Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 2, pp. 37-44, 2004

Review
The Effect of Na+, Mg2+, and Ca2+ Ions on the Formation of Eu(III)-Humate Complex
Yoshio Takahashi*,a,b and Yoshitaka Minai caDepartment of Earth and Planetary Systems Sci. Graduate School of Sci. Hiroshima University,
Higashi-Hiroshima, Hiroshima 739-8526, Japanb
Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE),
Hiroshima University, Hiroshima 739-8526, Japanc
Musashi University and Nezu Institute of Chemistry, Toyotama-kami, Nerima, Tokyo 176-8534, Japan
Received: August 9, 2004; In Final Form: November 8, 2004
Formation of Eu(III)-humate at various concentrations of Na+, Mg2+, and Ca2+ in a wide pH range (4 < pH < 8)
was studied by the solvent extraction method using 152Eu tracer. Concentrations of Na+, Mg2+, and Ca2+ ranged
from 0.020 to 1.0 M, 10−6 to 10−3 M, and 10−6 to 10−3 M, respectively. The dependence of the stability on pH andconcentrations of Na+, Mg2+, or Ca2+ was interpreted by considering the polyelectrolyte nature of humic acid.
Concentration ratios of Eu(III)-humate complex to free Eu(III) ion in fresh water and seawater was estimatedbased on the relations between the stability of Eu(III)-humate complex and concentrations of Na+, Mg2+, or Ca2+.
Speciation calculations of Eu(III) species dissolved in fresh water and seawater were conducted based on the relations. As a result, it was suggested that humate complex could be more important species of Eu(III) dissolved in
fresh water compared with inorganic complexes such as carbonate and hydroxide. In seawater, however, thecarbonate complex can be more important species, since Mg 2+ and Ca2+ in seawater at a 10−2 M level greatly
inhibit the formation of the humate complex of Eu(III).

1. Introduction
Humic substances like humic acid, fulvic acid, and humineinteract with metal ions in the environment.1, 2 Humic substances
are a general term for the heterogeneous polymeric organoacids
widely found in soils, sediments, fresh water, and seawater.3, 4
The carboxyl and phenolic groups are the main functional groupsof the polyacid. Humic substances form stable complexes withmetal ions by these functional groups in the environment. Since
humic and fulvic acids are soluble, they influence the migrationof cations in the environment.
The migration of actinide elements in the environment is ofparticular interest to the topic of radioactive waste problems.5
There are a lot of studies dealing with the speciation of actinide
elements (III–VII) to predict their environmental behavior,
where the discussion was based on each oxidation state.2, 6, 7
For the trivalent actinide, it has been suggested that humatecomplex is the predominant species in natural waters.2, 6−9 In
this work, we study the interaction between humic acid andEu(III) employed as an analogue of actinides(III). Since stability
constants of Eu(III) and Am(III) are similar for their various
complexes,8−10 it is expected that we can extend the results on
Eu(III) in this study to Am(III), or other actinides(III). In fact,
our previous study showed that the stability constants of humatecomplexes are similar between Eu(III) and Am(III) for the
humic acid also employed in this study.8
The stability of the humate complex apparently varies withpH, ionic strength (I s ), and concentrations of polyvalentcations.10−14 However, the stability constants of humate withlanthanides(III) and actinides(III) under environmental conditions such as a pH above 6, Is up to 0.7 M, and presence of
millimolar Ca2+ or Mg2+ have not yet been fully determined.
Therefore, the effects of pH and Na+, Mg2+, and Ca2+ ions on
the stability of Eu(III)-humate were studied in this work. In
particular, the complexation of Eu(III)-humate in the presence*Corresponding author. E-mail: takahasi@geol.sci.hiroshima-u.ac.
jp. FAX: +81-82-424-0735.

of millimolar Ca2+ or Mg2+ has not been thoroughly studied sofar, where divalent cations at millimolar level may considerably
influence the stability of humate with trace Eu(III) (< 10−9 M).
These parameters have wide ranges in natural aquifer; pH 8, Is= 0.7 M, [Mg2+] = 0.06 M, and [Ca2+] = 0.01 M in seawater,while pH 6 – 8, Is < 0.01 M, [Mg2+] = 0.1 mM, and [Ca2+] = 0.2
mM in fresh water.15 Although we have already reported apart of the results for pH and I s dependences (4 < pH < 6,0.020 M < [Na+] < 0.70 M),8 the main purpose of the present
study is to elucidate the effects of Na+, Mg2+, and Ca2+ ions on
the stability constants of Eu(III)-humate. Empirical relations
between stabilities of humate complexes and the concentrationof Ca2+ ions obtained in this study, were employed for estimating dissolved species of Eu(III) under various environmental conditions.
In determining stability constants, many methods have beenused such as radioisotope tracer technique,10, 16 spectroscopy,17
ion-selective electrode, 18 etc. Among them, we employedradioisotope tracer technique. For trivalent ions, it is necessary
to employ a phase separation method such as solvent extraction, ion exchange, or dialysis to distinguish free metal ionsfrom the metal ions bound to humate. However, in the environment, concentrations of dissolved actinide and lanthanide
ions are considered to be at trace level (at least below 10−9 M
for lanthanides). The stability constant of the humate complex
is a conditional value which is greatly affected by the degree ofmetal loading (= concentration ratio of metal ion to humic acid)due to the polyelectrolyte character of humic acid. Radioisotope
tracer technique is sensitive enough to detect metal ions below
10−9 M. Therefore, we can reduce the concentration ratio ofcomplexed metal ion to ligand of humate by the radioisotope
tracer technique lower than by any other methods. Therefore,
we employed the solvent extraction method using a radioisotope tracer, to determine the stability constants of Eu(III) withhumate. Solvent extraction is an excellent method to determine the concentration of free metal ions in the aqueous phase,compared with the ion exchange method, since humic aciditself can be adsorbed by the ion exchange resin. To discuss© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/24/2004

