THE EFFECTS OF pH AND Ca/P RATIO ON THE PRECIPITATION OF
CALCIUM PHOSPHATE
Y. SONG, H. H. HAHN, AND E. HOFFMANN*
Institute for Aquatic Environmental Engineering, University of Karlsruhe, Adenauerring 20, Karlsruhe, D-76128, Germany
Email: Erhard.Hoffmann@bau-verm.uni-karlsruhe.de
ABSTRACT
Phosphorus recovery from wastewater accords with the principle of sustainability and meets the needs of phosphate industry and water
industry. Studies aiming at understanding of the precipitation of calcium phosphate from wastewater are essential to the technologies of
phosphorus recovery. In the present paper, a chemically defined system with a low phosphate concentration of 20 miligrammes phosphorus per
liter and an ionic strength, expressed as conductivity of 2.0 milisiemens per centimeter, which was equivalent to wastewater was designed to
study the effects of pH value and Ca/P ratio on the precipitation of calcium phosphate. The precipitation experiments were undertaken in a
batch system at pH values ranging from 7.50 to 11.00 and initial Ca/P ratios ranging from 1.67 to 6.67 at a constant temperature of 21.5°C. It is
found that both the increasing of pH value and the increasing of initial Ca/P ratio of the precipitating system favour the precipitation efficiency
and precipitation rate of calcium phosphate. At initial Ca/P ratios of 1.67 and 3.33 the lowest pH values to achieve fast precipitation are 9.00
and 8.50, respectively; at initial Ca/P ratios above 5.00, fast precipitation can be achieved at pH 8.00. At the pH values ranging from 7.50 to 8.00,
after a reaction time of 90 minutes a special acceleration phenomenon was also observed, where a fairly high precipitation efficiency could be
achieved finally. The effects of pH value and initial Ca/P ratio on the precipitation of calcium phosphate are due to their influences on the
supersaturations of the precipitating systems.
Keywords: phosphorus recovery, precipitation, pH value, initial Ca/P ratio
INTRODUCTION
needed to remove carbon dioxide by addition of concentrated (96%)
sulphuric acid, accordingly the pH value of the influent is reduced to
Phosphorus is an essential element to all living organisms and its
3 to release carbon dioxide and then raised up to about 9 to
utilisation has promoted the development of agriculture and
precipitate. This is a necessary step to a full process, however this
industry, but the modern society does not use phosphorus resources
will increase the complexity and the cost of the process, especially in
in a sustainable way: phosphates are manufactured from phosphate-
the area where the influent contains much carbonate. Besides
containing rock,
then are consumed in agriculture and industry,
carbonate, magnesium, organic matter and other components may
finally go into soil, rivers and the sea. This, on one hand, is
also affect the precipitation or crystallisation process of calcium
exhausting the limited phosphate deposits which are unevenly
phosphate. It seems that a thorough understanding of the process
distributed in the world, on the other hand, has caused
conditions for the precipitation and crystallisation of phosphate is
eutrophication of water bodies. With the increasing concern at
indispensable to mature technologies for phosphorus recovery.
sustainability, more stringent nutrient discharge limits, and more
From the viewpoint of industry, to recover phosphorus as calcium
restrictions on sludge disposal, both wastewater treatment industry
phosphate is promising [2]. The calcium phosphate formation can be
and phosphate industry have to consider the alternatives of
induced with high calcium concentration and raised pH by adding
conventional phosphorus removal technologies and opportunities for
lime,
phosphorus recovery.
environmental problems. Calcium phosphate is just the component
Considerable world-wide researches have been undertaken on
phosphorus recovery technologies, of which some are already in
pilot-scale. From the viewpoint of availability, the crystallisation
which
is
inexpensive,
readily
available
and
free
of
of phosphate rock, so if it is recovered in a suitable physical form it
should be a good raw material both to industry and to agriculture.
