A radiochemical procedure for the determination and speciation of radiocobalt in environmental waters

The Science of the Total Environment, 130/131 (1993) 237-251

237

Elsevier Science Publishers B.V., Amsterdam

A radiochemical procedure for the determination and speciation of radiocobalt in environmental waters K.S. Leonard, D. McCubbin and B.R. Harvey Ministry of Agriculture, Fisheries and Food, Directorate of Fisheries Research, Fisheries Laboratory, Pakefieid Road, Lowestoft NR33 OHT, UK

ABSTRACT Radiocobalt can oceur in the aquatic environment in two different oxidation states. Under both fresh and saline conditions uncomplexed Co(II) normally predominates, however in certain circumstances anthropogenic Co(III) picolinate, and to a lesser extent, naturally occurring cyanocobalaw;.ae may also be preseat. Some complexed species are sufficiently stable to persist in the environment for considerable periods of time and thus influence the behaviour and distribution o~'radiocobalt. An analytical procedure has therefore been developed to permit the chemical separation and determination of the concentrations of Co01) and Co(Ill) radionuclide species in environmental waters. Details of the scheme are presented along with a discussion of the relative stabilities of the more important cobalt complexes. Decontamination from interfering radionuclides and the radiometric assay of three environmentally important cobalt radionuclides are also considered.

Key words: radiochemical procedure; determination; speciation; radiocobalt; environmental waters

INTRODUCTION

The behaviour of radionuclides in the environment will be dependent upon their physical and chemical forms, the source term and the prevailing conditions to which they are discharged. The interactions of radionuclides with particles are frequently described in terms of the distribution between the solid and dissolved solution phase (Kd) and provides a means for estimating sorption properties and hence the likely transport of radionuclides in the component phases. In the dissolved phase, physico-c~emical associations with other soluble components (e.g. colloidal organic matter) of different molecular size fractions may also contribute to the differential behaviour of radionuclides (Orlandini, 1990). In solution, certain radionuclides may also exist in more than one oxidation state such that their chemical behaviour in

238

K.s. LEONARD ET AL.

the aquatic environment will be dependent upon their relative distribution between the various oxidation state species. The importance of developing chemical methods for the separation of different oxidation states of plutonium in seawater has been demonstrated (Nelson and Lovett, 1978) and has provided a foundation for numerous studies of the environmental behaviour of transuranics in aquatic systems (Hamilton-Taylor et al., 1987; Nelson et al., 1989; Malcolm et al., 1990; and others). Cobalt occurs biologically in vitamin B~2 and prolonged deficiency results in deterioration in red blood cell formation in man and a disturbance of cell division in m~ny organisms. The occurrence and distribution of soluble vitamin B~2 in~ seawater has been previously reported (e.g. Carlucci and Silbernagel, 1966). Although radiocobalt is only a small component of radioactive waste discharges from various water cooled nuclear reactors it has important ~radiological significance (Preston and Dutton, 1970). Previous investigations !have chiefly been concerned with determining the retention and mobilisation characteristics of radiocobalt (and stable cobalt) with sediments. It has been shown that redox conditions are important in that mobilisation of ~:obalt occurs in sedimentary anoxic conditions but not in oxic layers (Heggie et al., 1986; and others). The behaviour of possible chemical forms of 6°Co on sediment/water systems has been evaluated under various environmental c~nditions in the laboratory by comparing dialysis and subsequent extraction techniques (Mahara and Kudo, 1981). The nuclide was categorised as mobile, exchangeable or irreversible depending on the magnitude of its mobility. Different chemical forms (anionic, cationic and neutral) have been observed in seawater and, to a greater extent, in pore waters by a voltametry technique but discrete species have not been identified (Huynh-Ngoc et al., 1989; Whitehead and Huynh-Ngoc, 1990). The present paper describes the development of a radioanalytical procedure designed to separate and determine radiocobalt in both its higher and lower oxidation states. The performance characteristics of the method are discussed and the extent to which it may contribute to the greater understanding of the speciation and distribution of radiocobalt in environmental waters is considered. ANALYTICAL PROCEDURE

