Journal of Nuclear and Radiochemical Sciences, Vol. 20, pp. 20-24, 2020

ArticlesThermodynamic interpretation of zirconium solubility in the presence of hydroxyacetic,
3-hydroxypropionic, and 2,3-dihydroxypropanoic acids
Taishi Kobayashia,* , Wang Penga, Takayuki SasakiaaDepartment of Nuclear Engineering, Kyoto University, Kyotodaigaku-katsura, Nishikyo-ku, Kyoto 615-8540,
Japan

Received January 14, 2020; Accepted March 18, 2020; Published online April 14, 2020
The solubility of zirconium hydroxide in the presence of hydroxyacetic (glycolic, O20), 3-hydroxypropionic(O21), and 2,3-dihydroxypropanoic (glyceric, O201) acids was investigated as a function of pHc = −log [H+] = 7−13
and total hydroxycarboxylic acid concentrations, [L]tot = 10 −4 −10 −1 mol/dm3. The Zr solubility dependences on
pHc and [L]tot indicated that ternary Zr(OH)4(L)22− complexes were the dominant soluble species under the experimental conditions. The formation constants of the Zr(OH)4(L)22− complexes were determined by least square fitting analysis of the solubility data and compared to the values for zirconium-poly(hydroxy)carboxylate analogs to
elucidate the complexation mechanism of Zr-hydroxycarboxylates.

1. Introduction
Tetravalent zirconium, Zr(IV), is known to undergo extensive hydrolysis in aqueous solution. Various mononuclear andpolynuclear hydrolytic species have been identified directly or
indirectly in acid solutions [1-5]. However, strong hydrolysis
reactions lead to the formation of a sparingly soluble amorphous hydroxide solid phase, Zr(OH)4(am), in neutral to alkaline pH range. Zr(IV) solubility in neutral to alkaline solutions
is controlled by Zr(OH)4(am) and limited to values of 10−8 mol/dm3 (M) or lower in the absence of complexing ligands [6,7].
In the presence of suitable inorganic or organic ligands, Zr(IV)solubility is increased by the formation of soluble complexes[8-10]. Enhanced Zr(IV) solubility is an issue in the environmental context of safe radioactive waste disposal, because thelong half-life fission product 93Zr (T1/2=1.53×106 y) in radioactive waste forms Zr(IV) and can migrate with groundwater
under the disposal conditions.
Zirconium(IV) solubility in groundwater is generally limited by its sparingly soluble Zr(OH)4(am), and a very few inorganic or organic ligands can compete with the stronghydrolysis reactions that occur in neutral to alkaline solutions.
For example, oxalic acid ((COOH)2, C20) forms stable Zr(IV)complexes via its two carboxylate groups in acidic to weakly
acidic solutions [11-13]. However, according to thermodynamic calculations [13], the hydrolysis reaction competes
effectively with C20 complexation which causes only a minorchange in Zr(IV) solubility in neutral to alkaline solutions.
We recently investigated the degree of Zr(IV) solubility in thepresence of gluconic (HOCH 2 (CHOH) 4 COOH, GLU) and
α-isosaccharinic (CH 2OHCHOHCH 2COH-(CH 2OH)COOH,
ISA) acids [14]. These poly(hydroxy)carboxylic acids are present at disposal sites since GLU is used as a cement additive
and ISA is a degradation product of cellulose found in lowand intermediate-level radioactive wastes [15-17]. They were
found to form stable complexes with Zr(IV) in neutral to alkaline solutions with high complexation constants, which suggests that the hydroxyl as well as the carboxyl groups of GLU
and ISA bind Zr(IV) [14].
In this work, we investigated the solubility of Zr(IV) in thepresence of three simple hydroxycarboxylic acids (hydroxy*Corresponding author. Email: kobayashi@nucleng.kyoto-u.ac.jp,
FAX: 075-383-3919