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Takahashi

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 20042+

the effect of Ca ion, the stability constant of Ca-humate wasalso determined using a 45Ca tracer in the presence of various
concentrations of Ca2+ ions to clarify the influence of divalentcations on the stability of Eu(III)-humate.
2. Experimental Details
The humic acid employed in this study was extracted frompaddy soil (Andosol) in Tochigi prefecture, Japan, and was
purified following conventional methods.3, 4 The characteristics of the humic acid were determined using UV/VIS spectroscopy, IR spectroscopy, thermal analysis, elemental analysis,
pH titration, and C-13 NMR.19, 20 All other chemicals used inthis study were of reagent grade or better.
pH titration. For pH titration of humic acid, back-titration
was employed, because the humic acid we extracted did notdissolve easily in water. Freezed-dried humic acid was dissolved
to a 0.100 M NaOH solution, where the concentration of humic
acid was about 200 mg/dm 3 . The solution was titrated by
0.0100 M HClO4 solution under N2 atmosphere at room temperature with an automatic titration system (autoburette: Metrohm
E552-20B; pH meter: Metrohm 654) controlled by a personal
computer (NEC PC-9801). The pH drift was monitored after
each addition of a small amount of the acid. The reading wasrecorded when the pH drift was less than 0.001 pH per 5 min.
Ionic strength (Is) was adjusted to 0.020, 0.50, 0.10, 0.20, 0.50,
0.70, and 1.0 M by NaClO4.
Solvent extraction to determine stability constants. The
aqueous phase was humic acid solution containing a buffer
reagent to adjust pH with NaClO4, MgClO4, or CaCl2 to adjustIs, [Mg(II)]t, or [Ca(II)]t (= total concentrations of Mg or Ca
species). When Mg or Ca salt was added, Is was adjusted by
NaClO4 to 0.020 M. The concentration of humic acid ranged
from 0 to about 1 × 10−3 eq/dm3 (≈ 150 mg/dm3). Four buffer
reagents were used to maintain pH: 3-chloropropionic acid (2.5mM) for pH around 4; acetic acid (1.0 mM) for pH around 5;hydroxylammonium chloride (0.010 M) for pH around 6; andTris buffer (0.010 M) for pH from 7 to 8. Each tracer (152Eu
and 45Ca) was prepared as perchloric acid solution (1.0 mM).
The extraction reagent was di-(2-ethylhexyl)-phosphoric acid
(DEHP), which was purified prior to use.21 The organic phase(DEHP/toluene solution) was pre-equilibrated by the buffer
solution in order not to alter the pH of the aqueous phase.
The organic phase and the aqueous phase with a small amountof tracer solution were mixed and shaken at room temperature inglass vials for three days, which was long enough to reach anequilibrium in the aqueous-organic distribution of the radioisotopes. The carrier concentration of Eu(III) in the solution was 1
× 10−9 M. After centrifugation, the two phases were separated
and distribution ratios were obtained by measuring radioactivities with a well-type NaI(Tl) counter for 152Eu(III) and with aliquid scintillation counter for 45Ca. The concentration of humic
acid was monitored by absorbance at 400 nm.
Experiments in the presence of Ca2+ ion were conducted asfollows. The stability constant of Ca-humate was determinedusing 45Ca as described above, where the carrier concentrationof Ca from the 45Ca solution was 2 × 10−9 M in the solvent
extraction system. A part of each phase was taken for thedetection of β-rays of 45Ca by a liquid scintillation counter.
Quenching of the luminescence of scintillater (PICO-FLUOR
40, Packard Instrument Company, Inc.) by humic acid in the
aqueous phase was corrected depending on the concentrationof humic acid. Afterwards, 152Eu tracer was injected into theresidual water-toluene solution. The injection of a small
amount of 152Eu tracer solution did not affect the humate equilibrium with Ca 2+ ions, since Eu(III) concentration and theacidity of the Eu tracer solution were low enough to keep theequilibrium between Ca2+ ions and humic acid. The pH values
showed little variation before and after the injection of the Eu

tracer, which suggests that the equilibrium was maintained.
The distribution ratio of Eu(III) was determined by detecting γrays of 152Eu by NaI(Tl) detector, while β-ray of 45Ca was notdetected by the NaI(Tl) detector. This procedure enables us todetermine the stability of Ca-humate and Eu(III)-humate underidentical conditions.
In this study, solvent extraction above pH 6 was also carriedout. Generally speaking, Eu(III) can be readily adsorbed on
the wall of glass vials above pH 6, which results in a loss ofradioactivity in the solvent extraction system. 2, 22 To avoid
such adsorption, silanols of the glass surface (Si-OH) were
substituted to methoxides by (CH3)2SiCl2 and methanol.
3. Principles and Data Treatment
Principles to obtain stability constants. The method todetermine the stability constants by solvent extraction follows thedescription in Reference 10. In the solvent extraction system,
D0, D1, and D2 were considered:
1/D0 = [Mz+]a / [M((DEHP)2) z]o(1)