Stability of cobalt species Cobalt has two commonly observed oxidation states, +2 and +3, in aqueous solution. The hydrated Co(Ill) ion is a strong oxidising agent and hence is thermodynamically unstable, as indicated by the standard electrode potential:

RADIOCHEMICAL DETERMINATION OF RADIOCOBALT

Co(H20)63+ 4- e- ~ C0(H20)62+

239

E 0 - 1.84 V

However, in the presence of complexing ligands the stability of the Co(III) species is greatly enhanced. Indeed this species forms numerous complexes with many ligands, some of which are stable in aqueous solutions. In general, thermodynamic formation constants of organo-Co(III) complexes are greater in magnitude than corresponding organo-Co(II) complexes. Kinetically, Co(III) complexes generally undergo ligand exchange reactions relatively slowly compared to Co(II) complexes. For example, although the thermodynamic formation constant of [Co(NH3)6] 3+ is small (log K = -25) it will persist for days in an acid medium because of its lack of lability (Cotton and Wilkinson, 1980). Therefore, it is reasonable to assume from thermodynamic and kinetic considerations that separation of the two oxidation states of cobalt in an environmental sample is possible. Because the ligand exchange of Co(II) is fast and ligand exchange of Co(III) is slow, separation of Co(II) from Co(III) is achieved with the minimum of disturbance to the equilibrium. In theory, quantitative recovery of the Co(III) complex is accomplished by creating strong reducing conditions in the analytical scheme by which the resultant reduced form I,~ndergoes rapid exchange of ligands.

Separation of chemicalforms of radiocobalt Several authors have reported the application of scavenging agents (e.g. iron hydroxide and manganese dioxide) for the assay of dissolved radiocobalt concentrations in aqueous solutions (Yamagata and Iwashima, 1963; Chakravarti et al., 1964; Bains, 1978). Concentration methods for isolating organically bound (cyanocobalamin) and ionic radiocobalt tracers in seawater have been studied to estimate the chemical yield (Lowman and Ting, 1973). It has also been observed that marked differences occur in the recovery of ~°Co from groundwaters derived from shallow land seepage pits containing the complexing agent EDTA (Olsen et al., 1983). Coprecipitating agents including ferric hydroxide were unable to remove significant amounts of the radionuclide whereas increased removal rates were observed after treatment witL, a reducing agent. This observation also supports the existence of a trivalent complex. For the procedure described here, it has been demonstrated that it is possible to distinguish between divalent cobalt and complexed tfivalent cobalt in aqueous solution in most cases by quantitatively scavenging the former species with ferric or nickel hydroxide precipitates. This is achieved by the addition of the scavenging agent to the filtered (0.45/~m) and acidified sample. The pH is then increased (to approx, pH 9) by the addition of sodium hydroxide solution. The complexed form is not coprecipitated under oxic

240

K.S. LEONARD ET AL.

conditions and remains in solution. This species may be recovered by introducing strong reducing conditions and rescavenging. In practice this is achieved by the addition of ammonium ferrous sulphate and sodium sulphite to the acidified sample to reduce any complexed cobalt to the divalent state. Hydrogen sulphide is generated in situ by the addition of sodium borohydride and sodium sulphite to the sample under acid conditions and the remaining cobalt is finally coprecipitated with iron sulphide under alkaline conditions. Ideally two samples are required for the chemical speciation procedure to ensure the precise determination of the yield recovery tracer. Complexed trivalent cobalt is determined as the difference between the assay of total dissolved cobalt (scavenged under strong reducing conditions) and the assay of divalent cobalt (scavenged under oxic conditions). It has not been possible to demonstrate the applicability of this separation procedure for all F~ssible complexes of cobalt. However, the separation of four different complex species has been investigated in the laboratory. Table 1 shows the separation of various cobalt species of environmental importance (humic material and cyanocobalamin) and of relevance to the environment as a result of use in the nuclear industry (EDTA and picolinate). The results demonstrate that the procedure quantitatively recovers uncomplexed Co(II) and Co(II) humate complexes and also separates Co(III) complexes of EDTA and picolinate. Only partial recovery is observed for cyanocobalamin presumably because the enhanced stability of the cobalt atom, which is central within the tetradendate ring structure, suppressed its release under reducing conditions. Under similar conditions, Lowman and Ting (1973) reported that cyanocobalamin recovery is poor but more success (51% yield) was observed by the oxidation of the complex. Laboratory experiments carried out by Santschi (1988) indicated that different characteristics of desorption from suspended particles could be created for radiocobalt

