PRECIPITATION OF CALCITE IN THE PRESENCE OF INORGANIC
PHOSPHATE
L.J. PLANT* AND W.A. HOUSE
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester,
Dorset DT2 8ZD, UK. Email: lop@ceh.ac.uk.
ABSTRACT
The precipitation of calcite from Ca(HCO3)2 solutions was studied over a wide range of dissolved
inorganic phosphorus concentrations (0- 500 mol dm-3 KH2PO4) and a dissolved calcium concentration
of 2.5 mmol dm-3 at 10 0C. At low phosphorus concentrations (< 20 mol dm-3), calcite growth was
retarded and phosphate ions were coprecipitated according to theory and gave a phosphate surface density
of coprecipitated phosphate of 0.150 mol m-2. As the phosphorus concentration was increased further,
calcite growth ceased and a separate calcium phosphate phase was formed. In these experiments CO32ion concentrations were < 190 mol dm-3. The calcite seed played no part in the nucleation of the
calcium phosphate phase as the growth rates were found to be independent of the amount of calcite seed
material used in the reactions. The boundaries for the homogeneous nucleation of calcium phosphate were
investigated by simple solution mixing of Ca(HCO3)2 and KH2PO4 over a wide range of supersaturations
and quantified in terms of calcium hydroxyapatite, tricalcium phosphate and octacalcium phosphate
saturation indices.
Keywords: Phosphorus, calcite precipitation, calcium phosphate minerals
INTRODUCTION
There is a great deal of information available in the literature about the precipitation of calcite and the
effects of inhibitors on the kinetics [1]. There have also been studies of the effects of inorganic phosphate
on the crystal growth of calcite in both laboratory studies [2, 3, 4] and in field studies of hardwater lakes
and rivers [5, 6, 7]. There is no doubt from these studies that phosphate in solution inhibits the formation
of calcite by retarding the precipitation reaction and is coprecipitated as part of the calcite lattice [3, 8]. At
low phosphate concentrations, typically < 20 mol dm-3, the coprecipitated phosphorus ions are
incorporated in the calcite surface with no evidence for a distinct calcium phosphate phase being formed
[8]. However, at higher phosphate concentrations there is the possibility that the calcite surface will
favour the formation of a distinct calcium phosphate phase and therefore promote the over-growth of a
calcium phosphate where calcite growth is impaired [9].
The present study extends previous seeded growth work on the coprecipitation of phosphate with calcite
at low phosphate concentrations (< 20 mol dm-3) to examining the effects of higher phosphate
concentrations (up to 500mol dm-3) more typical of the concentrations in treated sewage effluent.
EXPERIMENTAL
All chemicals were AR Grade unless otherwise specified. Solutions were prepared using ultra pure water
(Purite Select, Analyst HP) of conductivity < 1 S cm-1. The pH glass electrode (Radiometer GK2401C)
was calibrated before each experiment using freshly prepared NBS buffer solutions, KH 2PO4 / Na2HPO4
and Na2B4O7.10H2O [10]. The calcite seed material was AR Grade calcium carbonate, identified by XRay diffraction as calcite, with a specific surface area as determined by nitrogen gas adsorption and BET
analysis of 0.23 m2 g-1.
Free-drift experiments
Treated sewage effluents are best mimicked using a Ca(HCO3)2 solution as a base that often contributes
approximately 90 % of the ionic strength of real systems. A stock calcium bicarbonate solution (5 mmol
dm-3) was prepared by dissolving calcium carbonate in water in the presence of carbon dioxide gas. The
stock solution was diluted two-fold to give working solutions for the seeded growth experiments.
Appropriate volumes of a 0.01 mol dm-3 KH2PO4 were added to the calcium bicarbonate solution to give
concentrations of phosphorus between 3 and 18 mol
dm-3. In later experiments the phosphorus
concentration was increased to 50, 100, 250 and 500 mol dm-3 by addition of 0.1 mol dm-3 KH2PO4.
