Vol.9 No.1

原子力バックエンド研究

研究論文

フミン酸およびポリアクリル酸による鉄(II)の錯生成
ブディ・セティアワン＊ 田中紘一＊ 新堀雄一＊ 杤山修＊地下水中のフミン物質は，放射性核種と可溶性コロイド錯体を形成することにより，それらの移行挙動に影響を与えると考えられている．核種とフミン物質の相互作用を理解するためには，地下水中にもともと含まれている競争イオン
による影響をも評価しておくことが必要である．この目的で，Fe(II)とアルドリッチ社製フミン酸との錯生成をイオン交換法により検討した．フミン酸の組成不均質性と高分子電解質性による影響を分離して評価するために，組成が均質な
高分子弱酸であるポリアクリル酸（分子量 90000）についても同様の検討を行い，pH，イオン強度，金属イオン濃度が錯生成に及ぼす影響を比較した．みかけの錯生成定数をβα = [ML]/([M][R])（[M]：遊離 Fe2+イオン濃度，[ML]：錯生成している Fe2+イオン濃度，[R]は CRαで，CR はプロトン交換サイトの全濃度でαは解離度である）と定義し，pcH 4.6 から
5.5，イオン強度 0.1 および 1.0 M NaCl Fe(II) 濃度 ~10-8 から~10-4 M の溶液条件で logβαの値を求めた．ポリアクリル
酸錯体およびフミン酸錯体のいずれについても logβαは解離度とともに増加しイオン強度とともに減少した（I = 0.1 M
NaCl でポリアクリル酸錯体では 2.26 (α = 0.32) から 2.59 (α = 0.47), フミン酸錯体では，4.66 (α = 0.58) から 4.83 (α =0.70)，
I = 1.0 M NaCl でポリアクリル酸錯体では 0.53 (α = 0.49) から 0.98 (α = 0.71), フミン酸錯体では，
3.31 (α = 0.59) から 3.62 (α = 0.71)）．logβαの logαに対する依存性は両者で同程度であり，イオン強度依存性はフミン酸錯体の方がやや小
さい．ポリアクリル酸錯体の logβαは Fe(II)濃度の変化によらず一定であるが，フミン酸錯体の logβαは，フミン酸の組成の不均質性のために，Fe(II)濃度が低くなると著しく上昇した（I = 0.1 M NaCl, α = 0.68 で 3.21 (CFe = 4×10−4 M) から 4.79
(CFe = 6×10−8 M)，I = 1.0 M NaCl, α = 0.68 で 2.35 (CFe = 4×10−4 M) から 3.58 (CFe = 6×10−8 M)）．
Keywords: Fe(II)，錯生成，フミン酸，ポリアクリル酸，イオン交換
Humic substances in groundwater are expected to affect the migration behavior of radionuclides by forming soluble colloid
complexes with radionuclides. In order to understand the interaction between radionuclides and humic substances, it is also
important to estimate the effect of the competing cations originally present in the groundwater. In this connection, the complex
formation of Fe(II) with Aldrich humic acid has been investigated by ion exchange method. To evaluate the effect of heterogeneouscomposition and of polyelectrolyte nature of humic acid, the complex formation of Fe(II) with homogeneous polymeric weak acid,polyacrylic acid (MW = 90000 Dalton) has been also examined to compare the effect of pH, ionic strength and metal-ion
concentration on the complex formation. By defining the apparent formation constant as βα = [ML]/([M][R]), where [M] and [ML]are the concentrations of free and bound Fe2+ ion, and [R] = CRα (CR is the total concentration of proton exchange sites and α is thedegree of dissociation of humic or polyacrylic acid), the values of logβα have been obtained at pcH 4.6 to 5.5 in 0.1 and 1.0 M NaClat the concentration of Fe(II) from ~10-8 to ~10-4 M. For both humate and polyacrylate complexes, logβα increased with the degreeof dissociation and decreased with ionic strength (at I = 0.1 M NaCl, from 2.26 (α = 0.32) to 2.59 (α = 0.47) for Fe(II)-polyacrylate
and from 4.66 (α = 0.58) to 4.83 (α = 0.70) for Fe(II)-humate, and at I = 1.0 M NaCl, from 0.53 (α = 0.49) to 0.98 (α = 0.71) forFe(II)-polyacrylate and from 3.31 (α = 0.59) to 3.62 (α = 0.71) for Fe(II)-humate). The dependence of logβα of humate on logα
was similar to that of polyacrylate and that of humate on ionic strength was slightly less than that of polyacrylate. While thevariation of Fe(II) concentration had no appreciable influence on logβα of Fe(II)-polyacrylate, logβα of Fe(II)-humate appreciably
increased at lower Fe(II) concentrations due to the heterogeneous composition of the humic acid (at I = 0.1 M NaCl, α = 0.68, from
3.21 (CFe = 4×10−4 M) to 4.79 (CFe = 6×10−8 M) and at I = 1.0 M NaCl, α = 0.68, from 2.35 (CFe = 4×10−4 M) to 3.58 (CFe = 6×10−8
M)).
Keywords: Fe(II), Complex formation, Humic acid, Polyacrylic acid, Ion exchange

