it is at equilibrium with the river water entering the cell (after 6 i have been discharged; see Fig. 3). Thereafter the effluent pH is constant at the river water value. Only a small proportion of the total sediment-bound Pu was mobilised during the course of the continuous leaching experiment. If no redox transformations occurred (see Discussion), the remobilised fractions constitute ca. 3% of the original Pu(III, IV) content and ca. 60% of the much smaller Pu(V, VI) content of the sediments. Desorption into saline waters

Separate batch desorption experiments of 1-h duration were performed at various salinities. Figure 4a shows that the remobilisation of total Pu is maximal at low salinities and varies little above ca. 4%o. This behaviour is, in the main, attributable to the order of magnitude increase in Pu(III, IV) remobilisation below ca. 4%o salinity. In contrast, remobilisation into the more-saline waters results in Pu(V, VI):Pu(III, IV) ratios similar to those observed in the Irish Sea (Table 2). An equivalent experiment using Analar NaCl/river water solutions produced a similar Pu(III, IV) trend but with higher absolute activities (Fig.

Rapid remobilisation of plutonium from estuarinesediments

417

4b). At the same time, it failed to reproduce the low-salinity behaviour of Pu(V, VI). Two control experiments were also conducted using NaCl/river water solutions. The first was carried out in order to assess whether saltinduced coagulation of dissolved Pu contributes significantly towards the observed salinity effects. In this, River Esk water was equilibrated with contaminated sediment at a concentration of 864 mg litre -~ for 1 h, then filtered. Analar NaCI was added to the filtrates to give the same range of concentrations as in Fig. 4b. The solutions were stirred for a further 1 h and then re-filtered. Under these conditions, dissolved Pu(III, IV) and Pu(V, VI) activities exhibited no salinity dependence and were comparable with the pure River Esk desorption values. Taken together, these results suggest that the observed behaviour of Pu is not a function of salinity-dependent colloidal disaggregation-aggregation processes. The purpose of the second control was to assess the reversibility of the desorption reactions. As in the first control, River Esk water was equilibrated with contaminated sediment at a concentration of 866 mg litre-~. This was done in eight separate 1-1itre batches. Analar NaCI was then added in the same amounts as used previously, but this time the sediment was not removed beforehand by filtration. The samples were stirred for 1 h after NaCI addition and then filtered. Unlike the first control, this experiment did successfully reproduce the NaCI-dependent behaviour of Pu(II1, IV) seen in Fig. 4b. Thus Pu(III, IV) remobilised by low-salinity waters (<4%0) in the estuary around low tide will be readsorbed as mixing with more saline waters occurs on the flood.

DISCUSSION The Pu from Sellafield effluent, in its passage through the Irish Sea, is present in both oxidised and reduced states, the proportions of which are sensitive to the nature of their environment. The reduced state, generally considered to be Pu(IV) (Aston, 1980; Silver, 1983; Edgington & Nelson, 1984), is dominant in both the particulate and dissolved phases in the effluent itself (Pentreath et al., 1986) and in the marine porewater/sediment system (Harvey & Kershaw, 1984). In surface marine waters, the reduced state predominates only in the particulate phase (Nelson & Lovett, 1978), while the dissolved phase is mainly oxidised Pu(V), probably a s P u O 2 + (Edgington & Nelson, 1984; Orlandini et al., 1986; Pentreath et al., 1986). O u r data suggest that the known remobilisation of Pu in the Esk Estuary is related to rapid surface-exchange reactions involving competition between protons, major cations and Pu(III, IV) and Pu(V, VI) species for available surface sites, e.g. on oxyhydroxides--oxides, clay minerals and organic

418

J. Hamilton-Taylor, M. Kelly, S. Mudge, K. Bradshaw

matter. In the experiments, Pu-contaminated Esk Estuary sediments have been mixed with cation-poor, proton-rich River Esk water. Consequently, hydrogen-ion uptake and cation exchange (including Pu species) occur until a new equilibrium condition is established. These exchange reactions are frequently represented in terms of a surface complex model (Bourg, 1983): S O H + M z+ ~

