Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 127-131, 2007

Effects of Fulvic Concentration and Molecular Size on Complexation of Am and Pu with
Aquatic Fulvic Acids
S. Nagao,*, a Y. Sakamoto,b T. Tanaka,b Y. Nakaguchi,c and R. R. Raoda

Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
Tokai Research and Development Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japanc
School of Science and Technology, Kinki University, Higashi-Osaka, Osaka 577-8502, Japand
Chalk River Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario K0J1J0, CanadabReceived: December 19, 2006; In Final Form: July 13, 2007
The association properties of Am and Pu with aquatic fulvic acid in a 0.01 M NaClO 4 solution at pH 6–7 werestudied on the basis of molecular size distribution. Seven fulvic acids (FA) were isolated from river, lake, andgroundwater samples using XAD extraction technique. They were used for comparison of their effects on theassociation of Am and Pu. In the absence of FA, about 80% of Am and 75% of Pu were found to have molecularsize greater than 450 nm (particulate forms). On the other hand, in the presence of FA, particulate forms of Am
and Pu decreased with increasing FA concentration, and were negligible at the FA concentration of 10 mg/L. BothAm and Pu were associated mainly with aquatic FA having two size fractions of 30–10 kDa or less than 5 kDa.
The differences between Am and Pu were rather small and the association patterns were divisible into two groups:
size-selective and non-size-selective. Complexation of FA with Am and Pu was controlled by the amount of FA,
the percentage, and characteristics of each size fraction of FA.

1. Introduction

2. Experimental

The environmental behavior of actinides is dependent onwhether the actinides are injected in the environments assource-dependent or source-independent species.1 The mobilityof actinides is also controlled by complex physico-chemicalinteractions that depend largely on the properties and characteristics of actinides, the aquifer, and the suspended solids.
Association with colloidal organic materials such as humic
substances (humic and fulvic acids) is a well-recognized decisive factor for the fate of actinides in surface aquatic environments.2–4
Most organic materials in natural waters are humic substances that are polydispersed mixtures of natural organic
polyelectrolytes and brown or black complex organic acids ofwidely various molecular weights.5 These substances are heterogeneous mixtures of many molecules which vary fromregion to region. For that reason, they cannot be treated as
simple complex forming ligands.2,6–8 Studies of the chemical
characterization of humic and fulvic acids indicate that theamounts of different functional groups vary considerably
among humic substances of different origin.9–11 The associationof actinides-lanthanides with humic substances was found indifferent molecular size fractions through field observations12–15
and laboratory experiments.16–19 These molecular size effects ofhumic substances on the complexation affinity and sorption
ability might considerably affect the migration behavior of
actinides.17,18,20
This study reports association properties of Am and Pu withaquatic fulvic acid, the dominant fraction of humic substancesin natural waters, based on their molecular size distribution.
The aquatic fulvic acids were isolated from two shallow
groundwater samples and other samples obtained from threerivers and a lake surface. Those substances were then examined to compare their effects on the association of Am and Pu.

2.1. Environmental conditions of aquatic fulvic acids.
Fulvic acids (FA) were isolated from three river water samples(Yodo, Yamato, and Suwannee Rivers) and a Norwegiansurface water sample, each of different environmental
conditions. The Yodo River is a major river running throughthe Kinki region, which is a very heavily populated urban area
in Japan. The river water originates from Lake Biwa in Shiga
Prefecture, and is supplied from the lake at 40 m 3 /s.21 The
Yamato River is located in the Osaka Plain and Nara Basin in
the Kinki region and has a watershed area of 1070 km 2 . It
originates from the Kasachi mountain area, and runs through aheavily populated urban area. The Suwannee River rises in the
Okefenokee Swamp and flows through southern Georgia andnorthern Florida. Most of the Okefenokee Swamp drains into
the Suwannee River.22,23 The Nordic samples were obtained
from a reservoir of Vallsjorden, Norway. The reservoir is at
225 m above sea level and has a maximum depth of about 14
m. T he Suwa n nee a nd Nord ic waters a re brown ish.
Groundwater humic substances from the Glass Block site (GB)and Wallace Point (WP) in the Chalk River Laboratories ofAtomic Energy of Canada Limited were also used in this study.
The GB site is located in the recharge area for a shallow sandaquifer. That recharge area is a seasonal wetland and thesubsu r face residence ti me for t he g roundwater is ofapproximately one year. Wallace Point is located in the
discharge zone of an overburden flow system. Physicochemical properties of water samples are shown in Table 1.21–23
2.2. Isolation and preparation of fulvic acids. The Yodo
and Yamato FA were isolated from a 140-L river water sample
collected at a downstream site of the Yodo and Yamato Rivers.
From the GB and the WP sites, 20,000 L and 10,000 L, respectively, of shallow groundwater were collected. The isolationand purification of humic substances were carried out using
the column method with XAD-8 or DAX-8 resin. 2 5 The
amounts of humic substances were 0.04 g for the Yodo, 0.08 g
for the Yamato, 18.2 g for the GB, and 9.3 g for the WP.
Suwannee River and Nordic FA isolated by the XAD-8 extrac-*Corresponding author. E-mail: nagao@ees.hokudai.ac.jp.
Fax: +81-11-706-2349.