1/D1 = ([Mz+]a + [MBj]a) / [M((DEHP)2) z]o(2)

1/D2 = ([Mz+]a + [MAi]a + [MBj]a) / [M((DEHP)2) z]o= 1/D1 + [MAj]a / [M((DEHP)2) z]o(3)(M: Eu(III) or Ca; z: charge of the ion; o: organic phase; a: aqueous
phase, A: ligand of humic acid; B: other ligands such as OH−, CO32−,
and buffer anions).

The stability constant, βΜ, of humate with metal ion (Mz+) is
expressed as
βMi = [MAi] / ([M z+ ][A]i ),(4)

where i is the number of ligands bound to one metal ion.
Based on eqs 1 – 4, we can obtain
D0 (1/D2 – 1/D1) = ΣβMi [A]i.

(5)

D1 and various D2 were obtained according to the concentration of free ligands in humic acid in the unit of equivalentconcentration of the ligands (= [A] eq/dm3). The [A] is determined from the proton exchange capacity of humic acid (PEC)
and the degree of ionization (α) at each pH ([A] = α × PEC),
as determined by pH titration. When we obtain [A], we assume
that the trace amount of metal ions does not influence the equilibrium of proton dissociation of humic acid (however, thisassumption cannot maintained in the presence of larger [Ca]t as
we will discuss below). The extraction reaction of metal ions(Mz+) by DEHP can be written as+(Mz+)a + z ((DEHP)2)o →← z (H )a + (M((DEHP)2) z)o.

(6)

When [M]t (= total concentration of M in the aqueous phase) issmall enough, the concentration of free (DEHP)2 in the organic
phase does not change by the formation of M((DEHP) 2 ) z
species in the organic phase. In this case, D 0 in eq 5 (=[M((DEHP)2) z]o /[Mz+]a) can be obtained from the extraction
properties of DEHP aslog D0 = a pH + b log([(DEHP)2]o) + log Kex,(7)where a and b are expected to be equal to z and Kex an extraction constant defined as:
Kex = [H+]az [M((DEHP)2)z]o / ([Mz+]a [(DEHP)2]oz ).

(8)

The K ex and coefficients a and b in eq 7 were determined

The Effect of Na+, Mg2+, and Ca2+ Ions

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

pCH = pH (Is = 0.10 M) + 0.11.

(9)

At other Is, differences of pH at Is = 0.10 M were determined
experimentally by measuring the pH of NaOH-HClO4 solution
at each Is, where the degree of ionization of NaOH or HClO4 inaqueous solution was regarded as unity.
For the speciation calculation, we need the stability constantsof Eu(III) complexes at various Is found in natural water conditions. In general, the stability constant of metal complex in the
aqueous phase decreases with the increase in I s . SpecificInteraction Theory was often used to estimate the thermodynamic data at higher I s . 9, 24, 25 In this theory, the stability
constant at a certain Is can be obtained by the next equation:log β(Is) = log β(0) + ∆z 2 D + ∆ε Is,(10)where D is the Debye-Hückel term expressed as D = 0.51
Is1/2/(1 + 1.5 Is1/2), β(0) is the stability constant at Is = 0 M, andΣz 2 is equal to Σz 2(products) – Σz 2(reactants) where z is the
charge of each ion. The ∆ε is equal to ε(products) – ε(reactants), where ε is a parameter which is related to the specificinteraction between each pair of cation and anion in the corresponding complex. The parameters of the Specific Interaction
Theory and the stability constants of carbonate,26 hydroxide,7and chloride26 for Eu(III), or Am(III), are listed in Table 1,
where the stability constants calculated at a particular Is which
we used in the speciation calculations are also shown. For
sulfate and hydrophosphate, literature data of the stability
constants were used directly in the speciation calculations, due
to the absence of parameters in the Specific Interaction Theory.
4. Results and Discussion
pH titration. The proton exchange capacity of the humic
acid was 6.1 ± 0.4 meq/g (n = 10). The apparent pKa of humic
acid can be determined by the next equation where α is thedegree of ionization of humic acid:27
pKa = pH – log{α/(1 – α)}.