acetic (glycolic, O20), 3-hydroxypropionic (O21), and2,3-dihydroxypropanoic acid (glyceric, O201)) to clarify therole of the hydroxy groups in Zr complexation. The Zr(IV)solubility dependence on hydrogen ion concentrations (pHc)and hydroxycarboxylic acid concentrations ([L]tot) was examined to identify the dominant soluble species and to determine
their formation constants. The constants obtained were discussed and compared with the values for Zr-poly(hydroxy)carboxylate complexes.
2. Experimental methods and materials
2.1. Chemicals and analytical methods. The chemicals
used in this study were of reagent grade. Hydroxyacetic (glycolic, O20) acid was purchased from FUJIFILM Wako Pure
Chemical Corp. 3-Hydroxypropionic (O21) and 2,3-dihydroxypropanoic (glyceric, O201) acids were obtained from
Tokyo Chemical Industry Co., Ltd. A stock suspension ofZr(OH)4(am) was prepared from ZrCl4 (FUJIFILM Wako) as
described in our previous study [14]. A PIPES buffer(Dojindo) was used to adjust pHc in the neutral pH region.
Deionized purified water (Milli-Q, Millipore) was used in the
preparation of all solutions.
All reagent preparations and sample handling procedures
were performed in an Ar-filled glove box with O2 concentrations below 10 ppm at 25 °C. A combination glass electrode(9615-10D, Horiba Ltd.) was used to measure pHc. The reference Ag/AgCl electrode was filled with 3.6 M NaCl and 0.4 M
NaClO4 to prevent precipitation of KClO4 at the junction of theelectrode and sample solution. The electrode was calibrated
against standard HCl and NaOH solutions adjusted to 0.5 M
ionic strength (I) with NaClO4 to correct experimentally measured pHexp values to pHc.
2.2. Solubility experiments. Sample solutions were prepared by an undersaturation method [10,14]. An aliquot ofZr(OH)4(am) stock suspension was added to sample solutionsat selected pHc and [L]tot values within the pHc range of 7 to 13
and [L] tot range of 10 −4 to 10 −1 M. Total dissolution ofZr(OH) 4 (am) equated to a maximum Zr concentration of
0.01 M. The structures of the hydroxycarboxylic acids used inthis study and of GLU and ISA are shown in Figure 1, together
with their reported dissociation constants for the carboxylic© 2020 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 04/12/2020

J. Nucl. Radiochem. Sci., Vol. 20, 2020

21

Kobayashi

OH-2log [O20]tot
–2.0
–1.0

OH
O

OH

O

O

OH

-3

OH

-4
2,3-dihydroxypropanoic
3-hydroxy(Glyceric) acid;propionic acid;O201
O21
pKa = 4.61 [18]pKa = 3.61 [18]OH

log [Zr]

Hydroxyacetic(Glycolic) acid;O20
pKa = 3.83 [18]Zr(OH)4(am)Reference [6]Reference [7]-5-6

Slope 0

-7

OH
HO

HO

-8
HO
O

OH
OH

Isosaccharinic acid;
ISA
pKa = 3.27 [19]

OH
O

-9

OH

-10

Gluconic acid;
GLU
pKa = 3.52 [18]Figure 1. Hydroxycarboxylic acids in this study.

group [18,19]. The ionic strength of sample solutions was
adjusted to I = 0.5 M by adding an appropriate amount of
NaClO4. Sample solutions in polypropylene tubes were kept in
the Ar-filled glove box and in some cases manually shaken fora few minutes. After given aging periods, the pHc of the sample solutions was measured and a 0.5-mL aliquot of the supernatant was passed through an ultrafiltration membrane(Microcon, nominal molecular weight limit (NMWL) = 3 kDa,pore size ~2 nm, Millipore). The filtrate was acidified with 0.1
M nitric acid prior to the concentration measurements by
inductively coupled plasma mass spectrometry (ICP-MS)(ElanDRC II, PerkinElmer). The detection limit of Zr(IV) solubility was approximately 10 −8.5 M. Solubility measurementsof Zr were repeated several times to confirm the attainment ofa steady state. The standard error of ICP-MS measurements
was ±10%, which corresponded to an uncertainty of ±0.1 log
units for the Zr(IV) concentration.

