原子力バックエンド研究

Vol.9 No.1

研究論文

Effects of pH on the Speciation of Radionuclides in Groundwaterof Beishan Area, Gansu Province, China
Zhang Zhanshi*

Zhou Wenbin*

Fan Xianhua **Speciation of radioactive nuclides is one of the most important factors in the study of migration and precipitation of thenuclides related to deep geological disposal of high level radwastes (HLW). Theoretical speciation analysis with thermodynamic
equilibrium codes is an effective method. The authors have employed the EQ3NR, a computer program for geochemical aqueous
speciation-solubility calculation, to evaluate the speciation of the key nuclides in the groundwater of Beishan Area, a potential areafor hosting the HLW Repository of China. The calculated results showed that pH value as well as chemical composition of thewater had a great impact on the speciation of key nuclides, such as Np, Pu, Am, U and Th, which would be released from HLW tothe groundwater of Beishan Area. In addition, it was found that the speciation of nuclides varied from one to another. However,under alkaline conditions, the speciations were relatively simpler and hydroxyl complexes and carbonate complexes were dominant.
Key words: Geochemical modeling, Speciation, High level waste disposal, EQ3NR

1

3 CHEMICAL CHARACTERISTICS OF GROUNDWATER IN BEISHAN AREA

INTRODUCTION

The national project on deep geological disposal (DGD)
of HLW in China has been carried out for 15 years. After thenational, regional and area screening, Beishan Area of Gansu
Province in West China has been selected as a potential area for
hosting the repository of HLW in China. Now, preliminarysite characterization, including geological, hydrogeological,
and geotechnical investigations, is on its way[1].
The speciation of radioactive nuclides in groundwater is
one of the most important factors in the study of nuclide
migration, which is a key factor in performance assessment of arepository. On the basis of hydrogeological and hydrogeochemical survey in Beishan area, the effects of pH on thespeciation and solubility of several key radioactive nuclides are
simulated by using EQ3NR.
2

Beishan, a potential area for the HLW repository in
Gansu Province of West China, is located in a typical arid
climate region. As a part of the HLW DGD project of China,
a preliminary characterization of hydrogeology and chemistryof the groundwater for the area has been carried out. The pH
values for the shallow groundwater in this area are usually inthe range of 7.1 to 8.8, temperature from 8 to 14℃, and thetotal dissolved solid (TDS) from 0.3 to 12g/L. The majorityof shallow groundwater is of Cl-SO4-Na and SO4-Cl-Na type，
followed by Cl-SO4-Na-Ca type[4]. The water from Wuyi
well, a 50-meter-deep well in Beishan area, is considered as therepresentative of the deep groundwater for the area. The
chemical composition of the water, shown in Table 2, has beenused in the modeling [4].

CALCULATION CONDITIONS
4Speciation and solubility calculation of nuclides were
performed with the geochemical codes EQ3NR (Version 7.2a),acomputerprogramforgeochemicalaqueousspeciation-solubility calculation, developed by Wolery of
Lawrence Livermore National Laboratory (LLNL). It is partof the EQ3/6 software package for geochemical modeling [2-3].
The attached thermodynamic data file is employed in this study.
The numerical method employed is based on a hybrid
Newton-Raphson technique, and the B-dot equation has beenused for calculating activity coefficients.
The nuclide
concentrations adopted are 10-12mol/L and the temperature
adopted is 25℃.

RESULTS AND DISCUSSIONS

4.1 Speciation of Radionuclides in Beishan Groundwater
Table 3 shows the speciations of the critical radionuclides
in the groundwater of Beishan Wuyi well. The speciations ofAm, Np and U, which are most hazardous radionuclides in
HLW, are shown in Figs. 1 to 3.
Figure 1 indicates that in the water of Beishan Wuyi well,
the dominant species of Am is AmCO3+ (84.42%) and the otherspecies of Am are 10.15% of Am(CO3)2-, 2.58% of AmOH2+,
1.69% of AmSO4+and 0.941% of Am3+.
From Fig.2, the dominant species of Pu is PuO2(CO3)22(71.88%), and the others are PuO2+ (23.93%), PuO2OH+(2.13%), PuO2F3- (0.802%) and PuO2F2(aq) (0.667%).
Figure 3 indicates that the dominant species of Np in the
groundwater of Beishan Wuyi well is NpO2+(93.76%). The
minor species are NpO2CO3-(4.09%), NpO2OH(aq) (1.84%).