TABLE 1 Recovery of radiocobalt from various water soluble species Species

~°Co2+ ~°Co(II) humate 6°Co(Ill) EDTA 6°Co(Ill) picolinate Cyano[STCo]cobalamine

% Co recovered Co(ll) method

Total Co method

99.0 99.0 1.0 3.0 1.6

99.0 99.0 99.0 99.0 9.9 51.0

4. 1.0 -~- 1.0 4. 1.0 4. 0.3 4. 0.2

4. 4. 4. 4. 4. 4.

1.0 1.0 1.0 1.0 1.0 (with Co carrier) 7.0 (carrier free)

RADIOCHEMICAL DETERMINATION OF RADIOCOBALT

241

Co(II) and Co(II) humate species by lowering the pH of the sample. It is likely that cobalt is dissociated to some extent from the colloidal humate on acidification of the sample in the analytical scheme described here.

Purification and decontamination procedures For environmental water samples containing low levels of radiocobalt, as appropriate to this study, high purity counting sources are required to permit sensitive radiometric techniques such as end-window beta counting or liquid scintillation spectrometry to be used. Decontamination for the analytical method described here is achieved by a combination of anion exchange and volatilisation stages. Anion exchange is an excellent separative method for a large number of transuranium and lanthanide nuclides, elements of the natural decay series, and transition elements. For our procedure a typical elution pattern indicating the removal of possible interfering ions is given in Fig. 1. Cobalt is applied to an anion exchange column in 8 M HCI as an anionic chloride complex, and uncomplexed and weakly complexed interfering ions are removed by further washings with 8 M HCI. Cobalt (and plutonium) are eluted using 4 M HCI leaving strongly adsorbed ions, such as technetium and antimony, on the columr~. Separation of cobalt from plutonium is achieved by applying the cobalt fraction in 11.3 M HCI/0.1 M NH4I to a second anion column. 242pu is a potentially important interfering nuclide and under these cGaditions plutonium is reduced to the trivalent state and is not sorbed. Further washing with 8 M HCI removes any interfering ions carried over from the first ,:olumn. As before, cobalt is eluted with 4 M HCI. A partial separation of iron (introduced as the reducing agent), used in the total cobalt assay scheme, is required to prevent excessive loading on the column prior to the first anion exchange procedure. This is achieved by solvent extraction with di-isopropyl ether from 8 M HCI. Adequate decontamination is not achieved for ruthenium nuclides by anion exchange alone and it has been found necessary to further purify the sample by oxidation with perchloric acid to produce volatile ruthenium tetraoxide. To ensure sufficient purity of the counting source the separation of interfering nuclides of importance in environmental samples has been considered by incorporating these nuclides into the analytical scheme. Decontamination factors have been determined and are given in Table 2. The data show that significant removal of the majority of nuclides is achieved by applying the sample onto two successive anion exchange columns.

Yield determination The reliable chemical recovery of cobalt radionuclides may be achieved by the addition of stable cobalt as the yield recovery tracer. The us~eof radio-

K.S. LEONARD El" AL.

242

a) First anion column apply sample in 8M HCI.

wash with 8M HCt

elute Co with 4M HC[

nuclides strongly sorbed

Co Pu I

Fe Zn Tc U Sb

i C

•O

Cs

e-

Sr

O

Th Pb Ni

O

,'

i

I

0

r

2

4

-

-

6

-

-

-

8

-

-

r

10

~

. . . .

12

>

Column volumes b) Second anion column apply Co and Pu fraction in 11,3M HCl/0.1U NH41

washwith 8M HCl

elute Co with 4M HCI.

Co

8

I

=

U C 0 U

0

J 2

4

I 6

i 8

t ~' 10

/ 12

Column volumes Fig. 1. Separation of Co by anion exchange (Biorad AGI x 8, 100-200 mesh) using two successive columns.

