Physical, chemical, and biological properties of radiocerium relevant to radiation protection guidelines: Recommendations of the National Council on Radiation ... and Measurements (NCRP report ; no. 60)

NCRP REPORT NO. 60

PHYSICAL, CHEMICAL, A N D BIOLOGICAL PROPERTIES OF RADIOCERIUM RELEVANT T O RADIATION PROTECTION GUIDELINES Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS

Issued 15 December 1978 National Council on Radiation Protection and Measurements 7910 WOODMONT AVENUE

/ WASHINGTON, D.C. 20014

Copyright O National Council on Radiation Protection and Measurements 1978

AU rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews. Library of Congress Catalog Card Number 79-84486 International Standard Book Number 0-913392-44-8

Preface Cerium, an element in the lanthanide series, has a number of radioactive isotopes. Several of these are produced in abundance in nuclear fission reactions associated with nuclear industry operations or detonation of nuclear devices. This report summarizes our present knowledge of the relevant physical, chemical, and biological properties of radiocerium as a basis for establishing radiation protection guidelines. The first section of the report reviews the chemical and physical properties of radiocerium relative to the biological behavior of internally-deposited cerium and other lanthanides. The second section of the report gives the sources of radiocerium in the environment and the pathways to man. The third section of the report describes the metabolic fate of cerium in several mammalian species as a basis for predicting its metabolic fate in man. The fourth section of the report considers the biomedical effects of radiocerium in light of extensive animal experimentation. The last two sections of the report describe the history of radiatioh protection guidelines for radiocerium and summarize data required for evaluating the adequacy of current radiation protection guidelines. Each section begins with a summary of the most important findings that follow. No literature citations have been included in these s d e s since extensive documentation is contained in the main body of each section. This report also includes numerous citations of the most recent literature on radiocerium. In addition to these publications, there are a number of other publications on cerium that may be of interest to some readers. These are included within the comprehensive bibliography on cerium. The Council has noted the adoption by the 15th General Conference of Weights and Measures of special names for some units of the Systkme International d' Unitks (SI) used in the field of ionizing radiation. The gray (symbol Gy) has been adopted as the special name for the SI unit of absorbed dose, absorbed dose index, kenna, and specific energy imparted. The becquerel (symbol Bq) has been adopted as the special name for the SI unit of activity (of a radionuclide). One gray equals one joule per kilogram and one becquerel is equal to one second to the power of niinus one. Since the transition from the special units currently employed-rad and curie-to the new special names is expected to take some time, the Council has determined to continue, iii

iv

/

PREFACE

for the time being, the use of rad and curie. To convert from one set of units to the other, the following relationships pertain: 1 rad = 0.01 J kg-' = 0.01 Gy 1 curie = 3.7 x 10" s-' = 3.7 x 101° Bq (exactly). The report was prepared by the Council's Scientific Committee 30 on Physical, Chemical, and Biological Properties of Radionuclides. Serving on the Committee were: ROGER0.MCCLELLAN, Chairman Director, Inhalation Toxicology Research Institute Lovelace Biomedical and Environmental Research Institute Albuquerque, New Mexico JOHNE. BALLOU Biology Department Battelle Northwest Laboratory Richland, Washington RICHARD G. CUDDIHY Inhalation Toxicology Research Institute Lovelace Biomedical and Environmental Research Institute Albuquerque, New Meiico PATRICIAW.DURBIN Lawrence Radiation Laboratory University of California Berkeley. California Director, Radiobiology Laboratory School of Veterinary Medicine University of California Davis, California MARYJANE(COOK)HILYER 204 Norfolk Drive Concord, Tennessee BERNDKAHN Environmental Resources Center Georgia Institute of Technology Atlanta, Georgia ~ . E D E R K C KW.

LENGEMANN Department of Physical Biology New York State Veterinary College Cornell University Ithaca, New York

ARTHURLINDENBAUM Division of Biological and Medical Research Argonne National Laboratory Argonne, Illinois YOOKNG Bio-Medical Division Lawrence Radiation Laboratory University of California Livermore, California CHESTERR. RICHMOND Associate Director, Biomedical and Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, Tennessee JAMESS. ROBERTSON Diagnostic Nuclear Medicine Mayo Clinic Rochester; Minnesota BRUCE0.STUART Stouffer Chemical Company Farmington, Connecticut ROBERTG. THOMAS Group Leader, H-4 Biomedical Research Los Alamos Scientific Laboratory Los Alamos, New Mexico

NCRP Secretariat: JAMESA. SPAHN,JR.

The Council wishes to express its appreciation to the members of the Committee for the time and effort devoted to the preparation of this report. WARREN K. SINCLAIR Bethesda, Maryland September 15, 1978

President, NCRP

Contents ...

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lu 1. Chemical and Physical Properties of Radiocerium . . . . . 1 1.1 Chemistry of Cerium and Other Lanthanides . . . . . . . . . . . 1 1.2 Decay Schemes for Radioisotopes of Cerium . . . . . . . . . . . 5 2 . Sources of Radiocerium in the Environment . . . . . . . . . . . 9 2.1 Sources . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 9 2.2 Pathways to Man . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3 . Metabolism of Cerium in Mammalian Species . . . . . . . . . . 20 3.1 Gastrointestinal Absorption of Ingested cerium . . . . . . . . 21 3.2 Deposition and Retention of Inhaled Cerium . . . . . . . . . . . 24 3.3 Internal Organ Distribution of Absorbed or Injected Cerium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.4 Circulatory Transport of Radiocerium . . . . . . . . . . . . . . . . . 46 3.5 Variations in Deposition and Retention Patterns Among Individuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4 . Biomedical Effects of Radiocerium . . . . . . . . . . . . . . . . . . . . 55 4.1 Respiratory Tract, Tracheobronchial Lymph Nodes, and Heart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2 Liver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.3 Skeleton and Bone Marrow . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.4 Gastrointestinal Tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.5 Dose-Response Relationships . . . . . . . . . . . . . . . . . . . . . . . . . 67 5 . History of Radiation Protection Guidelines for Cerium Radionuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 6. Considerations for Establishing New Radiation Protection Guidelines for Radionuclides of Cerium . . . . . . . . . . . 73 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 The NCRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 NCRP Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

1. Chemical and Physical Properties of Radiocerium Cerium is a member of the lanthanide group of chemical elements. The elements of this group have similar chemical properties. In neneral, all lanthanides exhibit a principal oxidation state of (111) although some may also exist in the (11)and (IV) valence states.' Most lanthanide compounds are only sparingly soluble in aqueous solutions of nearly neutral pH. Their complexes with many organic and inorganic substances are often much more soluble. The radioactive isotopes of cerium of most concern to humans are 141Ce, '43Ce,and lMCe.These three isotopes, all of which are beta emitters, are abundant products of nuclear fission reactions and have moderately long radioactive halflives.