1the parameters affecting on the humic acids-radionuclides
interaction should be studied in detail for understanding of
migration behavior of radionuclides.
One of the main reasons for the difficulty in clarifying thisinteraction is that humic acids are mixtures of various
ill-defined organic macromolecules with different compositions,structures and molecular weights and their complex-forming
characteristics cannot be treated as those of simple
molecules[1,6-7]. Due to the complication introduced by theheterogeneous composition and polyelectrolyte nature of humic
acid, dependences of the interaction between metal ions andhumic acids upon ionic strength, pH and metal-ion
concentration are still unclear. Our previous studies on thecomplex formation of NpO2+[8], Eu3+[9], and Ca2+[10] with
Aldrich humic acid revealed that the apparent formation
constants of humate complexes increases with pH and decreases
with ionic strength mainly due to the polyelectrolyte nature of
the humic acid. More importantly, the constants considerablyincrease with the decrease in metal-ion concentrations. This
fact implies that the effect of foreign cations originally present
in groundwater on the complex formation is dependent not only

Introduction

In the safety assessment of underground disposal ofradioactive wastes, one of the key mechanisms in retarding thepossible transport of radionuclide from a disposal facility to thebiosphere through groundwater flow is the sorption ofradionuclide onto host rocks in geological barrier system. In
this process, however, when high concentrations of dissolved
organic materials exist in groundwater, they could have a strong
detrimental effect on the sorption. Since organic materials
such as humic (and fulvic) acids have high binding strengths tometal cations and tendency to exist as colloids in solution [1-3],
radionuclides sorbed onto host rocks will be redissolved into thegroundwater as complexes with the humic acids and mobilized
in the groundwater flow [4,5]. Because of the widespreadexistence of humic acids in groundwater (around 1ppm or more),
Complex Formation of Fe(II) with Humic Acid and Polyacrylic Acid, by:Budi Setiawan, Kouichi Tanaka, Yuichi Niibori and Osamu Tochiyama(osamu.tochiyama@qse.tohoku.ac.jp)＊東北大学大学院工学研究科量子エネルギー工学専攻 Department of
Quantum Science and Energy Engineering, Graduate School of Engineering,Tohoku University〒980-8579 仙台市青葉区荒巻字青葉 01

21

原子力バックエンド研究

distribution ratio rather limited, and as a result, theconcentration of the complexing ligand cannot be varied in awide range. However, when we have tried to apply the solvent
extraction method to the study of the complex formation of
Fe(II) with polyacrylic acid, some unknown side reaction
occurred at high ligand concentrations.
In the solvent
extraction system, the coexistence of the extracting reagent isunavoidable and this is undesirable to the present case where
the oxidation of Fe(II) is worried. For this reason, ion
exchange method has been applied because the method does not
introduce any foreign reagents other than the concerned metal
ion and ligand.

on the concentration of the concerned metal ion itself but also
on the concentrations of foreign cations. To understand theeffect of competing cations originally present in groundwater,
the complex formation of Fe(II) with Aldrich humic acid has
been investigated as a function of pH, ionic strength and Fe(II)concentration. In the anaerobic condition in deep underground,iron predominantly exists in divalent state.
Althoughconcentrations of iron in groundwater are low (around 10-7 to
10-4 M[1]), they are still comparable to the concentrations of
dissolved organic materials. To separately estimate the effectof heterogeneous composition and that of polyelectrolyte natureeach other, the complex formation of Fe(II) with thehomogeneous polymeric weak acid, PAA (polyacrylic acid,[-CH2CH(COOH)-]n), has been also examined and comparedwith the humic acid.
Usually, the formation constant of a metal complex isdefined as,β=[ML][M][L]

2[M][R]

Reagents
Radiotracer 59Fe (specific radioactivity 37MBq/ml, carrier
0.56mg Fe/ml, radionuclide purity 99.00%) obtained fromNEN® Life Science Products, Inc. was diluted to about 1.4x10-6
M by 0.01M HCl and kept in a polyethylene bottle as a stock
solution.
25wt% aqueous solution of polyacrylic acid
(MW=90000 Daltons) from Polysciences Inc. was used asreceived. Aldrich humic acid was purified by the procedure
based on the description given in the references[3, 22-23]. A
sample of 5g of Aldrich humic acid was dissolved in 5L
solution containing 0.1 M NaOH and 1g NaF. After adjusting
the pH of the solution to 7 by the addition of HCl, the solution
was stirred overnight with N2 bubbling. Solution containing(1)=