All experiments were carried out in a 1 litre polypropylene reaction vessel immersed in a water
thermostat (Heto BirkerØd). The vessel was continuously stirred at ca 200 rpm and left to equilibrate at
10 +/- 0.1 ºC (unless otherwise specified). The pH of the solution was adjusted by bubbling with nitrogen
gas to remove dissolved carbon dioxide to achieve a desired calcite supersaturation. For experiments at
lower phosphorus concentrations of 3 – 18 mol dm-3, an equilibrium period of approximately 20 minutes
was allowed. However at the higher phosphorus concentrations of 50, 100, 250 and 500 mol dm-3,
nitrogen gas was bubbled slowly through the solution for the entire experiment to maintain a low carbon
dioxide concentration and high supersaturation with respect to calcite growth.
The crystallisation of calcite was initiated by the addition of a known amount of calcite seed (ca 800 mg)
as a suspension of concentration of 400 mg g-1. The pH and conductivity were monitored in the solution
prior to seed addition, and for the duration of the experiment using a Radiometer PHM64 research pH
meter and a Wayne Kerr automatic precision bridge B905 respectively. At time intervals samples (40 ml)
were removed through a glass frit, filtered through a 0.45 m cellulose nitrate membrane, and analysed
for total dissolved calcium and phosphorus. The established ‘Murphy and Riley’ colorimetric method
[11], using a Beckman DU520 spectrophotometer, was employed for phosphorus determination (limits of
detection, 0.3 mol dm-3). Calcium was determined using bench-top EDTA titration, where the limit of
detection was controlled by one drop and was ca. 0.4 mol dm-3 [12]. A series of similar seeded growth
experiments were also performed at an initial dissolved phosphorus concentration of 100 mol dm-3 using
a range of seed amounts to promote crystal growth (0 to 4000 mg calcite seed).
Experiments without calcite seed
A series of experiments were carried out in test tubes at room temperature to investigate the conditions for
homogenous precipitation of calcium phosphate minerals. Small volumes of KH2PO4, (0.5 mmol dm-3 to
0.5 mol dm-3), were added to a calcium bicarbonate solution (2.5 mmol dm-3). The solutions were mixed
manually in the test tube and viewed using a Rank Brothers microelectrophoresis apparatus immediately
after mixing. The field of view was divided into 50 m grid enabling sub-micron particles to be detected.
The presence or absence of a precipitate was noted.
THEORY
Solution composition
The kinetics of precipitation of phosphate with calcium, either as a coprecipitate with calcite or the
formation of a distinct calcium phosphate mineral, is controlled by the solution composition at a fixed
temperature. The chemical speciation of solutions used in the present experiments were determined using
a speciation program, PHREEQ [13]. The program was modified to include other mineral phases likely to
form as a precursor to the thermodynamically stable calcium hydroxyapatite (HAP), such as dicalcium
phosphate dihydrate (DCPD), octacalcium phosphate (OCP) and crystalline tricalcium phosphate (TCP).
The temperature dependence for the solubility data of each mineral phase was obtained from studies
reported in the literature and entered using the format shown in Table 1. The solubility data for
amorphous phases of tricalcium phosphate was not suitable for a temperature of 10 ºC [18].
The saturation index (SI) and Ion Activity Product (IAP) were output from the speciation program, e.g.
for calcite:
SI  log 10
IAP
K calcite
IAP  aCa 2 aCO 2
(i)
(ii)
3
where ai is the activity of the ith ion, and Kcalcite is the solubility product of calcite at the temperature of
the reaction.
Coprecipitation kinetics.
A simple two-component adsorption model relating the simultaneous precipitation of calcite and
adsorption of inorganic phosphorus on the calcite surface can be used to describe the reaction
coprecipitation kinetics [19]. The literature proposes the combination of the relevant isotherm equation
and an equation describing the growth of calcite. Assumptions relating to precipitation are made in order
to derive a general coprecipitation equation [19]:
t
dnCaT

 N A   hsol 
dt
dt
0
dnPT
where nPT




(iii)
= Total number of moles of inorganic phosphorus coprecipitated.