1.1 Chemistry of Cerium and Other Lanthanides Cerium has 20 isotopes that range in mass from 129 through 148. Only four are naturally occurring (136, 138, 140, and 1421 and their abundances are 0.0019, 0.0025, 0.8847, and 0.1107, respectively, to give an average atomic weight of 140.12. Cerium occupies the position immediately following lanthanum in the periodic system and is the first member of the lanthanide group which encompasses atomic numbers 58 through 71. A thorough review of the inorganic, analytical, and radiochemistry of these elements was published by Stevenson and Nervik (1961). The chemistry of the lanthanides and yttrium has also been summarized by Yost et al. (1947), Vickery (19531, Eyring (1964, Moeller (19561, Kremers (1956), Quill (1956), Schweitzer (1956), Spedding and Powell (1956),Weaver (1956), and Leddicotte (1956). Unless

' Roman numerals in parentheses indicate the oxidation or valence states. These are the effective ionic charges of cerium atoms and ions or as they exist in chemical compounds. 1

2

/

1. CHEMICAL AND PHYSICAL PROPERTIES OF RADIOCERIUM

otherwise specified, the material given in the following discussion has been drawn from Stevenson and Nervik (1961). A predominant feature of the atomic structure of the lanthanide group is the sequential addition of 14 electrons to the 4f subshell (Table 1).The f electrons do not participate in bond formation and in ordinary aqueous solutions all of the lanthanides exhibit a principal (111) state. The common (111) state confers a similarity in chemical properties to all lanthanide elements. Some of the lanthanides can also exist in the (11)state (Nd, Sm, Eu, Tm, Yb) or in the (IV) state (Ce, Pr, Nd, Tb, Dy). Except for Ce(IV), Eu(II), and Yb(IT), these unusual lanthanide oxidation states can only be prepared under drastic redox pressure and temperature conditions, and they are not stable in aqueous solutions. Cerium (IV) is a strong oxidizing agent Ce(II1) = Ce(1V) - e-, E, = -1.61 V and this property has been used to separate Ce from lanthanide mixtures. However, Ce(IV) in solution reacts slowly with water and is partially reduced to Ce(II1). Although there is a uniformity of their chemical properties, the lanthanides do not behave identically. There is a gradual contraction of the ionic radii of the lanthanides as the nuclear charge increases (Goldschmidt and Lunde, 1925; Templeton TABLE1-Oxidation slates, electronic configurations, and radii of the (III) ion of the lanthanide elements and yttrium Abmic num-

ber

39 57 58 59 60 61 62 63 64 65 66 67

68 69 70 71

Element

Yttrium Lanthanum Cerium Praeseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Y tterbiurn Lutecium

Ra~,"(A".~!ll'

0.88 1.06 1.03 1.01 1.00 (0.98)d 0.96 0.95 0.94 0.92 0.91 0.89 0.88 0.87 0.86 0.85

Electronic configuration"

Oxidation atatea'

3 4f

4P 4P

5d1 6s" 6s" 6s2 6s2

4P

6s'

4p

6s' 6s2

4f7 4f7

4P

5d1 6s" 5d' 6s2

4f1" 4f" 4f" 4fI3 4fI4 4f1' 5d1

SS2

6s' 6s'

6s2 6s' 6s'

3 3, 4 3, 4 2,3,4 3 2,3 2, 3 3 3, 4 3, 4 3 3 2.3 2.3 3

" Ion radius of the cubic sesquioxide in Angstroms (lo-" m)calculated by Templeton and Dauben (1954). Ground state of neutral atom (Yost et aL, 1947). ' Italics indicate states that are stable in aqueous solution. Calculated from adjacent elements.

1.1

CHEMISTRY OF CERIUM AND OTHER LAMTHANIDES

/

3

TABLE2--Solubility of lmlhanide chlorides, sulfates, and hydroxides and the pH at which the hydroxidesprecipdate from M(N0d3 solutionrP Solubility

MCL?

Y La Ce Pr Nd Prn Sm Eu Gd Tb DY Ho Er Trn Yb Lu

M~(SOI)%

M(OH)asolubility product'

g/lW ml"."

g / l M ml

217 vs 100 334 246

9.8 3.9 26 20 8

-

-

-

s

2.7 2.6 3.3 3.6 5.1 8.2 16

2.0 1.4 1.4 -

s s s -

s vs

-

vs

34

-

66

h%?$;

1.6 8.5 4.4 5.4 2.7

-

0.8 0.6 0.5 0.5

8.0 X 1.0 x 1.5 x 2.7 x 1.9 X 1.0 x 6.8 x 3.4 X 2.1 x 2.0 x 1.4 X 5.0 x 1.3 x 3.3 x 2.9 X 2.5 x

lo-% lo-'g lo-"

70-" lo-" lo-"

at precipita. Lion incidence

7.4 8.4 8.1 7.4 7.0

-

6.9 6.8 6.8

lo-" lo-=

lo-24 lo-"

6.8 6.4 6.3 6.3

"Reproduced from Stevenson and Nervik (1961). In water at O°C to N°C. ' In water at 25°C. " Calculated by Latimer (1952). 'Soluble, s; very soluble, vs.

and Dauben, 1954). This so-called lanthanide contraction leads to decreasing basicity, increasing hydrolysis, and greater stability of complexes (see Tables 1-3). Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cupferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. Lanthanides form soluble complexes with many inorganic and organic substances; however, the nature of the bonding in these complexes has not been completely determined. There is evidence for either ionic or covalent bond formation or a combination of both. Lanthanides are complexed by inorganic ions, but not as readily as are the transition elements. The inorganic complexes are not as important

4

/

1. CHEMICAL AND PHYSICAL PROPERTIES OF RADIOCERIUM TABLE3-Stability constants of lanthanide chelatesa

Chem~calelement

Chelating agent

EDTAD

DCTA'

DTPA"

NTA"

Cilrnte'

AU determinations were made at 25'C. Ethylenediaminetetraacetic acid, EDTA; data of Mackey et al. (1962). 1,2-diaminocyclohexanetetraaceticacid, DCTA; data of Moeller and Hseu (1962). Diethylenetriaminepentaaceticacid, DTPA; data of Moeller and Thompson (1962). ' Nitrilotriacetic acid, NTA and citrate; data of Sillen and Martell (1964).