Experimental

2.1

where [L] is the concentration of free ligand, [M] and [ML]denote the concentrations of free and bound metal ion,
respectively. However, in the complex formation of metal ion
with humic acid or polyacrylic acid, functional groups whichbind to metal ions are fixed to a macromolecule and we do notknow how many functional groups coordinate to a single metal
ion. This makes the estimation of [L] impossible. Althoughvarious definitions have been used[11-21], the present study
takes the following definition of the apparent complex
formation constant.
[ML]
[ML]
(2)
βα =

September 2002

dissolved HA was passed through 0.45µm membrane filter(Advantec) to separate macro-size particles and then the filtratesolution was acidified with 2M HCl to pH1. After standing
the solution for several hours, the supernatant was discarded
and the precipitate was dissolved again into 0.1 M NaOH.
Then the solution was acidified again to pH1 and the procedure
was repeated for 3 times. The suspension was centrifuged(3000rpm, 15min.) and the precipitate was washed with 0.1 M[M] C R α

where [M] and [ML] are the concentration of free and boundFe2+ ion, and [R] is the concentration of free dissociated proton
exchanging sites. When the amount of dissociated proton
exchanging sites consumed in making the complex is negligible
as compared to the total amount of the dissociated functionalgroups, [R] can be approximated by CRα where CR is the total

HCl repeatedly until [Na+] becomes 10mg/l in supernatant.
The precipitate was then freeze-dried (Eyela FDU-810), washedwith distilled water and centrifuged. The precipitate waswashed repeatedly until ion meter reading of [Cl-] is less than
50mg/l in supernatant. The precipitate was freeze-dried andstored in a desiccator.
The synthetic organic cationic resin, Amberlite 200CT(Na-type) with exchange capacity 4.3meq/g was used becauseof its applicability to a broad pH range of 0 to 14. The resin
was settled in a column and purified by passing successively
1M HCl and 1M NaCl. The cycle was repeated three times.
Finally, the resin was equilibrated with 0.16 M NaCl. Afterrinsing with distilled water, the resin was spread in a thin layer
on a tray for air-drying.
Other chemicals were obtained from Dojindo Laboratories
or Wako Pure Chemical Industries and were used without
further purification.

concentration of proton exchange sites and α is the degree of
dissociation of humic or polyacrylic acid. The advantage byusing this definition is that the values of CR and α can be
independently determined by the potentiometric titration of
humic or polyacrylic acid. The effect of polyelectrolyte natureand heterogeneous composition can be disclosed by examining
the change in βα with pH, ionic strength and metal-ion
concentration.
Ion exchange method has been used to study the complex
formation of Fe(II) with humic and polyacrylic acids. As
compared with solvent extraction, ion exchange method has alimitation in the point that the ion exchanger (gel) phase cannot
be completely separated from the aqueous phase due to thewetting of the gel phase. Since the concentration of the metal
ion in the exchanger phase cannot be directly measured, it isobtained as a difference between the initial added concentrationof the metal ion and that remained in the aqueous phase atequilibrium.
This makes the range of the measurable

2.2
22

Procedures

フミン酸およびポリアクリル酸による鉄(II)の錯生成

Vol.9 No.1

Kd0 = [Fe2+]R/[Fe2+] = Kex([Na+]R/[Na+])2(8)
To check the applicability of the ion exchange system tothe study of complex formation, the effects of the shaking time,
the ratio of the resin mass to the solution volume, pcH and ionicstrength on the distribution of Fe(II) were examined. Kd0
values for the shaking time from 30 to 150minutes wereobtained and 90minutes was confirmed to be enough for the
Fe(II) distribution to reach equilibrium. The shaking time of
150minutes was adopted for the experiments. For 5ml of theaqueous solution, from 0.01 to 0.05g of the resin was confirmed
to give a constant Kd0 value and the experiments have been
carried out in this range of the mass to volume ratio. As for
pcH of the solution, Kd0 was constant in the range of pcH 4.2 –5.6 as suggested by eq.(6). Beyond pcH 5.6, the Kd0 value
increased with pcH probably due to the hydrolysis of ferrous
iron in the solution. For the present study, pcH range used forthe complexation experiments was 4.6 to 5.5. Figure 1 shows
the dependence of logKd0 on the ionic strength. At the ionicstrengths of 0.1, 0.4 and 1.0 M NaCl, values of logKd0 wereobtained to be 2.79, 1.78 and 1.09, respectively. These values