= Surface density of coprecipitated phosphorus (mol m-2).
NA
= Avagadros number (6.022 x 1023 mol-1)

= Molecular area of CaCO3 on the surface (20.101 x 10-20 m2)
nCa T
= Number of moles of calcium containing species
(total dissolved calcium)
h(sol) = Function describing how the form of the isotherm depends on the solution (sol) under
consideration.
t
= time / min
This can be further simplified to give:
n PT  N Ag (t )
(iv)
t
where
g (t )   h( sol )dnCaT
0
The function h(sol) illustrates how the form of the isotherm depends on the solution under consideration.
This can be determined by measuring the four independent variables, temperature, pH, total dissolved
phosphorus and total dissolved calcium, and adsorption constants K1 and K2 for PO43- and HPO42respectively as given in a two-component Langmuir adsorption isotherm:
h(sol) 
K1[PO 34 ]  K 2 [HPO 24 ]
1  K1[PO 34 ]  K 2 [HPO 24 ]
(v)
The surface density parameter, , (equation iii) is an important indicator of the amount of phosphorus
coprecipitated on the surface of calcite. The surface density parameter, , was calculated from equation
(iv), i.e. a plot of the amount of phosphorus coprecipitated Δ n PT against g(t) to yield a slope of NA.
RESULTS AND DISCUSSION
Seeded growth experiments at low phosphorus concentrations
The results from six experiments obtained using the seeded growth method in free-drift conditions are
summarised in Table 2. All experiments were carried out at 10 +/- 0.1 C, except for experiments 1 and 3
which were performed at 20 +/- 0.2 C. It is shown that the higher temperature results in a greater
saturation index with respect to HAP and a lower surface density of coprecipitated phosphorus on the
calcite surface in comparison to experiments at 10 +/- 0.1 C. The experiment without phosphate addition
gave the most rapid kinetics with a fast decrease in the solution pH at the beginning of the reaction as
calcite formed from the Ca(HCO3)2 solution, and then a much slower increase in pH as the reaction
slowed and carbon dioxide gas was lost from the reactor (Figure 1). In the other experiments as the initial
dissolved phosphorus concentration increased, the precipitation reaction became slower. To achieve a
reaction at the higher phosphorus concentrations, the initial pH (and saturation index) was increased as
shown in Table 2. In experiment 6, even with an initial saturation index of 1.63 with respect to calcite, the
reaction was relatively slow as shown by the gradual decrease in pH throughout the reaction (Figure 1).
These results are consistent with earlier findings that dissolved phosphorus inhibited calcite growth but
that the effects of the inhibition could be partly overcome by increasing the initial supersaturation prior to
calcite seed addition [2].
The results from the experiments were analysed using equation (iii) to obtain values of the surface density
of coprecipitated phosphorus, . Pearson’s correlation coefficients of between ca 0.8 and 0.98 were
calculated with values of the surface density given in Table 2. The mean value for all the experiments was
0.150 mol m-2 at 10 0C, which corresponds with 0.150 mol m-2 obtained by House and Donaldson [3]
and is similar to values calculated by other researchers at a similar temperature [4].