SOM(Z-I)+ + H +

(l)

where S = surface sites and M z+ -- seawater cations and Pu species. Such reactions are dependent on many variables, including pH, suspended matter concentration (as a result of mass-balance considerations), ionic strength and relative ionic composition (see Bourg, 1983). The consequences are therefore especially complex in the estuarine environment. The general trends observed in the experiments dealing with sediment concentration dependence (Fig. 1) and continuous leaching (Fig. 3), in principle, can be explained in terms of the exchange reaction and massbalance considerations. In addition, major cations will compete amongst themselves for the available surface sites and the effects of this competition will vary with both ionic strength and changing ionic ratios. The chemical speciation of dissolved Pu will also vary with salinity. Significantly, the salinity-dependent desorption effects (Fig. 4a) occur over that part of the salinity range (< ca. 4%) in which changes in relative ionic composition are especially abrupt resulting in equally sharp changes in thermodynamic equilibrium conditions (see Mantoura et al., 1978; Morris et al., 1978). The salinity effects are therelbre likely to be a function of all the factors discussed above. The observed kinetics of the initial desorption step (Pu release and proton uptake, see Figs 1 and 2) are comparable with those found for similar exchange reactions which have been proposed for marine clays (Li et al., 1984) and for Pu(IV) adsorption by geothite in simple ionic media (Sanchez et al., 1985). Sanchez and co-workers were able to provide a good fit of their data to a more specific surface complexation model involving the adsorption of individual Pu(IV) hydrolysis species. In addition, Edgington and Nelson (1984) have used a similar model to eqn 1 to describe the interaction of Pu(IV) ions with sediment surfaces. The near-stable Pu activities obtained in the batch experiments have been referred to as equilibrium values. This is not meant to imply complete redox equilibrium within the system, only that the rapid exchange reactions have approached an apparent equilibrium with respect to the initial sediment surface activities of Pu(III, IV) and Pu(V, V1). There is growing evidence that Pu reclox reactions occur preferentially at the solid surface (KeeneyKennicutt & Morse, 1985; Sanchez et al., 1985). Some preliminary

Rapid remobilisation of plutonium from estuarinesediments

419

experiments of our own similarly suggest the occurrence of redox transformations at the sediment surface and also indicate further slow changes in the dissolved Pu(III, IV) and Pu(V, VI) activities over a period of days. Nevertheless, under the existing conditions in the Esk Estuary, the rapid exchange reactions reported here are likely to be the major control on dissolved Pu activity over the time-scale associated with tidal cycling. The enhanced remobilisation of Pu(III, IV) at low salinities (Fig. 4a) is of particular interest since it is the first reported indication that the chemicallyreduced form can dominate Pu(V, VI) in the open waters of the Irish Sea area. Pu(III, IV) enrichment in non-saline waters has been reported from several other situations in the world, for example in shallow groundwaters (Rees & Cleveland, 1982) and freshwater lakes (Wahlgren & Orlandini, 1981; Bondietti, 1982; Nelson et al., 1985) but usually in association with high or significant dissolved organic carbon (DOC) concentrations. At this stage, it is not possible to rule out DOC entirely as an important factor in our experiments but a number of observations suggest otherwise. These include the presence of similar DOC concentrations in the river and seawater samples used (Table 1), the very abrupt change in Pu remobilisation at 10% seawater (Fig. 4a) and the absence of any removal of dissolved Pu via coagulation, brought about by the addition of NaC1 (see earlier description of salinity experiments). The lack of DOC involvement also appears reasonable in that the DOC concentrations in the River Esk waters (13 mg litre 1) are around the critical threshold value suggested by Nelson et al. (1985), below which no enhanced Pu(III, IV) remobilisation is observed. In spite of the obvious complexity in Pu solid-solution behaviour, simple distribution coefficients (i.e. Kd) are widely used in the literature to predict solution-phase activities. Previously observed Kd values for total 239"24°pu in the Esk Estuary show a wide range of values (Table 3). Figures 5a-c show how Kd varied under the experimental conditions. The Kd values of the two oxidation-state categories in saline waters are comparable to those given for the Irish Sea (Nelson & Lovett, 1978). The variation of total Pu Kd with respect to salinity is broadly similar to that observed in a series of comparable desorption experiments with Esk Estuary sediments (Burton, 1986) (Table 3). Burton interpreted the low total Pu Kd values (minimum 5"0 × 104, compared to our own of 7.2 x 104 litres kg-~), obtained at low salinities, as being related to an oxidative remobilisation reaction producing Pu(VI) in solution but did not measure the two oxidation-state categories separately over the appropriate salinity range. In reality, he was probably observing the same process as that described here. A n o t h e r problem is that Kd values are often applied to natural sediments without any attempt to differentiate between the different forms of sediment-bound Pu. Burton (1986) found total Pu Kd values of ca.

Kd

2

1

4

1

6

Volume discharged

//

A,'

/

J

/I

0

e~w,~ ~

/

I

/

1 t0

J._

Salinity/%

, I

-----~

o

210

I

l/,~/I

Y

30

Sediment

1'0

\

\

\

1=03

*3"

concentration

1102

\

/ mg 1-1

Fig, 5. Variation of Kd for Pu(II[, IV), Pu(V, VI) and total Pu (E) for I h desorption with (a) volume of river water eluted in continuous flow experiment; (b) salinity; and (c) sediment concentration.