The calibration of the electrode for pcH measurements(pcH is the negative logarithms of hydrogen ion concentrations)were conducted as described previously[24]. In order toestimate the concentration of dissociated functional groups, [R]in eq. (2), the titration curves of Aldrich humic acid andpolyacrylic acid were obtained at I = 0.1 and 1.0M NaCl in thesame way as described previously [24, 8]. Maximum proton
exchange capacities (CR) were obtained to be 4.95~5.10meq/g
for the humic acid and 12.88 meq/g for polyacrylic acid. The
concentration of dissociated functional group at each pcH was
calculated by applying the following relation to the titration
results:[H+] + [NaOH]added = [OH−]+[R][R] = CR

(3)
(4)

where [H+] = 10-pcH, [OH−] = 10pKw-pcH and pKw = 13.78 (I =0.1), 13.69 (0.4), 13.71 (1.0) [25].
For the determination of complex constants, the resin was
contacted in a glass tube with 5 ml of aqueous phase containinga variable concentration of humic or polyacrylic acid, 0.02M
hydroxyl ammonium chloride, 0.1 or 1.0M NaCl, buffer
reagents (a mixture of 0.02M of 2-(N-morpholino)ethanesulfonic acid and tris(hydroxymethyl) aminomethane,varied from pcH 4.6 to 5.5) and Fe(II) (varied from ~10-8 to~10-4M) labeled with 59Fe. The tube was capped with rubber
cap and shaken by using reciprocal shaker for 150min at 25 ±1oC.

indicate that ∂logKd0/∂log[Na+] ≅ -1.7, not exactly -2 as
expected from eq.(8). This is considered due to the changes in
the activity coefficients of Fe2+ and Na+ in the aqueous and resin
phases. Since the distribution of Fe(II) is considered to follow
eqs. (6) and (7), the result can be considered to be indicatingthat the ion exchange reaction between the iron ion of charge +2
and sodium iron of charge +1 given by eq. (6) is taking place
without any side reaction. Thus, it can be concluded that iron
ion exists predominantly as Fe2+ ion under the condition used in
the experiments.

After the centrifugation (2000rpm, 10min.,), γ-activityof 1 ml of the aqueous phase was measured with a well-type of
NaI(Tl) scintillation counter. The distribution coefficient Kd
was calculated by,
AT − Aeq V
(5)
Kd =

Aeq

m

3.2 Determination of logβα by the ion exchange method
In the presence of humic or polyacrylic acid, thedistribution coefficient of Fe(II) decreases from Kd0 to Kd due tothe complex formation of Fe(II) with humic or polyacrylic acid.

where AT and Aeq are the activities of 1 ml of the aqueous phasein the absence and presence of the ion exchange resin,
respectively, V is the volume (ml) of the aqueous phase, and mis the mass (g) of the resin. The remaining part of the aqueous
phase was used for the pH measurement.

3.0

3Results and discussions

3.1 Distribution of Fe(II) in the absence of humic or
polyacrylic acid
The ion exchange reaction of Fe(II) in the absence of any
complexing reagents can be described as,

2.5

log Kd0slope = -1.7

Fe2+ + 2RSO3Na ∩ (RSO3)2Fe + 2Na+
(6)
where Fe2+ and Na+ are free ions in the aqueous phase, RSO3Na
and (RSO3)2Fe are Na+ and Fe2+ ions bound to the resin.
Equilibrium constant for eq.(6) becomes,
Kex = ([Fe2+]R[Na+]2)/([Fe2+][Na+]R2)(7)
Brackets with subscript R indicate the concentration of
Fe(II) in the resin (mmol/g) and without subscript in thesolution (mmol/ml). Kex is the conditional constant whichchanges with the solution condition such as ionic strength (dueto the changes in the activity coefficients of the ionic species in
the aqueous and resin phases). The distribution coefficient, Kd,of a metal ion is defined as,

2.0

1.5

1.0
-1.0

-0.8

-0.6

-0.4

-0.2

0.0

+

log [Na ]Fig. 1 The distribution of Fe(II) as a function of ionicstrength in the absence of any complexing agents.
23

原子力バックエンド研究
Kd =[ Fe + ]R
K d0
K d0
=
≈
[Fe ] + [FeL] 1 +βα[R] 1 +βαCRα

September 2002

(9)

I = 0.1 M NaCl, C Fe = 8x10 -6 M

2+

1/{Kd/(ml/g)}

pcH and comparing them with Kd0, βα can be estimated. To
obtain a complex formation constant, eq.(9) can be transformed
into the following linear relationship famously called as
Schubert’s method [26],
βα
1
1
=
+
[ R]
K d
K d0
K d0