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

128 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Nagao

TABLE 1: Characteristics of water samples
SampleYodo River, Japan
Yamato River, Japanb
Suwannee River, U.S.A.c
Nordic, Norwegian surface waterd
Groundwater Glass Block site, Canada b
Groundwater Wallace Point, Canada baConductivity / mS m–1
17

pH
7.4
7.7
3.9
5.6
5.7
7.1

a

41
21

DOC / mg L –1
2.1
5.3
37.7
10.7
2.3
1.7

Water Quality Management Center for Osaka Pref.21 b This work. c Miles et al.22 d IHSS.23

TABLE 2: Elemental composition and functional groups of fulvic acids
Samplea,b

Suwannee FA
Nordic FAa,bGlass Block site HSc
Wallace Point HSc

C
53.0
52.3
53.2
52.8

Elemental composition / %
H
N
4.4
0.8
4.0
0.7
5.6
0.8
5.4
0.8

Functional groups / meq g–1
Carboxyl
Phenolic
6.5
1.6
6.4
1.6
7.8
0.86
9.9
1.9

O
43.9
45.1
40.3
41.0

The unit of elemental composition is expressed on an ash-free basis. The Yodo FA was not determined for the above parameters becauseof small sample amounts. The contents of functional groups were measured using potentiometric titration or 13C-NMR spectroscopy.
a
IHSS.23 bCalculated from data of Ritchie and Perdue.25 c This work.

100

Dissolved Am, Pu / %tion procedure were purchased from the International Humic
Substances Society (IHSS). The elemental composition of the
FA is given in Table 2.23,26 The chemical composition of GB
and WP humic substances indicates fulvic-like characteristics
and is consistent with the occupation of FA in natural waters.5
A stock solution of the FA was prepared by dissolving 10 mgof the purified FA in a dilute NaOH solution. Then the solution
was made up to 100 mL of 0.01 M NaClO4. The solution was
adjusted to pH 8 by adding dilute HClO4. All fulvic stock solutions were stored in a refrigerator at 4 °C.
2.3. Complexation experiments. Association experiments
were carried out in a medium of 0.01 M NaClO4 solution at thefulvic concentration of 1–20 mg/L and pH 6–7. A nitrate solution of 238 Pu and 241Am was used for experiments. A stock
solution containing 238Pu was prepared through dilution with
deionized water. The stock solution was adjusted to pH 4 using
NaOH solution. Valence of 238Pu in the stock solution was confirmed as Pu(IV) using TTA-xylene extraction method. A
stock solution containing 241Am was also prepared using a similar procedure. In addition, 100 µL of 238Pu and 241Am solution
was added to 10 mL of fulvic solution in a Teflon centrifugation tube. The initial concentrations of Am and Pu was respectively (1.0–7.6) × 10 –8 M and (3.0–7.6) × 10 –6 M. After 7 daysof shaking in an oven at 25 °C, the solution was filtered with
450 nm filters and measured for pH.
Ultrafiltration technique was used to size-fractionate Am
and Pu in the fulvic solutions because it is a non-destructive
and reagent-free method. The sample solution was sequentiallyultrafiltered with simple ultrafiltration filters (Ultrafree CL filters; Millipore Corp.) having nominal cut-off molecularweights of 100 kDa, 30 kDa (polyethersulfone membrane filters), 10 kDa, and 5 kDa (regenerated cellulose membrane filters). The radioactivity of 238Pu and 241Am was measured foreach molecular size fraction using liquid scintillation counting.
The percentage of Am and Pu in each molecular size range wascalculated from the radioactive concentration in each molecular size fraction. The molecular size distribution of the fulvic
acid was measured using ultrafiltration technique or high-performance size exclusion chromatography according to a method
described elsewhere.27

80

60

40

20

0

0

10

20

-1

30

FA concentration / mg L

Figure 1. Dissolved fraction of Am and Pu in the absence and presence of the Yodo and Yamato River fulvic acid (FA) as a function of
FA concentration: lAm-Yodo FA system at pH 5.8–6.2; n AmYamato FA system at pH 6.8–7.1; lPu-Yodo FA system at pH 6.0–
6.7.