Calcium
phosphates
occur
extensively
in
natural
aquatic
processes of calcium phosphate and magnesium ammonium
environments, biological fluids, and in processes of technological
phosphate hexahydrate (struvite) have been emphatically studied,
interests [3], and are studied in several fields: water geochemistry,
and a number of technologies such as the DHV CrystalactorTM
sedimentary mineralogy, fertilisers and food industry, osteology,
Pelletiser, the CSIR Fluidised Bed Crystallisation Column, the Kurita
urology, etc [4]. One active field for calcium phosphates studies is
Fixed Bed Crystallisation Column etc. have been obtained [1]. In
physiology, for these compounds are very important to the biological
these technologies, some techniques such as Phostrip are used to
systems like bone and teeth which consist mainly of an apatitic
extract phosphate from wastewater, then phosphate is precipitated
calcium phosphate [5]. Synthetic calcium hydroxyapatite has been
and/or crystallised in dedicated reactors such as fluidised bed, fixed
taken as an ideal biomaterial because of its bio-compatibility.
bed reactors, finally useful end products are obtained. Although
Numerous studies have been undertaken on the precipitation and
these technologies are successful from the viewpoint of research,
crystallisation of calcium phosphates [5-22]. Generally, when calcium
many problems on their application and popularization still remain.
phosphate precipitates from a system which contains total calcium
Taking the DHV CrystalactorTM Pelletiser, one of these technologies
and total phosphate each higher than 10 milimoles per liter (mmol.l-1
used to recover phosphate as calcium phosphate as an example, the
) and at pH values greater than 6.8, the precipitation is always
presence of carbonate can affect the crystallisation, so a degassifier is
preceded by the formation of an amorphous precursor, distinct both
Y. SONG, H. H. HAHN, AND E. HOFFMANN
chemically and structurally from the final product, hydroxyapatite
addition volume was recorded. Aliquots of 15 ml for calcium and
(HAP) Ca5(PO4)3OH [18], and this follows the Ostwald Rule of Stages
phosphate analyses were removed at frequent intervals after mixing,
in Precipitation [23]. Afterwards the precipitate transforms through
and filtrated with glassfibre filters (Sartorius AG). The calcium
some intermediate states to the thermodynamically stable product,
concentration of the filtrate was analysed by complexometric
HAP crystal. The formation and transformation may be influenced by
method, and the phosphate concentration was analysed by
supersaturation, pH values, magnesium [12,16,19], etc. Although so
molybdenum
many efforts have been made to elucidate the mechanisms of
wavelength of 700 nanometers with a Lambda 2 UV/VIS
formation, initiation of growth and transformations between
Spectrometer (Perkin Elmer).
different
mineral
forms,
very
few
mechanisms
have
heteropolyphosphate
complex
method
at
the
been
All the above reactant solutions were prepared from Analytical
unanimously accepted. Besides, most of these studies focused on the
Grade reagents. The water used in the experiments was deionized
physiological conditions, i.e. at pH 7.4 and temperatures around
water which has a conductivity of 0.7 microsiemens per centimeter.
37°C.
All the experiments were performed at 21.5 ± 0.5°C.
To recover phosphorus from wastewater as calcium phosphate, the
chemical and physicochemical factors which may influence the
precipitation process should be investigated. These factors may be:
CALCULATION OF SPECIES CONCENTRATION—
the compositions of the influent to a recovery process including
SUPERSATURATION
concentrations of phosphate, calcium, magnesium, organic matter
and carbonate, the pH value and temperature of the influent, etc.
Under proper physical and chemical environment, different kinds
Moreover it is noticeable that the composition of wastewater might
of calcium phosphate phases (Table 1) may precipitate from
be different in different areas. In the former studies of our group [1,
saturated solutions. The supersaturation (S) of a system with respect
24], valuable engineering experiences for phosphorus recovery as
to a given calcium phosphate phase indicates the thermodynamically
calcium phosphate were obtained and the wastewater characteristics
at Darmstadt-Eberstadt Sewage Treatment Works were investigated.
Table 1. Calcium phosphate phases
It has been found that further understanding of precipitation and
Phases
crystallisation of calcium phosphate from the system containing 10 to
Dicalcium phosphate
50 miligrammes phosphorus per liter (mg P.l-1) is of high importance
dihydrate
to improve and optimise the recovery process and equipment.
The present work has been undertaken in order to study the effects
of two operational factors, pH value and initial Ca/P ratio on the
Abbreviation
Composition
Ca/P
DCPD
CaHPO4.2H2O
1.00
Octacalcium phosphate
OCP
Ca4H(PO4)3.3H2O
1.33
Tricalcium phosphate
TCP
Ca3(PO4)2
1.50
Hydroxyapatite
HAP
Ca5(PO4)3OH
1.67
precipitation of calcium phosphate. Considering the characteristics of
wastewaters, a chemically defined system containing 20 mg P.l-1 was
driving force of the precipitation reaction. It is defined as
designed, within which practical knowledge on precipitation of
calcium phosphate has been obtained.