1: pcH = 5.44 (α = 0.47)
2: pcH = 5.27 (α = 0.42)
3: pcH = 5.01 (α = 0.35)
4: pcH = 4.88 (α = 0.32)

0.012

where βα is the apparent complex formation constant defined byeq.(2). By measuring Kd values as a function CR at a fixed

(10)

2
0.008

3
4

0.004

By plotting 1/Kd against [R], 1/Kd0 and βα/ Kd0 will beobtained as an intercept and a slope. This linear equation is
simple and easy to grasp the validity of the assumed reactions
from the obtained data. On the other hand, the measurementsof Kd were carried out so as to the errors in the values of logKd

0.000
0

4

(calculated from the γ-activities of 59Fe in the solutions in the

12-3[R]/(10 mol dm )Fig. 2 Distribution of Fe(II) as a function of polyacrylate
concentration at I = 0.1 M, CFe = 8×10-6 M.

logβα, an optimum set of logKd0 and logβα were obtained by anonlinear least-squares fitting which minimized the sum of thesquares of residuals between logKd(observed) and
logKd(calculated),S = Σ{logKd(observed) – logKd(calculated)}8-3

presence and absence of humic or polyacrylic acid) were nearly
equal in these experiments. Therefore, for the calculation of2

1

I = 1.0 M NaCl, C Fe = 4x10 -4 M
0.11

(11)

where

1/{Kd/(ml/g)}

log K d (calculated) = log K d0 − log(1 + 10log βα + log[R] ) (12)In practice, the fittings of the data to eq.(10) and to eq.(12)

1: pcH = 5.43 ( α = 0.69)
2: pcH = 5.20 ( α = 0.63)
3: pcH = 4.85 ( α = 0.52)
4: pcH = 4.63 ( α = 0.45)

1

0.10

2

did not give any appreciable differences in the calculated logβα.
Therefore, the results will be shown by the form of the plots of
1/Kd against [R].
Typical examples of the experimental results for thecomplex formation of Fe(II) with polyacrylic acid are given in
Figs. 2 and 3 in the form of the plots of 1/Kd against [R].
When pcH and ionic strength are fixed constant, the plots give a
linear relationship between Kd and [R] (the straight lines are theresult of the fitting by eqs.(11) and (12)), indicated the validityof the assumed reaction given by eq.(10). Figures 2 and 3 also
indicated that the slope varies with pcH and with ionic strength,

Fig. 3 Distribution of Fe(II) as a function of polyacrylate
concentration at I = 1.0 M, CFe = 4×10-4 M.

i.e. logβα defined by eq.(2) is not constant but varies with pcH
and ionic strength. Similar experiments were conducted at
different sets of pcH, ionic strength and Fe(II) concentration,and the results are summarized in Table 1. The deviations

Fe(II)-polyacrylate. This is reasonable because polyacrylicacid is a homogeneous polymeric acid and the ligand species in
the complex is the same at all Fe(II) concentrations. On the

0.09

3
4

0.08
0.0

0.5

[R]/(10

1.0
-2

1.5

2.0

-3

mol dm )other hand, logβα changes with α and with ionic strength.
Since, in the definition of eq.(2), the concentration of thegiven for the values of logβα and logKd0 are the standard
deviations σ which were estimated from the data. The
systematic errors arising from the flaw in the design of theexperiment, in the preparation of the reagents and solutions, indissociated monomeric unit, [R] = CRα, is simply used in placeof the concentration of the true ligand species, logβα would
increase with α if more than one carboxylate groups coordinate
to a single Fe2+ ion, because the dependence of the concentrationof multidentate unit (a set of two or more dissociated adjacent

the estimation of α, etc. are considered larger than these valuesof σ, and the uncertainties of the values of logβα are estimatedto be 0.05 to 0.1. As expected from the charge of Fe2+ ion, thevalues of logβα are not very large.

carboxylate groups) on pH (∂log[L]/ ∂pH) is larger than [R].
This may be understood when one considers, for example in the

3.3

oxalic acid, the increase in [C2O42−] with pH as compared to thatComplex formation of Fe(II) with polyacrylate
Table 1 shows the values of logβα of Fe(II)-polyacrylate
obtained at various conditions.
The variation of Fe(II)of the simple anionic charge. As a result, logβα increases withconcentration has no influence on the value of logβα ofeach metal ion. In Fig.4, the dependences of logβα on logα are