243

RADIOCHEMICALDETERMINATIONOF RADIOCOBALT

TABLE 2 Decontamination of potentially interfering radionuclides Radionuclide

Anion exchange decontamination factor (2 columns)

Overall decontamination factor

13~Cs 9°Sr 99mTc

3.8 x 10 8a 2.1 x l0 Ta

2t°pb 6SZn

7.7 9.0 3.6 1.0 2.2 2.1

238pU

2.5 X 10 4

2~l'h

2.6 x i0 5

5.2 x 10 5a

23SU

2.1 x

1.1 x

t°SRu

1.4 × 10 3

125Sb

x 10 5 X l0 4

X x x x

10 5 10 5 10 4 l0 s

l0 6

10 7a

6.8 X 10 4b

aFurther decontamination achieved by iron sulphide coprecipitation. bFurther decontamination achieved with perchloric acid volatilisation.

cobalt yield tracers, such as 6°Co, 5SCo and 57Co, is precluded because these may be present in environmental samples and are difficult to resolve by endwindow beta counting and liquid scintillation spectrometry. The environmentally unimportant radionuclide, 56Co (half-life, 77 days) is not suitable as a yield tracer because its decay process includes a positron emission and a number of gamma emissions which are difficult to resolve in the presence of the other cobalt radionuclides. Concentrations of stable cobalt in environmental waters are low; 0.2/~g/l has been reported in freshwaters (Martin and Meybeck, 1979), and 7-25 ng/l have been measured in British coastal waters (Harvey and Dutton, 1973). Therefore the addition of 5 mg of stable cobalt, equilibrated with the determinand, prior to chemical separation, is adequate to swamp any cobalt which may be present in the sample. The chemical recovery of the purified stable cobalt tracer is determined by atomic absorption spectrophotometry. Radiometric assay

Cobalt radionuclides of interest are given in Table 3, together with details of their half-lives, mode of decay and principle energies of radiation. Nuclides 6°C0, 5SCo and 57C0 are known to occur in the aquatic environment as a result of atmospheric testing of nuclear weapons whereas 6°C0 and 5SCo are known to occur from local sources associated with various

244

K.S. LEONARD El" AL.

TABLE 3 Nuclear characteristics of some cobalt radionuclides Nuclide

6°Co SSCo 5~Co S6Co

Half-life

5.27 y 70.8 d 270.9 d 78.8 d

Decay process

BEC/B + EC EC/~ +

Principle nuclear emissions MeV (% abundance) :/max

3,

0.318 (100) 0.475 (15) i.46 (19)

1.173 (100), 1.332 (I00) 0.511 (30), 0.811 (99) 0.122 (86), 0.137 (II) 0.511 (40), 0.847 (100), 1.238 (07), others

pressurised water and Magnox reactor effluents. The most common and convenient counting technique for the assay of cobalt radionuclides is gamma spectrometry. Relatively simple measurement of these radionuclides can be carried out simultaneously in the presence of other gamma-emitting nuclides without any radiochemical separation. Although these nuclides have been identified in the marine environment, few methods and results are available for environmental solutions by gamma spectrometry because of the insufficient sensitivity of this technique (Preston and Dutton, 1970). One alternative radiometric method which is more sensitive and provides lower detection limits than gamma counting is liquid scintillation spectrometry. It has been reported that both 6°Co and 57Co can be counted individually with high efficiencies by this ~echnique (Horrocks, 1974). The spectra for the simultaneous assay of the three environmentally important radiocobalt nuclides is given in Fig. 2. Unlike gamma spectrometry, a radiochemical separation procedure is required to purify the low level amounts of the radionuclides present in the sample and it is difficult to assay the individual nuclides simultaneously. The extent to which resolution can be achieved for the simultaneous determination of ~°Co and 57Co by liquid scintillation spectrometry has been considered for each of these two nuclides. It has been shown that the greatest statistical accuracy is obtainable in a given counting time for low sample count rates when the ratio E2/B (E and B are the counting efficiency and background count rate, respectively) is a maximum (Loevinger and Berman, 1951). Table 4 gives data of the relative efficiency of each radionuclide and an E2/B value for different channel set-

245

RADIOCHEMICAL DETERMINATION OF RADIOCOBALT

100 "

.~. 57C0 .~

80-

•

'

•

/~

°co

60-

8C °

:>

,'./ ,".'

