Abstract Concentrations of particulate and dissolved plu-tonium in seawater were determined together with strongorganic ligands (SOL) in particulate matter (PM). The con-centration of particulate 239,240Pu in surface waters of theWestern North Pacific ranged from 0.03 to 0.3 mBq m–3

during the period from 1987 to 1997. The percentage ofparticulate Pu to total Pu in seawater was less than 10% insurface waters, whereas its portions were less than 1% indeep waters. In order to characterize particulate Pu in sea-water, the relationships between particulate Pu, dissolvedPu and SOL concentrations in surface PM were exam-ined. The result reveals that particulate Pu was linearly re-lated to the SOL concentration independent of dissolvedPu concentration. Mass balance analysis suggests that adominant chemical form of Pu in surface PM, which mayexist as Pu(IV), is a complex with strong organic ligandsin PM.

Keywords Speciation · Plutonium · Seawater · Complexation · Particulate matter · Organic ligand

Introduction

Plutonium is released into the marine environment as a result of atmospheric nuclear weapons testing [1] and dis-charges from the nuclear fuel facilities [2]. Since pluto-nium is a highly toxic element, it is important to under-stand the biogeochemical behavior of plutonium in themarine environment [3]. The chemical form of plutoniumin the marine environment is very complicated becauseplutonium shows four oxidation states (III, IV, V, and VI)and complexation with inorganic and organic ligands inoxic waters [3, 4, 5]. Microbial processes play a signifi-

cant role in determining the chemical form of Pu in theenvironment [6]. The chemical speciation of plutonium isstill a significant topic in marine radioecology, although itsachievement is very difficult because 239,240Pu concentra-tions in seawater are extremely low (less than 10–17 mol L–1).

It is considered that plutonium in surface waters is re-moved by particle scavenging processes [7, 8]. In order tounderstand the oceanic cycling of plutonium and its fate,it is important to elucidate particle–plutonium interactionsin marine environments. Sequential leaching experiments[9, 10, 11, 12] have been developed for the chemical spe-ciation of particulate metals in seawater and have revealedthat trace metals, including Pu, in oceanic particulate mat-ter (PM) are associated with organic binding sites in PM.However, information on the characterization of particu-late plutonium is still poor.

In this paper, we describe the particulate 239,240Pu insurface waters of the Western North Pacific and marginalseas. We examine factors controlling the distribution ofparticulate plutonium in seawater based on mass balanceanalysis.

Experimental

Samples

Seawater samples were collected during cruises of the MS Ryofu,belonging to the Japan Meteorological Agency, during the winterseason and the MS Seifu belonging to the Maizuru Marine Obser-vatory, during the summer season. Large volumes of seawater weresampled with a pumping system and a 100 liters GoFlo sampler.Water samples were filtered through a membrane filter (diameter293 mm) with a pore size of 0.45 µm (Millipore HA). Residues onthe filter are defined as particulate matter. Filtered samples werestored frozen (–30°C) until analysis.

Apparatus

Radioactivity measurements were carried out by α-spectrometry.Eight samples were measured simultaneously with an α-spectrom-eter comprising eight solid-state detectors (Canberra Model PIPS)with a resolution of 25 keV FWNH at 5.486 MeV 241Am and amultichannel pulse height analyzer (Camberra 35+).

K. Hirose · M. Aoyama

Chemical speciation of plutonium in seawater

Anal Bioanal Chem (2002) 372 :418–420DOI 10.1007/s00216-001-1118-5

Received: 25 May 2001 / Revised: 30 July 2001 / Accepted: 10 August 2001 / Published online: 21 December 2001

SPECIAL ISSUE PAPER

K. Hirose (✉ ) · M. AoyamaGeochemical Research Department, Meteorological Research Institute, Nagamine 1–1, Tsukuba, Ibaraki 305–0052, Japane-mail: khirose@mri-jma.go.jp

© Springer-Verlag 2001

All reagents were analytical grade unless otherwise noted. Astock solution of Th4+ was prepared from reagent-grade ThO2. The232Th concentration in the stock solution was 10 Bq L–1 (about 10 µmol L–1). An anion exchange resin, Dowex 1×2, 50–100 mesh(BioRad Lab., USA), was used for ion-exchange columns. The ionexchanger was successively washed with NaOH solution and di-lute HCl solution before use.

Procedure

Plutonium analysis

Plutonium (239,240Pu) dissolved in seawater were coprecipitatedwith (Mg, Ca) hydroxides from 100 to 500 L seawater samples.Both dissolved and particulate plutonium were assayed using α-spectrometry following radiochemical separation with an anion-exchange resin, described in detail elsewhere [13]. The chemicalyield was determined by the addition of a known amount of 242Pu.