α, reflecting the number of carboxylate groups coordinating to24

フミン酸およびポリアクリル酸による鉄(II)の錯生成

Vol.9 No.1

Table 1 Apparent formation constant of Fe(II)-polyacrylate
complex
CFe/M

pcH

α

logβα

logKd0

0.1

-8

4.89

0.32

2.26 ± 0.03

2.79 ± 0.01

5.07

0.37

2.36 ± 0.05

2.79 ± 0.01

5.32

0.44

2.67 ± 0.02

2.79 ± 0.01

5.46

0.47

2.59 ± 0.04

2.79 ± 0.01

4.88

0.32

2.23 ± 0.01

2.79 ± 0.01

5.01

0.35

2.33 ± 0.01

2.79 ± 0.01

5.27

0.42

2.53 ± 0.01

2.79 ± 0.02

5.44

0.47

2.68 ± 0.02

2.79 ± 0.02

4.77

0.29

2.14 ± 0.03

2.79 ± 0.01

4.97

0.34

2.30 ± 0.03

2.79 ± 0.01

5.20

0.40

2.66 ± 0.03

2.79 ± 0.01

6×10

8×10-6

4×10-4

0.4

1.0

-6

8×10

6×10-8

-6

8×10

-4

4×10

5.46

0.48

2.97 ± 0.03

2.79 ± 0.02

4.87

0.44

1.23 ± 0.01

1.78 ± 0.01

5.05

0.49

1.38 ± 0.01

1.79 ± 0.01

5.31

0.57

1.54 ± 0.01

1.79 ± 0.01

5.48

0.62

1.63 ± 0.01

1.78 ± 0.01

4.76

0.49

0.53 ± 0.10

1.09 ± 0.01

5.01

0.57

0.72 ± 0.01

1.09 ± 0.01

5.28

0.65

0.86 ± 0.04

1.09 ± 0.01

5.49

0.71

0.98 ± 0.04

1.09 ± 0.01

4.72

0.47

0.54 ± 0.01

1.09 ± 0.01

4.96

0.55

0.67 ± 0.01

1.09 ± 0.01

5.26

0.64

0.89 ± 0.01

1.09 ± 0.01

5.44

0.70

0.98 ± 0.01

1.09 ± 0.01

4.63

0.45

0.45 ± 0.02

1.09 ± 0.01

4.85

0.52

0.59 ± 0.01

1.09 ± 0.01

5.20

0.63

0.93 ± 0.01

1.09 ± 0.01

5.43

0.69

1.25 ± 0.01

1.09 ± 0.01

7

Eu3+

6
5

logβα

I /M

8

4

NpO2+

Fe2+

3
2
1
0
-0.6

Fe2+
-0.5

-0.4

-0.3

-0.2

-0.1

0.0

logα
Fig. 4 Apparent formation constants of Eu(III), Fe(II)and Np(V)-polyacrylate as a function of logα.
□○◇： I = 0.1 M, ■●◆： I = 1.0 M,□■：Eu(III)-polyacrylate, ○●：Fe(II)-polyacrylate,◇◆：Np(V)-polyacrylatedensity. Thus, in the surrounding of dissociated polyacrylicacid, a part of the surface charge is neutralized by Na+ ionsattracted by the crowded negative charge. Therefore theconcentration of Fe2+, Eu3+ or NpO2+ in the close vicinity ofpolyacrylate is controlled by the competition between Na+ and
Fe2+, Eu3+ or NpO2+ to this zone. In other words, some part ofthe complex formation of metal ion-polyacrylate proceeds via
an ion exchange between Na+ and the metal ion. However, inthe present definition of βα, Na+ ions are assumed not to becomplexed no matter what they are close to or apart from thepolyanion.

As a result, logβα is expected to change with theconcentration of background salt. In Fig.4, logβα of Fe(II),
Eu(III) and Np(V)-polyacrylates are compared. As expectedfrom the above-considered ion exchange reaction in theelectrical double layer formed in the close vicinity of thepolymeric anion, the effect of ionic strength is in the order ofcompared for Fe(II), Eu(III) [9, 27] and Np(V)-polyacrylates [8,24]. The dependences are in the order of Eu3+ > NpO2+ ≅ Fe2+,
indicating that the number of carboxylates coordinating to asingle metal ion is somewhat larger for Eu(III) and similar for
Fe(II) and Np(V) despite the difference in the formal charge of
Fe2+ and NpO2+. In the case of NpO2+ ion, NpO2+ is the dioxo
ion and the charge density on the central neptunium atom ishigher than its formal charge of +1. Thus, the above ordersuggests that the higher charge density on the central neptunium
atom contributes to some extent to the complex formation of
Np(V) with polyacrylate.

the charge of the metal ions, i.e. Eu3+

Fe2+ > NpO2+.

