Solubility of UO2HPO4 in seawater

Article abstract of DOI:10.1134/S1066362206020123

The solubility of UO₂HPO₄ in normal seawater of 35‰ salinity was studied. The solubility of U(VI) phosphates in seawater increases nonlinearly with decreasing acidity (by two orders of magnitude per pH unit) due to formation of stron

Full text of DOI:10.1134/S1066362206020123

ISSN 1066-3622. Radiochemistry, 2006, Vol. 48, No. 2, pp. 167 169. Pleiades Publishing, Inc., 2006.
Original Russian Text A.V. Savenko, 2006, published in Radiokhimiya, 2006, Vol. 48, No. 2, pp. 150 152.
Solubility of UO2HPO4 in Seawater
A. V. Savenko
Geological Faculty, Moscow State University, Moscow, Russia
Received December 17, 2004
Abstract The solubility of UO2HPO4 in normal seawater of 35‰ salinity was studied. The solubility of
U(VI) phosphates in seawater increases nonlinearly with decreasing acidity (by two orders of magnitude per
pH unit) due to formation of strong hydroxo, carbonate and, probably, phosphate complexes. At pH > 7.5,
UO2HPO4 dissolves congruently. With decreasing pH to 6.9 7.2, UO2HPO4 is converted to UO2(H2PO4)2.
The solubility of U(VI) phosphates in seawater considerably exceeds the natural uranium concentrations in
seawater and pore solutions of the bottom sediments. Therefore, these compounds cannot form intrinsic min-
erals in the oceanic bottom sediments. The possibility of accumulation of U(VI) in sea phosphorites due to its
chemisorption in the form of surface uranyl phosphate complexes is discussed.
PACS numbers: 82.60.Lf
DOI: 10.1134/S1066362206020123
It is known that uranyl phosphates are poorly solu-
ble compounds naturally occurring as separate min-
erals. It is also believed that uranyl ions can be ac-
cumulated in phosphorites due to formation of strong
surface phosphate complexes.
studied in relation to the acidity of model seawater.
Model seawater samples of various acidity were pre-
pared by mixing normal seawater samples taken from
the Mediterranian Sea either with a natural seawater
sample preliminarily equilibrated with Mg(OH) , or
2
with model acidified seawater of 35‰ salinity in
which 0.1 M NaCl was partially replaced by HCl
(0.1 M). In the second series of experiments, we used
model seawater samples with variable phosphate con-
tent (40 100 M), prepared by adding small aliquots
of 0.01 M aqueous KH PO to normal seawater.
In spite of low solubility of uranyl phosphates and
their strong sorption on various phosphate-containing
minerals, the equilibrium concentrations of U(VI) in
natural water can be relatively high because of forma-
tion of strong hydroxo, carbonate, and phosphate
complexes. Unfortunately, the published stability con-
stants of these complexes vary in wide range, which
impedes their use for thermodynamic calculations of
solubility of uranyl phosphate in multicomponent
natural aquatic systems. In this connection, the direct
experimental determination of the solubility of U(VI)
minerals is of interest.
2
4
The solubility of uranyl phospate was determined
in hermetically stoppered Erlenmeyer flasks at a solid
to liquid phase ratio of 1 : 1250 at 22 2 C. The pH
of the model seawater samples was varied in the range
6.9 8.4 typical of seawater and pore solutions of
ocean bottom sediments. The samples were kept for
3.5 months with intermittent vigorous stirring and
then were filtered through a blue band filter. The pH
in the filtered solution were determined potentiomet-
rically. The uranyl and phosphate concentrations in
the filtered solution was determined colorimetrically
in the form of colored complexes with Arsenazo III
and ammonium molybdate (with reduction by ascorb-
ic acid), respectively.
In this study we determined the solubility of
UO HPO in seawater under various conditions.
2
4
EXPERIMENTAL
Solid UO HPO was prepared as follows. A boil-
2
4
ing 0.025 M aqueous UO (NO ) solution (0.5 l) was
2
3 2
added to a boiling 0.1 M aqueous Na HPO solution
2
4
(1 l). After cooling, the resulting precipitate was fil-
tered off on a Buchner funnel, washed with hot dis-
tilled water, and dried at 105 C. Solid UO HPO was
RESULTS AND DISCUSSION
2
4
crushed before experiments in an agate mortar.
Table and Fig. 1 show that, with increasing pH of
seawater, the solubility of uranyl phosphate gradually
increases. The pH dependence of the uranyl phosphate
Two different series of experiments were carried
out. In the first series, the solubility of UO HPO was
2
4
167