in either analysis or purification of mixtures as are the lanthanide complexes with polycarboxylic or aminopolycarboxylic acids. Citric acid and nitriloacetic acid (NTA) lanthanide complexes were used in the earliest ion exchange separations of lanthanides from fission product mixtures (Kf = 3.2 for Ce(H3CitJ3 and Kf = 10.8 for CeNTA2) (Sillen and Martell, 1964). More recently such polyarninopolycarboxylic acids as ethylenediaminetetraacetic acid (EDTA), 1,2diarninocyclohexaneacetic acid (DCTA), and diethylenetriaminepentaacetic acid (DTPA) have been prepared. Their lanthanide complexes are very stable (Table 3) and have been widely used in analysis and separation of lanthanide mixtures. They have also been used experimentally to remove internally-deposited l4"Ce and other radioactive lanthanide nuclides from animals and man (Foreman and Finnegan, 1957; Catsch, 1962; Balabukha et al., 1966; Palmer et al., 1968; among others). The trend toward greater complex stability with increasing lanthanide atomic number (see Table 3 for EDTA, DCTA, and DTPA complexes) has also been demonstrated for lanthanide complexes with Kr is the stability constant which is the negative logarithm of the dissociation constant, Ki. T h e dissociation constant is the product of the concentrations of the dissociated ions divided by the concentration of the parent molecule.

CHEMISTRY OF CERIUM AND OTHER LANTHANIDES

1.1

/

5

simpler ions. The lanthanide carbonates, oxalates, and potassium sulfates (K2M2 (SO4)a) are insoluble in water (Yost et al., 1947).

1.2

Decay Schemes for Radioisotopes of Cerium

Most of the radioactive isotopes of cerium have very short physical half-lives and do not normally represent a radiological hazard to humans. Only the three longer-lived isotopes, 14'Ce, '43Ce,and 14"Ce, TABLE4-Radioactive decay chains for the longer-lived radioisotopes of cerium

including those which have been identified in environmental studies Half-Life

kUpe

Ce-144 Ce-139 Ce-141 Ce-134 Ce-137m Ce-143 Ce-135

Mode of

Decsy

day

284 140 32.5 3 34 33 17

d d d d h h h

PECh

PEC

IT'

PEC

Pr-144 La-139 Pr- 141 La-134 Ce-137 Pr-143 La-135

Decay rod uct hd?-lifi

Mode of d e ~ y

Decay product'

17.3m

P-

Nd-144

6.7 m 9h 13.6 d 19.5 h

P+

Ba-134 La-137 Nd-143 Ba-135

EC

PEC

" All are stable except for La-137 which has a half-life of 6 x 10" years, but may be considered to be stable for purposes of radiation dosimetry. Electron capture. 'Isomeric transition. TABLE5-Radioactive decay scheme data for radionuclides of cerium observed in

previous environmental surveillance studies "'Ce

B Decay

Radiation

Energy

TVpe

(keV)

ce-L ce-MNO P-1 max avg P-'2 max avg total 8 avg x-ray L x-ray Kaz x-ray Ka, x-ray KJ3

138.605 143.929 434.6 129.6 580.0 180.7 144.7 5 35.55020 36.02630 40.7

145.440 " Auger-L = L-shell Auger electron ce-K = K-shell conversion electron yray ' A = Equilibrium absorbed - dose constant Y

lnremity (%)

2.57 0.73

A'(

red/ &h)

0.0076 0.0022

70.5

0.195

29.5 100 2.6 4.89 8.9

0.114 0.308 0.0003 0.0037 0.0069

3.34

48.4

0.0029

0.150

6

/

1. CHEMICAL AND PHYSICAL PROPERTlES OF RADIOCERIUM TABLE5-Continued '"Ce )¶- Decay Radiation Type

Inbnaity

A

(%)

?::?

Auger-L ce-K Auger-K ce-L ce-M ce-NOP ce-K ce-L p-1 max avg B 2 max avg fl-3 rnax avg p-4 max avg P-5 max avg 8-6 max avg total p- avg Eight weak fls omitted: E,g (avg)* = 151.4; ZIBD= 0.24% X-ray L X-ray Ka2 X-ray La, X-ray Kp Y Y Y

Y Y Y Y Y Y Y

Y Thirty-seven weak y's omitted;

E, (avg) = 629.4; XI, = 0.68%

Eg(avg) = average energy of the omitted radiations ' & = summed intensity of omitted radiations

(g-red/

&i-h)

1.2

DECAY SCHEMES FOR RADIOISOTOPES OF CERIUM

/

Ene

Radiation

Type

(key

Id3Pr8- Decay 932.0 avg 314.3 '"Ce p- Decay % Feeding to '"PI = 98.80 % Feeding to '44mFk = 1.20 Auger-L 4 ce-K 11.42 ce-L 26.74 Auger-K 29.4 ce-M 32.06 ce-L 31.10 ce-K 38.13 ce-M 39.42 ce-K 44.5094 ce-L 46.58 ce-K 49.0 W-L 73.29 ce-M 78.61 ce-L 79.6652 ce-L 84.2 ce-K 91.54 ce-L 126.70 ce-M 132.02 Fl m a x 181.9 avg 49.3 F 2 max 235.3 avg 65.3 P-3 max 315.4 avg 90.2 Total /T avg 81.0 x ray L 5 Y 1 33.57 x ray Ka, 35.55020 x ray Kal 36.02630 x -Y KB 40.7 Y 2 40.93 Y 4 53.41 Y 7 80.12 Y 8 86.5 Y 9 91.0 Y 11 133.53

p- 1 max

Five weak

7's

See also '"Pr IT Decay 11.3 0.0010 0.82 0.0002 0.93 0.0005 0.9 0.0006 0.196 0.0001 1.0 0.0007 3.5 0.0028 0.21 0.0002 0.61 0.0006 0.114 0.0001 0.53 0.0006 0.48 0.0008 0.101 0.0002 0.28 0.0005 022 0.0004 5.3 0.0104 0.73 0.0020 0.153 0.0004

omitted; E, (avg) = 80.4;XI1 = 0.09%

7

8

/

1. CHEMICAL AND PHYSICAL PROPERTIES OF RADIOCERIUM TABLE5-Continued Ene

Redialion

(key

Type

Intawity 1%)

Ar ( rad/

$I--h)

"'Pr IT Decay % IT Decay = 99.94 Feeds '"Pr % p- Decay = 0.06 Auger-L ce-K Auger-K ce-L ce-M ce-NOP x ray L x ray KaZ x ray Ka, x ray KB One weak y omitted: Ey (avg) = 59.0; 21"= 0.123%

IUPrp- Decay 11-1 max

avg P-2 max avg P-3

max

avg Total P- avg Y3 Y7 Y9

811 267.0 2301 894.8 2997 1221.8 1207.6 696.490 1489.15 2185.70 Seven weak y's omitted; E, (avg) = 10.286; XI, = 0.02%

have been identified among the nuclear wastes present in the environment. Several of the other radioactive isotopes of cerium are shown in Table 4 along with their decay products. The radioactive decay products generally have such short physical half-lives that, in radiation dosimetry calculations for biological tissues, they may be considered to be retained and decay in the organs in which they are formed. Further information on photon and particle emissions from the decay of '"Ce, 143Ce, and lUCeis summarized in Table 5 (Lederer et al., 1967; Nuclear Data Tables, 1970;Fasching et al., 1970;Kocher, 1977;NCRP, 1978).