Ionic activity product (IP) in solution
S = ,
Solubility product (Ksp)
(i)
EXPERIMENTS
where the following IP’s of different calcium phosphate are defined,
The precipitaion of calcium phosphate was initiated by the rapid
mixing of CaCl2 and K2HPO4 or KH2PO4 solutions. Experiments were
IPHAP
performed by adding 500 mililiters (ml) of 2.151, 4.296, 6.450, and
=
([Ca2+]f2)5([PO43-]f3)3(Kw/[H+])f1
(ii)
8.600 mmol.l-1 CaCl2 to 500 ml of 1.290 mmol.l-1 K2HPO4 or KH2PO4
IPOCP
within 30 seconds, in order to get Ca/P ratios of 1.67, 3.33, 5.00, and
=
([Ca2+]f2)4([PO43-]f3)3[H+]f1
(iii)
6.67, respectively. The reverse order of addition was also used with
IPTCP
no change in results, so the former order of addition was used
=
([Ca2+]f2)3([PO43-]f3)2
(iv)
throughout the experiments. Prior to mixing, the pH of each of the
IPDCPD
reagents was adjusted to a value between 7.50 and 11.00 at an
=
[Ca2+]f2[HPO42-]f2.
(v)
interval of 0.50 by the addition of NaOH or HCl, as needed. In order
to keep the ionic strength of the precipitation system similar to
Moreover, a saturation index (SI) is defined as
wastewater, 1.01 grammes KNO3 was added to the CaCl2 solution
prior to mixing, to arrive at a concentration of 0.01 mole per liter
SI = log(S)
(vi)
(mol.l-1) after mixing. The initial mixing was under strong stirring by
a HI 200M magnetic stirrer (Hanna Instruments) for 3 minutes (min),
to facilitate further discussion.
afterwards solutions were stirred at a constant reproducible rate. The
The calculation of ionic activity product necessitates a detailed
change in pH with time, after mixing, was monitored with a PH 191
knowledge of the chemical speciation of the ions in solution.
pH meter (WTW GmbH) and solutions were maintained at above
Assuming that no precipitation has taken place just after mixing, the
constant pH values by the addition of 0.45 mol.l -1 NaOH, and the
ions, ion-pairs and equilibria listed in Table 2 should be taken into
2
Y. SONG, H. H. HAHN, AND E. HOFFMANN
account in the present precipitating system. As an approxiamation,
written in FORTRAN, and the activity coefficients were calculated
the influence of the dissolved CO2 from atmosphere to the
according to Davies equation [5]
precipitating system is ignored. The thermodynamic dissociation and
-logf z
association constants are also given in Table 2. The calculations were
=
AZ 2 {I 1/2 /(1
+
I 1/2 )
–
0.3I},
–
9.221
(vii)
performed by an iterative method [5, 35] with a computer program
Table 2. Species, equilibria and constants used in the calculation of supersaturation a
Equilibrium
H3PO4 
pK
Reference
H+
pK = 799.31/T – 4.5535 + 0.013486T
[25]
H2PO4-  HPO42- + H+
pK = 1979.5/T – 5.3541 + 0.019840T
[26]
HPO42-  PO43- + H+
pK = 12.42  0.010 at 20°C
[27]
H2O  H+ + OH-
pKw = 4471.33/T + 0.017053T – 6.0846
[28]
H2PO4-

+
pK = -6.462 at 25°C
[29]
Ca2+ + HPO42-  CaHPO40
pK = 148.15 – 22188/T – 0.2554T
[30]
Ca2+ + H2PO4-  CaH2PO4+
pK = 53.335 – 8413.5/T – 0.08660T
[30]
Ca2+ + OH-  CaOH+
pK = -1.356
[31]
Ca5(PO4)3OH(s)  5Ca2+ + 3PO43- + OH-
pKHAP = 8219.41/T + 1.6657 + 0.098215T
[32]
Ca4H(PO4)3.3H2O(s) 
pKOCP = 50.606 at 20°C
[13]
Ca3(PO4)2(s)  3Ca2+ + 2PO43-
pKTCP = 45723.26/T – 287.4536 + 0.546763T
[33]
CaHPO4.2H2O(s)  Ca2+ + HPO42- + 2H2O
pKDCPD = 3649.6/T – 18.181 + 0.04203T
[34]
Ca2+
+
PO43-
CaPO4-
4Ca2+ + 3PO43- + H+ +
3H2O
a
T is the temperature (K), Kw is the ionic product of water, and KHAP, KOCP, KTCP, and KDCPD are the solubility
products for HAP, OCP, TCP, DCPD, respectively. (pK = - log10K).
where I is the ionic strength, and A has a value of 0.505 at 21.5°C.