(11)The apparent pKa increased with α and decreased with Is as
shown in Figure 1. The increase against α is partly induced bydifferent dissociation groups within humic acid molecules. In

8

I s = 0.020 M
I s = 0.10 M

7

I s = 0.70 M

pK aexperimentally by solvent extraction in the low pH region
where hydrolysis and carbonate complexation are negligible.23
Based on eqs 1 and 7, we can determine [Mz+]a in the aqueous
phase by the solvent extraction experiments.
Data treatment. Variation of Is (or ionic strength) essentially influences the experimental data of pH and stability
constants, since it affects the activities of chemical species in
the aqueous phase. We did not use proton activity but pCH(concentration of proton) to determine pH in this study. The
Debye-Hückel equation enables us to calculate pCH from pH
at Is = 0.10 M as following:

6

5

4

3

0.3

0.4

0.5

α

0.6

0.7

+

EuCO3
Eu(CO3)2−
EuOH2+
Eu(OH)2+
EuCl2+
EuSO4+
EuHPO4+

addition, the increase in pKa is also due to the increase of negative charges on humic acid molecule at larger α, where protons
are attracted more strongly to the macromolecule. Thisphenomenon was characteristic of polyelectrolyte like humic
acid.14, 28−30 On the other hand, supporting cations (= Na+ )shield the attraction between humic acid and proton, which
results in decrease of pKa at larger Is.14, 27−29
Extraction equilibrium of Eu(III) and Ca(II) by DEHP.
From D0 dependence on (i) pH (pH < 2) at constant [(DEHP)2]oand (ii) [(DEHP)2]o at constant pH in solvent extraction experiments, eq 7 was obtained experimentally for each Is by leastsquares analyses (r > 0.99990). We obtained a = 2.85 ± 0.04
and b = 2.88 ± 0.04 as averaged values for Eu(III). The Kex
apparently decreased with Is as shown in Figure 2. The data
were used for solvent extraction experiments at each Is fordetermining the stability constants. For Ca, a and b in eq 7
were 1.87 ± 0.03 and 1.80 ± 0.04, respectively, and log Kex of
Ca by DEHP was –3.80 at Is = 0.020 M.
Apparent stability constants of Eu(III)- or Ca(II)-humate
at various pH and I s . Plots of D0(1/D2–1/D1) against [A] at
constant Is as expressed by eq 5 have often been interpreted as
quadratic curves for Eu(III) or Am(III), which gives two
stability constants, β M1 and β M2 , corresponding to MA andMA2 complexes.10, 12, 19 However, the meaning of βM1 and βM2
is still not clear in terms of the stoichiometry of the complexation reaction. Since humic acid is polyacids having large
molecular weights, local structure around the metal ions inhumate complex would not change when the concentration of
humic acid (= [A]) increases at constant [M]t, because [M]t is
at trace levels. It is most likely that trace metal ion is bound tolog βi[References]

Is = 0.020 M

Is = 0.70 M

∆z 2

∆ε

8.1 (Is = 0 M)
12.9 (Is = 0 M)
–7.5 (Is = 0.10 M)–15.4 (Is = 0.10 M)–0.1 (Is = 1.0 M)
1.9 (Is = 0.50 M)
4.2 (Is = 0 M)*

[25]
[25]
[7]
[7]
[25]
[40]
[41]

7.4
12
–7.3
–15.4
0.37

6
10.1
–7.8
–15.8
–0.14

–12
–16
–4*
–6*
–6

0.25
0.2
–0.05*–0.01*–0.4*Data for Am complex are used.

0.8

Figure 1. Variation of apparent pKa at different degrees of ionization(α) of humic acid at various Is adjusted by NaClO4.

TABLE 1: Stability Constants Used for the Speciation Calculations and Parameters in Specific Interation Theory
Species39

40

Takahashi

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004
0.50

3
15

4

pH

5

6

0.6

0.8

Eu(III)-humate
Eu(III)-propionate

0.25

pKa of humic acidlog β Eu(III) or pKa

logK ex

12

0.0

-0.25

-0.50

9

6

0.0

0.20

0.40

0.60

0.80

3

1.0

Is /M

0.2

specific binding sites in the macromolecule even if [A]/[M] tchanges to some degree. Therefore, we think that βM1 and βM2
do not contain any structural information, but only distinguishthe metal-humate species from free metal ions.31 According toReference 8, the ratio βΜ2/βΜ1 {= [MA2]/([MA][A])} undernatural water conditions is less than 103 for Eu(III). Since
humic acid concentration in the environment is below about 10
mg/dm3 (ca. [A] = 0.5 × 10−4 eq/dm3), the ratio [MA2]/[MA] isless than 0.05. This shows that βΜ1, which gives [MA], is the
major parameter to describe the humate species of Eu(III) in
the environment. We will evaluate the metal- humate interaction through βΜ1 in this paper due to these reasons, as written
above, and hereinafter βΜ means βΜ1.
The dependences of the stability constants of Eu(III)-humate
on pH and α at Is = 0.10 M are shown in Figure 3, and indicatethat the stability constant increased with pH, or α. This trend
can also be explained by the polyelectrolyte effect, as
described in the pKa results. The increase in pH (or α) gives
more negative charges to humic acid molecule, which
enhances the strength of the electric field at the surface of thehumic acid molecule. Because of the electrostatic force, metal
ions are attracted to the humic acid molecule to a greater
degree when pH increases. The fraction of metal ions attracted
electrostatically to humic acid is theoretically distinguishedfrom another fraction which is directly bound to binding sites
in humic acid. In order to compare log βEu(III) of Eu(III)-humate
with simple monomeric acid, the stability constant of Eu(III)propionate was determined at several α by the solvent extraction method (Figure 3). There was little variation in thestability constant of the propionate at different α, suggestingthat pH dependence on the stability constants of humate ispeculiar to polyelectrolyte like humic acid.
In Figure 4, the pH dependence of log βCa is shown at Is =0.020 M, accompanied by log βEu(III) at Is = 0.020, 0.10, and0.70 M. The log βCa values are smaller than log βEu(III), and theslope of log βM versus pH is greater for Eu(III)-humate thanCa-humate, because electrostatic attraction to humic acid isgreater for trivalent ions than for divalent ions. The dependence of βM on pH (or α) was interpreted based on a simple
electrostatic model.29, 32−34 In the theory, pKa and log βM can be
written as a function of α:
pKa = pKint − 0.434 e φ(α)/kT