', ',/

'.,"

,,.i / ' , .

,,\

O0 40

/

-

,'\

',

/ ,/

0

I

50

I

ioo Channel number

I

Iso

I

200

Fig. 2. Liquid scintillation spectra of environmentally important cobalt radionuclides.

tings for the assay of 57Co and ~°Co. "I~e absolute efficiencies over the whole spectrum (256 channels) are 91.0 and 47.7 percent for 6°Co and 57Co, respectively. The optimum channels for the counting of 6°Co and 57Co individually, in terms of a maximum value of E2/B, are 5-150 and 5-90, respectively. From the data, given in Table 4, it is apparent that complefe resolution cannot be easily achieved for either nuclide. It is suggested that, to maintain a reasonably large value of E2/B and decrease the contribution of the interfering nuclide the optimum channel settings are 5-80 and 110-150 for the assay of 57Co and ~°Co, respectively. End-window ~ counting also provides a considerably more sensitive technique than gamma spectrometry. As with liquid scintillation counting, however, the occurrence of interfenng nuclides requires radiometric separation of the cobalt nuclides. Furthermore, it is not possible to resolve the energies for the simultaneous determination of cobalt radionuclides. The criterion for choosing between end-window ~ counting and liquid scintillation spectrometry for the determination of the three cobalt radionuclides has also been considered in terms of the statistical ratio E~/8 and these are given in Table 5. Using the counting systems available at DFR the preferred method for the assay of 6°Co and 58Co in environmental samples is end-window/7 counting whilst liquid scintillation spectrometry is the more sensitive assay method for 57Co= From our analyses, 10 mBq of ~°Co on the final counting source with a triplicate count time of 500 min, may be determined by end-

246

K.S. LEONARD ET AL.

TABLE 4 Resolution of 57C0 and 6°C0 using liquid scintillation spectrometry

(a) Resolution of 57C0 in the presence of 6°C0 Channel setting

5-60 5-70 5-80 5-90 5-95

E2/B (57C0)

Relative counts*

57Co

60Co

17.4 33.8 54.2 71.4 76.8

4.0 6.3 10.7 16.0 19.1

77 153 278 331 327

(b) Resolution of 6°Co in the presence of 57Co Channel setting

5-150 95-i50 105-150 110-150 115-150 120-150

E2/B (6°Co)

Relative counts*

6OCo

57Co

~4.9 66.8 57.7 53.5 48.0 41.8

100.0 22.0 13.4 10.7 8.4 7.0

321 255 209 197 152 148

*Relative counts for channels 0-256 = 100.

TABLE 5 Comparison of counting sensitivity (E2/B) for cobalt radionuclides using different counting systems Nuclide

6°Co 58Co 57Co

Assay method End-window counting (E2/B)

Liquid scintillation spectrometry (E2/B)

2600 180 145

321 109 331

247

RADIOCHEMICAL DETERMINATION OF RADIOCOBALT

window/~ counting giving a detection limit in the order of 0.2 mBq/l from a 50 l-sample. This is approximately two orders of magnitude more sensitive than reported for environmental solutions by gamma counting methods (Yamagata and Iwashima, 1963; Chakravarti e~ a!., 1964). The effect of the source weight of the stable cobalt yield recovery tracer (as cobalt hydroxide) on counting efficiency has been determined for the three cobalt radionuclides and data are provided in Fig. 3. Little variation in efficiency, and hence absorption within the source, is observed over the range of source weights produced by the analytical procedure. ENVIRONMENTAL APPLICATION

This speciation methodology has been applied chiefly to provide information regarding the environmental behaviour of 6°Co discharged, under authorization, by UKAEA (Winfrith) on the south coast of England. Decontamination to remove corrosion products from the reactor pipework of the Steam Generating Heavy Water Reactor involves the application of a complexing reducing solution known as the low oxidative-state metal-ion (LOMI) reagent (Bradbury et al., 1982). In addition to a reducing metal ion

40+ ~ 4 " ~ . i . .