2. Sources of Radiocerium in the Environment To develop a better understanding of the potential health consequences of radiocerium in our environment, it is important to know the possible sources and physical and chemical forms of its release. The metabolism and dosimetry of internally-deposited radiocerium are highly dependent upon the forms of the material presented to the body and the mode of exposure as discussed in Section 3-Metabolism of Cerium in Mammalian Species. The primary sources of environmental radiocerium in the past have been from nuclear explosive devices and nuclear power facilities. The associated high temperature reactions in nuclear explosive devices are generally thought to result in the release of refractory, insoluble chemical forms. The important isotopes of cerium released have been I4'Ce, L43Ce,and 144Ce.Most of these have been in insoluble forms. However, in some studies of environmental samples as much as onehalf of the radiocerium was in readily soluble forms. Radiocerium in the environment can be taken up by plants through roots or other plant surfaces. Plants have achieved concentrations up to 0.5 times the surrounding soil concentrations. Although there are many reports of radiocerium contamination of food crops, there are few measurements of its presence in animal products. This is due to poor absorption and transfer of cerium through biological food chains. Studies in human populations indicate that inhalation is the major route of entry into the body for radiocerium released into the atmosphere from the testing of nuclear explosive devices.

2.1

Sources

Large quantities of radiocerium have been produced and released to the atmosphere in nuclear weapons tests. For the fissioning of 235U, 238U,or 239pUby thermal neutrons, fission-spectrum neutrons, and high-energy (= 14.7 MeV) neutrons, the cumulative fractional yields of 141Ce,143Ce,and 14Ce range from 0.034 to 0.062 (Meek and Rider, 9

10

/

2. SOURCES OF RADIOCERIUM IN THE ENVIRONMENT

1974). All of these radionuclides can readily be detected in fresh nuclear debris, but I4'Ce and IMCeare measured more often than 143Ce in fallout because of their longer half-lives (Noyce et al., 1973; UNSCEAR, 1964). From the above fission isotope yield information and nuclear weapons test data, it is estimated that about 600 MCi of I4Ce and 8500 MCi of '"Ce have been produced in weapons tests and injected into the atmosphere through 1976. Both of these radionuclides were monitored in seawater, plankton, and marine foodstuffs in the Central Pacific after weapons tests (Held, 1963; Welander and Palumbo, 1963; Palumbo et al., 1963; Welander, 1969).Average monthly concentrations of 144Ce in surface air are reported at quarterly intervals in the Surface Air Sampling Program conducted by the Environmental Measurements L'aboratory of the Department of Energy (Volchok et al., 1976). The proposed peaceful uses of nuclear explosives include their use in large-scale excavation projects and in the stimulation of oil and gas reservoirs (UNSCEAR, 1972). The U.S. Nuclear Cratering Program has been described by Toman (1970). The Danny Boy experiment employed a 0.43-kt device to produce a crater in basalt (Bonner and Miskel, 1965). The total reported airborne activity contained about 200 Ci of 141Ceand about 5 Ci of 144Ceincluding both the activity deposited in fallout and that retained and transported to greater distances in the cloud. The Schooner experiment employed a 31-kt device to produce a crater in welded tuff (Tewes, 1970). About 300 Ci of 141Cewas transported from the crater site in the main cloud and about 24 Ci in the base surge cloud (Crawford, 1970). In contrast to nuclear cratering explosions that inevitably involve some venting of radioactivity to the atmosphere (Clemente et al., 1973), underground nuclear explosions for gas stimulation, as exemplified by the Gasbuggy and Rulison tests, did not release radiocerium into the atmosphere (UNSCEAR, 1972). In the high temperatures associated with nuclear detonations, the radiocerium formed is thought to exist as particles of oxides or other refractory forms (Palumbo, 1963). Most of the radiocerium in fallout was found to be insoluble; however, in some samples a substantial fraction was soluble. From weapons fallout collected in the USSR, more than 0.8 of the 144Cewas in the insoluble fraction (Zhilkina et al., 1973; Pavlotskaya et al., 1974). About 0.55 to 0.60 was extracted from the insoluble fraction by treatment with ammonium acetate, hydrochloric acid, and dilute nitric acid (Pavlotskaya et al., 1974). The authors suggested that much of the '44Cein fallout is not in the very insoluble dioxide form and that a portion is in the form of complexed compounds with different organic and inorganic ligands present in the

2.1 SOURCES

/

11

atmosphere (Pavlotskaya et al., 1974).Their results, however, do not preclude the possibility that the 14%e may have been in very fine oxide particles that could have been relatively soluble due to having very high particle surface to mass ratios. The distribution of soluble 144Ce among cationic, anionic, and neutral forms was 0.55, 0.18, and 0.27, respectively (Pavlotskaya et al., 1974). In New York City fallout in 1958, 0.42 of the 144Cewas water soluble (Welford and Collins, 1960). These observations lead to the conclusion that radiocerium in fallout exists in soluble and exchangeable forms as well as in relatively insoluble, refractory particles. Current attention is focused mainly on the formation and accumulation of radionuclides in nuclear power production. In a typical lowenrichment light-water reactor, '44Ceis produced at the annual rate of 5.3 kg/1000 MW of electric power (Holden and Walker, 1972) which is equivalent to about 17 MCi/1000 MW years of electric power. Other radioactive isotopes of cerium (14'Ce, 14%e, 145Ce,and 14%e) are produced from uranium fission but they have shorter radioactive halflives and do not accumulate to as great an extent as '44Ce.Cerium-141 (T1/2= 32.5 d) is about one-tenth as abundant as '44Cein irradiated uranium fuel after one year (Blomeke and Todd, 1958). The bulk of this activity is retained within the fuel elements until they are reprocessed. However, ' W e has been measured in the emissions and effluents from various nuclear facilities (Davis et al., 1958; Foster and Soldat, 1966; Heft et al., 1971; Mauchline and Templeton, 1963; Parker et al., 1966; Kahn et al., 1970). The U.S. Environmental Protection Agency is conducting comprehensive radiological surveillance studies at selected nuclear power stations and fuel-reprocessing facilities to characterize the releases of radionuclides to the environment and to evaluate related population exposures. Cerium-141, 14%e, and were identified and measured in the primary coolant from the Dresden Nuclear Power Station, a 210-MWe boihng water reactor, but only '44Ce was measurable in liquid wastes (Kahn et al., 1970). In most samples of liquid waste, a major fraction of the 144Ce was in particulate form. The average release rate of '44Cein high-conductivity liquid effluent was 2 x pCi/sec, and the ratio of the 14%e release rate in liquid effluent to the generation rate in the reactor core was estimated to be 4 x lo-'', among the lowest values for any fission product measured. This is due to the low volatility of cerium compounds and their low solubility in aqueous solvents near neutral pH. Stack releases of airborne particulate pCi/sec. were below the limit of detection, which was less than 3 x In similar studies conducted at the Yankee Nuclear Power Station (a 185-MWe pressurized water reactor), 14'Ce, 143Ce,and 144Cewere not