(12)

α

Figure 3. Apparent stability constants of Eu(III)-humate and Eu(III)propionate at various degrees of ionization (α) at I s = 0.10 M(NaClO4). Variation of apparent pKa was also shown.

Eu(III), I s = 0.70 M

20

Eu(III), I s = 0.10 M
Eu(III), I s = 0.020 M
Ca, I s = 0.020 M

16

log β Eu(III) or log βCa

Figure 2. Extraction constants (Kex) of Eu(III) by DEHP at various Is.

0.4

12

8

4
4

5

6

7

8

pH
Figure 4. The pH dependence of apparent stability constants ofEu(III)-humate at Is = 0.020, 0.10, and 0.70 M and Ca-humate at Is =0.020 M.

log βM = log βM-int − 0.434 z e φ(α)/kT(13)(k: Boltzman constant, e: elementary electric charge,
z: charge of metal ion)In these equations, pKint and log βM-int are the intrinsic pKa andlog βM, respectively, and φ(α) is the strength of the electricfield in the vicinity of negatively charged humic acid as a function of α. The region at the vicinity of humic acid is called the
Donnan phase in the theory. In this paper, only one binding site
was assumed in humic acid below pH 8 as the simplest case,

The Effect of Na+, Mg2+, and Ca2+ Ions

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

where the dissociation of the phenolate site can be negligible
below pH 8. In this case, the variation of pKint and log βM-int is
attributed to the electrostatic effect and that two species of metal
ions are considered. One species is directly complexed to thebinding site and the other is in the Donnan phase. From eqs 12
and 13,log βM = log βM-int + z ∫(dpKa /dα)dα(14)

was derived. The relation between pKa and α was obtained
empirically as a quadratic function by least squares analysis
for the experimental data shown in Figure 1. Based on thisrelation, the second term in the right hand of eq 14 can beobtained by assuming the valence of the metal ion. This allows
us to fit log βM variation against α. The fitted curves converted
to the relationship between log β M and pH are depicted in
Figure 4, showing that the curves derived by assuming z (2 for
Ca and 3 for Eu) in eq 14 can simulate the experimental data.
The log βEu-int and log βCa-int values obtained by the fittings
were 7.6 and 3.6 at Is = 0.020 M, respectively. The fact that we
can simulate the results by eq 14 for Eu(III) and Ca at various
Is suggests that electrostatic interaction is important to humate
formation.
The Is dependences of log βEu(III) and log βCa at constant α
was shown in Figure 5, where an example at α = 0.5 was indicated. This result shows that the stability constants apparently
decreased with Is. The decrease was possibly caused by theshielding effect of supporting cations (= Na +), showing thepresence of electrostatic interaction between metal ions andhumic acid. The I s dependence was also explained by thesimple electrostatic model. From the charge balance in the
Donnan phase, the next equation can be formed:+z[Na]D + [Mz+]B →← z[Na ]B + [M]D,log βM = –z log ([Na+]B) + log C' (C': constant).

(18)

(16)

(17)

TABLE 2: Estaimted Log([EuLi]/[Eu3+]) in Fresh Waterand Seawater Considering Carbonate, Hydrolysis, Chloride, Sulfate, and Hydrophosphate(15)

to give next equation:z log ([Na]D/[Na+]B) + log kNa = log ([M]D/[Mz+]B).

[Na]D is considered to neutralize the charge of humic acid.
Since [M]t and [A]t are below nM and meq/dm3, respectively,[Na]D is considered to be constant, when Is is above 0.020 M.
These facts lead to eq 18 at constant α, if we assume that [M]Dis approximately equal to the total concentration of complexed
species of M z+ (in other words, [MA] int directly bound tobinding site of humic acid is negligible compared with [M]Dunder the condition):