3.4 Complex formation of Fe(II) with humate
In the same manner as in the study of Fe(II)-polyacrylate,the values of logβα were obtained for Fe(II)-humate. Typicalexamples of the experimental results for the complex formationof Fe(II) with humic acid are given in Figs 5 and 6, and all the
results are summarized in Table 2. Similarly to Fe(II)polyacrylate, the plots give a linear relationship between Kd and[R], indicating the validity of this ion exchange method. The

The effect of ionic strength on logβα discloses thepolyelectrolyte nature of polyacrylic acid. The changes inlogβα of Fe(II), Eu(III) and Np(V)-polyacrylate with ionicstrength are much larger than expected from the changes in theactivity coefficients of Fe2+, Eu3+ and NpO2+ ions in the aqueous
solution. This may be explained by the consideration of thestructure of the electrical double layer around the polyacrylate[10]. Since anionic carboxylate groups in polyacrylate are
fixed to a macromolecular matrix, they give fairly high charge

value of logβα varies with α and with ionic strength in a similarmanner as in Fe(II)-polyacrylate. A distinct difference can beseen in the effect of Fe(II) concentration.
In Fig.7, the effect of Fe(II) concentration on logβα ofFe(II)-humate is compared with that of Fe(II)-polyacrylate in
the form of the plots of logβα against logα.
25

It clearly indicates原子力バックエンド研究
0.20

0.15

I = 0.1 M NaCl, CFe = 8x10-6 M

1

1: pcH = 5.44 (α = 0.69)
2: pcH = 5.24 (α = 0.65)
3: pcH = 5.04 (α = 0.60)
4: pcH = 4.89 (α = 0.57)

2
3

Table 2 Apparent formation constant of Fe(II)-humatecomplex
I /M
0.1

CFe/M
6×10-81/{Kd/(ml/g)}

40.10

8×10-60.05

4×10-4

6×10-8

1.0

0.00
0

1

2

3

4

[R]/(10-3 mol dm-3)
Fig. 5 Distribution of Fe(II) as a function of humate
concentration at I = 0.1 M, CFe = 8×10-6 M.
0.35

0.30

4×10-412

0.25

1/{Kd/(ml/g)}

8×10-6

I = 1.0 M NaCl, CFe = 6x10-8 M
1: pcH = 5.56 (α = 0.71)
2: pcH = 5.31 (α = 0.66)
3: pcH = 5.16 (α = 0.63)
4: pcH = 4.98 (α = 0.59)

4

0.15

logβα
4.66 ± 0.05
4.77 ± 0.05
4.79 ± 0.05
4.83 ± 0.04
4.38 ± 0.08
4.41 ± 0.05
4.46 ± 0.05
4.49 ± 0.05
3.17 ± 0.45
3.19 ± 0.44
3.21 ± 0.42
3.22 ± 0.41
3.31 ± 0.04
3.45 ± 0.03
3.49 ± 0.03
3.62 ± 0.02
2.88 ± 0.06
3.02 ± 0.05
3.15 ± 0.04
3.27 ± 0.04
2.04 ± 0.19
2.24 ± 0.15
2.33 ± 0.13
2.35 ± 0.12

α
0.58
0.62
0.68
0.70
0.57
0.60
0.65
0.69
0.62
0.63
0.68
0.71
0.59
0.63
0.66
0.71
0.54
0.59
0.64
0.68
0.53
0.58
0.64
0.68

pcH
4.93
5.12
5.39
5.48
4.89
5.04
5.24
5.44
5.10
5.15
5.39
5.56
4.97
5.16
5.31
5.56
4.74
4.96
5.20
5.42
4.69
4.92
5.20
5.42

logKd02.79 ± 0.13
2.79 ± 0.02
2.79 ± 0.02
2.79 ± 0.02
2.79 ± 0.13
2.79 ± 0.02
2.79 ± 0.02
2.79 ± 0.02
2.79 ± 0.05
2.79 ± 0.03
2.79 ± 0.03
2.79 ± 0.02
1.08 ± 0.06
1.08 ± 0.01
1.08 ± 0.01
1.08 ± 0.01
1.11 ± 0.03
1.11 ± 0.05
1.11 ± 0.05
1.11 ± 0.06
1.15 ± 0.01
1.15 ± 0.01
1.15 ± 0.01
1.15 ± 0.01

heterogeneous in composition, the increase of α of humic acidmeans not only the increase of the concentration of dissociatedfunctional groups but also the dissociation of weaker acidic sites(more basic and stronger complexing sites). Therefore, the

3

0.20

September 2002

increase in logβα with logα may be due both to the multidentate
coordination and the participation of more basic sites.
However, since the dependences of logβα on logα at threedifferent Fe(II) concentrations are similar, the influence of

