Interactions between calcium phosphate and heavy metal ions in aqueous
solution
Fernane Farida1, Boudia Saliha1, Saouli Hamida1
1
Laboratoire de Chimie Appliquée et de Génie Chimique (LCAGC), UMMTO, Algérie
Corresponding author email: [email protected]
Keyword: Calcium Phosphate; co-precipitation; hydroxyapatite, copper ions, nickel ions
Introduction
Several solid sorbents have been proposed or are
already used for the decontamination of industrial
liquid wastes for toxic or radioactive elements: ionexchange resins, charcoal, metallic iron, zeolites,
apatites, hexacyanoferrates, clays, etc... Calcium
hydroxyapatites have been proposed for the fixation
of several toxic divalent metals ions such as
cadmium, lead, copper, uranium, from aqueous
solutions [1−3].
The use of materials fixative such as apatites
showed that these last ones present a big efficiency
in the decontamination of industrial waste These
fixative mineral resist at radiations, remain stable in
a big domain of pH and present a big affinity for lot
of ions such as copper, lead , cadmium, nickel etc
[4-6].
Apatites constitute a family of compounds whose
general formula is Me10(XO4)6Y2, where Me is a
divalent metal, X is phosphorus, possibly
substituted by other elements such as As, V, C, Y is
constituted of hydroxide, fluoride, chloride or
carbonates ions. The basic crystal structure of these
compounds is hexagonal [1]. One of the most
common apatite is calcium hydroxyapatite (Hap),
whose theoretical formula is Ca10 (PO4)6(OH) 2
.Non stoichiometric Hap constitutes the mineral
component of hard tissues (bone, dental enamel,
etc.) of mammals [6−10].
Study interaction between metal ions such as
cadmium, zinc, copper and nickel with apatite is of
most interest both for the development of
decontamination methods for liquid wastes and for
the investigation of processes in human and animal
hard tissues [11-13].
It is generally agreed that heavy metal sorption
occurs on apatite at the surface of the particles and
that an ion exchange process leads to the fixation of
the toxic metal as an insoluble phosphate while a
calcium ion is released into solution. However, the
retention mechanisms are not fully elucidated and
may include ion exchange with diffusion into the
sample, or precipitation of new solid phases,
depending on the conditions and the reactivity of
the substituted Hap [4 - 7].
Synthetic and natural phosphates of calcium were
tested for reduction metallic pollution in aqueous
solution. Phosphates of Calcium with ratio Ca/P
understood between 1,33 and 1,67 are fluently
called apatites. They have a strong capacity to
immobilize metallic ions when they are put in touch
of aqueous solutions. Ca2+ ions can substituted
completely or partly by cations such as toxic
metallic ions (Ni2+; Cu2+; Co2+ and Cd2+).PO43− ions
can be replaced by anions such as AsO43-, CO32-,
…etc [7-10].
Sorption experiments of divalent metallic ions such
as Ni2+and Cu2+on apatites were led in static and
dynamic modes. Several operating parameters were
studied (pH, initial concentration in metallic cation,
contact time; temperature…).
Experimental
1-Materials and methods
The natural Hap used (NA) was constituted of a
variety of human teeth, extracted for prophylactic
reasons, and cleaned with a mixture of 10%
hydrogen peroxide and sodium hypochlorite during
24 hours, then rinsed with 1% nitric acid solution
followed by drying at 30°C during 24 hours. The
bleached teeth were ground in a mortar, and then
wet sieved to collect the fragments between 25 and
250 µm. This fraction was then dried at 60°C
during 8 hours.
The synthetic Hap (SA) was a Bio-Rad® calcium
phosphate referenced DNA Grade Bio-Gel HTP
130-0420.
The tricalcic phosphate (TCP) with theoretical
formula Ca3(PO4)2 was prepared by haste at neutral
pH. TCP was form by mixture of CaCl2 and
Na2HPO4 according the following equation:
3 CaCl2 + 2Na2HPO4 → Ca3(PO4)2 + 4NaCl + 2HCl
Chemical composition of apatite NA and SA was
determined by inductively coupled plasma atomic
emission spectrometry (ICP/AES), with a Varian
Vista spectrometer.
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70
60
50
T%
Some features evaluated for the two apatites
studied: specific surface evaluated by the N2-BET
method with a Coulter-SA3100 device, as well as
the formula determined by ICP/AES are given in
the Table 1.