Since [Na+]B is approximately equal to Is, the result in Figure 5
was fitted to eq 18, where the slopes obtained are –2.8 and–2.0 for Eu(III)- and Ca-humate, respectively. These results
suggest that most part of Eu(III) apparently bound to humic
acid is attracted electrostatically to the Donnan phase of humic
acid.
Speciation calculations of Eu(III) considering the pH
and Is effects. As shown in Table 2, carbonate species would
be the dominant dissolved species of Eu(III) both in fresh waterand seawater without taking account of humic substances. The
influence of humate formation was estimated based on thestability constants considering the pH and Is effects determined
above. From Figure 5, log βEu(III) under the conditions of pH
and Is in fresh water (pH = 7, Is = 0.020 M) and seawater (pH =8, Is = 0.70 M) were estimated to be 15.4 and 15.5, respectively. To take account of the influence of humic acid, theconcentration of humic acid, or humic substances, in natural
water is needed. In fresh water, the concentration of humic
substances is 2 – 10 mg/ dm3.33 In seawater, the concentration
is 0.06 – 2.5 mg/dm 3 . 34, 35 Therefore, the ranges of –6 <log{[A]total/(eq/dm3)} < –4 and –7 < log{[A]total/(eq/dm3)} < –4
are assumed for the concentrations of the ligand of dissolved
humic substances in fresh water and seawater, respectively.
The estimated values of dissolved species of Eu(III) are shown
against the concentration of humic substances for fresh waterand seawater in Figure 6, respectively, where the lined areashows the possible condition in natural waters. It is suggested
that humate complex may be the predominant species ofdissolved Eu(III) species in both fresh water and seawater, as

where D and B denote the Donnan phase and the bulk solution,
respectively. The equilibrium constant kNa can be written as(M = Eu(III) or Ca)([M]D/[Mz+]B) ([Na+]B/[Na]D)z = kNa

41

Fresh water (Is = 0.020 M, pH 7)
10

Ligand

[L]/M

CO32−

5.1 × 10−7OH−

1 × 10−7

Cl−
SO42−
HPO42−

1.6 × 10−7
1.1 × 10−7
2.1 × 10−10log β Eu(III) or logβ Ca

98Eu(III)7log([EuLi]/[Eu3+])
i=1
i=2
i=1
i=2
i=1
i=1
i=1

1.1
–0.7
–1.3
–3.1
–6.2
–5.1
–5.5

Ca

Seawater (Is = 0.70 M, pH 8)
6

Ligand

[L]/M

CO32−

2.4 × 10−3OH−

1 × 10−6

Cl−
SO42−
HPO42−

5.4 × 10−1
2.8 × 10−2
2.1 × 10−8

5

4

0.1

1

I s /M
Figure 5. Apparent stability constants of Eu(III)- and Ca-humates atvarious Is when α (degree of ionization) is 0.5.

log([EuLi]/[Eu3+])
i=1
i=2
i=1
i=2
i=1
i=1
i=1

3.5
4.9
0.2
0.2
–0.4
0.4
–3.5

Concentrations of the ligands (= L) are also shown.15

42

Takahashi

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004
8

100

80

EuCO3+

init[Ca] = 1.0 mMtinit[Ca] = 0.10 mM
t

Eu-HA

init[Ca] = 10 µMt60

init[Ca] = 1.0 µMt-

Eu(CO3)2

20
+

EuCO3
0
-15

init[Ca] < 0.10 µM
t

6

40

-10

(a)
-5

log{[HA]/(eq/dm3)}

(b)
0 -15

-10

-5

0

log{[HA]/(eq/dm3)}

Figure 6. Percentages of dissolved species of Eu(III) in (a) fresh
water (pH = 7, Is = 0.020 M) and (b) seawater (pH = 8, Is = 0.70 M) as
a function of the concentration of humic substances [A] (eq/dm3 )without considering the effect of Ca 2+ and Mg 2+ ions. Lined areashows the possible condition of [A] in each system.