12

/

2. SOURCES OF RADIOCERIUM IN THE ENVIRONMENT

detected in main coolant water (Kahn et al., 1970). The minimum detectable concentration was 10-"Ci/ml. Cerium-144 was also undetected in stack or liquid effluents. The I4'Ce and '44Ce observed in vegetation, precipitation, and soil of this area were attributed to fallout from weapons tests. In a comprehensive field study of liquid waste from the Nuclear Fuel Services (NFS) facility, the first commercial reprocessing plant in the United States, 9 Ci of '44Cewere discharged from the plant interceptor tanks to the holding ponds between April-August 1969 (Magno et al., 1970). During this period, 0.17 Ci or about 0.02 of the total '"Ce activity discharged from the plant was released from the terminal holding pond to the aquatic environment beyond the plant boundary. The '44Cein the holding ponds and that discharged to the aqueous environment was predominantly in suspended particles. The concentration of '44Cein air particulate samples from the NFS stack ranged from 1.2 x 10-l2 to 4.6 x 10-l2 pCi/cm3. However, this is below the federal regulation for an "allowable concentration for unrestricted area" (CFR, 1976; Cochran et al., 1970). I t is estimated that was discharged to the atmosphere from the NFS reprocessing plant at an Ci/MW of electric power (UNSCEAR, average annual rate of 1 x 1972). In aquatic environments, radiocerium readily forms chemical complexes in seawater and associates with particles by adsorption (Mauchline and Templeton, 1963). When radiocerium was added to natural seawater, it became associated with suspended matter, especially that with apparent particle diameters of 0.02 to 0.1 pm (Carpenter and Grant, 1967). When ionic radiocerium was added to filtered seawater a t pH > 6.0, it hydrolyzed and formed complexes with hydroxide, chloride, or other anions in seawater and went on to form particles (Hirano et al., 1973). Adsorption of radiocerium onto suspended particles has also been noted after its release to freshwater ecosystems (Beninson et al., 1966).

2.2

Pathways to Man

Contamination of food crops by radiocerium in fallout from nuclear weapons tests has been extensively documented in the worldwide literature (Chhabra and Hukkoo, 1962; Merk, 1967; Michelson et aL, 1962; Nezu et al., 1962; Sutton and Dwyer, 1964). The '44Ceconcentrations in spinach leaves and radish roots in Japan in 1960 were within a factor of two of the respective 90Srconcentrations (Nezu et al., 1962).

2.2 PATHWAYS TO MAN

/

13

The average 144Cecontent in the total diets of people in a number of U.S. cities in 1961 was 0.4 of the average '"Sr content and 0.08 of the average 137Cscontent (Michelson et al., 1962).The 14'Ce concentrations in wheat and various milling fractions in the U.S. in 1963were between 0.3 and 1.6 times that of 137Cs(Sutton and Dwyer, 1964). Deposition of airborne radiocerium on exposed plant parts can lead to contamination of food crops by retention on the plant surfaces or through absorption. Deposition of radiocerium on the ground can lead to contamination by absorption through the plant roots. In studies conducted in India, it was concluded that the elevated levels of 144Ce measured in tea during 1959 resulted from absorption of surface contamination and the levels measured in carrots were attributed to uptake through roots. The elevated concentrations of 141Cemeasured in vegetables during March 1960 were clearly due to surface contamination of aerial parts (Chhabra and Hukkoo, 1962). Another study in Switzerland showed that root uptake accounted for 0.1 to 0.3 of the 144 Ce measured in grass and cress during 1964 (Merk, 1967). Studies of the transfer of radiocerium into various plant parts via the soil-root pathway are summarized in Table 6. Other laboratory and field studies employing tracer radiocerium or nuclear weapons fallout simulants are summarized in Tables 7 and 8. In general, the cereal grains and vegetable pulp showed plant-to-soil concentration factors (radioactivity per gram of dry plant material/radioactivity per gram of TABLE6-Plant-to-soil concentration factors" for '"Ce a n d other rare earth isotopesh Plant type

Alfalfa Barley leaves Barley head Bean leaves Bean pods Brome grass Tomato leaves

Cmwth medium'

Sedan ejecta Bravo soil Bravo soil Bravo soil Bravo soil Blanca soil Jangle soil

Concentration factof'

0.06 0.002-0.007 0.001-0.002 0.007-0.03 0.002-0.003 0.02 0.009

Reference

Romney et al. (1966) Selders et aL (1956) Selders et al. (1956) Selders et al. (1956) Selders et al. (1956) Mills & Shields (1961) Selders et ak (1953)

" Activity per gram dry plant material/activity per gram dry soil. Plants grown in soil contaminated with nuclear debris. "Project Sedan was a Plowshare nuclear cratering experiment carried out at the Nevada Test Site (NTS) in 1962 (Nordyke and Williamson, 1965). T h e other soils originated b m nuclear weapons test sites (see U.S. Weather Bureau, 1964). The Bravo Test of Operation Castle was carried out at the Pacific Proving Ground in 1954. The Blanca Test of Operation Hardtack and Buster-Jangle series took place at NTS, the former in 1958 and the latter in 1951. In the case of alfalfa the concentration factor was actually determined for IMCe.For the other plants the concentration factors were determined for the rare-earth group of elements.