All the species of related nuclides in theattached data file are listed in Table 1. The emphasis is on theeffects of pH on the speciation and solubility of nuclides in thisstudy.
This article was presented in “Japan-China Workshop on Nuclear Waste
Management and Reprocessing”.
Effects of pH on the Speciation of Radionuclides in Groundwater of
Beishan Area, Gansu Province, China by Zhang Zhanshi (zszhang@
ecgi.jx.cn), Zhou Wenbin, Fan Xianhua＊East China Institute of Technology, Fuzhou, Jiangxi, 344000, China＊＊Radiochemistry Department, China Institute of Atomic Energy, Beijing,
102413, China

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Table 1 The species of related nuclides in attached data file in EQ3NR [2,3]3+Am
Am(H2PO4)2+
Am(H2PO4)3(aq)Am(H2PO4)4AmH2PO42+

AmF2+
AmF2+
AmF3(aq)
AmCl2+
AmCl2+AmO2HCO3(aq)
Am(CO3)33AmCO3+
Am(NO3)2+
AmNO32+

AmOH2+Am(OH)2+
Am(OH)3(aq)AmSO4+
Am(SO4)2-

NpO2+
Np(HPO4)44Np(HPO4)56Np(H2PO4)2+
Np(H2PO4)3(aq)Np(HPO4)2(aq)NpO2H2PO4(aq)NpO2H2PO4+
NpO22+

NpF3+
NpF22+
NpO2F(aq)
NpCl3+
NpCl22+NpO2Cl+NpH2PO42+
NpO2HPO4(aq)NpO2HPO4-(NpO2)3(OH)5+(NpO2)2(OH)22+
Np(OH)22+
Np(OH)3+
Np(OH)4(aq)
Np(OH)5NpO2OH(aq)NpO2OH+NpOH3+

NpO2CO3NpO2(CO3)22NpO2(CO3)34NpO2(CO3)23Np(CO3)56NpSO42+
NpO2SO4Np(SO4)2(aq)NpO2SO4(aq)

PuO2+
PuO2F+
PuF22+
PuF3+
PuF4(aq)
PuF3+
PuO2F2(aq)PuO2F3-

PuO22+
PuO2(CO3)22PuH2PO42+
PuHPO42+
PuO2H2PO4+
Pu(HPO4)32Pu(HPO4)44PuO2F42-

PuO2SO4(aq)PuSO4+
PuSO42+Pu(SO4)2(aq)Pu(SO4)2PuCl3+
Pu(OH)4(aq)PuO2OH(aq)PuO2OH+
Pu(OH)5Pu(OH)22+
Pu(OH)3+
PuOH3+
PuOH2+Th4+
ThF3+
ThF22+ThF3+
ThF4(aq)
ThCl3+

Th(H2PO4)22+
Th(HPO4)2(aq)
Th(HPO4)32ThH2PO43+
ThH3PO44+
ThCl4(aq)ThSO42+
Th(SO4)44Th(SO4)2(aq)
Th(SO4)32ThCl3+
ThCl22+Th(OH)22+
Th(OH)4(aq)Th2(OH)26+
Th4(OH)88+
Th6(OH)159+
ThOH3+

UO22+
UF3+
UF22+
UF3+
UF4(aq)
UF5UF62UO2F+
UO2F2(aq)UO2F3UO2F42UO2Cl+
UO2Cl2(aq)UO2ClO3+
UCl3+

U(CO3)44U(CO3)56UO2(CO3)22UO2(CO3)3UO2(CO3)34UO2CO3(aq)UO2(H2PO4)(H3PO4)+
UO2(H2PO4)(H3PO4)+
UO2(H2PO4)2(aq)UO2H2PO4+UO2H3PO42+
UO2HPO4(aq)
UO2PO4-UO2OH+UOH3+
U(OH)4(aq)UO2(OH)2(aq)UO2(OH)3UO2(OH)42UO2(SO3)22UO2(SO4)22UO2IO3+UO2(IO3)2(aq)UI3+
UO2S2O3(aq)UO2SO3(aq)UO2SO4(aq)U(SO4)2(aq)UO2(SCN)2(aq)UO2(SCN)3UO2SCN+U(SCN)22+
USCN3+UO2(N3)2(aq)UO2(N3)3UO2(N3)42UNO33+
U(NO3)22+
UO2N3+UO2NO3+UBr3+
UO2Br+UO2BrO3+