Seeded growth experiments at high phosphorus concentrations
In contrast to the previous experiments, the addition of the calcite seed did not produce a decrease in pH
during experiments 7-10. Instead the pH gradually increased during all the reactions as a result of the
continual nitrogen purging with no evidence of carbon dioxide production as a result of calcite
precipitation. As shown in Table 3, all the solutions were greatly supersaturated with respect to calcite
and also calcium phosphate minerals prior to any reaction. However it was noticed that the soluble
phosphorus and calcium concentrations measured in samples taken immediately before the calcite seed
was added to the reactor, were lower than the expected values. This is illustrated in Table 4 where the
differences are particularly marked for experiments 9 and 10, e.g. for the highest phosphorus
concentration over half of the dissolved phosphorus was missing from solution after equilibrating for
approximately 30 min. This indicated that precipitation of some phase was occurring prior to the calcite
seed addition. After the seed addition, the concentrations of calcium and phosphorus decreased slowly
(Figure 2). Changes in dissolved calcium were relatively small and difficult to measure at small time
intervals by the titration method. However, changes in the dissolved phosphorus and calcium over the
entire experiment, i.e. P and Ca respectively, gave much lower molar ratios of Ca/P than obtained
for calcite precipitation at phosphorus concentrations < 20 mol dm-3, i.e. predicted Ca/P ratios are
greater than 50 [20] and compare with an estimate from experiment 6 (Table 2) of Ca/P=90. The
ratios are much lower than this for experiments 7 and 8 (at initial phosphorus of 50 and 100 mol dm-3)
but not low enough to be consistent with the precipitation of a calcium phosphate phase alone. Also the
change in solution composition in these experiments was relatively small compared with experiments 9
and 10, causing larger uncertainties in the Ca/P ratios. The ratios for experiments 9 and 10 (Table 4)
are in close agreement to the ratio expected for calcium hydroxyapatite (HAP), viz 1.67 for HAP
compared with 1.50 for tricalcium phosphate (TCP). The precipitate was mixed with the calcite seed
material and therefore could not be identified separately by X-Ray diffraction. The application of the
results to the coprecipitation model (equation iii) failed to produce linear relationships that were observed
at the lower dissolved phosphorus concentrations.
As precipitation occurred prior to calcite addition (see Table 4), even in experiments at initial
phosphorus concentration of 100 mol dm-3, further experiments were performed to assess whether
precipitation was influenced by the presence of calcite seed. These experiments are listed in Table 3 and
include a reaction without seed addition (experiment 13) and with 800, 1600 and 4000 mg of seed for
experiments 8, 11 and 12 respectively. The reaction kinetics are summarised in Figure 3 with the initial
solution conditions given in Table 3. As shown, the reaction without seed addition gave a similar decrease
in dissolved phosphorus concentration in the reactor compared with experiment 8 with 800 mg of seed,
both starting at a similar saturation index (Table 3). Experiments 11 and 12 with 1600 and 4000 mg of
seed respectively, produced a similar decrease in dissolved phosphorus over a period of ca 2000 min
(Figure 3) and both experiments started with SI values for calcite, HAP and TCP in good agreement. The
initial growth rates for these experiments were calculated for t< 220 min using the soluble calcium
concentrations. Values varied between 0.35 to 0.78 mol min-1 but with no systematic trend with the
amount of seed used in the experiments, e.g. the initial calcium loss rate in the experiment without seed
addition was 0.36 mol min-1 which compares with 0.59, 0.78 and 0.35 mol min-1 for seed additions of
800, 1600 and 4000 mg respectively.
There is evidence from studies in solutions dominated by the Ca(HCO3)2 system that the precipitation of a
calcium phosphate (identified as an amorphous tricalcium phosphate) is possible at a SIHAP of ca. 14.4
corresponding to a solution of pH 8.6, calcium concentration 2.9 mmol dm-3 and a dissolved phosphorus
of 110 mol dm-3 (Temperature of 17 ºC) [21]. The saturation condition is similar to that described for
experiment 8 (Table 3). Other research in the absence of CO32- ions using Ca(NO3)2 and KH2PO4
solutions at 25 ºC, has also indicated the precipitation of an amorphous calcium phosphate phase at SIHAP
values of 14.5 and greater [22]. The amorphous phases subsequently transform to the more
thermodynamically stable phase of calcium hydroxyapatite (HAP), possibly through other precursor
phases [23].