C0
The calculated values for supersaturation help to understand and
–
Ct
=
7.659
ln(t)
(ix)
interpret the precipition of calcium phosphate
where C0 (mmol.l-1) is initial concentration of phosphate in solution
and Ct (mmol.l-1) is the concentration of phosphate in solution at
RESULTS
reaction time t (min). A transformation is obtained by differentiating
the above equation
The Effect of pH Value on the Precipitation of Calcium Phosphate
- dCt/dt = 7.659  1/t
(x)
For the convenience of discussion, a precipitation efficiency (t),
where 0  (t)  100%, is defined as
=
(C0
–
C t)
/
C0

100%
0.7
Precipitated P (mmol.l-1)
(t)
where –dCt/dt is the precipitation rate of phosphate, and 7.659 is a
(viii)
where Ct is the concentration of phosphate at reaction time t and C 0 is
the initial concentration of phosphate in solution. This ratio of
precipitated phosphate to the initial phosphate in solution also
indicates the extent of the precipitation reaction.
Figure 1 shows the effect of pH value on the precipitation of
0.6
0.5
pH 11.0
pH 10.5
pH 10.0
pH 9.50
pH 9.00
pH 8.50
pH 8.00
0.4
0.3
0.2
0.1
0
phosphate. In Figure 1a where the initial Ca/P ratio is 1.67, the
0
precipitated phosphate is plotted as a function of reaction time at
30
different pH values ranging from 8.00 to 11.00. At a given pH value
the precipitated phosphate increases with the proceeding of the
60
90
120 150
Reaction time (minutes)
a.
180
constant related to the reaction conditions including supersaturation,
reaction. With the increasing of pH value the (t) improves evidently.
Within a reaction time of 180 min, the higher the reaction pH value is,
the higher the (t) is. Considering the difference of the precipitation
Precipitated P (mmol.l-1)
0.7
processes, three situations can be distinguished. At pH 8.00 the (t) is
so low that it is less than 8.0% within a reaction time of 180 min. At
pH 8.50 the precipitation reaction can be precisely described by an
empirical kinetic equation as
0.6
pH 11.0
pH 10.5
pH 10.0
pH 9.50
pH 9.00
pH 8.50
pH 8.00
pH 7.50
0.5
0.4
0.3
0.2
0.1
0
3
0
30
60
90 120 150
Reaction time (minutes)
b.
180
Y. SONG, H. H. HAHN, AND E. HOFFMANN
pH value and temperature. It is found that the precipitation rate is
Figure 1. The effect of pH value on the precipitation of phosphate.
inversely proportional to the reaction time. At pH  9.00, the
(a), initial Ca/P ratio 1.67; (b), initial Ca/P ratio 3.33.
precipitation of phosphate is very fast at the beginning, but
afterwards the precipitation rate slows down although the (t) still
increases. Unlike the situation of pH 8.50 where the precipitation
reaction advances gradually, the precipitation reactions at pH values
from 9.00 to 11.00 are fast reactions, which arrive at high efficiencies
100
10 min
Precipitation efficiency (%)
Precipitation efficiency (%)
within 10 min, and the values of (t) depend on pH values.
180 min
80
60
40
20
0
11
10.5
10
9.5
pH value
a.
9
8.5
10 min
100
180 min
80
60
40
20
0
8
11
10.5
10
9.5
9
pH value
b.
8.5
8
7.5
Figure 2. The comparison of precipitation efficiencies at reaction time of 10 minutes and 180 minutes.
Ca/P ratio of the precipitate
Ca/P ratio of the precipitate
(a), initial Ca/P ratio 1.67; (b), initial Ca/P ratio 3.33.
4
pH 11.0
pH 10.5
pH 10.0
pH 9.50
pH 9.00
pH 8.50
3
2
1.67
30
60
90
120
150
pH 11.0
pH 10.5
pH 10.0
pH 9.50
pH 9.00
pH 8.50
pH 8.00
3
2
1.67
1
0
4
1
0
180
30
60
90
120
150
180
Reaction time (minutes)
b.
Reation time (minutes)
a.
Figure 3. The Ca/P ratios of the precipitate. (a), initial Ca/P ratio 1.67; (b), initial Ca/P ratio 3.33.