14

/

2. SOURCES OF RADIOCERIUM

IN T H E ENVIRONMENT

TABLE7-Plant-to-soil concentration factorsa of IJ4Cein crops determined from laboratory studies Plant type

Barley Barley Barley Barley

leaves leaves leaves shoots

Bean leaves Bean leaves Bean leaves Bean leaves Bean fruit Maize shoots Maize shoots Rice (flowering stage) Rice (flowering stage) Rice (6 weeks) Rice (6 weeks) Pea shoots

Growth medium

Concentration factors

Reference

Rediske et a 1 (1955) Rediske et al. (1955) Rediske et al. (1955) Molshanova (1968)

Ephrata sandy loam Wheeler silt loam Winchester fine sand "Meadow turf' sand and soil (1:2) Sassafras sandy loam Hanford sandy loam Sorrento loam S o ~ e n t oloam Sorrento loam Black clay loam Laterite Black clay loam

Nishita & Larson (1957) Nishita & larson (1957) Nishita & Larson (1957) Essington el al. (1963) Essington el al. (1963) Mistry el al. (1974) Mistry el al. (1974) Mistry el al. (1974)

Laterite

Mistry et al. (1974)

Black clay loam Laterite "Meadow turf' sand and soil (1:2)

D'Souza & Mistry (1973) D'Souza & Mistry (1973) Molshanova (1968)

"Activity per gram dry plant material/activity per gram dry soil. From measurements of "Y.

dry soil) between 5 x and 1 x Leafy vegetables had higher and 4 x lo-'. plant-to-soil concentration factors between 2 x These values are substantially higher than those for grains and vegetable meats and may indicate a greater amount of surface contamination through more contact with the soil or resuspended soil dusts per unit mass of plant material. Plant-to-soil concentration factors for "Sr and in most cases those for '37Cs can be expected to exceed those for '44Ce.The fractional uptake of 14%e by oat plants from nine agricultural soils ranged from 3 x to 3 x (Cummings and Bankert, 1971) and were generally two or more orders of magnitude lower than those of '37Cs and 85Sr (Cummings et al., 1969). The concentration factor for radiocerium in crop plants grown in nutrient solution (the activity per gram dry plant divided by the activity per milliliter of solution) varied between and 4 x lop3 (Rediske et al., 1955; D'Souza and Mistry, 1973). Radiocerium can also gain entry into food crops through irrigation or flooding of fields with waters containing these nuclides. However, only small amounts of radiocerium enter food crops by this route compared to the more soluble radioelements that have been studied. Cerium-144 originating from both the Hanford reactors and worldwide

TABLE 8-Pht-to-soil Plant type

Bean leaves Bean fruit Carrot meat Carrot tops

Corn leaves Corn kernel Lettuce head Lettuce leaves Potato leaves Potato meat Radish tops Tomato leaves Tomato leaves Tomato meat Tomato meat

concentration factors. of '"Ce in crops contaminated with fallout simulcrnts Growth medium

Sand Loam Clay Sand ham Clay Sand Loam Clay Sand ham Clay Clay loam Oakley sandy loam Pleasanton loam Clear lake clay Oakley sandy loam Pleasanton loam Clear lake clay Sand Loam Clay Sand Loam Clay Oakley sandy loam Pleasanton loam Clear lake clay Oakley sandy loam Pleasanton loam Clear lake clay Loam Sand ham Clay Oakley sandy loam Pleasanton loam Clear lake clay Sand Loam Clay Oakley sandy loam Pleasanton loam Clear lake clav

Concentration factor

Reference

Sartor et al. (1966) Sartor et al. (1966) Sartor et al. (1966) Sartor et al. (1966)

Sartor et al. (1968) Sartor et al. (1968) Sartor et al. (1966) Sartor et al. (1966) Sartor et al. (1968) Sartor et al. (1968) Sartor e t al. (1966) Sartor et al. (1966) Sartor et aL (1968) Sartor et al. (1966) Sartor et al. (1968)

16

/

2. SOURCES OF RADIOCERIUM IN THE ENVIRONMENT

Plant type

Wheat leaves

Wheat leaves Wheat grain Wheat grain

Gmwih medium

Sand Loam Clay Clay loam Oakley sandy loam Pleasanton loam Clear lake clay Clay Clay loam Oakley sandy loam Pleasanton loam Clear lake clay

Concentration fnrtnr

0.064 0.021 0.030 0.014 0.0091 0 . ~ ~ 7 0.0064 0.00014 0.00043 0.0015 0.0013 0.00077

Reference

Sartor el a1. (1966)

Sartor el al. (1968) Sartor el al. (1966) Sartor et al. (1968)

fallout has been measured in produce and forage irrigated with Columbia River water (Perkins and Nielsen, 1967).In a field study of fallout radionuclides in flooded rice fields during 1963-1964, not more than 0.28 of the 14'Ce in rice shoots was due to indirect contamination through water (Bourdeau et al., 1965).In experiments with potted soil, nutrient solutions, or model irrigation syste~ns(Myttenaere et al., 1967), the fractional uptake from water to rice grain was lowest for ld4Ce(Table 9) and increased in the order 60Co (Myttenaere et al., 1969a; 1969b);"Mn (Myttenaere et al., 1969~); and 137Cs(Myttenaere et al., 1969a; 1969b). The uptake of radiocerium by rice plants from water was comparable to that from soil, but uptake of nuclides of Mn, Co, Sr, and Cs from water exceeded that from soil (Verfaillie et aL, 1967). Relatively large quantities of radioceriurn have been deposited and retained on plants after nuclear weapons tests, but there are few reports of its presence in animal products. Cerium-141 and 14%e originating from Plumbbob Test Series3 fallout accounted for 0.01 or less of the total radioactivity in bone and muscle of jackrabbits (Larson et al., 1966). Cerium-144 was measured in cattle and horse bones in Japan in 1960 a t a concentration about one-tenth that of "Sr (Nezu et al., 1962). In wild mule deer collected in Colorado during 1963 and 1964, the concentration of '44Cein liver was three times that of 137Cs (Whicker et al., 1967). For caribou collected a t Anaktuvak Pass, Alaska in 1967, the muscle-to-lichen concentration ratio was less than onetenth that of '37Cs (Jenkins and Hanson, 1969). In contrast to 134Cs and 137Cs,14%e was not detected in wolves that prey on caiibou. The low tissue content of radiocerium s.ubsequent to its release to these The Plurnbbob Test Series was conducted at the Nevada Test Site in 1957 (see U.S. Weather Bureau, 1964).