0.10

4

0.05
0

2

4

6

8

[R]/(10-4 mol dm-3)
3Fig. 6 Distribution of Fe(II) as a function of humate
concentration at I = 1.0 M, CFe = 6×10-8 M.

logβαthat logβα of Fe(II)-humate is much higher than polyacrylate ata lower Fe(II) concentration and decreases with Fe(II)concentration.
Higher logβα values at lower metal ion
concentrations are in accordance with our previous results on
Np(V)-humate[8], Eu(III)-humate[9] and Ca(II)-humate[10].
This is considered due to the heterogeneous composition of thehumic acid. Since humic acids are mixtures of various organic
macromolecules with different compositions, they containweaker and stronger binding sites. At a lower metal-ion
concentration, minor but stronger sites are preferentially used toform stronger complex. With the increase in the metal-ion
concentration, these sites are occupied and weaker sites will be
used to form weaker complex.

2

PAA

1

0
-0.4

-0.3

-0.2

-0.1

logα
Fig. 7 The effect of Fe(II) concentration on the plots oflogβα vs. logα.

As shown in Fig.7, the effect of logα on logβα in
Fe(II)-humate is similar to Fe(II)-polyacrylate in contrast to alarge difference in the value of logβα.

HA■●▲：Fe(II)-humate, □○△：Fe(II)-polyacrylate□■：CFe = 6×10-8 M, ○●：CFe = 8×10-6 M△▲：CFe = 4×10-4 M

Since humic acid is26

フミン酸およびポリアクリル酸による鉄(II)の錯生成

Vol.9 No.1

heterogeneous composition is considered small and the increase

strengths of complexing sites in the humic acid. This effect ofin logβα with logα is supposed mainly due to the multidentate
coordination.

the metal ion concentration on logβα due to the heterogeneouscomposition of the humic acid disables the use of a single

In Fig.8, the effect of ionic strength on logβα ofFe(II)-humate is compared with that of Fe(II)-polyacrylate.
For both complexes, the effects are larger than expected fromthe change in the activity coefficient of Fe2+ ion in the aqueous
phase, indicating the polyelectrolyte effect in both polyacrylate
and humate.
The higher dependence of logβα ofFe(II)-polyacrylate than Fe(II)-humate suggests that the charge
density around polyacrylate is higher than around humate,which is consistent with their proton exchange capacities (CR =constant of logβ, that is, if we have a mixture of two complex
forming reagents, we have no way to represent the complex
formation of a metal ion with this mixture with a single
complex formation constant. Thus, in order to consider thecompetition of the radionuclides with the metal ions such as
Fe(II) originally existing in the underground water, we areconfronted with the formidable task of treating the variable(conditional) complex formation constants which change withthe metal ion concentration.

4.95∼5.10meq/g for the humic acid and CR = 12.88 meq/g forpolyacrylic acid).

References

4

Buffle, J.:Complexation Reactions in Aquatic System: An
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Radiochim. Acta 58/59, 113-120 (1992).
[3] Kim, J.I.: Actinide colloid generation in groundwater.
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[11] Moulin, V., Robouch, P., Vitorge, P.: Spectrophotometricstudy of the interaction between Americium(III) andhumic materials. Inorg. Chim. Acta 140, 303-306 (1987).
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The apparent formation constant (logβα) of Fe(II)-humate
and Fe(II)-polyacrylate were obtained at pcH 4.6 to 5.5 and I =0.1 and 1.0M NaCl by using the ion exchange method. The
result indicated that logβα of both complexes increased with thedegree of dissociation (α) and decreased with the ionic strength
due to their polyelectrolyte nature.
These effects are
considered to be the effects on a certain single complex
formation constant logβ and may be estimated if theirpolyelectrolyte nature and multidentate coordination areproperly taken into consideration. On the other hand, while
logβα of Fe(II)-polyacrylate is independent of the change inFe(II) concentration, logβα of Fe(II)-humate increases with thedecrease in Fe(II) concentration due to the existence of different
5
HA, I = 0.1

logβα

4

3

2

PAA, I = 0.1

HA, I = 1.0

1PAA, I = 1.0
0
-0.5

-0.4

-0.3

-0.2

-0.1

logα
Fig. 8 The effect of ionic strength on the plots of logβα vs.
logα.
Fe(II)-humate at I = 0.1 (□○△) and 1.0 M (■●▲);
CFe = 6×10-8 M (□■), 8×10-6 M (○●), 4×10-4 M (△▲).
Fe(II)-polyacrylate at I = 0.1 (◇) and 1.0 M (◆),
CFe = 4×10-4 M.
27

原子力バックエンド研究
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28

September 2002