SOL analysis

The SOL measurements in PM were carried out at ambient tem-perature in 0.1 mol L–1 HCl solution [14]. A portion of dried filtersamples was used for the SOL determination. PM on a filter wasequilibrated with 20-mL Th solution in a 50-mL vessel for 24 h.After equilibration, PM was separated from the Th solution by fil-tration using a membrane filter with a pore size of 0.2 µm (47 mmdiameter, Nucleopore). The residue was washed three times with 2 mL of 0.1 mol L–1 HCl solution. The residue with the filter wasdecomposed in conc. HNO3 on a hot plate. The Th fraction was pu-rified on anion-exchange resin after dissolution in 8 mol L–1 HNO3solution The Th fraction was eluted from the resin by 9 mol L–1

HCl, and electrodeposited onto a silver disc. Counting of radio-activity was carried out by α-spectrometry. The chemical yield forthe radiochemical analysis was determined with control runs byadding known Th concentrations. The SOL concentrations in PMcan be calculated from the equation:

CSOL = [ThL]{

1 + (Kc

ThL

[Th4+])−1

}(1)

where [ThL] and [Th4+] are the concentration of Th adsorbed ontoPM and in the solution, respectively, and Kc

ThL is the conditionalstability constant for Th complexation with the strong ligand onPM under the conditions of 0.1 mol L–1 HCl solution. We used avalue of 106.6 for Kc

ThL to calculate the SOL concentration in PM.Under the experimental conditions, the uncertainty of Kc

ThL valueled to a systematic error of less than 10% for the SOL concentra-tion. The SOL concentration in surface PM was reproducible toabout 5% for repeated runs.

Results and discussion

Dissolved 239,240Pu concentrations in surface waters of theWestern North Pacific have been reported (present level:around 3 mBq m–3, range: 1.5 –6.3 mBq m–3) [15]. Thedissolved 239,240Pu in surface waters of the Western NorthPacific showed a gradual decrease with time, although therates of decrease of surface 239,240Pu depend on sea area.On the other hand, levels of surface 239,240Pu in the Sea ofJapan have remained constant during the past two decades[16]. It has been considered that biogeochemical and phys-ical processes control the temporal change of 239,240Pu insurface waters of the ocean [7, 8]. It is, therefore, impor-tant to understand chemical interaction between particlesand Pu in the marine environment.

Concentrations of particulate 239,240Pu, defined by fil-tration using a fine membrane filter, ranged from 0.03 to0.3 mBq m–3 in the Western North Pacific surface waters[15], which occupies 1 to 10% of the total in surface wa-ters and less than 1% in deep waters. There is no definitetemporal change of particulate Pu in surface waters of theWestern North Pacific during the period from 1987 to 1997.Sequential leaching studies [11] suggest that 239,240Pu inPM forms a complex with organic ligands. In order to un-derstand the characteristics of particulate Pu, the relation-ship between particulate and dissolved 239,240Pu and or-ganic ligands in PM is examined on the basis of thermo-dynamic considerations. The mass balance analysis forparticulate U [12] is possible in seawater because chemi-cal compositions and concentrations of major ion and pHare almost constant in the surface seawater. The thermo-dynamic parameters, such as conditional stability constant,partition coefficients, and others, are a useful tool to char-acterize chemical forms of plutonium. Previous studies[11, 14, 17, 18] revealed that strong organic ligands (SOL),found in PM, dissolved organic matter (DOM) and marinemicroorganisms, is related to complexation with hard met-als such as Th, U, Fe(III) and Pu. It is therefore, importantto examine the relationship between Pu species and theSOL on the basis of mass-balance analysis.

According to mass-balance analysis [19], the followingequilibrium is established between Pu and SOL in PM:

Pud + Lp = PuLp (2)

where Pud and Lp are Pu dissolved and the SOL in PM, re-spectively. The apparent conditional stability constant (akind of the partition coefficient between PM and seawa-ter), Kp, is defined as:

Kp = [PuLp

][Pu]−1

d C−1SOL (3)

where CSOL is the concentration of the SOL in PM. Ifchemical equilibria between plutonium species withoutredox and irreversible processes are established in seawa-ter, the term [PuLp][Pu]d

–1 is linearly related to the con-centration of the SOL in PM. We examined the relation-ship between [PuLp][Pu]d

–1 and CSOL for seawater samplesbut could find none. The result suggests that particulatePu is not directly related to dissolved Pu.

When a dominant species of Pu in PM is Pu(IV), thefollowing equilibrium is established in seawater:

Pu4+ + Lp = PuLp (4)

and Pu4+ + 4OH− = Pu (OH)4,s (5)

The conditional stability constant of Pu(IV) complex (Eq. 4)under the conditions of seawater is defined as:

KPuL = [PuLp

] [Pu4+]−1 [

L′]−1(6)

where [Pu4+] and [L’] are the concentrations of free Pu(IV)ion and the SOL unbound with metals ( CSOL=[L’] αL(Mi);αL(Mi): the side reaction coefficient of the SOL (=1+ΣKMiL[Mi

n+]; Mi=Ca2+, Mg2+, Cu2+, ...)), respectively. Theconcentration of free Pu(IV) ion in seawater is controlledby the solubility product of Pu(IV) hydroxide due to the