Table 2Element
Na+
Ca2+
K+
Mg2+
Cu
Note:

Concentration
47.83
73.88
8.88
8.98
0.0001

Chemical composition of water from Wuyi well in Beishan Area[4]Element
NH4+
Al3+
Mn2+
Li+
Sr2+

Concentration
0.12
0.06
0.022
0.0112
0.715

Measured at the 50m deep in-situ condition, mg/L
68

Element
HCO3ClFBrSO42-

Concentration
Element
Concentration
12.91
103.7
SiO2（aq）
61.35
NO310.42
0.26
0.345
Eh（V）
0.0001
pH
7.24
161.8

Vol.9 No.1

Effects of pH on the Speciation of Radionuclides in Groundwater of Beishan Area, Gansu Province, China

1

2

4

5

3

1:AmCO3+ 2: Am(CO3)2- 3: AmOH2+
4: AmSO4+ 5: Am3+
Fig.1 Speciation of Am in the groundwater

1: NpO2+
Fig.3

2: NpO2CO3-

3

AmCO3

+

Speciation of Np in the groundwater

Am(CO3)2
2+

AmOH

-

Content(%)

Speciation
2-

84.42

PuO2(CO3)2

10.15

PuO2+
+

of Np is NpO2+ （93.76%）and the minor speciations are
NpO2CO3- and NpO2OH(aq). The speciation of Np greatly
varies with pH. In strong acid conditions, the dominant is
NpO2+and the minor ones are NpO22+ and NpO2SO4(aq); in
neutral conditions, the dominant is also NpO2+ ， but thecoexisting ones are NpO2CO3- and NpO2OH(aq); in weakalkaline to strong alkaline conditions, the dominant specieschange from NpO2CO3- to NpO2OH(aq)，and the minor ones
are Np（CO3）34- and NpO2（CO3）23-.

Figure 4 gives thedetailed evolution.
4.3.2 Speciation of Pu as a function of pH
The calculated species of Pu were PuO2(CO3)22-, PuO2+,
PuO2OH+, PuO2F3- and PuO2F2(aq). The speciation of Pu also
varies with pH. The dominant is PuO22+ and the minor ones
are PuO2SO4 (aq) and PuO2F+ in strong acid conditions.

Speciations of nuclides at pH 7.24

Content(%)
71.88

Speciation
2-

UO2(CO3)2

Content(%)
76.75

Speciation+

NpO2

Content(%)
93.76

23.93

UO2(CO3)3

4-

12.82

NpO2CO3

4.09

NpO2OH(aq)1.84

Th(OH)4aq

100

2.58

PuO2OH

2.13

UO2(OH)2(aq)

7.55

AmSO4

1.69

PuO2F3-0.802

UO2CO3(aq)

2.64

Am3+

0.941

PuO2F2(aq)

0.667

+

5

4.3 Effects of pH on the speciation of nuclide
The speciation of nuclides is greatly affected by the pH
value. In this study a great emphasis is put on the effects of
pH on the speciation and solubility of nuclides.
4.3.1 Speciation of Np as a function of pH
In the water of Beishan Wuyi well, the dominant species

4.2 Solubility of nuclides under in-situ condition
Saturation indices(SI’s) are important because they aremeasures of the thermodynamic driving forces behind theprogress of reaction. These quantities form an important
bridge between calculations that are purely thermodynamic andthose that pertain to dynamic evolution. The saturation index(SI) is defined as :SI=log Q/K, where Q is the activity productand K the equilibrium constant for a dissolution reaction [2,3].
Following the above definition, saturation indices are zero forthe case of exact saturation, negative for undersaturation, andpositive for supersaturation. So the SI of a mineral could be
used to discuss relative solubility of the nuclides in solution
even though the concentration of actinides is assumed to be
10-12 mol/L. The higher the SI of a mineral is, the lower thesolubility is.

Speciation

4

3

Table 4 shows all the SI values of minerals containing thenuclides in decreasing order. The SI values of PuO2,
Thorianite and NpO2 are higher than zero in Beishan Wuyi well.
Those minerals would be precipitated and should not dissolve
over 10-12 mol/L at that condition. Other minerals such as
AmOHCO3 are hardly formed and the solubility will be higher
than 10-12 mol/L.