7

3.1. Zr(IV) solubility in the presence of hydroxycarboxylic acids. Figure 2 shows the Zr(IV) solubility in the presence of 0.01 and 0.1 M glycolic acid (O20) as a function of pHc.
Zirconium solubility in the absence of O20 [6,7] is plotted in
the same figure for comparison. The solubility after aging for
20 to 323 d exhibited similar values, which indicated that thesolution mixture reached a steady state within several weeks.
The Zr(IV) solubility in the presence of 0.01 M O20 was close
to that in the absence of O20, although concentrations were
almost at the detection limit of ICP-MS. However, Zr(IV) solubility in the presence of 0.1 M O20 equaled 10 −7 M throughout the whole pHc range, and the solubility was about one orderof magnit ude higher than that in the absence of O20.
Zr(IV)-O20 complexes were considered to be the predominantZr(IV) species in 0.1 M O20 solution. Assuming that the solid
phase retained its Zr(OH)4(am) stoichiometry, the zero slope ofthe log [Zr] versus pH c plot indicated that the soluble
Zr(IV)-O20 complex contained four OH groups in its chemical formula.
Figure 3 shows Zr(IV) solubility in the presence of O20 as afunction of [O20]tot at effectively constant pHc levels of 7.2–7.9

9

10

11

12

13

Figure 2. Zr(IV) solubility in the presence of 0.01 and 0.1 M O20
after aging for 20–323 d. The ICP-MS detection limit is indicated by
the gray shading.

-2

(a)

pHc 7.2-7.9(b)

pHc 11.2-11.3

-3
-4
-5
-6

Slope 2
-7

Slope 2

-8
-9
-10
-5

3. Results and discussion8pHclog [Zr]

HO

-4

-3

-2

log [O20]tot

-1

0 -5

-4

-3

-2

log [O20]tot-1

0Figure 3. Zr(IV) solubility at two levels of pHc as a function of total
O20 concentration after aging for 84–323 d: (a) pHc = 7.2–7.9 and (b)pHc = 11.2–11.3. The ICP-MS detection limit is indicated by the grayshading.

and 11.2–11.3. At both pHc levels, Zr solubility increased
monotonically with a slope of two, indicating that two O20
ligands were present in the Zr(IV)-O20 complex. Therefore,
we assumed that Zr(OH)4(O20)22− was the major species in thesolution.
The formation of glycolate (O20) complexes with trivalentand tetravalent metal ions has been reported in the literature,but mainly in acidic solutions. Binary O20 complexes including polynuclear species have been proposed under conditionsof higher metal ion and O20 concentrations [20,21]. The concentration of free Zr(IV) is strictly limited by Zr(OH)4(am) at
neutral to alkaline pH. The difference in O20 behavior
between acidic and neutral to alkaline solutions may arise
from this condition. We noted in our previous study that ternary Zr(OH) 4(L)22− complexes are found in the presence of

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Thermodynamic interpretation of zirconium solubility in the presence of

-2
-3
-4

-4

log [Zr]

-6

-6
-7

-8

-8

-9

-9

-10

-10

8

9

10

11

12

Zr(OH)4(am)Reference [6]Reference [7]