This can be seen more clearly from Figure 2, in which the values of
11.00. With the increasing of pH value the (t) improves evidently,
(t) at reaction times of 10 min and 180 min are compared. Figure 1b
too. Here again three different situations can be distinguished. At pH
plots the precipitated phosphate as a function of reaction time, where
7.50 the (t) is low, only less than 5.0% within a reaction time of 180
the initial Ca/P ratio is 3.33 and the pH values ranging from 7.50 to
min. When pH  8.50, fast precipitation reactions occur and the
4
Y. SONG, H. H. HAHN, AND E. HOFFMANN
values of (t) also depend on pH values. An interesting phenomenon
Figure 4. The effect of the initial Ca/P ratio of solution on the
appears at pH 8.00: at first the precipitation advanced gradually, but
precipitation of phosphate. (a), pH 9.00; (b), pH 8.00; (c), pH 7.50.
after a reaction time of 90 min the precipitation reaction accelerated,
The effect of the initial Ca/P ratio of solution on the Ca/P ratio of
at the reaction time of 180 min the (t) became so high that it got
close to the reaction where pH value was 9.00. The above
phenomenon deserves special attention.
0.70
Precipitated P (mmol.l-1)
Besides the precipitated phosphate, the precipitated calcium and
the base consumption of the precipitation process versus reaction
time also show the same tendency as shown in Figure 1. This, on one
hand, confirms the result of the phosphate precipitation, on the other
hand, improves our understanding of the precipitation of calcium
phosphate. According to the precipitated calcium and phosphate, the
Ca/P ratio of the precipitate can be calculated, and the results are
0.60
0.50
0.40
0.30
Ca/P 1.67
Ca/P 3.33
Ca/P 5.00
0.20
0.10
0.00
illustrated in Figure 3. It can be found that both at initial Ca/P ratio
0
of 1.67 and of 3.33, if the pH value is as high as 9.00 and 8.50,
30
respectively, to realise fast precipitation, the precipitates will have
Ca/P ratios ranging from 1.2 to 2.0, and at the reaction time of 180
60
90
120
Reaction time (minutes)
a.
150
180
150
180
150
180
min will get close to 1.67, the Ca/P ratio of HAP. At a given pH
value, with the proceeding of the precipitation reaction, the Ca/P
0.70
Precipitated P (mmol.l-1)
ratio of the precipitate has an increasing tendency. But under the pH
value of fast precipitation, i.e. where the precipitation reaction
advances gradually and where acceleration phenomenon exists, the
Ca/P ratio of the precipitate may be higher than 2.0, even above 3.0
at the early stage of the reaction. With the proceeding of the
precipitation reaction, the Ca/P ratio goes down, and tends to get
close to 1.67. The above evolution phenomenon of Ca/P ratio means
that no matter what kind of precipitation process the reaction
follows,
the
final
precipitate
tends
to
transform
Ca/P
Ca/P
Ca/P
Ca/P
0.60
0.50
1.67
3.33
5.00
6.67
0.40
0.30
0.20
0.10
0.00
to
0
30
thermodynamically stable calcium phosphate, HAP.
Precipitated P (mmol.l-1)
The Effect of the Initial Ca/P Ratio of Solution on the Precipitation of
Calcium Phosphate
Figure 4 shows the effect of the initial Ca/P ratio of solution on the
precipitation of calcium phosphate at pH values of 9.00, 8.00, and
7.50. In Figure 4a where the pH value is 9.00, the precipitation
reactions are all fast. With the increasing of initial Ca/P ratios from
1.67 to 5.00 the precipitated phosphate increases, so the precipitation
60
90
120
Reaction time (minutes)
b.
0.70
Ca/P 5.00
0.60
Ca/P 6.67
0.50
0.40
0.30
0.20
0.10
0.00
efficiency (t) improves, but the efficiency is not simply directly
0
proportional to the initial Ca/P ratio. In Figure 4b where the pH
value is 8.0, the precipitation reactions show the typical acceleration
characteristic which has already been shown in Figure 1b. With the
30
60
90
120
Reaction time (minutes)
c.