3. Results and Discussion
3.1. Effects of fulvic concentration on complexation withAm and Pu. Figure 1 shows the percentage of dissolved Am
and Pu as a function of FA concentration. The dissolved Am
rapidly increases at the FA concentration of 5 mg/L. Based on
kinetic estimation, in the absence of FA, 77% of Am and 74%
of Pu were found in the molecular size greater than 450 nm(particulate forms) at pH 6–7 as a result of polymerization and/or precipitation. The percentage of dissolved Am and Pu
increases with increasing FA concentration, indicating thecomplexation of Am and Pu with the Yodo FA. Particulate
forms of Am and Pu are negligible at the FA concentration of
10 mg/L.
The molecular size distribution of dissolved Am and Pu in
the absence and presence of the Yodo and Yamato FA is shownin Figure 2. Size fractions of increased Am are less than 5 kDa

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 129

Effects of Fulvic Concentration and Molecular Size on Complexation80(a)450nm–100kDa
100k–30kDa
30k–10kDa
10k–5kDa
<5kDa

40

(a)

60

Am / %Am / %

60

40
20

20

0
80

(b)

>450nm450nm–100kDa
100kDa–30kDa
30kDa–10kDa
10kDa–5kDa
<5kDa

60

Pu / %

0
60

20
0
80

40

(c)

60

FA / %Am / %

(b)

40

20

40
20

Pu / %

060

0Suwannee FA

Nordic FA

Yodo FA

GB HS

WP HS

Sample(c)Figure 3. Molecular size distribution of (a) Am and (b) Pu in thepresence of aquatic fulvic acids and (c) fulvic acids themselves in a
0.01 M NaClO4 solution. The pH of sample solutions was pH 5.8–6.5
for Am-FA, pH 5.8–7.0 for Pu-FA, and pH 5.0–6.0 (pH 8 for the
Suwannee) for the FA. The error bars indicate deviation from meanvalue (n = 2).

40

20

0
0

10

20

30

FA concentration / mg L-1
Figure 2. Molecular size distribution of Am and Pu in the presenceof aquatic fulvic acids (FA) as a function of FA concentration: (a)Am-Yodo FA system; (b) Am-Yamato FA system; (c) Pu-Yodo FA
system.

at Am-Yodo FA and 100–10 kDa at the Am-Yamato FA system.
In the presence of FA, the percentage of Pu in the size fractionof 30–5 kDa increases with the FA concentration. On the other
hand, the variation in Pu is not great for the size fraction lessthan 5 kDa; it is nearly equal to that at the blank experiment.
However, we cannot identify whether Pu-fulvic complexes and/or Pu ionic species are present. Apparently, Pu is complexedwith FA having specific molecular size fractions.
3.2. Dissolved forms of Am and Pu in the presence of fulvic acid with different origin. The molecular size distributionof Am and Pu in the presence of FA with different origin isshown in Figure 3. In the presence of Suwannee and Nordic
FA, which are isolated from brownish water with high DOC
concentration (Table 1), the dominant size fraction of Am is
30–10 kDa (64–74%). On the other hand, the major part of
Am in the presence of FA isolated from uncolored natural
waters is found for molecular size less than 5 kDa at the AmYodo FA, Am-GB HS, and Am-WP HS systems. The dominant size fractions of Pu are less than 5 kDa for the Yodo FA,
GB, and WP humic substances (38–43%) and 30–10 kDa for
the Suwannee and Nordic FA (57–64%).
The molecular size distribution of FA is shown in Figure3(c). The dominant size of the FA is 30–10 kDa for the Nordic
sample, and less than 5 kDa for the Suwannee and the Yodo FA
and the groundwater humic substances. A similar size distribution was observed among Am and Pu in the presence of FA,
and FA itself. A discrepancy in the molecular size distribution
exists between Am and Pu in the presence of Suwannee-Nordic
FA and the fulvic acids themselves.
3.3. Complexation properties related to molecular size
distribution of fulvic acid. Figure 4 shows that a positive correlation exists between the percentage of Am and FA in eachsize fraction (correlation factor, r = 0.93) except for the size
fractions of 30 –10 kDa and less than 5 kDa at the AmSuwannee and Am-Nordic FA experimental systems. A similar positive correlation exists (r = 0.93) between Pu and FA,
except for the Suwannee and Nordic with 30–10 kDa and <5
kDa. Table 3 summarizes the percentages of Am and Pu in thedominant size fractions. We assumed that Pu in the size fraction less than 5 kDa was present as fulvic complexes on thebasis of the size distribution of Pu in soil solutions. 28 The
respective mean ratios of Am-FA/FA and Pu-FA/FA are 0.82 ±0.09 and 0.82 ± 0.19 for the size fraction of <5 kDa. On theother hand, the size fraction of 30–10 kDa for the Suwannee
and Nordic FA has different complexation affinities for Am
and Pu. The averaged Am-FA/FA and Pu-FA/FA ratios are,respectively, 2.5 ± 0.6 and 2.2 ± 0.5.
Size selective complexation of fulvic-like materials with Am
and Pu has been reported for laboratory experiments and fieldstudies.15,19,29 Matsunaga et al.15 showed that a size fraction of
100–10 kDa was dominant for 239,240Pu (67%) and 241Am (59%)
in surface water in the Chernobyl nuclear accident area. The
ratios of Am/DOC and Pu/DOC of the size-fractionation are
2.5–2.8 and are similar to our experimental data of the fractionof 30–10 kDa (Table 3). The different characteristics among