168
SAVENKO
Solubility of UO2HPO4 in seawater of 35‰ salinity at 22 C
Concentration,
M
[ UO2][ PO4] 1010,
[ UO2]
[ PO4]
pH
PO
4
2
UO2,
mol2 l
[
PO4]
equilibrium
initial
equilibrium
8.39
8.06
7.63
7.21
7.10
6.91
7.80
8.00
7.64
151
0
0
0
0
0
127
127
81.2
25.7
11.9
6.19
2.64
28.7
42.7
8.40
1.19
1.18
1.65
1.74
1.73
1.76
1.44
1.47
1.24
191
95.8
42.5
20.7
10.7
4.65
41.2
62.9
10.4
81.2
25.7
11.9
6.19
2.64
68.7
103
77.8
10.9
2.46
0.66
0.12
28.3
64.6
11.3
0
40
60
100
108
solubility is fitted by the logarithmic equation
log([ UO2][ PO4]) = 2.09pH 25.02, r = 0.980,
experimental points obtained in the second series of
experiments (in the presence of KH PO ) fall on the
2
4
same solubility curve as those obtained in the first
series. This means that the addition of KH PO up
(1)
2
4
to its concentration of 100 M does not affect the
solubility of UO HPO .
where [ UO ] and [ PO ] are the total (analytically
determined) uranyl and phosphate concentrations in
the equilibrium solution. Figure 1 shows that the
2
4
2
4
The increase in the solubility of UO HPO with
2
4
increasing pH can be caused by formation of hydroxo
2 n
2 2n]
[UO (OH) ] and carbonate [UO (CO )
com-
plexes. It is possible that the phosphate complexes
2
n
2
3 n
2 2n
2 3n
UO (HPO )
and UO (PO )
are also formed.
2
4 n
2
4 n
However, the lack of reliable data on the stability con-
stants of these complexes hampers evaluation of their
contribution to the solubility of UO HPO . The addi-
2
4
tional factor complicating the thermodynamic descrip-
tion of the solubility of UO HPO is the possibility of
changes in its composition induced by variations in
aqueous phase acidity.
Fig. 1. Product [ UO ][ PO ] vs. pH in seawater equi-
2
4
2
4
librated with UO HPO : (1) without addition of KH PO
and (2) after addition of KH PO (40 100 M).
2
4
2
4
2
4
To determine the chemical composition of species
formed in dissolution of UO HPO , we examined the
2
4
relationship between the uranyl and phosphate ion
concentrations in saturated solutions at various pH
(Fig. 2). Figure 2 shows that the experimental points
obtained in the second series of experiments (in the
presence of KH PO ) fall on the same curve as those
2
4
obtained in the first series. Since we have aimed to
elucidate the mechanism of dissolution of uranyl
phosphate proper, we will not consider further the
data on solubility of uranyl phosphates in the presence
of KH PO .
Fig. 2. Correlation between uranyl and phosphate ion con-
2
4
centrations in the saturated aqueous solution of UO HPO
2
4
Table shows that, in the solutions of UO HPO
2
4
(1) in the absence of KH PO and (2) in the presence of
2
4
with the highest pH (8.0 and 8.4) the phosphate con-
centrations are 103 and 127 M, respectively. These
KH PO (40 100 M) at various pH. Figures at points
2
4
denote equilibrium pH.
RADIOCHEMISTRY Vol. 48 No. 2 2006