compared with any other species. For the purpose of more
precise estimation of Eu(III) behavior, however, it is necessary
to consider the effect of Ca2+ and Mg2+ ions.
The influence of Ca2+ and Mg2+ ions on the stability ofEu(III)-humate. The stability of Eu(III)-humate in the presence of Mg2+ or Ca2+ ion was also determined by the solvent
extraction technique. In the experiments, it is possible thatextracted Mg or Ca in the organic phase may interfere with theextraction equilibrium of Eu(III) by DEHP, because theconcentration of free (DEHP) 2 in the organic phase wasassumed to be constant in the solvent extraction experiments
expressed in eq 6. In the extraction reaction by DEHP, Ca isextracted more readily than Mg, since log K ex of Mg wasreported as –4.45,23 whereas log Kex of Ca was determined tobe –3.80 in this study. Therefore, the extracted Ca in theorganic phase was monitored by adding 45Ca tracer to examine
the interference caused by the extracted Ca2+ ion. In the worst
case, about 8% of DEHP in the organic phase formed a complex
with Ca at [Ca]t = 1.0 mM (pH: 5.4, initial [(DEHP)2]o: 1.00 ×
10−3 M). DEHP bound with Ca in the organic phase was notincluded as the extracting reagent of Eu(III) and the fraction offree [(DEHP) 2 ] became 92% in the worst case, where thedecrease in concentration of free (DEHP)2 reduces log βEu(III) by
only 0.1, which can be neglected in this study. Hence, below
pH 5.4, Ca 2+ ions will not affect the extraction behavior ofEu(III) by DEHP in our experimental system.
As described in the experimental section, the stability
constants of Ca- and Eu(III)-humate were determined underidentical experimental conditions in the presence of various[Ca]t. However, it is difficult to determine [A] in the plot ofD0(1/D2–1/D1) versus [A] (eq 5), when [Ca]t is not negligible
compared with [A], because the presence of Ca2+ ions affectthe equilibrium of proton dissociation of humic acid between[HA] and [A] to a great degree at larger [Ca]t. This situation
inhibits the determination of stability constants at larger [Ca]t,because the ratio [Eu(III)A]/[Eu3+] or [CaA]/[Ca2+] obtained
from D0(1/D2–1/D1) must be normalized by [A] in order toobtain βM. The aim of the present study is the speciation ofEu(III) in the presence of [Ca]t above 10−4 M level as found innatural waters. For this purpose, it is enough to obtain theratio [Eu(III)A]/[Eu3+] in the presence of Ca2+ ions. Therefore,
we tentatively determined the ratios of [Eu(III)A]/[Eu3+] and[CaA]/[Ca2+] when the concentration of humic acid was 100
mg/dm3 (Figure 7). In this calculation, [Eu3+] and [Ca2+] in the
aqueous phase were determined based on the relations in eqs 1
and 7. As a result, it is clear that log([CaA]/[Ca2+]) decreased
with [Ca]t due to the saturation of binding sites and the shielding
effect of the electrostatic potential at the surface of humic acidby added Ca2+. Similarly, log([Eu(III)A]/[Eu3+]) decreased
with the increase in [Ca]t. In our experiments, if pH and [Ca]tare identical, α and [CaA] should be identical for the twolog([MA]/[M] free ) (M: Eu(III) or Ca)

Estimated dissolved species /%Eu-HA

Eu(III)-humate

4

2

0

Ca-humate

3.5

4

4.5

5

5.5

pH
Figure 7. The concentration ratio of Eu(III)- and Ca-humates and free
metal ions at various initial concentrations of Ca2+ ion (= init[Ca]t) in
the aqueous phase when the concentration of humic acid was fixed at
100 mg/dm3.

values, [Eu(III)A]/[Eu3+] and [CaA]/[Ca2+]. Thus, these twoseries of data ([Eu(III)A]/[Eu3+ ] and [CaA]/[Ca 2+ ]) can becompared directly, as shown in Table 3. It was found that there
is an empirical relationship between log([Eu(III)A]/[Eu3+]) andlog([CaA]/[Ca2+]) as shown in Figure 8, with which speciation
calculation in the presence of Ca 2+ and Mg 2+ ions will be
conducted. In Table 3, [CaA] must be lower than the total
concentration of ligands in humic acid, which was not the caseat initial [Ca]t = 1.0 mM and in pH 4.95 and 5.24. The reason
for this is not clear at present, but could be due to the contribution of additional ligands in humic acid, such as phenolate,which were not determined in pH titration.
For Mg2+ ions, we have only obtained the [Eu(III)A]/[Eu3+] ratio
at various [Mg]t (Figure 9), showing that log([Eu(III)A]/[Eu3+])
decreased with the increase in [Mg]t. From the data, it was
revealed that the effect of Mg2+ ion is similar to that of Ca2+.
Speciation calculations of Eu(III) considering the effectof Ca2+ and Mg2+ ions. For the purpose of more precise estimation of Eu(III) species in natural waters, the effect of Ca2+and Mg2+ ions are included in the speciation. In the calculation, it is regarded that the effect of the Mg2+ ion is identical tothat of the Ca2+ ion. First, the fresh water system is considered, where we assume following conditions; pH 7 and concentrations of [Mg 2+ ] and [Ca 2+ ] are 0.1 mM and 0.2 mM,
respectively. The lowest concentration of humic acid,log{[A]total/(eq/dm3)} = –6, within the range assumed in thisstudy (–6 < log{[A]total/(eq/dm3)} < –4) was employed in thecalculation. In this case, [Ca2+] + [Mg2+] (= 0.3 mM) is larger
than [A]total by more than two orders of magnitude, where wecan assume that all sites in humic substances are occupied by
Ca2+ and Mg2+ ions (i.e., [CaA] + [MgA] = 10−6 M). When we
assume that the effect of the Mg2+ ion is identical to that of theCa2+ ion, log([CaA]/[Ca2+]) becomes –2.5. Based on the empirical relations in Figure 8, log([EuA]/[Eu3+]) becomes 1.2, which

The Effect of Na+, Mg2+, and Ca2+ Ions

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

TABLE 3: Log([CaA]/[Ca2+]) and Log([EuA]/[Eu3+]) Values
for Various pH and Initial [Ca]t in the Aqueous Phase
initial [Ca]t log([CaA]/[Ca2+])[CaA]log([EuA]/[Eu3+])