130 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Nagao

TABLE 3: Dominant molecular size (MS) of Am and Pu in the presence of fulvic acid (FA)SampleSuwannee FA
Nordic FA
Yodo FA
Glass Block site HS
Wallace Point HS

Dominant MS
30–10 kDa
30–10 kDa
<5 kDa<5 kDa<5 kDaAm-FA / %
74
64
47
57
42

Pu-FA / %
64
57
38
66
43

Am-FA/FA ratio
3.1
1.9
0.84
0.90
0.72

Pu-FA/FA ratio
2.6
1.7
0.69
1.0
0.74

Am-FA (or Pu-FA) means Am (Pu) in the presence of FA.

Am in each size fraction / %80logic media. Fulvic acids in uncolored natural waters exhibit
size-dependent complexation with Am and Pu. On the other
hand, FA in colored natural waters show three complexation
properties as follows: 1) the size-dependent complexation at
450 nm–30 kDa and 10–5 kDa; 2) the size selective complexation at 300–10 kDa; 3) no complexation for the molecular size
less than 5 kDa. The molecular size of 30–10 kDa for the FA
has different characteristics such as functional groups in comparison with that of FA in uncolored waters. These results suggest that molecular size distribution and each size fraction of
FA are one of factors controlling the complexation.

(a)
60

40

20

Acknowledgements. This study was conducted partly
within the framework of the Cooperative Research Project
between the Japan Atomic Energy Research Institute (present:
Japan Atomic Energy Agency) and the Atomic Energy ofCanada Limited. This work was supported partially by aGrant-in-Aid (No. 18310002) for Scientific Research from the
Min ist r y of Education, Cult ure, Spor ts, Science a nd
Technology of Japan, and a research grant from the Radioactive
Waste Management Funding and Research Center.

Pu in each size fraction / %

0
80

(b)
60

References
40

20

0

0

20

40

60

80

FA in each size fraction / %Figure 4. Relationships in percentage between (a) Am and (b) Pu in
the presence of fulvic acids and fulvic acids in respective size fractions. The symbols indicate molecular size fractions: l 30–10 kDa
and n <5 kDa at the Suwannee-Nordic FA systems; l other size fractions at the Suwannee-Nordic FA systems and other systems.

size-fractionated humic and fulvic acids are presented.12,30–32
Consistent results for Am and Pu were obtained using 2-step
and/or 4-step size fractionation for river, lake, and soil solutions.15,28 Figure 4 shows that the slope (0.77) of the regression
line at the Am-FA experimental system was in agreement withthat (0.80) of the Pu-FA system. These results indicate thatsize-dependent complexation occurs in Am and Pu with FA in
a 0.01 M NaClO4 solution at pH 6–7 and fulvic concentrationof 10 mg/L.
Consequently, the association properties for Am-FA and PuFA complexes depend on molecular size distribution and characteristics in each size of surface water humic substances thatare supplied from the watershed of river systems and groundwater humic substances in different recharge areas and geo-

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