the precipitate is shown in Figure 5. In Figure 5a where pH value is
increasing of initial Ca/P ratio the (t) improves. At the initial Ca/P
9.00, it can be found that higher initial Ca/P ratio does not produce
ratio of 1.67 the (t) was low; at the initial Ca/P ratios  3.33,
precipitate with higher Ca/P ratio, but on the contrary. When the
acceleration phenomena appeared in all the reactions after the
initial Ca/P ratios are 1.67, 3.33, and 5.00, the corresponding
reaction time of 90 min, and finally comparatively high precipitation
precipitates at the reaction time of 180 min have Ca/P ratios of 1.85,
efficiencies were achieved. The outstanding situation appeared at the
1.73, and 1.51, respectively, which are still in the transformation
initial Ca/P ratio of 3.33: at the early stage of the reaction it fell far
process to HAP. In Figure 5b where pH value is 8.00, when the initial
behind the reactions which have higher initial Ca/P ratios, but after
Ca/P ratio is 3.33, the precipitate has the highest Ca/P ratio, and
the reaction time of 90 min it accelerated so much that the (t) at 180
when the initial Ca/P ratios are 5.00 and 6.67 the precipitates have
min got close to the reactions with higher initial Ca/P ratios. In
similar Ca/P ratios. At the reaction time of 180 min, all the
Figure 4c where the pH value is 7.50, the situations with initial Ca/P
precipitates obtained from reactions with different initial Ca/P ratios
ratios of 5.00 and 6.67 are shown. It is worth noticing that at the
have Ca/P ratios close to HAP. The above results indicate that initial
initial Ca/P ratio of 6.67, an obvious acceleration phenomenon is also
Ca/P ratio may influence the precipitation rate and efficiency, but the
observed. This is similar to the situation of pH 8.00.
final precipitates tend to have stoichiometric compositions of HAP.
5
Y. SONG, H. H. HAHN, AND E. HOFFMANN
except DCPD, which has a very low S value and the corresponding SI
Ca/P ratio of the precipitate
is not shown. The SI with respect to HAP is nearly directly
proportional to pH value. The SI with respect to OCP increases with
3
Ca/P 1.67
Ca/P 3.33
Ca/P 5.00
the increasing of pH value until pH 9.50, above which begins to
decrease slightly, and so is the situation of TCP, which arrives at a
highest SI value at pH 10.50. According to the Ostwald Rule of Stages
2
in Precipitation, although HAP is the most thermodynamically stable
1.67
species, it does not occur first and studies have shown that OCP is
the main precipitating phase at medium supersaturations [5]. In the
present studies the SI with respect to OCP is taken to discuss the
1
0
30
60
90
120
150
precipitation reactions. Comparing Figure 6a with Figure 1a, at pH
180
8.00 the SI is only 5.95, so (t) is low; at pH 9.00 the SI is 7.49 and fast
Reaction time (minutes)
a.
precipitation occurs; at pH 8.50 where SI is 6.86, precipitation
advances gradually. Comparing Figure 6b with Figure 1b, at pH 7.50
Ca/P ratio of the precipitate
where SI is 5.81, (t) is low; at pH 8.50 where SI is 7.78, fast
precipitation occurs; at pH 8.00 where SI is 6.93, a special acceleration
Ca/P 3.33
Ca/P 5.00
Ca/P 6.67
3
phenomenon appears.
Figure 6. The effect of pH value on the supersaturation. (a), initial
Ca/P ratio 1.67; (b), initial Ca/P ratio 3.33.
2
Through
1.67
the
above
comparisons,
can
be
found
that,
occurence and precipitation efficiency of calcium phosphate. It is
1
0
30
60
90
120
150
noticeable that after the fast precipitation of the early stage, the
180
following precipitation becomes very slow (Figure 2), this is because
Reaction time (minutes)
b.
the supersaturation of the solution has decreased to a low level, the
driving force is not so strong any more. The present experiment is
Figure 5. The effect of the initial Ca/P ratio of solution on the Ca/P
only a batch system without phosphate and calcium supplement
ratio of the precipitate. (a), pH 9.00; (b), pH 8.00.
25
Saturation index
DISCUSSION
The Effect of pH Value on the Supersaturation of Calcium Phosphate
The precipitation of calcium phosphate from solution is a base
uptake process. Because the process is related to the equilibria listed
in Table 2, the uptake of base promotes the dissociation of the
hydrogen-containing
phosphate
species,
then
promotes
HA P
OCP
TCP
20
15
10
5
0
the
7.5
8
8.5
precipitation of calcium phosphate. To a given precipitating system
without base addition, the pH value will decrease with the
proceeding of the precipitation reaction. In the present experiments,
9
9.5
pH value
b.
10
10.5
11
during the reaction, so the fast precipitation rate can not continue.
the pH value of the precipitating system is kept constant by base
However, at a continuous system the fast precipitation could be kept,
addition, so the pH value of the system affects the speciation of
provided that suitable pH value and concentrations of phosphate and
phosphate and the quantity of the base uptake during the
calcium permit. From the above discussion it can be found that the
precipitation. The effect of pH value on the precipitation of calcium
pH value of the precipitating system influences the supersaturation,
phosphate can be analysed by the calculation of the supersaturation
then the precipitation rate and efficiency.
of the precipitating system.