4.03
4.48
4.95
5.24

1.0 mM
1.0 mM
1.0 mM
1.0 mM

−0.35
0.09
0.61
0.76

0.291 mM
0.538 mM
0.789 mM
0.790 mM

3.50
4.30
4.95
5.36

4.06
4.59
5.03
5.36

0.10 mM
0.10 mM
0.10 mM
0.10 mM

0.11
0.58
1.22
1.45

53.3 µM
76.3 µM
91.2 µM85.2 µM

4.32
5.01
5.51
6.03

3.91
4.58
5.02
5.23

10 µM
10 µM
10 µM
10 µM

0.15
0.99
1.72
1.78

5.84 µM8.75 µM
9.65 µM9.45 µM

4.34
5.65
6.24
6.42

3.93
4.78
5.03
5.38

1.0 µM
1.0 µM
1.0 µM
1.0 µM

0.23
1.54
1.82
2.38

0.451 µM0.964 µM
0.969 µM
0.971 µM

4.78
6.25
6.67
7.26

3.91
4.78
4.73
5.41

0.10 µM
0.10 µM
0.10 µM
0.10 µM

0.29
1.63
1.38
2.49

0.0660 µM
0.0977 µM
0.0960 µM
0.0997 µM

5.13
6.25
6.22
7.15

8

7

log([Eu(III)A]/[Eu 3+])pHis slightly larger than log([EuCO 3 ]/[Eu ]), most important
inorganic species of Eu(III) in fresh water (Table 2). Considering that the concentration of humic substances assumed inthis calculation is at the lowest value, it is suggested that thehumate complex is predominant for Eu(III) species dissolvedin fresh water even considering the effect of Mg 2+ and Ca2+
ions. This estimation is consistent with the analyses of natural
waters which show that rare earth elements are associated withorganic materials in fresh water.36, 37
In seawater, the effect of Mg2+ and Ca2+ ions should be much
more intensive, since [Mg]t and [Ca]t in seawater are 0.053 M
and 0.013 M, respectively. In this case, we employed the largest
concentration of humic substances, log{[A]total/(eq/dm3)} = –4,
within the range considered in this study for seawater (–7 <
log{[A]total/(eq/dm3)} < –4). In this case, we assumed that all
the binding sites in the humic substances were occupied by
Ca2+ and Mg2+ ions (i.e., [CaA] + [MgA] = 10−4 M) due to themuch larger concentrations of these ions in seawater comparedwith those of humic acid and other polyvalent cations. When
we assume that the effect of the Mg2+ ion is identical to that of
the Ca2+ ion, log([CaA]/[Ca2+]) becomes –2.8. In this case,log([EuA]/[Eu3+]) becomes 0.84 according to the relation shownin Figure 8. In seawater, log([EuCO 3 + ]/[Eu 3+ ]) = 3.5 andlog([Eu(CO3)2−]/[Eu3+]) = 4.9, which shows that the carbonate
complex is much more important than humate for the dissolved
species of Eu(III) in seawater. In the marine geochemistry ofrare earth elements (REE), the REE pattern of seawater is
explained by the equilibrium between the carbonate complexation in the aqueous phase and the adsorption on inorganic
particulate matters.38, 39 By contrast to previous studies on thestability constants of Eu(III)-humate suggesting that humate
is the exclusively dominant species for actinides(III) andlanthanides(III) in seawater, 8, 10 the present results show theimportance of the carbonate complex for the dissolved speciesof lanthanides(III) and actinides(III) in seawater when taking
account of the effect of Mg2+ and Ca2+ ions, because the pres-

5

3
0

1

2

log([CaA]/[Ca 2+ ])Figure 8. Relationship between log([CaA]/[Ca 2 + ]) andlog([Eu(III)A]/[Eu3+]) at various initial [Ca]t.

7

init[Mg] = 1.0 mMtinit[Mg] = 0.10 mMt
init[Mg] = 10 µMtinit[Mg] = 1.0 µM
tlog([Eu(III)A]/[Eu 3+ ])

3+

6

4

*[CaA] + [Ca2+] is not equal to initial [Ca]t when Ca complexed to
DEHP in the organic phase is not negligible. Total concentrationof ligands in humic acid is 0.61 meq/dm3.
+43

init[Mg] < 1 µM
t

6

5

4

3.5

4

4.5

5

5.5

pH
Figure 9. The pH dependence of log([Eu(III)A]/[Eu3+]) in the presence of various initial concentrations of Mg2+ ion (= init[Mg]t) when theconcentration of humic acid was fixed at 100 mg/dm3.

ence of polyvalent cations inhibits the formation of humatecomplexes with trace metal ions.
5. Conclusions
The stability of Eu(III)-humate complexes at various pH in
the presence of Na+, Mg2+, and Ca2+ ions were evaluated. The
dependences on pH and concentrations of added cations can beinterpreted by considering the polyelectrolyte nature of humic
acid. From the stabilities, speciation calculations wereconducted for Eu(III) species dissolved in fresh water andseawater. The results showed that humate complex could be
the predominant form for lanthanides(III) and actinides(III) in
fresh water, where the effects of Mg2+ and Ca2+ ions are small.
In seawater, however, carbonate complex may be more important than humate for the dissolved species of lanthanides(III)44

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004
2+

2+

−2

and actinides(III), since Mg and Ca at 10 M level inhibit
the formation of humate complexes with trace metal ions.
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