SOLUBILITY OF UO2HPO4 IN SEAWATER
169
11
values are close to the solubility of calcium phosphate
prepared under laboratory conditions and then aged
to (0.00n 0.n) 10 . These values are by an order
of magnitude lower than those in aqueous solution
saturated with respect to the mixture UO HPO
for several months (100 300 M PO [1]), but are
4
2
4
noticeably lower than the solubility of freshly precipi-
UO (H PO ) .
2
2
4 2
tated octacalcium phosphate (400 500 M PO [2]).
4
Our results suggest that U(VI) phosphates cannot
form a separate mineral phase in bottom sediments
like phases of sea phosphorites. However, we do not
rule out that U(VI) can be accumulated in sea phos-
phorites by the mechanism of chemisorption in the
form of strong surface uranyl phosphate complexes.
We believe that chemisorption of uranyl phosphate
on sea phosphorites in the bottom sediments is
favored by the fact that the decreased pH of pore
solutions of bottom sediments decreases the product
[ UO ][ PO ]}.
The plot of uranyl ion concentration in saturated
solutions of UO HPO vs. the phosphate ion content
2
4
( [ PO ]) (Fig. 2) is well approximated by a combi-
4
nation of two linear equations
[ UO2] = k [ PO4] + b.
(2)
In this equation, k = 2 in the pH range 6.9 7.2, and
k = 1 in the pH range 7.8 8.4. The first pH range (k =
2) is typical for deep-sea water and pore solutions of
bottom sediments. The second pH range (k = 1) is
typical of surface sea and oceanic water. The tangent
lines with slopes k = 1 and 2 intersect at pH 7.5. These
data show that, whereas at pH > 7.5 UO HPO dis-
2
4
ACKNOWLEDGMENTS
2
4
This study was financially supported by the Rus-
sian Foundation for Basic Research (project no.04-05-
65051) and the Ministry of Education and Science
of the Russian Federation within the framework of the
World Ocean Federal Target Program.
solves congruently, at pH < 7.5 the dissolution of
UO HPO is incongruent. This change in the mech-
2
4
anism of dissolution is probably caused by the conver-
sion of UO HPO into UO (H PO ) , which is
2
4
2
2
4 2
manifested as an approximately twofold increase in
the uranyl to phosphate molar ratio in the saturated
solution.
REFERENCES
Thus, our experiments show that, in going to aque-
ous solutions with pH < 7.1 typical of pore solutions
of ocean bottom sediments [3], the solubility of uranyl
phosphate decreases. In this pH region the product of
the total uranyl and phosphate ion concentrations
([UO ][ PO ]) varies in relatively narrow limits
1. Savenko, V.S., Dokl. Akad. Nauk SSSR, 1978, vol. 243,
no. 5, pp. 1302 1305.
2. Golubev, S.V., Savenko, V.S., and Pokrovskii, O.S.,
Vest. Mosk. Univ., Ser. 4: Geol., 2000, no. 2, pp. 71
76.
2
4
11
11
3. Savenko, V.S., Fiziko-khimicheskii analiz protsessov
formirovaniya zhelezomargantsevykh konkretsii v oke-
ane (Physicochemical Analysis of Formation of Iron
Manganese Nodules in Ocean), Moscow: GEOS, 2004.
(1.2 10
6.6 10 ). These results suggest that
at pH < 7.1 the formation of uranyl hydroxo and car-
bonate complexes contributes insignificantly to the
solubility of uranyl phosphate. The field observations
reported in [4, 5] showed that, at phosphate concentra-
tions of 3 50 M typical of pore solutions of bottom
sediments really containing current phosphorites, the
uranium content does not exceed 0.0n M. Under
these conditions, the product [ UO ][ PO ] is equal
4. Baturin, G.N., Uran v sovremennom morskom osadko-
obrazovanii (Uranium in the Modern Sea Sedimenta-
tion), Moscow: Atomizdat, 1975.
5. Baturin, G.N., Fosfority na dne okeanov (Phosphorites
at Ocean Bottom), Moscow: Nauka, 1978.
2
4
RADIOCHEMISTRY Vol. 48 No. 2 2006