3: NpO2OH(aq)Table 3

2

1: PuO2(CO3)22- 2: PuO2+ 3: PuO2OH+
4: Pu O2F3- 5: PuO2F2(aq)Fig.2 Speciation of Pu in the groundwater

2

1

1

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原子力バックエンド研究

September 2002

Table 4 Saturation indices of some minerals under in-situ condition
MineralsSI

MineralsSI

MineralsSI

PuO2
Thorianite
NpO2
AmOHCO3
Pu(OH)4
Th(OH)4
Np(OH)4
Am(OH)3
UO2CO3
NpO2OH(am)Am(OH)3(am)UO3(gamma)
Th(SO4)2
UO3(beta)
UO3(alpha)
UO2.25
ThF4:2.5H2O
UO2FOH
ThF4
Uraninite
PuO2OH(am)PuO2(OH)2
NpO2(OH)2
UO2.6667
Uranophane
Np2O5
UO2F2
UO2(am)

3.056
2.038
0.801
-4.535-5.067-5.755-7.813-8.088-8.788-9.074-9.588-10.512-10.776-11.115-11.444-12.326-12.849-13.115-14.711-15.060-15.263-16.589-18.634-18.666-18.709-19.176-19.878-20.007

UO2SO4
Pu(OH)3
UOFOH
UO2.3333(beta)
UO2Cl
UOF2
PuF3
UO2SO3
U(OH)2SO4
UF4
PuF4
U(SO4)2
UOCl2
U(CO3)2
UClF3
Pu2O3(beta)
UO2Br2
UOF4
UOCl3
UF3
ThCl4
UOBr2
UCl2F2UOCl
U5O12Cl
UCl3F
U2O3F6
U(SO3)2-22.260-22.837-23.121-23.313-23.925-26.053-26.654-34.078-34.313-39.303-39.705-43.353-45.446-47.637-48.886-48.907-51.687-55.985-57.170-59.364-60.135-60.241-60.785-62.756-63.234-72.509-76.014-77.655

UCl4
UCl3
UOBr3
UBrCl3
UBrCl2
UF6
ThBr4
UO2(NO3)2
U2F9
UBr2Cl2
UCl5
UBr2ClU3O5F8U2O2Cl5
UBr3Cl
UBr3
UBr4
UCl6
UBr5
Th(NO3)4:5H2O
U4F17
ThS2
U2C3
Th2S3
U2S3
U3S5
Th7S12-82.060-85.468-86.516-89.749-93.106-93.144-94.923-95.388-98.153-98.580-101.941-102.520-102.885-103.799-107.619-111.104-115.931-120.674-136.954-159.130-175.896-183.636-289.920-335.694-356.296-387.750-1203.551

100

100

90

90

80

1

2

3

4

5

6

7

8

80

70

1

60

4

50

70

2

60

5

6

9

50

7

40

40

30

30

20

20

10

10

0

0
1

2

3

1: NpO2+
4: NpO2CO3-;
6: NpO2(CO3)34-;

4

5

6

7

8

pH
2: NpO22+;
5: NpO2OH(aq);
7: NpO2(CO3)23-;

9

10

11

12

1

2

3

1: PuO2+;
4: PuO2F+;
7: PuO2OH+;

3: NpO2SO4(aq);

4

5

6

7

pH
2: PuO22+;
5: PuO2F2(aq);
8: PuO2(CO3)22-;

8

9

10

11

12

3: PuO2SO4(aq);
6: PuO2F3-;
9: Pu(OH)5-;

Fig . 5 Speciation of Pu as a function of pH

Fig.4 Speciation of Np as a function of pH
70

Vol.9 No.1

Effects of pH on the Speciation of Radionuclides in Groundwater of Beishan Area, Gansu Province, China

100

100
1

90

2

5

80

90

6

80

8

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

1

2

3

4

5

6

7

8

9

10

0
1

2

3

1: Am3+ ;4: AmF2+;
7: AmOH2+;
Fig . 6

4

5

6

7

8

9

10

11

12

13

pH
2: AmSO4+ ;3: Am(SO4)2+
5: AmCO3 ;6: Am(CO3)-2;
8: Am(OH)3(aq)