Experiments without calcite seed
The experiments at the higher dissolved phosphorus concentrations indicated that precipitation was
occurring before calcite seed addition. There is evidence to therefore suggest that calcite is ineffective as
a nucleating material for calcium phosphate growth and that in experiments 7-10 homogenous nucleation
is occurring. This is contrary to experiments reported by Donnert [24], involving direct precipitation of
calcium phosphate induced by calcite as the seeding material. Use of calcite seed material was shown to
be effective as a nucleating media, particularly in solutions containing CO32- ions, and increasing the
calcite seed concentration was seen to improve the removal of phosphorus from a “German” waste-water
[25]. However it was also reported that according to recent results the calcite seed material contained
only some 6 % phosphorus after the application of calcite crystallisation [24]. The initial saturation of
initial solutions with respect to HAP was not reported.
The conditions for homogeneous precipitation of calcium phosphate phases is uncertain particularly in
solutions containing CO32- solutions. The results from mixing Ca(HCO3)2 and KH2PO4 solutions at room
temperature (ca 20 0C) are given in Figure 4. Each solution composition used in the experiments was
analysed using the speciation program to obtain values for the saturation indices with respect to calcite,
HAP, OCP, TCP and DCPD (see Table 1). The solutions tested included compositions that were
undersaturated with respect to calcite but supersaturated with respect to the calcium phosphate phases. A
clear division between solutions that spontaneously precipitated and those that did not was observed, and
is indicated by the open and filled symbols in Figure 4. The dashed “tramlines” shown in Figure 4 also
mark the limits of supersaturation with respect to HAP that were found in the seeded growth experiments
7-13 (Table 3). It was found that solutions with a saturation index greater than 9.4 with respect to HAP
(and 2.2 with respect to TCP) were unstable and precipitated a solid spontaneously on mixing the
solutions. As the supersaturations for the initial solutions conditions obtained in the experiments at the
higher phosphorus concentrations were above these values, homogeneous precipitation is expected. This
was confirmed by the initial decrease in dissolved phosphorus and calcium in experiments 8 to 10 (Table
4) measured immediately before seed addition and after mixing the KH2PO4 solution and allowing
thermal equilibration. The results are also consistent with the coprecipitation of phosphate with calcite at
the lower phosphorus concentrations, where initial saturation indices with respect to HAP formation were
< 8.05 at temperatures of 10 +/- 0.1 C (Table 2). Precipitation indicated by loss of dissolved phosphorus
and calcium, before seed addition was not found to occur.
CONCLUSION
At dissolved phosphorus concentrations less than 20 mol dm-3 calcite precipitation is able to occur as
long as the initial supersaturation with respect to calcite is sufficient to promote growth. During
precipitation from Ca(HCO3)2 solutions, inorganic phosphate ions are removed from solution and
coprecipitated with calcite with the kinetics corresponding to a theoretical interpretation proposed
previously. However, as the concentration of dissolved phosphorus increases, calcite precipitation ceases
and there is evidence from the changes in the solution composition, for the formation of a calcium
phosphate phase. This has been confirmed by simple solution mixing experiments with Ca(HCO3)2 and
KH2PO4 solutions covering a wide range of supersaturations in the absence of calcite seeds, but in the
presence of CO32- ions in alkaline solution. These results indicate that homogeneous nucleation of a
calcium phosphate phase occurs at SIHAP>9.4. Further research is needed to identify the phase formed and
how the presence of CO32- ions controls the reaction kinetics. A surprising result from the study is that
calcite was ineffective in promoting the nucleation of a calcium phosphate phase. Precipitation occurred
prior to the addition of the calcite seed material and the results were consistent with homogeneous
nucleation in solution.
ACKNOWLEDGMENTS
We thank Dr A.D. Pethybridge for his comments and help and CEEP for funding this research.
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Table 1. Temperature dependence for the solubility data for calcite and calcium phosphate mineral
phases. Solubility dependence expressed in following form:
log K  A1  A2T  A3/T  A4logT  A5/T 2
where K is the thermodynamic solubility product, T is the temperature (K) and Ai, are coefficients.