+ --- + 60Co

30-

0 C

.~_ 20 0

UJ

10-

0

0

e--e--

.--

----.--t:. . . . . . .

I

0.2

.1--

. . . . .

I

0.4

~

I

o.s

. . . . .

58C0 - ~ STCo

w

o.e

Source w e i g h t rng cm "2

Fig. 3. Variation of end-window beta counting efficiencies with source weight of stable cobalt yield recovery tracer.

248

K.s. LEONARD El" AL.

TABLE 6

Distribution of 6°C0 species in treated Winfrith effluent Sample collection date

% 6°C0

11 June 1987 16 May 1988 21 February 1989

6°Co(II)

6°Co(Ill) picolinate

24.6 4- 0.8 57.7 4. 1.3 93.8 4. 1.5

75.4 4- 1.1 42.3 4. 0.8 6.2 4. 0.4

(e.g. vanadium(II)) the LOMI reagent contains picolinic acid to chelate and solubilise metal ions. It has been possible to distinguish between 6°Co(II) and 6°Co(III) picolinate species by applying the above radiochemical procedure to treated reactor effluent prior to discharge to the sea. Some observed radiocobalt distributions are given in Table 6 and these results indicate that, although the percentage of inorganic and organic radiocobalt in samples collected from sea tanks varied considerably, it was likely that during 1987 and 1988 appreciable amounts of organically complexed radiocobalt were discharged to the sea. Over the past few years a number of seawater samples have also been examined from different locations. A few of the results taken from an annual survey of shoreline samples collected from Kimmeridge Bay and Poole Harbour during the period in which the majority of activity was discharged in 1987 are given in Table 7. For both sampling locations the major component TABLE 7

Distribution of 6°Co in shoreline seawat,~r samples collected at Kimmeridge Bay and Poole Harbour Location

Date

Total dissolved

% 6°Co

6°Co/mBq/l

Kimmeridge Bay 11/6/87

Poole Harbour

-4- 0.8 ± 1.8 4- 1.3 ± 2.2 ± 0.5 4. 0.5

6°Co(II)

Colll picolinate

92.0 91.4 83.9 94.6 88.0 88.2

8.0 8.6 16.1 5.4 12.0 11.8

23/7/87 19/8/87 16/9/87 10/6/87 22/7/87

17.6 38.5 29.9 46.9 10.8 10.2

± 0.4 ± 0.4 ± 0.4 -~- 0.4 4. 0.4 4. 0.5

19/8/87

10.8 4. 0.5

86.1 4. 0.4

16/9/87

25.1 4. 1.2

92.0 4- 0.5

± 0.1 ± 0.1 ± 0.1 ± 0.1 -~- 0.1 4. 0.1 13.9 4- 0.1 8.0 4- 0.1

249

RADIOCHEMICAL DETERMINATION OF RADIOCOBALT

of dissolved radiocobalt was the inorganic form. However, small amounts of the organic species were observed to persist in the marine environment. Further work which has been carried out includes a transect survey to determine the spatial behaviour and speciation of cobalt and laboratory experiments to determine the rate of breakdown of the effluent prior to discharge and upon discharge to the sea. These results will be discussed in detail elsewhere. The application of the radioanalytical procedure described here may also be of use for the study of radiocobalt behaviour in other environmental situations. For example, it has been suggested that a fraction of the mobile 6°Cc in ground waters in a formerly used seepage trench is chelated with EDTA (Olsen et al., 1986). Indeed cobalt has been shown to form very

TreatmentOotalCo~

I'

Sample

[

[ Filtrate [

filter 0.45wn acidify (pH 3) add stable Co

add (Wc14)2Fe (SO4)2' Na2SO~ then ada NaBH4, NH4OH filter precipitate

[

filter0.45I~m [

[

I

[

[

acidify(pH 3)

i

[ Totalradiocobalt ] ,I Iron sulphide coprecipitation

Tream~ent(Coll oniv~

]

CoIIonly ]

add stable Co

i

coprecipitationNiCkelhydr°xi[de

wet ash, dissolve in

8M HCI

extract Fe with di-isopropyl ether

I.