-5

-7

7

log [O201]tot
–2.0
–1.3

-3

Zr(OH)4(am)Reference [6]Reference [7]-5log [Zr]-2log [O21]tot
–2.0
–1.0

7

13

8

9

pHc

GLU and ISA at neutral and alkaline pH [14].
Figure 4 shows the solubility of Zr(IV) in the presence of
0.01 and 0.1 M 3-hydroxypropionic acid (O21) as a function of
pHc. Solubility was lower than 10 −8 M in both 0.01 and 0.1 M
O21, which was close to the value in the absence of O21 andclose to the ICP-MS detection limit. This result suggested thatcomplexation by O21 was not strong enough to solubilizeZ r ( I V ) u nde r t he p r e se nt ex p e r i me nt a l c ond it ion s.
Zirconium(IV) solubility also was examined as a function of[O21]tot, but no significant increase above the detection limit
was observed up to 0.1 M O21. A decrease in complexation
w ith increasing alkyl chain length is also observed for Zr–
dicarboxylates, wherein complexation by oxalate ((COOH)2,
C20) is stronger than that of malonate (CH2(COOH)2, C21) [9].
Similar to dicarboxylic acids, Zr(IV) chelation featuring
5-membered rings with O20 might be more stable than thatwith 6-membered rings with O21.
Zirconium(IV) solubility in the presence of 0.01 and 0.05 M
glyceric acid (O201) is plotted as a function of pHc in Figure 5.
At 0.01 M O201, the solubility was slightly higher than that in
the absence of O201. In 0.05 M O201, the solubility wasapproximately 10 −6 M at pH c 8 and 10 −6.5 M at pH c > 10.
Provided that the solid phase retained its Zr(OH)4(am) stoichiometry, the slope of −1 found at pHc 8–10 indicated that thesoluble Zr(IV)-O201 complex contained three OH groups inits chemical formula. On the other hand, the slope of log [Zr]against pHc was almost zero at pHc 10–12, similar to the caseof O20, indicating that four OH groups bound to Zr(IV) toform the Zr(IV)-O201 complex in this pH region. At pHc >12, the Zr(IV) was observed to increase with increasing pHc
at, suggesting that one additional OH− was involved in theZr(IV)-O201 complex in this pH region. This might be due tofurther coordination of an OH− to the Zr(IV)-O201 complex or
to deprotonation of the hydroxyl group of O201 as discussed
for GLU and ISA in our previous study [14].
Figure 6 displays the Zr(IV) solubility as a function of[O201] tot at pH c =6.0–6.9 and pH c = 9.2–10.6. Solubility
increased with increasing [O201]tot with a slope of two, whichindicated that the dominant Zr(IV)-O201 complex contained

10

11

12

13

pHc
Figure 5. Zr(IV) solubility in the presence of 0.01 and 0.05 M O201
after aging for 63–176 d. The ICP-MS detection limit is indicated by
the gray field.

-2

(a)

pHc 6.0-6.9

pHc 9.2-10.6

(b)

-3
-4

log [Zr]

Figure 4. Zr(IV) solubility in the presence of 0.01 and 0.1 M O21
after aging for 33–50 d. The detection limit of ICP-MS is indicatedby the gray shading.

22

-5
-6

Slope 2

Slope 2

-7
-8
-9
-10
-5

-4

-3

-2

log [O201]tot

-1

0 -5

-4

-3

-2

-1

0

log [O201]tot

Figure 6. Zr(IV) solubility at (a) pHc = 6.0–6.9 and (b) pHc = 9.2–
10.6 as a function of total O201 concentration after aging for 63–176
d. The ICP-MS detection limit is indicated by the gray shading.

two O201 ligands. Thus, we established that the ternary complexes, Zr(OH)3(O201)2− and Zr(OH)4(O201)22−, formed underthe experimental conditions. Although an additionalZr(IV)-O201 complex with five OH groups might exist in thehigh alkaline pH region at pHc > 12 as discussed above, thisspecies was not taken into consideration in the analysis due tothe lack of data for the Zr(IV) solubility dependence on[O201]tot.
3.2. Thermodynamic model for Zr(IV) hydroxycarboxylic acid complex formation. Assuming the solid phase to be
Zr(OH)4(am), the dependence of Zr(IV) solubility on pHc and[L]tot suggested the formation of ternary Zr(IV)-OH-L complexes (L = O20 or O201) as described below.

23

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Kobayashi

Zr + 3OH + 2L  Zr(OH)3 (L)(1)

logβ132 = log [Zr(OH)3(L)2–] – log [Zr4+]– 3 log[OH–] – 2log [L–](2)

Zr4++ 4OH– + 2L–  Zr(OH)4 (L)22–(3)

logβ142 = log [Zr(OH)4(L)22–] – log [Zr4+]– 4 log[OH–] – 2log [L–]

(4)