419

420

extremely low value (=10–55.15) (Eq. 5), whereas solubilityproducts of Pu(V) and Pu(VI) hydroxides and carbonatesare relatively high [20]. Therefore, the concentration offree Pu(IV) ion is maintained at a constant level in theocean because pH and ionic strength of seawater (pH 8.1,I 0.7) are kept constant. In this case, [PuLp] values in sur-face PM of the Pacific are linearly related to the SOL con-centration as shown by the equation:[PuLp

] = KPuL[Pu4+]

α−1L(mi)CSOL (7)

The plot of [PuLp] against CSOL is shown in Fig.1. Wehave a good linear relationship between [PuLp] and CSOL.The result suggests that Pu(IV) is associated with the SOLin PM and that the contents of particulate Pu are con-trolled by the SOL concentrations in PM. This findingmay be consistent with the result that oxidized Pu is re-duced to Pu(IV) in organic complexes [4]. A linear regres-sion slope is equal to the value of KPuL αL(Mi)

–1. The con-ditional stability constant of the Pu(IV) complex in PM isestimated to use the free Pu(IV) ion concentration and theside reaction coefficient of the SOL in PM. The SOLfound in PM, DOM and organisms is classified as a DTPAtype [14, 19, 21]. According to the coordination model[21], the concentration of the DTPA-type organic ligandnot bound with metals was estimated to be around 1% ofthe total, which means the side reaction coefficient of theligand is about 102. Therefore, the conditional stabilityconstant of Pu(IV) complex in PM under the conditions ofseawater is calculated to be about 1023.3 mol–1 L, which isgreater than that of Th(IV) (about 1021.1 mol–1 L) [21].The result is consistent with the assumption that the va-lence of Pu in PM is IV.

It is reported [4, 5] that dissolved Pu in seawater is pre-sent as the oxidized form (Pu(V) and/or Pu(VI)). We canprovide an estimate of organic Pu(IV) complex dissolvedin seawater because complexation ability of the SOL inDOM is the same as that in PM. Previous studies [18]suggest that the SOL concentration in DOM is less than 5 nmol L–1 for surface waters. This leads to an estimate of

less than 5% for the portion of the organic Pu(IV) com-plex to total dissolved Pu. This finding implies that the or-ganic Pu(IV) complex dissolved in seawater is only a mi-nor component within dissolved Pu species, which is con-sistent with previous results [5, 13].

Conclusions

The particulate Pu concentration in surface waters of theocean is controlled by the SOL concentration in PM, whichoriginates from marine microorganisms [18]. Microbialprocesses in the marine environment may play a signifi-cant role in determining the chemical forms of hard acidsincluding Pu. Mass balance analysis suggests that pluto-nium in PM forms the Pu(IV) complex with the SOL andthat the conditional stability constant of Pu(IV) complexwith the SOL in PM is about 1023.3 mol–1 L. The result re-veals that Pu(IV) complex with the SOL in DOM is onlya minor component within dissolved Pu species. How-ever, more detail speciation, including redox reactions, isneeded for understanding of the oceanic behavior of Pu.

References

1.Hirose K, Igarashi Y, Aoyama M, Miyao T (2001) Plutoniumin the environment. Elsevier Science, pp 251–266

2.Pentreath RJ (1988) Radionuclides: a tool for oceanography.Elsevier Applied Science, pp 13–34

3.Baxter MS, Fowler SW, Povinec PP (1995) App Radiat Isot46:1213–1223

4.Choppin GR, Morgenstern A (2001) Plutonium in the environ-ment. Elsevier Science, pp 91–104

5.Nelson DM, Carey AE, Bowen VT (1984) Earth Planet Sci Lett68:422–430

6.Francis AJ (2001) Plutonium in the environment. Elsevier Sci-ence, 201–219

7.Bowen VT, Noshkin VE, Livingston HD, Volchok HL (1980)Earth Planet Sci Lett 49:411–434

8.Hirose K (1997) Radioprotection-Colloq 32:225–2309.Chester R, Thomas A, Lin FJ, Basaham AS, Jacinto G (1988)

Mar Chem 24:261–29210.Hirose K, Sugimura Y (1991) J Radioanal Nucl Chem 149:

83–9611.Hirose K, Sugimura Y (1993) Sci Total Environ 130/131:

517–52412.Hirose K (1994) J Radioanal Nucl Chem 181:11–2413.Hirose K, Sugimura Y (1985) J Radioanal Nucl Chem 149:

83–9614.Hirose K, Tanoue E (1994) Geochim Cosmochim Acta 58:1–715.Hirose K, Aoyama M, Miyao T, Igarasihi Y (2001) J Radioanal

Nucl Chem 248:771–77616.Hirose K, Miyao T, Aoyama M, Igarasihi Y (2002) J Radioanal

Nucl Chem (in press)17.Hirose K (1996) J Radioanal Nucl Chem 204:193–20418.Hirose K, Tanoue E (1999) Fresenius J Anal Chem 363:531–

53319.Hirose K (1995) Sci Total Environ 173/174:195–20120.Coleman GH (1965) The radiochemistry of plutonium. AEC

Contract No. W-7405-eng-4821.Hirose K (1994) Anal Chim Acta 284:621–634

Fig.1 The relationship between particulate 239,240Pu and SOL con-centrations in PM