Etherextraction

add Ni2+. NaOH filter precipitate wet ash dissolvein 8MHC!

[

m

dry, dissolvein 8M HC! wash with 8M HCI elute Co with4M HCi

,dd. O ClO

[

proceed furtheras given for total Co method

I [' Anionexchange [

[

I

.

I

I I vo,..,,,,=oo I dissolve residuein

[

! 1.3MHCI/0.1M NH41

washwith8MHCi elute Co with4M HCI

I

I

Anionexchange [ I

dilute Co eluent

[ i

I

aliquot for atomic absorption analysis

[ YielddeterminationreCovery I

precipitateCo(OH)2 with NaOH

[

end window beta or liquid scintillation~counting

[

Radi°metric assay

Fig. 4. Schematic diagram of the analytical method.

Anionexchange I

I

250

K.S. LEONARD ET AL.

mobile complexes with EDTA and is the cause of major radiological concern at a number of the US radioactive waste disposal sites (Robertson, 1979). Also naturally occurring organic material is difficult to characterise and simple organic ligands are used as analogues for soluble organic matter. To this end, preliminary investigations with EDTA and acetate have been carried out to predict the behaviour of 5SCo tracers in groundwater of a confined aquifer in glacial sand (Williams et al., 1985). SUMMARY

The radioanalytical procedure developed for the determination and chemical characterisation of radiocobalt in environmental waters is summarised in Fig. 4. It has been shown that the analytical method is applicable to some types of cobalt complexes of environmental importance. To date, the method of separation is confined to distinguishing between the two most likely oxidation states of cobalt. Further consideration is required to adapt and improve the procedure to include the separation and determination of species of the same oxidation state (e.g. Co(III)oEDTA and Co(II1)picolinate or Co(II) and ColI-humate) by exploiting their individual physical and chemical properties. REFERENCES Bains, M.E.D., 1978. The radiochemical determination of cobalt-60 in urine. In: Symposium on the Determination of Radionuclides in Environmental and Biological Materials, CEGB Meeting, Sudbury House, London, 9-10 October 1978. Paper 5, 18 pp. Bradbury, D., M.G. Segal, R.M. Sellars, T. Swan and C.J. Wood, 1982. Decontamination systems of BWRs and PWRs based on LOMI reagents. In: Decontamination of Nuclear Facilities, Intern, Joint Topical Meeting ANS-CNA, Niagra Falls, pp. 21-35. Carlucci, A.F. and S.P. Silbernagel, 1966. Bioassay of seawater. In: Distribution of vitamin Bn in the northeast Pacific Ocean. Limnol. Oceanogr., 11: 612-616. Chakravarti, D., G.B. Lewis, R.F. Palumbo and A.H. Seymour, 1961. Analysis of radionuclides of biological interest in Pacific waters. Nature, 203: 571-73. Cotton, F.A. and G. Wilkinson, 80. Advanced Inorganic Chemistry. John Wiley and Sons, New York, 1396 pp. Hamilton-Taylor, J., M. Kelly, S. Mudge and K. Bradshaw, 1987. Rapid remobilisation of plutonium from estuarine sediments. J. Environ. Radioactivity, 5: 409-423. Harvey, B.R. and J.W.R. Dutton, 1973. The application of photo-oxidation to the determination of stable cobalt in sea water. Anal. Chim. Acta, 67: 377-385. Heggie, D., D. Kahn and K. Fischer, 1986. Trace metals in metalliferous seolments, MANOP Site M: interfacial pore water profiles. Earth Planet Sci. Lett., 80:106-116. Horrocks, D.L., 1974. Liquid scintillation counting of radionuclides used in radioimmunoassay. In: M.A. Crook and P. Johnson (Eds.), Liquid Scintillation Counting, Vol. 3, Heyden, pp. 28-33. Huynh-Ngoc, L., N.E. Whitehead, M. Boussemart and D. Calmet, 1989. Dissolved nickel and cobalt in the aquatic environment around Monaco. Mar. Chem., 26: i 19-132.

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