Figure 6 plots the initial SI values with respect to different kinds of
The Effect of the Initial Ca/P Ratio on the Supersaturation of Calcium
calcium phosphate phases. It is found that the present precipitating
Phosphate
systems are highly supersaturated with respect to HAP, OCP, TCP,
Figure 7.
20The effect of initial Ca/P ratio on the supersaturation. (a),
20
25
HA
HAPP
HA P
OCP
TCP
20
pH 8.00; (b), pH 7.50.OCP
OCP
TCP
TCP
15
15
Saturation
Saturation index
index
Saturation index
it
thermodynamically driving force, supersaturation accounts for the
15
10
5
6
0
7.5
8
8.5
9
9.5
pH value
a.
10
10.5
11
10
10
55
00
11
22
33
44
55
Initial
Ca/P
ratio
Initial Ca/P ratio
a.
b.
66
77
Y. SONG, H. H. HAHN, AND E. HOFFMANN
precipitation efficiency can be obtained at low pH values such as 8.00
Figure 7 shows the effect of initial Ca/P ratio of the solution on the
and 7.50.
supersaturation of calcium phosphate. The SI values are directly
proportional to the initial Ca/P ratio. At pH 8.00 the SI values with
The Composition of the Precipitate
respect to OCP are 5.95, 6.93, 7.45, and 7.77 at initial Ca/P ratios of
1.67, 3.33, 5.00, and 6.67, respectively. Comparing this with Figure 4b,
From Figure 3 and 5, it is found that the Ca/P ratio of the
it can be found that at initial Ca/P ratios of 5.00 and 6.67, the SI
precipitate continuously changes with the proceeding of the
values are high enough, so at first fast precipitations occurred, then
precipitation. If the reaction is a fast precipitation, the Ca/P ratio of
the reaction proceeded slowly until the reaction time of 90 min, after
the precipitate will increase slightly to arrive at a value close to 1.67;
which the acceleration phenomenon appeared. At pH 7.50 and initial
if the reaction advances gradually or accelerates after a second phase
Ca/P ratios  3.33, the SI values are so low that the values of (t) are
forms, the initial Ca/P ratio of the precipitate may be fairly high,
also low; at initial Ca/P ratios of 5.00 and 6.67, where the SI values
with the proceeding of the reaction, the Ca/P ratio decreases and has
are 6.35 and 6.70, respectively, acceleration phenomena appears, too.
the tendency to get to 1.67. The present precipitating system is a
The increasing of the initial Ca/P ratio increases the supersaturation
complex system, for several mineral phases with different Ca/P
of the precipitating system, so the thermodynamically driving force
ratios may occur in the precipitation process, and the formed phases
of the precipitation becomes stronger.
may transform to other phases, so the calculated Ca/P ratio is a
comprehensive reflection of the above reactions. For this reason, it is
difficult to distinguish the precipitated mineral phases only
The Accelaration Phenomena at pH 8.00 and 7.50
according to the Ca/P ratio. In the precipitating process the new
As stated before, at a precipitating system with pH 8.00 or 7.50, if
precipitates had developed surface area, in the gradually advanced
the SI value with respect to OCP is suitable, an acceleration
reaction or the reaction containing the acceleration phenomenon,
phenomenon will appear after the reaction time of 90 min. In the
excessive calcium ions might be adsorbed by the new precipitates,
present experiments, at pH 8.00 with initial Ca/P ratios from 3.33 to
thus causing the high Ca/P ratios of the precipitates, especially at the
6.67 (SI values from 6.93 to 7.77), and at pH 7.50 with initial Ca/P
early stage of the reactions. With the proceeding of the reaction, more
ratios from 5.00 to 6.67 (SI values from 6.35 to 6.67), acceleration
phosphate was combined to precipitate to the solid phase, then the
phenomena are observed after the reaction time of 90 min, and the
Ca/P ratio of the precipitate went down to get close to the
final precipitation efficiency after a reaction time of 180 min may get
stoichiometric value of the HAP.
close to or even surpass the reactions with high pH values (Figure
1b). Although the above phenomenon occurs at certain SI value, it
CONCLUSIONS
seems a characteristic of this special pH range, from 7.50 to 8.00. One
evidence is that at initial Ca/P ratio of 1.67, although the SI values
In the present paper, a chemically defined system with a low
with respect to OCP are 6.86, 7.48, and 7.72 at pH values of 8.50, 9.00,
phosphate concentration of 20 mg P.l-1 and an ionic strength
and 9.50, respectively, no acceleration phenomena are observed.