Calcium phosphate phase
Temperature dependence of solubility.
Ref
Hydroxyapatite (HAP)
Log K = -8219.41/T-1.6657–0.098215T
14
Dicalcium dihydrogen
Log K = -3649.5701/T+18.180752–0.0420307T
15
Log K = -45723.26/T+287.4536–0.546763T
16
Octacalcium phosphate
Log K = 1039082.71+196.004417T-(48333918.7/T) -
17
(OCP)
(388521.579logT)+(2303600498/T2)
Calcite (cal)
Log K = 13.543 - (0.0401T)-(3000/T)
phosphate (DCPD)
Crystalline Tricalcium
phosphate (TCP)
13
Table 2. Summary of results from the seeded growth experiments with phosphorus concentrations < 20
mol dm-3. Mass of seed in each experiment was ca 1000 mg. Initial data is representative of samples
taken immediately prior to seed addition.
Expt.
Temp ºC
Initial P
Initial pH
Initial SIcalcite
/mol dm-3
No.
Final P
Surface
Initial
/mol dm-3
density
SIHAP
/ mol m-2
1
19.8
0
8.34
1.26
0
-
-
2
9.9
3.3
8.54
1.27
0.17
0.159
4.51
3
19.8
4.7
8.46
1.45
0.03
0.27
9.97
4
10.1
8.3
8.45
1.24
4.0
0.139
5.50
5
10.0
11.4
8.70
1.47
6.1
0.123
6.77
6
9.9
18.4
8.95
1.63
11.5
0.221
8.05
Table 3. Results of the seeded growth experiments at higher dissolved phosphorus concentration at 10 +/0.1 C . Initial P concentrations are calculated from known standard additions. Saturation indices are
given for calcite, calcium hydroxapatite and crystalline tricalcium phosphate.
Expt. No.
Length
of Mass of seed/ mg
Initial pH
Initial P
SIcalcite
SIHAP
SITCP
/mol dm-3
expt./ min
7
2673
ca 800
9.00
50
1.70
13.9
3.76
8
7200
ca 800
8.83
100
1.52
14.3
4.10
9
1486
ca 800
8.67
250
1.38
14.9
4.59
10
5923
ca 800
8.80
500
1.44
16.2
5.41
11
3266
ca 1600
8.67
100
1.42
13.79
3.87
12
2755
ca 4000
8.72
100
1.44
13.92
3.93
13
1649
0
8.93
100
1.66
14.60
4.25
Table 4. Comparison of the seeded growth experiments with the same mass of calcite seed but high
dissolved phosphorus concentrations (50- 500 mol dm-3) at 10 +/- 0.1 C . Ca / P are the molar
amounts of calcium and phosphorus removed from the solution at the end of the experiment.
Expt No.
Nominal
P measured immediately
Initial
Ca measured
P
prior to seed addition
measured Ca
immediately prior to seed
/ mol
/ mol dm-3
/ mmol dm-3
addition
Ca / P
/ mol dm-3
dm-3
7
50
46.1
2.50
2.50
6.0
8
100
95.8
2.44
2.42
10.6
9
250
192
2.46
2.26
1.67
10
500
206
2.47
2.00
1.68
Figure legends
Figure 1. Changes in pH and dissolved phosphorus concentrations in experiments 1 and 6 (conditions
shown in Table 2).
Figure 2. Calcium and phosphorus concentration changes during seeded growth experiments 7-10 at 10
0
C. Time indicated as zero is the time of the calcite seed addition.
Figure 3. Reaction kinetics of seeded growth experiments (< 2000 min) using different seed
concentrations to initiate the reaction (experiments 8, 11-13). Conditions are shown in Table 3.
Figure 4. Calculated saturation indices for experiments without calcite seed, illustrating supersaturations
required for homogenous precipitation. Limits of the initial saturation indices, with respect to HAP, for
experiments 7-13 (Table 3) are indicated by the dashed “tramlines”.