2- Sorption experiments
Experiments were performed in polyethylene
bottles containing known amount of sorbent in the
suspension of 50 mL of cupric or nickel nitrate
solutions at range concentration between 2 and 800
mg/L and with constant agitation (120 rpm) in a
horizontal. The apatite suspensions were shaken at
20°C or 40°C and at pH 4 or 5 for times varying
from 2 to 600 min. The solutions were then filtered
on a 0.2 µm porosity filter and analyzed for Cu and
Ca by A.A.S. The pH of the solutions was
measured before and after the equilibration
experiments.
Other experiments were performed by dissolution precipitation of NA, SA or TCP apatites with metal
ions together.
40
30
20
10
0
4000
3500
3000
2500
2000
1500
1000
500
nombre d'onde (cm-1)
Figure 3- IRTF Specter of TCP apatite
Spectre IRTF presented on figures 1 to 3 shows the
presence of the main peaks of phosphates and
characteristic hydroxides of hydroxyapatites.
3- Results and discussion
3-1- Results material characterization
Table 1: Features of NA and SA apatites
Specific
area (m2/g)
Ca/P ration
Formula
Apatite NA
Apatite SA
1.5
1,48
Ca8,92 (PO4)6 Na 0. 32
77
1,37
Ca8.22(PO4)6 Na 0.48
Figure 4- MEB picture of NA apatite
Figure 5- MEB picture of SA apatite
Figure 1- IRTF Specter of NA apatite
Figures 4 and 5 show that powders of NA and SA
apatite are endowed with micro porous structure
3-2- Kinetic sorption
Kinetics sorption of Ni (II) and Cu (II) on SA and
NA apatites are similar to those presented on figure
6.
The steady state sorption is reached after 2 hours of
shaking. We note that the behavior of the two Haps
to the weak concentrations is nearly identical.
Figure 2- IRTF Specter of SA apatite
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Qe (mg/g apatite)
amount Ni sorbed (mg per g
apatite)
10
8
6
4
SA
NA
2
0
0
50
100
150
200
250
80
70
60
50
40
30
20
10
0
0
152
Time (mn)
The results confirm efficiency of these phosphates
calcium to decontaminate, by adsorption or coprecipitation, metallic ions such as Ni2+ or Cu2+
present in aqueous solution at concentrations
between 2 to 200 ppm. The returns on elimination
border 100 %.
3-3- Isotherm sorption
In order to compare the sorption capabilities of the
NA and SA apatite, we have drawn sorption
isotherms, which represent the sorbed concentration
Qe (in mg per g of apatite) of the metal after
equilibration versus the remaining concentration in
the liquid phase Ce (in mg.L-1), for initial
concentration varying from 2 to 800 ppm.
Several mathematical models have been developed
to quantitatively express the relationship between
the extent of sorption and the residual solute
concentration. The most widely used models are the
Langmuir and Freundlich adsorption isotherm
models.
The nickel and copper adsorption capacities of the
NA or SA apatite were calculated as the Eq. (1)
below:
(
)
Eq.(1)
Where m is amount of apatite and C0 initial
concentration of metal (Cu2+ or Ni2+)
Results are similar of those shown in figures 7 and
8.
Results allowed reporting that the model of
Langmuir describes better the isotherms of sorption
of Ni2+ but model of Freundlich for copper ions
NA, pH2
SA, pH1
SA,pH2
304
456
608
Ce (mg/L)
Figure 7 - Isotherm sorption of Cu on both
apatite SA and NA at 40°C for two value of pH
(pH1 = 4; pH2 = 5)
100
Qe (mg/g apatite)
Figure 6- Kinetic sorption of Ni(II) on synthetic
(SA) and natural (NA) apatite in batch
experiments (20°C, 60ppm and pH 4)
NA,pH1
80
60
40
NA;pH1
SA;pH1
20
NA;pH2
SA;pH2
0
0
165
330
495
Ce (mg/L)
660
Figure 8 - Isotherm sorption of Cu on both
apatite SA and NA at 20°C for two value of pH
(pH1 = 4; pH2 = 5)
Conclusion
This study confirmed the capacity
of
hydroxyalapatite (Hap) to sorption copper or nickel
ions from aqueous solutions under batch conditions.
Several features can be deduced from this study:
Copper and nickel ions are well sorbed, sorption is
pH and temperature dependent. Removal of copper
or nickel ions from aqueous solutions is efficient on
either synthetic or natural Hydroxyapatiteor by coprecipitation of metallic phosphate. The values
obtained for Qs confirm that Hydroxyapatite has
very favorable sorption efficiency towards copper
and nickel ions. These properties should lead to
practical
applications
in
waste
water
decontamination.
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