0

1 o 4_

o 8.

~o 7.

i

421

Rapid remobilisation o f plutonium from estuarine sediments TABLE 3 Distribution Coefficients (Kd) for 239.240pu in the Esk Estuary and the Irish Sea

Ka (x 10-4) Source

Irish Sea

Pu (111, IV)

Pu (V, VI)

Total Pu

34-600 a

0-6-2-8 ~

7-3-90 a

Esk Estuary Esk Estuary C

4-142 b 4.5-154 a

Nelson & Lovett, 1978 Assinder et al., 1985 Burton, 1986

a Kd values in litres kg -~.

b Ka values in kg kg -l. " Laboratory experiments.

1 x 106 litres kg -1 in desorption experiments with Esk Estuary sediment and North Sea water but values of ca. 1 × 104 litres kg -1 for adsorption experiments using contaminated saline waters and uncontaminated North Sea mud. These observations were explained by the existence of a sorbable and non-sorbable Pu species in solution. We propose an alternative hypothesis based on the presence of a large non-exchangeable fraction in the Esk Estuary sediments. The application of traditional sequential leaching procedures to Esk sediments indicates that the exchangeable fraction represents only a small proportion of the total Pu activity: < 3-5% according to Livens et al. (1986) and 2.5-12% (exchangeable and water soluble) according to Wilkins et al. (1987). Unpublished data of our own provide similar values (ca. 5%). If a figure of 5% for the exchangeable fraction is used, instead of the total Pu activity, to calculate a Kd for our seawater desorption experiment (5.5 mBq litre -~ total Pu desorbed in 100% seawater--see Fig. 4b), a value of 1 z 104 is obtained. Thus, the discrepancy disappears between the desorption and adsorption Kd values of Burton (1986). Caution is clearly required in applying Kd values for predictive purposes. The values will be a function of the many factors identified in this study and, by logical extension, of the environmental history of the sediment particles.

ACKNOWLEDGEMENTS This work was carried out as part of a contract with the UK Department of the Environment, and the results are' published with the agreement of the Department. The assistance of M. Gough, H. Vasey and J. Wrench is gratefully acknowledged.

422

J. Hamilton-Taylor, M. Kelly, S. Mudge, K. Bradshaw REFERENCES

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Morris, A. W., Mantoura, R. F. C., Bale, A. J. & Howland, R. J. M. (1978). Very low salinity regions of estuaries: important sites for chemical and biological reactions. Nature, 274,678-80. Nelson, D. M. & Lovett, M. B. (1978). Oxidation state of plutonium in the Irish Sea. Nature, 275,599-601. Nelson, D. M., Penrose, W. R., Karttunen, J. O. & Mehlhaff, P. (1985). Effects of dissolved organic carbon on the adsorption properties of plutonium in natural waters. Environ. Sci. Technol., 19, 127-31. Orlandini, K. A., Penrose, W. R. & Nelson, D. M. (1986). Pu(V) as the stable form of oxidised plutonium in natural waters. Mar. Chem., 18, 49-57. Pentreath, R. J., Harvey, B. R. & Lovett, M. B. (1986). Chemical speciation of transuranium nuclides discharged into the marine environment. In Speciation of Fission and Activation Products"in the Environment, ed. by R. A. Bulman & J. R. Cooper, Elsevier, London. Rees, T. F. & Cleveland, J. M. (1982). Characterization of plutonium in waters at Maxey Flats, Kentucky, and near the Idaho chemical processing plant, Idaho. In Proc. Symp. Environmental Migration of Long-lived Radionuclides, 27-31 July 1981, Knoxville, 1AEA-SM-257/66, 41-52. Sanchez, A. L., Murray, J. W. & Sibley, T. H. (1985). The adsorption of plutonium IV and V on geothite. Geochim. Cosmochim. Acta, 49, 2297-307. Silver, G. L. (1983). Comment on the evaluation of the chemical forms of plutonium in seawater and other aqueous solutions. Mar. Chem., 12, 91-6. Wahlgren, M. A. & Orlandini, K. A. (1981). Comparison of the geochemical behaviour of plutonium, thorium and uranium in selected North American lakes. In Proc. Symp. Environmental Migration of Long-lived Radionuclides, 27-31 July 1981, Knoxville, IAEA-SM-257/89, 81-96. Wilkins, B. T., Green, N., Stewart, S. P., Major, R. O. & Dodd, N. J. (1985). The disposition of caesium-137, iodine-129, ruthenium-106, strontium-90, americium-241 and plutonium in estuarine sediments and coastal soils. In Proc. CEC Seminar, Renesse, The Netherlands, 1984. CEC Publications XII/380/85 EN.