In a kinetic study on the precipitation of calcium phosphate, Van
equivalent to wastewater was designed to study the effects of the pH
Kemenade and De Bruyn [5] used a parameter, relaxation time t R, to
value and the initial Ca/P ratio of solution on the precipitation of
study the precipitation kinetics. To a single growing phase in a
calcium phosphate. It is found that both the increasing of solution pH
precipitating system, the relaxation time is the time from zero to the
value and the increasing of initial Ca/P ratio favour the precipitation
inflection point of the sigmoidal kinetic curve, where the
efficiency and precipitation rate of calcium phosphate. The
precipitating rate reaches its maximum value. In a comlex-
precipitating system with an initial Ca/P ratio of 1.67, the
precipitating system where the precipitation involves the growth of
stoichiometric Ca/P ratio of HAP, will realise fast precipitation at pH
more than one phase, the kinetic curve may have more than one
value of 9.00; if the initial Ca/P ratio increases to 3.33, the pH value
inflection to indicate the formation of different phases, so relaxation
to achieve fast precipitation will decrease to 8.50. At a given pH
time for each precipitating phase can be defined. To a given
value, the increasing of the initial Ca/P ratio increase the
precipitating system, the relaxation time of a certain phase is highly
precipitation efficiency and the precipitation rate. When the initial
reproducible. Analysing the kinetic curve of the present experiments
Ca/P ratio attains 5.00, the pH value to achieve fast precipitation can
at pH 8.00 or pH 7.50, where an acceleration phenomenon occurs
be lowered to 8.00, and at an initial Ca/P ratio of 6.67, the
after the reaction time of 90 min, it is found that the precipitation is a
precipitation efficiency at pH value of 7.50 can also reach a high
two-stages precipitation. The relaxation time for the second phase at
value of 83% after a reaction time of 180 min. It is noticeable that at
pH 8.00 and initial Ca/P ratio 3.33 is 115 min. The second phase is
the pH range from 7.50 to 8.00, the precipitation may accelerate with
also a fast growing phase, so the final precipitation efficiency is high.
a second phase formation after a reaction time of 90 min, and the
It is difficult to identify the phases in the process of precipitation only
final precipitation efficiency can reach a fairly high value. Because of
by the present Ca/P ratios of the precipitates, for the precipitates
this phenomenon, well designed precipitating system at low pH
may absorb some calcium and phosphate ions, instrumental methods
values could be considered. This, on one hand, can save the base and
are needed to distinguish different phases of the precipiates. In the
calcium needed for the recovery of phosphate, on the other hand,
process of phosphate precipitation, with a proper design the high
avoids the problem of the strong basicity of the effluent after the
phosphate recovery process.
7
Y. SONG, H. H. HAHN, AND E. HOFFMANN
The effects of pH value and initial Ca/P ratio on the precipitation
of
calcium
phosphate
are
due
to
their
effects
on
phosphate at defined conditions similar to the phosphate recovery
the
from wastewater.
thermodynamically driving force of the precipitation reactions. No
matter how the pH value and the initial Ca/P ratio of the
ACKNOWLEDGEMENTS
precipitating system vary, after a definite reaction time the final
precipitates tend to get close to the Ca/P ratio of HAP, this is the
The
reflection of the formation of calcium phosphates and transformation
authors
would
like
to
thank
Graduiertenkolleg
366
"Grenzflächenphänomene in aquatischen Systemen und wässrigen
to HAP.
The present paper focuses on the precipitation of a low phosphate
Phasen" for the financial support to Dr. Yonghui Song. Thanks also
concentration of 20 mg.l-1 at wide pH values ranging from 7.50 to 11.0
go to Prof. Fritz H. Frimmel of the Engler-Bunte-Institute,
and wide initial Ca/P ratio values ranging from 1.67 to 6.67. It differs
Department of Water Chemistry of the University of Karlsruhe, Dr.
from the traditional studies of the precipitation of calcium phosphate,
Dietfried Donnert of the Institute for Technical Chemistry of the
which were mainly dedicated to the physiological considerations, i.e.
Karlsruhe Research Center Technology and Environment, and Dr.
higher temperatures and pH value around 7.4. This study has
Ernst Antusch who was formerly a member of the authors’ institute
improved the understanding of the precipitaion of calcium
for their kind help to the research.
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9