1

2

3

5

6

7

8

9

10

11

12

13

pH
2: UO2SO4(aq); 3:UO2F;
4:UO2OH;
6:UO2(OH)2aq;
7:UO2(CO3)22-;
9:UO2(OH)3-;
10: UO2(OH)42-

1: UO22+;
5:UO2(CO3)aq;
8:UO2(CO3)34- ;Speciations of Am as a function of pH

4

Fig . 7 Speciations of U as a function of pH

In neutral conditions, the dominant is PuO2(CO3)22-, but thecoexisting ones are PuO2+, PuO2OH+, PuO2F3- and PuO2F2(aq).
In weak alkaline to strong alkaline conditions, thedominant species change from PuO2(CO3)22- to Pu(OH)5-.
Figuer 5 gives the detailed evolution.
4.3.3 Speciation of Am as a function of pH
In the water of Beishan Wuyi well, the calculated speciesof Am were AmCO3+, Am(CO3)2-, AmOH2+, AmSO4+and Am3+.
In strong acid conditions, the dominant species is Am3+; inweak acid conditions, the dominant one is AmSO4+; in neutral
conditions, the dominant one is AmCO3+; in weak alkaline thedominant one is Am(CO3)2- ; in strong alkaline the dominant
one is Am(OH)3(aq). Figure 6 gives the detailed evolution.
4.3.4 Speciation of U as a function of pH
In the groundwater, the calculated U species were
UO2(CO3)22-, UO2(CO3)34-, UO2(OH)2(aq) and UO2CO3(aq).

The speciations of Np, Pu, Am, U and Th are affected not
only by the chemical composition of the water but also by thepH value even with the same chemical composition.
Each of the five nuclides has its own speciation pattern,
but the speciation of Pu, Am, U and Th is very complicated inacid conditions.
The speciation of nuclides is simpler in alkalineconditions, and the dominant species are hydroxyl complexes
and carbonate complexes. It is thus important to study theadsorption and migration behavior of these species ingeological materials.
The SI values of PuO2, Thorianite and NpO2 are higher
than zero in Beishan Wuyi well. Those minerals would be
precipitated and should not dissolve over 10-12 mol/L at thatcondition. Other minerals such as AmOHCO3 are hardly
formed and the solubility will be higher than 10-12 mol/L.

When the pH changes from low to high，the dominant species
100of U is changed from UO22+ to UO2CO3(aq), UO2(CO3)22-,
UO2(CO3)34-, and finally to UO2(OH)3-.
The detailed
evolution is given in Fig .7.
4.3.5 Speciation of Th as a function of pH
The speciation of Th differs from the above nuclides.
Th (OH) 4(aq) is the dominant species of Th when the pH ishigher than 6. However in acid conditions, it changes with thepH, and the dominant species changes from ThSO42+ to Th(SO4)2(aq), then ThF22+, and finally to Th(OH)4(aq) withincreasing pH.
The speciation pattern of Th is different from otherActinides for the lack of thermodynamic data on Th carbonates
as shown in Table1. Figure 8 shows the detailed evolution.

90
80

1

2

3

4

5

6

70
60
50
40

7

30
20
10
0
1

2

3

4

5

6

7

8

9

10

11

12

pH
1: Th(SO4)2aq; 2: ThSO42+;
3: ThF3+
2+
4+
4: ThF2 ;5: Th ;
6: ThF3+ 7:Th(OH)4aq
Fig.8 Speciations of Th as a function of pH

5. CONCLUSIONS
The following conclusions can be derived from thecalculation:
71

原子力バックエンド研究

REFERENCES[1] Xu, G. Q., et al.: Deep geological disposal of high level
radioactive waste in China. In: Geological Problem inRadioactive Waste Isolation: Second Worldwide Review(Witherspoon, P.A. ed), pp.51-61(1996).
[2] Wolery, T. J.: EQ3/6, A software package for geochemical
modeling of aqueous systems: package overview andinstallation guide (version 7.0), (1992).
[3] Wolery, T. J.: EQ3NR, A computer program forgeochemical aqueous speciation-solubility calculations:
theoretical manual, user’s guide, and related documentation(version 7.0), (1992).
[4] Yang, T. X., Guo, Y.H.: The Ground Water Chemical
Characteristics of Beishan Area—The China’s Potential
High-Level Radioactive Waste Repository. Acta
Geoscientia Sinica. 20, 679-685(1999).

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