2.2 PATHWAYS TO M A N

/

17

TABLE 9-Fractional uptake of radionuclides from water into rice grain Fractional uptake x lo4 leotope

Observation

Whole

Hulled

nce

rice

"Mn

Flooded soil in pots

0.05

WCo

Model irrigation

0.003

1:17cs

Model irrigation

0.18

144Ce

IMCe

Lowland rice, nutrient 0.003 solution Upland rice, nutrient 0.003 solution Lowland rice, soil 0.002

'44Ce

Upland rice, soil

I4"Ce

0.002

HUll

Reference

Myttenaere et al. (1969~) Myttenaere et al. (1969a; 1969b) Myttenaere et al. (1969a; 196913) Myttenaere et al. (1967) Myttenaere et al. (1967) Myttenaere et al. (1967) Myttenaere et al. (1967)

environments is doubtlessly explained by low gastrointestinal absorption as has been observed in laboratory studies that are discussed in Section 3. Chertok and Lake (1971a, 1971b) fed atmospheric debris from Plowshare cratering events to peccary pigs; 14'Cewas not detected in urine and less than 0.005 of the ingested quantity remained in the animals at the end of 8 days. The potential uptake of 14%e from forage into muscle of commonly used food animals can be estimated. Assuming gastrointestinal absorption of 5 x of the ingested cerium in adult animals (see Table lo), deposition of 0.1 of the absorbed cerium in animal muscle (see Figure 5, page 30) and an average retention time of 400 days; then the total animal muscle mass would accumulate 144Ceup to an equilibrium level equal to 0.02 of the daily ingested amount. Radiocerium is poorly transferred from feed to milk. Although I4'Ce and 144Cefrom worldwide fallout were easily measured in forage, they were not detected in milk from cows feeding on the forage (Potter et al., 1967; 1969; Voilleque and Pelletier, 1974).After receiving oral doses or less into milk (Ekman and of tracer l4*CeCL,goats secreted 3 x Aberg, 1961; Stanchev et al., 1971) and cows secreted 1 x to 1.6 x lo-' (Garner et al., 1960). The transfer of radiocerium from forage to cow's milk is equivalent to about 2 x 10-%f the daily intake of radiocerium secreted in milk per liter at equilibrium. This transfer coefficient is low, but it is interesting that cerium seems to be concentrated during its transfer from plasma to milk. The average milk-toplasma ratio of 14Ce at 20 hours and beyond following intravenous administration of CeC13 to ewes was 3.4 (McClellan et al., 1962). The

18

/

2. SOURCES OF RADIOCERIUM IN T H E ENVIRONMENT

TABLE10--Gastrointestinal absorption of cerium in laboratory animals" Animab

Age at admin-

Chemical form

ktratiorr

chloride chloride in rats milkh nitrate

FracLional a& sorption

Mice Mice Rats Rats

0d 21 d 0-11 d 0d

Rats

7d

nitrate

0.015

Rats

14 d

nitrate

0.009

Rats

26 d

nitrate

l00 percent and have been corrected to 100 percent. "Data of S m l t et al. (1947) and Durbin el aL (1%): intramuscular (i.m.). ' W e c ~ t r a l ewas injected. Bath experiments were balance studies. Data have been corrected to 100 percent recovery and are expressed es p e ~ e n t a g e of absorbed activity.

144

Ce. The lower concentrations in the adrenal gland, testicle, ovary, and pancreas compared to data presented in Tables 18 and 19 are typical of intravenous rather than intraperitoneal injection. There are no available human data for transfer of cerium from the pregnant mother to the fetus. In studies with pregnant mice, Naharin et al. (1969) found that about 0.006 of the lMCecitrate injected into the mother was transferred to the fetus. These studies were in agreement with those of Sternberg (1962) in which 0.003 to 0.03 of the cerium injected into guinea pig mothers was transferred to their fetuses after 24 hours. The gastrointestinal absorption of cerium in the adult human is so small that inhalation is generally the exposure route of major concern. The limited available data on human inhalation exposures have been presented above. Organ distribution data for 144Ceinhaled by beagles are given in Tables 21 and 22 (Cuddihy et al., 1976). Turbinate, lung, and pulmonary lymph node concentrations are followed by liver, thyroid, kidney, and skeleton. Reproductive organs and endocrine glands, except for thyroid, are generally more than one order of magnitude lower in concentration. The sigmficance for man of the

46

/

3. METABOLISM OF CERIUM IN MAMMALIAN SPECIES

TABLE 2LDep0sition a n d concentration of "'Ce in the tissues of miniature swine 10 days after intravenous injection of carrier-free IUCe citrate" O q a n content (fractton of act~v~ty)

Tkue

-

Liver Kidneys Spleen Lungs Salivary Glands Heart Lymph Nodes Adrenals Testes Ovaries Thyroid G.I. TractC Pancreas Muscle Brain Skeleton Body Weight (kg)

Concentration" (fraction of activity/g wet wekht)

3 . 5 . ~lo-'" 4.2.x lo-" 1.2 X I O - : ~ 3.7 x lo-:"' 2.8 x 10-"' 1.9 x lo-" 2.3 x lo-:" 4.9 x lo-5c 1.7 x lo-"" 1.1 x 2.4 x lo-" 3.2 x lo-:" 2.1 x lo-'' 3.7 x lo-" ~ 1 . x3 lo-'' 3.95 x 10-ld 70

3x

lo-5

2.2 x lo-" 1.6 x

lo-"

1.3 x lo-" 1.0 x lo-" 7.9 x lo-" 9.3 x lo-" 9.3 x 105.7 x 10" 1 x lo-" 3.6 x 10" 2.1 2.1

x lo-" x lo-"

1.4 x lo-" t 1 . 4 x lo-" 3.4 x

" Data of McCleUan et al. (1965). Original concentrations (Co) were reported as Co = (pCi/g tisque)/(pCi adm/g body). Those values have been converted to C(Fractions of dose/g) using the relation, C = Co/700, assuming an average body weight of 70 kg for miniature swine. ' Based on concentration in small intestine. Reported by authors. " Calculated from concentration and assumed organ weights. '(Fraction of activity in total skeleton)/(.l@ x 70 kg).

high I4%e concentrations observed in dog's thyroid is unknown, particularly when it is recognized that the dog accumulatesother elements such as %'Am to a greater degree in its thyroid than do other animals (Taylor et ak, 1969). All of the longer-lived radioactive isotopes of cerium that are of significance for radiation protection (see Table 4) decay to isotopes of other lanthanide elements. Some of these daughter products are radioactive and have physical half-lives ranging up to 13.6 d. These other lanthanide nuclides are also retained for long times in the internal organs. For purposes of radiation dosimetry, it is reasonable to assume that all of the energy resulting from these decay chains will be deposited in the organs in which cerium decays. 3.4

Circulatory Transport of Radiocerium

Because of the small solubility products of their hydroxides, the lanthanides do not exist as ions even in dilute solutions.%t the pH of A solution of I pCi/ml of carrier-free IUCe is 2.2 x

lo-'

M.