4+

–

–
2

–

In addition to Zr(IV)-OH-hydroxycarboxylate complexes, freeZr4+ was assumed to hydrolyze to Zr(OH)m(4–m), so that the total
concentration of soluble Zr(IV), [Zr(IV)]tot, is expressed by thefollowing sum:[Zr(IV)]tot [Zr 4 ]  ¦ [Zr(OH)(4m  m ) ]  ¦ [Zr(OH)n (L) (42  n 2) ]
m

n 3,4

­
 m
 n  2½
®1  ¦ E1m 0 [OH ] ¦ E1n 2 [OH ] [L ] ¾ ,[OH  ]4 ¯ m
n 3,4
¿
Ksp

(5)

where Ksp, β1m0, and β1n2 are the solubility product, hydrolysis
constant, and formation constant, respectively, of the Zr(IV)hydroxycarboxylate system. Because the O20 and O201 carboxylate groups are deprotonated at the experimental pHc
range according to their dissociation constants [18,19], and[L]tot is much higher than [Zr(IV)]tot, the concentrations of freeO20 or O201 were assumed to have equal [L]tot. As in our previous study [14], Ksp, and β1m0 were taken from the literature[4,6], and corrections for ionic strength were applied using the° and β°specific ion interaction theory (SIT) [8]. Values of β132
142
at I = 0 determined by least square fitting analysis of the solubility data are summarized in Table 1, which includes resultsfor ISA and GLU [14].
Our log β °142 value for O20 was higher than the hydrolysis
constant log β °160 = 48.0 [4] and 50.95 [22] reported forZr(OH)62–, where six monodentate OH – ligands bound to Zr.
This possibly suggested that the deprotonated α-hydroxyl andcarboxylate groups of O20 (O20 –H ) combined to bind in abidentate manner as described below.
Zr(OH)4(O20)22–  Zr(OH)3(O20)(O20 –H)2– + H2O
 Zr(OH)2(O20 –H)22– + 2H2O

(6)

However, the difference between the formations of these species did not appear in the solubility experiment and analysis as
a function of pHc and [L]tot. In the present study, the expression Zr(OH)4(O20)22– was used for simplicity. The β°142 value
for O201 was one order of magnitude higher than that for O20.
Because steric hindrance makes it difficult for the α- andβ-hydroxyl groups of O201 to bind simultaneously to a single° value for O201 may point to the formaZr atom, the larger β142
tion of polynuclear species with the identical [Zr(OH) 4

2–(O201)2 ]n stoichiometry, as suggested for U(VI) [23]. The log
β °142 values for O20 and O201 in Table 1 are a few orders ofmagnitude smaller than those for GLU and ISA. Because
GLU and ISA have four (α-, β-, γ-, and δ-) hydroxyl groups on
their alkyl chains (Figure 1), the additional two γ- and δhydroxyl groups likely play a role in the formation of more stable Zr-OH-GLU and Zr-OH-ISA complexes.
4. Conclusion
Zirconium(IV) solubility in the presence of the O20, O21,
and O201 hydroxycarboxylic acids was investigated at pHc =7–13 and [L]tot = 10−4 –10−1 M. The dependence of Zr(IV) solubility on pH c and [L] tot suggested that Zr(OH) 3 (L) 2− andZr(OH)4(L)22− were the principal species under the experimental conditions. Formation constants determined by leastsquare fitting analysis of solubility data were compared withthe for mat ion const ants of zircon iu m-poly(hyd roxy)carboxylates to understand the mechanism of Zr(IV)hydroxycarboxylate complex formation. These results may
also help to identify some organic ligands which can compete
against the strong hydrolysis reactions of polyvalent radionuclides to enhance their solubilities under disposal conditions.
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TABLE 1: Formation constants of Zr(IV)-OH-hydroxycarboxylate complexes (I = 0)
Reactions
Zr

4+

Values–+ 4OH + 2O20

–

Zr

4+

Zr

4+

Zr

4+

–

–

–

–

–

–

+ 4OH + 2O201+ 4OH + 2GLU
+ 5OH + 2GLU

Referencelog β°142

51.8±0.1

Present study

Zr(OH)3(O201)2 –log β°132

48.2±0.2

Present study2–

log β°14252.7±0.1

Present study2–

log β°142

54.1

[14]

3–

log β°152

56.5

[14]

Zr(OH)4(O20)2

Zr 4+ + 3OH– + 2O201–

2–

Zr(OH)4(O201)2
Zr(OH)4(GLU)2
Zr(OH)5(GLU)2

Zr 4+ + 4OH– + 2ISA–

Zr(OH)4(ISA)22 –log β°142

54.2

[14]

Zr 4+ + 5OH– + 2ISA–

Zr(OH)5(ISA)23 –log β°152

55.6

[14]

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