TABLE 21-Tissue distribution of '4'Ce-'44Pr in beagle dogs after inhalation of '44CeC13 W

Percent sacrifice burden Tissue

2h

Male C.1. & Contents

U)

Lung Liver

31 0.54 0.1 0.045 0.m5 0.0020 0.0040 0.0007 0.0020 0.0025 0.0015

Skeleton Kidney Spleen Ti-acheobmnchial L.N.' Salivary Glands Thyroid Pancreav Prostate Testes

-

U ~ N S

2d

Female 24 20 0.14 0.12

0.05 0.002 0.001 0.0012 0.0011 0.0012 -

Poplitaal L.N. Adrenal Glands Hepatic L.N. Ovaries Pituitary Remaining Soft Tissue Exmral Nares and pelt

0.00002 4.8 27.0

O.M#Jl 0.m001 0.0002 0.0001 0.0001 0.00002 0.3 30.0

T

83.7

74.6

C

~

'L.N. indicates lymph nodes.

0.0004 0.0003 0.OOM -

Male 3.9 45 23 12 2.8 0.09 0.04 0.034 0.064 0.010 0.032 0.006

-

0.003 0.003 0.002

-

0.00012 2.6 4.4 94.0

B 8d

4d

Fede 2.3 40 24 10 1.8 0.17 0.06 0.04 0.062 0.015

-

0.0011 0.0000 0.0033 0.0045 0.0012 0.0002 1.3 5.4 85.2

Male 2 32

Female 3

35

23 1.6 0.14 0.07 0.047 0.032 0.010 0.011 0.0047 -

0.0071 0.0026 0.0004 -

0.00013 1.8 2.1 97.7

25 39 18 3.2 0.13 0.06 0.05 0.06 0.05 -

Male 1.4 21 43 29 2 0.13 0.084 0.042 0.040 0.017 0.019 0.010

0.003

-

0.006 0.007 0.002 0.0025 0.0002 3.5 5.0

0.M86 0 . W 0.0020

97.1

-

0.00023 2.0 3.2 102.0

32 d

Female 1.6 19 35 78

2.7 0.2 0.054 0.042 0.020 0.014

-

0.0022 0.0090 0.0053 0.0017 0.0022 0.00047 2.0 5.4 94.0

Male 0.4 14 58 24.2 0.70 0.062 0.070 0.033 0.0% 0.W 0.011 0.005 -

0.011 0.0034 0.0027 -

0.00014 1.2 1.3 100

.

Female 0.8 15 48 32 1.5

0.12 0.058

"lo 0.045

0.009 -

-

0.0016 0.014 0.005 0.003 0.0016 0.00021 1.6 1.7 101

E

4

8
7.2 and lUCe(N03)3 in solutions of pH > 6. In chloride solutions (4.5 < pH < 7.2) and in nitrate solutions (3.5 c pH < 6) was partly ionic and partly colloidal. The sizes of the particles, calculated from sedimentation rates, were 0.1 to 0.3 pm. Cerium-144 settled rapidly and completely from phosphate solutions of pH > 6. In a sodium citrate solution (mol ratio, citrate: Ce = 1000: l), '"Ce was present as a soluble complex at pH < 8. The influence of the pH on the physical state of '"Ce in simple salt solutions can also be demonstrated biologically. The distribution of intravenously injected l4CeCl3in rats was typical of ionic material for injected solutions of pH = 3 and typically that of colloidal material for injected solutions of pH > 9 (Aeberhardt, 1961; Moskalev, 1961a). Stem (1956) reviewed the early investigations of the interactions of lanthanides with proteins. These studies showed stable binding of carrier-free 90Y to a protein or proteins in plasma or ascites fluid that was not disturbed by the addition of a small amount of NTA, but was partially broken by a small amount of EDTA. They were unable to demonstrate any association of 9 with commercial bovine albumin. Using crude protein separation techniques (salting-out and dialysis), Durbin et al. (1956a) obtained results that suggested '"Ce, 1527'ME~, and 17Tmbinding to globulins in rat plasma. Lanthanide binding did not appear to be prevented when the mixture of lanthanide and plasma was 0.003 M in sodium citrate.

50

/

3. METABOLISM OF CERIUM IN MAMMALIAN SPECIES

Aeberhardt (1961) and Aeberhardt et al. (1961) demonstrated conclusively that canier-free '44Ceintroduced into rat or rabbit plasma as 144CeC4(pH = 4.5) or as 144Ce(N03)3 (pH = 3.5) either in vitro or in vivo was not colloidal, but was associated with protein. They concluded that the binding proteins were p2 and y-globulins. In other studies, depending upon the methods used, nearly all of the major proteins of plasma have been reported capable of binding lanthanide ions. Spencer and Rosoff (1963) and Puchkova (1969) implicated alpha and beta globulins. Ekman and Aberg (1961) reported binding to albumin and low molecular weight fractions in plasma. In one case, Kanapilly and Chimenti (1972) were unable to demonstrate any significant binding of the heavy lanthanide, 17'Tm, in their in vitro system, and the binding of "Y and of ' 9 was below expectation. To date, the only convincing demonstration of a genuine reaction between a nonessential multicharged cation and a specific serum constituent is that of Pu(1V) with transferrin, the betal globulin (molecular weight about 70,000~)that normally carries Fe(1II) in mammalian plasma (Popplewell and Boocock, 1967; Stover et al., 1968; Turner and Taylor, 1968a; 1968b; Stevens et al., 1968; Bruenger et al., 1969; 1971; Stevens and Bruenger, 1972). It is reasonable to suppose that the lanthanides may be bound to the same serum protein that binds Pu(IV) and which is also presumed to bind the trivalent actinides. In accord with theory (Martell and Calvin, 1952), several chelate compounds of the trivalent actinides (citrate, NTA, EDTA, DCTA) are somewhat more stable than the chelates formed by the lanthanides of the same ionic radius (Fuger, 1958; 1961; Krot et al., 1962; Mackey et al., 1962; Moeller and Hseu, 1962; Moeller and Thompson, 1962; Fuger and Cunningham, 1964; Sillen and Martell, 1964; Baybarz, 1965; 1966; Lebedev et al., 1968), and chelates of quadrivalent cations are much more stable than those of trivalent ions.of the same size (Fuger and Cunningham, 1 W ; Krot et al., 1962). Thus, one would expect the stability of protein binding of multicharged cations to be in the order of: Alkaline earths (11)